Photodecomposition of ammonia to dinitrogen and dihydrogen on platinized TiO2 nanoparticules in an aqueous solution

By using suspended platinized titanium dioxide (Pt-TiO2) as a photocatalyst in an NH3 aqueous solution, NH3 was photodecomposed into H2 and N2. The amount of loaded Pt was changed between 0 and 2.0 wt% and the reaction was conducted under irradiation in an Ar atmosphere. In the Pt loading between 0 and 0.5 wt% the H2 and N2 evolution increased linearly with the Pt amount and the H2/N2 molar ratio was about 3:1. The evolution of H2 and N2 reached a maximum at 0.5 wt% Pt, and then decreased probably due to a filter effect by the Pt. The effect of pH on the photodecomposition of NH3 in the presence of Pt-TiO2 under Ar was investigated at pH from 0.68 to 13.7. The evolution of H2 increased steeply at the pH from 9 to 10 showing that the dissociation of NH4+ to free NH3 is important for the photodecomposition (pKa of NH4+/NH3 is 9.24). The time-course of H2 and N2 evolution in a 59 mM NH3 aqueous solution during 53 h irradiation gave the photodecomposition yield of 21.6%. The effect of RuO2 loading as a co-catalyst for the TiO2 or Pt-TiO2 was investigated showing that the RuO2 does not work as a H+ reduction catalyst. The activity of SrTiO3 used instead of TiO2 was only small, and the H2/N2 ratio (0.64) was very small showing that the Pt-SrTiO3 is not a good photocatalyst to decompose NH3. A TiO2/FTO nanoporous film photoanode was used in combination with a Pt counter electrode instead of the Pt-TiO2 photocatalyst, leading also to photodecomposition of NH3 to produce H2 and N2 at 3:1 ratio.