Studies of a CuY zeolite as a redox catalyst

Abstract

The stability and behavior of CuY in the redox cycles with CO O 2 , H 2 O 2 , and CO NO have been studied using a microbalance operating in the flow mode and with a standard (BET) volumetric system. When CO was used as a reducing agent CO 2 was produced, thus removing oxygen from the zeolite lattice, but when H 2 was used only some of the H 2 consumed was evolved as H 2O. The rest was retained as lattice OH groups, but this was minimal when H 2 was used after treating the sample with CO. Oxidation with NO produced only N 2. At 500 °C the sample was stable and could be reversibly oxidized and reduced through many cycles using either CO O 2 or NO CO . In all cases the ratio O Cu was close to 0.5, i.e., 1e Cu . Treatment in CO at higher temperatures did not affect the reversible nature of the oxidation, but now the valence change was substantially larger; it approached 2e Cu . The crystallinity of the exchanged zeolite was studied using X-ray diffraction and by measurement of the pore filling with liquid N 2. No significant changes could be detected after the different treatments, even those performed at 750 °C. Temperature-programmed reduction, temperature-programmed oxidation, and X-ray diffraction studies were made and the different maxima are reported. CuO and Cu o appeared in the oxidized and reduced samples, respectively, after treatment at 750 °C in CO but not at lower temperatures. Subsequent redox cycles at 500 °C did not appear to affect the size or amount of Cu o crystallites. CuY was active in the oxidation of CO with O 2 or NO. Its activity was lower than that of FeY zeolite when it exhibited an oxygen-carrying capacity of 0.5 O Cu . Treatment with CO at 750 °C, however, reversed the situation. Kinetic results showed that the fresh CuY catalyst was close to zero order in CO and fractional order in O 2 with an activation energy of 15 kcal/mole. After treatment at 750 °C in CO, the rate law became dependent upon the CO O 2 ratio. It was close to first order in CO and zero order in O 2 under oxidizing conditions ( CO O 2 ≤ 2 ), but the orders were reversed under reducing conditions ( CO O 2 > 2 ). The activation energies were 12 and 15 kcal/mole, respectively. The data suggested that the Cu 2+ with bound oxygen are the species active in the oxidation reaction.