Pre-adsorbed Oxygen Research Articles

Adsorption of (S)-Histidine on Cu(110) and Oxygen-Covered Cu(110), a Combined Fourier Transform Reflection Absorption Infrared Spectroscopy and Force Field Calculation Study

Adsorption of (S)-histidine on Cu(110), and Cu(110) modified by adsorbed oxygen, has been studied by reflection absorption infrared spectroscopy (RAIRS). The molecular form and the orientation of the histidine molecule were deduced from experimental RAIRS data; moreover, vibrational spectra were simulated with a molecular mechanics force field (MMFF) calculation to address vibrational modes and the geometry of the adsorbate. The molecule preferentially binds to the surface in its HHis- molecular form. On the metallic Cu(110) surface, the adsorption occurs via the carboxylate group (COO-), with the two oxygen atoms equidistant from the surface, and via the dehydrogenated nitrogen of the imidazole group; the latter adopts an upright position with respect to the copper surface. The amino group (NH2) is likely to be maintained close to the surface, thus facilitating the interaction with the metal. Preadsorption of oxygen (θ ≈ 0.3) on the copper surface induces minor changes in the molecular orientation: the ring tends to be less upright than in the absence of oxygen; the ring now interacts via its NH group as deduced from RAIRS and MMFF calculations. The molecule does not reorient with increasing oxygen coverage (θ ≈ 0.6), and no surface blocking was observed upon oxygen adsorption; both the orientation and the amount of histidine only slightly vary when the oxygen coverage is increased.

Low-temperature partial dissociation of water on Cu(1 1 0)

The low-temperature reactivity of water (D 2O) adsorbed on clean and oxygen pre-covered Cu(1 1 0) was studied using high resolution X-ray photoelectron spectroscopy (HRXPS) and low energy electron diffraction (LEED). On the clean surface partial dissociation to hydroxyl was observed already at 95 K. Upon annealing to 220 K hydrogen bonded water–hydroxyl chains are formed. Upon further annealing water desorbs leaving behind a layer of hydroxyl, most of which desorbs recombinatively eventually. With pre-adsorbed oxygen water reacts to hydroxyl lifting the added-row reconstruction even below 225 K. Upon annealing this adsorbate layer passes through essentially the same stages as without pre-adsorbed oxygen.

Reversible, Band-Gap-Selective Protonation of Single-Walled Carbon Nanotubes in Solution

In acidic solution between pH 6 and 2.5, protons react reversibly and selectively in the presence of preadsorbed oxygen at the sidewall of aqueous dispersed single-walled carbon nanotubes suspended in sodium dodecyl sulfate. This reactive complex, which protonates the nanotube sidewall, reversibly diminishes absorption intensity, fluorescent emission, and resonant Raman scattering intensity. The results document the first evidence of electronic selectivity with metallic nanotubes reacting initially near neutral pH, followed by successive protonation of nanotubes with increasing band gap as the solution is increasingly acidified. Preadsorption of molecular oxygen is shown to play a critical role in the interaction, and its desorption kinetics is followed using UV irradiation. The role of the charged electric double layer of the surfactant is discussed. This chemistry, which proceeds under relatively mild conditions, holds promise for separating nanotubes by metal and semiconducting types.

Growth of NiO ultrathin films on Pd(1 0 0) by post-oxidation of Ni films: the effect of pre-adsorbed oxygen

