Complete CO Oxidation by O₂ and H₂O over Pt–CeO_2−δ/MgO Following Langmuir–Hinshelwood and Mars–van Krevelen Mechanisms, Respectively

Abstract

CO oxidation has attracted great attention in the automobile exhaust treatment and fuel cell industrial process, with Pt as one of the most promising catalysts. The efficiency of the catalyst is still below the requirement of the industry due to limited understanding about the reaction mechanism of CO oxidation by O2 or H2O, which were proposed to be following the similar/same reaction mechanism (the Mars–van Krevelen reaction mechanism). Our recent results indicate that this assumption might not be correct. Here, we design a catalyst with a combination of isolated platinum atoms (Pt1) and nanoparticles (Ptn) supported on MgO-dispersed CeO2−δ (CeO2−δ/MgO), named as 0.5Pt–xCeO2−δ/MgO (x = 0, 1, 2, 5, 10, 20) to establish two types of active sites, one is solely over Pt NPs (type-I) and the other is at the interface between Pt atoms and the reducible metal oxide support CeO2−δ (type-II), and we perform kinetic, thermodynamic, and in situ spectroscopy analysis on this catalyst to prove that CO oxidation by O2 undergoes the Langmuir–Hinshelwood reaction mechanism on type-I sites (Pt NPs), while water–gas shift (WGS) reaction undergoes the Mars–van Krevelen reaction mechanism at the interface between Pt atoms and the reducible support CeO2−δ (type-II) verified by activation energy assessment and the reactant and product pressure dependency studies applied, in which a systematic reduction of the reaction barrier of CO oxidation (by O2) was obtained once the size of Pt NPs increased and was independent of the changes in the size of CeO2−δ, while the reaction barrier of the WGS was very sensitive to the size of CeO2−δ and slightly inert against the size of Pt NPs. Additionally, there is competitive adsorption between CO and O2 over Pt–CeO2−δ/MgO, while there is no competitive adsorption between CO and H2O based on our pressure dependency studies. Collectively, our current work provides convincing evidence that the promotion of H2O on CO oxidation is the change of the reaction mechanism rather than the simple effect of hydroxyl dissociated by H2O dosing.