Scientific Publications 2005
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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. "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. "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. "Growth and Electrochemical Properties of Single-Crystalline V₂O₅ Nanorod Arrays." Japanese Journal of Applied Physics Part 1 - Regular Papers Short Notes & Review Papers 44(1B):662-668 . Abstract Growth and electrochemical properties of single-crystalline vanadium pentoxide (V₂O₅) nanorod arrays were investigated. Vanadium pentoxide nanorod arrays were grown by electrochemical deposition, surface condensation induced by a pH change and sol electrophoretic deposition. Uniformly sized vanadium oxide nanorods with a length of about 10 µm and diameters of 100 or 200 nm were grown over a large area with near-unidirectional alignment. Transmission electron microscopy (TEM) micrographs and electron diffraction patterns of V₂O₅ nanorods clearly show the single-crystalline nature of nanorods fabricated via all three growth routes with a growth direction of [010]. The growth mechanisms of single-crystal V₂O₅ nanorods have been discussed. Electrochemical analysis revealed that nanorod array electrodes possess significantly improved storage capacity and charge/discharge rate with approximately 5 times higher applicable current density than those of sol-gel derived films. Furthermore, for a given current density, the nanorod array electrode can intercalate up to 3.5 times higher concentration of Li⁺ intercalation. The relationships between electrochemical property, nano- and microstructure, and growth mechanisms have been discussed.
2005. "Synthesis of the H-Cluster Framework of Iron-Only Hydrogenase." Nature 433(7026):610-613. Abstract The reversible reduction of protons to dihydrogen is deceptively the simplest of reactions but one which requires multi-step catalysis to proceed at practical rates. How the metal-sulfur of the hydrogenases catalyse this interconversion has been the subject has been the subject of intensive structural, spectroscopic and mechanistic studies of the enzymes, of synthetic assemblies and of in silico models. Beyond the intrinsic desire to understand how metallo-sulfur clusters in biology catalyses a range of difficult chemistry, including nitrogen fixation, research on hydrogenase chemistry is particularly driven by the view that understanding active-site structure and function will inform the design of new materials for hydrogen production or uptake, pertinent to energy transduction technology and a hydrogen economy. Herein we report the assembly of the first materials with di-iron sub-sites linked by a thiolate bridge to a {4Fe4S} – cluster, as found at the active site of the iron-only hydrogenase, the H-cluster.
2005. "Threshold displacement energies in rutile TiO2: A molecular dynamics simulation study." Nuclear Instruments and Methods in Physics Research. Section 239(3):191-201. Abstract Threshold displacement energies are determined for Ti and O in rutile TiO2 using molecular dynamics simulations with an empirical model. The simulations involve the introduction of a primary knock-on atom (PKA) with a range of energies (30- 150 eV) in various crystallographic directions at 160 K. We observe the formation of stable Frenkel defects, as well as defect recovery via low-energy interstitial migration mechanisms. The latter causes significant statistical variation between simulation outcomes, which leads to the definition of a defect formation probability. This probability is characterized as a function of PKA energy in order to define the threshold displacement energy and compare with experimental results. Using a probability of 10%, the average threshold displacement energy is around 40 eV for oxygen (comparable to experiment) and 105 eV for titanium. Using a probability of 50%, the values are 65 eV and 130 eV respectively, which may be more appropriate for use in TRIM calculations. In addition, we run a parallel set of calculations using a second empirical model, finding that the detailed results are highly model-dependent, particularly the oxygen defect structures and energies, which are compared to new ab initio data.
2005. "A Mechanism of Photo-Induced Desorption of Oxygen Atoms From MgO Nano-Crystals." Surface Science 593(1-3):210-220. Abstract In a series of recent experimental and theoretical papers we reported the results of our studies of photo-induced hyper-thermal halogen atom desorption from alkali halide surfaces. There we demonstrated that the yield, electronic state and velocity distributions of desorbed atoms can be controlled by carefully choosing parameters of photo-irradiation such as laser photon energy and pulse power [ ]. To achieve laser control over desorption process one must have clear understanding of possible desorption mechanisms and parameters responsible for their selective excitation. For alkali halides, as it has been shown through a combination of theory and experiment, such selectively is observed if the laser energy is tuned to preferentially excite surface excitons. If similar mechanisms could be demonstrated for a wider variety of materials, this approach could become a new method for controlling surface processes and hence modifying surface structures on an atomic scale. In this paper we report the first experimental observation of the hyper-thermal oxygen atom emission from an of MgO nano-clusters and thin films using frequency selected laser pulses oxide surface and investigate theoretically the mechanisms of this process. On this way we demonstrate a new concept that can be applied to studying surface reactions and desorption of binary oxides.
2005. "Is There Stereoselectivity in Spin Trapping Superoxide by 5-tert-Butoxycarbonyl-5-methyl-1-pyrroline N-oxide?" Journal of Organic Chemistry 70(18):7093-7097. doi:10.1021/jo050692f Abstract Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 5, have been reported to be robust spin traps for superoxide (O2•-). Using a chiral column, we have been able to isolate the two enantiomers of nitrone 5. With enantiomerically pure nitrone 5a and 5b we explored whether one of these isomers was solely responsible for the EPR spectrum of aminoxyl 6. Data obtained demonstrate that the spin trapping of O2•- by nitrone 5a and nitrone 5b afford the identical EPR spectra and lifetimes in homogenous aqueous solution and exhibit the same ratio of cis and trans isomers. Quantum chemical modeling in vacuo also finds no difference, aside from the expected optical activity, arising from the difference in stereochemistry.
2005. "Carbon Nanotubes Based Nanoelectrode Arrays: Fabrication, Evaluation and Application in Voltammetric Analysis." Electroanalysis 17(1):79-84. Abstract Nanoelectrode arrays were fabricated based on controlled density aligned carbon nanotubes. Nanoelectrodes were demonstrated for voltammatric analysis of drug and metal ions.
2005. "The Singlet Electronic Ground State Isomers of DialuminumMonoxide: A1OA1, A1A1O, and the Transition State Connecting Them." Journal of Chemical Physics 122(9):094304. doi:10.1063/1.1850098 Abstract An abstract is not available at this time for this article.
2005. "The Singlet Electronic Ground State Isomers of Dialuminum Monoxide: AlOAl, AlAlO, and the Transition State Connecting Them." Journal of Chemical Physics 122(9):Art No. 094304. Abstract The abstract for this product is currently not available at this time.
