New Insight to the Effects of Heat Treatment in Air on the Permeation Properties of Thin Pd77%Ag23% Membranes

High-Performance Hydrogen-Selective Pd-Ag Membranes Modified with Pd-Pt Nanoparticles for Use in Steam Reforming Membrane Reactors

A unique method for synthesizing a surface modifier for metallic hydrogen permeable membranes based on non-classic bimetallic pentagonally structured Pd-Pt nanoparticles was developed. It was found that nanoparticles had unique hollow structures. This significantly reduced the cost of their production due to the economical use of metal. According to the results of electrochemical studies, a synthesized bimetallic Pd-Pt/Pd-Ag modifier showed excellent catalytic activity (up to 60.72 mA cm-2), long-term stability, and resistance to COads poisoning in the alkaline oxidation reaction of methanol. The membrane with the pentagonally structured Pd-Pt/Pd-Ag modifier showed the highest hydrogen permeation flux density, up to 27.3 mmol s-1 m-2. The obtained hydrogen flux density was two times higher than that for membranes with a classic Pdblack/Pd-Ag modifier and an order of magnitude higher than that for an unmodified membrane. Since the rate of transcrystalline hydrogen transfer through a membrane increased, while the speed of transfer through defects remained unchanged, a one and a half times rise in selectivity of the developed Pd-Pt/Pd-Ag membranes was recorded, and it amounted to 3514. The achieved results were due to both the synergistic effect of the combination of Pd and Pt metals in the modifier composition and the large number of available catalytically active centers, which were present as a result of non-classic morphology with high-index facets. The specific faceting, defect structure, and unusual properties provide great opportunities for the application of nanoparticles in the areas of membrane reactors, electrocatalysis, and the petrochemical and hydrogen industries.

Hydrogen Permeability of Composite Pd-Au/Pd-Cu Membranes and Methods for Their Preparation

Thin Pd-40%Cu films were obtained via the classical melting and rolling method, magnetron sputtering, and modified with nanostructured functional coatings to intensify the process of hydrogen transportation. The films were modified by electrodeposition, according to the classical method of obtaining palladium black and "Pd-Au nanoflowers" with spherical and pentagonal particles, respectively. The experiment results demonstrated the highest catalytic activity (89.47 mA cm^-2), good resistance to CO poisoning and long-term stability of Pd-40%Cu films with a pentagonal structured coating. The investigation of the developed membranes in the hydrogen transport processes in the temperature range of 25-300 °C also demonstrated high and stable fluxes of up to 475.28 mmol s^-1 m^-2 (deposited membranes) and 59.41 mmol s^-1 m^-2 (dense metal membranes), which were up to 1.5 higher, compared with membrane materials with classic niello. For all-metal modified membranes, the increase in flux was up to sevenfold, compared with a smooth membrane made of pure palladium, and for deposited films, this difference was manyfold. The membrane materials' selectivity was also high, up to 4419. The developed strategy for modifying membrane materials with functional coatings of a fundamentally new complex geometry can shed new light on the development and fabrication of durable and highly selective palladium-based membranes for gas steam reformers.

Experimental and Numerical Analysis of a Pd-Ag Membrane Unit for Hydrogen Isotope Recovery in a Solid Blanket

The interest of the fusion community in Pd-Ag membranes has grown in the last decades due to the high value of hydrogen permeability and the possibility of continuous operation, making it a promising technology when a gaseous stream of hydrogen isotopes must be recovered and separated from other impurities. This is the case of the Tritium Conditioning System (TCS) of the European fusion power plant demonstrator, called DEMO. This paper presents an experimental and numerical activity aimed at (i) assessing the Pd-Ag permeator performance under TCS-relevant conditions, (ii) validating a numerical tool for scale-up purposes, and (iii) carrying out a preliminary design of a TCS based on Pd-Ag membranes. Experiments were performed by feeding the membrane with a He-H₂ gas mixture in a specific feed flow rate ranging from 85.4 to 427.2 mol h^-1 m^-2. A satisfactory agreement between experiments and simulations was obtained over a wide range of compositions, showing a root mean squared relative error of 2.3%. The experiments also recognized the Pd-Ag permeator as a promising technology for the DEMO TCS under the identified conditions. The scale-up procedure ended with a preliminary sizing of the system, relying on multi-tube permeators with an overall number ranging between 150 and 80 membranes in lengths of 500 and 1000 mm each.

Gas-Transport Characteristics of PdCu–Nb–PdCu Membranes Modified with Nanostructured Palladium Coating

A method for obtaining composite gas-diffusion PdCu–Nb–PdCu membranes modified with a nanostructured crystalline coating was developed to increase the performance of Nb-based membranes. A modifying functional layer with a controlled size and composition was synthesized by electrochemical deposition, which made it possible to determine a certain geometric shape for palladium nanocrystallites. Developed PdCu–Nb–PdCu membranes have demonstrated flux values up to 0.232 mmol s⁻¹ m⁻² in the processes of diffusion purification of hydrogen at 400 °C. A very significant difference in the hydrogen fluxes through the modified and non-modified composite PdCu–Nb–PdCu membranes reached 1.73 times at the lower threshold temperature of 300 °C. Cu doping of protective layer did not affect the selective properties of the membranes, which was confirmed by the obtained high selectivity values up to 1323, and made it possible to reduce the noble metal content. The research data indicate that the modification of the membrane surface significantly accelerates the hydrogen transfer process at sufficiently low temperatures due to the acceleration of dissociative–associative processes on the surface. The reported approach demonstrates new possibilities for creating productive and cost-efficient membranes based on niobium.

