Hydrogen sorption in Pd monolayers in alkaline solution

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

Hydrogen sorption at Pd films of 20-80 nm 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 Au(1 1 1). The adsorption on polycrystalline Pd begins at potentials more negative than on 0.8 ML Pd on Au(1 1 1) and is not much affected by the nature of anion (sulfate or perchlorate), contrary to the thin layers on Au(1 1 1). The absorption charge in α-PdH phase in the potential range of 0.08-0.15 V was found to be similar to that at a 25 μm Pd foil in this potential range while at more positive potentials it is larger. In the presence of crystal violet which adsorbs at the electrode surface it was found that some residual H adsorption exists. There is more hydrogen absorbed in Pd in the presence of crystal violet in the hydrogen-poor α phase but in the hydrogen-rich β phase the amount of hydrogen is the same.     

Study of the hydrogen absorption in Pd in alkaline solution

Hydrogen adsorption/absorption in palladium thin deposits on gold electrode, in 0.1 M NaOH solution, was studied. The contributions of adsorption and absorption to the total charge of hydrogen are separated from the total isotherms at different deposit thicknesses. The adsorbed hydrogen charge increases to a plateau of ∼73.5 μC cm⁻², which corresponds to the surface coverage ratio by adsorbed hydrogen of 0.36. The absorbed hydrogen charge agreed with that obtained from the permeation experiments at 50 μm Pd foil, at potentials between +100 and +300 mV vs. RHE. EIS was carried out at thin Pd deposits. The kinetics of hydrogen sorption is slower in alkaline solutions than in acids and the isotherms are shifted towards more negative potentials.     

Preparation and electrochemical characterization of palladium single crystal electrodes in 0.1 M H2SO4 and HClO4: Part I. Low-index phases

The electrochemical properties of Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0) single crystal bead electrodes, prepared by a novel electron beam heating and inductive annealing technique, have been characterized in 0.1 M sulfuric acid and 0.1 M perchloric acid by cyclic voltammetry and chronoamperometry. Hydrogen and (hydrogen) sulfate adsorption as well as surface oxidation were found to depend strongly on the crystallographic orientation and the nature of the electrolyte. The combination of charge displacement and voltammetric experiments allowed the determination of the potentials of zero total charge (E_pztc) of Pd(1 1 1) and Pd(1 0 0). The values of E_pztc in sulfuric acid were found to be more negative than in perchloric acid. The estimation of E_pztc for Pd(1 1 0) was hampered by the superposition with hydrogen absorption. The electro-oxidation of irreversible adsorbed carbon monoxide monolayers was studied on the three low-index Pd electrodes. The onset potential of the CO oxidation reaction follows the sequence Pd(1 0 0) < Pd(1 1 0) < Pd(1 1 1). Chronoamperometric experiments revealed a pronounced structure sensitivity of the reaction kinetics. The processes involved are determined by nucleation of oxygen-containing species on defect (step) sites and by slow diffusion of CO_ads on (1 1 1) terrace sites.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.     

Effect of crystal violet on the kinetics of H sorption into Pd

Hydrogen insertion into Pd foil and Pd thin deposit on gold electrodes was studied in 0.1 M H₂SO₄ solution. Two paths are possible for hydrogen insertion: (a) indirect, with hydrogen adsorption at the surface followed by absorption and (b) direct absorption. The studies of hydrogen absorption in Pd foil and thin deposits were carried out using classical transient and steady-state techniques and the electrochemical impedance spectroscopy. Presence of dissolved hydrogen in solution at positive potentials may lead to errors in the determination of the isotherms. The aim of this work is to understand how a poison, the crystal violet, can influence hydrogen insertion. In the presence of crystal violet, hydrogen adsorption is inhibited but absorption becomes more reversible. Analysis of the electrochemical data raises the possibility of the direct absorption mechanism.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

Dependence of electrocatalytic activity on film thickness for the hydrogen evolution reaction of Pd overlayers on Au(1 1 1)

