Adsorption and isotopic exchange of nitrogen on polycrystalline iron films between 273 and 600 K

The adsorption of nitrogen on evaporated iron films under uhv conditions has been studied between 273 and 600 K by means of direct volumetric and pressure measurements. The dissociative adsorption occurs with sticking probabilities in the range of 10^?8 to 10^?7 and apparent activation energies between ?1 and 5 kJ mol^?1 depending on the structure of the film. A mechanism for the dissociative adsorption of nitrogen is proposed taking into account molecularly adsorbed nitrogen as a precursor. After the adsorption of nitrogen enriched in ¹⁵N₂ the establishing of the iso topic equilibrium was observed using isothermal desorption.     

Chemisorption of NO on Re studied by XPS

The adsorption of nitric oxide on clean rhenium has been studied by X-ray photoelectron spectroscopy. At room temperature only dissociative adsorption occurs; at 100 K, NO adsorbs molecularly at low coverages. Higher NO exposures lead to the formation of NO₂ which dissociates upon warming above 180 K.     

Adsorption,coadsorption, and reactivity of sodium and nitric oxide on Ag(111)

The adsorption, desorption, and surface structural properties of Na and NO on Ag(111), together with their coadsorption and surface reactivity, have been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption of NO into the a-state occurs at 300 K with an initial sticking probability of ~0.1, saturation occurring at a coverage of $\text{~}\text{1}\text{20}$. Desorption occurs reversibly without decomposition and is characterised by a desorption energy of E_d ~ 103 kJ mol^?1. In the coverage regime 0 < θ_Na < 1, sodium adsorbs in registry with the Ag surface mesh and the desorption spectra show a single peak corresponding to E_d ~ 228 kJ mol^?1. For multilayer coverages (1 < θ _Na < 5) a new low temperature peak appears in the desorption spectra with E_d ~ 187 kJ mol^?1. This is identified with Na desorption from an essentially Na surface, and the desorption energy indicates that Na atoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The LEED results show that Na multilayers grow as a (√7 × √7) R19.2° overlayer, and are interpreted in a way which is consistent with the above conclusion. Coadsorption of Na and NO leads to the appearance of a more strongly bound and reactive chemisorbed state of NO (β-NO) with E_d ~ 121 kJ mol^?1. β-NO appears to undego surface dissociation to yield adsorbed O and N atoms whose subsequent reactions lead to the formation of N₂, N₂O, and O₂ as gaseous products. The reactive behaviour of the system is complicated by the effects of Na and O diffusion into the bulk of the specimen, but certain invariant features permit us to postulate an overall reaction mechanism, and the results obtained here are compared with other relevant work.     

Calorimetric studies of NO on Ni{2 1 1}: criteria for switching from dissociative to molecular adsorption

The coverage-dependent heats of adsorption and sticking probabilities in the interaction of nitric oxide with clean and oxygen pre-covered Ni{2 1 1} surfaces have been measured at 300 K using single crystal adsorption calorimetry. The results are consistent with a switch from dissociative to molecular chemisorption at 1 ML of O plus N adatoms. Initial dissociative adsorption is attributed to step sites with a heat of 400 kJ mol⁻¹. When steps are saturated with adatoms, adsorption proceeds molecularly with a heat of 160 kJ mol⁻¹. With 0.24 ML oxygen adatom pre-coverage, the initial heat is only 250 kJ mol⁻¹ and with 0.6 ML oxygen adatom, NO adsorption is only molecular with an initial heat of 160 kJ mol⁻¹. The NO sticking probability behaviour is consistent with this picture, with successive precursor mediated adsorption at step and terrace sites. The inhibition of dissociation above O, or O plus N, adatom coverages of 1 ML is attributed to the strong lateral repulsive interactions between adatoms, which would drive the dissociative heat of adsorption below that of molecular adsorption at higher coverages.     

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).     

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.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

Adsorption and dissociation of water on LaB6(100) investigated by surface vibrational spectroscopy

The chemisorption of water (H₂O and D₂O) on a LaB₆(100) surface was studied with reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS). The clean surface was exposed to H₂O and D₂O at temperatures from 90 K to room temperature, and spectra were acquired after heating to temperatures as high as 1200 K. It was found that water molecularly adsorbs on the surface at 90 K as a monomer at low coverages and as amorphous solid water at higher coverages. Water adsorbs dissociatively at room temperature to produce surface hydroxyl species as indicated by OH/OD stretch peaks at 3676/2701 cm^?1. Room temperature adsorption also reveals low frequency loss features in HREEL spectra near 300 cm^?1 that are quite similar to results obtained following the dissociative adsorption of O₂. In the latter case, the loss features were attributed to the LaO stretch of O atoms bridge-bonded between two La atoms. In the case of dissociative adsorption of H₂O, the low frequency loss features could be due to either the LaO vibrations of adsorbed O or of adsorbed OH.     

