The conversion of methane to more valuable chemicals is one of the most intensively studied topics in catalysis. The direct conversion of methane is attractive because the process is simple, but unfortunately its products are chemicals that are more reactive than methane. The current status of this research field is discussed with an emphasis on C-H bond activation and future challenges.

The variation in the morphology and electronic structure of copper during the electroreduction of CO2 into valuable hydrocarbons and alcohols was revealed by combining in situ surface- and bulk-sensitive X-ray spectroscopies with electrochemical scanning electron microscopy. These experiments proved that the electrified interface surface and near-surface are dominated by reduced copper. The selectivity to the formation of the key C–C bond is enhanced at higher cathodic potentials as a consequence of increased copper metallicity. In addition, the reduction of the copper oxide electrode and oxygen loss in the lattice reconstructs the electrode to yield a rougher surface with more uncoordinated sites, which controls the dissociation barrier of water and CO2. Thus, according to these results, copper oxide species can only be stabilized kinetically under CO2 reduction reaction conditions.

Developing efficient catalytic processes to convert methane into useful feedstocks relies critically upon devising new coupling processes that use abundant, thermodynamically 'mild' oxidants together with selective catalysts. We report here on elemental sulfur as a promising 'soft' oxidant for selective methane conversion to ethylene over MoS(2), RuS(2), TiS(2), PdS and Pd/ZrO(2) catalysts. Experiments and density functional theory reveal that methane conversion is directly correlated with surface metal-sulfur bond strengths. Surfaces with weakly bound sulfur are more basic and activate methane C-H bonds more readily. In contrast, experimental and theoretical selectivities scale inversely with surface metal-sulfur bond strengths, and surfaces with the strongest metal-sulfur bonds afford the highest ethylene selectivities. High CH(4)/S ratios, short contact times and the provision of a support maximizes the coupling of CH(x) intermediates and selectivity to ethylene, because these conditions yield surfaces with stronger metal-sulfur bonding (for example, Pd(16)S(7)), which suppresses the over-oxidation of methane.