Blacks and Powders

Unsupported metals are found in a variety of forms wires, ribbons, single crystals, colloids, powders, blacks and the so called skeletal species. A black is a metallic powder obtained by reduction of a metal salt or by condensation of a metal vapor. Such materials are also sometimes referred to as powders. The blacks and powders are usually composed of relatively large particles having a low surface area. Skeletal metals are produced by leaching out one component of an alloy and leaving the active species behind in the form of a porous material having a high surface area.2 

Catalytic reactions can be run over large, massive metal particles as well as the much smaller, dispersed metal crystallites. The massive metal catalysts can be the single crystal catalysts such as those shown in Fig. 3.2 or polycrystalline forms of bulk metal such as wires, foils or ribbons. These latter materials were used somewhat routinely in the early catalytic research efforts that were involved with developing the mechanisms of vapor phase catalytic processes. These materials were considered to be analogs of the supported catalysts in which the effect of the support, if any, was eliminated. 

While these massive metal catalysts have been useful for basic mechanistic and theoretical studies, they are not amenable for use in liquid phase reactions. Since most synthetically useful catalytic reactions are run in the liquid phase these massive metal catalysts find little use in the study of synthetic catalysis. Instead, synthetic reactions use the more dispersed forms of the metal catalysts. 

Noble metal blacks are usually prepared by the reduction of aqueous solutions of their salts with hydrogen, formaldehyde, formic acid or hydrazine. These blacks are large metal particles having a surface composed primarily of large areas of 

Another form of active nickel catalyst has been prepared by the reaction of lithium iso-propoxide with nickel chloride to give nickel iso-propoxide. Heating this salt to 100°C in iso-propanol gives a black, finely divided nickel catalyst that is active for the hydrogenation of a variety of functional groups.  

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A. S. Diamond, Toner Black and Powder for an Exploding Market, Diamond Research Corp., Ventura, Calif., 1979. 

To improve the properties of the raw polymer (wear resistance, creep resistance, thermal and electrical conductivity), various fillers, such as glass fibers, powdered metals, and graphite, are combined with all three types of PTFE polymers, mostly by intimate mixing. Filled fine powders are produced mostly by adding fillers into a dispersion and then coagulating the mixture. Aqueous dispersions can also be modified by the addition of certain fillers, pigments, heat resistant dyes, carbon blacks, and powdered metals, especially when processed into films (see Chapter 6). 

Figure 3.9 Scanning electron micrograph (14,000X) of powdered rubber, carbon black and powdered rubber compound (top to bottom).

The compaction of the mixture of carbon black and powdered rubber proceeds primarily through a large deformation and fracture of rubber particles. The static pressure required is rather small it is two orders of magnitude smaller than the pressure generated by the action of the rotor blade against the chamber wall and one order of magnitude smaller than the ram pressure. 

Examples are provided by the work of Carman and Raal with CF2CI2 on silica powder, of Zwietering" with nitrogen on silica spherules and of Kiselev" with hexane on carbon black and more recently of Gregg and Langford with nitrogen on alumina spherules compacted at a series of pressures. In all cases, a well defined Type II isotherm obtained with the loose powder became an equally well defined Type IV isotherm with the compact moreover both branches of the hysteresis loop were situated (drove the isotherm for the uncompacted powder, but the pre-hysteresis region was scarcely affected (cf. Fig. 3.4). The results of all these and similar... 

Silver difluoride [7783-95-1], AgF2, is a black crystalline powder. It has been classified as a hard fluorinating agent (3) which Hberates iodine from KI solutions and o2one from dilute aqueous acid solutions on heating. It spontaneously oxidizes xenon gas to Xe(II) in anhydrous hydrogen fluoride solutions (20). 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

PyrogaHol oxidized, which is obtained by the action of ak and ammonia on pyrogaHol, is a brownish black to black lustrous powder and is almost insoluble in water, alcohol, or ether but is soluble in alkahes. Hexahydroxybiphenyl [4371-20-4] (diphenyUiexol), formed by... 

Potassium nitrate, essential in the manufacture of black gun powder, was produced by the Chinese, who had developed gun powder by the tenth century AD. The process involved the leaching of soil in which nitrogen from urine had combined with mineral potassium. By the early 1800s, potassium nitrate had become a strategic military chemical and was stiU produced, primarily in India, by using the ancient Chinese method. The caUche deposits in Chile are the only natural source of potassium nitrate (2). These deposits are not a rich source of potassium nitrate, purifying only to about 14% as K O. 

