Catalytic Activation of Nitrogen over Transition Metals

Abstract

Ammonia synthesis rates over metals are strikingly increased by addition of potassium as a promoter so that significant activities are obtained for the ammonia synthesis over rhodium and iridium, with which no synthesis activity has been known. The mechanism of enhancement by potassium has been investigated by means of kinetics of isotopic mixing in molecular nitrogen and of ammonia synthesis both over ruthenium. The kinetics of isotopic mixing is in conformity with the equation

$${{\text{R}}_{\text{E}}}{\text{=kP/}}{\left( {{\text{1+KP}}} \right)^2}$$

((1))

which is consistent with the dissociative chemisorption. From the temperature dependence of k and K at 320 to 380°C, the activation energy and the heat of adsorption are found to be 13 and 47 kcal/mol respectively. This is a remarkably high value as a heat of nitrogen adsorption on ruthenium, for which no nitride is known. The kinetics of ammonia synthesis is also in conformity with the equation

$${{\text{R}}_{\text{S}}}{\text{=k}}{{\text{P}}_{{{\text{N}}_2}}}{\text{/}}{\left( {{\text{1+K}}{{\text{P}}_{\text{A}}}{\text{/}}{{\text{P}}_{{{\text{H}}_{\text{2}}}}}^{\text{x}}} \right)^2}$$

((2))

The inverse isotope effect as found on iron catalysts is observed on Ru-AC-K, whereas, on pure ruthenium, no isotope effect was observed. These results demonstrate that the adsorption bond of nitrogen is strengthened by the potassium addition, thus promoting the noble metals for the ammonia synthesis.