Palladium catalyst for oxidation of methane and method of preparation and use thereof

Abstract

This invention relates to a novel palladium catalyst for the substantially complete oxidative removal of methane from exhaust streams at low operating temperatures compared to other current palladium catalysts and to methods of preparing the catalyst. Use of the catalyst to remove methane from vehicle exhaust streams, crude oil production and processing exhaust streams, petroleum refining exhaust streams and natural gas production and processing exhaust streams.

Claims

1. A catalyst for methane oxidation, comprising: nanoparticulate palladium species in the size of 1 nm to 100 nm; non-hierarchical ceria; and non-hierarchical alumina; wherein the catalyst is characterized by: light-off at approximately 200° C.; complete oxidation at approximately 430° C.; stability up to at least 800° C.; consistent catalytic activity to at least 800° C.; consistent catalytic activity for exhaust streams containing up to 15 wt % water to at least 800° C.; a Pd—O—Ce solid solution segregated on the surface of the alumina support; an X-ray photoelectron spectroscopy (XPS) of the Pd(3d) core level region that comprises doublets at least at the following binding energies: 335.4 eV, 336.8 eV, and 338.8 ev; and wherein the catalyst comprises 5 wt % nanoparticulate palladium species and 5 wt % ceria, both based on the quantity of alumina present.

2. The catalyst of claim 1, wherein the catalyst has a BET surface area of at least 88 m.sup.2/g.

3. The catalyst of claim 1, having an XPS of the Pd(3d) core level region as shown in FIG. 7.

4. The catalyst of claim 1, having an XPS of the Ce(3d) core level region as shown in FIG. 8.

Description

DETAILED DESCRIPTION

Brief Description of the Figures

(1) FIG. 1 shows a graph of the percent conversion of methane versus reaction temperature using a catalyst of this invention.

(2) FIG. 2 shows a light-off curve graph showing the percent conversion of methane versus reaction temperature during a full heating and cooling cycle using a catalyst of this invention.

(3) FIG. 3 shows the effect of wet-feed conditions on the percent conversion of methane using a catalyst of this invention.

(4) FIG. 4 shows the stability of a catalyst of this invention under dry-feed and wet-feed conditions

(5) FIG. 5 shows an XPS core-level spectrum of an IWI-prepared catalyst involving Pd.sub.3d 5PdI sample.

(6) FIG. 6 shows another XPS core-level spectrum of an IWI-prepared catalyst involving 5PdI Ce3d.

(7) FIG. 7 shows an XPS core-level spectrum of an SCS-prepared catalyst involving Pd.sub.3d 5P5CA.

(8) FIG. 8 shows an XPS core-level spectrum of an SCS-prepared catalyst involving Ce3d 5P5CA.

DISCUSSION

(9) It is understood that, with regard to this description and the appended claims, any reference to any aspect of this invention made in the singular includes the plural and vice versa unless it is expressly stated or unambiguously clear from the context that such is not intended.

(10) As used herein, any term of approximation such as, without limitation, near, about, approximately, substantially, essentially and the like means that the word or phrase modified by the term of approximation need not be exactly that which is written but may vary from that written description to some extent. The extent to which the description may vary will depend on how great a change can be instituted and have one of ordinary skill in the art recognize the modified version as still having the properties, characteristics and capabilities of the word or phrase unmodified by the term of approximation. In general, but with the preceding discussion in mind, a numerical value herein that is modified by a word of approximation may vary from the stated value by ±15%, unless expressly stated otherwise.

(11) As used herein, the use of “preferred,” “presently preferred,” “more preferred,” “preferably,” and the like refers to preferences as they exist at the time of filing of this application.

(12) As used herein, the term “substantially complete” refers to a process that achieves an end-result as close to, and preferably at, 100% completion of whatever the process is intended to accomplish, in the present instance conversion of methane to carbon dioxide and water. Thus, when it is claimed that a catalyst of this invention is capable of completely oxidizing methane or trace methane in an exhaust stream, what is meant is that methane is undetectable in the exhaust stream after treatment using the best current analytic methodology. With regard to self-sustained combustion, completion refers to the time at which the self-sustained temperature of the combustion reaction decreases from its highest sustained level.

(13) As used herein, “consistent catalytic activity” refers to a catalytic methane oxidation in which the light-off curve shows a steadily increasing percent conversion of methane until 100% conversion is achieved and then maintenance at the 100 percent conversion level for essentially the full useful lifetime of the catalyst in a continuous methane-containing exhaust stream or until all the methane has been converted in a batch-type waste stream.

(14) As used herein, “methane oxidation” refers to the degradation of methane to CO.sub.2 and water.

(15) As used herein, “nanoparticulate” refers to particles in the size range of approximately 1 nm to approximately 100 nm.

