X-ray photoelectron spectroscopy studies of the reduction of MoO3 thin films by NH3

X-ray photoelectron spectroscopy (XPS) has been used to gain insight into surface reaction pathways associated with the nitridation by NH3 of MoO3 thin films grown on metallic substrates. The samples formed can be seen as model hydrodenitrogenation (HDN) catalysts, but the more-controlled surface morphology allows spectral features to be studied in the absence of charging effects that can interfere with such measurements on high-area samples. Observed core level and valence spectra are consistent with the MoO3 being reduced, but the degree of reduction depends on the reaction temperature. Heating to 350°C indicates some conversion to Mo(+5) and `O-richʹ Mo(+4) components, while heating to 450°C and to 700°C give respectively a `N-richʹ Mo(+4) form and a Mo(+3) oxynitride as the dominant components. It is concluded that the nitridation of MoO3 by NH3 involves initial hydrogenation, with subsequent elimination of water, and the effective replacement of O by N as the reduction continues. Surface compositions determined here during the nitridation process contrast with conclusions reached previously for the evolution of bulk phases as deduced by X-ray diffraction.