Solid-State Ac-Conductivity Analysis For The Long Debated Catalytic Activity Of The Boralite Sieve
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The ac-phase dependent impedance, capacitance and dielectric loss of boralite are measured and correlated with those of a similar history HZSM-5 as a function of frequency (0.1-100 kHz) and temperature (373-638 K) at 0.8 V. The data illustrate distorted semicircle impedance Z"(Z') and linear permittivity e"(£') dependences throughout the temperature range. Below 373 K, the conduction is assisted by sorbed water, as its role diminishes with desorption and vanishes above 373 K. Thermally activated protonic conduction becomes more dominant at higher temperatures. Compared to the steady conduction in HZSM-5, the boralite permittivity reveals a conduction shifting across 573 K in temperature controlled regimes. Below 573 K the conduction is characterized by low activation energy of 0.09+0.01 eV, compared to the higher temperature conduction of the energy 0.37 eV. The latter conduction is sensitive to variation in the field frequency and irradiation dose of gamma rays. For instance, the 0.37 eV activation energy measured at 0.1 kHz falls to 0.19 eV if measured at 100 kHz and to 0.13 eV if exposed to 10 Mrad gamma dose. Such energy losses can be assigned to rises in the mobility of the conduction Bronsted sites. Compared to the steady rise in HZSM-5 conductivity, irradiation at higher doses reduces the boralite conductivity. This latter result suggests that the B-associated Bronsted sites are heterogeneous; the more mobile site is of so low stability that it cannot sustain the impact of integrated heat, irradiation and field frequency. They might undergo dehydroxylation or trapping as lattice defects under the impact of such reinforcing parameters. The apparently low activity of boralite can then be addressed to the low stability of its highly mobile B-associated Bronsted sites. Silicalite shows incomparable low conductivity that supports the more important role of the charge carriers, Bronsted sites in the conduction in both the boralite and HZSM-5 sieves.
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