Hydrogen Electro-Insertion into Pd/Pt(111) Nanofilms: An in Situ Surface X-ray Diffraction Study (original) (raw)
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Electrocatalysis, 2012
Ultrathin palladium films, at coverages that ranged from one half to eight monolayers, were prepared, one layer at a time, on a well-defined Pt(111) single-crystal electrode surface by surface-limited redox replacement reaction (galvanic exchange) of Cu initially coated via underpotential deposition. Adlayers produced by the galvanic-exchange method were completely free of any residual Cu after the displacement-by-Pd step. Analysis of the current-potential profiles in the hydrogen-adsorption region indicated that at one monolayer coverage, the film was essentially pure terrace, devoid of steps, which did not exhibit hydrogen absorption. At higher coverages, a Stranski-Krastanov growth mode was manifested by the emergence of a voltammetric peak characteristic of hydrogen adsorption-desorption at step sites. Peaks diagnostic of hydrogen adsorption on both terrace and steps persisted even at a Pd-film thickness of eight monolayers; the intensities were considerably diminished, however, which indicated that hydrogen absorption into the now-bulk-like film had become dominant.
Hydrogen Adsorption on Palladium and Platinum Overlayers: DFT Study
Advances in Physical Chemistry, 2011
Hydrogen adsorption on twenty different palladium and platinum overlayer surfaces with (111) crystallographic orientation was studied by means of periodic DFT calculations on the GGA-PBE level. Palladium and platinum overlayers here denote either the Pd and Pt mono-and bilayers deposited over (111) crystallographic plane of Pd, Pt, Cu, and Au monocrystals or the (111) crystallographic plane of Pd and Pt monocrystals with inserted one-atom-thick surface underlayer of Pd, Pt, Cu, and Au. The attention was focused on the bond lengths, hydrogen adsorption energetics, mobility of adsorbed hydrogen, and surface reactivity toward hydrogen electrode reactions. Both the ligand and strain effects were considered, found to lead to a significant modification of the electronic structure of Pd and Pt overlayers, described through the position of the d-band center, and tuning of the hydrogen adsorption energy in the range that covers approximately 120 kJmol −1 . Mobility of hydrogen adsorbed on studied overlayers was found to be determined by hydrogen-metal binding energy. Obtained results regarding Pd layers on Pt(111) and Au(111) surfaces, in conjunction with some of the recent experimental data, were used to explain its electrocatalytic activity towards hydrogen evolution reaction.
Investigation of Hydrogen Adsorption and Absorption in Palladium Thin Films
Journal of The Electrochemical Society, 2004
ABSTRACT The electrochemical impedance of palladium films of various thicknesses, deposited on a gold electrode, was measured in the potential range where hydrogen absorbs in palladium. The kinetic and diffusion parameters of the whole electrochemical process were determined. One of the main results was the observation of a maximum on the curves of charge-transfer resistance vs. electrode potential. At this maximum the charge-transfer resistance decreased with the increase of film thickness. This finding is in agreement with the model proposed in Part I, where such a behavior is found to be related to the trapping of the hydrogen atoms in a sublayer just under the electrode surface and a direct exchange of the trapped hydrogen with the electrolyte. (C) 2004 The Electrochemical Society.
Hydrogen adsorption/absorption on Pd/Pt(111) multilayers
Journal of Electroanalytical Chemistry, 2008
One to 20 monolayers of Pd were deposited on Pt(1 1 1) and their electrochemical behavior was studied by cyclic voltammetry and impedance spectroscopy. The hydrogen adsorption and absorption is more reversible at deposits on Pt(1 1 1) as compared to Au(1 1 1), probably due to less strain and/or electronic effects. The quantity of hydrogen adsorbed/absorbed at 10 ML are however comparable at both supports. The charge transfer resistance is lower at 1 Pd ML in perchloric and sulfuric acids than for thicker layers, and it is lower at Pd/Pt(1 1 1) than at Pd/Au(1 1 1). The adsorption pseudocapacitance reaches a maximum at 0.214 V in sulfuric acid for 1 ML and there are two peaks at 0.214 V and 0.255 V for thicker layers. Crystal violet increases the reversibility of hydrogen absorption and inhibits hydrogen adsorption although some residual adsorption remains.
