2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.
2009. "The Effect of the Incident Collision Energy on the Porosity of Vapor Deposited Amorphous Solid Water Films." Journal of Physical Chemistry B 113(13):4000-4007. Abstract Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mole. The effect of the incident angle and collision energy on the porosity and surface area of the vapor deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mole), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels-off at angles > 65°. This is in contrast to the nitrogen uptake data which display a maximum near 70° due to the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition-shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices.
2009. "Chemical Reactivity of Reduced TiO2(110): The dominant role of surface defects in oxygen chemisorption." Journal of Physical Chemistry C 113(28):12407-12411. doi:10.1021/jp901989x Abstract O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature programmed desorption and electron-stimulated desorption. On the annealed surface, 2 oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e. 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ~20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of two when the OHb concentration is enhanced – without changing the concentration of sub-surface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.
2009. "Reactivity of Fe-0 Atoms, Clusters, and Nanoparticles with CC14 Multilayers on FeO(111)." Journal of Physical Chemistry C 113(5):1818-1829. Abstract The interaction of Fe0 atoms and clusters with CCl4 multilayers was investigated using a novel "atom dropping" method at 30 K over a FeO(111) thin film. Temperature programmed desorption experiments over a range of Fe0 and CCl4 coverages demonstrate a rich surface chemistry with several reaction products (C2Cl4, C2Cl6, OCCl2, CO, FeCl2, FeCl3) observed. X-ray photoelectron spectroscopy data show that the initial reactive interaction occurs spontaneously at 30 K, with the experimentally observed reaction products formed at higher temperature, in agreement with the results of theoretical calculations. The formation of OCCl2 and CO is concluded to occur through abstraction of O atoms from the generally inert FeO(111) substrate. The buffer layer assisted growth technique is used to show that the reactivity, and interestingly the reaction products, is determined by the size of Fe0 nanoparticles which interact with CCl4.
2009. "Reactivity of Fe-0 atoms and clusters with D2O over FeO(111)." Journal of Physical Chemistry C 113(2):4960-4969. Abstract The interaction of Fe0 atoms with D2O layers on FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, Auger electron spectroscopy, and infrared absorption spectroscopy. The data demonstrate that isolated Fe atoms form DFeOD insertion species upon deposition at 35 K, which then dissociate into FeOD and a surface hydroxyl above 200 K. Interestingly, even at very low Fe0 coverages the D2O is perturbed by the presence of the Fe, but only D2O desorption is observed. At higher (≥ 0.5 ML) coverages, clusters of Fe form which have molecular D2O and OD species adsorbed on the surface. Both molecular and recombinative desorption are observed in TPD. In contrast to the low coverage data, a second reaction pathway emerges at high coverage which leads to desorption of D2 and the formation of stable substoichiometric oxide. The mechanism for this minor channel is concluded to involve a reaction between two (or more) DFeOD complexes.
2009. "Reactivity of C2Cl6 and C2Cl4 multilayers with Fe0 atoms over FeO(111)." Journal of Physical Chemistry C 113(23):10233-10241. Abstract The interaction of Fe0 atoms with C2Cl6 multilayers over FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature programmed desorption, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. The reactivity and reaction products are strongly dependent on the Fe0 coverage. Submonolayer Fe0 doses lead to high reactivity and primarily FeCl3 and C4Cl6, whereas multilayer Fe0 doses lead to the production of FeCl2 and C2Cl4 with much lower Fe0 reactivity. The data are consistent with a model where Fe atoms form intermediate species at low coverage, which consist of a Fe atom inserted into a C-Cl bond. When two Fe atoms react with C2Cl6, a different intermediate species is formed which produces the alternative reaction pathway and the formation of C2Cl4. Similar atom dropping experiments demonstrate that C2Cl4 is also reactive towards Fe0 atoms at low Fe0 dose, leading to the production of one FeCl2 molecule per C2Cl4 molecule reacted. At higher coverages, Fe atoms form clusters which are much less reactive toward C2Cl4.
2009. "No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene." Journal of the American Chemical Society 131(35):12838-12844. Abstract The structure of water at interfaces is crucial for processes ranging from photocatalysis to protein folding. Here, we investigate the structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate - graphene on Pt(111) - with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption, and ab-initio molecular dynamics. Unlike hexagonal ice, which consists of stacks of puckered hexagonal "bilayers", this new ice polymorph consists of two flat hexagonal sheets of water molecules in which the hexagons in each sheet are stacked directly on top of each other. Such two-layer ices have been predicted for water confined between hydrophobic slits, but not previously observed. Our results show that the two-layer ice forms even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a non-tetrahedral geometry with weakened bonds.
2009. "Competitive Oxidation and Reduction of Aliphatic Alcohols over (WO3)3 Clusters." Journal of Physical Chemistry C 113(22):9721-9730. Abstract The reactions of C1 to C4 aliphatic alcohols over (WO3)3 clusters were studied experimentally and theoretically using temperature-programmed desorption, infrared reflection-absorption spectroscopy and density functional theory. The results reveal that all C1 to C4 aliphatic alcohols readily react with (WO3)3 clusters by heterolytic cleavage of the RO-H bond to give alkoxy (RO ) bound to W(VI) centers and a proton (H+) attached to the terminal oxygen atom of a tungstyl group (W=O). Two protons adsorbed onto the cluster readily react with the doubly-bonded oxygen to from a water molecule that desorbs at 200-300 K and the alkoxy that undergoes decomposition at higher temperatures into the corresponding alkene, aldehyde, and/or ether. Our theory predicts that all three channels proceed over the W(VI) Lewis acid site with energy barriers of 30-40 kcal/mol, where dehydration is favored over the others. We also present further analysis of the yield and reaction temperature as a function of the alkyl substituents and discuss the origin of the reaction selectivity among the three reaction channels.
