Hydrogen is often regarded as an ideal energy carrier. Its use in energy conversion devices does in fact not produce any pollutants. However, due to challenges related to its transportation and storage, liquid hydrogen carriers are being investigated. Among the liquid hydrogen carriers, ammonia is considered very promising because it is easy to store and transport, and its conversion to hydrogen has only nitrogen as a byproduct. This work focuses on a review of the latest results of studies dealing with ammonia decomposition for hydrogen production. After a general introduction to the topic, this review specifically focuses on works presenting results of membrane reactors for ammonia decomposition, particularly describing the different reactor configurations and operating conditions, membrane properties, catalysts, and purification steps that are required to achieve pure hydrogen for fuel cell applications.

Energy need is predicted to increase by 48% in the next 30 years. Global warming resulting from the continuously increasing atmospheric CO₂ concentration is becoming a serious and pressing issue that needs to be controlled. CO₂ capture and storage/use (CCS/CCU) provide a promising route to mitigate the environmental consequences of CO₂ emission from fossil fuel combustion. In recent years, hollow fiber membrane contactors are regarded as an advanced technique with several competitive advantages over conventional technologies such as easy scale-up, independent control of flow rates, more operational flexibility, absence of flooding and foaming as well as high interfacial area per unit volume. However, many factors such as the membrane material selection, proper choice of solvent, and membrane module design are critical to success. In this regard, this paper aims at covering all areas related to hollow fiber membranes, including membrane material, membrane modification, membrane surface modification, shape, solvent characterization, operating parameters and costs, hybrid process, membrane lifetime, and energy consumption as well as commercially available systems. Current progress, future potential, and development of pilot-scale applications of this strategy are also assessed carefully. Furthermore, pore wetting as the main technical challenge in membrane contactor industrial implementation for post- and pre-combustion CO₂ capture processes is investigated in detail.

The use of ammonia as a hydrogen carrier requires efficient cracking technology. A promising solution is the use of a membrane reactor (MR), which enables both ammonia decomposition and hydrogen separation to take place within the same device, providing advantages in terms of efficiency and compactness compared to conventional systems. The literature reports that ceramic-supported double-skinned Pd-Ag membranes show outstanding performance for hydrogen separation as well as good stability of the separation layer during ammonia decomposition. However, their sealing in the reactor may result in leakage increase, while their mechanical stability remains an unresolved issue. To circumvent these limitations, the use of metallic supported Pd-based membranes is recommended, due to their higher mechanical stability and ease of sealing and integration in the reactor. In this work, we propose the development of robust metallic supported hydrogen-selective membranes for integration in membrane reactors for ammonia cracking. A conventional Pd-Ag membrane was prepared on a low-cost porous Hastelloy X tubular filter, modified with α-Al₂O₃/γ-Al₂O₃ to reach the desired surface quality. The membrane was then tested for ammonia decomposition in a MR configuration, showing the ability to reach >99% NH₃ conversion above 475 °C with H₂ feed recovery >60%. The results achieved pave the way towards a possible substitute for the ceramic-supported alternatives.

In this work, the fluidized bed membrane reactor (FBMR) technology for the direct dehydrogenation of propane (PDH) was demonstrated at a laboratory scale. Double-skinned PdAg membranes were used to selectively remove H₂ during dehydrogenation tests over PtSnK/Al₂O₃ catalyst under fluidization. The performance of the fluidized bed membrane reactor was experimentally investigated and compared with the conventional fluidized bed reactor (FBR) by varying the superficial gas velocity over the minimum fluidization velocity under fixed operating conditions (i.e., 500 °C, 2 bar and feed composition of 30vol% C₃H₈-70vol% N₂). The results obtained in this work confirmed the potential for improving the PDH performance using the FBMR system. An increase in the initial propane conversion of c.a. 20% was observed, going from 19.5% in the FBR to almost 25% in the FBMR. The hydrogen recovery factor displayed a decrease from 70% to values below 50%, due to the membrane coking under alkene exposure. Despites this, the hydrogen extraction from the reaction environment shifted the thermodynamic equilibrium of the dehydrogenation reaction and achieved an average increase of 43% in propylene yields.

H₂ production via membrane-assisted steam methane reforming (MA-SMR) can ensure higher energy efficiency and lower emissions compared to conventional reforming processes (SMR). Ceramic-supported Pd–Ag membranes have been extensively investigated for membrane-assisted steam methane reforming applications, with outstanding performance. However, costs, sealings for integration in the reactor structure, and resistance to solicitations remain challenging issues. In this work, the surface quality of a low-cost, porous Hastelloy-X filter is improved by asymmetric filling with α-Al₂O₃ of decreasing size and deposition of γ-Al₂O₃ as an interdiffusion barrier. On the modified support, a thin Pd–Ag layer was deposited via electroless plating (ELP), resulting in a membrane with H₂/N₂ selectivity >10,000. The permeation characteristics of the membrane were studied, followed by testing for membrane-assisted methane steam reforming. The results showed the ability of the membrane reactor to overcome thermodynamic conversion of the conventional process for all explored operating conditions, as well as ensuring 99.3% H₂ purity in the permeate stream at 500 °C and 4 bar.