2009. "Crystallographic Dependence of Visible-Light Photochemistry in Epitaxial TiO2-xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Art. No. 085401. Abstract All films were grown by plasma assisted molecular beam epitaxy (PAMBE) in a custom chamber described elsewhere (1). Epitaxial films of TiO2-xNx(001) (x ≤ ~0.02) anatase were grown by PAMBE on undoped or Nb-doped (0.02 at. %) SrTiO3(001) (STO) and undoped LaAlO3(001) (LAO). Similarly, TiO2-xNx(001) (x ≤ ~0.02) rutile epifilms were grown on rutile TiO2(110). The growth and physical properties of N-doped anatase on LAO(001) and N-doped rutile on TiO2(110) have been described in detail elsewhere (2-4). In what follows, we describe the growth details for N-doped anatase on STO(001). The PAMBE chamber is connected to an x-ray photoelectron spectrometer (XPS) chamber and a photodesorption chamber. The former is equipped with a Gamma Data/Scienta SES 200 analyzer and a monochromatic AlK x-ray source. The latter includes a molecular dosing apparatus for TMAA, a Hg arc lamp, and a quadrupole mass spectrometer. The STO substrates were etched in buffered HF and annealed in flowing O2 at 1 atm. at 950oC for 8 hours. The etch dissolved SrO terraces and the oxygen anneal resulted in mass transport of the discontinuous TiO2 microterraces, resulting in an atomically flat, TiO2 terminated surface with a minimum step height of 4 Å (5). This treatment left some residual fluorine on the surface which could not be removed by annealing. The measured F 1s binding energy was ~684.0 eV, which is close to that exhibited by SrF2 – 684.6 eV (6). Based on this binding energy and the high degree of thermal stability, we conclude that F substitutes for O in the lattice. Under this assumption and using atomic photoemission cross sections (7), the F mole fraction within the anion sublattice is estimated to be ~0.05 within the probe depth of XPS at normal emission (~45 Å).
2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.
2008. "Influence of O2-induced surface roughening on the chemistry of water on TiO2(110)." Surface Science 602(8):1507-1516. Abstract The impact of oxygen induced regrowth of TiO2 on the reduced rutile TiO2(110) surface has been studied using temperature programmed desorption (TPD) of adsorbed water multilayers. Pre-exposure of UHV annealed TiO2(110) surfaces to O2 at temperatures from 300 to 850 K induced changes in subsequent water TPDs that were interpreted in terms of the rougher surface morphologies resulting from the regrowth process. Water TPD from TiO2(110) previously oxidized at 300 K exhibited a new peak at *312 K due to reaction of water with O adatoms. These O adatoms were produced by dissociative adsorption of O2 at O-vacancy sites. Additionally, oxygen reacted (slowly) with surface Ti2O3 strands at RT. Water TPD from surfaces pre-oxidized at higher temperatures (P500 K) exhibited features reflective of desorption from rough surfaces, namely loss of peak resolution and eventual merger of the second layer and ice peaks, formation of a high temperature tail on the second layer peak, and broadening of the first layer TPD peak. The multiplicity of kinetically different adsorption sites on the roughened TiO2(110) surfaces contributed to the widening of the desorption features. Published by Elsevier B.V.
2008. "Acetaldehyde photochemistry on TiO2(110)." Surface Science 602(13):2238-2249. doi:10.1016/j.susc.2008.04.045 Abstract The ultraviolet (UV) photon induced decomposition of acetaldehyde absorbed on the oxidized retile TIO2(110) surface was studied with photon stimulated desorption (PSD) and theral programmed desorption (TPD). Acetaldehyde desorbs molecularly from TiO2(110) with minor decomposition channels yielding butene on the reduced TiO2 surface and acetate on the oxidized TiO2 surface. Acetaldehyde absorbed on oxidized TiO2(110) undergoes a facile thermal reaction to form a photoactive acetaldehyde-oxygen complex. UV irradiation of the acetaldehyde-oxygen complex resulting in the ejection of methyl radical into gas phase and conversion of the surface bound fragment to formate.
2008. "Hole-mediated Photodecomposition of Trimehtyl Acetate on a TiO2(001) Anatase Epitaxial Thin Film Surface." Journal of Physical Chemistry C 112(50):20050-20056. doi:10.1021/jp8077997 Abstract Surfaces of titanium dioxide in both rutile and anatase polymorphs have attracted significant attention in catalysis and photochemistry. The (110) orientation of rutile, and to a lesser extent other rutile orientations, have been studied on an atomic scale, yielding information on surface structure and chemical reactivity. In contrast, the thermal and photochemistry of well-defined, single-crystal anatase surfaces had not been investigated, largely because of the metastable nature of anatase , as well as the lack of availability of high-quality surfaces. Here we describe a study of the adsorption and photoreactivity of an organic adlayer, trimethyl acetate (TMA), on structurally-excellent anatase (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy (OPAMBE). High-resolution scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and photodesorption spectrometry have been used to study the chemisorptions and ultraviolet (UV) light-induced photodecomposition of TMA in ultrahigh vacuum. UV light promotes hole-mediated photodecomposition of TMA, resulting in decarboxylation to yield tert-butyl radical and CO2. The photochemical rate constant is equal to that measured for OPAMBE grown rutile TiO2(110) surfaces.
2008. "Relationship of O2 Photodesorption in Photooxidation of Acetone on TiO2." Journal of Physical Chemistry C 112(30):11433-11440. doi:10.1021/jp802551x Abstract Organic photooxidation on TiO2 invariably involves the coexistence of organic species with oxygen on the surface at the same time. In the case of acetone and oxygen, both species exhibit their own interesting photochemistry on TiO2, but interdependences between the two are not understood. In this study, a rutile TiO2(110) surface possessing 7% surface oxygen vacancy sites is used as a model surface to probe the relationship between O2 photodesorption and acetone photodecomposition. Temperature programmed desorption (TPD) and photon stimulated desorption (PSD) measurements indicate that coadsorbed oxygen is essential to acetone photodecomposition on this surface, however the form of oxygen (molecular and dissociative) is not known. The first steps in acetone photodecomposition on TiO2(110) involve thermal activation with oxygen to form an acetone diolate ((CH3)2COO) species followed by photochemical decomposition to adsorbed acetate (CH3COO) and an ejected CH3 radical that is detected in PSD. Depending on the surface conditions, O2 PSD is also observed during the latter process. However, the time scales for the two PSD events (CH3 and O2) are quite different, withthe former occurring at ~10 times faster than the latter. By varying the preheating conditions or performing pre-irradiation on an O2 exposed surface, it becomes clear that the two PSD events are uncorrelated. That is, the O2 species responsible for O2 PSD is not a significant participant in the photochemistry of acetone on TiO2(110) and likely originates from a minority form of O2 on the surface. The CH3 and O2 PSD events do not appear to be in competition with each other suggesting either that ample charge carriers exist under the experimental conditions employed or that different charge carriers or excitation mechanisms are involved.
