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Resolving the facet structure of supported Rh and Pt-Rh alloy nanoparticles during ammonia oxidation

  • Uta Hejral (Lund University, Sweden)
Date
Wednesday 1 April 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst

U. Hejrala, A. Restab, S. Albertina, K. von Allmena,  A. Vladb, A. Coatib, E. Lundgrena
aDivision of Synchrotron Radiation Research, Lund University, Lund, Sweden
bBeamline SixS, Synchrotron Soleil, St. Aubin, France

Fig. 1: 2D reciprocal space map centered around the Pt-Rh particle (111) Bragg peak. It contains particle facet signals that yield information on the quantitative particle shape.

Ammonia oxidation over Pt-Rh catalysts is an essential catalytic reaction that is widely used in chemical industry for the production of paints, preserving agents and fertilizers. Apart from the desired oxidation of ammonia (NH3) into nitric oxide (NO), there are two more competing reaction pathways, resulting in N2O and N2, which makes the possibility to select a distinct reaction pathway of highest scientific and economic interest [1].

In a previous surface x-ray diffraction study on epitaxial Pt-Rh nanoparticles during CO oxidation, we could correlate the surface structure of distinct nanoparticle facets to the catalytic activity [2]. In the investigation presented here, we used the same experimental approach to correlate the underlying facet surface structure of MgO(001)-supported Rh and Pt-Rh nanoparticles to the production of NO, N2O and N2 during ammonia oxidation, with the aim to shed light on the catalytic selectivity of the underlying catalyst structures towards the competing reaction pathways.

The experiment was carried out at beamline SixS at the Synchrotron Soleil by means of surface x-ray diffraction (photon energy: E= 11.2 keV) and in-situ mass spectrometry, using a dedicated in-situ catalysis chamber [3]. Measuring reference scans in reciprocal space allowed for tracking the formation of ultrathin Rh-O-Rh surface oxide trilayers on individual nanoparticle facets, where the probed reaction conditions were characterized by five different sample temperatures (450 K, 550 K, 650 K, 750 K, and 850 K) and five different NH3:O2 ratios (1:0, 1:0.5, 1:1, 1:2, and 1:8) at a constant total pressure of 300 mbar. The reference scans were complemented by high resolution line scans through the Pt-Rh and Rh particle Bragg peaks and by 2D reciprocal space maps (see Fig. 1), providing information on the particle shapes. X-ray reflectivity measurements yielded additional information on the reaction-induced particle sintering on the MgO(001) surface.

References

  1. G. Buscaa et al., Appl. Catal. B: Environ. 1998, 18, 1.
  2. U. Hejral et al., Phys. Rev. Lett. 2018, 120, 126101.
  3. R. Van Rijn et al., Rev. Sci. Instrum. 2010, 81, 014101.

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