*2.3. H2O<sup>2</sup> Production*

Figure 8a shows the concentration of H2O<sup>2</sup> production from NH2OH oxidation by O<sup>2</sup> on the ACF catalysts. For the ACF-0 catalyst without surface oxidation, the cumulative concentration of H2O<sup>2</sup> was very low and cannot be detected after reacting for 300 min. For the ACF-Rw catalyst, the concentration of H2O<sup>2</sup> increased to 55.9 mmol/L at 420 min and then increased slightly. Similar trends plots were observed on the ACF-R1, ACF-R2 and ACF-R4 catalysts, on which the H2O<sup>2</sup> concentration increased gradually with the increase of reaction time. When the reaction was conducted for 660 min, the concentration of H2O<sup>2</sup> approached 88.6 mmol/L and 112 mmol/L on the ACF-R1 and ACF-R4 catalyst, respectively. With increasing the H2SO4/HNO<sup>3</sup> (*v*/*v*) from 1 to 2, the maximum concentration of H2O<sup>2</sup> reached 123 mmol/L on the ACF-R2 catalyst, which clearly demonstrates more reactive species were generated on the ACF-R2 surface by an appropriately higher content of H2SO<sup>4</sup> in mixed acid. In order to explore the stability of ACF catalysts, the recycling tests of ACF-R2 were performed as shown in Figure 8b. After three cycles, there was almost

(123 mmol/L) after reacting for 660 min.

no decrease in the activity of the reused ACF-R2 catalyst with the yield of H2O<sup>2</sup> about 49% (123 mmol/L) after reacting for 660 min. of ACF-R2 were performed as shown in Figure 8b. After three cycles, there was almost no decrease in the activity of the reused ACF-R2 catalyst with the yield of H2O2 about 49% (123 mmol/L) after reacting for 660 min. of ACF-R2 were performed as shown in Figure 8b. After three cycles, there was almost no decrease in the activity of the reused ACF-R2 catalyst with the yield of H2O2 about 49%

**Figure 8.** (**a**) The concentration of H2O2 formation on the ACF catalysts, (**b**) the yield of H2O2 for the recycling tests of the ACF-R2 catalyst (pH = 8.6, temp. = 25 °C, the error bar calculated by STDEV method). **Figure 8.** (**a**) The concentration of H2O<sup>2</sup> formation on the ACF catalysts, (**b**) the yield of H2O<sup>2</sup> for the recycling tests of the ACF-R2 catalyst (pH = 8.6, temp. = 25 ◦C, the error bar calculated by STDEV method). **Figure 8.** (**a**) The concentration of H2O2 formation on the ACF catalysts, (**b**) the yield of H2O2 for the recycling tests of the ACF-R2 catalyst (pH = 8.6, temp. = 25 °C, the error bar calculated by STDEV method).

The selectivity of H2O2 along with the NH2OH conversion on the ACF catalysts at the reaction time of 180 min was shown in Figure 9a. The selectivity of H2O2 was only 46.0% on the ACF-Rw catalyst although the higher conversion of NH2OH (30%) was observed on it, which was possibly induced by the higher surface area and more carbonyl-quinone groups generating through the surface modification. In view of similar conversion toward NH2OH (~22%) consumption, the selectivity of H2O2 was 73.6% on the ACF-R1 catalyst prepared by an equal volume of H2SO4 and HNO3. Whereas the selectivity of H2O2 was greatly enhanced to 99.3% on the ACF-R2 catalyst obtained by further increasing the content of H2SO4. However, the selectivity of H2O2 decreased to 87.6% on the ACF-R4 catalyst by the increase of the H2SO4/HNO3 (*v*/*v*) from 2 to 4. Thus, the formation of more reactive nitrogen and oxygen containing groups on the ACF catalysts greatly enhanced the selectivity toward H2O2 formation. The selectivity of H2O<sup>2</sup> along with the NH2OH conversion on the ACF catalysts at the reaction time of 180 min was shown in Figure 9a. The selectivity of H2O<sup>2</sup> was only 46.0%on the ACF-Rw catalyst although the higher conversion of NH2OH (30%) was observed on it, which was possibly induced by the higher surface area and more carbonyl-quinone groups generating through the surface modification. In view of similar conversion toward NH2OH (~22%) consumption, the selectivity of H2O<sup>2</sup> was 73.6% on the ACF-R1 catalyst prepared by an equal volume of H2SO<sup>4</sup> and HNO3. Whereas the selectivity of H2O<sup>2</sup> was greatly enhanced to 99.3% on the ACF-R2 catalyst obtained by further increasing the content of H2SO4. However, the selectivity of H2O<sup>2</sup> decreased to 87.6% on the ACF-R4 catalyst by the increase of the H2SO4/HNO<sup>3</sup> (*v*/*v*) from 2 to 4. Thus, the formation of more reactive nitrogen and oxygen containing groups on the ACF catalysts greatly enhanced the selectivity toward H2O<sup>2</sup> formation. The selectivity of H2O2 along with the NH2OH conversion on the ACF catalysts at the reaction time of 180 min was shown in Figure 9a. The selectivity of H2O2 was only 46.0% on the ACF-Rw catalyst although the higher conversion of NH2OH (30%) was observed on it, which was possibly induced by the higher surface area and more carbonyl-quinone groups generating through the surface modification. In view of similar conversion toward NH2OH (~22%) consumption, the selectivity of H2O2 was 73.6% on the ACF-R1 catalyst prepared by an equal volume of H2SO4 and HNO3. Whereas the selectivity of H2O2 was greatly enhanced to 99.3% on the ACF-R2 catalyst obtained by further increasing the content of H2SO4. However, the selectivity of H2O2 decreased to 87.6% on the ACF-R4 catalyst by the increase of the H2SO4/HNO3 (*v*/*v*) from 2 to 4. Thus, the formation of more reactive nitrogen and oxygen containing groups on the ACF catalysts greatly enhanced the selectivity toward H2O2 formation.

