Gold supported yttrium oxide nanorods and methods of use thereof
10987657 ยท 2021-04-27
Assignee
Inventors
Cpc classification
Y02P20/52
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
C07C5/32
CHEMISTRY; METALLURGY
B01J35/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
A catalyst comprising Y.sub.2O.sub.3 nanorods and gold nanoparticles dispersed on a surface of the nanorods is provided. The gold is present at a concentration of 0.5-2 wt %. A method of forming olefins by oxidative cracking is also provided. The method includes reacting an alkane with a reactant gas mixture in the presence of a catalyst under conditions suitable for forming light olefins (ethtylene and propylene).
Claims
1. A catalyst, comprising: nanorods consisting of Y.sub.2O.sub.3; and gold nanoparticles dispersed on a surface of the nanorods, wherein the gold is present at a concentration of 0.5-2 wt %.
2. The catalyst of claim 1, wherein the Y.sub.2O.sub.3 nanorods have a diameter ranging from 10-20 nm.
3. The catalyst of claim 1, wherein the Y.sub.2O.sub.3 nanorods have a length ranging from 75-110 nm.
4. The catalyst of claim 1, wherein the gold nanoparticles are spherical in shape.
5. The catalyst of claim 4, wherein the gold nanoparticles have a diameter ranging from 20-50 nm.
6. A method of forming olefins by oxidative cracking, comprising: reacting an alkane with a reactant gas mixture in the presence of a catalyst according to claim 1 under conditions suitable for forming olefins.
7. The method of claim 6, wherein the alkane is n-propane and the olefins comprise ethylene and propylene.
8. The method of claim 6, wherein the reactant gas mixture comprises oxygen and argon.
9. The method of claim 8, wherein the reactant gas mixture comprises 15-25% oxygen and 75-85% argon.
10. The method of claim 6, wherein the reaction is performed at a temperature of 450 C. to 650 C.
11. The method of claim 6, wherein the reaction occurs under a gas hourly space velocity (GHSV) of 47000-49000 h.sup.1.
12. The method of claim 6, wherein at least 75% of the alkane is converted in the reaction.
13. The method of claim 6, wherein the reaction has olefins selectivity of at least 90%.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.
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DETAILED DESCRIPTION
(12) Embodiments of the disclosure provide gold supported yttrium oxide (Y.sub.2O.sub.3) nanorods that may be utilized as catalysts for oxidative cracking of alkanes such as n-propane to produce light olefins such as ethylene and propylene. Gold deposition provides a significant enhancement in both alkane conversion and olefins selectivity. This synergistic effect between the gold and Y.sub.2O.sub.3 nanorod support is due to a strong metal-support interaction.
(13) As used herein, nanorods are rod-shaped particles that have a length at least twice a radius or width and are typically 1 to 200 nm in length. The nanorods are solid structures composed, at least partially, of the metal oxide. The nanorods are single crystals.
(14) Y.sub.2O.sub.3 has been utilized as a catalyst or catalytic support in different reactions due to its basic nature. It was also found that Y.sub.2O.sub.3 is a suitable catalyst for oxygen activation [26], in addition it possesses excellent hydrothermal stability [27]. As described herein, Y.sub.2O.sub.3 is utilized to be a support to disperse gold nanoparticles. Yttrium oxide nanorods may be synthesized by methods known in the art e.g. by an alkali assisted hydrothermal synthesis method. For example, crystalline yttrium nitrate may be dissolved in double distilled water. To this solution, potassium hydroxide solution may be added under constant stirring to obtain a precipitate having a pH around 13. Then, the obtained precipitate subjected to a hydrothermal treatment, and subsequently cooled to obtain a white colored solid product. After washing, the cake is dried and then calcined to obtain yttrium oxide nanorods.
(15) Other methods for producing nanorods include, for example, controlled vapor-solid growth processes using a metal vapor source.
(16) The yttrium oxide nanorods may be loaded with gold nanoparticles using, for example, a chemical reduction method. For example, after dissolution of a gold precursor (e.g. HAuCl.sub.4) in water, an appropriate amount of a reducing agent (e.g. NaBH.sub.4) is added to reduce the Au precursor and produce gold nanoparticles. The Y.sub.2O.sub.3 nanorods are impregnated with the gold nanoparticles, washed, dried, then calcined to provide gold nanoparticles dispersed on the surface of Y.sub.2O.sub.3 nanorods.
