O-covered Surface Research Articles

T-Butyl Nitrite (TBN) and t-Butyl Alcohol (TBA) Reactions on Clean and O-Covered Rh

The reactions of t-butyl nitrite (TBN) and t-butyl alcohol (TBA) on clean and O-covered Rh foils were investigated using time-of-flight mass spectrometry temperature-programmed desorption and Auger electron spectroscopy. Although t-butoxy groups are formed when TBN and TBA dissociate on clean Rh and when TBA dissociates on O-covered Rh, there are significant differences in the thermal behaviors of these systems. For TBN on clean Rh, there is evidence that a monolayer decomposes to form NO and two oxygen-containing fragments, t-butoxy and a stabilized oxametallacycle. The proposed oxametallacycle decomposes at 350 K to acetone, whereas t-butoxy, relatively stable in the presence of N and NO, decomposes to isobutene at 500 K. For TBA dosed onto an O-covered surface, the OH bond breaks between 200 and 300 K to form t-butoxy. This occurs in competition with TBA desorption. The t-butoxy is stable only up to 380 K where, assisted by O, it decomposes to acetone and butene via a transient form of the oxametallacycle involved in TBN decomposition. Combustion reactions to form water, carbon monoxide, and carbon dioxide compete. For a monolayer of TBA dosed onto a clean Rh surface, H2, H2O, and CO, but no acetone or butene, are observed TPD, and carbon remains after TPD.

Potential intermediate in CH 4-assisted reduction of nitric oxide: the reactions of methyl nitrite on oxidized Mo(1 1 0)

The reactions of methyl nitrite (CH 3ONO) on O-covered Mo(1 1 0) were studied in order to determine whether methyl nitrite is a possible intermediate in CH 4-assisted reduction of NO. Methyl nitrite decomposes to NO and methoxy (CH 3O ads) upon adsorption at low temperature. Most NO is evolved directly into the gas phase at 110 K, although a small fraction is trapped on the surface and is identified using infrared spectroscopy. Methoxy remains on the surface until ∼600 K, at which point it decomposes to gas phase • CH 3 and adsorbed oxygen. Hence, the overall reaction is the reverse of the • CH 3+ NO reaction. However, no reduction products (N 2 or N 2O) are evolved. The absence of reduction may be due to the fact that all high coordination sites are occupied on the O-covered surface or to the low coverage of NO. These results are discussed in the context of • CH 3-assisted NO reduction.

Alkoxide Synthesis from Methyl and tert-Butyl Nitrite on Cu(100)

A one-step synthesis of adsorbed alkoxides from alkyl nitrites dosed on Cu(100) is illustrated using methyl nitrite (CH3ONO) to form methoxide (CH3O) and tert-butyl nitrite (t-C4H9ONO) to form tert-butoxide (t-C4H9O). Compared with an alternative, dosing CH3OH on an O-covered Cu surface, only minor differences in the concentration and local chemical environment of CH3O are evident. The potential energy profiles associated with the two synthesis methods are contrasted, and the key role played by the weak RO−NO bond is emphasized.

SURFACE STRUCTURE OF O/Cu(104) FACETS DETERMINED BY X-RAY DIFFRACTION

We have determined the surface structure of O/Cu(104) using X-ray diffraction. This surface was prepared by dosing Cu(115) with oxygen, transforming the clean surface into facets with {104} and {113} orientations. This method of preparation, in essence, naturally grows the (104)-oriented substrate concurrent with the O-covered surface, resulting in O/Cu(104) facets which are smooth and highly ordered. Our results indicate that the top three atomic rows significantly expand away from the bulk, but no Cu rows are missing. The Cu–O structures of this surface are similar to those present on other O on Cu surface reconstructions, but the adsorbed O inhabits two adsorption sites with notably distinct geometries. The relationship between the O/Cu(104) and O/Cu(001)[Formula: see text] structures, in particular, is discussed.

