Adsorbed azide anion (N3-) on Pt electrode surface during the electrochemical oxidation of ammonia in alkaline media was evident from surface enhanced Raman spectroscopic studies.1,2 It was also speculated that recombination of NH3 and N2H4,ads (and maybe other N2Hx (x = 2–4) species) lead to the formation of azide anion. The azide anion formation mechanism is still unclear and not studied in detail. In the present work, we examined possible pathways (represented as reactions (1)-(8)) for azide ion formation on Pt(100) surface at moderate surface potentials (from +0.3 V up to +0.6 V vs. RHE) using first-principles density functional theory/molecular dynamics study and a detailed atomistic mechanism of azide anion formation and its consequent evaluation during the electrochemical oxidation of ammonia on Pt(100) electrode in alkaline solution is provided. (1) NH3(aq) + N2Hx(bridge) + (x+3)OH- → N3-(ads) + (x+3)H2O + (x+2)e- , x = 1..4 (2) NH3(aq) + N2Hx(hollow) + (x+3)OH- → N3-(ads) + (x+3)H2O + (x+2)e- , x = 1..2 (3) NH3(aq) + N2(bridge) + 3OH- → N3-(ads) + 3H2O + 2e- (4) NH3(aq) + N2(hollow) + 3OH- → N3-(ads) + 3H2O + 2e- (5) NH3(aq) + N2(top) + 3OH- → N3-(ads) + 3H2O + 2e- (6) N2(top) + N(bridge) + e- → N3-(ads) (7) N2(bridge) + N(bridge) + e- → N3-(ads) (8) N2(desorbed) + N(bridge) + e- → N3-(ads) The studies reveal that reactions (1) - (7) are unfavorable because the energy of the product is higher than the energy of reactants and because of strong repulsion between the reactants. We found that desorbed molecule of N2 (formed via dimerization of bridging nitrogens) strongly interacts with the bottom layer of alkaline solution, may recoils from it and reacts with the sub-monolayer of adsorbed bridging nitrogens to form ontop azide anion, N3-(top) (reaction (8)). The N3-(top) may be transformed to N3-(bridge) or to N2(aq)+N(bridge).

Azide anion formation during electrochemical oxidation of ammonia on Pt(100) electrode: First-principles study

2019 Spring National Meeting

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