MODEL CATALYTIC-OXIDATION REACTIONS - OXYGEN WITH H-2, NH3, AND N2H4 ON RH(111)

To examine the role of oxygen in fuel decomposition during catalytic combustion reactions in the absence of carbon, the coadsorption and reaction of O-2 with H-2, NH3, and N2H4 on Rh(111) have been examined using TPD and AES. In particular, we consider whether the rate limiting step involves fuel dissociation or hydrogen abstraction by O adatoms and compare behavior on Rh with that on Pt(111). The adsorption of H-2 at 90 K on Rh(111) precovered with atomic O produces H2O desorption in a broad peak centered at similar to 330 K showing that H2O formation is limited by the reaction H + O --> OH which occurs with an activation energy of similar to 20 kcal/mol. This is significantly greater than the activation energies previously reported for Pt(111). While NH3 does not decompose significantly when adsorbed at 300 K on Rh(111), the adsorption of NH3 at 90 K on Rh(111) precovered with atomic O produces 80% NH3 decomposition to N-2 H2O and minor amounts of H-2. In this reaction H2O desorption occurs in peaks at similar to 380 and similar to 215 K. The similar to 380 K peak is formed by a hydrogen abstraction reaction occurring in the reactive decomposition of an NH3.3O complex. The similar to 215 K peak is due to the decomposition of an OH.(H2O)(x) complex formed in a second hydrogen abstraction reaction which probably involves reaction with NH2. H-2 desorption occurs at similar to 300 K due to the recombination of H adatoms and also at similar to 430 K due to the decomposition of NH. N-2 desorbs at similar to 600 K in a single broad peak from the recombination of N adatoms. In the reaction between coadsorbed N2H4 and O adatoms, both H2O and H-2 desorption are similar to that in the NH3 + O(a) system. Additionally, NH3 desorption caused by the scavenging of H adatoms by NH2 occurs between similar to 200 and similar to 425 K, and N-2 desorption occurs at similar to 200 K due to the decomposition of N2Hy and at similar to 600 K due to the recombination of N adatoms.