2009. "Two Pathways for Water Interaction with Oxygen Adatoms on TiO2(110)." Physical Review Letters 102(9):Art. No. 096102. doi:10.1103/PhysRevLett.102.096102 Abstract Scanning tunneling microscopy and density functional theory studies show that oxygen adatoms (Oa), produced during O2 exposure of reduced TiO2(110) surfaces, alter the water dissociation/recombination chemistry through two distinctive pathways. Depending on whether H2O and Oa are on the same or adjacent Ti4+ rows, Oa facilitates H2O dissociation and proton transfer to form a terminal hydroxyl pair, positioned along- or across-Ti row, respectively. The latter process has not been reported previously, and it starts from “pseudo-dissociated” state of water. In both pathways, the subsequent reverse proton transfer results in H2O recombination and statistical oxygen atom scrambling, as manifested by an apparent along- or across-row motion of Oa’s.
2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO₂(110): Identification of HO₂ and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. Abstract We report results of the combined experimental and theoretical investigation of the molecular oxygen reaction with a partially hydroxylated TiO₂(110) surface. The consecutive steps of both primary and secondary site-specific reactions have been tracked with high-resolution scanning tunneling microscopy (STM). For the first time, we have directly imaged stable, adsorbed hydroperoxyl (HO₂) species, which is believed to be a key intermediate in many heterogeneous photochemical processes but generally metastable and “elusive” until now. We also found terminal hydroxyl groups, another critical but never directly observed intermediates. A conclusive evidence that O₂ reacts spontaneously with a single bridging OH group as an initial reaction step is provided. The experimental results are supported by density functional theory (DFT) calculations that have determined species energies and configurations. Reported observations provide a basis for a consistent description of the elementary reaction steps and offer molecular-level insight into the underlying reaction mechanisms. In a broader perspective, the results are expected to have far reaching implications for various catalytic systems involving the interconversion of O₂ and H₂O.
2009. "Imaging Consecutive Steps of O2 Reaction with Hydroxylated TiO2(110): Identification of HO2 and Terminal OH Intermediates." Journal of Physical Chemistry C 113(2):666-671. doi: 10.1021/jp807030n Abstract The hydroperoxyl (HO2) species is believed to be a key intermediate in many heterogeneous photochemical processes, but generally metastable and thus hard to prove. We report here that for the first time, we directly imaged stable, adsorbed HO2 species during O2 reaction with a partially hydroxylated TiO2(110). We also found terminal hydroxyl groups, another critical but never directly observed intermediates. By imaging species and tracking site-specific reactions with high-resolution scanning tunneling microscopy, and determining the energies and configurations with density functional theory calculations, we provide molecular-level insight into the underlying reaction mechanisms. These results are expected to have far reaching implications for various catalytic systems involving the interconversion of O2 and H2O.
2009. "Infrared Spectroscopy and Optical Constants of Porous Amorphous Solid Water." Journal of Physical Chemistry B 113(13):4131-4140. Abstract Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20K on a Pt(111) substrate at various incidence angle (θBeam = 0-85o) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film-substrate and film-vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(ω) and k(ω)] for porous ASW in the 1000-4000cm-1 (10-2.5 μm) range. The behaviour of the optical properties of ASW in the intramolecular OH stretching region with increasing θBeam is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference (~200cm-1) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing θBeam in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.
2008. "Vacancy Assisted Diffusion of Alkoxy Species on Rutile TiO2(110)." Physical Review Letters 101(15):156103. doi:10.1103/PhysRevLett.101.156103 Abstract The catalytic and photocatalytic properties of TiO2 have attracted widespread interest in a variety of applications, such as air purification, self-cleaning glass, water splitting, solar cells and wastewater treatment. In many cases the catalytic chemistry of reducible oxides is dominated by oxygen vacancy sites. For reduced rutile TiO2(110)-1×1, the bridge-bonded oxygen (BBO) vacancies (BBOV’s) are the most prevalent surface defects and, as has been shown, they can readily dissociate small molecules such as H2O, O2, and alcohols.Here we demonstrate for the first time that BBOV’s can also catalyze the transport of adsorbed species which is a key ingredient in heterogeneous catalytic processes. Specifically, we show that at elevated temperatures (≥ 400 K), mobile BBOV’s can assist the diffusion of alkoxy groups formed by the dissociation of alcohols at BBOV’s. This type of mechanism is likely applicable to other adsorbates bound to BBO atoms of TiO2(110).
2008. "Intrinsic Diffusion of Hydrogen on Rutile TiO2(110)." Journal of the American Chemical Society 130(28):9080-9088. doi:10.1021/ja8012825 Abstract The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO2(110) is presented. The sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting a presence of long lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. The density functional theory calculations show that the lowest energy diffusion path involves a simple hydrogen motion along the BBO from one BBO to its neighbour. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via two step mechanism, the initial diffusion of localized charge followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.
2008. "Catalytic Dehydration of 2-Propanol on (WO3)3 Clusters on TiO2(110)." Journal of the American Chemical Society 130(15):5059-5061. doi:10.1021/ja800730g Abstract Monodispersed supported clusters of catalytically important materials provide an excellent platform for model catalytic studies. Ultimately such systems can provide the detailed understanding of the site specific reactivity and yield the structure-reactivity relationships that cannot be obtained otherwise. Unfortunately preparation of such model systems requires highly complex experimental apparatus and has been achieved only for limited number of clusters, primarily those of metals.[1] Our work in this area focuses on supported transition-metal oxide catalysts which are of particular technological importance and have broad applications in partial oxidation of alcohols, oxidative dehydrogenation of hydrocarbons, and selective reduction of nitric oxide
2008. "2-Propanol Dehydration on TiO2(110): The Effect of Bridge-Bonded Oxygen Vacancy Blocking." Surface Science 602(2):511-516. doi:10.1016/j.susc.2007.10.049 Abstract On 100 K rutile TiO2(110) surfaces with 3.5 0.5 % surface bridge-bonded oxygen vacancies (BBOV’s), propene formation by dehydration of 2-propanol was monitored by temperature programmed desorption (TPD). The BBOV’s were either filled or unfilled when the 2-propanol was dosed. Propene desorption rates exhibit two local maxima, nominally at 350 (LT) and 570 K (HT). The former is not altered by filling BBOV’s while the latter is reduced by factors of 2.5 and 5 when the vacancies are pre-filled with water (H-OH) and alcohol (R-OH), respectively. The HT process is attributed to a reaction of 2-propoxy groups located on surface BBO’s. To account for much of the HT C3H6 yield when BBOV’s are filled before (CH3)2CHOH dosing, we propose a model whereby, during TPD, vacancies form and are filled with 2-propoxy. The factor of two difference between titrating BBOV’s with H2O and alcohols is attributed to stoichiometry; twice as many OH groups form on surface BBO rows when H2O is used so twice as many vacancies are created when OH recombines.
