Electrocatalytic NH3 Oxidation

A primary and important conclusion from this experiment is that NH3 is converted to N2 at potentials lower than where surface oxygen species are generated due to the anodic oxidation of HgO. This implies that the initial step of NH3 activation is 

The activation of NH3 by the reaction with a co-adsorbed oxygen atom or hydroxyl group that occurs over the metal in the gas-phase reaction is now replaced by the acceptance of protons by H2O molecules from solution (Fig. 6.31). 

The protons are stabilized here by solvation in solution. The key initial elementary step is heterolytic activation of NH3 to give surface NH2 and an aqueous H3O+, a reaction that reminds us of the initial activation of NH3 adsorbed on Pt + by the basic 

This reaction towards N2 is of course assisted by the H2O phase that can accept the protons that are released in the N2 formation step. The rate of N2 formation is 0.01 N2 mol/s/Ptatom- Adsorbed nitrogen atoms are found to lead to catalyst poisoning. They are formed in the following reaction steps  

The recombination of nitrogen atoms does not occur under the conditions of the electrochemical experiment. At higher potentials other reactions tend to take place. Oxygen develops at the electrode and ammonia is oxidized to N2O, NO, NO2 and NO3. The selection of path (6.400 versus (6.420 correlates with the heat of adsorption of Nads. The governing mechanism over Pd, Rh, Ir and Ru metal surfaces is similar to that which is seen on Pt. The steps outlined in (6.40) do not appear to occur over Pd, Rh or Ru. Reaction (6.42) is much faster than reaction (6.40) and therefore tends to dominate over these metals. These catalysts appear to be selective catalysts for the oxidation of NH3 to the oxidized products. 

---

Mechanisms for Aqueous Phase Heterogeneous Catalysis and Electrocatalysis 303 6.4.2 Electrocatalytic NH3 Oxidation... 

---