Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Adsorption and reaction of oxygen and deuterium on a Pd(110) surface, studied by Δφ and TDS

The co-adsorption of oxygen and deuterium at 100 K on a Pd(110) surface has been studied by measurements of the change in work function (Δφ) and by thermal desorption spectroscopy (TDS). When the surface with co-adsorbed species is heated, the adsorbates O and D react to form D₂O which desorbs from the surface at T > 200 K. The D₂O desorption peaks shift continuously to lower temperatures as the surface D coverage (θ_D) increases. The maximum production of D₂O is estimated to be 0.26 ML (1 ML = 9.5 × 10¹⁴ atoms cm⁻²), resulting from reaction in a layer containing 0.65 ML D and 0.3 ML O. The maximum work function increase caused by adsorption of D to saturation onto oxygen precovered Pd(110) decreases almost linearly with Δφ_O of the oxygen precovered surface. On a surface with pre-adsorbed D however, the maximum Δφ increase contributed by oxygen adsorption decreases abruptly at Δφ_D > 200 mV. This sharp change occurs at θ_D > 1 ML and is believed to be associated with the development of the reconstructed (1 × 2) phase of D/Pd(110).     

Adsorption of Na onto γ-alumina studied by solid-state Na and Al nuclear magnetic resonance spectroscopy

The adsorption of Na⁺ on γ-alumina surfaces at four coverages of Na₂CO₃ [5, 10, 15 and 20% (w/w)] was characterized by solid-state ²³Na and ²⁷Al nuclear magnetic resonance (NMR) spectroscopy. The experimental results suggest that two distinct adsorbed species are present on the alumina surface: surface species and surface salts. At the lower coverages of Na₂CO₃ (5 and 10%), the surface species is predominant, in which the Na⁺ cations are associated with the oxygen atoms of γ-alumina. Increasing the loading level to 15% results in the appearance of a second adsorbed species that is attributed to the surface salt, Na₂CO₃, deposited on the solid surface. Further adsorption of Na₂CO₃ leads to an increase in the amount of surface salt while the amount of surface species remains unchanged. ¹H---²⁷Al Cross-polarization magic angle spinning (CP-MAS) experiments give the evidence that some Na⁺ cations in the form of surface species are coordinated with the Brönsted acid sites of γ-alumina. This may be the main driving force that improves appreciably the catalytic efficiency of an Na₂CO₃---Al₂O₃ catalyst.     

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.     

Investigation of the role of oxygen in NO reduction by C2H4 on the surface of stepped Pt(3 3 2)

The influence of pre-dosed oxygen on NO–C₂H₄ interactions on the surface of stepped Pt(3 3 2) has been investigated using Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). The presence of oxygen significantly suppresses the adsorption of NO on the steps of Pt(3 3 2), leading to a very specific adsorption state for NO molecules when oxygen–NO co-adlayers are annealed to 350 K (assigned as atop NO on step edges). An oxygen-exchange reaction also takes place between these two kinds of adsorbed molecules, but there appears to be no other chemical reaction, which can result in the formation of higher-valence NOx.

C₂H₄ molecules which are post-dosed at 250 K to adlayers consisting of ¹⁸O and NO do not have strong interactions with either the NO or the ¹⁸O atoms. In particular, interactions which may result in the formation of new surface species that are intermediates for N₂ production appear to be absent. However, C₂H₄ is oxidized to C¹⁸O₂ by ¹⁸O atoms at higher annealing temperature. This reaction scavenges surface ¹⁸O atoms quickly, and the adsorption of NO molecules on step sites is therefore quickly restored. As a consequence, NO dissociation on steps proceeds very effectively, giving rise to N₂ desorption which closely resembles that following only NO exposure on a clean Pt(3 3 2), both in peak intensity and desorption temperature. It is concluded that the presence of ¹⁸O₂ in the selective catalytic reduction (SCR) of NO with C₂H₄ on the surface of Pt(3 3 2) does not play a role of activating reactants.     

