Effect of the chemical nature and concentration of palladium (II) precursors on the palladium deposition by atomic hydrogen electrochemically assisted

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Effect of the chemical nature and concentration of palladium (II) precursors on the palladium deposition by atomic hydrogen electrochemically assisted Luis F. D0 Elia Camacho*, J. Moncada*, J. Caldero´n, Z. Puentes, K. Saavedra Petro´leos de Venezuela (PDVSA)-Intevep, Gerencia General de Refinacio´n e Industrializacio´n, Gerencia Departamental de Investigacio´n Estrate´gica, Apartado 76343, Caracas 1070-A, Bolivarian Republic of Venezuela

article info

abstract

Article history:

Pd deposits are obtained by applying 1.92 mA cm2 during 180 min, employing 0.06 M

Received 8 November 2010

PdCl2 þ 1 M HCl and 0.06 M PdCl2 þ 28% NH3 solutions. Some features on Pd depositions using

Received in revised form

x M PdCl2 þ 1 M HCl (x ¼ 0.130, 0.060, 0.030, 0.025, 0.020 and 0.010) solutions are detailed. Based

16 December 2010

on roughness factor (S ) and deposition efficiency (3Pd) values, Pd deposition is more efficient

Accepted 25 December 2010

using PdCl2 þ 1 M HCl than PdCl2 þ 28% NH3 (3Pd ¼ 99%, S ¼ 6 in 1 M HCl vs. 3Pd ¼ 17%, S ¼ 3 in

Available online 31 January 2011

28% NH3). A highly rough Pd deposit (S ¼ 357) is obtained, by applying 1.92 mA cm2 during 180 min, using a 0.020 M PdCl2 þ 1 M HCl solution.

Keywords:

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Atomic hydrogen

reserved.

Hydrogen permeation Palladium deposition Palladium black Chemical reduction

1.

Introduction

Palladium is usually deposited by different procedures; such as, chemical and electrochemical methodologies [1]. Depending on the preparation method, deposits with particular physicochemical and textural properties are obtained. These facts have important implications in hydrogen storage and sensing, catalyst and fuel cell applications [2e10]. Chemical Pd depositions involve, among others steps, reduction of metallic precursors by dissolved reducing agents (i.e. hydrazine) or

reducing agents adsorbed on specific surfaces [1]. An example of the second case is palladium black depositions using adsorbed atomic hydrogen on bare Pd (PdeH) or active hydrogen passing through a Pd membrane [11e16]. The reaction is performed in a reactor comprising two compartments separated by a Pd membrane: (i) generation of atomic hydrogen, adsorbed on bare Pd (PdeH) in the electrochemical compartment and (ii) Pd deposition in the chemical compartment. Atomic hydrogen, generated in the electrochemical compartment, diffuses or permeates across the membrane,

Abbreviations: S, roughness factor; 3Pd, Pd deposition efficiency; mPdexp, experimental Pd amount; mPdtheo, theoretical Pd amount; PdeH, adsorbed atomic hydrogen on Pd; Pdacid/Pd, palladium deposited in acid medium; Pdalk/Pd, palladium deposited in alkaline medium; PdAeH, adsorbed atomic hydrogen in compartment A; PdBeH, adsorbed atomic hydrogen in compartment B; a, oxidation state; Aeff, effective palladium surface area; Ageo, geometric surface area; jC, cathodic current densities, mA cm2; jp, permeation current densities, mA cm2; r, permeation yield; Eal, potential at the inflection point, mV. * Corresponding authors. Tel.: þ58 212 3307134; fax: þ58 212 3307230. E-mail addresses: [email protected], [email protected] (L.F. D0 Elia Camacho), [email protected] (J. Moncada). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.119

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thus Pd precursors are chemically transformed by permeated atomic hydrogen. In general, the following steps summarise the process: a. Electrochemical compartment: generation of adsorbed atomic hydrogen from water in alkaline electrolyte H2 O þ PdA þ e /PdA  H, þ OH

(1)

b. Diffusion or permeation of generated atomic hydrogen from electrochemical compartment to chemical compartment (across the Pd bulk membrane) Permeation

