利用原位APXPS与STM研究H2在ZnO(1010)表面的活化

In-situ APXPS and STM Study of the Activation of H_2 on ZnO(1010) Surface

Cu/ZnO/Al_2O_3是工业中最广泛使用的甲醇合成催化剂。然而该催化反应的活性位点和机理目前仍存争议。H_2作为反应物之一,研究其在ZnO表面的活化和解离对于弄清甲醇合成反应的催化机理具有重要的帮助。本工作利用近常压光电子能谱(APXPS)和扫描隧道显微镜(STM)原位研究了H_2在ZnO (1010)表面上的活化和解离。APXPS结果表明:在0.3mbar (1 mbar=100 Pa)的H_2气氛中,室温下ZnO表面形成羟基(OH)吸附物种。STM实验发现通入H_2后ZnO表面发生了(1×1)到(2×1)的重构。上述结果和原子H在ZnO (1010)表面的吸附结果一致。然而吸附H_2O可以导致同样的现象。因此,我们还开展了H_2O在ZnO (1010)表面吸附的对比实验。结果表明:H_2气氛中ZnO表面发生0.3 eV的能带弯曲,而H_2O吸附实验中几乎观察不到能带弯曲发生。同时,热稳定性实验表明H_2气氛中ZnO表面的OH不同于H_2O解离吸附产生的OH,前者具有更高的脱附温度。因此,本工作的结果表明常温和常压下H_2在ZnO (1010)表面发生解离吸附。这一结果和以往超高真空下未发现H_2在ZnO (1010)表面上的解离不同,说明H_2的活化是一个压力依赖过程。

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.H2 is one of the reaction gases in methanol synthesis.Studies of the activation and dissociation behaviors of H2 on ZnO surfaces are of great importance in understanding the catalytic mechanism of methanol synthesis.In this work,the activation and dissociation processes of H2 on a ZnO(101 0)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)H2 atmosphere.Meanwhile,STM measurements showed that the ZnO surface was reconstructed from a(1×1)to a(2×1)structure upon introduction of H2.These observations revealed adsorption behaviors of H2 the same as those of atomic H on a ZnO(101 0)surface as seen in previous studies,which could be evidence of the dissociative adsorption of H2 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(1×1)to a(2×1)structure on the ZnO(101 0)surface upon H2 introduction.Hence,it is necessary to exclude the influence of H2O in this work,because there may be trace amounts of H2O in the H2 gas.Therefore,we performed a comparative study of H2 and H2O on ZnO(101 0)single crystal surface.A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H2 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 H2 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 H2 on the ZnO(101 0)surface at room temperature and atmospheric pressure.This is in contrast to previous findings,in which no H2 dissociation on a ZnO(101 0)surface under ultra-high vacuum conditions was observed,indicating that the activation of H2 on ZnO surfaces is a pressure dependent process.