Hydrogen adsorption/absorption on Pd/Pt(111) multilayers (original) (raw)

Layer-by-Layer Deposition of Pd on Pt(111) Electrode: an Electron Spectroscopy–Electrochemistry Study

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.

The effect of specific chloride adsorption on the electrochemical behavior of ultrathin Pd films deposited on Pt (111) in acid solution

The electrochemical behavior of thin Pd films supported on a Pt(111) electrode is investigated by cyclic voltammetry (CV) and in-situ Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that in perchloric acid solution underpotential deposition of hydrogen (H upd ) and hydroxyl adsorption (OH ad ) is in strong competition with the adsorption of Clanions, the latter being present as a trace impurity in HClO 4 . The interaction of Clwith Pd is rather strong, controlling the adsorption of H upd and OH ad as well as the kinetic rate of CO oxidation. The microscopic insight (the binding sites) of the adsorbed CO (CO ad ) and the rate of CO oxidation (established from CO 2 production) on Pt(111) modified with a (sub)monolayer of Pd is elucidated by means of Fourier infrared (FTIR) spectroscopy. The appearance of both the characteristic Pt(111)-CO ad and Pt(111)1 ML Pd-CO ad stretching bands on a Pt(111) surface covered by 0.5 ML Pd confirms previous findings that the Pd atoms agglomerate into islands and that the bare Pt areas and the Pd islands behave according to their own surface chemistry. The systematic increase of the Pd surface coverage results in a gradual change in the catalytic properties of Pt(111)-xPd electrodes towards CO oxidation, from those characteristic of bare Pt(111) to those which are characteristic for Pt(111) covered with 1 ML of Pd.

Mechanism of hydrogen adsorption/absorption at thin Pd layers on Au(111)

Electrochimica Acta, 2007

ABSTRACT Hydrogen adsorption and absorption at thin palladium deposits of 0.8–10 monolayers (ML) on Au(1 1 1) was studied in 0.1 M H2SO4 and HClO4 using cyclic voltammetry, ac voltammetry, and impedance spectroscopy in the absence and in the presence of poison, crystal violet. Hydrogen adsorption on palladium is more reversible in sulfuric acid than in perchloric acid but it occurs at potentials 30 mV more positive in latter. The charge-transfer resistance exhibits a minimum at ∼0.27 V versus RHE and decreases with increasing in Pd deposit thickness in both acids. Adsorption capacitance at 0.8 ML Pd reaches maximum at the same potential. At other deposits the pseudo-capacitance starts to increase at lower overpotentials indicating the beginning of absorption, even at 2 ML Pd. The double layer capacitance is similar for all the deposits in sulfuric acid and it has a sharp maximum at 0.27 V versus RHE. In perchloric acid a broad maximum is observed. Crystal violet inhibits hydrogen adsorption but makes hydrogen absorption more reversible. The results suggest a fast direct hydrogen absorption mechanism that proceeds in parallel with slower hydrogen adsorption and indirect absorption.

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.

The effect of specific chloride adsorption on the electrochemical behavior of ultrathin Pd films deposited on Pt() in acid solution

Surface Science, 2003

The electrochemical behavior of thin Pd films supported on a Pt(111) electrode is investigated by cyclic voltammetry (CV) and in-situ Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that in perchloric acid solution underpotential deposition of hydrogen (H upd ) and hydroxyl adsorption (OH ad ) is in strong competition with the adsorption of Clanions, the latter being present as a trace impurity in HClO 4 . The interaction of Clwith Pd is rather strong, controlling the adsorption of H upd and OH ad as well as the kinetic rate of CO oxidation. The microscopic insight (the binding sites) of the adsorbed CO (CO ad ) and the rate of CO oxidation (established from CO 2 production) on Pt(111) modified with a (sub)monolayer of Pd is elucidated by means of Fourier infrared (FTIR) spectroscopy. The appearance of both the characteristic Pt(111)-CO ad and Pt(111)1 ML Pd-CO ad stretching bands on a Pt(111) surface covered by 0.5 ML Pd confirms previous findings that the Pd atoms agglomerate into islands and that the bare Pt areas and the Pd islands behave according to their own surface chemistry. The systematic increase of the Pd surface coverage results in a gradual change in the catalytic properties of Pt(111)-xPd electrodes towards CO oxidation, from those characteristic of bare Pt(111) to those which are characteristic for Pt(111) covered with 1 ML of Pd.

