AMMONIA PRODUCTION FROM NITRATE WASTE USING PtRu-BASED CATALYST
20250074779 ยท 2025-03-06
Inventors
Cpc classification
B01J2235/30
PERFORMING OPERATIONS; TRANSPORTING
B01J2235/00
PERFORMING OPERATIONS; TRANSPORTING
H01M8/06
ELECTRICITY
H01M8/1009
ELECTRICITY
C25B11/054
CHEMISTRY; METALLURGY
C01C1/026
CHEMISTRY; METALLURGY
International classification
C01C1/02
CHEMISTRY; METALLURGY
Abstract
Methods for electrocatalytic and thermocatalytic conversion of nitrate using PtxRuy/C catalysts are disclosed herein. The methods for electrocatalytic conversion of nitrate to ammonia can include contacting a nitrate containing source with an electrode comprising a PtxRuy/C catalyst while applying a potential sufficient to reduce nitrate to thereby convert nitrate present in the nitrate containing source to ammonia, wherein the PtxRuy/C catalyst comprises a carbon substrate having PtxRUy nanoparticles disposed thereon, and x is about 48 at % to about 90 at %, and y is 1x.
Claims
1. A method for electrocatalytic conversion of nitrate to ammonia, comprising: contacting a nitrate containing source with an electrode comprising a Pt.sub.xRu.sub.y/C catalyst while applying a potential sufficient to reduce nitrate to thereby convert nitrate present in the nitrate containing source to ammonia, wherein the Pt.sub.xRu.sub.y/C catalyst comprises a carbon substrate having Pt.sub.xRu.sub.y nanoparticles disposed thereon, and x is about 48 at % to about 90 at %, and y is 1x.
2. A method for electrocatalytic nitrate reduction in a flow reactor, comprising: flowing a nitrate containing source into a working electrode compartment of an electrochemical cell while applying a potential to the cathode, wherein: the electrochemical cell comprising a cathode electrode in the catholyte electrode compartment, and an anode electrode disposed in an anolyte electrode compartment, the anode electrode compartment being separated from the cathode electrode compartment by a membrane, the cathode electrode comprises a carbon substrate with a Pt.sub.xRu.sub.y nanoparticles disposed thereon to form a Pt.sub.xRu.sub.y/C catalyst, with x being about 48 at % to about 90 at %, and y is 1x, and upon contact with the Pt.sub.xRu.sub.y/C catalyst nitrate is converted to ammonia.
3. The method of claim 1, wherein the carbon substrate is carbon felt.
4. The method of claim 3, wherein the carbon felt is disposed on a graphite rod.
5. The method of claim 1, wherein the counter electrode comprises a carbon substrate having a conductive catalyst disposed thereon.
6. The method of claim 5, wherein the conductive catalyst is RuO.sub.2, IrO.sub.2 or mixtures.
7. The method of claim 1, wherein the nitrate source comprises an electrolyte and nitrate present in a concentration of about 1 mM to about 1 M.
8. The method of claim 1, wherein the nitrate source has a pH of about 5 to about 7.
9. The method of claim 1, wherein the cathode electrode has a catalyst loading of about 0.1 mg per cm.sup.2 to about 10 mg per cm.sup.2.
10. A method of thermocatalytic conversion of nitrate to ammonia comprising: generating H.sub.2 in an aqueous suspension of a Pt.sub.xRu/C catalyst, wherein x is about 48 at % to about 90 at % and y is 1x; flowing a nitrate containing source into the suspension containing the catalyst and generated H.sub.2, wherein upon contact with the catalyst nitrate in the nitrate containing source is converted to ammonia.
11. The method of claim 10, wherein the aqueous suspension has a pH of about 1 to about 5.
12. The method of claim 10, wherein the aqueous suspension is stirred at a rate of about 100 rpm to about 10,000 rpm, while flowing a nitrate containing source into the suspension.
13. (canceled)
14. The method of claim 10, wherein generating H.sub.2 comprising applying a potential to the aqueous suspension to generate H.sub.2 through water splitting or sparging the aqueous suspension with H.sub.2 gas.
15. (canceled)
16. The method of claim 10, wherein the suspension is maintained at a temperature of about 25 C. to about 90 C. while the nitrate containing source is flowed through the suspension.
17. The method of claim 10, wherein the catalyst is present in the aqueous suspension in an amount of about 1 mg catalyst per liter aqueous suspension to about 100 mg catalyst per liter aqueous suspension.
18. The method of claim 1, wherein the x is about 75 to 90.
19. The method of claim 1, wherein the Pt.sub.xRu.sub.y nanoparticles have an average diameter of about 2 nm to about 6 nm.
20. The method of claim 1, wherein the method has an ammonia Faradaic Efficiency of at least about 85%.
21. The method of claim 1, wherein the nitrate source is wastewater, agricultural runoff, refuse runoff, sewage waste, low-level nuclear waste, and urban drainage.
22. The method of claim 1, wherein the nitrate source comprises nitrate in a concentration of about 1 mM to about 1000 mM.
23. (canceled)
24. (canceled)
25. (canceled)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
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DETAILED DESCRIPTION
[0084] Thermocatalytic nitrate reduction (TNO.sub.3RR) and electrocatalytic nitrate reduction (ENO.sub.3RR) can address many of the concerns and limitations in current processes and promote the rapid conversion of nitrate to either N.sub.2 or NH.sub.3. The electrocatalytic nitrate reduction reaction (ENO.sub.3RR) uses protons and electrons, which removes the need for an external H.sub.2 stream and can be powered via renewable electricity. Referring to
[0085] A method for electrocatalytic conversion of nitrate to ammonia can include contacting a nitrate containing source with a working electrode comprising the Pt.sub.xRu.sub.y/C catalyst while applying a potential sufficient to reduce nitrate to thereby convert nitrate present in the nitrate containing source to ammonia. The method will also result in oxidation of water to oxygen.
[0086] The electrocatalytic conversion method can include flowing a nitrate containing source into a working electrode compartment of an electrochemical cell while applying a potential to the working electrode sufficient to reduce nitrate.
[0087] The working electrode can be the cathode and the counter electrode can be the anode. In such an arrangement, the working electrode compartment is the catholyte electrode compartment and the anode is disposed in an anolyte electrode compartment. The catholyte and anolyte compartments (or working and counter electrode compartments) can be separated by a membrane. The working electrode, which can be the cathode, includes the Pt.sub.xRu.sub.y/C catalyst. Upon contact with the catalyst, the nitrate present in the nitrate containing source is converted to ammonia.
[0088] The nitrate containing source in electrocatalytic methods can have a pH of about 5 to about 7. For example, the pH can be 5, 6, 7 or any ranges defined between such values.
[0089] The working electrode, which can be the cathode, can have a catalyst loading of about 0.1 mg per cm.sup.2 to about 10 mg per cm.sup.2. Other suitable values include about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 mg per cm.sup.2 or any values defined between such ranges.
[0090] A method of thermocatalytic conversion of nitrate to ammonia can include generating H.sub.2 in an aqueous suspension of the Pt.sub.xRu.sub.y/C catalyst and flowing a nitrate containing source into the suspension. Upon contact with the catalyst, nitrate in the nitrate containing source is converted to ammonia. The suspension can be maintained at a temperature of about 25 C. to about 90 C. For example, the suspension can be maintained at room temperature.
[0091] In methods of thermocatalytic conversion, the aqueous suspension can have a pH of about 1 to about 5. For example, the pH can be about 1, 2, 3, 4, or 5 or any ranges defined between such values.
[0092] The aqueous suspension can be stirred while flowing the nitrate containing source into the suspension. For example, the aqueous suspension can be stirred at a rate of about 100 rpm to about 10,000 rpm, about 1000 rpm to about 5000 rpm, about 1000 rpm to about 3000 rpm, or about 6000 rpm to about 9000 rpm. Other suitable rates, include, about 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, or 10,000 rpm or any ranges defined between such values.
[0093] The H.sub.2 can be generated by applying a potential to the aqueous suspension to generate H.sub.2 through water splitting. Alternatively or additionally, H.sub.2 can be generated by sparging the suspension with H.sub.2 gas.
[0094] The catalyst can be present in the aqueous suspension in an amount of about 1 mg catalyst per liter aqueous suspension to about 100 mg catalyst per liter aqueous suspension. Other suitable catalyst amounts per liter aqueous suspension include about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, and any ranges defined between and such values.
[0095] In any of the methods herein, the nitrate containing source can be wastewater, agricultural runoff, refuse runoff, sewage waste, low-level nuclear waste, and urban drainage. Other sources of nitrate can also be contemplated herein as suitable sources to be processed to ammonia. The nitrate can be present in the nitrate containing source in concentrations about 1 mM to about 1000 mM.
[0096] The nitrate containing source can undergo any suitable preprocessing if needed, for example, to concentrate or dilute the nitrate concentration. For example, the nitrate source can be pretreated by concentration to increase the nitrate concentration by various methods such as reverse osmosis, electrodialysis, or the like. Other pretreatments can include removal of heavy metal ions that may act as catalyst poisons.
[0097] Methods of the disclosure can have improved Faradaic Efficiency toward ammonia. For example, methods the disclosure can have an FE of ammonia of at least about 85%, at least about 88%, at least about 90%, or at least about 95%.
[0098] Methods for electrocatalytic conversion and thermocatalytic conversion utilize a Pt.sub.xRu.sub.y/C catalyst. The catalyst can include a carbon support having Pt.sub.xRu.sub.y nanoparticles disposed thereon, wherein x is about 48 at % to about 90 at % and y is 1-x. Other suitable values for x can include about 50 at % to about 80 at %, about 75 at % to about 90 at %, or about 65 at % to about 85 at %. For example, x can be about 48, 49, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90 at % or any ranges defined between such values.
[0099] The Pt.sub.xRu.sub.y nanoparticles can have an average diameter, as measured by transmission electron microscopy or estimated using extended X-ray absorption fine structure, of about 2 nm to about 6 nm, about 2 nm to about 5 nm, or about 2 nm to about 4 nm. For example, the average diameter can be about 2, 3, 4, 5, or 6 nm or any ranges defined between such values, as measured by transmission electron microscopy or estimated using extended X-ray absorption fine structure.
[0100] For electrocatalytic any suitable carbon support can be used. The support could be carbon black, for example. For example, the support can be a substrate such as carbon felt. The carbon felt can be disposed on a graphite rod for electrocatalytic conversion methods.
