Cooperative effect between copper and gold on ceria for CO-PROX reaction

Abstract

1.1 wt% Cu and 1 wt% Au/CeO 2 catalysts were prepared by impregnation of a high-surface area ceria (>200 m 2 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/CeO 2 and CuO x/CeO 2. The same procedure was used to prepare a composite catalyst by impregnation of gold on the CuO x/CeO 2 catalyst (Au-CuO x/CeO 2). XRD reveals that ceria is poorly crystallized (crystallite particle size of 11.4 nm) while Au and CuO x 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 Cu 0 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%O 2 + 70%H 2 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/CeO 2 is about 9 times more active than CuO/CeO 2 but it is much less selective (50–60% of selectivity for Au instead of 100% for CuO x). Au-CuO x/CeO 2 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 CO 2 and H 2. This behavior is more marked for Au than for CuO x and Au-CuO x catalysts, which shows that gold is very active in RWGS. The three catalysts are deactivated by CO 2 while only copper is very sensitive to steam. An interesting cooperative effect is shown for the Au-CuO x/CeO 2 catalyst: (i) moderate and totally reversible inhibition by CO 2 and (ii) absence of inhibition by steam.