(16) As used herein, “palladium species” refers, at least, to palladium metal, Pd.sup.0, palladium oxides involving Pd(II) and Pd(IV) species and Pd—O—CeO.sub.2.

(17) As used herein, “ceria” refers to cerium oxide, CeO.sub.2.

(18) As used herein, “alumina” refers to aluminum oxide, Al.sub.2O.sub.3.

(19) As used herein, “non-hierarchical” refers to ceria and alumina that has not pre-formed onto a physical superstructure comprising pores prior to introduction of Pd species into the catalyst preparation procedure.

(20) As used herein, “light-off” refers to the temperature at which catalytic oxidative decomposition of a material, in the present case methane, is initiated. The cool-down of the system is referred to as the “light-down” portion of the cycle.

(21) As used herein, an “exhaust stream” refers to any gaseous effluent from any manner of device, process or procedure, which effluent contains methane including, without limitation, motor vehicle exhaust, crude oil production and processing exhaust; petroleum refining exhaust; and natural gas production and processing exhaust.

(22) A catalyst of this invention is prepared by a one-step solution combustion synthesis (SCS) procedure in which a water soluble palladium salt, a water soluble cerium salt, a water soluble aluminum salt and an organic reductant are heated in air until auto-ignition occurs and the ensuing combustion is allowed to proceed to completion. After completion of auto-combustion, the resulting powder is calcined in air at 800° C. More specifically, Pd(NO.sub.3).sub.2, Ce(NO.sub.3).sub.3, Al(NO.sub.3).sub.3 and glycine are treated as indicated above. The result is a solid-state solution of palladium/ceria supported on alumina. Examination of the resulting catalyst by x-ray photoelectron spectroscopy (XPS) revealed that the SCS process results in the formation of a solid solution in which Pd ions are doped into a ceria lattice associated with the reduction of some Ce.sup.4+ to Ce.sup.3+, presumably with the formation of oxygen vacancies. A comparison of the XPS spectra of SCS-prepared catalysts with that of IWI-prepared catalysts of the same chemical composition are shown in FIGS. 5-8. As can be seen, the spectra are substantially different. The Pd/ceria segregates on the surface of the alumina support whereupon the intrinsic Ce.sup.3+ and oxygen vacancies are believed to play a major role in the resultant activity of the catalyst.

(23) Even more specifically, the catalyst of this invention was prepared by dissolving a selected quantity of Al(NO.sub.3).sub.3.9H.sub.2O in distilled water. An amount of Pd((NO.sub.3).sub.2.3H.sub.2O that will result in 5 wt % Pd based on the calculated amount of Al.sub.2O.sub.3 that will result from the combustion of the selected quantity of Al(NO.sub.3).sub.3.9H.sub.2O is added as is an amount of Ce(NO.sub.3).sub.3.6H.sub.2O that will result in 5 wt % CeO.sub.2 based on the calculated amount of Al.sub.2O.sub.3 that will result from the combustion of the selected quantity of Al(NO.sub.3).sub.3.9H.sub.2O. Glycine is then added such that the ratio of glycine to the total of Al(NO.sub.3).sub.3.9H.sub.2O, Pd((NO.sub.3).sub.2.3H.sub.2O and Ce(NO.sub.3).sub.3.6H.sub.2O is about 1:1.4. In a presently preferred embodiment, quantities of the above materials that will result in 5 wt % Pd and 5 wt % CeO.sub.2 based on weight of Al.sub.2O.sub.3 support present in the catalyst. Once dissolved, the solution is heated until auto-combustion is initiated at which time the heat source is removed and the reaction allowed to proceed to completion in auto-thermal mode. The powder obtained is then calcined in air at 800° C. to complete the synthesis.

(24) The material resulting from the above SCS preparation was tested as a catalyst for the oxidation of methane and was found to be extremely capable in all aspects. That is, it exhibited superior low temperature performance with a light-off temperature of approximately 200° C. followed by substantially complete oxidation at approximately 430° C. Stability of the catalyst in this temperature range was excellent, the catalyst showing no diminishment in activity throughout the test period which was approximately 60 hours. Further, when the temperature of the oxidation system was raised to approximately 800° C., no reduction in activity of the catalyst was observed for a substantial period of time, oxidation continuing unabated. Perhaps most importantly with regard to this SCS Pd-based methane catalyst, the presence of up to 15 wt % water in the exhaust stream begin treated had no observable effect on the efficiently of the catalytic process at all temperatures, 200° C. to 800° C.

(25) The above results were confirmed by various analytical methodologies. That is, the percent conversion of methane at temperatures ranging from 200° C., the light-off temperature, to 800° C. in a temperature programmed reaction (TPRS) run at a 2° C./min ramp-up heating regime showed a smooth and rapid increase from 200° C. to about 430° C. where substantially 100% conversion occurred, a degree of conversion that was maintained all the way to 800° C. This is shown in FIG. 1.