2008. "Ethyl Radical Ejection During Photodecomposition of Butanone on TiO2(110)." Surface Science 602(20):3188-3193. doi:10.1016/j.susc.2007.06.079 Abstract The photodecomposition of acetone and butanone were examined on the (110) surface of rutile TiO2 using temperature programmed desorption (TPD) and photon stimulated desorption (PSD). In both cases, photodecomposition was proceeded by a required thermal reaction between the adsorbed ketone and coadsorbed oxygen resulting in a diolate species. The diolate photodecomposed by ejection of an organic radical from the surface leaving behind a carboxylate species. In the acetone case, only methyl radical PSD was detected and acetate was left on the surface. In the butanone case there was a possibility of either methyl or ethyl radical ejection, with propionate or acetate left behind, respectively. However, only ethyl radical PSD was detected and the species left on the surface (acetate) was the same as in the acetone case. The preference for ethyl radical ejection is linked to the greater thermal stability of the ethyl radical over that of the methyl radical. Unlike in the acetone case, where the ejected methyl radicals did not participate in thermal chemistry on the TiO2(110) surface after photoactivation of the acetone diolate, ethyl radicals photodesorbing at 100 K from butanone diolate showed a preference for dehydrogenation to ethene through the influence of coadsorbed oxygen. These results reemphasize the mechanistic importance of organic radical production during photooxidation reactions on TiO2 surface. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2008. "Effect of Coadsorbed Water on the Photodecomposition of Acetone on TiO2(110)." Journal of Catalysis 256(2):287-292. doi:10.1016/j.jcat.2008.03.020 Abstract The influence of coadsorbed water on the photodecomposition of acetone on TiO2 was examined using temperature programmed desorption (TPD) and the rutile TiO2(110) surface as a model photocatalyst. Of the two major influences ascribed to water in the heterogeneous photocatalysis literature (promotion via OH radical supply and inhibition due to site blocking), only the negative influence of water was observed. As long as the total water and acetone coverage was maintained well below the first layer saturation coverage (‘1 ML’), little inhibition of acetone photodecomposition was observed. However, as the total water+acetone coverage exceeded 1 ML, acetone was preferentially displaced from the first layer to physisorbed states by water and the extent of acetone photodecomposition attenuated. The displacement originated from water compressing acetone into high coverage regions where increased acetone-acetone repulsions caused displacement from the first layer. The immediate product of acetone photodecomposition was adsorbed acetate, which occupies twice as many surface sites per molecule as compared to acetone. Since the acetate intermediate was more stable on the TiO2(110) surface than either water or acetone (as gauged by TPD) and since its photodecomposition rate was less than that of acetone, additional surface sites were not opened up during acetone photodecomposition for previously displaced acetone molecules to re-enter the first layer. Results in this study suggest that increased molecular-level repulsions between organic molecules brought about by increased water coverage are as influential in the inhibiting effect of water on photooxidation rates as are water-organic repulsions.
2007. "Direct Observation of Adsorption Evolution and Bonding Configuration of TMAA on TiO2(110)." Journal of Physical Chemistry C 111(11):4342-4346. doi:10.1021/jp067264d Abstract Trimethyl acetic acid (TMAA) adsorption evolution on the rutile TiO2(110) surface from submonolayer to saturation coverages was examined at the atomic level by scanning tunneling microscopy using the same area analysis approach. Upon TMAA deprotonation, no evidence of terminal OH group formation has been found. It has been suggested that uncommon geometry associated with detached hydrogen atom takes place instead, with proton bonding to pair bridging oxygen atoms. Such a configuration is likely to be stabilized by adjacent adsorbed TMA groups and, in turn, be a factor in the formation of TMA (2x1) reconstruction at saturation coverage. Our results indicate that TMAA adsorption on reduced TiO2 is virtually not affected by bridging oxygen vacancies or other surface defects.
2006. "The Partial Oxidation of Isobutene and Propene on TiO2(110)." Journal of Catalysis 238(1):111-121. doi:10.1016/j.jcat.2005.11.041 Abstract General techniques for the partial oxidation of alkenes by molecular oxygen are a goal for surface science and catalysis research as they may lead to more efficient and environmentallyfriendly industrial processes. In order to better understand the thermal surface chemistry of metal oxides toward alkene partial oxidation, the interactions of isobutene and propene on TiO2(110) were studied using temperature programmed desorption (TPD). Isobutene was found to adsorb and desorb molecularly below 250 K on the clean surface. With exposure to oxygen (>1000 L) and unknown quantities of water (< 10 L), isobutene monolayers on TiO2(110) react to form products that include methacrolein and isobutanal, as well as a third product that possesses a C4H8O stoichiometry. We tentatively assign this species to 2,2-dimethyloxirane (isobutene oxide). Structural conservation within this family of products points to a common surface intermediate which we propose to result from addition of O from a hydrogen peroxo (HOO) species to the C=C bond of isobutene. This hydrogen peroxo (HOO) species forms from the reaction of physisorbed water and oxygen assisted by partial charge transfer from the TiO2(110) substrate. Initial studies reveal a similar reaction pathway for the partial oxidation of propene on TiO2(110), yielding acetone and propanal. This work suggests that TiO2 surface sites on supported Au/TiO2 catalysts are active for partial oxidation of alkenes.
