Impact of Nonadiabatic Charge Transfer on the Rate of Redox Chemistry of Carbon Oxides on Rutile TiO2(110) Surface

Adsorption and dissociation of COCl2 on the rutile TiO2(110) surfaces: a systematic first-principles study

The adsorption and dissociation of phosgene (COCl₂) molecules on three kinds of rutile TiO₂(110) surfaces (stoichiometric: TiO₂-Sto; oxygen defective: TiO₂-Ov; and substoichiometric: TiO_1.875) were investigated based on density functional theory calculations. The nature of interactions between the COCl₂ molecule and rutile TiO₂(110) surfaces with different degrees of reduction was researched by the analysis of geometries, electron density difference, adsorption energies and density of states (DOS). Computational results show that COCl₂ indicates instability and will dissociate directly without the presence of transition states on a substoichiometric TiO_1.875(110) surface. The adsorption and dissociation behavior of COCl₂ on the rutile surface is not only helpful in providing theoretical support for the clean and efficient degradation of COCl₂, but also helpful in elucidating the role of COCl₂ as an intermediate product in the carbochlorination of titanium ore.

Surface chemistry of TiO2 connecting thermal catalysis and photocatalysis

Chemical reactions catalyzed under heterogeneous conditions have recently expanded rapidly from traditional thermal catalysis to photocatalysis due to the rising concerns about sustainable development of energy and the environment. Adsorption of reactants on catalyst surfaces, subsequent surface reactions, and desorption of products from catalyst surfaces occur in both thermal catalysis and photocatalysis. TiO₂ catalysts are widely used in thermal catalytic and photocatalytic reactions. Herein we review recent progress in surface chemistry, thermal catalysis and photocatalysis of TiO₂ model catalysts from single crystals to nanocrystals with the aim of examining if the surface chemistry of TiO₂ can bridge the fundamental understanding between thermal catalysis and photocatalysis. Following a brief introduction, the structures of major facets exposed on TiO₂ catalysts, including surface reconstructions and defects, as well as the electronic structure and charge properties, are firstly summarized; then the recent progress in adsorption, thermal chemistry and photochemistry of small molecules on TiO₂ single crystals and nanocrystals is comprehensively reviewed, focusing on manifesting the structure-(photo)activity relations and the commonalities/differences between thermal catalysis and photocatalysis; and finally concluding remarks and perspectives are given.

An essential descriptor for the oxygen evolution reaction on reducible metal oxide surfaces

The number of excess electrons (NEE), as a descriptor, perfectly reproduces the OER volcano curve of TiO₂(110) plotted using ΔG_O – ΔG_OH.

The development of a universal activity descriptor like the d-band model for transition metal catalysts is of great importance to catalyst design. However, due to the complicated electronic structures of metal oxides, the correlation of the binding energies of reaction intermediates (*OH, *O, and *OOH) in the oxygen evolution reaction (OER) with experimentally controllable properties of metal oxides has not been well established. Here we demonstrate that excess electrons are the essential factor that governs the binding properties of intermediates on the surfaces of reducible metal oxides. We propose that the number of excess electrons (NEE) is an essential activity descriptor toward the OER activities of these oxides, which perfectly reproduces the volcano curve plotted using the descriptor ΔG_O – ΔG_OH, so that tuning NEE can effectively tailor the OER activities of reducible metal oxide based catalysts. Guided by this descriptor, we predict a novel non-precious catalyst with an overpotential of 0.54 eV, which could be a potential alternative to current Ru or Ir based catalysts.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110)

The adsorption and photochemistry of CO on rutile TiO₂(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (V_O). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of V_O's does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either V_O or bridging hydroxyl).

Scenarios of polaron-involved molecular adsorption on reduced TiO2(110) surfaces

The polaron introduced by the oxygen vacancy (Vo) dominates many surface adsorption processes and chemical reactions on reduced oxide surfaces. Based on IR spectra and DFT calculations of NO and CO adsorption, we gave two scenarios of polaron-involved molecular adsorption on reduced TiO₂(110) surfaces. For NO adsorption, the subsurface polaron electron transfers to a Ti:3d-NO:2p hybrid orbital mainly on NO, leading to the large redshifts of vibration frequencies of NO. For CO adsorption, the polaron only transfers to a Ti:3d state of the surface Ti_5c cation underneath CO, and thus only a weak shift of vibration frequency of CO was observed. These scenarios are determined by the energy-level matching between the polaron state and the LUMO of adsorbed molecules, which plays a crucial role in polaron-adsorbate interaction and related catalytic reactions on reduced oxide surfaces.

Modulating the photocatalytic redox preferences between anatase TiO2{001} and {101} surfaces

The surface protonation/deprotonation can change the photocatalytic redox preferences of TiO₂{001} and {101} surfaces.