THE SELECTIVITY OF HYDRAZINE OXIDATION ON A POLYCRYSTALLINE RHODIUM SURFACE

[摘要] The surface reactions of coadsorbed hydrazine and atomic oxygen have been examined by means of temperature-programmed reaction spectroscopy (TPRS) on a clean polycrystalline rhodium surface over the 90 to 1450 K temperature range. Nitrogen and water are the primary oxidation products observed. In addition, the usual hydrazine decomposition products N2, NH3, H-2 and N2H2 are observed. Nitrogen oxides (NO and N2O) were not observed even in the presence of excess oxygen. Preadsorbed atomic oxygen initiates hydrazine oxidation below 140 K indicating that adsorbed hydrazine is quite reactive on a clean rhodium surface. Large initial coverages of coadsorbed hydrazine and atomic oxygen result in the formation of nitrogen and water below 200 K. We suggest that the N-N bond in hydrazine remains intact resulting in direct N2 formation during this low-temperature oxidation process. An analogous direct N2 formation process has been previously reported during hydrazine decomposition on iridium; we have also reported a similar process during hydrazine decomposition on rhodium. For lower hydrazine coverages, two higher temperature water desorption peaks at 270 and 360 K are observed. The water peak at 270 K is limited by reaction of H, O and OH while the water peak at 360 K appears to be limited by surface reaction between coadsorbed oxygen and a NH(y) surface intermediate. With increasing initial oxygen coverages the yield of water increases, while the yield of hydrazine decomposition products (NH3, H-2 and N2H2) decreases. The selectivity of nitrogen formation during hydrazine oxidation below 250 K is controlled by direct reaction processes which leaves the N-N bond in hydrazine intact. In this temperature range, hydrogen-oxygen reactions are the dominant mode of interaction with the adsorbed atomic oxygen. Above 250 K, nitrogen-nitrogen recombination is strongly favored over a broad range of temperatures and coverages. No evidence for a nitrogen-oxygen surface was observed even in the presence of excess adsorbed oxygen clearly suggesting the predominance of N atom recombination over N + 0 atom surface reaction even in the presence of excess adsorbed atomic oxygen.

[发布日期] 1991-11-01