In the high wavenumber region (> 650 cm -1 ), the vibrational modes of the intermediates were visible. While not every band could be unambiguously assigned, several species were tentatively assigned on the basis of previous studies using Raman spectroscopy on Cu-based catalysts. Initially, *NO2 could be seen through vibrations at 1240, 1330 and 1380 cm -1 , similarly indicating it to be an intermediate in the reaction pathway.