How surface species drive product distribution during ammonia oxidation, STM and AP-XPS study
- Oleksii Ivashenko (University of Oslo, Norway)
- Wednesday 1 April 2020
- 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)  and Ambient-Pressure X-rays Photoelectron Spectroscopy (AP-XPS) study  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).
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.
- 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.
- Ivashenko, O.; Johansson, N.; Pettersen, C.; Jensen, M.; Zheng, J.; Schnadt, J.; Sjastad, A. O., in preparation for submission.