Studies of molecular bonding, interactions and decomposition reactions on the (001) surface of ruthenium

The interactions of N₂, formic acid and acetone on the Ru(001) surface are studied using thermal desorption mass spectrometry (TDMS), electron energy loss spectroscopy (EELS), and computer modeling.

Low energy electron diffraction (LEED), EELS and TDMS were used to study chemisorption of N₂ on Ru(001). Adsorption at 75 K produces two desorption states. Adsorption at 95 K fills only the higher energy desorption state and produces a (√3 x √3)R30° LEED pattern. EEL spectra indicate both desorption states are populated by N₂ molecules bonded "on-top" of Ru atoms.

Monte Carlo simulation results are presented on Ru(001) using a kinetic lattice gas model with precursor mediated adsorption, desorption and migration. The model gives good agreement with experimental data. The island growth rate was computed using the same model and is well fit by R(t)^m - R(t₀)^m = At, with m approximately 8. The island size was determined from the width of the superlattice diffraction feature.

The techniques, algorithms and computer programs used for simulations are documented. Coordinate schemes for indexing sites on a 2-D hexagonal lattice, programs for simulation of adsorption and desorption, techniques for analysis of ordering, and computer graphics routines are discussed.

The adsorption of formic acid on Ru(001) has been studied by EELS and TDMS. Large exposures produce a molecular multilayer species. A monodentate formate, bidentate formate, and a hydroxyl species are stable intermediates in formic acid decomposition. The monodentate formate species is converted to the bidentate species by heating. Formic acid decomposition products are CO₂, CO, H₂, H₂O and oxygen adatoms. The ratio of desorbed CO with respect to CO₂ increases both with slower heating rates and with lower coverages.

The existence of two different forms of adsorbed acetone, side-on, bonded through the oxygen and acyl carbon, and end-on, bonded through the oxygen, have been verified by EELS. On Pt(111), only the end-on species is observed. On dean Ru(001) and p(2 x 2)O precovered Ru(001), both forms coexist. The side-on species is dominant on clean Ru(001), while O stabilizes the end-on form. The end-on form desorbs molecularly. Bonding geometry stability is explained by surface Lewis acidity and by comparison to organometallic coordination complexes.

A tale of two cobalts

There are important problems to overcome if solar energy or other renewable energy sources are to be used effectively on a global scale. Solar photons must not only be harvested and converted into a usable form, but they must also be efficiently stored so that energy is available for use on cloudy days and at night. In this work, both the energy conversion and energy storage problems are addressed. Specifically, two cobalt complexes were designed and their reactivity probed for applications in energy conversion and storage. The first chapter describes a cobalt complex that is the first example of a dimeric cobalt compound with two singly proton-bridged cobaloxime units linked by a central BO4--bridge. Using electrochemical methods, the redox properties of the dimer were evaluated and it was found to be an electrocatalyst for proton reduction in acetonitrile.

Because hydrogen gas is difficult to handle and store, the hydrogenation of CO2 and later dehydrogenation of the liquid product, formic acid, has been proposed as a hydrogen storage system. Thus, a second complex, described in chapter two, supported by a triphosphine ligand framework was used as a catalyst precursor for this key dehydrogenation step. The studies here demonstrate the efficacy of the complex as a precatalyst for the desired reaction, with good conversion of starting formic acid to CO2 and H2. In order to better understand the properties of the triphosphine cobalt complex, a synthetic procedure for substituting electron donating groups (e.g., methoxy groups) onto the ligand was investigated, yielding a novel diphosphine cobalt(II) complex.

