Pattern formation during the oxidation of CO on Pt{100}: a mesoscopic model

CO Oxidation over a Pt(100) Surface in a Ternary Gas Mixture of CO, NO, and Oxygen

Catalytic CO oxidation in a ternary gas mixture of CO, NO and O2 has been studied in the 10–5–10–4 mbar range using a Pt(100) sample as catalyst. Measurements of the reactive sticking coefficients of CO and NO are used to decompose the overall CO oxidation rate into the contributions from oxidation by NO and from oxidation by O2. The experiments are complemented by simulations with a realistic 7-variable model. The surface phase transition hex ↔ 1×1 and the inhibition of NO dissociation and O2 adsorption at high adsorbate coverages lead to significant hysteresis effects.

Theoretical study of pattern formation during the catalytic oxidation of CO on Pt{100} at low pressures

Theoretical studies have thus far been unable to model pattern formation during the reaction in this system on physically feasible length and time scales. In this paper, we derive a computational reaction-diffusion model for this system in which most of the input parameters have been determined experimentally. We model the surface on a mesoscopic scale intermediate between the microscopic size of CO islands and the macroscopic length scale of pattern formation. In agreement with experimental investigations [M. Eiswirth et al., Z. Phys. Chem., Neue Folge 144, 59 (1985)], the results from our model divide the CO and O(2) partial pressure parameter space into three regions defined by the level of CO coverage or the presence of sustained oscillations. We see CO fronts moving into oxygen-covered regions, with the 1 x 1 to hex phase change occurring at the leading edge. There are also traveling waves consisting of successive oxygen and CO fronts that move into areas of relatively high CO coverage, and in this case, the phase change is more gradual and of lower amplitude. The propagation speed of these reaction waves is similar to those observed experimentally for CO and oxygen fronts [H. H. Rotermund et al., J. Chem. Phys. 91, 4942 (1989); H. H. Rotermund et al., Nature (London) 343, 355 (1990); J. Lauterbach and H. H. Rotermund, Surf. Sci. 311, 231 (1994)]. In the two-dimensional version of our model, the traveling waves take the form of target patterns emitted from surface inhomogeneities.

Lifting of Ir{100} reconstruction by CO adsorption: An ab initio study

The adsorption of CO on unreconstructed and reconstructed Ir{100} has been studied, using a combination of density functional theory and thermodynamics, to determine the relative stability of the two phases as a function of CO coverage, temperature, and pressure. We obtain good agreement with experimental data. At zero temperature, the (5 x 1) reconstruction becomes less stable than the unreconstructed (1 x 1) surface when the CO coverage exceeds a critical value of 0.09 ML. The interaction between CO molecules is found to be weakly repulsive on the reconstructed surface but attractive on the unreconstructed, explaining the experimental observation of high CO coverage on growing (1 x 1) islands. At all temperatures and pressures, we find only two possible stable states: 0.5 ML CO c(2 x 2) overlayer on the (1 x 1) substrate and the clean (5 x 1) reconstructed surface.