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 i nto surface reaction pathways associated with the nitridation by NH3 o f MoO3 thin films grown on metallic substrates. The samples formed can be seen as model hydrodenitrogenation (HDN) catalysts, but the more-c ontrolled surface morphology allows spectral features to be studied in the absence of charging effects that can interfere with such measurem ents on high-area samples. Observed core level and valence spectra are consistent with the MoO3 being reduced, but the degree of reduction d epends on the reaction temperature. Heating to 350 degrees C indicates some conversion to Mo(+5) and 'O-rich' Mo(+4) components, while heati ng to 450 degrees C and to 700 degrees C give respectively a 'N-rich' Mo(+4) form and a Mo(+3) oxynitride as the dominant components. It is concluded that the nitridation of MoO3 by NH3 involves initial hydroge nation, with subsequent elimination of water, and the effective replac ement of O by N as the reduction continues. Surface compositions deter mined here during the nitridation process contrast with conclusions re ached previously for the evolution of bulk phases as deduced by X-ray diffraction. (C) 1998 Elsevier Science B.V. All rights reserved.