Binding states and decomposition of NO on single crystal planes of Pt

Nitric oxide desorption and reaction kinetics are compared on the (111), (110),and (100) planes of platinum using temperature programmed desorption mass spectrometry. NO exhibits large crystallographic anisotropies with the (100) plane having stronger bonding and much higher decomposition activity than the (110) or (111) planes. The desorption activation energies for the major tightly bound states are 36, 33.5, and 25 kcal mole^?1 on the (100), (110), and (111) planes respectively. Pre-exponential factors for these states on the (110) and (111) planes are 1 × 10^16±0.5s^?1. The major tightly bound state on the (100) plane dissociates to yield 50% N₂ and O₂, but all other states all planes desorb without significant decomposition. The fraction decomposed is less than 2% on the Pt(111) surface.     

Flash desorption and equilibration of H2 and D2 on single crystal surfaces of platinum

The adsorption and flash desorption of hydrogen and the equilibration of H₂ and D₂ has been studied on the (110), (211), (111) and (100) planes of platinum. Desorption from Pt (211), a stepped surface composed of (111) and (100) ledges, yields a desorption spectrum which apparently is a composite of desorption from the individual ledges. Pt (110) is quite similar to the tungsten structural analog, W (211), in that both yield two-peak desorption spectra, and on both planes adsorption kinetics are dramatically different for filling of the two states. On all four planes adsorption kinetics are apparently proportional to (1 ? θ)², and estimates of the initial sticking probabilities show them to decrease in the order: (110) > (211) > (100) > (111). Equilibration activity follows approximately the same order [(110) > (211) > (111) > (100)] with a factor of ~ 5 difference between the most and least active planes; no extraordinary activity is observed for the stepped surface, Pt(211). Below ~ 570 K equilibration of H₂ and D₂ is activated by less than 2 kcal/mole with the magnitude dependent on the specific face, and above this temperature the reaction is nonactivated. The non-activated case apparently results from absorption followed by statistical mixing on the surface. Calculated rates for HD production per cm² based on this model are in excellent agreement with the experimental values for Pt(110) and Pt(211), and in somewhat poorer agreement in the case of Pt (111) and Pt (100). This latter is probably due to the greater inaccuracy in the values of the sticking coefficients on these planes.     

Triple point of the first monolayer of nitric oxide adsorbed on the cleavage face of magnesium bromide

This paper is the first of three articles devoted to the CO/Mo(110) chemisorption. The experimental study of adsorption and desorption kinetics was performed by several methods: thermal desorption, low energy electron diffraction and Auger electron spectroscopy. The adsorption of CO on Mo(110) presents two different states. For these two states the desorption kinetics are first order ones, the desorption energies and frequency factors have been determined (E₁ = 99 kcal mole^?1, E₂ = 50 kcal mole^?1, v₁ = 10¹⁹ s^?1, v₂, = 5 × 10¹⁰ s^?1). The dependence of sticking coefficient on surface coverage θ was investigated and was found different for the two states of adsorption. LEED shows that the adsorption is not ordered. AES investigation suggests that in the two states C and O have different positions with respect to MO atoms.     

Field electron emission study of nitrogen adsorbed on individual tungsten planes

The adsorption of nitrogen on 211, 111, 100 and 110 tungsten planes has been studied by means of the probe-hole emission technique over a wide range of temperatures. The field emission tube was attached to a molecular beam system. This technique enabled deposition of strictly controlled doses of nitrogen. It has been found that on the 211 plane three states of nitrogen γ, α and β exist. In the γ state molecules of opposite polarity are present. These correspond to the γ⁺ and γ^?. The α state undergoes transformation at about 300 K to a more stable β state. β nitrogen leaves the 211 plane through surface diffusion in the temperature region 600–700K. Results obtained on the 111 plane in the low temperature region confirm previous findings on the existence of γ and α states. At higher temperatures the concentration of nitrogen in the β state increases as a result of migration from the 211 plane. There is some evidence as to the existence of two high energetic states of nitrogen on the 111 and 100 planes. On the 110 plane only partial results were obtained due to field desorption.     

Adsorption of CO on Pd single crystal surfaces

Studies of CO adsorption on Pd(110), (210) and (311) surfaces as well as with a (111) plane with periodic step arrays were performed by means of LEED, contact potential and flash desorption measurements. Isosteric heats of adsorption were evaluated from adsorption isotherms. Earlier work with Pd(111) and Pd (100) surfaces is briefly reviewed, yielding the following general picture: The initial adsorption energies vary between 34 and $\text{40}\text{kcal}\text{mole}$ and close similarities exist for the dipole moments, the maximum densities of adsorbed particles and for the adsorption kinetics. At low and medium coverage the adsorbed particles are located at highly symmetrical adsorption sites, whereas saturation is characterized by the tendency for formation of close-packed layers.     