[0101] Methods of electrocatalytic conversion in accordance with the disclosure can utilize electrochemical flow cells configurations. In such configurations, the counter electrode can include a carbon substrate having a conductive catalyst disposed thereon. The conductive catalyst can be, for example, RuO.sub.2, IrO.sub.2, and mixtures thereof.
Surface Characterization of the Supported Pt.sub.xRu.sub.y Alloys
[0102] In many Pt.sub.xRu.sub.y systems, changing the synthesis temperature or support can drastically alter the level of Pt-surface enrichment. Thus, alloys with the same bulk composition may have different levels of activity depending on the composition of the metals on the surface that catalyze the reaction.
[0103] Accurately determining the ECSA allowed for counting the number of surface Pt and Ru sites, which served to both normalize measured activity for qualitative comparison to theory and quantify the surface composition. Because each surface Pt atom adsorbs approximately one hydrogen atom, the charge associated with hydrogen adsorption and desorption is often used to calculate the ECSA. However, this well-known H.sub.upd technique was unsuitable for Ru-based materials due to overlapping hydrogen and ruthenium oxidation currents. Additionally, more than one monolayer of hydrogen may adsorb onto Ru sites. To overcome this challenge for Pt.sub.xRu.sub.y/C alloys, copper underpotential deposition (Cu.sub.upd) was used because there is roughly one Cu atom electrodeposited per surface Pt or Ru site (
[0104] Even though the selected deposition potential may slightly change the ECSA (10 mV in deposition potential is 0.014 cm.sup.2 in ECSA), it was not believed that it would significantly impact the changes observed in the measured activities of the alloy. After measuring the charge of the Cu.sub.upd peak, the ECSA is calculated by assuming that a single Cu atom will bind to Pt or Ru with a 1:1 ratio and that two electrons are transferred from Cu.sup.2+. The Cu.sub.upd values used to normalize the current activity are determined prior to kinetic experiments performed in fresh electrolyte solution. Due to the small differences in the amount of catalyst deposited on the glassy carbon electrode and contact with the electrolyte solution, the ECSA may vary up to 30% from run to run, so reported activities were normalized to the ECSA from a particular run. The normalized current densities for each catalyst were reproducible when normalizing to the ECSA for that deposition.
[0105] The measured ECSAs from both H.sub.upd and Cu.sub.upd are shown in
[0106] The commercial Pt/C and PtRu/C both had higher ECSA compared to the synthesized materials despite having the same metal loading. This may arise because the commercial catalysts had higher dispersion, therefore a smaller average particle size calculated from XRD. The ECSA of synthesized Pt.sub.xRu.sub.y/C ranged from 0.2-0.4 cm, regardless of measurement technique. The difference between the ECSAs of H.sub.upd and Cu.sub.upd increases as the Ru content increases because more than one hydrogen binds to Ru active sites. The Cu.sub.upd approach eliminates the over counted sites because only one Cu atom adsorbs per Ru site.
TABLE-US-00001 TABLE 1 Measured electrochemical active surface area from H.sub.upd and Cu.sub.upd technique for commercial and synthesized Pt.sub.xRu.sub.y/C catalysts. Catalysts H.sub.upd (cm.sup.2) Cu.sub.upd (cm.sup.2) Pt/Ccommercial 0.94 0.94 PtRu/Ccommercial 0.71 0.51 Pt.sub.100/C 0.36 0.36 Pt.sub.90Ru.sub.10/C 0.22 0.22 Pt.sub.78Ru.sub.22/C 0.25 0.20 Pt.sub.63Ru.sub.37/C 0.35 0.29 Pt.sub.48Ru.sub.52/C 0.34 0.22
[0107] The H.sub.upd and Cu.sub.upd ECSA measurements increasingly disagree as the bulk Ru at % increased. Without intending to be bound by theory, it is believed that this phenomenon is attributable to more than one hydrogen adsorbing per Ru site, such that H.sub.upd over-counts the ECSA when Ru is present on the surface, causing a disagreement between H.sub.upd and Cu.sub.upd that increases with increasing surface Ru. The increasing discrepancy between H.sub.upd and Cu.sub.upd charge ((Q.sub.HQ.sub.Cu)/Q.sub.Cu) is shown in
[0108] X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Ultra X-ray photoelectron spectrometer. While keeping the analysis chamber at 110.sup.9 Torr, a monochromatic Al X-ray source (10 mA and 12 kV) was used with a pass energy of 12 eV and step size of 1 eV. Collected spectra were calibrated by positioning the C(1s) peak at 248.8 eV. Survey scans ranged from 600-0 eV while the narrow scans were performed between 370-300 eV and 510-450 eV for Pt 4d and Ru 3p, respectively (
[0109] The data from Table 2 is plotted in
TABLE-US-00002 TABLE 2 Comparison between bulk Ru at % determined from ICP-MS and surface Ru at % determined from Ru XPS intensity. The naming convention of the catalysts are based on the bulk at % of the metals. Bulk Ru Surface Ru XPS Catalysts at % Ru at % Intensity Pt.sub.100/C 0 0 0 Pt.sub.90Ru.sub.10/C 10 12 2932.6 Pt.sub.78Ru.sub.22/C 22 25 6221.5 Pt.sub.63Ru.sub.37/C 37 55 17172.6 Pt.sub.48Ru.sub.52/C 52 58 18634.2
Density Functional Theory Modeling
[0110] All DFT calculations used the Vienna Ab Initio Simulation Package, version 5.4.4. Calculations used the projector-augmented wave method with an energy cutoff of 400 eV, the PBE functional, and Gaussian smearing of 0.2 eV. For surface calculations, the Brillouin zone was sampled with a 661 Monkhorst-Pack k-point grid. Self-consistent electronic calculations used a between-iteration tolerance of 10.sup.4 eV and ionic relaxation proceeded until all forces on atoms were less than 0.02 eV/A.
[0111] The alloy catalysts were constructed using the Atomic Simulation Environment software package, version 3.17.0. Nine random surface alloys were created based on a 344 supercell of Pt(211), using a Pt lattice constant which was optimized (3.97677 ) with the PBE functional on a 161616 k-point grid. For all simulations, the surface slab contained four layers of atoms, where the bottom two layers were constrained to their bulk positions and the top two layers could relax. Surface alloy models were prepared by randomly assigning each of the 12 atoms in the top surface layer as either Pt or Ru, resulting in surface compositions ranging from 0 at % Ru to 50 at % Ru. Surfaces were then geometry-optimized with a vacuum of at least 15 in the z direction.
[0112] The Pymatgen software package was used to locate unique adsorption sites. The electronic binding energy E.sub.A of species A was calculated with respect to the bare surface and the electronic energy of species A in the gas phase. Aqueous-phase NO.sub.3.sup. adsorption Gibbs free energies were obtained at 298.15 K and 0 V vs. RHE using a thermodynamic cycle.
[0113] The catalyst activity was predicted by relating the gas-phase electronic binding energies of atomic O and N (E.sub.O and E.sub.N) to the overall mean-field kinetics of the nitrate reduction reaction. This task was accomplished by using a theoretical volcano plot. The PBE functional and face-centered cubic (FCC) (211) facet were chosen to enable the comparison of results with its theoretical volcano plot. This was also considered an appropriate comparison to the synthesized Pt.sub.xRu.sub.y particles because only Ru compositions was considered for which Pt.sub.XRu.sub.y particles form in an FCC lattice.
[0114] The nitrate-to-nitrite dissociation barrier (NO.sub.3*+*NO.sub.2*+O*) for each random surface alloy slab was computed using the climbing-image nudged elastic band (CI-NEB) method. The band was formed with five interior images linearly interpolated between the initial and final endpoint geometry. CI-NEB relaxation used spring forces of 5 eV .sup.1 between images and the same electronic and force tolerance parameters as the adsorption calculations.
[0115] For each NEB calculation, the initial image was the relaxed geometry of NO.sub.3* at its optimal [OO]-chelating binding position on the third ridge of each FCC(211) material. The final endpoint was formed by assuming an elementary step in which one of the basal 0 atoms migrates to a neighboring bridge site up or down the third ridge, following which the remaining NO.sub.2 fragment rotates downward into a [NO]-chelating position.
[0116] All DFT-predicted energetics (adsorption energies, reaction energies, and activation energies) are done at low coverages (i.e., 1/12 ML for H, N, and 0 and ML for NO.sub.3) and neglect lateral adsorbate-adsorbate interactions due to high coverage of a single species or the presence of co-adsorbed species (e.g., co-adsorbed H affecting the adsorption strength of NO.sub.3.sup., which weakens adsorption strength of nitrate by 0.25 eV at 1/12 ML H coverage). Such shifts are typical of co-adsorption of H with small molecular adsorbates on metal surfaces. This effect would also similarly weaken adsorption energies for other NO.sub.3RR species, and thus would likely not qualitatively change trends. Neglecting co-adsorbate interactions on adsorption free energies is a common approximation when studying complex reaction networks such as electrocatalytic nitrate reduction because of the large computational expense to treat coverage-dependent interactions for all species in the model.
[0117] On pure transition metals, linear adsorbate scaling relations (among N, O, and other reaction intermediates) and Bronsted-Evans-Polanyi relations (between adsorption and activation energies) exist for the NO.sub.3RR. Consequently, a microkinetic model for NO.sub.3RR was found to be able to predict trends in the reaction rates, steady-state coverages, and degrees of rate control given only the N and O binding energies and an applied potential.
