Mathematical Modeling of Reactive Phase Separation in the System Rh(110)/K/O₂ + H₂

Abstract

The bistable behavior of the O 2 + H 2 reaction on Rh(110) is modified by the presence of coadsorbed potassium. Reaction fronts transporting potassium and the development of stationary Turing-like patterns have been observed. A realistic mathematical model is presented which reproduces qualitatively correct and, to a large part, even quantitatively correct the experimental results. Key factors of the model are the strong chemical affinity between coadsorbed oxygen and potassium, a reduced mobility of potassium on the oxygen covered surface, and a strongly reduced reactivity of oxygen toward hydrogen in the presence of coadsorbed potassium.

Spectromicroscopy of pulses transporting alkali metal in a surface reaction

“…8,10 Using the diffusion parameters from ref. 8 one calculates for a temperature of 500 K a diffusion constant D K for potassium which is 4.5 Â 10 À5 cm 2 s À1 for the oxygen free Rh(110) surface and only 5.2 Â 10 À9 cm 2 s À1 for a Rh(110) surface covered with 0.8 ML of oxygen, i.e. a reduction by four orders of magnitude.…”

“…9 Alkali promoted catalytic surfaces were shown to exhibit the occurrence of Turing-like stationary concentration patterns. 4,8,14,15 Such structures are non-equilibrium structures, i.e. they can only be sustained in the presence of an on-going chemical reaction.…”

“…Formally, diffusion is here no longer Fickian and mass transport is controlled by a gradient in the chemical potential. 7,8 Such patterns are often referred to as the result of a ''reactive phase separation''. [4][5][6] On catalytic surfaces, they have been first observed in the K-promoted Rh(110) and, later, on a Rh(110) surface alloyed with Pd/Au, both systems being exposed to the O 2 + H 2 reaction.…”

“…[4][5][6] On catalytic surfaces, they have been first observed in the K-promoted Rh(110) and, later, on a Rh(110) surface alloyed with Pd/Au, both systems being exposed to the O 2 + H 2 reaction. 4,8,[10][11][12] The formation of stationary K + O patterns is, however, not the only novelty occurring using alkali promoted systems but one may also observe pulse transporting potassium as seen in the NO + H 2 reaction on K/Rh(110). [13][14][15] When pulses coming from opposite directions collide a substantial accumulation of potassium takes place in the collision area, generating a laterally inhomogeneous K concentration.…”

“…This chemical attraction was the basis of the modeling of reactive phase separation in the system Rh(110)/K/O 2 + H 2 . 8 It turned out, however, that adding attractive energetic interactions between K + O to the mathematical model of the unpromoted system could not reproduce the mass transport of potassium by pulses. Only when repulsive interactions between K and coadsorbed nitrogen were added to the model the simulations yielded pulses which were transporting potassium as observed in the experiment.…”