The hydrogen evolution reaction has been studied for ultrathin Pd overlayers of various thickness on Au(1 1 1) in 0.1 M H₂SO₄. A clear correlation of the electrocatalytic activity as expressed by the exchange current density and the binding energy of adsorbed hydrogen has been found. While hydrogen is bound strongest on the second Pd monolayer (ML), the respective catalytic activity is poorest for all the surface structures under study. The exchange current density increases in the order 2 ML Pd < 1 ML Pd < bulk Pd (more than 2 ML). The electronic ligand effect, a geometric effect due to pseudomorphic growth and the surface defect density belong to the most crucial parameters in relations between structure of the electrode surface and its electrocatalytic activity. The experimental results are supported by an excellent agreement with theoretical predictions.     

Electrochemical reduction of nitrate on Pt(S)[n(1 1 1) × (1 1 1)] electrodes in perchloric acid solution

The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(1 1 1) and [n(1 1 1) × (1 1 1)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1 M HClO₄ + 10 mM KNO₃. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(1 1 1) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (1 1 1) step sites. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species does not adsorb on the (1 1 1) terraces but on the (1 1 1) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21 V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n = 2, abruptly increased with short terraces, i.e. n < 5, where the current wave of adsorbed hydrogen on the Pt stepped surface with comparatively narrow (1 1 1) terraces, denoted as H_nt, also appeared unmodified for n < 5 on voltammograms recorded in 0.1 M HClO₄ in the absence of nitrate.     

Formic acid electro-oxidation on carbon supported PdxPt1−x (0 ≥ x ≥ 1) nanoparticles synthesized via modified polyol method

Carbon supported nanoparticle catalysts of Pd_xPt_1−x (0 ≥ x ≥ 1) were synthesized using a modified polyol method and poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizer. Resulting nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperommetry (CA) study for formic acid electro-oxidation. Surface composition of the synthesized nanoparticles found from XPS revealed the Pt surface segregation even for the Pd-rich compositions. It is suggested that the surface segregation behavior in PdPt nanoparticles supported on carbon may be influenced, in addition to the difference in Pd and Pt surface energies, by particle size and particle interaction with the support. According to CA, the carbon supported Pd nanoparticles show the highest initial activity towards formic acid electro-oxidation at the potential of 0.3 V (RHE), due to the promotion of the direct dehydrogenation mechanism. However its stability is quite poor resulting in the fast deactivation of the Pd surface. Addition of Pt considerably improves the steady-state activity of Pd in 12 h CA experiment. CA measurements show that the most active catalyst is Pd_0.5Pt_0.5 of 4 nm size, which displays narrow size distribution and Pd to Pt surface atomic ratio of 27-73.     

Adsorption of phosphate species on poly-oriented Pt and Pt(1 1 1) electrodes over a wide range of pH

The adsorption of phosphate anions from phosphate solutions at poly-oriented and single-crystal platinum electrodes, primarily Pt(1 1 1), was studied over a wide range of pH by cyclic voltammetry. The features observed at the poly-oriented Pt electrode in phosphate solution may be related to the different crystalline facets, the (1 1 1) orientation presenting the most significant behavior in terms of phosphate adsorption. On the reversible hydrogen electrode (RHE) scale, the phosphate adsorption strength decreases with increasing alkalinity of the solution. Qualitatively, three different pH regions can be distinguished. At pH < 6 only a broad reversible peak is observed, corresponding to the adsorption of H₂PO₄⁻ and further deprotonation to adsorbed HPO₄⁻. For 6 < pH < 11 a butterfly feature followed by one or two anodic peaks (depending on scan rate) is observed, ascribed to the adsorption of HPO₄⁻ followed by its subsequent deprotonation to adsorbed PO₄³⁻. The splitting into two or three voltammetric features, and the irreversibility of the two features at more positive potential, is ascribed to the deprotonation reaction leading to a surface species (i.e. phosphate) which needs to change its surface coordination. At pH > 11 a reversible pre-wave and a sharp spike are observed, ascribed to the co-adsorption of phosphate and hydroxide.     

