Kinetic hysteresis in the CO oxidation reaction on Ir(111) surfaces

CO Adsorption and Disproportionation on Smooth and Defect-Rich Ir(111)

CO adsorption and dissociation on "perfect" and "defect-rich" Ir(111) surfaces were studied by a combination of surface-analytical techniques, including polarization-dependent (PPP and SSP) sum frequency generation (SFG) vibrational spectroscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. CO was found to be ordered and tilted from the surface normal at high coverage on the "perfect" surface (e.g., θ = 30° at 0.70 ML), whereas it was less ordered and preferentially upright (θ = 4-10°) on the "defect-rich" surface for coverages of 0.55-0.70 ML. SFG, LEED, and XPS revealed that CO adsorption at low pressure/high temperature and high pressure/low temperature was reversible. In contrast, upon heating to ∼600 K in near mbar CO pressure, "perfect" and even more "defect-rich" Ir(111) surfaces were irreversibly modified by carbon deposits, which, according to DFT, result from CO disproportionation.

Competing ternary surface reaction CO + O2 + H2 on Ir(111)

The CO oxidation on platinum-group metals under ultra-high-vacuum conditions is one of the most studied surface reactions. However, the presence of disturbing species and competing reactions are often neglected. One of the most interesting additional gases to be treated is hydrogen, due to its importance in technical applications and its inevitability under vacuum conditions. Adding hydrogen to the reaction of CO and O₂ leads to more adsorbed species and competing reaction steps towards water formation. In this study, a model for approaching the competing surface reactions CO+O ₂ + H₂ is presented and discussed. Using the framework of bifurcation theory, we show how the steady states of the extended system correspond to a swallowtail catastrophe set with a tristable regime within the swallowtail. We explore numerically the possibility of reaching all stable states and illustrate the experimental challenges such a system could pose. Lastly, an approximative first-principle approach to diffusion illustrates how up to three stable states balance each other while forming heterogeneous patterns.

Detailed analysis of transitions in the CO oxidation on palladium(111) under noisy conditions

It has been shown that CO oxidation on Pd(111) under ultrahigh vacuum conditions can suffer rare transitions between two stable states triggered by weak intrinsic perturbations. Here we study the effects of adding controlled noise by varying the concentrations of O₂ and CO that feed the vacuum chamber, while the total flux stays constant. In addition to the regime of rare transitions between states of different CO₂ reaction rates induced by intrinsic fluctuations, we found three distinct effects of external noise depending on its strength: small noise suppresses transitions and stabilizes the upper rate state; medium noise induces bursting; and large noise gives rise to reversible transitions in both directions. To explain some of the features present in the dynamics, we propose an extended stochastic model that includes a global coupling through the gas phase to account for the removal of CO gas caused by the adsorption of the Pd surface. The numerical simulations based in the model show a qualitative agreement with the noise-induced transitions found in experiments, but suggest that more complex spatial phenomena are present in the observed fluctuations.

Lévy noise-induced phenomena in CO oxidation on Ir(111) surfaces

The influences of external Lévy noise in CO oxidation on the Ir(111) surfaces are numerically examined. Depending on the control parameters, such as the temperature T  and the CO fraction Y  in the total reactant input gas flux, the reaction exhibits one or two stable stationary states in the absence of noise. Lévy noise is introduced in the reaction by randomly varying the quality of the influx mixture. We then observe a large variety of different types of behaviors depending on the initial CO fraction Y, in particular, the phenomenon of noise-induced shifts of the steady state in the monostable regime is regarded. We also show how the stationary probability density changes in terms of stochastic bifurcation when Lévy noise is applied in the bistable regime. The results provide a strong evidence for noise-induced transitions. We concentrate on the effects of the stability index α, the skewness parameter β,  and the noise intensity D  that emphasize the differences with Gaussian fluctuations.

Kinetic Analysis of High-Rate and of Low-Rate Regimes in CO Oxidation on Pt Group Metals: Evidence for Vibrational Excitation of the O-Rich Adlayer and for Thermal Equilibrium of the CO-Rich Phase

Proper reduction of available literature data on the heterogeneous CO+O2 reaction on Pt group metals allows one to describe the parallel process of CO desorption, in extended intervals of temperature and gas composition, both in the High-Rate regime of an oxygen rich surface and in the Low-Rate regime of a CO rich surface. The outcome of this analysis is the determination of “apparent” as well as of “true” activation energies for both CO oxidation and desorption and their dependence on surface coverage by O or by CO-adspecies. The conclusion is reached that non-equilibrium conditions of vibrational excitation of the adlayers prevail in the HR regions, while Boltzmann equilibrium characterizes LR regimes in extended temperature and pressure intervals. These results have been discussed in the framework of a theoretical model that predicts hyperthermal vibrational states of the adlayer, which depend on the efficiency of energy dissipation into the metal.

