Open AccessCCS ChemistryCOMMUNICATION1 Apr 2022

Atomically Dispersed Ru-Decorated TiO2 Nanosheets for Thermally Assisted Solar-Driven Nitrogen Oxidation into Nitric Oxide

    Thermally assisted photodriven nitrogen oxidation to nitric oxide (NO) using air as a reactant is a promising way to supersede the traditional NO synthesis industry accompanied by huge energy expenditure and greenhouse gas emission. Meanwhile, breaking the N≡N triple bond (941 kJ·mol−1) in nitrogen is still challenging, and the development of more efficient catalysts is necessary. Herein, Ru single atoms decorated TiO2 nanosheets (Ru SAs/TiO2) were constructed and achieved superior performance for NO photosynthesis with a product rate of 192 μmol g−1 h−1 and a quantum efficiency of 0.77% at 365 nm. Both 15N isotope labeling experiments and in situ near ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS) proved the origin of NO from N2 photooxidation. A series of in situ characterizations and theoretical calculations unveiled the reaction pathway of nitrogen photooxidation. Breaking the O–O bond to form (N–O)2–Ru intermediates was demonstrated as the rate-determining step. Importantly, a single-atomic structure was proven to inhibit the aggregation and inactivation of Ru, leading to outstanding durability.

    Introduction

    Nitric oxide (NO) has been widely used in the production of fertilizer, explosives, and fine chemicals.15 Presently, the NO synthesis industry involves four main processes (Scheme 1), including methane reforming, air separation, ammonia synthesis (i.e., 15–25 MPa, 673–873 K),6 and ammonia oxidation (i.e., 1073–1200 K).7 All the processes consume huge amounts of energy. Moreover, the reforming process for the preparation of hydrogen is accompanied by the release of greenhouse gas.8 Thus, seeking innovative strategies that allow energy-conserving and green synthesis of NO is of great significance. Considering the main components in air are nitrogen and oxygen, the development of an efficient strategy to directly drive N2 and O2 to synthesis NO is promising.

    Scheme 1

    Scheme 1 | Schematic illustration of a thermally assisted photodriven air-to-NO strategy and conventional industry synthesis routes for NO.

    Photocatalysis could produce various kinds of highly valued chemicals by converting inexhaustible and clean photoenergy to chemical energy.916 The theoretical potential for N2 direct photooxidation into NO is calculated to be 1.26 V versus normal hydrogen electrode (NHE).7 The photogenerated holes in many photocatalysts, such as TiO2,17,18 WO3,19 and ZnO,20 meet this requirement. Moreover, the temperature rise would improve the equilibrium concentration of NO owing to the positive standard enthalpy (90.3 kJ mol−1) and entropy (12.4 J K−1mol−1NO) of the reaction (N2(g) + O2(g) ⇌ 2NO(g)).8 Thus, our group recently proposed a strategy for NO synthesis via thermally assisted photooxidation of nitrogen with oxygen and experimentally proved the feasibility of this strategy over WO3/TiO2 heterostructure.21 However, breaking the N≡N triple bond (941 kJ·mol−1) in nitrogen is still challenging and hampers the output and quantum efficiency of NO photosynthesis. Furthermore, unveiling the reaction pathway of nitrogen photooxidation on the surface of catalytic materials is significant for the rational design and construction of efficient catalysts.

    Notably, isolated single-atomic catalysts have shown excellent performance in various areas owing to their high atom utilization and unique electronic structure.2225 Moreover, the anchored single atoms strongly interact with the surrounding supports, and thus the aggregation and inactivation of single atoms could be hindered.26,27 Herein, Ru single atom-decorated TiO2 nanosheets (Ru SAs/TiO2) were constructed and demonstrated to be efficient for thermally assisted NO photosynthesis in a continuous fixed-bed reactor (Scheme 1). The existence of Ru single sites was corroborated by X-ray absorption near edge structure (XANES) spectroscopy and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The NO yield rate achieved 192 μmol g−1 h−1 at 250 °C with a corresponding apparent quantum efficiency (AQE) of 0.77% at 365 nm. 15N isotope labeling experiments confirmed the generation of NO originated from N2 photooxidation. The combined results of in situ near ambient pressure X-ray photoelectron spectroscopy (in situ NAP-XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and density functional theory (DFT) calculations unveiled the reaction pathway and identified the rate-determining step of nitrogen oxidation on Ru SAs/TiO2. Additionally, compared with Ru nanoparticle-decorated TiO2 nanosheets (Ru NPs/TiO2), the single-atomic structure in Ru SAs/TiO2 efficiently hindered the aggregation and inactivation of active Ru sites, resulting in greatly improved durability.

