Photocatalytic Surface Chemistry On Anatase Tio₂?001?Single Crystal

TiO2 as a versatile material,has extensive applications in various fields,especially as a photocatalyst been attracting huge research interest.Exploring reactions of molecules on TiO2 surfaces is of great importance in both energy chemistry and photocatalysis.However,studies on fundamental mechanisms about detailed surface photocatalytic processes on TiO2 are still lacking.The fundamental understanding of both thermal-and photo-chemistry on TiO2,especially on rutile TiO2(110)surface,have been vastly investigated combining a variety of theoretical and experimental methods on single crystal surfaces under UHV conditions.Up to date,most of the investigations of anatase TiO2-based photocatalysts have been carried out in the nanocrystalline regime.And anatase TiO2 was observed to show greater photocatalytic activity than rutile TiO2 in most reported reactions.Furthermore,among various types of anatase TiO2 facets,anatase TiO2(001)is of particular interest.Several theoretical calculations suggested the anatase TiO2(001)surface to be very reactive,however,this reactivity has not been experimentally proved and identified.Meanwhile,experimental studies of its surface structure and reactivity are very limited.In this thesis,I have carried out comprehensive surface science studies of adsorption,thermocatalysis and photocatalysis of small molecules on mineral anatase TiO2(001)-(1×4)and rutile TiO2(110)-(1×1)single crystal surfaces,established structure-performance realtions and surface reaction mechanisms.The main results are as follows:1)Adsorption and thermocatalytic reactions of methanol on anatase TiO2(001)-(1×4)surface were studied.The fourfold-coordinated Ti4+(Ti4c)sites are very reactive to dissociate CH3OH to form strongly adsorbed CH3O species that undergoes dehydration coupling reaction to produce CH3OCH3(DME).These observations demonstrate the Ti4c site of TiO2 as an active site to catalyze the CH3OH-to-DME reaction and the enhanced catalytic activity of anatase TiO2(001)-(1×4)surface over other TiO2 facets.2)Photocatalytic reaction of methanol on anatase TiO2(001)-(1×4)surface was studied.Photocatalytic oxidation reaction was observed to exclusively occur for CH3O species at the Ti4c sites of the(1 × 4)added row but not for CH3OH species at the Tisc sites of the(1X1)basal surface.Density functional theory calculation results demonstrate that the valence band maximum is localized at the oxygen atoms of the methoxy species bonded to the Ti4c sites for methanol-covered anatase TiO2(001)-(1×4)surface.Meanwhile,the presence of Ti4c sites enhances the upward surface band bending to facilitate the photogenerated holes transfer to the Ti4,sites.Both lead to a Ti4c site-specific hole-transfer and subsequent oxidation of methoxy species during photocatalytic reaction.These results reveal a phenomenon of surface reconstruction-induced site-specific charge separation and photocatalytic reaction on oxide photocatalysts that will greatly deepen the understanding of the vital role of oxide surface structure in photocatalytic reactions.3)Intrinsic photocatalytic activity of anatase TiO2(001)-(1×4)and rutile TiO2(110)-(1×1 surfaces were comparatively studied employing photo stimulated desorption(PSD)of O2.Two types of photoreactive O2 species were observed on both surfaces,and the O2 species with high PSD reactivity on anatase TiO2(001)-(1×4)surface exhibits a larger PSD rate constant than that on rutile TiO2(110)-(1×1)surface.These results directly proves a higher intrinsic photocatalytic activity of anatase TiO2(001)-(1×4)surface than other TiO2 facets.DFT calculation results demonstrate that different O2 species exhibit different charges and Partial Density of State(PDOS)positions.Both high PSD-reactivity O2 species on anatase TiO2(001)-(1×4)and rutile TiO2(110)-(1×1)surfaces bear one negative charge,but that on anatase TiO2(001)-(1×4)surface exhibit a more positive PDOS position than that on rutile TiO2(110)-(1×1)surface and thus is more facile to accept the photogenerated holes to desrob.The O2 PSD results with co-adsorbed O2 and CH3OH demonstrate that electron-acceptor co-adsorbed species(O2)tends to strengthen the upward surface band bending of TiO2 and promote the transfer of photogenerated holes to the surface and the subsequent O2 PSD process while electron-donor co-adsorbed species(CH3OH)tends to weaken the upward surface band bending of TiO2 and suppress the transfer of photogenerated holes to the surface and the subsequent O2 PSD process.4)Hydrated protons were successfully prepared on a Pt/rutile TiO2(110)surface and demonstrated to be photocatalyzed to produce H2,proving hydrated protons as the active species of photocatalytic water reduction to H2.Pt-TiO2 interface was proved to be capable of dissociating water,forming hydroxyl groups that facilitate the formation of surface H-bonding network acting to stabilize hydrated protons to enhance photocatalytic H2 production.These results provide a novel insight in the role of Pt co-catalyst for photocatalytic water reduction.