Journal of Petroleum Science and Technology, Vol. 2, No. 2, 2012, 25-32 Developing of Ethylene Glycol as a New Reducing Agent for Preparation of Pd-Ag/PSS Composite Membrane for Hydrogen Separation L. Samiee*, M. Dehghani Mobarake, R. Karami, and M. Ayazi, Development and Optimization of Energy Technologies Research Division, Research Institute of Petroleum Industry (RIPI) samieel@ripi.ir Abstract I n t h e p r e s e nt wo r k, f or t he fir s t t i me , a pa l l a di u m s i l v e r me mbr a ne ha s be e n pr e pa r e d b y e l e c t r ol e s s plating on the surface of a porous stainless steel disk by using ethylene glycol as a new reducing agent and polyol process. The reducing action of ethylene-glycol in the presence of PVP as a protecting s ur f a c e a ge n t pr o duc e s a me mbr a ne wi t h fin e l y d i v i de d powde r a nd na n o s i z e d por e s . Fu r t he r mo r e , t he h ydr oge n s e p a r a t i on a bi l i t y of t he me mbr a ne c onfir me d t ha t t h e me mbr a ne wi t h Pd77Ag23 is highly selective towards hydrogen compared to other prepared membranes. This is mainly related to the f o r ma t i on of d e ns e a n d ho moge n ous mi c r os t r uc t u r e of t he c oa t i ng. Mo r e ov e r , a c c or di ng t o Gr a h a m’ s law, the tests showed that hydrogen purity rises by increasing the applying temperature. Keywords: Pd-Ag Membrane, Polyol Process, Ethylene Glycol, Hydrogen Separation Introduction Attempts to utilize pure palladium as a hydrogen separation membrane were severely hampered by the extremely high solubility of hydrogen in palladium below 300 °C, leading to the formation of a separate palladium-hydride phase, with a much larger lattice constant compared to pure Pd [1]. The subsequent distortion of the lattice is the cause of hydrogen embrittlement. Alloying of palladium with other metals, particularly silver, was investigated by Hunter and coworkers at the Atl a nt i c Re fini n g Co mpa n y i n c ol l a b or a t i on wi t h J . Bi s ho p Company which later became Johnson-Matthey [2,3]. In his patent published in 1956 [4], Hunter disclosed that alloying palladium with 27 wt% Ag not only prevented hydrogen embrittlement, but also improved the hydrogen permeability of the alloy by 70% compared to pure palladium at 450 °C. alkaline electrolysis processes have focused on the use of advanced anode materials in electrolyzers to reduce electrode polarization and thereby improving the overall e l e c t r i c a l e f fic i e nc y [ 5 ] . I t ha s b e e n a p pr e c i a t e d f o r ma n y years that electrolytic hydrogen will diffuse through the walls of the tubular palladium cathode against which it is generated. This neglected possibility of simultaneous ge n e r a t i on a nd pur i fic a t i on we r e ut i l i z e d i n t he de s i g n of some compact electrolytic cells which can yield pure hydrogen for laboratory or other small scale work. Better results were obtained with the silver-palladium alloy which expanded less when fully charged with hydrogen [6-7]. Several methods are available for the formation of hydrogen separation membranes such as phase inversion, rolling, electroless-plating, chemical vapor deposition (CVD), sputtering, spray pyrolysis, coatings, metal deposition, sol-gel, and so on [8]. Among other techniques, electroless deposition provides Historically, the production of high purity hydrogen has strong advantages such as the uniformity of deposits been based on the high temperature gas phase hydrogen even on very complex shapes, very simple equipment, pe r me a t i o n, but t h e be n e fic i a l a s pe c t s of l ow t e mpe r a t u r e and low cost. This technique can be discussed in three permeation of high purity hydrogen are very important. sections; surface activation, bath chemistry, and electroThe electrolysis of aqueous alkaline hydroxide soluless plating procedures [5]. tions has been one of the most popular routes for the The electroless plating solution generally consists of a production of hydrogen and oxygen from water. Nometallic salt, a complexing agent, a reducing agent, and table recent studies concerned with the improvement of 25 Vol. 2, No. 2 Developing of Ethylene Glycol as a New Reducing Agent ... 