Effects of Composition and Structure On the Basicity of Oxygen in Bismuth Molybdate and Bismuth Vanadomolybdate Thin Films

            Bismuth molybdate-based catalysts have been used for the production of acrolein and acrylonitrile from propylene for over 50 years.  While variants of this catalyst have been well studied in the intervening decades, the nature of the active sites involved in the partial oxidation or ammoxidation of propylene is still not fully understood [1].  Additionally, there is now growing interest in developing a direct route to acrolein and acrylonitrile from propane, a more abundant and less expensive feedstock.  Mixed bismuth vanadate-molybdate catalysts of the general composition Bi_1-x/3V_1-xMo_xO₄ have shown promise for this conversion, with maximum activity and selectivity to acrolein observed when x=0.45 [2].  However, the nature of the active sites present in this material, and the rationale for observed trends in activity and selectivity vs. composition, are even less well understood than in the parent bismuth molybdate.  In order to better understand the structure-function relationships in these catalysts, our group has developed a method for depositing thin films of bismuth molybdate or vanadomolybdate onto gold-coated silicon wafers, and then characterizing these thin films by x-ray photoelectron spectroscopy (XPS) and surface-enhanced Raman spectroscopy (SERS).  The position of the O1s peak in the XPS spectrum is used to determine the optical basicity of the metal oxide films.  Optical basicity is a quantitative measure of the Lewis base character of oxygen sites [3], and has been shown to correlate with activity of metal oxides for oxidation and oxidative dehydrogenation reactions [4].  SERS studies provide a second measure of the chemical environment of oxygen species in the catalyst:  the positions of V-O and Mo-O stretches in SERS spectra indicate the strength of V-O and Mo-O bonds, which in turn provides a measure of electron density on oxygen sites in the catalyst.  In Bi_1-x/3V_1-xMo_xO₄ films, end-member BiVO₄ (x=0) shows a strong Raman peak at 826 cm^-1, attributable to the symmetric vibration of VO₄^3- moieties.  As Mo⁺⁶ is increasingly substituted for V⁺⁵ up to the most active composition (Bi_0.85V_0.55Mo_0.45O₄, x=0.45), a second Raman peak at 880 cm^-1 appears and grows more prominent.  This peak is attributable to the symmetric stretch of the MoO₄^2- moiety, and suggests that only one type of molybdenum enviroment, and therefore two types of oxygen environments (associated to vanadate and molybdate) are present in the most active catalyst formulation.  This is in marked contrast to the x=1 end-member Bi₂Mo₃O₁₂, which has five different oxygen environments and exhibits v_Mo-O at 819, 844, 862, 903, 929, 961, and 995 cm^-1.  Development of a relationship between the structure of the bismuth vanadomolybdates and their Lewis basicity will provide a scientific basis for guiding the identification of such catalysts for the oxidation of propane. The methodology that we have developed should also be applicable to the study of a broad range of problems in metal oxide catalysis.

References

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2.  Kim Y.C.; Ueda W.; Moro-oka Y. Applied Catalysis 1991, 70, 175 – 187.

3.  Duffy J.A.; Ingram M.D. Journal of Non-Crystalline Solids 1976, 21, 373 – 410.

4.  Moriceau P.; Taouk B.; Bordes E.; Courtine P. Catalysis Today 2000, 61, 197 – 201.