Self-organization phenomena such as rate oscillations, chemical wave patterns, and precipitation of nanoparticles can be observed in the catalytic H2 + O2 reaction on a Rh(111) surface after alloying with Ni. The bimetallic Rh(111)/Ni surface has been studied in the 10–6–10–4 mbar range using PEEM (photoemission electron microscopy) and LEEM/SPELEEM (low energy electron microscopy and its spectroscopic variant) as the main analytical methods. The Rh(111)/Ni catalysts are prepared by thermal decomposition of Ni­(CO)4 on Rh(111), resulting in an alloyed surface with about 25% Ni in the topmost layers. One finds rate oscillations and chemical wave patterns comprising target patterns, pulse trains, and rotating spiral waves. The oscillatory behavior is attributed to periodic changes in the composition of the bimetallic surface alloy causing concomitant variations in catalytic activity. Under pattern-forming reaction conditions, three-dimensional NiO particles develop on top of the alloyed Rh/Ni surface, with dimensions ranging from <1 μm up to 50 μm. Their size which depends on the total pressure controls the Ni content in the surface alloy.

The bistable NH3 + O2 reaction over a Rh(110) surface was explored in the pressure range 10(-6)-10(-3) mbar and in the temperature range 300-900 K using photoemission electron microscopy and low energy electron microscopy as spatially resolving methods. We observed a history dependent anisotropy in front propagation, traveling interface modulations, transitions with secondary reaction fronts, and stationary island structures.

Chemical waves in the H + O reaction on a Rh(111) surface alloyed with Ni [Θ < 1.5 monolayers (ML)] have been investigated in the 10 and 10 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 H 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.

The dynamic behavior of the O 2 + H 2 reaction on a Rh(111) surface alloyed with Ni has been studied in the 10 À5 mbar range using photoemission electron microscopy (PEEM) as a spatial resolving method.