Chromium series

They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

FIGURE 25 Melt index and MW values of polymers made with three series of catalysts, showing the separate responses obtained by calcination of the silica, versus that of calcining the chromium. Series 1 Cr/silica calcined at temperature shown. Series 2 Silica calcined at temperature shown, aqueous impregnation of chromium, air 500 °C. Series 3 Silica calcined at temperature shown, chromium applied anhydrously, air 500 °C. 

In the chromium series, the reactive (salcn)chromium(V) oxo complex was isolated and X-ray characterized [3]. To explain why ( )-alkcncs arc epoxidized more effectively with the chromium-salen systems, the authors [ IS] proposed a bent geometry of the (salen)chromium(V) oxo intermediate however, this hypothesis has been neither confirmed nor denied. Our contributions to the discussion of the mechanism (namely, insight into the catalytic cycle) arc presented in this Chapter. (Salcn)chromium complexes arc known to catalyze other types of reactions (see (1,26] and references therein). 

The reactivity of the transition metals towards other elements varies widely. In theory, the tendency to form other compounds both in the solid state (for example reactions to form cations) should diminish along the series in practice, resistance to reaction with oxygen (due to formation of a surface layer of oxide) causes chromium (for example) to behave abnormally hence regularities in reactivity are not easily observed. It is now appropriate to consider the individual transition metals. 

Step 3 A series of redox reactions converts chromium from the 4+ oxidation state m HCr03 to the 3 + oxidation state... 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

Steels iu the AISI 400 series contain a minimum of 11.5% chromium and usually not more than 2.5% of any other aHoyiag element these steels are either hardenable (martensitic) or nonhardenable, depending principally on chromium content. Whereas these steels resist oxidation up to temperatures as high as 1150°C, they are not particularly strong above 700°C. Steels iu the AISI 300 series contain a minimum of 16% chromium and 6% nickel the relative amounts of these elements are balanced to give an austenitic stmcture. These steels caimot be strengthened by heat treatment, but can be strain-hardened by cold work. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

A series of nickel—chromium—iron alloys based on the soHd solution Inconel 600 alloy (see Table 4) was developed, initially depending on aluminum ... 

Solid-State Lasers. Sohd-state lasers (37) use glassy or crystalline host materials containing some active species. The term soHd-state as used in connection with lasers does not imply semiconductors rather it appHes to soHd materials containing impurity ions. The impurity ions are typically ions of the transition metals, such as chromium, or ions of the rare-earth series, such as neodymium (see Lanthanides). Most often, the soHd material is in the form of a cylindrical rod with the ends poHshed flat and parallel, but a variety of other forms have been used, including slabs and cylindrical rods with the ends cut at Brewster s angle. 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

At the end of the 72-h cycle, the cathodes are removed from the cells, washed in hot water, and the brittie deposit, 3—6 mm thick, is stripped by a series of air hammers. The metal is then cmshed by roUs to 50-mm size and again washed in hot water. The metal contains about 0.034% hydrogen and, after drying, is dehydrogenated by heating to at least 400°C in stainless steel cans. Composition limits for electrolytic chromium are shown in Table 4. 

Two classes of metals have been examined for potential use as catalytic materials for automobile exhaust control. These consist of some of the transitional base metal series, for instance, cobalt, copper, chromium, nickel, manganese, and vanadium and the precious metal series consisting of platinum [7440-06-4], Pt palladium [7440-05-3], Pd rhodium [7440-16-6], Rh iridium, [7439-88-5], Ir and mthenium [7440-18-8], Ru. Specific catalyst activities are shown in Table 3. 

F Embrittlement 700/1050°F Room temperature Ferritic chromium 400 Series ferritic Precipitation of a Do not use ferritic... 

C) 370/656X brittleness after exposure to temperatures between about 700 to 1. OSO-F. stainless steels. chromium stainless steels, over 13% Cr and any 400 Series martensitic chromium stainless steels low in carbon content (high Cr/C ratio). complex chromium compound, possibly a chromium-phosphorus compound. chromium steels at temperatures above about 700 F (370 C) keep carbon up in martensitic chromium steels and limit Cr to 13% max. 

L3M45M4 5 would often appear as L3M2,3V and L3W, respectively, and similarly 1 2,3 4,5 4,51 as M2,3W. In Fig. 2.22 the increase in the intensity of the L3W peak relative to the other two, upon going from chromium to iron, is because of the progressive increase in the electron density in the valence band. The characteristic doublet seen in the MNN series arises from the M4 5N4,sN4,5 transitions, in which the doublet separation is that of the core levels M4 and M5. 

Chromium Steels (400 Series), Low-Carbon Ferritic (Type 405) 12-13% Chromium... 

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