Structure Sensitivity of the Low-temperature Water-gas Shift Reaction on Cu–CeO2 catalysts
We have investigated the structure sensitivity of the water-gas shift (WGS) reaction on Cu-CeO{sub 2} catalysts prepared at the nanoscale by different techniques. On the surface of ceria, different CuO{sub x} structures exist. We show here that only the strongly bound Cu-[O{sub x}]-Ce species, probably associated with the surface oxygen vacancies of ceria, are active for catalyzing the low-temperature WGS reaction. Weakly bound CuO{sub x} clusters and CuO nanoparticles are spectator species in the reaction. Isolated Cu{sup 2+} ions doping the ceria surface are not active themselves, but they are important in that they create oxygen vacancies and can be used as a reservoir of copper to replenish surface Cu removed by leaching or sintering. Accordingly, synthesis techniques such as coprecipitation that allow for extensive solubility of Cu in ceria should be preferred over impregnation, deposition-precipitation, ion exchange or another two-step method whereby the copper precursor is added to already made ceria nanocrystals. For the synthesis of different structures, we have used two methods: a homogeneous coprecipitation (CP), involving hexamethylenetetramine as the precipitating agent and the pH buffer; and a deposition-precipitation (DP) technique. In the latter case, the ceria supports were first synthesized at the nanoscale with different shapes (rods, cubes) to investigate any potential shape effect on the reaction. Cu-CeO{sub 2} catalysts with different copper contents up to ca. 20 at.% were prepared. An indirect shape effect of CeO{sub 2}, manifested by the propensity to form oxygen vacancies and strongly bind copper in the active form, was established; i.e. the water-gas shift reaction is not structure-sensitive. The apparent activation energy of the reaction on all samples was similar, 50 {+-} 10 kJ/mol, in a product-free (2% CO-10% H{sub 2}O) gas mixture.