Complementary operando insights into the activation of multicomponent selective propylene oxidation catalysts

Two Bi–Mo–Co–Fe–O catalysts were synthesized by flame spray pyrolysis and tested for their catalytic performance in selective oxidation of propylene to acrolein. Pronounced structural changes during temperature–programmed oxidation and reaction were observed by operando X–ray absorption spectroscopy, X–ray diffraction, Raman spectroscopy, and thermogravimetric analysis. During oxidative treatment, mainly binary oxide phases (α–Bi$^{2}$Mo$^{3}$O$^{12}$, β–CoMoO$^{4}$, Fe$^{2}$ (MoO$^{4}$)$^{3}$) were observed, but single (MoO$^{3}$) or ternary (Bi$^{3}$ (FeO$^{4}$)(MoO$^{4}$)$^{2}$) oxides also formed depending on the relative elemental catalyst composition. During propylene oxidation, the reduction of Fe$^{3+}$ to Fe$^{2+}$ led to a strong rise in activity and induced further phase transformations. MoO$^{3}$ was found to be unselective towards acrolein but was essential in binding other single oxides. The formation of β–Co$^{0.7}$Fe$^{0.3}$MoO$^{4}$ and Bi$^{3}$ (FeO$^{4}$)(MoO$^{4}$)$^{2}$ as well as their synergistic interplay with α–Bi$^{2}$Mo$^{3}$O$^{12}$ are key factors for high performance. The combination of complementary operando methods was crucial to reveal new structure–activity/selectivity correlations, therefore bridging the knowledge gap between simplified model systems and complex applied catalysts.