Macroscopic and mesoscopic characterization of a bistable reaction system: CO oxidation on Pt(111) surface

The catalytic oxidation of CO by oxygen on a platinum (111) single-crystal surface in a gas-flow reactor follows the Langmuir–Hinshelwood reaction mechanism. It exhibits two macroscopic stable steady states (low reactivity: CO-covered surface; high reactivity: O-covered surface), as determined by mass spectrometry. Unlike other Pt and Pd surface orientations no temporal and spatiotemporal oscillations are formed. Accordingly, CO+O/Pt(111) can be considered as one of the least complicated heterogeneous reaction systems. We measured both the macroscopic and mesoscopic reaction behavior by mass spectrometry and photoelectron emission microscopy (PEEM), respectively, and explored especially the region of the phase transition between low and high reactivity. We followed the rate-dependent width of an observed hysteresis in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust conditions of the external control parameters which totally inhibited the motion of the reaction/diffusion front. By systematic variation of these conditions we could pinpoint a whole region of external control parameters in which the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, our experiments and simulation reveal the existence of an “experimental” bistable region inside the “computed” bistable region of the reactivity diagram (S-shaped curve) leading to a novel dollar ($)-shaped curve.

Direct multiquantum relaxation of highly vibrationally excited NO in collisions with O/Cu(111)

We present a new experimental approach to the study of vibrational relaxation of highly vibrationally excited molecules at solid surfaces. We observe NO in v=13 and 15 losing up to five vibrational quanta in collisions with an O-covered copper surface. The data indicate this vibrational relaxation occurs via a direct scattering mechanism.

Influence of Surface Modifiers on the Thermal Decomposition of Methanethiol on Fe(110)

The reactions of methanethiol (CH3SH) on clean, sulfur-covered, and oxygen-covered Fe(110) have been investigated by X-ray photoelectron spectroscopy, temperature-programmed reaction spectroscopy, and low-energy electron diffraction. On clean Fe(110), the S−H bond in methanethiol breaks below 100 K, affording methylthiolate (CH3Sa) and Ha. Heating to 600 K results in C−S bond cleavage near 290 K, leading to the evolution of methane and dihydrogen, and a p(2 × 2) sulfur overlayer. On the oxygen-covered surface (θO = 0.25 ML), adsorbed methylthiolate and hydroxyl are formed. The amount of irreversible thiol reaction is essentially the same on the O-covered and clean Fe(110) surfaces. The amount of irreversibly bound thiol is reduced by approximately half on the (2 × 2)−S overlayer (θS = 0.25 ML). The C−S bond in methylthiolate is stabilized more by surface sulfur than by surface oxygen, based on the methane formation temperature shifts of +55 and +20 K, respectively. Oxygen coverages greater than 1 monolayer form an FeO thin film, weakening the interaction of methanethiol with the surface, further reducing the methane yield which now reacts above 400 K.

Electron affinity and Schottky barrier height of metal–diamond (100), (111), and (110) interfaces

The electron emission properties of metal–diamond (100), (111), and (110) interfaces were characterized by means of UV photoemission spectroscopy (UPS) and field-emission measurements. Different surface cleaning procedures including annealing in ultrahigh vacuum (UHV) and rf plasma treatments were used before metal deposition. This resulted in diamond surfaces terminated by oxygen, hydrogen, or free of adsorbates. The electron affinity and Schottky barrier height of Zr or Co thin films were correlated by means of UPS. A negative electron affinity (NEA) was observed for Zr on any diamond surface. Co on diamond resulted in NEA characteristics except for oxygen-terminated surfaces. The lowest Schottky barrier heights were obtained for the clean diamond surfaces. Higher values were measured for H termination, and the highest values were obtained for O on diamond. For Zr, the Schottky barrier height ranged from 0.70 eV for the clean to 0.90 eV for the O-terminated diamond (100) surface. Values for Co ranged from 0.35 to 1.40 eV for clean- and O-covered (100) surfaces, respectively. The metal-induced NEA proved to be stable after exposure to air. For the oxygen-terminated diamond (100) surface a field-emission threshold of 79 V/μm was measured. Zr or Co deposition resulted in lower thresholds. Values as low as 20 V/μm were observed for Zr on the clean diamond (100) surface. Results for Zr or Co on H- or O-terminated surfaces were higher. H or O layers on diamond tend to cause an increase in the Schottky barrier height and the field-emission threshold field of Zr– and Co–diamond interfaces. The value of the electron affinity and Schottky barrier were correlated with work function and different initial surface preparation. The results were largely consistent with a model in which the vacuum level was related to the metal work function and the measured Schottky barrier.

