Field ion microscopic studies of the CO oxidation on platinum: Bistability and oscillations

The oscillating CO oxidation is investigated on a Pt‐field emitter tip by using the field ion mode of surface imaging of Oad sites with O2 as imaging gas. Based on data of the titration reactions [V. Gorodetskii, W. Drachsel, and J. H. Block, J. Chem. Phys. 100, C. E. UPDATE (1994)], external control parameters for the regions of bistability and of self‐sustained isothermal oscillations could be found. On a field emitter tip, oscillations can be generated in a rather large parameter space. The anticlockwise hysteresis of O+2 ion currents in temperature cycles occurs in agreement with results on single crystal planes. Unexpected regular oscillation sequences could occasionally be obtained on the small surface areas of a field emitter tip and measured as function of the CO partial pressure and of the temperature. Different stages within oscillating cycles were documented by field ion images. Oscillations of total ion currents are correlated with variations in the spatial brightness of field ion images. In the manifold of single crystal planes of a field emitter {331} planes around the {011} regions are starting points for oscillations which mainly proceed along [100] vicinals. This excludes the {111} regions from autonomous oscillations. With slightly increased CO partial pressures fast local oscillations at a few hundred surface sites of the Pt(001) plane display short‐living CO islands of 40 to 50 Å diameter. Temporal oscillations of the total O+2 ion current are mainly caused by surface plane specific spatial oscillations. The synchronization is achieved by diffusion reaction fronts rather than by gas phase synchronization

Macroscopic and mesoscopic characterization of a bistable reaction system: CO oxidation on Pt(111) surface

The catalytic oxidation of CO by oxygen on a platinum (111) single-crystal surface in a gas-flow reactor follows the Langmuir–Hinshelwood reaction mechanism. It exhibits two macroscopic stable steady states (low reactivity: CO-covered surface; high reactivity: O-covered surface), as determined by mass spectrometry. Unlike other Pt and Pd surface orientations no temporal and spatiotemporal oscillations are formed. Accordingly, CO+O/Pt(111) can be considered as one of the least complicated heterogeneous reaction systems. We measured both the macroscopic and mesoscopic reaction behavior by mass spectrometry and photoelectron emission microscopy (PEEM), respectively, and explored especially the region of the phase transition between low and high reactivity. We followed the rate-dependent width of an observed hysteresis in the reactivity and the kinetics of nucleation and growth of individual oxygen and CO islands using the PEEM technique. We were able to adjust conditions of the external control parameters which totally inhibited the motion of the reaction/diffusion front. By systematic variation of these conditions we could pinpoint a whole region of external control parameters in which the reaction/diffusion front does not move. Parallel model calculations suggest that the front is actually pinned by surface defects. In summary, our experiments and simulation reveal the existence of an “experimental” bistable region inside the “computed” bistable region of the reactivity diagram (S-shaped curve) leading to a novel dollar ($)-shaped curve

Efficient determiniation of multilayer relaxation in the Pt(210) stepped and densely kinked surface

The multilayer relaxation of the Pt(210) stepped and kinked surface is analyzed by low-energy-electron diffraction. This is the first application of the new real-space multiple-scattering theory of LEED, designed specifically for such open surfaces where conventional theories fail. Combined with an automated tensor LEED method, it efficiently detects nonalternating atomic relaxations which are oriented primarily perpendicular to the surface. These relaxations are in qualitative agreement with new embedded-atom-method results

Nature of phase transitions in a probabilistic cellular automaton with two absorbing states

We present a probabilistic cellular automaton with two absorbing states, which can be considered a natural extension of the Domany-Kinzel model. Despite its simplicity, it shows a very rich phase diagram, with two second-order and one first-order transition lines that meet at a tricritical point. We study the phase transitions and the critical behavior of the model using mean field approximations, direct numerical simulations and field theory. A closed form for the dynamics of the kinks between the two absorbing phases near the tricritical point is obtained, providing an exact correspondence between the presence of conserved quantities and the symmetry of absorbing states. The second-order critical curves and the kink critical dynamics are found to be in the directed percolation and parity conservation universality classes, respectively. The first order phase transition is put in evidence by examining the hysteresis cycle. We also study the "chaotic" phase, in which two replicas evolving with the same noise diverge, using mean field and numerical techniques. Finally, we show how the shape of the potential of the field-theoretic formulation of the problem can be obtained by direct numerical simulations.Comment: 19 pages with 7 figure