(17) In some embodiments, the gold is present at a concentration of 0.5-2 wt %, e.g. about 1.5% based on the weight of the total catalyst including the nanorods. In some embodiments, the Y.sub.2O.sub.3 nanorods have a diameter ranging from 10-20 nm, e.g. about 15 nm. In some embodiments, the Y.sub.2O.sub.3 nanorods have a length ranging from 75-110 nm. In some embodiments, the gold nanoparticles are spherical in shape. In some embodiments, the gold nanoparticles have a diameter ranging from 20-50 nm.
(18) Further embodiments provide a method of forming olefins by oxidative cracking comprising reacting an alkane with a reactant gas mixture in the presence of a catalyst as described herein under conditions suitable for forming olefins. In some embodiments, the alkane is n-propane. Other suitable alkanes include, but are not limited to, ethane and n-butane. Suitable olefins include, but are not limited to, ethylene, propylene, and butylene.
(19) The amount of catalyst added to the reaction is a catalytically effective amount. As used herein, the term catalytically effective amount refers to an amount of catalyst that provides a measurable conversion of alkane to olefins. The exact concentrations employed, of course, will depend on, among other factors, the desired catalytic properties. Examples of catalytic properties include, inter alia, operability (resistance to runaway), selectivity, activity, conversion, stability and yield. It is understood by one skilled in the art that one or more of the individual catalytic properties may be enhanced by the promoting amount while other catalytic properties may or may not be enhanced or may even be diminished. It is further understood that different catalytic properties may be enhanced at different operating conditions. For example, a catalyst having enhanced selectivity at one set of operating conditions may be operated at a different set of conditions wherein the improvement shows up in the activity rather than the selectivity. It may be desirable to intentionally change the operating conditions to take advantage of certain catalytic properties even at the expense of other catalytic properties. The exact operating conditions will depend upon, among other factors, feedstock costs, energy costs, by-product removal costs and the like.
(20) The conditions for carrying out an oxidative cracking reaction in the presence of the catalyst of the present disclosure broadly comprise those described in the prior art. This applies, for example, to suitable temperatures, pressures, residence times, diluent materials, the presence or absence of moderating agents to control the catalytic action, the desirability of employing recycle operations or applying successive conversion in different reactors to increase the yields of olefins, and any other special conditions which may be selected in processes for preparing olefins.
(21) In some embodiments, the reactant gas mixture comprises oxygen and argon. In some embodiments, the reactant gas mixture comprises 15-25%, e.g. about 20% oxygen and 75-85%, e.g. about 80% argon. In some embodiments, the reaction is performed at a temperature of 450 C. to 650 C., e.g. about 600 C. In some embodiments, the reaction occurs under a gas hourly space velocity (GHSV) of 47000-49000 h.sup.1, e.g. about 48000 h.sup.1. GHSV is the volume of gaseous reactant mixture, measured at 0 C. and 1 atm pressure, passed through a unit volume of reactor per hour.
(22) Total conversion of reactant (%)=percent of the reactant converted to all the products. Conversion of a reactant to a particular product=percent of the reactant converted to the particular product. In some embodiments, at least 75%, e.g. at least 80%, 85%, 90%, 95% or more, of the alkane is converted in the reaction.
(23) Selectivity for a particular product (%)=100[Conversion of reactant to the product (%)]/[Total conversion of reactant (%)]. In some embodiments, the reaction has a selectivity of at least 90%, e.g. at least 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or more.
(24) All the ratios of reactants or products described herein are mole ratios.
(25) Before exemplary embodiments of the present invention are described in greater detail, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
(26) Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
(27) 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 this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are now described.
(28) All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.
(29) It is noted that, as used herein and in the appended claims, the singular forms a, an, and the include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as solely, only and the like in connection with the recitation of claim elements, or use of a negative limitation.
(30) 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 invention. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.
(31) The invention is further described by the following non-limiting examples which further illustrate the invention, and are not intended, nor should they be interpreted to, limit the scope of the invention.
