Surface roughness improvement of Hastelloy X tubular filters for H2 selective supported Pd–Ag alloy membranes preparation

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Thin Pd–Ag layers have been successfully deposited on ceramic supports with controlled surface characteristics. The need for less fragile membranes and ease of sealing and connection leads to the study of metallic supports for thin Pd-based membrane development. Metallic supported membranes are prone to intermetallic diffusion issues so an interdiffusion barrier must be introduced. However, metallic supports with sufficient surface quality for direct membrane deposition are expensive and not readily available in the market. It is thus important to study how to improve surface roughness of commercially available rough metallic filters, in order to allow deposition of a smooth, delamination-free Pd–Ag layer.This work reports a first attempt towards a standardized preparation procedure for Pd-based membranes on cheap, rough, and unrefined Hastelloy X tubular filters. The focus is on surface roughness reduction, in order to allow the deposition of a smooth, uniform Pd–Ag selective layer. The surface roughness of the tubes is tuned via 1) polishing and 2) addition of a smoothening interdiffusion barrier layer based on a boehmite dip-coated dispersion. The polishing time was assessed by studying the average support's roughness variation, permeation behavior and ability to retain ceramic coating. It was found that the best trade-off between polishing extent and gas permeance of the support amounts to 6 h. Moreover, it was assessed that increasing the boehmite load in the interdiffusion barrier precursor solution leads to thicker layers and larger surface roughness reduction, but greater solution instability. 1,2%wt of boehmite load proved the best trade-off between layer reproducibility and support coverage. Different dipping-sintering routes were evaluated in order to improve surface's suitability for electroless plating: a single interdiffusion layer deposition route proved the most suitable for Pd–Ag deposition. The electroless plating performed onto the treated supports results in a continuous Pd–Ag layer, proving Pd–Ag deposition possible on the selected filters.