EMSL: Zdenek Dohnalek's Publications

Cholette F, T Zubkov, RS Smith, Z Dohnalek, BD Kay, and P Ayotte. 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.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 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.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 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.

Du Y, NA Deskins, Z Zhang, Z Dohnalek, M Dupuis, and I Lyubinetsky. 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.

Kim YK, Z Dohnalek, BD Kay, and RJ Rousseau. 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.

Kimmel GA, J Matthiesen, M Baer, CJ Mundy, NG Petrik, RS Smith, Z Dohnalek, and BD Kay. 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.

Parkinson GS, Z Dohnalek, RS Smith, and BD Kay. 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.

Parkinson GS, YK Kim, Z Dohnalek, RS Smith, and BD Kay. 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.

Parkinson GS, Z Dohnalek, RS Smith, and BD Kay. 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.

Petrik NG, Z Zhang, Y Du, Z Dohnalek, I Lyubinetsky, and GA Kimmel. 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.