Design of ammonia synthesis Ru catalysts with mesoporous structures through the Si-doping of cerium–lanthanum oxide

Abstract

Green ammonia has attracted growing attention as an energy carrier to facilitate the storage and transport of hydrogen that is derived from renewable energy sources. It is therefore necessary to develop catalysts that can efficiently synthesize ammonia under milder conditions (<10 MPa) than those required by conventional catalysts (15–30 MPa). In this study, Si is doped into Ru/CeO₂-La₂O₃ catalysts prepared via a polymerization complexation method to promote the synthesis of ammonia over the generated mesoporous structure. Scanning electron microscopy, X-ray diffractometry, N₂ adsorption, attenuated total reflection–Fourier transform infrared spectrometry, and CO adsorption measurements revealed that for the CeO₂-La₂O₃ support, Si doping at levels of ≤ 10 mol% created a mesoporous structure (10–50 nm range) through aggregation of the CeO₂-La₂O₃-SiO₂ solid solution with a smaller particle size than that of undoped particles. Consequently, this resulted in an enhanced Ru dispersion and an improved catalytic activity. However, Si doping at levels > 10 mol% led to the formation of isolated SiO₂ particles bearing inactive Ru species, thereby resulting in a lower catalytic activity. Notably, compared to the catalyst without Si doping, the 10 mol% Si-doped Ru/CeO₂-La₂O₃-SiO₂ catalyst exhibited a 3.5-fold higher rate for ammonia synthesis at a reaction pressure of 5.1 MPa. This system is therefore expected to promote ammonia synthesis under mild conditions, ultimately contributing to the transition from a fossil fuel-based society to a hydrogen-based society.