Metal Oxide Surfaces as a Support for Model Single-Atom Catalysis

Abstract

Supported metal nanoparticles on metal oxides are widely utilized in heterogeneous catalysis. Downsizing these precious metal nanoparticles enhances the fraction of active surface atoms, improving the per-atom efficiency. Further reducing the size of nanoparticles ultimately reaches the single-atom scale, where individual metal atoms are anchored directly to the support, resulting in so-called "single-atom"catalysts (SAC). SAC has emerged as a key strategy in the field of catalysis in the past decade, offering reduced precious metal usage and high selectivity, as the metal atoms coordinated to the support materials resemble organometallic complexes commonly used in homogeneous catalysis. This thesis focuses on surface science studies of model SAC, to overcome the largely unknown atomic structures of metal atoms on various metal oxide supports. Understanding these structures is essential for fine-tuning catalytic systems to enhance efficiency and selectivity.The work is divided into three main chapters, each addressing different metal oxide surfaces. The first chapter examines the influence of water vapour on the dispersion and sintering of Pt, Rh, Ni, and Ir adatoms on two well-defined TiO2 surfaces,rutile TiO2(110) and anatase TiO2(101), by STM and XPS. A trend in dispersion under UHV conditions, Ir; Ni; Pt; Rh, aligns with the oxygen affinity of the metals. However, the impact of water vapour varies significantly: water promotes dispersion of Ni adatoms, induces sintering for Ir, and has little effect on Rh and Pt. The behaviour also differs between the two TiO2 surfaces, highlighting the complexity of support-metal interaction.The second chapter investigates the Rh/α-Fe2O3(1102) system, where a previous study found that Rh forms clusters when deposited in UHV, but water vapour during deposition stabilizes Rh adatoms as single-atoms on the surface via coordination with OH ligands. For this thesis, nc-AFM with a CO-functionalized tip was utilized, identifying that two OH ligands stabilize the Rh adatoms, with occasional coordination of a third species, likely a water molecule.The final chapter focuses on the Rh/Fe3O4(001) system, aiming to understand the hydroformylation reaction catalysed by single Rh adatoms, by utilizing STM, TPD and XPS. Prior to the experiments with Rh, the interaction of C2H4 with the bare surface was analysed. Then the adsorption of the individual reactants (CO, C2H4 and H2) on the Rh adatoms was studied, along with the feasibility of co-adsorption of CO and C2H4. While co-adsorption was not observed at the given pressure regime, the study provides valuable insights into the interaction of reactants with Rh adatoms. Overall, these studies provide fundamental insights into the adsorption behaviour and reactivity of model SAC systems. They highlight the unpredictable yet crucial role of water in SAC, advance the understanding of metal-support interactions critical for optimizing catalytic systems, and analyse the fundamental steps of the industrially important hydroformylation reaction.