In-situ APXPS and STM Study of the Activation of H2 on ZnO(10(1)over-bar0) Surface

Cu/ZnO/Al2O3 is one of the most widely used catalysts in industrial methanol synthesis. However, the reaction mechanism and the nature of the active sites on the catalyst for this reaction are still under debate. Thus, detailed information is needed to understand the catalytic processes occurring on the surface of this catalyst. H-2 is one of the reaction gases in methanol synthesis. Studies of the activation and dissociation behaviors of H-2 on ZnO surfaces are of great importance in understanding the catalytic mechanism of methanol synthesis. In this work, the activation and dissociation processes of H-2 on a ZnO(10 (1) over bar0) single crystal surface were investigated in-situ using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM), two powerful surface characterization techniques. In the APXPS experiments, results indicated the formation of hydroxyl (OH) species on the ZnO single crystal surface at room temperature in 0.3 mbar (1 mbar = 100 Pa) H-2 atmosphere. Meanwhile, STM measurements showed that the ZnO surface was reconstructed from a (1 x 1) to a (2 x 1) structure upon introduction of Hz. These observations revealed adsorption behaviors of H-2 the same as those of atomic H on a ZnO(10 (1) over bar0) surface as seen in previous studies, which could be evidence of the dissociative adsorption of H-2 on a ZnO surface. However, H2O adsorption on ZnO surfaces can also result in the formation of OH species, which can be observed using XPS. The STM results show that the exposure of H2O also leads to the reconstruction from a (1 x 1) to a (2 x 1) structure on the ZnO(10 (1) over bar0) surface upon H-2 introduction. Hence, it is necessary to exclude the influence of H2O in this work, because there may be trace amounts of H2O in the H-2 gas. Therefore, we performed a comparative study of H-2 and H2O on ZnO(10 (1) over bar0) single crystal surface. A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H-2 atmosphere using APXPS, while negligible band bending was shown in the case of the H2O atmosphere. Moreover, thermal stability studies revealed that the OH group formed in the H-2 atmosphere desorbed at a higher temperature than the one resulting from H2O adsorption, meaning that the two OH groups formed on the ZnO surface were different. Results in this work provide evidence of the dissociative adsorption of H-2 on the ZnO(10 (1) over bar0) surface at room temperature and atmospheric pressure. This is in contrast to previous findings, in which no H-2 dissociation on a ZnO(10 (1) over bar0) surface under ultra-high vacuum conditions was observed, indicating that the activation of H-2 on ZnO surfaces is a pressure dependent process.