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Issues concerning the elementary steps which are important in hydrocarbon oxidation are reported. This research used oxidized Mo(110).

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed 16O with H218O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable—up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H218O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water. [ABSTRACT FROM AUTHOR]

We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF[sub 6] produces an absorptance of 0.9 for radiation in the 1.2-2.5 µm wavelength range, higher than any of the other gases. Use of Cl[sub 2], N[sub 2], or air produces surfaces with absorptances intermediate between that for microstructures formed in SF[sub 6] and that for flat crystalline silicon, for which the absorptance is roughly 0.05-0.2 for a 260 µm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration. [ABSTRACT FROM AUTHOR]

We report density functional theory calculations, using pseudopotentials with a plane wave basis, of methyl adsorbed on the MoO[sub 3](010) surface at 0.5 and 1.0 ML coverages. The MoO[sub 3] surface is modeled by a one-layer slab. Methyl is adsorbed most strongly over the terminal oxygen, where methoxy is formed. Over the bridging oxygens, qualitatively different results are obtained depending on the coverage of methyl. At 0.5 ML, methoxy is formed over the bridging oxygens, with an associated relaxation of the surface caused by a repulsion between the methyl and the terminal oxygen. For the 1.0 ML coverage, methyl is not stable and decomposes. Over the asymmetric bridging oxygen, methyl reacts to form formaldehyde and hydroxyl, while over the symmetric bridging oxygen a formyl species and water are produced. The hydroxyl and water are formed via hydrogen transfer to a terminal oxygen. The bonding and reaction of methyl over the various oxygen species are analyzed and discussed using charge density difference plots as well as the crystal orbital overlap projection (COOP). For the 0.5 ML methyl coverage, this analysis shows that the tilting of the terminal oxygen bond weakens this bond slightly. For the 1.0 ML methyl coverage, the COOP plots indicate that the decomposition of methyl over the bridging oxygens is not as stable as methyl over the terminal oxygen because the presence of the carbon fragments affects the electronic structure, and causes a weakening of the Mo-terminal oxygen bond. These results are discussed in the context of partial oxidation reactions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Shows that the structure and morphology of cobalt islands deposited on clean, sulfur- and oxygen-covered molybdenum depends on the nature of the interface. Use of scanning tunneling microscopy and low energy electron diffraction; Square lattice on the sulfur-covered surface; Microscopic bonding differences at the interface level related to the degree of charge transfer.

The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are observed at a binding energy of 282.8 eV, indicating that the kinetics for C–C and C–H bond activation in this system are similar. Decomposition to surface C and gaseous H2 is complete by 700 K. At 200 K, near-edge x-ray absorption fine structure measurements confirm the presence of molecular benzene in an orientation with the C6 plane parallel to the surface. After annealing to 450 K, the NEXAFS is complicated by the admixture of absorption structures from a surface intermediate and atomic carbon. Comparison of this NEXAFS with that for surface benzyne (C6H4) produced by annealing benzenethiol (C6H5SH) adsorbed on Mo(110) to 600 K suggests that the intermediate formed from benzene is C6H4 as well. [ABSTRACT FROM AUTHOR]

Chemisorbed phenyl thiolate (C6H5S) and surface benzyne (C6H4) formed during the temperature programmed reaction of benzenethiol on Mo(110) have been characterized using x-ray photoelectron and high resolution electron energy loss spectroscopies. Electron energy loss spectroscopy demonstrates that at all exposures the S–H bond of benzenethiol breaks upon adsorption at 120 K, while x-ray photoelectron spectroscopy confirms that the C–S bond is intact. The intermediate formed upon adsorption is assigned as a chemisorbed phenyl thiolate. At high exposures, approximately 40% of the phenyl thiolate reacts by way of hydrogenolysis at 350 K to form gaseous benzene and atomic sulfur, while 60% undergoes dehydrogenation at ≊370 K to form surface benzyne and atomic sulfur. Electron energy loss spectroscopy reveals that surface benzyne is aromatic in nature, and x-ray photoelectron spectroscopy confirms that the C–S bond is no longer intact. Surface benzyne is unusually stable on the sulfided Mo(110), decomposing at 680 K to gaseous dihydrogen and atomic carbon, with an activation energy of 42 kcal/mol in the limit of high coverage. At low coverages, the phenyl thiolate decomposes on Mo(110) to surface carbon, surface sulfur, and gaseous dihydrogen, with decomposition complete below 600 K. At low coverages, no formation of gaseous benzene or surface benzyne occurs. The coverage dependence of the reaction kinetics of the phenyl thiolate on Mo(110) are attributed, in part, to a change in the structure of the adsorbed thiolate at high coverages. [ABSTRACT FROM AUTHOR]

