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The action of ferric oxide as a catalyst is perceptible at 400°; below 600°, the ferric oxide has only a slight activity, and
above 620°, the activity decreased. The optimum temp, is between 600° and
620° ; and there is then approximately a 70 per cent, conversion. In practice,
however, a conversion of 40 per cent, with a burner gas containing 5-7 per cent,
of sulphur dioxide is considered to be a good result; but, according to F. Haber,
it is doubtful if the equilibrium yield has ever been attained because a long contact
with the catalyst is needed for equilibrium. G. Keppeler and J. d'Ans found the
optimum temp, to be 629° for a mixture containing 2 per cent, of SO2, and there
was a 72-5 per cent, yield ; with 3 per cent. S02, the optimum temp, was 640° ;
with 4 per cent., 650°; and with 7 per cent., 665°. The yields in the last two cases
were respectively 65-0 and 53-2 per cent. A number of attempts have been made
to improve the catalytic activity of ferric oxide. Thus, K. Albert heated a mixture
of ferric oxide with oxide of barium or strontium. The catalysis begins at 400°,
and gives a 94 per cent, conversion at 450° ; nor is the product so sensitive to
moisture as ferric oxide alone.
The catalytic action of ferric oxide has been explained by assuming that ferric
sulphate or a basic ferric sulphate is alternately formed and decomposed, since
ferric sulphate decomposes in the same temp, range as ferric oxide is active as a
catalyst:
2Fe2O3+6SO2+3O2 ---> 2Fe2(SO4)3 ;
Fe2(S04)3 ---> Fe2O3+3SO3.
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