Adsorption And Reaction Of Small Molecules On TiO₂ Single Crystal Surfaces

As a typical metal oxides,Ti02 has received remarkable interests for its important applications in photocatalytic oxidation,organic degradation and self-cleaning materials owing to its relatively high effciency,nontoxicity,low cost and high stability.Fundamental studies on well-defined single crystals using ultrahigh vacuum based surface science techniques can help us explore the reaction mechanism of Ti02-based catalysts at the atomic and molecular levels.Up to date,most of the research on TiO2 single crystal surface is on rutile TiO2(110)surface.In this thesis,by combining TDS and XPS,thermal-catalysis and photocatalysis of small molecules on rutile TiO2(011)-(2×1)surface and anatase TiO2(001)-(1×4)surface have been studied;meanwhile,Au growth and Au-TiO2 interaction have been studied on TiO2(011)-(2×1)surface.The main results are as the follwoing:1.The adsorption,thermo-catalytic surface reactions,and photo-catalytic surface reactions of formaldehyde on the rutile TiO2(011)-(2×1)surface have been studied.The dominant thermal-catalytic reaction is the formation of ethylene by a reductive carbon-carbon formation reaction of formaldehyde adsorbed at the oxygen vacancy sites,and the dominant photocatalytic reaction is the formation of formate,assisted by the bridge O2c sites,followed by carbon monoxide formation at elevated temperatures.The surface intermediates of formaldehyde reactions to ethylene and carbon monoxide on the rutile TiO2(011)-(2×1)surface were identified.The effect of the surface structure of the rutile TiO2(011)-(2×1)surface,particularly the oxygen vacancy,on the thermal-catalytic and photocatalytic activity toward formaldehyde was revealed by studying the coadsorption of water and formaldehyde.These results broaden our fundamental comprehension on the reaction mechanism of formaldehyde on the TiO2 surfaces.2.The adsorption,thermo-catalytic surface reactions,and photo-catalytic surface reactions of methanol on the rutile TiO2(011)-(2×1)surface have been studied.During the thermos-catalytic surface reactions,in addition to CH3OH and HCHO as products of disproportionated reaction of CH3O species,the formation of CH4 is observed as a product of CH3O species adsorbed at the Ti4c sites near O vacancies of rutile TiO2(011)-(2×1)surface.During the photo-catalytic reactions,the production of HCHO and HCOOCH3 were observed.Co-adsorption of CH3OH and HCHO with different orders were carried out to prove HCO as an intermediate of photo-catalytic coupling of CH3O to HCOOCH3.The structural effect of TiO2 surface on the photocatalytic reaction of CH3OH were compared between rutile TiO2(011)-(2×1)and rutile TiO2(110)surfaces.The structural effect of TiO2 surface on the competitive adsorption of CH3OH and H2O were comapered among TiO2 anatase(001)-(1×4),rutile(110)and rutile(011)-(2×1)single crystal surfaces.3.The growth,interaction and adsorption behaviors of Au on rutile TiO2(011)-(2×1)surface were investigated.The growth of Au on TiO2(011)-(2×1)surface adopts a SK mode of two-dimensional growth on surface oxygen vacancies followed by three-dimensional growth.Surface oxygen vacancies significantly enhance the stability of Au clusters.Size-dependent metal-support strong interaction of Au/rutile TiO2(011)-(2×1)wa observed and further verified by CO2 adsorption and photo-induced desorption beahviors.4.The adsorption,thermo-catalytic surface reactions,and photo-catalytic surface reactions of methanol on the anatse TiO2(001)-(4×1)surface have been studied.C3H6 molecules adsorbed at the Ti4c sites tend to undergo oxidation reactions to CO and CO2 while those at the Tis,sites can undergo epoxidation reaction to propene oxide.C3H6 molecules adsorbed at both Ti4,and Ti4c,sites undergo photo-induced desorption.CH3I molecules adsorb and desorb molecularly and reversibly,and both photo-induced desorption and photo-induced dissociation into CH3 and I of adsorbed CH3I molecules occur upon UV light illumination.