Surface Structure Dependent Activation of Hydrogen over Metal Oxides during Syngas Conversion

Abstract

Despite the extensive studies on the adsorption and activation of hydrogen over metal oxides, it remains a challenge to investigate the structure-dependent activation of hydrogen and its selectivity mechanism in hydrogenation reactions. Herein we take spinel and solid solution MnGaO_x with a similar bulk chemical composition and study the hydrogen activation mechanism and reactivity in syngas conversion. The results show that MnGaO_x-Solid Solution (MnGaO_x-SS) is a typical Mn-doped hexagonal close-packed (HCP) Ga₂O₃ with a Ga-rich surface. Upon exposure to hydrogen, Ga–H and O–H species are simultaneously generated. Ga–H species are highly active but unselective in CO activation, forming CH_xO, and ethylene hydrogenation, forming ethane. In contrast, MnGaO_x-Spinel is a face-centered-cubic (FCC) spinel phase featuring a Mn-rich surface, thus effectively suppressing the formation of Ga–H species. Interestingly, only part of the O–H species are active for CO activation while the O–H species are inert for olefin hydrogenation over MnGaO_x-Spinel. Therefore, MnGaO_x-Spinel exhibits a higher activity and higher light-olefin selectivity than MnGaO_x-SS in combination with SAPO-18 during syngas conversion. These fundamental understandings are essential to guide the design and further optimization of metal oxide catalysts for selectivity control in hydrogenations.