Application of modulation excitation-phase sensitive detection-DRIFTS for in situ/operando characterization of heterogeneous catalysts

This work describes the application of in situ/operando modulation excitation-phase sensitive detection-diffuse reflectance Fourier transform spectroscopy (ME-PSD-DRIFTS) for the characterization of heterogeneous catalysts. ME was enabled using a low void-volume diffuse reflectance cell which allowed rapid gas exchange (gas residence times <2 s) and by periodic feed concentration changes to the reaction cell by a simple switching valve system that provided quasi-square shaped modulation. PSD analysis of a relatively large data set of rapid scan spectra over many periodic cycles was done via a discrete Fourier transform (DFT)/frequency filtering/inverse discrete Fourier transform (IDFT) procedure. A general description of the ME-PSD principle, mathematical framework, guidelines for planning, running, and interpreting results was provided while focusing on ME-PSD-DRIFTS. Aspects such as the modulation frequency and amplitude, modulation waveform, sampling rate, in situ cell residence time, and limitations and future opportunities for ME-PSD-DRIFTS were also discussed. The proposed DFT/IDFT methodology uncovered the use of frequency magnitude plots for the evaluation of spectra baseline shifts, signal response to modulation, the response waveform type, noise, and signal decay/growth. Additionally, ethanol dehydration on gamma-Al2O3 was presented as an example of application of the ME-PSD-DRIFTS methodology.