2006. "Selectivity Changes During Organic Photooxidation on TiO2: Role of O2 pressure and Organic Coverage." Journal of Catalysis 238(1):153-164. doi:10.1016/j.jcat.2005.12.004 Abstract The selectivity of trimethyl acetate (TMA) photodecomposition on TiO2(110) as a function of O2 pressure and TMA coverage was probed at room temperature (RT) using isothermal mass spectrometry (ISOMS) and scanning tunneling microscopy (STM). The selectivity of TMA photodecomposition on TiO2(110) is sensitive to the initial TMA coverage and the O2 pressure. TMA bridge bonds to the surface via the carboxylate end of the molecule in a manner consistent with the binding of other carboxylate species (e.g., formate and acetate) on TiO2 surfaces. Under all conditions, photodecomposition of TMA was initiated via hole reaction with the electron in carboxylate’s system resulting in opening of the O-C-O bond angle, and formation of CO2 and a t-butyl radical by cleavage of the C-C bond between these groups. The CO2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O2 partial pressure is <1x10-10 torr, the TMA photodecomposition results in a near 1:1 yield of isobutene (i-C4H8) and isobutane (i-C4H10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO2(110) surface. In the presence of O2, the photodecomposition selectivity switches from initially i-C4H8 to a mixture of i-C4H8 and i-C4H10 and then back to predominately i-C4H8. The latter selectivity change occurs at the point at which void regions form and grow in the TMA overlayer. At this point, the photodecomposition rate accelerates and the reaction occurs preferentially at the interface between the TMA-rich and TMA-void regions on the surface. These results illustrate both the changing dynamics of a typical photooxidation reaction on TiO2, and how factors such as O2 pressure and TMA coverage, impact the photooxidation reaction selectivity. We also present results that suggest the rate of photodecomposition of monodentate carboxylates is greater than that of bidentate (bridging) carboxylates. This implies that the structural arrangement of Ti cation sites on the surface is an important issue that influences photocatalytic rates on TiO2.
2005. "Trimethyl Acetate on TiO₂(110): Preparation and Anaerobic Photolysis." Journal of Physical Chemistry B 109(25):12417-12430. Abstract The preparation and anaerobic ultraviolet photolysis of trimethyl acetate (TMA) on rutile TiO₂(110) have been examined with an emphasis on reaction paths. Substrates for photolysis were prepared by dosing trimethyl acetic acid at 100, 300 and 550 K. The chemistry was characterized by mass spectrometry during dosing, and by H₂O adsorption and temperature programmed desorption after dosing. Using temperature programmed desorption after photolysis and mass spectrometry during photolysis, the products ejected and retained during photolysis were sought. The photolysis results are interpreted using the following mechanistic model. Photons with energies exceeding 3 eV create electronhole pairs in the substrate. With probabilities of 10-₅ or lower, the holes initiate TMA chemistry by extracting an electron from the π orbital of the carboxylate moiety. The accompanying electrons are trapped at the surface and inhibit this chemistry. The electron deficient intermediate, TMA*, decarboxylates to form CO₂ and either chemisorbed t-butyl, (-C(CH₃)₃), or physisorbed i-butene. For photolysis at 100 or 200 K, the (-C(CH₃)₃) accumulates and there is a slow photon-driven secondary reaction that, with a source of H, hydrogenates adsorbed t-butyl to physisorbed i-butane. For photolysis at 300 K, (-C(CH₃)₃) thermally reacts to form and desorb i-butene and ibutane during photolysis.
2005. "Photooxidation of Acetone on TiO2(110): Conversion to Acetate via Methyl Radical Ejection." Journal of Physical Chemistry B 109(24):12062-12070. Abstract It is generally held that radicals form and participate in heterogeneous photocatalytic processes on oxide surfaces, although understanding the mechanistic origins and fates of such species is difficult. In this study, photodesorption and thermal desorption techniques show that acetone is converted into acetate on the surface of TiO₂(110) in a two step process that involves, first, a thermal reaction between acetone and coadsorbed oxygen to make a surface acetone-oxygen complex, followed second by a photochemical reaction that ejects a methyl radical from the surface and converts the acetone-oxygen complex into acetate. Designation of the photodesorption species to methyl radicals was confirmed using isotopically labeled acetone. The yield of photodesorbed methyl radicals correlates well with the amount depleted of acetone and with the yield of acetate left on the surface, both gauged using post-irradiation temperature programmed desorption (TPD). The thermal reaction between adsorbed acetone and oxygen to form the acetone-oxygen complex exhibits an approximate activation barrier of about 10 kJ/mol. A prerequisite to this reaction is the presence of surface Ti³⁺ sites that enable O₂ adsorption. Creation of these sites by vacuum reduction of the surface prior to acetone and oxygen co-adsorption results in an initial spike in the photodecomposition rate, but replenishment of these sites by photolytic means (i.e., by trapping excited electrons at the surface) appears to be a slow step a sustained reaction. Evidence in this study for the ejection of organic radicals from the surface during photo-oxidation catalysis on TiO₂ provides support for mechanistic pathways that involve both adsorbed and non-adsorbed species.
2005. "Acetone and Water on TiO₂(110): H/D Exchange." Langmuir 21(8):3451-3458. Abstract Isotopic H/D exchange between coadsorbed acetone and water on the TiO₂(110) surface was examined using temperature programmed desorption (TPD) as a function of coverage and two surface pretreatments (oxidation and reduction). Coadsorbed acetone and water interact repulsively on reduced TiO₂(110) based on results from the companion paper to this study, with water exerting a greater influence in destabilizing acetone and acetone having only a nominal influence on water. Despite the repulsive interaction between these coadsorbates, about 0.02 ML of a 1 ML d6-acetone on the reduced surface exhibits H/D exchange with coadsorbed water, with the exchange occurring exclusively in the high temperature region of the d₆-acetone TPD spectrum at ~340 K. The effect was confirmed with combinations of d₀-acetone and D₂O. The extent of exchange decreased on the reduced surface with water coverages above ~0.3 ML due to the ability of water to displace coadsorbed acetone from first layer sites to the multilayer. In contrast, the extent of exchange increased by a factor of 3 when the surface was pre-oxidized prior to coadsorption. In this case, there was no evidence for the negative influence of high water coverages on the extent of H/D exchange. Comparison of the TPD spectra from the exchange products (either d₁- or d₅-acetone depending on the coadsorption pairing) suggests that, in addition to the 340 K exchange process seen on the reduced surface, a second exchange process was observed on the oxidized surface at ~390 K. In both cases (oxidized and reduced), desorption of the H/D exchange products appeared to be reaction limited and to involve the influence of OH/OD groups (or water formed during recombinative desorption of OH/OD groups) instead of molecularly adsorbed water. The 340 K exchange process is assigned to reaction at step sites and the 390 K exchange process is attributed to the influence of oxygen adatoms deposited during surface oxidation. The H/D exchange mechanism likely involves an enolate or propenol surface intermediate formed transiently during the desorption of oxygen-stabilized acetone molecules.
