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
Metal-organic frameworks (MOFs) offer an attractive alternative to traditional hard and soft templates for nanocluster synthesis because their ordered crystalline lattice provides a highly controlled and inherently understandable environment. We demonstrate that MOFs are stable hosts for metal hydrides proposed for hydrogen storage and their reactive precursors, providing platform to test recent theoretical predictions that some of these materials can be destabilized with respect to hydrogen desorption by reducing their critical dimension to the nanoscale. With the MOF HKUST-1 as template, we show that NaAlH(4) nanoclusters as small as eight formula units can be synthesized. The confinement of these clusters within the MOF pores dramatically accelerates the desorption kinetics, causing decomposition to occur at approximately 100 degrees C lower than bulk NaAlH(4). However, using simultaneous thermogravimetric modulated beam mass spectrometry, we also show that the thermal decomposition mechanism of NaAlH(4) is complex and may involve processes such as nucleation and growth in addition to the normally assumed two-step chemical decomposition reactions.