2008. "Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2 (110)." Journal of Physical Chemistry C 112(7):2649-2653. doi:10.1021/jp077677u Abstract Tracking the same region of the reduced TiO2 (110) surface by scanning tunneling microscopy before and after oxygen exposure at room temperature confirms that O2 molecules dissociate only at the bridging oxygen vacancies, with one O atom healing a vacancy and other O atom bonding at the neighboring Ti site as an adatom. Majority (~81%) of O adatoms are found separated from the original vacancy positions, by up to two lattice constants along [001] direction. Since at room temperature the thermal diffusion of O adatoms has been found to be rather small, with experimentally estimated activation energy of ~1.1 eV, we conclude that observed lateral distribution of the oxygen adatoms is attained through a nonthermal, transient mobility in the course of O2 dissociation. Unlike for other known cases of the dissociation of the diatomic molecules where both “hot” adatoms accommodate at the equivalent sites, in the studied system the oxygen atoms filling the vacancies are locked into the bridging oxygen rows and only the O adatoms are relatively free to move. The transient motion of the hyperthermal oxygen adatoms on the TiO2 (110) surface occurs exclusively along the Ti troughs.
2008. "Understanding How Surface Morphology and Hydrogen Dissolution Influence Ethylene Hydrogenation on Palladium." Journal of Physical Chemistry C 112(40):15796-15801. doi:10.1021/jp803880x Abstract Ethylene hydrogenation is a prototypical reaction for catalytic hydrogenation of unsaturated hydrocarbons and as such it has been studied on a number of metals. On single crystalline Pd(111), Pd(110), and Pd(100) surfaces this reaction has been shown to be structure insensitive and to occur with extremely low yield (0.1%) (refs). Recent studies on support4ed Pd particles showed an approximately 10-folded increase in the ethane yield per surface Pd atom which was attributed to the increased surface to bulk Pd ratio on the particles thereby giving rise to reduced sorption of hydrogen into the bulk. The enhanced concentration of surface hydrogen is believed to result in the observed increase in catalytic activity. Even on these nanoparticles the C2H6 yield is relatively low (~2%)(refs).
2007. "Imaging Intrinsic Diffusion of Bridge-Bonded Oxygen Vacancies on TiO2(110)." Physical Review Letters 99(12):paper #126105. doi:10.1103/PhysRevLett.99.126105 Abstract Since oxygen atom vacancies play a central role in the behavior of oxide materials, determining their properties is widely pursued both experimentally and theoretically. As a function of temperature between 340 and 420 K, we report here the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen atom vacancies on a TiO2(110) rutile surface. Under conditions where interference by adsorbates was negligible, isothermal atomically-resolved scanning tunneling microscope images that track individual vacancies in real time show that, vacancies migrate exclusively along bridging oxygen atom rows. The hopping rate increases exponentially with increasing temperature with experimental activation energy of 1.15 eV. Density functional theory calculations are in very good agreement giving an energy barrier for hopping of 1.034 eV. Both theory and experiment, indicate a short-range repulsive interaction between vacancies on a given bridge-bonded oxygen atom row.
2007. "Direct Visualization of 2-Butanol Adsorption and Dissociation on TiO2(110)." Journal of Physical Chemistry C 111(7):3021-3027. doi:10.1021/jp067461c Abstract Atomically resolved scanning tunneling microscopy (STM) images of identical regions of a TiO2(110) surface were gathered before and after controlled doses of 2-butanol (CH3CH2CH(OH)CH3) at ambient temperature (~ 300 K). When dosing is initiated, 2-butanol preferentially adsorbs at bridge-bonded oxygen vacancy (BBOv) sites and dissociates via O—H, not C—O, bond scission to form paired 2-butoxy and hydroxyl species evidenced by two local maxima in STM line profiles. The measured separation is 0.4 nm, slightly larger than the measured separation (0.3 nm) between neighboring bridge-bonded oxygen anions in the surface unit cell of TiO2(110). As the dose increases, but before all the BBOv are occupied, there is direct STM evidence of hydroxyl proton hopping to an adjacent oxygen anion row. This process is facilitated by species bound to 5-coordinate Ti4+ rows, presumably undissociated 2-butanol, that hop slowly compared the STM imaging time scale. The backbones of these mobile species are centered over the Ti4+ rows with preference for lying parallel to these rows. On the other hand, the carbon backbones of the 2-butoxy species that fill BBOv's are centered over the O2- rows and prefer an orientation perpendicular to these rows. As the oxygen vacancy concentration increases from 0.4 to 11 % and 2-butanol is dosed the ratio of mobile species to 2-butoxy species decreases for doses that do not fill all the BBOv.
2007. "Crystalline Ice Growth on Pt(111) and Pd(111): Nonwetting Growth on a Hydrophobic Water Monolayer." Journal of Chemical Physics 126(11):Art. No. 114702. Abstract The growth of crystalline water films on Pt(111) and Pd(111) is investigated using temperature programmed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated (e.g. 20-155 K, for Pt(111)). However, crystalline ice films grown at higher temperatures (e.g. T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet exposing the water monlayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of ~14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).
2007. "Preparation and Characterization of Monodispersed WO3 Nanoclusters on TiO2(110)." Catalysis Today 120(2):186-195. doi:10.1016/j.cattod.2006.07.050 Abstract A procedure is described for preparing a novel model early transition metal oxide system for catalysis studies—direct sublimation of tungsten trioxide on TiO2(110). Isolated monodispersed cyclic trimers, i.e., (WO3)3, can be formed on TiO2(110) that are thermally stable up to at least 750 K. Although not readily generalizable to monodispersed (WO3)x clusters other than cyclic trimers, this protocol provides an ideal nanocluster platform for carrying out model system catalysis studies over a wide temperature range.