Synthesis, Electrocatalytic and Gas Transport Characteristics of Pentagonally Structured Star-Shaped Nanocrystallites of Pd-Ag

The method of synthesis of bimetallic Pd-Ag pentagonally structured catalyst "nanostar" on the surface of Pd-23%Ag alloy films has been developed. The resulting catalyst was studied as a highly active functional layer for methanol oxidation reaction (MOR) in alkaline media and the intensification of hydrogen transport through the Pd-23%Ag membrane in the processes of hydrogen diffusion purification. A modifying layer with a controlled size, composition and excellent electrocatalytic activity was synthesized by electrochemical deposition at a reduced current density compared to classical methods. The low deposition rate affects the formation of pentagonally structured nanocrystallites, allowing Pd and Ag particles to form well-defined structures due to the properties of the surfactant used. Electrochemical studies have demonstrated that the catalyst synthesized by the "nanostar" method shows better electrocatalytic activity in relation to MOR and demonstrates a higher peak current (up to 17.82 µA cm^-2) in comparison with one for the catalyst synthesized by the "nanoparticle" method (up to 10.66 µA cm^-2) in a cyclic voltammetric study. The nanostar catalyst electrode releases the highest current density (0.25 µA cm^-2) for MOR and demonstrates higher catalytic activity for the oxidation of possible intermediates such as sodium formate in MOR. In the processes of diffusion membrane purification of hydrogen, a multiple increase in the density of the penetrating flux of hydrogen through the membranes modified by the "nanostar" catalyst (up to 10.6 mmol s^-1 m^-2) was demonstrated in comparison with the membranes modified by the "nanoparticles" method (up to 4.49 mmol s^-1 m^-2). Research data may indicate that the properties of the developed pentagonally structured catalyst "nanostar" and its enhanced activity with respect to reactions involving hydrogen increase the desorption activity of the membrane, which ultimately accelerates the overall stepwise transfer of hydrogen across the membrane.

A Review for Consistent Analysis of Hydrogen Permeability through Dense Metallic Membranes

The hydrogen permeation coefficient (ϕ) is generally used as a measure to show hydrogen permeation ability through dense metallic membranes, which is the product of the Fick’s diffusion coefficient (D) and the Sieverts’ solubility constant (K). However, the hydrogen permeability of metal membranes cannot be analyzed consistently with this conventional description. In this paper, various methods for consistent analysis of hydrogen permeability are reviewed. The derivations of the descriptions are explained in detail and four applications of the consistent descriptions of hydrogen permeability are introduced: (1) prediction of hydrogen flux under given conditions, (2) comparability of hydrogen permeability, (3) understanding of the anomalous temperature dependence of hydrogen permeability of Pd-Ag alloy membrane, and (4) design of alloy composition of non-Pd-based alloy membranes to satisfy both high hydrogen permeability together with strong resistance to hydrogen embrittlement.

Pd3Ag(111) as a Model System for Hydrogen Separation Membranes: Combined Effects of CO Adsorption and Surface Termination on the Activation of Molecular Hydrogen

The co-adsorption of hydrogen and carbon monoxide on Pd3Ag(111) alloy surfaces has been studied as a model system for Pd-Ag alloys in membrane and catalysis applications using periodic density functional theory calculations (PW91-GGA). We explored the effects of Pd–Ag surface composition, since segregation of silver towards and away from the surface has been suggested to explain the experimentally observed changes in H2 activation, CO inhibition and reactivity. We found that CO pre-adsorbed on the surface weakens the adsorption of H on Pd3Ag(111) alloy surfaces irrespective of whether the surface termination corresponds to the bulk Pd3Ag composition, or is purely Pd-terminated. A higher coverage of H with CO present is obtained for the Pd-terminated surface; this surface also exhibits a larger range of chemical potentials for co-adsorbed hydrogen and CO. The barrier for H2 activation increases with increasing CO coverage, but the surface composition has the largest impact on H2 activation at intermediate CO coverage. The results imply that Pd-based membranes with typically ~ 23 wt% Ag are less prone to CO poisoning if the surface becomes Pd-terminated.

Pd-Based Membranes: Overview and Perspectives

Palladium (Pd)-based membranes have received a lot of attention from both academia and industry thanks to their ability to selectively separate hydrogen from gas streams. Integration of such membranes with appropriate catalysts in membrane reactors allows for hydrogen production with CO₂ capture that can be applied in smaller bioenergy or combined heat and power (CHP) plants, as well as in large-scale power plants. Pd-based membranes are, therefore, regarded as a Key Enabling Technology (KET) to facilitate the transition towards a knowledge-based, low carbon and resource-efficient economy. This Special Issue of the journal Membranes on “Pd-based Membranes: Overview and Perspectives” contains nine peer-reviewed articles. Topics include manufacturing techniques, understanding of material phenomena, module and reactor design, novel applications, and demonstration efforts and industrial exploitation.