Theoretical insight into NO formation and reduction at biochar N-sites: Influence of different oxygen-containing functional groups

Biochar is a cost-efficient and promising porous carbonaceous material for the efficient removal of flue gas NOx. The N-site located at the edge of biochar can be a crucial gateway for NO reduction to N2 or for NO formation by oxidation, which can be significantly affected by the oxygen-containing functional groups (OFGs) on the biochar edge. However, the roles of different OFGs in N-site-involved reactions remain elusive. Based on the biochar characterization results, reasonable theoretical models of biochar with different N-sites and OFGs were constructed. Subsequently, the effects of distinct OFGs on the interaction between NO and N-sites as well as the NO liberation from N-sites were elucidated, by employing density functional theory (DFT) and electronic structure analysis. The results reveal that all OFGs, encompassing -CHO, -COOH, and -OH, manifest their roles through the direct inherent electrostatic properties of O/H in OFGs (O*/H*) or altering the reactivity of edge atoms indirectly. For pyridinic nitrogen (N-6) at the zigzag edge, unsaturated (-CHO, -COOH)/saturated (-OH) OFGs inhibit/enhance NO reduction with N-6 by electrostatic properties of O*/H*, and all OFGs inhibit NO generation from N-6. Besides, all OFGs enhance the NO reduction with pyrrolic nitrogen (N-5) at the armchair edge while inhibiting the release of NO, by influencing the edge activity. The present study gives a mechanism insight into the roles of distinct OFGs in reactions involving biochar N-sites.