Enhanced Catalytic Hydrogen Peroxide Production from Hydroxylamine Oxidation on Modiﬁed Activated Carbon Fibers: The Role of Surface Chemistry

: Herein, direct production of hydrogen peroxide (H 2 O 2 ) through hydroxylamine (NH 2 OH) oxidation by molecular oxygen was greatly enhanced over modiﬁed activated carbon ﬁber (ACF) catalysts. We revealed that the higher content of pyrrolic/pyridone nitrogen (N5) and carboxyl-anhydride oxygen could effectively promote the higher selectivity and yield of H 2 O 2 . By changing the volume ratio of the concentrated H 2 SO 4 and HNO 3 , the content of N5 and surface oxygen containing groups on ACF were selectively tuned. The ACF catalyst with the highest N5 content and abundant carboxyl-anhydride oxygen containing groups was demonstrated to have the highest activity toward catalytic H 2 O 2 production, enabling the selectivity of H 2 O 2 over 99.3% and the concentration of H 2 O 2 reaching 123 mmol/L. The crucial effects of nitrogen species were expounded by the correlation of the selectivity of H 2 O 2 with the content of N5 from X-ray photoelectron spectroscopy (XPS). The possible reaction pathway over ACF catalysts promoted by N5 was also shown.


Introduction
Hydrogen peroxide (H 2 O 2 ), as a green chemical, attracts research attention in both energy and environmental related fields because it has the highest content of reactive oxygen among the common oxidants and the green by-product [1]. It has been widely used as a bleach in the paper and textile industry, an energy carrier in fuel cells, an oxidant in chemical production, wastewater treatment, hydrometallurgy and electronics industry [2]. Notwithstanding, the current industrial production of H 2 O 2 is mainly through the anthraquinone oxidation process [3], which involves multistep reactions, massive energy consumption and waste generation. Furthermore, the cost and safety problems are also raised ineluctably by the handle, transport and storage of high concentration H 2 O 2 . Nevertheless, in many practical applications, H 2 O 2 with only a low concentration could satisfy the demand in the reactions, such as selective oxidation, on-site degradation of dye, sewage treatment and disinfection (<30 mM) [4][5][6]. In this context, research on alternative production methods of H 2 O 2 and it's in situ use has been the research focus [5,[7][8][9][10][11][12][13][14][15]. The direct generation of H 2 O 2 by the reaction of molecular hydrogen (H 2 ) and oxygen (O 2 ) is considered the most promising method [16][17][18][19], but the industrial application is obscured by the dangers of the explosive reaction mixture and the insufficiency of catalysts with high selectivity without considering the reaction systems of O 2 and H 2 at high pressure [20][21][22][23][24][25]. In recent years, both photo-and electro-catalytic H 2 O 2 production techniques are in the process of research, but the former is still suffered from a low selectivity and yield of H 2 O 2 roughness was observed on the surface of ACF-R1. By increasing the value of H2SO4/HNO3 (v/v) to 2 and 4, some auricular-like sheet protrusions were both found on the surface of ACF-R2 and ACF-R4 samples. Consequently, the surface modification caused differently morphological changes on the ACF samples through the erosion of carbon surface by different H2SO4/HNO3 (v/v), and some microporous structures on the ACF samples might be destroyed through mixed acid treatment with higher content of concentrated H2SO4.  Table 1. Moreover, the ACF-0 sample displayed the features of type I isotherm, indicating the existence of micropores. There were two types of pores on the ACF-0 sample: micropore (1.41 nm and 1.13 nm) and supermicropore (0.78 and 0.57 nm) [41]. As for the ACF-R4 sample, the micropore content (1.14 nm) was greatly enhanced and some micropores enlarged to 1.69 nm while the supermicropore content was decreased compared with the ACF-0 sample. As displayed in Table 1, minor changes in the pore size and surface area were found between the ACF-0 and ACF-Rw samples but obviously decrease in micropore volume and surface area were observed on the ACF-R2 and ACF-R4 samples. As for the ACF-Rw sample, the surface area of micropores (Smic.), the total surface area (SBET) and the surface area of mesoporous (Smes.) was 895, 934 and 39 m 2 /g, respectively. Correspondingly, the micropore volume (Vmic.), the total pore