Adsorption behavior of H2O on clean and oxygen precovered Ni(s)(111)

Photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), isotope exchange experiments, work function change (δφ) and LEED were used to study the adsorption and dissociation behavior of H₂O on a clean and oxygen precovered stepped Ni(s)[12(111) × (111)] surface. On the clean Ni(111) terraces fractional monolayers of H₂O are adsorbed weakly in a single adsorption state with a desorption peak temperature of 180 K, just above that of the ice multilayer desorption peak (T_m = 155 K). In the angular resolved UPS spectra three H₂O induced emission maxima at 6.2, 8.5 and 12.3 eV below E_F were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C_2v symmetry of the H₂O gas-phase molecule is not conserved for H₂O(ad) on Ni(s)(111). Although the Δφ suggest a bonding of H₂O to Ni via the negative end of the H₂O dipole, the O atom, no hints for a preferred orientation of the H₂O molecular axes were found in the UPS, neither for the existence of water dimers nor for a long range ordered H₂O bilayer. These results give evidence that the molecular H₂O axis is more or less inclined with respect to the surface normal with an azimuthally random distribution. H₂O adsorption at step sites of the Ni(s)(111) surface leads in TDS to a desorption maximum at T_m = 225 K; the binding energy of H₂O to Ni is enhanced by about 30% compared to H₂O adsorbed on the terraces. Oxygen precoverage causes a significant increase of the H₂O desorption energy from the Ni(111) terraces by about 50%, suggesting a strong interaction between H₂O and O(ad). Work function measurements for H₂O+O demonstrate an increase of the effective H₂O dipole moment which suggests a reorientation of the H₂O dipole in the presence of O(ad), from inclined to a more perpendicular position. Although TDS and Δφ suggest a significant lateral interaction between H₂O+O(ad), no changes in the molecular binding energies in UPS and no “isotope exchange” between ¹⁸O(ad) and H₂¹⁶O(ad) could be observed. Also, dissociation of H₂O could neither be detected on the oxygen precovered Ni(s)(111) nor on the clean terraces.     

Adsorption and reactions of ethanol on Mo2C/Mo(1 0 0)

The adsorption, desorption and dissociation of ethanol have been investigated by work function, thermal desorption (TPD) and high resolution electron energy loss (HREELS) spectroscopic measurements on Mo₂C/Mo(1 0 0). Adsorption of ethanol on this sample at 100 K led to a work function decrease suggesting that the adsorbed layer has a positive outward dipole moment By means of TPD we distinguished three adsorption states, condensed layer with a Tp = 162 K, chemisorbed ethanol with Tp = 346 K and irreversibly bonded species which decomposes to different compounds. These are hydrogen, acetaldehyde, methane, ethylene and CO. From the comparison of the Tp values with those obtained following their adsorption on Mo₂C it was inferred that the desorption of methane and ethylene is reaction limited, while that of hydrogen is desorption limited process. HREEL spectra obtained at 100 K indicated that at lower exposure ethanol undergoes dissociation to give ethoxy species, whereas at high exposure molecularly adsorbed ethanol also exists on the surface. Analysis of the spectral changes in HREELS observed for annealed surface assisted to ascertain the reaction pathways of the decomposition of adsorbed ethanol.     

The coadsorption of CO and H2 on Fe(100)

The coadsorption of CO and hydrogen on an Fe(100) surface was studied by temperature programmed desorption and X-ray photoelectron spectroscopy. It was found that CO adsorption blocked the subsequent dissociative adsorption of H₂, although it did not seem to affect the hydrogen binding energy. Preadsorption of hydrogen was observed to reduce the binding energy of CO subsequently adsorbed and to inhibit the dissociation of CO. A new surface species was identified in a coadsorbed layer of CO and hydrogen. This species was evidenced by the formation of a desorption peak for H₂ at 475 K when CO was adsorbed subsequent to H₂ adsorption.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Quantum dynamics of the dissociation of H<Subscript>2</Subscript> on Rh(111)

The dissociative adsorption of H₂ on Rh(111) has been studied by high-dimensional quantum calculations using a coupled channel scheme. The potential energy surface was derived from ab initio total energy calculations using density functional theory together with the generalized gradient approximation to describe exchange-correlation effects. Experimentally, at high kinetic energy a step in the dissociative adsorption probability as a function of kinetic energy has been observed [M. Beutl et al., Surf. Sci. 429, 71 (1999)] which has been attributed to the opening up of a new adsorption channel. This feature in the dissociation probability is reproduced in the calculations for H₂ molecules initially in the ro-vibrational ground state but it is not related to the opening up of an additional dissociation channel. Instead, it is caused by purely dynamical effects. In addition, rotational effects in the H₂ dissociation are addressed.     

