The adsorption of Xe and CO on a Cu (311) single crystal surface

LEED, electron energy loss spectroscopy and surface potential measurements have been used to study the adsorption of Xe and CO on Cu (311). Xe is adsorbed with a heat of 19 ± 2 kJ mol^/t-1. The complete monolayer has a surface potential of 0.58 V and a hexagonal close-packed structure with an interatomic distance of 4.45 ± 0.05 Å. CO gives a positive surface potential increasing with coverage to a maximum of 0.34 V and then falling to 0.22 V at saturation. The heat of adsorption is initially 61 ± 2 kJ mol^?1, falling as the surface potential maximum is approached to about 45 kJ mol^?1. At this coverage streaks appear in the LEED pattern corresponding to an overlayer which is one-dimensionally ordered in the [011̄] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011̄] direction. At saturation the LEED pattern shows extra spots which are tentatively attributed to domains of a new overlayer structure coexisting with the first. Electron energy loss spectra (EELS) of adsorbed CO show two characteristic peaks at 4.5 and 13.5 eV probably arising from transitions between the electronic levels of chemisorbed CO.     

The adsorption of Xe and CO on Ag(111)

The adsorption of Xe and CO on Ag(111) in the range 66 to 123 K and 10^?7 to 10^?1 Pa has been studied by surface potential, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and electron energy loss spectroscopic (EELS) measurements. Isotherms derived from both surface potential and AES measurements for submonolayer Xe adsorption reveal successive stages and a two-dimensional phase change. Isosteric heats were 18 ± 1 kJ mol^?1. Temkin isotherms were observed for CO, the heat falling linearly with coverage from an initial value of 27 ± 1.5 kj mol^?1. No ordered CO overlayer structure could be detected. EEL spectra of clean Ag(111) agree with previous studies. Additional loss peaks were recorded for Xe and CO overlayers, and the assignment of the substrate loss features is discussed in relation to the effects of adsorption.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

The chemisorption of mercury on tungsten (100): Adsorption and desorption kinetics,equilibrium properties and surface structure

The adsorption of mercury on W(100) has been studied by Auger spectroscopy, LEED and by work function and thermal desorption measurements. Mercury adsorbs at room temperature to form a (1 × 1) monolayer, with a sticking probability of unity and a heat of adsorption of 208 ± 12kJ mol^?1. The coverage dependence of the work function change is interpreted according to an island growth mechanism which is shown to be consistent with the LEED observations. At higher temperatures, the equilibrium isotherms show evidence for attractive adsorbate-adsorbate lateral interactions. The isotherms were simulated using a localised adlayer model within the quasi-chemical approximation. This gave a nearest-neighbour pairwise interaction energy of 5.85 kJ mol^?1. The attractive interactions are shown to be consistent with the mechanism invoked to explain the desorption kinetics, which are zero order.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

Oxygen chemisorption and the carbon monoxide-oxygen interaction on Ru(101)

The adsorption of oxygen and the interaction of carbon monoxide with oxygen on Ru(101) have been studied by LEED, Auger spectroscopy and thermal desorption. Oxygen chemisorbs at 300 K via a precursor state and with an initial sticking probability of ~0.004, the enthalpy of adsorption being ~300 kJ mol^?1. As coverage increases a well ordered ¦11,30¦ phase is formed which at higher coverages undergoes compression along [010] to form a ¦21,50¦ structure, and the surface eventually saturates at 0 ~ $\text{8}\text{9}$. Incorporation of oxygen into the subsurface region of the crystal leads to drastic changes in the surface chemistry of CO. A new high; temperature peak (γ CO, E_d ~ 800 kJ mol^?1) appears in the desorption spectra, in addition to the α and β CO peaks which are characteristic of the clean surface. Coadsorption experiments using ¹⁸O₂ indicate that γ CO is not dissociatively adsorbed, and this species is also shown to be in competition with β CO for a common adsorption site. The unusual temperature dependence of the LEED intensities of the ¦11,30¦-O phase and the nature of α, β, and β CO are discussed. Oxygen does not displace adsorbed CO at 300 K and the converse is also true, neither do any Eley-Rideal or Langmuir-Hinshelwood reactions occur under these conditions. Such processes do occur at higher temperatures, and in particular the reaction CO(g) + O(a) → CO₂(g) appears to occur with much greater collisional efficiency than on Ru(001). The oxidation of CO has been examined under steady state conditions, and the reaction was found to proceed with an apparent activation energy of 39 kJ mol^?. This result rules out the commonly accepted explanation that CO desorption is rate determining, and is compared with the findings of other authors.     

The Pt(110) phase transitions: A study by rutherford backscattering,nuclear microanalysis,LEED and thermal desorption spectroscopy

The adsorbate induced (1×2) (1×1) (2×1)p1g1 phase transitions on Pt(110) have been studied by Rutherford backscattering (RBS), nuclear microanalysis (NMA), LEED and thermal desorption spectroscopy. RBS data indicate that any displacement of the surface atoms from their expected bulk-like lattice sites in the (1×2) phase is ? 0.002 nm laterally and ? 0.007 nm vertically. This contraint eliminates models for the reconstruction which involve significant lateral displacements (e.g., the paired-atom or hexagonal overlayer models). The RBS data are consistent with both the rumpled model with up/down displacements not exceeding ~0.007 nm and the missing row model with an unrelaxed surface in which the out-of-plane vibrational amplitude is slightly enhanced. A c(8×4) phase, produced by CO (or NO) exposure at T?250 K, has also been characterized by RBS which demonstrated that 0.92×10¹⁵ Pt cm^?2 move on average by ~0.017 nm laterally out-of-registry with the bulk upon formation of this phase. The values of the saturation adsorbate coverages at T?200K were determined by NMA to be 0.92 ± 0.05×10¹⁵, 1.0 ± 0.06×10¹⁵ and 1.07 ± 0.10×10¹⁵ CO molecules, NO molecules and D atoms, respectively, per cm². The value of the saturation coverage by CO (θ = 1.0) supports recent models of the (2×1)p1g1 overlayer. The isosteric heat of adsorption of CO is 160 ± 15 kJ mol^?1 in the range 0.2?θ?0.5.     