(26) FIG. 2 shows a typical light-off curve for methane oxidation using a catalyst of this invention. As can be seen, light-off begins at approximately 200° C. and continues increasing unabatedly as the temperature is ramped up at 2° C./min to about 430° C., at which temperature substantially complete oxidation of methane is observed to occur. Oxidation remains at the 100% level from 430° C. all the way to the top temperature recorded, 800° C. The system was then cooled down, likewise at 2° C./min. during which methane oxidation remained at 100% on the graph until the temperature again reached approximately 430-450° C. and then began to drop as the temperature was reduced back to 200° C. Only a slight hysteresis was observed between the heating and cooling curves indicating that the catalyst was relatively unaffected by the time spent at 800° C., which attests to the durability of the catalyst.

(27) FIG. 3 is another light-off curve diagram comparing the curve for methane oxidation under dry-feed conditions with the curve for methane oxidation under wet-feed condition. The dry-feed curve, of course, follows the previously observed path: light-off at 200° C., complete combustion from 400° C. upward followed by a similar curve for the cooling down cycle with only a slight hysteresis on the downward curve. The wet-feed curves are almost mirror images of the dry-feed curves but shifted very slightly to the right. Thus, as previously stated, the catalyst of the instant invention is virtually impervious to the presence of water, at least up to 15 wt % water in a wet-feed exhaust stream.

(28) FIG. 4 shows the stability of a catalyst of this invention over a 60-hour test period at 450° C. wherein the approximately 25 hours of the test were carried out under dry-feed conditions and then 12.16 wt % water was added to the feed and the test was continued for the remaining 30 hours. No change in the efficiency of the catalyst is observed in going from a dry-feed to a wet-feed condition; the conversion of methane remained as substantially complete level throughout.

EXPERIMENTAL

(29) Catalyst Synthesis

(30) As an example to prepare 1 g of 5 wt % Pd.5 wt % CeO.sub.2/Al.sub.2O.sub.3 by the solution combustion synthesis (SCS) technique, 0.125 g of palladium(II) nitrate trihydrate (Pd(NO.sub.3).sub.2.3H.sub.2O, BDH), 0.126 g of cerous (III) nitrate hexahydrate (Ce(NO.sub.3).sub.3.6H.sub.2O, Fluka-Garantie, >99.0%) and 6.6 g of aluminum nitrate hexahydrate (Al(NO.sub.3).sub.3.9H.sub.2O, Sigma Aldrich, 99.9%) precursor salts are dissolved in a 50 ml deionized water in a 250 ml capacity beaker and stirred well to get a homogeneous mixture. This was followed by the addition of 3.56 g glycine (Sigma Aldrich, 98.5%), to obtain fuel to oxidizer ratio of around 1/1.4 is added into the mixture as well. The resulting solution is heated over a hot plate for combustion. The reaction is exothermic in nature and once the combustion initiates, it proceeds in an auto-thermal mode without any external heating source. The synthesized nano-powder is then sintered in air by heating at a rate of 1° C./min till reaches 800° C. where stays for 3 hr, then cools down to room temperature, also, at a cooling rate of 1° C./min.

(31) The activity of the present SCS catalyst is benchmarked with a traditional palladia/ceria/alumina catalyst. In the two cases the palladium content is fixed at 5 Wt %. The traditional catalyst is prepared via a wet impregnation method. In this procedure, prior to metal loading 0.9 g alumina (SASOL) was treated with 10 ml aqueous solution containing 0.126 g of cerous (III) nitrate hexahydrate (Ce(NO.sub.3).sub.3.6H.sub.2O, Fluka-Garantie, >99.0%) solution for 5 hours followed by drying overnight at 120° C. and calcining at 800° C. 0.125 g of precursor salt (Pd(NO.sub.3).sub.2.3H.sub.2O, BDH) was dissolved in deionized water and introduced to the calcined support dropwise. The resultant slurry was stirred for 6 hours followed by drying at 120° C. and calcination in a tubular furnace at 800° C. for three with 1° C. heating and cooling rates. The impregnation method catalyst is denoted as 5PdI.
Catalytic Activity
Catalytic performances for methane oxidation were investigated in a U-shaped, quartz reactor connected with online Quadrapole mass spectrometer HPR20 [Hiden Analytical]. Light-off measurements were carried out with 30 mg of catalyst at a total gas pressure of 1 atm with CH.sub.4 (5% CH.sub.4/Ar) to O.sub.2 (1% O.sub.2/Ar) v/v ratio of 2, while maintaining a Gas Hourly Space Velocity (GHSV) of ˜20168.1 mLg.sup.−1h.sup.−1. The heating and cooling rates (800 to 200° C. and 200 to 800° C.) of the reactor are ramped at 2° C./min in all measurements. For experiments under wet conditions, 18% steam is introduced into the reaction mixture by passing the gases through a water saturator which was preheated to 50° C. Prior to each experiment, the catalyst is activated by treating with 5% O.sub.2/Ar at 30 mLmin.sup.−1 for 30 minutes at 500° C. This step is followed by flushing the catalyst bed with 20 mLmin.sup.−1 argon for 20 minutes and raising the furnace temperature to 800° C. to record the light-off curves. Light off curves are taken under heating and cooling conditions at rates of +2 and −2K min.sup.−1, respectively. The CH.sub.4% conversion is calculated using equation 1;

