Performance Of Nitrate Reduction Over Hexagonal Boron Nitride-based Photocatalysts

Photocatalytic reduction technology is always regarded as a strategy with great potential for treating nitrate in water because of its advantages of high activity and simple operation.However,this practice is still challenged,as the fact that the widely used metal-based catalysts inevitably lead to secondary pollution during photocatalytic nitrate reduction as the result of metal ion dissolution.Therefore,it seems urgent to develop a non-metal catalyst with high activity for photocatalytic denitrification.Hexagonal boron nitride（h-BN）,with the conduction band potential of-2.54 V,has the potential to achieve nitrate reduction over photoelectron(e_cb^-).However,to the best of our knowledge,there is no report on the nitrate reduction treatment using raw and modified h-BN.In this study,the feasibility of h-BN photocatalytic reduction of nitrate in water with formic acid（FA）as a hole scavenger was well evaluated.Moreover,to enhance the efficiency of photon-generated carriers separation,increase the number of nitrate adsorption sites and achieve both of the above two goals at the same time,copper-modified h-BN,thin h-BN and co-modified h-BN of-OH and-NH₂ groups were prepared,respectively.Moreover,the nitrate reduction efficiencies of these catalysts were investigated.Compared with a metal catalyst Ti O₂ and a non-metal catalyst g-C₃N₄,h-BN showed the higher reduction efficiency with NO₃^-conversion of 97.16%and N₂ selectivity of 95.42%.The initial p H significantly affected the NO₃^-reduction process,and the presence of anions decreased the NO₃^-conversion efficiency.The process of NO₃^-removal over h-BN fitted well with the Extended-Langmuir-Hinshelwood model.Exposed B atoms on the h-BN edges behaved as the adsorption sites of nitrogen oxide anions.The reductive species were identified by the analysis of the stoichiometric relationship between the amounts of consumed FA and removed NO₃^-,and quenching experiments.It was found that e_cb^- also took part in the NO₃^-reduction in addition to CO₂^·-radicals,and CO₂^·-radicals were mainly responsible for the conversion of NO₂^-to N₂.A wet chemical reduction method was adopted to prepare the modified h-BN catalysts with different metals（Ag,Cu and Ag-Cu,1wt%）.Among these catalysts,1%Cu/h-BN and0.5%Ag-0.5%Cu/h-BN exhibited much higher photocatalytic NO₃^-reduction efficiency.From an economic viewpoint,Cu was considered as an ideal metal for modifying h-BN.When the modification amount of Cu was 0.5%,h-BN obtained the highest catalytic activity with NO₃^-conversion of 98.15%and N₂ selectivity of 98.09%.Compared with h-BN,0.5%Cu/h-BN showed stronger light absorption performance,more negative CB position and more efficient photon-generated electron-hole pairs separation.This indicated that more high-reductive e_cb^-were generated by Cu modification on the surface of h-BN.Moreover,Cu⁺and Cu²⁺on the h-BN surface could act as adsorption sites for enhancing the adsorption of nitrogen oxide anions.The result of quenching experiments suggested that e_cb^- played a more pronounced role in the conversion of NO₂^-to N₂ in photocatalytic NO₃^-reduction over 0.5%Cu/h-BN.The thin layered h-BN was prepared by sonication with water,nitric acid solution and concentrated nitric acid,respectively.The layer number of h-BN was reduced significantly after sonication with each solvent,indicating the successful preparation of thin layered h-BN.In addition,-OH groups were formed on the h-BN surface after sonication.The highest catalytic activity with NO₃^-conversion of 98.71%and N₂ selectivity of 98.96%was obtained using h-BN-N that was prepared by sonication with concentrated nitric acid.The band structure and PL analysis showed that compared with h-BN,h-BN-N had higher photo-generated carriers separation efficiency and stronger reduction capability of e_cb^-.Moreover,the increased B atoms exposure and generation of-OH groups provided more adsorption sites for nitrogen oxide anions.The result of quenching experiments suggested that e_cb^- was also involved in the NO₃^-reduction in addition to CO₂^·-radicals.Moreover,e_cb^- was more conducive to the reduction of NO₂^-to N₂ compared with CO₂^·-radicals.Co-modified h-BN with-OH and-NH₂ groups was prepared by a dry ball milling method,and the ball milling time was optimized.The modification of-OH and-NH₂ groups on the surface of h-BN was confirmed by XPS and FT-IR.When the ball milling time was 4 h,the catalyst（h-BN-4）achieved the best photocatalytic performance of NO₃^-reduction,which NO₃^-conversion of 98.12%and N₂ selectivity of 97.31%were obtained without addition of hole scavengers.Results revealed that the modification of-NH₂ groups more efficiently separated electron-hole pairs to generate more reductive e_cb^- for NO₃^-reduction.Additionally,plenty of active sites,including B-terminated edges,-OH groups and-NH₃⁺,favored to bind nitrogen oxide anions onto the catalyst surface.Consequently,the e_cb^- mediated reduction process was promoted and the satisfactory removal of NO₃^-without additional hole scavengers was achieved,which would help simplify the reaction system and reduce the treatment cost.