Interaction of inert gases with a nickel (100) surface: I. Adsorption of xenon

The adsorption of Xe on a Ni(100) surface has been studied in UHV between 30 and 100 K using LEED, thermal desorption spectroscopy (TDS), work function (Δφ) measurements, and UV photoemission (UPS). At and below 80 K, Xe adsorbs readily with high initial sticking probability and via precursor state adsorption kinetics to form a partially ordered phase. This phase has a binding energy of ~5.2 kcal/mole as determined by isosteric heat measurements. The heat of adsorption is fairly constant up to medium coverages and then drops continuously as the coverage increases, indicating repulsive mutual interactions. The thermal desorption is first order with a preexponential factor of about 10¹² s^?1, indicative of completely mobile adsorption. Adsorbed Xe lowers the work function of the Ni surface by 376 mV at monolayer coverage. (This coverage is determined from LEED to be 5.65 × 10¹⁴ Xe molecules/cm^-2.) For not too high coverages, θ, Δφ(θ) can be described by the Topping model, with the initial dipole moment μ₀ = 0.29 D and the polarizability α being 3.5 × 10^?24 cm³. In photoemission, the $\text{Xe 5p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{5p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbitals show up as intense peaks at 5.56 and 6.83 eV below E_f which do not shift their position as the coverage varies. Multilayer adsorption (i.e. the filling of the second and third layers) can be seen by TDS. The binding energies of these α states can be estimated to range between 4.5 and 3.5 kcal/mole. The results are compared and contrasted with previous findings of Xe adsorption on other transition metal surfaces and are discussed with respect to the nature of the inert-gas-metal adsorptive bond.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.     

Unusually low stretching frequency for CO adsorbed on Fe(100)

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to ～ 350 K and low CO exposure (≦1 L), a predominant molecular species with an unusually low stretching frequency, 1180–1245 cm^?1, is detected. This unusual CO bond weakening is consistent with a “lying down” binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900–2055 cm^?1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.     

Formic acid desorption from graphitized Ni(110)

The adsorption/desorption behavior of formic acid from a monolayer of graphite carbon on Ni(110) was studied using AES, LEED and flash desorption spectroscopy. Formic acid adsorbed at 165 K did not form multilayers of adsorbate. Instead, due to strong hydrogen-bonding interactions the formic acid formed a two-dimensional condensed phase on the surface and exhibited zero-order desorption kinetics initially for a 30-fold change in initial coverage. The zero-order desorption rate constant was k_d = 10¹⁸ exp[?68.2 kJ mol^?1/RT]s^?1, suggesting a desorption transition state with nearly full translational and rotational freedom on the surface. The desorption kinetics and the coverage limit were consistent with the formation of a surface polymer-monomer equilibrium.     

On the structure of CO adlayers on Cu(100) and Cu(111)

Alternative models for the high coverage compressed overlayers of CO on copper (100) and (111) are proposed in which the LEED patterns previously attributed to out-of-registry, uniformly compressed structures are re-interpreted in terms of adsorption on linear and bridged sites. The models provide a simpler explanation for the small influence of compression on the frequency of the infrared adsorption bond near 2000 cm^?1 compared with the large change of surface potential.     

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.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

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.     

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 and thermal desorption spectroscopy. Molecular oxygen adsorbs mainly as a peroxo-like species at 100K with a heat of desorption of about 22 k^J/mol. Some of the molecular oxygen also adsorbs dissociatively at 100K. Atomic oxygen is adsorbed in three states. One state is due to adsorption on the terraces and another state is due to adsorption along the rough step sites. The heat of desorption of both of these states approximately equal and decreases from 290 k^J/mol to 195kJ/mol with increasing coverage. Atomic oxygen is also observed to adsorb in another state which is interpreted as adsorption at an on-top site.     

Chemisorption,surface structural chemistry,and electron impact properties of carbon monoxide on rhodium (110)

The adsorption, desorption, surface structural chemistry, and electron impact properties of CO on Rh(110) have been studied by LEED, Auger spectroscopy, thermal desorption, and surface potential measurements. At 300 K, CO adsorbs into a single chemisorbed state whose desorption energy (E_d) is ~130kJmol^-1. The initial sticking probability is unity, and at saturation coverage a (2 × 1)plgl ordered phase reaches its maximum degree of perfection, thus demonstrating that this CO structure is common to the (110) faces of all the cubic platinum group metals. The saturated adlayer corresponds to θ = 1 and shows a surface potential of Δ? = +0.97 V. Under electron impact, desorption and dissociation of CO occur with about equal probability, the relevant cross sections being ~10^-22 m² in each case. Slow thermal dissociation of CO occurs at high temperature and pressure, leaving a deposit of C and O atoms on the surface. The thermal, electron impact, and Δ? properties of Rh(110)CO resemble those of Ni(110)CO rather closely, and are very different from those of Pt(110)CO. Surface carbon is shown to inhibit CO chemisorption, whereas surface oxygen appears to lead to the formation of a new more tightly bound form of CO with a considerably enhanced desorption energy (E_d ~ 183 kJmol^-1). Similar oxygen-induced high temperature CO states have been reported recently on Co(0001) and Ru(101&#x0304;1).     

Interaction of O2 with Pt(100): I. Equilibrium measurements

Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10¹⁵ O atoms cm^?2), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (? 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.