Activity and stability of nanostructured gold-cerium oxide catalysts for the water-gas shift reaction

Advanced low-temperature water-gas shift (LTS) catalysts of high activity and stability are under development to produce essentially CO-free hydrogen to feed PEM fuel cells for power generation. Materials based on nanocrystalline cerium oxide (ceria) are among the most promising LTS catalysts. Understanding the structural properties relationship with the WGS activity is fundamentally important in order to rational design the catalysts.; Various gold structures, such as metallic gold nanoparticles, cluster and cations were found in gold-ceria sample containing 4–8 at% gold. To discriminate between the various gold species, leaching of the gold-ceria in sodium cyanide was conducted. The metallic gold and all other gold species not in close association with ceria were removed by leaching. A small amount of gold remained in the leached samples. The exact content of non-leachable gold was a function of the parent catalyst properties. Similar data were collected from Pt-ceria samples. STEM or HRTEM, coupled with EDX showed no gold or platinum particles remaining; only what appeared to be very fine clusters or atomically dispersed gold or platinum. Cationic gold or platinum was identified in these samples by XPS.; The unexpected finding was that the catalytic activity of the leached samples was similar or slightly better than that of the parent catalyst after removal of the metallic gold or platinum particles by cyanide leaching. Thus, metallic nanoparticles are not necessary; they are mere spectators in the water-gas shift reaction. Nonmetallic gold or platinum species strongly associated with surface cerium-oxygen groups are responsible for the activity, since the extra gold or platinum present in the parent material does not increase the reaction rate; nor does it change the activation energy of the reaction. The importance of the oxide support properties became clear by this work. The amount of gold or platinum retained in active form depends on the surface properties of ceria.; Addition of certain metals and dopants significantly enhances the reducibility of ceria. Temperature-programmed reduction of metal-ceria samples performed with H₂ or CO, identified that the surface oxygen of ceria is weakened by the presence of the metal. The reduction of oxygen of gold-ceria by CO begins at ambient temperatures, while the onset of surface reduction of the metal-free Ce(La)O_x is at 200–250°C. The WGS activity and reducibility both depend on the preparation of ceria, the dispersion of the metal additive and ceria particle size. However, no dependence on the metal particle size was found. (Abstract shortened by UMI.)...