8.2.1 Evaluating Candidate Catalysts
Haber’s results with van Oordt, indicated that to work at atmospheric
pressure, a temperature of 300 °C could not be exceeded, but at the time no
catalyst was known that operated efficiently at such low temperatures. The
most effective catalyst they discovered was manganese, and in practise temperatures between 500 and 700 °C had to be employed. Above 700 °C the
advantage of the increased rate of reaction was more than counterbalanced by
the low yield of ammonia. Below 500 °C, a much larger volume of ‘contact
substance and space’ was required to maintain a reasonable reaction rate, and
since a high-pressure apparatus is effected more easily at low volumes, temperatures below 500 °C could not be entertained.
The original apparatus for comparing the efficiency of various catalysts
consisted, in its simplest form, of a strong steel cylinder with a bore of about
6–7 mm heated externally in a bath of fused saltpetre.
15 Little other technical
detail is available, but this approach was soon abandoned in favour of a series
of more sophisticated apparatus which employed a convenient internal electrical heating mechanism. Now this might seem a little odd bearing in mind
Haber’s criticism of Nernst, but here of course the objective was to produce a
template for an industrial process and not simply the accurate measurement
of an equilibrium position. Electrical heating therefore had to be the more
practical option, and not only for the evaluation of the catalyst but also for the
continuous circulation apparatus. Haber and Robert’s original diagram
15
(Fig. 8.1) shows the apparatus they finally developed to compare the efficacy
of candidate catalysts.
Here, ‘Fig. 1’ in the diagram shows two views of the ‘furnace’ as it was then
called, where a strong steel outer tube enclosed a thin iron tube (9–13 mm
diameter), wrapped in asbestos paper and wound with a nickel wire so that
the whole could be heated electrically, the workers maintaining that ‘with
correct adjustment of the heating coil, a field of constant maximum temperature of 4–5 cm length can be obtained …’.
15 The catalyst, or ‘contact
substance’, was contained between asbestos plugs in an inner glass or quartz
tube, the same tube containing the wires for the thermocouple. The pressurised gases from the compressor entered the apparatus at the top and passed
down through the thin iron tube, over the catalyst and then out of the
apparatus at the top via a capillary tube emerging from the bottom of the
inner tube and running up through the annulus between the inner tube and
162
D. Sheppard
Haber’s results with van Oordt, indicated that to work at atmospheric
pressure, a temperature of 300 °C could not be exceeded, but at the time no
catalyst was known that operated efficiently at such low temperatures. The
most effective catalyst they discovered was manganese, and in practise temperatures between 500 and 700 °C had to be employed. Above 700 °C the
advantage of the increased rate of reaction was more than counterbalanced by
the low yield of ammonia. Below 500 °C, a much larger volume of ‘contact
substance and space’ was required to maintain a reasonable reaction rate, and
since a high-pressure apparatus is effected more easily at low volumes, temperatures below 500 °C could not be entertained.
The original apparatus for comparing the efficiency of various catalysts
consisted, in its simplest form, of a strong steel cylinder with a bore of about
6–7 mm heated externally in a bath of fused saltpetre.
15 Little other technical
detail is available, but this approach was soon abandoned in favour of a series
of more sophisticated apparatus which employed a convenient internal electrical heating mechanism. Now this might seem a little odd bearing in mind
Haber’s criticism of Nernst, but here of course the objective was to produce a
template for an industrial process and not simply the accurate measurement
of an equilibrium position. Electrical heating therefore had to be the more
practical option, and not only for the evaluation of the catalyst but also for the
continuous circulation apparatus. Haber and Robert’s original diagram
15
(Fig. 8.1) shows the apparatus they finally developed to compare the efficacy
of candidate catalysts.
Here, ‘Fig. 1’ in the diagram shows two views of the ‘furnace’ as it was then
called, where a strong steel outer tube enclosed a thin iron tube (9–13 mm
diameter), wrapped in asbestos paper and wound with a nickel wire so that
the whole could be heated electrically, the workers maintaining that ‘with
correct adjustment of the heating coil, a field of constant maximum temperature of 4–5 cm length can be obtained …’.
15 The catalyst, or ‘contact
substance’, was contained between asbestos plugs in an inner glass or quartz
tube, the same tube containing the wires for the thermocouple. The pressurised gases from the compressor entered the apparatus at the top and passed
down through the thin iron tube, over the catalyst and then out of the
apparatus at the top via a capillary tube emerging from the bottom of the
inner tube and running up through the annulus between the inner tube and
162
D. Sheppard