Atomically Dispersed Nickel Anchored on a Nitrogen‐Doped Carbon/TiO2 Composite for Efficient and Selective Photocatalytic CH4 Oxidation to Oxygenates

Abstract Direct photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen‐doped carbon/TiO2 composite (Ni−NC/TiO2) as a highly active and selective catalyst for photooxidation of CH4 to C1 oxygenates with O2 as the only oxidant. Ni−NC/TiO2 exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high‐performance photocatalysts. Experimental and theoretical investigations suggest that the single‐atom Ni−NC sites not only enhance the transfer of photogenerated electrons from TiO2 to isolated Ni atoms but also dominantly facilitate the activation of O2 to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.


Introduction
Methane is not only a highly available clean fuel from natural gas, shale gas and biogas, but also a very potent and important greenhouse gas with a warming potential more than 25 times that of CO 2 . [1][2][3] The catalytic conversion of methane to higher added-value chemicals, typically derived from petroleum and coal, is therefore attractive for reducing dependence on crude oil and mitigating global warming. The current industrial methane conversion technology is realized through an indirect route, associated with an energy-intensive syngas production process and the subsequent methanol or Fisher-Tropsch synthesis. [3,4] Direct conversion of methane to methanol and other oxygenates with molecular oxygen is one of the most ideal approaches to realize methane transformation more efficiently and cleanly. [5][6][7][8][9] The key challenge in direct methane conversion is the activation and selective oxidation of methane, because methane is a rather inert molecule and the desired products are more reactive than methane and is susceptible to overoxidation to CO 2 . [6,7] To minimize the overoxidation of oxygenates, the methane conversion reaction is generally conducted at relatively low temperatures (< 200°C), along with the utilization of corrosive or expensive oxidants (such as sulfuric acid, N 2 O, H 2 O 2 ) to replace O 2 to activate methane and/or the operation of a stepwise chemical looping process, which makes the process economically uncompetitive. [5-7, 10, 11] Compared with thermocatalytic methane conversion, photocatalytic methane oxidation can proceed at room temperature to achieve appreciable yields of oxygenates and has recently received great interest. [12][13][14][15][16][17][18][19][20][21][22][23] Cocatalysts play a vital role in semiconductor-based photocatalytic methane oxidation reactions, as they can not only promote the separation and transfer of photogenerated charge carriers, but also control the activation of reactants, thereby enhancing surface reaction rates and tuning product selectivity. Among various cocatalysts, noble metals generally exhibit the outstanding performance for photocatalytic methane oxidation. [15,16,19,20,[22][23][24][25] For example, our previous studies showed that noble metals (Pt, Pd, Au or Ag) decorated ZnO were active and selective for photooxidation of CH 4 with O 2 to oxygenates (CH 3 OOH, CH 3 OH and HCHO) and Ag/TiO 2 {001} enabled the selective production of CH 3 OH, while pristine ZnO and TiO 2 exhibited low activity and selectivity for the production of oxygenates. [20,23] Other researchers have also reported a series of good photocatalysts with noble metals as cocatalyst for photocatalytic aerobic oxidation of methane to oxygenates, such as AuÀ Pd/ TiO 2 , [26] Pd/In 2 O 3 , [16] Au/WO 3 , [25] Pt/WO 3 [24] and black phosphorous-supported Au single atoms. [19] Despite the promising results obtained in the abovementioned studies, the high-cost and limited-reserves of noble metals limits their applications. Therefore, there is a high demand to develop low-cost and high-performance alternative cocatalysts to noble metals. Atomically dispersed non-precious metal atoms anchored on N-doped carbon (M-NC) materials, which are generally considered as single atomic site catalysts, have been employed as cocatalysts for efficient photocatalytic reactions such as H 2 production and CO 2 reduction, [27][28][29] due to the highly exposed active metal sites and efficient transfer of charge carriers. Moreover, the electronic structure of atomic metal sites can be fine-tuned by changing the coordination environments, rendering M-NC active and selective for targeted catalytic reactions with favorable reaction kinetics. [27,30] In view of such distinctive characteristics, M-NC potentially have the capability to enhance the activity and selectivity of semiconductor-based photocatalysts in photooxidation of CH 4 . Nevertheless, to the best of our knowledge, there have been no studies reporting the utilization of M-NC as cocatalysts for photocatalytic CH 4 oxidation.
