The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω_CO = 1817 cm^?1. At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω_CO = 1910 cm^?1. At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm^?1. The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.     

Adsorption d'hydrogène entre 300 et 873 K sur un catalyseur Pt/MgO

Hydrogen adsorption on MgO-supported platinum was studied by thermal desorption and infrared spectroscopy at 300 and 800 K. For both temperatures, reversibly and irreversibly adsorbed species have been detected. At 300 K, reversible adsorption leads to the appearance of infrared bands at 2120 and 2060 cm^?1, attributed to terminal Pt-H species. Irreversibly adsorbed hydrogen has been detected by thermal desorption, whereas no infrared band was detected in the spectral range 4000–4750 cm^?1 for Pt/MgO sample. For hydrogen adsorption at 800 K, reversibly adsorbed hydrogen gave the same picture as for the 300 K adsorption. An additional form of irreversibly adsorbed hydrogen has been evidenced both by thermal desorption and infrared spectroscopy. This form corresponds to hydrogen strongly adsorbed on platinum and gives an infrared band at 950 cm^?1 which is characteristic of an hydrogen atom in interaction with more than one platinum atom (multicentered) species.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.     

Orientation and bonding of adsorbed CH3NCO on Pt{110} and Cu{110} from vibrational and photoelectron spectroscopies

The bonding and orientation of CH{in3}NCO on Pt{110} and Cu{110} are studied by HREELS, ARUPS, AES, and LEED. CH{in3}NCO is found to adsorb nondissociatively on both Cu{110} and Pt{110} at T = 160 K, bonding primarily through the 2π a′ orbital with the NCO group lying down on the surface and the methyl group largely unperturbed. We propose that the absence of a strong HREELS band at about 2290 cm^?1, which is the liquid phase frequency for the NCO asymmetric stretching mode, combined with the presence of strong bands between 1000 and 1450 cm^?1 provides a “finger-print” for NCO bonded to the surface in a lying-down configuration.     

Carbon monoxide adsorption on Pd-deposited Cu(1 1 0) surface: Infrared reflection absorption and temperature programmed desorption studies

We investigated carbon monoxide (CO) adsorption and desorption behaviors on 0.1-nm-, 0.15-nm-, and 0.3-nm-thick-Pd-deposited Cu(1 1 0) surfaces using infrared reflection absorption (IRRAS) and temperature-programmed desorption (TPD) spectroscopic methods. CO was exposed to the 0.1-nm-thick-Pd/Cu(1 1 0) surface at the substrate temperature of 90 K. The IR band attributable to CO bonded to Cu atoms emerged at 2092 cm⁻¹: the band was located at 2100 cm⁻¹ at saturation coverage, with a shoulder at 2110 cm⁻¹. In addition to these bands, weak absorptions attributable to the PdCO bonds appeared at 2050 and 1960 cm⁻¹. With increasing Pd thickness, the Pd related-bands became increasingly prominent. Particularly at the early stage of exposure, the band at 2115 cm⁻¹ became visible. The band at 2117 cm⁻¹ dominated the spectra all through the exposures for the 0.3-nm-thick-Pd surface. The TPD spectra of the surfaces showed two remarkable features at around 220-250 and 320-390 K, ascribable ,respectively, to CuCO and PdCO. The desorption peaks shifted to higher temperatures with increasing Pd thickness. Based on the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the Pd/Cu(1 1 0) surfaces.     

Theoretical studies of CO/Ni(100): Geometry,vibrational frequencies and ionization potentials for the on-top site

The chemisorption of CO on the (100) surface of Ni has been studied using an Ni₁₄ cluster and generalized valence bond (GVB) methods. CO is found to bond perpendicular to the Ni surface with optimized NiC and CO bond distances of 1.94 and 1.15 Å, respectively. The calculated NiCO bond strength is 29.7 kcal (experimental values 30–32 kcal). Vibrational frequencies are calculated to be 401 cm^?1 for NiC stretch, 327 cm^?1 for NiCO bend, and 2129 cm^?1 for CO stretch. This decrease of the CO frequency by 71 cm^?1 from the free molecule value is consistent with experiment based on self-consistent calculations of the positive ion states. We propose a new explanation for the loss of one PES peak upon chemisorption.     

