Ammonia (NH₃) is an important chemical raw material, playing an integral role in the agricultural, industrial and pharmaceutical fields. Industrially, NH₃ is still produced using the conventional Haber-Bosch process (developed at the start of the 20^th century): N₂ is reduced to NH₃ by H₂ using a metal-based catalyst, temperatures between 400 and 500°C and pressures between 150-250 atm. These harsh conditions make the process energetically expensive and environmentally polluting: H₂ mainly comes from the reforming of hydrocarbons (usually supplied through natural gas), and it produces large emissions of CO₂. [1,2]

Considering the global energy crisis and the devastating effect large emissions of greenhouse gasses have on the environment, it is of utmost importance to replace this production method with one that is greener and more sustainable, in both materials and conditions. Among many catalytic methods, photocatalytic nitrogen fixation (PNF) has been considered one of the best alternative candidates, even if it is less efficient than other photocatalytic process, because it’s powered by solar energy and both nitrogen fixation reactions (NRR and NOR) are carried out in mild conditions. [1,2]

In the search of novel and efficient photocatalysts, we investigated the potential use of heterojunctions based on graphitic carbon nitride nanosheets (gC₃N₄) coupled with BiOX (X=Br and I). By mapping the photocatalytic behavior of the heterojunction as a function of the weight relation between the two compounds, we found that the composite with a ratio equal to gCN(90)-BiOBr(10) produces the higher yield of NH₃ (~19 μmol/g/h).

We decided to further investigate these types of heterojunctions by keeping similar ratios between the two semiconductors, but changing the components: in tandem, we mapped the behavior of the same heterojunctions, swapping gCN with rice-husk Biochar (rBC) to explore sustainable materials. In this case too, we found that the higher yield (~10 μmol/g/h) is produced by rBC(90)-BiOBr(10) and that the heterojunction maintains its structure after it has been used in the PNF reaction. After mapping rBC-BiOBr, we decided to swap rBC with bran-BC (both non-activated and activated after pyrolysis).

We further investigated the potential of this heterojuction by playing on the band gap of the oxyhalide and to probe its effect on the photocatalytic behavior of the heterojunction. For this purpose we prepared BiOBr_1-xI_x samples and we correlated the composition to the photogenerated ammonia yields.