Halogen adsorption on crystallographic (1 1 1) planes of Pt, Pd, Cu and Au, and on Pd-monolayer catalyst surfaces: First-principles study

Halogen (Cl, Br and I) adsorption on crystallographic (1 1 1) planes of Pd, Pt, Cu, Au and on palladium monolayer catalysts surfaces was investigated by DFT calculations. Palladium monolayer catalyst here denotes either the Pd monolayer deposited over (1 1 1) crystallographic plane of Pt, Cu and Au monocrystals (Pd_ML/Me(1 1 1)), or the (1 1 1) crystallographic plane of Pd monocrystal with inserted one-atom thick surface underlayer of Pt, Cu and Au (Me_UND/Pd(1 1 1)). The adsorption on the 3-fold sites was found to be the strongest, and adsorption energies decreased if the size of the halogen atoms increased. For the case of Pd-monolayer catalysts it was demonstrated that energy of adsorption of halogen atoms could be correlated to the position of the d-band of surface atoms. Charge states of halogen adatoms and work function changes were evaluated. On the basis of calculated data and both experimental and theoretical data available in the literature, the changes in the catalytic activity toward oxygen reduction reaction of the Pd_ML/Pt(1 1 1) surface, caused by chloride adsorption, were discussed.     

Adsorption of camphor and 2,2′-bipyridine on Bi(1 1 1) electrode surface

Impedance spectroscopy and in situ STM methods have been used for investigation of the camphor and 2,2′-bipyridine (2,2′-BP) adsorption at the electrochemically polished Bi(1 1 1) electrode from weakly acidified Na₂SO₄ supporting electrolyte solution. The influence of electrode potential on the adsorption kinetics of camphor and 2,2′-BP on Bi(1 1 1) has been demonstrated. In the region of maximal adsorption, i.e. capacitance pit in the differential capacitance versus electrode potential curve, the heterogeneous adsorption and diffusion steps are the rate determining stages for camphor and 2,2′-BP adsorption at the Bi(1 1 1) electrode. It was found that for camphor | Bi(1 1 1) interface the stable adsorbate adlayer detectable by using the in situ STM method has been observed only at the positively charged electrode surface, where the weak co-adsorption of SO₄²⁻ anions and camphor molecules is possible. At the weakly negatively charged Bi(1 1 1) electrode surface there are only physically adsorbed camphor molecules forming the compact adsorption layer. The in situ STM data in a good agreement with impedance data indicate that a very well detectable 2,2′-BP adsorption layer is formed at Bi(1 1 1) electrode in the wide region of charge densities around the zero charge potential.     

Adenine adsorption on Au(1 1 1) and Au(1 0 0) electrodes: Characterisation, surface reconstruction effects and thermodynamic study

Adsorption of adenine on Au(1 1 1) and Au(1 0 0) electrodes is studied by cyclic voltammetry, impedance and chronoamperometric measurements in 0.1 M and 0.01 M KClO₄ and in 0.5 M NaF solutions. The experiments performed with flame-annealed electrodes at different contact potentials, scan potential limits and scan rates, suggest different adsorption behaviour on the unreconstructed and reconstructed surface domains. This is confirmed by comparing the results obtained with electrochemically annealed unreconstructed and with flame-annealed reconstructed surfaces. In both cases the initial electrode surface state is characterised by the E_pzc values. The adsorption on reconstructed surfaces takes place at more positive potentials than on the unreconstructed surfaces and induces the lifting of the reconstruction.The thermodynamic analysis is performed on the chronoamperometric data for adenine desorption on well characterised unreconstructed Au(1 1 1) surfaces. To this end a new methodology of the chronoamperometric experiments is introduced. Quantitative thermodynamic adsorption parameters such as surface tension, Gibbs surface excess, Gibbs energy of adsorption, potential versus Gibbs excess slope and electrosorption valency are determined. Weak chemisorption of adenine is inferred with a molecular orientation independent on the coverage and on the electrode potential. It is proposed that adsorbed adenine molecules adopt a tilted orientation at the surface to facilitate the coordination to the gold atoms.     