Mechanism of efficient carbon monoxide oxidation at Ru(0001)

We performed density-functional theory calculations using the generalized gradient approximation for the exchange-correlation functional to investigate the unusual catalytic behavior of Ru under elevated gas pressure conditions for the carbon monoxide oxidation reaction, which includes a particularly high CO2 turnover. Our calculations indicate that a full monolayer of adsorbed oxygen actuates the high rate, enabling CO2 formation via both scattering of gas-phase CO molecules as well as by CO molecules adsorbed at oxygen vacancies in the adlayer, where the latter mechanism is expected to be very efficient due to the relatively weak adsorption energy of both CO and O, as well as the close proximity of these reactants. We analyze the bonding and electronic properties associated with the reaction pathway for CO2 production via the scattering reaction. We find that the identified “bent” transition state is due to electron transfer into the unoccupied 2π orbitals of the CO molecule which reduces the Pauli repulsion between the impinging CO and the O-covered surface. Bond formation to CO2 then proceeds by electron transfer back from the CO 2π orbitals into the bonding region between CO and the adsorbed O atom.

The interaction of oxygen and hydrogen on a diamond C(111) surface: a synchrotron radiation photoemission, LEED and AES study

We report an investigation on the adsorption of atomic O and coadsorption with H on a diamond C(111) single crystal surface employing the core and valence level synchrotron radiation photoemission (SRPES) technique, Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Additional emission bands in the valence spectrum and a shoulder peak to the higher binding energy of the C 1s core level are found after oxygen exposure in the annealed C(111)-(2 × 1) reconstructed surface, indicating the formation of CO bonds on the surface. The CO reaction, however, does not convert the (2 × 1) reconstructed phase to the bulk-truncated (1 × 1) structure. Even at a saturation coverage of O, the LEED pattern shows the (2 × 1) structure, and the Auger and the photoemission spectra exhibit the characteristics of the reconstructed surface. This suggests a bonding of the O atoms to the (2 × 1) Π-bonded chain. The O-covered surface exposed to H atoms can drive the O from the surface, where it is replaced by H. Only a small amount of O is left on the surface. AES, LEED and SRPES data show the typical features of an H-terminated (1 × 1) state for the H/O/C(111) coadsorbed surface.

Thermal and Photoinduced Desorption and Decomposition of Fe(CO)5 on Clean and Oxygen-Modified Ru(001)

The thermal and photoinduced desorption and decomposition of Fe(CO)5 on clean and O-covered Ru(001) surfaces were studied. Adsorption of Fe(CO)5 on Ru(001) is associated with a partial decomposition, resulting in the formation of CO and Fe(CO)x fragments. In the thermal desorption spectrum for mass 28, the surface-stabilized decomposition product gives rise to a peak at 270 K, whereas the first and second monolayers of molecular Fe(CO)5 desorb at 190 and 160 K, respectively. The photochemical studies of Fe(CO)5 at mono- and multilayered molecular coverages on Ru(001) were carried out by UV irradiation at various wavelengths (290−450 nm). Irradiation at wavelengths > 370 nm resulted in photodesorption, while photodecomposition showed significant contribution at shorter wavelengths. The total cross sections for the photochemical process closely follow the UV absorption spectrum of Fe(CO)5 in the gaseous phase, suggesting that the photoreaction is mainly due to the direct absorption of UV photons by the adsorbed Fe(CO)5 molecule. The photodecomposition yields reactive intermediates that subsequently form Fex(CO)y clusters. These species thermally decompose, desorbing the CO moieties and depositing Fe atoms on the surface. Dissociative adsorption of Fe(CO)5 has also been observed on O-covered Ru(001). O adatoms create inhomogeneity in the adsorption sites for Fe(CO)5, as seen from the broadened desorption peak of the first molecularly adsorbed layer of Fe(CO)5. In contrast to the clean surface, irradiation of Fe(CO)5 adsorbed on O/Ru(001) at 290 nm produced relatively lower yields of photodecomposition and a higher extent of photodesorption attributable to the more effective quenching of electronically excited Fe(CO)5 by the O-covered surface.

Promotion and poisoning of the reaction of methanol on clean and modified platinum (100)

The reactivity of Pt(100) towards methanol depends on surface structure and the nature and amount of coadsorbed reaction modifiers. We studied the adsorption and reaction of CH 3OH in ultrahigh vacuum on the clean hex and (1 × 1) surfaces of Pt(100) and with coadsorbed O, Bi, and CO with thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. Methanol adsorbs molecularly on Pt(100) at 100 K and decomposes to adsorbed H and CO in the temperature range of 170–220 K. The vibrational spectra show evidence for surface interactions involving both the hydroxyl (O-H … Pt) and methyl (C-H … Pt) hydrogens of molecular methanol. Under static conditions (no desorption of reaction products) as much as 0.085 ± 0.01 monolayers of methanol react on the clean hex surface. Chemical saturation of the surface, which occurs when the total coverage of surface reaction products reaches about one-half of a monolayer, limits the maximum extent of reaction on the hex, (1 × 1), and O-covered surfaces. The (1 × 1) surface enhances methanol reaction by factors of 1–3.4 compared to reaction following the same exposure on the hex surface. Reaction with preadsorbed oxygen leads to a methoxy intermediate (CH 3O) and enhancement factors of 1–2.5 for oxygen coverages below 0.15 and above 0.3 monolayers. Coadsorbed oxygen enhances methanol reaction stoichiometrically, by scavenging H atoms to form OH and H 2O, and nonstoichiometrically, most likely by local lifting of the hex reconstruction. Oxygen coverages between 0.15 and 0.3 monolayers inhibit the reaction by factors of 1—0.85 through blocking of reactive sites. Both Bi and CO inhibit the reaction by factors of 1—0.05 through a combination of ensemble and site blocking effects. Methanol decomposition requires an ensemble of 5 adsorption sites, as determined by Bi coadsorption experiments. The adsorbate/substrate combinations in this study represent model electrochemical situations, and we briefly discuss the implications of these findings toward methanol electrocatalysis.

Measurement of the effect of pretreatment and adsorption on the electrical properties of ZnO powders using a microwave-Hall-effect technique.

