Platinum black reduction

Reduction of vanillin by means of platinum black in the presence of ferric chloride gives vanillin alcohol in excellent yields. In 1875, Tiemann reported the reduction of vanillin to vanillin alcohol by using sodium amalgam in water. The yields were poor, however, and there were a number of by-products. High yields of vanillin alcohol have been obtained by electrolytic reduction. 

Dihydrocholesterol has been prepared by the reduction of cholestenone with sodium and amyl alcohoP and by the hydrogenation of cholesterol. In the presence of platinum black or platinum oxide, yields varying from 6.5 per cent to 40 per cent have been obtained in ether, acetone, ethyl acetate, and acetic acid. ... 

Platinum Catalyst for Reductions (Coll. Vol. i, 452) Preparation from platinum black by heating with oxygen underpressure. Laffiteand Grandadam, Compt. rend. 200, 456 (193s). 

Its low specific gravity indicates that it is an open-chain compound, and from its easy reduction by sodium and alcohol, into dihydroaplotaxene, and by hydrogen and platinum black into normal heptadecane OjjHj,., it is evident that aplotaxene is a tetraolefinic normal chain hydrocarbon. 

The platinum black by reduction of the oxide may sometimes be used a second, third or even more times in the reduction of certain compounds merely by reactivating (Note 8) with air or oxygen. A spent catalyst must be reworked (Note 9)... 

The products prepared at temperatures below 450° and above 600 0 have a lower activity and require a greater length of time for reduction to platinum black than those prepared at temperatures between 450° and 6oo°. 

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... 

In certain reductions it is an advantage to reduce the platinum oxide to platinum black by shaking with hydrogen in the presence of solvent only, before the substance to be reduced is added to the mixture. More often the catalyst is reduced in the presence of the substance to be reduced with aldehydes for example the platinum black is usually more finely divided and generally more active if prepared in presence of the aldehyde. 

Platinum black has been prepared chiefly by the reduction of chloroplatinic acid with formaldehyde 5 the details have been improved by Willstatter and Hatt 6 Willstatter and Waldschmidt-Leitz,7 and by Feulgen.8... 

Vogel WM, Baris JM. 1977. Reduction of oxygen on platinum black in acid electrolytes. Electrochim Acta 22 1259-1263. 

Addition of platinum-black catalyst to ethanol caused ignition. Pre-reduction with hydrogen and/or nitrogen purging of air prevented this. 

Ti, or PEEK (polyether ether ketone) to allow measurements under very corrosive conditions. The separated phases pass AMX gadgets for on-line detection (radiometric, spectrophotometric, etc.) or phase sampling for external measurements (atomic absorption, spectrometric, etc.), depending on the system studied. The aqueous phase is also provided with cells for pH measurement, redox control (e.g., by reduction cells using platinum black and hydrogen, metal ion determination, etc.) and temperature control (thermocouples). 

Reactions. w-Hydroxybenzoic acid affords a variety of products, depending on the catalyst and conditions employed. Catalytic reduction over platinum black or platinum oxide in alkaline solution gives 3-hydroxycyclohexanecarboxylic acid [22267-35-2]. Reduction of a warm aqueous solution over platinum oxide or over colloidal platinum yields cyclohexanecarboxylic acid. w-Hydroxybenzaldehyde can be prepared by reducing ///-hydroxybenzoic acid with sodium amalgam. Finally, reduction over Raney nickel gives cydohexanol. 

Numerous methods for the synthesis of salicyl alcohol exist. These involve the reduction of salicylaldehyde or of salicylic acid and its derivatives. The alcohol can be prepared in almost theoretical yield by the reduction of salicylaldehyde with sodium amalgam, sodium borohydride, or lithium aluminum hydride by catalytic hydrogenation over platinum black or Raney nickel or by hydrogenation over platinum and ferrous chloride in alcohol. The electrolytic reduction of salicylaldehyde in sodium bicarbonate solution at a mercury cathode with carbon dioxide passed into the mixture also yields saligenin. It is formed by the electrolytic reduction at lead electrodes of salicylic acids in aqueous alcoholic solution or sodium salicylate in the presence of boric acid and sodium sulfate. Salicylamide in aqueous alcohol solution acidified with acetic acid is reduced to salicyl alcohol by sodium amalgam in 63% yield. Salicyl alcohol forms along with -hydroxybenzyl alcohol by the action of formaldehyde on phenol in the presence of sodium hydroxide or calcium oxide. High yields of salicyl alcohol from phenol and formaldehyde in the presence of a molar equivalent of ether additives have been reported (60). Phenyl metaborate prepared from phenol and boric acid yields salicyl alcohol after treatment with formaldehyde and hydrolysis (61). 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

