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.     

The study of hydrogen electrosorption in layered nickel foam/palladium/carbon nanofibers composite electrodes

In the present work, the process of hydrogen electrosorption occurring in alkaline KOH solution on the nickel foam/palladium/carbon nanofibers (Ni/Pd/CNF) composite electrodes is examined. The layered Ni/Pd/CNF electrodes were prepared by a two-step method consisting of chemical deposition of a thin layer of palladium on the nickel foam support to form Ni/Pd electrode followed by coating the palladium layer with carbon nanofibers layer by means of the CVD method. The scanning electron microscope was used for studying the morphology of both the palladium and carbon layer. The process of hydrogen sorption/desorption into/from Ni/Pd as well as Ni/Pd/CNF electrode was examined using the cyclic voltammetry method. The amount of hydrogen stored in both types of composite electrodes was shown to increase on lowering the potential of hydrogen sorption. The mechanism of the anodic desorption of hydrogen changes depending on whether or not CNF layer is present on the Pd surface. The anodic peak corresponding to the removal of hydrogen from palladium is lower for Ni/Pd/CNF electrode as compared to that measured for Ni/Pd one due to a partial screening of the Pd surface area by CNF layer. The important feature of Ni/Pd/CNF electrode is anodic peak appearing on voltammetric curves at potential ca. 0.4 V more positive than the peak corresponding to hydrogen desorption from palladium. The obtained results showed that upon storing the hydrogen saturated Ni/Pd/CNF electrode at open circuit potential, diffusion of hydrogen from carbon to palladium phase occurs due to interaction between carbon fibers and Pd sites on the nickel foam support.     

Hydrogen absorption in palladium films sensed by changes in their resistivity

The resistance of pure palladium films of 11, 30 and 54 nm thickness was monitored during their exposure to H₂ at pressures ranging between 1 and 20 Torr. After completing a cycle of H₂ absorption/desorption, the resistivity changed in a ``saw tooth' fashion similar to the changes observed in surface adsorption of hydrogen by other transition metals. Large resistance changes are observed in these studies, confirming that bulk hydrogen absorption occurs in Pd while hydrogen surface adsorption becomes dominant over bulk absorption in other metals such as niobium. The resistance change curves carry the information of the kinetics of absorption. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Mechanism of hydrogen adsorption/absorption at thin Pd layers on Au(1 1 1)

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.     

Palladium-hydrogen interaction on supported palladium catalysts of different metal dispersions

Palladium-hydrogen interaction on supported Pd catalysts of different metal dispersions has been studied by hydrogen chemisorption and back-sorption, and by temperature-programmed desorption (TPD) of either deuterium or hydrogen from the Pd surface after different initial gas dosages. Pd interacts with three hydrogen species whose amounts vary with the metal dispersion and with the dosage pressure. The amount of hydrogen strongly adsorbed on the Pd surface increases with the metal dispersion but is unaffected by the hydrogen pressure. The amount of hydrogen absorbed in the bulk of Pd is significant with a poorly dispersed catalyst, particularly when the hydrogen pressure is higher than about 10 Torr, but decreases with the metal dispersion. Hydrogen weakly bound with Pd shows two characteristic TPD peaks, whose intensity depends on the metal dispersion and the initial gas dosage. The peak at 270-330 K, assigned to the recombination of absorbed hydrogen with surface hydrogen, is large with a poorly dispersed catalyst and grows with the initial hydrogen dosage. The intensity of the peak at 215-226 K is not affected by the metal dispersion or by the hydrogen dosage. It has been concluded that an analysis of the intrinsic properties of Pd catalysts should be based on an understanding of the hydrogen species associated with the catalysts.     

Hydrogen spillover phenomena on Pt/ZrO2

A Pt/ZrO₂ catalyst has been investigated by temperature-programmed reduction and temperature-programmed desorption of hydrogen. Hydrogen spills over from Pt onto the ZrO₂ surface at about 550°C. One part of spillover hydrogen is consumed by a partial reduction of zirconia. The other part is adsorbed on the surface and is desorbed at about 650°C. This desorption is a reversible one, i.e. it can be followed by a renewed uptake of spillover hydrogen. No connection between dehydroxylable OH groups and spillover hydrogen adsorption has been observed. The adsorption sites for the reversibly bound spillover hydrogen were possibly formed during the reducing hydrogen treatment.     

