Inelastic neutron scattering of Na-zeolite A with in situ ammoniation: an examination of initial coordination

Abstract

The detection and rationalization of the coordination of low concentrations of ammonia within Na-zeolite A is carried out by the application of inelastic neutron scattering (INS), using inverse geometry time of flight spectrometry to study the partial phonon density of states of in situ ammoniated Na-zeolite A. The experimental spectra are subsequently characterised by density functional calculations, incorporating pre-optimisation by geometric simulation. We find that at a concentration of four ammonia molecules per alpha-cage, the ammonia molecule coordinates with extra-framework Na(+) cations and gives rise to three structured regions in the INS spectrum. We show that these regions correspond to translational, librational and tilting motions of the ammonia molecule. These results are in agreement structurally with previous studies of ammonia within a zeolite, and thus show that INS is a valid technique for such investigations.

Local probes show that framework modification in zeolites occurs on ammonium exchange without calcination

“…Our previous studies have shown that even low levels of ammonium exchange in zeolites lead to visible changes in INS spectra. [37][38][39]…”

Local and Average Structural Changes in Zeolite A upon Ion Exchange

“…For example, K-A is commonly used in the ethanol drying processes [11] and partially-exchanged K/Na-A is used to separate CO 2 from CO 2 /N 2 dry mixtures [12]. Ca-A zeolites are important in industry, where they selectively adsorb linear alkanes from a mixture of branched alkanes [13], and Na + /NH 4 + exchange is useful in minimising environmental pollution and eutrophication [14][15][16][17]. Li-A was proposed as a possible delivery material for pharmacological studies [18].…”

Inelastic Neutron Scattering Observation of Plasma-Promoted Nitrogen Reduction Intermediates on Ni/γ-Al2O3

“…Hydrogen atoms have a particularly high neutron scattering cross section (80 barns), and thus the technique is well suited to detecting hydrogen-containing adsorbates . The technique has been used successfully to observed hydrogen-containing surface species including hydrides, , peroxides, and NH 3 . , We use INS to interrogate the surface species (e.g., hydrides, NH x , ...) generated on a supported Ni catalyst after plasma treatments with N 2 and H 2 . Ni was chosen because it is inactive for thermal ammonia synthesis at 200 °C and ambient pressure but has been shown in previous studies to have appreciable NH 3 production rates under plasma stimulation. ,, …”