A study of air plasma in the presence of adjacent catalyst powder (γ-Al 2 O 3 ) improved air plasma energy efficiency for nitrogen oxides (NO x ) formation significantly.  However, the previous configuration (pin-to-pin) was poorly suited to identify the mechanisms of NO x formation on the catalyst surface. A more fully characterized plasma afterglow jet was designed for NO x formation coupled with heterogeneous catalysis in this report.  The energy cost for NO x production using this air plasma jet decreases with the increasing discharge current and tends to a stable value up to about 270 GJ/tN. However, the activated Al 2 O 3 (γ-Al 2 O 3 ) coupled with the afterglow plume of the air plasma jet does not obviously improve the energy efficiency for NO x production, even with higher gas flow rate or heated catalyst (up to 1000 °C). The overwhelming majority of the NO x species are apparently generated inside the plasma jet and most of the active intermediate species which could enhance the NO x production apparently do not survive to reach the catalyst surface in the afterglow plume. These results suggest that the catalyst should be placed as close to the plasma zone as possible. One option would be to 'fluidize' catalyst particles to improve NO x formation.