Palladium electrodeposits: Dependence of structure and sorption properties on the deposition potential

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 H₂SO₄. 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 thea and β hydrides is anomalously high for the deposits whose growth was accompanied by deep hydrogenation of Pd. At a given effective pressure, for the α-phase, this value is always substantially higher as compared with less defective materials. In the β-phase, the hydrogen concentration can be either lower or higher     

Electrolytic deposition of a cobalt-lead alloy with magnetic properties

Electrolyte for obtaining a cobalt-lead alloy was suggested. The electrochemical parameters at which cobalt and lead are simultaneously deposited from the pyrophosphate electrolyte were determined. The magnetic parameters of the alloy were measured.     

Electrochemical behavior of CO2 reduction on palladium nanoparticles: Dependence of adsorbed CO on electrode potential

The electrochemical reduction of CO₂ is strongly influenced by both the applied potential and the surface adsorption status of the catalyst. In this work a gas diffusion electrode (GDE) coated with Pd nanoparticles/carbon black (Pd/XC72) was used to study the electrochemical reduction of CO₂. Cyclic voltammetric (CV) analysis of Pd/XC72 between 1.5 V and − 0.6 V (vs. RHE) shows the formation of intermediates and the blocking of hydrogen absorption on the Pd nanoparticles (NPs) under a CO₂ atmosphere. The relationships between the Faradaic efficiency/current density and the applied potential reveal that the onset potential of CO formation is around − 0.4 V. Moreover, the presence of adsorbed CO was confirmed through CV analysis of Pd/XC72 under CO₂ and CO/He atmospheres. This demonstrates that H atoms and CO intermediates co-adsorb on the surface of the Pd NPs at an applied potential of around − 0.4 V. When the applied potential is more negative than − 0.6 V, adsorption of CO intermediates on the surface of the Pd NPs becomes dominant.     

Effect of quartz substrate on adsorption and texture properties of titanium-containing silica membranes

Sol-gel method with joint hydrolysis of silicon and titanium alkoxides were used to synthesize titanium-containing silica membranes on a macroporous quartz substrate. Isotherms of low-temperature adsorption-desorption of nitrogen by the samples obtained were measured. Characteristic features inherent in mesoporous samples were found in the isotherms. A change in the adsorption properties of the membranes was observed upon a preliminary treatment of the substrate with a mixture of hydrogen peroxide and concentrated sulfuric acid.     

Polyelectrolyte adsorption layers studied by streaming potential and particle deposition

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface zeta as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential zeta/zeta0 on the PAH coverage Theta(PAH), calculated using the convective diffusion theory. It was found that for the ionic strength of 10(-2) M, the dependence of zeta/zeta0 on Theta(PAH) can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ThetaPAH > 0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.     

Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ～32% (100), ～18% (111), ～34% (311), and ～15% (331)) at P _O2 ≤ 1.3 × 10^?5 Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O₂ adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O₂ adsorption rate is constant at ? ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an O_ads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ? ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (～800 K) O₂ desorption peak is observed for ? ≤ 0.25, which is due to O₂ desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E _des = 230 kJ/mol. A lower temperature (～700 K) O₂ desorption peak is observed for ? ≥ 0.25, which is due to O₂ released by the c(2 × 2) structure according to a first-order rate law with E _des = 150 kJ/mol. At ? ≥ 0.25, there are repulsive interactions between O_ads atoms on Pd(poly) (ε_aa = 5–10 kJ/mol).     

