Low temperature CO oxidation over Pt/CeO2

Licentiatavhandling, 2021

The oxidation of CO to CO2 is a widely studied reaction not only for its practical applications but also for its apparent simplicity.
CO oxidation is, in fact, often used as a model reaction for other oxidation reactions.
On metal surfaces, the reaction is known to follow a Langmuir-Hinsenlwood mechanism where CO and dissociated O2 react to form CO2 that desorbs. Despite the high cost of platinum catalysts, it is hard to match the activity of CO oxidation using other metals. Instead of changing the catalysts, the approach is to reduce the amount of necessary metal to carry out the reaction.
In this thesis, density functional theory (DFT) calculations and kinetic Monte Carlo (kMC) simulations are used to investigate the reaction energetics and kinetics on model systems, with a focus on the low temperature regime.
Stimulated by the experimental evidence that CO may dissociate at low temperatures, CO dissociation has been studied as a possible initial competing reaction to oxidation. Our results show that CO dissociation does not occur directly, even on stepped surfaces. Instead, we propose that dissociation is facilitated at high coverages by a Boudouard reaction path at undercoordinated sites.
In order to study the reaction kinetics, a complete description of the energy landscape is necessary. To describe the reaction landscape, scaling relations like the Bronsted-Evans-Polanyi and structure sensitive relations that link the adsorption energy of the reactants with a chosen descriptor, can be used to reduce the computational cost. Scaling relations are used in this work together with kinetic Monte Carlo to study CO oxidation over Pt nanoparticles. The sensitivity of the kinetic behaviours on scaling relations and on the oxygen sticking probability is investigated.
Our results show that varying the slope of the scaling relations changes the turnover frequency (TOF) of the reaction, but the general behaviour is maintained, and clear trends are established between the reaction kinetics and the slope of the scaling relations. 
It is known that reducible oxides like ceria can increase the reaction's activity at low temperatures, by allowing for a Mars-van Krevelen path. In this way, the issue with CO poisoning is reduced. We explore the effect of Mars-van Krevelen reaction steps for CO oxidation over Pt/CeO2. Additionally, we show that the common experimental assignment of XPS spectra shifts for the O 1s to the formation of oxygen vacancies might need reconsideration. Such shift could instead be due to OH groups adsorbed on the surface.