LaRuSi for ammonia synthesis
16 December 2018 - Chemical Zoo
Ammonia synthesis with electrides update. In 2012 Kitano et al. used a ruthenium doped Ca/Al/O zeolite (previous report) and in 2018 the catalyst of choice is La/Ru/Si (DOI).
Synthesis: combine elemental lanthanum, ruthenium and silicon by arc welding, repeat melting 5 times and anneal at 1000 degrees for 2 weeks. Key difference with the 2012 iteration: ruthenium nanoparticles on an electride support tend to clutter together with rapidly diminishing activity whereas the 2018 iteration is a intermetallic compound: an alloy but with a defined crystal structure. In this structure the La atoms form a distinct layer alternating with layers of Ru/Si atoms. Also accounted for are pseudo atom sites, the location of the anionic electrons. Hydrogenation is easy: there is room for one hydrogen atom in every La4 unit. Maximum ammonia production in a hydrogen/ nitrogen stream: 1760 micromole per gram per hour. For comparison, CaRuSi did nothing, as did LaCoSi (another 2018 iteration). The 2017 iteration LaScSi was not compared.
As for a mechanism ammonia synthesis works by hydrogenation, hydride exchange with the anionic electrons, followed by nitrogen adsorption and dissociation by a hot atom mechanism.