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. KEYWORDS: compensation effect, isokinetic temperature, metal hydride, hydrogen storage and production, nanoparticle nucleation, supported and stabilized nanocatalyst, layered double hydroxide derivative ■ INTRODUCTIONA main objective of heterogeneous catalysis is to improve reaction kinetics by adjusting the reaction activation energy E a through catalyst design. 1,2 When the turnover frequency (TOF) of the reaction becomes insensitive to the adjustment, the temperature under which the insensitivity incurs is referred to as the isokinetic temperature T i . T i originates from the compensation effect, 3−5 which manifests in a linear correlation between E a and the natural logarithm of the Arrhenius preexponential factor A (aka the Cremer−Constable relation): 5,6where α and β are constants. E a and A are related to TOF by the Arrhenius equation for zeroth-order reactions: 7,8where R is the gas constant and T is the absolute temperature. Combining eqs 1 and 2 giveswith T i = α −1 R −1 and TOF i = e β for T = T i . For reactions showing the compensation effect, estimates of T i almost exclusively lie outside the operational range of T, 6,9 giving scenarios of either T < T i or T > T i . To maximize TOF, catalysts should be designed to decrease E a if T < T i but increase E a if T > T i . Only a few excep...

Ammonia oxidation is operated at different temperatures over Pt catalysts of different structures to recover different products. In this work, we elucidate the dependency of ammonia oxidation rates and selectivities on both Pt structure and temperature. We perform density functional theory (DFT) computations to compare the reaction and activation energies of elementary reactions on Pt(211) and Pt(111). We develop a microkinetic model parametrized with the DFT results. We show that barriers to product formation are lower on stepped Pt than on terrace, leading to a much higher step rate at low temperature to selectively oxidize ammonia to nitrogen. At high temperature, however, both step and terrace perform comparably in rate to selectively produce nitric oxide. While N2 is always the thermodynamic product, relative N and O coverages interact to make NO the kinetic product at high temperature. The predicted rate and selectivity are consistent with experiments. We further show rate-controlling steps on the two Pt surfaces are different at low temperature but are the same at high temperature. The degrees of selectivity control for elementary reactions are comparable for the two surfaces. Finally, we demonstrate the flows of elementary reactions in the reaction network are also structure- and temperature-dependent.

This study investigated the cotransport of titanium dioxide nanoparticles (nTiO2) and fullerene nanoparticles (nC60), two of the most widely utilized nanoparticles, in saturated quartz sand under a series of ionic strengths in NaCl solutions (0.1-10 mM) at both pH 5 and 7. Under all examined ionic strengths at pH 5, both breakthrough curves and retained profiles of nTiO2 in the copresence of nC60 were similar to those without nC60, indicating that nC60 nanoparticles copresent in suspensions did not significantly affect the transport and retention of nTiO2 in quartz sand at pH 5. In contrast, under all examined ionic strengths at pH 7, the breakthrough curves of nTiO2 in the copresence of nC60 in suspensions were higher and the retained profiles were lower than those without nC60, which demonstrated that the presence of nC60 in suspensions increased the rate of transport (decreased retention) of nTiO2 in quartz sand at pH 7. Competition of deposition sites on quartz sand surfaces by the copresence of nC60 was found to contribute to the increased nTiO2 transport at pH 7. Under all examined ionic strength conditions at both pH 5 and 7, the breakthrough curves of nC60 were reduced in the copresence of nTiO2, and the corresponding retained profiles were higher than those without nTiO2, indicating that the presence of nTiO2 decreased the transport of nC60 in quartz sand. Co-deposition of nC60 with nTiO2 in the form of nTiO2-nC60 clusters as well as the deposition of nC60 onto previously deposited nTiO2 were responsible for the increased nC60 deposition in the presence of nTiO2 at pH 5, whereas deposition of nC60 onto surfaces of predeposited nTiO2 was found to be responsible for the increased nC60 deposition at pH 7.

Heterogeneous catalysts coupled with non-thermal plasmas (NTP) are known to achieve reaction yields that exceed the contributions of the individual components. Rationalization of the enhancing potential of catalysts, however, remains challenging because the background contributions from NTP or catalysts are often non-negligible. Here, we first demonstrate platinum (Pt)-catalyzed nitrogen (N2) oxidation in a radio frequency plasma afterglow at conditions at which neither catalyst nor plasma alone produces significant concentrations of nitric oxide (NO). We then develop reactor models based on reduced NTP- and surface-microkinetic mechanisms to identify the features of each that lead to the synergy between NTP and Pt. At experimental conditions, NTP and thermal catalytic NO production are suppressed by radical reactions and high N2 dissociation barrier, respectively. Pt catalyzes NTP-generated radicals and vibrationally excited molecules to produce NO. The model construction further illustrates that the optimization of productivity and energy efficiency involves tuning of plasma species, catalysts properties, and the reactor configurations to couple plasma and catalysts. These results provide unambiguous evidence of synergism between plasma and catalyst, the origins of that synergy for N2 oxidation, and a modeling approach to guide material selection and system optimization.

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