Electrochemical Properties of Gold Incorporated Inside FSM-16 and Y Zeolites

The dc and ac conductivities as well as the dielectric constant (ε) were
measured for different zeolites encapsulated gold (Au Cl3) catalysts at different
temperatures (300-500 K) as a function of frequency (5 kHz-1 MHz). The
conductivity values were found to be in the order Au/FSM-27 >Au/NaY > Au/FSM-
47. This result was confirmed by finding that the first ionization energy of Na+ ions
was much lower than that of Cl ones. The conductivity of Au either in FSM-27 or
FSM-47 was expected to be of electronic character where the hopping mechanism
is the most predominant. Whereas, in the case of Au/NaY; containing Na and Cl
ions, the conductivity was partially ionic partially electronic. Above 376 K, a
metallic behavior was obtained for Au catalysts. The IR study showed that the
exposure of Au zeolites to CO adsorption produced a characteristic band of Au+-
CO at 2180 cm-1 that tends to decrease with temperatures and even vanish at 376 K
in favor of the Auo-CO band at 2128 cm-1. Similarly, a phase transition at 338 K
that extends to 376 K was confirmed by DTA to further emphasize the temperature
region of the metallic Au formation. After the phase transition, the increase in
thermal energy caused an increase in the lattice scattering of the charge carriers
that produced a decrease in the mobility, polarizability and permitivity.