Platinum oxide impurities

Because of the presence of alkali in Raney nickel, ketones are hydrogenated over this catalyst to yield the more stable, equatorial alcohol e.g. 59) as the predominant product, Similar results can be expected with platinum in basic media or with platinum oxide in an alcoholic solvent since this catalyst also contains basic impurities. 

Commercial C.p. chloroplatinic acid varies somewhat in its purity. In this work that from the Mallinckrodt Chemical Works, St. Louis, was used and gave very satisfactory results. Since small amounts of impurities in the catalyst are important factors in the rate of reduction of certain types of compounds, this question of impurities in the chloroplatinic acid must be taken into account (Note 13). In a large proportion of the reductions studied, platinum oxide prepared from the chloroplatinic acid mentioned gave as good results as that from spectroscopically pure chloroplatinic acid made according to the directions of Wichers.1... 

FLINAK is purified by treatment with the HF released by ammonium bifluoride (NH4HF2) the HF converts oxide impurities in the melt to H20 [7]. In this purification procedure, the fluoride salt mixture is combined with 15 wt% NH4HF2 and heated to about 500°C in a graphite crucible. The molten mixture is poured into a platinum container and heated to 750°C. Hydrogen is passed through the molten mixture for approximately 2 days. Further purification can be achieved by con-trolled-potential electrolysis at an applied potential of about 3 V between a tungsten cathode and glassy carbon anode. 

At the impure Cu anode, copper is oxidized along with more easily oxidized metallic impurities such as zinc and iron. Less easily oxidized impurities such as silver, gold, and platinum fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions are reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution. 

Hydrogenations of cyclohexanones in basic media leads to the formation of the more stable, equatorial alcohol, 18, as the primary product (Eqns. 18.18-19). With basic Raney nickel catalysts, the predominance of equatorial alcohol has been shown to arise from the isomerization of the initially produced axial alcohol under hydrogenation conditions. The use of platinum oxide with its alkaline impurity also leads to extensive equatorial alcohol formation unless acetic acid is used as the solvent. This basic impurity was shown to be... 

The hydrogenation of aliphatic nitro groups takes place with retention of configuration when the reaction is run under the normal conditions. The basic nature of Raney nickel does not promote epimerization of the nitro group before hydrogenation at low temperatures but the alkaline impurity found in platinum oxide does. The use of platinum oxide in ethanol resulted in the formation of a mixture of product stereoisomers but, in acetic acid, retention of configuration was observed. Higher temperatures also promoted inversion (Eqn. 19.24).5>... 

The oscillatory kinetics of CO oxidation over Pt single crystals has been studied at atmospheric pressure by Yeates et al. (95). They present a model to explain rate oscillations that relies on the oscillatory formation of surface platinum oxide, which was observed in postreaction analysis and was related to the presence of silicon impurity. 

Figure 13.6.1 Cyclic voltammogram for a smooth platinum electrode in 0.5 M H2SO4. Peaks formation of adsorbed hydrogen. Peaks H oxidation of adsorbed hydrogen. Peaks Oq formation of adsorbed oxygen or a platinum oxide layer. Peak Oc reduction of the oxide layer. Point 1 start of bulk hydrogen evolution. Point 2 start of bulk oxygen evolution. The shape, number, and size of the peaks for adsorbed hydrogen depend on the crystal faces of platinum exposed (62), pretreatment of electrode, solution impurities, and supporting electrolyte. See also Figure 13.4.4.

Molten hydroxides Noble and precious metals Pure silver, gold, and platinum Reducing atmosphere usually corroded if oxidizing impurities are present such as nitrates. Melt resistance Ag > Au> Pt. 

The rate of platinum consumption has been found to accelerate in the presence of AC current ripples. Most wastage has been observed to occur with AC frequencies of less than 50 Hz. The repeated oxidation/reduction processes result in the formation of a brownish layer of platinum oxide. To avoid the occurrence of this phenomenon, a single- or a three-phase full-wave rectification is recommended. The consumption rate of platinized anodes is also adversely affected by the presence of organic impurities such as sugar and diesel fuel. 

CVs in Oj-saturated H SO solution containing MeN ", and EtN ions did not show significant changes from those in pure H SO, as long as the amount of impurity was not large (1% of H+). In the case of PrN" or BuN ", hydrogen adsorp-tion/desorption peaks as well as platinum oxide formation/reduction peaks were distorted even for the lower amount of 0.1%. 

Zirconium shows excellent corrosion resistance to hydrochloric acid and is superior to any other engineering metal for this application, with a corrosion rate of less than 0.125 mm y" at all concentrations and temperatures well in excess of the boiling point. Aeration does not affect its corrosion resistance, but the presence of oxidizing impurities such as cupric or ferric chlorides in relatively small amoimts will decrease it. Therefore, either these ions should be avoided, or suitable electrochemical protection should be provided. Zirconium also shows excellent corrosion resistance to nitric acid in all concentrations up to 90% and temperatures up to 200°C, with only platinum being equal to it for this service. Welded zirconium and its alloys retain this high corrosion resistance. In concentrated nitric acid, zirconium may exhibit SCC at nitric acid concentrations above 70%, if under high tensile stress. ... 

L. Ruthenia, Russia) Berzelius and Osann in 1827 examined the residues left after dissolving crude platinum from the Ural mountains in aqua regia. While Berzelius found no unusual metals, Osann thought he found three new metals, one of which he named ruthenium. In 1844 Klaus, generally recognized as the discoverer, showed that Osann s ruthenium oxide was very impure and that it contained a new metal. Klaus obtained 6 g of ruthenium from the portion of crude platinum that is insoluble in aqua regia. 

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

Neodymium oxide [1313-97-9] M 336.5, m 2320°. Dissolved in HCIO4, ppted as the oxalate with doubly recrystd oxalic acid, washed free of soluble impurities, dried at room temperature and ignited in a platinum crucible at higher than 850° in a stream of oxygen [Tobias and Garrett J Am Chem Soc 80 3532 1958]. 

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