PdA  H, ƒƒƒƒƒ!PdB  H,

(2)

c. Chemical compartment: Pd chemical deposition by PdeH 2PdB  H, þ PdðXÞ4aþ2 /Pd=PdB þ 4Xa þ 2Hþ 4



(3)

where a is the oxidation state of X (X ¼ Cl, NH3 and others).The process is continuous and limited by reaction (1). If hydrogen is not generated, in the electrochemical compartment, Pd depositions do not occur. Based on the facts stated above, palladium black has been deposited on bare Pd using 0.028 M PdCl2 þ 1 M HCl by applying 10 and 50 mA cm2 [11e16]. These Pd surfaces are very active for hydrogenation reactions of organic compounds; in this case, PdeH also plays a determinant role [11e16]. Similarly, novel applications for fuel and oil treatment have been reported by PDVSA Intevep (R&D oil institute of the Bolivarian Republic of Venezuela) [17e19]. Hydroconversion efficiencies, in these novel systems, are influenced by the following aspects: cathodic current density, temperature, solvent nature, effective palladium surface area (Aeff) or roughness factor (S ¼ Aeff/Ageo, Ageo is the geometric surface area) and others. The effect of S is the most notable aspect to be considered; for instance, the reaction rate for 4-ethyltoluene production, from 4-methylstyrene, increases 4 times with increasing S in a factor of 25 [16]. The aim of this paper is to address the influence of chemical nature and concentration of Pd precursors (PdCl2 in alkaline and acidic solutions) on morphology and roughness factor of palladium deposits obtained by reactions performed using adsorbed atomic hydrogen permeated trough palladium and (PdeH ). Therefore, optima conditions for preparing highly rough palladium deposits (palladium black) are proposed. These Pd deposits are usable, as active membranes, for fuel and oil assisted electrochemical reactions using atomic hydrogen permeated through palladium membranes [20,21]. 

2.

Experimental

Before experiments, Pd membrane surfaces (99.99%, thickness 125 mm, GoodFellow, Ageo ¼ 3.14 cm2) were polished with silicon carbide paper (1500, 3 M), etched in aqua regia for 30 s and finally washed with deionised water (18 MU). All experiments were carried out at 25  C. Before each experiment, solutions were deoxygenated by bubbling argon. All potentials

are referred to Standard Hydrogen Electrode (SHE) and applied currents were normalised by Ageo.

2.1.

Hydrogen permeation measurements

The two compartment cell used for electrochemical measurements was similar to that described by Devanathan and Stachurski [22]. 0.1 M NaOH (98.6%, Sigma) was employed as electrolytic medium in both compartments. Hydrogen was produced on the generation side of the palladium membrane by applying cathodic current densities ( jC) between 30.86 and 0.32 mA cm2 during 40 min. During this period of time (hydrogen charging), the currents produced by hydrogen oxidation in the consumption compartment, because of hydrogen permeation across the Pd membrane, were monitored by applying 444 mV. Thereafter, jC was set to zero while recording the currents produced (hydrogen discharging) by oxidation of the hydrogen stored in the Pd membrane by applying 444 mV. A detailed description on fundamental aspects of permeation measurements can be found elsewhere [22,23]. Before each permeation measurement, residual hydrogen was potentiostatically removed by applying 444 mV. The current density at the entry side was applied using a Power source/Multimeter Keithley 2400, and the hydrogen oxidation current (at the output side) was monitored employing a Potentiostat/Galvanostat Gamry PCI4-750. When Pd modified membranes were evaluated, the Pd modified surface is used as the hydrogen consumption side.

