Godiksen, Anita

Isaksen, Oliver

Mossin, Susanne

Rasmussen, Søren Birk

Vennestrøm, Peter N. R.

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Apr 10, 2018Single center reactivity of Cu(II) sites in copper substituted CHA zeolite followed by in-situ electron paramagnetic resonance spectroscopyMossin, Susanne; Isaksen, Oliver ; Godiksen, Anita; Rasmussen, Søren Birk; Vennestrøm, Peter N. R.Publication date:2016Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mossin, S., Isaksen, O., Godiksen, A., Rasmussen, S. B., & Vennestrøm, P. N. R. (2016). Single centerreactivity of Cu(II) sites in copper substituted CHA zeolite followed by in-situ electron paramagnetic resonancespectroscopy. Abstract from 17th Nordic Symposium on Catalysis 2016, Lund, Sweden.Single center reactivity of Cu(II) sites in copper substituted CHA zeolite followed by in-situ electron paramagnetic resonance spectroscopy Susanne Mossin1*, Oliver Isaksen1, Anita Godiksen1, Søren Birk Rasmussen2, Peter N. R. Vennestrøm2 1 – Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, Kemitorvet 207, 2800 Lyngby, Denmark   2 – Haldor Topsøe A/S, Haldor Topsøe alle 1, 2800 Lyngby, Denmark * slmo@kemi.dtu.dk  Copper exchanged zeolites are active for the selective catalytic reduction of nitrogen oxides with ammonia.[1] Recently small-pore zeolites and zeotypes with the CHA framework topology has attracted much attention and has been commercialized for use in the abatement of NOx in diesel exhaust due to favorable properties such as high stability towards hydrothermal ageing and high resistance towards the presence of hydrocarbons. At the same time the simple CHA zeolite structure greatly simplifies the spectroscopic characterization of the materials and allows investigation of one type of site at the time. Electron paramagnetic resonance spectroscopy (EPR) is very sensitive towards paramagnetic Cu2+ and offers the possibility for both quantification and speciation of Cu2+ sites with unrivaled sensitivity.[2] Cu+ is diamagnetic and does not give an EPR signal. Recent results obtained on a Cu-CHA zeolite material with Si/Al = 14 and Cu/Al = 0.46 shows that the elucidation of the different copper species is possible using EPR by exposing the Cu-CHA to different gas mixtures.[2]   The time-resolved development of the different Cu2+ sites was followed and quantified by EPR in order to follow the oxidation and reduction of the individual sites in the steps of the SCR cycle.[3] In this work Cu-CHA with low concentrations of copper (Cu/Al < 0.1) was exposed to different selective catalytic reduction (SCR) relevant gas mixtures under in-situ conditions at high space velocities (400,000 h-1) to resolve the reactivity of each specific copper site. It is shown that during exposure to SCR relevant gas mixtures the different extra-framework Cu species present in Cu-CHA react differently and several examples of this will be presented.   As one example, the rate of oxidation of Cu+ to Cu2+ is highly dependent of the coordination environment of Cu, and - for at least one of the sites - also on the concentration of NO in the gas phase, see Fig. 1.   The gas flow used in the time resolved experiment in Fig. 1 was switched between • a mixture of O2 (10 %), NO (in the interval 250 to 1000 ppm) and balance He, whereupon Cu+ is oxidized to Cu2+ giving an increase in the signal intensity  and • NO + NH3 (1000 and 1200 ppm) and balance He, which resulted in fast and almost complete reduction of Cu2+ to Cu+ and a decrease in the EPR signal intensity.  The Cu2+ species belonging to both site 1 and site 2 can be assigned and quantified on the basis of their EPR spectrum.                         Figure 1. EPR signal intensity followed as a function of time during an in-situ EPR experiment. The intensity is given relative to the signal of all Cu2+ in the sample.   The sample investigated was fractioned (150-300 µm) Cu-CHA (SSZ-13) with Si/Al = 14 and Cu/Al = 0.09. The sample was dehydrated at 250 °C in-situ and reduced in NO + NH3 at 200 °C before t = 0. The temperature was 200 °C and GHSV = 400,000 h-1 throughout the experiment.   Conclusions will be presented for the reaction with SCR relevant gases, NH3, NO and O2 of these materials and the results will be correlated with the recent suggestion for a consistent reaction cycle for the NH3-SCR reaction.[3]    [1]  U. Deka et. al. ACS Catal. 2013, 3, 413. [2]  A. Godiksen et. al J. Phys. Chem. C. 2014, 118, 23126.  [3]  T. V. W. Janssens et. al. ACS Catal. 2015, 5, 2832. 

Single center reactivity of Cu(II) sites in copper substituted CHA zeolite followed by in-situ electron paramagnetic resonance spectroscopy

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Single center reactivity of Cu(II) sites in copper substituted CHA zeolite followed by in-situ electron paramagnetic resonance spectroscopy

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