(32) $\mathrm{CH}4\mathrm{conversion}\left(\%\right)=\left[\frac{{\mathrm{CH}}_4\left(\mathrm{in}\right)-{\mathrm{CH}}_4\left(\mathrm{out}\right)}{{\mathrm{CH}}_4\left(\mathrm{in}\right)}\right]\times 100$

(33) For the wet-feed study, the reactant gas mixture was bubbled through a saturator heated to 50° C.

(34) XPS Analysis of SCS Method Leading to Formation of PD-O—Ce Solid Solution

(35) The surface chemical composition and distribution of palladium species laying on the surface of the calcined catalysts were investigated by means of XPS measurement. The Pd3d spectra (FIG. 5) of 5PdI (prepared by impregnation method) catalyst exhibited the presence of Pd.sup.2+ (PdO) and metallic Pd.sup.0. The XPS spectrum is characterized by two doublet Pd3d peaks: the binding energies 335.1 eV and 340.3 eV were assigned to PdO, whereas the second doublet at binding energies at 333.4 eV and 338.3 eV were assigned to Pd.sup.0. As can be seen in FIG. 6, in the XPS spectra f the Ce(3d) core level region of the 5PdI the Ce3d.sub.3/2 and 3d.sub.5/2 appears at 920 eV and 882 eV with well separated spin-orbit components (Δ=18.6 eV) having multiple splitting that are typical of Ce.sup.4+ in CeO.sub.2.

(36) Contrary to 5PdI, the XPS spectra of 5P5CA (prepared by solution combustion synthesis method) catalyst exhibited much broader and less symmetric peaks suggesting a larger heterogeneity in its palladium environments. The XPS of the Pd(3d) core level region are presented in FIG. 7. Six Pd(3d5/2,3/2) peak doublets were observed. The binding energies at 335.4 and 336.8 eV were assigned to Pd(3d5/2) in Pd metal and PdO, respectively. The third doublet with its Pd(3d5/2) peak at 338.8 eV was assigned to Pd ions which are much more ionic than those associated with PdO. This highly ionized Pd ions of the samples prepared by the SCS can be assigned to Pd ions which are inserted into the ceria lattice forming pallida/ceria solid solution. The appearance of the six Pd XPS doublets, comparing to only four Pd XPS doublets for the impregnation catalyst, strongly indicated the formation of Pd/ceria solid solution as has already been noticed in previous literature (Colussi et al., 2015; Colussi et al., 2009; Priolkar et al., 2002). In the theoretical study by Scanlon et al. (Scanlon, Morgan, & Watson, 2011), it was predicted that the ground state structure of Pd doped CeO.sub.2 is a square planar with Pd ions having d.sup.8 configuration; meaning that Pd.sup.2+ rather than Pd.sup.4+ is the predominant oxidation state in the Pd—O—Ce solid solution linkage. These Pd.sup.2+ ions have greater ionic property than the Pd.sup.2+ ions of normal PdO. Therefore, it is plausible to assign the high energy Pd XPS doublet at 338.8 eV to the strongly ionized Pd.sup.2+ ions inserted into the Pd—O—Ce solid solution linkage.

(37) On the basis of XPS results, it is reasonable to conclude that, in contrast to conventional method, during the solution combustion synthesis (SCS), Pd is inserted into the ceria lattice forming non stoichiometric Pd—O—Ce solid solution which is segregated on the surface of the alumina support. This explanation is in agreement with conclusions from the experimental work by Colussi et al. (Colussi et al., 2015; Colussi et al., 2009) and Priolkar et al. (Priolkar et al., 2002) as well as the theoretical prediction by Scanlon et al. (Scanlon et al., 2011). The insertion of Pd ion into the ceria lattice must be associated with oxygen vacancy formation; consequently, with the generation of Ce.sup.3+ ions. Indeed, XPS showed the formation of Ce.sup.3+ ions in all catalysts prepared by the SCS method, FIG. 8, though, these catalysts are formed under oxidation condition during the SCS. It is worth mentioning that the XPS of the catalyst 5PI, prepared by the impregnation, FIG. 6 does not show any peaks corresponding to Ce.sup.3+ species.