Ammonia has received much attention as a potential energy carrier and as a fuel for vehicles in addition to its use as a fertilizer because it is easily condensed into a liquid. Ammonia is conventionally synthesized by the Haber-Bosch process under high temperature and high pressure condition. Among the energy consumed in ammonia production, the portion consumed by hydrogen production from fossil fuels exceeds 90%. Therefore, the development of a novel synthetic process for ammonia production without hydrogen gas is needed[1]. The generation of ammonia from dinitrogen and water using sunlight is a preferred approach to synthesizing ammonia as an energy carrier. The co-catalyst is an important factor in determining the activity and selectivity of a photocatalytic reaction. We focused on the binding affinity of N and H adatoms on the surface of a co-catalyst. Density functional theory calculations on a broad range of transition-metal surfaces have predicted that the adsorption energy of N is smaller than that of H on the Zr surface, indicating that preferentially adsorbs N adatoms[2]. Here, we report the visible-light responsible ammonia synthesis from dinitrogen via plasmon-induced charge separation by using a SrTiO₃photoelectrode loaded with gold nanoparticles (Au-NPs) and a Zr thin film. We observed the simultaneous stoichiometric production of ammonia and oxygen from dinitrogen and water under visible-light irradiation.

[Experimental]

We designed an SrTiO₃ photoelectrode equipped with Au-NPs as the plasmonic nanostructure and Zr as a co-catalyst. The Au-NPs/niobium-doped strontium titanate (Nb-SrTiO₃)/Zr photoelectrode was fabricated as follows. Au-NPs were fabricated on a 0.05wt% Nb-SrTiO₃ single crystalline substrate using an annealing method. A Zr thin film was then deposited using the electron-beam evaporation method onto the opposite side of the Nb-SrTiO₃ substrate. The average size of the Au-NP and the surface coverage ratio of the Nb-SrTiO₃ substrate by Au-NPs were estimated to be 45 nm and 19.7% from a scanning electron microscope image. The X-ray photoelectron spectrum and X-ray reflection revealed that the surface of the Zr film deposited onto the SrTiO₃ was oxidized by air and that the thicknesses of the zirconium oxide (ZrO_x) and zirconium metal layers were 2.6 nm and 2.2 nm, respectively. The N₂ fixation device comprised sealed reaction cells with two reaction chambers separated by the SrTiO₃ substrate. The surface with Au-NPs was designated as the front of the photoelectrode, which was irradiated with visible light for the oxidation reaction, whereas the thin Zr film was used for NH₃ formation on the rear surface. The anodic chamber was filled with a KOH aqueous solution at pH 13; the cathodic chamber was filled with water-saturated N₂ gas and an HCl aqueous solution. The Au nanostructured Nb-SrTiO₃substrate was irradiated in an area of φ6 mm by a xenon lamp using an arbitrary light intensity and wavelength.

[Results and Discussion]

We attempted plasmon-induced NH₃ synthesis from N₂ and water by using Au-NPs/Nb-SrTiO₃/Zr/ZrO_x. As a result, the amount of formation of both NH₃ and O₂ increased linearly with increasing irradiation time, and the formation ratio between NH₃ and O₂ was almost 4 to 3.  This result indicates that dinitrogen reduction and water oxidation proceed stoichiometrically. NH₃ formation is strongly related to the localized surface plasmon resonance (LSPR) excitation because the value of apparent quantum efficiency of NH₃ formation was highly dependent on the position of the LSPR band. Additionally, NH₃ formation was observed in all visible wavelength regions, indicating that the plasmon-induced NH₃synthesis system can efficiently utilize the sunlight.

We also investigated NH₃ synthesis from isotopic N₂ gas. The photoelectrochemical reaction was promoted by the use of ^15,15N₂ gas instead of ^14,14N₂. The formed NH₃ was collected by distillation of the reaction liquid with KOH solution because the cathodic chamber is acidic, and the reaction product was obtained in the form of ammonium chloride. The collected NH₃ was identified by gas chromatography–mass spectrometry. In the case of the sample obtained from the reaction solution after 46 h of visible light irradiation, a peak derived from ¹⁵NH₃ (m/z = 18) was clearly observed at the same retention time as a standard ¹⁴NH₃ aqueous solution (m/z = 17). These results provide direct evidence that N₂ gas and water were converted into NH₃under visible-light irradiation.

[References]

[1] T. Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed. 2014, 53, 9802-9805

[2] E. Skulason, T. Bligaard, S. Gudmundsdottir, F. Studt, J. Rossmeisl, F. Abild-Pedersen, T. Vegge, H. Jonsson, J. K. Norskov, Phys. Chem. Chem. Phys. 2012, 14, 1235-1245