Particle adsorption and deposition: role of electrostatic interactions

This work demonstrates how electrostatic interactions, described in terms of the classical DLVO theory, influence colloid particle deposition phenomena at solid/liquid interfaces. Electrostatic interactions governing particle adsorption in both non-polar and polar media (screened interactions) are discussed. Exact and approximate methods for calculating the interaction energy of spherical and non-spherical (anisotropic) particles are presented, including the Derjaguin method. Phenomenological transport equations governing particle deposition under the linear regime are discussed with the limiting analytical expressions for calculating initial flux. Non-linear adsorption regimes appearing for higher coverage of adsorbed particles are analysed. Various theoretical approaches are exposed, aimed at calculating blocking effects appearing due to the presence of adsorbed particles. The significant role of coupling between bulk transport and surface blocking is demonstrated. Experimental data obtained under well-defined transport conditions, such as diffusion and forced convection (impinging-jet cells), are reviewed. Various experimental techniques for detecting particles at interfaces are discussed, such as reflectometry, ellipsometry, streaming potential, atomic force microscopy, electron and optical microscopy, etc. The influence of ionic strength and flow rate on the initial particle deposition rate (limiting flux) is presented. The essential role of electrostatic interactions in particle deposition on heterogeneous surfaces is demonstrated. Experimental data pertinent to the high-coverage adsorption regime are also presented, especially the dependence of the maximum coverage of particles and proteins on the ionic strength. The influence of lateral electrostatic interactions on the structure of particle monolayers is elucidated, and the links between colloid and molecular systems are pointed out.     

Catalytic activity of electrolytic palladium deposits on porous nickel substrates

Catalytic activity of palladium layers in reduction of atmospheric oxygen and oxidation of methanol and ethanol in an alkaline medium was studied for electrolytic palladium deposits on porous nickel substrates by cyclic voltammetry on a rotating disk electrode.     

Effect of post‐deposition annealing on surface morphology and gas sensing properties of palladium phthalocyanine thin films

The effect of post‐deposition annealing on surface morphology and gas sensing properties of palladium phthalocyanine (PdPc) nanostructured thin films has been studied. PdPc thin films were deposited on polyborosilicate substrate by thermal evaporation technique at room temperature. The surface morphology of thin films was investigated by SEM, X‐ray diffraction, and optical absorption. X‐ray diffraction patterns showed a phase transition from α to β based on post‐deposition annealing at temperatures above 200 °C. The SEM and optical absorption confirmed that annealing strongly influenced the surface morphology of nanostructured thin films. Sandwich devices (Au|PdPc|Al) were fabricated and exposed to different concentrations of NO₂ and NH₃ as oxidizing and reducing gases at different temperatures, and the sensitivity of devices were obtained versus gases. Obtained results showed α‐PdPc thin film devices had higher sensitivity in comparison with devices in β‐phase. In particular, it was found that the sensitivity of devices is temperature dependent and the best operating temperature range of devices was measured at about 90–100 °C. Devices showed good reversibility, response, and recovery time at room temperature. Finally, the stability of sensors was investigated for a period of about 1 year; results showed that the sensors were stable for 2 months and lost about 30% of their sensitivity after 1 year. Copyright © 2012 John Wiley & Sons, Ltd.     

Alloy formation at the deposition of palladium on gold at room temperature

Palladium is shown to be deposited underpotentially on gold electrodes in 1 M H₂OSO₄. The deposit forms an alloy according to a t^1/2 law. The rate of alloy formation is strongly dependent on the potential. The diffusion coefficient is about 10^?16 cm² s^?1. This value is discussed in comparison with earlier measurements on gold-palladium alloys at higher temperatures.     

Dependence of the heats of adsorption on the chemical potentials of adsorbent and adsorbate

The heats of adsorption of gases on metals show a rather complex dependence on the chemical potentials of the solid and gas participating in the process.

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Dependence of the adsorption and catalytic properties of a copper-platinum catalyst on the structure of metal particles and the composition of the catalyst surface

The kinetics of H₂ desorption from the surface of a copper-platinum catalyst deposited on silica gel ([1 wt % Pt + 0.15 wt % Cu]/SiO₂) and the kinetics of C₆H₁₂ dehydrogenation were studied. The effects of copper introduction in a platinum catalyst on the structural characteristics of platinum particles, the composition of their surface, and the effects of plasmochemical treatments on these parameters were studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The metal-H atom bond energies (E _Pt-H) and the catalytic activity were found to increase in the presence of Cu. This was explained by the formation of new hydrogen adsorption centers (due to the Cu^+δ adatoms) and catalytic centers composed of Cu^+δ adatoms and carbon atoms. The mean diameter of Pt particles (D) increased twofold. The microstresses (ɛ) in the particles increased after the catalyst was treated with glow discharge plasma in Ar and O₂ and with high-frequency plasma in H₂ (HF-H₂). The observed changes in the bond energy E _Pt-H and kinetic parameters were explained by the increase in microstresses in Pt particles.     