2007. "Inductive Effect of Alkyl Chains on Alcohol Dehydration at Bridge-Bonded Oxygen Vacancies of TiO2(110)." Catalysis Letters 119(1-2):1-4. doi:10.1007/s10562-007-9199-1 Abstract Reactivity of adsorbates with titania is typically influenced by the presence of surface oxygen vacancy defects. For alcohols, it is well-established that their dissociation on bridge-bonded oxygen vacancies (BBOV’s) of a prototypical TiO2(110) surface proceeds via heterolytic cleavage of the RO—H bond. The resulting intermediates are alkoxide (RO) that fills the vacancy and a proton that binds to a neighboring oxygen anion. Except for methoxide, these alkoxides dehydrate at elevated temperatures (well above 300 K) via β–hydride elimination to form alkenes.In some cases, the dehydration is accompanied by a minority dehydrogenation channel yielding aldehydes.
2007. "Alcohol Chemistry on Rutile TiO2(110): The Influence of Alkyl Substituents on Reactivity and Selectivity." Journal of Physical Chemistry C 111(49):18236-18242. doi:10.1021/jp075608+ Abstract Product yields and selectivities, based on ultra high vacuum temperature programmed desorption, are compared for ten C2 to C8 aliphatic alcohols dosed at 100 K on highly-ordered TiO2(110) with a 3.5 % concentration of surface oxygen vacancies. Dehydration to form an alkene and water typically dominates while two other channels, dehydrogenation to form aldehydes, and reformation of alcohol, make detectable contributions for primary alcohols. Depending on the alcohol there are two distinct dehydration pathways, one operative at low temperature (LT, 300 to 425 K) and the other at high temperature (HT, 480 to 650 K). The HT dehydration pathway is common, while the LT channel is not observed for tertiary butanol and 3- and 4-octanol. The observed trends are accounted for in terms of the inductive and steric effects of the alkyl substituents.
2007. "Reactive Ballistic Deposition of Porous TiO2 Films: Growth and Characterization." Journal of Physical Chemistry C 111(12):4765-4773. doi:10.1021/jp067641m Abstract Nanoporous, high-surface area films of TiO2 are synthesized by reactive ballistic deposition of titanium metal in an oxygen ambient. Auger electron spectroscopy (AES) is used to investigate the stoichiometric dependence of the films on growth conditions (surface temperature and partial pressure of oxygen). Scanning and transmission electron microscopy show that the films consist of arrays of separated filaments. The surface area and the distribution of binding site energies of the films are measured as functions of growth temperature, deposition angle, and annealing conditions using temperature programmed desorption (TPD) of N2. TiO2 films deposited at 50 K at 70º from substrate normal display the greatest specific surface area of ~100 m2/g. In addition, the films retain greater than 70% of their original surface area after annealing to 600 K. The combination of high surface area and thermal stability suggest that these films could serve as supports for applications in heterogeneous catalysis.
2007. "Surface Chemistry of 2-Propanol on TiO2(110): Low and High Temperature Dehydration, Isotope Effects, and Influence of Local Surface Structure." Journal of Physical Chemistry C 111(29):1105-11067. doi:10.1021/jp072298m Abstract Dosed on rutile TiO2(110) at 100 K, the thermal chemistry of 2-propanol in three forms—C3H7OH, C3D7OD and C3H7OD—was characterized using temperature programmed desorption. Only 2-propanol, propene and water desorb with no evidence for acetone. The propene forms and desorbs by two paths, a heretofore unreported low temperature path extending from 300 to 450 K and, concerning with prior work, a high temperature path peaking between 565 and 575 K. Both paths exhibit isotope effects. The high temperature path is interpreted in terms of decomposition of 2-propoxy species located on bridging oxygen atom rows. The low temperature path is attributed to 2-propanol dehydration on under-coordinated Ti4+ ions of the Ti4+ rows. The low temperature path characteristics vary with the long range order and bridge-bonded oxygen atom vacancy concentration.
2006. "Imaging Water Dissociation on TiO2(110): Evidence for Inequivalent Geminate OH Groups." Journal of Physical Chemistry B 110(43):21840-21845. doi:10.1021/jp063619h Abstract Identical regions of partially reduced TiO2(110) surfaces with bridge-bonded oxygen vacancy (BBOV) concentrations of ~ 10 % ML (1 ML = 5.2×1014 cm-2) were imaged using scanning tunneling microscopy (STM) before and after dosing H2O at ambient temperature (~ 300 K). Provided the H2O coverage is less than the BBOV concentration, atomically resolved images confirm that the titration of individual BBOV by dissociative adsorption of H2O is strongly favored. The products are two hydroxyl species, one positioned at BBOV and denoted OHV and the other, denoted OHB, formed by protonation at either of the two nearest-neighbor bridge-bonded oxygen atoms. Proton hopping along the [001] direction is observed at ambient temperature, with a strong preference for OHB (~ 10x) proton motion. This powerful imbalance demonstrates the inequality of OHV and OHB and suggests differences in their charge and/or binding configuration.
2006. "Imaging Adsorbate O-H Bond Cleavage: Methanol on TiO2(110)." Journal of the American Chemical Society 128(13):4198-4199. doi:10.1021/ja058466a Abstract We investigated methanol adsorption and dissociation on bridge-bonded oxygen vacancies of TiO2(110) (1×1) surface using in situ scanning tunneling microscopy. We provide the first direct evidence that methanol dissociates on oxygen vacancies via O-H bond scission rather than C-O scission. For CH3OH coverages lower than the oxygen vacancy concentration, stationary methoxy-hydroxyl pairs form. At CH3OH coverages close to the oxygen vacancy concentration undissociated mobile CH3OH interacts with methoxy-hydroxyl pairs and facilitates the movement of hydroxyl away from the methoxy group
2006. "n-alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters." Journal of Chemical Physics 125(23):Art. No. 234308. doi:10.1063/1.2400235 Abstract We have measured the desorption of seven small n-alkanes (CNH2N+2, N = 1-4, 6, 8, 10) from the Pt(111) and C(0001) surfaces by temperature programmed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption pre-exponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.
