INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

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1 INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 ISSN (Print) ISSN (Online) Volume 5, Issue 4, April (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJARET I A E M E HYDROGEN PERMEATION BEHAVIOR AND ANNEALING IN COMPOSITE PALLADIUM MEMBRANES AT HIGH TEMPERATURE Abubakar Alkali *1, Edward Gobina 1 1 Robert Gordon University, School of Engineering, Riverside East, Garthdee Road, Aberdeen, AB10 7GJ, United Kingdom. ABSTRACT The main purpose of this work is to investigate the hydrogen permeation behavior and also the effect of annealing in Pd and Pd/Ag composite membranes both of 2 µm thickness prepared on α- Al 2 O 3 support using electroless plating method. Pd and Pd/Ag membranes were prepared in a hydrazine based electroless plating bath. Single component hydrogen permeation tests were conducted to investigate the hydrogen permeation behavior of the membranes and the effect of annealing at different temperatures. The Palladium membrane displayed a H 2 flux of up to 4.32E + 01 cm 3 cm- 2 min -1 at 723 K. The Pd/Ag membrane displayed a slightly higher H 2 flux of up to 4.57E +01 at 723 K. Annealing the membrane greatly enhanced the H 2 flux to about two-fold from 4.32E +01 cm 3 cm -2 min -1 to 8.57E + 01 cm 3 cm -2 min -1 for the palladium membrane and up to 8.72E +01 cm 3 cm -2 min -1 for the Pd/Ag at 873 K. Keywords: Hydrogen Flux, Electroless Plating, Palladium Membranes, Palladium/Silver Membranes, Annealing, Activation Energy. INTRODUCTION Hydrogen separation and purification technologies are becoming increasingly popular as a result of the importance of hydrogen as a clean energy carrier (1). Hydrogen is used in several industrial processes such as petroleum refining, production of ammonia, production of methanol, petrochemical industries and semi conductor industries (2). The demand for high purity hydrogen is rising especially when juxtaposed with the importance of hydrogen as an alternative source of energy in view of the current global challenges of energy insecurity and climate change. It is in the light of these challenges that interest has spiked up in polymer electrolyte membrane fuel cell (PEMFC) due to their environmental friendliness and economic viability 3. Moreover the development of hydrogen fuel cell vehicles has boosted motivation in hydrogen separation and purification processes using inorganic membranes (4). 205

2 It is generally acknowledged that the world is now in a transition from a fossil fuel based to a hydrogen energy system and albeit it will take more years to complete this transition to hydrogen based global energy system, this interim period should be used to develop or optimize technologies for hydrogen separation and purification (5). Presently, steam methane reforming is the most widely used method for hydrogen production but the method is not in tandem with the much envisaged global clean energy future for several reasons such as high energy consumption, threat of impurities and cost (6). More critically, greenhouse gases such as CO 2 are produced as end products in steam methane reforming which recycles back to the same problem of carbon emission (6). Palladium membranes are the membranes of choice for hydrogen separation, purification and production due to their infinite selectivity to hydrogen when defect free (7). Palladium can be used to optimize the steam reforming process by selectively extracting high purity hydrogen from the products such that there are no greenhouse gases such as CO and CH 4 (6). There are several methods for the preparation of palladium based membranes by deposition of palladium films over porous supports such as chemical vapor deposition, electroplating, electroless plating, physical vapor deposition, magnetron sputtering (8). However, electroless plating has been identified as the preferred method due to the several advantages it has compared to other methods (8). Some of these advantages include easiness of coating over any surface of any shape, low energy consumption, uniformity of coating and simple equipment which makes it less prone to errors and complexities (9). Electroless plated palladium membranes also have excellent resistance to corrosion and high mechanical stability (9). Several decades back, thin palladium films were used in hydrogen separation and purification processes. However, these thin films lack the mechanical and thermal stability to withstand harsh operating conditions. They suffer from high cost and are prone to cracks and breaks (9). To address these problems associated with thin Pd films, the concept of composite membranes was developed in which thin, defect free palladium films are deposited over porous support. These composites have shown to achieve higher hydrogen flux and can withstand harsh operating conditions at low cost (8). The primary objective of this work is to investigate the hydrogen permeation behavior and the effect of annealing at higher temperature in Pd and Pd/Ag. This will provide a better understanding on hydrogen permeation behavior in palladium and palladium-alloy membranes. EXPERIMENTAL In the electroless plating of Pd and Pd/Ag membranes, porous ceramic α-alumina supports of 30 nm average pore size supplied by ceramiques techniques et industrielles (CTI SA) France were used onto which thin Pd and Pd/Ag films were deposited. The porous alumina support used for both the Pd and Pd/Ag membranes has the specification i.d= 7 mm, o.d=10 mm, effective length= 340 mm. The same procedure was used in plating both Pd and Pd/Ag membranes only that a different plating bath composition in a separate plating bath was used for the Pd/Ag membrane. The support was first dried at 65 0 C in an oven for 2 hours to remove any moisture and calcined in air at 873 K for 24 hours. The alumina support was then modified prior to the electroless plating through a 2 step sensitization and activation procedure in order to seed it with Pd nuclei to create catalytic sites and ensure a uniform deposition of the metallic layer. A M Sn(11) solution and M Pd(11) solution were used as sensitization and activation solutions respectively (10). The support was sealed at both ends to prevent internal deposition and immersed in the sensitization solution for 5 minutes followed by rinsing in distilled water. The support was then immersed in the activation solution for another 5 minutes and again rinsed in distilled water. This procedure was repeated 10 times to obtain a more uniformly seeded support. After the sensitization and activation procedure, the seeded support was stored overnight at room temperature. 206

