GOLD CATALYST SUPPORTED IN CUO/ZNO/AI203, PRODUCTION METHOD AND USE THEREOF

Abstract

The present invention relates to the synthesis and application of gold catalysts supported in mixed CuO/ZnO/Al.sub.2O.sub.3 oxides prepared on the basis of their corresponding solids with a hydrotalcite structure as catalysts in the water-gas shift reaction, for use in fuel processors coupled to fuel cells.

Claims

1. A catalyst comprising gold supported on CuO/ZnO/Al.sub.2O.sub.3 wherein the catalyst comprises between 10% and 80% of Al.sub.2O.sub.3 and between 90% and 20% of CuO/ZnO.

2. The catalyst, according to claim 1, wherein the CuO/ZnO/Al.sub.2O.sub.3 support precursor has a hydrotalcite structure.

3. The catalyst, according to claim 1, wherein the Cu+Zn/Al ratio is comprised between 0.5 and 3.

4. The catalyst, according to claim 1, wherein the Cu/Zn ratio is comprised between 1 and 6.

5. The catalyst, according to claim 1, wherein the catalyst comprises: between 0.5% and 4% w/w of Au between 10% and 90% w/w of CuO/ZnO

6. A method for preparing a catalyst, as defined in claim 1, comprising the following operations: synthesis of hydrotalcites as CuO/ZnO/Al.sub.2O.sub.3 mixed oxide precursors, and deposition of gold on the CuO/ZnO/Al.sub.2O.sub.3 substrate.

7. The method, according to claim 6, wherein the synthesis of hydrotalcites takes place by means of co-precipitation at a low supersaturation of Cu, Zn and Al salts at a pH comprised between 7 and 10 and temperatures comprised between 20 C. and 80 C.

8. The method, according to claim 7, wherein Cu(NO.sub.3).sub.2.2H.sub.2O, Zn(NO.sub.3).sub.2.6H.sub.2O and Al(NO.sub.3).sub.3.9H.sub.2O are used as precursors and Na.sub.2CO.sub.3 1M as a precipitation agent, said precipitation being maintained for a period of 48 hours.

9. The method, according to claim 7, wherein precipitation is followed by drying at a temperature comprised between room temperature and 100 C., followed by subsequent calcination at 300 C. for 4 hours with a ramp of 10 C./min.

10. The method, according to claim 7, wherein the deposition of Au is performed by means of direct anionic exchange assisted by NH.sub.3.

11. The method, according to claim 10, wherein it is based on an aqueous HAuCl.sub.4 solution to which the support is added.

12. The method, according to claim 7, wherein the deposition of Au is performed by means of deposition-precipitation.

13. The method, according to claim 12, wherein Au is deposited in the form of auric hydroxide on the oxide layers under agitation at a constant pH.

14. A process for a water-gas shift reaction comprising the use of a catalyst, as defined in claim 1.

15. The process, according to claim 14, wherein the reaction takes place in reactive streams having a composition comprising: between 4.5% and 9% of CO, between 0% and 11% of CO.sub.2, and between 30% and 50% of H.sub.2O.

16. The process, according to claim 14, wherein the reaction takes place at a temperature comprised between 140 and 350 C. and at a spatial velocity between 4,000 and 8,000 h.sup.1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0036] FIG. 1.X-Ray diffraction patterns [0037] A) X-Ray diffraction patterns of the non-calcined samples. [0038] B) X-Ray diffraction patterns of the calcined sample. [0039] C) Comparison of the diffraction patterns of a pre-reaction calcined sample with the same sample after use thereof in WGS. [0040] D) Comparison of a calcined mixed oxide with its corresponding gold-containing catalyst.

[0041] FIG. 2.Results of catalytic activity in WGS of the systems where M.sup.2+/M.sup.3+ remained constant at 1 and the gold was deposited by means of the assisted anionic exchange method. [0042] A) Comparison of the activity of the mixed oxides with those of their corresponding gold catalysts supported under ideal conditions (mixture of the described model). [0043] B) Comparison of the activity of the mixed oxides with those of their corresponding gold catalysts supported under the described industrial conditions.

