The Catalytic Activities of Metals

Strength of adsorption is conveniently measured by the heat released when adsorption takes place, and for several molecules there is evidence to show that the strength decreases on passing from left to right across each of the three transition series with some molecules (hydrogen, alkenes) 

For example, maximum activity for alkane hydrogenolysis is to be found in Group 8 (Ru, Os) rather than in Group 9 or 10, because the hydrocarbon intermediates have to be multiply bonded to the surface, and metals in the later Groups have insufficient unpaired electrons for this purpose. Palladium is outstandingly the best metal for hydrogenating alkynes, but is of little use for hydrogenating aromatics. 

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In order that the possibility of contamination of catalysts with traces of oxides could be eliminated Campbell and Emmett (51) studied the catalytic activity of metallic films of nickel and its alloys with copper or gold. They were deposited under a high vacuum and then sintered (alloys also homogenized) in hydrogen at 5 cm Hg pressure at 350°C or 500°C. The films were subsequently allowed to cool to room temperature and only... 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

The following general points may be made about the catalytic activity of metals for the exchange of hydrocarbons ... 

Some evidence to support this scheme has been obtained. Thus the catalytic activity of metals has been found to be associated with the formation of soluble metal-thiol complexes (13), and the geometric configuration of thiols has been found to affect the over-all rate of oxidation,... 

As described above, the catalytic activity of metal ion-exchanged zeolites for aniline formation has a good correlation with electronegativity and with the formation constant of ammine complexes of metal cations. The order of the activity agrees with the Irving-Williams order. These facts give irrefutable evidence that the transition metal cations are the active centers of the reaction. 

This section describes a simple model that enables evaluation of the influence of charge effects on the catalytic activity of metallic nanostructures. Also, the results of experiments performed with nanostructured catalysts synthesized by laser electrodispersion are discussed. These results demonstrate a relationship between the catalytic activity and charge density in the... 

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). 

In some cases (e.g., gasoline), autoxidation of hydrocarbons is undesirable, and trace amounts of metal catalysts may often be deactivated by the addition of suitable chelating agents. The latter affect the catalytic activity of metal complexes by hindering or preventing the formation of catalyst-hydroperoxide or catalyst-substrate complexes by blocking sites of attack or by altering the redox potential of the metal ion. 

Although it is generally agreed that sulfur can have devastating effects on the catalytic activity of metals, it is now being recognized that a careful and well... 

All results reviewed herein demonstrate that the microgel particles may serve as nanoreactors for the immobilization of catalytically active nanostructures, namely for metal nanoparticles and enzymes. In both cases, the resulting composites particles are stable against coagulation and can be easily handled. Moreover, the catalytic activity of metal nanoparticles can be modulated through the volume transition that takes place within the thermosensitive microgel carrier system. Similar behavior has been also observed for the temperature dependence of enzymatic activity. Thus, the microgel particles present an active carrier system for applications in catalysis. 

A catalyst is usually requires in the formation of H2S. Metal sulfides are the most common catalysts used in the laboratory or in large-scale production of H2S. The catalytic activity of metal sulfides is linked to the metal-sulfur bond strength. 

From catalysis it is well-known that the metal-substrate interaction influences the reactivity of supported nanoparticles. For instance, for noble metal particles on oxidic supports, the hydrogenation and hydrogenolysis activity is much greater if the support has a higher acidity (high concentration of acidic —OH groups at the surface) than for neutral or alkaline oxidic supports. The influence of the presence of a support on the catalytic activity of metal nanoparticles has been ascribed to [70, 75-79] ... 

Much is being done to increase the catalytic activity of metals. Certain binary alloys seem to have higher catalytic activity than either of their components separately. Oxides and oxide mixtures have been tried, and electrodes modified by organic or metal-organic molecules hold hope for better catalytic properties. An engineering approach, that has been quite successful (although some may consider it a "brute force" approach) is to increase the roughness of the surface, thus... 

The results seen in Tables X and XI show that the reaction always proceeds at a considerable rate when the interlayer spacing of Cu " -TSM is expanded. This means that only a part of the metal ions is distributed on the rim of interlayer space, and the observations give a guide for improvement of the catalytic activity of metal ion-exchanged layer lattice silicates. 

The catalytic activities of metal ion-exchanged TSMs for the methanol conversion are individually different, depending on the metal ion. Figure 8 shows the attractive reactions found in this study. It is of particular interest that the catalytic activity of copper varies depending on the oxidation state. Cu -TSM is inactive, whereas Cu " - and Cu°-TSMs catalyze selectively the dehydrogenation of methanol to methyl formate and to formaldehyde, respectively. Cu -TSM has potential for practical use because of its high stability and high selectivity for the methly formate formation. 

While the incorporation of transition metal oxides into complexes with materials such as alumina can lower their volatilities by factors from 10 (CuO) to 1000 (BaO) depending primarily upon the heat of reaction between the two oxides, it is also likely that formation of very stable complex metal oxides, such as aluminates, can also greatiy lower the chemical activity of the transition metal. As mentioned above, Mn, Ni, and Co may requite stabilization in complex oxides for long catalyst life, but the complex oxides generally have inferior activity. The most active transition metal oxides (Ru and Cu) may still have unacceptable volatility as relatively active complex oxides. As a consequence, there may be a technology-limiting trade-off between the catalytic activity of metals and metal oxides and their chemical and thermal stability in combustion environments. 

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