Type of Active Sites

2 Type of Active Sites. - In heterogeneous catalysis the following type of actives sites can be distinguished (i) metallic, (ii) acid-base, (iii) red-ox type, and (iv) anchored metal-complex. The catalytic sites may contain one of the above types of active sites or can include several types of sites. In case of different type of sites the catalysts are bifunctional or multifunctional. For instance, Pt/Al203 and Pt/mordenite are typical bifunctional catalysts containing both metallic and acidic types of active sites. On the other hand, Pt or Pd supported on silicon carbide, nitride, or Pt/L-zeolite are mono-functional catalysts. There are important industrial reactions, such as isomerization and aromatization of linear hydrocarbons, which requires bifunctional catalysts, such as chlorinated 

Pt/AlzOs. In these catalysts the two types of sites have to be located sufficiently close to each other so that transport between the sites would not be rate limiting in the overall process. 

Metal catalysed reactions are differentiated introducing the concept of facile and demanding reactions. In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form multiplets or ensembles. In other words, some reactions depend on the surface geometry e.g. hydrogenolysis of hydrocarbons), while other may not e.g. hydrogenation of olefinic double bond). 

Red-ox type catalysts are mostly used in oxidation or related types of reactions. For instance, vanadium catalysts containing ions of different valence state are used in the oxidation of benzene to maleic anhydride. Bismuth molybdate catalyst can be used both for the oxidation or ammoxidation of propene. Anchored metal-complex catalysts combine the advantage of both homogeneous and heterogeneous catalysts, however in these catalysts the molecular character of the active sites is maintained. In the last generation of this type of catalysts, heteropolyacids are fixed first to the support and in the second step different metal-complexes are anchored to the heteropolyacid. In this way highly active and stable catalyst have been prepared for different reactions.  

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Wuerges J, J-W Lee, Y-I Yim, H-S Yrm, S-O Kang, KD Carugo (2004) Crystal structure of nickel-containing superoxide dismutase reveals another type of active site. Proc Natl Acad Sci USA 101 8569-8574. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

These observations could be interpreted as a pore mouth catalysis. It was suggested that EU-1 fresh catalyst comprises two types of active sites, inner and external acid sites, the first ones which are non selective to isomerization and sensitive to deactivation, the second ones selective to isomerization but non sensitive to deactivation. Selectivity of inner acid sites could be estimated by difference between results obtained after 45 minutes and those obtained after 8 hours. These results are shown on table 3. 

The active site of GO contains a covalently modified tyrosyl radical coordinated to a Cu(II) ion constituting thereby a novel mononuclear active site in which both the metal ion and a ligand are redox active. Recently, evidence was provided for the same type of active site in glyoxal oxidase (82), which catalyzes the oxidation of aldehydes to carboxylic acids, Eq. (3). 

Anyway, all results have a common point. They suggest that there are two types of active sites on the catalyst surface (1) one site exhibiting affinity forTA where e.d. hydrogenation (e.d. site) is supposed to take place and (2) a second site without affinity for TA where racemic products are produced (non-e.d. site). 

R. J. S. Ducan, Aldehyde Dehydrogenase. An Enzyme with Two Distinct Catalytic Activities at a Single Type of Active Site , Biochem. J. 1985, 230, 261-267. 

Evidence tom a variety of sources indicates that the active site of tyrosinase is very similar to that of hemocyanin, a dioxygen-binding protein found in molluscs and arthropods (15,16). This type of active site contains two copper ions, which are cuprous in the deoxy state, and which reversibly bind dioxygen, forming the oxy form of the enzyme or protein in which a peroxy ligand bridges between two cupric ions. 

Aramendia et al. (22) investigated three separate organic test reactions such as, 1-phenyl ethanol, 2-propanol, and 2-methyl-3-butyn-2-ol (MBOH) on acid-base oxide catalysts. They reached the same conclusions about the acid-base characteristics of the samples with each of the three reactions. However, they concluded that notwithstanding the greater complexity in the reactivity of MBOH, the fact that the different products could be unequivocally related to a given type of active site makes MBOH a preferred test reactant. Unfortunately, an important drawback of the decomposition of this alcohol is that these reactions suffer from a strong deactivation caused by the formation of heavy products by aldolization of the ketone (22) and polymerization of acetylene (95). The occurrence of this reaction can certainly complicate the comparison of basic catalysts that have different intrinsic rates of the test reaction and the reaction causing catalyst decay. 

We studied the following system (let us call it the Basic Case) corresponding to the single-route mechanism of catalytic reaction with the single type of active sites... 

We consider the systems in which each intermediate contains the same number of active sites, e.g. AZ, BZ (Z is the catalytic site) or PtO, PtCO include only one catalyst site. This assumption simplifies the analysis. However, our results can be generalized to systems, in which surface intermediate include more that one active site or the catalyst surface is characterized by more than one type of active sites. 

For so-called multi-site catalysts, Ziegler catalysts for example, kinetic constants might vary with the type of active sites, see [10], In such a case, one should summarize Eqn.(5.4-1) over all contributions of different active sites, or kp is interpreted as an average value of all contributing sites. 

This diversity of sites explains why the molecular weight distribution (MWD) of polymers produced by Cr/silica is broad (71). Model calculations which assume a single type of active site usually predict Mw/Mn 2,4 but in reality Mw/Afn = 6-15 is common, and 20-30 can be achieved with catalyst modifications. The distribution is also broader than that generally obtained from Ziegler catalysts, for which Mw/Afn = 3-6 under similar conditions. Experience with organometallic compounds suggests that a broad MWD may be a general feature of catalysts which terminate by -elimination. 

Therefore, the catalyst possesses different types of active sites one site that can activate the paraffin and oxide hydrogenate it to the olefin, and one that (amm) oxidizes the adsorbed olefin intermediate. 

Here bi = the number of j-type active sites entering into the ith substance. In most cases it is assumed that there exists only one type of active site, hence the linear law of conservation, eqn. (35), is unique. 

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