Ce-containing Mordenites: Synthesis, structure and reactivity towards NO and CO gases

Abstract

Aqueous solutions of cerium nitrate of increasing concentrations (2.5, 5 and 7.5 wt.% Ce) have been contacted with the components forming Mordenite zeolite; during forming the gel under hydrothermal conditions, for allowing the accessibility of Ce ions to proceed into compensating positions in Mordenite structure. These materials were characterized by the methods of FTIR, XRD, N 2 adsorption and UV–vis diffuse reflectance spectroscopy. The interaction of NO and CO adsorptions; at room temperature, on thermally pre-treated (300 °C, 10 −5 Torr, 3 h) as well as pre-reduced (50 Torr, 500 °C, 1 h) samples were studied by in situ Fourier transform infrared spectroscopy. XRD and FTIR results indicate that the Ce atoms are mostly present in internal surfaces in Mordenites for 2.5 and 5CeMOR samples whereas for 7.5CeMOR, a decrease in diffusion of Ce to be in compensating positions is perceived; as conceived from lowering the lattice volume, pointing to the presence of discrete amounts of CeO 2 (582 cm −1) and cerium silicate (Si–O–Ce; 797 cm −1) species. All the samples indicate intra-crystalline mesopores as depicted from V l– t plots particularly the 7.5CeMOR sample that showed the highest wide-pore volume (0.073 cm 3/g), lowest pore radius (21 Å) and thus, revealed the highest S BET between all samples (363 m 2/g). UV–vis characterization of 7.5CeMOR sample shows octahedral Ce species (345, 360 and 390 nm) in small clusters inside zeolite channels and most probably originated from cerium silicates having different coordination with NaMOR along with discrete amounts of CeO 2 (420 nm) species. CO readily adsorbs on the Ce 3+ sites of the pre-reduced 7.5CeMOR catalyst, rather than those on Ce 4+, to display minor amounts of carboxylate and dominant amounts of monodentate carbonate that were amenable to decompose to produce CO 2 gas (2335 cm −1). On the other hand, the in situ interaction of nitric oxide (NO) gas on the 7.5CeMOR catalyst led to the formation of a series of nitrosyl species: N 2O (2240 cm −1), NO (1908 cm −1), N 2O 3 (1880 cm −1) and (NO) 2s,as (1844, 1734–1720 cm −1). Such nitrosyl complexes were favorably formed on Ce 3+ in 7.5CeMOR those exchanged Na ones.