Errors and Artifacts in Infrared Bands of CO Chemisorbed on Supported Metals

Nearly three decades have passed since Eischens et al. ¹ measured the infrared bands of CO chemisorbed on Cu, Ni, Pt and Pd, and their assignments, unscathed by time and criticism, have prompted a great deal of infrared work on the interactions of CO with a wide variety of adsorbents. CO adsorption, followed by a variety of infrared techniques, has now become a valued conventional tool for surface studies. The C-O stretching bands of chemisorbed CO yield data about band frequency, half-width, contour and intensity, as well as band shifts and intensity changes when some parameter is changed, which provide information about the structure of the chemisorbed species and permit deductions to be made concerning the adsorbate-adsorbent interaction and the nature and properties of the surface. However, we now find and show that under certain conditions such data, measured by the conventional transmission/absorption (T/A) techniques, can be susceptible to errors and artifacts stemming from the measurement technique which is employed.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm^?1, 1855 cm^?1 (shifting to 1944 cm^?1 with increasing coverage) and at 1960 cm^?1 (shifting to 2016 cm^?1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.     

Reflection-absorption infrared and thermal desorption studies of carbon monoxide and hydrogen adsorbed on rhodium

Reflection-absorption infrared spectroscopic and thermal desorption techniques have been used to study the interaction of mixtures of carbon monoxide and hydrogen with evaporated rhodium films. For equimolar mixtures near 10^?9 Torr, hydrogen adsorbed much more rapidly, but long exposure times or increases in CO pressures to 10^?6 Torr led to its partial, but never complete, displacement by adsorbed carbon monoxide. Hydrogen desorption spectra taken during the displacement process showed two peaks which was consistent with a cooperative interaction between adsorbed CO and H species. In contrast to previous transmission studies of CO adsorption on small rhodium particles, the present reflection—absorption infrared study of the film system showed a single absorption band at 2075 ±10 cm^?1. While explanations for the discrepancy in terms of particle size effects are possible it is considered more likely that all CO molecules are linearly bound to individual Rh atoms in the present situation. In our work, increases in CO pressure (especially above 10^?6 Torr) were accompanied by an upward frequency shift (from 2065 cm^?1 to 2085 cm^?1) and a narrowing in half width (from 25 to 17 cm^?1). Several possible explanations for the latter unusual effect are discussed.     

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni₂CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the NiNi distance fixed at the bulk value. The minimum energy is found at a NiC distance of 1.72 Å and a CO bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm^?1) shows reasonable agreement with EELS data (1810, 1870 cm^?1), whereas the (Ni₂)C frequency of 495 cm^?1 is remarkably higher than the experimental values (380, 400 cm^?1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a₁orbital manifold exhibits strong antibonding effects.     

Normal Unenhanced Raman Spectra of CO and CH4 Adsorbed on Cobalt(poly)

Abstract

Normal unenhanced Raman spectra (NURS) of low-polarizability CO molecules were observed for the first time on cobalt at R. T. and residual gas pressure. We assign five bands observed between 2030–2130 cm^?1 to linear chemisorbed CO species, while those observed between 1840–2010 cm^?1 have been ascribed to the 2-fold chemisorbed species. The three bands observed between 1740–1830 cm^?1 we believe are due to the 3-fold species. The corresponding fourteen Co-C stretches were observed and assigned. A model based upon electron backdonation is proposed for each of the three structures. NURS were also observed at R. T. for physisorbed CH₄ and assignments are made to the four frequencies of CH_4.     

Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts

Pt, Ru and Pt/Ru nano-particles, synthesized in ethylene glycol solutions, are studied using infrared (IR) spectroscopy and high resolution transmission electron microscopy (HRTEM). The synthesis method allows the control of the mono- and bi-metallic catalyst particle sizes between 1 and 5.5 nm. The IR spectra of CO adsorbed (CO_ads) on the Pt, Ru and bi-metallic Pt/Ru colloids are recorded as a function of the particle size. The stretching frequency of CO_ads depends on the particle size and composition. Strong IR bands due to the stretching vibration of CO_ads are observed between 2010 and 2050 cm⁻¹ for the Pt nano-particles, while two IR bands between 2030 and 2060 cm⁻¹ for linear bonded CO_ads, and at lower wavenumbers between 1950 and 1980 cm⁻¹ for bridged bonded CO_ads, are found for the Ru particles. The IR spectra for the Pt/Ru nano-sized catalyst particles show complex behaviour. For the larger particles (>2 ± 0.5 nm), two IR bands representative of CO_ads on Ru and Pt-Ru alloy phases, are observed in the range of 1970-2050 cm⁻¹. A decrease in the particle size results in the appearance of a third band at ∼2020 cm⁻¹, indicative of CO_ads on Pt. The relative intensity of the band for CO_ads on the Pt-Ru alloy vs. the Pt phase decreases with decreasing particle size. These results suggest that Ru is partially dissolved in the Pt lattice for the larger Pt/Ru nano-particles and that a separate Ru phase is also present. A Pt-Ru alloy and Ru phase is observed for all Pt/Ru particles prepared in this work. However, a decrease in particle size results in a decrease of the number of Pt and Ru atoms in the Pt-Ru alloy phase, as they are increasingly present as single Pt and Ru phases.     

