The present invention provides compositions, systems, and methods for achieving high efficiencies of nitrogen oxide (NOx) removal from exhaust gases while minimizing ammonia slip and sulfur dioxide oxidation. In one embodiment, a method of removing nitrogen oxides from an exhaust gas comprises providing a first catalyst layer, adding an ammonia-containing compound to the exhaust gas upstream of the first catalyst layer in excess of the stoichiometric equivalent of the nitrogen oxides in the exhaust gas, flowing the exhaust gas through the first catalyst layer, selectively catalytically decomposing ammonia in excess of the stoichiometric reaction equivalent of the nitrogen oxides in the exhaust gas, and catalytically reducing the nitrogen oxides by reaction with ammonia not selectively catalytically decomposed.