Oxidation of Methane to Methanol by Water Over Cu/SSZ-13: Impact of Cu Loading and Formation of Active Sites

Cu/SSZ-13 catalysts with different Cu loadings (1-5 wt.%) were prepared by conventional impregnation via a slow step-dry process for the direct conversion of methane to methanol. Catalytic performance of the catalysts is found to depend on the Cu loading in both stepwise and continuous processes and 2 wt.% Cu presents the best performance. CuO nanoparticles, CuxOy clusters and monomeric [CuOH](+) species are found on Cu/SSZ-13 based on HRTEM, HAADF-STEM, DR UV-vis and FTIR spectroscopies. The high coverage of CuO nanoparticles on 5 wt.% Cu/SSZ-13 results in the loss of catalytic reactivity in spite of the existence of active CuxOy and [CuOH](+) sites. Besides, it is proposed that the formation of [Cu2O2](2+) and [Cu2O](2+) active sites involves [CuOH](+) intermediates. We further found that improved catalytic performance in cyclic stepwise process involves the re-dispersion of CuO nanoparticles and/or large clusters as well as the re-oxidation of Cu(I) cations from self-reduction of [CuOH](+) species.