Fem and volumetric studies of silver-nitrous oxide and silver-oxygen interactions

The non-dissociative and the dissociative adsorption of nitrous oxide and the adsorption of oxygen on silver have been studied by field-emission microscopy using whiskers and epitaxial layers on tungsten tips and volumetrically, with the aid of ultraclean thin films. At 77 K non-dissociative adsorption of nitrous oxide takes place, leading to a decrease in work function. At 273–473 K slow face-specific dissociative adsorption of nitrous oxide occurs, which causes an increase in work function and proceeds with an activation energy at low coverages of 29 ± 5 kJ mol^?1. The adsorption of oxygen in this temperature range is more than 10⁴ times faster and for low coverages work function-oxygen exposure plots yield an activation energy of 16 ± 3 kJ mol^?1. The coverages reached above 1 Pa are constant and occur in the ratio 1:2:3.5 at 296, 373 and 473 K, the corresponding increases in work function being approximately 0.4, 0.6 and 0.8 eV. The oxygen adsorbed at low temperatures (≈ 273 K) is bound more loosely than that adsorbed at higher temperatures, which is shown by the partial desorption upon evacuation to low pressures (10^?8 Pa) at 273 K and application of high electric fields (5 V/nm). The adsorbate formed in the presence of oxygen at 273 K can further be distinguished from the adsorbates formed in the presence of nitrous oxide at 273 K and oxygen at 473 K (both probably O⁼_ads) by the higher reactivity towards hydrogen reduction and the easier thermal desorption, indicating that at 273 K molecular adsorption (O^?_{2, ads}) occurs.     

The chemisorption of N2 on the (110) surface of iridium

The molecular chemisorption of N₂ on the reconstructed Ir(110)-(1 × 2) surface has been studied with thermal desorption mass spectrometry, XPS, UPS, AES, LEED and the co-adsorption of N₂ with hydrogen. Photoelectron spectroscopy shows molecular levels of N₂ at 8.0 (5σ + 1π) and 11.8 (4σ) eV in the valence band and at 399.2 eV with a satellite at 404.2 eV in the N(1s) region, where the binding energies are referenced to the Ir Fermi level. The kinetics of adsorption and desorption show that both precursor kinetics and interadsorbate interactions are important for this chemisorption system. Adsorption occurs with a constant probability of adsorption of unity up to saturation coverage (4.8 × 10¹⁴ cm^?2), and the thermal desorption spectra give rise to two peaks. The activation energy for desorption varies between 8.5 and 6.0 kcal mole^?1 at low and high coverages, respectively. Results of the co-adsorption of N₂ and hydrogen indicate that adsorbed N₂ resides in the missing-row troughs on the reconstructed surface. Nitrogen is displaced by hydrogen, and the most tightly bound state of hydrogen blocks virtually all N₂ adsorption. A p1g1(2 × 2) LEED pattern is associated with a saturated overlayer of adsorbed N₂ on Ir(110)-(1 × 2).     

The adsorption of CO on evaporated iridium films,studied with infrared reflection-absorption spectroscopy

The adsorption of ^12CO on Ir films evaporated under ultrahigh vacuum (UHV) conditions was studied using infrared reflection-absorption spectroscopy (IRAS). Only a single absorption band was observed at 300 K, shifting continuously from the “singleton” value ~2010 cm^?1 at very low coverages to 2093 cm^?1 at saturation coverage. This band is attributed to CO adsorbed on top of the surface atoms. Synchronously with this shift the bandwidth at half maximum intensity $\text{Δv}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ decreases from ~30 to 8 cm^?1. The integrated peak area increases linearly with coverage up to a relative coverage (θ_r) of approximately 0.4, then the increase levels off and a maximum is observed. Upon continuing adsorption the intensity decreases slightly. In addition results are presented on adsorption at 300 K of ¹²CO?¹³CO isotopic mixtures. The coverage induced frequency shift is discussed in terms of a dipole-dipole coupling mechanism and it is concluded that intermolecular coupling can explain the shift (~83 cm^?1) observed. The decrease in intensity at coverages > 0.4 is attributed to the formation of a compressed overlayer with part of the CO molecules adsorbed in a multicentre position with different spectral properties. No infrared bands of nitrogen adsorbed at 78 K could be detected at pressures up to 6.7 kPa (1 Pa = 0.0075 Torr, 1 Torr = 133.32 Pa).     

