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.