The elementary processes during the fixation of nitrogen by plasma catalysis are studied in a low-pressure plasma experiment with N$2$ and O$_2$ as source gases. The formation of surface groups on an iron oxide foil is monitored with infrared reflection absorption spectroscopy. Surface nitrates (NO$_3^-$) are formed when the substrate is exposed to a 1:1 N$_2$:O$_2$ plasma, as well as N$_2$O(g), NO(g), NO$_2$(g), and O$_3$(g) in the gas phase. It is postulated that NO${1,2}$(g) species created by the plasma, adsorb at the surface and create these nitrates. This constitutes an intermediate step for nitrogen oxidation by plasma catalysis.