Metal-catalyzed hydrolysis is an important reaction for releasing hydrogen stored in ammonia borane, a promising fuel form for the future hydrogen economy, under ambient conditions. A variety of catalysts made of different transition metals have been investigated to improve the efficiency of hydrogen generation; however, little attention has been given to the possible influence of the compensation effect on catalyst design. Using face-centered cubic (FCC) packed ruthenium (Ru) nanoparticles supported on layered double oxide nanodisks, we show that the compensation effect produces an isokinetic temperature at Ti = 17.5(±1.6) °C within the operational range of hydrogen generation. We further show that the turnover frequency (TOF) of the reaction can be maximized for operations performed below Ti by reducing the size of Ru-FCC nanoparticles, which increases the fraction of edge and corner atoms and lowers the activation energy. At 15 °C, TOF can reach more than 90% of the theoretical maximum (0.72 mol m-2 h-1) using Ru nanoparticles having an average diameter of 2 nm and giving an activation energy of 17.7(±0.7) kJ mol-1. To generate hydrogen above Ti, TOF is maximized by using enlarged Ru nanoparticles with a diameter of 3.8 nm, giving an activation energy of 87.3(±5.8) kJ mol-1. At 25 °C, these nanoparticles produce a TOF of 1.8(±0.3) mol m-2 h-1, representing at least an 81% increase in comparison to the highest TOF reported for elemental catalysts. Our results suggest that controlling the reaction activation energy by adjusting nanoparticle size represents a viable strategy for designing catalysts that can maximize TOF for ammonia borane hydrolysis operated both below and above the isokinetic temperature.
- compensation effect
- hydrogen storage and production
- isokinetic temperature
- layered double hydroxide derivative
- metal hydride
- nanoparticle nucleation
- supported and stabilized nanocatalyst