CONTROL OF A BIPHASIC SURFACE-REACTION BY OXYGEN COVERAGE - THE CATALYTIC-OXIDATION OF AMMONIA OVER PT(100)

The ammonia oxidation reaction on Pt{100} has been investigated over t he temperature range 300-800 K, using molecular beams under UHV condit ions. The reaction is biphasic, with N-2 being the major product below 600 K and NO being the major product above 600 K, It is found that pr oduct selectivity can be controlled by varying the beam composition as well as by varying the surface temperature. The efficiency of the rea ction to NO can be significantly increased by preadsorption of oxygen on the crystal. Coadsorption and isothermal experiments translate the beam composition dependence into a surface oxygen coverage dependence, with high oxygen coverages resulting in the suppression of N-2 produc tion. N-2 is believed to be produced mainly from the dissociation of N O produced by oxidation of adsorbed NH3. The observed oxygen coverage dependence of product formation is explained by a sharp fall in the he at of adsorption of the dissociated N(a) and O(a) with increasing O(a) coverage. At high surface oxygen coverages the suppression of N-2 pro duction arises from the resulting inhibition of NO dissociation. The o bserved surface temperature dependence of product formation is explain ed by competition between NO desorption and NO dissociation.