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 reaction of nitrous oxide with rhodium

A clean rhodium filament at room temperature is highly reactive towards nitrous oxide. The oxygen atom of the N₂O molecule is adsorbed with a sticking probability of 0.45 whilst the nitrogen atoms appear in the gas phase as molecular nitrogen. The room temperature uptake of oxygen is about 5 × 10¹⁴ atom cm^?2 and is independent of nitrous oxide pressure in the range 3.5 × 10^?8 to 1.1 × 10^?6 torr. The adsorption curve is of typical form with an initial region of essentially constant sticking probability. For the first 80% of adsorption at room temperature the shape is satisfactorily accounted for if molecules are able to visit 4–5 adsorption sites whilst held in a weakly-bonded precursor state.     

Adsorption of carbon monoxide on polycrystalline nickel films

The adsorption states of carbon monoxide on polycrystalline nickel films have been investigated by measuring the thermal desorption, the heat of adsorption, the change in resistivity, and the change in work function in dependence on coverage and temperature. It can be shown that there are two chemisorbed (β₂, β₂) and one weakly bound (γ) species. Desorption peaks appear at 170K, 310–360 K, and 460–490 K. The differential heat of adsorption is $\text{30}\text{kcal}\text{mole}$ at low coverages and approximately $\text{25}\text{kcal}\text{mole}$ between 0.3 and 0.6 monolayers. The resistivity of the nickel film is characteristically changed with increasing coverage, and there is a maximum of resistivity at half a monolayer. At low coverages the increase in the work function is proportional to the amount adsorbed; at a monolayer the total increase is 1.26 eV at 77 K and 1.46 eV at 273 K. The two chemisorbed species differ only in the structures they form in the adsorption phase, β₂ being the species that is stable at low coverages, β₁ being the species that is stable at high coverages. These results are in good agreement with those recently found for CO adsorption on single crystal surfaces.     

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.     

Physical adsorption of nitric oxide on metal oxides

Adsorption isotherms have been measured at 77.5 K for nitric oxide and nitrogen on Al₂O₃, MgO, ZnO and NiO, and at 90.2 K. for nitric oxide on A1₂O₃ and NiO. Three isotherm measurements at 77.5 K were made on the Al₂O₃ sample for each adsorbate to examine the effect of different degrees of surface dehydroxylation. The latter was assessed by means of infrared absorption studies on an Al₂O₃ disc. Isosteric heats for NO adsorption on Al₂O₃ and NiO increase from ca. 8 kJ mol^?1 and 6 kJ mol^?1 (respectively) at half monolayer coverage to near the value of the enthalpy of sublimation (16.6 kJ mol^?1) at monolayer completion. These results are discussed in terms of adsorbate dimerisation. Anomalous adsorption-desorption behaviour for the NONiO system is discussed. Effective adsorbate molecular cross-sectional areas and results for N2 adsorption on preadsorbed NO do not support the existence of either localisation or micro-porosity.     

Elucidation of the nature of active oxygen in the reaction of low-temperature oxidation of CO on single crystal surfaces platinum and palladium

The temperature-programmed reaction (TPR) method, high-resolution electron energy loss spectroscopy (HREELS), and molecular beam method were used to elucidate the role surface reconstruction, subsurface oxygen (O_subs), and CO_ads concentration play in the low-temperature oxidation of CO on the Pt(100), Pt(410), Pd(111), and Pd(110) surfaces. The possibility of the formation of so-called hot oxygen atoms, which arise at the surface at the instant of dissociation of O_2ads molecules and can react with CO_ads at low temperatures (～150 K) to form CO₂, was examined. It was revealed that, when present in high concentration, CO_ads initiates the phase transition of the Pt(100)-(hex) reconstructed surface into the (1 × 1) non-reconstructed one and blocks fourfold hollow sites of oxygen adsorption (Pt₄-O_ads), thereby initiating the formation of weakly bound oxygen (Pt₂-O_ads), active in CO oxidation. For the Pt(410), Pd(111), and Pd(110) surfaces, the reactivity of O_ads with respect to CO was demonstrated to be dependent on the surface coverage of CO_ads. The ¹⁸O_ads isotope label was used to determine the nature of active oxygen reacting with CO at ～150–200 K. It was examined why a CO_ads layer produces a strong effect on the reactivity of atomic oxygen. The experimental results were confirmed by theoretical calculations based on the minimization of the Gibbs energy of the adsorption layer. According to these calculations, the CO_ads layer causes a decrease in the apparent activation energy E _act of the reaction due to changes in the type of coordination and in the energy of binding of O_ads atoms to the surface.     

