Water-gas shift reaction on Au/CeO2 catalytic material

Journal of Chemistry, Vol. 41 (1), P. 119 - 123, 2006<br /> <br /> <br /> Water-gas shift reaction on Au/CeO2 catalytic<br /> material<br /> Received 17 January 2005<br /> La The Vinh, La Van Binh<br /> Hanoi University of Technology<br /> <br /> <br /> Summary<br /> Nanostructured Au-Ceria has been known as a promising catalyst for the low-temperature<br /> water-gas shisft reaction. The catalyst prepared by coprecipitation method with the gold loading<br /> various between 2 - 3 at.% with the crystallite size of 2.9 nm has been used to study some factors<br /> that effect to catalytic activity and long-term stability of this material.<br /> <br /> <br /> I - Introduction dried in an exicator under vacuum over night<br /> and then stored in the dark. Before use the<br /> CO oxidation over catalyst was studied and catalyst was conditioned by heating it up to<br /> there were also a lot of catalytic materials used 200oC in N2 and kept at this temperature for 30<br /> for this purpose. This report focus on a new minutes in N2 flow. Then the gas flow was<br /> material based on Au/CeO2, which has been switched for 45 minutes to a 10% H2 in N2<br /> known as the new catalytic material for the low mixture and finally it was kept another 30<br /> temperature water-gas shift (LTS). The catalyst minutes in a N2 flow. After the conditioning<br /> was prepared by co-precipitation (CP) from treatment the catalyst is cooled down to reaction<br /> cerium-nitrat solution and gold acid by pH = 6.5 temperature.<br /> ÷ 7.0. The gold loading was varied between 2 ÷ 2. Kinetic measurements<br /> 3 at.% with the crystallite size of 2.9 nm. The<br /> catalytic activity correlates well with structure, The kinetic measurements were carried out<br /> characterization of gold as well as dispersion of in a quartz tube reactor which is heated by a<br /> gold on ceria. ceramic tube furnace. For the analysis of the gas<br /> flow we used an online gas chromatograph from<br /> II - Experimental DANI. In order to work under differential flow<br /> conditions the catalyst was diluted with -<br /> 1. Catalyst preparation Al2O3. We used a gas flow rate of 60 Nml/min.<br /> Water was added to the gas mixture by bubbling<br /> The support is suspended in water at 60oC the gas through a temperatured water bath.<br /> and at pH of 6.5 - 7. The pH was controlled by<br /> adding a Na2CO3 solution. We added goldacid III - Results and discussion<br /> (HAuCl4) and kept the pH between 6.5 and 7 by<br /> adding more Na2CO3 solution. After 30 min of 1. Catalyst characterization<br /> stirring, the solution is cooled down to room<br /> temperature and filtered with a Rotbandfilter. With BET we found a surface area of 188<br /> The catalyst is resuspended twice in warm water m2/g on both the support and the catalyst. In<br /> to diminish the natrium amount, after that it is XPS measurements of the fresh sample we<br /> <br /> 119<br /> found the Au 4f7/2 peak at 85.1 eV. After from the peak area ratio that after conditioning<br /> reductive conditioning the peak shifted to a the Ce3+ has a higher value.<br /> binding energy of 84.4 eV. The Au 4f7/2 peak for<br /> 2. Kinetic data<br /> Au0 was reported at 83.9 eV, for Au3+ at 86.3 eV<br /> (Au2O3) and 87.7 eV (Au(OH)3). In figure 1 we can see the temperature<br /> Cerium has a rather complicated spectrum dependency of the water-gas shift reaction. The<br /> with shakeup and shakedown effects. We find measurement was stoped at 190oC because the<br /> both Ce4+ and Ce3+ in the system. We can say conditioning was at 200oC.<br /> <br /> 6e-6<br /> Reactionrate, mol/gkats<br /> <br /> <br /> <br /> <br /> 5e-6<br /> Au/CeO2 Catalyst<br /> CeO2 Support<br /> <br /> 4e-6<br /> <br /> <br /> 3e-6<br /> <br /> <br /> 2e-6<br /> <br /> <br /> 1e-6<br /> 4e-8<br /> 2e-8<br /> 0<br /> 60 80 100 120 140 160 180 200<br /> Temperature, oC<br /> <br /> Figure 1: A comparison of the reaction rate of the water-gas shift reaction over<br /> the pure support and the catalyst<br /> <br /> It is well known that CeO2 can catalyze the surface and the peak at 2093 cm-1 is the Ce3+<br /> water-gas shift reaction, but as one can see in adsorption site. On the catalyst the peak at 2118<br /> figure 1 the pure CeO2 support is not active for cm-1 dominates the spectrum. It is well known<br /> the water-gas shift reaction in the temperature that the CO adsorption on Gold yields to a peak<br /> region we use. in this region. As soon as CO is introduced into<br /> the system we find CO2 and some C-H<br /> 3. Adsorption experiments<br /> stretching bonds on the surface.