(625h) Cu Reduction Mechanism in H2-TPR and Implications for Cu Distribution in Cu-CHA

Cu-exchanged chabazite (CHA) zeolites are catalytically active for the selective catalytic reduction of NO_x and partial methane oxidation, two reactions that consume O₂ over nominally atomically dispersed Cu sites. The identity of exchanged Cu sites is observed to be a function Si/Al ratio, zeolite synthesis conditions, and Cu loading. Various techniques have been used to identify and quantify Cu sites in Cu-CHA. In the 2+ state, monomeric Cu²⁺ and [CuOH]⁺ sites are believed to coexist with Cu dimers (e.g., [CuOCu]⁺) and even higher nuclearity sites. Spectroscopic quantification is challenged by the silence of some site types and overlapping bands of others. H₂-TPR experiments do reveal distinct features that have been attributed to the reduction of various Cu dimers and Cu monomers ([CuOH]⁺ and Cu²⁺) from Cu²⁺ to Cu⁺. However, these features are sensitive to sample composition (Si/Al ratio) and Cu loading, complicating assignment of observed features to specific Cu motifs. The mechanism by which H₂ reduces the diversity of Cu²⁺ sites remains unknown, hindering a complete characterization of Cu-CHA, as the reduction of Cu²⁺ to Cu⁺ consumes a single electron. Here we use density functional theory (DFT) calculations to explore this question. We consider both the initial H₂ activation step over a range of candidate monomeric and dimeric Cu²⁺ sites, as well as the subsequent H migration steps, either framework- or extraframework-mediated, that ultimately close the hydrogen balance. Results indicate that H migration steps are important to the overall kinetics of Cu²⁺ to Cu⁺ reduction and that their rates are a function of Cu²⁺ proximity. We use these insights combined with statistical models to predict TPR results as a function of Cu²⁺ site distribution and compare with experimental observations, showing the influence of Cu and Al density on the H₂-TPR profile of Cu-CHA samples.