In this work, a single-atom NiÀ NC/TiO 2 composite is prepared and used as a photocatalyst for direct CH 4 oxidation with O 2 to produce liquid oxygenates. We found that owing to the unique structural properties, the atomically dispersed NiÀ NC sites not only promote the carrier separation and transfer efficiency, but also enable the controlled activation of O 2 to * OOH radicals, a key intermediate for the formation of the primary product CH 3 OOH that can be readily transformed into CH 3 OH and HCHO. As a result, a high C1 oxygenates yield of up to 198 μmol with 93 % selectivity is achieved after 4 h of irradiation, superior to most previously reported photocatalysts using noble metals as cocatalysts. Figure 1a illustrates the synthetic process for the preparation of NiÀ NC/TiO 2 via a facile one-pot solvothermal method. [31] Briefly, TiO 2 (P25) and Ni precursor (NiCl 2 ) were first dispersed in formamide (HCONH 2 ). Then, the mixed solution was solvothermally heated at 180°C for 12 h. During the solvothermal process, formamide can be easily transformed into nitride-doped carbon (NC) material on the surface of TiO 2 ; meanwhile, considering the strong interaction of NiÀ N coordination, NiÀ N bond was formed in the presence of Ni 2 + . Finally, the resulting sample was washed with diluted acid and water for several times to yield TiO 2 loaded with NC coordinated Ni catalyst (denoted as NiÀ NC/ TiO 2 ). The color of the material after solvothermal reaction changed from white to black ( Figure S1), indicative of loading of Ni and CN on TiO 2 . The Fourier-transform infrared (FT-IR) spectra show a new peak at 1386 cm À 1 on NiÀ NC/TiO 2 ( Figure S2), confirming the presence of CÀ N groups. Inductively coupled plasma optical emission spectrometry shows that the weight amount of Ni in NiÀ NC/ TiO 2 is 0.5 wt %. For comparison, TiO 2 decorated with Ni nanoparticles (NPs) with a loading amount of 0.5 wt % (denoted as Ni NPs/TiO 2 ) was prepared via an impregnation method followed by H 2 reduction at 400°C for 1 h.

Results and Discussion
X-ray diffraction (XRD) patterns ( Figure 1b) show that all diffraction peaks are associated with TiO 2 (anatase and rutile) and no peak of any likely Ni species is observed on NiÀ NC/TiO 2 and Ni NPs/TiO 2 . [32] Transmission electron microscopy (TEM) and high-resolution TEM images (Figure 1c-e) show that the surface of TiO 2 is wrapped by a thin amorphous layer in NiÀ NC/TiO 2 and no sign of remarkable Ni NPs is detected, while small Ni NPs with size of 2-3 nm were formed on TiO 2 surface in Ni NPs/TiO 2 ( Figure S3). Two lattice fringes with interplanar distances of 0.352 and 0.325 nm agree well with the crystal parameters of anatase (101) and rutile (110) planes, respectively, implying that solvothermal treatment did not alter the crystal structure of TiO 2 (Figure 1d and e). Aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM) image shows many isolated bright spots with no observed clusters or subnanoparticles in NiÀ NC/TiO 2 (Figure 1f), which directly validates the formation of atomically dispersed Ni sites. The energy dispersive X-ray (EDX) spectroscopy elemental mapping analysis (Figure 1g) demonstrates that elemental Ni is uniformly dispersed throughout the entire structure of NiÀ NC/TiO 2 .
The surface compositions and chemical states of NiÀ NC/ TiO 2 were investigated by X-ray photoelectron spectroscopy (XPS). The high-resolution Ni 2p XPS spectrum of NiÀ NC/ TiO 2 (Figure 2a) displays the binding energy of Ni 2p 3/2 peak at 855.2 eV, which is higher than that of Ni 0 (853.5 eV) and slightly lower than that of Ni 2 + (855.8 eV), [33,34] suggesting the formation of positively charged Ni species. The highresolution N 1s spectrum of NiÀ NC/TiO 2 is deconvoluted into three characteristic peaks at 398.8 eV, 399.7 eV and 400.7 eV ( Figure S4), which could be assigned to pyridinic-N, NiÀ N and pyrrolic-N, [35] respectively. The presence of NiÀ N species indicates that Ni atoms are adequately coordinated with N sites.
