Activity of metals

Contaminant coke is produced by catalytic activity of metals such as nickel, vanadium, and by deactivation of the catalyst caused by organic nitrogen. 

Finally, it is necessary to observe that the values of activities and fugacities calculated are thermodynamic quantities that cannot always be realised in practice, e.g. very high activities of metal ions cannot be attained because of solubility consideration and very low activities have no physical significance. 

Pourbaix has evaluated all possible equilibria between a metal M and HjO (see Table 1.7) and has consolidated the data into a single potential-pH diagram, which provides a pictorial summary of the anions and cations (nature and activity) and solid oxides (hydroxides, hydrated oxides and oxides) that are at equilibrium at any given pH and potential a similar approach has been adopted for certain M-H2O-X systems where A" is a non-metal, e.g. Cr, CN , CO, SOj , POj", etc. at a defined concentration. These diagrams give the activities of the metal cations and anions at any specified E and pH, and in order to define corrosion in terms of an equilibrium activity, Pourbaix has selected the arbitrary value of 10 ° g ion/1, i.e. corrosion of a metal is defined in terms of the pH and potential that give an equilibrium activity of metal cations or anions > 10 g ion/1 conversely, passivity and immunity are defined in terms of an equilibrium activity of < 10 g ion/1. (Note that g ion/1 is used here because this is the unit used by Pourbaix in the S.I, the relative activity is dimensionless.)... 

Thus the diagram shows the solid phases of iron, the activities of metal ions and the pressures of hydrogen and oxygen gas that are at equilibrium at any given potential and pH when pure iron reacts with pure water. 

The equilibrium potentials and E, can be calculated from the standard electrode potentials of the H /Hj and M/M " " equilibria taking into account the pH and although the pH may be determined an arbitrary value must be used for the activity of metal ions, and 0 1 = 1 is not unreasonable when the metal is corroding actively, since it is the activity in the diffusion layer rather than that in the bulk solution that is significant. From these data it is possible to construct an Evans diagram for the corrosion of a single metal in an acid solution, and a similar approach may be adopted when dissolved O2 or another oxidant is the cathode reactant. 

It should be noted that whereas a completely soluble hydroxide (e.g. NaOH) will give a solution of high pH in which the pH will increase with concentration of the hydroxide, the pH of a solution of a sparingly soluble hydroxide will depend upon the equilibrium constant for hydrolysis and the activity of metal ions. 

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... 

Habid and Malek49 who studied the activity of metal derivatives in the catalyzed esterification of aromatic carboxylic acids with aliphatic glycols found a reaction order of 0.5 relative to the catalyst for Ti(OBu)4, tin(II) oxalate and lead(II) oxide. As we have already mentioned in connection with other examples, it appears that the activation enthalpies of the esterifications carried out in the presence of Ti, Sn and Pb derivatives are very close to those reported by Hartman et al.207,208 for the acid-catalyzed esterification of benzoic and substituted benzoic acids with cyclohexanol. These enthalpies also approach those reported by Matsuzaki and Mitani268 for the esterification of benzoic acids with 1,2-ethanediol in the absence of a catalyst. On the other hand, when activation entropies are considered, a difference exists between the esterification of benzoic acid with 1,2-ethanediol catalyzed by Ti, Sn and Pb derivatives and the non-catalyzed reaction268. Thus, activation enthalpies are nearly the same for metal ion-catalyzed and non-catalyzed reactions whereas the activation entropy of the metal ion-catalyzed reaction is much lower than that of the non-catalyzed reaction. 

The effects of aluminium, zinc, iron, nickel and copper powders on the thermal degradation of waste PS were studied. The results showed that the catalytic effects of metal powders were related to their activities. The catalytic effects increased with increasing activities of metals. It was suggested that PS degraded through a transient intermediate in the presence of metal powders and that the degradation of the transient intermediate was the rate-determining step. 10 refs. 

As shown in Table IV, the highest catalytic activity of metal halides used as Lewis acid for the alkylation reaction of ferrocene with 2 was observed in methylene chloride solvent. Among Lewis acids such as aluminum chloride, aluminum bromide, and Group 4 transition metal chlorides (TiCl4, ZrCU, HfCU), catalytic efficiency for the alkylation decrea.ses in the following order hafnium chloride > zirconium chloride > aluminum chloride > aluminum bromide. Titanium chloride... 

Because of the many examples of such activation of metal powders by TCS 14 only a limited and arbitrary number will be discussed here. The Clemmensen-type reduction of ketones such as cyclohexanone with Zn powder in the presence of TCS 14 affords, via 2082, 2084, and 2085, cyclohexene and, via 2082, O-silylated pinacol 2083 [19, 20]. Ketones such as 5a-cholestan-3-one 2086 are reduced by Zn dust-TCS 14 in TFIF, in ca 65-70% yield, to give 5a-cholest-2-ene 2087 and ca 5% 5a-cholest-3-ene [21] (Scheme 13.8). 

Correlation Between Catalytic Activity of Metals and Their Bulk Properties... 

Summarizing the discussion above, we can only say that all the relations observed experimentally between the parameters of catalytic activity of metals and parameters of their physicochemical bulk properties are secondary. The primary factor influencing the catalytic activity of all metals and most of their physicochemical parameters are... 

Formylisoquinoline S-methyldithiocarbazate, 19, forms green [Cu(19-H)Cl], and its magnetic moment of 1.80 B.M. and v(d-d) = 16000 cm are cited as evidence for planar stereochemistry [146]. It and the analogous zine complex showed the greatest antitumor activity of metal eomplexes tested. 

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

This suggestion is not necessarily intended to imply that the bicarbonate intermediate of the carbonic anhydrase cycle must exhibit unidentate coordination in the ground state. Rather, it is intended to imply that a unidentate species should be readily accessible. Moreover, it is not intended to suggest that this factor alone is responsible for influencing the activity of metal-substituted carbonic anhydrases,... 

As part of a study into the activation of metal catalysts in thiol or thiolate-rich environments, X-substituted coenzyme M and thioglycolate derivatives were investigated in a Ni-catalyzed crosscoupling reaction with a zinc co-factor the role of zinc was shown to be in a transmetallation process.571... 

NishiokaH. 1975. Mutagenic activities of metal compounds in bacteria. MutRes 31 185-189. 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

The activity of metals other than platinum for skeletal reactions of larger molecules is not well documented, particularly in a mechanistic sense. Carter, Cusumano, and Sinfelt (157) have recently studied the reaction of n-heptane on a series of group VIII metals in the form of hydrogen-reduced (300°C) metal powders. The nature of the reaction pathways is summarized in Table IX. Although many metals have been... 

Fig. 1. Catalytic activities of metals for ethane hydrogenolysis in relation to the percentage d character of the metallic bond. The closed points represent activities compared at a temperature of 205°C and ethane and hydrogen pressures of 0.030 and 0.20 atm, respectively, and the open points represent percentage d character. Three separate fields are shown in the figure to distinguish the metals in the different long periods of the periodic table.

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

Studies on the electrocatalytic activity of metal porphyrins are limited in comparison with those on other classes of macrocyclic metal complex. Among the few porphyrin complexes tested, cobalt porphyrins have been demonstrated to be efficient electrocatalysts for the reduction of C02 to CO... 

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