The activity of H2O2 decomposition over ACF catalysts was shown in Figure 9b. According to the reference [59], the activity toward H2O2 decomposition was directly related

The activity of H2O2 decomposition over ACF catalysts was shown in Figure 9b. According to the reference [59], the activity toward H2O2 decomposition was directly related

**Figure 9.** (**a**) The selectivity of H2O2 and conversion of NH2OH on the ACF catalysts (pH = 8.6, temp. = 25 °C, time = 180 min.), (**b**) the decomposition of H2O2 over ACF catalysts (the error bar calculated by STDEV method). **Figure 9.** (**a**) The selectivity of H2O2 and conversion of NH2OH on the ACF catalysts (pH = 8.6, temp. = 25 °C, time = 180 min.), (**b**) the decomposition of H2O2 over ACF catalysts (the error bar calculated by STDEV method). **Figure 9.** (**a**) The selectivity of H2O<sup>2</sup> and conversion of NH2OH on the ACF catalysts (pH = 8.6, temp. = 25 ◦C, time = 180 min.), (**b**) the decomposition of H2O<sup>2</sup> over ACF catalysts (the error bar calculated by STDEV method).

The activity of H2O<sup>2</sup> decomposition over ACF catalysts was shown in Figure 9b. According to the reference [59], the activity toward H2O<sup>2</sup> decomposition was directly related to the basic sites (chromene groups) on the AC materials surface, while the formation of surface carboxylic groups (–COOH) will accordingly retard the catalytic decomposition of H2O2. It was also found that the acidic function groups of AC materials treated by HNO<sup>3</sup> would suppress the H2O<sup>2</sup> decomposition rate. As shown in Figure 9b, almost no decomposition of H2O<sup>2</sup> was detected on the ACF-R2 and ACF-R4 catalyst during the first 60 min. After reacting for 420 min, the concentration of H2O<sup>2</sup> in the ACF-R1, ACF-R2 and ACF-R4 catalyst system only decreased to 246 mmol/L, 247 mmol/L and 248 mmol/L, respectively. As for the ACF-Rw catalyst, with the smallest amounts of carboxylic groups, the concentration of H2O<sup>2</sup> quickly decreased to 245 mmol/L only within 180 min. Therefore, the modified ACF catalysts with large amounts of carboxylic groups by mixed acids retarded the catalytic decomposition of H2O<sup>2</sup> and exhibited a higher activity of H2O<sup>2</sup> generation.

