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Reaction driven ordering of the surface of a PtRh alloy model catalyst

  • Helen Edström (Lund University, Sweden)
Date
Tuesday 31 March 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst
Room
Hotel NH Noordwijk Conference Centre Leeuwenhorst

H. Edströma, U. Hejrala, S. Albertina, K. von Allmena, B. Hagmana, E. Lundgrena, A. Stierleb, C. Seitzb, V. Vonkb, A. Schaeferc, and J. Gustafsona
aSynchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden
bDeutsches Elektronen-Synchrotron (DESY), D-226 03 Hamburg, Germany
cDepartment of Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

To better understand the catalytic reactions on the atomic level for the PtRh catalyst, we investigated a Pt25Rh75(100) single-crystal using high-energy surface X-ray diffraction (HESXRD) at beamline P21.2 at PETRA III, DESY, in Hamburg, Germany. Under oxidizing conditions, we found two Rh-O structures that have been observed previously, namely a p(3×1) reconstruction with chemisorbed O [1–3] and a c(8×2) surface oxide [2–4]. Under reducing conditions, we found an unexpected c(2×2) structure, see Figure 1a. On Rh(100), CO adsorbs in this structure [2, 5], but the scattering from the adsorbate molecules is usually too weak to be observed in HESXRD. This indicates that we have an ordered surface of the substrate rather than only an adsorbate structure. We tried to recreate the c(2×2) structure by exposing the crystal to a CO pressure ranging from 10-7 to 10-1 mbar. We were not able to recreate the structure this way, but after annealing the crystal in ultra-high vacuum (UHV), the c(2×2) structure was found. Our hypothesis is that the gas exposure resulted in a Rh surface layer with a Pt layer beneath. During UHV annealing, the Pt atoms approached the surface, resulting in a well-ordered c(2×2) structure, see Figure 1b-c. We see this as a new route to create systems similar to so-called single atom alloy catalysts, avoiding evaporation that is often very time consuming, especially during beamtimes.

Figure 1. a) An HESXRD plot of Pt25Rh75(100) during reducing conditions and high temperature reveals the additional spots from the c(2×2) structure, marked by white rings. The black and red lines in the center of the plot mark the unit cells of p(2×2) and c(2×2), respectively. b) Gas exposure pulls out Rh atoms (magenta) to the surface, causing a second layer of Pt atoms (cyan). c) During annealing, Pt atoms segregate towards the surface and a mixed layer is formed, resulting in a well-ordered c(2×2) structure.

Acknowledgements

Support by the Swedish Research Council and the Knut and Alice Wallenberg Foundation is gratefully acknowledged.

References

  1. M. Sporn et al., Surf. Sci., 1998, 416, 384-395.
  2. J. Gustafson et al., Phys. Rev. B, 2005, 71, 1-9.
  3. R. Westerström et al., J. Phys. Cond., Mat., 2008, 20, 6-12.
  4. J. Gustafson et al., Cat. Tod., 2009, 145, 227-235.
  5. A. Baraldi et al., Appl. Surf. Sci., 1996, 99, 1-8.
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