Pd-based composite membranes have received great attention from both academia and industry as they exhibit significant potential in several important fields, such as hydrogen purification and small-scale hydrogen generation as well as CO₂ capture. Thin and defect-free Pd-based membranes must have high hydrogen flux, strong thermal stability and resistance to poisoning species. This work critically reviews both the experimental and modeling studies of Pd-based composite membranes over the last few decades, particularly focusing on the critical issues of thermal and chemical stability, which are the main obstacles blocking their commercial applications.

Within the last decade, the application of gallium-based liquid metals in catalysis has received great attention from around the world. This article provides an overview concerning Ga-based liquid metals (LMs) in energy and environmental applications, such as the catalytic synthesis of ethylene by non-petroleum routes via Pd–Ga liquid catalysts, alkane dehydrogenation via Pd–Ga or Pt–Ga catalysts, CO₂ hydrogenation to methanol via Ni Ga or Pd/Ga₂O₃ catalysts, and catalytic degradation of CO₂ via EGaIn liquid metal catalysts below 500 °C, where Ga-based liquid metal catalysts exhibit high selectivity and low energy consumption. The formation of isolated metal sites in a liquid metal matrix allows the integration of several characteristics of multiphase catalysis (particularly the operational friendliness of product separation procedures) with those of homogeneous catalysis. In the end, this article sheds light on future prospects, opportunities, and challenges of liquid metal catalysis.

Pd membranes act in an important role in H2 purification and H2 production in membrane reactors. Pd-Ag alloy membranes fabricated by consecutive electroless- and electroplating process on alumina tubes exhibited good stability under stringent heating/cooling cycles at a ramp rate of 10 K/min, imitating practical fast initiation or emergency shutdown conditions. Bilayer Pd-Ag membranes can form dense and uniform alloy after thermal treatment for 24 h at 823 K under H2 atmosphere, despite a porous structure due to the development of liquid-like properties above Tamman temperature to enforce the migrativity. On the contrary, alloying under N2 atmosphere resulted in a Pd-enriched layer. This led to a lower H2 flux but superior thermal stability compared to that alloying under H2 atmosphere. The trilayer approach of electroless-plated Pd, electro-polated Ag and electroless-plated Pd is not suitable to achieve homogeneous Pd-Ag alloys, which, on the other hand, presented the occurrence of a small gap between top Pd layer and middle Ag layer, probably due to insufficient wetting during plating process. An on-site repair treatment in analogous to MOCVD (Metal-organic Chemical Vapor Deposition) process was first proposed to extend the lifetime of Pd-Ag membrane, i.e., by vaporizing, and subsequent decomposition of Ag(OOCC2F5) powders to “preferentially” block the pinholes under vacuum and at working temperature of ca. 473–673 K, which effectively reduced the N2 flux by 57.4% compared to the initial value. The H2 flux, however, declined by 16.7% due to carbon deposition on the membrane surface, which requires further investigation. This approach shows some potential for on-site repair without disassembly or cooling to room temperature.

For H₂ separation by Pd-based composite membranes, the pore mouth size distribution of the porous support immediately affects the quality of the deposited layer, including continuity and defect/pinhole formation. However, there is a lack of convenient and effective methods for characterization of pore mouth size of porous supports as well as of defect distribution of dense Pd-based composite membranes. Here we introduce a novel method by modifying conventional liquid–liquid displacement porometry. When the pore tunnels are filled with Liquid B and the outer surface is occupied by Liquid A, the reopening of the pore mouth depends on the pressure of Liquid B and the interfacial tension at the position of the pore mouth, from which the pore mouth size can be determined according to the Young–Laplace equation. Our experimental tests using this method with model samples show promising results, which are well supported by those obtained using FESEM (fild emission scanning electron microscope), AFM (atomic force microscope), and conventional liquid–liquid displacement porometry. This novel method can provide useful information for not only surface coatings on porous substrates but also for modification of dense membrane defects; thus, broad utilizations of this technique can be expected in future study.

Sulfur-tolerant Pd-based composite membranes were produced by coating a thin layer of MoS₂/γ-alumina on the membrane surface.

It is known that hydrogen embrittlement could result in warping and destruction of pure Pd membranes, which limits the working temperatures to be above 293 °C. This study attempted to investigate the relationship between hydrogen embrittlement resistance and membrane geometry of ultrathin pure Pd membranes of 2.7–6.3 μm thickness. Thin tubular Pd membranes with an o.d. of 4 mm, 6 mm and 12 mm immediately suffered from structural destruction when exposed to H₂ at room temperature. In contrast, thin hollow fiber membranes (outer diameter, 2 mm, thickness < 4 μm) exhibit strong resistance against hydrogen embrittlement at temperatures below 100 °C during repeated heating/cooling cycles at a rate up to 10 °C min⁻¹ under H₂ atmosphere. This is ascribed to reduced lattice strain gradients during α–β phase transition in cylindrical structures and lower residual stresses according to in situ XRD analysis, which shows a great prospect in low temperature applications.