The catalytic performance in the reaction of H2O<sup>2</sup> production from NH2OH oxidation over modified ACF catalysts was listed and compared to those of previously reported catalysts in Table 7. The modified ACF catalysts showed a higher formation concentration of H2O<sup>2</sup> than the Au/MgO and Pd/Al2O<sup>3</sup> system with a longer reaction time. The ACF-R2 and ACF-R4 catalysts exhibited similarly catalytic performance with the ACH system but with higher selectivity toward H2O2. As for the homogeneous Mn (II/III)-complex system, both the concentration and the yield of H2O<sup>2</sup> were higher than all heterogeneous catalysts systems without considering their separating and recycling problems. Meanwhile, the concentration of H2O<sup>2</sup> over ACF-R2 and ACF-R4 catalysts was higher than the most reactive carbon supported Au and Pd catalysts, which were used in the direct H2O<sup>2</sup> production process from H<sup>2</sup> and O<sup>2</sup> at high pressure. Thus, compared with the Au-Pd/C catalyst, the ACF catalysts system had a longer reaction time (>9 h) while the supported Au and Pd catalysts system only took 0.5 h to obtain a similar concentration of H2O2. Considering the practical application, the reaction of the ACF catalysts system was easy to handle at atmospheric pressure whereas the high pressure was necessary for the supported Au and Pd catalysts system.



<sup>a</sup> The third cycle of the ACF-R2 catalyst. <sup>b</sup> The [Na]5[Mn(3,5-(SO3)2-Cat)2]·10H2O complex with addition of Tiron as catalyst. <sup>c</sup> Reaction conditions: 10 mg catalyst in 5.6 g methanol and 2.9 g water solvent, 420 psi 5% H2/CO<sup>2</sup> + 160 psi 25% O2/CO2, with stirring 1200 rpm at 2 ◦C.

## *2.4. Effect of Surface Nitrogen- and Oxygen-Containing Groups*

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 13 of 18

Obviously, there was no direct correlation between the selectivity of H2O<sup>2</sup> with the surface area or the microporous volume of ACF catalysts. That is, the H2O<sup>2</sup> formation was affected little by the microporous structure. With the aim of exploring the reactivity and the surface chemistry of ACF catalysts, we correlated the selectivity of H2O<sup>2</sup> with the percentage of N5 (pyrrolic/pyridone) from XPS spectra, and the concentration of H2O<sup>2</sup> on the specific surface area of ACFs with the amounts of desorbed carboxyl-anhydride groups over the ACF catalysts from TPD results, as shown in Figure 10. Clearly, there was a perfectly positive correlation between the selectivity and the percentage of N5 on the ACF catalysts shown in Figure 10a. It has been considered that, for nitrogen doping, the wholeness of the π conjugate system was broken by the higher electronegativity on the N atom doped in the carbon basal framework of ACF. Moreover, this could induce charge redistribution, which changes the adsorption performance of the reactive intermediates over the carbon materials [57,58]. Thus, compared with pyridine, pyrrolic/pyridone structure in the carbon skeleton possessing more electronegativity was beneficial for the effective adsorption of reactants, which greatly enhanced the selectivity of H2O<sup>2</sup> on ACF-R2 with higher content of N5. Obviously, there was no direct correlation between the selectivity of H2O2 with the surface area or the microporous volume of ACF catalysts. That is, the H2O2 formation was affected little by the microporous structure. With the aim of exploring the reactivity and the surface chemistry of ACF catalysts, we correlated the selectivity of H2O2 with the percentage of N5 (pyrrolic/pyridone) from XPS spectra, and the concentration of H2O2 on the specific surface area of ACFs with the amounts of desorbed carboxyl-anhydride groups over the ACF catalysts from TPD results, as shown in Figure 10. Clearly, there was a perfectly positive correlation between the selectivity and the percentage of N5 on the ACF catalysts shown in Figure 10a. It has been considered that, for nitrogen doping, the wholeness of the π conjugate system was broken by the higher electronegativity on the N atom doped in the carbon basal framework of ACF. Moreover, this could induce charge redistribution, which changes the adsorption performance of the reactive intermediates over the carbon materials [57,58]. Thus, compared with pyridine, pyrrolic/pyridone structure in the carbon skeleton possessing more electronegativity was beneficial for the effective adsorption of reactants, which greatly enhanced the selectivity of H2O2 on ACF-R2 with higher content of N5.

**Figure 10.** The relationship between (**a**) the selectivity of H2O2 and percentage of N5, (**b**) the concentration of H2O2 and the amounts of carboxyl-anhydride groups on the ACF catalysts (pH = 8.6, temp. = 25 °C, time = 180 min.). **Figure 10.** The relationship between (**a**) the selectivity of H2O<sup>2</sup> and percentage of N5, (**b**) the concentration of H2O<sup>2</sup> and the amounts of carboxyl-anhydride groups on the ACF catalysts (pH = 8.6, temp. = 25 ◦C, time = 180 min.).

