Surface characterization and methane activation on SnO_x/Cu₂O/Cu(111) inverse oxide/metal catalysts

To activate methane at low or medium temperatures is a difficult task and a pre-requisite for the conversion of this light alkane into high value chemicals. Herein, we report the preparation and characterizations of novel SnO_x/Cu₂O/Cu(111) interfaces that enable low-temperature methane activation. Scanning tunneling microscopy identified small, well-dispersed SnO_x nanoclusters on the Cu₂O/Cu(111) substrate with an average size of 8 Å, and such morphology was sustained up to 450 K in UHV annealing. Ambient pressure X-ray photoelectron spectroscopy showed that hydrocarbon species (CH_x groups), the product of methane activation, were formed on SnO_x/Cu₂O/Cu(111) at a temperature as low as 300 K. An essential role of the SnO_x-Cu₂O interface was evinced by the SnO_x coverage dependence. Systems with a small amount of tin oxide, 0.1-0.2 ML coverage, produced the highest concentration of adsorbed CH_x groups. Calculations based on density functional theory showed a drastic reduction in the activation barrier for C-H bond cleavage when going from Cu₂O/Cu(111) to SnO_x/Cu₂O/Cu(111). On the supported SnO_x, the dissociation of methane was highly exothermic (ΔE∼-35 kcal mol^-1) and the calculated barrier for activation (∼20 kcal mol^-1) could be overcome at 300-500 K, target temperatures for the conversion of methane to high value chemicals.