Catalytic mechanism of nitrogen-doped biochar under different pyrolysis temperatures: The crucial roles of nitrogen incorporation and carbon configuration

Highlights

• Pyrolysis temperature regulated carbon configuration and nitrogen speciation.

• The role of carbon configuration and nitrogen speciation in catalysis was unveiled.

• The PMS activation pathway of NBCs converts from ¹O₂ to electron transfer.

• Pyridinic N and oxygen groups enabled the generation of singlet oxygen (¹O₂).

• Graphitic N and increased graphitization degree induce electron transfer pathway.

Abstract

To scrutinize the crucial role of carbon configuration and nitrogen speciation in peroxymonsulfate (PMS) activation, nitrogen-doped biochars (NBCs) were prepared at different pyrolysis temperatures (700, 800 and 900 °C) and named NBC700, NBC800 and NBC900, respectively. Nitrogen doping introduced many nitrogen-containing groups into NBCs and the carbon configuration and nitrogen speciation of NBCs were regularly changed by the pyrolysis temperature. Compared to the phenol (PN) removal in the pristine biochar (BC)/PMS system that mainly depended on adsorption, NBCs showed excellent PMS activation activity for efficient PN degradation and the PMS activation activity was highly dependent on the carbon configuration and nitrogen speciation of NBCs. Furthermore, the PMS activation pathways of NBCs were unveiled to convert ¹O₂ to electron transfer with increasing pyrolysis temperature, which was ascribed to the variation of active sites on NBCs caused by the regular changes in carbon configuration and nitrogen speciation. Pyridinic N and oxygen groups (CO, CO and O-C=O) were proposed as potential active sites on NBC700 and NBC800 for ¹O₂ generation via PMS activation. Differently, the highly sp²-hybridized carbon skeleton and graphitic N of NBC900 played an important role in the electron transfer pathway by acting as a carbon bridge to accelerate electron transfer from PN to PMS. This study provides new insight into the effects of carbon configuration and nitrogen speciation on PMS activation mechanism of NBCs and identifies opportunities for the subsequent catalyst design in a specific degradation pathway.

Introduction

The safe disposal and utilization of biomass residues have been widely studied due to their huge annual production (J. Chen et al., 2019; X. Zhang et al., 2020). Among them, the conversion of crop residues to biochar is an appealing strategy, which not only reduces environmental pollution but also achieves waste reclamation. Biochar has been widely applied in soil amendment and contaminants adsorption for containing porous structure and rich surface functional groups (L. Wang et al., 2020; Yang et al., 2021). Recently, biochar has attracted considerable attention as an emerging and eco-friendly catalyst in advanced oxidation processes (AOPs) (Duan et al., 2018a; Yu et al., 2020a). Among various AOPs, persulfate-based AOPs via peroxymonosulfate (PMS) have wider application prospect owing to the generation of powerful reactive oxygen species (ROS) with a higher redox potential (2.5–3.1 V) than other AOPs (Duan et al., 2018b).

Various functional structures of biochar, including oxygen-containing groups, defects and persistent free radicals have been reported to serve as active centers for PMS activation (Fang et al., 2014; Meng et al., 2020; Zhou et al., 2020). However, the catalytic efficiency of pristine biochar was insufficient, and the PMS activation was mainly mediated by radical pathway (Kemmou et al., 2018; Liang et al., 2020; R. Zhang et al., 2020), which was susceptible to interference by environmental substances and self-quenching in the reaction process (Oh et al., 2016). In contrast, non-radical pathways, including ¹O₂ and electron transfer showed superior anti-interference to complex environmental substances (Hu et al., 2021; Ye et al., 2020). Therefore, it is necessary to develop methods to improve the catalytic efficiency and increase the contribution of nonradical pathways in PMS activation for subsequent biochar-based catalyst design.

Heteroatom doping, especially nitrogen doping has been proposed as a reliable strategy for improving the catalytic efficiency of biochar and inducing nonradical reactions, which could break the inertia of carbon plane and introduce more active sites for catalytic oxidation (Ding et al., 2020; Ren et al., 2020). However, the correlation between different nitrogen species (i.e., pyridinic N, pyrrolic N and graphitic N) and different nonradical pathways (¹O₂ or electron transfer) remains unclear. Some studies reported that the edge nitrogen configuration (pyridinic N and pyrrolic N) was responsible for the electron transfer pathway, which could change the charge distribution of pristine biochar, further facilitating electron transfer from pollutants to surface-bonding complexes (Wang et al., 2019). Another study reported that the increased positive charge of carbon atoms caused by graphitic N is beneficial for forming metastable PMS in the electron transfer pathway and generating ¹O₂ through decomposition of absorbed PMS (Qi et al., 2020). Therefore, the correlation between nitrogen speciation and the nonradical activation pathway requires further exploration.

In addition, carbon configuration including graphitization, functional groups and defects of biochar is vital for PMS activation. Previous studies have reported that biochar with high graphitization (sp²-hybridized carbon) and abundant functional groups (such as CO) were favorable for PMS activation, whereby the graphitic structure could promote electron directional transfer, and CO with lone-pair electrons was considered as an active site for PMS activation via nucleophilic reaction. (Liu et al., 2021; Ye et al., 2020; Yu et al., 2020b). The carbon configuration of biochar could be adjusted by changing the pyrolysis temperature during biochar preparation, and previous studies found that biochar pyrolyzed at high temperature (>700 °C) has a graphitic carbon framework, and that a higher degree of graphitization would be induced at the higher pyrolysis temperatures (Zhu et al., 2018). However, the catalytic performance would be reduced when a remarkable loss of total N was caused by the exorbitant pyrolysis temperature. Moreover, the nitrogen speciation of nitrogen-doped biochar (NBC) was also significantly affected by pyrolysis temperature that the transformation of nitrogen speciation generally occurs from unstable N speciation (pyridinic N and pyrrolic N) to stable N speciation (graphitic N) (Chen et al., 2017; Leng et al., 2020). However, the specific influence degree and manner of both carbon configuration and nitrogen speciation in NBC on PMS activation has not been ascertained, which is important for the subsequent catalyst design of NBC.

In this study, various NBCs were prepared by pyrolyzing rice straw and urea at different temperature (700, 800 and 900 °C). The effects of the pyrolysis temperature on the intrinsic properties (degree of graphitization, nitrogen speciation, etc.) of NBCs were characterized, and the catalytic performance was evaluated based on the removal efficiency of phenol (PN), a typical refractory pollutant. Moreover, the PMS activation pathway was determined by a series of experiments, including quenching, electron paramagnetic resonance (EPR) and electrochemical experiments. Furthermore, the PMS activation mechanism was concluded by analyzing the XPS spectra of the catalysts before and after catalysis. Overall, this work was aimed at exploring the specific effects of carbon configuration and nitrogen speciation of NBCs on regulating PMS activation mechanism, as well as on analyzing the active sites of the prepared NBCs in PMS activation and providing more information for efficient biochar-based catalyst design.