Hydrocarbon specificity in the selective catalytic reduction of NOx over Cu-ZSM-5 and Co-ZSM-5 catalysts

Abstract

Transformation of surface nitrates under CI-I4 (CH4 + 02) was found to ensure steady-state activity of Co-ZSM-5 in the selective catalytic reduction of nitrogen oxides by methane (CI-I4-SCR). For Cu-ZSM-5, such species are mainly converted into NO. Kelaxation of the coordination sphere due to oxygen and NO adsorption, stability of C,N-containing intermediates and activation routes of hydrocarbons (methane, propane) were analyzed as factors determining catalytic properties of Cu and Co cations.

IR studies of the transformations of nitrogen-containing organic intermediates during selective reduction of nitrogen oxides by hydrocarbons

“…These bands can be assigned to a compound of the acetamide (CH3-CO-NH2) type, resulting from water addition to acetonitrile. These results agree with available data [3,4,7,8], indicating the participation of nitriles in the HC-SCR reaction schemes for Co-ZSM-5, while for Cu-ZSM-5 only such species as ketenes or acetone oximes [5,7,8] are the main candidates for intermediates. The results obtained in this work allow further elucidation of some mechanistic details of the HC-SCR reactions.…”

“…For the NO• selective reduction by heavier hydrocarbons, such as propane, etc., the present results are very important for understanding the nature of differences in the reaction mechanism on Cu-and Co-ZSM-5 suggested earlier [4]. Low reactivity of organic nitriles bound to Cu 2+ explains the absence of such pathway for Cu-ZSM-5.…”

“…In this case, contrary to earlier suggested schemes [7,8,12], gas phase NO is not required to produce N2. From this point of view, the high selectivity of CI-L-SCR on Co-ZSM-5 can be explained not only by the efficient pathway ofnitromethane transformation into nitriles [4], but also by the high thermal stability ofnitrosyls [3,4].…”

“…All rights reserve& 298 BELOSHAPKIN et aL: NIq~OGEN OXIDES experiments with Co-ZSM-5, the stages of the reaction of surface nitrates with propane [3] and methane [4] were also found to be rate-limiting. At these stages surface C, N-containing compounds are formed.…”

“…[5], acetone oxime was suggested as an intermediate in the NO• reduction by propane on Cu-ZSM-5, and a scheme of the reaction mechanism including this species was drawn. In our studies [3,4], the isotope shifts of the absorption bands of intermediates observed after the substitution of nitrogen and oxygen atoms in the NO molecule were analyzed. This allowed identifying these species as organic nitrates and ketenes (for Cu-ZSM-5), organic nitrates, nitriles or isocyanates (for Co-ZSM-5), and schemes of mechanisms including these intermediates have been suggested.…”