Modification of the ZnO(0001)-Zn surface under reducing conditions

The ZnO(0001)-Zn terminated crystal face was studied after reduction at high temperatures by combination of STM, STS, XPS and TDS. The clean ZnO(0001)-Zn surface exhibits triangular reconstruction in UHV, while after exposure to 10(-5) mbar H-2 at RT this reconstruction is lifted and a rough surface has formed. The roughness as well as the metallic character of the surface increased with the applied low-pressure reduction temperature up to 800 K. XPS revealed that exposure to 1 bar H-2 at RT led to the formation of OH groups; at higher temperatures progressive metallization of the ZnO surface was found to occur. Analysis of the thermal desorption results showed that huge amounts of H-2 dissolved into the ZnO crystal. The results obtained under these conditions were in good accordance with thermodynamic calculations. The experimental ratio between the absorbed amount of H-2 at RT and 800 K amounts to 1000. The ratio calculated from increasing diffusion coefficients with temperature only amounts to 6. This emphasizes the importance of ZnO as a H supplier by spillover, and proves that metallic Zn boosts dissociative adsorption of H-2. This surface modification of the ZnO structure during the reduction promotes an enhanced activity of the Cu/ZnO catalyst at elevated temperatures.