Surface Diffusion Of Hydrogen Research Articles

Reduction of iron oxide by hydrogen spillover over Pt/TiO2 and Pt/Al2O3 surfaces

A two-dimensional model is developed to predict the reduction of iron oxide via hydrogen spillover in Pt/TiO2 and Pt/Al2O3 catalytic systems. The reduction over TiO2 is predicted to be independent of the distance between Pt and iron oxide particles, and is explained by the high surface diffusivity of hydrogen on TiO2. In contrast, the characteristic time for diffusion of hydrogen on Al2O3 is similar to the time scales of the experiment and is strongly dependent on the inter-particle distance, which explains the lower and distance-dependent reduction over Al2O3. It is inferred that hydrogen reaches the TiO2/iron-oxide interface in a short time period, and the reduction rate is primarily governed by the reduction kinetics. The kinetics-governed reduction explanation is also valid for Al2O3, however at longer times when hydrogen has reached the Al2O3/iron-oxide interface. For early time periods, the surface diffusion of hydrogen governs its availability at the Al2O3/iron-oxide interface.

Assessment of multiscale hydrogen desorption models from (0001) Be surfaces

Beryllium is a proposed neutron multiplier material in future fusion devices like ITER and DEMO. As such, numerous studies of neutron-irradiated beryllium specimen have shown the appearance of facetted bubbles with considerable tritium accumulation at their (0001) surfaces. In this work, we thus simulate hydrogen isotope desorption from (0001) beryllium surfaces using a multiscale modeling approach. We first establish that very fast surface diffusion of hydrogen adsorbed in FCC and HCP sites can be studied independently from the bulk by means of static DFT calculations, as diffusion into the bulk is inhibited by multiple diffusion processes with an accumulated energy barrier of almost 3 eV. Static saddle point searching dimer DFT calculations also reveal energy barriers for associative desorption in a range between 0.94 eV and 1.52 eV due to varying amounts of co-adsorbed hydrogen isotopes in the immediate vicinity of the desorbing pair. A simple rate equation-based desorption model incorporating these associative desorption processes is constructed and compared to thermal programmed desorption (TPD) spectra of earlier exposure experiments. It is found that the simple rate equation-based desorption model seemingly already explains some characteristic aspects of the TPD spectra like a peak shift to lower temperatures by almost 100 K and considerable broadening with increasing coverage. To asses the merit of this model, a refined model effectively taking the repulsion among adsorbed hydrogen isotopes into account by means of relevant correlations from kinetic Monte Carlo (KMC) simulations using a cluster expansion (CE) truncation is considered as well. It is found that the simple model’s reproduction of characteristic features of experimental spectra is the result of error cancellation from considering an incomplete set of desorption processes on one hand and neglecting repulsion among adsorbed hydrogen on the other hand. We conclude that the experimental TPD spectra can not be fully explained by either of the two models and further research particularly considering additional adatoms like oxygen are necessary to understand the TPD spectra of the corresponding experiments.

Extraordinarily large kinetic isotope effect on alkene hydrogenation over Rh-based intermetallic compounds

ABSTRACT A series of Rh-based intermetallic compounds supported on silica was prepared and tested in alkene hydrogenation at room temperature. H2 and D2 were used as the hydrogen sources and the kinetic isotope effect (KIE) in hydrogenation was studied. In styrene hydrogenation, the KIE values differed strongly depending on the intermetallic phase, and some intermetallic compounds with Sb and Pb exhibited remarkably high KIE values (>28). An extraordinarily high KIE value of 91, which has never been reported in catalytic reactions at room temperature, was observed particularly for RhPb2/SiO2. RhPb2/SiO2 also showed high KIE values in the hydrogenation of other unsaturated hydrocarbons such as phenylacetylene and cyclohexene. The density functional theory calculation focused on the surface diffusion of hydrogen suggested no contribution of the quantum tunneling effect to the high KIE values observed. A kinetic study revealed that the dissociative adsorption of H2 (D2) was the rate-determining step in the styrene hydrogenation over RhPb2/SiO2. We propose that the large KIE originates from the quantum tunneling occurring at the hydrogen adsorption process with the aid of the specific surface structure of the intermetallic compound and adsorbate alkene.