[0118] N and O binding energies were also shown to serve as NO.sub.3RR activity descriptors on Pt.sub.xRu.sub.y alloys because similar free energy scaling relations hold on the model Pt.sub.xRu.sub.y surfaces. Examining the sites of strongest binding energy (
[0119] Linear adsorbate scaling relationships between adsorbates were predicted to exist on Pt.sub.xRu.sub.y alloys. The data in
Binding Energy Trends of O and N on Pt.sub.xRu.sub.y
[0120] DFT modeling was used to examine how adsorption strength of O and N depends on Pt.sub.xRu.sub.y surface alloy composition. The atomic distribution of Pt and Ru in each alloy's surface was generated using random assignment (
[0121] The N atom prefers to adsorb in hollow sites, but also in locations that maximize its coordination with surface Ru atoms (
[0122] NO.sub.3.sup. adsorption free energies were predicted at 298.15 K using a thermodynamic cycle to avoid error in predicting ion energies using periodic DFT calculations. For NO.sub.3 binding, only sites in which NO.sub.3.sup. binds in an O,O-bidentate chelating fashion to two consecutive atoms on the same vertical FCC(211) ridge were considered. Such binding positions were tested only for the middle and rightmost ridges, as the binding on the leftmost (lowest) ridge was found to be unfavorable. For all surfaces, NO.sub.3.sup. prefers to bind on the rightmost (highest) ridge and to as many Ru atoms on that ridge as possible at once (
[0123] On pure Pt(211) facets (denoted as s-Pt.sub.100), H prefers an atop site at the top ridge (FIG. 28Error!Reference source not found.). As Ru surface atoms become available, H prefers to adsorb at sites near the top ridge and which increase the coordination of H with Ru. For most sites, H adsorbs at a bridge position in the top ridge with at least one Ru atom in its first coordination sphere. For surfaces where Ru is available only in the bottom ridge (e.g., s-Pt.sub.92Ru.sub.8 and s-Pt.sub.83Ru.sub.17), H adsorbs at a position between the top ridge and the bottom ridge immediately next to it, such that it is as close to a Ru atom as possible.
[0124] As the Ru content of the computational model alloy catalyst (denoted s-Pt.sub.xRu.sub.y) increased, both N and O bind more strongly (
[0125] In a related way, the O and N binding energies for Pt.sub.xRu.sub.y alloys of intermediate compositions can also be rationalized by ensemble effects at the surface of each model slab. N, O, H, and NO.sub.3.sup. usually prefer bridge binding positions between two atoms in the highest FCC(211) ridge or a hollow position inside three atoms on the catalyst surface (
[0126] The data in
[0127] Some of the model alloys (s-Pt.sub.17Ru.sub.83 and s-Ru.sub.100) adsorb nitrate more strongly than Ru(211). Here, the Ru(211) surface was generated by optimizing the lattice constant of FCC Ru, whereas all the model alloys surfaces (including s-Pt.sub.17Ru.sub.83 and s-Ru.sub.100) are FCC(211) surfaces constrained to the Pt lattice constant, which is slightly larger than that of Ru. Thus, the alloy surface atoms are under a slight biaxial tensile strain, which raises the average d-band center of the surface with respect to the Fermi level, increasing the overall adsorbate-surface bonding interaction. In reality an alloyed surface would have a different lattice constant between that of its constituent metals. Nonetheless, strain effects have a much smaller perturbation on the nitrate binding energy than change in adsorption site (i.e., from interacting directly with a Pt atom to a Ru atom) and the qualitative trends match with experiment.
Rationalizing Activity Trends with Alloy Composition by Microkinetic Modeling
[0128] The NO.sub.3RR activity was rationalized as a function of surface composition (
[0129] The contours in
[0130] The magnitude of the current density from
[0131] A volcano in activity with alloy composition occurred because alloying tunes the binding energies of reactants and key intermediates, and these binding energies are related to the barriers of individual elementary steps through free energy relations. The activity is maximized at some intermediate binding energy of O and N (
[0132] The degree to which any elementary step in the reaction mechanism determines the total activity can be estimated by computing the degree of rate control (DRC) for that reaction. DRC analysis in this work at 0.1 V vs. RHE predicted that for surfaces with low Ru content, nitrate dissociation is rate-limiting (DRC1), and increasing the adsorption strength of nitrate increases the rate (
[0133] In particular,
[0134] Experiments show that maximum NO.sub.3RR current density was achieved at 0.1 V vs. RHE when using a Pt.sub.5Ru.sub.25/C catalyst. For the five regions mentioned above in which a single elementary step controls the overall reaction rate, the s-Pt.sub.75Ru.sub.25 point lies at or very close to the boundary of each region. DRC analysis also predicts that none of the other elementary steps becomes rate-limiting at the N and O binding energies of s-Pt.sub.75Ru.sub.25. These results suggest that s-Pt.sub.75Ru.sub.25 exhibits near-optimal N and O binding energies for which no single elementary step in the mechanism is rate-limiting. Under these conditions, one would expect the overall reaction rate to reach a local maximum, which rationalizes the observation that Pt.sub.25Ru.sub.25/C produces the highest NO.sub.3RR current density of all the Pt.sub.xRu.sub.y catalysts.
[0135] Although the computational results predicted that N.sub.2 is the dominant species forming at high Ru contents and strong O and N adsorption, experimental selectivity results showed that NH.sub.3 is the dominant product for all the alloy catalysts tested. Therefore, it is unlikely that the new rate-determining step is the association of nitrogen, but rather another step on the ammonia production reaction pathway. It was observed that NH.sub.2*+H.sup.++e.sup.NH.sub.3* was also rate-determining for surfaces with similar adsorption energies to s-Pt.sub.75Ru.sub.25 (, which is in line with the experimental observations and previous reports of this step being rate controlling for CuNi alloys. Without intending to be bound by theory, it is believed that this DRC discrepancy is attributable to uncertainties in the linear scaling relationships for alloys and to the fact that activity trends are easier to predict with microkinetic modeling compared to selectivity trends. Nevertheless, the switch from one rate-limiting step to another at the binding energies of s-Pt.sub.75Ru.sub.25 rationalizes the experimentally observed local maximum in activity at that composition.
Thermocatalytic Nitrate Reduction (TNO.sub.3RR) and Electrocatalytic Nitrate Reduction (ENO.sub.3RR)
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[0137] Pt/C, PtRu/C, and Pt.sub.25Ru.sub.25/C for TNO.sub.3RR and ENO.sub.3RR under various operating conditions (i.e., pH, hydrogen partial pressure, nitrate concentration, applied potential) were analyzed to compare thermocatalytic and electrocatalytic approaches for nitrate reduction. It was observed that that increasing the hydrogen (electro)chemical potential (0.1 to 1 atm H.sub.2 and 0.15 to 005 V vs. RHE) increases the rate of nitrate conversion and that the ranking of catalyst activity is the same for ENO.sub.3RR and TNO.sub.3RR, that is, Pt/C<<PtRu/C<Pt.sub.78Ru.sub.25/C. This change in activity from increasing Ru content in the alloy is believed to be attributed to increasing the adsorption strength of nitrate, hydrogen, and intermediates. Similarly, increasing the nitrate concentration increased reaction rates in ENO.sub.3RR and TNO.sub.3RR for PtRu/C. However, at concentrations above 0.5 M NO.sup., ENO.sub.3RR activity decreased due to surface poisoning by nitrate. Unlike hydrogen driving force and nitrate concentration, which similarly affect catalyst activity, the effect of the pH and the apparent activation energies were different for ENO.sub.3RR and TNO.sub.3RR on the PtRu/C catalyst. This finding implies that pH has a more complex role in the nitrate reduction mechanism than previously developed microkinetic models based on Langmuir-Hinshelwood surface reactions might suggest, and that there are fundamental differences between the two reactions. Despite these differences, certain catalyst properties (such as stronger nitrate adsorption) or reaction conditions (more available adsorbed hydrogen) increase the TNO.sub.3RR and ENO.sub.3RR rates in a way that is qualitatively captured by the existing theoretical volcano plot. TNO.sub.3RR and ENO.sub.3RR performance were compared on PtRu/C to rates and operating costs for industrial ammonia synthesis to evaluate the feasibility of both systems. It has been shown that TNO.sub.3RR on PtRu/C at pH 1 produces NH.sub.3 at comparable rates to the Haber-Bosch process and, depending on the regional cost of H.sub.2, can have lower operational costs than the USDA standard cost per tonne of NH.sub.4NO.sub.3.
EXAMPLES
Example 1: Catalyst Preparation
[0138] Referring to
[0139] The final Pt and Ru loadings were determined by using a PerkinElmer NexION 2000 ICP-MS after digesting 1 mg of the catalyst in aqua regia (3:1 molar HCl:HNO.sub.3). The sample solutions were co-fed along with a 20-ppb bismuth internal standard. X-ray diffraction (XRD) analysis indicated the presence of a separate Ru hexagonal phase instead of the bimetallic phase for Ru compositions above 60 at %. Therefore, Pt.sub.xRu.sub.y alloys in the catalysts herein were limited to bulk Ru concentrations of 0-52 at %.
[0140] Commercial 30 wt % Pt/C, 30 wt % Pt.sub.50Ru.sub.50/C, and 20 wt % Rh/C were also purchased from Fuel Cell Store for comparison.
Material Characterization:
[0141] X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were taken at the Sector 20 bending-magnet beamline of the Advanced Photon Source at Argonne National Laboratory. Catalyst samples were loaded into 1.5 mm glass capillaries for measurement in transmission mode at the Pt L.sub.3-edge. To take the spectra at the Ru K-edge, the catalyst samples were also measured in the glass capillaries using transmission mode, except for the lowest Ru weight loading sample, for which the sample was filled into a Kapton tube to allow a longer path distance to increase the signal to noise ratio. All measurements were taken of samples exposed to air (ex-situ). For the Ru K-edge the harmonic rejection mirror was set to 3.9 mrad, whereas for Pt L.sub.3-edge it was 4.1 mrad. Catalyst samples were measured in transmission mode at the Pt L.sub.3-edge and Ru K-edge. The p(E) data was processed using the ATHENA software with a Fourier cutoff of R.sub.bkg=1.0 and a k range from 3 to 16 .sup.1. Structural parameters were derived from the experimental data using FEFF9 theoretical standards as input to the ARTEMIS software. Two 14-min scans were taken for each sample at each edge and co-added to generate the spectrum. Pt and Ru reference foils were located downstream and taken concurrently with the sample for energy calibration and to verify monochromator stability. The data was processed using ATHENA software with a Fourier cutoff of R.sub.bkg=1.0 and a k range of 3 to 16 .sup.1. Structural parameters were derived from the experimental data by fitting using FEFF9 theoretical standards as inputs to the ARTEMIS software package. Fits included first PtPt or PtRu and Pt-O paths including the 3.sup.rd cumulant to account for asymmetry. The spectra were obtained by merging two scans of each catalyst. The raw data obtained at the Pt L.sub.3-edge is presented in
[0142] ICP-MS measurements determined the bulk weight and atomic loading of Pt and Ru in the alloys. The data in Table 3 shows that a smaller wt % (weight %) of Ru than intended was incorporated into the catalyst. The deviations between the target and actual composition are likely due to the precision of the weighing scale and different reactivities of the two types of precursors upon reduction with NaBH.sub.4. The ICP-MS measured actual atomic percentage of Ru (with the balance Pt) were for the naming convention of the catalysts. In Table 3, atomic and weight percent loading of Ru in Pt.sub.xRu.sub.y/C (x=48-100%) catalysts from ICP-MS. Target Ru wt % reflected the calculated amount of RuCl.sub.3 precursor added during synthesis. All values were with respect to the total metal loading, not including carbon, such that the balance is Pt. The total target metal loading on carbon was 30 wt %.
