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How surface species drive product distribution during ammonia oxidation, STM and AP-XPS study

  • Oleksii Ivashenko (University of Oslo, Norway)
Date
Wednesday 1 April 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst

Oleksii Ivashenkoa, Niclas Johansonb, Christine Pettersena, Martin Jensena, Jian Zhenga, Joachim Schnadtb, Anja O. Sjåstada
a Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
b  Division of Synchrotron Radiation Research, Department of Physics, and NanoLund, Lund University, Lund, Sweden,  2 MAX IV Laboratory, Lund University, Lund, Sweden

Oxidation of ammonia is an essential chemical reaction used for production of artificial fertilizers (Ostwald process, preferred product NO) and in environmental applications for reducing dangerous emissions in diesel engines (NH3 slip reaction, preferred product N2). For both processes, PtRh alloys are active catalysts and are commonly used. Although reaction mechanism and kinetics for the oxidation process was described, a direct operando observation of the routes towards N2 and NO was lacking. In this contribution we present a combined Scanning Tunneling Spectroscopy (STM) [1] and Ambient-Pressure X-rays Photoelectron Spectroscopy (AP-XPS) study [2] of catalytically active PtRh alloys prepared on Pt(111) and Rh(111) surfaces, measured operando during NH3 oxidation at 1 mbar.

Correlation of mass spectrometry and AP-XPS data allowed establishing how the surface species coverage drive the product distribution in the gas phase. Specifically, oxygen excess (100:1 mixing) produces high coverage of O-, which facilitates rapid oxidation of N- to NO followed by desorption. In contrast, in NH3-rich environment (1:1 mixing), atomic N- was a predominant species, which recombines and desorbs as N2.

Finally, by varying the Rh enrichment at the Pt(111) surface we were able to tune the abundancy of surface N- and O-, resulting in corresponding changes in the product distribution. These findings provide a direct fundamental insight into how PtRh alloys can be further optimized for desired products (N2 vs NO).

Figure 1. Left: UHV STM topography of 1.0 ML Rh/Pt(111) used to study NH3 oxidation. Ut = -0.5 V, It = -0.2 nA, image size 120×120 nm2. Middle: AP-XPS spectra of Rh/Pt(111) during dosing 1 mbar of O2 at RT, followed by heating to 325 °C, showing formation of surface oxide Rh2O3 (307.9 eV). Right: Schematic summary of the findings, showing preferential formation of N2 as a result of recombination of surface N- species obtained while dosing 0.5 mbar O2 and 0.5 mbar NH3; and formation of NO in excess oxygen conditions of 0.5 mbar O2 and 0.005 mbar NH3, studied at 325 °C.

Acknowledgements

OI, MJ and JZ are grateful to the industrial Catalysis Science and Innovation Centre (iCSI) and the ASCAT-project, which receives financial support from the Research Council of Norway (contract no. 237922 and 247753). JZ acknowledges support of InterReg.

References

  1. Zheng, J.; Ivashenko, O.; Fjellvag, H.; Groot, I. M. N.; Sjastad, A. O., Roadmap for Modeling RhPt/Pt(111) Catalytic Surfaces. Journal of Physical Chemistry C 2018, 122 (46), 26430-26437.
  2. Ivashenko, O.; Johansson, N.; Pettersen, C.; Jensen, M.; Zheng, J.; Schnadt, J.; Sjastad, A. O., in preparation for submission.

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