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

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.