ELECTROCHEMICAL HYDROGEN INSERTION INTO PALLADIUM AND PALLADIUM-NICKEL THIN FILMS
Electrochimica Acta, 1991
Abatraet-A generator-detector mode with the rotating nng-dlsk electrode has been used to determme the &fference between Pd and Pt electrode behavior and the ongm of the larger overpotentials at Pd-hydrogen evolving cathodes The use of thm film Pd disk electrodes has allowed full hydrogen or deutenum chargmg of the metal phase m short ties The electrode system has made feasible emultaneous measurements of the hydrogen uptake m PdH, lilms where the-mum composition reached was PdH, 8, Alloymg NI mto Pd decreases the maxlmum hydrogen absorption by sohd solution Pd-Nl films to near zero at 17% NI The outgassmg of hydrogen on open cxcmt from vanously charged electrodes can be followed m real time and shows the losses associated with transfer of specimens to post-analysis (w 6% m 5 mm)
Russian Journal of Electrochemistry, 2000
A series of Pd electrodeposits (edPd) on Pt substrates is prepared at deposition potentials of-0.05 to 0.55 V with respect to a reversible hydrogen electrode in 0.5 M H2SO 4. Their nanostructure is characterized by scanning tunneling microscopy. The size distribution of particles is estimated, and dependences of its maximum and half width on the deposition potential are determined. A comparative coulometric study of adsorption of copper and oxygen on edPd shows that real surface areas determined from these data substantially differ. The average size of particles for edPd, estimated within the model of equal-size spheres, is shown to be incorrect. The assumption that particles in the deposits essentially coalesce is substantiated. It is shown that the equilibrium hydrogen content in the tx and 13 hydrides is anomalously high for the deposits whose growth was accompanied by deep hydrogenation of Pd. At a given effective pressure, for the or-phase, this value is always substantially higher as compared with less defective materials. In the I~-phase, the hydrogen concentration can be either lower or higher.
Hydrogen absorption in Pd thin-films
International Journal of Hydrogen Energy, 2014
Hydrogen absorption isotherms for Pd thin films were modeled at atomistic scale by Monte Carlo (MC) simulation in the TPmN ensamble and by Molecular Dynamics (MD) simulations at 300 K. The interaction among atoms was modeled by embedded atom method (EAM) potentials. Simulated samples consisted of monocrystalline nanofilms with different thickness (2e8 nm) and two crystallographic surface orientations, (001) and (111). The isotherms were compared to bulk Pd and a few available experimental results. Instead of the plateau corresponding to the a-b PdH equilibrium in the bulk, the isotherms at nanofilms show a two-plateaux behavior: a small one corresponding to a surfaceesubsurface hydride formation, and a larger one for the subsequent bulk hydride formation. This is strongly correlated with the atomic stress distribution induced within the thin film. The equilibrium pressures at the isotherms depend on the thin-film thickness, with pressure being larger for thicker films. The isotherms of the (001) films display lower equilibrium pressures than those for (111) films.
Nanocrystalline Palladium Thin Films for Hydrogen Sensor Application
Sensor Letters, 2009
We report the application of palladium nanoparticles and thin films for hydrogen sensor. Electrochemically grown palladium particles with spherical shapes deposited on Si substrate and sputter deposited Pd thin films were used to detect hydrogen at room temperature. Grain size dependence of H 2 sensing behavior has been discussed for both types of Pd films. The electrochemically grown Pd nanoparticles were observed to show better hydrogen sensing response than the sputtered palladium thin films. The demonstration of size dependent room temperature H 2 sensing paves the ways to fabricate the room temperature metallic and metal-metal oxide semiconductor sensor by tuning the size of metal catalyst in mixed systems. H 2 sensing by the Pd nanostructures is attributed to the chemical and electronic sensitization mechanisms.
Engineering Palladium Surfaces to Enhance the Electrochemical Storage of Hydrogen
ECS Transactions, 2017
Materials can be engineered to have enhanced hydrogen storage capabilities for a given electrolysis working condition by modifying the composition of the first few atomic layers. The changes in composition of the near surface can affect the balance between the Volmer, Tafel, and Heyrovsky reactions, which changes the chemical potential of the adsorbed hydrogen, and ultimately controls the electrochemical insertion of hydrogen. To this end, the hydrogen stored under galvanostatic conditions was investigated after altering the composition of the Pd surface with various combinations of Pb, Bi, and Pt. It was found that the addition of an underpotential deposition (UPD) of Bi on the Pd cathode increases the hydrogen content from PdH 0.77 to PdH 0.81 at −10.9 mA cm −2 , and the addition of a small amount of Pt to the UPD Bi (Pt/Bi atomic ratio of 0.13 ± 0.01) further increased the hydrogen content to PdH 0.87. For comparison, the same change in hydrogen content from pressurized gas loading experiments would require an increase in hydrogen fugacity from about 16 to 2400 atm. This work provides a fundamental basis for the future design of surface alloys yielding enhanced electrochemical hydrogen storage in Pd and other hydrogen absorbing materials.
Surface Science, 2003
The morphology of electrochemically deposited Pd films on the Pt(0 0 1) electrode surface has been examined through the combination of cyclic voltammetry (CV) and in situ surface X-ray scattering (SXS). Analysis of SXS measurements has indicated that the Pd grows via pseudomorphic island formation, with the partial occupation of successive layers occurring at a first layer occupation of 0.8 ML. Further Pd deposition sees the formation of larger islands built onto the now complete monolayer, characteristic of pseudomorphic Stranski-Krastanov (SK) growth. In the H UPD potential region the effect of CO on the surface expansion of the multilayer Pd film is negligible. In the hydrogen evolution region, however, the effect of the adsorption of CO has been shown to produce surface normal expansion and in-plane disorder of the Pd film. It is suggested that hydrogen permeation into the Pd film is enhanced on the CO-poisoned surface.