The structures of absorbed acetonitrile and the surface intermediate resulting from the formation of an N–H bond and the cleavage of one C–H bond on W(100)-(5×1)-C under ultrahigh vacuum conditions have been investigated using high resolution electron energy loss and x-ray photoelectron spectroscopies. At temperatures below 200 K, a weakly bound molecular state and a species with a strongly perturbed C–N bond are present on the surface. The molecular state coordinates to a single tungsten atom via the nitrile nitrogen and undergoes competing desorption and reaction below 400 K. Acetonitrile is hydrogenated at the nitrile nitrogen and dehydrogenated at the methyl carbon to form a CH2CNH surface intermediate in the temperature range of 200–400 K in the competing reaction. The C–N stretching frequency for the RCNH intermediate is approximately 1400 cm-1 corresponding to a weakening of the C–N bond proposed to result from bonding of both the nitrile carbon and nitrogen to the surface. The RCNH intermediate reacts further above 400 K. Gaseous acetonitrile is reformed in the range of 400–600 K with competing irreversible decomposition yielding gaseous H2, HNC( HCN) and surface carbon and nitrogen. The acetonitrile formation is proposed to result from an intramolecular isomerization of the CH2CNH on the surface. Only atomic carbon and nitrogen are present on the surface after heating to 700 K. [ABSTRACT FROM AUTHOR]

The adsorption and reaction of n-butane have been investigated on clean and carbided W(100) surfaces using temperature programmed reaction spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and isotopic exchange methods. For the clean surface, the initial adsorption probability is 1.0 at a crystal temperature of 130 K. At low coverages, butane adsorbed at 130 K converts to a dissociated state with unit probability below 200 K during temperature programmed reaction. Higher exposures result in a decrease in the adsorption probability and the onset of molecular butane desorption in the range of 150–200 K. A weakly adsorbed molecularly bound butane state is observed for all coverages on the [[sub 1][sup 3] [sub -1][sup 0]] and (5×1) carbide surfaces. Studies using n-butane-d10 show a dramatic isotope effect. The low coverage dissociation probability of n-C4D10 is significantly less than unity and the saturation amount of decomposition in a temperature programmed reaction experiment is less than for n-C4H10. This is evidence for a kinetic isotope effect in the decomposition process, with the activation barrier for C–D bond scission greater than the barrier for C–H bonds. For butane exposures made above 200 K, the initial sticking probability is lowered, and the saturation value for decomposition is also decreased. Preadsorption of deuterium suppresses greater than 90% of the butane decomposition, while postadsorption of D2 under the same conditions blocks less than 5%. These observations suggest that for low butane exposures, there is a substantial amount of dehydrogenation upon adsorption which eventually leads to complete decomposition. As the crystal temperature is increased, buildup of surface hydrogen and possibly carbon from further decomposition rapidly inhibits dissociation of additional butane and results in molecular desorption. A limited amount of butane decomposition occurs on the intermediate coverage... [ABSTRACT FROM AUTHOR]

The adsorption and decomposition of Fe(CO)5 on the Si(111)-(7×7) surface has been investigated using multiple internal reflection Fourier transform-infrared, Auger electron, and temperature programmed desorption spectroscopies and low energy electron diffraction under ultrahigh vacuum conditions at 120 K. Resonant decomposition of adsorbed Fe(CO)5 via a multiphoton electronic excitation of the molecule was observed using laser photolysis. Iron deposition was induced efficiently by ultraviolet but not visible photons with high cross sections, in other words, only in the wavelength region where excitation of metal–ligand charge transfer bands and d–d transitions occur. In addition, Fe(CO)5 decomposition was induced using 1.6 keV electrons. No thermal reaction was apparent in temperature programmed desorption experiments. Only molecular Fe(CO)5 desorption was observed at temperatures of 150 and 170 K. Significant amounts of carbon were deposited from the electron induced decomposition while no residual oxygen or carbon were detected in the photodeposited Fe. No partially decarbonylated Fe(CO)x, x<5, fragments were detected subsequent to exposure to ultraviolet photons using infrared spectroscopy. In addition, no new features were observed in a temperature programmed desorption experiment after laserphotolysis. These data suggest that there are no surface stable Fe(CO)x, x<5, species in the photodeposition process. Instead, photolysis yields adsorbed Fe atoms without trapping of iron carbonyl fragments, even at low temperatures. Multiple photons were required to induce the Fe(CO)5 decomposition based on the fluence dependence of the photodecomposition yield. These data are consistent with gas phase photodecomposition energetics. The two mechanisms proposed are rapid sequential ejection of CO into the gas phase with loss of the first CO being rate limiting or simultaneous ejection of all CO. An unresolved low frequency shoulder did appear in the... [ABSTRACT FROM AUTHOR]

