Catalysts, theoretical design

In this chapter, we will mainly address the vital topics in theoretical membrane research. Specifically, we will consider aqueous-based proton conductors. Our discussion of efforts in catalyst layer modeling will be relatively brief. Several detailed accounts of the state of the art in catalyst layer research have appeared recently. We will only recapitulate the major guidelines of catalyst layer design and performance optimization and discuss in some detail the role of the ionomer as a proton-supplying network in catalyst layers with a conventional design. 

Theoretical Design of Catalysts An area where work is needed in the field of selective oxidation is the theoretical design of catalysts. Although there has been some work in this area in the past thirty years 54,55,56) it has not... 

In spite of the widely recognized importance of an advanced catalyst layer design, detailed structural data for catalyst layers are still scarce in the open literature on fuel cells [116, 117]. In one of the rare experimental studies, Uchida et al. showed the effect of the variation of the PFSI (and PTFE) content on catalyst layer performance [101]. An attempt to rationalize the experimentally observed composition dependence theoretically was first undertaken in Ref. 17. The prerequisites for an adequate theoretical study... 

Phase-transfer catalysis (PTC) is the most widely synthesized method for solving the problem of the mutual insolubility of nonpolar and ionic compounds. The liquid-solid-liquid phase-transfer catalysis (LSLPTC) can overcome the purification of product and the separation of reactant and catalyst in the liquid-liquid phase-transfer catalytic reaction. The main structure of LSLPTC discussed in this study was focused the quaternary ammonium poly(mcthylstyrene-resin system. The reaction mechanism, catalytic activity, characterization of catalyst, theoretical modeling, mass transfers of reactant and pnxluct. and reactor design of LSLPTC were investigated. 

A molecular perspective of reactions from quantum chemistry calculations is the first step toward a theoretical design of new electrodes (e.g. binary or even ternary alloys). While reaction mechanisms on Pt and Pt alloy surfaces are getting clearer, details of these mechanism still remain elusive. For example, bifunctional mechanism of CO oxidation on PtSn and PtMo has received very little theoretical attention. Loading effects (CO, OH or specifically adsorbed anions) on CO oxidation is also poorly understood and requires further investigation. Theoretical calculations are also required to understand catalyst reorganization. Details of these calculations are required for accurately modeling the macroscopic kinetics on well-defined electrode surfaces and ultimately designing nanocatalyst particles. 

In a recent theoretical study, solvation effects that influence the energy of the reaction have been taken into accormt by a water cluster. A similar approach can be used to theoretically design bimetallic catalysts for the ORR Two metallic components are coupled and, based on their thermodynamics, their efficiency for the rate-determining step (4) and therefore their activity in the ORR is estimated. 

Catalyst molecular design in theory and application rely mainly on the following developments Theoretical chemistry and simulations, analytical instruments, surface chemistry and physics, organometallic chemistry, molecular sieve sciences and reaction engineering. 

Fig. 7.2 Partial serine protease mimics theoretical design catalyst 31 by Cram and realized hosts 32, 33 and 34...

Much progress has been made ia understanding how to create and use catalysts, but the design and preparation of practical catalysts stUl rehes on a substantial amount of art that is, the appHcation of known facts and iatuition to trial and error methods. General principles are described ia a number of texts (18—21). Very few completely new catalyst systems have been designed from first principles or completely theoretical considerations. New catalysts are much more likely to be discovered as a result of an adventitious observation than designed by iatent. 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

Initially, most theoretical methods calculated the properties of molecules in the gas phase as isolated species, but chemical reactions are most often carried out in solution. Biochemical reactions normally take place in water. Consequently, there is increasing interest in methods for including solvents in the calculations. In the simplest approach, solvents are treated as a continuum, whose average properties are included in the calculation. Explicit inclusion of solvent molecules in the calculation greatly expands the size of the problem, but newer approaches do this for at least those solvent molecules next to the dissolved species of interest. The detailed structures and properties of these solvent molecules affect their direct interaction with the dissolved species. Reactions at catalytic surfaces present an additional challenge, as the theoretical techniques must be able to handle the reactants and the atoms in the surface, as well as possible solvent species. The first concrete examples of computationally based rational catalyst design have begun to appear in publications and to have impact in industry. 

Progress in understanding FT reaction mechanisms shall be useful for theoretical calculations, catalyst design, and reaction engineering. 

Catalyst Design from Theoretical Principles Chemzymes for Peptide Synthesis from Theozyme Blueprints... 

Initial theoretical studies focused on steps (1) and (2). Several model systems were examined with ab initio calculations.1191 For the reaction of methyl amine with methyl acetate, it was shown that the addition/elimi-nation (through a neutral tetrahedral intermediate) and the direct displacement (through a transition state similar to that shown in Figure 5a) mechanisms for aminolysis had comparable activation barriers. However, in the case of methyl amine addition to phenyl acetate, it was shown that the direct displacement pathway is favored by approximately 5 kcal/mol.1201 Noncovalent stabilization of the direct displacement transition state was therefore the focus of the subsequent catalyst design process. 

How can the use of modem quantitative theoretical approaches help in designing new and better WGS catalysts ... 

In view of the reported theoretical studies, the regio- and diastereoselectivies should be the focus of the future theoretical investigation. Insights into the factors influencing the selectivities are badly required for designing better and useful catalysts. 

To examine the nature of the enatioselectivity of the catalysis we have examined the catalytic cycle with two substrates, styrene which leads to predominantly the R form of the product (64% ee) and 4-(dimethylamino)styrene which gives predominantly the S form of the product (67% ee). Our simulations suggest that the t 3-allylic coordination of the styrene substrate plays an important role in defining the enatioselectivity of the hydrosilylation. As a first step, this theoretical study constitutes a valid contribution in rationalizing the enantioselective determining factors and possibly in designing a new catalyst with improved enantioselective properties. We are currently examining nature of the enantioselectivity in more detail as well as the dependence of the enantioselectivity on the electronic nature of the substrates [58]. 

Research tools and fundamental understanding New catalyst design for effective integration of bio-, homo- and heterogeneous catalysis New approaches to realize one-pot complex multistep reactions Understanding catalytic processes at the interface in nanocomposites New routes for nano-design of complex catalysis, hybrid catalytic materials and reactive thin films New preparation methods to synthesize tailored catalytic surfaces New theoretical and computational predictive tools for catalysis and catalytic reaction engineering... 

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