Three pathways to selective catalytic reduction of NO over Pt/Nb-AlMCM-41 under H2 with excess O2

Abstract

We selectively reduced NO under hydrogen with excess oxygen (H₂-SCR) on a Pt-loaded AlMCM-41 (Pt/AlM41) catalyst over two temperature regions, around 373 and 500 K. At 500 K, H₂-SCR was observed only when the catalyst contained aluminum ions. Addition of niobium to Pt/AlM41 (Pt/Nb-AlM41) expanded the active temperature region at 373 K and greatly improved NO conversion at 500 K. The NO conversion level at 373 K was maintained even at 80 000 h⁻¹ space velocity (SV), while conversion at 500 K decreased with increasing SV. Conversion at both temperatures was observed only in the presence of H₂. Reaction without O₂ produced N₂O below 373 K and NH₃ at 373 K and above. XAFS measurements indicated the formation of a niobium silicate-like species on AlM41, which stabilized metallic platinum even under oxidative conditions. In situ FT-IR spectra revealed NH₄⁺ species on Pt/Nb-AlM41 during H₂-SCR at 373–523 K. The catalytic activity, characterization, and surface adsorbates indicated three types of reaction in H₂-SCR on Pt/Nb-AlM41: (1) below 373 K, dissociative adsorption of NO on Pt occurred, and the liberated nitrogen atom reacted with NO to form N₂O. The remaining oxygen atom was removed with H₂. (2) At 350–450 K, NH₄⁺ was generated on Nb or Al by reaction of acidic hydroxyl groups with NH₃ produced from NO + H₂ on Pt. Ammonia-SCR then proceeded under NO + H₂ + O₂. (3) At 450–650 K, ammonia was selectively oxidized to N₂ or N₂O on Al or Nb–O–Al sites. The formation of an ammonia intermediate and subsequent SCR or oxidation of ammonia to N₂ or N₂O, depending on the reaction temperature, expand the utility of H₂-SCR.