Carbon Molecular Sieve Membrane Reactors for Ammonia Cracking

The utilization of ammonia for hydrogen storage relies on the implementation of efficient decomposition techniques, and the membrane reactor, which allows simultaneous ammonia decomposition and hydrogen recovery, can be regarded as a promising technology. While Pd-based membranes show the highest performance for hydrogen separation, their applicability for NH3-sensitive applications, such as proton exchange membrane (PEM) fuel cells, demands relatively thick, and therefore expensive, membranes to meet the purity targets for hydrogen. To address this challenge, this study proposes a solution involving the utilization of a downstream hydrogen purification unit to remove residual ammonia, thereby enabling the use of less selective, therefore more cost-effective, membranes. Specifically, a carbon molecular sieve membrane was prepared on a tubular porous alumina support and tested for ammonia decomposition in a membrane reaction setup. Operating at 5 bar and temperatures ranging from 450 to 500 °C, NH3 conversion rates exceeding 90% were achieved, with conversion approaching thermodynamic equilibrium at temperatures above 475 °C. Simultaneously, the carbon membrane facilitated the recovery of hydrogen from ammonia, yielding recoveries of 8.2–9.8%. While the hydrogen produced at the permeate side of the reactor failed to meet the purity requirements for PEM fuel cell applications, the implementation of a downstream hydrogen purification unit comprising a fixed bed of zeolite 13X enabled the production of fuel cell-grade hydrogen. Despite performance far from being comparable with the ones achieved in the literature with Pd-based membranes, this study underscores the viability of carbon membranes for fuel cell-grade hydrogen production, showcasing their competitiveness in the field

Plasma assisted non-oxidative methane coupling over Ni-Fe mixed metal oxides

In this work, Ni-Fe mixed metal oxides with different Fe/Ni molar ratios of 0.33, 1 and 3 were synthesized by hydrothermal method at a constant pH. The obtained catalysts were characterized via XRD, TEM, ICP-OES, XPS and nitrogen physisorption. The catalysts were tested at room temperature for plasma-assisted non-oxidative coupling of methane with a mixture of 20 vol% CH4 in Ar in a planar DBD reactor with an electrode gap of 1.5 mm. The Fe3Ni showed the highest methane conversion of 7.4% with a 26% selectivity towards ethylene and an energy consumption as low as 0.55 MJ·molC2H4-1. The XPS spectra of Fe3Ni showed a surface enrichment in Ni3+ and Fe2+ promoting ethylene desorption rate as compared with the other two synthesized catalysts. The CH4 conversion increases up to 5 times with decreasing the CH4/Ar molar ratio in the feed at constant flow rate of 100 ml∙min-1. All the catalysts showed a similar relative decrease of conversion and increase of energy consumption. Negligible carbon deposition rate was observed and the total selectivity to C2 and C3 hydrocarbons was close to 100%, indicating the advantages of the catalytic formulation proposed for methane activation in plasma environment.</p

Comparison of Pt/C electrocatalyst deposition methods for PEM fuel cells

In recent years, in order to solve the environmental problems associated with electricity production, alternative systems to traditional methods have been developed. In this contest, fuel cells are widely considered to be an efficient and non-polluting power source and therefore, they are considered to be promising energy devices for the transport, mobile, and stationary sectors. At the current stage of technology, among different types of developed fuel cells, proton exchange membrane fuel cells (PEM) deserve major attention. Pt/C deposited on Nafion membrane is the most used electrocatalyst for PEM due to the highest oxygen reduction catalytic activity. The most used conventional deposition methods on Nafion polymeric membranes for commercial PEM are screen printing and transfer printing. In order to improve the specific activity of Pt, alternative and cheap techniques are under investigation. Therefore, in this work the performances of Pt/C samples deposited by dipping method and spray coating method have been compared, using a commercial system, as reference. Experimental tests were performed using a commercial Pt/C catalyst (20 wt. % Pt on Carbon Vulcan XC-72) deposited on Nafion membrane and tested in the 100 cm2single-cell testing kits. At the same cell voltage (0.3 V) and Pt loading (0.12 mgPt cm-2), the results showed that the spray coating provided both a higher current density and a higher maximum power density (90 mW cm-2) compared to the dipping method (38 mW cm-2). Subsequently, the influence of Pt/C electrocatalyst load on PEM by spray coating method has been investigated and a Pt loading equal to 0.12 mgPt cm-2has guaranteed the best performances. Moreover, the performances of a commercial PEM containing a Nafion membrane functionalized by screen printing method with 0.5 mgPt cm-2has been compared to the PEM with the membrane prepared by spray coating method loaded with 0.12 mgPt cm-2. Experimental results showed that, at the same cell voltage, the PEM prepared with spray coating method allowed to obtain an electrical power higher than that obtained using commercial PEM. Finally, the use of PEM at 0.12 mgPt cm-2obtained by spray coating method showed a remarkable stability because there was no decrease in the generated current also for long test time (higher than 30 h)