[0044] FIG. 3.Au/HT_2 catalyst [0045] A) Continuous stability under real operating conditions. [0046] B) Start/stop cycles under industrial mixture.

DETAILED DESCRIPTION OF THE INVENTION

[0047] The present invention relates to the synthesis and application of gold catalysts supported on mixed CuO/ZnO/Al.sub.2O.sub.3 oxides prepared from their corresponding solids with a hydrotalcite structure as catalysts in the WGS reaction.

[0048] Firstly, the invention describes the synthesis of mixed copper zinc and alumina oxides (CuO/ZnO/Al.sub.2O.sub.3) in an extraordinarily controlled manner through the use of a defined hydrotalcite-type structure as a precursor thereof. As a result, these solids are stable with respect to the significant sintering, which is reflected in a constant and long-lasting catalytic activity.

[0049] The addition of a minimum quantity of gold gives rise to a maximum increase (maximum permitted by the thermodynamics in the temperature window of 140-250 C.) of the activity, to which other advantages are added such as the omission of the catalyst pre-conditioning stage, reduction of its deactivation, increase in its durability and significant stability in the event of changes in flow or temperature or start/stop cycles.

Synthesis of Hydrotalcites as a CuO/ZnO/Al.sub.2O.sub.3 Precursor

[0050] The synthesis of hydrotalcites as CuO/ZnO/Al.sub.2O.sub.3 mixed oxide precursors takes place by means of co-precipitation under oversaturation, where the salts and alkaline solution are slowly added, ensuring that the pH and temperature remain constant. The most widely used conditions are: between 10-80% in w/w of Al.sub.2O.sub.3 and 90-20% in w/w of CuO/ZnO; pH between 7 and 10 (pH at which most hydroxides precipitate); temperatures between 20 and 80 C.; under concentrations and flows of reagents; washing after filtration with hot water, to fully remove the sodium ions; and drying at low temperatures (maximum 120 C.).

[0051] First, an aqueous Na.sub.2CO.sub.3 solution with a concentration of 1 M is prepared as a precipitation agent. Next, the necessary quantity of each of the precursors used is deposited in a 1 L beaker. In all cases, the precursors used were the nitrates of said metals, Cu (NO.sub.3).sub.2.2H.sub.2O, Zn (NO.sub.3).sub.2.6H.sub.2O, Al (NO.sub.3).sub.3.9H.sub.2O, since it does not generate solid residue and makes it possible to obtain cleaner hydrotalcite. These were dissolved in a volume of 0.8 L of distilled water and magnetically agitated throughout the precipitation phase. The resulting colour of the dissolution is sky blue, characteristic of copper hydrotalcites. The precipitation of the hydroxides is maintained for a period of 48 hours. Next, it is left to dry at a temperature below 100 C., to which end a heater may be used, or left to dry at room temperature for a couple of days. Lastly, after the drying phase, the sample is calcined. Lastly, the samples were calcined at a temperature of 300 C. and a heating ramp of 10 C./min for a period of 4 hours.

[0052] This process has two major advantages: on the one hand, it makes it possible to integrate a large group of anions and cations in the structure and, on the other, the large-scale preparation thereof is less complex.

Gold Deposition Method

[0053] Two different gold impregnation methods were used. In both gold deposition methods, loads of 0.5%-4% (w/w) of gold were swept. [0054] 1) Ammonia-assisted direct anionic exchange method [S. Ivanova, C. Petit, V. Pitchon, A new preparation method for the formation of gold nanoparticles on an oxide support Applied Catalysis AGeneral Volume 267, 2004, Page 191-201]. [0055] Said method consists of taking an aqueous solution of HAuCl.sub.4, with concentrations of 10.sup.4 M, which is heated to a temperature of approximately 70 C. Once said temperature is reached, the support is added to the solution and the mixture is left in agitation for 20 minutes. After said period of time has elapsed, the solution is cooled to approximately 40 C. and 20 mL of NH.sub.3 30% (v/v) are added. It is agitated again for 20 minutes, filtered and the solid collected. Lastly, it is dried and calcined at 300 C. for 4 hours. [0056] 2) Deposition-precipitation method [D. Andreeva, T. Tabakova, V. Idakiev, P. Christov, R. Giovanoli, Au/alpha-Fe.sub.2O.sub.3 catalyst for water-gas shift reaction prepared by deposition-precipitation Applied Catalysis AGeneral Volume 169, 1998, Page 9-14]. [0057] Precipitation was performed by means of an automated system (Contalab) that makes it possible to control all the precipitation parameters (pH, temperature, agitation speed, reagent feed flow, etc.). The gold is deposited in the form of Auric Hydroxide, Au(OH).sub.3, on the oxide layers, under vigorous agitation, maintaining a constant pH of 7. After filtration and thorough washing, the precursors were vacuum-dried and calcined in air at 400 C. for 2 hours.