Investigations on the adsorption and electrocatalytic oxidation of carbon monoxide on gold + palladium alloys

A bifunctional mechanism of electrocatalysis was found for the CO-oxidation at Au + Pd alloys. The synergetic effect works best at the 20% Au alloy. The different behaviour of An + Pd and Ag + Pd alloys was assigned to the formation of CO-induced ensembles of Pd-atoms at Au + Pd alloys with high gold content, which is possible due to a high mobility of Pd-atoms at the surface. As CO could be shown to react preferably with the Au-oxide species, the course of the energy of activation for the gas phase reaction might be understood on the-basis of geometric effects.     

Surface properties of platinum and palladium

Surface energies, defect formation heats and activation energies of atom self-diffusion on Pt and Pd (111), (100) and (110) planes have been estimated by the interacting bond method.

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Hydrogen/deuterium adsorption and absorption properties on and in palladium using a combined plane wave and localized basis set method

Detailed information on the H/D isotope effects for adsorption on the surface and absorption in the bulk is important for understanding the nuclear quantum effect. To achieve this, we developed a new theoretical approach, namely, the combined plane wave and localized basis set (CPLB) method. By using the multicomponent quantum chemical method, which takes into account the quantum effect of a proton or deuteron, with the localized part of the CPLB method, direct analysis of the H/D isotope effect about adsorption and absorption is carried out. In this study, we performed a theoretical investigation of the H/D isotope effects for adsorption on a Pd(111) surface and absorption in bulk Pd. We clearly showed an H/D isotope effect on geometry during adsorption and absorption. Our developed CPLB approach is a powerful tool for analyzing the quantum nature of H/D in surface, bulk, and inhomogeneous systems.     

新型Pd系聚吡咯配合物的合成、结构和吸附行为的研究

本文首次报道了一种新的二氯-二聚吡咯合钯(简称DCBPP)配合物的合成、结构和吸附H~2行为的实验和理论研究。确定了DCBPP的稳定性结构,探究了吸附过程的表面行为和微观机制,理论计算和实验测定的结果符合较好,圆满合理地说明一些实验现象。     

Hybrid polymer-immobilized palladium nanoparticles: Preparation and catalytic properties

A new approach to the synthesis of mixed-type immobilized catalysts was developed: the frontal polymerization of a metal-containing monomer in the presence of a highly dispersed mineral support. The synthesis of an acrylamide complex of Pd(II) nitrate on the surface of SiO₂, Al₂O₃, or C and its subsequent polymerization and reduction resulted in the formation of an organic-inorganic composite that included nanosized Pd particles stabilized by a polymer matrix and an inorganic support. The resulting hybrid nanocomposites are efficient and selective catalysts for the hydrogenation reactions of cyclohexene and alkene and acetylene alcohols.     

Dependence of single-walled carbon nanotube adsorption kinetics on temperature and binding energy

We present results for the isothermal adsorption kinetics of methane, hydrogen, and tetrafluoromethane on closed-ended single-walled carbon nanotubes. In these experiments, we monitor the pressure decrease as a function of time as equilibrium is approached, after a dose of gas is added to the cell containing the nanotubes. The measurements were performed at different fractional coverages limited to the first layer. The results indicate that, for a given coverage and temperature, the equilibration time is an increasing function of E/(k(B)T), where E is the binding energy of the adsorbate and k(B)T is the thermal energy. These findings are consistent with recent theoretical predictions and computer simulations results that we use to interpret the experimental measurements.