H2 分子在Li3N(110)表面吸附的第一性原理研究

采用第一性原理方法研究了H₂分子在Li₃N(110)晶面的表面吸附. 通过研究H₂/Li₃N(110)体系的吸附位置、吸附能和电子结构发现: H₂分子吸附在N桥位要比吸附在其他位置稳定,此时在Li₃N(110)面形成两个-NH基,其吸附能为1.909 eV,属于强化学吸附;H₂与Li₃N(110)面的相互作用主要是H 1s轨道与N 关键词： 第一性原理 3N(110)')" href="#">Li₃N(110) 2')" href="#">H₂ 吸附和解离     

A vibrational and TDS study of sulfur adsorbates on Cu(100): Evidence for CH3S species

Vibrational (EELS) and TDS data for methyl mercaptan (CH₃SH), dimethyl sulfide (CH₃)₂S and dimethyl disulfide (CH₃S)₂ are analyzed to determine the nature of the adsorption states on Cu(100). Dimethyl sulfide is reversibly adsorbed on Cu(100); no dissociation (CS bond breaking) was found. By contrast, methyl mercaptan and dimethyl disulfide dissociate below 300 K to form adsorbed CH₃S (methyl mercaptide) species. Depending on the coverage, two orientations of methyl mercaptide are found: linear and bent. The two different orientations can be distinguished via the surface dipole selection rule by different intensities of the methyl rocking and deformation vibrations. By contrast with the methoxy species, which on Cu(100) decomposes to formaldehyde, no H₂C=S is liberated during decomposition of CH₃S. The mercaptide is stable to ∼ 350 K, but decomposes at higher temperatures to form adsorbed sulfur and recombinant methane, hydrogen and ethane. The methane appears to be formed by methyl-hydrogen recombination when the C-S bond scission occurs. TDS results show that sulfur released from the decomposition poisons the surface toward further adsorption. In addition, the selectivity toward methane versus ethane can be altered by pre-titrating the adsorbed hydrogen with oxygen, thereby changing the relative methyl-hydrogen and methyl-methyl recombination probabilities.     

Adsorption de l'éthyléne et de l'acétylène sur des films de rhénium evaporés sous ultra-vide; Hydrogénation de l'éthylène: II. Discussion des résultats expérimentaux et interprétation

A qualitative model is proposed in order to explain our experimental results on ethylene chemisorption on evaporated rhenium films and hydrogenation of ethylene (part I). The surface must present at least two kinds of surface sites (A and B). The second type (B), either preexists on the surface, or is induced by the adsorption phenomenon itself. On the most energetic ones (A), dissociation of ethylene and hydrogen is complete. Adsorption of ethylene is characterized by a sticking coefficient value of 0.1 if they are free and 1 if they are hydrogen covered. On sites B, ethylene is adsorbed without full dissociation (sticking coefficients equal to 0.015). independent on adsorption temperature. Hydrogen desorption is due to full dissociation of ethylene on the surface and a displacement reaction while ethane is produced by reaction between non-dissociated adsorbed ethylene and hydrogen in the gas phase. The same Rideal-Eley mechanism applies for hydrogenation of ethylene in quasi-stationary conditions, along with a self-poisoning mechanism involving dehydrogenation leading to C₂H₂ non-hydrogenable adsorbed species.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Adsorption structure and work function of dicarboxylic acid on Cu(1 1 0) surface

We investigated the relation between work function and the adsorption structure of dicarboxylic acids (organic molecules) such as succinic acid (HOOC-CH₂-CH₂-COOH) and an adipic acid (HOOC-(CH₂)₄-COOH) on a Cu(1 1 0) surface (electrode) as a function of the surface temperature using a Kelvin probe (KP). The work function changes of the two acids are similar. The work function increases by adsorption at room temperature due to ionization of molecules and then decreases with increasing temperature until 450 K due to the effects of change in the dipole moment of the conformational change of the molecule. From 450 to 600 K, the work function is constant because of competition between desorption and change in the dipole moment of molecules. It then reached the clean-surface value. Experiments clarified that the work function was affected by the adsorbed difference in conformation of molecules.     

