Catalytic reactions of NO over 0-7 mol% Ba/MgO catalysts: II. Reduction with CH4 and CO

Article abstract of DOI:10.1006/jcat.1999.2649

Barium oxide supported on magnesium oxide (Ba/MgO) is an effective catalyst at elevated temperatures for the selective catalytic reduction (SCR) of nitric oxide, with methane as the reductant. The rate of the SCR reaction is greater than either the rate of reduction with CH₄, in the absence of O₂, or the rate of decomposition of NO. At 650°C the rate of the SCR reaction over 1 mol% Ba/MgO was 0.19 μmol g^-1s^-1 with 1% NO, 0.25% CH₄, and 0.5% O₂. The SCR reaction is believed to occur via gas-phase CH₃· radicals that are formed at the surface. First-order kinetics with respect to CH₄ is consistent with radical formation being the rate-determining step. The activities of the catalysts and the overall reaction order are influenced by CO₂, which is a product of the reaction, but also would be present in an exhaust stream. The inhibiting effect of CO₂ is much more significant for a catalyst containing 4 mol% Ba than for one containing 1 mol% Ba. In the latter case, the MgO support modifies the basicity of the BaO. In the absence of O₂, CO also is a reductant for NO, but if O₂ is present, the nonselective oxidation of CO occurs. That is, CO reacts preferentially with O₂ instead of NO.