X-ray absorption fine structure spectroscopy (XAFS) analysis was further performed to investigate the coordination environment of Ni in NiÀ NC/TiO 2 using Ni foil, NiO and nickel phthalocyanine (NiPc) as references. The Ni Kedge X-ray absorption near-edge structure (XANES) spec-tra ( Figure 2b) show that the absorption edge position of NiÀ NC/TiO 2 is between those of Ni foil and NiO, revealing the cationic Ni sites, in good consistency with the result of XPS analysis. Additionally, NiÀ NC/TiO 2 has a similar preedge profile to NiPc with a peak at 8340 eV, which is attributed to the transition of 1s to 4p and is the feature of planar NiÀ N 4 moiety. [33] Compared with NiPc, the slight decrease in light intensity of NiÀ NC/TiO 2 probably results from the distorted NiÀ N 4 structure. As shown in the Fourier transformed (FT) Ni K-edge extended XAFS (EXAFS) spectra (Figure 2c), the prominent peak at ca.1.40 Å for NiÀ NC/TiO 2 corresponds to first shell coordination of NiÀ N bond, [36] contributing similarly to the NiPc reference, and no obvious NiÀ Ni peak at 2.19 Å is detected, revealing the negligible presence of metallic Ni species. These results confirm the presence of atomic dispersion of Ni species in NiÀ NC/TiO 2 in the form of NiÀ N coordination, in accordance with the result of dispersed Ni atoms from HAADF-STEM image. To precisely quantify the coordination microenvironment of Ni site, the curve fitting for EXAFS spectra was performed (Figure 2d, Figure S5, and Table S1). As shown in Figure 2d, the fitting results of the first coordination shell verify that the Ni site in NiÀ NC/TiO 2 is fourcoordinated by N atoms, matching well with a NiÀ N 4 site configuration. In addition, the wavelet transform EXAFS (WT-EXAFS) spectra (Figure 2e) show that NiÀ NC/TiO 2 and NiPc exhibit similar contour plot with only one intensity maximum at 6.5 Å À 1 instead of the NiÀ Ni interaction (c.a. 8.4 Å À 1 ), [37] This further manifests the formation of dispersed Ni atoms with NiÀ N coordination. All of above characterizations demonstrate that Ni species are atomically dispersed in NiÀ NC/TiO 2 with NiÀ N 4 moiety.
Photocatalytic CH 4 oxidation performance was evaluated in a batch reactor at room temperature using only O 2 as the oxidant. [22,23] As shown in Figure 3a, only HCHO was detected in the liquid phase over pristine TiO 2 after 4 h of irradiation with a yield of 140 μmol, accompanied by 33 μmol of CO 2 . For Ni NPs/TiO 2 , the yields of HCHO and CO 2 slightly decreased to 135 and 26 μmol, respectively, with the production of a small amount of CH 3 OH (19 μmol). The selectivity for C1 oxygenated products increased from � 81 % over pristine TiO 2 to � 86 % over Ni NPs/TiO 2 . Due to CH 3 OH being the precursor of HCHO and CO 2 in the photocatalytic CH 4 oxidation, [22,23] the trace or small amount of CH 3 OH observed over TiO 2 and Ni NPs/TiO 2 suggests the facile overoxidation of CH 4 . By comparison, a much higher yield of primary products CH 3 OOH (55 μmol) and CH 3 OH (29 μmol) together with 114 μmol of HCHO were produced over NiÀ NC/TiO 2 , and the amount of CO 2 decreased to 16 μmol. This leads to a remarkable � 93 % oxygenates selectivity and the corresponding apparent quantum efficiency (AQE) for oxygenates at 360 nm was determined to be 1.9 %. The yield and selectivity of liquid oxygenates of NiÀ NC/TiO 2 are higher than those of NiÀ NPs/ TiO 2 and TiO 2 , demonstrating the superiority of single atom NiÀ NC cocatalysts for the photocatalytic CH 4 oxidation. The excellent photocatalytic performance observed over NiÀ NC/TiO 2 is comparable to or even outperforms most reported photocatalysts decorated with either noble metal or non-noble metal cocatalysts under similar experiment conditions (Table S2). [12,13,16,17,19,20,[22][23][24][25][38][39][40] Reactions without photocatalyst, without light or replacing CH 4 with Ar did not yield any product. Isotope labelling experiment using 13 CH 4 was performed to elucidate the source of carbon atoms of the products. 