2006. "Layer-by-Layer Growth of Thin Amorphous Solid Water Films on Pt(111) and Pd(111)." Journal of Chemical Physics 125(4):044713-1 - 044713-12. doi:10.1063/1.2218844 Abstract The growth of amorphous solid water (ASW) films on Pt(111) is investigated using rare gas (e.g. Kr) physisorption. Temperature programmed desorption of Kr is sensitive to the structure of thin water films and can be used to assess the growth modes of these films. At all temperatures that are experimentally accessible (20 – 155 K), the first layer of water wets Pt(111). Over a wide temperature range (20 – 120 K), ASW films wet the substrate and grow approximately layer-by-layer for at least the first 3 layers. In contrast to the ASW films, crystalline ice films do not wet the water monolayer on Pt(111). Virtually identical results were obtained on epitaxial Pd(111) films grown on Pt(111). The desorption rates of thin ASW and crystalline ice films suggest that the relative free energies of the films are responsible for the different growth modes. However at low temperatures, surface relaxation or “transient mobility” is primarily responsible for the relative smoothness of the films.
2006. "Physisorption of N2, O2, and CO on Fully Oxidized TiO2(110)." Journal of Physical Chemistry B 110(12):6229-6235. doi:10.1021/jp0564905 Abstract Physisorption of N2, O2 and CO was studied on fully oxidized TiO2(110) using beam reflection and temperature programmed desorption (TPD) techniques. Sticking coefficients for all three molecules are nearly equal (0.75 ± 0.05) and approximately independent of coverage suggesting that adsorption occurs via a precursor mediated mechanism. Excluding multilayer coverages, the TPD spectra for all three adsorbates exhibit three distinct coverage regimes that can be interpreted in accord with previous theoretical studies of N2 adsorption. At low coverages (0 to 0.5 N2/Ti4+), N2 molecules bind head-on to five-coordinated Ti4+ ions. The adsorption occurs preferentially on the Ti4+ sites that do not have neighboring adsorbates. This arrangement minimizes the repulsive interactions between the adsorbed molecules along the Ti4+ rows resulting in a relatively small shift of the TPD peak (105 → 90 K) with increasing coverage. At higher N2 coverages (0 to 1.0 N2/Ti4+) the nearest-neighbor Ti4+ sites become occupied. The close proximity of the adsorbates results in strong repulsion thus giving rise to a significant shift of the TPD leading edges (90 → 45 K) with increasing coverage. For N2/Ti4+ > 1, an additional low temperature peak (~ 43 K) is present and is ascribed to N2 adsorption on bridge-bonded oxygen rows. The results for O2 and CO are qualitatively similar. The repulsive adsorbate-adsorbate interactions largest for CO, most likely due to aligned CO dipole moments. The coverage dependent binding energies of O2, N2, and CO are determined by inverting TPD profiles.
2006. "Growth of Epitaxial Thin Pd(111) Films on Pt(111) and Oxygen-Terminated FeO(111) Surfaces ." Surface Science 600(17):3461-3471. doi:10.1016/j.susc.2006.06.036 Abstract Ultra-thin Pd films (1-10 monolayers) were deposited at 30 K on a Pt(111) single crystal and on an oxygen-terminated FeO(111) monolayer supported on Pt(111). Low energy electron diffraction, Auger electron spectroscopy, and Kr and CO temperature programmed desorption techniques were used to investigate the annealing induced changes in the film surface morphology. For growth on Pt(111), the films order upon annealing to 500 K and form epitaxial Pd(111). Further annealing above 900 K results in Pd diffusion into the Pt(111) bulk and Pt-Pd alloy formation. Chemisorption of CO shows that even the first ordered monolayer of Pd on Pt(111) has adsorption properties identical to bulk Pd(111). Similar experiments conducted on FeO(111) indicate that 500 K annealing of a 10 ML thick Pd deposit also yields ordered Pd(111). In contrast, annealing of 1 and 3 ML thick Pd films did not result in formation of continuous Pd(111). We speculate that for these thinner films Pd diffuses underneath the FeO(111).
2006. "Formation of Monodisperse (WO3)3 Clusters on Ti02(110)." Angewandte Chemie International Edition 45(29):4786-4789. doi:10.1002/anie.200600837 Abstract Monodisperse, adsorbed cyclic trimers of WO3 have been prepared and characterized. Powdered WO3 was sublimed onto TiO2(110) at 300 K. After annealing to 600 K, scanning tunneling microscopy (STM) images indicated monodispersity, X-ray photoelectron spectroscopy indicated fully oxidized W6+, and mass microbalance, coupled with STM, indicated that each nanocluster was (WO3)3. Within the STM image of each adsorbed nanocluster, there was structure ascribed, on the basis of density functional theory, to characteristic low-lying unoccupied molecular orbitals of the cyclic isomer of (WO3)3.
2005. "n-Alkanes on MgO(100). II. Chain Length Dependence of Kinetic Desorption Parameters for Small n-Alkanes." Journal of Chemical Physics 122(16):4708 (13 pages). Abstract Coverage-dependent desorption kinetics parameters are obtained from high quality temperature programmed desorption (TPD) data for seven small n-alkane molecules on MgO(100). The molecules, CNH2N+2 (N = 1-4, 6, 8, 10), were each studied for a set of 29 initial coverages at a heating ramp rate of 0.6 K/s as well as at a set of nine ramp rates in the range 0.3 to 10.0 K/s. The inversion analysis method with its least-squares prefactor optimization discussed in the accompanying article is applied to these data. This method allows for accurate determination of prefactors and coverage-dependent desorption energies. The pre-exponential factor for desorption increases dramatically with chain length from 1013.1 to 1019.1 s-1 over the range N = 1-10. We show that this increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gas-like transition state for desorption. The desorption energy increases with chain length as Ed(N) = 6.5 + 7.1 N, which implies an incremental increase of 7.1±0.2 kJ/mol per CH2.
2005. "n-Alkanes on MgO(100). I: Coverage-Dependent Desorption Kinetics of n-Butane." Journal of Chemical Physics 122(16):4707 (9 pages). Abstract High quality temperature programmed desorption (TPD) measurements of n-butane from MgO(100) have been made for a large number of initial butane coverages (0-3.70 ML) and a wide range of heating ramp rates (0.3-10 K/s). We present a TPD analysis technique which allows the coverage-dependent desorption energy to be accurately determined by mathematical inversion of a TPD spectrum, assuming only that the prefactor is coverage-independent. A variational method is used to determine the prefactor that minimizes the difference between a set of simulated TPD spectra and corresponding experimental data. The best fit for butane desorption from MgO is obtained with a prefactor of 1015.7±1.6 s-1. The desorption energy is 34.9±3.4 kJ/mol at 0.5 ML coverage, and varies with coverage approximately as . Simulations based on these results can accurately reproduce TPD experiments for submonolayer initial coverages over a wide range of heating ramp rates (0.3-10 K/s). Advantages and limitations of this method are discussed.
