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 chemisorption of CO,CO2, C2H2, C2H4, H2 and NH3 on the clean Fe(100) and (111) crystal surfaces

The chemisorption of small molecules (CO, CO₂, C₂H₂, C₂H₄, H₂ and NH₃) has been studied on the clean Fe(110) and (111) crystal faces by low-energy electron diffraction (LEED) and thermal desorption. C₂H₄ and C₂H₂ yield the same sequence of surface structures that change with temperature and crystal orientation. CO and CO₂ chemisorption similarly results in the formation of the same types of surface structures that change with surface temperature and crystal orientation. Ammonia forms several ordered surface structures on both iron crystal faces. All of the molecules decompose as a function of temperature on the iron surfaces as indicated by the Auger and thermal desorption spectra.     

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.     

Interaction of PH3 coadsorbed with H2, D2, O2 and H2O on Rh(100)

The coadsorption of PH₃ with H₂, D₂, O₂ and H₂O on Rh(100) has been studied using temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The adsorption and molecular desorption of PH₃ is not affected by preadsorbed H₂, D₂ and O₂. Preadsorbed PH₃ blocks H₂ desorption sites while postdosed PH₃ displaces H₂ (D₂₁) from the Rh(100). When D₂ and PH₃ are coadsorbed, no D appears in desorbed phosphine. Preadsorbed O₂ reduces the amount of H₂ desorption (from PH₃ decomposition) and increases the H₂ desorption temperature. There is also some reaction between O(a) and H(a) to form water. Preexposure to H₂O decreases the extent of PH₃ adsorption and of PH₃ decomposition.     

The CO coadsorption and reactions of sulfur,hydrogen and oxygen on clean and sulfided Mo(100) and on MoS2(0001) crystal faces

The chemisorption and reactivity of O₂ and H₂ with the sulfided Mo(100) surface and the basal (0001) plane of MoS₂ have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10^?8–10^?5Torr) and high (1 atm) pressures of O₂ and H₂. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of ¹⁸O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S₂ and O, but not SO₂. A pressure of 1 × 10^?7 Torr of O₂ was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS₂ was unreactive in the presence of 1 atm of O₂ at temperatures of 520 K. Sputtering of the MoS₂ produced a marked uptake of oxygen and the removal of sulfur under the same conditions.     

Kinetics of the rehydration of Fe(NH4)2(SO4)2. 4H2O

The kinetics of the reaction of ferrous ammonium tetrahydrate with water has been studied at room temperature using the Mössbauer effect. Data were fitted to two equations and from the results it is concluded that the reaction is diffusion controlled.     

H2S在Fe(100)面吸附的第一性原理研究

基于密度泛函理论第一性原理, 在广义梯度近似下, 研究了表面覆盖度为0.25 ML (monolayer)时硫化氢分子在Fe(100)面吸附的结构和电子性质, 并与单个硫原子吸附结果进行了对比. 结果表明: 硫化氢分子吸附在B2位吸附能最小为-1.23 eV, 最稳定, B1位吸附能最大为-0.01 eV, 最不稳定; 并对硫化氢分子在B1位和B2位吸附后的电子态密度进行了分析, 也表明了吸附在B2位稳定, 且吸附在B2位后硫化氢分子几何结构变化不大; 将硫化氢中硫原子吸附与单个硫原子吸附的电子性质进行了比较, 发现前者吸附作用非常微弱; 同时对吸附后的Fe(100)面进行了对比, 单个硫原子吸附的Fe(100)面电子态密度出现了一系列峰值且离散分布, 生成了硫化亚铁, 表明在硫化氢环境下, 主要是硫化氢析出的硫原子发生了吸附. 关键词： 第一性原理 Fe(100)表面 吸附能 硫化氢     

Total cross sections for scattering of electrons from O2, H2O, H2, O3, CO and CO2 at 100－2000eV

A complex optical model potential rewritten by the concept of bonded atom, which considers the overlap of electron clouds, is employed to calculate the total cross sections for electron scattering from several simple molecules (O_2, H_2O, H_2, O_3, CO and CO_2) consisting of C, H and O atoms in an incident energy range of 100－2000eV by the use of the additivity rule at Hartree－Fock level. In the study, the complex optical potential composed of static, exchange, correlation polarization plus absorption contributions firstly uses the bonded-atom concept. The quantitative molecular total cross section results are compared with experimental data and with the other calculations wherever available and good agreement is obtained. It is shown that the additivity rule along with the complex optical model potential rewritten by the concept of bonded atom can be used successfully to calculate the total cross section of electron－molecule scattering above 100eV, whereas the rule together with the complex optical model potential not rewritten by the concept of bonded atom is only successfully used above 300－500eV. So, the introduction of the bonded-atom concept in the complex optical potential can improve the accuracy of the total cross section calculations.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

The vibrational spectrum of H2O on Si(100)

Recent EELS measurements for H₂O adsorbed on the Si(100) reconstructed surface have been interpreted on the basis of dissociation of H₂O into OH plus H. We attempt to construct a valence force potential which yields harmonic vibrational frequencies in agreement with the EELS experiments. There is some consideration of experimental intensities in the achievement of this agreement. The experimental data cannot be fitted without introducing interaction force constants into the potential energy expression, such as the force constant coupling the silicon-oxygen stretching motion and the silicon-oxygen-hydrogen bending motion.     