Microwave-Hall-effect (MHE) and electrical conductivity measurement techniques can now be used to obtain absolute values of the electrical properties of semiconductor powders under different controlled conditions. A commercial ESR spectrometer was modified to conduct MHE experiments and a network analyzer was utilized to tune the microwave cavity and obtain Q factors. A sample of ultrahigh-purity ZnO powder (30 ${\mathrm{m}}^{2}$/g) was prepared and studied. Evacuation of ZnO at 673 K decreased the electron Hall mobility but increased the conductivity and electron density at 300 K, and typical values for this high-surface-area powder were \ensuremath{\mu}=4 ${\mathrm{cm}}^{2}$/V s, \ensuremath{\sigma}=0.05 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and ${\mathit{N}}_{\mathit{e}}$=2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$/g. Oxygen chemisorption at 300 K increased the mobility but decreased the conductivity and electron density, and after this step typical values were \ensuremath{\mu}=50 ${\mathrm{cm}}^{2}$/V s, \ensuremath{\sigma}=0.001 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and ${\mathit{N}}_{\mathit{e}}$=2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$/g. These changes were reversible. The \ensuremath{\mu} values varied more than an order of magnitude; therefore, the assumption of constant mobilities cannot be made safely even at these low carrier densities. Three different ZnO powders were studied to observe the effect of surface area (i.e., particle size) on electrical properties.Lattice scattering was the dominant mechanism determining electron mobility for large particles, where the particle radius was much greater than the electron mean free path; however, as the ZnO particle size approached that of the estimated electron mean free path, defect scattering in the surface region became the dominant controlling mechanism. On an evacuated ZnO surface, ${\mathrm{CO}}_{2}$ adsorption had a significant effect on the electrical properties while CO adsorption had little effect, whereas on an O-covered ZnO surface CO adsorption had a noticeable effect but ${\mathrm{CO}}_{2}$ adsorption produced a much smaller change. Hydrogen adsorption on either surface gave little change in electrical properties.

Oxygen adsorption on Ni3Al(001) by low-energy ion beams

The interaction of O with the Ni3Al(001) surface has been studied using low-energy He+ scattering. Oxygen adsorption at an exposure of 10 L and a temperature of 700°C results in the total disappearance of the Ni ISS signal, indicating the formation of a monolayer AlOx overlayer. This oxide AlOx overlayer has been examined by measuring the azimuthal angle dependence of the scattered ion yield at fixed scattering angle under different coverages of oxygen. The azimuthal variations of the Ni and Al ISS signals from the O-covered surface are compared with those of Ni and Al from the clean surface, and the results used to derive a value for the fractional coverage of oxygen on the surface. The analysis supports the model that the oxide initially grows in the form of islands, with no influence on remaining areas of the crystal. No evidence of the ordering of the oxide islands was observed. Further evidence of the growth of the AlOx surface. The results indicate that the remaining (uncovered) Ni3Al surface retains the crystal structure of a clean surface. The rate of growth of the AlOx islands on the Ni/3Al(001) surface was also found to be related to temperature.

Dynamic behavior of a Pt 0.25Rh 0.75(100) single crystal surface during NO + H 2 reaction

The catalytic reduction of NO by H 2 on a Pt 0.25Rh 0.75(100) alloy single crystal was investigated by monitoring the surface species while the reaction was proceeding. Adsorption of NO at 500 K formed an O-covered surface with p(3 × 1) structure. Enrichment of Rh on the surface induced by adsorption of O was observed, and a Rh-rich alloy surface forms the p(3 × 1)-O faster. The nature of the p(3 × 1) overlayer is discussed. The NO + H 2 reaction formed a c(2 × 2) overlayer of N(a). The hydrogenated adsorbate NH x (a) was observed under the presence of gas-phase H 2. The hydrogenation reaction of N(a) to NH x (a) was reversible, and faster than complete hydrogenation to NH 3(g).

Effect of adsorbate proximity on surface reactions: Synthesis and decomposition of the formate intermediate in UHV from coadsorbed CO, H2O, and O on Rh(100)

The coadsorption of CO, H2O, and O on the Rh(100) surface has been studied using temperature programmed electron energy loss spectroscopy (TP-EELS), temperature programmed reaction spectroscopy (TPRS), and low energy electron diffraction (LEED). Following exposure at 90 K both H2O and CO are adsorbed without decomposition on the O-covered surface. As the temperature is increased to between 150 and 200 K, TP-EELS reveals that hydroxyl (OH) is formed (with bending mode at 114 meV and stretching mode at 394 meV) and disappears as gas phase water is evolved at 260 K. Beginning at 220 K and continuing to 260 K, two new modes develop at 94 and 164 meV which we identify as the scissor [δ(OCO)] and symmetric stretch [νs(OCO)] modes of bidentate formate (HCOO). TP-EELS and TPRS establish that the formate decomposes near 290 K with gas phase CO2 and H2O as products. Based on EELS intensity vs temperature, the kinetics of formate production (activation energy E=8±2 kcal⋅mol−1, and preexponential ν=103±2 cm2⋅s−1) and decomposition (E=26±3 kcal⋅mol−1, ν=1017±2 s−1) were determined. The effect of varying the initial reactant concentrations on the rate of formate production indicates that adsorbate concentrations high enough to force CO and OH into adjacent sites are required for the reaction to proceed. The role of preadsorbed O is both to facilitate production of OH through reaction with coadsorbed H2O and to help provide the crucial crowding of the surface necessary for HCOO formation.

