Low CO Coverage Research Articles

Influence of the thermal treatments of a (Ni+Mg)/Al layered double hydroxide in the hydrogenation of acetonitrile

A Ni/Mg(Al)O catalyst was obtained from a composite NiMgAl lamellar double hydroxide precursor. This material was tested in the gas-phase hydrogenation of acetonitrile between 353 and 423 K, after calcination and reduction at various temperatures. The highest selectivity to monoethylamine, 94.6% at 47% reactant conversion, was reached after calcination at 623 K and reduction at 723 K. FTIR spectroscopies of adsorbed CD 3CN and CO were carried out on the reduced sample. The adsorption of CD 3CN provided evidences that the acidity of Ni/Mg(Al)O was the lowest after calcination at 623 K and reduction at 723 K. Depending on the activation treatment, the ν CO of (Ni) x CO, at low CO coverage, took different values reflecting some modifications of the Ni d-band. The increased selectivity in monoethylamine in the hydrogenation of acetonitrile on Ni/Mg(Al)O was correlated with the decrease of acidity of the sample, and not with a modification of the electronic properties of Ni. The decrease of the acidity of the support lowered the acid-catalysed transamination reaction leading to secondary and tertiary amines.

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm −1 for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.

Electrochemical Promotion by Na of the Platinum-Catalyzed Reaction between CO and NO

The Pt-catalyzed reduction of NO by CO exhibits strong electrochemical promotion (EP) by backspillover Na supplied from β′′ alumina under appropriate conditions of temperature, gas composition, and catalyst potential. At high CO partial pressures (PCO) the EP effect is attenuated by CO island formation and limited availability of chemisorbed NO. At lowPCO, the CO+O reaction is limited by the low coverage of CO and the EP effect is again relatively small. In the intermediatePCOregime, Na pumping strongly accelerates the reaction: rate gains of up to 1300% for N2production, relative to the unpromoted rate are observed. This can be understood in terms of Na-induced dissociation of chemisorbed molecular NO, which is thought to be the reaction initiating step. The overall kinetic behavior and the dependence of N2/N2O selectivity on catalyst potential are in good accord with this hypothesis. Thus the EP system also provides direct information about the nature of the reaction mechanism under conditions of elevated pressure.

Vibrational Stark effect of adsorbates at electrochemical interfaces

Electrochemical experiments find that an adsorbate's vibrational frequency ν varies as a function of electrode potential φ. This has been attributed both to the effect of φ on the molecule's bonding and to the vibrational Stark effect—the molecule's interaction with local electrostatic field F. Both theories are shown to describe the same effect. In application, however, they involve different approximations. For CO on Pt, ( dν dφ ) with aqueous electrolyte is the same as ( Δν ΔΦ ) in vacuum, where ΔΦ is the coadsorbate-induced change of the metal's work function. This is explained in terms of CO's bonding. With a nonaqueous electrolyte, the effect of cation size on ( dν dφ ) is explained by the vibrational Stark effect. At low CO coverage in vacuum, a semiclassical theory accurately predicts ( dν dF ). Ab initio calculations of ( dν dφ ) are also discussed. For CO on two surfaces, Pt(111) and Pt(335), both ( dν dφ ) with aqueous electrolyte and ( dν dF ) in vacuum have been measured. If local F is assumed to be screened the same by the metal's electrons in electrolyte and vacuum, a comparison gives ( dF dφ) = 0.59 ± 0.07 and ∼ 0.85 A ̊ −1 for Pt(111) and Pt(335), respectively. This is significantly different from the ( dF dφ ) ∼ 0.28 A ̊ −1 predicted by simple models of the double layer.

