Cooperative effect between copper and gold on ceria for CO-PROX reaction
Résumé
1.1 wt% Cu and 1 wt% Au/CeO2 catalysts were prepared by impregnation of a high-surface area ceria (>200 m2 g−1) with copper nitrate and dimethyl(acetylacetonate) gold(III), respectively. They were dried and calcined in air at 300 °C. They were denoted Au/CeO2 and CuOx/CeO2. The same procedure was used to prepare a composite catalyst by impregnation of gold on the CuOx/CeO2 catalyst (Au-CuOx/CeO2). XRD reveals that ceria is poorly crystallized (crystallite particle size of 11.4 nm) while Au and CuOx cannot be detected. Redox properties were evaluated by temperature-programmed reduction (TPR) and oxygen storage capacity (OSC) measurements. While gold is easily reduced below 100 °C, copper oxide reduces to Cu0 at 266 °C. Copper and gold promote the surface reduction of ceria. OSC measurements confirm that about one monolayer of ceria can be reduced at 400 °C. The performances of the catalysts in PROX reaction (2%CO + 2%O2 + 70%H2 in He) were measured between room temperature and 300 °C. At low temperature (below 120 °C), there is an increase in CO conversion up to 100%. Au/CeO2 is about 9 times more active than CuO/CeO2 but it is much less selective (50-60% of selectivity for Au instead of 100% for CuOx). Au-CuOx/CeO2 exhibits an intermediary behavior with both good activity and good selectivity. A decrease in CO conversion is observed above 120 °C for all the catalysts. It is due to the reverse water gas shift reaction (RWGS) tending to reform CO from CO2 and H2. This behavior is more marked for Au than for CuOx and Au-CuOx catalysts, which shows that gold is very active in RWGS. The three catalysts are deactivated by CO2 while only copper is very sensitive to steam. An interesting cooperative effect is shown for the Au-CuOx/CeO2 catalyst: (i) moderate and totally reversible inhibition by CO2 and (ii) absence of inhibition by steam.