26 a stabilizing agent. Electroless plating process occurs on the solid liquid interface between support and the plating solution. It involves the reduction of a metal salt on a catalytic surface. Several reductants are available for the electroless plating process such as dialkylamine-borane, borohydride, hypophosphite, formaldehyde, and hydrazine. Hypophosphite is commonly used, but deposits contain phosph or u s , r e d uc i ng t he qu a l i t y of t h e fil m [ 5] . For ma l d e hyde is not a good reducing agent, especially in the case of Pd deposition since H2 gas is released during plating and cracks the membrane. Also, the reducing effect of formaldehyde is very low (Ohno et al., 1985). Hydrazine is the most commonly used reducing agent. The disadvantages of hydrazine and related compounds mainly include toxicity and carcinogenic effects. Hydrazine is toxic if inhaled, and it is an eye and skin irritant. Besides, the decomposition of hydrazine is directly responsible for the low p l a t i n g e f fic i e n c y o f t h e h y d r a z i n e b a s e d p l a t i n g b a t h [ 5 ] . Recently, several studies have been reported related to the fabrication of Pd-Ag alloy particles with uniform microstructure by using ethylene glycol as a solvent and a reductant agent [9-12]. In the present study, we propose the preparation of PdAg/PSS composite membranes by using electroless deposition and employing ethylene glycol as a reductant and polyol process. By considering the ethylene glycol as a reducing agent, we were therefore stimulated to study the hydrogen separation activity of the new prepared Pd-Ag membrane and investigate the effect of the ratio of Pd to Ag on the hydrogen separation activity. ox i di z e d i n a hi gh t e mpe r a t ur e f ur n a c e und e r a i r flo wi n g at 550 °C for 10 hrs. Table 1: Chemical composition of the alkaline solution Compound Amount Na3PO4.12H2O 45 g Na2CO3 65 g NaOH 45 g DI. Water 1 liter The rate of heating and cooling during the oxidation treatment were kept constant at 3 °C.min-1. Activation Process of PSS Supports The next step in membrane preparation is the surface activation for seeding the PSS surface with palladium nuclei, which helps to achieve a better autocatalytic process of reduction of a metastable metal salt complex on the target surface during electroless plating. Solutions for the activation were prepared using tin (II) chloride dihydrate and palladium (II) chloride. Two solutions were prepared by dissolving 1 g.l-1 SnCl2 and 0.1 g.l-1 PdCL2 salts in 1 ml.l-1 of HCl solution. The activation process consisted of the immersion of the supports in an acidic SnCl2 solution (sensitizing) followed by an acidic PdCl2 solution at room temperature. After the immersion in the SnCl2 solution, a gentle rinsing with deionized water was used. Finally, the samples were rinsed with 0.01 M HCl and then with water. This cycle was repeated normally six to nine times. Electroless Plating of Pd and Ag A typical experimental procedure for electroless plating is described as follows: First, 0.1331 gr of PdCl2 and 0.7980 gr of PVP were disExperimental Procedures solved in 15 ml of ethylene glycol as a solvent, and the Membrane Supports formed suspension was mechanically stirred for 10 min Por o us 316 L s t a i n l e s s s t e e l ( PSS) s up por t s ( 0 . 2 μm to allow the dissolution of the precursors. Then, 0.1274 grade) were purchased from Mott Metallurgical Corpogr of AgNO3 and 0.8090 gr of PVP were added to the ration. PSS plates were prepared by cutting 1 dm2 PSSabove suspension under agitation for 1 hour. The total 316L sheets into 53.066 cm2 circular pieces. concentration of metal ions and the ratio of PVP to the metal were kept at 0.05 M and 10 (w/w) respectively. Cleaning of the Supports The surface of the PSS supports was cleaned prior to electroless deposition to remove contaminants such as oil, grease, and dirt. The plating was done at 120 °C after 5 hours. Finally, the samples were rinsed in hot water and dried at 120 °C for 3 hours. The membranes were prepared for the different ratios of Pd to Ag. The composition of the alkaline solution used is given in Table 1. Then, the samples were thoroughly washed with high purity water (DI water) to remove the basic solution trapped in the pores of the supports. The last cleaning step involved a 10 min immersion in iso-propanol to easily remove the water inside the pores. All cleaning steps were carried out in ultrasonic bath at 60 °C. Finally, the supports were