2.2. Pd deposition using PdCl2 in 1 M HCl and 28% NH3 solutions PdCl2 (99.999%, SigmaeAldrich) solutions, at different concentrations, in 1 M HCl (37%, Riedel-de Hae¨n) and 28% NH3 (28%, Riedel-de Hae¨n) were used for depositions; since they are the most common media used in Pd deposits preparations [24]. In order to know the chemical nature of palladium precursors, UVeVisible spectra of PdCl2 solutions were recorded. Pd depositions were carried out using the reactor previously described in the Introduction Section. The applied current density in the electrochemical compartment, which contains 0.1 M NaOH, was 1.92 mA cm2 during 180 min. Pd deposits were obtained in the chemical compartment; in other words, on the Pd surface of the membrane exposed to PdCl2 solutions. Deposits were characterised evaluating: (i) morphology by means of Scanning Electron Microscopy (SEM, JEOL JSM-6490); (ii) hydrogen permeation by applying 1.92 mA cm2 on the generation side, following the procedure described before and (iii) effective surface area by cyclic voltammetry. Voltammetric responses of Pd in 0.5 M H2SO4 (95e97%, Fluka) were recorded between 444 and 1268 mV at 25 mV s1 using a Potentiostat/Galvanostat Gamry PCI4-750. Experiments were performed in a cell of one compartment, employing Pd as working electrode and a platinum gauzed (99.999%, GoodFellow) as counter electrode. Based on the charge for PdO monolayer formation (424 mC cm2) [25,26] and the measured charge for PdO reduction, Pd effective surface area was calculated.

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Fig. 1 e Galvanostatic permeation and discharging curves applying different jC (during 40 min) at 25  C. Pd membrane (thickness 125 mm) in 0.1 M NaOH.

3.

Results and discussion

3.1.

Hydrogen permeation measurements

In order to set up the optimum jC value for Pd depositions, permeation studies were initially carried out for a Pd membrane (bare Pd). Fig. 1 shows galvanostatic permeation and discharging curves for different jC, and their steady-state permeation current densities values ( jp). It may be noted that a permeation yield (r ¼ jp/jC) of 0.96 is achieved (maximum value, r ¼ 1) for 1.92 mA cm2. It is important to know that, at the generation side, hydrogen absorption/diffusion and chemical/electrochemical desorption of hydrogen atoms (hydrogen evolution reaction) compete with each other. Beyond 1.92 mA cm2, hydrogen evolution reaction may become predominant and the permeation yield decreases. The applied current density (at the generation side) generates atomic hydrogen, which is partially inserted (absorbed) in the membrane and partially evolved as H2. This behaviour agrees with existing reports, regarding permeation measurements of Pd membranes under similar conditions [23,27]. r is not reported for jC ¼ 30.86 mA cm2, since it was not possible to measure the steady-state permeation current ( jp measurements are unreliable due instability produced by the vertiginous hydrogen evolution). Ideally, under specific border conditions, discharging curves should fit exponential decaying function [28]; nevertheless, this is not the case for curves obtained beyond 1.92 mA cm2. At high jC values (>1.92 mA cm2), the generated hydrogen amount is very high, and b-PdH phase formation is promoted instead of a-PdH phase [27]. It is well known that b-PdH formation does not involve reversible hydrogen absorption, thus modifications of the Pd bulk microstructure are expected [27]. In fact, it was observed that the Pd membrane was seriously deteriorated after tests carried out by applying current densities beyond 1.92 mA cm2. Pd membrane deterioration, due to bPdH (irreversible phase) formation, explains why the discharging curves do not follow an exponential decaying pattern. jC ¼ 1.92 mA cm2 can be established as the optimum current density value, due to high permeation yields (0.96). This means that collateral reactions (i.e. hydrogen evolution)

may be minimised; in other words, atomic hydrogen is efficiently produced and inserted to the Pd bulk. At this condition, Pd membrane bulk microstructure is not negatively affected, a-PdH reversible phase formation is being favoured.

3.2. Pd deposition using PdCl2 in 1 M HCl and 28% NH3 solutions Fig. 2 shows that two different species are formed for 0.06 M PdCl2 in 1 M HCl and 28% NH3. Peaks at 471 and 294 nm are 2þ assigned to PdCl2 4 and Pd(NH3)4 respectively [24]. Based on 2 ligand field theory, Pd in PdCl4 is more labile than in Pd (NH3)2þ 4 ; in other words, Pd depositions may be less energy consuming or demanding in chloride than in ammoniacal solution. This is confirmed by the experimental evidences found for depositions carried out using 0.06 M PdCl2 solutions: a. Pd deposition efficiencies (3Pd ¼ mPdexp  100/mPdtheo, mPdexp and mPdtheo are the experimental and theoretical deposited Pd amount respectively) are higher in acidic

Fig. 2 e UVeVisible spectra for 0.06 M PdCl2 solutions in 1 M HCl and 28% NH3 at 25  C.

