Modulated excitation Raman spectroscopy of vanadia-based catalysts

We report the increased level of mechanistic information that Raman spectroscopy can provide when combined with modulated excitation experimentation. A V₂O₅/TiO₂ catalyst for the selective catalytic reduction (SCR) of NO_x was subjected to periodic NH₃ pulses while recording time-resolved Raman spectra simultaneously. Through phase sensitive detection, we confirmed unambiguously that both VO_x and TiO₂ play a dynamic role in the SCR process. Only a well-defined portion of VO_x species (i.e., those that are coordinatively unsaturated) act as catalytic active centers while the majority remain largely unresponsive to the NH₃ pulses. Although TiO₂ is not directly involved in the SCR reaction, the phase-resolved Raman spectra demonstrate that it acts as a reservoir of NH₃ molecules that can be mobilized to the active sites. The inherent depth of these molecular insights goes beyond what is currently achievable with Raman experiments under steady-state conditions. Modulated excitation Raman spectroscopy can therefore advance the fundamental understanding of SCR and other metal oxide-catalyzed reactions.