Structural and vibrational properties of molybdena catalysts supported on alumina and zirconia studied by in situ Raman and FTIR spectroscopies combined with ¹⁸O/¹⁶O isotopic substitution

Molybdena/alumina and molybdena/zirconia catalysts with Mo surface densities in the range 1.1-5.2 Mo/nm² were studied by in situ vibrational (Raman and FTIR) spectroscopies and ¹⁸O/¹⁶O isotopic exchange experiments combined with in situ Raman spectra at 450 °C. The vibrational isotope effects and the combined interpretation of Raman and IR band wavenumbers and characteristics suggest a mono-oxo configuration for the deposited molybdena phase on both catalytic systems at low (∼1.5 Mo/nm ²) as well as at high (4-5 Mo/nm²) coverage, irrespective of extent of association (polymerisation). Isolated mono-oxo monomolybdates predominate on alumina at low loadings, while increasing relative amounts of mono-oxo polymolybdates are formed at high loadings. Mono-oxo polymolybdates and monomolybdates occur on zirconia, with the polymolybdates predominating already from low coverage (1.7 Mo/nm²). A "next-nearest-neighbor ¹⁸O/¹⁶O substitution" vibrational effect is observed, resulting in small red shifts (2-5 cm^-1) of the Mo¹⁶O Raman band wavenumber. The catalysts' behavior for the oxidative dehydrogenation of ethane is addressed and discussed with relevance to the configuration of deposited molybdena.