On the other hand, the correlation between the concentration of H2O2 on a specific surface area of ACFs with the amounts of CO2 desorbed from carboxyl-anhydride groups demonstrated that the yield of H2O2 increased in a positive correlation way with the increment of carboxyl-anhydride groups on ACF catalysts, as shown in Figure 10b. This could be ascribed to the more hydrophilic surface on ACFs induced by the formation of large quantities of carboxyl-anhydride species, which are in favor of both effective contact with the hydrophilic reactant and maintaining the existence of H2O2. Therefore, the highest selectivity of the ACF-R2 catalyst can be sensibly and directly ascribed to the great quantity of surface oxygen-containing groups and nitrogen-containing groups, particularly the pyrrolic/pyridone nitrogen groups. On the other hand, the correlation between the concentration of H2O<sup>2</sup> on a specific surface area of ACFs with the amounts of CO<sup>2</sup> desorbed from carboxyl-anhydride groups demonstrated that the yield of H2O<sup>2</sup> increased in a positive correlation way with the increment of carboxyl-anhydride groups on ACF catalysts, as shown in Figure 10b. This could be ascribed to the more hydrophilic surface on ACFs induced by the formation of large quantities of carboxyl-anhydride species, which are in favor of both effective contactwith the hydrophilic reactant and maintaining the existence of H2O2. Therefore, the highest selectivity of the ACF-R2 catalyst can be sensibly and directly ascribed to the great quantity of surface oxygen-containing groups and nitrogen-containing groups, particularly the pyrrolic/pyridone nitrogen groups.

For the sake of further clarifying the crucial function of the surface nitrogen, a possible promotion mechanism is proposed. Scheme 1 shows the possible reaction pathway of H2O2 production from NH2OH and O2 on ACF catalysts promoted by N5. Similar to the reaction mechanism proposed in our previous work [60], NH2OH loses protons and electrons when contacted with the quinone species on the ACF surface, forming the HNO intermediate. Then the HNO reacts with NH2OH, producing N2 and H2O. The quinoid groups subsequently transfer the protons and electrons to O2 through the redox cycles of quinone and hydroquinone, completing a typical process of H2O2 formation. The role of N5 can be explained from two aspects, namely pyrrolic nitrogen and pyridone structure. For the sake of further clarifying the crucial function of the surface nitrogen, a possible promotion mechanism is proposed. Scheme 1 shows the possible reaction pathway of H2O<sup>2</sup> production from NH2OH and O<sup>2</sup> on ACF catalysts promoted by N5. Similar to the reaction mechanism proposed in our previous work [60], NH2OH loses protons and electrons when contacted with the quinone species on the ACF surface, forming the HNO intermediate. Then the HNO reacts with NH2OH, producing N<sup>2</sup> and H2O. The quinoid groups subsequently transfer the protons and electrons to O<sup>2</sup> through the redox cycles of quinone and hydroquinone, completing a typical process of H2O<sup>2</sup> formation. The role of N5 can be explained from two aspects, namely pyrrolic nitrogen and pyridone structure.

The pyrrolic nitrogen doped on a carbon structure with more electronegativity formed in

The pyrrolic nitrogen doped on a carbon structure with more electronegativity formed in the edges of the carbon basal plane on ACF, which promotes the electrons transfer between O<sup>2</sup> and NH2OH. Thus, the adsorbed O<sup>2</sup> species on the ACF surface received the electrons transferred easily from the nitrogen species with extra electrons, and then formed HO<sup>2</sup> • intermediates [53]. For the pyridone structure, the NH group is considered a portion of the six-membered ring on the brink of an extended carbon basal plane. The electronic surrounding of the NH species is thought similar to that of pyrrole because the excess electrons of the N atom could be delocalized among the condensed aromatic system and entrapped at defects on the carbon basal layer [40]. Meanwhile, the pyridone structure is usually in presence of two tautomeric structures including 2-hydroxypyridine and α-pyridone. Usually, these two tautomeric forms are transformed to each other by the intramolecular proton transfer, which may facilitate the protons transfer to the HO<sup>2</sup> • intermediates, forming H2O2. Therefore, the higher selectivity of H2O<sup>2</sup> can be attributed to the higher content of N5 on the ACF catalyst. the edges of the carbon basal plane on ACF, which promotes the electrons transfer between O2 and NH2OH. Thus, the adsorbed O2 species on the ACF surface received the electrons transferred easily from the nitrogen species with extra electrons, and then formed HO2• intermediates [53]. For the pyridone structure, the NH group is considered a portion of the six-membered ring on the brink of an extended carbon basal plane. The electronic surrounding of the NH species is thought similar to that of pyrrole because the excess electrons of the N atom could be delocalized among the condensed aromatic system and entrapped at defects on the carbon basal layer [40]. Meanwhile, the pyridone structure is usually in presence of two tautomeric structures including 2-hydroxypyridine and α-pyridone. Usually, these two tautomeric forms are transformed to each other by the intramolecular proton transfer, which may facilitate the protons transfer to the HO2• intermediates, forming H2O2. Therefore, the higher selectivity of H2O2 can be attributed to the higher content of N5 on the ACF catalyst.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 14 of 18