Hydrogen sorption in Pd monolayers in alkaline solution

2009

Hydrogen adsorption/absorption at palladium monolayers (ML) deposited on monocrystalline Au(1 1 1) electrode was studied in 0.1 M NaOH solution. H charge isotherms demonstrated that adsorption started at potentials more positive than at thicker nanometric Pd/Au(polycrystal) deposits. Due to 3-dimensional deposit growth, absorption could be seen at all deposits thicker than 1 ML. Besides, H sorption at Pd/Au(1 1 1) monolayers was more reversible than at nanometric Pd/Au(polycrystal) deposits. Strong geometric and electronic effects due to the Au substrate were observed up to 5 Pd ML. Influence of benzotriazole (BTA) on H sorption was also investigated. BTA blocked H adsorption above 250 mV vs. RHE. At less positive potentials adsorbed BTA layer seemed to undergo a reorientation allowing H adsorption. Stationary and dynamic electrochemical impedance spectroscopy was used to obtain double layer capacitance and charge transfer resistance. BTA also promoted kinetically H sorption into Pd/Au(1 1 1) monolayer and Pd/Au(polycrystal) nanometric deposits.

Separation of hydrogen adsorption and absorption on Pd thin films

Electrochimica Acta, 2008

Hydrogen sorption at Pd films of 20–80nm deposited on a polycrystalline gold electrode was studied in sulfuric and perchloric acid. Assuming that the hydrogen adsorption does not vary with the Pd films thickness, hydrogen adsorption/absorption charges in Pd were separated in the two contributions in the hydrogen-poor α-Pd–H phase. The results are compared to those obtained at Pd monolayers on

Electrochemical properties of palladium adlayers on Pt(110) substrates

Journal of Electroanalytical Chemistry, 2011

Palladium deposition on Pt(1 1 0) electrodes was studied. Differently from Pt(1 1 1) and Pt(1 0 0), first and further layers cannot be distinguished in this case only through the voltammetric behavior of hydrogen and anion adsorption. The potential of zero total charge (pztc) was determined as a function of the amount of deposited Pd using CO charge displacement experiments and voltammetric curves. The variation of the voltammetric charge due to hydrogen and anion adsorption has been followed during Pd deposition. The voltammetric charge between 0.06 and 0.4 V first decreases to a minimum, then increases and finally becomes stable, suggesting that the surface is finally covered with a Pd multilayer. The pztc and CO oxidation charge show similar behavior. CO oxidation, NO reduction and Cu UPD were used as probes to monitor Pd coverage. The potential of CO oxidation increases with Pd coverage while the potential of NO reduction decreases. Using the information obtained from Cu UDP and FTIR experiments it has been possible to determine when the first Pd single layer was completed and when a second (further) layer(s) starts to grow.

Surface (electro-) chemistry on Pt (111) modified by a pseudomorphic Pd monolayer

The formic acid and methanol oxidation reaction are studied on Pt(1 1 1) modified by a pseudomorphic Pd monolayer (denoted hereafter as the Pt(1 1 1)-Pd 1 ML system) in 0.1 M HClO 4 solution. The results are compared to the bare Pt(1 1 1) surface. The nature of adsorbed intermediates (CO ad ) and the electrocatalytic properties (the onset of CO 2 formation) were studied by FTIR spectroscopy. The results show that Pd has a unique catalytic activity for HCOOH oxidation, with Pd surface atoms being about four times more active than Pt surface atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO 2 is observed at low potentials. This indicates that the reaction can proceed on Pd at low potentials without the typical ''poison'' formation. In contrast to its high activity for formic acid oxidation, the Pd film is completely inactive for methanol oxidation. The FTIR spectra show that neither adsorbed CO is formed on the Pd sites nor significant amounts of CO 2 are produced during the electrooxidation of methanol.