Heterogeneous and Framework-Bound Copper Species Contribute to Catalytic Partial Methane Oxidation in Cu–Chabazite Zeolites

The relationship between continuous partial methane oxidation (PMO) rates and Cu site speciation in Cu-CHA zeolite catalysts is explored through differential rate measurements across a series of samples of varying compositions combined with density functional theory, first-principles thermodynamics, and statistical models that characterize Cu speciation. Under continuous PMO conditions (573 K, 0.07 kPa O2, 3 kPa H2O), Cu ions are shown to anchor to the CHA framework in both monomeric (Z2Cu and Z2CuH2O) and dimeric (O- and OH-bridged Cu) forms that are sensitive to the identity of the local framework anchoring site. Consequently, across the studied compositional range, Cu-CHA catalysts are predicted to contain a mixture of monomeric and dimeric Cu sites. Cu-normalized CH3OH formation rates extrapolated to zero conversion reflect contributions from multiple site types. Predicted CH3OH formation rates indicate that the Cu site reactivity toward CH4 is influenced by zeolite composition and is likely limited by the reduction half-cycle.