EXAMPLE
(32) Summary
(33) An alkali assisted hydrothermal method was utilized to synthesize Y.sub.2O.sub.3 nanorods. Gold (0.5, 1.0, 1.5 and 2.0 wt. %) supported Y.sub.2O.sub.3 nanorods samples were prepared by chemical reduction method and the synthesized AuY.sub.2O.sub.3 nanomaterials were utilized as catalysts for oxidative cracking of n-propane. The bulk Y.sub.2O.sub.3 nanorods showed n-propane conversion of 20% with 25% light olefins (ethylene and propylene) selectivity at 600 C. Significant enhancement in both n-propane conversion and olefins selectivity were observed after gold deposition. Gold (1.5 wt. %) supported Y.sub.2O.sub.3 nanorods (1.5AuY.sub.2O.sub.3NR) sample offered highest activity (75.5% conversion of n-propane and 90.3% olefins selectivity) at 600 C. Different techniques such as elemental analysis, powder X-ray diffraction, FT-IR, FE-SEM, DR UV-vis, N.sub.2-physisorption, XPS and H.sub.2-TPR measurements were used to determine physico-chemical properties of the synthesized catalysts. The characterization results revealed an existence of synergistic effect between the gold and Y.sub.2O.sub.3NR support due to strong metal-support interaction. The superior catalytic activity of 1.5AuY.sub.2O.sub.3NR sample is due to the fact that this catalyst possessed more number of mobile oxygen species, large number of Lewis acidic sites and also Au.sup.+OY species. Time on stream analysis indicated that gold supported Y.sub.2O.sub.3 nanorod catalysts exhibited considerable stable activity for 24 h with slight loss of performance due to agglomeration of gold particles.
(34) Materials and Methods
(35) Synthesis of Catalysts
(36) Yttrium oxide nanorods: Yttrium oxide nanorods were synthesized by adapting alkali assisted hydrothermal synthesis method. In a typical synthesis, 7.66 g of crystalline yttrium nitrate was dissolved in 100 mL of double distilled water. To this solution, 2.0 M potassium hydroxide solution was added dropwise under constant stirring to obtain a precipitate (pH is around 13). Then, the obtained precipitate was transferred into a Teflon lined stainless steel vessel and it was heated at 160 C. for 48 h. After the hydrothermal treatment, the vessel was brought to 25 C. and the obtained white colored solid product was collected and washed with distilled water for five times and finally with diluted HCl to remove any remained KOH. The washed cake was first dried in an oven over for 120 C. for 24 h and then material was calcined at 650 C. for 6 h in air to obtain the yttrium oxide nanorods. The calcined product was labelled as Y.sub.2O.sub.3NR.
(37) Gold supported yttrium oxide nanorods: Gold (0.5, 1.0, 1.5 and 2.0 wt %) supported Y.sub.2O.sub.3NR catalysts were prepared by simple chemical reduction method. The calculated amount of HAuCl.sub.4 corresponding to gold loading was dissolved in 50 mL of double distilled water. After complete dissolution of Au precursor, an appropriate amount of aqueous 0.1 M NaBH.sub.4 solution (molar ratio of NaBH.sub.4/Au=10) was added to reduce the Au precursor and the resulting nanoparticles are kept under stirring for 30 minutes. Then, calculated amount of Y.sub.2O.sub.3NR to obtain 0.5, 1.0, 1.5 and 2.0 wt % of Au in the final samples. The excess water was separated by centrifugation and the residual solid product was washed with distilled water for five times, then the material was dried at 110 C. for 12 hours. Finally, the dried materials were calcined at 300 C. for 4 hours in an electric furnace.
(38) Characterization of Synthesized Materials
(39) The elemental composition of the synthesized materials was determined by using ICP-AES, Optima 7300DV (Perkin-Elmer) instrument. The XRD patterns of the powders were collected by using PANalytical XpertPro diffractometer. The crystallite size of obtained materials was determined by applying the Debye-Scherer equation The SEM analysis of the samples was carried out using JEOL Model JSM-6390LV microscope. The FT-IR spectra of calcined materials were obtained using Bruker vertex 70 FT-IR spectrometer. The acidic character of the samples was investigated by pyridine adsorption measurements using FT-IR spectroscopy; the analysis was performed over calculated amount of catalyst, which was treated at 100 C. under vacuum for 5 h. Then, the sample was treated with pyridine vapor and finally heated at 100 C. under vacuum for 30 min to remove physically adsorbed pyridine [30]. FT-IR spectra were collected at room temperature. XPS spectra of the materials were collected by using Kratos Axis Nova spectrometer. The textural properties of the samples were obtained from the N.sub.2-physisorption experiments, which were conducted using Quantachrome ASiQ adsorption system. The H.sub.2-TPR and O.sub.2-TPD experiments were performed by using Quantachrome CHEMBET-3000 system. For the O.sub.2-TPD experiments, a known amount catalyst was first pretreated in helium gas flow at 120 C. for 2 h. After the temperature of the sample cooling to 25 C., the sample was exposed to O.sub.2 gas for 1 h and then flushed the sample with helium gas [31]. Finally, the O.sub.2-TPD patterns were obtained by heating the sample to 700 C., with a heating rate of 10 C. min.sup.1. The dispersion of the gold nanoparticles over the Y.sub.2O.sub.3NR support was determined by using CO pulse chemisorption measurements. Prior to analysis, known amount of the sample (100 mg) was reduced at 350 C. under H.sub.2 flow (40 mL min.sup.1) for 60 min and then cooled to 25 C. under the flow of helium gas. Then, the CO pulse injection was conducted in a flow of helium gas stream. The gold dispersion was evaluated from the amount of CO consumption (assuming CO/Au=1).
