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.     

Thermal and low energy electron bombardment induced oxygen loss of V2O5 single crystals: Transition into V6O13

Thermal treatment in UHV of clean V₂O₅ single crystals results in homogeneous oxygen loss, involving a rate-limiting surface reaction. Depending upon the pretreatment, aircleaved samples transform topotactically into V₆O₁₃, or into what we call a phase Q of probable composition V₄O₉ or V₆O_13.5. Low energy electron bombardment of clean UHV-cleaved V₂O₅(010) surfaces produces the transition V₂O₅ → V₆O₁₃ at room temperature. This effect is attributed to electron beam stimulated reactions. The influence on the transition of carbon-containing impurities is discussed. The nucleation of V₆O₁₃ on V₂O₅ is explained by a model based on a surface reaction, the rate of which is enhanced by the interaction with contaminating molecules and low energy electron bombardment. The presence of shear planes at the boundary between V₂O₅ and the V₆O₁₃ nuclei locally enhances the oxygen loss rate and allows the V₆O₁₃ nuclei to grow into the bulk.The enhanced mobility of the oxygen at these boundaries is thought to influence favorably the oxidation-regeneration rate of the V₂O₅-catalyst.     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

Ion survival probabilities for 3 keV Ar+ scattering from La,Yb, and chemisorbed H2, O2, and H2O on La surfaces

TOF spectra of scattered primary and surface recoiled neutrals and ions for 3 keV Ar⁺ bombardment of clean La and Yb and H₂, O₂, and H₂O saturated La surfaces are presented. The spectra are analyzed in terms of single (SS) and multiple (MS) scattering of the primary ions and surface recoiling (SR) of adsorbate atoms. Measurement of spectra of neutrals + ions and neutrals alone allows determination of scattered ion fractions Y. The Y values for the SS event are high for clean La (37%) and lower for adsorbate covered La (32% for H₂, 13% for O₂, and 8% for H₂O); Yb exhibits a similar behavior, i.e. 16% for clean Yb and 5% for O₂ + H₂O covered Yb. Photon emission accompanying the scattering collision has been observed from clean La and Yb and adsorbate covered La. A preferential inelastic energy loss of 15 ± 3 eV for the SS event has been observed for scattered neutrals as opposed to ions for La and H₂ saturated La at 135°. These results are interpreted within the models for Auger and resonant electronic charge exchange transitions during approach or departure of an ion with a surface and the electron promotions occuring during close atomic encounters where the electron shells are interpenetrating.     

Density functional theory study of the interaction of H2 with pure and Ti-doped WO3 (002) surfaces

Density functional theory (DFT) calculations are conducted to explore the interaction of H₂ with pure and Ti-doped WO₃ (002) surfaces. Four top adsorption models of H₂ on pure and Ti-doped WO₃ (002) surfaces are investigated respectively, they are adsorption on bridging oxygen O_1c, absorption on plane oxygen O_2c, absorption on 5-fold W_5c (Ti), and absorption on 6-fold W_6c. The most stable and H₂ possible adsorption structure in the pure surface is H-end oriented to the surface plane oxygen O_2c site, while the favourable adsorption sites for H₂ in a Ti-doped surface is not only an O_2c site but also a W_6c site. The adsorption energy, the Fermi energy level E_F, and the electronic population are investigated and the H₂-sensing mechanism of a pure-doped WO₃ (002) surface is revealed theoretically: the theoretical results are in good accordance with our existing experimental results. By comparing the above three terms, it is found that Ti doping can obviously enhance the adsorption of H₂. It can be predicted that the method of Ti-doped into a WO₃ thin film is an effective way to improve WO₃ sensor sensitivity to H₂ gas.     