volume (Vtotal) and the micropore width (dpore.) of ACF-Rw were calculated to be 0.361, 0.395 cm 3 /g and 0.85 nm. These results were similar to those of the ACF-0 sample. With the increase of H2SO4/HNO3 (v/v), the values of VTotal and SBET initially reduced to 0.323 cm 3 /g and 686 m 2 /g on the ACF-R1 sample, then declined to 0.229 cm 3 /g and 481 m 2 /g on the ACF-R2 sample. Moreover, the dpore for ACF samples by mixed acid oxidation increased slightly from 0.87 to 0.95 nm, with increasing H2SO4/HNO3 (v/v) from 0.5 to 4. Notably, the mesopores of the ACF samples were not altered much through the modification by mixed acid, whereas the micropores decreased remarkably by higher values of H2SO4/HNO3 (v/v), especially in the ACF-R2 and ACF-R4 samples. This suggests that the higher content of H2SO4 caused severe destruction of the The pore size distribution of ACF-0 and ACF-R4 samples, and nitrogen adsorptiondesorption isotherms of the ACF-0 sample without surface modification is shown in Figure 2. The detailed texture parameters for the ACF samples were shown in Table 1. Moreover, the ACF-0 sample displayed the features of type I isotherm, indicating the existence of micropores. There were two types of pores on the ACF-0 sample: micropore (1.41 nm and 1.13 nm) and supermicropore (0.78 nm and 0.57 nm) [41]. As for the ACF-R4 sample, the micropore content (1.14 nm) was greatly enhanced and some micropores enlarged to 1.69 nm while the supermicropore content was decreased compared with the ACF-0 sample. As displayed in Table 1, minor changes in the pore size and surface area were found between the ACF-0 and ACF-Rw samples but obviously decrease in micropore volume and surface area were observed on the ACF-R2 and ACF-R4 samples. As for the ACF-Rw sample, the surface area of micropores (S mic. ), the total surface area (S BET ) and the surface area of mesoporous (S mes. ) was 895, 934 and 39 m 2 /g, respectively. Correspondingly, the micropore volume (V mic. ), the total pore volume (V total ) and the micropore width (d pore. ) of ACF-Rw were calculated to be 0.361, 0.395 cm 3 /g and 0.85 nm. These results were similar to those of the ACF-0 sample. With the increase of H 2 SO 4 /HNO 3 (v/v), the values of V Total and S BET initially reduced to 0.323 cm 3 /g and 686 m 2 /g on the ACF-R1 sample, then declined to 0.229 cm 3 /g and 481 m 2 /g on the ACF-R2 sample. Moreover, the d pore for ACF samples by mixed acid oxidation increased slightly from 0.87 to 0.95 nm, with increasing H 2 SO 4 /HNO 3 (v/v) from 0.5 to 4. Notably, the mesopores of the ACF samples were not altered much through the modification by mixed acid, whereas the micropores decreased remarkably by higher values of H 2 SO 4 /HNO 3 (v/v), especially in the ACF-R2 and ACF-R4 samples. This suggests that the higher content of H 2 SO 4 caused severe destruction of the microporous structures while the higher content of HNO 3 preserved the textual characteristics of the ACF sample. microporous structures while the higher content of HNO3 preserved the textual characteristics of the ACF sample.

Surface Properties
The Fourier transformation infrared (FTIR) spectra of ACF samples is shown in Figure 3a. According to the references, the peak at 1225 cm −1 is originated from the stretching mold of C-N and C-O in carboxylic anhydrides, ethers, lactones and phenols [42,43]. The peak at 1405 cm −1 and 1580 cm −1 is respectively related to the nitrogen groups and the double bond of C=C in quinoid structure. Meanwhile, the peak at 1730 cm −1 is owing to the stretching vibration of the C=O band in carboxyl and lactones groups attached to the aromatic rings, and the peak at 910 cm −1 is related to the anhydride groups [41,43,44]. After modification by different values of H2SO4/HNO3 (v/v), the intensities of the above peaks were wholly enhanced to different extents, suggesting the formation of large quantities of oxygen-containing species on ACF surface. For ACF-R1, ACF-R2 and ACF-R4 samples, the intensities of the peaks at 910 cm −1 , 1225 cm −1 , 1580 cm −1 and 1730 cm −1 were all greatly increased, indicating the enlargement in phenols, quinones, lactones, carboxyls and anhydrides. The highest peak intensity was found on ACF-R2 and ACF-R4 samples, especially at the position of 1730 cm −1 , which confirms the further enrichment of anhydride and carboxylic groups by a higher content of H2SO4 in mixed acid.

