Although the thermocatalytic ammonia decomposition reaction (ADR) is an effective way to obtain clean hydrogen, it relies on the use of expensive and rare ruthenium (Ru)-based catalysts, making it not sustainable or economically feasible.

Now, a team from the University at Buffalo, Southern Illinois University, University of South Carolina and Brookhaven National Laboratory reports a highly active and stable Ru-free catalyst from earth-abundant elements for efficient carbon-free hydrogen generation via ammonia decomposition.

A complete ammonia conversion to hydrogen was achieved at an economically feasible 450 ˚C over the inexpensive catalyst. An open-access paper describing their work is published in the RSC journal Energy & Environmental Science.

The heterostructured Ru-free catalyst consists of CoNi alloy nanoparticles well-dispersed on MgO-CeO₂-SrO mixed oxide support with further potassium promotion.

Schematic illustration for the decorated CoNi_alloy on the oxide support of MgO-CeO₂-SrO. Tabassum et al.

The catalyst presents 97.7% and 87.50% NH₃ conversion efficiency at 450 ˚C in GHSV of 6000 mL h^-1 g_cat^-1 and 12000 mL h^-1 g_cat^-1, respectively. At 500 ˚C, the hydrogen production rate (57.75 mmol g_cat^-1 min^-1) becomes comparable to most of the reported Ru-based catalysts.

The team demonstrated catalyst performance in a membrane reactor prototype, showing excellent stability up to 600 hours at 500 ˚C and 1.5 bar without apparent degradation.

… this is the first Ru-free catalyst that exhibited outstanding performance, approaching complete NH₃ conversion at economically feasible 450 ˚C for clean on-site hydrogen generation. This work would stimulate more research on developing advanced ammonia cracking technologies using earth-abundant materials, which is the key to the sustainable hydrogen economy.

Further engineering the catalyst nanostructures and morphologies (e.g., porosities, sizes, and surface areas) is crucial for achieving complete NH₃ conversion at higher GHSVs and higher pressures (up to 40 bar) for practical application in the future. Scaling up the catalyst synthesis from grams to industrially relevant quantities while maintaining the ideal dispersion and other characteristics remains a grand challenge. Low-Co or Co-free catalysts are desirable due to the relatively high cost of Co and a major human rights issue associated with its production at present.

—Tabassum et al.

Resources

• H. Tabassum, S. Mukherjee, J. Chen, D. Holiharimanana, S. Karakalos, X. Yang, S. Hwang, T. Zhang, B. Lu, M. Chen, T. Zhong, E. Kyriakidou, Q. Ge and G. Wu, (2022) “Hydrogen Generation via Ammonia Decomposition on Highly Efficient and Stable Ru- free Catalysts: Approaching Complete Conversion at 450 ˚C”Energy Environ. Sci. doi: 10.1039/D1EE03730G.