M. Ehsasi

Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction: Oxidation of CO on platinum, experiments and simulations

The rate of reaction for oxidation of CO over (210) and (111) single‐crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation. Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface. At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2). The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena. A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al. was used for the simulation of the...

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 wh...

A reactive phase diagram of CO oxidation on Pd(110): Steady and oscillatory states

The steady and oscillatory regions of the CO oxidation reaction on the Pd(110) surface have been determined as a function of externally controlled parameters (flow rate, CO and oxygen partial pressures, temperature) over a wide range. At constant sample temperature and flow rate, the experiments yield a characteristic cross‐shaped phase diagram separating regions of monostability, bistability, and oscillatory behavior. The existence of a cross‐shaped phase diagram indicates the operation of a slow feedback process, which could be traced back to the (experimentally verified) formation and retarded removal of subsurface oxygen during the reaction. The diagram reflects one of the first well‐defined oscillatory systems in heterogeneous catalysis and may provide a general basis for mechanistic studies and models of oscillatory surface reactions.

Hydrogen on rhodium (110); the formation of a high-density chemisorbed phase

Abstract The adsorption of H (D) on Rh (110) at 80 K leads to a sequence of lattice gas phases: p1 × 3 at θ = 1 3 , p1 × 2 at θ = 1 2 , 1 × 3-2H at θ = 2 3 and 1 × 2-3H at θ= 3 2 ; and also the formation of a 1 × 1 -2H phase at θ = 2, the first time that a chemisorbed H layer with a density nearly 2 × 1015 atoms/cm2 is reported for a fcc surface. LEED investigations suggest the absence of surface reconstruction due to adsorbed hydrogen.

Coupled chemical oscillators in catalytic oxidation of CO on Pd(110) surfaces

Abstract Gas phase coupling between two separate (110)-oriented palladium single crystals is reported during reaction rate oscillations of CO oxidation. The oscillations were monitored by simultaneous measurement of work function changes on each crystal and of the combined CO 2 production rates. With the onset of the oscillations, synchronization between the two samples occurred rapidly via the gas phase. Under strong coupling conditions and identical sample temperatures, both surfaces exhibited identical amplitudes and frequencies. Variation of the coupling intensity led to phase locking, entrainment, bead formation, composed oscillations and enhancement. The communication between the two crystals occurs entirely via the CO pressure. The coupled crystals system can serve as a model for the study of the dynamics of coupling in heterogeneous reactions as well as in oscillating reactions generally.

Reaction rate oscillation of CO oxidation on Pt(210)

We report on the occurrence of kinetic instabilities and regular oscillations in the rate of CO2 formation as it results from the CO oxidation reaction carried out in ultrahigh vacuum and high vacuum on a platinum (210) surface. While the surface properties are characterized and controlled by combined low‐energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and ΔΦ measurements, the rate of reaction is monitored by measurements of the partial pressures of the reactants and/or of the work‐function change of the Pt(210) surface. We have followed the reaction kinetics as a function of ‘‘external’’ parameters (PCO, PO2, T) and have investigated the conditions under which oscillations occur. Likewise, the influence of some of the ‘‘internal’’ parameters (sample cleanliness, gas purity, surface structure) has been worked out. Finally, possible mechanisms for the occurrence of periodic oscillations are discussed, including the local oscillation as well as the process of synch...

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 t...

UV-photoemission electron microscopy investigation of pattern formation during oxidation of CO on a platinum(210) surface

Abstract The formation of spatiotemporal patterns during catalytic CO oxidation on a Pt(210) surface was followed using photoemission electron microscopy (PEEM). Depending on the choice of reaction parameters (flow rate, P CO , P O 2 , sample temperature) the reaction exhibited both steady and oscillatory rates. In the steady state, the surface was covered by either oxygen or carbon monoxide. Oscillatory behaviour occurred over a narrow range of parameters and O 2 pressures > 10 -4 Torr in the transition region between the two steady states. The appearance of oscillations was preceded by the nucleation of small oxygen islands and the formation of reactive wavefronts which frequently led to target or spiral patterns. Interestingly, the formation of spirals is often preceded by the rupture of one of the inner target pattern rings. Preferential nucleation of oxygen islands was observed close to surface defects on both a microscopic and a macroscopic scale (scratches etc.). For most cases the velocity of the reactive fronts was isotropic and was not correlated with any particular azimuthal symmetry direction of the (210) surface. Our results underline the usefulness of PEEM in the study of pattern formation and reactive diffusion processes on surfaces, and can thus help to clarify the microscopic reaction mechanisms.

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H2 and D2 on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases atθH=0.5 and atθH=1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (α1,α2 andβ) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of theβ state is invariant with the hydrogen isotope, the D α1 state appears at a ∼8 Klower and theD α2 state at a ∼5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D2 adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases.Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.

Spatial self-organization during catalytic reaction of Co and oxygen on a Pd(110) surface

The formation of spatial patterns during the catalytic oxidation of CO on a Pd(110) surface has been studied using a photoelectron emission microscope (PEEM). The reaction exhibited both steady state and oscillatory reaction rates over a wide pressure range of reactants (10−6 < Po2 < 10−1 torr). The coupling of reaction and surface diffusion of reactants resulted in the formation of spatial structures that were made visible due to differences in the work function of adsorbed CO and oxygen. Regions covered by CO had a higher work function, they therefore appeared darker than oxygen-covered areas. The temporal oscillations in the CO2 reaction rate were correlated with alternate switching between the high CO-covered and high oxygen-covered phases. However, in some cases spatial patterns such as waves, target patterns and spirals were formed, while the temporal CO2 reaction rate remained constant. The effect of the elongation of the structures was attributed to two different reaction-diffusion rate processes along and across the [110] troughs. The damping of large amplitude temporal oscillations synchronized by the gas phase was accompanied by a gradual development of structural patterns (in this case target patterns). The oscillations of the pacemakers responsible for these target patterns were, for the most part, out of phase