Surface Properties
The Fourier transformation infrared (FTIR) spectra of ACF samples is shown in Figure 3a. According to the references, the peak at 1225 cm −1 is originated from the stretching mold of C-N and C-O in carboxylic anhydrides, ethers, lactones and phenols [42,43]. The peak at 1405 cm −1 and 1580 cm −1 is respectively related to the nitrogen groups and the double bond of C=C in quinoid structure. Meanwhile, the peak at 1730 cm −1 is owing to the stretching vibration of the C=O band in carboxyl and lactones groups attached to the aromatic rings, and the peak at 910 cm −1 is related to the anhydride groups [41,43,44]. After modification by different values of H 2 SO 4 /HNO 3 (v/v), the intensities of the above peaks were wholly enhanced to different extents, suggesting the formation of large quantities of oxygen-containing species on ACF surface. For ACF-R1, ACF-R2 and ACF-R4 samples, the intensities of the peaks at 910 cm −1 , 1225 cm −1 , 1580 cm −1 and 1730 cm −1 were all greatly increased, indicating the enlargement in phenols, quinones, lactones, carboxyls and anhydrides. The highest peak intensity was found on ACF-R2 and ACF-R4 samples, especially at the position of 1730 cm −1 , which confirms the further enrichment of anhydride and carboxylic groups by a higher content of H 2 SO 4 in mixed acid. As shown in Figure 3b, Raman spectroscopy detection was conducted to investigate the defects on carbon structure of ACF samples with surface modification. Usually, carbon fiber mainly has two characteristic peaks, one of which is the D peak at the position of 1350-1375 cm −1 , and the other is the G peak at the position of 1580-1603 cm −1 [45]. The D peak is related to amorphous and defects of carbon structure while the G peak is related to graphite crystal structure. Generally, the calculation of ID/IG ratio from integral areas values of D and G peak was used to measure the structural defects of carbon materials [46]. It is widely known that ID/IG value increases with more structural defects generated on the carbon material. Obviously, the intensity of Raman spectra on ACF samples gradually increased, meanwhile the ID/IG values of all the ACF samples increased from 0.95 to 1.07 with increasing H2SO4/HNO3 (v/v) from 0.5 to 2. Whereas the ID/IG values of ACF-R4 decreased to 1.06 with increasing H2SO4/HNO3 (v/v) from 2 to 4. Therefore, it is believed that there were more surface defects and structural changes on the ACF carbon framework according to the surface modification with more H2SO4 contents in mixed acid. These results were well matched with the textural characteristics in ACF samples shown in Table  1.
The narrow scan of C 1s regions in X-ray photoelectron spectroscopy (XPS) of ACF samples is exhibited in Figure 4a. Moreover, the deconvolution results of the C 1s spectrum are given in Table 2. For modified carbon materials, the C 1s spectra usually involved graphitic carbon (C-graphite, Peak I), ether, alcohol or phenolic groups (C-O, Peak II), carbonyl or quinone groups (C=O, Peak III), carboxylic groups (-COO-, Peak IV) and Peak V for the satellite peak from the π-π* electron shake-up [47][48][49]. The intensities of peak I was decreased by the oxidation of mixed acid, whereas the intensities for the peaks attributed by C-O groups were increased [49]. However, the areas of peak II of all ACF samples by acid oxidation increased not so obviously compared to that of peak III or peak IV, which indicated phenolic groups may not be tailored much by adjusting different values of H2SO4/HNO3 (v/v). Similarly, the integral area of peak IV for both ACF-Rw (4.5%) and ACF-R1 sample (5.4%) was more than two times larger than that of the ACF-0 (2.0%). Notably, the area of peak IV increased to 7.9% and 8.7% on the ACF-R2 and ACF-R4 samples respectively, clearly confirming the generation of a large amount of surface carboxylic groups by the higher content of H2SO4. As shown in Figure 3b, Raman spectroscopy detection was conducted to investigate the defects on carbon structure of ACF samples with surface modification. Usually, carbon fiber mainly has two characteristic peaks, one of which is the D peak at the position of 1350-1375 cm −1 , and the other is the G peak at the position of 1580-1603 cm −1 [45]. The D peak is related to amorphous and defects of carbon structure while the G peak is related to graphite crystal structure. Generally, the calculation of I D /I G ratio from integral areas values of D and G peak was used to measure the structural defects of carbon materials [46]. It is widely known that I D /I G value increases with more structural defects generated on the carbon material. Obviously, the intensity of Raman spectra on ACF samples gradually increased, meanwhile the I D /I G values of all the ACF samples increased from 0.95 to 1.07 with increasing H 2 SO 4 /HNO 3 (v/v) from 0.5 to 2. Whereas the I D /I G values of ACF-R4 decreased to 1.06 with increasing H 2 SO 4 /HNO 3 (v/v) from 2 to 4. Therefore, it is believed that there were more surface defects and structural changes on the ACF carbon framework according to the surface modification with more H 2 SO 4 contents in mixed acid. These results were well matched with the textural characteristics in ACF samples shown in Table 1.