    Results and Discussion

    TiO2 nanosheets were synthesized through a typical solvothermal process. Then, Ru SAs/TiO2 and Ru NPs/TiO2 were obtained by photochemical reduction under an ice-bath.25 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed the two-dimensional (2D) morphology of TiO2 nanosheets ( Supporting Information Figure S1). In the SEM ( Supporting Information Figure S2a) and TEM images (Figure 1a) of Ru SAs/TiO2, only 2D nanosheets could be found. The high-resolution TEM (HRTEM) image displayed its crystalline plane of 0.35 nm, corresponding to the (101) plane of TiO2 (Figure 1b). For Ru NPs/TiO2 sample, Ru nanoparticles owned an average size of 1.22 nm ( Supporting Information Figures S2b–S2d). The mass percentage of Ru was calculated to be 0.42% and 4.57% in Ru SAs/TiO2 and Ru NPs/TiO2, respectively ( Supporting Information Figure S3a). The characteristic X-ray diffraction (XRD) peaks of anatase TiO2 (JCPDS no. 84-1286) could be seen in all samples ( Supporting Information Figure S3b). Metallic Ru peaks (JCPDS no. 06-0663) were presented in Ru NPs/TiO2 but absent in Ru SAs/TiO2, indicating high dispersion of Ru in Ru SAs/TiO2.28 HAADF-STEM image of Ru SAs/TiO2 exhibited many bright points in yellow circles (Figure 1c), indicating the atomically dispersed Ru sites. Energy-dispersive X-ray (EDX) mapping images (Figures 1d1f) showed the even dispersion of Ru sites in Ru SAs/TiO2. As shown in the XANES spectroscopy (Figure 1g), the absorption threshold of Ru was located between Ru foil and RuO2. The Ru–O bond at ca. 1.5 Å was observed while the metallic Ru–Ru bond (at ca. 2.5 Å) was absent in the extended X-ray absorption fine structure (EXAFS) spectrum of Ru SAs/TiO2 (Figure 1h), confirming the successful synthesis of atomically dispersed Ru sites decorated TiO2 nanosheets.28 Notably, the valence state of Ru was between 0 and +4, and the metallic Ru–Ru bond at ca. 2.5 Å was identified for Ru NPs/TiO22830 (Figures 1g and 1h), which could be ascribed to the partial surface oxidation of metallic Ru nanoparticles. UV–vis absorbance spectroscopy demonstrated that the loading of Ru single atoms could extend the absorption range of TiO2 ( Supporting Information Figure S4a). The bandgap energies (Eg) of TiO2 and Ru SAs/TiO2 were calculated to be 3.08 and 2.95 eV, respectively ( Supporting Information Figure S4b). The valence band (VB) positions of Ru SAs/TiO2 and TiO2 were estimated to be +2.43 and +2.73 V (vs. NHE) ( Supporting Information Figures S4c and S4d). Based on these results, bandgap structure diagrams were depicted, demonstrating the thermodynamic possibility of N2 photooxidation on Ru SAs/TiO2 ( Supporting Information Figure S5). Photoanodes of Ru SAs/TiO2 displayed significant enhancement in photocurrent response compared with TiO2 ( Supporting Information Figure S6a). Meanwhile, the photoluminescence intensity of Ru SAs/TiO2 ( Supporting Information Figure S6b) obviously decreased compared with TiO2, indicating a reduced recombination of electron–hole pairs in Ru SAs/TiO2.

