Cao Yunjun, Luo Jie, Huang Wugen, Ling Yunjian, Zhu Junfa, Li Wei-Xue, Yang Fan, Bao Xinhe
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
School of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at Microscales, University of Science and Technology of China, Hefei 230026, China.
J Chem Phys. 2020 Feb 21;152(7):074714. doi: 10.1063/1.5138372.
The catalytic properties of metal oxides are often enabled by surface defects, and their characterization is thus vital to the understanding and application of metal oxide catalysts. Typically, surface defects for metal oxides show fingerprints in spectroscopic characterization. However, we found that synchrotron-radiation photoelectron spectroscopy (SRPES) is difficult to probe surface defects of ZnO. Meanwhile, CO as a probe molecule cannot be used properly to identify surface defect sites on ZnO in infrared (IR) spectroscopy. Instead, we found that formaldehyde could serve as a probe molecule, which is sensitive to surface defect sites and could titrate surface oxygen vacancies on ZnO, as evidenced in both SRPES and IR characterization. Density functional theory calculations revealed that formaldehyde dissociates to form formate species on the stoichiometric ZnO(101¯0) surface, while it dissociates to formyl species on Vo sites of the reduced ZnO(101¯0) surface instead. Furthermore, the mechanism of formaldehyde dehydrogenation on ZnO surfaces was also elucidated, while the generated hydrogen atoms are found to be stored in ZnO bulk from 423 K to 773 K, making ZnO an interesting (de)hydrogenation catalyst.
金属氧化物的催化性能通常由表面缺陷赋予，因此对其进行表征对于理解和应用金属氧化物催化剂至关重要。通常，金属氧化物的表面缺陷在光谱表征中会显示出特征。然而，我们发现同步辐射光电子能谱（SRPES）难以探测ZnO的表面缺陷。同时，在红外（IR）光谱中，CO作为探针分子不能适当地用于识别ZnO上的表面缺陷位点。相反，我们发现甲醛可以作为探针分子，它对表面缺陷位点敏感，并且可以滴定ZnO上的表面氧空位，这在SRPES和IR表征中都得到了证实。密度泛函理论计算表明，甲醛在化学计量的ZnO(101¯0)表面解离形成甲酸盐物种，而在还原的ZnO(101¯0)表面的Vo位点上解离形成甲酰基物种。此外，还阐明了甲醛在ZnO表面脱氢的机理，同时发现从423 K到773 K产生的氢原子存储在ZnO体相中，这使得ZnO成为一种有趣的（脱）氢化催化剂。