Effect of palladium loaded activated carbons on hydrogen storage

Pd-loaded high surface area activated carbon (BAC-Pd) was produced from bamboo by carbonization and activation using potassium hydroxide with subsequent loading of palladium. The palladium loaded onto BACs appears to exist more in micropores. The Pd compounds exist mainly as amorphous PdCl₂ with Pd (0) structures in the whole surface of BAC-Pd. The hydrogen adsorption capability of BAC-Pd at 0.1?MPa and room temperature was evaluated for hydrogen storage. The amount of adsorbed hydrogen on BAC-Pd was the maximum among three Pd-loaded activated carbons because the physical properties and the Pd content are the highest among them. According to the effect of Pd content, the amounts of adsorbed hydrogen on BAC-Pd increased linearly as the Pd content increased, and the BET surface area of BAC-Pd decreased. BAC, which had the highest Pd contents in micropores, exhibited excellent adsorption ability for hydrogen at 298?K and 0.1?MPa. The amounts of chemisorbed hydrogen of BAC-Pd increased along with the increase in Pd content. Furthermore, the amount of adsorbed hydrogen of BAC-Pd is expected to increase in conditions with pressure higher than 0.1?MPa.     

Effect of metal‐support interface on hydrogen permeation through palladium membranes

Thin palladium membranes of different thicknesses were prepared on sol‐gel derived mesoporous γ‐alumina/α‐alumina and yttria‐stabilized zirconia/α‐alumina supports by a method combining sputter deposition and electroless plating. The effect of metal‐support interface on hydrogen transport permeation properties was investigated by comparing hydrogen permeation data for these membranes measured under different conditions. Hydrogen permeation fluxes for the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes are significantly smaller than those for the Pd/YSZ/α‐Al₂O₃ membranes under similar conditions. As the palladium membrane thickness increases, the difference in permeation fluxes between these two groups of membranes decreases and the pressure exponent for permeation flux approaches 0.5 from 1. Analysis of the permeation data with a permeation model shows that both groups of membranes have similar hydrogen permeability for bulk diffusion, but the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes exhibit a much lower surface reaction rate constant with higher activation energy, due possibly to the formation of Pd‐Al alloy, than the Pd/YSZ/α‐Al₂O₃ membranes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Study of hydrogen surface mobility and hydrogenation reactions over alumina-supported palladium catalysts

Alumina-supported Pd catalysts with different particle surface densities have been prepared using incipient wetness impregnation of aqueous solution of a palladium nitrite complex. Buta-1,3-diene and orthoxylene hydrogenation reactions were performed both in a batch and a fixed bed reactor. Hydrogen surface mobility was studied using H₂–D₂ isotopic exchange. The influence of (i) the particle surface density and (ii) the surface area of the support on the catalytic properties are discussed. The turnover frequency (TOF) of the but-1,3-diene hydrogenation was highly sensitive to the surface density of Pd particles (D_sp). Moreover, for a given surface density, TOF also depend on the nature of the alumina support. For a given support, modifications of the electronic properties of palladium can explain the increase of the reaction rate with D_sp while changes in the kinetics of hydrogen surface diffusion are proposed to explain the support effect.     

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.     

Synthesis of palladium nanoparticles on carbon nanotubes and graphene for the chemoselective hydrogenation of para-chloronitrobenzene

We have studied the synthesis of palladium nanoparticles over carbon nanotubes (Pd/CNT) and graphene (Pd/G) and we have tested their catalytic performance in the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The catalysts were characterized by N₂ adsorption/desorption isotherms, TEM, X-ray diffraction, infrared and X-ray photoelectron spectroscopy and ICP-OES. The palladium particle size on Pd/G (3.4 nm) and Pd/CNT (2.8 nm) was similar though the deposition was higher on Pd/G. Pd/CNT was more active which can be ascribed to the different surface area and electronic properties of the Pd nanoparticles over CNT, while the selectivity was 100% to the corresponding haloaniline over both catalysts and they were quite stable upon recycling.     