2005. "Methane Adsorption and Dissociation and Oxygen Adsorption and Reaction with CO on Pd Nanoparticles on MgO(100) and on Pd(111)." Surface Science 591(1-3):90-107. Abstract We present measurements of the desorption kinetics and dissociative sticking probability of methane on the surfaces of Pd(111) and Pd nanoparticles supported on MgO(100). A molecular beam system was used to directly probe the fraction of methane molecules that dissociate at the Pd surfaces as a function of the molecular beam energy and incident angle. Measurements on the Pd(111) surface confirm a “normal energy scaling” of dissociative sticking, consistent with an activation barrier normal to the surface. Sticking measurements on supported Pd particles (~3 nm wide) with the methane beam directed normal to the MgO(100) surface results in a large fraction of the methane / Pd collisions occurring on regions of the particles where the beam direction is far from the local particle surface normal, resulting in lower sticking probability. We attempt to decouple this effect from the measured sticking probabilities in order to compare the intrinsic reactivity of the Pd particles with Pd(111). We find that the sticking probability on ~3 nm Pd particle surfaces is at most twice as large as on Pd(111). This result depends on our assumption that these annealed Pd particles have the known equilibrium particle shape (truncated half octahedron). We also discuss the need for detailed structural knowledge of the particles and careful geometric analysis when probing direct collisional activation barrier crossing using molecular beams. Temperature programmed desorption studies of physisorbed (not dissociated) methane reveal that the Pd particles bind methane more strongly than Pd(111). Oxygen adsorbs on the Pd nanoparticles via a mobile, molecular O2 precursor state which is transiently adsorbed on the MgO(100) surface. An induction period is observed on Pd nanoparticles for the titration of adsorbed O by CO gas to make CO2 which is not observed on Pd(111). This is attributed to inhibition by adsorbed O, whose saturation coverage on the Pd particles is 41 % greater than on Pd(111).
2005. "Crystalline Ice Growth on Pt(111): Observation of a Hydrophobic Water Monolayer." Physical Review Letters 95(16):166102. Abstract The growth of crystalline water films on Pt(111) is investigated using rare gas physisorption. The water monolayer wets Pt(111) at all temperatures investigated (20-155 K). At low temperatures (T ≤ 120 K) where the water mobility is limited, additional water layers kinetically wet the monolayer surface. However, crystalline ice films grown at higher temperatures (T > 135 K) do not wet the water monolayer. These results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on Pt(111).
2005. "Structural Characterization of Nanoporous Pd Films Grown Via Ballistic Deposition." Surface Science 586(1-3):137-145. Abstract Nanoporous Pd films were vapor-deposited onto an oxidized Mo(100) substrate at 22 K and various dosing angles. Temperature programmed desorption (TPD) of N2 was used to determine the surface area of the Pd films immediately after deposition at 22 K, and following annealing. The surface area of Pd films was found to depend dramatically on the Pd dosing angle and annealing temperature. Pd films grown at 22 K with a 85o deposition angle exhibit the highest surface area of 120 m2/g. Ex situ SEM imaging reveals that these films consist of a tilted array of nanocrystalline filaments. The annealing studies show that the films densify upon annealing and lose approximately 50% of their surface area by 300 K and are almost completely dense by 500 K. Pd deposition at elevated temperatures (< 300 K) produces denser Pd films compared to those grown at 22 K. Growth above 300 K leads to dewetting of Pd from the substrate.
2005. "Cryogenic CO2 Formation on Oxidized Gold Clusters Synthesized via Reactive Layer Assisted Deposition." Journal of the American Chemical Society 127(42):14592-93. Abstract We report studies where gas-phase Au atoms are deposited onto a multilayer film of molecular oxygen and demonstrate that this technique produces oxidized gold clusters. This method of producing oxidized gold clusters is an extension of our earlier work synthesizing MgO nanostructures using a technique termed Reactive Layer Assisted Deposition (RLAD). Emp[loying a combination of temperature programmed desorpotion (TPD) and molecular beam techniques we show that the oxidized gold clusters readily react with CO to produce CO2. At present, the structure and size distribution of these clusters are unknown. Nevertheless, these oxidized gold clusters readily catalyze the formation of CO2 at temperatures as low as 35K. Furthermore, above 120K the reaction runs to completion during exposure to gas-phase CO with initial reaction yields as high as 50
2005. "Water Adsorption, Desorption, and Clustering on FeO(111)." Journal of Physical Chemistry B 109(20):10362-10370. Abstract The adsorption of water on FeO(111) is investigated using temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Well-ordered 2 ML thick FeO(111) films are grown epitaxially on a Pt(111) substrate. Water adsorbs molecularly on FeO(111) and desorbs with a well resolved monolayer peak. IRAS measurements as a function of coverage are performed for water deposited at 30 and 135 K. For all coverages (0.2 ML and greater) the adsorbed water exhibits significant hydrogen bonding. Differences in IRAS spectra for water adsorbed at 30 and 135 K are subtle but suggest that water adsorbed at 135 K is well ordered. Monolayer nitrogen TPD spectra from water covered FeO(111) surfaces are used to investigate the clustering of the water as a function of deposition or annealing temperature. Temperature dependent water overlayer structures result from differences in water diffusion rates on bare FeO(111) and on water adsorbed on FeO(111). Features in the nitrogen TPD spectra allow the monolayer wetting and 2-dimensional (2D) ordering of water on FeO(111) to be followed. Voids in a partially disordered first water layer exist for water deposited below 120 K and ordered 2D islands are found when depositing water above 120 K.
