Activity, catalytic

A key aim of QM modelling in microporous solids is to understand the mechanisms of catalytic conversions. Intermediates and transition states in catalytic reactions are short lived and difficult to observe experimentally, so that modelling their stability may be the only way to establish a reaction pathway. 

The first step in the catalytic conversion of methanol to olefins or gasoline, for example, has been extensively studied by both cluster and plane wave methods. The first reaction is the formation of dimethylether by the apparent 

Two mechanisms have been proposed for this reaction. The first includes a step in which the first methanol is protonated and dehydrated to give a reactive methoxy group attached to the framework, whereas the second involves two methanol molecules undergoing an Sn2 reaction inside the zeolite cage, catalysed by protonation of one of the methanols and with the transition state solvated by the zeolite cage. The conclusion from QM studies (using both cluster and plane wave approaches) is that the reaction proceeds through the latter pathway, in which the zeolite stabilises the transition state. 

The next step in the methanol-to-hydrocarbons reaction, and in fact the crucial one for the generation of hydrocarbon products is C-C bond formation. Very many proposed mechanisms exist for potential routes at the acid sites of the zeolites, but recent evidence suggests that the reaction instead proceeds via a reactive hydrocarbon pool (See Chapter 8). In fact, an extensive series of high-level theoretical calculations suggests that no single combination of direct reaction steps can link methanol to ethene, and so provides strong indirect evidence that the hydrocarbon pool mechanism is the correct one. 

Use has also been made of QM methods in determining the transition state 

A cathode material for SOFC should meet various requirements in catalytic activity, thermodynamic stability, and compatibility. The following are the requirements for the cathode materials and the proposed approach to find and design a suitable material for high stability and a high-performance cathode. 

For further improvement, the reaction mechanism should be clarified. Although many reports have been published so far on the reaction kinetics, information obtained from the experiments is limited, and because of the variety of reaction models the reaction mechanism still remains unclear. The development of in situ observation technique will be necessary. Recently, some efforts were reported on in situ techniques. Lu et al. [6] applied infrared emission spectroscopy to observe the adsorbed species on a (Sm,Sr)Co03 cathode under operation. They suggested 02 is the most probable adsorbate (Fig. 7.2). Murai et al. [7] employed polarization-modulated IR reflection absorption spectroscopy and found response in a similar frequency region. Quantum mechanical calculations are also made by several researchers [8]. 

An electrode transfers electrons from the current collectors to the reaction sites. The importance of electronic conductivity depends on the structure of the cell stack. For a porous electrode that is fabricated on electrolyte as a thin layer, the lateral current transport often becomes a serious problem. 

Especially, segment-in-series type stacks will have large current collection loss if the electrode has low conductivity. Even for planar stacks, the electrons may not be supplied sufficiently to the place if the current collection points are separated. Generally, electronic conductivity higher than 100 S cm is preferred for a SOFC electrode. If the electronic conductivity is 10 S cm and the electrode thickness is 50 pm, the resistance to transport electrons to the distance of 1 mm is as high as 2 cm Because area-specific resistance of a practical cell is below 1 Q cm , it will cause constriction of the current into the vicinity of the current collector. 

Particle size effects of highly dispersed supported catalysts (Pt) on the hydrogen oxidation reaction were evaluated by the same authors using this technique (97). Moreover, electrocatalysts for oxidation of methanol were screened using a technique called scanning differential electrochemical mass spectrometry (98, 99). This method uses a capillary probe scanned over the array that allows the intake and detection by mass spectrometry of products generated locally on each electrode. 

---

Enzymes are important catalysts in biological organisms and are of increasing use in detergents and sensors. It is of interest to understand not only their adsorption characteristics but also their catalytic activity on the surface. The interplay between adsorption and deactivation has been clearly illustrated [119] as has the ability of a protein to cleave a surface-bound substrate [120]. 

Fig. XVIII-17. Correlation of catalytic activity toward ethylene dehydrogenation and percent d character of the metallic bond in the metal catalyst. (From Ref. 166.)...

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

Valden M, Lai X and Goodman D W 1998 Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties Science 281 1647... 

Most reactions in cells are carried out by enzymes [1], In many instances the rates of enzyme-catalysed reactions are enhanced by a factor of a million. A significantly large fraction of all known enzymes are proteins which are made from twenty naturally occurring amino acids. The amino acids are linked by peptide bonds to fonn polypeptide chains. The primary sequence of a protein specifies the linear order in which the amino acids are linked. To carry out the catalytic activity the linear sequence has to fold to a well defined tliree-dimensional (3D) stmcture. In cells only a relatively small fraction of proteins require assistance from chaperones (helper proteins) [2]. Even in the complicated cellular environment most proteins fold spontaneously upon synthesis. The detennination of the 3D folded stmcture from the one-dimensional primary sequence is the most popular protein folding problem. 

