Abstract: The intrinsic electrocatalytic activity of materials is critically dependent on their composition and surface orientations. It is well known that the addition of a second element (bimetallic catalysts) to a metal has a profound effect on the activity and stability of the resulting materials through ensemble, ligand, and strain effects. Likewise, it has been known for a long time that an element’s electrocatalytic activity is critically dependent upon surface orientation and atomic surface arrangement. For example, the following electrocatalytic reactions occur more easily at metals that exhibit perfect two-dimensional (100) terraces: (i) reduction of CO on copper [[i],[ii],[iii]]; and (ii) ammonia oxidation on platinum [[iv]]. Other examples abound in the literature. Accordingly, it is of the utmost importance to control both the composition and surface orientation of electrodes to determine as precisely as possible the intrinsic electrocatalytic activity of materials.
Pulsed laser deposition (PLD) was used to prepare epitaxial thin films with well-defined composition and surface orientation. PLD is a particularly attractive deposition method for the formation of epitaxial layers, since it has an unmatched ability to vary the kinetic energies of the species impinging on the substrate, along with the nature and the pressure of the gas in the deposition chamber. Most importantly, almost all elements can be deposited by PLD. Moreover, PLD is considered a non-equilibrium deposition technique and several metastable alloys have been prepared so far, allowing the investigation of bimetallic compounds that are kinetically stable but would be difficult to obtain by following a heat-treatment step, as the elements are not miscible, or only partially so.
In this presentation, we will focus on Pt alloys thin films with (100) orientation and investigate the effect of the alloying element (Ir, Ru, Rh and Pd) on the electrocatalytic activity for the oxidation of ammonia (NH3). We first undertook a detailed investigation of the growth and the surface arrangement of Pt thin films deposited on (001)-oriented MgO substrates as a function of the deposition conditions. It will be shown that the width of the (100) terraces vary with the deposition temperature and shows a maximum value corresponding to 9 atoms at 325° C. Then, we will look at the effect of the alloying element on both the growth mode and the electrocatalytic activity. We will show that all Pt alloys thin films investigated here exhibit only one diffraction peak (the (001) peak) and a 4-fold in-plane symmetry, indicating epitaxial growth is achieved independently of the deposition temperature. The pole figure indicates that all films exhibit a Pt(001)//(001)MgO cube-on-cube epitaxial relationship, consistent with the AFM images. The effect of the alloying element (Ir, Ru, Rh and Pd) on the electrocatalytic activity for the oxidation of NH3 will be discussed.
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[[iv]] F. J. Vidal-Iglesias, J. Solla-Gullón, V. Montiel, J. M. Feliu and A. Aldaz, ²Ammonia Selective Oxidation on Pt(100) Sites in an Alkaline Medium² J. Phys. Chem. B 109 12914 (2005).