(40) Catalytic Oxidative Cracking of n-Propane
(41) Oxidative cracking of n-propane measurements over synthesized catalysts were performed using a fixed bed quartz reactor. The reactor was loaded with weighed catalyst pellets (200 mg), which were diluted with unreactive quartz particles. The reactant gas mixture, which contained n-propane (20 mL min.sup.1), 20% oxygen-80% argon (100 mL min.sup.1) and argon (40 mL min.sup.1) was used to perform the catalytic tests. Different reaction temperatures were used to investigate the effect of reaction temperature on the catalyst performance. The composition of product gas mixture was continuously analyzed with assistance of Agilent 6890 A gas chromatograph equipped with flame ionization and thermal conductivity detectors.
(42) Results and Discussion
(43) Powder X-ray diffraction (XRD) patterns of synthesized gold supported Y.sub.2O.sub.3 nanorod samples and uncalcined Y.sub.2O.sub.3 sample are shown in
(44) The average crystallite sizes of Y.sub.2O.sub.3 nanotubes [intense reflection due to (222) plane] and Au particles [intense reflection due to (111) plane] were calculated using Scherer's formula. The Au metal particle size was observed as 12.3 nm, 18.5 nm, 30.2 nm and 47.5 nm for samples with Au loading of 0.5, 1.0, 1.5 and 2.0 wt % respectively. It is observed that the crystallite size of Y.sub.2O.sub.3 phase (15 nm) have not changed considerably with increase of Au loading.
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(46) It was previously reported that these two bands appears due to the absorption of CO.sub.2 on surface of Y(OH).sub.3 and presence of some unconverted hydroxynitrate [34]. The bands at 570 cm.sup.1 and 733 cm.sup.1 could be assigned to stretching vibrations of yttrium bonded with hydroxide and nitrate ions [35].
(47) It is clear from the FTIR spectrum Y.sub.2O.sub.3NR sample that after the thermal treatment at 650 C., the bands due to hydroxyls, carbonate and hydroxynitrate species disappeared and a new band appeared at 560 cm.sup.1, which could be assigned to the stretching vibration of YO bond of Y.sub.2O.sub.3 nanorods [36]. The FT-IR spectra of gold supported Y.sub.2O.sub.3NR samples exhibited stretching vibration of YO bond along with multiple bands in the range of 1060-1230 cm.sup.1, which are related to different vibrations of carboxylate groups generated due to CO.sub.2 absorption [37]. It is clear that the intensity of these bands increased with increase of gold loading from 0.5 to 2.0 wt. %, revealing more gold nanoparticles are available on the surface of the 2.0AuY.sub.2O.sub.3NR catalyst.
(48) The morphology of the synthesized gold supported Y.sub.2O.sub.3NR materials was investigated by FE-SEM analysis (
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(50) The textural properties of gold supported Y.sub.2O.sub.3NR samples are determined from N.sub.2-physical adsorption experiments. The materials showed Type-III isotherms as per IUPAC classification and it is clear that isotherms have not shown any curvature at lower pressures [40]. This observation indicating that the materials possessed either macro pores or no pores at all, as a weak nitrogen-material interaction is clearly evident. It is also clear that the isotherms exhibited Type H3 hysteresis loop, which is an indication that aggregated particles of the materials were arranged randomly to form macro size pores [40]. To get detailed information of the pore structure of the samples, the pore size distribution patterns were obtained. The synthesized materials exhibited meso and macro pores. The intensity of the pore size distribution peaks were decreased in case of gold deposited Y.sub.2O.sub.3NR samples. This is probably due to the deposition of gold nanoparticles on the surface as well as inside of the Y.sub.2O.sub.3 agglomerates. The BET surface area, pore volume and pore diameter of the synthesized materials are shown in Table 1. The results clearly indicate that the BET surface area and pore volume of materials were decreased gradually with increase of gold loading from 0.5 wt % to 2.0 wt %. The bulk Y.sub.2O.sub.3 NR sample calcined at 650 C. exhibited surface area of 59 m.sup.2g.sup.1 and pore volume of 0.142 cm.sup.3g.sup.1, after 2.0 wt % gold deposition, the surface area and pore volume decreased to 39 m.sup.2g.sup.1 and pore volume of 0.127 cm.sup.3g.sup.1 respectively. However, it is interesting to note that the average pore diameter of the samples has not changed significantly. The bulk Y.sub.2O.sub.3NR sample possessed a pore size of 15.4 nm, while it was decreased to only 14.4 nm after 2.0 wt % gold deposition. This is possibly due to the fact that the deposited gold nanoparticles are partially blocking pores of Y.sub.2O.sub.3 nanorods, which resulted in a decline in surface area and pore volume. The reason for the slight change in the average pore diameter is that the majority of the pores are formed due to the presence of large voids among the Y.sub.2O.sub.3 nanorods and they were not blocked by the gold particles. These observations are corroborated with results obtained in SEM analysis of the samples.
