Exploration of reaction pathway of cerium dioxide-based Pt catalyst for $CH_4$ oxidation

Abstract

Identifying the key reaction site/pathway in oxide-supported metal nanoparticles is critical in designing high-performance catalysts for industrial chemical reactions, such as catalytic oxidation of methane. However, the high temperatures (> 673 K) required for $CH_4$ conversion cause the metal nanoparticles to aggregate into larger crystallite, which make it difficult to understand fundamental catalysis at a given condition. In this study, Pt nanoparticle catalysts are prepared via a colloidal synthesis to make the nanoparticles uniform, and then individual nanoparticles are encapsulated in porous oxide shells ($CeO_2$ or $SiO_2$) to allow chemical reaction through pores and to prevent agglomeration of nanoparticles at high temperatures. The key reaction sites are defined as a two-phase boundary (2PB) where gas and metal contact and a three-phase boundary (3PB) at which the gas, metal, and oxide support phases are in contact simultaneously. Each reaction site is then quantified through electron tomography and carbon monoxide oxidation/adsorption experiments as a 2PB area and 3PB length per gram of Pt. The density of each active site shows a different particle-size-dependency: 2PB density $\propto d^{-1}$, 3PB density $\propto d^{-2}$. By using these scaling relations, the dominant active site of the methane oxidation was investigated with respect to the Pt size, reaction temperature, and reactant concentration. Moreover, theoretical interpretation and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) analysis provide a further understanding of the reaction mechanism and key surface-adsorbed species during methane oxidation. Based on the understanding of the active site and reaction pathway, the methane oxidation catalyst with improved activity was demonstrated. The results of this study provide a new approach on which to explore reaction pathways and mechanisms applicable to multiple reactions and materials.