Adsorption and thermal chemistry of acrolein and crotonaldehyde on Pt(111) surfaces

The surface chemistry of acrolein and of crotonaldehyde on Pt(111) single-crystal surfaces was investigated under vacuum by temperature-programmed desorption (TPD) and reflection-absorption infrared (RAIRS) spectroscopies. The main thermal decomposition path seen for both compounds was the expected decarbonylation of the unsaturated aldehyde to carbon monoxide and the corresponding olefin (ethene and propene, respectively), but small amounts of propene and ketene were detected in the case of acrolein as well. The RAIRS data indicate that while acrolein initially adsorbs with its plane parallel to the surface and interacts mainly via the carbonyl group, crotonaldehyde adopts a more complex geometry where the main interaction to the metal is via a rehybridization of the C=C double bond. It is suggested here that the changes in adsorption geometry induced by substitutions in the C=C double bond may be responsible for the observed changes in the subsequent reactivity of the adsorbed unsaturated aldehydes.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

The coadsorption of I and Cl on Pt(111)

Using LEED and AES, the coadsorption of iodine and chlorine on Pt(111) was studied. Five surface structures were observed: (1 × 1), (√3 × √3)R30°, (√7 × √7)R±19.1°, (3 × 3) and c(2 × 4). Fractional monolayer coverages for both Cl and I are assigned to these structures. The importance of the adatom-adatom steric interactions is discussed.     

Anisotropic interface magnetoresistances in Pt(111)/Co n /Pt(111)

It is shown in terms of a fully relativistic spin-polarized ab initio-type approach that in Pt/Co/Pt trilayers two types of anisotropic magnetoresistance (AMR) have to be distinguished: an in-plane and an out-of-plane AMR. The obtained results, namely the magnetic field dependence as well as the thickness dependence of both AMR types are in very good agreement with a very recent experimental study, in which the in-plane as well as the out-of-plane AMR was reported for this system. The difference between the two types of AMR is visualized in terms of layer-resolved resistivities. In particular, it is confirmed that the anisotropic interface magnetoresistance (AIMR) introduced in the recent publication mainly originates in the vicinity of the Co/Pt interfaces.     

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.     

Thermodynamic measurements of Xe-Adsorptton on Pt(111)

The adsorption of xenon on a nearly defect free Pt(111) surface has been studied by elastic and inelastic He-scattering. With decreasing temperature the sequential of 2D-gas, monolayer, bilayer and multilayer films are observed. The thermodynamic phase diagram and the latent heats of adsorption are determined.     

Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

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.     

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.     

Auger spectra of carbon monoxide chemisorbed on Pt(111) and Cu(111)

The Auger spectra of carbon monoxide adsorbed on Pt(111) and Cu(111) are compared. The characteristic features now regarded as a fingerprint of this adsorbed species are observed, even for the weakly adsorbed CO on copper which gives complex X-ray photoelectron spectra. No coverage dependence of the spectra was observed on either substrate. The C lsVV spectrum of CO/Cu(111) is dominated by transitions involving the “screening” electron in the 2π orbital.     

NO在Pt（111）表面的振动和离解

基于密度泛函理论和周期平板模型研究了NO在Pt（111）表面的吸附,通过扫描隧道显微镜（STM）的理论计算分析了吸附的结构特征．计算得到的Pt-NO伸缩振动（圪）频率基本保持不变,受阻平动（v3,v4）和受阻转动（us,v6）完全是简并的．采用CI-NEB方法讨论了NO在Pt（111）表面的离解过程,研究结果表明NO在Pt（111）表面的离解比较困难,必须克服2．29eV的能垒．     

The formation and decomposition kinetics of alkylidynes on Pt(111)

The formation and decomposition kinetics of ethylidyne and propylidyne on Pt(111), were studied using static secondary ion mass spectrometry and temperature programmed desorption. For the maximum amounts of dissociatively adsorbed ethylene and propylene formed during adsorption at 200 K and subsequent temperature programmed desorption, the following activation energies (E) and pre-exponential factors (A) are determined: (a) for ethylidyne formation: E = 17±1 kcal mol⁻¹ and A = 1×10^12±1 s⁻¹; (b) ethylidyne decomposition: E = 27 ±2 kcal mole⁻¹ and A = 6×10^11±1 s⁻¹; (c) propylidyne formation: E = 17.5 ± 2 kcal mol⁻¹ and A = 7×10^12±1 s⁻¹; and (d) propylidyne decomposition: E = 22.5±2 kcal mol⁻¹ and A = 4×10^{11 ± 1} s⁻¹.     

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.     

Comparative study of photochemistry of methane on Pt(111) and Pd(111) surfaces

Adsorption states and photochemistry of methane physisorbed on Pd(111) have been investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy and compared with those on Pt(111). On both of the surfaces, methane is either dissociated into a hydrogen atom and a methyl radical or molecularly desorbed by 6.4 eV photon irradiation. In the photochemistry, the direct electronic excitation of the adsorbate-substrate complex plays an important role. Different features observed for Pd(111) compared with Pt(111) are: (1) the adsorbate-substrate interaction is slightly stronger; (2) methane adsorbates show a (√3√3)R30° LEED pattern at 40 K; (3) the photochemical cross-section is larger by 60%; and (4) the photochemistry is not self-quenched at prolonged irradiation. The origins of these features are discussed in terms of the differences in the electronic structure between the two surfaces.     

Small rafts of xenon physisorbed on Pt(111)

The behavior of small rafts of xenon (<50 particles) is illustrated. Events which involve collective motion of raft atoms are shown to occur. These events are persistent below a transition temperature and are not persistent above this temperature. A video of the events is available.