dried at 120 °C for 3 hrs. The t he r ma l t r e a t me nt o f a s d e pos i t e d Pd–Ag fil ms wa s carried out in a high temperature furnace under an N2 atmosphere. The temperature was increased at a rate of 1 °C.min-1 until it reached 550 °C and then was kept constant at this temperature for 10 hrs. After cooling the samples, they were put in a reactor and their permeation tests were carried out. Formation of an Oxide Barrier Layer After cleaning, the porous stainless steel supports were Membrane Characterization Surface characterization was performed using a Philips XL30, 17 kV equipped with an EDX light element detec 27 Samiee et al. Journal of Petroleum Science and Technology tor. The X-ray diffraction (XRD) patterns of all the sampl e s we r e c ol l e c t e d i n a 2θ r a n ge be t we e n 3 0 ° a nd 60 ° by us i ng Phi l i p s PW184 0 di f f r a c t ome t e r ( Cu Kα1 r a di a t i on, Kα1 = 1. 54 06Å) Mi c r o g a s c h r oma t og r a ph y( mode l Agilent 3000A) was used to analyze the outlet gases of the membrane. pe a k s a t 2θ =39 . 92 ° a n d 2 θ =46. 29° a r e r e l a t e d t o Pd Ag alloy phase corresponding to 111 and 200 respectively. These results strongly indicate that the membrane obtained is an alloy of Pd and Ag [13]. The average particle size of the Pd-Ag alloy nanoparticles (d) was estimated by using Scherrer equation after background subtract i on f r o m ( 1 1 1) pe a k a t 2 θ =39 . 42° . I n t he be s t s a mpl e Single Gas Permeation Test of the Membranes (Pd77Ag23), the average particle size of the nanoparticles was around 11 nm. Figure 3 shows the typical scanning A shell-and-tube apparatus shown in Figure 1 has been electron micrograph of the PSS coupons that were plated e qu i ppe d wi t h ma s s flow c ont r ol l e r s , p r e s s ur e c on t r ol with Pd-Ag intermetallic alloy. valves, and a temperature indicator to measure the permeation of hydrogen and nitrogen through the present In the present work, the polyol process has been used for membranes. Both ends of the membrane tube were concoating the PSS membrane. The polyol process has been nected to inlet and outlet tubes and placed in the center of r e c e n t l y p r op os e d a s a me t h od f o r t h e p r e pa r a t i o n of fin e furnace. Hydrogen and nitrogen gases with a ratio of 50 ly divided powders of easily reducible metals [10,11]. A to 50 were introduced into the membrane tube from outer suitable solid inorganic compound is dissolved or suss he l l t hr o ugh t he ma s s flo w c ont r o l l e r a nd t h e ga s pe r me pended into a liquid polyol (e.g., ethylene-glycol, diethated through the membrane. Online gas chromatography ylene-glycol or a mixture of both). Afterward, the system was used to analyze the outlet gas of the membrane; it is stirred and heated to a given temperature, which can should be noted that the calibration of the chromatograreach the boiling point of the polyol for less easily rephy was already performed with hydrogen gas (99.999% ducible metals. The reduction of the starting compound purity). yi e l ds t h e me t a l a s a fin e l y d i vi de d po wde r . The s t a r t i n g compound may be a hydroxide, an oxide or a convenient A single point calibration was done by using the micro salt. The main feature of the reaction mechanism is that TCD’ s l i ne a r d yna mi c r a n ge of 10 pp m t o 1 00 pe r c e nt . the reduction reaction proceeds via the solution rather The system has a repeatability of 0.5 percent relative than the solid phase. Therefore, the metal particles are standard deviation and a linear detector response of formed by nucleation and growth from solution. Accord±5 pe r c e n t o ve r five o r de r s of ma gn i t u de . I n f or ma t i o n ing to this mechanism, the polyol acts as a solvent for is displayed in up to six display windows that support the starting inorganic compound due to the rather high r e a l t i me mo ni t or i ng o f t he de fine d s i gna l s o f ni t r og e n dielectric constant of these organic media. In particuand hydrogen compounds. Data is also sent to a CSV lar, silver nitrate and palladium chloride are soluble in fil e f or l a t e r p r oc e s s i ng i n Mi c r os of t Ex c e l , f o r e xa mpl e . ethylene-glycol to such an extent that a complete dissoThe me a s ur e me n t s of hyd r og e n a n d ni t r oge n p ur i fic a t i o n l ut i on i