Sensitivity enhancement and dynamic behavior analysis by modulation excitation spectroscopy: Principle and application in heterogeneous catalysis

Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

“…Whilst the mathematical basis of this approach [184][185][186][187] is well beyond the scope of this review the major benefits of this method may be summarized as follows. In the first instance it permits a considerable enhancement in the sensitivity of any given technique to changes that are related to the applied perturbation and can therefore reveal aspects of the chemistry at hand that are otherwise invisible.…”

“…MES, using phase sensitive detection (PSD), is founded upon the principle that the behavior of a system that may be reversibly perturbed (for instance redox cycling) can then be analyzed on the basis that an applied square wave perturbation may be expressed as Fourier series wherein the square wave is viewed as an infinite summation of odd-frequency sine functions [184][185][186][187]. Almost by definition, therefore, any decent (i.e., continually reformed) catalytic system should be amenable to this form of experimentation and subsequent analysis.…”

“…The infrared spectroscopic community were the first to adopt this type of approach to studying catalysis [183][184][185][186][187][188][189] and hence the soubriquet MES. However, this is an a posteriori method that is not limited to spectroscopic techniques; providing the requisite reversible modulation may be realized, any data, be it spectroscopic or scattering in nature, may be subsequently handled in this manner.…”

Trends and advances in Operando methodology

“…Among them, steady-state isotopic transient kinetic analysis (SSITKA) is an experimental technique to follow the kinetics of catalytic reactions by varying reactant concentration in the form of isotopomers without disturbing the chemistry [40]. Another approach to add selectivity with drastic improvement in detection sensitivity is modulation excitation spectroscopy (MES) [41]. MES uses periodic change of an external parameter (e.g.…”

“…Spectra are continuously measured over a number of stimulus-relaxation cycles and averaged into one cycle (first S/N improvement) and then the one-cycle data are further processed by phase-sensitive detection (PSD) to filter out static signal (e.g. due to solvent and catalyst support) and noise (second S/N improvement) and to kinetically differentiate evolving signals in the so-called phase domain [41]. MES can be combined with any techniques such as IR [42] and XAFS [43].…”