The narrow scan of C 1s regions in X-ray photoelectron spectroscopy (XPS) of ACF samples is exhibited in Figure 4a. Moreover, the deconvolution results of the C 1s spectrum are given in Table 2. For modified carbon materials, the C 1s spectra usually involved graphitic carbon (C-graphite, Peak I), ether, alcohol or phenolic groups (C-O, Peak II), carbonyl or quinone groups (C=O, Peak III), carboxylic groups (-COO-, Peak IV) and Peak V for the satellite peak from the π-π* electron shake-up [47][48][49]. The intensities of peak I was decreased by the oxidation of mixed acid, whereas the intensities for the peaks attributed by C-O groups were increased [49]. However, the areas of peak II of all ACF samples by acid oxidation increased not so obviously compared to that of peak III or peak IV, which indicated phenolic groups may not be tailored much by adjusting different values of H 2 SO 4 /HNO 3 (v/v). Similarly, the integral area of peak IV for both ACF-Rw (4.5%) and ACF-R1 sample (5.4%) was more than two times larger than that of the ACF-0 (2.0%). Notably, the area of peak IV increased to 7.9% and 8.7% on the ACF-R2 and ACF-R4 samples respectively, clearly confirming the generation of a large amount of surface carboxylic groups by the higher content of H 2 SO 4 .   Figure 4b exhibits the narrow scan of XPS spectra in O 1s regions of the ACF samples. Moreover, the deconvolution results of the O 1s spectrum are displayed in Table 3. As shown in Figure 4b, the O 1s XPS spectra can be deconvoluted into three main peaks, namely Peak I, Peak II and Peak III, which are associated with the C=O group, C -O group and adsorbed H2O or O2, respectively [42]. The adsorbed CO or CO2 in the ACF surface can be attributed to the minor Peak IV, the binding energy of which was at 536.9-537.0 eV. Obviously, the intensities of both Peak III and Peak IV decreased by surface modification, whereas the peaks corresponding to C=O groups increased evidently. As for Peak I, the intensities increased from 25.5 to 30.5% by surface modification with increasing the   Figure 4b exhibits the narrow scan of XPS spectra in O 1s regions of the ACF samples. Moreover, the deconvolution results of the O 1s spectrum are displayed in Table 3. As shown in Figure 4b, the O 1s XPS spectra can be deconvoluted into three main peaks, namely Peak I, Peak II and Peak III, which are associated with the C=O group, C-O group and adsorbed H 2 O or O 2 , respectively [42]. The adsorbed CO or CO 2 in the ACF surface can be attributed to the minor Peak IV, the binding energy of which was at 536.9-537.0 eV. Obviously, the intensities of both Peak III and Peak IV decreased by surface modification, whereas the peaks corresponding to C=O groups increased evidently. As for Peak I, the intensities increased from 25.5 to 30.5% by surface modification with increasing the content of H 2 SO 4, and similar results were obtained on ACF-R2 and ACF-R4 samples. Additionally, the atomic ratio of surface O/C in the ACF samples by acid oxidation was enhanced significantly from 21.3 to 32.9% with increasing the H 2 SO 4 /HNO 3 (v/v) from 0.5 to 4. The above results suggested that more carboxylic species were generated by mixed acid oxidation with higher content of H 2 SO 4 , being consistent with the results of FTIR measurement. For the sake of examining the crucial role of the intrinsic nitrogen doped in ACF samples, the deconvolution results of N 1s XPS profiles of ACF samples are exhibited in Figure 5. Moreover, the corresponding results of the deconvolution are displayed in detail in Table 4. According to the curve fitting results and references, five distinct types of nitrogen contained species were deconvoluted from the N 1s spectra: NX (-NO 2 ), N4 (pyridine-N oxide), NQ (quaternary N), N5 (pyrrolic/pyridone) and N6 (pyridine) [50][51][52][53]. It was evident that the content of N6 in ACF-0 was highest among all