Ac-conductivity analysis of the effects of partial dealumination and γ-irradiation on the electric features of HZSM-5

Abstract

For illustrating such titled effects, analysis of the electric impedance, capacitance, and dielectric loss dependences on the frequency W (0.1–100 kHz) and temperature T (373–673 K) is correlated at 0.8 V for γ-irradiated HZSM-5 deal samples with reference to a well identified HZSM-5 sample. The dielectric loss spectra (e″ versus W ) reveal conduction mechanisms separated at 473 K for HZSM-5 deal . Below this temperature, low frequency dispersion (LFD) dominates the conduction via highly polarized dipoles, probably at the dealumination-induced nest silanols. LFD diffuses into a more steady conduction at higher temperatures as in HZSM-5 solely conducting via proton transfer. The ac-conductivity σ ac is of 1–2 orders of magnitude higher for HZSM-5 deal than HZSM-5. γ-Irradiation activates the low dipole conduction as shown by rising σ ac at reducing activation energy. At variance, the γ effect at higher temperatures is rather controversial. The impedance shows complementary data, based on which the dc-conductivity σ dc is derived. σ dc leads σ ac by 3–4 orders of magnitude. Regardless of the controversial effects of irradiation above 473 K, the relaxation and activation energies rise steadily with the irradiation dose. σ ac and σ dc drop then rise at higher irradiation doses but to still lower values compared with the fresh HZSM-5 deal sample. The earlier σ drop can be due to loss in the population (by dehydroxylation) or mobility (by trapping) of the highly mobile protons. The subsequent σ rise may then be associated with lattice defects forming at higher irradiation doses. These data ascribe free charge carriers over strongly polarized surfaces. The nest silanols form a lake over which the zeolite protons exhibit the extraordinary acidity established over dealuminated surfaces.