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

Transcription

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 IN COMPOSITE PALLADIUM MEMBRANES AT HIGH TEMPERATURE Abubakar Alkali *1, Edward Gobina 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 of Pd and Pd/Ag composite membranes 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 at different temperatures. The rate of H 2 permeation through the membrane at different temperatures and the effect of annealing on H 2 permeation were investigated. Palladium membranes of about 6 µm thickness µm show a H 2 flux of up to 3.68E + 01 cm 3 cm- 2 min -1 at 723 K. The Pd/Ag membrane of about the same thickness show a higher H 2 flux of up to 4.22E +01 at 723 K. Annealing enhanced the H 2 flux from 3.68E +01 cm 3 cm -2 min -1 to 4.76E + 01 cm 3 cm -2 min -1 for the palladium membrane at 723 K and up to 6.53E +01 cm 3 cm -2 min -1 for the Pd/Ag at 723 K. The activation energies of both membranes indicate that the temperature effect on H 2 permeation was more pronounced in the Pd membrane than the Pd/Ag membrane and also more for the annealed membrane than before the annealing. Key words: Hydrogen Flux, Electroless Plating, Palladium, 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 based on the use 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 205

2 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). 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 and CH 4 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 also 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 in Pd and Pd/Ag membranes at high temperature. This will provide a better understanding on the how hydrogen permeation can be enhanced in Pd-based 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 has the specification i.d= 7 mm, o.d=10 mm, effective length= 340 mm. 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 Table 1: Composition of Pd and Pd/Ag plating bath Pd Plating Bath Pd/Ag plating bath PdCl 2 = 2.7 g PdCl 2 = 2.4 g N 2 H 4 = 10 ml AgNO 3 = 0.3 g NH 4 OH = 440 ml N 2 H 4 = 6.5 ml Na 2 EDTA = 70 g 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 then inserted into the membrane reactor in the permeation test plant shown in Fig. 1. Hydrogen was passed through the membrane at 673 K for 2 hours to activate the Pd membrane. 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. 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) and energy dispersive x-ray analysis (EDXA) to identify both the morphological features and elemental composition. 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. Fig. 1: Concept Schematic of a permeation test plant 207

4 Fig. 2: A Picture of the coated Pd Membrane Fig. 3: SEM Micrograph of the cross area of the coated membrane RESULTS AND DISCUSSION Fig. 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 y = 63.04x y = x R 2 = R 2 = y = x R 2 = K 673 K 623 K Linear (723 K) Linear (673 K) P1 - P2 (Bar) Linear (623 K) Fig. 4: H 2 flux at different temperature for the Pd membrane 4.50E E E+00 y = x E- R 2 = E- 1.45E- 1.50E- 1/ T ( K) 1.55E- 1.60E- 1.65E- n=1 Linear (n=1) Fig. 5: Arrhenius plot of temperature dependance for the Pd membrane Fig. 4 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 hence 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 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 effect of pressure on permeation. For the Pd membrane, a H 2 flux of up to 3.68E + 01 cm 3 cm -2 min -1 was observed at 723 K. H 2 flux increased with pressure for both Pd and Pd/Ag membranes. Fig. 5 represents the Arrhenius plot of Ln(M) against the inverse temperature. 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 permeance at different temperature was used to determine the activation energy from the slope (M) in Fig 5. The activation energy was calculated as 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. 6 for the 6 µm Pd/Ag membrane, the H 2 flux was up to 4.22E+01 at 723 K. This indicates a higher H 2 flux for the Pd/Ag membrane compared to the Pd membrane despite the fact that the membranes are of the same thickness. The high H 2 flux could be attributed to the alloying factor in the Pd/Ag membrane which not only prevents hydrogen embrittlement but also enhances 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 this membrane achieved a higher H 2 flux compared to the Pd-only 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 8.87 kj/mol for the Pd/Ag membranes as shown in Fig. 9. Both activation energy values for the Pd and Pd/Ag membranes are within those reported in literature. However, the lower activation value for the Pd/Ag membrane compared to the Pd membrane indicates that temperature effect on H 2 permeation was less than that in the Pd membrane. 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 H2 Flux (cm3 cm -2 min-1) y = x R 2 = y = x R 2 = y = 54.23x R 2 = K 773 K 673 K Linear (673 K) Linear (873 K) Linear (773 K) Ln (M ) 4.60E E E E E E E+00 y = x R 2 = n=1 Linear (n=1) P1 - P2 3.90E E- 1.20E- 1.40E- 1.60E- 1/T (K) Fig. 6: H 2 flux at different temperatures for the annealed Pd membrane Fig. 7: Arrhenius plot for the annealed Pd membrane y = x R 2 = y = x R 2 = y = x R 2 = K 673 K 623 K Linear (723 K) Linear (673 K) P1 - P2 Linear (623 K) Fig. 8: H 2 flux at different temperatures for the Pd/Ag membrane 4.20E E E E E E E E+00 y = x R 2 = E E- 2.00E- 2.50E- Series1 Linear (Series1) Fig. 9: Arrhenius plot for the Pd/Ag membrane H2 Flux (cm 3 cm -2 m in-1) y = 124.9x R 2 = y = x R 2 = P1 - P2 y = x R 2 = K 773 K 673 K Linear (673 K) Linear (873 K) Linear (773 K) 4.50E E E- y = x R 2 = E- 1.45E- 1.50E- 1/ T (K) 1.55E- 1.60E- 1.65E- n=1 Linear (n=1) Fig. 10: H 2 flux at different temperatures for the Annealed Pd/Ag Fig. 11: Arrhenius plot for the annealed Pd/Ag membrane 210

7 As shown in Fig. 6, results for the annealed Pd membrane indicate an increase in H 2 flux of up to 4.76E + 01 cm 3 cm 2 min -1 at 723 K which is higher than that of the Pd membrane prior to the annealing. In Fig 8, the Pd/Ag membrane also shows a marked improvement in the H 2 flux from 4.22E +01 cm 3 to 6.53E +01 cm 3 cm -2 min -1 at 723 K when annealed. 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. As shown in Figs. 7, 9 and 11 the annealed Pd, Pd/Ag and annealed Pd/Ag membranes gave activation energies of 12.57, 8.87 and 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 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 temperature increased H 2 permeation for both the Pd and Pd/Ag membranes and also the Pd/Ag alloy membrane showed higher H 2 flux compared to the Pd-only membrane. Alloying palladium with silver enhances the rate of H 2 permeation compared to the Pdonly 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. This work provides a better understanding of the significance of annealing in Pd-based membranes to achieve high purity hydrogen. ACKNOWLEDGEMENT Sincere thanks to PTDF 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, 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:

8 7. 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: