Highly Selective Methane to Methanol Conversion on Inverse SnO₂/Cu₂O/Cu(111) Catalysts: Unique Properties of SnO₂ Nanostructures and the Inhibition of the Direct Oxidative Combustion of Methane

Abstract

Direct methane to methanol (CH4 → CH3OH) conversion in heterogeneous catalysis has been a long-standing challenge due to the difficulties in equalizing the activation of methane and protection of the methanol product at the same reaction conditions. Here, we report an inverse catalyst, consisting of small structures of SnO2 (0.5–1 nm in size) dispersed on Cu2O/Cu(111), for highly selective CH3OH production from CH4. This system was investigated by combining theoretical [density functional theory calculations (DFT) and kinetic Monte Carlo simulations (KMC)] and experimental methods [scanning tunneling microscopy (STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS)]. The DFT and AP-XPS studies showed that on SnO2/Cu2O/Cu(111), the conversion of CH4 by oxygen (O2) preferred complete combustion to carbon dioxide (CO2). The addition of water (H2O) enhanced the production of CH3OH to nearly 100% selectivity in KMC simulations. This trend was consistent with the results of AP-XPS. The presence of water in the reaction environment rendered an extremely high amount of methoxy species (*CH3O), a precursor for CH3OH production. The high CH3OH selectivity of SnO2/Cu2O/Cu(111) reflected the unique atomic and electronic structure of the supported SnO2 nanoparticles. As a result, the O2 adsorption and dissociation, and thus the full combustion of CH4 to CO2, were completely suppressed, while the H2O dissociative adsorption was still feasible, providing active hydroxyl species for a truly selective CH4 to CH3OH conversion