Amy Linsebigler

The adsorption and photodesorption of oxygen on the TiO2(110) surface

We have investigated the adsorption and thermal conversion of molecular oxygen (O2) states on the TiO2(110) surface by making use of the distinct photodesorption behavior of each adsorption state. Oxygen chemisorbs at the oxygen vacancy defect sites on the annealed TiO2(110) surface at 105 K to a saturation coverage of less than 0.12 monolayers (ML), producing mostly the α‐O2 species which is observed to undergo slow photodesorption. Upon heating this surface to above 250 K, the α‐O2 is converted to the β‐O2 state which can photodesorb at a significantly higher rate. The β‐O2 species dissociates above 400 K to produce atomic oxygen, eliminating the oxygen anionic vacancies. Both the α‐ and β‐photodesorption processes have a threshold energy at the TiO2 band gap (3.1 eV), indicating a substrate excitation mediated process. The photodesorption time‐profile is fitted with an exponential decay function with a cross section of ∼8×10−17 cm2 for the α‐O2 and ∼1.5×10−15 cm2 for the β‐O2 species at a photon energy...

CO chemisorption on TiO2(110): Oxygen vacancy site influence on CO adsorption

CO chemisorption has been studied on TiO2(110) under surface conditions where oxygen anion vacancy sites are not present (oxidized surface), compared to conditions where the vacancy sites are present (annealed surface). The binding energy of CO on the nondefective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. CO...CO repulsive interactions have been observed at higher coverages. When anion vacancy sites are produced under controlled annealing conditions in vacuum at 900 K, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed. This observation, coupled with measurements showing that defective TiO2(110) does not have enhanced CO chemisorption capacity, suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites. It is postulated that enhanced CO bonding occurs via the interaction of the O moiety of CO with the anion vacancy site while primary adsorbate bonding occurs via the C moiety to Ti lattice sites. Neither CO2 production nor oxygen exchange in CO occurs when CO desorbs from defective TiO2(110).

The photochemical identification of two chemisorption states for molecular oxygen on TiO2(110)

We report the first experimental observation of two chemisorption states for molecular oxygen on a TiO2(110) surface containing anion vacancy sites. The first molecular species can be photoactivated to oxidize coadsorbed CO to CO2 (α channel) and undergoes slow photodesorption. The second molecular oxygen species only undergoes fast photodesorption (β channel). Conversion from α‐O2, to β‐O2 occurs upon heating the surface to above 200 K.

The bimetallic Pt/Mo(110) surface: Structural and CO chemisorption studies

Abstract Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature-programmed desorption (TPD) measurements were performed to monitor the growth, thermal stability, and CO adsorption characteristics of Pt layers on Mo(110). Pt grows in a layer-by-layer or Frank-van der Merwe fashion on the molybdenum surface below 600 K. The first monolayer of Pt is stable and no significant structural change is observed at higher substrate temperatures. A monolayer of Pt eliminates the dissociative adsorption of CO observed on the Mo(110) surface. The thermal desorption of CO from this Pt overlayer occurs at lower temperatures (in comparison to CO desorption from the bulk Pt(111) and Mo(110) surfaces), indicating a withdrawal of electron density from the Pt monolayer due to interactions with the Mo(110) surface. As the Pt coverage is increased above a monolayer, LEED measurements indicate the growth of Pt(111) layers at 300 K. The multilayers of Pt are thermally unstable and agglomerate into three-dimensional (3D) clusters upon heating to above ~ 600 K. The 3D clusters contribute little to the AES signal. The CO desorption spectra from the annealed layers are very similar to the CO desorption spectra from one monolayer of Pt on the Mo(110) surface, suggesting that Pt clustering occurs on top of a single Pt monolayer on Mo(110). The spatial distribution of Pt clusters is estimated from Auger intensity changes following annealing of a 6 monolayer Pt film.

Elimination of serious artifacts in temperature programmed desorption spectroscopy

Regurgitation and wall displacement processes are often encountered in temperature programmed desorption measurements. Such effects can lead to misleading interpretations of the surface reaction and desorption products. A simple and useful pumping scheme is described which overcomes this problem by using a cryogenically cooled titanium sublimation pump (TSP) for differential pumping of the mass spectrometer. It is demonstrated that minor wall displacement effects (mainly CO, H2O, and H2) are present in temperature programmed desorption spectroscopy and are insensitive to the mode of differential pumping. In comparison to ion pumping, however, serious methane (CH4) regurgitation and displacement effects can be completely eliminated when the TSP is employed.

Surface Science and Extreme Pressure Lubrication — CCl 4 Chemistry on Fe(110)

The application of surface science methods to the investigation of model lubricant-metal surface chemistry processes is described. The chemistry of CC14 on an atomically clean Fe(110) surface has been investigated and the lubricant, FeC12, has been postulated to nucleate as clusters on defect sites. A method to quench this extreme pressure lubricant nucleation and hence to promote lubricant dispersion is described.