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Ind. Eng. Chem. Res. 2010, 49, 10254–10261
Toward an Integrated Ceramic Micro-Membrane Network: Effect of Ethanol Reformate on Palladium Membranes Daejin Kim, David Donohue, Bhanu Kuncharam, Christine Duval, and Benjamin A. Wilhite* Department of Chemical, Materials and Biomolecular Engineering, and Center for Clean Energy Engineering, UniVersity of Connecticut, Storrs, Connecticut 06269-3222
Our research group has developed a cartridge-based, ceramic microchannel system capable of integrating multiple unique chemical processes within a single monolithic system, for rapid heat and mass transfer. In this manuscript, the authors report on the performance of palladium thin films incorporated within this ceramic microchannel system and their chemical compatibility with ethanol reforming chemistry. A dense, ∼9-µmthick palladium membrane for hydrogen purification from ethanol reformate was developed on a cordierite extruded ceramic support coated with successive alumina layers, and its compatibility was investigated via exposure to carbon dioxide, carbon monoxide, oxygen, water, and ethanol. The hydrogen permeability was determined to be 1.73 × 10-9 mol m-1 s-1 Pa-0.5 at 350 °C with an activation energy of 7.3 kJ mol-1 over the range 350-550 °C. Exposure to carbon dioxide and oxygen had no effect on hydrogen permeation, while carbon monoxide and water exposure resulted in a 12% and 14% decrease in hydrogen flux, which was fully recovered upon the removal of contaminants. Exposure to ethanol vapor caused a 41% drop in hydrogen flux, which was restored to 91% of the initial steady-state value upon ethanol removal, indicating an irreversible surface modification of the palladium film, in addition to competitive adsorption. The hydrogen/helium selectivity of the membrane remained in excess of 1000:1 throughout all exposure tests, verifying the suitability of this system for integrated hydrogen purification and ethanol reforming. 1. Introduction Shrinking fossil-fuel resources, rising environmental concerns, and a rapidly diversifying renewable-fuels portfolio have led to the emergence of hydrogen (H2) as a potential fuel of the future. Hydrogen can be produced on a large scale via the catalytic reforming of natural gas, coal, petroleum distillates, or renewable hydrocarbons, thus providing a universal energy currency from diverse fuel resources. This universal fuel can then be utilized over a broad range of scale in fuel cell systems at greater efficiency than competing internal combustion engines.1 Microreactors are a promising means to produce hydrogen both on a small scale for portable applications, as well as at high efficiency on larger scales, because of inherently high heat- and mass-transfer rates, compact design, and potentials for process intensification.2,3 Among the various hydrogen sources proposed for portable-power systems, such as methane,4 methanol,5-7 ethanol,8-10 ammonia11 and sodium borohydride,12 ethanol is a leading candidate, because it is environmentally friendly, relatively benign/safe, and allows utilization of renewable resources.13,14 Ethanol (C2H5OH) can be catalytically converted to hydrogen via three main processes: steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR).7,9,13,15 Effluents resulting from all three contain carbon monoxide (CO), because of the incomplete oxidation of carbon, as well as carbon dioxide (CO2), unreacted steam, and ethanol. Hydrogen yields can be further boosted by a subsequent water-gas shift (WGS) reaction, with the additional benefit of reducing CO content. The resulting hydrogen-rich reformate can then be purified using a permselective membrane to reduce CO levels to below ∼10 ppm for use in a proton exchange membrane (PEM) fuel cell.1 Hydrogen purification approaching infinite selectivities can be achieved using dense palladium membranes.16,17 These * author to whom correspondence should be addressed. Tel.: 860486-3689. Fax: 860-486-2959. E-mail:
[email protected].
palladium films can be deposited via electroless plating, electroplating, physical vapor deposition, sputtering, chemical vapor deposition, or spray pyrolysis.17 Hydrogen transport through the palladium membrane is accomplished in three steps: (i) dissociative adsorption of hydrogen molecules on the metal surface, (ii) diffusing atomic hydrogen through bulk metal, and (iii) associative desorbing hydrogen molecules from the metal surface.18,19 The hydrogen flux (FH2) through the palladium membranes is governed by eq 1: FH2 )
P p n - pH2,sn) t ( H2,f
(1)
where P is the permeability, t is the membrane thickness, and pH2,f and pH2,s are the respective partial pressures of hydrogen at the feed side and the sweep side. Assuming that the solidstate diffusion of hydrogen is rate-determining, and that H atoms form an ideal solution in the metal, the hydrogen pressure exponent (n) is equal to 0.5, which corresponds to Sieverts’ law.18 Deviations from Sieverts’ law (i.e., when n > 0.5) may be attributed to the accumulation of impurities on the membrane surface20 or pinholes or microcracks in the membranes,21 or it may be attributed to when porous supports generate substantial mass-transport resistance.22 Among these considerations, surface contamination plays a significant role in reducing H2 permeation flux through palladium membranes upon exposure to reforming chemistries.23 Contaminants such as hydrocarbons,24,25 carbon monoxide,26-28 sulfur,29,30 chlorine,30,31 ammonia,32 and water33-35 can inhibit the H2 dissociation and recombination reactions on the palladium membrane surface, thus decreasing hydrogen permeation. Hydrocarbons inhibit hydrogen permeation by either adsorbing on the surface or reacting to form carbonaceous layers at higher temperatures.24,25 The decrease of H2 permeation in the presence of CO is commonly attributed to competitive adsorption.26 CO has significant inhibitive effects on hydrogen permeation at
10.1021/ie100548b 2010 American Chemical Society Published on Web 06/28/2010
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010
temperatures of 400:1 800-1000:1 >1000:1 4500:1 ∞ 1200:1 ∞ >1000:1 n.r.a 1000:1 1000:1 n.r.a n.r.a
53 54 55 56 57 58 59 60 61 62 this work 63 64 65 36 66
n.r. ) not reported.
Figure 7. SEM images of the palladium membrane: (a) surface before exposure tests, (b) surface after exposure tests, (c) cross section before exposure tests, and (d) cross section after exposure tests.
H2O, with full recovery of permeability upon removal of contaminants. Ethanol exposure resulted in an ∼40% reduction of hydrogen permeation rates, including the irreversible adsorption of carbon and oxygen on the membrane surface, which was responsible for an irreversible 9% loss in permeability after ethanol was removed from the feed flow. Permeation data and preliminary SEM analysis of membranes before and after exposure indicate the need for a moredetailed materials stability analysis of the palladium membrane under light alcohol exposure. These findings provide the basis for further fundamental analysis aimed at (i) determining reversible adsorption coefficients for all species and (ii) understanding surface and bulk film mechanisms for irreversible permeation losses observed under ethanol exposure. This latter issue also indicates the need for corrosionmitigation strategies (e.g., composite catalytic-permselective membrane designs)6 to ensure optimal performance of the palladium thin films for hydrogen extraction from ethanol. Our current research efforts are aimed at addressing these challenges to enhance the long-term stability of palladium membranes under ethanol reforming conditions. Lastly, ongoing efforts are aimed at new external sealing and/or packaging methods, to reduce hydrogen losses. Acknowledgment
Figure 8. EDS analysis of the palladium membranes before and after exposure tests.
observed only in the case of ethanol exposure, which suggests that contamination of the membranes was primarily caused by the ethanol exposure. 4. Conclusions The presented work demonstrates the viability of using palladium thin films within the ceramic microchannel network for integrating hydrogen purification with multistage ethanol reforming to achieve compact, cartridge-based reformers. The hydrogen selectivity of the membrane was greater than 1000:1 under all conditions, and the hydrogen flux through the membrane decreased
