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

Abstract

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 NO_x reduction, especially at low temperatures, compared with the Pd/TiO₂ catalyst. However, the addition of Cu resulted in a negative effect on the NO_x 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 NO_x 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 NO_x 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., NH_x, which were crucial for the enhancement of H₂-SCR catalytic activity at low temperatures. The spillover hydrogen, although required for the formation of NH_x 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 NH_x intermediates.