2005. "Acetone and Water on TiO₂ (110): Competition for Sites." Langmuir 21(8):3443-3450 . Abstract The competitive interaction between acetone and water for surface sites on TiO₂ (110) was examined using temperature programmed desorption (TPD). Two surface pretreatment methods were employed, one involving vacuum reduction of the surface by annealing at 850 K in ultrahigh vacuum (UHV) and another involving surface oxidation with molecular oxygen. In the former case the surface possessed about 7% oxygen vacancy sites and in the latter reactive oxygen species (adatoms and molecules) were deposited on the surface as a result of oxidative filling of vacancy sites. On the reduced surface, excess water displaced all but about 20% of a saturated d6-acetone first layer to physisorbed desorption states, whereas about 40% of the first layer d6-acetone was stabilized on the oxidized surface against displacement by water through a reaction between oxygen and d6-acetone. The displacement of acetone on both surface is explained in terms of the relative desorption energies of each molecule on the clean surface and role of intermolecular repulsions in shifting their respective desorption features to lower temperatures with increasing coverage. Although first layer water desorbs from TiO₂ (110) at slightly lower temperature (275 K) than submonolayer coverages of d6-acetone (340 K), intermolecular repulsions between d6-acetone molecules shift its leading edge for desorption to 170 K as the first layer is saturated In contrast, the desorption leading edge for first layer water (with or without coadsorbed d6-acetone) was at 210 K. This small difference in the onsets for d6-acetone and water desorption resulted in the majority of d6-acetone being compressed into islands by water and eventually displaced from the first layer when excess water was adsorbed. On the oxidized surface the species resulting from reaction of d6-acetone and oxygen was not influence by increasing water coverages. This species was stable on the clean surface up to 375 K (well past the first layer water TPD feature) where it decomposed mostly back to d6-acetone and atomic oxygen. These results are discussed in terms of the influence of water in inhibiting acetone photo-oxidation on TiO₂ surfaces.
2005. "Structure, Band Offsets and Photochemistry at Epitaxial ⍺-Cr₂O₃/⍺-Fe₂O₃ Heterojunctions." Surface Science 587(3):L197-L207. Abstract We test the hypothesis that electron-hole pair separation following light absorption enhances photochemistry at oxide/oxide heterojunctions which exhibit a type II or staggered band alignment. We have used hole-mediated photodecomposition of trimethyl acetic acid chemisorbed on surfaces of heterojunctions made from epitaxial ⍺-Cr₂O₃ on ⍺-Fe₂O₃(0001) to monitor the effect of UV light of wavelength 385 nm (3.2 eV) in promoting photodissociation. Absorption of photons of energies between the bandgaps of ⍺-Cr₂O₃ (Eg = 4.8 eV) and ⍺-Fe₂O₃ (Eg = 2.1 eV) is expected to be strong only in the ⍺-Fe₂O₃ layer. The staggered band alignment should then promote the segregation of holes (electrons) to the ⍺-Cr₂O₃ (⍺-Fe₂O₃) layer. Surprisingly, we find that the ⍺-Cr₂O₃ surface alone promotes photodissociation of the molecule at hv = 3.2 eV, and that any effect of the staggered band alignment, if present, is masked. We propose that the inherent photoactivity of the ⍺-Cr₂O₃ (0001) surface results from the creation of bound excitons in the surface which destabilize the chemisorption bond in the molecule, resulting in photodecomposition.
2004. "Thermal Chemistry of Trimethyl Acetic Acid on TiO₂(110)." Journal of Physical Chemistry B 108(11):3592-3602. Abstract Based on temperature programmed desorption and isothermal reaction mass spectrometry, the thermal surface chemistry of trimethyl acetic acid, (CH₃)₃CCOOH, dosed onto a well characterized single crystal TiO₂(110) surface is described. Deprotonation occurs at or below 300 K to form trimethyl acetate, (CH₃)₃CCOO-, and hydroxide, OH-. (CH₃)₃CCOO- is bound to exposed Ti₄⁺ cations and OH- involves a bridging oxygen atom of the substrate. Based on temperature programmed desorption and isothermal reaction mass spectrometry, the desorbing products include (CH₃)₃CCOOH, isobutene (i-C₄H₈), carbon monoxide and water accompanied by smaller amounts of other products including methyl isopropenyl ketone (CH₂=C(CH₃)C(=O)CH₃), isobutane (i-C4H10), and di-t-butyl ketone, (CH₃)₃CC(=O)C(CH₃)₃. Much of the (CH₃)₃CCOO- is relatively stable and decomposes to release mainly carbon monoxide and isobutene above 550 K with a maximum rate at 660 K. Thermal desorption to 750 K leaves a carbon-free surface that is indistinguishable from the initially clean surface. During dosing at 550 K, a steady-state reaction condition is realized with about half the adsorption sites being occupied at any instant.
2004. "Photoinduced Redox Reaction Coupled with Limited Electron Mobility at Metal Oxide Surface." Journal of Physical Chemistry B 108(30):10621-10624. Abstract Photoinduced oxidation of an organic adsorbate was examined on an atomically flat surface of titanium dioxide by scanning tunneling microscopy combined with macroscopic analysis of desorbed products. The rate of oxidation was coupled with limited surface mobility of photoexcited electrons, and thereby spatially modulated on the surface exposed to oxygen gas. The dimension of the modulations as small as a few lattice constants of the oxide demonstrates that photoexcited electrons are localized to this extent on the surface covered by adsorbed chemicals.
2004. "Formate Adsorption on the (111) Surface of Rutile TiO2 ." Journal of Physical Chemistry B 108(36):13706-13710. Abstract The (111) surface of rutile TiO2 was prepared with argon-ion sputtering and vacuum annealing at 970 K. Scanning tunneling microscope observation revealed nanometer-scale domains of (1 x 1) and (1 x 2) phases covering the surface. Formic acid was dissociatively adsorbed on the surface at room temperature. The recombination and decomposition temperatures of adsorbed formate were similar to those reported on rutile surfaces of different orientations. The CH-related and COO-related stretching modes probed by high-resolution electron energy loss showed two types of formate. The wavenumbers of the vibrational modes can be interpreted with a mixture of monodentate, bidentate, or bridge form of adsorption. The number density and limited mobility of formate evaluated in microscope topography supports monodentate and bidentate forms.
