Bebensee, F.

Nefedov, A.

Schöttner, L.

Wöll, C.

Yang, C.

KITopen

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Chengwu Yang,1,2  Ludger Schöttner,1,2 Alexei Nefedov,1 Fabian Bebensee,1 Christof Wöll1,2 NEXAFS studies of N2O conversion on reduced ceria surface 1 Institute of Functional Interfaces (IFG) 2 Helmholtz Research School Energy-Related Catalysis NEXAFS/PES Endstation at HE-SGM Beamline Motivation N2O together with other nitrogen oxide species (NO, NO2) is formed in the engine and also in the catalytic converter by the reaction of N2 with adsorbed oxygen atoms. The destruction of these gases (DeNOx process) via reduction of NOx to N2 is of great significance due to their environment and health impact. Especially for nitrous oxide (N2O) has been considered as the single most important ozone-depleting substance emitted and is expected to remain the largest throughout the 21st century. Specifically, NO and NO2 have to be firstly converted into N2O characterized by the high inertness, and then N2O further liberates oxygen to finally generate environmentally benign N2, where the latter is the rate determining step of DeNOx process. Reducible oxides, such as TiO2, CeO2 and Fe2O3, have shown the reactivity to convert N2O to N2 presumably at oxygen vacancy sites.  In this study, we monitor the conversion of N2O to N2 over the model catalyst oxide support, namely CeO2(111), and ceria powders by using photoemission spectroscopy (PES) and near edge x-ray absorption fine structure spectroscopy (NEXAFS), which also enables us to determine the kinetics of this process. Experimental The NEXAFS and PES measurements were carried out with a synchrotron radiation source in our own UHV-apparatus on the HE-SGM beamline at BESSY II. CeO2 single crystal was mounted on the sample holder with electron beam heating. Cooling was made with copper braids connected to the sample holder and a liquid helium cryostat. The CeO2(111) single crystal surface was prepared by repeated cycles of sputtering with 1 keV Ar+ and annealing at 800 K for 15 min in an O2 atmosphere of 1×10-5 mbar for forming a stoichiometric surface, or alternately without O2 to create a reduced one. For temperature monitoring, a K-type thermocouple was directly attached on the sample surface. For preparation of ceria powders, the sample was annealed stepwise from 800 K to 1000 K for 30 min.  NEXAFS measurements only were performed after desired Ce oxidation state as judged by PES. Exposure to 50 Langmuir N2O at sample temperatures at 120 K was achieved by backfilling the analysis chamber up to 2,6 × 10-7 mbar for 250 seconds. Typical base pressures during acquisition of NEXAFS and valence band (VB) spectra were 2 × 10-10 mbar. After N2O exposure NEXAFS spectra were recorded at 120 K. Due to strong grounding effects, PES measurements cannot be performed at that low temperature. For that reason the temperature was elevated to a given set of temperatures. The VB data were gathered for evaluation of the ceria oxidation state. *ludger.schoettner@partner.kit.edu N2O dosing experiments on CeO2-x(111) N2O dosing experiments on CeO2-x powder Conclusions References Proposed mechanism 12 10 8 6 4 2 0 -20,02,0x1054,0x1056,0x1058,0x1051,0x106Intensity (cps)Binding energy (eV)Excitation energy 153 eV 230 K clean reduced CeO2(111) 200 K after interaction with 50 L N2OCe 4fO 2pVB12 10 8 6 4 2 0 -20,02,0x1054,0x1056,0x1058,0x105O 2pIntensity (cps)Binding energy (eV)Excitation energy 153 eV Take-off angle 0 deg (after dosage of 50 L N2O) Take-off angle 45 deg (after dosage of 50 L N2O)VBCe 4f12 10 8 6 4 2 0 -201x1052x1053x1054x1055x105Intensity (cps)Binding energy (eV)Excitation energy 220 eV 300 K clean reduced ceria powders 300 K after interaction with 50 L N2OVBCe 4fO 2pFigure 1: N K-edge NEXAFS spectra after dosing 50 Langmuir N2O on a reduced CeO2(111) surface (left) at 120 K are depicted. Two X-ray resonances at 401,2 eV (Ce-O-N-N) and 404,8 eV (Ce-O-N-N) were observed. The signal decay for both photon energies is given in a time dependent plot (right). The measuring time between both signals in one spectrum was estimated to 1,5 min, between stacked spectra it was set to 3 min. Kinetic predictions should be valid under the consideration that both signals have equal peak areas on a simultaneous time recording scale.  