    Figure 1

    Figure 1 | Characterizations of Ru SAs/TiO2. (a) TEM and (b) HRTEM images of Ru SAs/TiO2. (c) HADDF-STEM image of Ru SAs/TiO2. Single-atom Ru was highlighted by yellow circles. (d–f) EDX mapping images of Ru SAs/TiO2. (g) Ru K-edge XANES spectra and (h) Fourier transforms of k3-weighted Ru K-edge EXAFS of different samples.

    The photocatalytic activities of Ru SAs/TiO2 for N2 oxidation were tested in a home-made flow reactor using a 300 W Xe lamp as the light source ( Supporting Information Figure S7). In all experiments, meticulous pretreatments were taken to avert NH3 and NO2/NO3 contaminants. Under irradiation and 150 °C, Ru SAs/TiO2 could catalyze the reaction between N2 and O2 to produce NO (Figure 2a). When light, catalysts, N2, or O2 was removed, no NO was generated, excluding the existence of impurities in the reaction system. As shown in Figure 2b, the NO yield rate over Ru SAs/TiO2 improved rapidly with temperatures ranging from 150 to 300 °C. However, the time-dependent yield rate exhibited a rapid shrink during the long-time test at 300 °C ( Supporting Information Figure S8). Therefore, 250 °C was chosen as the optimal operating temperature for the subsequent studies. The obvious photoresponsive phenomenon for NO production proved that the reaction was driven by photon illumination (Figure 2c).31 With the increase of temperature from 25 to 150 °C ( Supporting Information Figure S9), an obvious decay of photoluminescence intensity was observed in Ru SAs/TiO2, indicating the reduced electron–hole recombination at higher temperatures. Moreover, the NO-temperature-programmed desorption (TPD) profile ( Supporting Information Figure S10) showed that NO on Ru SAs/TiO2 desorbed at 110 and 246 °C. Thus, the external heating promoted photodriven nitrogen oxidation via the increase of NO equilibrium concentration,8 the suppression of electron–hole recombination, and the enhancement of product desorption.3237 With increasing monochromatic light wavelength, the AQE value decreased, which was consistent with the absorption spectra of Ru SAs/TiO2 (Figure 2d). Furthermore, the NO yield rate showed no obvious decay over eight cycles ( Supporting Information Figure S11). The morphology and structure was maintained well after long-term testing ( Supporting Information Figure S12). To the best of our knowledge, the sample of Ru SAs/TiO2 showed the best performance for NO photosynthesis using N2 and O2 as reactants in a flow reactor (NO yield rate: 192 μmol g−1 h−1; AQE: 0.77% at 365 nm) and even comparable with the best photocatalysts for nitrogen fixation using N2 and H2O as reactants in a batch reactor ( Supporting Information Table S1). 15N isotope-labeling experiments (Figure 2e) showed that 15NO (m/z = 31) could be detected only when 15N2 was used as the N-source, and no 15NO (m/z = 31) could be detected in 15N2 feeding gas ( Supporting Information Figure S13), demonstrating that the generation of NO originated from N2 photooxidation. Moreover, the photocatalytic in situ-generated NO could react with styrene to produce β-nitrostyrolene with 100% selectivity (Figure 2f and Supporting Information Figures S14 and S15), indicating the wide application potential of this N2 photooxidation strategy.

    Figure 2

    Figure 2 | Performances and verification experiments of NO production via the photooxidation of N2 over Ru SAs/TiO2. (a) Quantitative determination of generated NO over Ru SAs/TiO2 under different conditions at 150 °C. (b) Photocatalytic yield rate of NO over Ru SAs/TiO2 at different temperatures. (c) NO yield rate on the Ru SAs/TiO2 catalyst with and without light irradiation at 250 °C. (d) Wavelength-dependent AQE and light absorption of Ru SAs/TiO2. (e) Mass spectra of gas chromatography–mass spectrometry (GC–MS) analysis in the 15N2 isotope-labeling experiment. (f) Synthesis of β-nitrostyrolene using in situ-synthesized NO.

