Heterogeneous model systems

Even the simplest detersive system is surprisingly complex and heterogeneous. It can nevertheless be conceptually resolved into simpler systems that are amenable to theoretical treatment and understanding. These simpler systems are represented by models for substrate-soHd soil and substrate-Hquid sod. In practice, many sod systems include soH—Hquid mixtures. However, removal of these systems can generally be analyzed in terms of the two simpler model systems. Although these two systems differ markedly in behavior and stmcture, and require separate treatment, there are certain overriding principles that apply to both. 

In this seetion we begin the diseussion with some basie thermodynamie eonsiderations whieh, as we hope, will show elearly why the efleets of surfaee heterogeneity make the study of surfaee phase transitions so diflieult. Then we shall eoneentrate mainly on the results of the reeent eomputer simulation studies of phase transitions in model systems exhibiting various forms of disorder. 

In the following we shall study a model system for the synthesis of hydrogen peroxide (H2O2) over a heterogeneous catalyst containing a hypothetical metal M. It is proposed to split the reaction into the following elementary steps, which are all assumed to be in quasi-equilibrium except for step 3, which is assumed to be the rate-limiting step ... 

In the recent past much ejperimental and theoretical effort has been undertaken to understand the microsoopic steps of heterogeneous surface reactions. Ihe main problem oonsists of evaluating the total energy of the reacting coponents (including tiie surface atoms ) as a function of all nuclear coordinates at any reaction time. The solution of this problem is extremely difficult. Detailed studies with model systems, however, can shed same light ipon the various steps of the interaction pattern. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

In addition, it sustains CO electro-oxidation at relatively low overpotential, and there are crystal face dependences for both the ORR and CO oxidation. Since Au is also a system that exhibits both particle size and support effects in heterogeneous catalysis, it provides an interesting model system for smdying such effects in electrocatalysis. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

R. R. Vang, E. Laesgaard and F. Besenbacher, Bridging the pressure gap in model systems for heterogeneous catalysis with high-pressure STM, Phys. Chem. Chem. Phys., to be published. 

Malmstrom, M. E., G. Destouni and P. Martinet, 2004, Modeling expected solute concentration in randomly heterogeneous flow systems with multicomponent reactions. Environmental Science and Technology 38,2673-2679. 

As an example, C2-symmetric isospecific models for homogeneous catalytic systems based on the (R, / -coordinated isopropyl-bis(l-indenyl) ligand and for heterogeneous catalytic systems based on TiCLt supported on MgCl2 are compared in Figure 1.19. These models correspond to minimum-energy preinsertion intermediates calculated to be suitable for primary propene insertion... 

Rather than limit ourselves to a specific component technology, we then introduce the port-connector model of component architectures. We discuss a typical example of such an architecture and show how to specify and design with components in this architecture. Then we show how even ad hoc and heterogenous component systems are amenable to systematic development in Catalysis. 

The photochemistry of four triphenylmethane acid dyes was studied in poly(vinyl alcohol), methylcellulose and gelatin films. These model systems were chosen with a view to elucidating the complex free-radical reactions taking place in the heterogeneous dyed wool/ water/air system on exposure to UV radiation. The dye fading mechanism seems to involve an excited triplet state of the dye molecule [ 164] The rate of fading is governed by ... 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

Besides the practical application, the diversity of nanostructured carbon allotropes makes nanocarbon also an ideal model system for the investigation of structure-function correlations in heterogeneous catalysis. Nanocarbons can be tailored in terms of their hybridization state, curvature, and aspect ratio, i.e., dimensions of stacks of basic structural units (BSU), Chapters 1 and 2. The preferred exposition of two types of surfaces, which strongly differ in their physico-chemical behavior, i.e., the basal plane and prismatic edges, can be controlled. Such controlled diversity is seldom found for other materials giving carbon a unique role in this field of basic research. The focus of this chapter is set on the most prominent representatives of the... 

The relative simplicity of CO oxidation makes this reaction an ideal model system of a heterogeneous catalytic reaction. Each of the mechanistic steps (adsorption and desorption of the reactants, surface reaction, and desorption of products) has been probed extensively with surface science techniques, as has the interaction between O2 and CO " . These studies have provided essential information necessary for understanding the elementary processes which occur in CO oxidation. 

Micro structures in heterogeneous catalysts are closely related to the catalytic properties. TEM and related microanalytic techniques are powerful tools in characterising catalysts at atomic level. The obtained structural information is essential to the understanding of correlations between microstructures and catalytic properties. In this lecture note, the general principle of characterization of catalysts by TEM is introduced and the applications on Pt/Si02 model system and on VPO catalysts are intensively described. 

---