Non-thermal plasma assisted effective ammonia decomposition on highly monodisperse ruthenium-based catalysts: approaching complete conversion at 400 °C

Ammonia (with hydrogen content of 17.7 wt%) is a promising hydrogen storage carrier because of its ease of storage and transportation. Herein, a series of highly monodisperse Ru/X (X = MgTiO₃, TiO₂, Al₂O₃, SiO₂) catalysts were employed for efficient decomposition from ammonia (NH₃). As functional supports, four oxide supports have been demonstrated to be capable of dispersing and stabilizing nanometer Ru nanoparticles. To certify the structure-performance relationship of Ru/X catalysts, systematic techniques including X-ray diffraction (XRD), N₂ physisorption measurements, X-ray photoelectron spectroscopy (XPS), aberration-corrected transmission electron microscopy (AC-TEM), energy dispersive Spectrometer (EDS) maps, hydrogen temperature-programmed reduction (H₂-TPR) and NH₃ temperature-programmed desorption (NH₃-TPD) were performed. Plasma-catalytic decomposition of NH₃ was carried out in a specially designed coaxial dielectric barrier discharge (DBD) reactor. Compared with thermal catalysis, plasma-assisted catalysis can improve the discharge characteristics of the catalysts and promote the ammonia conversion. The plasma-assisted NH₃ decomposition performance followed the order of Ru/MgTiO₃ > Ru/TiO₂ > Ru/Al₂O₃ > Ru/SiO₂. With the assistance of plasma, the NH₃ achieved complete conversion over Ru/MgTiO₃ at 400 °C and 38 W. The Ru/MgTiO₃ catalyst achieved a H₂ generation rate of 654 mmol·g-1 Ru·min⁻¹ at 450 °C and 1445 mmol·g-1 Ru·min⁻¹ at 400 °C with plasma assistance, which is superior to some reported pure Ru-based catalysts without additive. The effect of monodisperse Ru nanoparticles (∼1 nm) on the physical properties of the discharge is negligible, which indicates that the catalytic effect provided by the supports is more dominant in plasma assisted NH₃ decomposition. The comprehensive characterizations indicates that the unique highly monodisperse Ru/X nanocomposite structure with heightened metal-support interaction, not only provided a rich Ru-X interface to stabilize Ru NPs, but also strengthening electronic metal-support interaction. The systematic researches of kinetics, temperature-programmed reaction and DFT calculations demonstrated strong metal support interaction including electronic effects, favored NH₃ enrichment and N [tbnd]N bond formation. The work emphasizes the importance of plasma-assisted catalytic enhancement, which providing a new idea for the diversification of hydrogen generation from ammonia using Ru-based monometallic catalyst.