The direct oxidation of methane (CH₄) to methanol (CH₃OH) at low temperatures has long been one of the holy grail reactions in C1 chemistry. To generate CH₃OH as a stable product, one must find catalytic materials which can bind and activate CH₄ at temperatures below 500 K, because at temperatures higher than 600 K, the CH₄+ ½ O₂ → CO+2H₂ reaction is more favorable than the CH₄+ ½ O₂ → CH₃OH one. Here, we show that catalysts consisting of Ni nanoparticles (< 0.2 monolaters, ML) on ceria are not only active for the cleavage of the first C-H bond in CH₄ and O-H bonds in H₂O, but also for the full dissociation of the CH₃ species to C [1,2]. We further show that a proposed linear scaling relation to predict the energy barrier for the activation of methane does not generally hold for ceria-supported metal nanoparticles [3]. Strong interactions between the small metal particles in direct contact with the ceria support lead to the stabilization of both the CH₄ molecule and the CH₃ + H dissociation product, producing active and stable catalysts for methane activation under very mild conditions. For achieving a selective direct oxidation of methane to methanol (DMTM), the further decomposition of the methyl species has to be controlled. We here report on two possible strategies, namely, the addition of water as reactant over Ni-ceria systems or the addition of Cu, forming ceria-supported bimetallic NiCu nanoparticles. The emphasis is here put on theoretical studies in combination with experiments using ambient pressure X-ray photoelectron spectroscopy and catalytic studies. We demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose over Ni-ceria and enhances the selectivity toward methanol formation [4], and suggest that mixing Cu with Ni in ways that the surface Ni ensembles in the NiCu bimetallic nanoparticles are isolated, would have a similar effect.

Feng Zhang

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