Influence of the OH groups of hydroxylated rutile (110) surface on the Lewis acidity: an investigation of CO adsorption by quantum-mechanical simulations

The adsorption of carbon monoxide on a clean and hydroxylated rutile (110) surface has been investigated using a periodic approach at DFT/B3LYP level. The hydroxylated surface was modelled by considering both terminal and bridging OH groups. The variation of the electrophilicity of the Lewis acid site near these groups was evaluated by taking into account the adsorbate–substrate distance, the magnitude of the interaction energy and the blue-shift of the adsorbed CO stretching frequency. The results obtained suggest that the electrophilicity increases in proximity to OH terminal groups, and decreases near the OH bridging groups.     

13C NMR and Fukui function analysis on C82 hydroxylated fullerene through density functional theory

Density functional theory calculations were performed on C₈₂ hydroxylated fullerene. B3LYP and PBE0 functionals with 6-31G** basis set were utilised to get chemical shieldings, chemical shifts and the isotropic Fermi contact coupling on each atomic site. A relation between nuclear magnetic resonance (NMR) properties and reactivity of the molecule, obtained through the electronic Fukui function, was observed. Interestingly, the most stable configurations of OH groups adsorbed on C₈₂ surface were obtained when the hydroxyl groups are adsorbed on deshielded (isotropically and anisotropically) sites. For open-shell systems, a relation between isotropic Fermi contact, spin density and average Fukui function was found, that is, sites with a great amount of Fukui function (analytical and the one obtained through finite difference) and spin density have the largest isotropic Fermi contact coupling data. With the adsorption of the first hydroxyl molecules, spin densities and Fukui functions show preferential sites to adsorb the following OH groups close to previously adsorbed. Additionally, theoretical spectra of chemical shifts of C₈₂(OH)_n (n = 1, 2, 3 and 4) were obtained and they were compared with experimental reports, getting a reasonable comparison. For example, regarding ¹³C NMR chemical shifts obtained in C₈₂OH molecule, 80 ppm (B3LYP) and 79 ppm (PBE0) were calculated on hydroxylated carbon, which is in good agreement with experimental results in C₆₀ fullerols.     

Frequency shifts in the spectra of molecules adsorbed on metals,with emphasis on the infrared spectrum of adsorbed CO

Two processes are considered which cause the frequency of the band maximum in the infrared absorption spectrum of molecules adsorbed on metals to shift with increasing coverage. The first arises from the coverage-dependent modification of the local field acting on an adsorbed molecule while the second originates from vibrational coupling arising from a through-metal interaction between molecules adsorbed on different metal atoms. Infrared absorption strengths are discussed and the method used heretofore to obtain the extinction coefficients of adsorbed molecules is questioned. In particular, a factor of 2 which results from the constructive superposition of the incident and reflected electric fields is shown to be often ignored. We show that dipolar (through-space) coupling is too weak to account for the magnitudes of the shifts recently observed for CO on single-crystal Pt and Pd as coverage progressed from low to high values. Vibrational coupling is also shown to account for the disparity in the intensities of the two bands observed when ¹²CO and ¹³CO are coadsorbed on supported metals. Similar experiments on single-crystal surfaces are suggested to help determine to what extent the observed spectral frequency shifts are derived from adsorbate—adsorbate coupling as opposed to other mechanisms.     

Surface‐enhanced Raman spectra of dimethoate and omethoate

Surface‐enhanced Raman scattering (SERS) spectra are presented and analyzed for two important organophosphate pesticides, dimethoate (DMT) and omethoate (OMT). Very detailed SERS spectra were obtained by aggregated Ag hydrosols, both in aqueous suspension and dried on a glass substrate. The SERS and ordinary Raman spectra of DMT do not resemble each other, suggesting that a chemical reaction immediately occurs when DMT is adsorbed onto the metal surface. We propose that the reaction product is OMT, which is the oxygen analog of DMT, on the basis of the Raman and SERS spectra of OMT. Further support is derived from the calculated Raman spectra of DMT and OMT. Minor wavenumber and intensity differences that are observed between the SERS spectra of DMT reaction product and those of OMT could be related to different metal/adsorbate interaction modes. The results can be useful in the development of new analytical methods for the determination of pesticide residues in food. Copyright © 2010 John Wiley & Sons, Ltd.     

