Desorption related to adsorption of hydrogen via detailed balance on the Si(1 0 0) surfaces

Recent progress on desorption and adsorption dynamics of hydrogen (deuterium) on monohydride and dihydride Si(1 0 0) surfaces is reviewed and discussed. The dynamics experiments reveal that the desorption dynamics of hydrogen is well related to the adsorption dynamics via detailed balance. Dependence of time-of-flight (TOF) distributions of desorbed molecules on H(D) coverage is noticed to be important in understanding the kinetics mechanism of the adsorption/desorption reactions of hydrogen on the Si(1 0 0) surface. The desorption dynamics varies from the situation of strongly translational heating to the other situation of less translational heating with D coverage. This trend seems to be consistent with the 2H/3H/4H interdimer mechanism. However, despites by far the richest 4H configuration at high H coverage, the 2H desorption prevails over the 4H desorption already at 0.8 ML. To reconcile this unexpected desorption kinetics, a diffusion-promoted desorption mechanism is proposed. Height of the adsorption barriers for the 2H and 3H pathways could be reduced by the H-atom diffusion along the Si dimer rows, but that for the 4H pathway could not be the case because of no capability of diffusion on the H saturated surface. The desorption dynamics of hydrogen from the (3 × 1) dihydride surface is also reviewed and compared with the case on the monohydride surface. The sticking coefficients of hydrogen molecules onto the monohydride surfaces are evaluated from the TOF curves and found to be strongly activated by the kinetic energy. Not only the degrees of freedom of the molecules but also the vibrational degrees of freedom of substrate Si atoms determine the barrier height for adsorption. The desorption dynamics of hydrogen from the monohydride and dihydride surfaces appears to be quite similar, but the dynamics of substrate Si atoms is expected to be quite dissimilar between the two desorption pathways.     

Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR

Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO₄. We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H_ad). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H_ad is in fact a spectator in the HOR.     

Nickel-underpotential deposition on Pt(1 1 0) in sulphate-containing media

Nickel-underpotential deposition (UPD) on platinum electrodes has been studied in sulphuric media using cyclic voltammetry. The process seems structure-related: among the surfaces we did study, only (2 × 1)-reconstructed Pt(1 1 0) surface yields nickel-UPD. At low pH values, nickel-UPD peaks overlap with the hydrogen adsorption/desorption region. At higher pH (2–3) the overlapping is less severe: nickel coverage increases and can be estimated. Nickel-UPD on platinum (1 1 0) is a slow and irreversible process. The co-adsorption of sulphate or hydrogen-sulphate anions is probable. Nickel-UPD on Pt(1 1 0) occurs even in the presence of CO. Nickel submonolayer on Pt(1 1 0) exhibit remarkable activity towards CO-oxidation.     

Preparation and electrocatalytic activity of Rh adlayers on Pt(1 0 0) electrodes: reduction of nitrous oxide

Rhodium adlayers (submonolayer range) have been prepared on Pt(1 0 0) electrodes by electrodeposition from acidic solutions containing an excess of chloride. These Rh/Pt(1 0 0) electrodes give a well-defined voltammetric signal in the hydrogen adsorption region, which gives evidence of a high level of order in the Rh adlayer and allow a reliable estimation of the coverage. The voltammetric behavior of the Rh/Pt(1 0 0) electrodes points to an epitaxial growth with formation of rhodium islands. The well-ordered bimetallic surfaces freshly prepared were tested as electrocatalysts for nitrous oxide reduction and the responses were compared with those of the bulk Pt(1 0 0) and Rh(1 0 0) electrodes. The voltammogram for the bimetallic surface showed well separated N₂O reduction signals for Rh and Pt surface zones. An exceptionally high electrocatalytic activity for the Rh adlayer was found for low coverages. This behavior is explained on the basis of a high activity of the rhodium adatoms in the periphery of the islands.     

