Photocatalysis of the reaction between ammonia and nitric oxide on TiO2 surfaces

The photocatalytic reaction between ammonia and nitric oxide on TiO₂ wafers under near ultraviolet illumination has been investigated. The reaction produces nitrogen and nitrous oxide with a ratio slightly above two. When ¹⁵NH₃ and ¹⁴NO are used, ¹⁴N¹⁵N and ¹⁴N¹⁴NO are formed almost exclusively. Comparative measurements have been made for the related photocatalytic reaction between ammonia and oxygen, and for nitric oxide photodecomposition. The rate of the latter is approximately one-sixth that of the ammonia plus nitric oxide reaction. It yields nitrogen and nitrous oxide with a ratio less than 0.5 and is accompanied by some deposition of nitrates on the TiO₂ wafer. The ammonia plus oxygen reaction is of intermediate rate and is also accompanied by nitrate formation. The nitrogen/nitrous oxide ratio is approximately four when using 10 Torr of each reactant. Both reactions involving ammonia exhibit a small deuterium kinetic isotope effect, the rate ratio for NH₃ versus ND₃ being about 1.3. Nitrous oxide production is zero order in ammonia and shows a Langmuir-like dependence on oxidant pressure. Both oxidation reactions exhibit similarly curved Arrhenius plots with low apparent activation energies. The dependencies on light intensity are nonlinear, with an apparent order of approximately 0.6 in both cases. A model which can explain the overall characteristics of the three reactions is presented.