ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al 2 O 3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al 2 O 3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al 2 O 3 catalysts can be easily regenerated by oxidation in air at 420 ºC followed by re-exposure to reaction conditions at 250 ºC, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 ºC) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO 2 selectivity.

The adsorption and bonding configuration of CO on clean and Zn-covered Pd(111) surfaces was studied using Low Energy Electron Diffraction (LEED), Temperature Programmed Desorption (TPD) and High Resolution Electron Energy Loss Spectroscopy (HREELS). LEED and TPD results indicate that annealing at 550 K is sufficient to induce reaction between adsorbed Zn atoms and the Pd(111) surface resulting in the formation of an ordered surface PdZn alloy. Carbon monoxide was found to bond more weakly to the Zn/ Pd(111) alloy surfaces compared to clean Pd(111). Zn addition was also found to alter the preferred adsorption sites for CO from threefold hollow to atop sites. Similar behavior was observed for supported Pd-Zn/Al 2 O 3 catalysts. The results of this study show that both ensemble and electronic effects play a role in how Zn alters the interactions of CO with the surface.

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