Hydrogen selective palladium-alumina composite membranes prepared by Atomic Layer Deposition

Hydrogen Separation Membranes: A Material Perspective

The global energy market is shifting toward renewable, sustainable, and low-carbon hydrogen energy due to global environmental issues, such as rising carbon dioxide emissions, climate change, and global warming. Currently, a majority of hydrogen demands are achieved by steam methane reforming and other conventional processes, which, again, are very carbon-intensive methods, and the hydrogen produced by them needs to be purified prior to their application. Hence, researchers are continuously endeavoring to develop sustainable and efficient methods for hydrogen generation and purification. Membrane-based gas-separation technologies were proven to be more efficient than conventional technologies. This review explores the transition from conventional separation techniques, such as pressure swing adsorption and cryogenic distillation, to advanced membrane-based technologies with high selectivity and efficiency for hydrogen purification. Major emphasis is placed on various membrane materials and their corresponding membrane performance. First, we discuss various metal membranes, including dense, alloyed, and amorphous metal membranes, which exhibit high hydrogen solubility and selectivity. Further, various inorganic membranes, such as zeolites, silica, and CMSMs, are also discussed. Major emphasis is placed on the development of polymeric materials and membranes for the selective separation of hydrogen from CH4, CO2, and N2. In addition, cutting-edge mixed-matrix membranes are also delineated, which involve the incorporation of inorganic fillers to improve performance. This review provides a comprehensive overview of advancements in gas-separation membranes and membrane materials in terms of hydrogen selectivity, permeability, and durability in practical applications. By analyzing various conventional and advanced technologies, this review provides a comprehensive material perspective on hydrogen separation membranes, thereby endorsing hydrogen energy for a sustainable future.

Membrane Fabrication and Modification by Atomic Layer Deposition: Processes and Applications in Water Treatment and Gas Separation

Membrane-based separation processes are part of most water purification plants worldwide. Industrial separation applications, primarily water purification and gas separation, can be improved with novel membranes or modification to existing ones. Atomic layer deposition (ALD) is an emerging technique that is proposed to upgrade certain kinds of membranes independent of their chemistry and morphology. ALD deposits thin, defect-free, angstrom-scale, and uniform coating layers on a substrate's surface by reacting with gaseous precursors. The surface-modifying effects of ALD are described in the present review, followed by a description of various types of inorganic and organic barrier films and how these can be used in combination with ALD. The role of ALD in membrane fabrication and modification is categorized into different membrane-based groups according to the treated medium, i.e., water or gas. In all membrane types, the ALD-based direct deposition of inorganic materials, mainly metal oxides, on the membrane surface can improve antifouling, selectivity, permeability, and hydrophilicity. Therefore, the ALD technique can broaden the applications of membranes to the treatment of emerging contaminants in water and air. Finally, the advancement, limitations, and challenges of ALD-based membrane fabrication and modification are compared to provide a comprehensive guideline for developing next-generation membranes with improved filtration and separation performance.

Tunable TiO2-BN-Pd nanofibers by combining electrospinning and atomic layer deposition to enhance photodegradation of acetaminophen

The demand for fresh and clean water sources is increasing globally, and there is a need to develop novel routes to eliminate micropollutants and other harmful species from water. Photocatalysis is a promising alternative green technology that has shown great performance in the degradation of persistent pollutants. Titanium dioxide is the most used catalyst owing to its attractive physico-chemical properties, but this semiconductor presents limitations in the photocatalysis process due to the high band gap and the fast recombination of the photogenerated carriers. Herein, a novel photocatalyst has been developed, based on titanium dioxide nanofibers (TiO2 NFs) synthesized by electrospinning. The TiO2 NFs were coated by atomic layer deposition (ALD) to grow boron nitride (BN) and palladium (Pd) on their surface. The UV-Vis spectroscopy measurements confirmed the increase of the band gap and the extension of the spectral response to the visible range. The obtained TiO2/BN/Pd nanofibers were then tested for photocatalysis, and showed a drastic increase of acetaminophen (ACT) degradation (>90%), compared to only 20% degradation obtained with pure TiO2 after 4 h of visible light irradiation. The high photocatalytic activity was attributed to the good dispersion of Pd NPs on TiO2-BN nanofibers, leading to a higher transfer of photoexcited hole carriers and a decrease of photogenerated electron-charge recombination. To confirm its reusability, recycling tests on the hybrid photocatalyst TiO2/BN/Pd have been performed, showing a good stability over 5 cycles under UV and visible light. In addition, toxicity tests as well as quenching tests were carried out to check the toxicity of the byproducts formed and to determine active species responsible for the degradation. The results presented in this work demonstrate the potential of TiO2/BN/Pd nanomaterials, and open new prospects for the preparation of tunable photocatalysts.

Nanoparticles functionalized ceramic membranes: fabrication, surface modification, and performance

Membrane technologies are used intensively for desalination and wastewater treatment. Water filtration using ceramic membranes exhibited high performance compared with polymeric membranes due to various properties such as high resistance to fouling, permeability, rejection rate, and chemical stability. Recently, the performance of nanocomposite ceramic membranes was improved due to the development in nanotechnology. This article focusses on the development of porous ceramic membranes and nanomaterial functionalized ceramic membranes for water filtration applications. At the beginning, various fabrication methods of ceramic membranes were described, and the effect of surface modification techniques on the membrane intrinsic properties was reviewed. Then, the performance of nanoparticles functionalized ceramic membranes was evaluated in terms of physicochemical properties, rejection rate, and water permeability. This work can help new entrants and established researchers to become familiar with the current challenges and developments of nanoparticle-incorporated ceramic membranes for water filtration applications.

Palladium/Carbon Nanofibers by Combining Atomic Layer Deposition and Electrospinning for Organic Pollutant Degradation

As organic dyes are a major source of pollution, it is important to develop novel and efficient heterogeneous catalysts with high activity for their degradation. In this work, two innovative techniques, atomic layer deposition and electrospinning, were used to prepare palladium nanoparticles (Pd NPs) supported on carbon nanofibers (CNFs). The sample morphology was investigated using scanning and transmission electron microscopy. This showed the presence of nanofibers of several micrometers in length and with a mean diameter of 200 nm. Moreover, the size of the highly dispersed Pd NPs was about 7 nm. X-ray photoelectron spectroscopy visually validated the inclusion of metallic Pd. The prepared nano-catalysts were then used to reduce methyl orange (MO) in the presence of sodium borohydride (NaBH₄). The Freundlich isotherm model was the most suitable model to explain the adsorption equilibrium for MO onto the Pd/CNF catalysts. Using 5 mL MO dye-solution (0.0305 mM) and 1 mL NaBH₄ (0.026 mM), a 98.9% of catalytic activity was achieved in 240 min by 0.01 g of the prepared nano-catalysts Pd/C (0.016 M). Finally, no loss of catalytic activity was observed when such catalysts were used again. These results represent a promising avenue for the degradation of organic pollutants and for heterogeneous catalysis.