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Fig. 3 e SEM microphotographs (55003) of Pd deposits obtained at 25  C by applying L1.92 mA cmL2 during 180 min, using: a) 0.060 M PdCl2 D 1 M HCl and b) 0.060 M PdCl2 D 28% NH3. Pd membrane (thickness 125 mm) in 0.1 M NaOH. S [ roughness factor.

(chloride) than in alkaline media (ammoniacal), 98 and 17% respectively. mPdexp is obtained by weighing the obtained deposit and mPdtheo is calculated using the atomic hydrogen generated by applying 1.92 mA cm2 during 180 min (corrected by r ¼ 0.96). b. A three-dimensional Pd nucleation is promoted for depositions in 1 M HCl (Fig. 3). Similar results have been reported by Inoue and co-workers for Pd deposition in 1 M HCl [16]. The deposit obtained in acid (Pdacid/Pd) is more porous than the obtained in alkaline (Pdalk/Pd). Pdacid/Pd (S ¼ 6, Aeff ¼ 19 cm2) is rougher than Pdalk/Pd (S ¼ 3, Aeff ¼ 11 cm2). Fig. 4 shows that galvanostatic permeation and discharging curves for Pd modified membranes (Pdacid/Pd and Pdalk/Pd) are very similar, as well as, r values. This could be understood considering that the presence of rough Pd layer overlying a bare Pd foil should not influence the hydrogen permeation yield (same material means similar hydrogen diffusion

coefficient). In contrast, the permeation rate (P) should be influenced by the Pd deposit, due to the fact that P is inversely proportional to the membrane thickness (L) at constant diffusion coefficient (D) and surface coverage (q) [22]. This last was not evidenced; the new interface, between bare Pd and porous Pd, may not hinder the atomic hydrogen diffusion. The obtained Pd deposit may be very thin; therefore, the possible effect on permeation rate is not observed. In terms of S and 3Pd values, Pd deposition in acid (0.1 M HCl) is the most convenient medium for preparing palladium black or highly rough palladium deposits. Based on the results described above, Pd depositions are more efficient in 0.1 M HCl than in 28% NH3 (considering that the highest Pd deposition efficiency and roughness factor are achieved in 0.1 M HCl). Pd depositions in 1 M HCl were also performed using xM PdCl2 solutions (x ¼ 0.130, 0.060, 0.030, 0.025, 0.020 and 0.010). Effective surface area calculations are based on PdO charge reduction measurements. Correia and co-workers had reported that voltammetric conditions (for effective surface area

Fig. 4 e Galvanostatic permeation and discharging curves for Pdacid/Pd and Pdalk/Pd at 25  C and jC [ L1.92 mA cmL2 during 40 min. Pd membrane (thickness 125 mm) in 0.1 M NaOH.

Fig. 5 e Voltammetric responses of Pd in 0.5 M H2SO4, 25 mV sL1, 25  C at different anodic potential limits (Eal). Inset: PdO reduction charge (q) as function of Eal.

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Fig. 6 e Roughness factor (S ) and Pd deposition efficiency (3Pd) curves for Pd depositions performed, by applying L1.92 mA cmL2 during 180 min, using different PdCl2 concentration in 1 M HCl at 25  C. Pd membrane (thickness 125 mm) in 0.1 M NaOH.

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calculations) must be selected to avoid hydrogen adsorption/ absorption, PdO2 formation and Pd dissolution [26]. Moreover, measurements must be performed under steady-state conditions, thus it is suggested to use a sweep rate of 25 mV s1 [26]. Cathodic potential limit was set up at 444 mV, for preventing the hydrogen adsorption/absorption. In order to obtain accurate effective Pd surface area values, avoiding PdO2 formation, the anodic potential limit (Eal) needs to be appropriately chosen. Fig. 5 shows voltammetric responses of Pd in 0.5 M H2SO4 at different Eal. It may be noted a cathodic wave, due to PdO2 reduction, between 1000 and 1400 mV [25]. Current density increases with increasing Eal and a cathodic peak, due to PdO reduction between 500 and 800 mV. Inset, in Fig. 5, shows the relationship between PdO reduction charge (q) and Eal. The optimum Eal value is the potential at the inflection point, where a PdO monolayer is formed [26]. The calculated charge, at the inflection point (Eal ¼ 1268 mV), is about 436 mC cm2. This value is very close to the reported one (424 mC cm2) [26], suggesting a high accuracy of the values obtained by the used methodology. Consequently, potential must be swept between 444 and 1268 mV for effective surface area calculations. Fig. 6 shows the variation of S and 3Pd for depositions performed, by applying 1.92 mA cm2 during 180 min, using

