Three pathways to selective catalytic reduction of NO over Pt/Nb-AlMCM-41 under H-2 with excess O-2

摘要

We selectively reduced NO under hydrogen with excess oxygen (H-2-SCR) on a Pt-loaded AlMCM-41 (Pt/AlM41) catalyst over two temperature regions, around 373 and 500 K. At 500 K, H-2-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 80000 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 H-2. Reaction without O-2 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 H-2-SCR at 373-523 K. The catalytic activity, characterization, and surface adsorbates indicated three types of reaction in H-2-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 H-2. (2) At 350-450 K, NH4+ was generated on Nb or Al by reaction of acidic hydroxyl groups with NH3 produced from NO + H-2 on Pt. Ammonia-SCR then proceeded under NO + H-2 + O-2. (3) At 450-650 K, ammonia was selectively oxidized to N-2 or N2O on Al or Nb-O-Al sites. The formation of an ammonia intermediate and subsequent SCR or oxidation of ammonia to N-2 or N2O, depending on the reaction temperature, expand the utility of H-2-SCR.