Low temperature CO oxidation: Effects of support, composition and structure

Doktorsavhandling, 2023

Carbon monoxide is a tasteless, colorless and in-odorous toxic gas, which is formed during incomplete combustion. Due to its toxicity, efficient methods for CO oxidation to CO2 are required. One of the most active catalysts for this reaction is platinum, and the technological catalyst is generally based on nanosized platinum, yielding a high atom efficiency.
In this thesis, density functional theory (DFT) calculations, mean field micro kinetic models and kinetic Monte Carlo (kMC) simulations are used to investigate the reaction energetics and kinetics on model systems, to understand and enhance the low temperature reaction regime.
Experimental evidence of CO disproportionation stimulated the study of CO dissociation as a possible competing reaction to oxidation at low temperature. Our results show that dissociation of CO is facilitated at high coverages by a Boudouard reaction path at under-coordinated sites.
To study the reaction kinetics over nanoparticles, a complete description of the energy landscape is necessary. In order to reduce the computational cost, it is possible to describe the energy landscape through scaling relations like the Brønsted-Evans-Polanyi and structure sensitive relations that link the adsorption energy of the reactants with a chosen descriptor. The sensitivity of scaling relations and sticking coefficient on CO oxidation is investigated, showing that varying the slope of the scaling relations results only in minor modifications of the reaction kinetics.
It is known that reducible oxides like ceria can increase the reaction's activity at low temperatures, by allowing for a Mars-van Krevelen (MVK) path. The effect of Mars-van Krevelen reaction steps for CO oxidation over Pt/CeO2 is explored. Our results show that the high activity for CO oxidation at low temperatures is due to the possibility of following the MVK mechanism at low temperatures and not to overall lower reaction barriers. The nature of oxygen vacancies in ceria is furthermore investigated, and we show that the common assignment of XPS spectra shifts for the O 1s to the formation of oxygen vacancies might need reconsideration. Such shift could be instead due to the presence of adsorbed OH groups on the surface. More on the fundamental description of CO oxidation, we investigate the possibility of predicting reaction paths from the experimental measurements of reaction orders. Exploring different reaction pathways by mean field simulations shows that reaction orders alone do not reveal the reaction mechanism. Lastly, we investigate CO oxidation on dilute Pt-Au alloy nanoparticles.