3 Pd Plating Bath PdCl 2 = 2.7 g N 2 H 4 = 10 ml NH 4 OH = 440 ml Na 2 EDTA = 70 g Table 1: Composition of Pd and Pd/Ag plating bath Pd/Ag plating bath PdCl 2 = 2.4 g AgNO 3 = 0.3 g N 2 H 4 = 6.5 ml NH 4 OH = 350 ml Na 2 EDTA = 31 g A plating bath was prepared into which the seeded support was inserted at 328 K for 30 minutes. The composition of the Pd and Pd/Ag plating baths are shown in Table 1. Plating commences after the addition of the hydrazine reducer into the plating bath. The seeded support laps straight up during plating so as to avoid tilting sideways which could lead to uneven coating. After deposition, the wet membrane was dried overnight at room temperature and a membrane of ~2 µm was obtained for both the Pd and Pd/Ag membranes. The membranes were then inserted into the membrane reactor in the permeation test plant and Hydrogen was permeated through the membrane at 673 K for 2 hours to activate the Pd layer. Hydrogen permeation in both the palladium and palladium alloy membranes was investigated at 623, 673 and 723 K using a permeation set up as shown in Fig. 1 comprising of a stainless steel shale and tube membrane reactor module. After the permeation test at different temperatures, the effect of annealing on hydrogen permeation was also tested by annealing the membrane at 673, 773 and 873 K for 10 hours each. After annealing at each of these temperatures, permeation test was carried out at 673 K after the membrane was allowed to cool down to 673 K. The feed pressure was controlled through back-pressure regulators and metering valves monitored with a pressure gauge. The temperature was measured using a thermocouple inserted in the membrane unit and monitored using certified thermometer. The flow rate was measured using a mass flow meter and gas separation data collected online using a Varian HP 3800 Gas Chromatograph interfaced to a PC and equipped with a T.C.D and F.I.D detectors in series. Membrane characterization was carried out with a scanning electron microscopy (SEM). Fig. 3 shows the SEM micrograph of cross sectional area of a homogenous and uniformly coated metallic palladium film over the porous ceramic alumina support. Figure 1: Concept Schematic of a permeation test plant 207

RESULTS AND DISCUSSION Figure 2: SEM micrograph of the cross section area of the Pd layer Hydrogen flux was measured for both the Pd and Pd/Ag membrane at 723, 673 and 623 K for transmembrane