This paper reports on the optimisation of the growth parameters of NiO ultrathin films on Pd(1 0 0). Growth is performed by means of UHV metal deposition and post-oxidation cycles. Chemical and structural characterisation of the deposits is achieved by means of electron spectroscopy (X-ray photoelectron spectroscopy, XPS; angle resolved XPS) and electron diffraction techniques (low energy electron diffraction, LEED; X-ray photoelectron diffraction, XPD). Three growth procedures have been investigated, which differ for the particular growth parameters adopted in each case. We demonstrate that post-oxidation is effective in order to obtain epitaxial NiO only if the initial dose of Ni evaporated on the clean Pd(1 0 0) substrate exceeds a critical value, corresponding approximately to two equivalent monolayers. However, the overlayer thus obtained is strongly understoichiometric in oxygen close to the metal/oxide interface and poorly ordered on the long range. When a Ni dose below this limiting first value is used, the layer evolves toward polycrystalline NiO, due to substantial oxidation of the Pd substrate promoted by the presence of Ni, very likely through a work function decrease upon direct metal/metal interface formation. On the contrary, epitaxial NiO(1 0 0) layers of good structural quality, with limited oxygen deficiency at the interface, with negligible substrate oxidation and with a good degree of long-range order are obtained if deposition and post-oxidation cycles are initiated on an oxygen pre-saturated Pd surface, characterised by the (√5×√5)- R27° O/Pd(1 0 0) LEED pattern. We therefore demonstrate that oxygen can act either as an inhibitor or as a promoter of NiO epitaxial growth on Pd(1 0 0), depending on the way it is used.

Mechanisms of deep benzene oxidation on the Pt(1 1 1) surface using temperature-programmed reaction methods

The catalytic oxidation of benzene on the Pt(1 1 1) surface has been characterized using temperature-programmed reaction spectroscopy (TPRS) over a wide range of benzene and oxygen coverages. Coadsorbed atomic oxygen and benzene are the primary reactants on the surface during the initial oxidation step. Benzene is oxidized over the 300–500 K range to produce carbon dioxide and water. Carbon–hydrogen and carbon–carbon bond activation are clearly rate-limiting steps for these reactions. Preferential oxidation causes depletion of bridge-bonded benzene, suggesting enhanced reactivity in this bonding configuration. When oxygen is in excess on the surface, all of the surface carbon and hydrogen is oxidized. When benzene is in excess on the surface, hydrogen produced by dehydrogenation is desorbed after all of the surface oxygen has been consumed. Repulsive interactions between benzene and molecular oxygen dominate at low temperatures. Preadsorption of oxygen inhibits adsorption of less reactive benzene in threefold hollow sites. The desorption temperature of this non-reactive chemisorbed benzene decreases and overlaps with the multilayer desorption peak with increasing oxygen exposure. The results presented here provide a clear picture of rate-limiting steps during deep oxidation of benzene on the Pt(1 1 1) surface.

An infrared vibrational spectroscopic study of the interaction of methanol with oxygen-covered Cu( [formula omitted

Reflection–absorption infrared spectroscopy has been used to characterise further the formation of the surface methoxy species, CH 3O–, on Cu(1 1 1) through the interaction with pre-adsorbed oxygen. Using a simple titration experiment, the relative reaction probability at 300 K has been determined as a function of oxygen pre-coverage. The experiments show a low oxygen coverage of approximately 0.04 ML or less to produce the most reaction at this temperature, although higher coverages appear to be favoured at lower reaction temperatures. The results support the earlier suggestion of Russell et al. [Surf. Sci. 163 (1985) 516] that the reaction occurs at ‘perimeter sites’ but also indicate the existence of a transiently adsorbed methanol precursor state on the clean surface. Coverage-dependent shifts in the C–O stretching frequency are shown to be attributable almost entirely to dynamic dipole coupling, with at most very small contributions from chemical shifts due to co-adsorbed oxygen or changing methoxy coverage, although chemical shifts may contribute in the C–H-related vibrational modes.

Low-Temperature Ammonia Oxidation Over Pt/γ-Alumina: The Influence of the Alumina Support

Deactivation of a Pt/γ-alumina catalyst used for low-temperature ammonia oxidation was studied using positron-emission profiling (PEP). Evidence is presented that irreversibly adsorbed nitrogen and oxygen species deactivate the catalyst. This deactivation is further accelerated by preadsorption of oxygen on the catalyst. Ammonia strongly interacts with the alumina support. A steady state of the ammonia oxidation reaction is reached after the alumina sites are saturated with ammonia. PEP experiments show that spillover of oxygen to the support is not significant during the ammonia oxidation.