The influence of structural defects on the adsorption of simple molecules (H2, C2H2, C2H4, C2H6, CO,NO) on rhenium single crystals

Using Thermal Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) we have studied the adsorption of hydrogen-containing molecules (H₂, C₂H₂, C₂H₄, C₂H₆) and oxygen-containing molecules (CO and NO) on two vicinal planes of the Re(0001) surface. The two surfaces are designated thus: ReS ¦14(0001)(101̄1)¦, ReS |6(0001)(167̄1) | . The structural defects have little effect on the adsorption of hydrogen and the hydrocarbons. They are more influential in the case of the oxygen-containing molecules. This is particularly true for CO; on the kink sites the CO molecules can completely dissociate whereas only a partial dissociation is possible on the steps. These results should be viewed in relation to the strong bond energy between carbon and oxygen in a CO molecule of 256 kcal/mole and the great affinity of oxygen for rhenium; E_Re?O = 127 kcal/mole.     

The role of adsorbed sulfur in the H2D2 equilibration reaction on Pt single crystals

The H₂D₂ equilibration on Pt single crystals was investigated under intermediate pressure (100–400 Torr) and temperature (50–250°C), as a function of sulfur coverage. On Pt(110) and Pt(111), adsorbed sulfur modifies the kinetic parameters, activation energy and pre-exponential factor; the latter depends on the temperature on Pt(110) only. The clean Pt(110) face was found to be 5 times more active than the clean Pt(111). On both faces, adsorption of sulfur induces electronic effects on the neighbouring reactional sites. The difference in the behaviour of the two faces and a clear influence of the arrangement of the adsorbed sulfur atoms, deduced from LEED diagrams, tend to prove the structure dependency of the H₂D₂ reaction. A consistent reaction mechanism could be proposed, involving the dissociative adsorption and surface recombination of hydrogen and deuterium, and the reaction between adsorbed molecules for high sulfur coverages. The value of the sulfur coverage which makes the platinum inactive towards H₂D₂ is lower for the (111) than for the (110) orientation; this is in correlation with the roughness of the surface; the denser at atomic scale a surface is, the further is the extent of the lateral interactions due to adsorbed sulfur.     

Pressure broadening and collisional shift of the Rb D2 absorption line by CH4, C2H6, C3H8, n-C4H10, and He

The pressure broadening and shift rates of the rubidium D2 absorption line 5²S_1/2→5²P_3/2 (780.24 nm) with CH₄, C₂H₆, C₃H₈, n-C₄H₁₀, and He were measured for pressures ≤80 Torr using high-resolution laser spectroscopy. The broadening rates γ_B for CH₄, C₂H₆, C₃H₈, n-C₄H₁₀, and He are 28.0, 28.1, 30.5, 31.3, and 20.3 (MHz/Torr), respectively. The corresponding shift rates γ_S are −8.4, −8.8, −9.7, −10.0, and 0.39 (MHz/Torr), respectively. The measured rates of Rb for the hydrocarbon buffer gas series of this study are also compared to the theoretically calculated rates of a purely attractive van der Waals difference potential. Good agreement is found to exist between measured and theoretical rates.     

LEED and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 and C) chemisorbed on the stepped rhodium (755) and (331) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₂, C₂H₄ and C has been studied on the clean stepped Rh(755) and (331) surfaces. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) were used to determine the size and orientation of the unit cells, desorption temperatures and decomposition characteristics for each adsorbate. All of the molecules studied readily chemisorbed on both stepped surfaces and several ordered surface structures were observed. The LEED patterns seen on the (755) surface were due to the formation of surface structures on the (111) terraces, while on the (331) surface the step periodicity played an important role in the determination of the unit cells of the observed structures. When heated in O₂ or C₂H₄ the (331) surface was more stable than the (755) surface which readily formed (111) and (100) facets. In the CO and CO₂ TDS spectra a peak due to dissociated CO was observed on both surfaces. NO adsorption was dissociative at low exposures and associative at high exposures. C₂H₄ and C₂H₂ had similar adsorption and desorption properties and it is likely that the same adsorbed species was formed by both molecules.     