2004. "Acetone Chemistry on Oxidized and Reduced TiO2(110)." Journal of Physical Chemistry B 108(49):18932-18941. Abstract The chemistry of acetone on the oxidized and reduced surfaces of TiO₂(110) was examined using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The reduced surface was prepared with about 7% oxygen vacancy sites by annealing in ultrahigh vacuum (UHV) at 850 K, and the oxidized surface was prepared by exposure of the reduced surface to molecular oxygen at 95 K followed by heating the surface to a variety of temperatures between 200 and 500 K. Acetone adsorbs molecularly on the reduced surface with no evidence for either decomposition or preferential binding at vacancy sites. Based on HREELS, the majority of acetone molecules adsorbed in an ƞ¹ configuration at Ti⁴⁺ sites through interaction of lone pair electrons on the carbonyl oxygen atom. Repulsive acetone-acetone interactions shift the desorption peak from 345 K at low coverage to 175 K as the first layer saturates with a coverage of ~ 1 ML. In contrast, about 7% of the acetone adlayer decomposes when the surface is pretreated with molecular oxygen. Acetate is among the detected decomposition products, but only comprises about 1/3rd of the amount of acetone decomposed and its yield depends on the temperature at which the O₂ exposed surface was preheated to prior to acetone adsorption. Aside from the small level of irreversible decomposition, about 0.25 ML of acetone is stabilized to 375 K by coadsorbed oxygen. These acetone species exhibit an HREELS spectrum unlike that of ƞ¹-acetone or of any other species proposed to exist from the interaction of acetone with TiO₂ powders. Based on the presence of extensive ¹⁶O/¹⁸O exchange between acetone and coadsorbed oxygen in the 375 K acetone TPD state, it is proposed that a polymeric form of acetone forms on the TiO₂(110) surface through nucleophilic attack of oxygen on the carbonyl carbon atom of acetone, and is propagated to neighboring ƞ¹-acetone molecules. This process is initiated at temperatures as low as 135 K based on HREELS. Although the dominant thermal pathway of this surface species is to liberate acetone in UHV, it may be a key intermediate in acetone thermal and photolytic chemistry on TiO₂ surfaces.
2003. "The Photon-Driven Hydrophilicity of Titania: A Model Study Using TiO₂(110) and Adsorbed Trimethyl acetate." Journal of Physical Chemistry B 107(34):9029-9033. Abstract The behavior of H₂O on clean and trimethylacetate (TMA)-covered TiO₂(110)-(1x1), prepared with or without oxygen vacancies and associated Ti₃⁺, reveals the hydrophilic nature of clean surfaces and the hydrophobic nature of TMA-covered surfaces. UV irradiation of a hydrophobic surface in the presence of 10-6 Torr of O₂ removes TMA with a cross section of at least 10-17 cm₂ photon-1 and rapidly restores hydrophilicity. The presence of oxygen vacancies does not detectably increase the hydrophilicity of either clean or TMA-covered TiO₂(110).
2003. "Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO₂ (111)." Surface Science 526(1-2):1-18. Abstract We present x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Å epitaxial CeO₂ (111) film grown on yttria-stabilized ZrO₂ (111). Oxidation with O₂ at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO₂ (111). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce₃d₃/₂ 4f0 photoemission peak at 917 eV which results primarily from Ce₄⁺ sites. As expected, the level of surface reduction (generation of Ce₃⁺ sites) increased with increasing temperature. These Ce₃⁺ sites were primarily in the first layer based on the fact that exposure of the film to O₂ at RT resulted in nearly complete conversion of Ce₃⁺ to Ce₄⁺. Annealing at 773 K led to a surface in which approximately 40% of the surface Ce₄⁺sites were reduced to Ce₃⁺, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O₂.
2003. "Photochemical Charge Transfer and Trapping at the Interface Between an Organic Adlayer and an Oxide Semiconductor." Journal of the American Chemical Society 125(49):14974-14975. Abstract In this study, we identify surface sites associated with charge transfer and trapping during photo-decomposition of an organic adsorbate on the TiO2(110) surface using scanning tunneling microscopy, electron energy loss spectroscopy and photodesorption. Trimethyl acetic acid was selected because it decomposes on TiO2(110) at room temperature to form a densely packed trimethyl acetate adlayer in which each TMA group bridges two TI4+ sites and the acid proton is transfer to a bridging O2 site.
2003. "Insights into Photoexcited Electron Scavenging Processes on TiO₂ Obtained from Studies of the Reaction of O₂ with OH Groups Adsorbed at Electronic Defects on TiO₂ (110) Defects on TiO2(110)." Journal of Physical Chemistry B 107(2):534-545. Abstract In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups that are formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO₂ (110). The electronic structure of an oxygen vacancy defect on TiO₂ (110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO₂ photocatalysts, being Ti₃⁺ in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O₂ adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO₂ (110) to form an unidentified intermediate that decomposes to generate terminal OH groups at non-defect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti₃⁺) is oxidized. Both vibrational and electronic EELS results indicate that the reaction between O₂and OHbr occurs at about 230 K. Detailed TPD experiments in which the precoverage of water was varied indicate that O₂ need not chemisorb to cation sites on the TiO₂ (110) surface in order for the reaction between O₂ and OHbr to occur, which implies a direct interaction between weakly bound O₂ and the OHbr groups. In agreement with this conclusion, we find that second layer water, which selectively hydrogen-bonds to bridging O and OH groups, blocks the reaction of O₂with OHbr groups and prevents oxidation of the vacancy-related Ti₃⁺ electronic state.
2003. "Conversion of N₂O to Nֿ² on TiO₂ (110)." Catalysis Today 85 (2-4):251-266. Abstract In this study we examine the interaction of TiO₂ with TiO₂ (110) in an effort to better understand the conversion of NOx species to N₂ over TiO₂-based catalysts. The TiO₂ (110) surface was used as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂ (110) in vacuum at high temperature (above 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti₃⁺sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂ (110) are weakly held, and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂ (110) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti₄⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule, which permits the O-end of the molecule to interact with an adjacent Ti₄⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on this result, it appears that the decomposition of N₂O to N₂ requires trapping of the Molecule at vacancies and that the lifetime of the N₂O-vacancy interaction may be key to the conversion of N₂O to N₂.
2002. "The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited ." Surface Science Reports 46(1-8):5-308. Abstract Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled "The Interaction of Water with Solid Surfaces: Fundamental Aspects" in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists, such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry, and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces, such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated, and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.
