Ind. Eng. Chem. Res. 2007, 46, 6313-6319
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Experimental and Computational Prediction of the Hydrogen Transport Properties of Pd4S Bryan D. Morreale,* Bret H. Howard, Osemwengie Iyoha,† Robert M. Enick,† Chen Ling,‡ and David S. Sholl§ US DOE National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236; NETL Research Fellow, Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261; Department of Chemical Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213; and NETL Research Fellow, Department of Chemical Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213
Computational and experimental methods were used to quantify the apparent influence of a Pd4S corrosion product resulting from flux testing of 100-micron thick pure palladium membranes in a 0.1%H2S-10%HeH2 retentate gas mixture. The permeability of Pd4S was estimated to be approximately 20 times less than that of pure palladium from the results obtained through sulfide growth kinetics using gravimetric methods and the observed H2 flux decay during permeability characterization from 623 to 908 K. To complement experimental analysis, density functional theory was used to predict the hydrogen permeability of Pd4S by examining diffusivity and solubility of H in bulk Pd4S. Results are in good agreement between the experimental and computational prediction of the activation energy of permeation, while only in moderate agreement when comparing the hydrogen permeability of Pd4S. The permeability values obtained through experimentation were approximately 7 times greater than the computational predictions. Introduction Pd-based dense metal membranes are a well-established technology for separating and purifying hydrogen from mixedgas streams. Diffusion experiments with palladium began in the mid-1800s, with commercialization of Pd-based membranes being realized approximately a century later.1-10 The interest in Pd-based membranes is a result of palladium’s relatively high permeability compared to other metals and its excellent catalytic activity for dissociating molecular hydrogen. The latter property is important for producing atomic H that can permeate through a metal membrane via interstitial diffusion. Unfortunately, the cost and chemical instability of Pd limits the widespread application of Pd-based membranes. Efforts to circumvent these difficulties have focused on reducing the amount of Pd required in membrane fabrication through alloying with low-cost metals and developing methods to fabricate thinner membranes.11-14 There has been progress in the development of Pd alloys that have enhanced mechanical properties and provide reasonable hydrogen permeability values. The chemical stability of Pd in the presence of contaminants, however, remains problematic. One target application for the integration of membrane technologies has been the treatment of syngas streams from gasification processes. The gasification effluent stream typically contains hydrogen mixed with carbon dioxide, carbon monoxide, and steam, with trace amounts of hydrogen sulfide, ammonia, mercury, and other impurities. Unfortunately, trace gases such as hydrogen sulfide can have deleterious effects on the chemical and mechanical stability of Pd-based membranes.14-17 The impact of H2S on Pd-based membranes can be discussed in terms of two phenomena. First, sulfur can deactivate the membrane surface by adsorbing on surface sites and diminishing * To whom correspondence should be addressed. Tel.: (412) 3865929. E-mail:
[email protected]. † University of Pittsburgh. ‡ Carnegie Mellon University. § NETL Research Fellow, Carnegie Mellon University.
the catalytic activity of the membrane surface. Because catalytic H2 dissociation is a fundamental step in the overall transport mechanism of hydrogen through dense palladium, this phenomenon negatively impacts membrane performance. In this mechanism, sulfur only needs to be chemisorbed or physisorbed on the surface of the membrane. This phenomenon will be referred to as a “catalytic poisoning” mechanism. Second, the sulfurcontaining gas can chemically react with the membrane to produce a corrosion product on the surface. This corrosion product can exhibit characteristics that would limit the catalytic activity required for transport and/or reduce the overall performance by forming a layer with low permeability for hydrogen. Depending on the growth rate of the surface scale and exposure duration, the entire membrane can potentially be converted to the corrosion product. This phenomenon will be referred to as “corrosive decay”. The aim of this study was to use a combination of experimental and theoretical methods to gather quantitative information regarding the influence of corrosion products on the hydrogen permeability of palladium membranes. Experiments at NETL provide clear evidence that palladium membranes exposed to gas mixtures containing H2S for prolonged periods under certain conditions develop thick layers of Pd4S. Thus, when examining the effect of H2S, corrosive decay must be considered rather than simply assuming catalytic poisoning. Experimental studies provide information on the growth of Pd4S scale as a function of operating conditions and also provide an indirect means to estimate the permeability of hydrogen through Pd4S. To obtain an independent estimate of this permeability, first-principles density functional theory (DFT) calculations were applied to describe H solubility and transport through Pd4S. Experimental Section Membrane Fabrication, Testing, and Analysis. Palladium membranes were fabricated from a 100 micron thick, 99.99% purity foil purchased from Alfa Aesar. Once cut to the proper
10.1021/ie070461u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/11/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Figure 1. Schematic of the membrane assembly used in this study.
dimensions (∼19 mm disks) using surgical stainless steel scissors, the membrane foils were mounted utilizing Swagelok VCR fittings, illustrated in Figure 1. The mounting employed a porous Hastelloy support to reduce deformation of the membrane under the severe test conditions, an Anodisc membrane layer to reduce intermetallic diffusion between the membrane and the porous support, and a platinum washer to reduce intermetallic diffusion between the VCR fittings and the membrane. Additionally, a quartz liner and feed tube were employed in the feed gas assembly in an effort to reduce the interaction and consumption of the H2S in the feed gas by the stainless steel reactor materials. The characterization of the hydrogen flux of the palladium membrane in the presence and absence of H2S was conducted in NETL’s Hydrogen Membrane Test (HMT) unit, which has been described in detail previously.15,18 The mounted membrane assembly was placed in the HMT unit and heated under inert gas flows. Following performance verification in the presence of a 10%He-H2 gas mixture, testing proceeded by introducing a 0.1%H2S-10%He-H2 gas mixture to the membrane (in all permeability tests, an Ar sweep gas was used and H2 concentrations in the permeate were kept at
