Structural aspects of synthetic zeolite: A comparative assay through positron annihilation and gas adsorption methods

Abstract

Structural arrangement in alumino-silicate compounds (synthetic zeolite species), which encompasses large cavities and channels, have been discussed on the basis of their molecular organization and is further illustrated with the help of micrographs as perceived through transmission electron microscopic studies. The crystallinity aspects and structural defects were investigated through line broadening in x-ray diffraction and the grain sizes through the Debye-Scherrer method. Further, assays of the void size (micro- as well as mesopores) have been performed through positronium annihilation lifetime spectroscopy (PALS) and Brunauer-Emmett-Teller adsorption-desorption isotherm and compared. Despite of some of the limitations of the latter, the final results are in close agreement. The utility of PALS as a microprobe is illustrated through our models. Finally, it has been justified that PALS not only probes the subnanometer micropore sizes efficiently but it also senses the mesopores.

Effect of Confinement on the Phase Transition of Benzene in Nanoporous Silica: A Positron Annihilation Study

“…27 The total annihilation rate, however, does not depend on the geometric shape of the pore 27,28 and the calibration evolved (λ ) 0.3/R′ +1/140) has worked well for silica and zeolite materials. 27,29 In addition to the evaluation of pore size, positron annihilation technique is very sensitive to phase transitions that invariably involves alteration in the electron density/momentum distribution at the annihilation site.…”

“…For larger pores (pore radius R ≥ 1 nm) the contribution of o-Ps intrinsic annihilation (three gamma) becomes significant along with o-Ps pick-off annihilation; hence, the lifetime becomes larger and the annihilation rate is given by λ = λ p + λ 3 γ = ς true( δ R ′ true) + λ 3 γ where, λ 3γ (=1/142 ns −1 ) is the intrinsic 3γ annihilation rate of o-Ps and ς includes the parameters such as Z eff as well as the density (ρ < 1) of the porous material, R′ is the size parameter for the large pore and δ is the diffusivity parameter at the surface of the mesopore . The total annihilation rate, however, does not depend on the geometric shape of the pore , and the calibration evolved (λ = 0.3/ R′ +1/140) has worked well for silica and zeolite materials. , In addition to the evaluation of pore size, positron annihilation technique is very sensitive to phase transitions that invariably involves alteration in the electron density/momentum distribution at the annihilation site. In the context of freezing/melting of liquids confined in nanopores, the pick-off rate of o-Ps is expected to vary with changes in density of liquid as well as free-volume following condensation of the liquid inside the pore.…”

A hybrid model for estimation of pore size from ortho-positronium lifetimes in porous materials

“…( 6) and ( 7) produce similar results if λ T is relatively small compared to λ SE and λ T E . four decades [5,11,19,25,27,28,29,30,31,32,33]. In Fig.…”