**Scheme 1.** The possible reaction pathway of H2O2 production from NH2OH and O2 on ACF catalysts promoted by N5. **Scheme 1.** The possible reaction pathway of H2O<sup>2</sup> production from NH2OH and O<sup>2</sup> on ACF catalysts promoted by N5.

#### **3. Materials and Methods 3. Materials and Methods**

#### *3.1. Surface Modification of ACF*

*3.1. Surface Modification of ACF*  Ten grams of PAN-based ACF (Jilin, Jiyan high-tech Fibers) were put into 100 mL of concentrated hydrochloric acid (HCl, 37%) and mixed for removing the possible impurities including ashes or inorganic substances. The mixtures were firstly stirred for 3 h at ambient temperature, then Cl− was thoroughly removed from the filtrate by washing with hot water (detected with AgNO3). The obtained sample was put into a vacuum oven and dried at 80 °C overnight, which was christened ACF-0. Then, the ACF-0 (0.5 g) was mixed and stirred in 50 mL of concentrated sulfuric acid (H2SO4, 98%) and concentrated nitric acid (HNO3, 68%) at 60 °C for one hour with a volume ratio of 0.5, 1, 2 and 4, respectively. The oxidized ACF was washed by hot water in order to obtain nearly neutral pH of the filtrate and put into a vacuum oven, then dried at 80 °C overnight. The samples as pre-Ten grams of PAN-based ACF (Jilin, Jiyan high-tech Fibers) were put into 100 mL of concentrated hydrochloric acid (HCl, 37%) and mixed for removing the possible impurities including ashes or inorganic substances. The mixtures were firstly stirred for 3 h at ambient temperature, then Cl− was thoroughly removed from the filtrate by washing with hot water (detected with AgNO3). The obtained sample was put into a vacuum oven and dried at 80 ◦C overnight, which was christened ACF-0. Then, the ACF-0 (0.5 g) was mixed and stirred in 50 mL of concentrated sulfuric acid (H2SO4, 98%) and concentrated nitric acid (HNO3, 68%) at 60 ◦C for one hour with a volume ratio of 0.5, 1, 2 and 4, respectively. The oxidized ACF was washed by hot water in order to obtain nearly neutral pH of the filtrate and put into a vacuum oven, then dried at 80 ◦C overnight. The samples as prepared thus were noted as ACF-Rw, ACF-R1, ACF-R2 and ACF-R4, respectively.

#### pared thus were noted as ACF-Rw, ACF-R1, ACF-R2 and ACF-R4, respectively. *3.2. Characterization of the ACF Catalysts*

*3.2. Characterization of the ACF Catalysts*  Field-emission scanning electron microscopy (FE-SEM) images were recorded on a Philips Fei Quanta 200F instrument operating at 20 kV, while elemental mapping images of ACF-0 were obtained on a Hitachi SU8220 SEM instrument working at 15 kV. Nitrogen adsorption-desorption detection was measured by a Micrometrics ASAP 2460 instrument Field-emission scanning electron microscopy (FE-SEM) images were recorded on a Philips Fei Quanta 200F instrument operating at 20 kV, while elemental mapping images of ACF-0 were obtained on a Hitachi SU8220 SEM instrument working at 15 kV. Nitrogen adsorption-desorption detection was measured by a Micrometrics ASAP 2460 instrument under −196 ◦C. Moreover, the ACF catalysts were outgassed at 250 ◦C overnight before

under −196 °C. Moreover, the ACF catalysts were outgassed at 250 °C overnight before the

