Palladium-Alloy Membrane Reactors for Fuel Reforming and Hydrogen Production: A Review

Abstract

Hydrogen has a potential to be a clean energy carrier that emits only water after combustion and can be produced from diverse feedstocks. Hydrogen has much better combustion characteristics in conventional combustion systems and higher energy efficiency when used with fuel cells. More than 75 million tons of hydrogen are currently produced primarily using fossil fuels as feedstock via steam methane reforming processes. Steam methane reforming is the mature technology for producing hydrogen and when coupled with CO2 capture can help address climate challenges. Inorganic palladium (Pd) membranes have demonstrated great potential to separate hydrogen due to their stability and high selectivity for hydrogen. In this review, several methods of fabricating Pd-alloy membranes are discussed and compared in terms of membrane stability and selectivity of hydrogen. Such methods include electroless plating (ELP), chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating deposition (EPD). The permeability of hydrogen in different Pd-based alloy membranes are presented and compared. Focus has been made, in this review, on Pd–Ag, Pd–Cu, Pd–Au, and Pd–Ru alloys. The effects of impurities (H2S, CO, O2, and CO2) on performance of different Pd-based alloy membranes are also investigated. Moreover, the subject of using Pd-membrane reactors for fuel reforming and H2 production is investigated in detail based on numerous experimental and numerical studies in the literature, considering different membrane reactor designs: axial-flow tubular, radial-flow tubular, axial-flow spherical, packed-bed, fluidized bed, and slurry bubble column. The performance of Pd-membranes in such reactors for hydrogen production is compared, and the effects of temperature, pressure, H2O/CH4 ratio, and residence time on reformer performance are also investigated. Finally, the use of computational methods, particularly, density functional theory (DFT), to complement well-established experimental methods for studying the diffusion of H and its isotopes in different metals is reviewed. The review concludes with some insights into future work to bring Pd-membrane reactors to the level required for hydrogen production at the commercial level.

Recent Advances in Catalysts and Membranes for MCH Dehydrogenation: A Mini Review

“…Dense ceramic oxides materials with high proton and electronic conductivity (mostly perovskite-type oxides but also pyrochlorates, niobates, tantalates, and tungstates with high proton and electronic conductivities) are also capable of providing high purity hydrogen separation, but they are still limited to lab scale use and in only few applications. Recent review papers reported advances in the fabrication of various types of membranes for the separation of hydrogen and other gases [37,38,[63][64][65][66][67][68].…”

An Enhanced Sherwood Number to Model the Hydrogen Transport in Membrane Steam Reformers

“…When the membrane permeability is sufficiently low as to become the limiting factor, no radial concentration gradients are established within the packed bed, and a traditional 1D model is sufficient. It should be noted that this scenario is becoming of increasing interest as membranes characterized by significantly high hydrogen permeability are being developed [18].…”

On the Potential of Gallium- and Indium-Based Liquid Metal Membranes for Hydrogen Separation

“…The relatively small size of the interstitial sites through which the permeating species should diffuse, along with favourable activity for the dissociation of hydrogen compared to other molecules enables this high selectivity for hydrogen [1,2]. Many alternative metals have been proposed, but palladium continues to be the metal of choice in this regard, as its catalytic activity and observed hydrogen flux have not been matched by other metals [3,4]. The alloying of palladium with metals such as gold, silver and copper has been studied in detail as this allows for an increase in permeability (in case of silver at specific concentrations), stability and chemical resistance of the membrane (in case of copper and gold), increasing the potential for the application of membrane separators in cost-effective production of ultra-pure hydrogen [5][6][7].…”

“…The influence of crystal geometry on diffusion can be expressed through the parameter γ D . Values of 4 6 and 1 12 , are selected for gallium and indium, respectively, [28,40]. As for the diffusive jump distance, this parameter is related to the lattice constant, which is in turn calculated from the temperature-dependent density.…”