2002. "Surface stabilization of organics on hematite by conversion from terminal to bridging adsorption structures." Geochimica et Cosmochimica Acta 67(5):1055. Abstract The chemistry of methanol on the R-cut surface of hematite was studied in ultrahigh vacuum (UHV) using temperature programmed desorption (TPD), and electron energy loss spectroscopy in both the electronic (EELS) and vibrational ('high resolution' - HREELS) regimes. The R-cut hematite surface, also referred to as the alpha-Fe2O3(012) surface has two stable surface structure in UHV. The (1x1) surface is obtained after oxidation with O2 at 750K, and has a bulk terminated structure possessing undercoordinated Fe3+ and O2- surface sites. The phonon and electronic spectra of this surface are consistent with those of hematite. In contrast, the (2x1) reconstructed surface, obtained by annealing the (1x1) surface at 950 K, possesses Fe2+ surface sites based on the electronic spectrum, but is essential hematite in structure based on the phonon spectrum. Although the atomic arrangements of the (2x1) surface are not known, the reconstruction modifies every other row of cation and anion sites that run parallel to the surface along the [1 2 1] direction.
2002. "Insights into the (1x1)-to-(2x1) Phase Transition of the alpha-Fe2O3(012) Surface Using EELS, LEED and Water TPD." Surface Science 515(1):253-262. Abstract The (1x1)-to-(2x1) surface reconstruction of alpha-Fe2O3(012) (also known as the R-cut or (011- 2) surface) was examined using low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). The (1x1) surface is generated by heating in 5 x 10-7 torr O2 at 750 K, followed by cooling in O2. The surface prepared in this manner exhibits a p(1x1) LEED pattern consistent with a bulk-terminated structure. EELS analysis of this surface shows a 2 eV bandgap consistent with that of hematite and little or no evidence for Fe2+ surface sites. In contrast, a (2x1) LEED pattern is observed after annealing the (1x1) surface in UHV at 950 K. The EELS spectrum of the (2x1) surface exhibits a prominent loss feature at about 1 eV, which is consistent with Fe2+ sites. The (2x1) surface prepared by annealing at 950 K does not consist of a termination film of Fe3O4, as has been reported in the literature for the (001) surface, since the phonon spectrum remains consistent with that of alpha-Fe2O3. Both LEED and EELS detect the onset of the (1x1)-to-(2x1) reconstruction process to be at 700 K, whereas water TPD, which shows distinctively different desorption features for those two surface phases, detects the onset to occur at about 600 K. The (1x1)-to-(2x1) surface reconstruction process appears to be highly nucleated based on observations that the (2x1) LEED spots grow in during annealing without streaking and that coverage-dependent water TPD for a half (1x1) - half (2x1) surface shows simultaneous filling of both (1x1) and (2x1) binding sites. Given that the reconstruction process is accompanied by reduction of Fe3+ surface sites to Fe2+ sites, the preference for growth of existing (2x1) domains over initiation of new (2x1) domains implies that the kinetics for hematite reduction (i.e., Fe2+ formation) are more favorable in the vicinity of other Fe2+ sites than in regions rich in Fe3+.
2001. "Self-Diffusion in Ceria." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 19(4 PT 2):1942-1946. Abstract Ceria (CeO₂) is an oxygen storage material vital to the proper functioning of automobile three-way catalysts and is typically viewed as an anion conductor. Prior experimental work using temperature programmed static secondary ion mass spectrometry (TPSSIMS) has indicated that for rutile TiO₂, a prototypical oxide, the mobile species are Ti cations rather than O anions. To further expand on the mobile species in CeO₂ we have investigated the diffusion of both cerium and oxygen ions by TPSSIMS. The CeO₂(111) film was heteroepitaxially grown by molecular beam epitaxy on a yttria stabilized zirconia substrate. Although high quality LEED patterns and AES spectra free of impurity signals were obtained after just a few sputtering and annealing cycles, further cleaning was necessary to remove intense alkali and alkaline earth signals observed in SSIMS. The CeO₂(111) surface was slightly enriched in 18O by first annealing the film in UHV at 830 K and then exposing the 130 K crystal to 18O₂. TPSSIMS data in conjunction with temperature programmed desorption data demonstrate that surface oxygen atoms begin to diffuse into the bulk around 550K. Physical deposition of sub-monolayer amounts of isotopically enriched cerium (136Ce) in an 18O₂ background allowed the simultaneous study of the diffusion of both cerium and oxygen ions. Surface cerium cations were found to be immobile with no diffusion into the bulk for temperatures up to 900K, the highest temperature studied.
2001. "Photodesorption and Trapping of Molecular Oxygen at the TiO₂(110)-Water Ice Interface." Journal of Physical Chemistry B 105(18):3856-3863. Abstract By means of temperature programmed desorption (TPD), static secondary ion mass spectroscopy (SSIMS), and electron loss spectroscopy (EELS) we have investigated further the states of oxygen adsorbed on rutile TiO₂. Previous work has shown that annealing the (110) surface in vacuum produces isolated bridging oxygen vacancies are intimately connected with molecular and dissociative oxygen adsorption channels. We find that at 115K illumination of the oxygen exposed surface with 4.1eV photons results in photodesorption of the oxygen associated with a TPD state centered on 410 K, in contrast to the remaining oxygen destined for a dissociative channel. An unusual effect of water overlayers on the photochemical properties of the O/TiO₂(110) system is explored. For thick overlayers, it is possible to generate via UV irradiation a previously unobserved oxygen TPD state. Evidence is presented for this new Ox TPD state originating from the photolysis of an O₂-water adduct.
