S. A. ISAAND J. M. SALEH
2530
Oxidation of Metal Films by Nitrous Oxide by S. A. Isa and J. M. Saleh* Department of Chemistry, College of Science, University of Baghdad, Baghdad, Republic of Iraq (Received November 6, 1971) Publication costs assisted by the Universitg of Baghdad
The oxidation, by NzO, of evaporated films of Fe, Ni, W, Pd, and Pb has been investigated in the temperature range -78 to 250". Rapid dissociative chemisorption, which was accompanied by Nz evolution, occurred on the first four metals at -78". The completion of the surface layer on Fe film at -78" was followed by the formation of bulk oxide. The oxidation of Fe and Ni films by NzO gas became extensive above 30", but the oxidation of W film was slow and far less extensive than of the former two metals. The oxidation of Pd film was confined exclusively to the surface phase and the incorporation reaction became extremely slow even at temperatures as high as 200". Pb film, on the other hand, did not take up NzO irreversibly at any temperature in the range -78 to 250". Both the activation energy (Ex)and the preexponential factor (A,) increased as the extent of the oxidation (X) of each of the Fe, Ni, W, and Pd films increased, so that the experimental rate remained constant; a marked compensation effect existed for the oxidation processes. The effect was likely to arise from energetically heterogeneous reacting sites. The slow step of the oxidation process was assumed to be the dissociative chemisorption of NzO. The deposition of a Pb film on a film of Fe caused a substantial decrease in the surface area of the latter film. The behavior of the resulting Pb-Fe film toward NtO in the temperature range -78 to 100" was very similar to that of the Pb film; reaction occurred only at T 2 100" and was associated with high values of both the energy of activation and the preexponential factor compared with those for the Fe film.
Introduction There have been no previous investigations on the interaction of KzO, at low pressures (10-z-8 nm-2) and over the temperature range -78 to 250", with evaporated metal films, compared with numerous investigations on bulk metals, oxides, reduced powders, and metal The published information refers mostly to adsorption studies which were carried out at high N2O pressures (>lo nmV2)and elevated temperatures (>200"). We have selected Fe, Ni, W, Pd, and P b as typical metals upon which to investigate the initial chemisorption of Nz0 and the subsequent incorporation of oxygen. A P b film did not react with NzO, but a thick film of P b deposited on a sintered Fe film was active in the temperature range -78 to +SO". Experimental Section The apparatus, the preparation of metal films, and the general procedure have been described.6 The films were always sintered a t 70" for 20 min. After degassing of the glass and the metal filaments, the rate of degassing with the reaction vessel a t 400" was nm-z hr-l. During the deposition of each film the reaction vessel was maintained open to the pumps and the pressure was always
