Isokinetic Temperature and Size-Controlled Activation of Ruthenium-Catalyzed Ammonia Borane Hydrolysis

Abstract

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...

Highly active layered double hydroxide-derived cobalt nano-catalysts for p-nitrophenol reduction

“…Different from Co-containing LDH in which cobalt is incorporated in the brucite layers by replacing magnesium, anion-coordinated palladium cations are intercalated between the brucite layers[80,81], which may interfere with the phase transformation process. Palladium nanoparticles can be attached on LDH or LDO surfaces by adsorption with or without the use of polymer linkers; however, this methodof direct decoration is unlikely to result in an improvement of reactivity compared to well-dispersed dendrimerstabilized palladium nanoparticles because the nanoparticles created by the direct decoration method can only create loosely attached nanoparticles[70] that can still fall off the support and aggregate.…”

Cobalt-Nickel-Boron Supported over Polypyrrole-Derived Activated Carbon for Hydrolysis of Ammonia Borane

“…XPS analysis also demonstrated that the surface molar composition of Co-Ni-B was Co 47. 5 [6], which indicates that PAC was successfully doped into the Co-Ni-B catalyst. Figure 4c shows the high-resolution XPS spectrum of Co 2p; the peaks at 780.1 and 795.8 eV correspond to Co 2p3/2 and Co 2p1/2, respectively.…”

“…The XPS results confirm the presence of Ni, Co, B, C, N, and O in the Co-Ni-B/PAC composite. The binding energy at 398.8 eV was attributed to uncharged pyrrolic nitrogen ( Figure 4b) [6], which indicates that PAC was successfully doped into the Co-Ni-B catalyst. Figure 4c shows the high-resolution XPS spectrum of Co 2p; the peaks at 780.1 and 795.8 eV correspond to Co 2p3/2 and Co 2p1/2, respectively.…”

“…The specific surface area drastically increased to 151 m 2 •g −1 after doping with PAC derived from PPy nanofibers. The increase in surface area has to be attributed to the presence of the porous PAC [6], which indicates that PAC was successfully doped into the Co-Ni-B catalyst. Figure 4c shows the high-resolution XPS spectrum of Co 2p; the peaks at 780.1 and 795.8 eV correspond to Co 2p3/2 and Co 2p1/2, respectively.…”

“…However, AB decomposition is sluggish without a catalyst [3][4][5]. Thus, it is necessary to choose suitable materials to accelerate the AB hydrolysis reaction; noble metal-based catalysts, such as Ru, Pd, and Pt, are commonly chosen [6][7][8]. However, limited resources and high cost hinder their widespread application, so considerable effort has been devoted to the exploration of efficient alternatives based on non-noble transition metals.…”