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Chen C-FI, Washburn N and Gewirth A A 1993 In situ atomic force microscope study of Pb underpotential deposition on Au(111) Structural properties of the catalytically active phase J.Phys. Chem. 97 9754-60... 

Although there are examples of enzymes which maintain their catalytic activity even when ciystallized, they normally work in their natural (i.e., aqueous) environment. This is the reason why the majority of the simulations are carried out applying a technique that accounts for solvent effects. But what is the effect of a solvent ... 

Surprisingly, the highest catalytic activity is observed in TFE. One mi t envisage this to be a result of the poor interaction between TFE and the copper(II) cation, so that the cation will retain most of its Lewis-acidity. In the other solvents the interaction between their electron-rich hetero atoms and the cation is likely to be stronger, thus diminishing the efficiency of the Lewis-acid catalysis. The observation that Cu(N03)2 is only poorly soluble in TFE and much better in the other solvents used, is in line with this reasoning. 

The rate constants for the catalysed Diels-Alder reaction of 2.4g with 2.5 (Table 2.3) demonstrate that the presence of the ionic group in the dienophile does not diminish the accelerating effect of water on the catalysed reaction. Comparison of these rate constants with those for the nonionic dienophiles even seems to indicate a modest extra aqueous rate enhancement of the reaction of 2.4g. It is important to note here that no detailed information has been obtained about the exact structure of the catalytically active species in the oiganic solvents. For example, ion pairing is likely to occur in the organic solvents. 

In the previous section efficient catalysis of the Diels-Alder reaction by copper(II)nitrate was encountered. Likewise, other bivalent metal ions that share the same row in the periodic system show catalytic activity. The effects of cobalt(II)nitrate, nickel(II)nitrate, copper(II)nitrate and zinc(ll)nitrate... 

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

It turned out that the dodecylsulfate surfactants Co(DS)i Ni(DS)2, Cu(DS)2 and Zn(DS)2 containing catalytically active counterions are extremely potent catalysts for the Diels-Alder reaction between 5.1 and 5.2 (see Scheme 5.1). The physical properties of these micelles have been described in the literature and a small number of catalytic studies have been reported. The influence of Cu(DS)2 micelles on the kinetics of quenching of a photoexcited species has been investigated. Interestingly, Kobayashi recently employed surfactants in scandium triflate catalysed aldol reactions". Robinson et al. have demonshuted that the interaction between metal ions and ligand at the surface of dodecylsulfate micelles can be extremely efficient. ... 

In contrast to the situation in the absence of catalytically active Lewis acids, micelles of Cu(DS)2 induce rate enhancements up to a factor 1.8710 compared to the uncatalysed reaction in acetonitrile. These enzyme-like accelerations result from a very efficient complexation of the dienophile to the catalytically active copper ions, both species being concentrated at the micellar surface. Moreover, the higher affinity of 5.2 for Cu(DS)2 compared to SDS and CTAB (Psj = 96 versus 61 and 68, respectively) will diminish the inhibitory effect due to spatial separation of 5.1 and 5.2 as observed for SDS and CTAB. 

Many superacid-catalyzed reactions were found to be carried out advantageously not only using liquid superacids but also over solid superacids, including Nafion-H or certain zeolites. We extensively studied the catalytic activity of Nafion-H and related solid acid catalysts (including supported perfluorooctanesulfonic acid and its higher ho-... 

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

Biological catalysts — enzymes — are usually proteins. The development of new protein syntheses is nowadays dominated by genetic protein engineering (see section 4.1.2.6). Bio-organic approaches towards novel catalytically active structures and replicating systems try to manage without biopolymers. 

Phosphites, such as triisopropyl and triphenyl phosphite, are weaker electron donors than the corresponding phosphines, but they are used in some reactions because of their greater rr-accepting ability. The cyclic phosphite trimethylol-propane phosphite (TMPP) or 4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]oc-tane (8), which has a small cone angle and small steric hindrance, shows high catalytic activity in some reactions It is not commercially available, but can be prepared easily[27]. 

Carbonyiation of butadiene gives two different products depending on the catalytic species. When PdCl is used in ethanol, ethyl 3-pentenoate (91) is obtained[87,88]. Further carbonyiation of 3-pentenoate catalyzed by cobalt carbonyl affords adipate 92[89], 3-Pentenoate is also obtained in the presence of acid. On the other hand, with catalysis by Pd(OAc)2 and Ph3P, methyl 3,8-nonadienoate (93) is obtained by dimerization-carbonylation[90,91]. The presence of chloride ion firmly attached to Pd makes the difference. The reaction is slow, and higher catalytic activity was observed by using Pd(OAc) , (/-Pr) ,P, and maleic anhydride[92]. Carbonyiation of isoprcne with either PdCi or Pd(OAc)2 and Ph,P gives only the 4-methyl-3-pentenoate 94[93]. 

---