(51) TABLE-US-00001 TABLE 1 Elemental composition and textural properties of gold supported Y.sub.2O.sub.3 nanorods. Bulk elemental Surface elemental Textural properties analysis (ICP-AES) analysis (XPS) Surface Pore Pore (wt. %) (wt. %) area volume diameter Sample Au Y O Au Y O (m.sup.2g.sup.1) (cc g.sup.1) (nm) Y.sub.2O.sub.3NR 85.2 14.8 84.9 15.1 65 0.142 15.4 0.5Au 0.43 84.1 15.4 0.38 83.8 15.8 48 0.140 14.5 Y.sub.2O.sub.3NR 1.0Au 0.68 81.2 18.1 0.53 80.8 18.6 34 0.136 14.5 Y.sub.2O.sub.3NR 1.5Au 1.27 79.7 19.0 1.14 79.0 19.8 22 0.131 14.4 Y.sub.2O.sub.3NR 2.0Au 1.70 78.3 20.0 1.36 77.5 21.1 14 0.127 14.4 Y.sub.2O.sub.3NR
(52) The surface properties such as composition and electronic states of elements in the synthesised samples was studied by means of XPS analysis (data not shown). The Y 3d XP spectra for bulk Y.sub.2O.sub.3NR sample exhibited two contributions at 157 eV (3d.sub.5/2) and 159 eV (3d.sub.3/2). It was reported that bulk Y.sub.2O.sub.3 sample showed the XP peak at 156.8 eV (3d.sub.5/2) and the fitted Y3d spectrum showed a doublet with a separation binding energy of 2.06 eV [41]. The shape and positions of the Y3d peak observed in the samples are in accordance with reported observations in the literature. The Y3d spectra for gold deposited samples also has been fitted with two peaks at 156 eV and 158 eV and these peaks are at lower binding energy compared to bulk Y.sub.2O.sub.3NR sample. It was previously reported that presence of highly electronegative ions such as CO.sub.3.sup.2 and (OH)CO.sub.3.sup. could result an increase of binding energy values of Y species [42]. However, peaks are shifted to lower values in spectra for gold supported samples. The peak shift may therefore be due to presence of YOAu.sup.+ or YOAu.sup.3+ species resulting shift in binding energy to lower values.
(53) It was previously reported that the variations in the chemical state or structural aspects of the Au surface could be studied by comparing the core-level shifts in the Au4f spectra of the samples [43] It is clear from the XPS analysis that the gold deposited samples showed clear Au4f XP peaks attributed to Au4f.sub.7/2 and Au4f.sub.5/2 contributions. It was reported that the 4f.sub.7/2 binding energy values for Au.sup.0, Au.sup.+ and Au.sup.3+ oxidation states are 84.0 eV, 85.5 eV and 86.4 eV respectively [44]. Deconvolution of core-level Au 4f spectrum of 1.0AuY.sub.2O.sub.3NR sample showed a small tail peak at 83.0 eV and major peak at 86.2 eV; indicating that this sample majorly contained surface Au.sup.3+ species with minor Au.sup.0 species. The binding energy of the Au4f.sub.7/2 peak is shifted to 85 eV (lower value) as gold loading raised to 1.5 wt %. Increase of gold deposition to 2.0 wt % resulted lowering of binding energy further for Au4f.sub.7/2 peak to 84.3 eV. It is clear that the binding energy of the tail peak have not changed considerably and the intensity of this peak increased gradually with increase of gold loading. These observations are indicating that the amount of surface Au.sup.0 species were increased with increase of gold loading, as the nature of gold-support interactive species also changed. Therefore, it is possible to argue that the oxidized Au species (Au.sup.+) might have electronic state between +1 and +3. A similar observation reported in the literature that the Au nanoparticles could anchor with defective sites of metal oxide support [45]. The XRD patterns and DR UV-vis spectra of the materials clearly revealed the presence of Au.sup.0 species and the amount of these species increased with increase of gold loading. Interestingly, the XPS spectra of samples revealed the presence of more oxidized Au species than metallic Au species on surface of the catalysts. This is possibly due to the fact that XPS is a surface sensitive technique compared to XRD and DR UV-vis methods.
