Design of Stable Palladium-Based Zeolite Catalysts\nfor Complete Methane Oxidation by Postsynthesis Zeolite Modification

Abstract

Catalytic methane\noxidation is used in exhaust gas aftertreatment\nto reduce methane emissions from lean-burn natural gas vehicles as\nwell as in stationary combustion processes. Pd/zeolite catalysts provide\nhigh activity for this reaction, but they deactivate rapidly under\nthe reaction conditions and in the presence of steam due to extensive\npalladium nanoparticle sintering, which is a common deactivation pathway\nfor supported catalysts. Understanding the origin of this phenomenon\nis crucial for improving the performance of such materials. In this\nwork, we identify all stability and activity descriptors of Pd/zeolites\nfully exchanged with sodium. On the basis of these descriptors we\ndesign an active and stable catalyst using a synthetic approach which\ncomprises the formation of mesopores in the zeolite by mild desilication,\nremoval of surface and extraframework aluminum by selective dealumination,\nand complete sodium postexchange. This allows unstable Pd/H-ZSM-5\nto turn into a highly active sintering-resistant hierarchical Pd/Na-ZSM-5\nfor the demanding reaction of complete methane oxidation. This synthetic\nprocedure can be applied to other zeolites to enhance the stability\nof supported catalysts that are prone to sintering.