The effect of sulfur on co adsorption/desorption on Pt(S)-[9(111) × (100)]

CO adsorption/desorption on clean and sulfur covered Pt(S)-[9(111) × (100)] surfaces was studied using AES, TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface — a low temperature state associated with the (111) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption significantly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was

$\text{1 × 10}{}^{15}\text{s}{}^{\mathrm{?1}}\text{exp (}\text{?36.2 kcal/mole}\text{RT}\text{)}$

Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic of the step/defect sites. For the surface on which step/defect sites were blocked by sulfur the effective desorption rate constant was

$\text{k}{}_{\mathrm{eff}}\text{= 1 × 10}{}^{13}\text{s}{}^{\mathrm{?1}}\text{exp (?}\text{27.5 kcal/mole}\text{RT}\text{)}$

indicating an appreciable decrease in CO binding on the terraces, though sulfur-CO repulsive interactions had probably made k_eff larger than the true rate constant for desorption from clean (111) planes. The results showed clearly a compensation effect in activation energy and preexponential factor.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

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).     

Errata

The rate of NH₃ decomposition on (111), (100), (110) and (210) planes of Pt have been measured between 600 and 1400 K and between 10^?2 and 1 Torr in a low conversion flow system with mass spectrometric gas analysis. Surfaces were in the form of thin discs which were heated by focussed light beams to achieve uniform temperatures. The open (210) plane has almost the same activity as polycrystalline wires or foils, and rates decrease in the order (210) > (110) > (111) > (100). Temperature and pressure dependences agree quite well with a Langmuir-Hinshelwood (LH) expression from which reaction and adsorption activation energies on each plane are obtained. On (100) and (111) the rate is 10 times less than on high activity planes and this rate may be largely due to reaction on the edges of the crystals. From AES analysis of surfaces and reproducibility of results it is suggested that surfaces are free of contaminants in these experiments.     

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.     

Adsorption of nitrogen on platinum

The adsorption and desorption of nitrogen on a platinum filament have been studied by thermal desorption techniques. Nitrogen adsorption becomes significant only after any carbon contamination is removed from the surface by heating the platinum filament in oxygen, and after the CO content in the background gas is reduced substantially. At room temperature nitrogen populates an atomic tightly bound β-state, E^≠ = 19 kcal mole^?1. The saturation coverage of the (3-state is 4.5 × 10¹⁴ atoms cm^?2. Formation of the (β-state is a zero order process in the pressure range studied. At 90 K two additional α₁- and α₂-desorption peaks are observed. The activation energy for desorption for the α₂-state is 7.4 kcal mole^?1 at low coverage decreasing to 3 kcal mole^?1 at saturation of this state, 6 × 10 molecules cm^?2. The maximum total coverage in the α-states was 1.2 × 10¹⁵ molecules cm^?2. A replacement process between the β- and α-states has been observed where each atom in the (β-state excludes two molecules from the α-state.     

Kinetics of adsorption and desorption using Auger electron spectroscopy: Application to xenon covered (0001) graphite

Three regimes of condensation have been observed between 74 and 80 K in the adsorption and desorption of a submonolayer film of xenon. The first one corresponds to thej condensation or evaporation of a two-dimensional (2D) ‘gas’, the second one to the growth of 2D crystal in the presence of the 2D gas, and the third one to the completion of the 2D crystal on the (0001) graphite face. Zero order kinetics for both adsorption and desorption is found in the large range of coverage (0.3 < θ <0.9) where the two phases coexist on the surface. The activation energy of desorption of the 2D crystal is measured; its value (～6 kcal mole^?1) is in fair agreement with the value of the latent heat of evaporation of this phase (5.5 or 5.7 kcal mole^?1) determined previously. No activation energy of nucleation has been observed during the adsorption process. The growth rate is controlled by the incident flux only.     

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.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

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.     