Investigation of the hydrogen evolution reaction at a 10 wt% palladium-dispersed carbon electrode using electrochemical impedance spectroscopy

The hydrogen evolution reaction (h.e.r.) at a 10 wt % palladium-dispersed carbon (Pd/C) electrode in 0.1 m NaOH solution has been investigated with reference to that on carbon (Vulcan XC-72) and palladium foil electrodes by analysing the a.c.-impedance spectra combined with cyclic voltammograms. From the coincidence of the maximum charge transfer resistances and the minimum hydrogen evolution resistances for the h.e.r. at the respective electrode potential for the Pd/C, carbon and Pd foil electrodes, it is suggested that the h.e.r. at the Pd/C electrode takes place along with the absorption and diffusion of hydrogen above –1.10 V vs SCE, whereas the former dominates over the latter below –1.10V vs SCE. In the case of the Pd foil electrode the transition of absorption and diffusion to evolution occurs at –0.96V vs SCE. In contrast to the Pd/C and Pd foil electrodes the h.e.r. occurs strongly at the carbon electrode below –1.20V vs SCE. The hydrogen evolution overpotential on the Pd/C electrode is decreased by 0.10 V in comparison to the carbon electrode due to the larger electrochemical active area of the finely dispersed Pd particles.     

Surface restructuring of palladium particles induced by CO adsorption

In order to characterize chemisorption induced reconstruction the surface of supported palladium particles (model catalyst) has been investigated during CO adsorption by SSIMS (static secondary ion mass spectrometry). The SSIMS signal ratio _n Pd _n CO⁺/Pd _n ⁺ has been used to monitor the CO adsorption kinetics. The relative occupancy of linear and bridged CO sites has been determined by the value of PdCO⁺/( _n Pd _n CO⁺) and the variation of the Pd-Pd next neighbour distance by plotting Pd ₂ ⁺ /Pd⁺ during CO exposure. It has been shown that CO chemisorption induces a surface restructuring by increasing the Pd-Pd distance in the particle surface. This phenomenon has the features of a cooperative phenomenon. On the reconstructed particle one bonding state has been identified which appeared to be a precursor of the CO dissociation.     

Hydrogen absorption by palladium electrode polarized in sulfuric acid solution containing surface active substances—II. The anodic region

The hydrogen pressure equivalent to hydrogen overpotential on Pd electrode was determined under anodic polarization by combining the data of the electric resistance of a Pd foil electrode observed at various overpotentials with the electric resistance vs hydrogen pressure relationship which was obtained in a separate, gas phase experiment. The equivalent hydrogen pressure under anodic polarization was less diminished as compared with the value calculated by the Nernst equation from the total overpotential. Results are consistent with the conclusion reported previously for the cathodic region (Part I) that the Tafel—Volmer route is followed on the Pd hydrogen electrode with comparable value of the constituent step rates. Effect of addition to the solution of tetrabutylammonium hydroxide was also examined. Hydrogen content in Pd at the equilibrium hydrogen electrode potential under 1 atm hydrogen pressure was evaluated electrochemically to be H/Pd = 0.691. Hydrogen content in Pd under anodic polarization was evaluated.     

Reaction pathway studies of C6 hydrocarbons on palladium model catalysts: effects of hydrogen/hydrocarbon ratio, molecular structure, surface structure and temperature

Reaction pathways of C₆ hydrocarbons, such as hydrogenolysis, dehydrogenation, aromatization and dehydrocyclization, have been studied on small surface area palladium model catalysts, including Pd(111) single crystal and polycrystalline palladium foil. These reactions were performed in a batch reactor, at atmospheric pressure and at two different temperatures (573 and 673 K). This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies of carbon monoxide and hydrogen adsorption on nickel and cobalt foils aimed at gaining a better insight into the mechanism of hydrocarbon formation

