Vladimir P. Zhdanov

Mechanism and kinetics of the NOCO reaction on Rh

Abstract During the past 15 years, the NOCO reaction on Rh has attracted considerable attention of the researchers working in academic and applied surface science. The practical importance of this reaction is connected with its relevance for environmental chemistry. From the point of view of academic studies, the NOCO reaction on Rh is of interest because it represents one of the simplest examples from the class of catalytic reactions occurring via decomposition of adsorbed species. At present, the detailed kinetic data for this reaction are available both for single-crystal and supported Rh, at ultrahigh vacuum (UHV) conditions and also at realistic pressures. For this reason, the NOCO reaction on Rh has become one of the major testing platforms for a microscopic, surface-science based approach to heterogeneous catalysis. The present review shows how far the progress in this field has come. In particular, the review describes in detail the evolution of the ideas for the mechanism of the reaction and also presents the data for the elementary reaction steps, obtained primarily on Rh(1 1 1) at UHV conditions. Then, the possibility of using these data for simulation of the reaction kinetics at moderate pressures, P NO ⋍ P CO ⋍ 0.01 bar, is discussed. The technological aspects of application of Rh in the automotive exhaust systems are surveyed as well, but only briefly.

Kinetic phase transitions in simple reactions on solid surfaces

Abstract In analogy to real thermodynamic phase transitions, the term “kinetic phase transition” physically means that the kinetic behaviour of the system under consideration changes qualitatively when a control parameter (e.g., temperature or pressure) passes through a critical point. Mathematically, this means that a bifurcation occurs at this point. If the change in the reaction rate is stepwise at the critical point, the kinetic phase transition belongs to the first-order class. If the change is softer, the transition is continuous. The present review is primarily focused on the first-order kinetic phase transitions connected with bistability and resulting in chemical waves. Transitions of this type, predicted and often well described by common, mean-field kinetic equations, are experimentally observed in rapid surface reactions such as CO or hydrogen oxidation on transition metals under UHV conditions, and at atmospheric pressure as well. Continuous kinetic phase transitions in heterogeneous reactions have been predicted by Monte Carlo simulations for systems with a high reaction rate, provided that the adsorbed species are immobile. In real systems, however, surface diffusion is usually rapid compared to reaction steps. Perhaps this is the main reason why continuous kinetic phase transitions have not been observed experimentally so far. Nevertheless, such transitions are of general theoretical interest, and they are also discussed in this review. The material presented may be useful for interpreting how the steady-state kinetics of surface reactions vary with the control parameters and also for understanding more complex time-dependent critical kinetic phenomena, such as oscilations and chaos, which may take place in strongly nonequilibrium heterogeneous systems.

Kinetics of the hydrogen-oxygen reaction on platinum

Abstract A kinetic model has been constructed primarily to describe the rates of water and hydroxyl desorption during the hydrogen-oxygen reaction on Pt at high temperatures, ∼ 1000 K, and pressures in the range 1–1000 m Torr. The calculated results are compared with experimental observations. The model is based on dissociative sticking of H 2 and O 2 and hydrogen addition to oxygen to form OH. For water formation the two alternative routes OH + H → H 2 O and OH + OH → H 2 O + O are considered. OH decomposition, OH → O + H, is found to be an important reaction step. Using available literature data and results from the model calculations, an enthalpy diagram for the reaction is constructed. It is concluded that a unique enthalpy diagram for the H 2 + O 2 reaction on Pt is still lacking, even at nearly zero coverage. Adsorbate-adsorbate interactions are expected to modify the enthalpy diagram at high coverages. The kinetic equations for various reaction steps have been formulated assuming random distribution of adsorbed species on a uniform surface. At fixed temperature, both routes for H 2 O formation mentioned above can give a reasonably good quantitative description of the OH and H 2 O desorption rates as functions of gas mixture and pressure in the regimes where experimental data are available. However, a closer analysis shows that the relative importance of the two water formation routes could depend sensitively on temperature and gas-phase H 2 /O 2 ratio. High temperature and hydrogen excess favors the OH + H → H 2 O route, while oxygen excess and low temperatures may favor the OH disproportionation reaction. The model has also been used to predict the reaction kinetics at high pressures up to 105 Torr. The latter results may be useful as guides to high-pressure experiments and in calculations of catalytic combustor performance.

Simulation of adsorption kinetics of lipid vesicles

Employing the Monte Carlo technique, we study the kinetics of vesicle adsorption at a solid–liquid interface. The proposed model combines a treatment of the surface kinetics, including spontaneous, adsorption- and lipid-membrane-induced decomposition of adsorbed vesicles, and limitations of the adsorption rate by vesicle diffusion in the solution. With this model, we demonstrate different kinetic cases, corresponding to various scenarios of vesicle adsorption and decomposition. The general results are employed to simulate recent experimental kinetic data for adsorption of small phospholipid vesicles at a SiO2 surface.

