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REACTION BETWEEN KITRICOXIDE AND AMMONIA We would like to thank Professor P. Le Goff for his enthusiastic support, Dr. C. Rosenthal for discussions
on the cascade, and D. Chery for helpful discussions on the TiOz probe.
Studies of Surface Reactions of Nitric Oxide by Isotope Labeling.
IV.
The
Reaction between Nitric Oxide and Ammonia over Copper Surfaces at 150-200"
by K. Otto* and M. Shelef Fuel Sciences Department, Ford Motor Company, Dearborn, Michigan 48121
(Received M a y 28, 1971)
Publication costs assisted by the Ford Motor Company
Mixtures of 14N0and 15NH8,diluted by Ar, were circulated over unsupported cupric oxide in a closed reaction loop to study the product distribution as a function of oxidation state. The CuO was reduced along with the nitric oxide, and a t first the only product in the gas phase besides water was mixed nitrogen 14NW. As the reaction proceeded, three other products were formed, 15N14N0,I4Nz,and I4N2O. Under steady-state conditions, using a slightly reducing mixture of the reactants, the ratio of nitrogen-containing products was 14N16N: 15N14NO:14NzO:l4N2= 54: 16:9:21. At this stage the catalyst is practically reduced to metallic copper, although some residual oxygen cannot be removed from the solid. ,4comparison with earlier results on Pt (J.Phys. Chem., 75, 875 (1971)) shows that the reaction rate a t 200" per metal atom in the surface is 0.2 N O molecule/sec for Cu and 0.05 molecule/sec for Pt. The formation of unmixed nitrogen is more pronounced in the case of Cu than on a Pt catalyst. The experimental findings can be rationalized within the framework of the general mechanism proposed earlier for the reaction on Pt, with certain modifications.
Introduction The catalytic interaction between ammonia and nitric oxide over supported Pt has been studied previously in this laboratory.'V2 The study of the same reaction was extended presently to a base metal, or metal oxide catalyst. It was motivated by the following reasons. First, there are several patents describing the use of this reaction for selective purification of X O containing effluents where base metal oxides are used as the catalyst^.^-^ Therefore, it was of practical interest to compare the reaction mechanism on two different classes of catalysts. Second, it was deemed important to explore the effect of oxidation state changes, which base metal catalysts experience in the course of the reaction, on the reaction mechanism. Copper was chosen as a representative base metal.
Experimental Section Catalyst, Purity of Gases, and Apparatus. I n the majority of the experiments a single sample of 0.185 g of "specpure" cupric oxide (Johnson Matthey Chemicals, Ltd.) was used. The BET surface area before use was 0.88 m2/g. It mas determined by Kr adsorption a t liquid nitrogen temperature. A few runs were carried out on copper metal powder (B & A, reagent grade) which had a very small surface area
(