s ob s e r v e d a s t he fir s t s t e p of t he r e a c t i o n. Nuc l e level were made at different temperatures (25-120 °C) ation and growth must be completely separated in order and at a pressure difference of 400 kPa. to explain the fact that the obtained metal suspension is a n a l mos t ho mod i s pe r s e s y s t e m. Thi s c ond i t i o n, fir s t e x Results and Discussion pressed by La Mer [12, 13] in his study about sulfur sols Structural Study of Pd/Ag Membrane f r o m a c i di fie d t hi os ul f a t e s ol ut i o n, a pp e a r s a s a g e ne r a l The phase properties of the Pd-Ag alloy membrane were condition to obtain a homodisperse system. The metal is investigated using XRD measurements. The results from slowly provided in solution by the progressive reduction XRD analysis are shown in Figure 2. The diffraction of the dissolved species. Figure 1: Schematic presentation of gas permeation apparatus Developing of Ethylene Glycol as a New Reducing Agent ... 28 Vol. 2, No. 2 2 θ ( °) Figure 2: XRD pattern of Pd-Ag alloy A B D C E Figure 3: Sc a n ni n g e l e c t r on mi c r o gr a ph s o f o xi d i z e d PSS a f t e r p l a t i ng a t di f f e r e n t Pd t o Ag r a t i os ; a : 50 : 5 0 , b : 60 : 40 , c : 7 7: 23 , d: 8 8: 1 2, e : 9 8: 2 29 Samiee et al. Journal of Petroleum Science and Technology In particular, the reducing action of ethylene-glycol can be monitored by analyzing the volatile compounds resulting from its oxidation. The formation of the main product, i.e. diacetyl, can be explained in terms of a double oxidation of acetaldehyde, previously produced by the dehydration of ethylene-glycol [11], as follows: CH2OH-CH2OH H O CH3-CHO (1) 2CH3CHO M+2H++CH3-CO-CO- CH3 (2) 2 During reaction (2), the metal concentration rises to the saturation where the nucleation can occur. This condition results in the formation of many nuclei in a short burst. They grow rapidly and the metal concentration is lowered to a point below the nucleation concentration, but high enough to allow particle growth to occur at a rate that just consumes all the generated metal. An envelopment of the reduced metal atoms within a suitable layer could act as a protection for the nanometric particles during their growth [14, 17]. A number of organic materials are known to work as protective agents for preventing colloid sintering [18]. Among them, poly (N-vinylpyrrolidone) (PVP) exhibits the best protecting properties. The prevention of particle sintering can be achieved by adding a critical dosage of an organic protective agent which covers the particles and inhibits any possibility of metal-metal particle bond formation. Such coordination interaction and the repulsion between polymer chains of PVP make the adsorption of PVP to be or i e n t e d i n s ome de fini t e di r e c t i ons , whi c h l e a ds t o t he growth of particles occurring in the preferred directions and being inhibited in other directions. This factor also c on t r i but e s t o pa r t i c l e s h a pe a nd t h e fin a l s ha p e d i s t r i bu tion. An increase in the concentration of the protecting agents suppresses the growth of metal particles, leading to smaller metal nanoparticles [12, 23]. Because of the relatively high temperature used in the annealing process, wide Brownian motions characterize the particles, and the atoms on their surface have an elevated mobility. Consequently, the probability of particle collision, adhesion, and subsequent coalescence by sintering is enhanced. Figure 3 shows that the deposits are a mixture of individual particles. EDS analysis (Figure 4) also proves that these particles are composed of Pd and Ag metals. With a further annealing step, these deposits make the Pd-Ag alloy. Moreover, it can be clearly seen that a change in the me t a l r a t i o i n t he ba t h ha s a s i gn i fic a n t e f f e c t on t h e d e posit concentration; an increase in the concentration of Ag in the solution results in an increase in the deposit concentration. Due to the very thin layers of Pd and Ag, the pores of the substrate can clearly be seen in Figure 3.e. As can be inferred from Figure 3, Ag shows a preferential deposition onto Pd. The deposits obtained are bimetallic; therefore, an increase in the Ag concentration in the deposits means relatively more surface coverage by Ag deposits than Pd deposits. The lower catalytic