The Thermal Decomposition of Ammonia upon Mixed Surfaces of Tungsten and Platinum

PROCEEDINGS OF THE

NATIONAL ACADEMY OF SCIENCES Volume 14

August 15, 1928

Number 8

THE THERMAL DECOMPOSITION OF AMMONIA UPON MIXED SURFACES OF TUNGSTEN AND PLATINUM BY ROBIRT U. BuRK* DsPARTM1ENT oF CEMmisTRY, CoRNEJL UNIvIRsITY Communicated July 5, 1928

It is well recognized' that the lowering of the heat of activation is a probable way in which contact catalysts can accelerate reactions. The writer2 has recently discussed two definite ways in which the catalyst could do this: (1) by distorting the atoms with a resulting weakening of the relevant bond, (2) by bodily separating the atoms forming the bond. In the case of the thermal decomposition of ammonia on tungsten, the lowering of the heat of activation seems to be the only r6le played by the catalyst. This reaction offers, therefore, a possible means of studying the mechanisms (1) and (2) and, therefore, of estimating their relative importance. Some previous experiments2 designed to investigate the contribution of mechanism (1), where ammonia was decomposed in strong electric fields, could not be carried through with complete satisfaction, because for fields greater than about 150,000 volts per cm., the fine wires (necessary in order to get the strong fields) were fused by ionic bombardment. However, the present experiments, designed to bring out the second mechanism, have met with more success. If the bonds of ammonia are weakened by stretching, particularly active reaction centers should be produced by a solid whose surface contains a mixture of atoms, some of which adsorb the hydrogen atoms of ammonia very strongly, and others adsorb the nitrogen atom very strongly, provided, of course, that their spacing is suitable. If some factor other than the heat of activation does not operate, such a surface would exhibit "promoter action," i.e., its activity would be greater than an equal area of surface of any of the constituents. If, on the other hand, the first mechanism (atomic distortion) were the only effect of the catalyst, one would not be justified in expecting promoter action with any specific mixture of catalysts, and the promoter could not be chosen "theoretically" in the present state of knowledge.

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CHE MIS TR Y.- R. E. B URK

PROC. N. A. S.

On the basis of these considerations a mixed surface of tungsten and platinum was chosen as the prospective "promoted" catalyst, previous work4 having given the kinetic characteristics of the decomposition of ammonia upon the separate metals, and together with general knowledge of the chemical and adsorptive properties of the metals, having rendered it plausible that tungsten adsorbs nitrogen more strongly than hydrogen, and that the reverse is true for platinum. "Surface alloys" were produced by evaporating one metal from a spiral on to a filament of the other metal mnounted axially within it.' Not only did the mixed surface cause a greater velocity of decompbsition tfiin the same area of either platinum or tungsten alone, over the whole range of temperature investigated, but the temperature coefficient was decidedly smaller for the mixed surfaces, in complete agreement with expectations on the basis of mechanism (2). EAT OP ACTIVATION

CATALYST

Tungsten (wire A) Platinum Tungsten-platinum (wire X) Tungsten (wire C)

Tungsten-platinum (wire C)

(CALORXES/GM. MIoL.) 47,000

130,000 34,600 48,000 36,000

While the temperature measurements were inaccurate due to a small progressive alteration in the standard resistance of the wire, no possible method of calculating the results could obscure them, as was found by trial. In the case of wire C, the temperature coefficient was measured on the pure, well-degassed tungsten wire; the platinum was then put on the identical wire, which was never out of the vacuum after the preceding ammonia decomposition, and was not heated at any temperature except that used in the decomposition of ammonia. There was thus little opportunity for the structure of the underlying metal to change, and the possible spacing factor to vary. Both synthetic ammonia and that made from Kahlbaum's pure ammonium chloride- and pure calcium oxide, using all glass apparatus, gave the same results. * NATIONAL RESEARcH FELLow. 1 See, e.g., Hinshelwood, Kinetics of Chemical Change in Gaseous Systems, 1926. Rideal and Taylor, Catalysis in Theory and Practice, p. 46 (second edition). 2 These PRocEEDINGs, 13, 719 (1927); J. Phys. Chem., 30, 1134 (1926). 3 The argument for this will be developed in the forthcoming "Sixth Report of the Committee on Contact Catalysis," and also in the more complete account of these experiments, which is to be published in the Journal of Physical Chemistry. 4 Hinshelwood and Burk, J. Chem. Soc., 127, 1105 (1925). 65Full experimental details will be published. For the general procedure apart from the evaporation see Burk, these PROCEEDINGS, 13, 67 (1927).