Oxidation of CO on a Pt/Al 2O 3 catalyst: from the surface elementary steps to light-off tests : V. Experimental and kinetic model for light-off tests in excess of O 2

Abstract

This article is the final part of a study on the CO/O 2 reaction over a 2.9% Pt/Al 2O 3 in line with the microkinetic approach of the heterogeneous gas–solid catalysis. Mainly, the kinetic parameters of each elementary step of two kinetic models (Models M1 and M2) determined previously are used to explain the evolution of the coverage of the adsorbed CO intermediate species (a strongly adsorbed linear CO species on Pt 0, denoted by L) as well as the turnover frequency (TOF in s −1) during light-off tests (increase in the reaction temperature T r) using 1% CO/ x% O 2/He gas mixtures with x⩽50. Model M1 involves a L-H elementary step between L CO species and a weakly adsorbed oxygen species (O wads). It is operative (a) whatever T r in excess CO and (b) only at low T r values in excess O 2. Model M2 involves a L-H elementary step between a L CO and a strongly adsorbed oxygen species: O sads is operative at high T r values in excess O 2. It is shown that the switch for M1 to M2, at the ignition process, during the heating stage occurs for a high CO conversion (>60%) at a specific T r value (denoted by T i) depending on the oxygen partial pressure. Similar to the observations on Pt single crystals, it is shown that the ignition process is associated with a surface-phase transformation from a Pt surface mainly covered by L CO species (denoted by Pt–CO) to a Pt surface mainly covered by O sads (denoted by Pt–O). The Pt–CO → Pt–O transformation is due to an oxidative removal of the adsorbed L CO species into CO 2 and not to a competitive chemisorption. The high CO conversion associated with the Pt–CO → Pt–O transformation indicates that mass-transfer processes contribute to the ignition process. During a cooling stage from T r> T i, the switch from M2 to M1 (extinction process) is associated with the surface-phase transformation Pt–O → Pt–CO at a reaction temperature T e< T i.