2004. "Interaction of CH4, CH3Cl, CH2CI2, and CCl4 with O-terminated FeO(111)." Journal of Physical Chemistry B 108(11):3644-3650. Abstract Well-ordered FeO(111) thin films are epitaxially grown on a Pt(111) substrate. A series of molecules including CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 are used as probes to test the chemical reactivity of the FeO(111) surface. The temperature programmed desorption spectra show no evidence of dissociative adsorption or chemical reaction between the substrate and the adsorbates. The desorption kinetics studies reveal that all the molecules are physisorbed and have desorption kinetics with an order between 0 and 1. Kinetic analysis is conducted, assuming both zero and first order desorption, and shows that an uncertainty in the desorption order introduces an error in determination of the terrace site desorption energies ( = 0.5 ML) of only ~ 2%. The desorption energies for the series of molecules increase with the number of chlorine atoms in the molecule. The increase of desorption energies is not linear with the molecular polarizability and the deviations from linearity are attributed to the permanent dipole in some of the molecules. We conclude that the oxygen terminated FeO(111) surface is in general unreactive toward both C-H and C-Cl bonds.
2004. "Reactive Growth of Nanoscale MgO Films by Mg Atom Deposition onto O2 Multilayers." Journal of Physical Chemistry B 108(31):11666-11671. Abstract Nanometer thick layers of Mg metal vapor deposited onto a polycrystalline gold substrate at 22 K with and without O2 multilayers were examined after annealing. Auger electron spectroscopy and temperature programmed desorption of N2 were used to determine the Mg oxidation state and the surface area of the deposits immediately after deposition at 22 K, and following annealing. Deposited on 20 layers of O2, Mg oxidizes and forms an MgO film having a high surface area (~700 m2/g). The surface area decreases upon annealing, approaching that of a well-ordered MgO(100) surface by 500 K. On thinner O2 multilayers, both oxidized and metallic Mg are observed at 22 K. Mild thermal annealing (200 to 400 K) results in transport of the excess metallic Mg to, and alloy formation with, the underlying Au substrate. MgO films annealed to 1000 K display a narrow distribution of N2 binding sites, similar to well-ordered MgO(100). The film sublimes above 1000 K when heated.
2003. "Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water." Chapter 14 in Water in Confining Geometries, ed. V. Buch and J.P. Devlin, pp. 337-357. Springer-Verlag, New York, NY. Abstract In this article, we discuss the use of nanoscale thin films to explore the structural and physical properties of amorphous solid water.
2003. "Temperature Independent Physisorption Kinetics and Adsorbate Layer Compression for Ar Adsorbed on Pt (111)." Journal of Chemical Physics 119(13):6776-6783. Abstract The influence of adlayer compression on the physisorption of Ar on Pt(111) is investigated using temperature programmed desorption and modulated molecular beams. We find that the difference in coverage between the compressed and uncompressed first laysers is ~10-15%. For coverages near one monolayer, this compression causes nearly temperature independent desorption kinetics over a wide temperature range (32K-42K). We present a theory that includes the effects of the compression on the desorption kinetics and explains the observed kinetics in terms of a competition between adsorbate-substrate and adsorbate-adsorbate interactions resulting in a continuous increase in the chemical potential near the completion of each successive layer.
2003. "Experimental Investigation on the Interaction of Water and Methanol with Anatase-TiO2(101)." Journal of Physical Chemistry B 107:2788-2795. Abstract The interaction of water and methanol with well defined (1x1) terminated surfaces of anatase-TiO2(101) were investigated with temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). For water, three desorption states were observed in the TPD spectra at 160 K, 190 K, and 250 K. The three desorption peaks were assigned to multilayer water, water adsorbed to 2-fold coordinated O, and water adsorbed to 5-fold coordinated Ti, respectively. The TPD spectra for methanol were more complicated. For methanol five desorption peaks were observed in the TPD spectra at 135 K, 170 K, 260 K, 410 K, and 610 K. The five desorption peaks were assigned to multilayer methanol, methanol adsorbed to 2-fold coordinated O, methanol adsorbed to 5-fold coordinated Ti, methoxy adsorbed to 5-fold coordinated Ti, and methoxy adsorbed to Ti at step edges, respectively. The XPS results indicated that the adsorbed water and methanol were predominantly bound to the surface in a molecular state, with no evidence for dissociation. Furthermore, the O 1s core-level binding energy for water and methanol were found to shift to ~0.75 eV lower binding energy, for coverages before multilayer desorption is observed in the TPD spectra. The O 1s core-level binding energy shift appears to be linear in this region, and corresponds to water and methanol bonding to Ti cation and O anion sites on the surface. The C 1s core-level binding energy for methanol was found to remain approximately constant in the same coverage regime.
2003. "The Deposition Angle-Dependent Density of Amorphous Solid Water Films." Journal of Chemical Physics 118(1):364-372. Abstract The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz-Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are fully dense (0.94 g/cm3). At glancing incidence (86?) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle dependent film density is in good quantitative agreement with the results of ballistic deposition simulations of ASW film growth.
2002. "Structural and chemical characterization of aligned crystalline nanoporous MgO films grown via reactive ballistic deposition ." Journal of Physical Chemistry B 106(14):3526-3529 . doi: 10.1021/jp013801c Abstract Highly-porous (~ 90%), high-surface area (~ 1000 m2/g), thermally stable (1200 K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition technique. The film consists of a tilted array of porous nanoscale crystalline filaments. Surprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO(100). However the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "Structural and Chemical Characterization of Aligned Crystalline Nanoporous MgO Films Grown via Reactive Ballistic Deposition." Journal of Physical Chemistry B 106(14):3526-3529. Abstract Highly-porous (~90%), high-surface area (~1000 m2/g), thermally stable (1200K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition techniques. The film consists of a tilted array of porous nanoscale crystalline filaments. Suprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO (100). However, the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "Adsorption Dynamics and Desorption Kinetics of Argon and Methane on MgO(100) ." Journal of Physical Chemistry B 106(33):8360-8366. Abstract The adsorption dynamics and desorption kinetics of Ar and CH4 on MgO(100) are studied using a combination of molecular beam scattering and temperature programmed desorption (TPD). Both Ar and CH4 exhibit an initial trapping probability that decreases dramatically with increasing kinetic energy and is independent of incident angle indicating a barrier-less process obeying total energy scaling. The trapping probability for both systems increases roughly linearly with increasing adsorbate coverage in the first layer. Analysis of the TPD spectra yields desorption energies of 8.5 and 13 kJ/mole for Ar and CH4, respectively.
