A fast transient kinetic study of the effect of H2 on the selective catalytic reduction of NOx with octane using isotopically labelled 15NO

The H₂-assisted hydrocarbon selective catalytic reduction (HC-SCR) of NO_x was investigated using fast transient kinetic analysis coupled with isotopically labelled ¹⁵NO. This allowed monitoring of the evolution of products and reactants during switches of H₂ in and out of the SCR reaction mix. The results obtained with a time resolution of less than 1 s showed that the effect on the reaction of the removal or addition of H₂ was essentially instantaneous. This is consistent with the view that H₂ has a direct chemical effect on the reaction mechanism rather than a secondary one through the formation of "active" Ag clusters. The effect of H₂ partial pressure was investigated at 245 °C, it was found that increasing partial pressure of H₂ resulted in increasing conversion of NO and octane. It was also found that the addition of H₂ at 245 °C had different effects on the product distribution depending on its partial pressure. The change of the nitrogen balance over time during switches in and out of hydrogen showed that significant quantities of N-containing species were stored when hydrogen was introduced to the system. The positive nitrogen balance on removal of H₂ from the gas phase showed that these stored species continued to react after removal of hydrogen to form N₂.