Investigating the mechanism of the H2-assisted selective catalytic reduction (SCR) of NOx with octane using fast cycling transient in situ DRIFTS-MS analysis

A mechanistic study of the H2-assisted Selective Catalytic Reduction (SCR) of NOx with octane as reductant over a Ag/Al2O3 catalyst was carried out using a modified DRIFTS cell coupled to a mass spectrometer. Using fast transient cycling switching of H2, with a time resolution of a few seconds, it was possible to differentiate potential reaction intermediates from other moieties that are clearly spectator species. Using such a periodic operation mode, effects were uncovered that are normally hidden in conventional transient studies which typically consist of a single transient. In experiments based on a single transient addition of H2 to, or removal of H2 from, the SCR feed, it was found that the changes in the concentrations of gaseous species (products and reactants) were not matched by changes at comparable timescales of the concentration of surface species observed by IR. This observation indicates that the majority of surface species observed by DRIFTS under steady-state reaction conditions are spectators. In contrast, under fast cycling experimental conditions, it was found that a surface isocyanate species had a temporal response that matched that of 15N2. This suggests that some of the isocyanate species observed by infrared spectroscopy could be important intermediates in the hydrogen-assisted SCR reaction although it is emphasised that this may be dependent on the way in which the infrared spectra are obtained. It is concluded that the use of fast transient cycling switching techniques may provide useful mechanistic information under certain circumstances.