Coverage-, temperature-, and frequency-dependent broadband infrared reflectance change induced by CO adsorption on Pt(111)

Abstract We present an experimental study of the broadband IR reflectance change Δ R R induced by CO adsorption on a Pt(111) single crystal, over the frequency range 425–2800 cm −1 . CO's modification of the near-surface conductivity of the metal causes an increase in the absorption of light by the metal. Coverage-, temperature-, and frequency-dependent data are compared to a model of conduction electron scattering from randomly positioned adsorbates. The maximum magnitude of Δ R R , at room temperature and high frequency, is 0.28% ± 0.03%, a factor of three smaller than is observed for oxygen or CO on Cu(100). The scattering cross section per CO molecule is about 1 A 2 at low coverage. After increasing with coverage for low CO coverages, the magnitude of Δ R R peaks around a coverage of ϑ = 0.33 monolayers, and then decreases toward saturation coverage. Possible explanations for the nonmonotonic dependence include coherent scattering from a partially ordered overlayer or a coverage-dependent scattering cross section of the CO. The fractional reflectance change decreases when the temperature is lowered to 90 K, even though theory predicts an increase in ΔR because of the increase in the electron mean-free-path. We attribute the decrease in | Δ R R | to an increase in the clean-surface reflectance R , which is in agreement with theory. The frequency dependence is roughly consistent with predictions, but a rigorous test was not possible.

The adsorption of CO on Ir(111) investigated with FT-IRAS

Abstract The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm−1, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm−1. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm−1 for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm−1 at a coverage of 0.5 ML.

Local structure determination for low-coverage CO on Ni(111)

Scanned-energy-mode photoelectron diffraction has been used to determine the local adsorption structure of CO on Ni(111) at low temperature at a coverage of approximately 0.25 ML, corresponding to a nominal phase. CO molecules are found to occupy the same two (`hcp' and `fcc') inequivalent hollow sites as in the 0.5 ML phase, and while the results do indicate a preference for the hcp hollow site directly above a second-layer Ni atom in the low-coverage state, this preference is of marginal statistical significance. The similarity of this parameter in both the and phases can only be reconciled by assuming significant local disorder. It does, however, indicate that the local binding energies at the two hollow sites are very similar and that the presence of both sites in the phase is not simply a consequence of near-neighbour adsorbate - adsorbate repulsion.

CO Adsorption and Oxidation on Pd(110)-c(2 × 4)−O by Reflection−Absorption Infrared Spectroscopy

Adsorption and oxidation of CO on Pd(110)-c(2 × 4)−O were investigated by reflection−absorption infrared spectroscopy (RAIRS), low-energy electron diffraction (LEED), and temperature-programmed reaction (TPR). High-frequency CO at 2140 cm-1, which was greatly influenced by adsorbed oxygen atoms, was observed by RAIRS even at low CO coverage when the Pd(110)-c(2 × 4)−O surface was exposed to CO at 100 K. This peak developed along with a broad band around 2000 cm-1 from low CO coverage to the saturation. The reaction of CO and an oxygen atom to form CO2 on the c(2 × 4)−O surface was observed in the range 100−450 K. Below 250 K, two desorption peaks of CO2 were observed in TPR spectra and the corresponding RAIR spectra showed that linear CO species (2140−2070 cm-1) disappeared by desorption as CO or reaction with an oxygen atom to form CO2, while there was little decrease of bridge CO species (2020−1880 cm-1). It is likely that linear CO species mainly reacted to form CO2 on Pd(110)-c(2 × 4)−O below 250 K. We successfully followed the constant-flow reaction of CO and O2 on the Pd(110)-c(2 × 4)−O at 220 K; however, the reaction ceased with an increase of bridge CO at 1992 cm-1. RAIR spectra during the reaction of CO and O above 250 K suggest that CO began to form domains separated from oxygen atoms and the substrate structure of (1 × 2) was gradually converted into (1 × 1) according to the local coverage of CO.

