Ac-conductivity analysis of the effects of partial dealumination and γ-irradiation on the electric features of HZSM-5
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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-5deal samples with reference to a well identified HZSM-5 sample. The dielectric loss spectra (ε″ versus W) reveal conduction mechanisms separated at 473 K for HZSM-5deal. 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 σac is of 1–2 orders of magnitude higher for HZSM-5deal than HZSM-5. γ-Irradiation activates the low dipole conduction as shown by rising σac 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 σdc is derived. σdc leads σac 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. σac and σdc drop then rise at higher irradiation doses but to still lower values compared with the fresh HZSM-5deal 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.
- Chemistry & Earth Sciences [103 items ]