Time-of-flight SIMS study of heterogeneous catalysts based on praseodymium and molybdenum oxides

Abstract

Time-of-flight secondary ion mass spectrometry (TOF SIMS) analyses have been performed on MoO₃–Pr₆O₁₁ mixtures before and after their use in the selective oxidation of isobutene to methacrolein at 673 K. The three pure molybdate phases (Pr₂MoO₆, Pr₂Mo₃O₁₂, Pr₆MoO₁₂) obtained independently by the citrate route have also been examined to determine their respective SIMS fragmentation patterns. Although the Pr₂MoO₆ and Pr₂Mo₃O₁₂ phases were found to give the same molecular fragments, these phases could be distinguished by the relative intensity ratios of some particular species. Identification of Pr₆MoO₁₂ was easier owing to the presence of characteristic high-mass mixed Pr–Mo fragments. A linear relationship was observed when plotting the experimental intensity ratios of several molecular fragments against the Pr/Mo bulk composition. The same behaviour was also noticed for a series of Pr₆O₁₁–MoO₃ mixtures. Moreover, the examination of the 3-D crystal structure of the pure molybdate phases indicates that the fragmentation patterns of these phases were directly related to their structure. As far as the used catalysts were concerned, the SIMS results showed that: (i) the experimental intensity ratios of given fragments were much smaller in the used 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.