The selectivity of hydrazine oxidation on a polycrystalline rhodium surface

Abstract

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, H2 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 NHy surface intermediate. With increasing initial oxygen coverages the yield of water increases, while the yield of hydrazine decomposition products (NH3, H2 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 + O atom surface reaction even in the presence of excess adsorbed atomic oxygen.