Catalysts caught in the act – What in situ spectroscopy can tell us about active sites and mechanisms
Prof Angelika Brückner
Rational catalyst design beyond empirical trial-and-error approaches is the ultimate goal of research in catalysis which, however, requires detailed knowledge on the role of distinct catalyst building blocks in the different steps of catalytic reactions. This can be obtained best when catalysts are monitored in their active working state by a dedicated combination of spectroscopic methods, each of which contributes a particular facet of information to the global view of the system under study. This will be illustrated for two processes comprising global challenges, namely sustainable energy supply and environmental protection.
Sustainable production of hydrogen as a major energy carrier is a future challenge that might be solved by photocatalytic water splitting with sun light when sufficiently active catalysts were available. Their effective design requires rational improvement of charge separation, stabilization and transport, which can only be understood in detail by monitoring such processes with suitable spectroscopic in situ methods. In this example the potential of in-situ EPR, in-situ XANES and in-situ UV-vis spectroscopy for analyzing structure-reactivity relationships in plasmonic M/TiO2 (M = Au and/or Cu) water reduction catalysts is demonstrated. 1,2,3 EPR evidenced that the SPR effect promotes transfer of electrons from the Au conduction band to the TiO2 support surface where they are trapped and provided in vacancies close to the Au-TiO2 interphase for proton reduction. XANES and UV-vis spectroscopy revealed that the formation of metal particles by in-situ photoreduction of the precursors depends crucially on the catalyst synthesis route.
While selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR of NOx) is an established technique for cleaning exhaust gases from stationary sources, problems exist with vehicles, since the used zeolite-based catalysts are not active enough at low exhaust temperature. This came to light by the recent diesel scandal and might boost research activities for low-temperature NH3-SCR. Promising catalysts for this purpose are based on ceria. In this example it will be shown how operando EPR, simultaneous TPR/UV-vis spectroscopy, quasi-in-situ XPS and in-situ FTIR can be utilized to elucidate structural features responsible for high catalytic performance in supported VOy/CexM1-xO2 (M = Zr, Ti, Mn) catalysts.4,5 In catalysts with highest activity, special –O–Ce–O–V(=O)–O–M–O– surface moieties were identified. They expose an exclusive V5+/V4+ redox shuttle when M=Zr, while both Ce4+/Ce3+ and V5+/V4+ redox cycles operate during SCR over V/CexTi1-xO2 with even higher activity. In-situ FTIR revealed important mechanistic differences, namely a Langmuir-Hinshelwood mechanism for bare CexM1-xO2 supports and an Eley-Rideal mechanism for VOy/CexM1-xO2 catalysts.
- J. B. Priebe, M. Karnahl, H. Junge, M. Beller, D. Hollmann, A. Brückner, Angew. Chemie Int. Ed. 52 (2013) 11420-11424.
- J. B. Priebe, J. Radnik, A. J. J. Lennox, M.-M. Pohl, M. Karnahl, D. Hollmann, K. Grabow, U. Bentrup, H. Junge, M. Beller, A. Brückner, ACS Catalysis 5 (2015) 2137−2148.
- J. B. Priebe, J. Radnik, C. Kreyenschulte, A. Lennox, H. Junge, M. Beller, A. Brückner, ChemCatChem 9 (2017) 1025–1031.
- T. H. Vuong, J. Radnik, E. V. Kondratenko, M. Schneider, U. Armbruster, A. Brückner, Appl. Catal. B: Environm. 197 (2016) 159-167.
- T. H. Vuong, J. Radnik, J. Rabeah, U. Bentrup, M. Schneider, H. Atia, U. Armbruster, W. Grünert, and A. Brückner, ACS Catalysis 7 (2017) 1693-1705.