Journal
ACS CATALYSIS
Volume 12, Issue 10, Pages 6255-6264Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.2c00978
Keywords
ZnO; hydride; hydroxyl; H-2 activation; ZnCrOx; IR
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Funding
- National Key R&D Program of China [2021YFA1502800]
- National Natural Science Foundation of China [91945302, 21688102, 21825203]
- Photon Science Center for Carbon Neutrality, LiaoNing Revitalization Talents Program [XLYC1902117]
- Dalian National Laboratory for Clean Energy (DNL) Cooperation Fund [DNL201907]
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This study investigates the surface hydrogen species of ZnO and ZnCrOx catalysts using transmission infrared spectroscopy and temperature-programmed desorption. The results show that the surface geometry of metal oxide plays a crucial role in the evolution of hydrogen species.
Formation and evolution of active hydrogen species from H-2 on a metal oxide is of great importance in many hydrogenation reactions, while understanding the nature of surface hydrogen species remains a great challenge. Herein, high-vacuum-based transmission infrared spectroscopy coupled with temperature-programmed desorption has been applied to study surface hydrogen species over ZnO and ZnCrOx catalysts. We show that D-2 dissociates readily to form Zn-D and O-D on ZnO at 153 K, and simultaneous Zn-D and O-D desorption in the form of D-2 occurs at 223 K. This corroborates reversible dissociation and recombination of D-2 on ZnO. ( Cr-)Zn-D and (Cr-)O-D species appear on ZnCr2O4 in D-2 at 273 K, indicating a higher barrier for D-2 activation than that on ZnO. (Cr-)Zn-D can diffuse to form (Cr-)O-D at 573 K, and then desorption of surface O-D groups requires a higher temperature of 943 K. Theoretical studies demonstrate that surface geometry plays a crucial role in the evolution of the surface hydrogen species. The polar ZnCr2O4(111) surface can stabilize metal-H species due to the high barrier for its surface diffusion and the thermodynamic obstruction for its association with O-H, in contrast with the nonpolar ZnO(10 (1) over bar0) surface. These results provide insights into the tuning of the stability of surface metal-H species on metal oxide catalysts, which will contribute to tailoring the hydrogenation processes in the H-involved catalytic reactions.
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