Bismuth Triiodlde. Bismuth(Ill) iodide is a greenish black crystalline powder. The iodines are in a hexagonal close-packed array with each bismuth having six nearest-neighbor iodines at 0.32 nm (21). This suggests that the lone pair on bismuth is stereochemicaHy inactive and that the compound is largely ionic in character. 

Chromium (II) also forms sulfides and oxides. Chromium (II) oxide [12018-00-7], CrO, has two forms a black pyrophoric powder produced from the action of nitric acid on chromium amalgam, and a hexagonal brown-red crystal made from reduction of Cr202 by hydrogen ia molten sodium fluoride (32). Chromium (II) sulfide [12018-06-3], CrS, can be prepared upon heating equimolar quantities of pure Cr metal and pure S ia a small, evacuated, sealed quartz tube at 1000°C for at least 24 hours. The reaction is not quantitative (33). The sulfide has a coordination number of six and displays a distorted octahedral geometry (34). 

Chemical explosives detonate, or deflagrate. Detonating explosives (e.g., TNT or dynamite) rapidly decompose to produce high pressure and a shock front (travels faster than the velocity of sound). Deflagrating explosives (e.g., black and smokeless powders) bum fast, prodr er... 

The determination of precise physical properties for elemental boron is bedevilled by the twin difficulties of complex polymorphism and contamination by irremovable impurities. Boron is an extremely hard refractory solid of high mp, low density and very low electrical conductivity. Crystalline forms are dark red in transmitted light and powdered forms are black. The most stable ()3-rhombohedral) modification has mp 2092°C (exceeded only by C among the non-metals), bp 4000°C, d 2.35 gcm (a-rhombohedral form 2.45gcm ), A77sublimation 570kJ per mol of B, electrical conductivity at room temperature 1.5 x 10 ohm cm- . 

The brown oxide is a heavy granular powder which settles to the bottom of the solution in the bottle in which the reduction is carried out (p. 10). It must be reduced to platinum black before it becomes a catalyst for the reduction. When the hydrogen is admitted and the bottle shaken the brown oxide becomes black and whips up into a fine suspension. The time necessary for the change of the oxide to platinum black is called the lag. The time of lag varies usually from several seconds to two or three minutes, depending upon the conditions under which... 

A suspension of chloroboron 2.9.16-triiodosubphthalocyanine (210 mg, 0.26 mmol) in H20 (50 mL) was refluxed with vigorous stirring for 24 h. After this time, the solid was filtered and dried to give a black-violet powder yield 112 mg (55%). 

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

As far as conducting fillers are concerned, we have rather a wide range of choice. In addition to the traditional and long used fillers, such as carbon black and metal powders [13] fiber and flaky fillers on organic or metal bases, conducting textures, etc recently appeared and came into use. The shape of the filler particles varies widely, but only the particle aspect ratio, the main parameter which determines the probability... 

From 775 to 865°, small amts of N02 and N20 are also formed. The residue of the decompn is Na20. For DTA TGA data see Ref 2 Uses. Na nitrate is an oxidizing agent containing more available oxygen per unit wt than K nitrate, and for this reason it is a suitable ingredient of expls such as black blasting powders (introduced by duPont) and some Dynamites (see Vol 6,... 

The existence of materials now included among the conducting polymers has long been known. The first electrochemical syntheses and their characterization as insoluble systems took place well over a century ago. In 1862 Letheby reported the anodic oxidation of aniline in a solution of diluted sulphuric acid, and that the blue-black, shiny powder deposited on a platinum electrode was insoluble in HjO, alcohol, and other organic solvents. Further experiments, including analytical studies, led Goppelsroeder to postulate in 1876 that oligomers were formed by the oxidation of aniline. 

The black explosive powder previously described as Se4N2 is now assigned this structure. It is very sensitive when dry and handling it damp with an hydrocarbon is recommended. There is brown dimer, also explosive [2]. 

Initially we tried the standard approach, reduction of NiL, NiB, or NiC with 2.0 equivalents of potassium in refluxing THF. Finely divided black nickel powders were obtained however, they showed rather limited reactivity toward oxidative addition with carbon-halogen bonds. Similar results were found for palladium and platinum. 

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