Signatures of multiple jumps in surface diffusion on honeycomb surfaces

The jump distribution, a property of the motion of adsorbates on a corrugated surface, contains crucial information on adsorbate-substrate energy dissipation processes. To provide a means to study jump distributions in a honeycomb array of adsorption sites, we derive analytical expressions for the intermediate scattering function (ISF) describing jump diffusion taking into account jumps up to fourth-nearest neighbor in length. To enable testing the analytical expressions against experimental or simulated data, we develop a global fitting routine that can be applied to experimental or simulated ISFs to infer multiple jumps. We demonstrate the analysis method by studying the jump distribution arising from classical Langevin molecular dynamics simulations of two model systems, cyclopentadienyl (Cp) on Cu(111), and deuterium (D) on Pd(111). The simulations and analysis confirm that diffusion of Cp/Cu(111) at a surface temperature ${T}_{s}=135$ K takes place in a regime of predominantly single jumps. Classical simulations of D/Pd(111) at ${T}_{s}=350$ K, with a realistic Langevin friction, suggest that the diffusion of D/Pd(111) involves a high proportion of multiple jumps. The parameters that apply to D/Pd(111) are typical of the interaction of hydrogen atoms with close-packed transition metal surfaces, suggesting that long jumps are a general feature of the high temperature surface diffusion of hydrogen.

The role of fluctuations in bistability and oscillations during the H2 + O2 reaction on nanosized rhodium crystals.

A combined experimental and theoretical study is presented of fluctuations observed by field ion microscopy in the catalytic reaction of water production on a rhodium tip. A stochastic approach is developed to provide a comprehensive understanding of the different phenomena observed in the experiment, including burst noise manifesting itself in a bistability regime, noisy oscillations, and nanopatterns with a cross-like oxidized zone separating the surface into four quadrants centered on the {111} facets. The study is based on a stochastic model numerically simulating the processes of adsorption, desorption, reaction, and transport. The surface diffusion of hydrogen is described as a percolation process dominated by large clusters corresponding to the four quadrants. The model reproduces the observed phenomena in the ranges of temperature, pressures, and electric field of the experiment.

Impedance Study of Anodic Properties of Ni-Fe Impregnated SDC

Samaria-doped ceria (SDC) possesses a higher oxide ionic conductivity compared to YSZ, furthermore it exhibits a high electronic conductivity under a reducing condition. Therefore SDC can form electron conduction paths instead of Ni particles, which means Ni concentration can be suppressed to avoid their aggregation. As a result of suppressing Ni aggregation, the 20wt% Ni/SDC anode exhibited the higher performance compared to that containing lower or higher concentration of Ni. This is due to the increase in the number of reaction site, i.e., interfaces between Ni particles and SDC. Ni-Fe anodes with the high Ni-Fe concentrations have been already reported. Lu et al. evaluated the 50wt% Ni1-xFex/SDC anode in hydrogen fuel and reported their best performance at x=0.25. They assigned the modified performance to the electronic interaction between Ni and Fe. In the present study, we newly prepared Ni-Fe/SDC anode containing 20wt% Ni-Fe alloy, and their anodic properties were investigated by the impedance analyses. The Ni1-xFex/SDC particles were prepared by the impregnation method using the nitrates of the corresponding metals. The precursors were calcinated at 700oC for 10h. The obtained powers were deposited on the dense 10Sc1CeSZ disk and sintered at 900oC. The cathode was formed with Pt paste on the opposite surface of the 10Sc1CeSZ disk. 100%H2 and air were supplied on the anode and cathode, respectively. All the impedance spectra of the Ni1-xFex/SDC anodes were measured at 800oC with the three-terminal configuration under the OCV condition. The impedance spectra of the anodes exhibited an ohmic component and two depressed arcs. Therefore, the spectra were fitted by assuming the equivalent circuit of R ohm(RHFQHF)(R LH Q LF), where Q is a phase constant element. The R HF component hardly influenced by P H2, and on the other hand, the R LF component was proportional to P H2 0.5. From these results, we assigned the R HF and R LF to charge transfer process of hydrogen and surface diffusion of the hydrogen atoms, respectively. Furthermore, the R LF component sensitively decreased by the humidification of hydrogen. Therefore it was found that the adsorbed water molecule or associated species helped hydrogen atom diffuse on the anode. The i-v characteristics of the cells with the Ni1-xFex/SDC anodes were resulted in order of 0.5 ≥ 0.2 > 0.8 ≥ 0. The crystallite sizes of the Ni1-xFex/SDC were dependent on x, and minimum at x=0.5. The i-v characteristics correlated with the crystallite size. The impedance spectra of the Ni1-xFex/SDC anodes were also fitted by assuming the equivalent circuit of R ohm(RHFQHF)(R LH Q LF). The R LF, which can be assigned to the dissociative adsorption and surface diffusion of hydrogen, was most variable among the resistance components. The x=0.5 anode exhibited the R LF minimum value. Therefore we tentatively conclude that Fe addition to Ni reduces the potential for the hydrogen dissociation and the surface diffusion processes.

Electrochemical Behavior of Ni-CZO Cermet Anodes Prepared by Glycine-nitrate-process

The impact of H2O for H2 oxidation on Ni-Ce0.75Zr0.25O2 (Ni-CZO) anode material synthesized by glycine-nitrate-process has been successfully investigated under dry and wet H2 fuel atmospheres over a range of operating conditions using yttria-stabilized zirconia (YSZ) electrolytes. Using electrochemical impedance spectroscopy (EIS), the polarization resistance (comprised of high frequency (HF) and low frequency (LF) features) of Ni-CZO symmetrical cells (Ni-CZO/YSZ/Ni-CZO) has been studied. The LF arc was almost negligible at all the operating temperatures in wet condition, and the HF arc was linearly increased with time at 800oC only in dry condition. The EIS response in dry and wet H2 conditions clearly exhibited that, water vapor enhanced the dissociative adsorption and surface diffusion of hydrogen and achieved the stable electrochemical performance at 3PB on the Ni-CZO cermet.

Three steps in the anode reaction of the polymer electrolyte membrane fuel cell. Effect of CO

We studied the influence of CO poisoning of the anode in the polymer electrolyte membrane fuel cell (PEMFC) using electrochemical impedance spectroscopy (EIS). The anode impedance was found by first feeding neat hydrogen gas and next hydrogen with CO into one of the electrodes, keeping neat hydrogen gas on the other electrode as a reference. The electrodes were E-TEK Elat gas-diffusion electrodes with 0.5 mg Pt/cm 2 , and the membrane was Nafion � 117. The CO concentration was 103 ppm, and the total pressures were 1, 2.5 and 4 bar. Operating temperatures were kept constant, 30.0 � C or 50.0 ± 0.1 � C. Bias voltages of 0 and 0.05 V were used. Three steps were revealed in the reaction mechanism, the slow adsorption/diffusion step, the charge transfer step and the proton hydration step, confirming earlier results. Carbon monoxide affects the charge transfer step by blocking active sites and by affecting the surface polarisation. We further conclude that CO adsorbs to the porous carbon matrix, and reduces significantly the rate of surface diffusion of hydrogen to the surface. � 2007 Elsevier B.V. All rights reserved.

Quantum Diffusion of H on Pt(111): Step Effects

Using a linear optical diffraction technique, we have systematically investigated the defect effects on quantum surface diffusion of hydrogen on Pt(111) surfaces. The quantum tunneling effect was clearly observed for hydrogen diffusion at low temperatures as manifested by a leveling off of the diffusion coefficient on flat surfaces. The strong influence of surface defects on the quantum diffusion is in good agreement with the creation of an inhomogeneous surface with adsorption sites of different binding energies.