The adsorption of xenon by W(111), and its interaction with preadsorbed oxygen

The physisorption of Xe on W(111) and of Xe on partial layers of oxygen chemisorbed on W(111) has been studied using flash desorption and work function methods. It has been found that xenon adsorbs up to monolayer coverages at 104K. Xenon desorbs from W(111) as a single binding state following first order kinetics. At low coverages (θ_Xe < 0.07) the binding energy decreases with increasing coverage possibly because of the presence of high energy adsorption sites due to crystal imperfections and edge effects. For θ_Xe > 0.07 the desorption data fit a first order rate expression with a desorption energy of 9.3 kcal/mol and preexponential ν = 10¹⁵s^?1. The observed work function change of ?1.1 ± 0.1 eV is consistent with monolayer estimates reported in field emission studies of physisorbed xenon on tungsten. The effect of preadsorbed oxygen layers on the physisorption of xenon on this surface is very striking. The energy of desorption shifts as much as 50% higher for a moderate exposure of oxygen. Several physisorption models are explored along with estimates of dispersion and electrostatic interaction contributions.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

CO adsorption isotherms on Cu(100) at elevated pressures and temperatures using infrared reflection absorption spectroscopy

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10⁻⁶ to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δE_ads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δE_ads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δE_ads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures (<10⁻⁴ Torr) and temperatures (<275 K). At substrate temperatures above 240 K and at pressures > 10⁻⁴, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.     

Vibrational spectroscopy of oxygen on the (100) and (111) surfaces of lanthanum hexaboride

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O₂ produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ～ 734 cm^?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm^?1 with full widths at half maxima of only 12 cm^?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB₆. On the boron-terminated LaB₆(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O₂.     

The water-forming reaction on palladium

The water-forming reaction on Pd has been studied on a PdSiO₂Si (Pd-MOS) structure in the temperature range 323–473 K. The reaction is found to be of the Langmuir-Hinshelwood type with the formation of OH beeing rate limiting. Since the Pd-MOS structure works as a sensitive hydrogen detector unique information on the behaviour of hydrogen during this catalytic reaction has been obtained. The reaction can be described in a model where the hydrogen atoms on the Pd surface have a large temperature activated lateral mobility and with no evidence of beeing in hot precursor states. At T = 473 K this means that for oxygen coverages ? 0.01 monolayers all hydrogen adsorbed will also react with oxygen. For smaller oxygen coverages unreacted hydrogen will not initially desorb towards the vacuum but towards the internal Pd surface of the Pd-MOS structure. Futhermore, hydrogen adsorption is blocked by adsorbed oxygen. The sticking coefficient for hydrogen on the bare Pd surface is, however, close to one and only weakly temperature dependent. An effect giving rise to a hysteresis in the work function versus oxygen coverage curve during oxygen adsorption - desorption is also discussed.     

Structural and reactive properties at the metal-halogen interface: The interaction of bromine with chromium (110)

The chemisorption and subsequent reaction of bromine on Cr(110 has been studied by Auger spectroscopy, LEED, Δφ, and thermal desorption measurements. For gas doses of < 7.5 × 10¹⁸ molecules m^?2, very efficient dissociative chemisorption leads to a series of well-ordered, out-of-registry compression structures. Uniquely, however, the overlayer falls back into registry at saturation coverage; at this point the appearance of glide symmetry indicates that the three-fold coordinated adsorption sites are occupied exclusively. Bromine → metal charge transfer occurs during adsorption (in contrast to Cr(100)). On raising the temperature at low coverages, the surface phase decomposes by evaporation as CrBr molecules; at higher coverages the desorption product switches to CrBr₂. Continuous growth of bulk CrBr₂ sets in at high gas exposures, this corrosion reaction proceeding at a rate which is ten times slower than the rate of overlayer formation. The chromium dibromide layer also evaporates as CrBr₂(g). Structural relationships with related metal-halogen systems are discussed.