Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

High coverage states of oxygen adsorbed on Pt(100) and Pt(111) surfaces

Oxygen adsorption on the Pt(100) and Pt(111) surfaces was investigated using X-ray photo-emission and thermal desorption spectroscopies. Low pressure (ca. 10^?5 Pa) oxygen dosing at near ambient crystal temperature resulted in the formation of dissociated adsorbed species at saturation coverages of nominally 0.2–0.25 monolayer on both surfaces. The combination of higher pressure (ca. 10^?3 Pa) and higher surface temperature (570 K) dosing produced a three to five times higher saturation coverage than the low pressure dosing. The effect of dosing condition on the saturation coverage appears to reconcile apparent discrepancies for the Pt(100) surface in the literature. Characterization by XPS of the higher coverage state for oxygen showed that it is in the same chemical state as the oxygen adsorbed at very low coverage. Angle-resolved XPS has shown that in all cases the oxygen appears to reside on the surface with no significant penetration of oxygen into the bulk, as would be characteristic of oxidation. However, some penetration on the surface by oxygen, such as by a place-exchange type restructuring of the first two atomic layers, cannot be entirely ruled out.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

Surface (electro-)chemistry on Pt(1 1 1) modified by a Pseudomorphic Pd monolayer

The formic acid and methanol oxidation reaction are studied on Pt(1 1 1) modified by a pseudomorphic Pd monolayer (denoted hereafter as the Pt(1 1 1)-Pd_1 ML system) in 0.1 M HClO₄ solution. The results are compared to the bare Pt(1 1 1) surface. The nature of adsorbed intermediates (CO_ad) and the electrocatalytic properties (the onset of CO₂ formation) were studied by FTIR spectroscopy. The results show that Pd has a unique catalytic activity for HCOOH oxidation, with Pd surface atoms being about four times more active than Pt surface atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO₂ is observed at low potentials. This indicates that the reaction can proceed on Pd at low potentials without the typical “poison” formation. In contrast to its high activity for formic acid oxidation, the Pd film is completely inactive for methanol oxidation. The FTIR spectra show that neither adsorbed CO is formed on the Pd sites nor significant amounts of CO₂ are produced during the electrooxidation of methanol.     

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.     

Electron energy loss spectrum of cyanogen on Pt (100)

Electron energy loss spectra for adsorbed cyanogen on Pt(100) are presented, and discussed in terms of possible models suggested by other techniques. Adsorbate induced loss features are found at 11 and 14 eV, and these are associated with levels below the Fermi level as observed in ultra-violet photoemission. As in the case of CO adsorption losses in the adsorbed phase occur at energies significantly larger than in the gas phase, indicating an upward shift of the final 2π¹ state due to mixing with metal orbitais. Thermal desorption studies of C₂N₂ on Pt clearly resolve α and β phases, but there is some controversy over whether the β phase involves mainly single CN units bonding to the metal, whether C₂N₂ is molecularly adsorbed, or whether paracyanogenlike structures form at the surface. The electron spectroscopic evidence is examined, and is shown on balance to support adsorption in some molecular form.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

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.     

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.     

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.     

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.     

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)

The synchrotron radiation from BESSY has been used to measure the photoemission from CO orbitals adsorbed as ordered overlayers on Ni(100) c(2 × 2), Pt(111) c(4 × 2) and Pt(110) (2 × 1)p2mg. Angular distribution patterns of photoelectrons from CO orbitals were recorded with a display-type analyzer. The data were compared with differential photoionization cross sections calculated for free and oriented molecules. The results demonstrate the upright orientation of CO on Ni(100) and Pt(111), while CO on Pt(110) shows a marked difference which can be explained by assuming that the CO molecules are tilted in the [001] directions of Pt(110), yielding a (2 × 1)p2mg superstructure observed in LEED. The tilt angle is estimated to about 20°. The structure model is supported by the shape resonances of the 4σ (5σ) orbitals of CO/Pt(110) as compared to CO/Pt(111).     

