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Acta Phys. -Chim. Sin.  2018, Vol. 34 Issue (12): 1366-1372    DOI: 10.3866/PKU.WHXB201804161
Special Issue: Surface Physical Chemistry
ARTICLE     
In-situ APXPS and STM Study of the Activation of H2 on ZnO(10${\rm{\bar 1}}$0) Surface
Qiang LIU1,4,Yong HAN1,3,Yunjun CAO2,Xiaobao LI1,4,Wugen HUANG2,Yi YU3,Fan YANG2,Xinhe BAO2,Yimin LI1,3,*(),Zhi LIU1,3,*()
1 State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China
2 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning Province, P. R. China
3 School of Physical Science and Technology, Shanghai Tech University, Shanghai 201203, P. R. China
4 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
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Abstract  

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(10${\rm{\bar 1}}$0) single crystal surface were investigated in-situ using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM), two powerful surface characterization techniques. In the APXPS experiments, results indicated the formation of hydroxyl (OH) species on the ZnO single crystal surface at room temperature in 0.3 mbar (1 mbar = 100 Pa) H2 atmosphere. Meanwhile, STM measurements showed that the ZnO surface was reconstructed from a (1×1) to a (2×1) structure upon introduction of H2. These observations revealed adsorption behaviors of H2 the same as those of atomic H on a ZnO(10${\rm{\bar 1}}$0) surface as seen in previous studies, which could be evidence of the dissociative adsorption of H2 on a ZnO surface. However, H2O adsorption on ZnO surfaces can also result in the formation of OH species, which can be observed using XPS. The STM results show that the exposure of H2O also leads to the reconstruction from a (1×1) to a (2×1) structure on the ZnO(10${\rm{\bar 1}}$0) surface upon H2 introduction. Hence, it is necessary to exclude the influence of H2O in this work, because there may be trace amounts of H2O in the H2 gas. Therefore, we performed a comparative study of H2 and H2O on ZnO(10${\rm{\bar 1}}$0) single crystal surface. A downward band bending of 0.3 eV was observed on the ZnO surface in 0.3 mbar H2 atmosphere using APXPS, while negligible band bending was shown in the case of the H2O atmosphere. Moreover, thermal stability studies revealed that the OH group formed in the H2 atmosphere desorbed at a higher temperature than the one resulting from H2O adsorption, meaning that the two OH groups formed on the ZnO surface were different. Results in this work provide evidence of the dissociative adsorption of H2 on the ZnO(10${\rm{\bar 1}}$0) surface at room temperature and atmospheric pressure. This is in contrast to previous findings, in which no H2 dissociation on a ZnO(10${\rm{\bar 1}}$0) surface under ultra-high vacuum conditions was observed, indicating that the activation of H2 on ZnO surfaces is a pressure dependent process.



Key wordsH2      ZnO(10${\rm{\bar 1}}$0)      Activation      Dissociative adsorption      APXPS      STM     
Received: 14 March 2018      Published: 16 April 2018
MSC2000:  O647  
Fund:  The project was supported by the National Natural Science Foundation of China(11227902);the Ministry of Science and Technology of China(2017YFB0602205);the Ministry of Science and Technology of China(2016YFA0202803);the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB17020200)
Corresponding Authors: Yimin LI,Zhi LIU     E-mail: liym1@shanghaitech.edu.cn;zliu2@mail.sim.ac.cn
Cite this article:

Qiang LIU,Yong HAN,Yunjun CAO,Xiaobao LI,Wugen HUANG,Yi YU,Fan YANG,Xinhe BAO,Yimin LI,Zhi LIU. In-situ APXPS and STM Study of the Activation of H2 on ZnO(10${\rm{\bar 1}}$0) Surface. Acta Phys. -Chim. Sin., 2018, 34(12): 1366-1372.

URL:

http://www.whxb.pku.edu.cn/10.3866/PKU.WHXB201804161     OR     http://www.whxb.pku.edu.cn/Y2018/V34/I12/1366

Fig 1 XPS O 1s (a) of ZnO(10${\rm{\bar 1}}$0) surface under UHV (bottom) and 0.3 mbar H2 (top) at RT, STM images of as prepared ZnO(10${\rm{\bar 1}}$0) surface under UHV (b) and after H2 exposure (c), schematic of H adsorption sites on ZnO(10${\rm{\bar 1}}$0) surface (d).
Fig 2 O 1s spectra of ZnO(10${\rm{\bar 1}}$0) surface under different conditions after H2 or H2O exposure; relative area changing of adsorbates on ZnO(10${\rm{\bar 1}}$0) surface under different conditions (c).
Fig 3 Zn 2p and O 1s spectra of ZnO(10${\rm{\bar 1}}$0) surface under different conditions after exposure to H2 or H2O.
Fig 4 Corresponding relationship between the extent of band bending caused by H2 (red) or H2O (blue) exposure and amount of OH produced by H2 or H2O on ZnO(10${\rm{\bar 1}}$0) surface.
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