Figure 2: Valence band spectra on partial reduced ceria and photorelated ceria reoxidation by N2O (left). Angle-resolved -PES measurements exhibit the oxidation process close to the surface (right). [1] A. R. Babbin, D. Bianchi, A. Jayakumar, B. B. Ward, Science, 2015, 348, 6239. [2] D. R. Mullins, Surface Science Reports, 2015, 70, 42. [3] J. A. Rodriguez, Environmental Catalysis, 2005, 211, Taylor & Francis Group. [4]  G. Ceballos, H. Wende, K. Baberschke, D. Arvanitis, Surface Science, 2001, 482-485, 15.  [5] C. N. Rusu, J. T. Yates Jr., Journal of Physical Chemistry B, 2001, 105, 2596. [6]  C. Yang, L. Yin, F. Bebensee, M. Buchholz, H. Sezen, S. Heissler, J. Chen, A. Nefedov, H. Idriss, X. Gong,       C. Wöll, Physical Chemistry Chemical Physics, 2014, 16, 24165.  Figure 3: NEXAFS spectra  were recorded in the N K-edge region after interaction with 50 L N2O on reduced ceria powder at an ambient temperature of 120 K. The corresponding kinetic profile is illustrating the signal decay over time (right). Expontential fitting reveals a first order kinetic for N2O splitting on reduced ceria. Figure 4: PES-VB measurements before and after the dosage of 50 L N2O on reduced ceria powder for different excitation energies. For powders, ceria reoxidation is not observable by means of valence band spectra. Lowering the excitation energie will result in a more surface sensitive measurement, which can be derived from the higher ratio between Ce4f and O2p peak (right). In this work we presented the in-situ reaction monitoring of laughing gas (N2O) to N2 on reduced ceria substrates by NEXAFS and PES experiments. After deposition of 50 L N2O at 120 K, two apparent signals were observed for photon energies of 401,2 eV and 404,8 eV in NEXAFS spectra inside the N K-edge region on CeO2-x(111) and CeO2-x powder, respectively. For the single crystal surface the oxidation process could be directly followed by the shrink of the Ce4f shell signal in valence band photoemission spectra, which is a clear indication for replenishing the voids on reduced surfaces with atomar oxygen. Hence the chemical conversion from N2O to molecular nitrogen can be assumed. In comparison to ceria powder the situation seems to be more complicated, because the reoxidation is concealed in VB measurements. The amount of adsorbed N2O molecules on powders is much higher than on single crystals, which can be read out in the kinetic profiles. The nitrogen signal decay in NEXAFS occurs much more rapid than for single crystals, so the conversion seems to be more effective on powders for multilayer dosages due to much higher surface area. This is the reason why we cannot detect the reoxidation via PES, because the number of unoccupied adsorption sites (vacancies) on reduced ceria powder is vast referring the „small“ gas dosage of 50 L. Figure 5: Schematic view on N2O conversion for ceria and preparation conditions. 12 10 8 6 4 2 0 -201x1052x1053x1054x105Intensity (cps)Binding energy (eV)Excitation energy 153 eV 250 K clean reduced ceria powder 250 K after interaction with 50 L N2OVBCe 4fO 2p0 5 10 15 20 25 300,00,20,40,60,81,0 Experimental values at 401,2 eV Experimental values at 404,8 eV Exponential fitIntegrated peak area (normalized)Time (min)0 5 10 15 20 25 300,00,20,40,60,81,0 Experimental values at 401,2 eV Experimental values at 404,8 eV Exponential fitIntegrated peak area (normalized)Time (min)400 402 404 406 40801x10-72x10-7 TimeIntensity (a.u.)Photon energy (eV)N K-edge1s->3*Time400 402 404 406 4081,2x10-71,6x10-72,0x10-7TimeIntensity (a.u.)Photon energy (eV)TimeN K-edge1s->3*

NEXAFS studies of N₂O conversion on reduced ceria surface

https://publikationen.bibliothek.kit.edu/220102034/3816900

NEXAFS studies of N₂O conversion on reduced ceria surface

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