However, NO production increases significantly upon placing a Pt catalyst in the middle of the heating mantle (Fig. ). NO yields exceed thermal equilibrium across the entire gas composition range explored and vary non-linearly with O 2 mole fraction, maximizing near 5 × 10 -4 and decreasing at lower or higher O 2 concentrations. The results provide unambiguous evidence of NO production dependent upon both plasma and Pt catalyst and sensitivity to exact plasma composition. X-ray diffraction (Supplementary Fig. ) and Xray photoelectron spectroscopy (Supplementary Fig. ) observations confirm that the bulk and surface of the Pt catalyst are unmodified by plasma exposure.