TIME-OF-FLIGHT SIMS STUDY OF HETEROGENEOUS CATALYSTS BASED ON PRASEODYMIUM AND MOLYBDENUM OXIDES

Time-of-flight secondary ion mass spectrometry (TOF SIMS) analyses hav e been performed on MoO3-Pr6O11 mixtures before and after their use in the selective oxidation of isobutene to methacrolein at 673 K. The th ree pure molybdate phases (Pr2MoO6, Pr2Mo3O12, Pr6MoO12) obtained inde pendently by the citrate route have also been examined to determine th eir respective SIMS fragmentation patterns. Although the Pr2MoO6 and P r2Mo3O12 phases were found to give the same molecular fragments, these phases could be distinguished by the relative intensity ratios of som e particular species. Identification of Pr6MoO12 was easier owing to t he presence of characteristic high-mass mixed Pr-Mo fragments. A linea r relationship was observed when plotting the experimental intensity r atios of several molecular fragments against the Pr/Mo bulk compositio n. The same behaviour was also noticed for a series of Pr6O11-MoO3 mix tures. Moreover, the examination of the 3-D crystal structure of the p ure molybdate phases indicates that the fragmentation patterns of thes e phases were directly related to their structure. As far as the used catalysts were concerned, the SIMS results showed that: (i) the experi mental intensity ratios of given fragments were much smaller in the us ed catalysts than in the fresh ones; (ii) fragments characteristic of the Pr6MoO12 phase were absent; and (iii) differences in the relative SIMS intensities of certain fragments showed the dominant presence of the Pr2Mo3O12 phase.