Metal hydrides activity

The precipitated brown / -TiCl3 undergoes, in certain spots of its surface, an alkylation reaction with the organoaluminium compound present in the reaction system, which results in the formation of active species with a Ti-C bond (or with a Ti-H bond in the case of a metal hydride activator (in these spots, scheme (8) shows the activation of a catalyst precursor such as TiCl3 with AIR3 as the catalyst activator ... 

Catalysts Based on Transition Metal Salts without an Organometallic or Metal Hydride Activator... 

Chain transfer to hydrogen leads to the production of a dead chain with a saturated chain end, Dr, and a metal hydride active site that can be initiated with monomer according to Eq. (5). 

The metal hydride active material and constmction also have similar special design options. The active materials may be adjusted to influence one or more of the factor of capacity, power, and/or cycle life. For the AB5 system, typical formulas include ... 

Special electrolytes are also used in NiMH batteries to enhance high temperature operation. Instead of binary KOH/LiOH electrolytes, it is also possible to substitute a portion of the KOH with NaOH. The ternary KOH/NaOH/LiOH electrolyte is used at a high concentration of about 6 Molar, but the contribution of NaOH promotes high temperature operation charging efficiency although it is typical for this electrolyte to decrease cycle life by increased corrosion of the metal hydride active materials. 

A crucial factor in high power NiMH batteries has been the development of highly catalytic metal hydride active materials. In particular, the interface between the metal hydride itself and the electrolyte has been identified as essential for low voltage loss under pulse discharge. The surface oxide thickness and microporosity influence the reaction of H+ and OH, and a critical observation has been made that within the high power surface oxide metallic catalysts of enriched nickel having sizes less than 70 A are especially important for reducing activation polarization. "- ... 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

Trunsition-MetnlHydrides, Tiansition-metal hydiides, ie, inteistitial metal hydrides, have metalhc properties, conduct electricity, and ate less dense than the parent metal. Metal valence electrons are involved in both the hydrogen and metal bonds. Compositions can vary within limits and stoichiometry may not always be a simple numerical proportion. These hydrides are much harder and more brittie than the parent metal, and most have catalytic activity. 

Although estrone and estradiol (26) have both been isolated from human urine, it has recently been shown that it is the latter that is the active compound that binds to the so-called estrogen receptor protein. Reduction of estrone with any of a large number of reducing agents (for example, any of the complex metal hydrides) leads cleanly to estradiol. This high degree of stereoselectivity to afford the product of attack at the alpha side of the molecule is characteristic of many reactions of steroids. 

Nickel hydroxides have been used as the active material in the positive electrodes of several alkaline batteries for over century [1], These materials continue to attract much attention because of the commercial importance of nickel-cadmium and nickel-metal hydride batteries. In addition to being the cathode active material in nickel-metal hydride batteries, Ni(OH)2 is an important corrosion product of the anode during cycling. There are several reviews of work in the field [2-10],... 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

In acidic electrolytes only lead, because it forms passive layers on the active surfaces, has proven sufficiently chemically stable to produce durable storage batteries. In contrast, in alkaline medium there are several substances basically suitable as electrode materials nickel hydroxide, silver oxide, and manganese dioxide as positive active materials may be combined with zinc, cadmium, iron, or metal hydrides. In each case potassium hydroxide is the electrolyte, at a concentration — depending on battery systems and application — in the range of 1.15 - 1,45 gem"3. Several elec-... 

V. Catalytic Activity of Other Metal Hydrides in Test Reaction of Hydrogen. 283... 

There are, however, cases when the hydrogen pretreatment results in an enhanced catalytic activity of an alloy. The phenomenon may be explained also in connection with a metal-metal hydride transformation, namely as a post-hydride effect. 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

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