Sum-frequency generation at electrochemical interfaces: Cyanide vibrations on Pt(111) and Pt(110)

We use optical sum-frequency generation to investigate the stretching vibrations of cyanide (CN^–) molecules chemisorbed from aqueous electrolytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. For clean and well-ordered Pt(111) electrodes, a single vibrational band between 2080 and 2150 cm^–1 with a nonlinear frequency dependence on the potential is observed and assigned to the CN stretching vibration of chemisorbed cyanide. A second band between 2145 and 2150 cm^–1 with very weak potential dependence appears on a surface which was subjected to oxidation-reduction cycles and is attributed to cyanide associated with a microscopically disordered surface. This assignment is supported by preliminary results for a Pt(110) single-crystal electrode. On a well-ordered (110) surface a single and potential-dependent cyanide vibration between 2070 and 2112 cm^–1 is observed. After oxidation of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm^–1 which is essentially not potential-dependent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Oxygen-exchange reaction between O2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N¹⁶O and ¹⁸O₂ coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N¹⁸O and ¹⁸O¹⁶O are desorbed from Pt(111) initially saturated with ¹⁸O₂ at 94 K followed by exposure of N¹⁶O. Three distinct desorption peaks are observed in N¹⁸O TPD spectra at 145, 310, and 340 K, and two peaks in ¹⁸O¹⁶O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO₂ or NO₃ is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.     

Anomalous adsorption of Cs on Pt(1 1 1)

The adsorption of Cs on Pt(1 1 1) surfaces and its reactivity toward oxygen and iodine for coverages θ_Cs?0.15 is reported. These surfaces show unusual “anomalous” behavior compared to higher coverage surfaces. Similar behavior of K on Pt(1 1 1) was previously suggested to involve incorporation of K into the Pt lattice. Despite the larger size of Cs, similar behavior is reported here. Anomalous adsorption is found for coverages lower than 0.15 ML, at which point there is a change in the slope of the work function. Thermal Desorption Spectroscopy (TDS) shows a high-temperature Cs peak at 1135 K, which involves desorption of Cs⁺ from the surface.The anomalous Cs surfaces and their coadsorption with oxygen and iodine are characterized by Auger Electron Spectroscopy (AES), TDS and Low Electron Energy Diffraction (LEED). Iodine adsorption to saturation on Pt(1 1 1)(anom)-Cs give rise to a sharp LEED pattern and a distinctive work function increase. Adsorbed iodine interacts strongly with the Cs and weakens the Cs-Pt bond, leading to desorption of Cs_xI_y clusters at 560 K. Anomalous Cs increases the oxygen coverage over the coverage of 0.25 ML found on clean Pt. However, the Cs-Pt bond is not significantly affected by coadsorbed oxygen, and when oxygen is desorbed the anomalous cesium remains on the surface.     

STM investigation of the adsorption and temperature dependent reactions of ethylene on Pt(111)

The adsorption and reactions of ethylene adsorbed in UHV on Pt(111) have been studied as a function of temperature by STM. The STM images taken at 160K show an ordered structure of adsorbed ethylene. Annealing to 300 K produces ethylidyne (C-CH₃) irreversibly, as has been demonstrated by a wide variety of surface science techniques. The ethylidyne on Pt(111) is not visible to the STM at room temperature. Cooling the sample allows direct observation of the ethylidyne ordered structure by STM. Annealing above 430 K results in further dehydrogenation, eventually leaving only carbon on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt(111) surface. The annealed graphite islands exhibit several supersturctures with lattice parameters of up to 22 Å, which are thought to result from the higher order commensurability with the Pt(111) substrate at different relative rotations.     

