Understanding the promotional effect of 3d transition metals (Fe, Co, Cu) on Pd/TiO2 for H2-SCR

The addition of 3d transition metal (Fe, Co, Cu) oxides to Pd/TiO2 catalysts was investigated for the selective catalytic reduction of NO with H2 in the presence of O2. It was found that the addition of Fe and Co resulted in a promotional effect on the NOx reduction, especially at low temperatures, compared with the Pd/TiO2 catalyst. However, the addition of Cu resulted in a negative effect on the NOx reduction. Transient reaction experiment results showed that the amounts of stored H2 on the 1Pd–5Fe/TiO2 and 1Pd–5Co/TiO2 catalysts were similar, which were twice that on the 1Pd–5Cu/TiO2 catalyst, suggesting that the stored hydrogen alone was not the crucial factor for the effect of the 3d transition metal additives on the H2-SCR reaction. Operando diffuse reflectance infrared spectroscopy (DRIFTS) results showed that addition of 3d transition metal oxides affected the formation and distribution of stored NOx species. Moreover, the bridging nitrates, monodentate nitrates and bidentate nitrates played different roles in the various Pd–M/TiO2 catalysts. For the 1Pd–5Cu/TiO2 catalyst, most of the stored NOx species were only spectator species which were not active for the H2-SCR reaction. However, for the 1Pd–5Fe/TiO2 and 1Pd–5Co/TiO2 catalysts, the monodentate nitrates were the main active species that were involved (along with spiltover hydrogen) in the formation of intermediates, i.e., NHx, which were crucial for the enhancement of H2-SCR catalytic activity at low temperatures. The spillover hydrogen, although required for the formation of NHx intermediates, is abundant with or without promoters. This work shows that the formation of monodentate nitrates is the rate limiting step in the formation of the active NHx intermediates.