(560ip) Metal-Organic Framework Templated Catalysts for Hydrogen Generation from Low-Temperature Ammonia Decomposition

The pressing escalation of atmospheric CO₂ concentration has raised world-wide concerns nowadays because of its adverse consequences mainly in climate changes and ocean acidification. A common consensus that has been reached is to capture and utilize CO₂ as an immediate solution. Hydrogen has been recognized as the ultimate fuel to solve this issue because of its high energy density and clean water by-products after combustion, but its storage and transportation remain as a major obstacle to be tackled. Ammonia has been presented as an attractive alternative to hydrogen storage due to its high hydrogen content both volumetrically (81 g L^-1) and gravimetrically (17.6 wt.%) and a narrow flammable range (16 ~ 25 %). In this context, it has been regarded as an attractive alternative to the current lack of efficient and economically viable methods to store hydrogen in a compact, safe and cost-effective manner. According to the 2020 US Department of Energy report, the deployment of hydrogen as energy vector lies on achieving a high-density storage (9 wt.% gravimetrically and 75 g L^-1 volumetrically in capacity) and its on-demand supply at temperatures below 100 ^â—¦C for proton exchange membrane fuel cells (PEMFCs). The key for such a system being feasible is to develop a highly efficient and stable catalyst capable of decomposing ammonia gas to hydrogen under mild conditions.

Metal-organic frameworks (MOFs), constructed from coordination bonds between metal cations and organic ligands, have emerged as promising adsorbent and catalytic materials. The last two decades have witnessed ample studies of MOFs and their applications in gas storage and separation, because of their ultra-high porosities, tunable pore size and structures, and versatile chemical functionalities. During the last few years, use of ammonia as a hydrogen vector has demonstrated the release of hydrogen at temperatures ~250 ℃ using ruthenium-based catalysts supported on carbon nanotubes (CNT). It is believed that this high activity is related to the ruthenium particle size (~3 - 5 nm) facilitated by the CNT support where the concentration of B₅ active sites is maximized. In addition, metal-support interactions have been reported of playing a vital role in enhancing the activity and stability of supported catalysts. However, there lack of efficient methods to enhance the metal-support interactions to improve the efficiency and stability of metal nanoparticle sites. I have developed a MOF-templated approach to synthesize ruthenium nanoparticle catalysts with cesium promoters supported on mesoporous crystalline zirconia for ammonia decomposition to produce hydrogen.