2001. "Electron Source in Photoinduced Hydrogen Production on Pt-supported TiO₂ Particles." Journal of Physical Chemistry B 105(2):595-596. Abstract Abe et al. illuminated an aqueous suspension of TiO₂/Pt with a Hg arc lamp and observed H₂ production in the absence of O₂ production. An unspecified elemental analysis excluded carbonaceous contaminants, and the source of the electron donor was concluded to be Ti₄+ cations. We can suggest at least three more likely (than Ti₄⁺) sources for their mysterious electron donor: 1)an overlooked inorganic species, 2) Ti₃⁺, and/or 3)organic impurities. The authors excluded the latter in their paper, but we view this conclusion as suspect given the lack of information on the limits of detection of the carbon assay, and the fact that the choice of argon(thermal conductivity = 41.33 x 10-6 cal cm-2s-1 (C cm-1)-1)3 as a carrier gas in their thermal conductivity detector4 probably precluded the observation of CO₂ (thermal conductivity = 37.61 x 10-6 cal cm-2s-1 (C cm-1)-1)3, which is a common product of organic photochemical oxidations. No mention is made of an analysis for inorganic impurities. It is widely accepted that nanometer scale TiO₂ particles can have significant fractions of undercoordinated Ti₃⁺ sites which can be easily oxidized to Ti₄⁺, the highest valence of Ti observed in condensed phases. Here we point out that oxidation of Ti₃⁺ to Ti₄⁺ is much more physically realistic than oxidation of Ti₄⁺ to Ti₅⁺. Removal of a fifth electron from titanium requires ~56 eV5.Ti₅⁺, if it were to be generated in a solid oxide, would extract an electron from the valence band composed mainly of oxygen 2s and 2p states rather than remain in the ⁺5 valence state. This unrealistically high oxidation state seems even more unlikely given the fact that at the room temperatures used in the authors' reactions Ti₅⁺ would have been generated and remained at the surface (at room temperature the diffusion of Ti cations in TiO₂ is minimal), and the fact that the surface Madelung potential is smaller than that of the bulk6.
2000. "The Influence of the Bulk Reduction State on the Surface Structure and Morphology of Rutile TiO₂(110) Single Crystals." Journal of Physical Chemistry B 104(20):4944-4950. Abstract We have investigated the relationship between different types and amounts of bulk defects and the surface morphology of TiO₂(110) single crystals prepared by annealing in ultrahigh vacuum and in oxygen. Rutile TiO₂(110) specimens were cut from the same crystal, and were heated in a furnace to different temperatures which resulted in different states of reduction (colors of the crystals). After characterization of the bulk defects with electron paramagnetic resonance (EPR) the specimens were studied with scanning tunneling microscopy (STM), low-energy He⁺ ion scattering (LEIS) and work function measurements. EPR reveals that darker rutile crystals exhibit higher concentrations of extended Ti₃⁺ related bulk defects such as crystallographic shear planes (CSP), with a decrease in substitutional and interstitial defects as compared to lighter crystals. Surface structures with (1x2) features are preferably formed upon UHV annealing on these darker crystals. LEIS measurements show that all the crystals' (110) surfaces are reoxidized upon annealing in 18 O₂ (573 K, 1x10 -6 mbar, 10 min) and that the 18O surface content is proportional to bulk reduction state. UV-Visable adsorption spectra and resistivity measurements also scale with the reduction states of crystals. Only the (1x1) structure is observed on the surface of slightly reduced crystals. Annealing in oxygen induces additional metastable structures, i.e., TiO₂ clusters on blue crystals and rosette networks on dark blue crystals.
2000. "HREELS, TPD and XPS Study of the Interaction of Water with the Alpha-Cr₂O₃(001) Surface." Surface Science 449(1-3):135-150. Abstract The interaction of water with the (001) surface of alpha-Cr₂O₃ was examined with TPD, HREELS and XPS. Two alpha-Cr₂O₃(001) surfaces were examined, both of which were grown on alpha-Al₂O₃(001) substrates using oxygen-plasma assisted MBE. The two surfaces differed in that one was grown with alpha-Fe₂o3 interlayers whereas the other was grown directly on alpha-Al₂O₃(001). The in-plane lattice spacing of the alpha-Cr₂O₃(001) surface on alpha-Fe₂O3/alpha-Al₂O₃(001) was 2% expansively strained and unstrained alpha-Cr₂O₃(001) surface grown directly on alpha-Al₂O₃(001). Both the strained and unstained surfaces exhibited similar water TPD behavior with the possible exception that the desorption states of water on the strained surface were shifted slightly to lower temperatures relative to those on the unstained surface. Water adsorbs on alpha-Cr₂O₃(001) in both molecular and dissociative states, with the former desorbing in TPD at 295 K and the latter at 345 K. TPD uptake measurements and XPS data suggest that each surface Cr₃+ atom has the capacity to bind two water molecules, one in a molecular state and one in a dissociative state. Water in the dissociative state is comprised of two distinct OH groups based on HREELS, one of which is a terminal group with a v(OH) mode at 3600 cm -1 and the other of which is presumably bridging group with a v(OH) mode at 2885 cm-1. These losses shift to 2645 and 2120 cm-1 with D₂O adsorption. The low loss energy for the bridging OH/OD group indicates it involvement in a very strong hydrogen-bonded interaction with another species, presumably the oxygen atom of the terminal OH group. This pairing behavior is likely responsible for the first-order desorption kinetics observed for the recombinative desorption state at 345 K. The hydrogen-bonding interaction is unusually strong, as exemplified by the very low v(OH) stretch energy for the bridging OH group.
2000. "Mechanistic Study of Metalorganic Chemical Vapor Deposition of (Ba,Sr)TiO₃ Thin Films." Journal of Applied Physics 87(10) (May 15 2000):7430-7437. Abstract The metalorganic precursor chemistry was studied on Pt(111) surfaces in an 16 O₂ and 18 O₂ backgrounds. Using temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SSIMS). The precursor chemistry of Sr(thd)2 was found to be different on oxide covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, TPD showed at least four different reaction processes which involved the removal of carbon from the precursor ligands on oxide covered Pt(111). In two of these, gas phase oxygen was incorporated in the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments have also been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba,Sr)TiO₃ (BST) films. Time-of-flight SIMS and nuclear reaction analysis (NRA) reveal that the oxygen in the BST films originates from both the gas phase oxidants (18 O) and the precursor ligands (16 O). The ligands substitution by gas phase O₂ plays a more prominent role in the film-formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking in the O-C bonds than substituting the precursor ligands for the film formation. Use of the 50%18 O2-50%N₂ 16 O mixture results in a reduction of 18 O incorporation in the BST films, indicative of the direct involvement of N₂O in the film-formation reactions. The mechanistic studies are essential for understanding the new BST precursors used in this study, and provide useful information to correlate the film microstructure, step coverage, and dielectric properties with the precursors properties.
