Kinetic phase diagram for CO oxidation on Pt(210): Pattern formation in the hysteresis and oscillation regions

The reactive behavior of catalytic CO oxidation on Pt(210) is studied by means of combined reaction rate measurements and photoelectron emission microscopy (PEEM). These methods allow an investigation of the phenomena at macroscopic and mesoscopic level, respectively. The external control parameters (flow rate, CO and oxygen partial pressures, surface temperature and scanning rates of pressure and temperature) are systematically varied to reveal various reactive regions in parameter space. The macroscopic measurements for a given temperature and flow rate (under isothermal conditions) show that lower pressures lead to a pronounced clockwise hysteresis in the production rate of CO2, while increasing pressures cause a systematic narrowing leading to a crossing of the two hysteresis branches into a region of counterclockwise hysteresis. A further pressure increase leads to macroscopic temporal oscillations. Mesoscopic spatiotemporal oscillations appear at the same conditions. The resulting macroscopic isothermal kinetic phase diagram exhibits a cross-shaped characteristic similar to that previously obtained for the Pd(110) surface. The mesoscopic lateral distribution of CO and oxygen adsorbed on the surface is monitored with the photoelectron emission microscope during the reaction at isothermal conditions and different constant oxygen pressures. The observed mesoscopic spatiotemporal patterns, such as islands, waves, target patterns and spirals, are correlated via the external control parameters with different regions in the macroscopic isothermal phase diagram. The results are compared with previous data of CO oxidation on other surfaces, like Pd(110) and Pt(110)

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

Numerical study of a first-order irreversible phase transition in a CO+NO catalyzed reaction model

The first-order irreversible phase transitions (IPT) of the Yaldran-Khan model (Yaldran-Khan, J. Catal. 131, 369, 1991) for the CO+NO reaction is studied using the constant coverage (CC) ensemble and performing epidemic simulations. The CC method allows the study of hysteretic effects close to coexistence as well as the location of both the upper spinodal point and the coexistence point. Epidemic studies show that at coexistence the number of active sites decreases according to a (short-time) power law followed by a (long-time) exponential decay. It is concluded that first-order IPT's share many characteristic of their reversible counterparts, such as the development of short ranged correlations, hysteretic effects, metastabilities, etc.Comment: 17 pages, 10 figure

Oscillating Catalytic CO Oxidation on a Platinum Field Emitter Tip: Determination of a Reactive Phase Diagram by Field Electron Microscopy

A [111]-oriented Pt field emitter tip is used to study bistability and oscillations of CO oxidation by field electron microscopy (FEM) under low-pressure conditions (pO2 = 4 × 10−4 Torr, pCO = 8 × 10−8 to 4 × 10−5 Torr) in the temperature range 240 to 450 K. A kinetic phase diagram can be established, obtained on an FEM level with ≍2 nm lateral resolution. Below T ≍ 300 K a region of bistability prevails, and above T ≍ 300 K an oscillatory regime is obtained, thus yielding a cross-shaped phase diagram. These phenomena are surface plane specific and visible at the {011} planes of Pt and their surroundings

CO oxidation on a copper-modified Pt(111) surface

The catalytic oxidation of carbon monoxide is studied in situ by means of reaction-rate measurements on a macroscopic scale and photoelectron emission microscopy (PEEM) on a mesoscopic scale on a Pt(111) single-crystal surface partially modified by evaporated Cu. By using a special preparation technique it is possible to cover selectively certain parts of the Pt(111) surface with copper, thus enabling a simultaneous PEEM investigation of the behavior of each separate catalyst component as well as their mutual influence. The objective of the work is to investigate the possible influence of deposited copper on CO oxidation on Pt(111) and to compare the results with pure Pt(111). Cu-modified regions preferentially form oxygen adsorption layers under the reaction conditions. The well-documented behavior of CO oxidation on Pt(111) is modified by the presence of the Cu domains. This modification concerns the transition between the GO-covered and oxygen-covered state of the Pt surface. The macroscopic effect of the modification by Cu is to shift the whole hysteresis in the reaction rate to higher CO pressures. On the mesoscopic scale the nucleation of oxygen islands at the beginning of the phase transition takes place on Cu-modified areas. On a Pt(111) sample on which submonolayer quantities of copper were uniformly deposited, the propagation velocity of the reaction/diffusion front of a growing oxygen island increases with the amount of deposited Cu. These experiments demonstrate the possibility of creating special geometric adsorbate patterns during CO oxidation on the Pt(111) surface, and of modifying the velocity of propagation of the reaction/diffusion front. (C) 1997 Elsevier Science B.V