Reactions of azomethane on a clean, H-covered, and O-covered Pt(111) surface

The reactions of azomethane were studied on a clean, H-covered, and O-covered Pt(111) surface at different coverages of azomethane, and ratios of hydrogen or oxygen to azomethane, by temperature programmed desorption. On a clean surface, dehydrogenetion, together with breaking of the NN bond to produce HCN and H 2 were the major reactions. Small amounts of methylamine and cyanogen were also observed. On an H-covered surface at high hydrogen coverage, methane was the dominant product, in addition to H 2. This adsorbed hydrogen promoted breaking of the CN bond. In addition, the dehydrogenation product HCN was also observed, as was methylamine. On the O-covered surface, CO, CO 2, H 2O, and small amounts of NO were observed together with the products typical for a clean surface. Comparison of the results with those for acetylene, ethylene, and diazomethane is made.

Temperature programmed desorption study of acetylene on a clean, H-covered, and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH 4, C 2H 6, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO 2, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.

Reactions of diazomethane on a clean, H-covered and O-covered Pt(111) surface

The reactions of diazomethane on a clean, a H-covered, and an O-covered Pt(111) surface were studied by temperature programmed desorption. Methane, ethylene, and hydrogen were the desorption products for the clean surface. The same products were observed together with nitrogen for the H-covered surface. Comparison of the desorption profiles on the clean and H-covered surfaces with those for ethylene and acetylene showed that methane was formed from the hydrogenation of surface methylene species that were formed by the dissociation of diazomethane on adsorption. Some of the surface methylene species also combined to form ethylene before undergoing further reaction. On the O-covered surface, products characteristic of the clean surface were observed together with the oxidation products water, CO and CO 2. Some of the water was formed by the reaction of hydrogen released in the decomposition of methylene. The remaining water, CO, and CO 2 were formed by the oxidation of an intermediate of stoichiometry CH, similar to the oxidation of ethylene and acetylene.

Atom superposition and electron delocalization (ASED) theory for catalysis. Dissociative properties of acetylene on Fe and Ni(100) with coadsorbed O, S, Se and implications for Te

Using an atom superposition and electron delocalization (ASED) theory, acetylene is found to bond to the fourfold site on Ni(100) more strongly and with greater CC bond lengthening distortion than on Ni(111). Half-monolayer c(2 × 2) O, S, Se, and Te coverages poison the surface toward acetylene chemisorption. Quarter-monolayer (2 × 2) O and S allow weakened adsorption. A specific destabilization of an acetylene π orbital when bridging two O atoms activates CC bond scission barrier reduction. This is of catalytic significance. Increasing or decreasing the surface Ni charge, as occurs when bonding to more electronegative or electropositive atoms or on an electrode in a dielectric medium, also serves to weaken the barrier. The bonding of S and Se to Fe(100) is treated. The fourfold site is preferred because a high-lying orbital loses its unpaired electron in this site. On quarter-monolayer covered Fe(100), acetylene CC bond scission is accelerated in chalcogen bridging sites by the same wedging effect even though the chemisorption energy is weakened. Chemisorption occurs only on the half-monolayer c(2 × 2) O-covered surface if a high-lying orbital loses its unpaired electron. This possibility is discussed. The relationship of the theory and photoemission spectra is discussed.