(54) The O1s XP spectrum of bulk Y.sub.2O.sub.3NR sample exhibited two major peaks at 529 eV and 531 eV, which could be assigned to the lattice oxygen in Y.sub.2O.sub.3NR support and H.sub.2O/OH/O.sup. species on the surface of the support respectively [46]. Existence of high intense peak at 531 eV due to H.sub.2O/OH/O.sup. species in case of Y.sub.2O.sub.3NR, 1.0AuY.sub.2O.sub.3NR and 1.5AuY.sub.2O.sub.3NR samples could be witnessed from the XP spectra. It is clear that intensity of the peak at 529 eV decreased gradually with increase of gold loading, indicating that the composition of surface lattice oxygen species was lowered due to increase in the concentration YOAu species. It is clear that 2.0AuY.sub.2O.sub.3NR sample showed two peaks at 531 eV and 533 eV. The appearance of XP peak at 533.8 eV could be attributed to interactive (YOAu) species revealing that at high gold loading resulted surface coverage of interactive YOAu species.
(55) The reducibility of bulk Y.sub.2O.sub.3NR and gold supported Y.sub.2O.sub.3NR samples was investigated using H.sub.2-TPR analysis (
(56) The H.sub.2-TPR pattern of 1.0AuY.sub.2O.sub.3NR sample exhibited a new reduction peak at 675 C., along with the original reduction peak. An increase in gold content to 1.5 wt % caused appearance of additional reduction peak at 740 C. (total three peaks). Further gold loading to 2.0 wt % lead to form to intense broad peaks centred at 700 C. and 750 C. The observed new reduction peaks displayed substantial broadening and this might be due to random distribution of Au ions on the surface of Y.sub.2O.sub.3NR support. This observation indicates that deposition of Au with Y.sub.2O.sub.3 nanorods results in an increase in reducibility of the sample.
(57) FT-IR spectra of pyridine adsorbed gold supported Y.sub.2O.sub.3NR samples were obtained to study the acidity of samples. The spectra clearly revealing that the samples showed bands due to pyridine molecules bounded to both Brnsted (B) and Lewis (L) acid sites [49]. Table 2 shows the number of acid sites presented in the synthesized materials. Deposition of gold nanoparticles lowered the number of Brnsted acid sites and increased the quantity of Lewis acid sites. Enhancement in Lewis acid sites is clearly due to the presence of more Au.sup.0 nanoparticles on the surface of Y.sub.2O.sub.3NR support. It is well known that material which possesses Brnsted and Lewis acid sites could effectively activate CH bond in n-alkanes [50]. It is clear from Table 2 that the 1.5AuY.sub.2O.sub.3NR sample possessed a large quantity of Lewis acid sites.
(58) TABLE-US-00002 TABLE 2 Oxidative cracking of n-propane performance of gold supported Y.sub.2O.sub.3-NR catalysts at reaction temperature of 600 C. and GHSV of 48,000h.sup.1. Conversion Number of acid sites of Lewis Brnsted B/L n-propane Selectivity of products (%) Catalyst (L) (B) ratio (%) C.sub.2H.sub.4 C.sub.3H.sub.6 CH.sub.4 C.sub.2H.sub.6 CO.sub.x Y.sub.2O.sub.3-NR 26.5 16.3 38.1 4.5 6.2 34.9 0.5Au 9.4 2.3 0.244 49.6 23.4 52.5 2.2 1.9 20.4 Y.sub.2O.sub.3-NR 1.0Au 14.3 2.7 0.174 56.8 25.1 56.2 1.9 2.3 14.5 Y.sub.2O.sub.3-NR 1.5Au 19.7 3.3 0.167 75.5 26.2 64.1 1.6 1.7 6.4 Y.sub.2O.sub.3-NR 2.0Au 14.5 2.8 0.197 59.5 24.3 55.8 1.8 2.2 15.9 Y.sub.2O.sub.3-NR
(59) Initially, the influence of the gold loading and reaction temperature on the oxidative cracking activity was investigated (
(60) It was observed that oxidative cracking performance was improved after gold deposition over the Y.sub.2O.sub.3NR support. It was reported that activation of the CH bond is the rate limiting step in n-propane oxidative cracking [51]; presence of gold nanoparticles on the surface of Y.sub.2O.sub.3NR is responsible for substantial increase in n-propane conversion and olefins yield. The physico-chemical characterization results indicate that interactions existed between Y.sub.2O.sub.3NR and gold particles. Conversion of n-propane and product distribution at 600 C. in oxidative cracking of n-propane over synthesized catalysts are presented in Table 2. Highest selectivity for CO.sub.x formation (34.9%) was noticed in case of Y.sub.2O.sub.3NR sample. Deposition of gold over Y.sub.2O.sub.3NR support resulted in suppression in CO.sub.x formation. The lowest CO.sub.x selectivity (6.4%) was detected in case of 1.5AuY.sub.2O.sub.3NR catalyst. The increase of reaction temperature from 450 C. to 600 C. caused a surge in olefins selectivity (formation of more propylene in all the catalysts), but further raises in reaction temperature to 650 C. led to decrease of olefins selectivity probably due to decomposition of olefins at high temperatures [52].
