Photo-Ionisation and Density Functional Theory Studies of Gold Doped Cerium Oxide Clusters

Abstract

This thesis presents experimental and theoretical work on small cerium oxide (CemOn, m=2,3; n=0-2m) and gold-doped cerium oxide (AuCemOn, m=2,3; n=0-2m) clusters. These cluster systems are considered as simple models for larger gold-ceria systems which have shown potential for use as catalysts in low-temperature CO oxidation processes. Experimental work is also presented for the Ce4On (n=0-5) clusters without any supporting computational data. CemOn and AuCemOn clusters are prepared in the gas phase via dual laser ablation and detected experimentally using Time-of-Flight Mass Spectrometry (TOFMS) coupled with threshold laser ionisation. Photo-Ionisation Efficiency (PIE) spectroscopy is performed on the cluster ions detected in the mass spectra following photo-ionisation (which include the Ce2, Ce2O, Ce2O2, Ce3, Ce3O, Ce3O2, Ce3O3, Ce3O4, Ce4, Ce4O, Ce4O2, Ce4O3, Ce4O4, Ce4O5, AuCe2, AuCe2O, AuCe2O2, AuCe3, AuCe3O, AuCe3O2 and AuCe3O3 species) over the 3.92 – 5.17 eV (240 – 315.75 nm) energy range to record the depletion of each cluster ion with decreased photon energy. Experimental PIE spectra are presented for these species. The highly oxidised Ce2O3, Ce2O4, Ce3O5, Ce3O6, AuCe2O3, AuCe2O4, AuCe3O4, AuCe3O5 and AuCe3O6 cluster ions – which approach the stochiometric n=2m ratio of bulk ceria - are either not detected or are detected in trace quantities in all photo-ionisation mass spectra recorded during this work. DFT calculations are used to identify the lowest energy structures for each cluster species and their corresponding cationic states. Potential ionisation transitions for each cluster are investigated using Zero Electron Kinetic Energy simulations which are subsequently combined and integrated to produce a calculated PIE spectrum. The calculated and experimental PIE spectra are then compared to (i) ascertain the adiabatic ionisation energy (IE) for each cluster species and (ii) verify that the geometric and electronic properties of the relevant cluster species can be inferred from their DFT-calculated structures. Excellent agreement is found between the experimental and calculated PIE spectra for all CemOn and AuCemOn (m=2,3; n=0-2m) cluster species detected in this work. Moreover, DFT calculations predict high ionisation energies for all cluster species not detected in the photo-ionisation mass spectra. The interactions between cerium oxide and adsorbed gold atoms – and their subsequent effects on catalytic CO oxidation – are explored via calculations which involve comparison of the undoped and gold-doped geometric structures, Hirshfeld charges, HOMO/LUMO energies and bonding energies (involving O, Au and CeO2 fragments). The preferential site of Au deposition on the CemOn cluster is found to vary with oxidation; the Au atom adsorbs to an oxygen vacancy site on highly reduced CemOn clusters [AuCe2On (n=0-3); AuCe3On (n=0-4) and CeO3 and CeO2 vacancy sites on moderately reduced and stoichiometric clusters, respectively (i.e. AuCe2O4, AuCe3O5, AuCe3O6). Both Au→Ce and Ce→Au charge transfer mechanisms are calculated; the former occurs when Au adsorbs to an oxygen vacancy site while the latter occurs when Au adsorbs to a CeO2/CeO3 vacancy site. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCemOn cluster by (i) stabilising the negatively charged Au atom on reduced AuCemOn clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCemO2m species; (iii) destabilising the electronic structure of the AuCemO2m cluster to potentially reduce the energy cost associated with a catalytic redox cycle; and (iv) facilitating the release of additional oxygen atoms to adsorbed gas phase reactants while having minimal negative effect on subsequent oxygen vacancy healing processes.