Mechanism of electrochemical oxidation of ammonia

The electrochemical oxidation of ammonia has been studied in the most detail in alkaline solution at platinized platinum. Almost all work supports the essence of a mechanism first proposed by Gerischer and Mauerer (1970) 19], in which elemental nitrogen is formed at mildly positive potentials with near quantitative current efficiency through dimerization of partly dehydrogenated ammonia molecules NH_x(ads). The major intermediate, NH₂(ads), is formed at Pt(1 0 0) domains on the metal surface, where it dimerizes to hydrazine, and rapidly oxidizes to N₂. At somewhat more positive potentials, the formation of adsorbed nitrogen atoms poisons the anode, and nitrogen evolution ceases. At potentials where water is oxidized, the Pt anode is modified by a surface oxide; under these conditions, nitrogen evolution is accompanied by nitrogen oxides and oxyanions. Similar mechanisms are most probably followed on other noble metals and their alloys, although there is less experimental information. In the past decade there has been preliminary study of other anode materials, such as Ni/Ni(OH)₂, Ti/IrO₂, and boron-doped diamond, with a view to finding inexpensive and long-lasting anodes for ammonia oxidation, but so far, little is known about the mechanism of oxidation at these materials.