HREELS investigation of CO and K coadsorption on Fe(110)

The coadsorption of CO and K on Fe(110) has been studied using HREELS. Three adsorption states of CO with distinct C-O stretch frequencies have been found. These states, which are occupied sequentially during the exposure to CO and named as ₁,₂ and ₃, have strong (₁), intermediate (₂) and weak (₃) interaction respectively between the adsorbed CO molecule and the coadsorbed K. The distance between the CO molecule and the nearest K neighbor is the shortest for the ₁ state and the longest for ₃. The distance for the latter is estimated to be larger than 6 Å.     

Analysis of the paramagnetic susceptibility of Fe(HCOO)2·2H2O

The paramagnetic susceptibility of powdered Fe(HCOO)₂·2H₂O is analysed by the combined method of high temperature expansion and molecular field approximation. The experimental data are well explained by the consistent model obtained in the previous specific heat analysis.     

Adsorption induced backscattering enhancement — CO and C2N2 on Pt(100)

Enhancement of the elastic backscattering coefficient is observed in the range 50–200 eV when CO or C₂N₂ are adsorbed on Pt(100). The general trends are insensitive to the state of order on the surface, and may be understood in terms of an incoherent scattering model, in which the backscattering power of the light elements is greater than that of the substrate at low energy. At higher energies bulk elastic scattering becomes relatively more prominent, the mean free path in the metal becomes greater and the enhancement is destroyed.     

Adsorption of H2S,H2O and O2 on Si(111) surfaces

Electron energy-loss spectroscopy has been applied to the study of Si(111) surfaces covered with H₂S, H₂O and O₂ at room temperature and the surfaces annealed at ～ 600°C. The experimental results strongly suggest that H₂S and H₂O adsorb in the molecular states at room temperature. It is proposed that O₂ is first adsorbed in a molecular state, then adsorbs as atoms, and finally oxidizes forming SiO₂.     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

Temperature induced effects in the coadsorption system: CO/D2/Ni(100)

The temperature dependent changes in the CO/D/Ni(100) system have been examined using thermal desorption spectroscopy, work function change measurements, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. Three forms of CO(a) have been found, one of which exists only in the intermediate temperature range of 130–215 K. This form is believed to be a CO(a) tilted with respect to the surface normal, with significant ONi interaction. In addition, a new interpretation of several UPS parameters measured for CO(a) is presented. It is proposed that for CO(a), the 5σ?1π separation is a probe of bonding geometry, the 4σ?1π separation reflects COCO repulsion, and the $\text{4σ}\text{5σ}$ intensity ratio reflects the NiCO bond strength.     

The interacting states (020), (100), and (001) of H217O and H218O

A fit of about 350 rotational levels of the (020), (100), and (001) vibrational states has been performed for H₂¹⁷O as well as for H₂¹⁸O leading to the determination of 51 rotational and coupling constants for each isotopic species. The Fermi-type interaction and the two Coriolis-type interactions have been taken into account by appropriate rotation-vibration operators and the v-diagonal part of the Hamiltonian is, for each vibrational state, a Watson-type Hamiltonian. The results are very satisfactory since 87% of the experimental levels are reproduced within 15 × 10^?3 cm^?1.     

RHEED and Auger study of the MoO3(010) surface interaction with H2S/H2 mixture and pure H2S

Pure single crystals of MoO₃ were carefully grown by physical vapor transport. The initial stages of the interaction of H₂S with a MoO₃ (010) surface were studied by RHEED and AES at low pressure p <133 Pa and at T < 700 K. Three main stages were found: (a) The formation of either a completely disordered sulphur adiayer (with pure H₂S) or a superstructure MoO₃(010)-[2 × 1]S (with H₂S/H₂). (b) A reduction of MoO₃ producing oriented three-dimensional MoO₂ islands, (c) An epitaxial overgrowth of MoS₂ from the MoO₂ three-dimensional crystallites.     

The adsorption of H2O on TiO2 and SnO2(110) studied by first-principles calculations

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H₂O adsorption on the (110) surface of TiO₂ and SnO₂. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H₂O→OH⁻+H⁻) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO₂ and SnO₂ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO₂ and SnO₂ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed.