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.

Dual-Element Modulation Excitation XAS to Probe the Redox Surface Chemistry of a Ni–Fe Dry Reforming Catalyst

“…Under the imposed modulations, Ni thus largely retains its metallic Ni 0 state, unlike Fe. Collectively, these analyses confirm the lower affinity of Ni toward CO 2 , yet higher affinity to H 2 , compared to Fe, in line with the results of De Coster et al…”

“…The relatively larger contribution of Ni–O within Ni/MgFeAlO 4 post-TPR, 16 ± 3 versus 10 ± 1% in Ni/MgAl 2 O 4 , can be ascribed to the larger metal–metal oxide interfacial area and/or stronger MSI when Ni is deposited onto MgFeAlO 4 . This hypothesis is in line with the transmission electron microscopy (TEM) analysis of as-prepared Ni/MgAl 2 O 4 and Ni/MgFeAlO 4 , which revealed nanoparticle (NP) sizes of ∼6.3 and ∼4.3 nm, respectively. Since both samples were synthesized through the same stratagem using the deposition of presynthesized, well-defined Ni NPs, it is plausible to associate the larger NP sizes of Ni/MgAl 2 O 4 with less Ni–O interface sites than in Ni/MgFeAlO 4 , hence less anchoring of the NPs, leading to enhanced sintering upon calcination at 800 °C.…”

“…Per anti-correlated pair, it suffices to examine one of the two constituent spectra, in view of their mutual symmetry. For Fe, the in-phase (Δϕ PSD = 148°) demodulated spectrum (Figure S5) resembles that reported in prior work, wherein this was associated with concurring Fe 3+ ↔ Fe 0 and Fe 2+ ↔ Fe 0 changes. Comparison of this spectrum with the difference spectra (Figure S5 and associated text) confirms the occurrence of such redox events: the WL and pre-edge features correlate to the Fe 3+/2+ –Fe 0 difference spectra.…”

“…Fe K edge XANES spectra of activated “monometallic” MgFeAlO 4 (Figure S2A) are virtually identical to those of Ni/MgFeAlO 4 . This implies that, like in Ni/MgFeAlO 4 , part of Fe in MgFeAlO 4 has also segregated out of the spinel, leading to Fe 0 and MgFe 2+/3+ AlO 4 formation, which is confirmed by EXAFS analysis in Figure S2C (fit parameters in Table S1).…”

“…The resulting powder was then dried at 120 °C for 3 h and finally calcined at 800 °C (3 °C/min) for 1 h. The obtained catalysts are labeled “Ni/MgAl 2 O 4 ” and “Ni/MgFeAlO 4 ”, and their state after synthesis is termed “as prepared”. Inductively coupled plasma optical emission spectrometry (ICP-OES) indicated 9.6 wt % Fe in MgFeAlO 4 , 9.8 wt % Ni in Ni/MgAl 2 O 4 , and 8.9 wt % Fe and 9.0 wt % Ni for Ni/MgFeAlO 4 (i.e., Ni/Fe ratio ∼1/1), as published elsewhere …”