TABLE-US-00003 TABLE 3 Catalysts Target Ru wt % Actual Ru wt % Actual Ru at % Pt.sub.100/C 0 0 0 Pt.sub.90Ru.sub.10/C 12.5 6 10 Pt.sub.78Ru.sub.22/C 25 13 22 Pt.sub.63Ru.sub.37/C 37.5 23 37 Pt.sub.48Ru.sub.52/C 50 36 52
[0143] To confirm that Pt.sub.xRu.sub.y/C alloys are synthesized, ex-situ EXAFS was employed to measure the local coordination of Pt and Ru atoms. The EXAFS spectra of the Pt L.sub.3-edge for Pt.sub.xRu.sub.y/C in real space are shown in
[0144] The measured spectra and fittings for the Pt foil and each of the five compositions of the Pt.sub.xRu.sub.y/C are shown in
[0145] By fitting the EXAFS data using PtPt, PtO, and PtRu paths (
TABLE-US-00004 TABLE 4 Tabulated fitting results for Pt foil and Pt.sub.xRu.sub.y/C catalysts. Material Pt foil Pt.sub.100/C Pt.sub.90 Ru.sub.10/C Pt.sub.78Ru.sub.22/C Pt.sub.63Ru.sub.37/C Pt.sub.48Ru.sub.52/C Pt-Pt R () 2.756 0.01 2.743 0.03 2.740 0.03 2.735 0.04 2.738 0.04 2.745 0.03 Path CN 12 7.5 0.5 5.8 0.6 4.4 0.6 5.6 0.1 7.3 0.6 .sup.2 (.sup.2) 0.005 0.0001 0.006 0.0004 0.007 0.0007 0.006 0.0010 0.007 0.0007 0.006 0.0006 Pt-O R () 1.995 0.04 1.995 0.03 1.992 0.05 1.987 0.05 1.983 0.05 Path CN 1.28 0.18 1.34 0.20 1.77 0.21 1.22 0.18 0.79 0.19 .sup.2 (.sup.2) 0.006 0.0026 0.005 0.0026 0.004 0.0020 0.004 0.0024 0.005 0.0043 Pt-Ru R () 2.792 0.03 2.785 0.03 2.776 0.02 2.758 0.00 Path CN 1.19 0.51 1.35 0.55 1.44 0.44 1.28 0.42 .sup.2 (.sup.2) 0.009 0.0038 0.008 0.0034 0.008 0.0025 0.007 0.0028
[0146] The presence of PtRu first-shell coordination by EXAFS indicates these materials are alloys, rather than separate phases of Pt and Ru. Because there was less Ru than Pt in the alloys, the Ru K-edge EXAFS data had low signal and was too noisy to accurately fit (
[0147] The XRD patterns for different compositions of the Pt.sub.xRu.sub.y/C displayed a shift in the Pt(111) diffraction patterns to higher 29 as the Ru at % increased (
[0148] XRD analysis was conducted using a Rigaku Miniflex XRD with Cu K radiation and a Ni filter (=1.5418 ). The 26 range (10<2<90) was scanned at a rate of 5/min with a 0.02 step size. Crystallite sizes were estimated using the Scherrer equation. The average size of the synthesized nanoparticles was calculated using Scherrer's equation:
where is the average size of the crystalline particles, K is the shape factor (0.89), is the wavelength of the X-ray (1.54056 ), is the full width of the peak at half maximum, and e is the Bragg angle of the peak. Error bars were determined by using the standard deviation across four different Pt diffraction peaks. The average particle sizes from XRD were compared with the average particle sizes measured from TEM images (Table 5). The particle sizes from both characterization techniques agreed within error for all studied catalysts.
[0149] The total coordination number (CN) from the PtPt and PtRu paths ranged from 6-9 for all the samples. From established relationships between metal nanoparticle size and first shell CN, these values correspond to nanoparticles between 1.5-5 nm, which is within the range of XRD calculations and TEM imaging (Table 5Error!Reference source not found.). Nanoparticle sizes estimated from CN were lower than sizes extracted from TEM images, which may arise because the CN from EXAFS fittings estimates of size exclude the oxide layer around each nanoparticle, as only the metal-metal bonds of the metallic core were counted.
[0150] The raw data obtained at the Ru K-edge is presented in
TABLE-US-00005 TABLE 5 Particle sizes from XRD using Scherrer equation, TEM, and EXAFS from first shell Pt-metal coordination number. XRD particle TEM particle EXAFS particle Catalyst size (nm) size (nm) size (nm) Pt.sub.100/C 3.5 0.6 1.5-5.0 for all catalysts Pt.sub.90Ru.sub.10/C 5.0 1.0 4.3 1.4 Pt.sub.78Ru.sub.22/C 5.7 1.0 4.0 1.1 Pt.sub.62Ru.sub.37/C 3.2 0.7 3.9 1.0 Pt.sub.48Ru.sub.52/C 4.7 1.3 3.6 0.9
[0151] Pt and Ru peaks were referenced to #04-0802 and #06-0663, respectively.
[0152] Transmission electron microscopy (TEM) was performed on a JEOL 201OF electron microscope operating with 200 kV accelerating voltage. The samples were made by adding 1 mg of catalyst into isopropanol. One drop of this suspension was deposited on a gold grid. The isopropanol was dried before imaging of the sample. The TEM images in
[0153] X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Ultra X-ray photoelectron spectrometer. While keeping the analysis chamber at 110.sup.9 Torr, a monochromatic Al X-ray source (10 mA and 12 kV) was used with a pass energy of 12 eV and step size of 1 eV. Collected spectra were calibrated by positioning the C(1s) peak at 248.8 eV. The resulting Pt 4d and Ru 3p peaks were fitted with the Shirley-type background with the CasaXPS software.
Example 2Rotating Disk Electrode Preparation
[0154] The catalyst ink was prepared by adding 3 mg of the supported catalyst in 5 mL of water and isopropanol (1:1 molar ratio). 17.5 L of Nafion (5% in 95% isopropanol, Sigma Aldrich) was added to the solution to act as a binder and sonicated for at least 120 min. A glassy carbon rotating disk electrode (5 mm in diameter) was polished with 0.05 m alumina suspensions before sonication in Millipore water to remove trace surface contaminants. The catalyst ink was sonicated for at least 30 min before depositing 8 L of the ink onto the surface of the clean glassy carbon electrode. The deposition was kept in closed containment as the ink dried and repeated once more. The total loading was 9.6 pg of catalyst, including carbon.
[0155] The prepared electrodes were placed into the electrolyte solution and cycled from hydrogen evolution to Pt oxidation potentials (0.17 to 1.23 V vs. RHE) at least 50 times at 100 mV s.sup.1 before conducting electrochemical measurements. ICP-MS experiments of the solution before and after the electrocatalyst pretreatment process for a commercial PtRu/C showed 8% of Pt and Ru in the electrolyte solution. It is believed that presence of Pt and Ru in the electrolyte solution was attributable to catalyst powder that was not adequately bound to the surface of the glassy carbon. Following this pretreatment, stable CVs for all reported Pt.sub.xRu.sub.y/C was obtained, implying no further loss of catalyst.
Example 3Electrochemical Measurements
[0156] The electrochemical experiments were conducted in either a single compartment, three-electrode glass electrochemical cell (for steady-state activity measurements) or a two-compartment, three-electrode cell (to enable product quantification for selectivity measurements) using a VSP potentiostat (Bio-Logic Science Inst.). All measurements were taken at room temperature (23.3 C.). A graphite rod (AGKSP grade, ultra F purity, Alfa Aesar) and Ag/AgCl (4 M KCl, Pine Research Inst., Inc.) were used as the counter and the reference electrode, respectively. Before electrochemical experiments, the Ag/AgCl reference electrode was calibrated against a Pt wire with 1 bar H.sub.2 in the electrolyte solution. All reported potentials were referenced to RHE. The sulfuric acid electrolyte was prepared by adding concentrated H.sub.2SO.sub.4 (99.999%, Sigma Aldrich) to Millipore water. Before electrochemical measurements, N.sub.2 gas (Ultra-high purity grade, 99.999%, Cryogenic Gases) was sparged through the electrolyte for at least 45 min to remove dissolved O.sub.2 from the solution. Throughout the experiment, N.sub.2 also blanketed the electrolyte solution to prevent O.sub.2 from reaching the electrolyte.
[0157] The data in
[0158] The intrinsic activities of commercial Pt/C and PtRu/C were comparable to those of the synthesized Pt.sub.100/C and Pt.sub.48Ru.sub.52/C samples, respectively (
[0159] The steady-state current densities for Pt.sub.100/C were comparable with other Pt/C reports and reach a maximum activity at 0.1 V vs. RHE. This maximum in activity arose from the competition between adsorbed nitrate and hydrogen, with 0.1 V vs. RHE being the potential when both species were considerably present on the surface. Below 0.1 V, the reaction rate decreased because there was a low coverage of nitrate on the Pt, and surface sites were blocked by adsorbed hydrogen. Above 0.1 V, the reaction rate decreased because there was not enough hydrogen available on the surface. Unlike the Pt.sub.100/C, none of the Pt.sub.xRu.sub.y/C alloys exhibited a maximum activity at 0.1 V vs. RHE. Without intending to be bound by theory, it is believed that this is because, similar to Rh, these Pt.sub.xRu.sub.y alloys bind nitrate more strongly than pure Pt, which shifts the maximum activity to a more negative potential. The stronger adsorption of nitrate and shift in potential of maximum activity of the Pt.sub.xRu.sub.y alloys was expected because Ru is less noble than Pt and was supported by the DFT calculations. Importantly, the Pt.sub.xRu.sub.y/C alloys were more active than Pt.sub.100/C at all eight applied potentials, confirming DFT modeling predictions that Pt.sub.3Ru would be more active than Pt for NO.sub.3RR.