(61) The n-propane conversion and product formation rates were normalized with the total surface area (mol m.sup.2s.sup.1) for all synthesized samples (
(62) The influence of the gas hourly space velocity (GHSV) over catalytic oxidative cracking performance for synthesized gold supported Y.sub.2O.sub.3NR samples was also studied.
(63) It is known that the oxygen mobility of the catalyst is an important factor in an oxidation reaction given that if the oxygen species presents sufficient mobility, the lattice oxygens can become involved in the catalytic process as well [54]. The oxygen adsorption capacity for each sample was calculated based on the oxygen uptake in the O.sub.2-TPD analysis. These values allow us to estimate the total amount of oxygen available in the catalyst. The 1.5AuY.sub.2O.sub.3NR sample exhibited high O.sub.2-TPD peak area indicating that this sample has a high amount of mobile oxygen species, as also indicated by the characterization results discussed previously favoring high catalytic oxidative cracking activity and there is a correlation between O.sub.2-TPD peak area and the catalytic activity for the catalysts (
(64) To determine TOFs based on Au dispersion, first we determined the gold dispersion using CO chemisorption measurements and then measured the TOF values. It is clear that the Au dispersion is gradually decreased with increase of gold loading from 0.5 wt % to 2.0 wt %. It has been found that n-propane conversion and TOF values has a different trend to Au dispersion as the highest gold dispersion was observed for 0.5AuY.sub.2O.sub.3NR sample but the highest oxidative cracking activity was observed for 1.5AuY.sub.2O.sub.3NR sample. Therefore, dispersion of gold particles on the support surface is not a major influencing factor for oxidative cracking ability of synthesized catalysts.
(65) It is known that an increased number of mobile surface oxygen species enhances the rate of oxidation reactions tremendously [55]. It is clear from H.sub.2-TPR results that the movement of oxygen species in Y.sub.2O.sub.3NR support is lower than gold supported Y.sub.2O.sub.3NR samples. The observed high catalytic performance of the gold supported Y.sub.2O.sub.3NR catalysts indicate that mobility of the oxygen species of these catalysts could be the key factor. The time on stream analysis for Y.sub.2O.sub.3NR and 1.5AuY.sub.2O.sub.3NR samples was performed at a reaction temperature of 600 C. Catalytic oxidative cracking activity of Y.sub.2O.sub.3NR and 1.5AuY.sub.2O.sub.3NR samples was tested for 24 hours without any interruption, and activity results are presented in
(66) The observed results clearly indicated that addition of gold significantly enhanced the performance of Y.sub.2O.sub.3NR sample for oxidative cracking of n-propane. It is clear from the characterization results that the total acidity increases with the gold deposition on Y.sub.2O.sub.3NR support, suggesting that Au ions are the additional source of acidity on AuY.sub.2O.sub.3 NR catalysts. Martins et al [56] reported that catalysts having acidic properties could promote the activation of alkanes through the formation of carbenium intermediates. Accordingly, it is reasonable to argue that the n-propane is protonating to form C.sub.3H.sub.9.sup.+, which decomposes into H.sub.2 and C.sub.3H.sub.7.sup.+ or CH.sub.4 and C.sub.2H.sub.5.sup.+ on the surface of AuY.sub.2O.sub.3 catalysts. Propene and ethene are formed after deprotonation of C.sub.3H.sub.7.sup.+ and C.sub.2H.sub.5.sup.+, respectively. Previously, V.sub.2O.sub.5/SiO.sub.2 and AuV.sub.2O.sub.5/SiO.sub.2 catalysts were tested for oxidation of n-propane at 420 C. [57]. The authors observed that addition of gold resulted in an increase in the conversion of n-propane. The same effect was observed when gold was added to AuMoO.sub.3/SiO.sub.2 catalyst. Sa et al [58] reported that the active site is believed to be a combination between cationic gold and a rare earth oxide (La.sub.2O.sub.3), which activates the n-butane at low temperatures. The catalyst is selective towards alkene formation and does not provide sites for combustion reactions. The addition of gold also appears to stabilize lanthanum oxycarbonate species that may play a role in enhancing the performance of the gold-promoted oxide catalysts. Our previous studies also revealed a similar observation that deposition of gold enhanced the oxidative cracking ability of CeO.sub.2ZrO.sub.2 and La.sub.2O.sub.3 supports [21]. Recently, Wang et al [59] used ab initio molecular dynamics simulations and reported that the charge state of the supported Au particle is dynamically changing during the catalytic cycle, where the charging/discharging of Au particle not only controls the amount of O.sub.2 adsorbed at the Au particle/oxide interface but also strongly influences the energetics of all the redox steps. Therefore, it is clear that the presence of nanosized gold is helpful to obtain better performance in the catalytic oxidative cracking process.