4 RESULTS AND DISCUSSION Figure 2: SEM micrograph of the cross section area of the Pd layer Hydrogen flux was measured for both the Pd and Pd/Ag membrane at 723, 673 and 623 K for transmembrane pressure difference of 0.05 to 0.40 bar. The effect of annealing was investigated at and 673 K for the same transmembrane pressure difference of 0.05 to 0.40 bar. Figures 3 and 4 show the H 2 flux for single gas permeation at different temperatures for the Pd membrane and the Arrhenius plot for temperature dependence on H 2 permeation. H 2 Flux (cm 3 cm -2 min -1 ) y = x R 2 = y = x R 2 = y = x R 2 = K K K 10 Linear (723 K) 5 Linear (623 K) P1 - P2 (Bar) Figure 3: H 2 flux at different temperature for the Pd membrane Figure 4: Arrhenius plot of temperature dependance for the Pd membrane Fig. 3 shows the hydrogen flux for the Pd membrane at 723, 673 and 623 K. It can be observed that the hydrogen flux is directly proportional to the difference in the downstream and upstream hydrogen partial pressures and the H 2 flux increased with increase in the feed pressure. Pressure is the most important driving force in the permeation of hydrogen through the membrane. The permeation of hydrogen through palladium membranes is governed by the solution-diffusion mechanism based on the following steps (11) : 1. External mass transfer of H 2 molecules through internal diffusion from the bulk of the gas phase onto the membrane surface on the high pressure side. 2. Dissociative adsorption of the H 2 molecules into atoms on the high pressure side. 3. Reversible dissolution process where the H 2 atoms are dissolved into the bulk palladium layer

5 Diffusion of the H 2 atoms into the bulk palladium layer. 5. Reversible movement of the H 2 atoms from the bulk metallic layer to the membrane surface. 6. Reversible recombination desorption of the H 2 molecules at the low pressure side. 7. External mass transfer of H 2 molecules on the membrane surface at the low pressure side. The rate of H 2 permeation is therefore an interplay of the hydrogen diffusion through the metallic bulk and the difference in the H 2 concentration in the upstream and the downstream sides. Thus, the concentration of H 2 in the film is influenced by both the H 2 solubility and its partial pressure which implies that the rate of H 2 permeation through the palladium membrane can be expressed based on Fick s first law (12) : J = Q (P h n P l n )/L (1) Where J = H 2 flux, Q is the coefficient of H 2 permeation, L is the thickness, P h and P l are the H 2 partial pressure difference in the feed and permeate sides and n is the exponential factor indicating the rate limiting step in hydrogen permeation through the palladium membrane. The effect of n value is explained as follows: 1) When n = 0.5, the rate limiting step is the bulk diffusion of hydrogen. 2) When n= 1, the rate limiting step is the surface processes such as hydrogen dissociative adsorption and/or hydrogen recombination and desorption at the permeate side. 3) When 1> n > 0.5, then both bulk diffusion and surface processes will constitute the rate limiting steps in hydrogen permeation through the palladium membrane. For the Pd membrane, a H 2 flux of up to 4.32E + 01 cm 3 cm -2 min -1 was observed at 723 K. H 2 flux increased with increasing transmembrane pressure difference for both Pd and Pd/Ag membranes. Permeability depends on temperature and the Arrhenius equation enables the estimation of the activation energy at different temperatures as described by the equation (12) : J = A o exp (-E a /RT) (2) Where A o is the exponential factor, R is the gas constant, and T is the operating temperature. The experimental data of the H 2 flux at different temperature was used to determine the activation energy from the slope (M). In Fig 4, the activation energy was calculated as 8. kjmol -1 and it represents the effect of temperature on H 2 permeation. The higher the activation energy, the more the resistance to H 2 permeation (13). As shown in Fig. 7 for the Pd/Ag membrane, the H 2 flux was up to 4.57E+01 cm 3 cm -2 min -1 at 723 K. This indicates a slightly higher H 2 flux for the Pd/Ag membrane compared to the Pd membrane despite the fact that both membranes are of the same thickness. This could be attributed to the alloying factor in the Pd/Ag membrane which enhanced the permeation of hydrogen through the membrane (6). The alloying with silver in the Pd/Ag membrane provided less resistance to permeation by enhancing the solubility and subsequent diffusion of hydrogen through the membrane hence the Pd/Ag membrane achieved a higher H 2 flux compared to the Pd membrane (6). The temperature dependence on hydrogen permeation across the membranes was also investigated at different pressures and temperatures. A plot of Ln (M) against 1/T for the Pd/Ag membrane gave activation energy of kj/mol for the Pd/Ag membranes as shown in Fig. 8. Both activation energies for the Pd and Pd/Ag membranes are within those reported in literature. The membranes were annealed at high temperatures in order to investigate the effect of annealing on the membrane permeation behavior and also to activate the Pd metal and the alloy in the Pd membrane and the Pd/Ag membrane respectively. 209

