Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV–Vis Spectroscopies

Catalytic reaction systems for the direct conversion of methane to methanol have been previously developed using Cu zeolites. Among these materials, the Cu-CHA has been reported to show relatively high catalytic performance during the reaction of CH4–O2–H2O mixtures, although this catalytic activity varies with composition. In the present study, four Cu-CHA catalysts having different compositions and catalytic activity levels were prepared, and the redox properties and local structures of these specimens were analyzed using in situ X-ray absorption fine structure and UV–vis diffuse reflectance spectroscopies in conjunction with a CH4–O2–H2O reaction mixture. The relationships between the redox rates of the materials and catalytic activities (as reflected in the turnover frequency (TOF) during CH4 oxidation) were assessed, and this analysis showed that the Cu-CHA reduction rate was highly correlated with activity. The data also suggested that the Cu2+ reduction is associated with the activation of C–H bonds in CH4, which is the rate-determining step for the overall reaction. The effects of the local structure of the Cu-CHA on selectivity for CH3OH, the TOF value, and the Cu2+ reduction rate were studied. Those Cu-CHA samples having high proportions of Cu2+ ions coordinated to six-membered rings (Z2Cu) in the CHA framework exhibited higher selectivity than specimens having high proportions of [CuOH]+ ions coordinated to eight-membered rings (ZCuOH), although the TOF values of the former Cu-CHA specimens were not higher than those of the latter because of the slower reduction rates of Cu2+ species. The different catalyses can be attributed to the difference in Cu2Ox active structures formed in the Z2Cu-rich samples and the ZCuOH-rich samples.