Pattern formation during the CO-oxidation involving subsurface oxygen

Travelling fronts of the CO oxidation on Pd(111) with coverage-dependent diffusion

In this work, we study a surface reaction on Pd(111) crystals under ultra-high-vacuum conditions that can be modeled by two coupled reaction-diffusion equations. In the bistable regime, the reaction exhibits travelling fronts that can be observed experimentally using photo electron emission microscopy. The spatial profile of the fronts reveals a coverage-dependent diffusivity for one of the species. We propose a method to solve the nonlinear eigenvalue problem and compute the direction and the speed of the fronts based on a geometrical construction in phase-space. This method successfully captures the dependence of the speed on control parameters and diffusivities.

Effect of Subsurface Oxygen on the Reactivity of the Ag(111) Surface

Periodic, self-consistent, density functional theory calculations have been performed to demonstrate that subsurface oxygen (O(sb)) dramatically increases the reactivity of the Ag(111) surface. O(sb) greatly facilitates the dissociation of H2, O2, and NO and enhances the binding of H, C, N, O, O2, CO, NO, C2H2, and C2H4 on the Ag(111) surface. This effect originates from an O(sb)-induced upshift of the d-band center of the Ag surface and becomes more pronounced at higher O(sb) coverage. Our findings point to the important role that near-surface impurities, such as O(sb), can play in determining the thermochemistry and kinetics of elementary steps catalyzed by transition metal surfaces.

Stationary fronts due to weak thermal effects in models of catalytic oxidation

We analyze the possible existence of an infinite number of stationary front solutions in a microkinetic model of a catalytic reaction coupled with weak enthalpy effects in the domain of kinetics bistability. The kinetic model incorporates three steps: dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. The infinitude of stationary front solutions emerges due to the lack of intercrystallites communication of surface species in supported catalysts; thermal conductions and gas-phase diffusion are the only means of interaction. Incorporation of surface species diffusion leads to a very slow front motion. We complement this analysis with simulations of stationary states on one- (wire and ring) and two-dimensional (disk) systems which may be subject to control or to fluid flow. These results account for certain experimental results and may have implications for various technological problems.

Moving waves and spatiotemporal patterns due to weak thermal effects in models of catalytic oxidation

We analyze the behavior of a microkinetic model of a catalytic reaction coupled with weak enthalpy effects to show that under fixed gas-phase concentrations it can produce moving waves with an intrinsic length scale, when the underlying kinetics is oscillatory. The kinetic model incorporates dissociative oxygen adsorption, reactant adsorption and desorption, and surface reaction. Three typical patterns may emerge in a one-dimensional system (a long wire or a ring): homogeneous oscillations, a family of moving waves propagating with constant velocities, and patterns with multiple source/sink points. Pattern selection depends on the ratio of the system length to the intrinsic wave length and the governing parameters. We complement these analysis with simulations that revealed a plethora of patterned states on one- and two-dimensional systems (a disk or a cylinder). This work shows that weak long-range coupling due to high feed rates maintains such patterns, while low feed rates or strong long-range interaction can gradually suppress the emerging patterns.