Embodiment of the Invention

[0058] By way of example, following is a description of the most representative results of a series of gold catalysts supported on mixed CuO/ZnO/Al.sub.2O.sub.3 oxides, which are not intended to be representative of their scope.

Chemical Composition

[0059] The elemental analysis was performed using X-Ray micro fluorescence spectrometry (XRMFS) in an EDAX Eagle III spectrometer with a rhodium source of radiation.

[0060] In order to synthesize a series of precursor hydrotalcites of the CuO/ZnO/Al.sub.2O.sub.3 mixed oxides, M.sup.2+/M.sup.3+ ratios from 1 to 6 were swept. Additionally, for each one of these M.sup.2+/M.sup.3+ ratios, the M.sup.2+/M.sup.2+ ratio was varied between 1 and 6.

[0061] Table 1 includes the ratios of some of the representative M.sup.2+/M.sup.3+ ratios.

TABLE-US-00001 TABLE 1 M.sup.2+/M.sup.3+ ratios for the synthesis of hydrotalcites (Cu + Zn)/Al Molar Ratio 0.5 2 3

[0062] As with the M.sup.2+/M.sup.3+ ratio, some of the different Cu.sup.2+/Zn.sup.2+ molar ratios that were prepared are exemplified in Table 2.

TABLE-US-00002 TABLE 2 M.sup.2+/M.sup.2+ ratios for the synthesis of hydrotalcites Cu/Zn Molar Ratio 1.4 2.8 5.6

[0063] The objective pursued is to perform a representative sweep of the ratios in order to find or at least come as close as possible to the ideal M(II)/M(II) ratio. Table 3 shows the composition of some of the solids prepared (HT) maintaining the M.sup.2+/M.sup.3+ ratio constant at 1, where M.sup.3+ is Al.sup.3+ and M.sup.2+ is a mixture of Cu.sup.2+/Zn.sup.2+ whose Cu.sup.2+/Zn.sup.2+ ratio varies between 1.4 and 5.6.

TABLE-US-00003 TABLE 3 Chemical composition of some of the solids prepared, maintaining M.sup.2+/M.sup.3+ constant. CuO ZnO Al.sub.2O.sub.3 Theoretical Real Sample (%) (%) (%) Cu/Zn Ratio Cu/Zn Ratio HT 1.4 35.82 25.97 38.21 1.4 1.38 HT 2.8 17.40 47.73 34.87 2.8 2.74 HT 5.6 51.95 10.34 37.71 5.6 5.02

[0064] In all cases, a Cu/Zn ratio close to the targeted ratio in the synthesis was achieved.

[0065] The X-Ray diffraction analysis (XRD) was performed on an X'Pert Pro PANalytical X-Ray diffractometer. The diffraction patterns were recorded using the K radiation of Cu (40 mA, 45 kV) in a range of 20 comprised between 3 and 80 and a sensitive detector position using a sieve size of 0.05 and a passage time of 240 s. The XRD patterns of the synthesized solids are shown in FIG. 1.