The role of co-adsorbed metal atoms in the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury and dimethylzinc

The chemistry of methyl species resulting from the decomposition of dimethylmercury (DMM) and dimethylzinc (DMZ) on Pt(111) in the range 300–400 K has been investigated by temperature prograrnmed desorption (TPD) and Auger electron spectroscopy (AES). In each case at 300 K, dissociative adsorption of the precursor results in the formation of an adlayer of methylmetal (CH₃M) moieties. These species are thermally stable to around 350 K before decomposing to yield mainly gaseous products, methane and hydrogen, and surface bound metal atoms. For DMM, subsequent heating to 400 K or direct dissociative adsorption at 400 K results in the formation of ethylidyne species. Ethylidyne formation is not observed in the thermal chemistry of DMZ at temperatures below 400 K and only transiently in the chemistry at 400 K. Complementary TPD and AES data indicate that, for DMM, desorption of the mercury atoms produced by CH₃Hg decomposition is the limiting factor in allowing the prevailing C₁ species to couple to form ethylidyne. In contrast, AES evidence indicates that zinc atoms remain on the surface to temperatures in excess of 750 K and hence prevent C---C coupling by blocking surface sites.     

The interaction of oxygen and potassium on the Ru(001) surface

The interaction of oxygen and different coverages of potassium on Ru(001) has been investigated by thermal desorption spectroscopy (TDS), metastable quenching spectroscopy (MQS), electron stimulated desorption spectroscopy (ESD), and work-function change measurements. The results show that this is a complex surface system with several different oxides forming, depending on the surface stoichiometry and temperature. While we cannot uniquely identify all the surface species, our interpretation of the present data combined with previous information is as follows. For potassium coverages up to about three monolayers (θ_K ≈ 1), exposure to oxygen initially gives oxygen atoms on the surface. Further exposure produces some surface monoxide ions O²⁻, which are converted with additional exposure to Superoxide ions O⁻₂ and possibly peroxide ions O²⁻₂. Thermal annealing causes strong changes in the surface oxide composition, and with potassium multilayers (θ_K ≈ 10) all the oxides diffuse beneath the K surface layer with annealing to only 300 K. K₂O and K₂O₂ are found to desorb together in the 600–700 K region.     

Monte Carlo simulation for temperature dependence of Ga diffusion length on GaAs(0 0 1)

Diffusion length of Ga on the GaAs(0 0 1)-(2×4)β2 is investigated by a newly developed Monte Carlo-based computational method. The new computational method incorporates chemical potential of Ga in the vapor phase and Ga migration potential on the reconstructed surface obtained by ab initio calculations; therefore we can investigate the adsorption, diffusion and desorption kinetics of adsorbate atoms on the surface. The calculated results imply that Ga diffusion length before desorption decreases exponentially with temperature because Ga surface lifetime decreases exponentially. Furthermore, Ga diffusion length L along and [1 1 0] on the GaAs(0 0 1)-(2×4)β2 are estimated to be and L_[110]200 nm, respectively, at the incorporation–desorption transition temperature (T860 K).     

The adsorption and decomposition of NO on Pd(110)

The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N₂) and nitrous oxide (N₂O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal.     

NMR study of solid C60(γ-cyclodextrin)2

A solid complex of C₆₀ with γ-cyclodextrin (γ-CyD) was examined with NMR spectroscopic methods in order to understand the dynamics of C₆₀, and the interaction between C₆₀ and γ-CyD. A ¹³C solid-state cross-polarization magic angle spinning (CP/MAS) NMR spectra shows C₆₀ resonance at 142.6 ppm. This provides the evidence of interaction between ¹³C spins in C₆₀ and ¹H spins in the γ-CyD host. Ambient temperature experiments on the ¹³C CP/MAS NMR, with varying contact time, shows that the water associated with γ-CyDs plays an important role in the nuclear relaxation processes. The dynamics of C₆₀ in γ-CyD was investigated using temperature and field-dependent ¹³C spin-lattice relaxation time measurements. The influence of water on the dynamics of C₆₀ was less significant below 250 K.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Reactions of π-allyl with atomic oxygen and hydroxyl on Ag(110)