Study by Fourier Transform Infrared Spectroscopy of the Electroadsorption of CO on the Ferrous Metals. 1. Iron

The electroadsorption and electrooxidation of CO on Fe have been studied at the following pH values: 14, 9.2, 6.9, and 3. In spite of the high oxidizability of Fe, a cathodic cleaning sufficed to produce an unoxidized surface on which CO could chemisorb. Over the whole pH range 3−14 chemisorbed CO was detected on the electrode surface as evidenced by an IR band at 1916−1993 cm-1, which was assigned to linear (on-top) CO. Its intensity was about 0.2%, from which a surface coverage of 5−10% was estimated. This low CO coverage inhibited both hydrogen evolution and the electrooxidation of Fe itself, which was interpreted by assuming that the cathodic cleaning reduced the native Fe oxides only over a small fraction of the surface and that CO chemisorption, H2 evolution, and start of Fe electrooxidation take place only on this small fraction of the surface.

Coadsorption of K and CO on Cu(111) surfaces

It is well established that for submonolayer coverages the strong interaction between CO + K is not only influenced by the adsorptives but also by the substrate itself. We report about investigations of CO + K on the Cu(111) surface, where the CO/Cu(111) interaction is rather weak. The analytical techniques employed include thermal desorption spectroscopy, LEED and work function change measurements. We find the interaction between K + CO on Cu(111) to be very complex. Three different regimes of CO/K interaction can be distinguished. (1) At very low ΘK (0.0–0.02) TDS measurements show a shift of the CO desorption maximum (for low CO coverages) towards lower temperature. ΔΦ indicates a decrease and finally a vanishing of the effective negative CO dipole moment. (2) At ΘK = 0.02−0.155 a new CO desorption maximum appears, shifting continuously from 165 to 270 K with increasing K coverage. ΔΦ indicates a positive effective dipole moment (increasing 2π∗ backdonation). Between ΘK = 0.13 and 0.18 around 300 K a very sharp and highly asymmetric CO desorption peak can be detected with no coincident K desorption. (3) At ΘK> 0.2, where the transition from “ionic-” to “metal-like” K occurs, the strong interaction between CO/K is evidenced by a thermal stabilization of the adlayer to temperatures> 700 K. Under these conditions several additional desorption peaks are detected between 400 and 800 K. Desorption of CO2 was detected under these ΘK conditions, probably due to a partial dissociation and recombination reaction.

Model Electrochemical Interfaces in Ultrahigh Vacuum: Solvent Effects upon Coverage-Dependent Infrared Spectra of Carbon Monoxide on Platinum(111)

The influences of the solvating medium on a model electrochemical chemisorbate system-carbon monoxide on Pt(111)-have been explored by means of the infrared spectral responses in the C-O stretching (ν CO ) region to the dosage of various solvents onto CO adsorbed on Pt(111) in ultrahigh vacuum at 100 K. Measurements were made for a range of preadsorbed CO coverages (θ CO ) as a function of the solvent dosage. The molecules selected-methanol, acetonitrile, acetone, benzene, and ammonia-span a range of solvating and dielectric properties. At low CO coverages (θ CO ≤ 0.25), even submonolayer solvent dosages induced in most cases a near-complete shift in the CO binding geometry from atop to doubly bridging coordination, as seen from a displacement of the sharp vco band at 2090-2095 cm -1 by a markedly weaker feature at ca. 1790-1820 cm -1 . Additional (multilayer) solvent dosing yielded essentially no further spectral changes, other than small additional frequency downshifts. Submonolayer ammonia dosage triggered more substantial frequency decreases, yielding vco bands at ca. 1640 and 1530-1565 cm -1 . At saturated (or near-saturated) CO coverages (θ CO ∼ 0.65), solvent dosages yield only milder (≤50 cm -1 ) frequency downshifts in the atop and bridging ν CO features that are present in the absence of solvent. The marked ν CO spectral changes occurring at low (and intermediate) Oco values are interpreted in terms of CO-solvent coadsorption, involving short-range dipolar interactions and through-metal charge polarization. The marked solvent-induced attenuation in the ν CO band intensities seen at low (θ CO ) are discussed in terms of dielectric screening. The conventional dipole-coupling treatment is able to describe approximately the nature and solvent-dependent magnitude of the effect, including the severe intensity attenuation observed with the polarizable solvent benzene. The milder ν CO frequency downshifts observed at higher solvent dosages, as well as for high CO coverages, are consistent with Stark-tuning effects exerted by overlayer (rather than coadsorbed) solvent. Comparisons are made with in-situ infrared spectra for corresponding electrochemical interfaces. Attention is called to the value of this infrared-based UHV electrochemical modeling approach for elucidating interfacial solvation effects.

