ABSTRACT
Reactive nanoparticles are of great interest for applications ranging from catalysis to energy storage. However, efforts to relate cluster size to thermodynamic stability and chemical reactivity are hampered by broad pore size distributions and poorly characterized chemical environments in many microporous templates. Metal hydrides are an important example of this problem. Theoretical calculations suggest that reducing their critical dimension to the nanoscale can in some cases considerably destabilize these materials and there is clear experimental evidence for accelerated kinetics, making hydrogen storage applications more attractive in some cases. However, quantitative measurements establishing the influence of size on thermodynamics are lacking, primarily because carbon aerogels often used as supports provide inadequate control over size and pore chemistry. Here, we employ the nanoporous metal-organic framework (MOF) Cu-BTC (also known as HKUST-1) as a template to synthesize and confine the complex hydride NaAlH(4). The well-defined crystalline structure and monodisperse pore dimensions of this MOF allow detailed, quantitative probing of the thermodynamics and kinetics of H(2) desorption from 1-nm NaAlH(4) clusters (NaAlH(4)@Cu-BTC) without the ambiguity associated with amorphous templates. Hydrogen evolution rates were measured as a function of time and temperature using the Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry method, in which sample mass changes are correlated with a complete analysis of evolved gases. NaAlH(4)@Cu-BTC undergoes a single-step dehydrogenation reaction in which the Na(3)AlH(6) intermediate formed during decomposition of the bulk hydride is not observed. Comparison of the thermodynamically controlled quasi-equilibrium reaction pathways in the bulk and nanoscale materials shows that the nanoclusters are slightly stabilized by confinement, having an H(2) desorption enthalpy that is 7 kJ (mol H(2))(-1) higher than the bulk material. In addition, the activation energy for desorption is only 53 kJ (mol H(2))(-1), more than 60 kJ (mol H(2))(-1) lower than the bulk. When combined with first-principles calculations of cluster thermodynamics, these data suggest that although interactions with the pore walls play a role in stabilizing these particles, size exerts the greater influence on the thermodynamics and reaction rates.
ABSTRACT
Through the use of simultaneous thermogravimetry modulated beam mass spectrometry, optical microscopy, hot-stage time-lapsed microscopy, and scanning electron microscopy measurements, the physical and chemical processes that control the thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine (RDX) below its melting point (160-189 degrees C) have been identified. Two gas-phase reactions of RDX are predominant during the early stages of an experiment. One involves the loss of HONO and HNO and leads to the formation of H2O, NO, NO2, and oxy-s-triazine (OST) or s-triazine. The other involves the reaction of NO with RDX to form NO2 and 1-nitroso-3,5-dinitrohexahydro-s-triazine (ONDNTA), which subsequently decomposes to form a set of products of which CH2O and N2O are the most abundant. Products from the gas-phase RDX decomposition reactions, such as ONDNTA, deposit on the surface of the RDX particles and lead to the development of a new set of reaction pathways that occur on the surface of the RDX particles. The initial surface reactions occur on surfaces of those RDX particles in the sample that can accumulate the greatest amount of products from the gas-phase reactions. Initial surface reactions are characterized by the formation of islands of reactivity on the RDX surface and lead to the development of an orange-colored nonvolatile residue (NVR) film on the surface of the RDX particles. The NVR film is most likely formed via the decomposition of ONDNTA on the surface of the RDX particles. The NVR film is a nonstoichiometric and dynamic material, which reacts directly with RDX and ONDNTA, and is composed of remnants from RDX and ONDNTA molecules that have reacted with the NVR. Reactions involving the NVR become dominant during the later stage of the decomposition process. The NVR reacts with RDX to form ONDNTA via abstraction of an oxygen atom from an NO2 group. ONDNTA may undergo rapid loss of N2 and NO2 with the remaining portion of the molecule being incorporated into the dynamic NVR. The dynamic NVR also decomposes and leads to the formation of H2O, CH2O, N2O, NH2CHO, (CH3)2NCHO, (CH3)2NNO, C2H2N2O, and (CH3)3N or CH3NCH2CH3. The competition between reaction of the dynamic NVR with RDX and its own thermal decomposition manifests itself in a rapid increase in the rate of evolution of the NVR decomposition products as the amount of RDX remaining in the sample nears depletion. The reactions between the NVR film and RDX on the surface of the RDX particles leads to a localized environment that creates a layer of molten RDX on the surface of the particles where reactions associated with the liquid-phase decomposition of RDX may occur. The combination of these reaction processes leads to an acceleration of the reaction rate in the later stage of the decomposition process and creates an apparent reaction rate behavior that has been referred to as autocatalytic in many previous studies of RDX decomposition. A reaction scheme summarizing the reaction pathways that contribute to the decomposition of RDX below its melting point is presented.