Temperature and pressure effects on formation and decomposition of phenylvinylperoxy radicals in the C6H5C2H2 + O2 reaction

The effects of temperature and pressure on the formation and decomposition of C₆H₅C₂H₂O₂ in the C₆H₅C₂H₂ + O₂ reaction have been investigated at temperatures from 298 to 378 K by directly monitoring the C₆H₅C₂H₂O₂ radical in the visible region by cavity ringdown spectrometry (CRDS). The rate constant for the C₆H₅C₂H₂ + O₂ association and that for fragmentation of C₆H₅C₂H₂O₂ were found to be k₁ (C₆H₅C₂H₂ + O₂ → C₆H₅C₂H₂O₂) = (3.20 ± 1.19) × 10¹¹ exp(+760/T) cm³ mol⁻¹ s⁻¹ and k₂ (C₆H₅C₂H₂ O₂ → C₆H₅CHO + HCO) = (1.68 ± 0.13) × 10⁴ s⁻¹, respectively. Additional kinetic measurements by pulsed laser photolysis/mass spectrometry show that C₆H₅CHO was produced in the C₆H₅C₂H₂ + O₂ reaction as predicted and the formation of C₆H₅CHO from the decomposition of C₆H₅C₂H₂O₂ is temperature-independent, consistent with the CRDS experimental data.     

Auger electron spectroscopic study of the surface oxidation of uranium-niobium alloy {U-6 wt.% Nb} in a UHV environment containing primarily H2, H2O and CO

Auger electron spectroscopy has been used to study the surface oxidation of a uranium-niobium alloy, U-6 wt.% Nb, in a UHV environment containing primarily H₂, H₂O and CO. For comparison, the reaction of the oxidative species with pure uranium and niobium metals has also been investigated. The observed changes of the shape and position of the Auger transition peaks for U, Nb and O following exposures this UHV environment up to 200 L, and under a continuous electron bombardment indicated that oxidation of the alloy leads, first, to the formation of a surface oxide consisting of UO_2.0. The formation of this oxide was followed by the development of Nb-oxides (NbO, NbO₂ and Nb₂O₅) at the metal-oxide interface. Pure uranium was found to exhibit faster oxidation kinetics than the alloy, while pure niobium oxidised at a much lower rate than the uranium based materials.     

Effect of Ta buffer and NiFe seed layers on pulsed-DC magnetron sputtered Ir20Mn80/Co90Fe10 exchange bias

A systematic investigation has been done on the correlation between texture, grain size evolution and magnetic properties in Ta/Ni₈₁Fe₁₉/Ir₂₀Mn₈₀/Co₉₀Fe₁₀/Ta exchange bias in dependence of Ta buffer and NiFe seed layer thickness in the range of 2-10 nm, deposited by pulsed DC magnetron sputtering technique. A strong dependence of 〈1 1 1〉 texture on the Ta/NiFe thicknesses was found, where the reducing and increasing texture was correlated with exchange bias field and unidirectional anisotropy energy constant at both NiFe/IrMn and IrMn/CoFe interfaces. However, a direct correlation between average grain size in IrMn and H_ex and H_c was not observed. L1₂ phase IrMn₃ could be formed by thickness optimization of Ta/NiFe layers by deposition at room temperature, for which the maximum exchange coupling parameters were achieved. We conclude finally that the coercivity is mainly influenced by texture induced interfacial effects at NiFe/IrMn/CoFe interfaces developing with Ta/NiFe thicknesses.     

Tuning the dispersion of 4f bands in the heavy-fermion material YbRh2Si2

Localized Yb 4f and itinerant Rh 4d states are subject to substantial hybridization effects in the heavy-fermion material YbRh₂Si₂. The proximity to the Fermi level and the high anisotropy in k space naturally raise questions regarding the role of these hybridization effects for the observed, unusual physical properties. Using angle-resolved photoemission spectroscopy (ARPES) we found that the non-dispersive behavior of the localized Yb f states is broken around the Γ point due to interaction with approaching Rh 4d bands. The intriguing point here is that the hybridization strength turns out to be systematically tunable by electron doping of the material. Gradual deposition of silver atoms onto the atomically clean, silicon terminated surface of YbRh₂Si₂ leads to transfer of Ag 5s charge into the Rh 4d bands. This substantially changes the energy overlap, and thus the hybridization strength, between the interacting Yb 4f and Rh 4d bands in the surface and subsurface region. The shown possibility to control the variation of the f-d hybridization at the surface of heavy-fermion materials may also be helpful for other ARPES studies on the diverse phenomena in electron-correlated materials.     

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.     

