## 3 Results and Discussion
### XPS Results  
According to the above experimental data, the optimum denitration rates of RHA, BC, and SC were 850 \({}^{\circ}\)C, 900 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C, respectively. Under the protection of N\({}_{2}\), the three materials were held at their optimum denitration temperature for 10 min, and then we tested the type and relative content of chemical bonds on the surface of their samples by XPS. XPS allowed the quantitative analysis at the micrometer-scale of the surface C1s of the samples, which contained both organic and inorganic C element; in contrast, EA tested the organic C element of the whole sample. Therefore, the rests of the two analyses were very different, as shown in Tables 3 and 4. The chemical bond species of the surface C1s of the three materials as well as their relative contents are shown in Figure 6 and Table 4. The relative content was obtained by dividing the area of each single peak area by the total peak area.  
The content of functional groups of C is determined by the formula  
\[C_{i}=(C_{o}\div C1s)\times 100\%\]  
in which \(C_{o}\) is the original relative content determined by the XPS test, and C1s is total carbon content.  
Figure 6 and Table 4 show that the three biochars contain multiple carbon-containing functional groups. The relative content of graphite carbon C-C bond (284.5-284.9 eV) in RHA was larger with respect to the content in the other two biochars, which may partly explain why the denitration rate of RHA decreased slowly with time at different reaction temperatures. However, due to their high contents of (NCO)\({}_{\text{x}}\) bonds (292.1-297.2 eV) which can catalyze the reduction reaction of C-NO and increase the denitration rate1[17; 18], also BC and SC showed a high denitration rate, although less C-C bonds were observed in BC and SC than in RHA. According to published studies [12; 17; 18; 19], COOH (289.2-289.8 eV), an acid group in the reaction process, will inhibit the reduction reaction. For the relative content of C-O bonds (286.0-286.4 eV), BC has the highest, followed by RHA and lastly by SC. This order is consistent with the optimal denitration rates of the three biochars. According to the experimental study of [PERSON] and [PERSON] [17; 18; 19], C-O bonds are reactive sites and play a very important role in the reaction. On the one hand, the binding energy of the C-O bond is low, so it can easily break and participate in the reaction; on the other hand, the C-O bond can play a reducing role in the reaction process. There are other functional groups in SC, as shown by XPS of C1s, such as the C-N bond  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample** & **C-C** & **C-N** & **C-O** & **C=O** & **COOH** & **\(\pi\)-\(\pi\)** & **(NCO)\({}_{1}\)** & **(NCO)\({}_{2}\)** & **C1s** \\ \hline BC & 63.79 & - & 7.69 & - & 5.84 & - & 3.06 & 0.79 & 81.17 \\ RHA & 68.85 & - & 5.78 & - & 4.96 & - & 0.60 & - & 81.13 \\ SC & 53.80 & 7.12 & 3.29 & 3.38 & 4.42 & 2.28 & 6.88 & - & 81.19 \\ \hline \hline \end{tabular}
\end{table}
Table 4: C1s fitting peaks of three biochars.  
Figure 6: XPS fitting peaks of BC, RHA, and SC.  
(285.0-286.1 eV), C=O bond (287.6-288.0 eV), and \(\pi\)-\(\pi\) bond (291.3 eV) [19; 23]. The binding energy of the C-N bond is between those of the C-C and C-O bonds, so it could also be involved in the reduction of C-NO.  
These data indicate that the C-C bond, (NCO)x bond, and C-O bond promote or catalyze the reduction reaction of C-NO, while the COOH bond has an inhibitory effect. The mechanism is presented in Figure 7. At high temperature in the presence of active gas, the surface of biochar contains a large number of chemical bonds that participate in the process of C-NO reduction, promoting or inhibiting the reduction of nitrogen oxides.