The bonding of water molecules to platinum surfaces

Using high resolution electron energy loss spectroscopy in ultra-high vacuum we have studied the vibrational spectrum of submonolayer and multilayer quantities of water adsorbed on platinum (100) surfaces. For adsorbed multilayers the spectrum resembles the spectrum of ice I. For submonolayer quantities of H₂O we find three different OH stretching vibrations, 2850, 3380, and 3670 cm^?1. The highest frequency is attributed to free OH groups. The vibration around 3380 cm^?1 indicates H bonding between oxygen atoms. It is therefore concluded that the water molecules cluster even at low coverage. The lowest OH stretching frequency is attributed to H bonding to platinum. We find also evidence for additional oxygen lone pair orbital bonding to the surface which disappears however when the first monolayer is completed. The relation to currently considered models in electrochemistry of aqueous solutions is discussed.     

Vibrational properties of the adsorbed monolayer on face-centered cubic crystals

Calculations of mean-square displacements 〈u²〉 of the atoms in adsorbed monolayers on fcc crystals are presented and compared with LEED experimental results. This text is restricted to the case of a C(2 × 2) adsorbed layer on a (100) surface [experimental case of Ni(100) with adsorbed sulfur, sodium, cesium or oxygen]. 〈u²〉's perpendicular to and parallel to a (100) surface are calculated for the adsorbed atoms and the atoms of the first surface layer of the crystal. The values obtained are compared with those for a clean (100) surface and the volume of the crystal. Every possibility for force constants between adsorbate and substrate atoms is examined. It is shown that the measurement of 〈u²〉 perpendicular to the (100) surface yields the adsorbate-substrate force constants and that 〈u²〉 parallel to the (100) surface yields the adsorbate-adsorbate force constants.     

Evidence for Eley-Rideal abstraction mechanism in reactive ion scattering of Cs from Pt(1 1 1) adsorbed with CO and CO2

We studied the reactive ion scattering (RIS) of Cs⁺ from a Pt(1 1 1) surface adsorbed with CO and CO₂ leading to the emission of CsCO⁺ and . The RIS products were measured as functions of adsorbate coverage and ion incidence energy for the range of 10-60 eV. The yield and cross-section for the RIS processes were extracted from these measurements. The RIS cross-section is higher for weakly adsorbed CO₂ than for more strongly bound CO. The RIS energy-dependence shows a maximum at 15-20 eV and a decrease at higher energy. These observations provide evidence for the theoretically proposed mechanism of RIS, in which a slow Cs⁺ picks up an adsorbate in an Eley-Rideal abstraction reaction.     

Hydrogen bonds of extremely high polarisability between molecules adsorbed on silica gel — an IR investigation

The adsorption on silica gel of a series of electron pair donors with a variation of the pKa value from 12 up to the range of negative values was subjected to IR spectroscopic investigation. Adsorption is effected via hydrogen bonds with the OH groups of the surface, indicated by the disappearance of the band of the free OH groups (3750 cm⁻¹) and by the occurrence of a broad band at smaller wave numbers. The shift is roughly proportional to the pKa value. Furthermore, a continuous absorption is observed as the pKa value increases. This shows that protons become detached from the SiOH groups and hydrogen bonds of type NH⁺…N form between the adsorbed molecules. A double minimum potential well occurs in these hydrogen bonds, which are extremely easily polarisable. The anomalous proton conductivity to be expected sometimes in the presence of such groupings is discussed.     

Interaction of 1-(β-oxyethyl)-2-methyl-5-nitroimidazole with the surface of highly dispersed silica

Using the IR spectroscopy method, it has been established that formation of a monolayer 1-(β-oxyethyl)-3-methyl-5-nitroimidazole (metronidazole) on the surface of nonporous amorphous highly dispersed silica (S_sp = 230 m²/g) occurs during its absorption in an amount of 0.6 mmole/g. Such a content of adsorbate corresponds to the concentration of free silanol groups that are main active centers of the surface. Based on the analysis of the IR spectra of the specimens obtained and the dependence of the degree of perturbation of silanol groups on the content of adsorbed metronidazole, the structure of the surface adsorption complexes is suggested. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 717–721, November–December, 2005.     

