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How the anisotropy of surface oxide formation influences the transient activity of a surface reaction

Oxide surfaces are important in many areas of technology, including fuel and energy generation/storage (reforming, syngas, fuel cells, electrolysers and batteries), corrosion, sensors, exhaust gas cleaning, and others. Intensive experimental and theoretical research during the last decade has led to the discovery that the transition from the metal to the bulk oxide may proceed via the formation of ultrathin oxide films, which are termed surface oxides. Such surface oxides, with the topmost metal layer sandwiched between two atomic layers of oxygen (O-Me–O trilayer), can even be considered a new class of materials, as they may exhibit novel unexpected properties. 

However, surface oxidation exhibits anisotropy, so that the local oxidation rates on crystallographically different and technologically relevant less-ideal surfaces vary substantially. To explore anisotropy effects, usually time-consuming sequential one-sample-after-another experiments on multiple single crystal samples are performed. In the present work, surface structure libraries are for the first time applied to examine the anisotropy of surface oxidation, exploiting their essential advantage, namely guaranteeing the same oxygen exposures, temperatures and temperature ramps for differently oriented domains. On a surface of a polycrystalline Rh foil, used as a surface library, several stepped high index domains are present, resembling rough surfaces or nanoparticles in typical catalytic applications. These µm-sized stepped Rh surfaces are in the focus of the present work.
The studies of the anisotropy of surface oxide formation on Rh and its influence on catalytic H2 oxidation presented here, have been performed by the team of the Institute of Materials Chemistry of TU Wien lead by Günther Rupprechter and Yuri Suchorski. A powerful combination of two surface-imaging techniques: SPEM, performed at the Escamicroscopy beamline of Elettra, and PEEM was applied presenting the resolved comparative study of the oxidation of high-Miller-index Rh surfaces. Figure 1 presents main results of the study showing the Electron BackScattered Diffraction (EBSD) map of a polycrystalline Rh foil, consisting of well-defined Rh(hkl) domains (Fig.1a) and the corresponding SPEM chemical map (Fig.1e) acquired at Elettra directly illustrating the anisotropy of the surface oxidation of different stepped Rh(hkl) surfaces. Figure 1c shows an example of the XPS-spectrum deconvolution for the Rh(11 11 7) domain with 26%, 64 and 10% contributions by metallic, oxidic and interface Rh, respectively. Corresponding results for the Rh(13 9 1) domain with 10%, 78% and 12% of Rh, RhOx and interfacial Rh, respectively, are shown in Fig. 1d.

 

Figure 1. Mapping anisotropic Rh surface oxidation:a) EBSD image of the polycrystalline Rh foil (512 × 600 µm); b) crystallographic orientations (Miller indices) of the Rh(hkl) domains indicated in a) and e); c,d) examples of local XPS spectra and ball models of the metallic and oxidised Rh(11 11 7) and Rh (13 9 1) domains, respectively. Squares: measured values; solid lines: sum of the deconvoluted components; e) oxidation map of the region displayed in a) after oxidation in O2(T = 623 K, pO2= 2.5 × 10−4 mbar, t = 90 min), with the right edge of the map displaying the colour scale for the RhOxcontribution.
 

The impact of surface oxidation anisotropy on catalytic Hoxidationwas then investigated by in situ PEEM imaging of the ongoing reaction. Catalytic experiments both on metallic and oxidised Rh always started from the inactive O-covered steady state. Increasing pH2 at constant pO2 and T leads to a kinetic transition to the active state at particular pH2 value. Such transitions are always accompanied by reaction fronts spreading over the entire sample as can be imaged by PEEM. Reducing the Hpartial pressure causes the reverse kinetic transition, however at much lower pH2, i.e. one observes a pronounced hysteresis between the kinetic transitions indicating a bistability of the reaction. Bistability means that the system can exist either in its active or inactive state, depending solely on the sample prehistory. Due to the fast diffusion of atomic hydrogen on metallic and oxidic Rh surfaces, the reaction is not confined to particular domains (in contrary to CO oxidation on Pt and Pd), the reaction fronts cross grain boundaries. From the time dependence of the front position, the local front velocity can be evaluated and a front velocity map can then be constructed, with the velocity values associated with the particular Rh(hkl) domains. Such velocity map clearly shows a pronounced structure-sensitivity of the front propagation velocity ranging between 0.77 μm/s for Rh(19 5 0) and 4.40 μm/s for Rh(18 1 1) at present conditions. Comparing the results for metallic and oxidised surfaces reveals that the front propagation velocity correlates with the presence of surface oxide: the higher the extent of the surface oxide is, the faster the reaction front propagates over the domains. This provides adirect visualisation of the “transient” surface oxide activity.
In the present study the combined power of two surface-imaging techniques, SPEM and PEEM is convincingly demonstrated: when applied to a polycrystalline sample, representing a library of different well-defined surface structures, this specific combination enables a direct correlation between initial surface structures, the formation of surface oxides, and their resulting catalytic properties.This allows to reveal the intrinsic relation between the presence and extent of Rh surface oxide and the reaction front propagation, the latter transmitting kinetic transitions between the catalytically active and inactive states of the catalyst surface.


 

This research was conducted by the following research team:

P. Winkler1, J. Zeininger1, Y. Suchorski1, M. Stöger-Pollach2, P. Zeller3, M. Amati3, L. Gregorattiand G. Rupprechter1

Institute of Materials Chemistry, TU Wien, Vienna, Austria. 
University Service Center for Transmission Electron Microscopy, TU Wien, Vienna, Austria.
Elettra–Sincrotrone Trieste S.C.p.A., Trieste, Italy. 


Contact persons:

Günther Rupprechter, email:


Reference

P. Winkler, J. Zeininger, Y. Suchorski, M. Stöger-Pollach, P. Zeller, M. Amati, L. Gregoratti and G. Rupprechter,“How the anisotropy of surface oxide formation influences the transient activity of a surface reaction”, Nature Communications 12, 69 (2021), DOI: 10.1038/s41467-020-20377-9,

Last Updated on Wednesday, 27 January 2021 12:57