[0160] In
[0161] Cyclic voltammograms of the alloy catalysts remained consistent after multiple cycles, suggesting that the alloy catalysts were stable prior to steady-state measurements. For comparison, the last three H.sub.upd CVs of Pt.sub.xRu.sub.y/C catalyst after 50 cycles of pretreatment are included in
[0162] Rh/C, the most active pure metal standard, was four times more active than Pt.sub.78Ru.sub.22/C (
Underpotential Deposition
[0163] After compensating for 85% of the solution resistance using electrochemical impedance spectroscopy (EIS), H.sub.upd in the hydrogen desorption region was used as one method to determine the ECSA of the Pt.sub.xRu.sub.y/C alloys. The average charge density of Pt (210 C cm.sup.2) was employed to calculate the ECSA. A slanted baseline, representing the double-layer charging current, was taken by subtracting half of the double-layer charging current measured at 0.35 V vs. RHE.
[0164] All Cu.sub.upd experiments were conducted in 0.1 M H.sub.2SO.sub.4 for an initial H.sub.upd baseline before adding 2 mM CuSO.sub.4 into the solution. The electrodes were polarized at 1.0 V vs. RHE for 2 min to ensure no Cu ions adsorbed to the surface of the electrode. Deposition potentials from 0.28-0.48 V vs. RHE were applied for 100 s to deposit a monolayer of Cu.sup.2+ on the surface of the catalyst. After, a linear voltammetric scan was performed at 100 mV s.sup.1 from the applied potential to 1.0 V vs. RHE, in which all the underpotential-deposited copper has been oxidized. Charges obtained from the copper stripping were corrected by subtracting the double-layer charge obtained in the absence of cupric ions in the solution.
Steady-State Current Measurements for Nitrate Reduction
[0165] H.sub.upd and baseline chronoamperometric measurements were performed in 100 mL of 1 M H.sub.2SO.sub.4 solution. The rotating disk electrode (RDE) was held at each potential for 5 min while rotating at 2500 rpm to eliminate mass transfer limitations. The absence of external mass transfer limitations was confirmed by verifying that the current densities were independent of rotation rate at 2500 rpm or above. The film drop-cast method was used to deposit a thin layer of catalyst onto the glassy carbon electrode to avoid sources of internal diffusion limitations. The measured currents in the last 20 s were averaged and reported accordingly. After adding 20 mL of 6 M NaNO.sub.3 (Sigma Aldrich, 99.0%) to reach 1 M nitrate, the electrolyte solution was sparged with N.sub.2 for 15 min to remove trace oxygen. The chronoamperometric measurements were repeated with nitrate in the solution.
Example 4Selectivity Measurements
[0166] A working electrode having a catalyst disposed thereon was prepared by depositing Pt and Ru precursors via the same NaBH.sub.4 reduction method as described in Example 1, on 2.52.5 cm.sup.2 pieces of carbon felt (6.35 mm thick, 99.0%, Alfa Aesar). The carbon felts (CFs) were attached to a graphite rod (AGKSP grade, ultra F purity, Alfa Aesar) for use as the working electrode.
[0167] Before electrochemical measurements, N.sub.2 (Ultra-high purity grade, 99.999%, Cryogenic Gases) was sparged through the electrolyte for at least 45 min to remove O.sub.2 from the solution. Throughout the experiment, N.sub.2 blanketed the electrolyte solution to prevent O.sub.2 from reaching the electrolyte. The carbon felt was treated in 1 M H.sub.2SO.sub.4 solution by cycling from hydrogen evolution to Pt oxidation (0.17 to 1.23 V vs. RHE) at least 35 times at 100 mV s.sup.1 to remove oxygenated species from the surface of the metal nanoparticles. H.sub.upd experiments were conducted after compensating for 85% of the solution resistance.
[0168] The Pt.sub.xRu.sub.y/CF (Pt and Ru alloys supported on carbon felt) was transferred to a two-compartment, three-electrode glass electrochemical cell with 150 mL of 0.1 M HNO.sub.3 (sparged with N.sub.2) as the electrolyte solution in the cathodic compartment. The electrolyte for selectivity measurements was 0.1 M HNO.sub.3 (rather than 1 M H.sub.2SO.sub.4 and 1 M NaNO.sub.3) to avoid possible sodium and sulfate interference in the ion chromatograph used for product quantification. Again, 85% of the solution resistance was compensated using EIS before running a 4-hr steady-state measurement at 0.1 V vs. RHE. Only 85% was directly compensated to avoid instability of the potentiostat controller.
[0169] An ion chromatography (Agilent), equipped with AS9-HC column (Dionex) with 9 mM sodium carbonate eluent, was used to quantify the amount of nitrate and nitrite in the electrolyte solution. For anion measurements, sodium nitrate (Sigma Aldrich, 99.0%) and sodium nitrite (Sigma Aldrich, 99.999% trace metal basis) were used to prepare the standard solutions for the calibration curve. To prevent oversaturating the system with anions, 0.1 mL of the electrolyte solution was extracted every hour and diluted by a factor of ten with Millipore water to measure the change in nitrate concentration. Separately, 0.5 mL of the electrolyte solution was extracted and neutralized with 0.1 M NaOH (Sigma Aldrich, 99.99%) to inhibit the decomposition of nitrite in acidic media. However, the measured values of the nitrite concentration may be lower than the actual values due to the decomposition of nitrite during the extraction of the reactor aliquots.
[0170] NH.sub.3 was quantified by using the indophenol blue test. An aliquot of 1 mL of electrolyte solution was extracted from the cathodic side of the two-compartment cell every hour. 1 M NaOH (Sigma Aldrich, 99.99%) was added to the electrolyte solution to neutralize the acid to a pH of 12. After, 122 L of sodium salicylate (Sigma Aldrich, >99.5%), 27.3 L of sodium nitroprusside dihydrate (Sigma Aldrich, >99%), and 40 L of sodium hypochlorite solution (Sigma Aldrich, 4.00-4.99%) were sequentially added to the electrolyte solution and manually stirred together. The solution was covered and left for 40 min. Afterward, a UV-vis spectrometer (Thermo Fischer, Evolution 350) was used to obtain spectra between 400-1000 nm. The indophenol peak was identified as the maximum absorbance between 650-700 nm. A fresh 0.1 M HNO.sub.3 electrolyte solution prepared with the indophenol blue method was used as the background and subtracted from the sample spectra. If the concentration of NH.sub.3 was too high and oversaturated the detector, the solution was diluted and retested. A calibration curve was created using known concentrations of NH.sub.4Cl (Sigma Aldrich) in 0.1 M HNO.sub.3, and unknown NH.sub.3 concentrations were calculated using the Beer-Lambert law. The faradaic efficiency (FE) was calculated by dividing the charge required to form the total NH.sub.3 measured by the total charge passed during the steady-state experiments. The total charge passed was calculated by integrating the reduction current over the duration of the experiment and the charge required from NH.sub.3 was calculated by assuming that eight electrons are required to form one molecule of NH.sub.3 from one molecule of nitrate.
[0171]
[0172] The faradaic efficiencies (FE) and total charge (in C) of the Pt.sub.xRu.sub.y/CF towards NH.sub.4.sup.+ over seven hours at an applied potential of 0.1 V vs. RHE are shown in
[0173] The total current density for the Pt.sub.xRu.sub.y/CF shown in
Example 5H.SUB.upd .and Cu.SUB.upd .Experiments
[0174] To perform the H.sub.upd experiments, the Pt.sub.xRu.sub.y/C catalysts were first pretreated by cycling from hydrogen evolution to Pt oxidation (0.17 to 1.23 V vs. RHE) at least 50 times or until the CVs were stable. This pretreatment ensured that surface oxides were reduced before taking measurements. CV scans between 0.08 and 1.23 V vs. RHE were used to obtain H.sub.upd peaks (
[0175] For the Cu.sub.upd measurements, the scan ranges were kept the same as H.sub.upd and 2 mM CuSO.sub.4 was added into the solution. The first Cu desorption peak at 0.3 V vs. RHE corresponded to bulk Cu stripping, and the smaller peaks that follow from 0.3-0.8 V vs. RHE corresponded to a monolayer of Cu stripping from the catalyst surface. The charges obtained from Cu stripping were subtracted by the double layer baseline obtained in the H.sub.upd experiments in the absence of Cu.sup.2+ ions in the solution. To further ensure that the Cu.sub.upd total charge was only from the stripping of a monolayer of Cu, experiments were performed to determine the appropriate deposition potential. The electrodes were first polarized at 1.0 V vs. RHE for two minutes so that no Cu ions remained on the surface. Deposition potentials from 0.28-0.48 V vs. RHE were applied for 100 seconds to deposit a monolayer of Cu.sup.2+ on the surface before applying a linear voltammetric scan (LSV) at 100 mV s.sup.1 from the deposition potential to 1.0 V vs. RHE. The ratio of copper to hydrogen stripping charge as a function of the deposition potential is shown for Pt.sub.100/C in
Example 6Thermocatalytic and Electrocatalytic Reduction Experiments
[0176] A NaBH.sub.4 reduction synthesis was used to synthesize Pt.sub.25Ru.sub.25/C. The carbon black (Vulcan XC 72; Fuel Cell Store) was pretreated at 400 C. for 2 hrs to remove surface impurities. Afterwards, the support was suspended and sonicated in Millipore water (18.2 Mcm, Millipore MilliQ system) for 15 min. Measured concentrations of RuCl.sub.3 (38% Ru; Alfa Aesar) and H.sub.2PtCl.sub.e(38-40% Pt; Sigma Aldrich) in Millipore water were added to the solution and stirred for another 15 min before 40 mg of NaBH.sub.4 (Sigma Aldrich) dissolved in 25 mL of Millipore water were added to accelerate the reaction. The final solution was stirred for 2 hrs before centrifuging three times at 3000 rpm for 8 min each and washed with Millipore water. The recovered solid was dried overnight in an oven at 80 C. in air. All commercial catalysts (Pt/C and PtRu/C) were purchased from Fuel Cell Store. For the nitrate concentration and pH effect studies, the commercial PtRu/C was used instead of the most active synthesized Pt.sub.25Ru.sub.25/C because a single batch of commercial PtRu/C was sufficient to perform all studies. Using Pt.sub.25Ru.sub.25/C for these studies would require multiple batch syntheses and introduce batch-to-batch variations in the measurements.