Accurate wavenumbers of the AΣ → XΠ (0, 0) and (1, 0) bands of OH and OD

The important A²Σ → X²Π (0, 0) absorption band of OH has been remeasured from spectra obtained by flash photolysis of H₂O₂ vapor. Accurate wavenumber measurements using thorium standards show that the previous rotational line measurements are about 0.1 cm^-1 too large. Measurements are also given for the (1, 0) band of OH and the (0, 0) and (1, 0) bands of OD.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Summary abstract: Vibrational excitations of hydrogen and oxygen on Pd(100)

EMIRS spectra for ¹²CO-¹³CO mixtures on platinum, HSO₄^? and acrylonitrile on gold and water on silver are discussed. These examples illustrate the adsorbate identification, bonding, and orientation information which EMIRS data offer to complement electrochemical Raman data and infrared reflection absorption and EELS results. The advantages of experimental fine control of the electronegativity of the metal are shown with application to studies of coupling mechanisms between adsorbed species.     

A computational DFT analysis of OH chemisorption on catalytic Pt(111) nanoparticles

In this article, various energies and geometries of pure platinum nanoparticles and those of platinum nanoparticles with adsorbed OH were investigated. Fourteen different platinum clusters of 3–40 atoms were studied using spin-unrestricted density functional theory (DFT) with a double numerical plus polarization basis set. This range of sizes gave enough information for establishing the general trends that were the primary goal of the study. Three different shapes of platinum clusters were presented, and the effect of cluster size on binding energy, total energy, and HOMO–LUMO energy gap was investigated. Subsequently, same calculations were performed for those selected Pt clusters with OH adsorbate on the surface. The results show that the stability of both the pure clusters and the clusters with adsorbed OH molecule increases with an increase of cluster size. This fact indicates that direct influence of the size of Pt cluster on the reaction rate is possible, and the understanding of how cluster size would affect binding energy is important. As expected, the effect of cluster size on total energy of molecule was shown to be a linear function independent of cluster type. We also found that optimized (stable) Pt clusters were bigger in size than that of the initial clusters, or clusters with bulk geometry.     

Reactive and inhibiting adsorbates for the catalytic electrooxidation of carbon monoxide on gold (210) as characterized by surface infrared spectroscopy

Potential-dependent sequences of surface infrared spectra have been obtained during the voltammetric electrooxidation of solution carbon monoxide on ordered Au(210), in acidic, neutral, and alkaline aqueous media. Under optimal catalytic conditions, only a reactive form of adsorbed CO is detected, yielding a CO stretching frequency (v_CO) at ∼ 2100–2115 cm⁻¹. The occurrence of reaction inhibition is accompanied by the displacement of this adsorbate by a lower v_CO form, located at ∼1900–2000 cm⁻¹, its appearance being favored by increasing surface exposure time and more negative dosing potentials. The mechanistic implications of such combined real-time infrared/voltammetric results are noted.     

Numerical experiments on low-energy ion scattering at a surface containing adsorbed impurity atoms

The energy distributions of Cs⁺ and W⁺ ions scattered, respectively, at Mo and W single crystals, the surfaces of which contain adsorbed oxygen, nitrogen, and cesium atoms, are calculated via the molecular dynamics method. It is demonstrated that the energy spectra are sensitive to the impurity atoms of a surface. When compared with the energy distributions corresponding to a pure surface, even a partial monolayer of these atoms can modify their shape. A peak in the scattered-ion intensity is commonly shifted to the lowerenergy region if the adsorbate is light and to higher energies if the adsorbate is heavy.     

Adsorption on the carbon nanotubes

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi-conductor transitions can be achieved by the doping of alkali atoms.     

Inelastic electron tunneling spectroscopy of carboxylic acids on alumina at low coverage

The difficulties in assigning certain modes of carboxylic acids adsorbed on alumina are discussed. The study of the spectra of marked acetic acid CH₃C¹⁸O¹⁸OH adsorbed on alumina has allowed us to clarify the assignments of the acetate ion formed. Then an examination of the spectra at very low coverages has enabled us to propose a model for the adsorption, which occurs on (at least) two different sites.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.