Laser diagnostics for catalytic reaction

Background:

Catalytic systems play a central role in reducing greenhouse gas emissions and local pollutants across the chemical, energy, and transportation sectors. Their performance, however, is governed by a complex interplay of processes occurring over multiple spatial and temporal scales. At the microscopic level, the atomic structure and chemical state of the active sites determine the intrinsic catalytic activity, whereas at the meso- and macroscale, catalyst architecture affects heat and mass transport. At the reactor scale, flow conditions (laminar or turbulent), temperature fields, and the spatial distribution of reactants and products further influence overall reaction rates. Under realistic operating conditions, catalytic systems exhibit strongly dynamic behavior. Catalyst activity evolves in both space and time due to variations in flow composition, temperature, and catalyst aging. Consequently, local reaction rates are often not solely controlled by intrinsic surface kinetics but are significantly influenced, or even limited, by mass and heat transfer processes.

Conventional diagnostic techniques, such as sampling with thin capillaries, already provide insights into the spatial evolution of substance concentrations in the direction of flow. However, such methods are invasive, can potentially disturb the flow field and concentration profile, and usually have limited temporal resolution. Consequently, they are often not well-suited for capturing the full spatial and temporal complexity of transient phenomena in reactive flows over catalytic surfaces. This creates a significant need for non-invasive, spatially and temporally resolved diagnostic techniques that allow the composition of the gas phase and potentially also the state of the catalyst to be directly investigated under in-situ conditions.

Project: Our research aims to deepen the understanding of the interplay between catalytic kinetics and mass transport by measuring spatially and temporally resolved reactant and product concentration fields using non-invasive laser-based diagnostics such as planar laser-induced fluorescence (PLIF) and Raman spectroscopy. For this purpose, a reactor with excellent optical access is essential. In PLIF measurements, a thin laser sheet is introduced into the reactor to excite selected target species, for example NO, OH, or HCHO. Since the laser sheet is aligned parallel to the flow direction and perpendicular to the catalytic surface, two-dimensional concentration distributions above the catalyst can be visualized directly. Alternatively, Raman spectroscopy might be employed to obtain spectrally resolved one-dimensional species profiles of major species. In this case, the laser is focused into a narrow line, enabling the determination of 1D concentration profiles. Information on concentration gradients normal to the catalytic surface is crucial, as under many operating conditions the overall reaction rate is often governed not by intrinsic surface kinetics, but by mass transfer limitations. Without wall-normal concentration data, the catalytic activity under transport-limited conditions can easily be underestimated. By combining spatially and temporally resolved laser diagnostics with operando reactor studies, we can obtain a more comprehensive understanding of catalyst behavior under defined flow conditions.

Co-workers: Thomas Häber, Olaf Deutschmann

Funding: Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)

Selected publications:

T. Häber, S. Wan, S. Struzek, C. Cárdenas, A. Zimina, F. Maurer, P. Lott, R. Suntz, J.-D. Grunwaldt, O. Deutschmann, Novel advanced channel reactor for spatio-temporal activity and catalyst state correlations applied for the reduction of NO by CO over Pt/Al₂O₃, Appl. Catal., A 712 (2026) 120748. https://doi.org/10.1016/j.apcata.2025.120748.

S. Wan, K. Keller, P. Lott, A.B. Shirsath, S. Tischer, T. Häber, R. Suntz, O. Deutschmann, Experimental and numerical investigation of NO oxidation on Pt/Al₂O₃- and NOx storage on Pt/BaO/Al₂O₃-catalysts, Catal. Sci. Technol. 12 (2022) 4456–4470. https://doi.org/10.1039/D2CY00572G.

S. Wan, T. Häber, P. Lott, R. Suntz, O. Deutschmann, Experimental investigation of NO reduction by H2 on Pd using planar laser-induced fluorescence, Appl. Energ. Comb. Sci. (2023) 100229. https://doi.org/10.1016/j.jaecs.2023.100229.

S. Wan, B. Torkashvand, T. Häber, R. Suntz, O. Deutschmann, Investigation of HCHO Catalytic Oxidation over Platinum using Planar Laser-Induced Fluorescence, Appl. Catal., B 264 (2020) 118473. https://doi.org/10.1016/j.apcatb.2019.118473.

A. Zellner, R. Suntz, O. Deutschmann, Two-Dimensional Spatial Resolution of Concentration Profiles in Catalytic Reactors by Planar Laser-Induced Fluorescence: NO Reduction over Diesel Oxidation Catalysts, Angew. Chem., Int. Ed. 54 (2015) 2653–2655. https://doi.org/10.1002/anie.201410324.