6 H 2 Flux (cm 3 cm -2 min -1 ) y = x R 2 = y = x R 2 = y = x R 2 = K 773 K 673 K Linear (773 K) Linear (873 K) P1 - P2 (Bar) Figure 5: H 2 flux at different temperature for the annealed Pd membrane Ln (M ) 5.00E E E E E E E- 1.10E- 1.20E- 1.30E- 1/T (K) 1.40E- 1.50E- y = x R 2 = E- n=1 Linear (n=1) Figure 6: Arrhenius plot for the annealed Pd membrane H 2 Flux (cm 3 cm -2 min -1 ) P1 - P2 (Bar) y = x R 2 = y = x R 2 = y = x R 2 = K 673 K 623 K Linear (623 K) Linear (723 K) Figure 7: H 2 flux at different temperature for the Pd/Ag membrane Ln (M ) 4.30E E E E E+00 y = -1462x R 2 = E- 1.40E- 1.50E- 1.60E- 1.70E- 1/T (K) Series1 Linear (Series1) Figure 8: Arrhenius plot for the Pd/Ag membrane H 2 Flux (cm 3 cm -2 min -1 ) P1 - P2 (Bar) y = x R 2 = y = x R 2 = y = x R 2 = K 773 K 673 K Linear (873 K) Linear (773 K) Figure 9: H 2 flux at different temperature for the Annealed Pd/Ag Ln (M) 5.00E E E+00 y = x R 2 = E E- 1.15E- 1.30E- 1.45E- 1.60E- 1/T (K) n=1 Linear (n=1) Figure 10: Arrhenius plot for the annealed Pd/Ag membrane 210

