Metal oxidative addition

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

Zero-valent transition metals, oxidative addition, 227... 

Amiridis, M.D., Duevel, R.V., Wachs, l.E. 1999. The effect of metal oxide additives on the activity of VjOj/TiOj catalysts for the selective catalytic reduction of nitric oxide by ammonia. Appl Catal B Environ 20 111-122. 

Effect of Metal Oxide Additives on Au/Ti02, Catalyst Pretreated 200°C CaO, ZnO... 

Influence of Surface Metal Oxide Additives upon Oxidation Reactions... 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Poisons. - Unlike secondary surface metal oxide additives that indirectly interact with the surface vanadia sites via lateral interactions, poisons are surface metal oxide additives that directly interact with the surface vanadia sites and decrease the TOF. For example, the addition of surface potassium oxide to supported vanadia catalysts results in both a structural change and a reactivity change of the surface metal oxide species.50 This interaction, at submonolayer coverages, reflects the attractive interaction between these two surface metal oxide species. The presence of the surface potassium oxide poison alters the V-O bond lengths and the ratio of polymeric and isolated surface vanadia species (favoring isolated surface vanadia species). The interaction of the surface potassium oxide poison with the surface vanadia species is schematically shown in Figure 5. 

As for the complete oxidation of propene, propane and methane, Nieuwenhuys and coworkers studied the influence of metal oxides additives on the catalytic activity of Au/Al203 [109-115], The addition of 3d transition metal oxides (MnOx, CoOx or FeOx), which were active by themselves, or ceria that was poorly active by itself promoted the catalytic activity of Au/Al203 in the total oxidation of propene [112]. The most active catalyst was Au/Ce0x/Al203, with a T95 at 497 K and with a high stability. In these cases, ceria and the transition metal oxides may act as co-catalysts and the role is twofold it stabilizes the Au NPs against sintering (ceria)... 

Sloczynski J, et al. Effect of metal oxide additives on the activity and stability of Cu/ZnO/ Zr02 catalysts in the synthesis of methanol from C02 and H2. Appl Catal A. 2006 310 127-37. 

Methyl formate is expected to be one of the intermediates of the methanol-based industries that produce dimethylformamide, acetic acid, pure hydrogen, carbon monoxide, etc. A number of patented catalysts (J8) are therefore composed of copper oxide as a main ingredient and include a variety of metal oxide additives. In our comparative studies, some of these catalysts exhibit fairly good activity but do not exceed Cu " -TSM in activity and selectivity (26). The results of the reactions over these patented catalysts, which have relatively high performance levels, are illustrated in Fig. 4 together with the result obtained with Cu -TSM the yield of methyl for-... 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

These reactions, as the name suggests, involve an increase in both the formal oxidation state and the coordination number of the metal. Oxidative addition (OA) reactions are among the most important of organometallic reactions and are essential steps in many catalytic processes. The reverse type of reaction, designated reductive elimination (RE), is also very important. These reactions can be described schematically by the equation ... 

An important property of Pd(0) complexes is their reaction with a broad spectrum of halides having proximal TC-bonds to form a-organopalladium(II) complexes. This process is known as oxidative addition because the metal becomes formally oxidized from Pd(0) to Pd(II) while the substrate R-X (R = alkenyl, aryl X = Br, I, OTf) adds to the metal. Oxidative addition is believed to occur on the 14-electron Pd(0) species resulting from ligand (L) dissociation in solution. This coordinatively unsaturated Pd(0) is then the active species undergoing oxidative addition with the substrate to form the 16-electron Pd(II)complex. 

Figure 4.37. Effect of 1 wt% of various metal oxide additives on the silicothermal formation of X-sialon at 1500°C monitored by Si NMR. As the reaction proceeds, the shape of the spectra change according to changes in the various regions corresponding to particular structural units as marked = silicon oxynitrides, O = Si3N4, = Si02, = mullite. Note that not all oxide additives produce the characteristic X-sialon spectrum at 1500°C. Adapted from Sheppard and...

Figure 5.36. Modification of the silicothemial synthesis of X-sialon by metal oxide additives monitored by A. changes in the tetrahedral Al shift as a function of synthesis temperature, and B. changes in the tetrahedralroctahedral Al ratio as a function of synthesis temperature. The heavy line in both graphs refers to the additive-free control sample. From Sheppard...

For the representative metals, oxidative additions to the metals themselves provide a powerful synthetie teehnique to Grignard reagents, alkyllithium reagents and other compounds. Some of the heavier metals also have a stable lower oxidation state which can undergo oxidative addition reactions. 

It was suggested that the role of the partly reducible metal oxide additive is to contribute to the formation of new active sites and increase N2O dissociation. On the other hand, alkali and alkaline earth metal oxides stabilise gold particles against sintering. 

In the binary glass-forming systems of alkali metal borates, it is possible to observe change in the trend of a number of physico-chemical properties in the concentration range of approximately 20 mole % of alkali metal oxide. This phenomenon is known in the literature as boric acid anomaly , and it is due to the change in the structure of the B2O3 melt caused by the alkali metal oxide addition and is related to the ability of boron to change its coordination number. 

Although the above-described examples have all been based on relatively simple nanocrystalline metal oxides, additional phases might have been introduced. The use of more complex metal oxides has also been investigated, with nanocrystalline thick films of both barium titanate [96] and cobalt titanate [97] having been considered as possible sensor materials. When the response of such barium titanate films doped with 10% CuO and 10% CdO was studied with respect to CO, LPG, H2S, and H2 [96], sensor selectivity was improved for LPG over the other gases at 250 °C. However, the addition of 0.3 wt% Pd resulted in an even greater selectivity to LPG at a lower temperature, of 225 °C. 

Expansivity, as a function of metal oxide addition, for liquid silicates, 740... 

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