Isokinetic temperature and size-controlled activation of ruthenium-catalyzed ammonia borane hydrolysis

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 T_i = 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 T_i 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 T_i, 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.