<br /> In CO adsorption experiments we find Figure 3 shows the dominating peaks at<br /> different adsorption sites on the pure support 2833 cm-1 and at 1586 cm-1. As both peaks<br /> than on the Au/CeO2 catalyst (Fig. 2). In both behave in the same way we conclude that this is<br /> cases we have a peak at 2142 cm-1. On the pure one species. Both can be assigned to format so<br /> support there is also a peak at 2093 cm-1 and a there is a format species on the surface. It is<br /> shoulder at 2115 cm-1. produced by the reaction of adsorbed CO with<br /> Since CO adsorbs on Ce4+ at higher wave OH groups on the surface. Format is an<br /> numbers than on Ce3+, we assume the peak at intermediate in the water-gas shift reaction in<br /> 2142 cm-1 is the adsorption peak on the Ce4+ our system. CeO2 can store a lot of oxygen so<br /> <br /> 120<br /> the production of CO2 can be explained by a CO diminished. Carbonate species could be the<br /> oxidation with the stored oxygen. reason for the deactivation. We find a peak at<br /> The CO oxidation with the stored oxygen is 1427 cm-1 which is growing during the reaction.<br /> the reason for the high catalytic activity at the For the reaction at 80oC we found a very low<br /> beginning (see figure 4). After 200 minutes reaction rate and the DRIFTS spectrum at this<br /> most of the stored oxygen is depleted and the temperature looks nearly similar to the spectrum<br /> water-gas shift reaction is becoming the on the pure support. The peak of the CO<br /> dominant reaction. The reaction rate is quite adsorption on gold at 2118 cm-1 was only<br /> constant from there on in a similar Au/CeO2 viewable as a small shoulder in the CO<br /> system. Figure 4 also shows the direct adsorption on Ce3+ peak at 2093 cm-1.<br /> connection between the CO2 concentration on Something must block the CO adsorption on<br /> the surface and the reaction rate. The reaction gold at these low temperatures. We did water<br /> rate decreases with the same gradient as the CO2 adsorption experiments at 80oC and found a<br /> concentration on the surface. What we can see peak at 1616cm-1. When we did the same<br /> as well is that the format concentration on the experiment at 180oC the peak at 1616 cm-1 could<br /> surface goes along with the reaction rate after not be found. So the CO adsorption on gold is<br /> the first 200 minutes. As we think format is an hindered by water and that is one reason for the<br /> intermediate of the water-gas shift reaction on low reaction rate and it will be difficult to<br /> the Au/CeO2 catalyst, we propose that format is reduce the temperature of a water-gas shift<br /> built at the beginning then with the deactivation reactor much more.<br /> of the catalyst the amount of format is<br /> <br /> Intensity Intensity<br /> 0.0160 0.028<br /> <br /> <br /> <br /> <br /> 0.0155 0.026<br /> <br /> 2093<br /> 2142 2118<br /> 2142 2115<br /> 0.024<br /> 0.0150<br /> <br /> <br /> <br /> 0.022<br /> 0.0145<br /> <br /> <br /> 0.020<br /> 0.0140<br /> <br /> <br /> 0.018<br /> <br /> 0.0135<br /> <br /> 5000 ppm CO on the support 5000 ppm CO on the catalyst<br /> 0.016<br /> 10000 ppm CO on the support 10000 ppm CO on the catalyst<br /> 0.0130<br /> 2160 2140 2120 2100 2080 2060<br /> 2160 2140 2120 2100 2080 2060<br /> 1<br /> Wave number, cm-1 Wave number, cm-1<br /> Figure 2: CO adsorption on the pure support and on the catalyst<br /> <br /> <br /> <br /> 121<br /> Intensity Intensity<br /> 0.16 5<br /> <br /> 5000 ppm CO on the catalyst<br /> 0.14 10000 ppm CO on the catalyst<br /> <br /> 4<br /> 0.12<br /> <br /> <br /> 0.10 3<br /> <br /> <br /> 0.08<br /> 2<br /> <br /> 0.06<br /> <br /> 1<br /> 0.04<br /> <br /> <br /> 0.02<br /> 0<br /> <br /> 0.00<br /> 3000 2900 2800 2700 2600<br /> 1700 1600 1500 1400 1300 1200<br /> -1 -1<br /> Wave number, cm Wave number, cm<br /> Figure 3: Spectra of format species on the surface<br /> <br /> 2.4e-4<br /> Reactionrate, mol/gkats<br /> <br /> <br /> <br /> <br /> 2.2e-4<br /> <br /> <br /> <br /> <br /> 2.0e-4<br /> <br /> <br /> <br /> <br /> 1.8e-4<br /> <br /> 4<br /> <br /> <br /> <br /> <br /> 3<br /> Peakarea<br /> <br /> <br /> <br /> <br /> 1586cm-1<br /> 2833cm-1<br /> 2 2332+2363<br /> <br /> <br /> <br /> <br /> 1<br /> <br /> <br /> <br /> <br /> 0 200 400 600 800 1000 1200<br /> Time, min<br /> Figure 4: Reaction rate and peak area of formate and CO2 against time<br /> <br /> 122<br />  References 4. 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