CO 2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO 2 chemisorption on carbonaceous surfaces. The differential heat of CO 2 adsorption at low and high coverages was determined in the temperature range 553–593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO 2 chemisorption on carbonaceous surfaces was performed. Several of the carbon–oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO 2 adsorption. In the high-coverage region, our theoretical results suggest that CO 2 molecules are likely to adsorb on surface oxygen complexes and on graphene planes.

Kinetic evidence for the identical nature of active centers for partial and deep oxidation of ethylene on silver

Kinetics of interaction of ethylene with pre-adsorbed oxygen on a silver film was studied at 473 K and different initial values of surface coverage with oxygen. Dependencies of the initial rates of partial and deep ethylene oxidation on the oxygen surface coverage manifest themselves as peaked curves with coinciding maxima. The results are considered as evidence that the processes of partial and deep ethylene oxidation on silver, when they occur at optimal conditions, proceed viaidentical active centers.

Interaction of NH3 with oxygen-pretreated Ni at 600 K

The aim of the present study was to investigate the reaction between ammonia and pre-adsorbed oxygen on Ni at 600 K using metastable impact electron spectroscopy (MIES) and UPS techniques. For a given oxygen-pretreated nickel surface, the changes in the MIES spectra along with ammonia adsorption can be decomposed into three different steps: a first domain without significant changes, a second domain corresponding to drastic changes in the intensity of a signal at 15.8 eV; and a third domain where no more change is detected. The first domain may correspond to the ammonia chemisorption on the surface, whereas the changes in the second domain would be attributed to surface dehydroxylation. The existence of NHx groups on the surface has been detected by UPS; they result from ammonia chemisorption. The changes in the MIES spectra have also been studied versus the initial oxygen uptake on Ni. It emerged that the first domain is larger with increasing initial oxygen exposure: this may be explained by the poorer surface reactivity of NiO, contrary to the oxygen-adsorbed phase. Copyright  2002 John Wiley & Sons, Ltd.

Au/TiO2 Nanosized Samples: A Catalytic, TEM, and FTIR Study of the Effect of Calcination Temperature on the CO Oxidation

Three Au/TiO2 catalysts, with the same Au loading and with different particle sizes, were prepared by the deposition–precipitation method followed by calcination at three different temperatures, 473, 573, and 873 K. The mean diameters of Au particles were 2.4, 2.5, and 10.6 nm, respectively. On all the samples the CO adsorption and different CO–O2 interactions were examined by FTIR at 90 K and at room temperature. The higher catalytic activity on CO oxidation found for the samples calcined at 473 and 573 K is related to the higher concentration of step sites over the Au surfaces and to a higher concentration of step sites at the borderline with the support. At 90 K, CO and molecular oxygen are competitively adsorbed on step sites. By CO preadsorption on hydrated catalysts, the reaction with O2 gives CO2 already at 90 K, while by oxygen preadsorption the reaction is completely inhibited, unless moisture is present in the gas phase. An effect of CO coadsorption has been evidenced on water dissociation on gold sites or at the interface with the support, producing atomic hydrogen. The hydrogen reacts with the oxygen, producing a reactive species, quickly dissociated in nascent oxygen and OH groups. Moreover, the reaction with 18O2 at 90 K in the presence of moisture produces only C16O18O, giving evidence that there is no direct participation of oxygens of the support and of the water in the reaction. At room temperature, other reaction channels become operative, involving oxygen species activated on the support, as shown by the extensive exchange reactions occurring with the support oxygen.

Thermal rearrangement of oxygen adsorbed on oxygen-rich Ru(0 0 0 1)

Atomic oxygen adsorbed on oxide domains terminating an oxidized Ru(0 0 0 1) surface is marked by its unusually high chemical reactivity as compared to other known oxygen species. We applied thermal desorption spectroscopy employing isotopic labeling to characterize the thermally driven incorporation of these oxygen atoms into the subsurface region. This O ad→O sub conversion takes place in temperature region below 600 K where the pre-adsorbed oxygen becomes removed via the associative oxygen desorption. The efficiency of the process depends linearly upon the occupation of the available adsorption sites. The conversion probability reaches a value of 0.25 per adatom. The rather low activation energy of 0.22 eV for this oxide mediated oxygen incorporation promises a high efficiency of this reaction pathway in the high temperature regime. We propose a model based on a thermal transformation of Ru x O y domains as the decisive step for the oxygen conversion.

