Partial Oxidation of Methane to Syngas Over γ‑Al₂O₃‑Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

Abstract

A series of supported Rh/γ-Al₂O₃ catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al₂O₃ with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al₂O₃ and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H₂ are mainly formed through reforming reactions of CH₄ with CO₂ and H₂O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH₄, O₂, and CO₂ with Rh/γ-Al₂O₃ as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of CH₄, O₂, and CO₂ activation decrease with an increase in the size of supported Rh NP thus influencing both primary (methane combustion) and secondary (reforming of methane) pathways within the course of POM