[0066] FIG. 1A shows the X-Ray diffraction patterns of the non-calcined samples. In all cases the typical hydrotalcite signals are obtained and a certain proportion of malachite phase appears. After calcining the sample, FIG. 1B, the CuOZnOAl.sub.2O.sub.3 mixed oxide is obtained, as shown by the reflections of the diffractogram. FIG. 1C shows a comparison between the diffraction patterns of a pre-reaction calcined sample and the same sample after its use in WGS. It can be clearly observed how the copper oxide is reduced to metal copper during the reaction. The latter is the active species in the water-gas shift. Lastly, FIG. 1D shows a comparison between a calcined mixed oxide and its corresponding gold-containing catalyst. Typical reflections of metal gold or of any other gold species were not observed, which indicates that the gold nanoparticles are small (they are smaller than 5 nm in size, which is the detection limit of the diffractometer) and are well dispersed on the CuOZnOAl.sub.2O.sub.3 mixed oxide.

[0067] The WGS reaction was carried out in a proprietary design diffractometer. The typical gas mixtures used for the water-gas shift reaction were:

Model Mixture:

[0068] 4.5% CO in Ar (Abell Linde); 30% H.sub.2O (0.024 mL/min H.sub.2O (I).

Industrial Conditions (Imitating the Outlet of an Ethanol Reformer):

[0069] 9% CO (Abell Linde); 11% CO.sub.2 (Abell Linde); 50% H.sub.2 (Abell Linde); 30% H.sub.2O (0.024 mL/min H.sub.2O (I).

[0070] FIG. 2 shows the results of catalytic activity of the systems where M.sup.2+/M.sup.3+ remained fixed at 1 and the gold was deposited by means of the assisted anionic exchange method. FIG. 2A shows a comparison of the activity of the mixed oxides with that of their corresponding gold catalysts supported under ideal conditions (previously described model mixture). Within the mixed oxides, CuO/ZnO=2.8 and 5.6 seem to be the optimum ratios, reaching equilibrium conversions of 330 C. The activity of the gold systems is very superior to that of their corresponding supports. Equilibrium reductions are achieved practically from the start of the reaction, making these systems very promising and superior catalysts to the current low-temperature water-gas shift industrial systems. In FIG. 2B, the supported gold systems are tested under realistic operating conditions (previously described industrial conditions). The solids with Cu/Zn ratios=1.4 and 2.8 reached equilibrium at 270 C. and the Au_HT 2.8 system was the most active in the entire temperature range studied.

[0071] Frequently, from the industrial viewpoint, the stability of the catalyst is more important than the catalytic activity itself and this stability under operating conditions is a determining factor when selecting a catalyst. FIG. 3A shows the stability of one of the Au/CuOZnO/Al.sub.2O.sub.3 systems synthesized by means of the deposition-precipitation method under industrial conditions. Specifically, it is a system with a (Cu+Zn)/Al ratio=2 and a Cu/Zn ratio=5.6, called Au/HT_2.

[0072] The catalyst slightly loses activity in the first 14 hours of operation (from 70% to 65% of CO conversion). After said period has elapsed, the stationary state is reached and CO reduction remains high and stable. It should be noted that the temperature of the stability test is very low (220 C.) and, therefore, the performance of this catalyst, considering the high activity and good stability shown at such a low temperature, is excellent and exceeds that of the CuO/ZnO-based catalysts currently used in the industry for the low temperature water-gas shift reaction.

[0073] In addition to continuous stability, it is of vital importance to verify catalyst resistance in start/stop cycles. Start/stop cycles imply lowering the temperature of the reaction mixture to room temperature (approximately 30 C.) and maintaining said situation for 40 minutes (always with the gases and water flowing through the catalytic bed). After said period, the system is heated again up to the stability test temperature (220 C.). This implies that the catalyst will come into contact with liquid water. The selected catalyst was capable of successfully tolerating up to four start/stop cycles without showing any deactivation. This result is quite promising and makes the catalysts studied very adequate for WGS processes in both stationary and portable applications.

[0074] It should be noted that in all the catalytic tests the reaction mixture, once stabilised, is made to flow directly over the catalyst. In other words, no treatment is performed prior to activation. The catalysts are activated directly in the reaction mixture, which implies an additional advantage with regard to possible applications in fuel processors that work in continuous mode.

[0075] The overall activity and stability results are quite satisfactory taking into account the characteristics offered by the catalysts currently available in the market, making these systems promising candidates for direct application thereof in real hydrogen stream purification processes.