13 C NMR spectrum shows three obvious peaks assigned to CH 3 OOH, CH 3 OH and HCHO (Figure 3b), suggesting that the produced oxygenates originated from methane, instead of carbon materials in NiÀ NC/TiO 2 . In addition, no liquid products were detected without the introduction of O 2 ( Figure S6), which indicates that O 2 is the necessary for photocatalytic CH 4 oxidation. Isotopic experiments with oxygen revealed that O 2 molecules was the oxygen source of the produced oxygenates ( Figure S7). The overall yield of oxygenates was increased with the reaction time, and the formation of CH 3 OH was observed by extending the irradiation time over 3 h (Figure 3c). Increasing the water amount was conducive to promoting the production of oxygenates and suppressing the overoxidation of CH 4 to CO 2 ( Figure S8). There was marginal loss in the photocatalytic performance and selectivity for oxygenates after consecutive five runs ( Figure S9), and the morphology and structure of catalyst remained unchanged (Figures S10 and S11). These results confirm the good stability of NiÀ NC/TiO 2 . Increasing the amount of Ni from 0.5 wt % to 1.1 wt % did not noticeably improve the performance of NiÀ NC/TiO 2 ( Figure S12), because excessive loading amount of NiÀ CN can shield light absorption of TiO 2 ( Figure S13). When Ni was replaced with Co and Fe, the total amounts of oxygenates were reduced, due to no detectable formation of CH 3 OOH (Figure 3d). This demonstrates that the isolated Ni site in NiÀ NC with unique properties play an important role for efficient photooxidation of methane to oxygenates.
To understand the role of cocatalysts in photocatalytic reaction, the photoluminescence (PL) spectra of the samples were performed to study the photogenerated charge separation efficiency (Figure 4a). Bare TiO 2 shows an intensive emission peak at 400-440 nm upon excitation at 320 nm. After the introduction of Ni NPs or NiÀ NC on TiO 2 , the PL intensity is remarkably decreased, and NiÀ NC/TiO 2 exhibits a lower emission peak than Ni NP/TiO 2 , indicating that NiÀ NC favorably prevents the recombination of charge carriers compared to Ni NPs cocatalysts. The time-resolved PL spectra were carried out to investigate the dynamics of charge carriers (Figure 4b). The average lifetime of NiÀ NC/ TiO 2 (0.9 ns) is shorter than those of Ni NP/TiO 2 (1.9 ns) and TiO 2 (4.5 ns), in line with the typical cocatalyst/semiconductor systems in which the facile electrons transfer from semiconductors to cocatalysts leads to fast fluorescence decay, [41,42] revealing the excellent ability of NiÀ NC to accelerate the transfer of photogenerated electrons. These results demonstrate the positive role of NiÀ CN in efficiently separating electrons and holes, thereby leading to the enhanced performance of photocatalytic CH 4 oxidation.
Generally, for photocatalytic CH 4 aerobic reaction in aqueous solution, the CÀ H bond of CH 4 is oxidized by photo-generated active oxygen species to form * CH 3 radicals, which would react with oxygen-derived free radicals to produce oxygenates. [23] To elucidate the reaction mechanism of photocatalytic CH 4 oxidation, electron paramagnetic resonance (EPR) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was conducted. As shown in Figure 4c, * CH 3 radicals are detected on both NiÀ NC/TiO 2 and Ni NP/TiO 2 , and the intensities of * CH 3 radicals of NiÀ NC/TiO 2 is slightly higher than that of Ni NP/TiO 2 , revealing that the activation of CH 4 to * CH 3 radicals occurs in selective photo-oxidation of CH 4 in aqueous solution. Figure S14 shows that strong EPR signals assigned to * OH radicals are observed without the introduction of CH 4 . The decreased intensity of * OH radicals in the presence of CH 4 may be due to the highly active * OH radicals participating in CH 4 oxidation, such as deep oxidation of CH 4 to HCHO and CO 2 . [22] For the intermediates in photocatalytic O 2 reduction, one set of EPR signals that are assigned to * OOH radicals appear upon illumination (Figure 4d). The observed * OOH radicals are easily produced from O 2 reduction with protons by photogenerated electrons. Clearly, the signal intensity of * OOH radicals of NiÀ NC/TiO 2 is higher than that of Ni NP/ TiO 2 , indicating that NiÀ CN cocatalyst can facilitate the formation of * OOH radicals compared with Ni NP cocatalyst. The high amount of * OOH radicals probably lead to the enhanced production of CH 3 OOH and other oxygenates.