7 As shown in Fig. 5, results for the annealed Pd membrane indicate an increase in the H 2 flux of up to 8.57E + 01 cm 3 cm 2 min -1 at 873 K which is two-fold higher than that of the Pd membrane prior to the annealing. In Fig 9, the annealed Pd/Ag membrane also displayed marked improvement in the H 2 flux from 4.57E +01 cm 3 to 8.72E +01 cm 3 cm -2 min -1 at 873 K. These results indicate that annealing increased the hydrogen flux by two- fold for both the Pd and Pd/Ag membranes. The increase in H 2 flux for the annealed membranes is attributed to the removal of surface contaminants and also the formation of hydride phases 6 From Figs. 6, 8 and 10, the activation energies of annealed Pd, Pd/Ag and annealed Pd/Ag were calculated as 9.12, and 9.96 kj/mol respectively. These activation energies are within the values reported in literature (4, 14). The activation energy indicates that the effect of temperature on hydrogen permeation was more pronounced for the palladium membrane compared to the Pd/Ag membrane and also this effect was more significant when the membranes were annealed compared to hydrogen permeation before the annealing. CONCLUSION Palladium and palladium alloy membranes prepared through the electroless plating method show good promise in hydrogen separation and purification. In this work, results for the hydrogen permeation behavior of a Pd and Pd/Ag membranes prepared through the electroless plating method were presented. It was observed that hydrogen permeation increased with temperature and the transmembrane pressure difference. The Pd/Ag alloy membrane also displayed slightly higher H 2 flux compared to the Pd-only membrane. Alloying palladium with silver enhances the rate of H 2 permeation compared to the Pd-only membrane. Investigations on the effect of annealing also show that annealing both Pd and Pd/Ag membranes at higher temperatures decreased the permeation resistance of the membrane and enhanced the H 2 flux through the membranes by about two-fold. This work provides a better understanding of the significance of alloying with silver and annealing at high temperature in Pd membranes to achieve high purity hydrogen. Specifically, it has been shown in this work that annealing both Pd and Pd/Ag membranes at high temperature up to 837 K or above could enhance hydrogen permeation through the membrane by two-fold. ACKNOWLEDGEMENT Sincere thanks to Petroleum Technology Development Fund (P.T.D.F) Nigeria for funding this research. REFERENCES 1. Lu, G. Q., Diniz da Costa, J.C., Duke, M., Giessler, S., Socolow, R., Williams, R.H. & Kreutz, T. (2007). Inorganic membranes for hydrogen production and purification: A critical review and perspective. Journal of colloid and interface science, 314: Nowotny, J., Sorrell, C.C., Sheppard, L.R. & Bak, T. (2005). Solar hydrogen: Environmentally safe fuel for the future. International Journal of hydrogen energy, 30: Balamurali K.R.N., Choi, J., Harold, P.M. (2006). Electroless plating and permeation features of Pd and Pd/Ag hollow fiber composite membranes. Journal of Membrane Science 288: Chee, C. & Gobina, E. (2010). Ultra-thin palladium technologies enable future commercial deployment of PEM fuel cell systems. Membrane technology, Vol. 2010, Issue 3,

8 5. Feroz, E. H., Raab, R.L., Ulleberg, G, T. & Alsharif, K. (2009). Global warming and environmental production efficiency ranking of the Kyoto protocol nations. Journal of environmental management, 90: Pizzi, D., Worth, R., Baschetti, M, G., Satti, G, C. & Noda K-I. (2008). Hydrogen permeability of a 2.5 µm palladium-silver membranes deposited on ceramic supports, Journal of membrane science, 325: Wang, L., Yoshiie, R. & Uemiya, S. (2007). Fabrication of novel Pd-Ag-Ru/Al 2 O 3 ternary alloy composite membrane with remarkably enhanced hydrogen permeability. Journal of membrane science, 306: Yun, S. & Oyama, T.S. (2011). Correlations in palladium membranes for hydrogen separation: A review. Journal of membrane science, 375 (1-2): David, E. & Kopac, J. (2010). Development of palladium/ceramic membranes for hydrogen separation. International journal of hydrogen energy, 36: Cheng, Y.S., Pena, M.A., Fierro, J.L., Hui, D.C.W. & Yeung, K.L. (2002). Performance of alumina, zeolite, palladium, Pd-Ag alloy membranes for hydrogen separation from towngas mixture. Journal of Membrane Science 204: Gabito, J. & Tsouris, C. (2008). Hydrogen transport in composite inorganic membranes. Journal of membrane science 312: Wu, L-Q, Xu, N. & Shi, J. (2000). Preparations of a palladium composite membrane by an improved electroless plating technique. Ind. Eng. Chem. 39: Lee, H-J., Suda, H. & Haraya, K. (2005). Gas permeation properties in a composite mesoporous alumina ceramic membrane. Korean Journal of Chemical Engineering. 22(5), Zeng, G. Shi, L., Liu, Y., Zhang, Y. & Sun, Y. (2014). A simple approach to uniform Pd/Ag alloy membranes: Comparative study of conventional and silver controlled co-plating. International Journal of Hydrogen Energy B. Chirsabesan and M.Vijay, Membrane Assisted Electro Chemical Degradation for Quinoline Yellow, Eosin B and Rose Bengal Dyes Degradation, International Journal of Design and Manufacturing Technology (IJDMT), Volume 4, Issue 2, 2013, pp , ISSN Print: , ISSN Online:

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 ISSN 0976-6480 (Print) ISSN