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

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 with a similar bulk chemical composition and study the hydrogen activation mechanism and reactivity in syngas conversion. The results show that MnGaO-Solid Solution (MnGaO-SS) is a typical Mn-doped hexagonal close-packed (HCP) GaO 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 CHO, and ethylene hydrogenation, forming ethane. In contrast, MnGaO-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-Spinel. Therefore, MnGaO-Spinel exhibits a higher activity and higher light-olefin selectivity than MnGaO-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.