First principles mechanistic study of borohydride oxidation over the Pt(1 1 1) surface

The mechanism of borohydride oxidation and the competing hydrolysis reaction are examined over Pt(1 1 1) using density functional theory (DFT) methods. Adsorption of BH₄⁻ over Au(1 1 1) and Pt(1 1 1) is examined. Adsorption over Pt(1 1 1) is dissociative and extremely exothermic at potentials of interest, leading to a high surface coverage of H^* for which gaseous hydrogen evolution is competitive with oxidation. Elementary surface reactions oxidizing B-containing intermediates are favorable over Pt(1 1 1) at −0.85 V (SHE), consistent with experimental voltammetry results in the literature. The energetics of the initial adsorption step dictate the activity limitation of gold anodes and the selectivity limitation of platinum electrodes. This adsorption energy can be rapidly calculated with DFT methods, enabling screening of pure metals, alloys, poisons, and promoters to optimize borohydride oxidation catalyst design.     

Reactivity of monolayers and nano-islands of palladium on Au(1 1 1) with respect to proton reduction

Electrochemical reactivity regarding hydrogen reduction was studied at epitaxially grown Pd monolayers and sub-monolayers on Au(1 1 1) in 0.1 M HClO₄ solution. The rate of hydrogen evolution increases with decreasing numbers of layers, and it is considerably higher for sub-monolayers, i.e. the fewer Pd islands are on the surface the higher is the catalytic activity. No clear dependence of the reactivity on the ratio of Pd edge atoms to terrace atoms was found. Possible mechanisms explaining the experimental results are discussed.     

XPS study of irreversibly adsorbed arsenic on a Pt(1 1 1) electrode

The chemical composition of an irreversibly adsorbed layer of arsenic on Pt(1 1 1) in sulfuric acid solution has been studied by X-ray photoelectron spectroscopy (XPS). From the chemical shift of the As 3d level, a change in the valence state from As(0) to As(III) with positive-going electrode potential is deduced, the total amount of As on the surface remaining constant. The As coverage derived from XPS is around 0.33 ML, which is in agreement with the charge under the current peak in the cyclic voltammogram. From the coadsorption of anions, accompanying the valence transition of As, As(III) is assumed to exist as As(OH)₃ on the surface.     

Influence of temperature on hydrogen electrosorption into palladium-noble metal alloys. Part 1: Palladium-gold alloys

Hydrogen electrosorption into Pd-rich (>70 at.% Pd in the bulk) Pd-Au alloys obtained by electrodeposition was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. The influence of temperature (in the range between 283 and 313 K) on the amount of absorbed hydrogen, the potential of the α-β phase transition, absorption-desorption hysteresis and the potential of absorbed hydrogen oxidation was examined. It was found that for the temperature range studied the potentials of the α → β and β → α phase transitions are shifted negatively with increasing temperature and positively with decreasing Pd content in the alloy bulk. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of the β-phase formation and decomposition were determined from the temperature dependence of the potential of the α-β phase transition. The absolute values of enthalpy and entropy of the β-phase formation and decomposition increase with the decrease in Pd bulk concentration in Pd-Au alloys. The maximum hydrogen absorption capacity of Pd-Au alloys slightly decreases with increasing temperature. The potential of absorbed hydrogen oxidation peak is shifted negatively with increasing temperature and decreasing Pd bulk content.     

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid-liquid interface: Pt(3 1 0) = 3(1 0 0)-(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)-(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO₄. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)-(1 1 1) and Pt(3 1 1) = 2(1 0 0)-(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d₁₂ is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d₁₂ is expanded after CO adsorption.