CI Study of CO adsorption on MgO(1 0 0)

Using CI embedding method, we have studied the adsorption of CO on MgO(1 0 0). The MgO(1 0 0) substrate is described by a Mg₉O₉ (3 × 3 × 2) core cluster, embedded in ionic (Mg²⁺/O²⁻) core potentials. The adsorption energy is calculated to be 0.11 eV at the CI level with a blue shift of 19 cm⁻¹ for CO stretching on MgO(1 0 0). The dispersion accounts only 35% of the total binding energy of CO on MgO(1 0 0). The CO/MgO(1 0 0) interaction is weak and mainly of the van der Waals type with only slight chemical bonding characters.     

A study of the growth and CO electrooxidation behaviour of PtRh alloys on Pt{1 0 0} single crystals

The formation of quasi-crystalline PtRh two dimensional films supported on Pt{1 0 0} is described. For the first time, the voltammetry of PtRh{1 0 0} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{h k l} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{1 0 0} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a “PtRhPt sandwich” precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{1 0 0} annealed in the absence of oxygen. In addition, an excess of Rh in the “sandwich structure” from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems.     

First-principles study of hydrogen adsorption on Mo(1 1 0)

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed for hydrogen (H) adsorption on Mo(1 1 0) surface. For various coverages, the hollow (hol) site was found to be the most stable binding site. The adsorption energy of this site was slightly increased as the increasing of hydrogen coverage. Subsurface (sub) occupation at low and medium coverages was ruled out while it became to be stable at the coverage of 1 ML. This is also supported by the potential energy surface (PES) study for hydrogen diffusing from hol to sub site. It’s interesting to find a surface reconstruction at the coverage of 1 ML, which is characterized by the lateral shift of the topmost layer for the sub adsorption. At higher coverage, the local density of states (LDOS) analysis showed that a new peak was clearly visible which was ascribed to a surface state induced by hydrogen adsorption. This surface state was mostly localized on the hydrogen atom and the first Mo layer, implying the hybridization of the hydrogen 1s states and the Mo metal states.     

Comparative theoretical study of the structure and bonding of propyne on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of propyne with the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied by means of the generalised gradient approach of density functional theory using periodic slab models. For both surfaces, the most stable adsorption mode of propyne is di-σ/π mode where the hydrocarbon is σ-bonded to two metal atoms with some additional π bonding to a third adjacent surface atom. The adsorption geometry is a highly distorted propyne with the C₁ and C₂ in a nearly sp² hybridisation. Two equivalent surface structures have been found on Pt and Pd. These correspond to the adsorption on the fcc or hcp hollow sites. The adsorption energies on Pt(1 1 1) and Pd(1 1 1) are predicted to be ∼−197 and −161 kJ mol⁻¹, respectively. The electronic factors that control the chemisorption have been analysed by means of the projected density of states.     

A spectroscopic proof of a surface equilibrium between on top and bridge bonded CO at Pt(1 1 0) in acid solution

According to most of works in the literature, adsorbed carbon monoxide at Pt(1 1 0) electrodes in acid media presents only linear bonded (CO_L) so-called, atop geometry. In the present work, the formation of bridge bonded carbon monoxide (CO_B) is shown via in situ infrared FT spectra, measured on a Pt(1 1 0) electrode covered with 25% CO, in HClO₄ solutions. For the first time, the inter conversion between atop and bridge bonded CO at potentials in the hydrogen adsorption region is reported in acid solution. Band intensity and band center frequency indicate dipole–dipole coupling effects in spite of the low CO total coverage.     

Adsorption of the first row of transition metals on the perfect and defective MgO(1 0 0) surface

We present DFT calculations for adsorption of the first row of transition metal atoms on a MgO(1 0 0) surface and on a surface exhibiting defects. Some atoms exhibit a high adsorption energy on the defect (e.g. Co, Ni and Cu), but others (Ca, Sc) rather adsorb on a clean surface and another set is indifferent to the presence of defect. The adsorption becomes energetically unfavorable when the σ anti-bonding orbitals become populated; this is worse on a defective surface than on a terrace. The π back-donation to the surface contributes to favor the adsorption on the center.     

