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.     

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.     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

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.     

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.     

Vibration spectroscopy of benzene adsorbed on Pt(111) and Ni(111)

High resolution electron energy loss spectroscopy has been applied to study the adsorption of benzene (C₆H₆ and C₆D₆) on Pt(111) and Ni(111) single crystal surfaces between 140 and 320 K. The vibrational spectra provide evidence that benzene is chemisorbed with its ring parallel to the surface, predominantly π bonded to the platinum and nickel surface respectively. A significant frequency increase of the CH-out-of-plane bending mode, largest in the case of platinum, is observed compared to the free molecule. On both metals two phases of benzene exist simultaneously, characterized by a different frequency shift. The shifts are explained by electronic interaction between the metal d-orbitals and molecules adsorbed in on top and threefold hollow sites respectively. The vibrational spectra of the multilayer condensed phase of benzene exhibit the infrared active modes of the gasphase molecule as expected.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

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.     

The mechanism and kinetics of water formation on Pt(111)

We use modulated molecular beam relaxation spectroscopy (MMBRS) to identify the sequence of elementary reaction steps and to quantify the rate parameters for catalytic water (D₂O) formation on Pt(111). This investigation is restricted to surface temperatures in the range 373 ⩽ T_S ⩽ 723 K and oxygen and deuterium pressures on the order of 10⁻⁵ mbar, where coverages of oxygen-containing species are low (ϑ ⩽ 0.04 monolayer), and rate parameters can be assumed coverage-independent. Under these conditions, we find that adsorbed hydroxyl (OD_a) formation is rapid and (nearly) irreversible; the rate-limiting, majority pathway for water production is D_a + OD_a → D₂O (E_a = 16 kcal/mol); and water production by OD_a disproportionation. i.e. 2 OD_a → D₂O + O_a, contributes as a minority pathway (E_a = 18 kcal/mol). From the results we construct potential energy diagrams that account for the energetics of nearly all elementary steps in these reactions.     

Evidence for two different adsorption sites of CO on Pt(111) from infrared reflection spectroscopy

The adsorption of CO on Pt(111) surfaces has been studied under clean conditions by a highly surface sensitive double-beam infrared reflection spectroscopy (IRS). In contrast to results of other authors two stretching vibrations of adsorbed CO rather than one are detected near 2100cm⁻¹ and 1870cm⁻¹. This is in agreement with recent findings in high-resolution electron energy loss spectroscopy (ELS). The results are discussed in terms of two adsorption sites: CO adsorbed in on-top positions and double coordinated on bridging sites, respectively. Furthermore, a precursor state and a preferential adsorption in islands at low coverage is taken into account.     

Structure and bonding of water on Pt(111)

We address the adsorption of water on Pt(111) using x-ray absorption, x-ray emission, and x-ray photoelectron spectroscopy along with calculations in the framework of density functional theory. Using the direct relationship between the electronic structure and adsorbate geometry, we show that in the first layer all the molecules bind directly to the surface and to each other through the in-layer H bonds without dissociation, creating a nearly flat overlayer. The water molecules are adsorbed through alternating metal-oxygen (M-O) and metal-hydrogen (M-HO) bonds.     

Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry

The adsorption and desorption of the system CO/Pt(111) and C₆H₆/Pt(111) at 300 K has been investigated with a pulsed molecular beam method in combination with a microcalorimeter. For benzene the sticking probability has been measured in dependence of the coverage θ. For coverages θ > 0.8 transient adsorption is observed. From an analysis of the time-dependence of the molecular beam pulses the rate constant for desorption is determined to be 5.6 s^? 1. With a precursor-mediated kinetic adsorption model this allows to obtain also the hopping rate constant of 95.5 s^? 1. The measured adsorption enthalpies could be best described by (199 ? 77θ ? 51θ²) kJ/mol, in good agreement with the literature values. For CO on Pt(111) also transient adsorption has been observed for θ > 0.95 at 300 K. The kinetic analysis yields rate constants for desorption and hopping of 20 s^?1 and 51 s^?1, respectively. The heats of adsorption show a linear dependence on coverage (131 ? 38θ) kJ/mol between 0 ≤ θ ≤ 0.3, which is consistent with the desorption data from the literature. For higher coverage (up to θ = 0.9ML) a slope of ?63 kJ/mol describes the decrease of the differential heat of adsorption best. This result is only compatible with desorption experiments, if the pre-exponential factor decreases strongly at higher coverage. We found good agreement with recent quantum chemical calculations made for (θ = 0.5ML).     

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.     

Superlattices formed by interaction of hydrogen bromide and hydrogen chloride with Pt(111) and Pt(100) studied by LEED,Auger and thermal desorption mass spectroscopy

HBr and HCl react with Pt(111) and Pt(100) surfaces to form adsorbed layers consisting of specific mixtures of halogen atoms and hydrogen halide molecules. Exposure of Pt(111) to HBr yielded a (3×3) LEED pattern beginning at $\text{Θ}{}_{\mathrm{Br}}\text{=}\text{2}\text{9}$ and persisting at the maximum coverage which consisted of $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{=}\text{1}\text{3}$ plus $\text{Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{9}$. The most probable structure at maximum coverage, Pt(111)[c(3 × 3)]-(3 Br + HBr), nas a rhombic unit cell encompassing nine surface Pt atoms, and containing three Br atoms and one HBr molecule. On Pt(100) the structure at maximum coverage appears to be Pt(100)[c(2√2 × √2)]R45°-(Br + HBr), $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{= Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{4}$; the rectangular unit cell involves four Pt atoms, one Br atom and one HBr molecule. Each of these structures consists of an hexagonal array of adsorbed atoms or molecules, excepting slight distortion for best fit with the substrate in the case of Pt(100). Treatment of Pt(100) with HCl produced a diffuse Pt(100)(2 × 2)-(Cl + HCl) structure at the maximum coverage of Θ_Cl = 0.13, Θ_HCl = 0.11. Exposure of Pt(111) to HCl produced a disordered overlayer. Thermal desorption, Auger spectroscopy and mass spectroscopy provided coverage data. Thermal desorption data reveal prominent rate maxima associated with the structural transitions observed by LEED. Br and HBr, Cl and HCl were the predominant thermal desorption products.     

C60 adsorption on Cu(110) studied by optical spectroscopy

The formation of the interface between C₆₀ thin films and the Cu(110) surface has been investigated in situ using reflectance difference spectroscopy (RDS). The electronic interaction between C₆₀ molecules in the first monolayer and the substrate inhibits low‐energy intramolecular transitions, whereas the C₆₀ molecules above the first monolayer are effectively decoupled from the substrate. The morphology of C₆₀ thin films prepared at room temperature is thermally stable up to 500 K. Above this threshold, optical spectroscopy and low energy electron diffraction (LEED) indicate the formation of rather large three dimensional C₆₀ islands on a one monolayer thick wetting layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)