activity of Ag for the ethylene glycol oxidation means that as the Ag concentration in the deposits increases, the electroless deposition potential rises too. The high overpotential associated with the Pd deposition also means that when the Ag concentration is 2 wt% in the solution, the electroless deposition potential decreases to an extent that the Pd deposition is not possible anymore [24]. Ag has a passivating effect on the Pd deposition. The Ag passivating effect was explained on the basis of the electrode potential of the metal ions in the bath. Figure 3 shows that the electrode potential of the Ag ions is less than that of Pd up to 23 wt% of Ag and the sample containing 77 wt% of Pd and 23 wt% of Ag (Pd77Ag23) has a u ni f or m s ha pe d mi c r os t r uc t u r e ; ho we v e r , a f t e r 2 wt % Ag in the bath, covering tendency is coming down. The worst condition can be easily seen in the sample containing 98 wt% of Pd and 2 wt% of Ag (Pd98Ag2). Table 2: Comparison between the initial ratio of Pd to Ag and the obtained ratio of Pd to Ag by EDS Initial Ratio of Pd to Ag Obtained Ratio of Pd to Ag by EDS 50:50 46:54 60:40 62.7:37.3 77:23 76.3:23.7 88:12 88.5:11.5 98:2 96:4 Hydrogen Permeation Behavior of the Membrane With respect to using the prepared membrane as a hydrogen diffusion cathode in electrolyzing systems, the membranes were tested for H2 and N2 permeation at the range of 25-120 °C. Table 3 shows the hydrogen and nitrogen purities after passing through the membrane at the applied temperatures. As can be seen in Table 3, the hydrogen passed through the Pd77Ag23 membrane , which has a homogenous and dense alloyed layer, has a maximum purity, whereas the hydrogen passed through the Pd98Ag2 membrane (with the least coating has less purity. This problem can be possibly due to the formation of a nonhomogeneous alloy with layers that contained higher Pd content than the desired composition. Also, the characterization of this membrane shows that the maximum H 2 purity can be achieved for the membrane with reduced support porosity. (3) diffusion In addition, since a constant volume of different gases contains the same number of particles, the number of moles per liter at a given temperature (T) and pressure (P) is constant. Therefore, the density of a gas is directly proportional to its molar mass (MM): (4) Thus, it can be concluded that H2 diffuses more rapidly than N2 by increasing temperature [25]. Vol. 2, No. 2 Developing of Ethylene Glycol as a New Reducing Agent ... 30 Figure 4: EDS An a l ys i s o f o x i d i z e d PSS a f t e r p l a t i ng a t di f f e r e n t Pd t o Ag r a t i o s ; a : 50 : 5 0 , b : 6 0: 40 , c : 7 7: 23 , d: 8 8 : 1 2, e : 9 8 : 2 31 Samiee et al. Journal of Petroleum Science and Technology Table 3: Hydrogen purity (%) Applied temperature (ºC) Hy dr o ge n pu r i fic a t i o n ( %) t hr o ug h me mbr a ne wi t h d i f f e r e n t Pd t o Ag r a t i o s 98:2 88:12 77:23 60:40 51:49 25 87.3571 92.1472 99.9847 93.8153 90.3591 40 87.8462 93.4253 99.9891 94.2684 91.1473 60 88.4392 94.0026 99.9903 95.1275 92.8735 80 89.0173 94.8472 99.9918 96.0073 95.4263 100 89.8637 95.7361 99.9924 96.8165 95.8462 120 90.2755 96.0752 99.9936 97.5739 96.2639 The comparison results between the initial ratio of Pd to Ag and the obtained ratio of Pd to Ag by EDS also c on fir ms t h e r e l i a bi l i t y of t hi s r e duc i n g a g e nt ( Ta bl e 2 ) . Conclusion A palladium-silver membrane was developed by depositing Pd-Ag onto a PSS substrate by a polyol process. A s moot h , fla t , a nd de n s e Pd Ag l a ye r i s o bt a i ne d a t f or a combination of 77 wt% Pd and 23 wt% Ag (Pd77Ag23). Due to the passivating effect the porous structure increases remarkably as the ratio of Pd to Ag is increased, Hydrogen purity measurements in the temperature range of 25 to 120 °C shows that the composite membrane has a high selectivity for hydrogen. The permeation tests shows that hydrogen purity level rises by increasing temperature, which represents a good reliability of the prepared membrane for using as a hydrogen diffusion cathode in the operating temperature range of electrolyzer systems. References [1]. Akis B. C., Preparation of Pd-Ag/PSS Composite Membranes for Hydrogen Separation, Phd Thesis, WORCESTER POLYTECHNIC INSTITUTE, 2004. [2]. Hunter J. 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