REDOX PROCESSES IN FINE-PARTICLE
1325
0
ment with experimental data. The ∆G of reaction (4) the presence of rutile, the oxidation of chromium(III)
switches sign in the range 30–630°C, as does that of oxide is insignificant.
reaction (3). The forward reaction (4) is possible above
1
20°C.
ACKNOWLEDGMENTS
Reactions (3) and (4) provide qualitative explana-
This work was supported by P.I. Sumin, the Gover-
nor of Chelyabinsk oblast, grant no. 29/MO6/A.
tion for the influence of the particle size of the starting
oxides on the efficiency of the oxidation of Cr é in the
2
3
TiO –Cr é system. Increasing the specific surface of
2
2
3
REFERENCES
TiO or Cr é promotes the surface hydration of the
2
2
3
crystallites, thereby increasing the extent of oxidation
1. Bol’shoi entsiklopedicheskii spravochnik. Khimiya (Big
Handbook of Chemistry), Moscow: Bol’shaya Rossi-
iskaya Entsiklopediya, 1998.
of Cr é . The hydration of TiO and Cr é crystallites,
2
3
2
2
–
3
leading to the formation of reactive éç groups, is pos-
sible at room temperature under ordinary conditions.
Cr –éç and Ti –éç surface bonds persist at tem-
peratures above 730°C [8]. Calcination in air leads to
dehydration of the crystallites and Cr formation. It
also cannot be ruled out that the Cr
2
. Deren, J. and Haber, J., Studies on the Physical-Chemi-
cal and Surface Properties of Chromium Oxides, Solid
State Ionics, 2003, vol. 157, no. 1, pp. 386–397.
3+
–
4+
–
3
. Viktorov, V.V., Gladkov, V.E., Fotiev, A.A., and Ivashni-
kov, V.T., Reactions of Fine-Particle Al O –Sr O
6
+
2
3
2
3
3
+
6+
Cr oxida-
Oxides between 400 and 800°C, Izv. Akad. Nauk SSSR,
Neorg. Mater., 1983, vol. 19, no. 6, pp. 930–933.
tion involves protons. Storage of the calcined mixtures
is accompanied by rehydration of the crystallite surface
4
. Viktorov, V.V., Gladkov, V.E., and Fotiev, A.A., Effect of
TiO on the Oxidation of Cr O in Fine-Particle Al O –
6
+
3+
and Cr
Cr reduction since, at room tempera-
2
2
3
2
3
ture, the reverse reactions (3) and (4) take place.
Cr O Oxides, Izv. Akad. Nauk SSSR, Neorg. Mater.,
2
3
1
984, vol. 20, no. 4, pp. 686–689.
5
6
. Belen’kii, E.V. and Riskin, N.V., Khimiya i tekhnologiya
pigmentov (Chemistry and Technology of Pigments),
Leningrad: Khimiya, 1974.
. Zolotov, Yu.A., Osnovy analiticheskoi khimii (Funda-
mentals of Analytical Chemistry), Moscow: Vysshaya
Shkola, 1999.
CONCLUSIONS
We studied redox processes in fine-particle TiO2–
Cr é oxides during calcination in air and subsequent
storage under normal conditions.
2
3
The results indicate that, in the TiO –Cr é system
7. Arteminko, A.I., Tikunova, I.V., and Malevannyi, V.A.,
Spravochnoe rukovodstvo po khimii (Handbook of
Chemistry), Moscow: Vysshaya Shkola, 2002.
2
2
3
6
+
in air, the Cr ion is more stable between 800 and
3
+
1
000°C, whereas the Cr ion is more stable near room
8
. Morrison, S., The Chemical Physics of Surfaces, New
York: Plenum, 1977.
temperature.
6
+
3+
During storage, Cr to Cr reduction occurs. This
process is fully reversible.
9. Kovel’, N.S., Viktorov, V.V., Evdokimov, I.V., and
Fotiev, A.A., Formation of (CrxAl1 – x)2é3 Solid Solu-
tions during Cocalcination of Aluminum and Chromium
Hydroxides, Izv. Akad. Nauk SSSR, Neorg. Mater., 1989,
vol. 25, no. 7, pp. 1160–1164.
Anatase and rutile react with Cr O by different
2
3
mechanisms. Calcination of Cr O with anatase leads to
2
3
3
+
active oxidation of Cr and the formation of metastable 10. Viktorov, V.V., Evdokimov, I.V., and Kovel’, M.S., For-
Cr(VI) compounds. The highest content of such com-
pounds was reached by calcination at 850°C for 1 h. In
mation Kinetics of Al O –Cr O Solid Solutions, Zh.
2 3 2 3
Fiz. Khim., 1990, vol. 64, no. 7, pp. 1820–1824.
INORGANIC MATERIALS Vol. 43 No. 12 2007