MAS NMR Studies on the Formation and Catalytic Activity of Oxygen Vacancy on the ZnO {100} Surface

Abstract

ZnO is an important component of the Cu/ZnO catalyst for catalyzing the carbon dioxide hydrogenation, which is used not only as a support to stabilize other metal species loaded on its surface but also its surface oxygen vacancies provide adsorption and catalytic active sites for the reactants as well as intermediates. In order to investigate the generation and nature of surface oxygen vacancies on ZnO crystalline surfaces and the activation of CO₂ and H₂, ZnO nanorods with exposed {100} surfaces were treated at different temperatures under vacuum conditions. The surface oxygen vacancies were characterized by ³¹P MAS NMR with trimethylphosphine (TMP) as the probe molecule, ¹H–³¹P CP/MAS NMR, and pyridine adsorption infrared spectroscopies. Based on the characterization results, the adsorption model of the probe molecule TMP on the surface of ZnO was established, and the density of the oxygen vacancies on the surface was quantitatively measured. The concentration of surface oxygen vacancies reached the maximum value when the treatment temperature was 350 °C under vacuum conditions. On such surface, the activation mechanism of CO₂ and H₂ was proposed based on the surface structure of ZnO nanorods with exposed {100} surfaces before and after the adsorption and activation of H₂ as well as the relevant generated intermediates on the surface during CO₂ hydrogenation characterized by ¹H, ³¹P, and ¹³C MAS NMR techniques.