Adsorption of H2S at Stone–Wales defects of graphene-like BC3: a computational study

We employed density functional theory to characterise H₂S adsorption, and dissociation on the pristine and Stone–Wales (SW) defected BC₃ graphenes. H₂S is predicted to be weakly adsorbed on the pristine graphene with the adsorption energy of about 7.11 kcal/mol. Two types of SW defects were generated by rotating a C–C bond (SW-CC) or a B–C bond (SW-BC) by about 90°. We predict that, in contrast to SW-BC, dehydrogenation of H₂S is energetically more favourable on the SW-CC compared to the associative adsorption. It is also found that SW-CC formation is more favourable than the formation of SW-BC. Molecular adsorption of H₂S on both of the SW defected sheets is more favourable than that on the pristine sheet. The preferable adsorption process on the SW-BC and SW-CC defected graphene sheets is via associative and dissociative mechanisms, respectively. Furthermore, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap of the SW-BC defected sheet is highly sensitive to the adsorption process which may be used for the detection of H₂S.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

The interaction of hydrazine with an Fe(111) surface

The interaction of hydrazine with a clean and nitrogen precovered Fe(111) surface was investigated in the temperature range of 126–600 K by means of UV and X-ray photoelectron spectroscopy (PES). At temperatures below 170 K the molecular adsorption of hydrazine is followed by multilayer condensation. In going from adsorbed to condensed hydrazine the valence and core levels shift in different directions relative to the vertical gas phase ionisation energies indicating strong interactions via hydrogen bonding in the condensed phase. Dissociative adsorption of N₂H₄ was observed at temperatures above 220 K. At room temperature no difference in the photoelectron spectra following the adsorption of N₂H₄ or NH₃ was observed indicating the presence of the same surface species, predominantly being -NH₂ radicals. Preadsorbed nitrogen stabilizes N₂H₄ against decomposition. The results will be discussed in view of possible intermediates in the ammonia-synthesis reaction on iron. Simple thermochemical arguments are presented to explain the observed difference in the heterogeneous dissociation mechanism of hydrazine on transition metals. General conclusions on the mechanism of ammonia synthesis on various transition metals can also be derived from these thermodynamic considerations.     

H2S在氧化石墨烯表面的吸附和分解的理论研究

用周期性密度泛函方法对H₂S在氧化石墨烯(GO)上的吸附和分解进行了理论计算, 讨论了H₂S和GO上的羟基和环氧基团的反应过程．结果表明,反应过程是通过H₂S或-SH上的H转移使得GO的环氧基开环和羟基氢化,当GO相反面存在羟基时有助于环氧基团的开环和羟基氢化反应．H₂S在GO上吸附和分解到S原子的反应机理中引入了相应的中间态,计算两次脱氢过程能垒分别为3.2和10.4 kcal/mol,第二个H原子的转移是GO还原过程的速率决定步骤．结果还表明GO上的羟基和环氧基团有助于加强S原子和石墨烯的结合．     

The effect of defects on the hydrogenation in Mg (0 0 0 1) surface

Density-functional theory was presented to investigate the hydrogen dissociation on a pure, Pt-doped, vacancy and oxide Mg(0 0 0 1) surface. Our results show that the energy barriers are 1.05, 0.39, 0.93 and 1.33 eV for H₂ dissociation on the pure, Pt-doped, vacancy and oxide Mg surface, respectively. The calculation results imply that the initial dissociation of H₂ is enhanced significantly for the Pt-doped Mg(0 0 0 1) surface, negligible for the vacancy model and weekend for the oxide model. The density of state results shows that, following the dissociation reaction coordinate, the H–H interactions are weeker for the Pt-doped model while interactions become stronger for the oxide model. It is suggested that the dissociation process is facilitated when Pt atom acts as catalyst and oxide overlayers delay hydrogen adsorption on the Mg layer. The present study will help us understand the defect role being played for the improvement or opposition effect in absorption kinetics of H₂ on the Mg(0 0 0 1) surface.     

Adsorption and decomposition of formic acid on the NiO(111)-p(2 × 2) surface: TPD and steady state kinetics studies

The adsorption and decomposition of formic acid on NiO(111)-p(2 × 2) films grown on Ni(111) single crystal surface were studied by temperature-programmed desorption (TPD) spectroscopy. Exposure of formic acid at 163 K resulted in both molecular adsorption and dissociation to formate. The adsorbed formate underwent further dissociation to H₂, CO₂ and CO. H₂ and CO₂ desorbed at the same temperatures of 340, 390 and 520 K, while CO desorbed at 415 and 520 K. The desorption features varied with the formic acid exposure. Two reaction channels were identified for the decomposition of formate under equilibrium with gas-phase formic acid with a pressure of 2.5 × 10⁻⁴Pa, one preferentially producing H₂ and CO₂ with an activation energy of 22 ± 2 kJ mol⁻¹ and the other preferentially producing CO and H₂O with an activation energy of 16 ± 2 kJ mol⁻¹. The order of both reaction paths was 0.5 with respect to the pressure of formic acid.     

