Preferential CO oxidation on Pt-Cu/Al_2O_3 catalysts with low Pt loadings

摘要

This work demonstrates that a small loading of Pt (0.2-0.5 wt.%) selectively located at the top of the Cu/Al_2O_3 solid is enough to obtain active catalysts for the COPrOx reaction. The characterizations of the catalysts prepared in this work using different Pt:Cu ratios show that the phases formed strongly depend on the Cu and Pt loadings. For the samples with 4 wt.% of Cu, the formation of defective CuAl_2O_4-like species is predominant, with Cu~2' occupying distorted sites in the alumina surface. For the samples with 8 wt.% of Cu, CuO and metallic Cu in the form of small crystals are observed besides Cu~2. For both 0.2 and 0.5 wt.% of Pt, some proportion of Cu-Pt alloy is found which increased with the Cu content. Metallic Pt particles are only observed by EXAFS for the samples with a higher loading of the noble metal. The higher CO conversions at low temperatures were obtained for the Pt_0.5Cu_8/Al_2O_3 sample, probably due to the simultaneous formation of small Pt crystals in close contact with CuO. The XPS results show that a strong surface enrichment in Pt takes place in all the samples. Thus, a small loading of the noble metal is enough to obtain effective catalysts. The surface Pt enrichment observed is most probably due to the impregnation of the Pt salt onto the Cu/Al_2O_3 solid, with a relatively low calcination temperature (300 ℃) that prevents the formation of bulky CuAl_2O_4 spinel crystals.