(690h) Presence of Multiple Types of Cu Active Sites and Their Connections to Framework Composition and Methane Oxidation Reactivity in Cu-CHA

Partial methane oxidation (PMO) to methanol can be carried out both step-wise and continuously over Cu-Chabazite (Cu-CHA) catalysts. A wide variety of Cu species have been proposed as possible PMO active sites. Experimental observations show that methanol production in step-wise processes is always less than the stoichiometry suggested by the Cu loading, suggesting the presence of multiple Cu exchange sites, only some of which are active for PMO. To aid in rationalizing these observations, we compute the free energies of formation of the most commonly suggested Cu dimers (O-bridged, O₂-bridged, OH-bridged, and (OH)₂-bridged) and monomers at different locations in CHA, relative to isolated Cu species, using supercell density functional theory. We find that formation energies are a strong function of both Al anchoring sites and activation conditions, and that at some sites, free energy differences between different Cu species are small, which may account for apparent differences in observed PMO activity. We use a titration procedure to populate exchange sites for a given distribution of Al. We find a large fraction of 8MR-3NN sites that have a preference to condense O-bridged Cu dimers. However, a non-negligible fraction of other PMO-active Cu species at other locations are also expected to be present, many of which may contribute to methane activation. These results are consistent with prior calculations and explain the range of Cu active sites and condensation sites reported in the literature. These results highlight how the active sites are influenced by the CHA topology, and subsequently impact on the catalytic activity.