H₂ Activation Mechanisms on ZnO-Based Catalysts

Abstract

ZnO is extensively applied in a variety of fields because of its unique electronic and structural properties and plays crucial roles in industrially important catalytic processes involving H 2 such as methanol synthesis from H 2 /CO/CO 2 and syngas conversion to light olefins. In general, H 2 can readily dissociate at Zn–O pairs via a heterolytic pathway, generating hydroxyl and zinc hydride species that are active in the hydrogenation of polar groups in hydrocarbons. On specific ZnO surfaces with structural defects or reconstructions, the homolytic dissociation of H 2 takes place at elevated temperature. For metal supported ZnO catalysts, metal nanoparticles can dissociate H 2 homolytically along with hydrogen spillover. In contrast, the formation of an abundant metal–support interface is beneficial for heterolytic H 2 dissociation at Lewis acid–base pairs, which not only facilitates H 2 dissociation but also improves the hydrogenation activity. In addition, H 2 dissociation is mostly dictated by the experimental conditions, including reaction temperature, gas pressure, high or low vacuum, and specific catalytic system. In this review, we briefly summarize H 2 dissociation through heterolytic and homolytic pathways, the progresses on H 2 activation mechanisms over ZnO-based catalysts both in experiment and calculation, as well as the role of H 2 dissociation on subsequent hydrogenation reaction in heterogeneous catalysis. H 2 activation reactions over single/faceted/supported ZnO catalysts are discussed through this review with emphasis on the effect of surface structure, exposure of facets, reaction conditions (e.g., temperature and time), catalyst components, the formation of metal–support interface and Lewis acid–base sites on H 2 dissociation, and possible hydrogenation reactions. Based on our previous work for H 2 adsorption on ZnO nanorods under different conditions, the detailed activation mechanisms of H 2, the fate of generated hydrogen species, and the relevant changes of surface structure of ZnO during the catalytic reaction are discussed. This review of the H 2 activation mechanisms on ZnO-based catalysts covers what is known and what questions remain to be addressed to support our understanding of the catalytic reaction in hydrogenation and advance the rational design of this important catalytic system.

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