Traces of Potassium Induce Restructuring of the Anatase TiO2(001)-(1×4) Surface from a Reactive to an Inert Structure

Reconstruction of solid surfaces is generally accompanied by changes in surface activities. Here, via a combined experimental and theoretical study, we successfully identified that a trace amount of potassium dopant restructures the mineral anatase TiO2(001) single-crystal surface from an added molecule (ADM) termination to an added oxygen (AOM) one without changing the (1×4) periodicity. The anatase TiO2(001)-(1×4)-ADM surface terminated with 4-fold coordinated Ti4c and 2-fold coordinated O2c sites is (photo)catalytically active, whereas the anatase TiO2(001)-(1×4)-AOM surface terminated with O2c and inaccessible 5-fold coordinated Ti5c sites is inert. These results unveiled a mechanism of dopant-induced transformation from a reactive to an inert TiO2(001)-(1×4) surface, which unifies the existing arguments about the surface structures and (photo)catalytic activity of anatase TiO2(001)-(1×4).

Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals

Photocatalytic oxidation of methanol on various anatase TiO₂ nanocrystals was studied by in situ and time-resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk-to-surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO₂ nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti_4c ) at which CH₃ OH molecules dissociate to form the CH₃ O adsorbate (CH₃ O(a)_Ti4c ). CH₃ O(a)_Ti4c localized density of states are almost at the valence band maximum of TiO₂ surface, facilitating the interfacial hole transfer process; CH₃ O(a)_Ti4c with a high coverage promotes upward surface band bending, facilitating bulk-to-surface hole migration. CH₃ O(a)_Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO₂ nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.

Size-dependent interaction of plasma with anatase TiO2 nanoparticles

We study the particle size distribution and phase changes of the anatase TiO2 nanopowder samples when they are subject to the plasma treatments of three different kinds of gases as nitrogen (N2), oxygen (O2), and argon (Ar). The plasma gas pressures vary as 0.1, 0.3, and 0.6 Torr. We demonstrate that the plasma treatments have an effect neither on the phase structure nor on the mean nanocrystalline size. The phase and size invariances of the samples are attributed to their nanoscale thermodynamic aspects. We find out that elevating the gas pressure in some cases creates fine-size amorphous nanoparticles with a narrow distribution. Our findings authenticate that plasma treatment affects the amorphous phase with etching particles down to a mean value of ∼3 nm. The small-angle X-ray scattering (SAXS) technique was utilized to obtain the size distribution of the nanoparticles, and the wide-angle X-ray scattering (WAXS) technique was used to probe the phase and size changes of the crystalline structure.

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.

Adsorption and reaction of methanol on Fe3O4(001)

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe₃O₄(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe₃O₄(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.