Bimetallic Au–Cu supported on ceria for PROX reaction: Effects of Cu/Au atomic ratios and thermal pretreatments

Abstract

The influence of pretreatment (calcination or reduction), on bimetallic Au–Cu/CeO2 catalysts with a fixed Au loading of 2wt.% and an atomic Cu/Au ratio of 1/3, 1/1 or 3/1 was studied. The catalysts were prepared by direct anionic exchange of HAuCl4 followed by a Cu(NO3)2 impregnation. Both pretreatment methods led to catalysts of significantly different structures and surface compositions. For the calcined samples, the presence of a copper oxide phase can be observed when the Cu/Au is 3/1, whereas, for the reduced catalysts, there is the formation of an Au–Cu alloy and of metallic copper due to the immediate decomposition of the copper precursor, which is able to be partially alloyed with gold. XPS reveals that the calcination led to the segregation of a CuO-rich phase on the surface, while in the case of reduction, the surface is not so Cu-enriched. All of the catalysts were evaluated by preferential oxidation of CO in the presence of hydrogen and compared with monometallic 2% Au/CeO2 and 2% Cu/CeO2. For all catalysts, it was found that irrespective of the atomic Cu/Au ratio, a reduction improves the CO conversion and CO2 yield compared with monometallics while enhanced catalytic performances are only observed for the 1/3 Cu/Au atomic ratio of the calcined samples. The reduced Au–Cu bimetallics are stable with time on-stream contrary to the calcined one where the formation of carbonate-like species, responsible of a deactivation, is observed. Furthermore the reduced catalyst remained active even in the presence of CO2 and H2O in the gas stream