Coadsorption of hydrogen and CO on Pt(335): Structure and vibrational Stark effect

We have studied CO and H coadsorbed on the stepped Pt(335) surface in vacuum to learn about their interaction at steps and to compare with previous electrochemical studies. Both electroreflectance and conventional reflectance absorption vibrational spectra were obtained of atop bonded CO. Its stretch vibrational frequency ν, Stark tuning rate (dν/dE), and integrated absorbance S were all studied as functions of CO and H coverage. With CO only on step edges, S for atop CO decreases to zero with increasing H coverage. The CO affected by H is in a mixed phase and is bridge bonded. Atop CO’s (dν/dE) decreases with increasing CO coverage but is unaffected by coadsorbed H. In the low CO coverage limit (dν/dE)=88±9 cm−1/(V/Å), in agreement with theory and with previous measurement. The standard model of dipole–dipole screening is unable to explain the CO coverage dependence of both (dν/dE) and S. The vibrational polarizability of CO increases with coverage. We also compare our results with spectroelectrochemical data from CO on Pt(335) in aqueous electrolyte. Our measured (dν/dE) is too small to explain the variation of CO’s ν with electrode potential φ in the electrochemical experiments if (dE/dφ) in the double-layer is taken from conventional models. The effect of coadsorbed H is also different; in vacuum H has no effect on CO’s (dν/dE) but in electrolyte, at low CO coverage, (dν/dφ) drops to zero at the potential where H adsorption begins.

High resolution helium scattering studies of inelastic interference structures of the frustrated translational mode of CO on Cu(001).

The angular distributions of multiquantum excitations of the lowest energy, frustrated translational vibrations of a low coverage of CO chemisorbed on Cu(001) have been investigated by high resolution helium scattering time-of-flight measurements. The elastic and the one-phonon creation and annihilation cross sections show several interference undulations as a function of parallel momentum transfer, and up to four multiquantum overtones. All the experimental observations are in reasonable agreement with a multiquantum scattering theory, which also provides information on the height and width of the adsorbed CO molecules.

Diagnosis of industrial catalyst deactlvation

The coadsorption of CO and methylamine on Ru(001) has been investigated by LEED, TPD and HREELS to clarify coadsorbate-induced reaction phenomena in ordered coadsorbed layers. CO and methylamine formed mixed adsorption layers accompanied with the change of adsorption site for CO by the interaction with methylamine. The dissociation process of methylamine changed as a function of partial CO coverage. For methylamine alone on Ru(001), the CH, NH, and CN bonds dissociate simultaneously at 280–320 K. In the presence of low-coverage CO molecules (θ ≈ 0.3), NH2 species was formed and persisted on the surface up to 380 K, whereas the coadsorbed CO molecules behaved as a spectator for the dissociation of the CH bonds. In the presence of high-coverage (0.5–0.68) CO, a well ordered (2 × 2) phase was formed and the new partial dehydrogenation of methylamine takes place to produce a species containing a CN double bond as a stable intermediate, followed by complete dissociation at higher temperatures. The control of reaction path by coadsorbates is discussed.

Substrate induced interactions between water and carbon monoxide on Ni(100) and Cu(100)

The interaction between carbon monoxide and water was studied on two different metal surfaces, Ni(100) and Cu(100), using high resolution electron energy loss spectroscopy (HREELS). In the case of Ni(100) it was found that water could displace CO from on-top sites to both bridge and four-fold sites. The effect is strongest when the CO coverages are low and the water coverages are high. In the limit of very low CO coverages, all the CO can be displaced to four-fold sites, and the C-O stretching frequency falls as low as 1410 cm −1. In sharp contrast, virtually no change in the C-O stretching frequency is seen for any combination of relative CO and water coverages on Cu(100). These results are compared to other UHV and electrochemical measurements.