1999. " Model Catalyst Studies with Single Crystals and Epitaxial Thin Oxide Films." Catalysis Today 51(3-4):513-519. Abstract The paper describes recent results from our relatively new program to perform detailed studies of the catalytic properites of metal-oxide materials; in particular, to effect a determination of the active catalytic site(s) and the mechanism for reactions over this especially important class of heterogeneous catalysts. Issues of structure-sensitivity, poisioning and promotion, and competing reaction mechanisms are critical questions that need to be addressed in a detailed manner for catalysis by oxides. As just one important example, both surface (Langmui-Hinhelwood) and direct (Eley-Rideal) reaction mechanism have been proposed for the selective catalytic reduction (SCR) reaction of nitrogen oxides (NOx) over vanadia/titania catalysts. For this program, we are using a number of unique, state-of-the-art capabilities available in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory; for example, the first molecular beam epitaxy (MBE) system dedicated to the growth of model metal-oxide films, and a unique moderate-pressure catalytic reactor/surface science apparatus. We describe the growth, characterization, and water adsorption properties of a thin Fe3O4(001) film grown on a lattice-matched MgO(001) substrate. Because our moderate pressure catalysis studies are preliminary at this point, we instead describe our previous results on the CO oxidation reaction over a Ru(0001) model catalyst to demonstrate the utility of the experimental approach. We specifically discuss the possibility that this reaction occurs by an Eley-Rideal mechanism.
1999. "Oxygen-induced restructuring of the TiO 2 (110) surface:a comprehensive study ." Surface Science 437(1-2):173-190. Abstract We report a comprehensive experimental and theoretical study of the effect of oxidizing a TiO 2 (110) surface at moderate temperatures. The surfaces are investigated with scanning tunneling microscopy (STM), low-energy He + ion scattering (LEIS) and static secondary ion mass spectroscopy (SSIMS). Flat (1?1)-terminated TiO 2 (110) surfaces are obtained by sputtering and annealing in UHV at 880 K. These surfaces are exposed to oxygen gas at elevated temperatures in the range 470-830 K. Formation of irregular networks of pseudo-hexagonal rosettes (6.5 A ? ?6 A ? ) and small (11:0] oriented (1?1) islands along with {001}-oriented strands is induced at temperatures from 470 to 660 K. After annealing above 830 K, only regular (1?1) terraces and white strands are observed. The composition of these oxygen-induced phases is quantified using 18O 2 gas in combination with LEIS and SSIMS measurements. The dependence of the restructuring process on annealing time, annealing temperature, and sample history is systematically investigated. Exposure to H 2 18O and air in the same temperature regime fails to induce the restructuring. UHV annealing of restructured, oxygen-enriched TiO 2 (110) surface smooths the surfaces and converts the rosette networks into strands and finally into the regular (1?1) terraces. This is reported in an accompanying paper [ M. Li, W. Hebenstreit, U. Diebold, Phys. Rev. B (1999), submitted]. The rosette model is supported by first-principles density functional calculations which show a stable structure results, accompanied by significant relaxations from bulk-truncated positions. A mechanism for the dynamic processes of the formation of rosettes and (1?1) islands is presented and the importance of these results for the surface chemistry of TiO 2 (110) surfaces is discussed.
1999. "Mass Spectroscopy of Recoiled Ions, Secondary Ion Mass Spectroscopy, and Auger Electron Spectroscopy Investigation of Y2O3-Stabilized ZrO2(100* and (110*." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):939-944. Abstract Studies the (100) and (110) surfaces of yttria-stabilized cubic ZrO2, using Auger electron spectrosocpy (AES), low energy electron diffraction (LEED), direct recoil spectrosocpy (DRS), mass-spectrosocpy of recoiled ions (MSRI), and secondary ion mass spectrosocpy (SIMS). Results show that yttria-stabilized cubic ZrO2 surfaces may not be suitable for detailed studies on molecular level surface interactions.
1999. "The Chemistry of Methanol on the Ti02(110) Surface: The Influence of Vacancies and Coadsorbed Species." Faraday Discussions 114:313-329. Abstract The chemistry of methanol was explored on the vacuum annealed Ti02(110) surface, with and without the presence of coadsorbed water and oxygen, using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED). The higher surface coverage of methozyl then resulted in a disproportionation reaction to form CH3OH and H2CO above 600 K. In contrast, low temperature exposrue of the vacuum annealed TiO2 surfacte to )2 resulted in low temperature state of O2 (presumably an O2 species) that oxidized of CH3OH to H2CO by C-H bond cleavage.
1999. "Interaction of Molecular Oxygen with the Vacuum Annealed TiO2 (110) Surface: Molecular and Dissociative Channels." Journal of Physical Chemistry B 103(25):5328-5337. Abstract We have examined the interaction of molecular oxygen with the TiO2 (110) surface using temperature programmed desorption (TPD), isotopic labeling studies, sticking probability measurements and electron energy loss spectroscopy (ELS). Based on the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical photochemical oxidtion processes on TiO2 surfaces.
1999. "A Surface Perspective on Self-Diffusion in Rutile TiO2." Surface Science 419:174-187. Abstract Although there has been considerable debate in the solid state literature regarding oxygen vacancy versus titanium interstitial models for point defect diffusion in rutile TiO2, ultrahigh vaccum (UHV) surface scientists have all but unanimously adopted the oxygen vacancy model in explaining mass transport phenomena occurring between the bulk and surface. This unanimity, however, has not arisen from detailed mechanistic studies, but from a general misunderstanding that surface oxygen and titanium concentration changes measured by techniques like XPS and AES can be interpreted mechanistically. This thinking is especially pervasive in investigations of the bulk-assisted reoxidation of ion sputtered TiO2 surfaces. It is will known that ion sputtering TiO2 preferentially depletes the surface of oxygen until a steady-state level of surface reduction has been established. The surface stoichiometry of the sputtered TiO2 surface can be restored when annealing in UHV by transport of sputter damage to the bulk.
1998. "Interaction of Water with the (1x1) and (2x1) Surfaces of alpha-Fe2O3(012)." Surface Science 417(1):66-81. Abstract Examines the interaction of water with temperature programmed desorption, static secondary ion mass spectrometry, low energy electron diffraction and high resolution electron energy loss spectroscopy.
1998. "Chromyl Chloride Chemistry on the TiO2(110) Surface." Journal of Physical Chemistry B 102:111-122. Abstract Examines the surface chemistry of chromyl chloride on TiO2(110). Results suggest that the reduction and potential immobilization of Cr(VI) species on TiO2 materials may occur thermally if the appropriate surface defect sites are present.