The oxidation of the adsorbed π-allyl (η³-C₃H₅), prepared on atomic oxygen- and hydroxyl-covered Ag(110) by dissociation of allyl chloride (C₃H₅Cl), is investigated with temperature-programmed desorption and high-resolution electron energy loss spectroscopy. Allyl chloride adsorbs molecularly on oxygen-covered Ag(110) at 110 K. Upon heating to 180 K, some allyl chloride dissociates to form π-allyl and atomic chlorine, and the remainder desorbs molecularly. The π-allyl undergoes combustion to form hydroxyl or carbonate until all of the free oxygen is consumed by 200 K. Migratory insertion of hydroxyl into excess π-allyl commences near 220 and finishes by 250 K, forming adsorbed allyl alcohol (C₃H₅OH), which reacts either with excess hydroxyl near 240 K to form allyl alkoxy (η¹(O)-C₃H₅O) and water, or with π-allyl at 250 K to form allyl alkoxy and propylene (C₃H₆). Th allyl alkoxy evolves acrolein (C₃H₄O) by β-hydrogen elimination near 285 K, and propylene is evolved concurrently as the hydrogen released by this reaction rapidly scavenges π-allyl. Finally, the remaining π-allyl dimerizes to form 1,5-hexadiene (C₆H₁₀), which desorbs at 315 K. The gross observations of reaction pathways and temperatures are used to evaluate important aspects of the thermochemistry of these reactions.     

Defects in CdS: In detected by perturbed angular correlation spectroscopy (PAC)

The local lattice environment of the donor In in CdS is investigated measuring the electric-field gradient at the site of the radioactive probe atom ¹¹¹In by the perturbed γγ angular correlation technique. It is shown that implantation of In into CdS with subsequent annealing drives 100% of the In atoms to Cd lattice sites. Diffusion of In into CdS under S overpressure results in the formation of In_Cd-V_Cd pairs which seem to be responsible for the self-compensation of In donors in CdS.     

The interaction of C2Cl4 with fe(110)

Quantitative adsorption studies, temperature programmed desorption (TPD) and Auger spectroscopy have been used to study the interaction of C₂Cl₄ with Fe(110) at 90 and 325 K. At 90 K, multilayer C₂Cl₄ adsorption occurs. The following desorption products are observed in the temperature range of 90–1050 K: C₂Cl₄ from the multilayer and the monolayer, FeCl₂, and a high mass iron chloride species with mass spectrometer cracking products FeCl⁺₂, FeCl⁺, and Fe⁺. Irreversible dissociative C₂Cl₄ adsorption occurs at 325 K and the only desorption product which is observed is the high mass iron chloride species. Auger spectroscopy shows that surface carbon from C₂Cl₄ starts to diffuse into the bulk of the crystal at ˜ 480 K while small coverages of chlorine remain on the surface of the crystal even after heating to 1050 K. Comparison of the behavior of C₂Cl₄ and CCl₄ on Fe(110) indicates that radical products (·CCl₃ and :CCl₂) observed to be produced from CCl₄ adsorption are not produced from C₂Cl₄ adsorption. This difference is probably due to the enhanced surface reactivity of the C=C bond in C₂Cl₄. A special reactivity of iron defect sites for C₂Cl₄ is observed through the production of associated FeCl₂ species which desorb via zero-order kinetics with an activation energy of 44.8 ± 8.5 kcal/mol, the sublimation enthalpy of FeCl₂.     

Adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface

The adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface has been studied with scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 0.1 ML Cs coverage the whole surface exhibits a mixture of (1 × 2) and (1 × 3) reconstructed structures, indicating that Cs atoms exert a cooperative effect on the surface structures. Real-time STM observation shows that silver atoms on the Cs-covered surface are highly mobile on the nanometer scale at 300 K. The Cs-reconstructed Ag(1 1 0) surface alters the structure formed by dissociative adsorption of oxygen from p(2 × 1) or c(6 × 2) to a p(3 × 5) structure which incorporates 1/3 ML Ag atoms, resulting in the formation of nanometer-sized (10–20 nm) islands. The Cs-induced reconstruction facilitates the adsorption of CO₂, which does not adsorb on unreconstructed, clean Ag(1 1 0). CO₂ adsorption leads to the formation of locally ordered (2 × 1) structures and linear (2 × 2) structures distributed inhomogeneously on the surface. Adsorbed CO₂ desorbs from the Cs-covered surface without accompanied O₂ desorption, ruling out carbonate as an intermediate. As a possible alternative, an oxalate-type surface complex [OOC–COO] is suggested, supported by the occurrence of extensive isotope exchange between oxygen atoms among CO₂(a). Direct interaction between CO₂ and Cs may become significant at higher Cs coverage (>0.3 ML).     