(67) The physico-chemical characterization results measured from the different techniques are useful to realize the reasons for superior activity of gold supported Y.sub.2O.sub.3NR samples in n-propane oxidative cracking. The FE-SEM results revealed that prepared gold supported Y.sub.2O.sub.3NR samples possessed Y.sub.2O.sub.3 naosized rods and gold nanoparticles. It is widely reported that highly dispersed nanosized gold catalysts show superior catalytic performance compared to catalysts with micro-sized particles [60]. It was previously observed that morphology of catalytically active component and support play an important role to obtain superior catalytic performance. The H.sub.2-TPR results revealed that gold supported Y.sub.2O.sub.3NR catalysts are more easily reducible than bulk Y.sub.2O.sub.3NR sample and it was also reported in the literature that gold supported Y.sub.2O.sub.3 catalysts possessed more oxygen storage capacity than the bulk Y.sub.2O.sub.3 material [61]. It is clear that the existence of gold-Y.sub.2O.sub.3 interactive species could enhance the redox ability of the catalysts. The characterization results also revealed that gold supported Y.sub.2O.sub.3NR catalysts consisted of small particle size, considerably large specific surface area, large quantity of Lewis acid sites and easy reducibility; the combination of these properties are essentially responsible for the superior catalytic oxidative cracking ability.
Conclusions
(68) A simple alkali assisted hydrothermal method was used to prepare Y.sub.2O.sub.3 nanorods sample. The synthesized Y.sub.2O.sub.3 nanorods was used as a support to prepare gold (0.5, 1.0, 1.5 and 2.0 wt %) supported Y.sub.2O.sub.3 nanorods samples by chemical reduction method. Powder XRD, FT-IR, FE-SEM, DR UV-vis, N.sub.2-physisorption, XPS, H.sub.2-TPR and O.sub.2-TPD techniques were used to characterize the synthesized catalysts. The obtained results indicated the existence of a synergistic effect between gold species and Y.sub.2O.sub.3 NR support due to strong metal-support interaction. The synthesized AuY.sub.2O.sub.3NR samples were used for oxidative cracking of n-propane to produce light olefins. The bulk Y.sub.2O.sub.3 nanorods offered 20% of n-propane conversion and 25% of light olefins (ethylene and propylene) selectivity at 600 C. Deposition of gold nanoparticles over the Y.sub.2O.sub.3 nanorods resulted in a significant enhancement in both conversion of n-propane and selectivity to olefins. The 1.5AuY.sub.2O.sub.3NR catalyst showed highest activity (75.5% conversion of n-propane and 90.3% olefins selectivity) at 600 C. due to the presence of more Lewis acid sites, mobile oxygen species and also Au.sup.+OY species in the sample. Time on stream analysis indicated that gold supported Y.sub.2O.sub.3 nanorod catalysts exhibited continuous stable activity for 24 h with a slight loss of activity due to agglomeration of gold particles.
Acknowledgements
(69) This project was funded by the research and development office (RDO) at the Ministry of Education, Kingdom of Saudi Arabia: Grant no (HIQI-20-2019). The inventors also acknowledge with thanks research development office (RDO-KAU) at King Abdulaziz University for technical support.
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(71) While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. Accordingly, the present invention should not be limited to the embodiments as described above, but should further include all modifications and equivalents thereof within the spirit and scope of the description provided herein.