Cu-exchanged chabazite (Cu-CHA) zeolites are prominent catalysts for NOx selective catalytic reduction (SCR) and partial CH₄ oxidation. The types and distributions of Cu active sites depend sensitively on zeolite composition, synthesis, environmental conditions, and treatment history. While various species like monomeric (Z2Cu2+, ZCuOH+) and dimeric ([CuOCu]2+) Cu sites are proposed, their spectroscopic distinction is complicated by overlapping signals and detection limitations. H2 temperature-programmed reduction (H2-TPR) reveals distinct features often attributed to different Cu species. However, observed H2-TPR profiles, even in samples presumed to contain only monomeric Z2Cu2+, are sensitive to composition. This sensitivity suggests complex site distributions and raises questions about the mechanism of one-electron Cu²⁺ reduction by the two-electron reductant H2.T Here, we investigate Cu²⁺ reduction mechanisms in Z2Cu-only CHA zeolite samples, comparing isolated Z2Cu2+ sites and pairs of Z2Cu2+ sites. Using statistical titration simulations, we quantified these two site distributions as a function of Si/Al and Cu/Al ratios, finding that proximal Z2Cu populations increase with Cu density, while isolated Z2Cu sites remain relatively constant. Density functional theory (DFT) and microkinetic modeling explored the reduction mechanisms, considering H2 activation and subsequent framework- or extra-framework-mediated H migration. Our results indicate that the framework-mediated H migration mechanism is promoted by closer Cu2+ inter-site proximity. By combining simulated H2 consumption rates with statistical site quantification, we successfully reproduced experimental H₂-TPR profiles across experimentally relevant Cu-CHA compositions. This work rationalizes observed H2-TPR behavior and provides crucial insights into Cu site distribution and its impact on reactivity in these catalytic materials.
