Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH3

The structure of iron and copper sites during the selective catalytic reduction (SCR) of NOx by NH3 and related reactions (NH3 adsorption/oxidation, NOx oxidation) has been elucidated by spatially- and time-resolved X-ray absorption spectroscopy (XAS) along the catalyst bed over Fe-containing BEA and ZSM-5 zeolites as well as Cu-SAPO-34 silicoaluminophosphate. Strong gradients of the Fe and Cu oxidation state are present along the catalyst bed for the processes involving NH3 and NOx (SCR) and less pronounced for NH3 oxidation, whereas the catalyst state in the NOx containing feed resembles that of a catalyst exposed to air. The variation in the oxidation state is strongly correlated to the concentration of NH3 and is more pronounced in the presence of NOx. For temperatures higher than 250 degrees C the Fe and Cu sites at the beginning of the catalyst bed stay in partially reduced state, whereas they are more oxidized in the later zones where NH3 and NOx concentrations decrease. For temperatures lower than 250 degrees C a reverse effect is seen for Fe-zeolites where the beginning of the catalyst bed contained more oxidized iron species than at the end of the catalyst bed. This is attributed to NH3 inhibition. The obtained data allow to conclude that both NH3 and NOx are involved in a reaction over the corresponding transition metal site, and its reoxidation is a rate-limiting step of the NH3-SCR for these catalysts.