Catalytic ignition in the COO2 reaction on platinum: experiment and simulations

Abstract Catalytic ignition temperatures for the CO+O2 reaction on a Pt wire, and the corresponding transient ignition curves, have been measured and analyzed theoretically for nonflammable reactant mixtures in Ar and He at atmospheric pressure. At very low relative CO concentrations (

Solvent-Assisted Lipid Bilayer Formation on Silicon Dioxide and Gold

Planar lipid bilayers on solid supports mimic the fundamental structure of biological membranes and can be investigated using a wide range of surface-sensitive techniques. Despite these advantages, planar bilayer fabrication is challenging, and there are no simple universal methods to form such bilayers on diverse material substrates. One of the novel methods recently proposed and proven to form a planar bilayer on silicon dioxide involves lipid deposition in organic solvent and solvent exchange to influence the phase of adsorbed lipids. To scrutinize the specifics of this solvent-assisted lipid bilayer (SALB) formation method and clarify the limits of its applicability, we have developed a simplified, continuous solvent-exchange version to form planar bilayers on silicon dioxide, gold, and alkanethiol-coated gold (in the latter case, a lipid monolayer is formed to yield a hybrid bilayer) and varied the type of organic solvent and rate of solvent exchange. By tracking the SALB formation process with simultaneous quartz crystal microbalance-dissipation (QCM-D) and ellipsometry, it was determined that the acoustic, optical, and hydration masses along with the acoustic and optical thicknesses, measured at the end of the process, are comparable to those observed by employing conventional fabrication methods (e.g., vesicle fusion). As shown by QCM-D measurements, the obtained planar bilayers are highly resistant to protein adsorption, and several, but not all, water-miscible organic solvents could be successfully used in the SALB procedure, with isopropanol yielding particularly high-quality bilayers. In addition, fluorescence recovery after photobleaching (FRAP) measurements demonstrated that the coefficient of lateral lipid diffusion in the fabricated bilayers corresponds to that measured earlier in the planar bilayers formed by vesicle fusion. With increasing rate of solvent exchange, it was also observed that the bilayer became incomplete and a phenomenological model was developed in order to explain this feature. The results obtained allowed us to clarify and discriminate likely steps of the SALB formation process as well as determine the corresponding influence of organic solvent type and flow conditions on these steps. Taken together, the findings demonstrate that the SALB formation method can be adapted to a continuous solvent-exchange procedure that is technically minimal, quick, and efficient to form planar bilayers on solid supports.

Determination of Exosome Concentration in Solution Using Surface Plasmon Resonance Spectroscopy

Exosomes are cell-secreted nanometer-sized extracellular vesicles that have been reported to play an important role in intercellular communication. They are also considered potential diagnostic markers for various health disorders, and intense investigations are presently directed toward their use as carriers in drug-delivery and gene-therapy applications. This has generated a growing need for sensitive methods capable of accurately and specifically determining the concentration of exosomes in complex biological fluids. Here, we explore the use of label-free surface-based sensing with surface plasmon resonance (SPR) read-out to determine the concentration of exosomes in solution. Human mast cell secreted exosomes carrying the tetraspanin membrane protein CD63 were analyzed by measuring their diffusion-limited binding rate to an SPR sensor surface functionalized with anti-CD63 antibodies. The concentration of suspended exosomes was determined by first converting the SPR response into the surface-bound mass. The increase in mass uptake over time was then related to the exosome concentration in solution using a formalism describing diffusion-limited binding under controlled flow conditions. The proposed quantification method is based on a calibration and control measurements performed with proteins and synthetic lipid vesicles and takes into account (i) the influence of the broad size distribution of the exosomes on the surface coverage, (ii) the fact that their size is comparable to the ∼150 nm probing depth of SPR, and (iii) possible deformation of exosomes upon adsorption. Under those considerations, the accuracy of the concentration determination was estimated to be better than ±50% and significantly improve if the exosome deformation is negligible.

Monte Carlo simulation of the kinetics of rapid reactions on nanometer catalyst particles

Abstract Using an original Monte Carlo algorithm, we have analyzed the kinetics of the 2 A + B 2 →2 A B reaction, occurring via the standard Langmuir–Hinshelwood (LH) mechanism, in the two limits corresponding to the infinite surface and to interacting facets (by surface diffusion) of a nanometer-sized crystallite, respectively. The latter mimics possible kinetics on real supported catalysts, and on recent nanofabricated model catalysts. The simulations were carried out with a realistic ratio between the rates of the elementary steps (the LH step is rapid compared to A and B 2 adsorption and slow compared to A diffusion, and B particles are immobile). The results demonstrate that the kinetics for nanometer-sized particles can be remarkably different compared to those for the infinite surface. Depending on the values of model parameters, the reaction window (along the A + B 2 gas mixture scale) on the nanometer catalyst particle may be wider or narrower than on the infinite surface, and its position may be shifted. These findings have implications for interpretation of experimental data and for the design of real catalysts and also have important consequences for the continuing efforts to bridge the so-called pressure and structure “gaps” in catalysis.

Hydride formation thermodynamics and hysteresis in individual Pd nanocrystals with different size and shape

Physicochemical properties of nanoparticles may depend on their size and shape and are traditionally assessed in ensemble-level experiments, which accordingly may be plagued by averaging effects. These effects can be eliminated in single-nanoparticle experiments. Using plasmonic nanospectroscopy, we present a comprehensive study of hydride formation thermodynamics in individual Pd nanocrystals of different size and shape, and find corresponding enthalpies and entropies to be nearly size- and shape-independent. The hysteresis observed is significantly wider than in bulk, with details depending on the specifics of individual nanoparticles. Generally, the absorption branch of the hysteresis loop is size-dependent in the sub-30 nm regime, whereas desorption is size- and shape-independent. The former is consistent with a coherent phase transition during hydride formation, influenced kinetically by the specifics of nucleation, whereas the latter implies that hydride decomposition either occurs incoherently or via different kinetic pathways.

High sintering resistance of size-selected platinum cluster catalysts by suppressed Ostwald ripening.

Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.