[0177] The final metal loadings were determined by using thermogravimetric analysis (TGA) on a Shimadzu TGA-50H in a quartz pan. All catalyst samples were pretreated under He at 100 C. for 30 min to remove surface contaminants and adsorbed water. Samples were heated to 700 C. at 10 C./min in air to oxidize all the carbon. The metal weight loading was determined by dividing the final weight by the initial weight prior to the temperature ramp. X-ray diffraction (XRD) analysis was conducted using a Rigaku Miniflex XRD with Cu K radiation and a Ni filter (=1.5418 ). The 2e range (10<2<90) was scanned at 5/min with a 0.02 step size. Crystallite sizes were estimated using the Scherrer equation and the Pt and Ru peaks were referenced to #04-0802 and #06-0663, respectively, from JADE XRD processing software. Imaging and chemical characterization of the catalysts were performed with scanning electron microscopy (Nova 200 Nanolab; Thermo Fisher) coupled with energy-dispersive X-ray spectroscopy (SEM/EDX).
Thermocatalytic Nitrate Reduction Experiments
[0178] Thermocatalytic nitrate reduction activity was measured in a 125 mL 3-neck jacketed flask (ChemGlass) at atmospheric pressure. For all experiments, 10 mg of catalyst was suspended in 100 mL of Millipore water and stirred at 500 rpm. The solution was sparged with H.sub.2 (Cryogenic Gases) for at least 30 min to remove dissolved oxygen and reduce the catalyst. The H.sub.2 partial pressure (0.1-1 atm) was adjusted accordingly by co-feeding Ar (Cryogenic Gases) while keeping the total flow rate consistent at 250 mL/min. The temperature (20-50 C.) of the reactor was controlled via a refrigerated/heated bath circulator (Fisher Scientific). Desired concentrations of nitrate (1-100 mM NaNO.sub.3) were added to the reactor at the beginning of the reaction after H.sub.2 pretreatment. For lower concentrations of nitrate (510 mM NaNO.sub.3), a sample was collected every 3 min for the first 15 min. At higher nitrate concentrations (>10 mM NaNO.sub.3), a sample was collected every 15 min to ensure accurate rate quantifications under differential conditions. In all cases, a 1 mL syringe was used to extract the sample from the reactor before centrifuging at 3000 rpm for 5 min to separate the aliquot solution and catalyst particles. Nitrate, nitrite, and ammonia concentrations were measured using a UV-Vis spectrometer (Thermo Fischer, Evolution 350). The activity is reported as a turnover frequency (TOF) in moles of aqueous products (e.g., ammonia, nitrite) per mole of surface metal per minute.
Electrocatalytic Reduction Experiments
[0179] A single-compartment, 3-electrode, glass electrochemical cell (Pine Research) was used for electrochemical measurements with a clean graphite rod (Alfa Aesar, Ultra F purity) as the counter electrode. A single junction reference electrode (Pine Research, in 4 M KCl) was used in solutions with pH less than or equal to 7, and a double-junction reference electrode (Pine Research, in 10% KNO.sub.3) was used in pH 10. Both reference electrodes were calibrated at 1 atm of H.sub.2 (Cryogenic Gases) in different pH solutions. The cell initially contained 100 mL of electrolyte solution (pH 0: 1 M sulfuric acid; pH 1: 0.1 M sulfuric acid; pH 3: 0.1 M sodium citrate+0.1 M citric acid; pH 5: 0.2 M sodium acetate+0.2 M acetic acid; pH 7: 0.2 M sodium phosphate+0.1 M citric acid; pH 10: 0.1 M sodium carbonate+0.1 M sodium bicarbonate; Sigma Aldrich) with all anions in the solution confirmed to not react at the operating potentials. The selected buffers were chosen from those that have previously been used to study pH effects for electrochemical reactions where anion adsorption was not reported to significantly impact the results. Prior to electrochemical experiments, N.sub.2 (Cryogenic Gases) was sparged through the solution with a stir bar for at least 45 min to remove traces of dissolved O.sub.2. Cyclic voltammogram (CV) scans after sparging confirmed the absence of dissolved O.sub.2 from the solution and stability of the working electrode.
[0180] The working electrode was prepared and tested as described previously. Briefly, a catalyst ink was prepared with a Nafion binder and deposited onto a glassy carbon rotating disk insert (Pine Research) to result in a total loading of 9.6 g of catalyst, including carbon. The prepared electrodes were cleaned by cycling 50 times between hydrogen evolution and oxidation potentials (from 0.1 to 1.2 V vs. RHE) at 100 mV s.sup.1. Both hydrogen underpotential deposition (H.sub.upd) and copper underpotential deposition (Cu.sub.upd) were used to accurately evaluate the electrochemically active surface area (ECSA) of the catalysts as described previously. After an 85% compensation for internal solution resistance as measured by electrochemical impedance spectroscopy, H.sub.upd was determined by cycling between the onset of HER to Pt oxidation (pH 0: 0.06-1.3, pH 1: 0.07-1.3, pH 3: 0.05-1.3, pH 5: 0.05-0.8, pH 7: 0.06-1.3, and pH 10: 0.04-1.3 V vs. RHE), at a scan rate of 100 mV s.sup.1 until the cyclic voltammograms were stable. The background-corrected hydrogen desorption charge and the average charge density of Pt (210 C cm.sup.2) were used to determine the ECSA.
[0181] All chronoamperometry measurements were taken after an 85% compensation for internal solution resistance as measured by electrochemical impedance spectroscopy. The rotating disk electrode (RDE) was held at a rotation rate of 2500 rpm to eliminate mass transfer limitations and minimize differences in the pH between the bulk solution and at the electrode surface. A rotation rate of 2500 rpm was selected, as it was sufficiently high where the reaction rates did not change with further increase in rotation rate. During the measurements, the bulk pH of the solution did not vary by more than a pH of 0.1. Currents were measured at four different applied potentials (0.05, 0.075, 0.1, 0.15 V vs. RHE) and recorded as the average current in the final 20 s. A baseline current was recorded in the electrolyte solution at each applied potential without the presence of nitrate. For ENO.sub.3RR experiments, 20 mL of dissolved sodium nitrate in electrolyte solution was added to reach the desired concentration (0.01, 0.03, 0.1, 0.5, 1 M NaNO.sub.3) before measuring the current at each applied potential.
Apparent Activation Energy Measurements
[0182] For ENO.sub.3RR measurements, reduction currents were recorded for 10 min at two applied potentials (0.05 V and 0.1 V vs. RHE) and four different temperatures (T=10, 20, 25, 30 C.) after compensating for 85% of the internal solution resistance. The TNO.sub.3RR experiments were prepared using similar methods as previously described and operated at four different temperatures (T=20, 30, 40, 50 C.). A heating/cooling jacket was used with a refrigerated/heated bath circulator (Fischer Scientific) to maintain the desired temperature. The difference in the temperature ranges selected were due to limitations of the experimental setup. For ENO.sub.3RR experiments above 30 C., thermal expansion caused the glassy carbon electrode to pop out of the Teflon holder. A wider range of temperatures was used for thermocatalytic measurements to reduce the influence of experimental error on the results. The apparent activation energy (E.sub.a) was evaluated from an Arrhenius plot of the current density or TOF.
Selectivity Measurements
[0183] ENO.sub.3RR measurements from depositing catalysts onto glassy carbon did not generate high enough currents to allow for product quantification. Thus, 10 mg of powder Pt.sub.xRu.sub.y/C catalysts were directly deposited on 2.52.5 cm.sup.2 pieces of carbon felt (6.35 mm thick, 99.0%, Alfa Aesar) in 40 mL of 1 M H.sub.2SO.sub.4. To ensure all of the catalyst was deposited onto the carbon felt, the solution was mixed for 30 min with bubbling H.sub.2 at 80 C. In a two-compartment electrochemical cell separated by a Nafion 117 membrane, these carbon felts (CFs) were attached to a graphite rod (AGKSP grade, ultra F purity, Alfa Aesar) for use as the working electrode for ENO.sub.3RR selectivity experiments.
[0184] Nitrate and select liquid-phase products (i.e., NO.sub.2.sup. and NH.sub.3) were measured using UV-vis spectrometer (Thermo Fischer, Evolution 350). Nitrate was quantified using standard spectrometry techniques. 10 L from the sample aliquot was acquired and diluted to 2 mL using Millipore water. 1 mL of this resulting, well-mixed solution was further diluted to 3 mL in a quartz cuvette (Fisher Scientific, Azzota Corp 10 mm). UV-Vis measurements were taken between 190-300 nm, and the nitrate concentrations were calculated via the adsorption peak at 220 nm. Millipore water was used as the background and subtracted from the sample spectra, and a calibration curve was created using known concentrations of NaNO.sub.3 in solution.
[0185] Nitrite (NO.sub.2.sup.) was quantified via a modified Griess diazotization reaction. 0.3 mL of the extracted sample aliquot was diluted to 1 mL and neutralized with 1 M NaOH. 40 L of the Griess color reagent, which consisted of 2% sulfanilamide (Fischer Scientific, >98%) and 0.2% N-(1-napthyl)-ethylenediamine (Sigma Aldrich, >98%) in phosphoric acid (Acros Organics; 85%) diluted to 0.1 M, was added. The resulting solution was left in the dark for 30 min before measuring absorbances at 543 nm. Known concentration of calibration standards were made from NaNO.sub.2 (>99.0%, Sigma Aldrich).
[0186] Ammonia was quantified by using the indophenol blue test with 1 mL of the sample aliquot. 1 M NaOH (Sigma Aldrich, 99.99%) was added to the electrolyte solution to neutralize the acid to a pH of 12. This was followed by sequentially adding 122 L of sodium salicylate (Sigma Aldrich, >99.5%), 27.3 L of sodium nitroprusside dihydrate (Sigma Aldrich, >99%), and 40 L of sodium hypochlorite solution (Sigma Aldrich, 4.00-4.99%) to the electrolyte solution and manually stirred together. The solution was covered and left for 40 min. The indophenol peak was identified as the maximum absorbance between 600-700 nm. A fresh 0.1 M HNO.sub.3 electrolyte solution prepared with the indophenol blue method was used as the background and subtracted from the sample spectra. If the concentration of NH.sub.3 was too high and oversaturated the detector, the solution was diluted and retested. A calibration curve was created using known concentrations of NH.sub.4Cl (99.99%, Sigma Aldrich) and unknown NH.sub.3 concentrations were calculated using the Beer-Lambert law.
