Tight-binding study of hydrogen adsorption on palladium decorated graphene and carbon nanotubes

In this work we report a theoretical study on the atomic and molecular hydrogen adsorption onto Pd-decorated graphene monolayer and carbon nanotubes by a semi-empirical tight-binding method. We first investigated the preferential adsorption geometry, considering different adsorption sites on the carbon surface, and then studied the evolution of the chemical bonding by evaluation of the overlap population (OP) and crystal orbital overlap population (COOP). Our results show that strong C–Pd and H–Pd bonds are formed during atomic hydrogen adsorption, with an important role in the bonding of C 2p_z and Pd 5s, 5p_z and 4d_z² orbitals. The hydrogen storage mechanism in Pd-doped carbon-based materials seems to involve the dissociation of H₂ molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface.     

In situ deposition of Pd nanoparticles with controllable diameters in hollow carbon spheres for hydrogen storage

A novel in situ synthesis of Pd nanoparticles supported in hollow carbon spheres (HCS) is reported. The size of the nanoparticles can be tuned via application of different Pd precursors. The hydrogen storage properties of Pd supported in HCS under room temperature were examined at partial pressures. We observed significant difference between the storage capacities of two samples containing Pd nanoparticles with different diameter distributions. The results showed that the sample with suitable diameters of Pd nanoparticles was more favorable for the H₂ storage, even lower mass of Pd was used. The maximum hydrogen storage of 0.36 wt % exhibited the sample with Pd nanoparticles with the diameter of 11 nm (measured at 298 K and 24 bar) and it was enhanced by the factor of two in respect to the pristine HCS. The enhanced storage capacity is due to cumulative hydrogen adsorption by HCS and Pd nanoparticles. We also propose the mechanism of hydrogen storage in our material.     

Performance and economics of a Pd-based planar WGS membrane reactor for coal gasification

Conceptual 300 tonne per day (tpd) H₂-from-coal plants have been the subject of several major costing exercises in the past decade. Incorporating conventional high- and low-temperature water-gas-shift (WGS) reactors, amine-based CO₂ removal and PSA-based H₂ purification systems, these studies provide a benchmark against which alternative H₂-from-coal technologies can be compared. The catalytic membrane reactor (CMR), combining a WGS catalyst and hydrogen-selective metal membrane, can potentially replace the multiple shift and separation stages of a plant based on conventional technology. CMR-based shift and separation offers several major advantages over the conventional approach, including greater-than-equilibrium WGS conversion, the containment of the CO₂ at high-pressure and a reduction in the number of unit processes.     

Room-temperature optical detection of hydrogen gas using palladium nano-islands

Palladium thin films of different thicknesses have been processed by oxidation and subsequent reduction in hydrogen atmosphere. Hydrogen optical sensing properties of as-deposited (Pd) and processed (r-Pd) samples have been experimentally tested with a H₂ concentration of 5% and 1%, discovering that the reduced films show improved performances in term of response/recovery time. As reveled by SEM images, the oxidation/reduction process modifies the surface appearance, which assumes a nano-islands structured morphology. The porosity of the processed films may explain the reduction of the response/recovery time, while the larger effective sensing surface in thinner samples justifies the responsivity performances.     

Hydrogen adsorption on palladium dimer decorated graphene: A bonding study

We report a density-functional theory study of dihydrogen adsorption on a graphene sheet functionalized with palladium dimers considering different adsorption sites on the carbon surface and both molecular and dissociative Pd₂H₂ coordination structures. Our results show that a (PdH)₂ ring without an H–H bond and not dissociative Pd₂(H₂) complexes are stable adsorbed systems with more elongated Pd−Pd and Pd–H bonds compared to the unsupported configurations caused by C–Pd interactions. In contrast, individual Pd atoms supported on graphene react with H₂ to form only a Pd(H₂) complex with a relaxed but not dissociated H–H bond. We also performed the Mulliken analysis to study the bonding mechanism during the adsorption process. In most cases, we found donor-acceptor C−Pd and Pd−H interactions in which C 2p, Pd 5s, and H 1s orbitals played an important role. We also found that the adsorption of a second Pd atom close to a PdH₂ system destabilizes the H−H bond. In this work we contribute to shed more light on the relation between Pd clustering and the possibility of hydrogen storage in graphene-based materials.     