ACFs. Moreover, the content of N5 significantly increased to 33.2%, 43.4%, 50.5% and 46.3% for the ACF-Rw, ACF-R1, ACF-R2 and ACF-R4, respectively. As shown in Table 4, the content of N6 on the ACF-0 decreased from 15.6 to 5.2% corresponding to the ACF-R2. Moreover, the content of both NQ and N4 on the ACF sample decreased nearly one half by surface modification. It was reported that −NO 2 and pyridine were the main forms of the nitrogen introduced from HNO 3 oxidation and different forms of nitrogen can be transformed to each other [42,54]. The content of NX initially reached the maximum (32.3%) on ACF-Rw, then decreased to 22.3%, 15.3% and 19.0% on ACF-R1, ACF-R2 and ACF-R4, respectively. No content of NX can be observed on the ACF-0 sample without surface modification. Accordingly, when the nitrogen form predominated in the ACF sample was quaternary N, the mixed acid modification transformed them to −NO 2 with the higher content of HNO 3 . Meanwhile, more pyrrolic nitrogen species were generated by a higher content of H 2 SO 4. In addition, the atomic ratios of surface N/C on ACF samples were gradually enhanced from 1.5 to 2.4 with the increase of H 2 SO 4 /HNO 3 (v/v) from 0.5 to 2, whereas that on ACF-R4 sample decreased to 2.0 by increasing the value of H 2 SO 4 /HNO 3 (v/v) to 4. These results suggest that the surface N-containing groups could be effectively tuned by mixed acid oxidation with different volume ratios of concentrated H 2 SO 4 and HNO 3 .   The temperature-programmed desorption (TPD) results of the ACF samples were shown in Figure 6. After being heated, carbon oxides were the main decomposition products of surface oxygen-containing functional groups [55][56][57][58]. As shown in Figure 7, the anhydrides and carboxylic acids usually decomposed into CO2 at relatively lower temperatures while the lactones decomposed into CO2 at higher temperatures. Meanwhile, the carboxylic anhydrides, ethers, phenols, carbonyl-quinones generally decomposed into CO [58]. Only little quantities of COx were obtained on the ACF-0 sample while significant quantities of COx were obtained on the other three ACF samples. For the ACF samples modified by mixed acid, the data of COx gradually rose with increasing the H2SO4/HNO3 (v/v) from 0.5 to 4. Especially, the desorption quantity of CO from the ACF-R1 sample was almost five-fold larger than that of the ACF-0 sample, illustrating the formation of large quantities of phenol and carbonyl-quinone groups. On the flip side, the desorption amount of CO2 from the ACF-R2 sample was almost more than 15 times greater than the  The temperature-programmed desorption (TPD) results of the ACF samples were shown in Figure 6. After being heated, carbon oxides were the main decomposition products of surface oxygen-containing functional groups [55][56][57][58]. As shown in Figure 7, the anhydrides and carboxylic acids usually decomposed into CO 2 at relatively lower temperatures while the lactones decomposed into CO 2 at higher temperatures. Meanwhile, the carboxylic anhydrides, ethers, phenols, carbonyl-quinones generally decomposed into CO [58]. Only little quantities of CO x were obtained on the ACF-0 sample while significant quantities of CO x were obtained on the other three ACF samples. For the ACF samples modified by mixed acid, the data of CO x gradually rose with increasing the H 2 SO 4 /HNO 3 (v/v) from 0.5 to 4. Especially, the desorption quantity of CO from the ACF-R1 sample was almost five-fold larger than that of the ACF-0 sample, illustrating the formation of large quantities of phenol and carbonyl-quinone groups. On the flip side, the desorption amount of CO 2 from the ACF-R2 sample was almost more than 15 times greater than the ACF-0 sample, primarily owing to the remarkable generation of lactones, anhydrides and carboxylic acids. The quantities of CO x obtained from ACF-R4 were very similar to the ACF-R2 sample.
