Methods and systems are provided for selective catalytic reduction of NOx with a low molecular low molecular weight aldehyde, e.g., acetaldehyde, as a reductant using a catalyst system including two catalysts. An exhaust stream containing an amount of NO from a combustion operation is provided. A portion of the exhaust stream is introduced to a first catalyst to convert the NO to NO2. The exhaust stream from the first catalyst with NO2 and a reductant stream containing a low molecular weight aldehyde, e.g., acetaldehyde, are introduced to the second catalyst to convert the NO2 to N2. The first catalyst can be bulk Co3O4, Ru or Pt loaded on alumina; the second catalyst can be various zeolites, or zeolites loaded with potassium.