Abstract:
Operando X-ray absorption spectroscopy (XAS) is a powerful technique for probing the structure of heterogeneous catalysts under realistic reaction conditions. However, extracting mechanistic insight from operando XAS remains challenging because the measured spectra represent ensemble-averaged responses of dynamically evolving catalysts and often contain overlapping contributions from active intermediates, catalyst precursors, and spectator species. As a result, identifying the true catalytic active sites requires experimental strategies that directly correlate spectroscopic observations with catalytic performance.
In this talk, I will present original approaches for designing operando XAS experiments that distinguish active species from spectators and reveal the dynamic structure of catalytic active sites. These approaches combine the design of catalytic materials, steady-state and transient operando measurements, chemometric analysis, and kinetic modeling. Several case studies will illustrate how these methodologies provide mechanistic insight beyond conventional operando characterization.
For preferential CO oxidation over supported Pt–FeOₓ catalysts, we demonstrated that the most active and selective sites consist of Fe²⁺–O species located in direct contact with metallic Pt⁰ nanoparticles, and showed how their concentration and catalytic performance can be enhanced through tailored hydrogen–water pretreatments [1,2]. For CO oxidation on Pt nanoparticles on ceria-based oxides, we demonstrate how the nature of the rate-determining step influences the apparent kinetics and the redox state of active cerium atoms.[3] In the selective oxidation of alcohols over sub-monolayer VOₓ catalysts supported on TiO₂ and CeO₂, we revealed fundamentally different promotion mechanisms: while TiO₂ primarily enhances the reducibility of V⁵⁺ species, CeO₂ actively participates in the catalytic cycle together with vanadium, forming mixed active sites whose structure and reactivity can be systematically tuned by the vanadium loading.[4,5]. For the Pd–Zn catalytic system for CO2 hydrogenation to methanol, we established a comprehensive structure–function map. Using XAS, we deconvoluted the distinct spectral signatures of all major structural components. Statistical analyses, supported by kinetic measurements, enabled us to show that methanol and CO are produced through parallel pathways on different active sites.
These examples demonstrate that combining operando XAS with reaction kinetics enables quantitative determination of active-site concentrations and their intrinsic catalytic activity. Direct comparison of catalytic turnover rates with the rates of catalyst-site transformations establishes causal relationships between catalyst structure and function. Such quantitative methodologies are essential for advancing operando XAS from a predominantly descriptive characterization technique toward a predictive tool for catalyst design and mechanistic understanding.
Keywords – heterogeneous catalysis, operando XAS, active sites, structure-activity relationships.
References
[1] Sadykov, I. I.; Sushkevich, V. L.; Krumeich, F.; Nuguid, R. J. G.; van Bokhoven, J. A.; Nachtegaal, M.; Safonova, O.
V. Platinum-Iron(II) Oxide Sites Directly Responsible for Preferential Carbon Monoxide Oxidation at Ambient Temperature: An Operando X-Ray Absorption Spectroscopy Study**. Angew. Chem. Int. Ed. 2023, 62 (1)
[2] Sadykov, I. I.; Palagin, D.; Krumeich, F.; Plokhikh, I. V.; van Bokhoven, J. A.; Nachtegaal, M.; Safonova, O. V. Water-Assisted Generation of Catalytic Interface: The Case of Interfacial Pt-FeOx(OH)y Sites Active in Preferential Carbon Monoxide Oxidation. J. Catal. 2024, 429, 115263.
[3] Kopelent, R.; Tereshchenko, A.; Guda, A.; Smolentsev, G.; Artiglia, L.; Sushkevich, V. L.; Bugaev, A.; Sadykov, I. I.; Baidya, T.; Bodnarchuk, M.; Van Bokhoven, J. A.; Nachtegaal, M.; Safonova, O. V. Enhanced Reducibility of the Ceria–Tin Oxide Solid Solution Modifies the CO Oxidation Mechanism at the Platinum–Oxide Interface. ACS Catal. 2021, 11 (15), 9435–9449.
[4] Zabilska, A.; Clark, A. H.; Moskowitz, B. M.; Wachs, I. E.; Kakiuchi, Y.; Copéret, C.; Nachtegaal, M.; Kröcher, O.; Safonova, O. V. Redox Dynamics of Active VO x Sites Promoted by TiO x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS. Jacs Au 2022, 2 (3), 762–776.
[5] Zabilska, A.; Zabilskiy, M.; Nuguid, R. J. G.; Clark, A. H.; Sadykov, I. I.; Nachtegaal, M.; Kröcher, O.; Safonova, O.
V. Origin of the Activity Trend in the Oxidative Dehydrogenation of Ethanol over VOx/CeO2**. Angew. Chem. Int. Ed. 2023, 62 (18), e202301297.