Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. I. Photoelectron emission microscopy

Stationary stripe patterns and chemical waves on the bimetallic Rh(110)/Ni surface during the H2 + O2 reaction

Chemical wave patterns that develop in the O₂ + H₂ reaction on a bimetallic Rh(110)/Ni surface have been studied with photoelectron emission microscopy (PEEM) in the 10^-6 to 10^-4 mbar range. The bifurcation diagram for Ni coverages up to 3 monolayers (ML) was mapped out for T = 770 K. Stationary concentration patterns of macroscopic stripes as well as target patterns and irregular chemical waves were observed.

Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation

The growth of ultrathin layers of VOx (< 12 monolayers) on Pt(111) and the activity of these layers in catalytic methanol oxidation at 10−4 mbar have been studied with low-energy electron diffraction, Auger electron spectroscopy, rate measurements, and with photoemission electron microscopy. Reactive deposition of V in O2 at 670 K obeys a Stranski–Krastanov growth mode with a (√3 × √3)R30° structure representing the limiting case for epitaxial growth of 3D-VOx. The activity of VOx/Pt(111) in catalytic methanol oxidation is very low and no redistribution dynamics is observed lifting the initial spatial homogeneity of the VOx layer. Under reaction conditions, part of the surface vanadium diffuses into the Pt subsurface region. Exposure to O2 causes part of the V to diffuse back to the surface, but only up to one monolayer of VOx can be stabilized in this way at 10−4 mbar.

Chemical waves in the O2 + H2 reaction on a Rh(111) surface alloyed with nickel. II. Photoelectron spectroscopy and microscopy

Chemical waves in the H₂ + O₂ reaction on a Rh(111) surface alloyed with Ni [Θ_Ni < 1.5 monolayers (ML)] have been investigated in the 10^-7 and 10^-6 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior. The coincidence of a high oxygen signal with a low Ni 2p intensity, which seemingly contradicts the chemical attraction between O and Ni, has been explained with a phase separation of the oxygen covered Rh(111)/Ni surface into a 3D-Ni oxide and into a Ni poor metallic phase. Macroscopic NiO islands (≈1 μm size) formed under reaction conditions have been identified as 2D-Ni oxide. Titration experiments of the oxygen covered Rh(111)/Ni surface with H₂ demonstrated that the reactivity of oxygen is decreased by an order of magnitude through the addition of 0.6 ML Ni. An excitation mechanism is proposed in which the periodic formation and reduction of NiO modulate the catalytic activity.