Systematic studies of adsorption of hydrogen and carbon monoxide on polycrystalline surfaces of nickel and cobalt have been carried out. The aim of these studies was to gain a better insight into the catalyzed formation of hydrocarbons from H₂-CO mixtures. We have studied the adsorption of these gases by means of thermal desorption spectroscopy (TDS) on nickel foils and powders. More recently, we have obtained desorption spectra of hydrogen adsorbed on cobalt foils and powders. In this work we described desorption spectra of carbon monoxide on cobalt foils. Carbon monoxide desorption from cobalt foils was studied in a similar way as prior work, using a mass spectrometric method in an ultra high vacuum system. Two carbon monoxide desorption peaks were observed. These two states correspond to molecular CO and presumably dissociated CO, as it is observed in the case of stepped surfaces of Ni and Co single crystals. An activation energy of around 4.0 kcal/mol is obtained for the molecular state while for the dissociated state the energy is coverage-dependent with a value between 8.0 and 15.0 kcal/mol. The carbon monoxide desorption peaks were fitted with near Gaussian curves which facilitates the analysis of the data to obtain activation energies for desorption. Kinetic parameters for carbon monoxide and hydrogen desorption from nickel and cobalt foils are provided and compared with already published data involving single crystals.     

Influence of adsorbed carbon dioxide on hydrogen electrosorption in palladium-platinum-rhodium alloys

Carbon dioxide electroreduction was applied to examine the processes of hydrogen electrosorption (adsorption, absorption and desorption) by thin electrodeposits of Pd-Pt-Rh alloys under conditions of cyclic voltammetric (CV) experiments. Due to different adsorption characteristics towards the adsorption product of the electroreduction of CO₂ (reduced CO₂) exhibited by the alloy components hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. Reduced CO₂ causes partial blocking of hydrogen adsorbed on surface Pt and Rh atoms, without any significant effect on hydrogen absorption into alloy. It reflects the fact that adsorbed hydrogen bonded to Pd atoms does not participate in CO₂ reduction, while hydrogen adsorbed on Pt and Rh surface sites is inactive in the absorption reaction. In contrast, CO is adsorbed on all alloy components and causes a marked inhibition of hydrogen sorption (both adsorption and absorption)/desorption reactions.     

A kinetic study of methanol decomposition catalyzed over plate-type palladium catalyst

A kinetic study of the catalytic methanol decomposition to carbon monoxide and hydrogen has been carried out in the pressure range of methanol up to 8 atm at 200 and 250°C over a palladium catalyst supported on an oxidized aluminum plate. The reaction pathway can be proposed as (i) dissociative adsorption of methanol to methoxyl groups and hydrogen adsorbed on palladium sites, (ii) decomposition of the methoxyl groups to carbon monoxide and hydrogen adsorbed, and (iii) desorption of the surface carbon monoxide and hydrogen species. It is suggested that the second step is rate-determining and the surface hydrogen species enhance the decomposition of the methoxyl groups. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of palladium on sulfur resistance in Pt–Pd bimetallic catalysts

The interactions of H₂ and H₂S molecules with Pt–Pd bimetallic catalysts were investigated at the molecular level using a DFT (density functional theory) approach to better understand the structures and properties of active sites, and the relations between structural changes and sulfur resistance. It was found that when alloying the Pt catalyst with a small amount of Pd at a particular surface atomic ratio range, both H₂ and H₂S showed different adsorption properties compared to those on monometallic Pt or Pd catalyst. The adsorptions of both H₂ and H₂S were enhanced, but the adsorption energy of H₂ increased more than that of H₂S, indicating that the adsorption of H₂S became less favorable compared with H₂ on the bimetallic Pt–Pd catalyst surface. The desorption energy of hydrogen from monometallic Pt or Pd, as well as bimetallic Pt–Pd supported on zeolite, were calculated by temperature-programmed desorption (TPD), the values were compared against the DFT results to explain experimentally and theoretically why the bimetallic Pt–Pd catalyst has better sulfur resistance than monometallic Pt catalyst.