Mechanism of Oxygen Poisoning of Ammonia Synthesis Catalyst

V. L. Kuchaev^†, E. N. Shapatina, and A. K. Avetisov^z

Russian Federation Scientific Center “Karpov Institute of Physical Chemistry”
ul. Vorontsovo Pole 10, Moscow, 105064 Russia

Received May 13, 2008

Abstract—The effect of oxygen adsorbed on the surface of a commercial catalyst from a mixture of hydrogen
with water vapor on the steady–state and nonsteady–state ammonia synthesis kinetics is studied under gradi-
entless conditions at the pressures of the stoichiometric nitrogen–hydrogen mixture below the atmospheric
pressure and at the temperatures of 285 and 240°C. The results obtained are discussed on the basis of the con-
cepts of the ammonia synthesis theory of Temkin. The poisoning effect of oxygen on the reaction rate is
explained by an increase in the activation energy of the rate constant k₊ in the Temkin–Pyzhev equation, i.e., an
increase in the activation energy of the rate constant of nitrogen adsorption at the fixed nitrogen adsorption heat.
This conclusion agrees with the concepts of Ertl et al., according to which the activation energy of nitrogen
adsorption on iron changes in symbasis with the variation of the electronic work function. Oxygen adsorption
on the catalyst surface increases the electronic work function. Thus, the mechanism of the catalyst poisoning
by oxygen (at its low surface coverage) consists in an increase in the electronic work function. Assumptions are
stated as to the role of chemical promoters of iron catalysts.

Key words: ammonia synthesis, catalyst poisoning, poisoning mechanisms

DOI: 10.1134/S1023193509090031

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