Vibrational excitations and structure of CO adsorbed on Ni(100)

Vibrational excitations and structure of CO chemisorbed on the clean Ni(100) surface have been investigated by high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). The Ni(100) c(2 × 2) CO structure produces two vibrational losses at 59.5 and 256.5 meV interpreted as due to excitations of the Ni-C and C-O stretching vibrations of CO linearly bonded to the Ni atoms. At higher coverages a quasihexagonal CO structure develops which produces vibrational losses that are substantially shifted and spread in energy because of the incoherency of this CO structure with respect to the Ni(100) net. At low CO coverage, where no ordered structure forms at the temperatures investigated (173, 293 K), the vibrational loss spectrum reveals a temperature dependent mixing of two bond configurations, one of which is the linear. The other, which is dominating at 173 K produces vibrational losses at 44.5, 81.5 and 239.5 meV which are suggested to be due to excitations of the symmetric and asymmetric Ni-C and the C-O stretching vibrations of CO bridge bonded to two nearest neighbour Ni atoms.

Orientational study of low coverage CO and NO on Ni(111)

The molecular axis orientation of CO and NO molecules adsorbed at room temperature on Ni(111) at low coverage has been studied by the XPD technique. The CO and NO uptake versus exposure has been followed by XPS. Our results show that CO molecules are perpendicular to the surface, while NO molecules are strongly tilted or lying down.

Orientational study of low coverage CO and NO on Ni(111)

The molecular axis orientation of CO and NO molecules adsorbed at room temperature on Ni(111) at low coverage has been studied by the XPD technique. The CO and NO uptake versus exposure has been followed by XPS. Our results show that CO molecules are perpendicular to the surface, while NO molecules are strongly tilted or lying down.

Coadsorption of CO and methylamine on Ru(001): reaction paths of methylamine induced by CO in ordered coadsorbed structures

The coadsorption of CO and methylamine on Ru(001) has been investigated by LEED, TPD and HREELS to clarify coadsorbate-induced reaction phenomena in ordered coadsorbed layers. CO and methylamine formed mixed adsorption layers accompanied with the change of adsorption site for CO by the interaction with methylamine. The dissociation process of methylamine changed as a function of partial CO coverage. For methylamine alone on Ru(001), the CH, NH, and CN bonds dissociate simultaneously at 280–320 K. In the presence of low-coverage CO molecules ( θ ≈ 0.3), NH 2 species was formed and persisted on the surface up to 380 K, whereas the coadsorbed CO molecules behaved as a spectator for the dissociation of the CH bonds. In the presence of high-coverage (0.5–0.68) CO, a well ordered (2 × 2) phase was formed and the new partial dehydrogenation of methylamine takes place to produce a species containing a CN double bond as a stable intermediate, followed by complete dissociation at higher temperatures. The control of reaction path by coadsorbates is discussed.

Coadsorption of CO and methylamine on Ru(001): reaction paths of methylamine induced by CO in ordered coadsorbed structures

Abstract The coadsorption of CO and methylamine on Ru(001) has been investigated by LEED, TPD and HREELS to clarify coadsorbate-induced reaction phenomena in ordered coadsorbed layers. CO and methylamine formed mixed adsorption layers accompanied with the change of adsorption site for CO by the interaction with methylamine. The dissociation process of methylamine changed as a function of partial CO coverage. For methylamine alone on Ru(001), the CH, NH, and CN bonds dissociate simultaneously at 280–320 K. In the presence of low-coverage CO molecules ( θ ≈ 0.3), NH 2 species was formed and persisted on the surface up to 380 K, whereas the coadsorbed CO molecules behaved as a spectator for the dissociation of the CH bonds. In the presence of high-coverage (0.5–0.68) CO, a well ordered (2 × 2) phase was formed and the new partial dehydrogenation of methylamine takes place to produce a species containing a CN double bond as a stable intermediate, followed by complete dissociation at higher temperatures. The control of reaction path by coadsorbates is discussed.