A technique for extending the precision and the range of temperature programmed desorption toward extremely low coverages

In this paper, an improvement of the temperature programmed desorption (TPD) technique is introduced, which facilitates fully automated sampling of TPD spectra with excellent reproducibility, especially useful for extremely low coverages. By averaging many sampled TPD spectra, the range of the TPD technique can be extended toward lower coverages, as well as the quality of the spectra can be improved. This allows for easy extraction of information about the adsorbate-surface bond. A state of the art TPD apparatus with a two chamber setup and a high quality quadrupole mass spectrometer was extended by automated components. These are an automated gas dosing system, ensuring precise dosing of gas, combined with a motor driven sample manipulation unit and a liquid nitrogen cryostat with automatic refilling. In addition all components were controlled by a computer. A large number of TPD cycles could be sampled without the need of interaction of an operator. Here, it is shown for up to more than 400 TPD cycles. This opens a wide range of new interesting applications for the TPD technique, especially in the limit of zero coverage. Here, basic experiments on well known adsorbate systems are shown to view the ability and limit of this approach.

Interplay of first-order kinetic and thermodynamic phase transitions in heterogeneous catalytic reactions

Using the rate constants obtained on the basis of independent transient measurements and density functional theory calculations, we perform Monte Carlo (MC) simulations of the bistable kinetics of the N2O-CO reaction on Pd(110) at 450 K. In the absence of lateral interactions, the MC technique predicts a wide hysteresis loop in perfect agreement with the mean-field analysis. With attractive substrate-mediated lateral interactions resulting in the formation of (1 x 2) O islands and reducing the reaction rate inside islands, the hysteresis is found to be dramatically (about 5 times) narrower. This finding explains why the first-order kinetic phase transition experimentally observed in this reaction is not accompanied by hysteresis.

Noise-induced spatiotemporal patterns in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling of the oxidation reaction on Ir(111)

We use photoelectron emission microscopy (PEEM) measurements to study the spatiotemporal patterns obtained for the CO oxidation reaction on Ir(111) as a function of the noise strength we superpose on the CO and the oxygen fractions of the constant total reactant gas flux. The investigations are focused on the bistable regime this reaction displays including its monostable vicinity. Simultaneously we analyze numerically the underlying reaction-diffusion (RD) equations in two spatial dimensions. For intrinsic and/or small strength of the external noise we find transitions from the locally stable to the globally stable branch via slow nucleation and growth of islands of the globally stable state: oxygen or CO, respectively. With increasing noise strength the number of islands as well as their growth rate increases. These phenomena are very well reproduced by numerical calculations of the RD model. For sufficiently large noise strength we observe bursts from CO rich to oxygen rich and back as well as switching between the two states. While such phenomena are also obtained from the model calculations, their experimentally observed spatial scales were not satisfactorily reproduced using the same approach as for the lower noise strengths.

Spatiotemporal patterns of external noise-induced transitions in a bistable reaction-diffusion system: photoelectron emission microscopy experiments and modeling

The rate of CO oxidation on Ir(111) surfaces exhibits bistability at T=500 K in a range of the CO fraction Y in the CO+O reactant gas flux. Measured CO2 rates as a function of the noise strength imposed on Y are well reproduced by parameter-free modeling. We present photoelectron emission microscopy measurements and 2D calculations of the spatiotemporal patterns of CO- and O-rich domains. The role of combined multiplicative and additive noise on Y for CO and O domain wall motion and island nucleation-growth-coalescence processes is analyzed.

Fluctuations and Bistabilities on Catalyst Nanoparticles

We show that coverage fluctuations on catalyst particles can drastically alter their macroscopic catalytic behavior. Scrutinizing the occurrence of kinetic bistabilities, it is demonstrated by molecular beam experiments on model catalysts that macroscopically observable bistabilities vanish completely with decreasing particle size, as previously predicted by theory. The effect is attributed to fluctuation-induced transitions between two kinetic reaction regimes, with a transition rate controlled by both particle size and surface defects. These results suggest that fluctuation-induced effects represent a general phenomenon affecting the reaction kinetics on nanostructured surfaces.

External noise imposed on the reaction-diffusion system CO+O2-->CO2 on Ir(111) surfaces: experiment and theory

We study experimentally and theoretically the influence of noise on the fractions of CO and oxygen in the constant gas flow directed at an Ir(111) surface during CO oxidation. Depending on the noise strength and the fraction Y of CO we observe in the deterministically bistable region a large variety of different types of behavior. These include bistable behavior for small noise intensities, transitions from the upper to the lower branch of the bistable loop and vice versa, island nucleation and growth and noise-induced switching. Near the boundary of the bistable region and in the presence of noise the transition between the two branches takes place via very slow domain wall motion with time scales of the order of 10(4)-10(5) s. The experiments were carried out in an UHV system for which the mass flow could be controlled very precisely. The modeling was using the reaction-diffusion system underlying the reaction studied for which all the kinetic coefficients are known rather precisely. Our numerical analysis was performed for one and two spatial dimensions showing qualitatively similar behavior. The comparison between the experimental results and the modeling shows semiquantitative to quantitative agreement.