Effect of Adsorbed Oxygen on Initial Growth of Fe on Cu(111)

Epitaxial growth mode and morphology of Fe on Cu(111), affected by the pre-adsorbed oxygen in the form of a “29” structure of the Cu(111) surface, were investigated by scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). According to RHEED observation, the γ-Fe structure was kept to more than 3 monoatomic layers (ML) and fcc and bcc structures coexisted at 5 ML . In contrast, the RHEED pattern of 1 ML Fe on a clean surface already shows the coexistence of fcc and bcc structures. By STM observation, at about 0.4 ML of Fe on the oxygen absorbed Cu(111) surface, small islands appeared on the surface with a high density instead of decorating the step edges with large islands about 6 ML high for the iron growth on a clean Cu(111) surface. As the surface diffusion was suppressed due to oxygen adsorption, the growth mode was significantly changed. However, no strong surfactant effect was observed as in Fe growth on oxygen-adsorbed Cu(001) surface where the γ-Fe structure was kept to 45 ML with a surface O (\\sqrt2×\\sqrt2)R45° reconstruction and all the oxygen floated to the surface.

Investigation of electrochemical promotion using temperature-programmed desorption and work function measurements

The origin of electrochemical promotion was investigated via temperature-programmed desorption (TPD) of oxygen from polycrystalline Pt and Ag films deposited on YSZ under high-vacuum conditions and via measuring the work function of a Pt film deposited on YSZ under catalytic reaction conditions at atmospheric pressure. It was found that electrochemical O 2− pumping to Pt and Ag catalysts in the presence of pre-adsorbed oxygen causes backspillover of large amounts of oxygen on the catalyst surface and leads to the formation of two oxygen adsorption states, i.e. strongly bonded anionic oxygen along with weakly bound atomic oxygen. Furthermore, the desorption activation energy of oxygen adsorbed on Pt and Ag catalyst films was found to decrease linearly with increasing catalyst potential. Finally, by increasing the Pt-catalyst potential from −0.9 to +1.3 V during C 2H 4 oxidation on Pt, i.e. under electrochemical promotion conditions, a 1.42 eV increase in catalyst work function was observed. These results provide a straightforward explanation of the effect of electrochemical promotion on Pt and Ag deposited on O 2− conducting solid electrolytes.

Reaction between CO and a pre-adsorbed oxygen layer on supported palladium clusters

The transient kinetics of reaction between carbon monoxide and an oxygen monolayer pre-adsorbed on palladium clusters supported on MgO(100), has been studied for various cluster sizes (4–15 nm), in the temperature range 120–400°C, using molecular beam and mass spectrometry under ultrahigh vacuum. When the CO beam is opened, the CO 2 production rate first increases instantaneously, and then increases slowly to its maximum, before decreasing to zero due to the lack of oxygen. The period of slow increase of the reaction rate, namely the induction period, appears at about 200°C and becomes longer when temperature increases. Although it is not observed on Pd(111), this peculiar reaction kinetics does not depend on cluster size, and is attributed to a precursor state of CO chemisorption. The oxygen coverage at saturation is found equal to 0.4. At this high oxygen coverage, gaseous CO physisorbs above the oxygen adlayer. At high temperature, the precursor is more likely to desorb, which reduces its chemisorption probability, and thus the CO 2 production rate. The temperature-dependant kinetics of CO adsorption and reaction with oxygen has been simulated thanks to a simple kinetic model accounting for the precursor mechanism.