the start of measurement. The multipoint Braunauer–Emmett–Teller (BET) analysis was used to calculate the specific surface area (SBET). Fourier transformation infrared (FTIR) spectra of the ACF catalysts were conducted on an IR spectrometer (Bruker Vector 22) by making KBr pellets containing 0.5 wt.% of ACF. The Raman spectra of ACF catalysts were obtained on a Horiba LabRAM HR Evolution Raman spectrometer by using a 532 nm laser. The measurements of X-ray photoelectron spectroscopy (XPS) were carried out on an ES-CALAB MK-II spectrometer (VG Scientific Ltd., West Sussex, UK) with an Al Kα radiation source under an accelerated voltage of 20 kV. For correcting the charge effect, the binding energy (BE) of C1s was adjusted to 285.0 eV. The sensitivity factors and the peak areas of the elements were used to calculate the surface atomic ratio of O/C [61]. Temperatureprogrammed desorption (TPD) was accomplished in a quartz tubular reactor, which was linked to a quadrupole mass spectrometer (Omnistar, Balzers). After the ACF catalyst (40 mg) was filled in the reactor, the temperature was increased to 900 ◦C with a heating rate of 10 ◦C/min in helium flow of 30 mL/min. The mass spectrometer was used to monitor the outlet gas.

#### *3.3. Catalyst Testing*

The general reaction of NH2OH with O<sup>2</sup> was performed in a 100 mL of jacketed glass reactor by stirring at room temperature under atmospheric pressure, as reported elsewhere [31]. In a typical reaction process, 0.15 g of ACF catalyst was put into the aqueous solution of reactant, which was made of hydroxylammonium chloride (NH2OH•HCl, 1.74 g) and 50 mL of deionized water. Before adding the ACF catalyst, the pH value of NH2OH•HCl aqueous solution was regulated to 8.6 by the solution of 1 M NaOH. Moreover, O<sup>2</sup> was bubbled into the reaction mixtures at a constant flow rate of 25 mL/min, which was tailored by a mass flow controller. Samples of the reactants were taken out periodically in order to analyze the concentration of H2O<sup>2</sup> by the colorimetric method, which was based on the titanium (IV) sulfate [62]. Similarly, the colorimetric method with the Fe (III)-1,10-phenanthroline complexes was used to detect the concentration of NH2OH•HCl [63]. The recycling tests of ACF catalysts were performed with the same conditions mentioned above. For each cycle, the used ACF catalyst was washed with hot water and dried at 80 ◦C in a vacuum oven overnight. The tests of H2O<sup>2</sup> decomposition were carried out in similar reaction conditions only without feeding NH2OH•HCl and O2. The initial concentration of H2O<sup>2</sup> was 0.25 M without adjusting the pH value. The dosage of ACF catalyst for each decomposition test was 0.15 g. The yield toward H2O<sup>2</sup> formation was calculated in accordance with the stoichiometric ratio of the reaction (2NH2OH + O2= H2O<sup>2</sup> + 2H2O + N2), as the following equation:

$$\mathrm{H\_2O\_2\text{ Yield (\%)}=2} \times n(\mathrm{H\_2O\_2})/n(\mathrm{NH\_2OH}\bullet \mathrm{HCl}) \times 100\% \tag{1}$$

where *n*(H2O2) is the moles of H2O<sup>2</sup> generated in the reaction, and *n*(NH2OH•HCl) is the moles of NH2OH•HCl in feed.

#### **4. Conclusions**

Proper tuning of the surface chemistry of ACFs with intrinsic nitrogen content could expeditiously promote the selectivity of H2O<sup>2</sup> production through NH2OH oxidation. Mixed acid oxidation of ACF under mild reaction conditions effectively increased the surface oxygen groups and tailored the pyrrolic/pyridone nitrogen doped on a carbon structure, which then accelerated the selectivity for H2O<sup>2</sup> over 99.3% on ACF-R2 catalyst. The higher content of H2SO<sup>4</sup> in the mixed acid created more pyrrolic/pyridone nitrogen, carboxyl and anhydride groups, enhancing the selectivity and yield toward H2O<sup>2</sup> formation. In our present work, both an easy and low-priced synthetic process for H2O<sup>2</sup> generation was described, while a new comprehension on the conception and mechanistic examination of metal-free N- and O-doped carbon materials were also provided.

**Author Contributions:** Conceptualization, W.S.; methodology, W.S. and L.Y.; formal analysis, R.Z.; data curation, X.X. and M.S.; writing-original draft preparation, W.S. and R.Z.; writing-review and editing, X.X. and M.S.; supervision, Y.L.; funding acquisition, W.S. and Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (21603039, 51678160).

**Conflicts of Interest:** The authors declare no conflict of interest.
