Transition Metal ons

Effects of various combinations of 3c(-transition metals on the formation of SW tubes have been studied by Seraphin and Zhou[41]. They reported that mixed metals enhanced the production of SW tubes in particular, a 50% Fe -I- 50% Ni combination performed much better than Fe, Co, or Ni alone. It was also shown that the addition of some metals, such as Cu, to these metals poisoned their catalytic action. 

The formation of surface compounds of low-valent ions of transition metals on the surface of the support. In particular, fixing organometallic compounds on the support surface, it may be possible to stabilize coordi-natively insufficient complexes of transition metals and to obtain highly active catalysts. In the ideal case a complete use of the transition metal in the formation of the propagation centers can be achieved. 

Examples of reverse spillover (or backspillover) are the dehydrogenation of isopentane and cyclohexane on active carbon. Deposition of a transition metal on the active carbon accelerates the recombination of H to H2 due to a reverse spillover or backspillover effect.72... 

The basic structure of the catalysts was not changed on the conditions of modification from the XRD patterns shown in Fig. 2. From the ICP analysis, it was observed that impregnated concentration of transition metals on the surface of Ti02 were consistent with leached solution concentration. 

The performance of the catalysts doped with transition metals on 20 mol% NiO/Ti02 catalyst for hydrogen production at 998 K and GHSV of 2700 h" at steady state. 

Apart from d- and 4f-based magnetic systems, the physical properties of actinides can be classified to be intermediate between the lanthanides and d-electron metals. 5f-electron states form bands whose width lies in between those of d- and 4f-electron states. On the other hand, the spin-orbit interaction increases as a function of atomic number and is the largest for actinides. Therefore, one can see direct similarity between the light actinides, up to plutonium, and the transition metals on one side, and the heavy actinides and 4f elements on the other side. In general, the presence or absence of magnetic order in actinides depends on the shortest distance between 5f atoms (Hill limit). 

Figure 11.15. Promotional effect of different transition metals on initial NOx storage/reduction activity for 0.5Pt/7.5Ba/2.5 Promoter catalyst [90].

The effects of transition metals on the photochemical reduction of C02 to formaldehyde (0.1 %), formaldehyde to methanol (6-8%), and methanol to methane (ca. 10 5%) were examined172 in aqueous solutions, but the yields were very low as shown in parentheses for each reaction. 

The increased stability of ruthenium carbene complexes towards oxygen-containing compounds might be because later transition metals, having more d-electrons, are softer and hence react better with soft bases, e.g. olefins. The early transition metals, on the other hand, having few d-electrons, are generally harder and react preferentially with hard bases, such as water or carbonyl compounds. 

Commonly, transition metals on non-reducible supports are used for the selective hydrogenation of aromatic ketones, with most research to date having been done on Pt-based catalysts. In particular, we will discuss here some interesting results published on this subject. 

Karim, Z. (1984a) Influence of transition metals on the formation of iron oxides during the oxidation of Fe(II)Cl2 solution. Clays Clay Min. 32 334-336... 

Takematsu, N. (1979) Sorption of transition metals on manganese and iron oxides and silicate minerals. J. Oceanogr. Soc. Japan 35 ... 

Working with diazo compounds, known since the early 1900s to undergo loss of dinitrogen when treated with copper or copper salts, Yates described in 1952 the possibility that transition metals could form an intermediate that combined units of the diazo compound and the metal (Eq. 1, L = ligand) and acted like a carbene in addition and insertion reactions. Somewhat later, but independently, E. O. Fischer isolated and characterized stable metal carbenes that could also undergo cyclopro-panation reactions." They were derived from transition metals on the left side of the... 

These examples all involve group IVa metal complexes, suggesting that the desired behavior might be possibly an attribute of early transition metal hydride complexes, an area that has been studied considerably less than complexes of metals that are further to the right of the early transition metals on the periodic table. Indeed, as discussed in this volume (see Chapter 10), a zirconium hydride complex recently has been found to reduce CO to methanol among other products. 

The promotional effect of first-row transition metal on the activity of molybdenum-based catalysts has also been correlated with a similar activity... 

Why do we separate clusters into two classes rather than deal with them as u single group of compounds It is primarily because they have unrelated chemistry. Metal atoms in class I have low formal oxidation states, -1 to +1. while those in class II are found in higher formal oxidation stales. +2 to +3. The transition metals on the right side of the periodic tabic (late transition metals) typically form class I clusters, while those on the left-hand side (early second and third row transition metals) tend to form class II clusters. 

In general, the catalytic active centers possess at least one atom of a transition metal, on which the monomer is chemisorbed before the polymerization (76, 88), and at least a metal-carbon bond. Although catalytic centers might contain only one transition metal atom (29), many experimental findings seem to be better explained by admitting that the catalytic centers essentially consist of at least two equal or different metal atoms, one of which is bound to the growing chain (91). 

In fact it can be assumed that, in the catalytic system TiCl4-bis[(S)-2-methyl-butyl]-zinc, dialkyl zinc alkylates the titanium atom (19) and that the titanium alkyl thus formed gives more stable complexes with the (S) olefin than with the (R) olefin, thus favouring the adsorption and polymerization of the (S) antipode (104). The influence exerted by the asymmetric groups bound to transition metals on the type of complexes formed by olefins with the same metal atom, has been recently investigated by Pajaro, Corradini, Palumbo and Panunzi (90). 

An influence by optically active groups bound to the transition metals on the complexation of the monomers with the same atoms might probably explain also the asymmetric polymerization of diolefins (95). 

The effects of such oxidants as a mixture of potassium iodate and dichromate in concentrated sulfuric and phosphoric acids (van Slyke reagent), hot solutions of chromic acid, and acidic solutions of ceric sulfate, permanganate, periodate, and hyperoxidized transition metals on a number of sugar derivatives has been described (Ref. 1, pp. 1151-1153). 

Photochemistry of these compounds in solution and the catalytic effects of transition metals on oxidation rates. 

Effect of Transition Metal on Activity of Carbon Catalyst and Conversions and Yields of Methanol-Ethanol Reactions ... 

G. Liang, 1. Huot, S. Boily, A. Van Neste, R. Schulz, Catalytic effect of transition metals on hydrogen sorption in nanocrystalline ball milled MgH2-Tm (Tm = Ti, V, Mn, Fe and Ni) systems, . /. Alloys Compd. 292 (1999) 247-252... 

Reynolds, C. H. (1980). Phosphoenolpyruvate carboxykinase from the rat and from the tapeworm Hymenolepis diminuta. Effects of inhibitors and transition-metals on the carboxylation reactions. Comparative Biochemistry and Physiology, 65B 481-7. 

---