Fig. 7 e SEM microphotographs (55003) of Pd deposits prepared, by applying L1.92 mA cmL2 during 180 min, using xM PdCl2 D 1 M HCl solutions at 25  C: a) 0.130 M, b) 0.060 M, c) 0.030 M, d) 0.025 M, e) 0.020 and f) 0.010 M. Pd membrane (thickness 125 mm) in 0.1 M NaOH. For comparison purposes, bare Pd is shown g).

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different PdCl2 solutions. S values decrease with increasing concentration, this behaviour agrees with evidences found by SEM technique (Fig. 7). Nonetheless, S values slightly decrease for diluted PdCl2 solutions (<0.020 M). In this case, the atomic hydrogen recombination to form H2 may predominate instead of Pd reduction. The highest S value is obtained for experiments performed using 0.020 M PdCl2 þ 1 M HCl. For higher concentrations (0.030 M), Pd nucleation may take place instantaneously and non porous or compact Pd deposits are obtained. Nevertheless, for Pd diluted solutions nucleation may occur progressively and a three-dimensional coral-like shape Pd deposit is formed (Fig. 7). Pd deposition and atomic hydrogen diffusion, on the formed deposit, occur simultaneously; consequently, the formation Pd deposits takes place (non porous or coral-like shape). Hydrogen diffusion/absorption in Pd is the key factor that allows Pd nucleation on freshly formed Pd deposits. Additionally, it can be noted that 3Pd is constant between 0.130 M and 0.030 M (z98%). However, 3Pd falls down to 72 and 31% for 0.020 and 0.010 M respectively (Fig. 6). For Pd depositions performed using diluted solutions (i.e. 0.010 M PdCl2 þ 1 M HCl), atomic hydrogen recombination to form H2 may predominate instead of Pd reduction by PdeH. In other words, PdCl2 4 low concentration nearby the Pd membrane surface and the mass transport limitation effect produced by the consumption of this low surface concentration increases the probability for hydrogen recombination. Thus, 3Pd tends to be very low.

4.

Conclusions

Electrochemically assisted Pd deposition performed, by applying 1.92 mA cm2 during 180 min, in 1 M HCl is more efficient than in 28% NH3. This is based on the high values achieved for Pd deposition efficiency (3Pd ¼ 99% in 1 M HCl vs. 3Pd ¼ 17% in 28% NH3) and roughness factor (S ¼ 6 in 1 M HCl vs. S ¼ 3 in 28% NH3). Pd depositions carried out, using solutions of different PdCl2 concentrations in 1 M HCl, yield rough three-dimensional coral-like shape Pd deposits. The effect is more remarkable for concentrations below to 0.030 M, S values sharply increase in about 7 times. Deposition efficiencies were also affected by the concentrations of PdCl2 solutions. 3Pd is constant for concentrations between 0.130 and 0.030 M (z98%). Nevertheless, 3Pd values decrease in 0.3 times for 0.020 M and 0.7 times for 0.010 M. When PdCl2 diluted solutions are employed (<0.020 M), atomic hydrogen recombination to form H2 may predominate, instead of the chemical of Pd. Pd deposition, using 0.020 M PdCl2 þ 1 M HCl, yields a highly rough Pd deposit (S ¼ 357, Aeff ¼ 1122 cm2) with an acceptable 3Pd value (72%). Therefore, this is the most proper condition for preparing high roughness Pd modified bare Pd membranes by applying 1.92 mA cm2 during 180 min.

Acknowledgements Authors would like to thank PDVSA Intevep for the permission of publishing this work. Special mention to Mr. Henry Tovar

(PDVSA Intevep, Gerencia Departamental de Materiales y Confiabilidad Operacional) for his collaboration in SEM analyses.

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