Photocatalytic reduction of molecular nitrogen to ammonia catalysed by dinitrogen complexes of ruthenium aminipolycarboxylates

Abstract

The reduction of molecular nitrogen to ammonia in aqueous solution under ambient conditions (30°C, 1 atm N₂) is reported in the presence of the illuminated (λ=505 nm) particulate system Pt---CdS---RuO₂. The reaction is catalysed by the ruthenium(II) dinitrogen complexes [Ru(EDTAN)N₂]₂₋ (1) (EDTA), ethylenediaminetetraacetato ion), [Ru(HEDTA)N₂]⁻ (2) (HEDTA, N-hydroxyethylethylenediamine-N,N'N'-triacetato ion), [Ru(PDTA)N₂]²⁻ (3) (PDTA, 1,3-propylenediaminetetraacetato ion), [Ru(CDTA)N₂]²⁻ (4) (CDTA, cylohexanediaminetetraacetato ion), [Ru(H₂DTPA)N₂]⁻ (5) (H₂DTPA, dianion of diethylenetriaminepentaacetic acid) and [Ru(HIMDAN)N₂] (6) (HIMDA, N-hydroxyethyliminodiacetato ion). The photolysed reaction mixture also contains traces of hydrazine. The catalytic activities of these complexes decreases in the order 1 > 2 > 5 > 4 > 3 > 6. Photodecomposition of H₂O takes place at the illuminated Pt---CdS---RuO₂ system. The rate of nitrogen reduction is first order with respect to the concentration of the ruthenium dinitrogen complexes 1-6. The pseudo-first-order rate constant decreases in the order 1 > 2 > 5 > 4 > 3 > 6 and correlate with the increasingly negative E^½ values for the Ru^II/Ru^I couples of these complexes. This trend is further reflected by the increasing stability of the complexes. Isotopic studies with ¹⁵N₂ gas confirm that molecular nitrogen is the source for NH₃.