XPS STUDIES OF THE NITRIDATION OF MOO3 THIN-FILMS ON ALUMINA AND SILICA SUPPORTS

X-ray photoelectron spectroscopy has been used to characterize thin fi lms of MoO3 formed on planar oxidized supports, namely AlOx (from alum inum strip) and SiO2 (from silicon wafer). Comparisons are made with b ehaviour for MoO3 on metallic Mo substrate. It is observed that on cal cination at 450 degrees C, the Mo 3d spectral features shift to higher binding energy for MoO3/AlOx and to lower binding energy for MoO3/SiO 2, On nitridation by heating in NH3, it is found that the samples on t he oxide supports show easier O-N replacement compared with the MoO3/M o system. In general, the nitridation behaviour for MoO3/AlO, is simil ar to that of MoO3/Mo, but Mo species in MoO3/SiO2 seem to be more eas ily reduced (Mo(0) is detected for the SiO2 system but not for AlOx). Comparisons of heating rates for the second nitridation step from 350 to 450 degrees C were made for the MoO3/Mo and MoO3/AlOx samples. Diff erences between the high heating rate (100 K/h) and the low heating ra te (40 K/h) are incremental but definite. The lower heating rate is fa vourable both for the O-N replacement and for the metal reduction. For example, more Mo(+3) is present after the nitridation to 450 degrees C when the low-heating rate regime is used, compared with that formed when the heating is done at the higher rate. (C) 1998 Elsevier Science B.V. All rights reserved.