NO x reduction by hydrocarbons and hydrogen on metal oxide and zeolite based catalysts in lean conditions (2007)

The removal of nitrogen oxides (NOx) from exhaust and flue gases is a demanding technical problem particularly in the presence of excess oxygen. The reduction of nitrogen oxides by hydrocarbons (HC-SCR) is a promising but also challenging method to utilize in applications, where fuels like diesel oil or natural gas might be used as a reductant. In this work new high activity HC-SCR catalysts were developed based on metal oxides (cobalt, indium, gallium) supported on sol-gel aluminum oxide or ZSM-5 zeolite. The interaction of indium and cobalt with aluminum oxide was studied by the catalysts prepared by impregnation, sol-gel, ion exchange and mechanical mixing methods. The active compounds were Co2+ on Co/Al2O3, In2O3 on In/Al2O3 and intrazeolitic InO+ on In/ZSM-5.&#13;\n&#13;\nA significant promotion in NOx conversions was reached by mechanically mixing a small amount (5%) of Mn3O4 with In/Al2O3 or Ga/Al2O3 in propene-SCR and Mn3O4 or Pt/Al2O3 with In/ZSM-5 in methane-SCR. Gaseous or mobile surface intermediates are necessary to explain the observed promotion. Pt/alumina and Mn3O4 seemed themselves to catalyze the N2 formation step in addition to promotion on HC and NO oxidation.&#13;\n&#13;\nSurface intermediates were detected by in situ FTIR in the presence of flowing reaction gas mixtures. Before the reaction initiation the catalyst surface was usually covered with the inhibiting compounds (nitrates, carbonates), which disappeared at higher reaction temperatures (300-400°C) and the amount of surface reactants remained low in HC-SCR operation window (&gt; 300°C). The derived reaction mechanisms for SCR by light hydrocarbons (C1-C3) included six potential surface reductants (H2NCO, HNCO, NCO, NH2, NH, N) for NO. The reductants with NHi functional groups were found to be final reductants and the adsorbed NH2 (amine) was the most probable in a single surface reaction.&#13;\n&#13;\nThe micro-kinetic modeling based on surface reactions was used parallel to other methods to evaluate the reaction routes and dynamics. The kinetic models for propene-SCR with Co/alumina and methane-SCR with In/ZSM-5 were based on the assumption that adsorbed H2NCO (amide) acts as the reductant for NO. The improved model for methane-SCR on In/ZSM-5 was based on a mechanism where NO is reduced by NH2. The binary catalyst combination (In/ZSM-5+Pt/Al2O3) was modeled using two different NO reduction mechanisms assuming NH2 path on indium sites and NO decomposition to nitrogen on platinum sites. The models were able to quantitatively describe the concentrations of all observed gaseous reactants and products and as well as the defined surface coverages in propene- and methane-SCR. Finally, a generic reaction mechanism for HC-SCR was proposed.&#13;\n&#13;\nThe analysis of NO-H2 reactions in transient conditions on Pt, Pt-Rh and Rh three-way catalysts was used as a tool to categorize the NOx reduction mechanisms. The NOx reduction studies resulted in four mechanisms to form nitrogen: 1) N route, 2) N2O route, 3) ONNO route and 4) NHi route. In fact, nitrogen formation on three-way and HC-SCR catalysts has similar final steps but requires different catalysts due to diverse conditions.