Transition metals on the (0 0 0 1) surface of graphite: Fundamental aspects of adsorption,diffusion, and morphology

In this article, we review basic information about the interaction of transition metal atoms with the (0 0 0 1) surface of graphite, especially fundamental phenomena related to growth. Those phenomena involve adatom-surface bonding, diffusion, morphology of metal clusters, interactions with steps and sputter-induced defects, condensation, and desorption. General traits emerge which have not been summarized previously. Some of these features are rather surprising when compared with metal-on-metal adsorption and growth. Opportunities for future work are pointed out.     

Synthesis and characterization of preferentially oriented (1 0 0) Pt nanowires

Two types of platinum deposits were obtained by potentiostatic deposition onto Ti substrates, namely platinum black (Pt B) and platinum nanowires (Pt NW) with the latter being achieved through a porous anodic aluminum oxide (AAO) membrane at the solution/substrate interface. Surface characterization of these deposits was performed using scanning electron microscopy (SEM) and cyclic voltammetry (CV) in sulphuric acid solution. Surface properties for Pt NW revealed a predominant presence of (1 0 0) crystallographic planes, not present in Pt B deposit grown in the same conditions. Also, Pt NW exhibits an increased resistance to electrochemically active surface area (EASA) loss upon potential cycling in acidic solution, as compared to Pt B.     

Nitrate reduction on Pt(1 1 1) surfaces modified by Bi adatoms

This work presents the effect of Bi modification of Pt(1 1 1) electrodes on the electroreduction of nitrate anions by using voltammetric and FTIR experiments. On Pt(1 1 1) nitrate consumption occurs at potentials lower than 0.35 V, but with Pt(1 1 1)/Bi this process is shifted to significantly higher potentials (0.6–0.7 V). In the latter surface N₂O was observed as the main product in solution. Different forms of adsorbed NO were detected on the adatom covered surfaces as well as on clean Pt(1 1 1).     

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.     

Adsorption of alkali metals on Ge(0 0 1)(2 × 1) surface

Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(0 0 1)(2 × 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge–Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.     

CO electrooxidation on a polycrystalline Pt electrode: A wall-jet EQCN study

Pre-adsorbed and bulk (continuous) CO oxidation on a polycrystalline Pt electrode were examined in a wall-jet electrochemical quartz crystal nanobalance (EQCN) setup, using both differential and integral evaluation of the EQCN data, to get further insights into the kinetics and mechanism of this important fuel-cell related electrocatalytic reaction. The hydrogen underpotential adsorption–desorption features in the base cyclic voltammogram of a Pt film are accompanied by significant changes in the electrode mass due H-upd induced desorption–adsorption of anion. In the double-layer region small capacitive currents are accompanied by comparatively large reversible mass changes indicating anion adsorption/desorption (96.5 g mol⁻¹ assigned to bisulfate). OH and oxygen electrosorption from water at potentials more positive of 1.0 V result in relatively small variations in the electrode mass (16 g mol⁻¹ for PtOH and ca. 9 g mol⁻¹ for PtO formation, respectively). The CO-adlayer stripping first leads to the electrode mass decrease in the “pre-peak” region, followed by a fast mass increase within the main stripping peak due to re-adsorption of bisulfate anion (91 g mol⁻¹). A mass-transport limited current for bulk CO oxidation under continuous flow of CO-saturated electrolyte leads to negligible mass changes (0–1 g mol⁻¹) in the PtO region, suggesting that bulk CO oxidation is mediated by electroformed PtO.     

Surface structure for Au on (0 0 1) SrTiO3

First-principle calculations are performed to study the crystal structure, formation energies, and electronic structures of (0 0 1) SrTiO₃ surfaces with/without Au covered. The initiative Au additive layer is crystallized in a fcc structure with (0 0 1) face on SrO-terminated surface. The bimodal growth trend of Au on TiO₂-terminated surface is qualitatively consistent with the experimental observations. The defect structure of Au occupying the oxygen (O) vacancies of TiO₂-terminated surface is energetically favorable under oxygen-poor conditions, and a feature corresponding to gap states appear and the occupied Ti 3d states disappear.