[0187] The faradaic efficiency (FE) for ENO.sub.3RR was calculated by dividing the charge required to form the total NH.sub.3 measured by the total charge passed during the steady-state experiments. The total charge passed was calculated by integrating the reduction current over the duration of the experiment and the charge required from NH.sub.3 was calculated by assuming that eight electrons are required to form one molecule of NH.sub.3 from one molecule of nitrate.
[0188] The weight loading of the catalysts was determined by TGA (
[0189] X-ray diffraction (XRD) spectra for Pt.sub.xRu.sub.y/C catalysts and the corresponding Pt and Ru powder diffraction files are provided in
[0190] The average nanoparticle sizes of Pt/C, PtRu/C and Pt.sub.75Ru.sub.25/C were 2.6, 2.4, and 3.7 nm, respectively (Table 6). The crystallite sizes and weight loading of the catalysts were calculated by applying the Scherrer equation (Table 6). The different catalysts have roughly the same average particle sizes. Additionally, the particle size of Pt.sub.75Ru.sub.25/C matched previously synthesized materials.
TABLE-US-00006 TABLE 6 Crystallite sizes and metal weight percent loading for platinum-ruthenium catalysts. Catalysts Crystallite Size (nm) Weight Loading (%) Pt/C 2.6 0.6 27.3 PtRu/C 2.4 0.3 32.2 Pt.sub.75Ru.sub.25/C 3.7 1.0 25.1 Ru/C 2.9 0.5 28.7
[0191] Scanning electron microscopy (SEM) images and elemental analysis from energy dispersive spectroscopy (EDX) are shown in
[0192] SEM images of the Pt.sub.xRu.sub.y/C catalysts are provided in
[0193] As a result, a 500-rpm stir rate was used throughout TNO.sub.3RR experiments to ensure no external mass transport limitations. Without the presence of metals on the Vulcan carbon support, no catalytic activity is recorded (
[0194] For TNO.sub.3RR measurements for Pt/C, there was no observed nitrate conversion and ammonia production activity. To ensure that this result was due to a catalytic effect rather than experimental design issue, the amount of Pt/C was increased in the reactor from 10 mg to 50 mg.
[0195] Assuming ENO.sub.3RR follows a Langmuir-Hinshelwood model, both a single site model (SSM) and multisite model (MSM) were considered to model the reaction. SSM assumes a homogeneous electrode surface, the rate r can be derived as shown is Eq. S1 by inserting expressions for the coverages into Eq. 1. Both nitrate and H.sup.+ adsorb onto this single site and competitively inhibit the other species. The adsorption equilibrium constants K.sub.N and K.sub.H refer to the adsorption of nitrate and H.sup.+, respectively; C; refers to the bulk concentration of species i; k.sub.SSM denotes the rate constant of the surface reaction between adsorbed nitrate and hydrogen for the SSM.
[0196] If TNO.sub.3RR follows a surface reaction RDS, it will obey the same rate equation.
[0197] The main assumptions of the proposed multisite kinetic model (MSM) were as follows: 1) There are two adsorption sites on the catalyst surface, *.sub.1,*.sub.2; 2) The reaction only occurs between NO.sub.3*.sub.1 and H*.sub.2 and thus each species competitively inhibits the other on the opposite site. Adsorption equilibrium constants K.sub.1, K.sub.2, K.sub.3, K.sub.4, refer to Eqs. S2-5, respectively.
[0198] The rate determining step is seen in Eq. S6, resulting in the corresponding rate law shown in Eq. S7.
[0199] In Eq. S7, .sub.i*j; refers to the surface coverage of the species i on site j. From these assumptions, and assuming quasi-equilibrium in the adsorption reactions in Eqs. S2-S5, a rate law (Eq. S8) is derived relating reaction rate with bulk concentration of nitrate (C.sub.N) and H.sup.+ (C.sub.H), a constant of proportionality k.sub.MSM [M s.sup.1 m.sup.2], and using a site balance in Eq S9.
[0200] .sub.*1 and .sub.*2 are the coverage of open sites on site 1 and 2, respectively. This MSM rate law is compared to that of the SSM for accuracy in predicting the nitrate reduction reaction rate. A nonlinear least-square regression was performed on MATLAB version R2020b, relating current density to concentration of H.sup.+. The SSM can be reduced to a two-parameter fit (Eqs. S10, S11), and the MSM to a three-parameter fit (Eqs. S12, S13). The independent variable, x, may refer to C.sub.H, or 10.sup.pH depending on context.
[0201] The nitrate conversion TOF (for TNO.sub.3RR) and current density (for ENO.sub.3RR) was studied as a function of H.sub.2 pressure and applied potential, respectively, for the Pt/C, PtRu/C, and Pt.sub.75Ru.sub.25/C materials in
[0202] The activity of the catalysts followed the order Pt.sub.75Ru.sub.25/C>PtRu/C>Pt/C for both TNO.sub.3RR (
[0203] While the behavior of TNO.sub.3RR and ENO.sub.3RR with hydrogen pressure/applied potential and catalyst alloying are qualitatively the same, there were differences in the reactions when considering the quantitative activity of the catalysts. One difference in the behavior was that for Pt/C there was no measured activity during TNO.sub.3RR, even with increasing the amount of catalyst in the reactor (
Nitrate Concentration on PtRu/C
[0204] The data in
[0206] The ENO.sub.3RR activity as a function of nitrate concentration is rationalized using the Langmuir-Hinshelwood model used to generate the rate law in Eq. 1 (
[0207] When fitting the LH models to nitrate concentration and experimental data, the B and C fit parameters become equivalent, rendering the MSM mathematically identical to the SSM. The data in
[0208] Initially, both the SSM and MSM were fit to a pH range of 0-7 to describe the C.sub.H effect on rate (
[0209] The kinetic models explored were simplistic and only capture direct effects of C.sub.H and C.sub.N, and thus cannot provide a comprehensive understanding for the effect of pH on reaction rate. For example, pH affects the adsorption equilibria of both nitrate and protons while the model assumes these equilibria to be fixed.
[0210] The fitted rate law captures that the activity for PtRu/C in pH 7 increased with nitrate concentration up to 0.4 M, but decreased at higher nitrate concentrations. For a surface reaction involving adsorbed hydrogen and adsorbed nitrate, increasing the nitrate concentration had a similar effect as increasing the nitrate adsorption strength, as both lead to higher nitrate coverages. The model provided a qualitative description of the relationship between ENO.sub.3RR activity, nitrate adsorption energy, and nitrate concentration. Although, there was the possibility of a bifunctional (multi-site) mechanism on alloys, there was no conclusive evidence that this was the case from the kinetic modeling and thus it was postulated that only the simplest model that qualitatively describes the data.
[0211] The results in
pH Effects on Rate and Apparent Activation Energy of PtRu/C
[0212] Despite the similar effect of hydrogen chemical potential, alloying, and nitrate concentration between TNO.sub.3RR and ENO.sub.3RR, there were distinct differences when considering the effect of pH and apparent activation energies (E.sub.3),
[0213] Without intending to be bound by theory, it is believed that the higher activity and lower E.sub.a observed for TNO.sub.3RR at lower pH was most likely because it is easier for nitrite to either decompose or hydrogenate to other products in acidic conditions. At low pH, literature has indicated higher nitrite hydrogenation TOF rates through increased surface coverage of reaction intermediates, such as *NO and *HNO..sup.45 The E.sub.a for TNO.sub.3RR at pH 7 was 45 kJ mol.sup.1, similar to that of measurements of Pt group metals in neutral solution. The lower E.sub.a at pH 1 than pH 7 may arise from more favorable intermediate conversion to ammonia at low pH. It is also possible that the pH (and corresponding changes in the electrochemical double layer) affects the adsorption of nitrate, which would influence the rate.
[0214] The shift in activity and E.sub.a for ENO.sub.3RR with pH was more challenging to deconvolute than for TNO.sub.3RR. This change in activity may either be due to a different RDS entirely at the different pH values or the same RDS, but with different coverages of the intermediates. Although the pH may affect nitrate adsorption energy and thus the reaction rate because the effect of pH is opposite for TNO.sub.3RR than ENO.sub.3RR, other pH effects likely play a role in the reaction. Similarly, the conversion of nitrite being faster at lower pH values (as described above for TNO.sub.3RR) does not explain the trend in pH for ENO.sub.3RR.
[0215] Previous reports hypothesize a mechanistic shift occurs with an increase in the pH of the electrolyte solution for ENO.sub.3RR. In acidic media, the concentration of H.sup.+ correlated to the nitrate reduction activity. As the pH increased, the reaction stopped being dependent on H.sup.+, and the hydrogen source was provided from H.sub.2O. Similarly, in the results, the FE for ENO.sub.3RR changed from 93% at pH 1 to 54% at pH 7 (
[0216] The FE of PtRu/C towards NH.sub.3 at pH 1 and pH 7 are shown in
[0217] The PtRu/C current densities for ENO.sub.3RR for pH 0-10 at four different operating potentials vs. RHE are shown in
[0218] The effect of pH on other electrocatalytic reactions has been studied extensively, and some of the findings for other reactions may be applied to ENO.sub.3RR. Hydrogen binding energy is one proposed factor in which pH influences catalytic activity, but other effects, such as the ionic strength of the buffer, hydrogen equilibrium potential, point of zero free charge (pzfc), and water orientation and reorganization energy, can also influence the activity. For hydrogen evolution, the activities for Pt group metals are much higher at lower pH values, but the reason is debated in several recent reviews and publications. This enhancement is the opposite direction of what was observed for ENO.sub.3RR. Oxygen reduction reaction (ORR) is more complicated, with ORR activity on Pt(111) increasing as the pH increases from 1 to 6 and decreasing with increasing pH past 11 and a predicted maximum at pH 9. This trend is attributed to the ORR onset potential being positive and negative with respect to the pzfc of the electrode in acidic and basic solution, respectively, causing the switch in pH dependence. For ENO.sub.3RR on PtRu/C, there seems to be a maximum with pH similar to ORR, but the ENO.sub.3RR maximum occurs at pH 5 (excluding potential oxide effects at pH 10). Thus, one possible cause of the pH dependence of ENO.sub.3RR could be differences in the surface charge of the electrode.
