Ruthenium-based catalysts supported on carbon xerogels for hydrogen production via ammonia decomposition

Abstract

Carbon xerogels were synthesised and used as support of ruthenium-based catalysts for the ammonia decomposition reaction. To improve their physical-chemical properties, carbon xerogels were either activated in carbon dioxide atmosphere (for 1 and 5 h), or doped with nitrogen via co-precursor method, using urea as nitrogen source. Un-promoted and sodium-promoted ruthenium catalysts were prepared by incipient wetness impregnation. All catalysts were tested during the ammonia decomposition reaction (1 atm, 100–600 °C), showing high catalytic activity. The 5 h carbon dioxide activation treatment resulted in a decrease in oxygen surface groups (i.e. by 11 wt%) on carbon xerogels surface, and in an increase in the structure crystallinity (i.e. by 15% in the TBurn) of carbon xerogels, resulting in a higher ammonia decomposition reaction rate (i.e. 3.5-fold at 450 °C). Similarly, nitrogen addition to carbon xerogels had a positive effect on the catalysts basicity, enhancing their catalytic performance (i.e. triple reaction rate at 450 °C). The addition of sodium conferred an enhancement in the performance of each catalyst (i.e. reaction rate up to 9 times higher at 450 °C). Two ammonia decomposition reaction runs were performed for all catalysts, to test the performance reproducibility of the catalysts. It was found that un-promoted catalysts exhibited higher reaction rates (i.e. up to 3.5 times at 450 °C) during the second run of reaction due to the larger ruthenium particle size, whereas sodium-promoted catalysts exhibited similar catalytic activity in both reaction runs due to the presence of sodium oxide avoiding the sintering of ruthenium particles.