The adsorption of CO,H2, H2, CO2 and H2O on carburized and graphitized Ni(110)

A study of the adsorption/desorption behavior of CO, H₂O, CO₂ and H₂ on Ni(110)(4 × 5)-C and Ni(110)-graphite was made in order to assess the importance of desorption as a rate-limiting step for the decomposition of formic acid and to identify available reaction channels for the decomposition. The carbide surface adsorbed CO and H₂O in amounts comparable to the clean surface, whereas this surface, unlike clean Ni(110), did not appreciably adsorb H₂. The binding energy of CO on the carbide was coverage sensitive, decreasing from 21 to 12 $\text{kcal}\text{mol}$ as the CO coverage approached 1.1 × 10¹⁵ molecules cm^?2 at 200K. The initial sticking probability and maximum coverage of CO on the carbide surface were close to that observed for clean Ni(110). The amount of H₂, CO, CO₂ and H₂O adsorbed on the graphitized surface was insignificant relative to the clean surface. The kinetics of adsorption/desorption of the states observed are discussed.     

The adsorption of hydrogen on iron; A surface orbital modified occupancy — bond energy bond order calculation

A Surface Orbital Modified Occupancy — Bond Energy Bond Order (SOMO-BEBO) model calculation of hydrogen adsorption on iron is presented. This calculation represents a novel approach to the CFSO-BEBO method in that the calculation is correlated in a consistent way with the thermal desorption spectra of the hydrogen-iron system. Heats of molecular adsorption calculated are ?32.88, ?35.68 and ?49.57 kJ/mol for the iron (110), (100), and (111) surfaces, respectively. Heats of dissociative adsorption calculated are ?54.40, ?75.30 and ?87.90 kJ/mol for the three states on the iron (111) surface; ?51.21 and ? 73.62 kJ/mol for the two states on the iron (100) surface; and ?63.78 kJ/mol for the one state on the iron (110) surface. Activation energies for dissociative adsorption were found to be small or zero for the iron (111) surface while non-zero activation energies of 49.27 and 45.05 kJ/mol were calculated for the iron (100) and (110) surfaces, respectively. The FeH single-order bond energy has been calculated to be 298.2 kJ/mol. The radius of the hydrogen surface atom has been estimated to be 1.52 × 10^?10 m consistent with the expected size of an H^? ion. The elimination of certain surface sites for molecular adsorption as a result of the ferromagnetism of iron is suggested by the calculation. The reason for the absence of well defined LEED patterns for hydrogen adsorption on the iron (111) and (100) surfaces [Bozso et al., Appl. Surface Sci. 1 (1977) 103] is explained on the basis of the size of the H^? surface ion. The adsorption of hydrogen on the iron (110) surface is consistent with a relatively stable, small-sized H⁺₂ surface ion giving, therefore, a regular LEED pattern and a positive surface potential upon adsorption of hydrogen on this surface.     

Adsorption of molecular nitrogen by the W(110) plane

Characteristics of the adsorption of nitrogen on the (110) plane of tungsten were determined by thermal desorption and work function measurements. The low temperature γ-N₂ state desorbs with first order kinetics and an activation energy of 6 kcal mole^?1. The absence of isotope mixing between ¹⁴N₂ and ¹⁵N₂ demonstrates γ-N₂ is adsorbed molecularly. Monolayer coverage shows a decrease of 0.19 eV in work function. A Topping model plot indicates the layer is immobile at 123 K.     

The adsorption of carbon monoxide,ammonia, and wet air on gold

The work functions of gold films which were deposited on glass substrates in UHV were 0.5–0.9 eV higher than the work function of a well-baked gold sheet. The contact potential difference between a film and the sheet was reduced by wet air admitted to both surfaces at room temperature. Carbon monoxide admitted to both surfaces reduced the contact potential difference reversibly at pressures from 1 × 10^? to 2 × 10^?2 torr, and the evidence suggested that most of the change was owing to a reduction in the work function of the gold film. This reduction varied linearly with the gas pressure; it also depended on the temperature; decreasing from 2.8 eV torr^? at 17°C to < 0.25 eV torr^? at 72°C. The results for CO fitted a simple classical model, from which the mean adsorption energy for CO/Au was estimated as 11.3 ± 0.3 kcal mole^?. Ammonia at 17°C caused a similar reduction of work function at much lower pressures, ～ 10^?4 torr, and its adsorption energy was estimated as 13.6 kcal mole^?1. The films and the sheet gold were polycrystalline with their crystal orientations random in two directions, but their {100} planes were preferentially parallel to the exposed surface. The films were rougher than the sheet. The positive surface potentials for CO/Au and NH₃/Au seem to be due either to weakly bound electropositive states, or to their molecules penetrating into the sub-surface region of the film.     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.