Kinetic investigations of heterogeneously catalyzed reactions on the Ru(001) and Pt(110)-(1x2) surfaces

The initial probabilities of activated, dissociative chemisorption of methane and ethane on Pt(110)-(1 x 2) have been measured. The surface temperature was varied from 450 to 900 K with the reactant gas temperature constant at 300 K. Under these conditions, we probe the kinetics of dissociation via trapping-mediated (as opposed to 'direct') mechanism. It was found that the probabilities of dissociation of both methane and ethane were strong functions of the surface temperature with an apparent activation energies of 14.4 kcal/mol for methane and 2.8 kcal/mol for ethane, which implys that the methane and ethane molecules have fully accommodated to the surface temperature. Kinetic isotope effects were observed for both reactions, indicating that the C-H bond cleavage was involved in the rate-limiting step. A mechanistic model based on the trapping-mediated mechanism is used to explain the observed kinetic behavior. The activation energies for C-H bond dissociation of the thermally accommodated methane and ethane on the surface extracted from the model are 18.4 and 10.3 kcal/mol, respectively.

The studies of the catalytic decomposition of formic acid on the Ru(001) surface with thermal desorption mass spectrometry following the adsorption of DCOOH and HCOOH on the surface at 130 and 310 K are described. Formic acid (DCOOH) chemisorbs dissociatively on the surface via both the cleavage of its O-H bond to form a formate and a hydrogen adatom, and the cleavage of its C-O bond to form a carbon monoxide, a deuterium adatom and an hydroxyl (OH). The former is the predominant reaction. The rate of desorption of carbon dioxide is a direct measure of the kinetics of decomposition of the surface formate. It is characterized by a kinetic isotope effect, an increasingly narrow FWHM, and an upward shift in peak temperature with Ɵ_T, the coverage of the dissociatively adsorbed formic acid. The FWHM and the peak temperature change from 18 K and 326 K at Ɵ_T = 0.04 to 8 K and 395 K at Ɵ_T = 0.89. The increase in the apparent activation energy of the C-D bond cleavage is largely a result of self-poisoning by the formate, the presence of which on the surface alters the electronic properties of the surface such that the activation energy of the decomposition of formate is increased. The variation of the activation energy for carbon dioxide formation with Ɵ_T accounts for the observed sharp carbon dioxide peak. The coverage of surface formate can be adjusted over a relatively wide range so that the activation energy for C-D bond cleavage in the case of DCOOH can be adjusted to be below, approximately equal to, or well above the activation energy for the recombinative desorption of the deuterium adatoms. Accordingly, the desorption of deuterium was observed to be governed completely by the desorption kinetics of the deuterium adatoms at low Ɵ_T, jointly by the kinetics of deuterium desorption and C-D bond cleavage at intermediate Ɵ_T, and solely by the kinetics of C-D bond cleavage at high Ɵ_T. The overall branching ratio of the formate to carbon dioxide and carbon monoxide is approximately unity, regardless the initial coverage Ɵ_T, even though the activation energy for the production of carbon dioxide varies with Ɵ_T. The desorption of water, which implies C-O bond cleavage of the formate, appears at approximately the same temperature as that of carbon dioxide. These observations suggest that the cleavage of the C-D bond and that of the C-O bond of two surface formates are coupled, possibly via the formation of a short-lived surface complex that is the precursor to to the decomposition.

The measurement of steady-state rate is demonstrated here to be valuable in determining kinetics associated with short-lived, molecularly adsorbed precursor to further reactions on the surface, by determining the kinetic parameters of the molecular precursor of formaldehyde to its dissociation on the Pt(110)-(1 x 2) surface.

Overlayers of nitrogen adatoms on Ru(001) have been characterized both by thermal desorption mass spectrometry and low-energy electron diffraction, as well as chemically via the postadsorption and desorption of ammonia and carbon monoxide.