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm^?1, 1855 cm^?1 (shifting to 1944 cm^?1 with increasing coverage) and at 1960 cm^?1 (shifting to 2016 cm^?1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

Quasi-elastic neutron scattering of benzene in Na-mordenite

The rotational and translational dynamics of benzene adsorbed in Na-mordenite have been studied by incoherent quasi-elastic neutron scattering. The measurements were performed at two benzene coverages at 300, 400 and 450 K. The observed quasi-elastic broadenings are described by a uniaxial rotational model about the six-fold axis of benzene. The mean time between successive jumps, at 300 K, is τ=1.45 × 10^?12 s at low coverage and 2.05×10^?12 s at high coverage. The correlation times follow an Arrhenius law with E_A=4.51 kJ mol^?1, at both coverages. The translational diffusion coefficient has been measured at 300 K and was found to be 0.67 × 10^?6 cm²s^?1.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

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.     

The adsorption of water vapor on clean cleaved germanium: II. Electrical properties

Clean germanium surfaces obtained by cleavage in ultrahigh vacuum were exposed to water vapor in the range from 10^?9 up to 1 Torr min. Surface state properties have been derived from measurements of surface conductivity and field effect mobility. At coverages close to a monolayer a steplike change in surface state density indicates a change of the adsorption state similar to a phase transition. The results are described quantitatively by assuming that each structure has its characteristic surface state distribution.     

A proposed experiment to measure surface roughness in Si/SiO2 system

The adsorption of oxygen on clean cleaved (111) silicon surfaces has been investigated by high resolution electron spectroscopy (HRES), Auger electron spectroscopy (AES) and ellipsometry. Localized vibrations ($\text{h}\text{?}\text{ω = 94}$, 130 and 175 meV) which are related to the binding state band of oxygen are identified with HRES. AES measures the concentration of adsorbed atoms basically independent of their binding state while ellipsometry refers additionally to the optical properties of the adsorbed layer. The same adsorption kinetics was found with the three methods. Oxygen therefore adsorbs in a single likely molecular state. The sticking coefficient S increases exponentially with the surface step concentration. S is also enhanced by the presence of nude ion gauges. Depending on these parameters sticking coefficients between 2 × 10^?4 and 10^?1 have been obtained. This result might contribute to an explanation of the large differences in earlier works.     

Ellipsometric study of oxygen adsorption and the carbon monoxide-oxygen interaction on ordered and damaged Ag(111)

The adsorption of oxygen on Ag(111) has been studied by ellipsometry in conjunction with AES and LEED. The oxygen pressure varied between 10^?5 and 10^?3 Torr and the crystal temperature between room temperature and 250° C. Changes in the Auger spectrum and the LEED pattern upon oxygen adsorption are very small. Oxygen coverages were derived from the changes in the ellipsometric parameter Δ. At room temperature a maximum coverage is reached within a few minutes. Its value increases with the damage produced by the preceding argon ion bombardment. The sticking coefficient derived from the initial rate of Δ-change amounts to 3 × 10^?5 for well-annealed surfaces and 2.5 ? 5 × 10^?4 for damaged surfaces. After evacuation no desorption takes place. Other types of adsorption, associated with much larger changes in Δ, were observed upon bombardment with oxygen ions and with oxygen activated by a hot filament. The reaction of CO with adsorbed oxygen was studied ellipsometrically at room temperature in the CO pressure range 10^?7–10^?6 Torr. The initial reaction rate is proportional to the CO pressure. The reaction probability (number of oxygen atoms removed per incident CO molecule) is 0.36.     