The interaction of CO and O2 with the (111) surface of Pt3Ti

The electronic properties of clean and partly oxidized Pt₃Ti(111) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt₃Ti(111) is quasi-pure platinum. Scarcely any (θ ≈ 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(111) and a p(2 × 2) “reconstruction” of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm⁻¹ which coincides with infrared absorption spectra of TiO₂. Flashing with CO shifts the band to 500 cm⁻¹. Correlated with this shift we observe (i) CO₂ desorption at a temperature well above that observed for Pt(111)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same c(4 × 2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates “islands of TiO₂”, segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti⁴⁺ ions on the surface of these islands are reduced by CO to pentacoordinated Ti³⁺ species. The vibrational shift, 650 to 500 cm⁻¹, can be understood by the dipole active bands of a triatomic O−Ti⁴⁺ −O vibrator compared to a diatomic Ti³⁺−O vibrator.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

Inhibition par le cuivre de l'adsorption du monoxyde de carbone sur les alliages nickel-cuivre

The evolution of the relative concentration of CO bridged species with the NiCu alloys composition allows to estimate the copper efficiency to inhibit this kind of chemisorption. This efficiency decreases as the copper content increases. One copper atom inhibits almost nine nickel atoms for a copper concentration of 5%; this number decreases below two nickel atoms for a copper content greater than 40%.     

Interactions of CO molecules adsorbed on oxidised Cu(111) and Cu(110)

Reflection absorption infrared spectroscopy has been used in conjunction with LEED and surface potential measurements to study low temperature CO adsorption on the oxidised Cu surfaces Cu(111)O|$\text{3}\text{2}\text{?}\text{2}$|, Cu(110)O(2 × 1) and Cu(110)Oc(6 × 2). On all three surfaces adsorption at 80 K yields surface potential changes in excess of 0.6 V and does not lead to the formation of an ordered overlayer. At high coverages the adsorption enthalpy is lower than on the clean surfaces. Infrared spectra show the growth of a doublet band with components initially at 2100 and 2117 cm^?1 on the oxidised Cu(111) surface. Similar features seen on the oxidised Cu(110) surfaces are accompanied by a band at 2140 cm^?1: a very weak band at the same frequency on oxidised Cu(111) is attributed to defect sites. Studies of the temperature dependence of the spectrum from oxidised Cu(111) lead to the conclusion that two different binding sites are occupied. Spectra of ${}^{12}\text{CO}{}^{13}\text{CO}$ mixtures show that the molecules occupying these sites are in close proximity to each other, and that the spectrum is subject to large but opposing coverage-dependent frequency shifts.     

Yet another vibrat1onal study of the adsorption system Pt{111}-CO

Infrared spectra of CO adsorbed on Pt{111} indicate only one linear and one bridging species below θ = 0.5. The linear band is subject to a positive dipole-coupling shift and a negative chemical one, both of which exhibit a maximum when plotted as a function of coverage. When q > 0.50, a complex temperature-dependent pattern of shifts and splittings arises in both bands. The temperature-dependent behaviour of the bridging band reported in an earlier paper by Hayden and Bradshaw was not observed. In addition to the bands characteristic of the perfect surface, a second linear band, about 15 cm⁻¹ below the first, is produced by defect sites: we show that such defects are likely to have produced very significant effects in earlier studies of the Pt{111}-CO system. We also propose a real-space interpretation of the low-coverage LEED patterns first reported by Steininger et al.