Nitrogen doping turns carbonaceous materials into fast-reacting catalysts for reductive dechlorination

Nitrogen (N) doping of biomass prior pyrolysis has been identified as an effective approach for enhancing biochar catalytic reactivity. However, high-temperature pyrolysis of N-rich biomass may produce N-devoid biochars with high reactivity, calling for attention to the true causes of the reactivity increases and the role of nitrogen. In this study, N-doped wheat straw biochar (N-BC) materials were produced using urea as N dopant and different pyrolysis conditions, and their catalytic reactivity assessed for the reduction of trichloroethylene (TCE) by green rust (GR), a layered Fe(II)Fe(III) hydroxide. Overall, the extent and rate of TCE reduction increased from ∼8 to ∼90 % and 0.064 to 0.299 h^-1, respectively, with increasing urea dosage from 0 to 25 wt%. A similar increase in catalytic activity was also seen with an increase in pyrolysis temperature during N-doping, from 600 to 800 or 1000 °C, while increasing pyrolysis duration had a lower impact. Principal component analysis and Pearson correlation analysis demonstrated some correlation between TCE catalytic reactivity and structural properties of N-BC materials. However, little correlation was seen between catalytic reactivity, and N content and N surface functional groups of N-BC materials; even though N surface functional groups are often described as the key redox active sites in these dechlorination reactions, and also the reason why N-doping is applied. For comparison, N-doping and TCE catalytic reactivity with GR was also performed with a highly conductive graphene (GP) substrate. N-doping of GP led to a similar increase in the rate and extent of TCE as was observed for N-BC, demonstrating the importance of N-doping to create catalytically active sites in these carbonaceous materials. Surprisingly, highly reactive N-GP materials produced at 800 and 1000 °C exhibited no N surface functional groups but instead, N-doping created structural defect sites. Overall, this study demonstrates that N-doping of the bio-substrate may increase catalysis of reductive dechlorination by a factor of ∼5 times but this is not due to the formation of catalytically active N-functional groups, rather we attribute it to changes of the carbonaceous material structure. With this understanding we can then embark on the production of catalytic active biochar materials also from other substrates than wheat straw, forming the basis for optimizing other reductive contaminant degradation systems.