Surface\nStructure Dependent Activation of Hydrogen\nover Metal Oxides during Syngas Conversion

Despite the extensive\nstudies on the adsorption and activation\nof hydrogen over metal oxides, it remains a challenge to investigate\nthe structure-dependent activation of hydrogen and its selectivity\nmechanism in hydrogenation reactions. Herein we take spinel and solid\nsolution MnGaO<sub><i>x</i></sub> with a similar bulk chemical\ncomposition and study the hydrogen activation mechanism and reactivity\nin syngas conversion. The results show that MnGaO<sub><i>x</i></sub>-Solid Solution (MnGaO<sub><i>x</i></sub>-SS) is\na typical Mn-doped hexagonal close-packed (HCP) Ga<sub>2</sub>O<sub>3</sub> with a Ga-rich surface. Upon exposure to hydrogen, Ga–H\nand O–H species are simultaneously generated. Ga–H species\nare highly active but unselective in CO activation, forming CH<sub><i>x</i></sub>O, and ethylene hydrogenation, forming ethane.\nIn contrast, MnGaO<sub><i>x</i></sub>-Spinel is a face-centered-cubic\n(FCC) spinel phase featuring a Mn-rich surface, thus effectively suppressing\nthe formation of Ga–H species. Interestingly, only part of\nthe O–H species are active for CO activation while the O–H\nspecies are inert for olefin hydrogenation over MnGaO<sub><i>x</i></sub>-Spinel. Therefore, MnGaO<sub><i>x</i></sub>-Spinel exhibits a higher activity and higher light-olefin selectivity\nthan MnGaO<sub><i>x</i></sub>-SS in combination with SAPO-18\nduring syngas conversion. These fundamental understandings are essential\nto guide the design and further optimization of metal oxide catalysts\nfor selectivity control in hydrogenations.