Sustainable ammonia synthesis on TiO2-based photo-thermo catalysts

The industrial production of ammonia (NH3) by the Haber-Bosch process operates at high temperature and pressure, and remains highly energy-incentive. Hence, a sustainable strategy integrating sunlight as renewable energy input is a high-prospect alternative. We shows here the ability of Ru/TiO2 and Fe-TiO2 catalysts to benefit from a dual photonic/thermal excitation for boosting the NH3 production rates from molecular N2 and H2 under UV-A light at ambient pressure. The synergistic effect evidenced allowed also to drive the synthesis of NH3 at a lower temperature. The Ru/TiO2 and Fe-TiO2 photo-thermo catalysts were prepared by wet impregnation of a photoactive TiO2 support, but differed in terms of structure and morphology. Ru/TiO2 consisted in Ru nanoparticles (1.5 ± 0.3nm) highly dispersed at the surface the TiO2 support, while Fe-TiO2 are described as a composite material formed by uniformly distributed submicrometric-scale aggregates of Fe and TiO2 nanoparticles. The dual-mode excitation lowered the apparent activation energy from for Ru/TiO2, while no significant change was observed for Fe-TiO2, suggesting different light-induced reaction mechanisms. An optimum Fe content of 10wt.% was observed, with the highest initial NH3 production rate of 93 μmol g-1 h -1 at 350 °C under UV-A light that overcomes that achieved with Ru/TiO2. In addition, the Fe-TiO2 composite catalyst showed a better stability with time on stream than its Ru/TiO2 counterpart that suffered from deactivation.