Modulation Engineering: Stimulation Design for Enhanced Kinetic Information from Modulation-Excitation Experiments on Catalytic Systems

Abstract

Modulation excitation (ME) with phase-sensitive detection (PSD) is an emerging strategy to selectively characterize catalytic species that actively participate in a chemical reaction. The commonly applied square-wave (SW) modulations, however, contain a limited frequency content, impeding rigorous kinetic analysis of short-lived reaction intermediates through PSD analysis by considering higher-order harmonics. To overcome this bottleneck, a “modulation engineering” approach is designed, whereby stimulation shapes with a complementary frequency content are superposed onto a base modulation, thus subjecting the system to a more complex frequency pattern in a single experiment. Building on practical and mathematical considerations, this design scheme’s feasibility is demonstrated using a superposition of SW and rectangular wave stimulations, applied to H2/CO2 concentration modulation-excitation X-ray absorption spectroscopy of a Ni/MgFeAlO4 methane dry reforming (DRM) catalyst at the Fe and Ni K edge. Under redox conditions, PSD evidences Ni ↔ Ni2+ and Fe0 ↔ Fe2+ ↔ Fe3+ redox events, wherein Fe2+ ↔ Fe3+ transitions exhibit faster kinetics, adding insight into this material’s redox functionalities under DRM conditions. This approach is extendable to other ME-based characterization techniques and provides a general, time-efficient framework to expand the transient kinetic insights that can be obtained for catalytic systems through ME with PSD.