Particle surfaces catalysis

We have discussed here, very briefly, some recent observations of small particle surfaces and how these relate to geometrical catalytic effects. These demonstrate the general conclusion that high resolution imaging can provide a direct, structural link between bulk LEED analysis and small particle surfaces. Apart from applications to conventional surface science, where the sensitivity of the technique to surface inhomogenieties has already yielded results, there should be many useful applications in catalysis. A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why results comparable to those described cannot be obtained from commercial catalyst systems. 

The Michaelis-Menten equatioa 10.2-9, is developed in Section 10.2.1 from the point of view of homogeneous catalysis and the formation of an intermediate complex. Use the Langmuir-Hinshelwood model of surface catalysis (Chapter 8), applied to the substrate in liquid solution and the enzyme as a colloidal particle with active sites, to obtain the same form of rate law. 

The dynamics of particles, especially the role of particle-particle interactions (coagulation) is critically assessed. The effects of particle surfaces on the catalysis of... 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

Special reference should be made for the last section of Chapter 3 Particle analysis. Everything in connection with particle properties and basic calculations, irrespective of its specific use, is presented from particle surface area to calculations regarding its terminal velocity and diffusion coefficients. Furthermore, concerning materials used in adsorption, ion exchange, and catalysis, special paragraphs are included in Chapters 4 and 5 as well as the management of spent materials. 

The theoretical approach to the subject of surface catalysis was first considered in a series of classical papers by Langmuir (1), who suggested that the adsorbed particles are held to the surface by chemical forces, and applied the theory to interaction of adsorbed species at adjacent adsorption sites on the surface. Langmuir pointed out that steric hindrance effects between molecules might play a prominent part, and the role of the geometric factor in catalysis was greatly emphasized by Balandin and others. The importance of this factor has already been reviewed in this series by Trapnell (2) and Griffiths (3). 

The ratio of surface area to bulk volume of the reinforcing particles can have important implications on optical properties, where the contribution of surface states can result in unique properties.56,57 These surface states cause shifts in the plasmon absorption frequencies and can be manipulated by use of different combinations of metals and ceramics.56 Another possibility due to the high surface area of the metal particles is catalysis applications, provided the ceramic matrix contains open pores.19... 

Homogeneous catalysis is, of course, a major field in it s own right, as catalytic transformations are important synthetic tools. However, catalysis is also a potentially sensitive probe for nanoparticle properties and surface chemistry, since catalytic reactions are ultimately carried out on the particle surface. In the case of bimetallic DENs, catalytic test reactions have provided clear evidence for the modification of one metal by another. DENs also provide the opportunity to undertake rational control experiments not previously possible to evaluate changes in catalytic activity as a function of particle composition. 

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

These three aspects in catalysis by metals enter in the general frame of structure-activity relationships. They have been the subject of reviews dealing with the (1) particle-size and plane-structure sensitivities [10], (2) ensemble-size sensitivity [11], and (3) metal-support interaction [9]. Depending on whether or not the turnover frequency (TOF), or rate per unit surface area or per accessible metal atom, is affected by the structure of the particle surface, the reactions have been called structure-sensitive or structure-insensitive [12]. The structure-activity relationships... 

This equation explains the observed rate dependency in Eq. (9.41), where the rate constant in the observed reaction, k, is equal to the product, kK, K 2- The surface catalysis of manganese oxidation is veiy effective in natural waters. The strength of the catalysis as indicated by the catal5Tic turnover number (CTN) in Table 9.7 is less than some of the other catalysts listed, but particle surfaces are ubiquitous. 

Catalysis. Many reactions are catalyzed, i.e., increased in rate, by a compound in solution (homogeneous catalysis) or a group at the surface of a particle (heterogeneous catalysis), where the catalyst is not consumed itself. Examples are various hydrolyzing reactions, like the ester hydrolysis mentioned above, that are catalyzed by H+ as well as OH ions. In such a case the reaction rate greatly depends on pH, though the ions themselves do not appear as reactants in the overall reaction scheme. Ubiquitous in natural foods are enzyme-catalyzed reactions. The simplest case leads to Michaelis-Menten kinetics, but several complications may arise. 

Other exciting frontier areas of research in chemical engineering include molecular and nanoscale engineering, molecular simulation, surface modification, protein separation processes, supercritical fluid extraction, fluid particle systems, catalysis and reaction engineering, biochemical engineering, and computer-aided design, see also Careers in Chemistry. 

Two types of mass- transfer can be distinguished for catalysis with heterogeneous catalyst particles. External mass transfer refers to molecular transport between the bulk reaction mixture and the surface of the enzyme particle through a boundary layer. Internal mass transfer is the molecular transport inside the solid enzyme phase. Internal mass transfer occurs within the pores of the catalyst particle to and from the particle surface. Figure 4.9-4 illustrates the definitions of external and internal mass transfer. 

In a comparative study [81], we have been able to show that - according to XANES and XPS studies - Pt is predominantly present in the zerovalent state (Fig. 2.7). However, there is a strong indication that a small layer at the particle surface is slightly oxidized. For most appUcations in catalysis this is only a minor drawback because re-reduction of the oxidized surface occurs easily in the temperature range... 

The construction of LbL multilayer films of biomaterials on colloid particles is of particular interest in applications where a microscopic contact is essential, such as protein interaction and cell communication, and where high surface area is desirable, such as in catalysis. For example, enzyme LbLs on particle surfaces are useful for biorelated catalysis since microscopic objects of higher surface area can potentially yield higher enzymatic reaction efficiencies than their planar film counterparts. If the LbL assembly is conducted on microparticles with certain functions, we can... 

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