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 18 ACF-0 sample, primarily owing to the remarkable generation of lactones, anhydrides and carboxylic acids. The quantities of COx obtained from ACF-R4 were very similar to the ACF-R2 sample.  . Deconvolution of the TPD profiles for ACF samples, in which peak a, peak b, peak c from the CO 2 desorption of the carboxyl, anhydride, lactone groups while peak d, peak e, peak f from the CO desorption of the anhydride, phenol, carbonyl groups on ACF samples. Tables 5 and 6 show the detailed data of CO and CO 2 desorbed from specific surface groups on ACF samples. The desorption quantities of CO and CO 2 on ACF-0 sample were 244 µmol/g and 65 µmol/g, severally, and they were raised to 1008 µmol/g and 400 µmol/g on ACF-Rw sample. Upon increasing the H 2 SO 4 /HNO 3 (v/v) from 0.5 to 1, the desorption quantities of CO and CO 2 on the ACF-R1 sample remarkably raised to 1207 µmol/g and 678 µmol/g, respectively. Nevertheless, the amounts of CO desorbed from carbonyl-quinone groups on the ACF-R2 sample decreased to 125 µmol/g. While compared with ACF-Rw, more than two times larger amounts of CO 2 desorbed from carboxyl and anhydride groups were also found on the ACF-R2 sample. The desorption quantities of CO 2 and CO on the ACF-R4 sample were very similar to the ACF-R2 sample. Considering all these examinations, it could be deduced that the largest amounts of carboxyl (407 µmol/g) and anhydride (425 µmol/g) were obtained on the ACF-R2 and ACF-R4 samples while the most enrichment of phenol groups was detected on the ACF-R1 sample. This means that the moderate content of H 2 SO 4 produced more phenol groups while the higher content of H 2 SO 4 in the mixed acid created more carboxylic and anhydride groups. These results were consistent with the FTIR and XPS results of ACF samples.   Figure 8a shows the concentration of H 2 O 2 production from NH 2 OH oxidation by O 2 on the ACF catalysts. For the ACF-0 catalyst without surface oxidation, the cumulative concentration of H 2 O 2 was very low and cannot be detected after reacting for 300 min. For the ACF-Rw catalyst, the concentration of H 2 O 2 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 H 2 O 2 concentration increased gradually with the increase of reaction time. When the reaction was conducted for 660 min, the concentration of H 2 O 2 approached 88.6 mmol/L and 112 mmol/L on the ACF-R1 and ACF-R4 catalyst, respectively. With increasing the H 2 SO 4 /HNO 3 (v/v) from 1 to 2, the maximum concentration of H 2 O 2 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 H 2 SO 4 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 no decrease in the activity of the reused ACF-R2 catalyst with the yield of H 2 O 2 about 49% (123 mmol/L) after reacting for 660 min. 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 selectivity of H 2 O 2 along with the NH 2 OH conversion on the ACF catalysts at the reaction time of 180 min was shown in Figure 9a. The selectivity of H 2 O 2 was only 46.0% on the ACF-Rw catalyst although the higher conversion of NH 2 OH (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 NH 2 OH (~22%) consumption, the selectivity of H 2 O 2 was 73.6% on the ACF-R1 catalyst prepared by an equal volume of H 2 SO 4 and HNO 3 . Whereas the selectivity of H 2 O 2 was greatly enhanced to 99.3% on the ACF-R2 catalyst obtained by further increasing the content of H 2 SO 4 . However, the selectivity of H 2 O 2 decreased to 87.6% on the ACF-R4 catalyst by the increase of the H 2 SO 4 /HNO 3 (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 H 2 O 2 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 H 2 O 2 decomposition over ACF catalysts was shown in Figure 9b. According to the reference [59], the activity toward H 2 O 2 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 H 2 O 2 . It was also found that the acidic function groups of AC materials treated by HNO 3 would suppress the H 2 O 2 decomposition rate. As shown in Figure 9b, almost no decomposition of H 2 O 2 was detected on the ACF-R2 and ACF-R4 catalyst during the first 60 min. After reacting for 420 min, the concentration of H 2 O 2 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 H 2 O 2 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 H 2 O 2 and exhibited a higher activity of H 2 O 2 generation.

