Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.
We address the formation of the energetically most favourable single oxygen vacancies in ceria nanoparticles (CeO 2 ) n focusing on their size dependence. We study a series of structures with increasing number of CeO 2 units (n ¼ 21, 30, 40 and 80) that, according to well tested interatomic-potential calculations, approach the global minima for these particle sizes. The structures thus obtained are refined by means of density functional (DF) methods, modified by the on-site Coulomb correction. Subsequent DF calculations are performed to quantify and analyse the depletion of atomic O from the nanoparticles that results in the formation of a vacancy O vac . We show that (i) removal of a low-(two-)coordinate O atom from ceria species requires the lowest energy, in line with evidence from other metal oxides; (ii) the depletion of such O atoms from the nanoparticles is strongly facilitated compared to extended (even irregular) surfaces; (iii) increase of the particle size is accompanied by a dramatic decrease of the O vac formation energy, implying that at certain sizes this energy should reach a minimum; (iv) the size dependence of the O vac formation energy is driven by the electrostatics, thus enabling the prediction of the most easily removable O atoms by analysing the distribution of the electrostatic potential in the pristine stoichiometric (vacancy-free) ceria systems. Our findings provide a key to rationalize the observed spectacularly enhanced reactivity of ceria nanostructures.
The formation of oxygen vacancies in nanoparticles Ce(n)O(2n) (n < or = 80), studied using density-functional calculations, is found to be greatly facilitated compared to extended surfaces, which explains the observed spectacular reactivity of nanostructured ceria.
Photocatalytic and photovoltaic activity depends on the optimal alignment of electronic levels at the molecule/semiconductor interface. Establishing level alignment experimentally is complicated by the uncertain chemical identity of the surface species. We address the assignment of the occupied and empty electronic levels for the prototypical photocatalytic system of methanol on a rutile TiO 2 (110) surface. Using many-body quasiparticle (QP) techniques we show that the frontier levels measured in ultraviolet photoelectron and two photon photoemission spectroscopy experiments can be assigned with confidence to the molecularly chemisorbed methanol, rather than its decomposition product, the methoxy species. We find the highest occupied molecular orbital (HOMO) of the methoxy species is much closer to the valence band maximum, suggesting why it is more photocatalytically active than the methanol molecule. We develop a general semi-quantitative model for predicting manybody QP energies based on the appropriate description of electronic screening within the bulk, molecular or vacuum regions of the wavefunctions at molecule/semiconductor interfaces. M olecular energy levels are strongly renormalized when molecules are brought into contact with surfaces. 1 The energy positions of the frontier highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the adsorbate with respect to the valence band maximum and conduction band minimum (VBM and CBM) of photocatalytic substrates define the potentials for electron transfer across a molecule/semiconductor interface. Photoelectron spectroscopy can accurately determine the alignment of the frontier orbitals of the adsorbate with respect to the substrate bands provided that the chemical state of the chemisorbed molecule is known. The chemical assignment and correct description of the molecule-photocatalyst interaction require theory to accurately predict the electronic structure of the coupled system.We consider the electronic structure of methanol chemisorbed intact or in its partially dissociated methoxy form on the stoichiometric rutile TiO 2 (110) surface. Methanol is well established as a sacrificial hole scavenger in the photocatalytic splitting of H 2 O by UV light excitation of TiO 2 nanocolloids. 2,3 Experimentally, the electronic structure and photocatalytic activity of methanol on the single crystal rutile TiO 2 (110) surface under ultra-high vacuum (UHV) conditions has been investigated by ultraviolet, X-ray, and two photon photoelectron spectroscopy (UPS, XPS, and 2PP), 4-7 scanning tunnelling microscopy 7-9 (STM), and mass spectrometric analysis of reaction products. 10,11 These experiments have reached contradictory conclusions regarding whether the empty "wet electron" level 12 that is observed in 2PP spectra of methanol covered TiO 2 surfaces should be assigned to the methanol or methoxy species. 6,7 Furthermore, although it is clear that the methoxy species is more photocatalytically active than the methanol molecule, there is stil...
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