Hydrogen-induced low temperature CO displacement from the Pt(111) surface

A new low temperature displacement mechanism for CO on the Pt(111) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr H₂). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K (E_d = 10.6 kcal/mol) instead of near the desorption temperature of 400 K (E_d = 26 kcal/mol). The large decrease in CO desorption energy appears to be caused by substantial repulsive interactions in the compressed monolayer induced by coadsorbed hydrogen. The new low temperature CO desorption channel appears to be caused by displacement of the compressed CO adlayer by coadsorbed hydrogen. In addition, the desorption activation energy for the main desorption channel of CO near 400 K is lowered by ~ 1 kcal/mol for hydrogen pressures in the 0.001 to 0.1 Torr range. These new results clearly emphasize the importance of in-situ methods capable of performing kinetic experiments at high pressures on well characterized adsorbed monolayers on single crystal surfaces. High coverages of coadsorbed hydrogen resulting from substantial overpressures may substantially modify desorption activation energies and thus coverages and kinetic pathways available even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.     

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10¹⁴ S atoms cm^?2 ($\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ structure) oxygen exposures of 5 × 10^?5 Torr s do not result in the adsorption of oxygen nor in the formation of SO₂. At concentrations lower than 3.8 × 10¹⁴ S stoms cm^?2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO₂ and O₂. With decreasing sulfur concentration the amount of desorbing O₂ increases and that of SO₂ passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO₂ is adsorbed with high sticking probability and can be desorbed nearly completely as SO₂ with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO₂ is highly sensitive to hydrogen. Small H₂ doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO₂ on an oxygen predosed surface small amounts of SO₃ were desorbed in addition to SO₂ and O₂ during heating. Preadsorbed oxygen produces variations of the SO₂ peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.     

Molecular orbital study of co chemisorption and oxidation on a Pt(111) surface

A molecular orbital study was made, using an atom superposition and electron delocalization (ASED) technique, of the structures and energy levels of CO on Pt(111) surface. CO is predicted to be preferentially adsorbed at a height of 2.05 Å from the surface at a 1-fold position with the carbon end down. The calculated binding energy (1.7 eV) is in good agreement with the recent experimental result (1.5 eV) of Campbell et al. Calculated binding energies for bridging (1.3 eV) and high coordinate (1.1 eV) sites are predicted to be smaller in magnitude. Calculated results are used to discuss the ordering of energy levels of adsorbed CO. The interaction between CO (adsorbed) and O (adsorbed) has been studied to estimate the energy of activation for the oxidation of CO on Pt(111) surface. The calculated activation energy (1.6 eV) is in reasonable agreement with the recent experimental result (1 eV) of Campbell et al. The Langmuir-Hinshelwood mechanism is found to be favored. We predict CO₂ bonds vertically.     

Adsorption and decomposition of methyl iso-cyanide on Pt(111)

CH₃NC adsorption and thermal decomposition on a Pt(111) surface has been studied by high resolution EEL and TD spectroscopies. At 90 K, CH₃NC adsorbs initially in a terminal-bonded configuration characterized by a blue-shifted iso-cyanide stretch at 2265-2240 cm^?1. At higher coverages this form co-exists with a second form characterized by an imine-like stretch at 1600–1770 cm^?1, increasing with coverage. This form is associated with bridge bonding to adjacent surface platinum atoms. Adsorption is irreversible and. except for multilayer desorption at 135 K, only reaction-limited H₂ (T_p = 440–460 K) andHCN (T_p = 420–610 K) desorption and. at high coverages, isomerization to CH₃CN (T_p = 430 K) was seen. EEL spectra recorded after heating the adsorbed layer indicated that at lower coverages, the molecular integrity of the adsorbed CH₃NC was completely lost before dehydrogenation occurred. On the other hand, at saturation structural changes in the adsorbed layer corresponded firstly to the onset of dehydrogenation and then. at higher temperatures, to HCN evolution. No spectroscopic evidence for an η²-bonding configuration was found either at low temperatures or during thermal decomposition. The terminal- and bridged-bonded configurations adopted by CH₃NC have been compared and contrasted with those found with the isoelectronic CO and the isomeric CH₃CN by reference to the chemically important frontier orbitals of these ligand molecules.