Progress in the construction strategy of noble metal active sites for zeolite-based PNA and VOCs catalysts

Zeolite-loaded noble metal catalysts have demonstrated excellent performance in addressing cold-start automotive exhaust NO_x emissions and catalytic oxidation of VOCs applications. Pd and Pt are the most commonly used active metals in PNA and VOC catalysts, respectively. However, despite the same metal/zeolite composition, the efficient active sites for PNA and VOC catalysts have been viewed as mainly Pd²⁺ and Pd⁰, respectively, both of which are different from each other. As a result, various methods need to be applied to dope Pd and Pt in zeolitic support respectively for different usages. No matter which type of metal species is needed, the common requirement for both PNA and VOC catalysts is that the metal species should be highly dispersed in zeolite support and stay stable. The purpose of this paper is to review the progress of synthetic means of zeolite-coated noble metals (Pd, Pt, etc.) as effective PNA or VOC catalysts. To give a better understanding of the relationship between efficient metal species and the introduced methods, the species that contributed to the NO_x adsorption (PNA) and VOCs deep catalytic oxidation were first summarized and compared. Then, based on the above discussion, the detailed construction strategies for different active sites in PNA and VOC catalysts, respectively, were elaborated in terms of synthetic routes, precursor selection, and zeolite carrier requirements. It is hoped that this will contribute to a better understanding of noble metal adsorption/catalysis in zeolites and provide promising strategies for the design of adsorption/catalysts with high activity, selectivity and stability.