Based on the above results, a plausible photocatalytic CH 4 oxidation mechanism on NiÀ NC/TiO 2 is depicted in Figure S15. Under light irradiadtion, electrons and holes are generated on TiO 2 . The photogeneratred electrons are transferred to single NiÀ NC sites to promote the reducion of O 2 to form * OOH radicals, while the powerful holes are left on the surface of TiO 2 to initiate the CH 4 oxidation to produce * CH 3 radicals. These two radicals can easily combine to form the primary product CH 3 OOH, which can be subsequently transformed into CH 3 OH and HCHO. The single NiÀ NC sites guarantee the efficient separation of photogenerated electrons and holes and the favourable formation of * OOH radicals by mild reduction of O 2 , ultimately leading to excellent performance of photocata- lytic CH 4 oxidation with O 2 . To demonstrate this hypothesis, the detailed reaction pathways were calculated by density functional theory (DFT) calculations. The optimized structural models of NiÀ NC/TiO 2 and Ni NPs/TiO 2 are given in Figure S16.
The energy profiles of the O 2 reduction and CH 4 activation reactions are illustrated in Figure 5a and b, with the corresponding structures of reaction intermediates and transition states shown in Figure 5c and d. The activation of O 2 to form *OOH species is an exothermic reaction on NiÀ NC/TiO 2 and Ni NP/TiO 2 , with a reaction energy of À 0.79 and À 2.85 eV, respectively. The comparatively unfavourable formation of *OOH species indicates the weak surface adsorption of NiÀ NC because of its unique electronic structure. This results in the preferential desorption of *OOH species to generate * OOH radicals that can participate in the production of CH 3 OOH, instead of the subsequent dissociation of *OOH to form * OH radicals due to the large reaction energy (2.00 eV). By contrast, the desorption energy of *OOH species on Ni NPs is as high as 2.62 eV. Compared with *OOH desorption, the dissociation of *OOH to *O + *OH is more preferred on Ni NPs, with a reaction energy of À 2.25 eV and an energy barrier of 0.12 eV, and the produced *OH species on Ni NPs could desorb to form * OH radicals for oxidizing oxygenates to CO 2 . These results indicate that NiÀ NC cocatalyst is beneficial for the production of * OOH radicals in O 2 reduction, consistent with the EPR results, which contributes to the production of oxygenates. The different behaviors of Ni NPs and NiÀ NC on O 2 activation probably because *OOH is very unstable on metallic Ni NPs and is easily dissociated to form strong NiÀ O bonds, as indicated by the large reaction energy (À 2.25 eV), while NiÀ NC is stabilized by N coordination, thus unfavorable for further dissociation of *OOH.
For CH 4 activation, the reaction energy for the cleavage of the first CÀ H bond of 1.14 eV on NiÀ NC/TiO 2 is quite similar to that on Ni NPs/TiO 2 (1.15 eV), with a relatively lower energy barrier (1.46 eV vs. 1.54 eV). Likewise, the reaction energies for the subsequent *CH 3 desorption to * CH 3 radicals on NiÀ NC/TiO 2 and Ni NPs/TiO 2 are similar (1.14 eV vs. 1.13 eV). It clearly shows that there is no significant difference in the activation of methane over NiÀ NC and Ni NPs cocatalysts. As a result, the different pathways of O 2 reduction over NiÀ NC/TiO 2 and Ni NP/TiO 2 primarily contribute to the differences of activity and selectivity in photocatalytic CH 4 oxidation.

Conclusion
In summary, atomically dispersed NiÀ NC/TiO 2 has been developed by a facile one-pot solvothermal method for room-temperature photocatalytic CH 4 oxidation with O 2 to C1 oxygenates with 93 % selectivity. The single-atom NiÀ NC sites function as electron capture centers to achieve efficient separation of charge carriers in NiÀ NC/TiO 2 . Moreover, the isolated Ni atoms are active for the favorable formation and desorption of * OOH radicals in O 2 reduction, rather than being active for the production of * OH radicals that are more likely to facilitate the overoxidation of oxygenates to CO 2 . Such unique properties of NiÀ NC results in a prominent C1 oxygenates productive rate and high selectivity. This work is the first case of single metal atoms anchored N-doped carbon material as a cocatalyst to promote the performance of photocatalytic aerobic oxidation of CH 4 , which may drive the discovery of more earth-abundant and low-cost photocatalysts for efficiently and selectively oxidizing CH 4 to solar fuels and chemicals.