Dynamics of hydrogen permeation across metallic membranes

Thin and supported palladium membranes can be coupled to gas reformers to produce purified hydrogen. Such systems can potentially be used in the automotive industry to feed PEM fuel cells. However, for such applications, the membrane design must be optimized to meet some specific requirements, in particular to allow fast accelerations. The purpose of this paper is to take advantage of the possibility offered by pneumato-chemical impedance spectroscopy to analyze the dynamics of hydrogen permeation in transient conditions of flow, to determine the conditions for which shifts in rate-determining step (rds) between surface and bulk rate contributions are observed. Results reported in this paper have been obtained using a Pd₇₇Ag₂₃ metallic membrane. The gas-phase impedance of this 50 μm thick membrane has been measured. A model has been developed to evaluate separately surface and bulk rate contributions. It is shown that in a typical permeation experiment performed in transient conditions of flow, the surface step is rate-determining in the early stages of the experiment (highly transient conditions of flow) whereas the bulk diffusion step becomes rate-determining at longer time (quasi-stationary conditions of flow). The relationship between membrane characteristics, experimental conditions and the time at which the shift in rds is observed are determined, opening the way to the development of customized membranes for operation in transient conditions of flow.     

Experimental and theoretical studies of hydrogen generation by binary metal (oxide)-graphene oxide composite materials

Theoretical and experimental studies with focuse on hydrogen generation through the hydrolysis of graphene oxide (GO) functionalized with magnesium (GOMg), titanium (GOTi), and niobium (GONb) were performed. Thermogravimetric (TGA) results reveal variable thermal decomposition profiles for the composites, in agreement with the decomposition of the labile oxygenated groups of GO. X-ray photoelectron spectroscopy (XPS) results show variable oxygen content (%) for the composites and the formation of MgO, TiO₂, and NbO₂ layers on the GO surface. The hydrogen generation studies suggest that the formation of a Mg(OH)₂ layer on the GO surface is a critical limiting factor for the hydrolysis of GOMg, whereas TiO₂ and NbO₂ catalyze the hydrolysis of GONb and GOTi. The hydrogen generation results reveal that GONb produced the highest H₂ yield of 2 L in 2 h compared to 1.5 L for GOTi and 1.3 L for GOMg. The results support the claim that the hydrolysis of GO functionalized with niobium and titanium are promising candidates for on-board H₂ generation applications.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

CFD simulation of Pd-based membrane reformer when thermally coupled within a fuel cell micro-CHP system

In this work, a bi-dimensional CFD simulation investigates a fuel processor for hydrogen production from natural gas or biogas composed by a steam methane reformer coupled with a palladium-based hydrogen permeable membrane, the so-called “membrane reformer” (MREF). The heat required for the endothermic reforming reaction taking place on the MREF is supplied by a stream of hot gas coming from an external source, typically represented by a combustor burning the unconverted fuel and the unpermeated hydrogen. The resulting fuel processor arrangement, which has already been simulated by the point of view of energy and mass balances, may achieve a very high efficiency and is particularly suited for integration with fuel cells. The interest on this configuration relies on the possibility to implement this technology within a PEMFC-based micro-cogenerator (also micro-Combined Heat and Power, or m-CHP) with a net electrical power output in a range of 1–2 kW. In particular, the work focuses on the temperature profiles along the membrane, which should be kept as close as possible to 600 °C to favourite permeation and avoid any damages, and examines the advantages of hot gas on co-current direction vs. counter-current with respect to the reformer flux direction.     

Tube-wall catalytic membrane reactor for hydrogen production by low-temperature ammonia decomposition

Ammonia has attracted great interest as a chemical hydrogen carrier. However, ammonia decomposition is limited kinetically rather than thermodynamically below 400 °C. We developed a tube-wall catalytic membrane reactor that could decompose ammonia with high conversion even at temperatures below 400 °C. The reactor had excellent heat transfer characteristics, and thus nearly 100% conversion for an NH3 feed of 10 mL/min at 375 °C was achieved with a 2-μm-thick palladium composite membrane, and hydrogen removal from the decomposition side resulted in a large kinetic acceleration.