There have been used essentially only three catalysts foi the hydrogenation of ethylene oxides nickel, palladium on charcoal, and platinum black. Solvent normally employed include ethanol wait nickel, and ethanol, ethyl acetate, or acetic acid with the other. Reduction over platinum or palladium catalysts is usually conducted at room temperature and low pressure, whereas nickel catalysth Imvi-been employed in autoclaves at temperatures ranging from 3fT to nearly 200° and high pressures. For excellent general discussions ol catalytic redaction any of several outstanding sources14" 11-ltni m.i> be consulted. 

When the bis(isopropylamino)iodocyclopropenylium iodide is reacted with platinum black in acetonitrile, the reaction takes a different course, affording mainly the trans-bis[bis(diisopropylamino)cyclopropenylidene] diiodoplatinum complex (equation 277)351. A plausible pathway for this reaction involves two consecutive oxidative additions to platinum leading to the hexacoordinated intermediate Ptlv-complex [ -Pr2N)2C3]Ptf4, followed by reductive elimination of I2 to form the product (cf Section VI. A. 1. a). 

The pressed-salt method can be used to overcome this difficulty because it can be used to observe spectra of carbon monoxide on platinum particles which are not supported on silica or alumina (14)- It is possible to use commercial platinum black, but best results are obtained if the platinum particles are formed by reduction of chloroplatinic acid while it is dispersed on the powdered salt. After reduction in hydrogen at 300° C., the sample was treated with CO and transferred to the die. An essential feature of this technique is a final treatment with CO after the sample is in the die. The die is sealed with Apiezon Q wax to prevent exposure to the atmosphere between the time of the final treatment with CO and the application of pressure to form the salt disk. 

Alcohols. Trifluoroethanol has been obtained by catalytic reduction of trifluoroacetic anhydride on platinum black.88 This reaction, which... 

Hydrogen gas is bubbled over a platinum surface that is coated with platinum black, an electrolytically deposited coating of colloidal platinum, which is an excellent catalyst for the above equilibrium. The hydrogen electrode has been selected as the standard against which the potentials of other electrodes are measured. Equations of the type of reaction (I) are called half-cell reactions, because they include electrons. Reaction I is a reduction half-cell reaction. 

If the diacetyl-D-pseudoglucal is hydrogenated with very active catalysts (palladium black of Wieland or platinum black of Willstatter) the reaction proceeds in the main somewhat differently. Four atoms of hydrogen in total are added, but the pyranose ring is retained the cyclic form apparently is reduced first at carbon atom 2l and the subsequent reduction produces the 4,6-diacetate of the 1,5-anhydride of d-erythro-1,4,5,6-hexanetetrol (XXXII). The same compound is produced even more easily from triacetyl-D-pseudoglucal (XXV) in the same manner. 

If any cation can exist in several states of oxidation its reduction at the cathode may be accompanied by several limiting currents. For instance, if for the reduction of vanadic acid dissolved in concentrated sulphuric acid platinum electrode coated with platinum black is used, three steps in the reduction process can be easily distinguished, namely Vv -> VIV, then Vlv -> Vm, and finally the stage of hydrogen liberation. 

In the majority of cases, the last step in the preparation of catalytically active metals is a reduction. The precursor is very frequently an oxide. An oxychloride is the real precursor of active platinum and some noble metals if chlorometal complexes (e.g. chloroplatinic acid) are used. It may be advantageous to use still other precursors and to reduce them directly without any intermediary transformation to oxide. On the other hand, nearly all catalytic metals are used as supported catalysts. The only notable exception is iron for ammonia synthesis, which is a very special case and then the huge body of industrial experience renders scientific analysis of little relevance. The other important metals are Raney nickel, platinum sponge or platinum black, and similar catalysts, but they are obtained by processes other than reduction. This shows the importance of understanding the mechanisms involved in activation by reduction. 

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