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.     

The adsorption of CO on Pt(111) studied by infrared-reflection-adsorption spectroscopy

The adsorption of CO on Pt(111) between 85K and 300K has been studied by infrared-reflection-absorption spectroscopy together with TPD and LEED. The intensity of the absorption band due to the CO stretch of the linear species shows a maximum at the formation of the (√3 × √3)R30° LEED pattern followed by a minimum at the c(4×2) structure during the adsorption of CO at low temperatures (150K). The absorption band due to the C-O stretch of the bridging species appears only after the formation of the (√3 × √3)R30° pattern and reaches maximum intensity at the c(4×2) structure. Adsorption of CO to higher coverages (corresponding to the compression structures) broadens and shifts this absorption band. At higher temperatures (150K) a third peak is observed at 40cm⁻¹ below the peak due to the bridging species and is attributed to adsorption in the three-fold sites. At 300K both peaks in this region are very broad. The intensity data differs from that measured with EELS (ref.1) and favors a “faultline” structure of the type proposed by Avery (ref.2). Together with the additional information from bandwidths it is possible to distinguish between the various structural models. The results obtained here may also be important in explaining data from other systems such as CO/Cu.     

Adsorption sites of CO on Pt (111)

The surface vibrations of CO adsorbed on Pt(111) single crystal surfaces at 320 K have been studied by electron-energy-loss spectroscopy. At low coverages two vibration modes at 58 and ∼260 meV are observed. For exposures >0.2 Langmuir two additional modes at 45 and 232 meV develop. Considering also the observed LEED structures these vibrations are attributed to CO molecules being adsorbed upright in on-top and bridge sites, respectively.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Infrared spectra of high coverage CO adsorption structures on Pt(111)

The adsorption of carbon monoxide on Pt(111) was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and sum frequency generation (SFG) spectroscopy. Two CO on-top signals at 2110 cm^? 1 and 2097 cm^? 1 have been detected under continuous CO exposure in a pressure range from 10^? 7 to 100 mbar and at temperatures between 200 K and 300 K. The formation of the higher wavenumber signal is found to be kinetically limited below 200 K and by the presence of a stable c(4 × 2) adlayer in UHV. On the basis of the results presented in this study and previous experimental findings the two on-top signals are related to different CO compression layers on Pt(111) with θ > 0.5, hexagonal Moiré lattices and rectangular coincident site lattices.     

Ethylene and acetylene adsorption on Cu(111) and Pt(111) studied by Auger spectroscopy

High resolution, electron impact excited, carbon Auger spectra of ethylene and acetylene adsorbed on Cu(111) and Pt(111) are compared. The spectra of ethylene on the two metals provide the first example of the sensitivity of AES to the nature of metal-adsorbate bonding for molecular adsorbates. The acetylene spectra are identical on the two metals. The changes in the carbon Auger spectra resulting from thermal decomposition of the two adsorbates on Pt(111) are discussed in the context of results from electron energy loss spectroscopy.     

Surface vibrations and the nature of the adsorption state: C2H2 on Pt(111)

Molecular vibrations of C₂H₂ and C₂D₂ adsorbed on Pt(111) at 140 K and ∼300K have been measured by high resolution electron energy loss spectroscopy. The comparison of C₂H₂ and C₂D₂ spectra allows an unambiguous assignment of the observed losses to the excitation of C−H bending, C−H stretching, and C−C stretching modes of nondissociatively adsorbed acetylene. From the relative intensities of losses the hybridisation state is determined to be nearsp ². The C−C stretching frequency indicates a C−C bond order of ∼1.8.     

CO adsorption isotherms on Cu(100) at elevated pressures and temperatures using infrared reflection absorption spectroscopy

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10⁻⁶ to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δE_ads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δE_ads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δE_ads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures (<10⁻⁴ Torr) and temperatures (<275 K). At substrate temperatures above 240 K and at pressures > 10⁻⁴, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.     

The adsorption of water on Pt(111) studied by irreflection and UV-photoemission spectroscopy

IR-reflection spectroscopy (IRS) under grazing incidence and UV-photoemission (UPS) with Hel and Hell radiation were used to study the adsorption of D₂O and H₂O, respectively, on Pt(111) under UHV conditions. In IRS three different vibrational bands in the OD stretch region yield information about the orientation of hydrogen-bonded water molecules and the formation of water clusters. Initially formed water multimers grow to clusters and finally build up multilayers. From UPS the adsorption of molecular water in the whole coverage range is confirmed. Both UPS and IRS show that the water molecules in the “first layer” are bound through their oxygen end to the surface.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

Identification of adsorbed species at metal surfaces by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a surface analysis method for measuring vibrational spectra of adsorbed species on metal surfaces. This paper summarizes recent work on the study of bonding of simple adsorbates on metal surfaces, and the identification of new chemical intermediates in reactions between two or more species in the adsorbed monolayer. The spectra of atomic oxygen, di-oxygen, water and ammonia adsorbed on platinum, copper and silver are discussed with emphasis on identification of the adsorbed species and their orientations relative to the surface plane. Surface reactions between atomic oxygen and water, methanol and formic acid yield the new surface intermediates hydroxyl (OH), methoxy (CH₃O) and formate (HCOO), respectively, on copper and silver surfaces. Each species was identified by comparison of surface spectra with known infrared spectra and through the use of deuterium isotopic shifts. The ability to identify and distinguish between chemical species at surfaces with high sensitivity will allow direct correlation of low pressure UHV surface experiments with high pressure surface reactions on catalysts and liquid-solid interfaces.     

Vibrational EELS,XPS and UPS study of CO chemisorbed on Pt10Ni90(111): Evidence for the existence of different sites

CO adsorption on the (111) face of a Pt₁₀Ni₉₀ alloy single crystal has been investigated at room temperature by vibrational electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (XPS and UPS). Two well separated CO stretching modes develop at 2070 and 1820 ± 10 cm^?1, with their intensities reaching 64 and 36% respectively of the total intensity at saturation coverage. They are attributed to CO adspecies in terminal and bridge bonded configuration respectively. The UPS spectra of 4σ, 5σ and 1π molecular orbitais of adsorbed CO show complex features which may be resolved into two components having the main characteristics of CO adsorbed on pure Pt(111) and Ni(111) respectively. Such behaviour is also observed by XPS on C 1s on O 1s peaks. Their respective contributions, in both XPS and UPS spectra are about 64 and 36% of the whole spectrum. Finally compared to Ni(111) — on which CO adsorbs mainly in bridge configuration — the alloying with 10% Pt has generated the appearance of a large number of new sites for CO chemisorption associated with the presence of Pt atoms at the surface. The large amount of terminal CO adspecies is interpreted in terms of considerable surface enrichment of the alloy in platinum.