Impact of Surface Composition of SrTiO3 Catalysts for Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) to C-2 hydrocarbons (C2H6 and C2H4) have regained much attention due to the shale gas revolution. Perovskite catalysts have shown promising activity and selectivity to C-2 hydrocarbons. Here, we investigate the effect of surface reconstruction (leading to different surface compositions) of perovskites on the OCM by using SrTiO3(STO) as a model catalyst. Different surface densities of Sr (25-96 %) were attained via various treatments of STO. Low energy ion scattering (LEIS) and UV-Raman results are in good agreement on the surface and subsurface composition of the reconstructed STO. From H-2-TPR, the same H-2 consumption of STO samples allows relating their catalytic performances with surface acid-base properties (quantified by NH3-/CO2-TPD). At 600-800 degrees C, the surface Sr enrichment was found to enhance CH4 conversion, C-2 selectivity and the ratio C2H4/C2H6 up to Sr/(Sr+Ti) of 0.66 and then levels off. Furthermore, the relative concentration of basic sites, base/(base+acid), is found as a better descriptor for STO catalytic performances. This work shows the clear correlation between surface reconstruction, relative basicity/acidity and OCM catalytic performance over perovskite catalysts. The trends here are similar to those for CH4 combustion over the reconstructed STO in our recent work. Overall, we suggest that tuning surface reconstruction/composition of perovskites can be an effective approach to control CH4 activation and conversions.