Simultaneous neutron diffraction and microwave dielectric characterisation of ammine materials – a non-destructive, non-contact characterisation tool for determining ammonia content in solids

Abstract

We have investigated ammonia adsorption in group two halides (MgI2 and CaBr2) using custom-built apparatus that permits simultaneous neutron diffraction, microwave dielectric characterisation and out-gas mass spectroscopy of solid state materials during ammonia adsorption. Deuterated ammonia was flowed over the sample and the uptake - as measured by mass flow meters, mass spectroscopy and structure - compared with the change in dielectric constant. An excellent correlation between ammonia content and dielectric property was observed and, when linked to diffraction, mass flow and mass spectroscopy data, could be used to determine the amount of ammonia present within the solid. The combination of these techniques could also be used to differentiate physisorbed and metal-coordinated ammonia and explain subtleties in the observed structural transformations.

Simultaneous neutron powder diffraction and microwave dielectric studies of ammonia absorption in metal–organic framework systems

“…While ammonia may be stored easily and safely under pressure, 2,6 leaks from such systems can cause widespread environmental problems, especially in aquatic environments. Consequently, there has been much interest recently in the development of ammine materials -solid-state ammonia storage media [7][8][9] -that demonstrate high gravimetric ammonia densities and low partial pressures of ammonia at room temperature. Furthermore, ammines have applications as ammonia filters and ammonia detectors for associated energy-vector technologies.…”

“…Furthermore, ammines have applications as ammonia filters and ammonia detectors for associated energy-vector technologies. Current studies of solid-state ammonia storage materials have predominantly focussed on ionic salts systems 1,9 or complex hydrides 10,11 due to their low cost and high gravimetric density, respectively.…”

“…9,[41][42][43] This technique provides a non-invasive, nondestructive method of measuring the intrinsic dielectric properties of solid-state materials as they absorb ammonia gas, and is effective as ammonia is a highly polar molecule with an electric dipole moment of 1.47 D. 44 The absorption of ammonia within the solid matrix causes a change in the polarisation (quantified by e 1 ) of the sample, which can be seen as a shift in the resonant frequency (Df 0 ) of the microwave cavity resonator (MCR) from when it is empty ( f 0 ), with chemisorbed (strongly bound to metal centres) ammonia causing the greatest perturbation in polarisation. 9 Changes in dielectric loss (quantified by e 2 ), are most closely associated with physisorbed (weakly bound to surfaces) ammonia 9 and cause a change in the resonant bandwidth of the system (Df B ). A combination of these effects show the complex relative permittivity of the sample (e = e 1 À je 2 ) and may be calculated using the approximate formulae (eqn (1) and (2)) derived from the cavity perturbation equations.…”

“…Here, we have utilised a modified microwave cavity resonator (MCR) 9 operating at a nominal frequency of 2.5 GHz to measure the dielectric properties of MOF samples under ammonia flow while simultaneously performing powder neutron diffraction and mass spectrometry. The cavity is constructed from aluminium, a highly conductive material resulting in a high Q factor of the empty cavity.…”

“…Schematic diagram of the experimental housing for the MCR in the POLARIS neutron diffractometer (left) and the MCR alone, showing internal position of components (right) 9. …”