Structural and textural characteristics of Ce-containing mordenite and ZSM-5 solids and FT-IR spectroscopic investigation of the reactivity of NO gas adsorbed on them

Abstract

The in situ interaction of nitric oxide (NO) gas on the cerium framework-substituted ZSM-5 and mordenite zeolites was studied by FT-IR spectroscopy. Samples of ZSM-5 and mordnite-containing cerium (7.5 wt.% expressed as CeO 2) have been hydrothermally synthesized from starting gel upon which the introduction of cerium is being made during synthesis of zeolites. The morphological and textural characteristics of cerium-free and cerium-containing zeolites were studied using XRD, FT-IR in the T O range, and N 2 adsorption at −196 °C. The results revealed that the insertion of cerium in both zeolites led to a decrease in their degree of crystallinity. Such a decrease was 36% for Ce-ZSM-5 while it was 18% for Ce-mordenite. The results indicated a significant decrease in nitrogen sorption capacity (BET) by 31% from 624 to 431 m 2/g and also a decrease of the micropore volume by 27.8% from 0.579 to 0.418 cm 3/g for Ce-ZSM-5. These results were found on the other extreme in case of Ce-mordenite. The presence of Ce did not affect the cell volume of ZSM-5 much, while it increased that of mordenite. Most of Ce IV ions were embedded in the framework of mordenite, where they exposed as extra-framework CeO 2 and cerium silicate in ZSM-5. The in situ interaction of NO on Ce-zeolites was studied using an FT-IR quartz cell. The adsorption of NO gas led to the formation of a series of nitrosyl species: N 2O (2245 cm −1), NO + (2160 cm −1), NO (1910 cm −1), N 2O 3 (1880, 1580 cm −1), (NO) 2s,as (1844, 1734–1720 cm −1), NO 2 (1630 cm −1) and ionic compounds were stable upon evacuation, i.e. nitrato and nitrito NO x − ( x = 2–3) (1300–1500 cm −1). Such nitrosyl complexes were favorably formed on Ce-ZSM-5 than on Ce-mordenite due to facilitated intervention of the cerium couple (Ce III/Ce IV) on the former than on the latter.