[0219] Although a Langmuir-Hinshelwood model describes some of the reaction data, it does not adequately capture the effects of pH and the buffer solutions on the activity (
[0220] For all the ENO.sub.3RR measurements in different pH, different buffer solutions were prepared to ensure that the pH of the solution remains constant throughout the reaction. However, the ionic strength of the solution can also influence the reduction currents.
Net Changes in PH During Reaction
[0221] The balanced full-cell nitrate to ammonia reaction for ENO.sub.3RR is:
[0222] Here it is assumed that oxygen evolution is the anodic reaction. The net reaction is:
For TNO.sub.3RR, if the hydrogen is produced from water electrolysis the reaction is:
[0223] The TNO.sub.3RR using this hydrogen is:
[0224] Thus, the net reaction is the same as ENO.sub.3RR if H.sub.2 comes from water electrolysis:
[0225] Therefore, in both cases one net proton would be consumed per ammonia produced, requiring a balancing to maintain a constant pH. Although more than one proton is required for ENO.sub.3RR half-cell reaction, all but one proton is provided from the anodic reaction, which in a commercial system would be via a proton conducting membrane. Without a sufficiently conductive or selective membrane, a local pH gradient may build up at the cathode compartment in a commercial system.
[0226] Many modifications and other embodiments disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain having the benefit of the teachings presented in the foregoing descriptions. Therefore, it is to be understood that the disclosures are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
[0227] It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. As used in the specification and in the claims, the term comprising can include the aspect of consisting of. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined herein.
[0228] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.
[0229] The use of the terms a, an, the, and similar referents in the context of the disclosure herein (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated. Recitation of ranges of values herein merely are intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended to better illustrate the disclosure herein and is not a limitation on the scope of the disclosure herein unless otherwise indicated. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure herein.
Example 7Electrochemical Reactor
Electrocatalyst Preparation.
[0230] Supported nanoparticle catalysts were prepared as inks and deposited on a glassy carbon rotating disk electrode (RDE, Pine Research Inst., Inc) or a carbon felt (6.35 mm thick, 99.0%, Alfa Aesar). 30 wt % Pt/C, 20 wt % Rh/C, and 30 wt % PtRu/C were used as catalysts. For all inks, a 5 wt % Nafion in alcohol solution (Sigma Aldrich) was used as the binder. The catalyst was pre-treated before being used for flow cell measurements.
Electrochemically Active Surface Area.
[0231] The electrochemically active surface area (ECSA) was measured using hydrogen underpotential deposition (H.sub.upd) on Pt/C, Rh/C, and PtRu/C. Copper underpotential deposition (Cu.sub.upd) was only measured on PtRu/C in the batch cell configuration. The ECSA for normalizing current was measured prior to each electrochemical measurement (each current density has an ECSA value). The potential range was 0.05 to 0.8 V vs. RHE for Rh/C and 0.05 V to 1.0 V vs. RHE for Pt/C and PtRu/C.
Electrocatalytic Nitrate Reduction Reaction Measurements.
[0232] Nitrate reduction measurements were taken at a constant potential for 20 minutes on a VSP or SP 150 potentiostat using EC-Lab software (BioLogic, Inc.). In the both the batch and flow cell, the current density reported was from the average current during the last 5 minutes of the measurement. In the batch cell with the catalysts deposited on a RDE, 85% iR compensation was applied during the measurement and the correction was less than 1 mV. iR correction for the flow cell measurements was applied after measurement; the series resistance was 0.05 for all catalysts in 0.1 M H.sub.2SO.sub.4 and 0.1 M HNO.sub.3. For the flow cell, a sample of the outlet solution was taken during the last 5 minutes of the measurement. The electrolytes for the working and counter electrode compartments were held in 60 mL plastic syringes and flowed through the electrochemical cell using a syringe pump (LongerPump model LSP02-1B). The electrolyte was sparged with N.sub.2 for 1 hour prior to being drawn into the syringe. The electrolyte in the syringe was degassed by drawing a slight vacuum by pulling the plunger, tapping on the side of the syringe, and releasing the gases that accumulated at the opening of the syringe. Degassing was performed until the amount of gas at the opening of the syringe was negligible. The solution in the syringes was degassed to prevent formation of gas pockets (of O.sub.2 or H.sub.2) within the electrochemical cell or carbon felt during operation which could lower the available surface area for reaction or increase the cell potential.
Product Quantification and Faradaic Efficiency Towards Ammonium.
[0233] The samples for constant potential measurements were collected in a scintillation vial at the outlet of the flow cell over the last 5 minutes of the measurement. For Faradaic efficiency analysis, the currents from one residence time prior to sample collection time were used. UV-Vis spectroscopy was used for ammonium (NH.sub.4.sup.+) quantification. Ammonium itself is not active in the UV-Vis range, therefore the salicylate colorimetric method was used.
Flow Reactor Setup and Testing
[0234] The electrochemical flow reactor had two compartments (working and counter/cathode and anode) and a connection for a reference electrode (RE) (working electrode (WE) compartment) as shown in
[0235] A IrO.sub.2 paper was also used as a CE alongside the unmodified carbon felt to lower the overpotential for oxygen evolution (and total cell potential). The carbon felt and IrO.sub.2 paper CE was loaded into the counter electrode compartment and then covered with a Nafion 117 membrane which had been stored in Millipore water for at least 24 hours prior. The working electrode compartment was then constructed similarly using the prepared WE felt. The electrolyte was flowed through the flow cell for twice the residence time prior to electrochemical measurements. From open circuit voltage (700 mV vs. RHE for Pt/C and Rh/C in 0.1 M H.sub.2SO.sub.4), for the first measurement, the potentiostat had difficulty applying a potential or performing a cyclic voltammogram. To avoid this, the catalyst was pre-conditioned before use.
Load PtRu Catalyst onto High Surface Carbon Felt
[0236] Measuring the electrochemical surface area is imperative to compare the catalyst performance in the batch RDE system and the carbon felt flow cell system. ECSAs were determined for catalysts in the batch RDE system and the flow cell system while loaded on a carbon felt using the method developed here.
[0237] The fraction of catalyst that is electrochemically active was higher on the RDE than on the carbon felt for all catalysts tested (Table 7). The catalysts deposited onto the felt have lower ECSA per mass loaded when deposited due to catalyst loss on the felts, parts of the catalyst may not be accessible to the electrolyte due to packing of the catalyst on the felt, and underestimation of ECSA using H.sub.upd for catalyst in the felts. The felt loading is estimated by subtracting the mass of catalyst that does not adhere to the felt (collected after deposition) by the total mass of catalyst attempted to be deposited onto the felt. Though an ionomer binder was used during catalyst deposition on to the carbon felts, the catalyst nanoparticles can be knocked loose from the felt during the reaction and were observed in the outlet collection reservoir after the experiment. Catalyst loss on the felts would underestimate the ECSA per mass for the carbon felts with catalyst. Additionally, over-packing electrocatalyst ink on the carbon felt could reduce the active material available to the solution by creating catalyst ink layers, where the bottom layer does not contact the solution.
TABLE-US-00007 TABLE 7 Catalyst loading (metal only) onto RDE and felt, electrochemically active surface area from H.sub.upd or Cu.sub.upd, and ECSA per mass loaded. RDE geometric area is 0.196 cm.sup.2 and felt geometric area is 5.52 cm.sup.2. RDE ECSA ECSA Felt ECSA ECSA Catalyst loading from RDE per mass loading from felt per mass 20 wt % Rh/C 2.9 g 2.3 cm.sup.2 78.1 m.sup.2/g 2.0 mg 142 cm.sup.2 34.6 m.sup.2/g 30 wt % Pt/C 1.9 g 2.0 cm.sup.2 105.3 m.sup.2/g 5.5 mg 1012 cm.sup.2 60.6 m.sup.2/g 30 wt % PtRu/C 1.9 g 1.2 cm.sup.2 63.7 m.sup.2/g 3.3 mg 374 cm.sup.2 37.8 m.sup.2/g
[0238] All three catalysts lose ECSA during nitrate reduction, but PtRu/C was observed to especially undergo preferential Ru dissolution.
[0239] Without intending to be bound by theory, it is believed that the ECSA loss may be related to poisoning of intermediates, surface restructuring, or mechanical loss. To address the issue of varying ECSA for kinetic measurements, the ECSA was measured before and after kinetic measurements to account for the decrease in available surface area. Between prior to and between sequential nitrate reduction measurements, the electrodes were cycled between oxidative and reductive potentials to clean the electrode surface, which for PtRu/C would cause Ru dissolution during the cleaning procedure.
Initial Ammonia Production Quantification as a Function of Potential in Flow Conditions
[0240] The current densities at a given potential did not match for all catalysts between those in the RDE and those in the flow cell. Within the flow cell activity sets, there was large uncertainty in the value reported (e.g., Rh/C at 0.1 V vs. RHE at 2 mL/min is 4-5 times greater in
[0241] The activity trends on an RDE (Pt/C<PtRu/C<Rh/C) match those in the flow cell. In
Results for Ammonia Production
[0242] The highest rate of ammonia production in the flow cell was with Rh/C at 0.05 V vs. RHE with a flow rate of 2 mL/min with a nitrate reduction current density to ammonia of 0.18 mA/cm.sup.2 normalized to the Rh surface area and 105 mA/cm.sup.2 normalized to the geometric surface area (electrolyzer area).
[0243] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the disclosure may be apparent to those having ordinary skill in the art.
[0244] All patents, patent applications, government publications, government regulations, and literature references cited in this specification are hereby incorporated herein by reference in their entirety. In the case of conflict, the present description, including definitions, will control.
[0245] Throughout the specification, where the compounds, compositions, methods, and/or processes are described as including components, steps, or materials, it is contemplated that the compounds, compositions, methods, and/or processes can also comprise, consist essentially of, or consist of any combination of the recited components or materials, unless described otherwise. Component concentrations can be expressed in terms of weight concentrations, unless specifically indicated otherwise. Combinations of components are contemplated to include homogeneous and/or heterogeneous mixtures, as would be understood by a person of ordinary skill in the art in view of the foregoing disclosure.
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