Kinetics of the decomposition of ammonia on platinum at low pressures

Abstract

The decomposition into nitrogen and hydrogen on heated filaments was studied at pressures of 10^–4 torr and less with both conventional and ultra-high vacuum (u.h.v.) techniques. The reaction probability P of an ammonia molecule at a collision with the platinum was determined under various conditions. Particular attention was given to initial values of P after flashing, to steady values of P after some reaction, and to the effect on P of various filament pre-treatments. Exposure of an incandescent filament to hydrogen at 10^–2 torr and to residual gases revealed important effects. Complex results were observed in the conventional apparatus, including a negative temperature coefficient above about 900°K, and a variable order of reaction, which could be less than unity, depending on the pre-treatment. In the absence of residual gases (u.h.v.), simple kinetics were observed above 700°K. The conventional vacuum results were reproduced by a wire pre-treated with ammonia containing hydrocarbon (2 : 2 : 4-trimethylpentane) but not with a wire pretreated with hydrogen only. It is suggested that ammonia on platinum rapidly forms a surface nitrogen-containing layer which may be a surface nitride. This is removed by reaction with carbon diffusing from the interior. Surface carbon is also removed by reaction with ammonia. Excessive carburization gives an inactive surface. Equations are deduced on this basis. The rate-determining step at high temperatures is postulated to be formation of NH₂(ads) from NH₃(ads). This is supported by rate-theory calculations and hydrogen retardation studies.