A Gerischer Phase Element in the Impedance Diagram of the Polymer Electrolyte Membrane Fuel Cell Anode

We study the isothermal hydrogen adsorption and reaction at the E-TEK electrode of a polymer electrolyte fuel cell with a Nafion 117 membrane by impedance spectroscopy at 30 degrees C. We find that the impedance diagram must include a Gerischer phase element. Constant phase elements are not sufficient to describe the experimental data. This means that an adsorption reaction takes place in combination with surface diffusion of hydrogen in the carbon layer located before the platinum surface, separate from the charge transfer step at the platinum particle surface. We are not able to distinguish between molecular or atomic hydrogen diffusion on carbon. We predict and find that the relaxation time of the adsorption step is independent of the applied potential. Water may also enter rate-limiting steps in the electrode reaction, but its role needs further clarification.

Step effects and coverage dependence of hydrogen atom diffusion onPt(111)surfaces

Step effects on surface diffusion of hydrogen on stepped $\mathrm{Pt}(111)$ have been studied by linear optical diffraction technique over a temperature range from 90 to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Diffusion anisotropy on stepped $\mathrm{Pt}(111)$ surfaces has been observed: the unexpected enhanced diffusion perpendicular to steps cannot be explained within the lattice gas model on stepped substrates, indicating a nonlocal and directional step effect. The coverage-dependent diffusion coefficient on flat $\mathrm{Pt}(111)$ surface was also measured over a wide coverage range from 0.1 to $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$. They were analyzed within the framework of the lattice gas model using quasichemical approximation, indicating that $\mathrm{H}\text{\ensuremath{-}}\mathrm{H}$ repulsive interaction can significantly affect the energy of saddle point as well as that at the adsorption sites.

An Intermediate-Temperature Solid Oxide Fuel Cell Providing Higher Performance with Hydrocarbons than with Hydrogen

The promotion of direct electrochemical oxidation of hydrocarbons in a solid oxide fuel cell was investigated using a ceria-based electrolyte with different noble metals-containing anode at 600°C. The objective was to avoid interference from a large amount of steam and being produced by discharging the cell, because these gases degrade the anode performance, especially at a high fuel utilization. Ru was an effective catalyst for removing these gases from the anode surface due to its high catalytic activity for the steam and reforming of hydrocarbons. The resulting peak power densities reached 750 mW cm−2 with dry methane, which was comparable to the peak power density of 769 mW cm−2 with wet hydrogen. The cell performance was maintained at a high level regardless of the change in methane utilization from 12 to 46% but was significantly reduced by increasing hydrogen utilization from 13 to 42%. The anodic reaction of hydrogen was controlled by the slow surface diffusion of hydrogen, while the anodic reaction of methane was not subject to the onset of such a gas-diffusion process. © 2002 The Electrochemical Society. All rights reserved.

Diffusion on a stepped surface: H and D on Rh[lcub]332[rcub

Surface diffusion of hydrogen and deuterium on Rh[lcub]332[rcub] has been studied using laser-induced thermal desorption. The recently developed boundary method was applied to measure diffusion at saturation coverages both parallel and orthogonal to the step direction. Activation energies E ∥ = 20.5 ± 3.0 kJ mol −1 and E ⊥ = 22.0 ± 2.1 kJ mol −1 and E ∥ = 16.1 ± 4.4 kJ mol −1 and E ⊥ = 20.9 ± 1.7 kJ mol −1 were found for D and H, respectively. Due to a compensation effect, the diffusion coefficients only differ by a factor of ⩽ 2 in the temperature range 150–250 K studied. Activation energies are somewhat higher than previously reported for Rh[lcub]111[rcub]. The results can be rationalised by a small additional energy barrier associated with the steps (the Schwoebel effect) and possibly an influence of kink sites.

