Coupling of morphological instability and kinetic instability: Chemical waves in hydrogen oxidation on a bimetallic Ni/Rh(111) surface

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming ${\mathrm{O}}_2+{\mathrm{H}}_2$ reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in ${\mathrm{H}}_2$ produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the ${\mathrm{O}}_2+{\mathrm{H}}_2$ reaction involve a cyclic transformation between 3D-NiO and 2D-NiO.

Figure 1

Formation of three phases upon exposure of 0.8 ML Ni on Rh(111) to oxygen at $T=780\mathrm{K}$ as demonstrated by a LEEM image (middle). Selected areas of approximately 1 μm in diameter indicated by red lines are characterized by μLEED. The LEED patterns show a (1 × 1) (left), a (1 × 1) with additional spots belonging to the NiO(100) surface (top right, rotational domains are indicated), and a (6 × 1) of 2D-NiO (bottom right). Illumination apertures are used to minimize surface damage by the probing electron beam. Adsorption of ${\mathrm{O}}_2:p\left({\mathrm{O}}_2\right)=5\times {10}^{-8}\mathrm{mbar}$. $t=1500\mathrm{s}$; LEEM field of view (FOV): 7.5 μm, $E=12\mathrm{eV}$; μLEED: $E=51\mathrm{eV}$.

Figure 2

Ni $3p, \mathrm{O}1s$, and Rh $3d_{5/2}$ XPEEM images from a Rh(111) surface promoted with 0.8 ML Ni, which was oxidized at 780 K. The images display a different surface region to that shown in Fig. 1. The bright objects in the O 1s (top left) and Ni 3p (bottom left) images are 3D-NiO particles. The particles are surrounded by a Ni-depleted region, appearing darker in the Ni 3p image. The 2D-NiO regions show up in an intermediate intensity level. An enlarged section of the Ni 3p image (bottom right) shows a shadowing of the photon beam by a tall 3D-NiO island. The ratio of the Ni 3p XPS intensity, normalized to the Rh 3d signal, from different regions on the surface is 3D-NiO: 2D-NiO: Ni-depleted area $=44\pm 9:6.6\pm 2.2:1.3\pm 0.6$ . Photon energy: 150 eV (Ni 3p) and 650 eV (O 1s and Rh 3d).

Figure 3

Spreading of a 2D metallic Ni film during the reduction of 3D-NiO with hydrogen. Prior to hydrogen exposure, the 0.8 ML Ni film was oxidized at 800–830 K in $5\times {10}^{-8}\mathrm{mbar}$ oxygen. Shown are LEEM images recorded during heating the sample up in $5\times {10}^{-8}\mathrm{mbar} {\mathrm{H}}_2$ at (a) 540–570 K, (b) 590–620 K, and (c), (d) 640–670 K. LEEM $E=12\mathrm{eV}$. (Video in the Supplemental Material S2).

Figure 4

Reversible transformation of 3D-NiO into 2D-NiO during chemical waves. (a) Variation of the LEEM intensity averaged over the FOV during propagation of chemical waves. The arrows mark the times where the snapshots in (b) and (c) were taken. LEEM images of the oxidized (b) and reduced state (c). (d) LEEM intensity profiles showing intensity changes for some of the 3D-NiO particles during chemical waves. (e) Enlarged sections of the LEEM images in (b) and (c) with the profiles used for (d) indicated. Note that the 3D-NiO imaged here as bright particles appears in Figs. 1 and 3 as dark islands due to different focusing adjustments. Experimental conditions: $\mathrm{p}\left({\mathrm{O}}_2\right)=1\times {10}^{-6}\mathrm{mbar}$; $\mathrm{p}\left({\mathrm{H}}_2\right)=9\times {10}^{-7}\mathrm{mbar}, T\approx 740–770\mathrm{K}$; LEEM $\mathrm{E}=12\mathrm{eV}$.