The nitrogen-adatom overlayer was prepared by decomposing ammonia thermally on the surface at a pressure of 2.8 x 10^(-6) Torr and a temperature of 480 K. The saturated overlayer prepared under these conditions has associated with it a (√247/10 x √247/10)R22.7° LEED pattern, has two peaks in its thermal desorption spectrum, and has a fractional surface coverage of 0.40. Annealing the overlayer to approximately 535 K results in a rather sharp (√3 x √3)R30° LEED pattern with an associated fractional surface coverage of one-third. Annealing the overlayer further to 620 K results in the disappearance of the low-temperature thermal desorption peak and the appearance of a rather fuzzy p(2x2) LEED pattern with an associated fractional surface coverage of approximately one-fourth. In the low coverage limit, the presence of the (√3 x √3)R30° N overlayer alters the surface in such a way that the binding energy of ammonia is increased by 20% relative to the clean surface, whereas that of carbon monoxide is reduced by 15%.

A general methodology for the indirect relative determination of the absolute fractional surface coverages has been developed and was utilized to determine the saturation fractional coverage of hydrogen on Ru(001). Formaldehyde was employed as a bridge to lead us from the known reference point of the saturation fractional coverage of carbon monoxide to unknown reference point of the fractional coverage of hydrogen on Ru(001), which is then used to determine accurately the saturation fractional coverage of hydrogen. We find that ƟSAT/H = 1.02 (±0.05), i.e., the surface stoichiometry is Ru : H = 1 : 1. The relative nature of the method, which cancels systematic errors, together with the utilization of a glass envelope around the mass spectrometer, which reduces spurious contributions in the thermal desorption spectra, results in high accuracy in the determination of absolute fractional coverages.

Rydberg states of diatomic and polyatomic molecules using model potentials

A simple model potential is used to calculate Rydberg series for the molecules: nitrogen, oxygen, nitric oxide, carbon monoxide, carbon dioxide, nitrogen dioxide, nitrous oxide, acetylene, formaldehyde, formic acid, diazomethane, ketene, ethylene, allene, acetaldehyde, propyne, acrolein, dimethyl ether, 1, 3-butadiene, 2-butene, and benzene. The model potential for a molecule is taken as the sum of atomic potentials, which are calibrated to atomic data and contain no further parameters. Our results agree with experimentally measured values to within 5-10% in all cases. The results of these calculations are applied to many unresolved problems connected with the above molecules. Some of the more notable of these problems are the reassignment of states in carbon monoxide, the first ionization potential of nitrogen dioxide, the interpretation of the V state in ethylene, and the mystery bands in substituted ethylenes, the identification of the R and R’ series in benzene and the determination of the orbital scheme in benzene from electron impact data.

Investigations of nonhistone chromosomal proteins

The major nonhistone chromosomal proteins (NHC proteins) are a group of 14-20 acidic proteins associated with DNA in eukaryotic chromatin. In comparisons by SDS gel electrophoresis (molecular weight sieving) one observes a high degree of homology among the NHC protein fractions of different tissues from a given species. Tissue-specific protein bands are also observed. The appearance of a new NHC protein, A, in the NHC proteins of rat liver stimulated to divide by partial hepatectomy and of rat ascites cells suggests that this protein may play a role in preparing the cell for division. The NHC proteins of the same tissue from different species are also very similar. Quantitative but not qualitative changes in the NHC proteins of rat uterus are observed on stimulation (in vivo) with estrogen. These observations suggest that the major NHC proteins play a general role in chromatin structure and the regulation of genome expression; several may be enzymes of nucleic acid and histone metabolism and/or structural proteins analogous to histones. One such enzyme, a protease which readily and preferentially degrades histones, can be extracted from chromatin with 0.7 N NaCl.

Although the NHC proteins readily aggregate, they can be separated from histone and fractionated by ion exchange chromatography on Sephadex SE C-25 resin in 10 M urea-25% formic acid (pH 2.5). Following further purification, four fractions of NHC protein are obtained; two of these are single purified proteins, and the other two contain 4-6 and 4-7 different proteins. These NHC proteins show a ratio of acidic to basic amino acids from 2.7 to 1.2 and isoelectric points from apparently less than 3.7 to 8.0. These isolated fractions appear more soluble and easier to work with than any whole NHC protein preparation.