A Combined First-Principles Molecular Dynamics/Density-Functional Theory Study of Ammonia Electrooxidation on Pt(1 0 0) Electrode

The electrocatalytic oxidation of ammonia on Pt(1 0 0) electrode was studied by first-principles direct molecular dynamics and density functional theory. The mechanism of ammonia electrooxidation on Pt(1 0 0) depends strongly on the surface potentials. The modeling study supports the mechanism posited by Oswin and Salomon for moderate potentials (≥ +0.5 V vs. RHE), and the Gerischer and Maurer's mechanism for lower potentials (< +0.5 V vs. RHE).

At moderate potentials (≥ +0.5 V vs. RHE), the electrooxidation consists of ammonia adsorbing to the surface, and subsequent oxidations of NH_3(top), and the intermediates NH_2(bridge) and NH_(bridge), forming adsorbed bridging nitrogen.  Subsequent steps are high submonolayer coverage of N_(bridge), followed by spontaneous N_(bridge) dimerization.  On highly covered Pt(1 0 0) surface, the bridging nitrogen atoms spontaneously dimerize and desorb into solution.  The rate-determining step at moderate potentials is NH_2(bridge) oxidation.  Dimerization of NH_2(top)forming hydrazine would be a rare event. 

At low potential region (< +0.5 V vs. RHE), oxidation of ammonia becomes a very sluggish process.  Two competing pathways for the lower potentials are 1) NH_2(top) dimerization forming hydrazine and subsequent oxidation to form molecular nitrogen, and 2) oxidation of NH_2(bridge), forming bridging nitrogen N_(bridge), followed by N_(bridge) dimerization. The rate determing steps at the lower potentials are NH_2(top) dimerization and NH_2(bridge) oxidation.

Catalytic inactivity of Pt(1 1 1) and Pt(1 1 0) surfaces is attributed to poisoning of the surfaces by strongly adsorbed N_(hollow).  On both Pt(1 1 1) and Pt(1 1 0) surfaces, nitrogen atoms are strongly bound at hollow sites and dimerization of N_(hollow) would not occur because of high activation barrier.

Acknowledgements

           The authors gratefully acknowledge financial support from the National Science Foundation.