The adsorption structure of carbon monoxide on W(100)-(5×1)–C has been characterized using high-resolution electron energy loss spectroscopy and near edge x-ray absorption fine structure measurements. A single C–O stretch of 2100 cm-1 is observed at a surface temperature of 130 K. The C–O bond vector is determined to be normal to the surface on the basis of near edge x-ray absorption fine structure measurements. The position in energy of the C(1s)→σ* resonance in CO was measured to be 304.2 eV. The empirical relation between the σ* resonance energy and bond length yields a bond length of 1.12 Å. All spectroscopic results are consistent with CO bound atop a single tungsten atom, normal to the surface plane with minimal C–O bond weakening. These results correlate with the known difference in the reactivity of the W(100)-(5×1)–C surface compared to clean W(100). The increased barrier for CO dissociation on the W(100)-(5×1)–C surface is explained on the basis of reduced backdonation of electron density from the tungsten d band to the CO 2π* orbitals. Tight-binding slab calculations for CO on W(100) and W(100)-(5×1)–C which support this contention are discussed. [ABSTRACT FROM AUTHOR]

The structure and reactivity of methoxide adsorbed on Mo(110) was investigated using temperature programmed reaction, x-ray photoelectron, high resolution electron energy loss and infrared reflection absorption spectroscopies. Methanol decomposes through a methoxy surface intermediate on Mo(110), with dehydrogenation and carbon–oxygen bond scission occurring at ∼400 K. The structure of the methoxy moiety is dependent on coverage, by reference to data obtained using surface infrared spectroscopy in combination with selective isotopic labeling. We demonstrate that methoxy exhibits C3v symmetry, i.e., the C–O bond is normal to the surface, at coverages below 0.17 ML. However, the C–O axis begins to tilt towards the surface at higher coverages, so that at saturation coverage (0.25 ML), two distinct methoxy species with Cs symmetry are observed with an average tilt angle of 25°±15° from the surface normal. In addition, we conclusively show that the intense features at ∼2910 cm-1 in the infrared spectrum of adsorbed methoxide are due to overtones of the methyl deformation modes, gaining intensity by Fermi resonance with the symmetric carbon–hydrogen stretching mode, in contrast to previous assignments to the in-plane asymmetric C–H stretch which, we demonstrate, occurs at above 2935 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactions of 2-propanol on Mo(110) were investigated using temperature programmed reaction, high resolution electron energy loss, and x-ray photoelectron spectroscopies. 2-Propanol forms 2-propoxide upon adsorption at 120 K on Mo(110). The 2-propoxide intermediate deoxygenates via selective γ C–H bond scission to eliminate propene as well as C–O bond hydrogenolysis to form trace amounts of propane. The C–O bond of 2-propoxide is estimated to be nearly perpendicular to the surface. Selective isotopic labeling was used to establish the coupling between the C–O stretch and modes associated with the hydrocarbon framework. The degree of coupling was strongly affected by bonding to the surface, primarily due to weakening of the C–O bond when 2-propoxide is bound to Mo(110). Selective isotopic labeling was, therefore, essential in making vibrational assignments and in identifying key reaction steps. Only a small kinetic isotope effect was observed during reaction of (CD3)(CH3)CHOH, consistent with a substantial component of C–O bond breaking in the transition state for propene elimination. Coupling of the C–O stretch to motion of the methyl group is also suggested to be important in the transition state for propene elimination. [ABSTRACT FROM AUTHOR]