The adsorption of water on Ni(110): Monolayer, bilayer and related phenomena

The adsorption of water of Ni(110) has been studied by nuclear reaction analysis (NRA), thermal desorption spectroscopy (TDS), LEED and work function measurements (Δφ). The major findings of this study are: (1) the saturation coverage of the first chemisorbed layer of water is slightly less than 0.5 water molecules per surface Ni atom or 0.5 ML (1 ML = 1 MONOLAYER = 1.14 × 10¹⁵ molecules cm⁻²) and the layer exhibits a c(2 × 2) LEED pattern; (2) this water desorbs in three separate desorption states; (3) the slightly less strongly bound, second layer of water can be distinguished from subsequent “ice” layers by a discrete work function change. These results are discussed in terms of a recently published model of Benndorf and Madey [C. Benndorf and T.E. Madey, Surf. Sci. 194 (1988) 63].     

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH₃Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH₃Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH₃Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH₃Cl caused a significant work function decrease, Δ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH₃Cl at lower concentration, prevented the adsorption of CH₃Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.     

The interaction of D2O with Zr(0001) at 80 K

The adsorption of D₂O on Zr(0001) at 80 K and its subsequent reactions at higher temperatures have been studied by thermal desorption spectroscopy (TDS), work-function measurements (Δф), nuclear reaction analysis (NRA), LEED, infrared reflection spectroscopy (FTIR-RAS), Auger electron spectroscopy (AES), and static secondary ion mass spectroscopy (SSIMS). D₂O adsorption on Zr(0001) at 80 K is accompanied by a Δф of −1.33 eV. The adsorbed D₂O can be characterized into three layers by TDS: a chemisorbed layer (up to 0.23 ML), a second adsorbed layer, and an ice layer. The chemisorbed D₂O dissociates into OD_ad and D_ad at 80 K (possibly also into O_ad) and no desorption products could be detected, implying that the reaction products dissolved into the zirconium at temperatures appropriate for each component. The ice layer and most of the second adsorbed layer desorb as molecular water during heating. The water adsorbed at 80 K did not form any long-range ordered structure, but a (2 × 2) LEED pattern that was formed by heating the sample to temperatures above 430 K is believed due to be an ordered oxygen superstructure.     

Adsorption of D(H) atoms on Ar ion bombarded (0 0 0 1) graphite surfaces

Adsorption of thermal (2000 K) D (H) atoms on HOPG surfaces prior to and after bombardment with 500 eV Ar ions was studied with thermal desorption and vibrational spectroscopies. Ion bombardment of HOPG generates vacancy (VD, displaced surface C atoms) and interstitial (ID, Ar captured between 1st and 2nd C plane) defects. These defects remove the ability of the surface to adsorb D like on virgin HOPG surfaces and to form C_gr–D bonds. After a dose of 0.1 Ar per C surface atom, D adsorption is markedly suppressed. Annealing of bombarded surfaces at 1350 K, connected with desorption of trapped Ar and removal of ID, recovers a large fraction of the adsorption capacity for D. Therefore, the long range stress in the surface plane introduced by ID must be responsible for a significant fraction of D adsorption blocking. It is suggested that ID prevent reconstruction of the C surface which is required for the formation of C_gr–D bonds. For ion doses above 0.5 Ar/C, adsorption of D on the surface is negligible. After annealing at 1350 K, D can be adsorbed in quantities comparable to the virgin HOPG surface, however forming C–D bonds which are similar to those observed in hydrogenated amorphous carbon instead of those which are normally formed on HOPG. Instationary etching via release of deuterocarbon species occurs primarily in the C₁ and C₂ channels. It is only observed at bombarded HOPG prior to annealing and probably due to the presence of isolated C₁ and C₂ species on the surface generated upon VD formation.