Mechanism of the low-temperature interaction of hydrogen with α-oxygen on FeZSM-5 zeolite

The mechanism of a low-temperature reaction of hydrogen with the radical anion surface oxygen species (alpha-oxygen, O-alpha) formed by decomposing N2O over FeZSM-5 zeolite was studied using kinetic and isotope techniques; It was found that the reaction is of first order with respect to hydrogen and the rate of the reaction is proportional to the concentration of O-alpha. The activation energy of the reaction, which was measured for H-2 or D-2 over a temperature range from +20 to -100degreesC, is equal to 3.2 or 5.3 kcal/mol, respectively. The reaction occurs with a. considerable kinetic isotope effect (k(H)/k(D)) which varies over the range of 3.4-41 depending on the temperature. This fact indicates that the rate-limiting step of the reaction includes the dissociation of the hydrogen molecule. The temperature dependence of-the isotope effect gave a value of 2.1 kcal/mol, which is close to the difference between the zero bond energies in the molecules of H-2 and D-2; this fact suggests that a tunnel effect does not significantly contribute to the reaction. The dissociative mechanism is consistent with data obtained by in situ IR spectroscopy. The interaction of hydrogen with alpha-oxygen is accompanied by the formation of new hydroxyl groups OalphaH (absorption bands at 3635 and 3674 cm(-1)) at the surface of the zeolite. The identification of these groups was supported by an isotope shift either on the replacement of H-2 by D-2 or on the replacement of O-16(alpha) by O-18(alpha). The stoichiometric ratio H-2 : O-alpha is consistent with the previously drawn conclusion on the paired arrangement of a-sites.