2002. "A Beaker without Walls: Formation of Deeply Supercooled Binary Liquid Solutions of Alcohols from Nanoscale Amorphous Solid Films." Physical Review Letters 88(24):art. no. 245505. Abstract Layered nanoscale amorphous solid films of methoanol and ethanol undergo complete intermixing prior to the onset of measurable desorption at 120 K. This intermixing precedes and inhibits crystallization. Subsequent desorption of the film is described quantitatively by a kinetic model describing evaporation from a continuously remixing ideal binary liquid solution. This occurs at temperatures below the melting point of the binary mixture indicating ideal behavior for the supercooled liquid solution. This approach provides a new method for preparing and examining deeply-supercooled solutions.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - II: Ballistic Deposition Simulations ." Journal of Chemical Physics 114(12):5295-5303. Abstract Ballistic deposition simulations of thin film growth were performed. The results of the simulations are compared to experiments of N2 adsorption by porous amorphous solid water thin films. The simulations are in qualitative agreement with the experimental observations: The porosity of the thin films is controlled by using a collimated beam to vapor deposit the films. Films with normal or near normal growth angles (q ~ 0?) are relatively dense and smooth. Films with larger growth angles are highly porous and the average pore size increases as the growth angle increases. The simulations indicate that for growth angles greater than ~70?, adsorption into the largest pores is not possible leading to the experimentally observed maximum in N2 adsorption by porous amorphous solid water at q = 70?.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - I: Experimental Results ." Journal of Chemical Physics 114(12):5284-5294. Abstract The adsorption of N2 was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of N2 adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of N2. For larger growth angles, the films are porous and adsorb large quantities of N2 with apparent surface areas as high as ~2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.
2001. "Physisorption of CO on The MgO(100) Surface." Journal of Physical Chemistry B 105(18):3747-3751. Abstract The ability to grow thin MgO(100) films of quality approaching that of vacuum cleaved MgO(100) is demonstrated using low energy electron diffraction and temperature programmed desorption of CO. The highly ordered MgO(100) surfaces are used to study adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ? 0.03 to 0.90 is observed for relative CO coverages less than 0.8 ML. This is a consequence of constant but different sticking coefficient on bare (0.47 ? 0.03) and CO covered (0.90) MgO(100) surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from the defects. At intermediate coverages (0.25 - 0.8 ML) the CO desorbs via first order desorption with the desorption energy of 17 ? 2 kJ/mol and preexponential factor of 1x10(superscript)15?2 s(superscript)-1 at 0.8 ML when the peak saturates. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above Theta = 0.8 ML the adsorption occurs on fully CO covered MgO(100) surfaces and further increase in Theta is achieved by compression of the CO layer. This results in a sharp decrease in desorption energy which upon saturation of the first CO layer (Theta = 1 ML) and formation of c(4x2) ordered phase has value of ~9 kJ/mol.
2001. "Effect of Porosity on the Adsorption, Desorption, Trapping and Release of Volatile gases by Amorphous Solid Water." Journal of Geophysical Research. Planets 106(E12):33,387-33,392. Abstract We compare the adsorption, desorption, trapping, and release of Ar, N2, O2, CO, and CH4, by dense (non-porous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques were used to control the porosity of the vapor deposited ASW thin films. Experiments where the gas species was deposited on top and underneath of dense and porous ASW were conducted. The porous films were found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas was also dependent on the porosity of the ASW film. The differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results were largely independent of the gas studied confirming that the adsorption and trapping of gases is dominated by the ASW porosity. These findings show that laboratory studies must account for the growth conditions and their effects on ASW morphology in order accurately predict the properties of astrophysical ices.
2000. "The Self-diffusivity of Amorphous Solid Water Near 150 K." Chemical Physics 258(3):291-305. Abstract Molecular beam techniques are used to create nanoscale thin films composed of different isotopes of amorphous solid water (ASW). The metastable ASW composites are then heated above the glass transition temperature, Tg, and the extent of isotopic intermixing is determined using temperature programmed desorption (TPD). The observed self-diffusion in the 150-160 K range is roughly a million-fold greater than that expected for crystalline ice. The magnitude and temperature dependence of the self-diffusivity are consistent with an amorphous solid that melts into a deeply supercooled liquid prior to crystallization. The overall temperature dependence for the diffusivity of liquid water, supercooled liquid water (238-273 K), and of ASW (150-160 K) is well described by the Vogel-Fulcher-Tamman equation. These results suggest that ASW above its Tg is a deeply supercooled metastable extension of normal liquid water prior to crystallizing near 160 K
2000. "The Effect of Underlying Substrate on the Crystallization Kinetics of Dense Amorphous Solid Water Films." Journal of Chemical Physics 112(13):5932-5941. Abstract The crystallization kinetics of thin, dense amorphous solid water films deposited on crystalline ice and Pt(111) substrates are investigated. A dramatic acceleration of the crystallization rate is observed for amorphous films on crystalline ice substrates. The acceleration originates from the absence of the nucleation process on the crystalline ice substrate which serves as a 2-dimensional nucleus for the growth of the crystalline phase. This contrasts with the crystallization on a Pt(111) substrate which proceeds via bulk nucleation and 3-dimensional growth. Activation energies for growth and nucleation are determined to be 56 and 140 kJ/mol, respectively using the apparent activation energies measured on crystalline ice and Pt(111) substrates. For amorphous films deposited on crystalline ice substrates, the crystallization rate decreases rapidly with increasing distance from the crystalline ice substrate most probably due to crystallization induced cracking of the films.
1999. "Controlling the Morphology of Amorphous Solid Water." Science 283:1505-1507. Abstract The morphology of amorphous solid water grown by vapor deposition depends strongly ont he angular distribution of the water molecules incident from the gas phase...
1999. "Substrate Induced Crystallization of Amorphous Solid Water at Low Temperatures." Journal of Chemical Physics 110(12):5489-5492. Abstract We show that N2 monolayer desorption from ice surfaces is a quantitive, highly sensitive method for following the surface crystallization kinetics at low temperatures...