Thermodynamics of xenon adsorption on Pd(s)[8(100) × (110)]: From steps to multilayers

The thermodynamic properties of the adsorption of xenon on the stepped Pd(s)[8(100)×(110)] surface have been studied over a wide range of pressure (5×10^?11 to 1×10^?4 Torr) and temperature (40–140 K). We have measured adsorption isobars using AES in order to evaluate the surface coverage. By choosing pressure and temperature we have studied under equilibrium conditions, the successive adsorption of xenon on the steps and on the terraces until the first layer is formed, the condensation of the second layer as well as the formation of xenon multilayers. For a small range of pressure and temperature, adsorption takes place only on the atomic steps. The LEED pattern shows that only every other site along the steps is occupied. The extrapolated initial heat of adsorption for steps is E_ad^S = 10.2 kcal/mol, decreasing monotonically by about 2 kcal/mol as the relative coverage of the step sites increases. The dipole moment of the Xe atoms adsorbed on steps is 1.12 D. During adsorption on the terraces the LEED observations suggest that the xenon adlayer is non-localized up to completion of the hexagonally close packed monolayer. The initial heat of adsorption on the terraces, E_ad^T is 8.2 kcal/mol and decreases continuously to a value of 6.9 kcal/mol for a complete monolayer due to lateral repulsive interactions between the adsorbed xenon atoms. The induced dipole moment of Xe on terraces is reduced to 0.49 D. The 5p_{$\text{1}\text{2}$} binding energy of Xe adsorbed on terrace sites is 0.3 eV smaller than that of Xe occuping step sites. The differential molar entropy of the adsorbed layer on the terraces as a function of coverage compares fairly well with the calculated value for an ideally mobile two-dimensional gas. No indication of the growth of two-dimensional xenon islands has been found under these conditions. The isosteric heat of adsorption for the second layer is E_ad^sec = 5.8 kcal/mol independently of the coverage. The condensation of the second layer is a first order two-dimensional gas ? two-dimensional solid phase transition in opposition to the continuous nature of the adsorption of the first layer (extending over a wide range of temperature for a given pressure). The induced dipole moment is further reduced for the Xe second layer to a value of 0.11 D. Finally, the condensation of multilayers proceeds with a latent heat of transformation of E_cond = 3.8 kcal/mol in excellent agreement with the known bulk value for the heat of sublimation of xenon. The line shape of the NVV low energy Auger transitions of xenon or the UPS binding energies of the Xe 5p_{$\text{3}\text{2}$,$\text{1}\text{2}$} spectra allow a clear distinction between first, second and higher layer Xe atoms. We have also established the temperature/pressure conditions for equilibrium between first, second and bulk xenon layers, i.e. a so-called “roughening point”.     

Effective dipole moment of rotational transitions of X 2 Y molecules

Contributions determining the rotational dependence of the effective dipole moment of molecules are calculated for the ground state of H₂S and H₂O molecules. The calculation is carried out in various ordering algorithms of perturbation theory. It is shown that the convergence of the effective dipole moment for the ground state of an H₂O molecule in the polynomial representation is rather slow in the rotational operator J _z (the convergence radius is K*≤17). Nonpolynomial forms of the dipole moment as a function of rotational operators are discussed.     

DFT study of isocyanate chemisorption on Cu(100): Correlation between substrate–adsorbate charge transfer and intermolecular interactions

The adsorption of isocyanate (? NCO) species on Cu(100) was studied using the density functional theory (DFT) and the periodic slab model. The calculations indicate that at low and intermediate coverages NCO adsorbs preferentially on bridge and hollow sites. Work function and dipole moment changes show a significant negative charge transfer from Cu to NCO. The resulting charged NCO species interact repulsively among themselves being these dipole–dipole interactions particularly intensive when they are adsorbed in adjacent sites. Consequently, isocyanates tend to be separated from each other generating the vacant sites required for the dissociation to N and CO. This condition for NCO dissociation has been suggested in the past from experimental observations. A comparison was also performed with the NCO adsorption on Pd(100). In particular, the calculated minimal energy barrier for NCO dissociation was found to be higher on Cu(100) than on Pd(100) in accord with the well known higher NCO stability on Cu(100).