Stability of hydroxy-formate species at a Pb(110) surface

Chemisorbed surface hydroxy-formate species on Pb(110), formed by reacting gaseous HCOOH with pre-adsorbed atomic oxygen, yields O(1s) and C(1s) photoelectron peaks at 531.3 and 288.3 eV, respectively. The three desorption features can be assigned as multilayer HCOOH at 190 K, a monolayer formic acid at 230 K and a metastable state (hydroxy-formate) at 300 K. Heating the overlayer in vacuum to 450 K results in removal of all surface hydroxyl although the higher temperature peak which corresponds to the recombination of surface protons with formate species remains stable. The formate species desorbs partially from the metal surface with extensive decomposition as shown by a large C(1s) residue at ∼285.0 eV.

Ab initio pseudopotential study of dehydrogenation of methanol on oxygen modified Ag(110) surface

The adsorption and dehydrogenation of methanol on oxygen modified Ag(110) surface have been studied by ab initio pseudopotential total energy method based on density functional theory. The roles of surface and subsurface pre-adsorbed oxygen in dehydrogenation of methanol were investigated. A new pathway for dehydrogenation of methanol was suggested.

Nucleation and Crystal Growth of Cu on Ir Tips Under Ultra-High Vacuum and in the Presence of Oxygen

Results concerning the morphology of Cu adsorption layers deposited from vapor under ultra-high vacuum on Ir tip and the influence of oxygen on this morphology are reported. The method employed was field electron emission microscopy. It was found that the presence of oxygen decreases the copper wettability of iridium. Preadsorption of oxygen on the Ir surface is followed by an increase in cohesion interaction between atoms of the Cu deposited onto the tip at room temperature. Coadsorption of Cu and O on the Ir tip surface at liquid nitrogen temperature, when followed by gradually heating the adlayer, results in crystallization of the deposit in the temperature range from 430 K to about 700 K. Some evidence indicates the formation of Cu2O with a high degree of crystallinity under these conditions. Cu and O coadsorption on the Ir surface at a temperature higher than 1090 K leads to selective accumulation of Cu on the {111} faces and to formation of epitaxial crystals which are oriented to the substrate in the same manner as the Cu crystals grown at ultrahigh vacuum from Cu flux containing no oxygen. Oxygen incorporated into the Cu beam interact preferentially with {011} and {001} Ir faces, where it can produce oxide layers.

Energetic and Entropic Contributions to Surface Diffusion and Epitaxial Growth

For Pd/Pd(111) an exceptionally high barrier (350 meV) for surface self-diffusion and a negative additional energy DeltaE = -53 meV for step-down diffusion are measured. Both findings agree with the proposed role of free-electron-like surface states. Despite the negative value of DeltaE layer-by-layer growth is not observed. This is related to the low preexponential factor for step-down diffusion. Preadsorption of oxygen increases DeltaE but flattens the films. Again this is due to the prefactors for diffusion. The present results demonstrate the importance of entropic effects for diffusion and growth.

Adsorption of carbon dioxide on Cu(110) and on hydrogen and oxygen covered Cu(110) surfaces

The interaction of CO2 with a clean Cu(110) surface and with pre-adsorbed oxygen and hydrogen on this surface has been studied in ultra-high vacuum at temperatures between 20 and 500 K with temperature programmed thermal desorption, low-energy electron diffraction, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy and work function change measurements. CO2 adsorbs only molecularly on the clean and on the hydrogen(1×2) and oxygen(2×1) reconstructed Cu(110) surface, respectively. The initial sticking probability of CO2 is not affected by co-adsorption of oxygen or hydrogen, although the CO2 adsorption is energetically stabilised in this case by 1.3 and 5.4 kJ mol-1, respectively. On clean Cu(110), the isosteric heat of adsorption rises with coverage from ∽13 to 25 kJ mol-1 at saturation. High-resolution electron energy loss spectroscopy suggests that the isolated carbon dioxide molecule is adsorbed in a linear configuration on the clean and on the reconstructed surfaces, while for coverages >0.1 three-dimensional clustering occurs. Our experiments reveal that neither dissociation into oxygen and carbon monoxide nor hydrogenation of carbon dioxide occurs under the experimental conditions.