(631d) Effect of Metal Loading and Support Particle Size on the Catalytic Performance of Cs-Ru/CeO2 in a Microwave Reactor for Ammonia Synthesis

Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber–Bosch process, but this process requires high temperatures (450-520 ^o C) and pressures (200-300 bar) and consumes more than 1% of the world’s power production. Therefore, the search is on for a more environmentally benign process that occurs under mild conditions. One method is to use microwave reactor and optimize the already developed ruthenium catalyst. Microwave heating is a selective heating mechanism that reduces the activation energy and increases the catalytic activity. Here, we report that a Cs promoted Ru catalyst supported on ceria with different metal loading and support size is studied. The Cs- Ru metal loading ranging from 2-4 % to 12-24% supported on 50nm cerium oxide and Cs-Ru(2-4%) catalyst supported on cerium oxide with three different particle sizes, one in the micro- and the other two in the nano-range(25nm and 50nm) were investigated for their activity towards ammonia synthesis at low temperature and pressure in a microwave reactor. The smaller size of the ceria support enhanced the adsorption of hydrogen and nitrogen and led to desorption of surface hydrogen in the form of H₂ resulting the highest activity. In the 5-micrometer ceria support H₂ and N₂ adsorption was less favorable plausibly due to the large particle size of Ru species and low dispersion of Ru. While proper metal loading governs the cost, the optimal Ru loading was found to be around 4wt.%. Furthermore, peak activity was obtained with 10–20% Ru dispersion, higher dispersion values leading to less performing catalysts and economically not feasible.