Surface diffusion of hydrogen on a stepped Ru(001) surface

The surface diffusion of hydrogen on Ru(S)-[15(001) × 2(100)] was studied using laser-induced thermal desorption (LITD) techniques. Hydrogen diffusion on this stepped Ru(001) surface displayed a dramatic anisotropy. At 230 K, the diffusion coefficient parallel to the steps edges ( D ∥) was approximately 50 times greater than the diffusion coefficient perpendicular to the step edges ( D ⊥). Hydrogen surface diffusion coefficients across a terrace ( D t) and over a step ( D s) were obtained from the measured values of D ∥ and D ⊥ at θ H = 0.60 ML. The activation energy for diffusion along a terrace was E t = 3.2 ± 0.3 kcal/mol and the corresponding diffusion preexponential was D o,t = 3.9 × 10 −4 ± 0.1cm 2/s. For diffusion over a step, the kinetic parameters were E s = 5.9 ± 0.6 kcal/mol and D o,s = 3.8 × 10 −3 ± 0.2cm 2/s. Although the hydrogen surface mobility was highly anisotropic, the surface diffusion coefficient was observed to be coverage independent. The coverage independent diffusion suggests that hydrogen adatoms are not trapped by the step edges. The dramatic anisotropy of the diffusion coefficient and the coverage independent diffusion indicate that the step edges on Ru(S)-[15(001) × 2(100)] act as a repulsive barrier to hydrogen diffusion. The good agreement between the surface diffusion kinetic parameters for hydrogen diffusion on Ru(001) and hydrogen diffusion on the terraces of Ru(S)-[15(001) × 2(100)] demonstrates that surface steps did not dominate the earlier LITD measurements on Ru(001).

Surface diffusion of hydrogen on Ni(100): An experimental observation of quantum tunneling diffusion.

We studied the surface diffusion of hydrogen on Ni(100) by linearly diffracting a probe laser beam from a monolayer grating of adsorbed hydrogen. In the temperature range from 110 to 161 K the Arrhenius plot shows a significant leveling off of the diffusion coefficient D(T) at around 144 K. Such a deviation from the behavior of thermally activated diffusion suggests the onset of quantum tunneling diffusion.

Transport and Related(Gas and Vapour) Separation in Ceramic Membranes

The main transport mechanisms in pores with a size of 3 to 100 nm are Knudsen and surface diffusion. According to Knudsen diffusion the transport of gases is inversely proportional to the square root of the molar mass. Surface diffusion is an additional mechanism to Knudsen diffusion and decreases, in most cases, the separation possibilities. Only surface diffusion of hydrogen (on metals) can increase the separation factors. Multilayer adsorption and capillary condensation can be very selective mechanisms for the separation of condensable gases from gas mixtures in pores of 2 to 10 nm. The application is limited because of the temperature/pressure regime which is needed for condensation and which may lead to technical inconveniences. In pores smaller than 2 nm and in zeolites micropore diffusion occurs. Transport of gases is activated with activation energies smaller than 25 kJ/mol for amorphous microporous systems. In silica-type of microporous toplayers H2,He and CO2 are transported much faster than any other type of gas. At 200°C a separation factor of over 200 is found for the mixture H2/C3H6. The transport mechanism in amorphous microporous materials is still under discussion. In this contribution the mentioned transport mechanisms will be discussed and their possibilities in gas and vapour separation will be illustrated.

Coverage dependence of surface diffusion of hydrogen and deuterium on Rh{111} by laser induced thermal desorption

