Iron oxide and chromia supported on titania-pillared clay for selective catalytic reduction of nitric oxide with ammonia

TiO₂-pillared clay (PILC) with high surface area, large pore volume, and large interlayer spacing was used as the support for mixed Fe₂O₃ and Cr₂O₃ as the catalyst for selective catalytic reduction (SCR) of NO with NH₃. The Fe/Cr ratio was varied at a fixed total amount of oxide dopant of 10% (wt). The Fe-Cr/TiO₂-PILC with Fe/Cr = 3 showed the highest activity. Compared with commercial V₂O₅/TiO₂ catalysts, the activity (on a per gram basis) of the doped pillared clay was approximately twice as high under H₂O- and SO₂-free conditions and was approximately 40% higher under conditions with H₂O and SO₂. In addition, its activity for SO₂ oxidation was only 20%-25% of that of the V₂O₅-based catalysts. TPD of NH₃ on the Fe-Cr/TiO₂-PILC catalyst showed that both M=O and M-OH (M=Fe or Cr) were necessary for the SCR reaction. In situ IR spectra of NH₃ showed that there was a higher Brønsted acidity than the Lewis acidity on the surface under reaction conditions and that there existed a direct correlation between the SCR activity and the Brønsted acidity among pillared clays with different Fe/Cr ratios. These results, along with the transient response to O₂, indicated that a similar mechanism to that on the V₂O₅ catalyst was operative. The TiO₂-pillared clay used as the support also contributed to the high activity of the Fe-Cr catalyst. The TiO₂ pillars combined with the tetrahedral SiO₂ surfaces of the clay apparently gave rise to a high dispersion of Fe₂O₃.