The use of short time-on-stream insitu spectroscopic transient kinetic isotope techniques to investigate the mechanism of hydrocarbon selective catalytic reduction (HC-SCR) of NOx at low temperatures

The problem of differentiating between active and spectator species that have similar infrared spectra has been addressed by developing short time-on-stream insitu spectroscopic transient isotope experimental techniques (STOS-SSITKA). The techniques have been used to investigate the reaction mechanism for the reduction of nitrogen oxides (NOx) by hydrocarbons under lean-burn (excess oxygen) conditions on a silver catalyst. Although a nitrate-type species tracks the formation of isotopically labeled dinitrogen, the results show that this is misleading because a nitrate-type species has the same response to an isotopic switch even under conditions where no dinitrogen is produced. In the case of cyanide and isocyanate species, the results show that it is possible to differentiate between slowly reacting spectator isocyanate species, probably adsorbed on the oxide support, and reactive isocyanate species, possibly on or close to the active silver phase. The reactive isocyanate species responds to an isotope switch at a rate that matches that of the rate of formation of the main product, dinitrogen. It is concluded that these reactive isocyanates could potentially be involved in the reduction of NOx whereas there is no evidence to support the involvement of nitrate-type species that are observable by infrared spectroscopy.

A short time-on-stream SSITKA method can discriminate between active and inactive surface species under conditions where conventional experimental methods fail.Figure optionsDownload high-quality image (87 K)Download as PowerPoint slideHighlights
► Quick SSITKA identifies possible reaction intermediates under real conditions.
► The technique has been applied to (HC-SCR) on Ag catalysts.
► Nitrate-type species do not satisfy the criteria for an intermediate.
► Most isocyanates seen by infrared are spectators.
► An active isocyanate species that may be a reaction intermediate has been identified.