14 Application of Differential Thermal Analysis to the Study of Solid Catalysts -- Systems Cr2O3, Fe2O3, and Cr2O3-Fe2O3

The dissolution behavior of precipitated hydrous chromium oxide in perchloric acid solutions containing SO24-or Cl-was studied and the dissolution mechanism was discussed from the point of view of electrochemistry.The dissolution rate in acid solutions obeyed a linear rate law and was expressed asThe dissolution rate is determined by superimposing the following two coupling and coupled reactions whose rates depend upon the potential difference at the hydrous oxide-solution interface:where H. O. denotes hydrous chromium oxide. An electrochemical dissolution mechanism is proposed which involves as an intermediate complex CrSO4+ for perchloric acid solutions containing SO24-, CrCl2+ or Cr(OH)Cl+ for those containing Cl-, and Cr(OH)2+ for pure perchloric acid solutions.( Received December 25, 1974) It has been considered that the corrosion of passive metals and alloys is not caused by the active metal-dissolution but the dissolution of a nonporous passive oxide film in aqueous solution. The corrosion current density of iron(1)(2), chromium(3) and their alloys(4) is independent of the potential in all the range of passivity, although depending upon the pH of solution. Vetter(1) explained such a corrosion behavior by assuming that the dissolution rate of the passive oxide film is electrochemically controlled by the potential difference at the filmsolution interface, which is determined soley by the pH of solution.Recent studies of passivated stainless steels in acid solutions by means of X-ray photo electron spectroscopy(5) and Auger electron spectroscopy(6) have proved the high enrichment of chromium component in the passive oxide film, which seems to explain the high corrosion resistivity of stainless steels in acid solutions. The passive oxide film has an amorphous structure with a chemically bound water (7).Hydrous chromium oxide prepared by precipitation in aqueous solution according to the X-ray diffraction data(8), has also an amorphous structure. Therefore it is expected that the structure of hydrous chromium oxide is similar to that of the passive chromium oxide on stainless steels. In this study, hydrous chromium oxide prepared by precipitation is adopted as a model material of the passive oxide film on stainless steels and the dissolution of hydrous chromium oxide in acid solutions is investigated and the dissolution mechanism is discussed from the electrochemical point of view.

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“…(7) is given by eq. (8) because the reaction is controlled by the (8) determining step (7) and is in the range of Introducing eq. (5) …”

Section: Mechanism Of Electrochemical Dissolution Of Hydrous Chromiummentioning

confidence: 99%

“…Hydrous chromium oxide prepared by precipitation in aqueous solution according to the X-ray diffraction data (8), has also an amorphous structure. Therefore it is expected that the structure of hydrous chromium oxide is similar to that of the passive chromium oxide on stainless steels.…”

mentioning

confidence: 99%

Dissolution of Hydrous Chromium Oxide in Acid Solutions

et al. 1975

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“…The temperatures corresponding to the initiation and the peak in the DTA curves for the crystallizations reactions "The data are for freshly prepared samples. 1 Freshly prepared Ti02(anatase) gives a single exothermic peak at ~305°C. With aging, another peak appears at ~230°C.…”

Section: Resultsmentioning

confidence: 99%

“…Exothermic crystallization reactions associated with large heats have been observed in the differential thermal analysis curves of amorphous ferric oxide and chromia (1,7), titania (11), and alumina (2). After the exothermic reactions these oxides show high crystallinity as evidenced by x-ray analysis.…”

mentioning

confidence: 99%

Heats of crystallization of some amorphous oxides

Natarajan¹,

Chandrashekhar²,

Rao³

1968

J. Chem. Eng. Data

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“…Exothermic reactions associatetl with large heats have beell observed in the differential thermal ailalysis of hydrates of chromic and ferric hydroxides (4,5) and titaniuin peroxide gel (6). T h e exothermic reactions are foui~d just after the endotheriiiic pealrs due t o tlie decomposition of the hydrate or tlie peroxide.…”

Section: Calories Per Gram) the Heats T H U S~e S T I~n A T E Dmentioning

confidence: 99%