The coverage dependence of the surface diffusion of hydrogen and deuterium on Rh{111} has been studied by laser-induced thermal desorption. Diffusion across a step-like boundary created by a rapid interlaced laser raster was monitored at a series of temperatures by an orthogonal laterally resolved laser-induced desorption scan. Relative surface coverages as a function of distance were estimated from the integrated time-of-flight desorption yield and the coverage dependence of the surface diffusion coefficient calculated via a Boltzmann-Matano analysis. The activation energy for deuterium diffusion was found to be 13.8 ± 2 kJ mol -1, independent of coverage within the accuracy of the analysis in the range 0.3 ⩽ θ D ⩽ 0.9 ML. Corresponding pre-exponentials varied from 5.7 × 10 -4 cm 2 s -1 at θ D = 0.4 ML to 7.1 × 10 -4 cm 2 s -1 at θ D = 0.8 ML. Hydrogen exhibited similar values with activation energies and pre-exponentials from E d = 11 ± 2 kJ mol -1, D 0 = 5.9 × 10 -4 cm 2 s -1 at θ H = 0.8 ML to E d = 13.4 ± 2 kJ mol -1, D 0 = 6.5 × 10 -3 cm 2 s -1 at θH = 0.3 ML. The “hole-refilling” method was also used to study diffusion, and similar values were obtained for E d and D 0. The weak lateral adatom-adatom interactions apparent in the diffusion data are suggestive of the coverage dependent population of two distinct binding sites to explain the pronounced shift to lower temperature of peak maxima in thermal desorption spectroscopy.

A new ab initio potential energy surface for hydrogen atom on ruthenium(0001) and its use for variational transition state theory and semiclassical tunneling calculations of the surface diffusion of hydrogen and deuterium

We have applied variational transition state theory with a semiclassical tunneling method to calculate the rates of hydrogen and deuterium diffusing on a Ru(0001) surface. The hydrogen-metal interaction is modeled by a pairwise potential which has been fitted to electronic structure calculations based on a local density approximation and a pseudopotential. We present diffusion coefficients and kinetic isotope effects fort the temperature range from 80 to 800 K using the fitted potential and also for two modified potentials ― one in which the potential along the minimum-energy path is scaled by a constant and one in which the pairwise potential parameters are modified to change the barrier height. The results for all three potentials are compared to experimental rate coefficients determined recently by laser-induced thermal desorption

KINETICS OF FIELD EVAPORATION OF GALLIUM PHOSPHIDE IN THE PRESENCE OF HYDROGEN

Field evaporation of gallium phosphide and field desorption of hydrides from different gallium phosphide surfaces were studied in the temperature range of 26 to 340K. Field evaporation rates spanning up to 3 orders of magnitude were measured as a function of temperature and at different partial pressures of hydrogen. The temperature dependence of the evaporation rate showed a pronounced minimum at 200K. Below 200K the evaporation rate was found to be proportional to the hydrogen gas pressure. The evaporation rate was proportional to the hydrogen coverage. The temperature dependence of the evaporation rate was fitted by Langmuir- and Volmer-isobars. From these fits, a binding energy of 45 meV was evaluated. Above 200K the activation energy for field evaporation was determined to be 662meV. Two models describe the occurrence of different activation energies. In the first case field desorption of gallium phosphide hydrides are influenced by an increased hydrogen supply by surface diffusion of hydrogen from the shank to the apex of the tip or by impingement from the gas phase. Conversely, the second model neglects the influence of hydrogen and regards the onset of field evaporation of gallium phosphide, as usually observed for metals and semiconductors.

Effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001)

The effects of coadsorbed carbon monoxide on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. The surface mobility of hydrogen was measured as a function of CO surface coverage at 260 and 280 K. At both temperatures, the surface diffusion of hydrogen displayed an abrupt reduction at a coadsorbed CO coverage of ΘCO =0.12 ML. LEED studies revealed that a CO coverage of ΘCO =0.12 ML corresponded to the onset of the formation of ordered √3×√3 CO islands at 260 and 280 K. Temperature programmed desorption results demonstrated that a lateral repulsive interaction exists between hydrogen and carbon monoxide on Ru(001). Assuming that the lateral repulsive interaction leads to a hydrogen exclusion area around each CO admolecule, a hydrogen exclusion radius of rCO =2.2–2.7 Å was determined. The lateral repulsive interaction would also result in hydrogen exclusion from the interior of ordered √3×√3 CO islands and provide an explanation for the hydrogen surface diffusion results. According to this interpretation, the rigid √3×√3 CO islands obstruct the hydrogen surface diffusion pathways and thereby reduce the hydrogen surface mobility on Ru(001).