Surface Chemistry On Rutile TiO₂ And NiO/Au（111） Model Catalysts

TiO2 is one of the most widely studied oxide materials,and its applications include catalysis,solar cells,sensors,anticorrosion,and thin-film MOS transistors.Fundamental understanding of TiO2-based heterogeneous catalysis is of great importance.Surface chemistry studies of single crystal model catalysts under UHV conditions is a useful approach.The rutile phase is the most stable TiO2 phase,and rutile TiO2 has been extensively studied as catalysts for thermal catalysis and photocatalysis.In this thesis,thermal catalysis and photocatalysis on various rutile TiO2 single crystal surfaces were studied by means of surface-sensitive techniques under UHV conditions,with focus on the effects of surface structure and defects.Meanwhile,the role of water on O2 activation and CO oxidation on a NiO/Au(111)inverse model catalyst was also studied under UHV conditions.The main results are summarized as follows:1)Effects of subsurface interstitial Ti3+defects on adsorption and photochemistry of small molecules on TiO2(001)surface are identified.H2O dissociatively adsorb at the surface Ti sites close to the Ti interstitials;CO species adsorbed at the surface Ti sites close to the Ti interstitials is photo-active,while those at the normal surface Ti sites is not;CO2 only adsorbs at the normal surface Ti sites and the resulting adsorbed CO2 species is photo-inactive;methanol dissociates to form methoxy species at the surface Ti sites close to the Ti interstitials,and the resulting methoxy species undergo the deoxygenation and dehydrogenation reactions respectively to produce CH4 and HCHO upon subsequent heating,while under subsequent UV light illumination,it undergoes the photogenerated electrons-mediated C-O bond cleavage into methyl fragment in addition to photogenerated holes-mediated oxidation reactions,and the resulting methyl fragment undergoes hydrogenation and coupling reactions respectively to produce methane and ethane upon subsequent heating.2)Adsorption and photochemistry of small molecules on TiO2(110)surfaces with various types of defects are studied and TiO2 islands formed by oxidation of bulk interstitial Ti3+defects are proposed as a novel type of defect on TiO2(110)surface.CO and CO2 adsorbed on oxidized TiO2(110)surface without defects are photo-inactive.On TiO2(110)surface with surface oxygen vacancies and bulk interstitial Ti3+defects prepared by annealing oxidized TiO2(110)surface at 1000 K,CO and CO2 preferentially adsorb at surface oxygen vacancy sites,and the resulting species are photo-active;On TiO2(110)surface with TiO2 islands and bulk interstitial Ti3+defects prepared either by annealing oxidized TiO2(110)surface at 800 K or by repeated O2TDS experiments of TiO2(110)surface with surface oxygen vacancies and bulk interstitial Ti3+defects,CO and CO2 preferentially adsorb at the perimeter sites of TiO2 islands,and the resulting species are photo-active.CO adsorbed at surface oxygen vacancies exhibits the highest photodesorption probability,while CO2 adsorbed at the perimeter sites of TiO2 islands exhibits the highest photodesorption probability.3)A synergistic effect of photoexcited electrons and holes on photocatalytic converting methanol to CO and H2 is discovered on rutile TiO2(100)surface.Methanol dissociates at the fivefold coordinated Ti4+ sites to methoxy species.Upon Hg light irradiation,photoexcited holes mediate the oxidation of methoxy species sequentially to formaldehyde and formate intermediates,while photoexcited electrons mediate the reduction of associated Ti4+ sites to Ti3+ sites.Upon subsequent heating,the formate intermediate at the Ti3+ sites decomposes to produce CO and H2.4)OH and H2O species are controllably prepared on a NiO/Au(111)inverse model catalyst and their roles on O2 activation and CO oxidation are identified.Via combined experimental and density functional theory calculation studies,molecularly adsorbed O2 species was found active for CO oxidation at low temperature;the moderately adsorbed O2 species stabilized by water exhibits 18O2/H2O exchange reactivity and promotes low-temperature CO oxidation reaction,whereas OH-stabilized strongly adsorbed O2 species is too stable to participate low-temperature CO oxidation reaction.The above results broaden the fundamental understanding of surface chemistry and photochemistry of TiO2 surfaces,and clarify the roles of OH and water in O2 activation and CO oxidation on gold catalysts.