Surface structure sensitivity of manganese oxides for low-temperature selective catalytic reduction of NO with NH3

Hollandite-type manganese oxides with K+ or H+ cations in the tunnels (K-Hol or H-Hol) were successfully synthesized and investigated in low-temperature selective catalytic reduction of NO by NH3. The results of the catalytic tests revealed that both K-Hol and H-Hol with almost same catalytic activities had much higher reaction rates than beta-MnO2 under the same conditions, and even at a high gas hourly space velocity of 160,000 h(-1), K-Hol obtained more than 90% conversions of NO in a wide temperature window of 100-300 degrees C. The high resolution transmission electron microscopy observations showed that both K-Hol and beta-MnO2 were tetragonal prism-shaped nanorods with same exposed {110} planes, and the atoms in the {110) planes of K-Hol arranged to form semitunnel structures, while the {110) planes of beta-MnO2 were relatively smooth surfaces. The temperature-programmed reduction by H-2 and thermal gravimetric analyses indicated that active surface lattice oxygen atoms of K-Hol were around 1.6% with respect to total lattice oxygen atoms. The transition reactions of NH3 demonstrated that K-Hol with special semitunnel structured surface and active surface lattice oxygen showed much stronger ability to efficiently adsorb and activate NH3 molecules than beta-MnO2. Hence, both efficient semitunnel structured external surfaces and high active surface lattice oxygen atoms predominantly accounted for the high catalytic activities. (C) 2010 Elsevier B.V. All rights reserved.