Methane-selective oxidation to methanol and ammonia selective catalytic reduction of NOx over monolithic Cu/SSZ-13 catalysts: Are hydrothermal stability and active sites same?

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This study investigated and compared the reactivities of NH3-SCR and methane-to-methanol reactions over the monolithic Cu/SSZ-13 catalysts with different zeolite crystal sizes before and after hydrothermal aging. It was found that the methanol yield decreases with the increase in crystal size in methane-to-methanol, and a severe deactivation was detected after aging. However, the NH3-SCR performance presents a higher thermal stability and even an enhanced low-temperature reactivity after aging. The fresh and aged catalysts were comprehensively characterized by XRD, SEM, N2 physisorption, NH3-TPD, H2-TPR, XPS, UV–vis-NIR and in-situ DRIFTS measurements. The results indicate that the isolated Cu2+ species, including those that interacted with two Al in 6MR and [CuII(OH)]+ motifs, and acid sites are main active centers for NH3-SCR. While CuxOy clusters, [CuII(OH)]+ motifs and Brønsted acid sites in the Cu-CHA frameworks could be important active sites for methane-to-methanol. Besides, the stable SCR performance is ascribed to the stable textural property, higher crystallinity of zeolite, increased NH3 storage capacity and isolated Cu2+ active sites. The loss of the catalytic activity in methane-to-methanol is mainly attributed to the decrease of Brønsted acid sites and CuxOy concentration and the break of zeolite crystal structure. This work concludes the differences in active sites and hydrothermal stability between NH3-SCR and methane-to-methanol reactions, it is very helpful to understand the two reactions.