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Uncovering dynamic structure of active sites in selective oxidation catalysts using time-resolved X-ray absorption spectroscopy

Applicant Safonova Olga
Number 179132
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Physical Chemistry
Start/End 01.10.2018 - 30.09.2022
Approved amount 559'928.00
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All Disciplines (2)

Discipline
Physical Chemistry
Inorganic Chemistry

Keywords (10)

selective oxidation of alcohols; X-ray absorption spectroscopy; transient kinetics; bimetallic nanoparticles; structure of active sites; vanadium; sub-second time resolution; CO oxidation; operando spectroscopy; heterogeneous catalysis

Lay Summary (German)

Lead
Aufdeckung der dynamischen Struktur von aktive Zentren in selektive Oxidationskatalysatoren mittels Zeit-aufgelöste Röntgenabsorptions-Spektroskopie
Lay summary

Zeit-aufgelöste Röntgenabsorptions-Spektroskopie (XAS) ist eine Methode womit die Redox –Aktivität von einem spezifischen metallischen Zentrum in ein Katalytische Zyklus quantitativ untersucht werden kann. In dieses Projekt werden solche XAS Methoden, mit verbesserte chemische Selektivität, entwickelt um die Struktur von den katalytischen aktiven Zentren in zwei viel gebrauchte selektive Oxidations- Katalysatoren auf zu decken: (i) metallisch V, Mo und W auf einem oxydischen Träger für die selektive Oxidation von Alkohole und (ii) bi-metallische Nanopartikeln für die selektive CO Oxidation bei niedrige Temperaturen. Beide Systemen sind intensiv erforscht in der Literatur. Jedoch, eine detaillierte Kenntnis der Redox Kinetik von dem katalytischen aktiven Zentrum während der katalytische Zyklus fehlt. In dieses Projekt werden wir Zeit-aufgelöste XAS messen um die Redox Rate von die Metall Zentren zu messen während eine schnelle Änderung von der Gas Zusammenstellung damit der Gleichgewichtszustand vom katalytische System kurzfristig verstört wird. Wir werden die anfängliche oxidations- oder reduktions- Rate von Metallen die möglicherweise involviert sind in der Struktur von den katalytischen aktiven Zentren korrelieren zu der katalytischen Gesamtreaktionsrate. Wann der anfängliche Redox Rate von das Metallische Zentrum gleich ist zu der Gesamtreaktionsrate wird das Zentrum als ‘Aktiv’ betrachtet und auch der geometrischen Struktur mittels XAS identifiziert. Für dieses Projekt werden gut definierte Katalysatoren basiert auf V, Mo und W geträgert auf oxydische Träger (Al2O3, CeO2 und TiO2) und Pt-basierte geträgerte bi-metallische Nanopartikeln synthetisiert. Der Erfolg von diesem Projekt verlässt sich auf das gelingen von der Synthese von Katalysatoren mit fein abgestimmte Struktur und der Entwicklung von sensitive Zeit-aufgelöste XAS Methoden. Das so geschaffene Wissen und neue experimentelle Methoden werden für eine grosse wissenschaftliche Gemeinschaft bereitgestellt.

Direct link to Lay Summary Last update: 22.01.2019

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Abstract

It was recently demonstrated that time-resolved X-ray absorption fine structure (XAFS) methods can quantitatively discriminate the involvement of the redox activity of specific metal sites in the catalytic cycle distinguishing true intermediates from spectators. The goal of the proposed research is to apply these methods for well-known selective oxidation catalysts: (i) oxide-supported V, Mo and W species used for selective oxidation of alcohols and (ii) bimetallic nanoparticles selectively oxidizing CO at low temperatures. Both systems are intensively studied in literature; however the detailed knowledge about redox kinetics in the active sites during the catalytic cycle is still missing.Selective oxidation or oxidative hydrogenation of light alcohols on supported V, Mo, and W catalysts presumably proceeds via the Mars van Krevelen mechanism involving lattice oxygen and the redox activity of V, Mo, and W centers. CeO2 and TiO2 supports can strongly enhance the activity of supported metals suggesting the involvement of oxygen from the interface sites (M-O-support, M = V, Mo, W) in the catalytic mechanism. However, till now it was impossible to look directly at the dynamic structure of active sites during the catalytic cycle simultaneously probing the reactivity of the support. For the bimetallic nanoparticles selectively oxidizing CO the situation is similar. Reactivity of Pt nanoparticles can be significantly enhanced by transition metal promoters (Fe, Co, Ni, Sn) either forming alloys with Pt or segregating on the surface as MOx islands. The activity of these catalysts strongly depends on the preparation method and reaction conditions suggesting that only a part of transition metal promoters participate in activation of oxygen. This complicates understanding of oxygen activation mechanism by standard spectroscopic methods, including element specific XAFS methods under static conditions, not allowing efficient discrimination of true intermediates from spectators. The ideal spectroscopic method has to be element specific, quantitative and sensitive to the activity of a small fraction of atoms in the catalyst forming active sites. In addition, the method should be applied under operando conditions and provide sufficient time-resolution to allow correlations between the overall reaction rate and the rates of structural transformations in the active sites during the catalytic cycle. In this proposal we plan to develop such methods based of operando time-resolved XAFS with enhanced chemical sensitivity. The time-resolved approach will be based on rapid deviation of the catalytic system from the steady state conditions (by fast gas switching) and quantitative comparison of the initial rates of oxidation or reduction of metals potentially involved in structure of active sites to the overall reaction rate controlled by mass spectrometry. Metal species changing their state with the rate similar to the rate of the overall catalytic reaction will be considered as active and by this criterion discriminated from inactive spectators. Well-defined catalysts based on V, Mo, and W species supported on Al2O3, CeO2, and TiO2 and Pt based supported bimetallic nanoparticles will be prepared by standard incipient wetness impregnation methods as well as atom layer deposition and chemical vapor deposition methods. The success of the research proposal relies on the synthesis of catalytic materials with finely tuned structure and the development of highly sensitive time-resolved XAFS methods. Created knowledge and new experimental methods will become available to a wide community of scientists.
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