Decomposition of NO on clean Pt near atmospheric pressures: II. Adsorbate coverages

Temperature programmed desorption (TPD) and temperature programmed adsorption (TPA) have been used to characterize adsorbate coverages during and after NO decomposition on polycrystalline Pt foils at pressures between 10^?4 and 30 Torr. The densities and stoichiometries of tightly bound species were determined after reaction by TPD of NO, N₂, and O₂ following cooling and pumpdown to <10^?8 Torr. For characterization during reaction at pressures up to 10^?3 Torr the ribbon was flashed inside a 35 cm³ reaction cell, and desorption and adsorption spectra of all species were recorded. Using digital acquisition of pressures versus time, peaks as small as 10^?3 of the background pressure could be analyzed. By flashing to different fixed temperatures, adsorption isobars during reaction were determined. These measurements show that there is a tightly bound stoichiometric layer of N and O (perhaps undissociated) and that the reactive state is weakly bound and appears to be strongly inhibited by molecular oxygen. This model also agrees with reaction rate measurements at these pressures.     

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen $\text{(1,}\text{1}\text{2}\text{) LEED}$ beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is $\text{13.5 ± 0.5}\text{kcal}\text{mole}$. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10^?3 cm² s^?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

Adsorption von Wasserstoff an platin: I. Adsorbierte menge,kinetik der ad- und desorption

Chemisorption of hydrogen on a platinum-foil has been studied by the flash-filament technique and by the capillary-flow method. Isobars were determined at temperatures between 140 and 600° K and at pressures between 10⁻⁴ and 10⁻⁶ torr. The dependence of the isosteric heat of adsorption E on the coverage σ is given by E = 16.3 −1.4 × 10⁻¹⁴σ (kcal/mole) with σ<2.5 × 10¹⁴ molecules/cm². At higher coverages E reaches values smaller than 5 kcal/mole. Adsorption and desorption at low coverages are second order processes. The initial sticking probability is 4.5 × 10⁻³. The results are discussed in terms of an atomic and a molecular state of the adsorbed hydrogen.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

Impact of potassium on the heats of adsorption of adsorbed CO species on supported Pt particles by using the AEIR method

The heats of adsorption at several coverages of the linear and bridged CO species (denoted L and B, respectively) adsorbed on the Pt⁰ sites of the 2.9 wt% Pt/10% K/Al₂O₃ catalyst are determined using the Adsorption Equilibrium Infrared spectroscopy method. The addition of K on 2.9% Pt/Al₂O₃ modifies significantly the adsorption of CO on the Pt particles: (a) the ratio L/B is decreased from 8.4 to 1, (b) a new adsorbed CO species is detected with an IR band at 1763 cm⁻¹, (c) the heats of adsorption of L and B CO species are significantly altered and the positions of their IR bands are shifted. The heats of adsorption of L CO species are decreased: i.e. 206 and 105 kJ/mol at low coverages on Pt/Al₂O₃ and Pt/K/Al₂O₃ respectively. Two B CO species denoted B1 and B2, with different heats of adsorption are observed on Pt/K/Al₂O₃. The heats of adsorption of B2 CO species (major B CO species) are significantly larger than those measured in the absence of K: i.e. 94 and 160 kJ/mol at low coverages on Pt/Al₂O₃ and Pt/K/Al₂O₃ respectively, whereas those of B1 CO species (minor species) are similar: 90 kJ/mol at low coverages. These values are consistent with the qualitative High Resolution Electron Energy Loss Spectrometry literature data on Pt(1 1 1) modified by potassium.     

The adsorption of nitric oxide on W(110) at 300 K: An electron spectroscopic study

Nitric oxide adsorbs dissociatively onto W(110) at 300 K. The simplest interpretation requires adsorption into three states. In the first of these N and O penetrate the surface. In the second state N and O form an overlay er on the W(110) surface. The third state also involves surface penetration but this is deeper than for state 1. The adsorbate stoichiometry was 1:1 throughout adsorption at 300 K. The occupancy of each state was similar after a 300 L exposure when the total amount adsorbed exceeded 17 (±2) × 10¹⁸ atoms m^?2.     

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.