Interlayer Properties of In-Situ Oxidized Porous Stainless Steel for Preparation of Composite Pd Membranes

Abstract

Abstract:Hydrogen is considered as a real alternative for improving the current energy scenario in the near future and separation processes are a crucial step for the economy of the process in both centralized and distributed production systems. In this context, Pd-based composite membranes appear as an attractive technology trying to reduce the Pd thickness by modifying the commercial supports, mainly formed by metals to fit properly in conventional industrial devices. In most cases, a final calcination step is required and hence, the metallic support can be oxidized. This work analyzes in detail the properties of intermediate layers generated by in-situ oxidation of tubular PSS supports as a crucial step for the preparation of Pd/PSS membranes. The oxidation temperature determines the modification of original morphology and permeability by increasing the presence of mixed iron-chromium oxides as temperature rises. A compromise solution need to be adopted in order to reduce the average pore mouth size and the external roughness, while maintaining a high permeation capacity. Temperature of 600 • C lets to reduce the average pore size from 3.5 to 2.1 µm or from 4.5 to 2.3 µm in case of using PSS supports with 0.1 or 0.2 µm porous media grades, respectively but maintaining a hydrogen permeation beyond targets of United States of America Department of Energy (US DOE). Lower temperatures provoke an insufficient surface modification, while greater values derive in a drastic reduction of permeability. In these conditions, two composite membranes were prepared by ELP-PP, obtaining 14.7 and 18.0 µm thick palladium layers in case of modifying PSS tubes of 0.1 or 0.2 µm media grades, respectively. In both cases, the composite Pd membranes exhibited a hydrogen perm-selectivity greater than 2000 with permeances ranged from 2.83 to 5.84·10 −4 mol m −2 s −1 Pa −0.5 and activation energies of around 13-14 kJ mol −1 .

Versatile and Resistant Electroless Pore-Plated Pd-Membranes for H2-Separation: Morphology and Performance of Internal Layers in PSS Tubes

“…However, it was still possible to find some pore mouths larger than 15 μm that could compromise the formation of a fully dense Pd-film of a few micrometers. These values are quite similar to those obtained on the external surface of analogous PSS supports after their calcination in air, and only represent a slight reduction compared with uncalcined ones [ 35 ]. Therefore, it can be stated that the generated oxide layer around each stainless steel particle was thin enough to avoid a drastic modification of the original surface morphology on the internal and external sides and further modifications are required to fabricate composite membranes with limited palladium thickness.…”

“…Then, clean supports were dried overnight at 110 °C, and their weight was taken as reference for the subsequent preparation steps. These supports were then calcined in air at 600 °C for 12 h with a heating rate of 1.0 °C/min to generate a first intermediate barrier between the PSS support and the Pd film made of Fe–Cr oxides coming from the steel [ 35 ].…”

Effective H2 Separation through Electroless Pore-Plated Pd Membranes Containing Graphite Lead Barriers

“…As can be seen, the calcined supports ( Figure 2 a,b) present a relatively high roughness with a wide pore-size distribution, at least in their pore mouths. This morphology is very similar to commercial substrates despite generating oxide around the stainless-steel particles by the thermal treatment of calcination [ 31 ]. However, this morphology noticeably changes after incorporating the graphite lead ( Figure 2 c,d).…”

“…After that, the general procedure followed for the preparation of the membranes consisted of five successive steps: (i) preliminary cleaning, (ii) support calcination (12 h at 600 °C), (iii) incorporation of graphite as an intermediate layer, (iv) seeding with homogeneously distributed Pd nuclei, and (v) deposition of a palladium film by electroless pore-plating (ELP-PP). Details about the first two general steps, in which the supports were cleaned and calcined, can be found in previous works [ 17 , 31 ].…”

“…In the last years, a modification of the traditional electroless plating alternative in which the main reactants are separated by the wall of the porous support during the entire deposition process has exhibited very promising results in terms of Pd-thickness reduction and mechanical stability [ 17 , 22 , 30 ]. This alternative, denoted as electroless pore-plating (ELP-PP), has been satisfactorily applied onto both raw ceramic and metallic porous substrates or modified ones with different intermediate layers such as mixed iron–chromium oxides [ 31 ], ceria [ 22 ], or SBA-15 [ 32 ].…”