Kinetics of Hydrocarbon Oxidation on Noble Metal – Ceria Catalysts

Abstract

This work focuses on the catalytic oxidation of hydrocarbons on ceria-supported noble metal catalysts: Pt/CeO2 and Pd/CeO2. The study aims to understand three key aspects: i) the synthesis methods and characteristics of highly active and stable catalysts, ii) the reaction mechanism and minimum energy pathways under reductive and oxidative environments, and iii) the kinetics of the oxidation process. To achieve this, a combined approach, involving experimental analysis, molecular modelling using density functional theory (DFT), and thermodynamic-consistent micro-kinetic analysis, is employed. The synthesised catalysts with 1%wt of Pt or Pd doping exhibit significant catalytic activity for CH4 oxidation. The nanoclusters formed on the CeO2 surface, containing metallic and oxidized ions, interact with the lattice oxygen, resulting in their unique catalytic activity. The CH4 oxidation on the Pt/CeO2 and Pd/CeO2 is modelled using DFT. The reaction happens at the boundary between the cluster and CeO2, where the CHx species are adsorbed on the CeO2, and hydrogen atoms are extracted by clusters. The rate-limiting step in this process is identified as the first dehydrogenation from CH4 to CH3, with significant energy barriers observed on both Pt/CeO2 and Pd/CeO2. Micro-kinetic models based on DFT results accurately predict the apparent activation energies of CH4 oxidation on Pt/CeO2 and Pd/CeO2. The micro-kinetic analysis reveals that, compared to conventional noble metal catalyst like Pt, the catalytic activity on ceria-based catalysts is promoted by accelerating the CH4 oxidative dehydrogenation with the participation of CeO2 lattice oxygen. The reactivity of oxygen on catalyst surfaces plays a crucial role in determining the efficiency of hydrocarbon oxidation. The interaction between noble metals and CeO2 improves the reactivity of lattice oxygen, thus facilitating hydrocarbon decomposition and product formation.