Étude par spectroscopie auger,diffraction d'électrons lents et rapides des premiers stades de la sulfuration de la face (100) de NiO par H2S

A structural study of the different stages during NiO(100) sulphurization by H₂S was carried out by RHEED, LEED and AES. On exposure to H₂S (PH₂S < 10^?5 Torr) The “clean” surface, obtained by UHV cleavage, was found to react with H₂S to produce islands of Ni(100) covered with an ordered c(2 × 2) S structure up to 300°C. Growth of Ni₃S₂ islands occurs on increasing the temperature and the exposure to H₂S.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

The interfacial properties of MgCl2 films grown on a flat SiO2/Si substrate. An XPS and ISS study

The interaction between MgCl₂ and SiO₂ was investigated by X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and contact potential difference (CPD) measurements. A thin SiO₂ layer was grown for this purpose on a Si(1 0 0) wafer and MgCl₂ was applied on this support at room temperature by evaporation under UHV conditions. It was found that magnesium chloride is deposited molecularly on the SiO₂ substrate, growing in layers and covering uniformly the oxide surface. The interaction with the substrate is initially very weak and limited to the interfacial layer. Above 623 K, most of the molecular MgCl₂ is re-evaporated and the interfacial interaction becomes stronger, as Mg-Cl bonds in the remaining sub-monolayer chloride break and Cl atoms desorb. This leaves on the surface sub-stoichiometric MgCl_x, which interacts with oxygen atoms from the substrate to form a complex surface species. At 973 K all Cl atoms desorb and Mg remains on the surface in the form of an oxide.     

三甲基镓在Pd(111)表面吸附解离及表面预吸附H和O的影响

利用X-射线光电子能谱（XPS）和程序升温脱附谱（TPD）研究了三甲基镓在Pd（111）表面的吸附和解离行为,并考察了表面预吸附H和O的影响。结果表明,在吸附温度为140 K时,三甲基镓在Pd（111）上主要为解离吸附,此时表面物种为Ga（CH₃）_x （x=1,2,3）和CH_x物种。加热将导致Ga的甲基化合物中的Ga-C键发生分步断裂,在不同温度下产生CH₄和H₂从表面脱附。同时,XPS结果证实了在275~325 K的温度区间内存在Ga甲基化合物的分子脱附。退火至更高温度,表面只观察到积碳和金属Ga物种,这二者随着温度的继续升高逐渐向体相扩散。在Pd（111）表面预吸附O和H对上述吸附和解离行为存在显著的影响。当表面预吸附H时,脱附产物CH₄和H₂的脱附主要位于315 K,可归属为一甲基镓的解离脱附。当表面预吸附O时,只在258 K观察到CH₄和H₂的脱附峰,可能来自于Pd-O-Ga（CH₃）₂吸附结构的解离.     

Adsorption of H2O on clean and on boron-contaminated Rh surfaces

The adsorption and dissociation of H₂O on Rh(111) and Rh foil surfaces have been studied in UHV using Auger electron, electron energy loss (in the electronic range) and thermal desorption spectroscopy. H₂O adsorbs weakly on clean Rh samples at 110 K. The adsorption is accompanied by the appearance of a broad loss feature at 14–14.5 eV. At higher exposures new losses appeared at 8.6 and 10.5 eV. The desorption of H₂O took place in two stages, with T_p = 183 K (β, chemisorption) and 158 K (α, multilayer formation). There was no indication of dissociation of H₂O on a clean Rh(111) surface. Similar results were obtained for a clean Rh foil. However, when small amounts of boron segregated on the surface of Rh, they exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociation of H₂O. This was exhibited by the formation of H₂, by the buildup of surface oxygen, by the appearance of an intense new loss at 9.4 eV, identified as B-O surface species, and by the development of “boron-oxide”-like Auger fine structure.     

Magnetocaloric effect in high-energy ball-milled Gd5Si2Ge2 and Gd5Si2Ge2/Fe nanopowders

Evolution of structure and magnetocaloric properties in ball-milled Gd₅Si₂Ge₂ and Gd₅Si₂Ge₂/0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (ΔS_M) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd₅Si₂Ge₂ alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.