Bimetallic RuCo and RuCu catalysts supported on y-Al_2O_3.A comparative study of their activity in hydrolysis of ammonia-borane

摘要

Bimetallic-based RuCo and RuCu catalysts, supported on y-Al_2O_3 (1.5 wt% Ru as theoretical value), were synthesized by polyol method. Ru, Co, and Cu acetylacetonates were used as precursors and ethylene glycol as reducing agent. The as-synthesized catalysts were characterized by SEM, TEM, XRD and XPS, and tested in ammonia-borane (NH_3BH_3) hydrolytic dehydrogenation at variable amount of catalyst (10-30 wt%), concentration of NH_3BH3 (1.0-0.65 M), and temperatures (50-65 ℃). The reactions were monitored by volumetric (inverted burette) and spectroscopic methods (~(11)B and ~(11)B{~1H} NMR). It was found that the best bimetallic catalysts are those having a molar ratio Ru:Co and Ru:Cu of 1:1 such as RuCo > RuCu ~ Ru. They, i.e. RuCo and RuCu, consist of nanosized spherical particles of Ru℃o(OH)_2 and Ru ~0Cu~0, respectively. Kinetic investigation highlights similar rate laws with activation energies of 47 kj mol~(-1) and 52 kj mol~(-1), respectively, and, for both, reaction orders of 1 versus both the NH_3BH3 and the catalytic free sites concentrations. ~(11)B and ~(11)B{~1H} NMR investigation confirmed (i) a more effective NH_3BH3 hydrolytic dehydrogenation in the presence of RuCo catalyst even though a loss of activity after the first run was observed for both catalysts, and (ii) a rapid NH_3BH_3 hydrolysis with initial formation of B(OH)_4~-, which besides favors equilibriums of formation of polyborates. These results are reported and the reaction mechanism discussed herein.