Active sites metal catalysts

Molecular design of the active site on catalyst surfaces regarded as a reaction intermediate for a target catalytic reaction is also a way to provide efficient catalysis [22], In order to achieve the catalyst design, the reaction mechanism including the structural and electronic change of active metal sites must be known at a molecular level. 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

It is the need for improvements in the HDS and HDN processes or the development of new methods for the removal of sulfur and nitrogen from fuels that has attracted the interest of inorganic and organometallic chemists. Their investigations have been directed toward understanding how organosulfur and organonitrogen compounds bind in transition metal complexes as models for their adsorption on active sites of catalyst surfaces such studies have also provided... 

Bivalent cations also affect the proton distribution if small cages have been filled, e.g., with Mg ions, the protons that are created during the reduction of a transition metal ion will predominantly stay in the supercages, where they interact with the metal clusters, as will be described in more detail below. Because such adducts can act as very active sites, a catalyst promoter effect of Ca and Mg on reduced Pd has therefore been attributed, in part, to the enhanced concentration of metal-proton adducts in accessible supercages (765). 

This principle appears amenable to generalization active sites and catalyst promoters can be positioned in the same cage in order to systematically study catalyst promoter effects due to direct interaction of metal particles and metal ions. Quantum chemical calculations by van Santen et al. have resulted in detailed predictions, e.g., of the effects of Mg ions, that are in direct contact with zeolite-encaged Ir4 tetrahedra, on the adsorption of H2 (i72) or CO 373) on these clusters. These theoretical results should be verified experimentally, as they could form a basis for general predictions on the action of ionic promoters on chemisorbing transition metals. 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

Even though the component and size of metals and metal oxide support are defined, the catalytic activity for CO oxidation often markedly changes depending on the contact structure of noble metal particles with the supports. In particular, Pd, Ir, and Au exhibit high catalytic activity when they are deposited on reducible metal oxides by coprecipitation, deposition-precipitation, and grafting. Goulanski has classified supported metal catalysts for low-temperature oxidation into three groups [72], There are three possible active sites metal surfaces with metal oxide as a simple support metal oxide thin layer underneath of which metal particles are buried and the perimeter interfaces around noble metal particles. 

Transition metal complexes of phthalocyanine encaged in faujasite type zeolites have been reported as efficient catalysts in the oxidation of alkanes at room temperature and atmospheric pressure [6-13]. These catalysts constitute potential inorganic mimics of remarkable enzymes such as monooxygenase cytochrome P-450 which displays the ultimate in substrate selectivity. In these enzymes the active site is the metal ion and the protein orientates the incoming substrate relative to the active metal center. Zeolites can be used as host lattices of metal complexes [14, 15]. The cavities of the aluminosilicate framework can replace the protein terciary structure of natural enzymes, thus sieving and orientating the substrate in its approach to the active site. Such catalysts are constructed by the so-called ship in a bottle synthesis the metal phthalocyanine complexes are synthesized in situ within the supercages of the zeolite... 

CH4 reactions with CO2 or H2O on group VIII or noble metals (Ru, Rh, Pd, Ir, Pt) [1] form synthesis gas which is the precursor to valuable fuels and chemical compounds, as lirst shown by Fischer and Tropsch [2]. Due to the cost and availability of the nickel, compared to noble metals, Ni catalysts are used industrially. However, Ni-based catalysts tend to form inactive carbon residues that bloek the pores as well as the active sites of catalyst, and whose main activity is die formation of carbon filaments [3]. Therefore, the industrial methane steam reaction is usually performed under an excess of water to maintain the catalyst activity. Another alternative is the modification of the composition of the catalyst (generally Ni/Al203) by addition of a basic compound like MgO [4]. It is well known that the formation of NiO-MgO solid solution is easily favoured by calcining the mixed oxide at high temperatures [5] and much attention was devoted to its specific properties [6]. Parmaliana and al. 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

From this short survey, it should become clear that metal alkoxide, beside their working considerations, have very large prospective applicability in different area of organic reaction catalysis which must be investigate more comprehensively to show their properties more clearly. Steric and electronic possessions have crucial effect on catalytic properties of metal alkoxides like many other metal complexes. In one hand, electronic properties of different substituents on alkyl R group of metal alkoxides determine their metal core Lewis acidic property and/or Lewis basic character of alkoxy oxygen atom which both have substantial effect on their catalytic behavior. In addition, steric properties of alkyl R group may hinder substrate to come closer or to attack by active site of catalyst. 

It is seen from Fig. 6.24 that the alkali metal hydroxide can be easily absorbed by carbon support due to its low melting point and its large fluidity. Therefore, only with enough quantity, the alkali metal can be accreted on the interface between ruthenium and carbon support and then plays the promotional roles effectively. As alkaline earth metal oxides have high melting point and poor fluidity, small amounts of them can be accreted on the interface between ruthenium and carbon support, which can produce effective active sites. The excessive promoters might cover the active sites of catalyst smface, which can influence the effective contact between active sites of ruthenimn smface and reactant gases and therefore decrease the catalytic activity. 

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