(598f) Intermediate Species, Active Sites, and Reaction Rate Constants in Heterogeneous Catalysis from Modulation Excitation Spectroscopy

In this presentation, we will discuss a general application and implementation of modulation excitation (ME) of diffuse reflectance UV-visible (DR-UV-vis) and infrared spectroscopies (DRIFTS) for the in situ characterization of heterogeneous catalysts. The modulation method is not limited to sine and square perturbations commonly reported in the literature and can be applied to any waveform, and whose time domain spectra is analyzed by phase sensitive detection (PSD) using well-known Fourier and inverse Fourier transform (FT/IFT) software algorithms. Examples will be presented where intermediate species and active sites are discriminated from spectator species. Additionally, we will describe a general methodology, modulation excitation spectrokinetic analysis (MESKA), by which reaction rate constants can be determined for elementary steps in simple kinetic models from modulation excitation experiments. The methodology is based on the measurement of phase shifts (i.e., argument) in transient periodic data of reaction (gas and surface) species, which upon solution of material balance equations for the proposed catalytic cycle leads to a matrix with relationships between the tangent of the argument differences (TAD) among all species and the modulation frequency (ω). Such relationship pairs predict the phase shifts of species required to be determined experimentally via MES for the determination of specific reaction rate constants. Overall, the MESKA methodology in combination with ME-PSD of in situ spectroscopic data constitute a powerful set of tools to further advance our understanding of surface catalyzed reactions.