H 2 O 2 Production
The catalytic performance in the reaction of H 2 O 2 production from NH 2 OH 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 H 2 O 2 than the Au/MgO and Pd/Al 2 O 3 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 H 2 O 2 . As for the homogeneous Mn (II/III)-complex system, both the concentration and the yield of H 2 O 2 were higher than all heterogeneous catalysts systems without considering their separating and recycling problems. Meanwhile, the concentration of H 2 O 2 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 H 2 O 2 production process from H 2 and O 2 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 H 2 O 2 . 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.

Effect of Surface Nitrogen-and Oxygen-Containing Groups
Obviously, there was no direct correlation between the selectivity of H 2 O 2 with the surface area or the microporous volume of ACF catalysts. That is, the H 2 O 2 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 H 2 O 2 with the percentage of N5 (pyrrolic/pyridone) from XPS spectra, and the concentration of H 2 O 2 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 H 2 O 2 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. 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.
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. The pyrrolic nitrogen doped on a carbon structure with more electronegativity formed in On the other hand, the correlation between the concentration of H 2 O 2 on a specific surface area of ACFs with the amounts of CO 2 desorbed from carboxyl-anhydride groups demonstrated that the yield of H 2 O 2 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 H 2 O 2 . 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 H 2 O 2 production from NH 2 OH and O 2 on ACF catalysts promoted by N5. Similar to the reaction mechanism proposed in our previous work [60], NH 2 OH loses protons and electrons when contacted with the quinone species on the ACF surface, forming the HNO intermediate. Then the HNO reacts with NH 2 OH, producing N 2 and H 2 O. The quinoid groups subsequently transfer the protons and electrons to O 2 through the redox cycles of quinone and hydroquinone, completing a typical process of H 2 O 2 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 edges of the carbon basal plane on ACF, which promotes the electrons transfer between O 2 and NH 2 OH. Thus, the adsorbed O 2 species on the ACF surface received the electrons transferred easily from the nitrogen species with extra electrons, and then formed HO 2 • 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 2 • intermediates, forming H 2 O 2 . Therefore, the higher selectivity of H 2 O 2 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. Scheme 1. The possible reaction pathway of H2O2 production from NH2OH and O2 on ACF catalysts promoted by N5.

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 prepared thus were noted as ACF-Rw, ACF-R1, ACF-R2 and ACF-R4, respectively.

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 under −196 °C . Moreover, the ACF catalysts were outgassed at 250 °C overnight before the Scheme 1. The possible reaction pathway of H 2 O 2 production from NH 2 OH and O 2 on ACF catalysts promoted by N5.

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 AgNO 3 ). 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 (H 2 SO 4 , 98%) and concentrated nitric acid (HNO 3 , 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.

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 under −196 • C. Moreover, the ACF catalysts were outgassed at 250 • C overnight before the start of measurement. The multipoint Braunauer-Emmett-Teller (BET) analysis was used to calculate the specific surface area (S BET ). 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.

Catalyst Testing
The general reaction of NH 2 OH with O 2 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 (NH 2 OH•HCl, 1.74 g) and 50 mL of deionized water. Before adding the ACF catalyst, the pH value of NH 2 OH•HCl aqueous solution was regulated to 8.6 by the solution of 1 M NaOH. Moreover, O 2 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 H 2 O 2 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 NH 2 OH•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 H 2 O 2 decomposition were carried out in similar reaction conditions only without feeding NH 2 OH•HCl and O 2 . The initial concentration of H 2 O 2 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 H 2 O 2 formation was calculated in accordance with the stoichiometric ratio of the reaction (2NH 2 OH + O 2 = H 2 O 2 + 2H 2 O + N 2 ), as the following equation: where n(H 2 O 2 ) is the moles of H 2 O 2 generated in the reaction, and n(NH 2 OH•HCl) is the moles of NH 2 OH•HCl in feed.

Conclusions
Proper tuning of the surface chemistry of ACFs with intrinsic nitrogen content could expeditiously promote the selectivity of H 2 O 2 production through NH 2 OH 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 H 2 O 2 over 99.3% on ACF-R2 catalyst. The higher content of H 2 SO 4 in the mixed acid created more pyrrolic/pyridone nitrogen, carboxyl and anhydride groups, enhancing the selectivity and yield toward H 2 O 2 formation. In our present work, both an easy and low-priced synthetic process for H 2 O 2 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.