Enhancement of decane-SCR-NO_x over Ag/alumina by hydrogen.Reaction kinetics and in situ FTIR and UV-vis study

摘要

Decane-SCR-NO_x,oxidation of NO and decane by molecular oxygen,and these reactions enhanced by hydrogen over Ag/alumina were investigated by monitoring of gaseous product composition,surface intermediates (in situ FTIR),and the state of silver (in situ UV-vis) under realistic reaction conditions in the steady-state and transient modes.It has been shown that oxidation of NO or decane by oxygen is greatly affected by the presence of decane or NO.Monodentate nitrates are formed preferentially and are more reactive compared with the bidentate species.Oxidation of decane mostly yields surface acetates,and the presence of NO_x favors the formation of formates (acrylates).The reaction steps most enhanced by the addition of hydrogen to the SCR-NOjt reaction are the transformations of the intermediate -CN species into -NCO and oxidation of the hydrocarbon to formates (acrylates).Although NO-NO_2 oxidation is also enhanced by hydrogen,this effect does not contribute to the increased rate of the SCR-NO_x reaction.A small part of very reactive Ag~+ (estimated to be < 5%) is reduced to metallic charged Ag_n~(delta+) clusters (n<=8) during both the decane- and H_2/decane-SCR-NO_x reactions.The number of Ag_n~(delta+) clusters formed depends mainly on the level of NO conversion to nitrogen,regardless of whether the conversion level is attained by the addition of hydrogen or by an increased concentration of decane or oxygen in the feed.The time-resolved responses of NO_x-N_2 conversion and of the number of Ag_n~(delta+) clusters to the addition/removal of hydrogen from the reactants indicate that the Ag_n~(delta+) clusters are mainly formed because of the reducing effect of adsorbed CH_xO-containing reaction intermediates.The active sites are suggested to be single Ag~+ ions or small Ag_2O species.Hydrogen itself takes part in the SCR-NO_x reaction.It is supposed that it dissociates,forms Ag-hydride,and enhances SCR-NO)x by initiation radical reactions.