    Theoretical simulations were performed to reveal the reaction pathway (Figure 3a). All results were obtained based on optimized surface structures of Ru SAs/TiO2 ( Supporting Information Figures S16 and S17). When N2 and O2 co-existed in the system, two oxygen molecules were first adsorbed on the interface of Ru and TiO2 to form *O2 intermediates. Next, the third O2 molecule adsorbed at the top of Ru, forming a new unstable *O2 intermediate. Then, N2 molecules attacked the third-adsorbed *O2 with the broken O–O bond and the formation of (N–O)2–Ru five-membered ring (ΔG = 0.79 eV). Finally, with the release of two NO molecules step by step, the Ru–O octahedra was restored. Along this reaction pathway, the formation of (N–O)2–Ru intermediates with five-membered ring configurations served as the rate-determining step. To confirm the reaction pathway obtained by DFT results, a series of in situ characterizations were carried out. Under the ultrahigh vacuum, two typical Ti2p XPS peaks maintained the same with and without light irradiation ( Supporting Information Figure S18). When 0.4 mbar simulated air was introduced, upon exposure to irradiation, Ti2p peaks shifted to higher binding energy, indicating an electron donation from Ti sites to absorbed O2 to form *O2 ( Supporting Information Figure S19 and Figure 3b).29 As shown in Supporting Information Figure S20, no characteristic N1s XPS peaks were observed in the ultrahigh vacuum either under dark or irradiation, demonstrating no N-containing impurities absorbed on the surface.38 When 0.4 mbar-simulated air was introduced, the XPS peak for gaseous N2 (g-N2) appeared at 404.5 eV in the dark (Figure 3c).15 Upon exposure to illumination, the characteristic peak of g-N2 shifted to lower binding energy, indicating that electrons injected into the Π antibonding of N2 molecular to form N–O–Ru species.39 Along with the irradiation time, *NO species emerged in the XPS spectra with binding energy at 399.9 eV (Figure 3c).4043 In situ DRIFTS were further applied to capture the intermediates adsorbed on the catalyst surface. In the control experiment performed in the Ar atmosphere under irradiation, no obvious bands were observed (Figure 3d). The absence of N-containing species excluded the contaminants once again. When simulated air was purged into the cell, two characteristic peaks of adsorbed *NO species centered at 1620 and 1540 cm−1 appeared and the peak intensity gradually increased along with irradiation time (Figure 3e).4446 The aforementioned results unveiled the reaction pathway of photocatalytic air-to-NO and identified the rate-determining step of the total reaction, which was critical for the rational design and construction of efficient photodriven N2 oxidation catalysts.

    Figure 3

    Figure 3 | Theoretically simulated reaction pathway and intermediate analyses of N2 photooxidation over Ru SAs/TiO2. (a) Reaction free-energy diagram of N2 oxidation on Ru SAs/TiO2. (b) Ti2p in situ NAP-XPS spectra of Ru SAs/TiO2 under illumination. (c) N1s in situ NAP-XPS spectra of Ru SAs/TiO2 under 0.4 mbar simulated air. (d and e) Time-dependent in situ DRIFTS spectra on Ru SAs/TiO2 in Ar atmosphere and simulated air.

    TiO2 nanosheets presented negligible activity for N2 photooxidation, while the NO yield rate of Ru NPs/TiO2 was higher than that of Ru SAs/TiO2 at the initial time (Figure 4a). Notably, the activity of Ru NPs/TiO2 exhibited a rapid decline, and the performance of was Ru SAs/TiO2 maintained, indicating the superior durability of single-atom Ru. After the performance test, there were no apparent changes in the 2D morphology of Ru SAs/TiO2 (inset in Figure 4b), and the HAADF-STEM image still showed the isolated single Ru sites (Figure 4b). XANES (Figure 4c) and EXAFS (Figure 4d) spectra of Ru SAs/TiO2 demonstrated the preservation of Ru single atoms during the test.45 As for Ru NPs/TiO2, the nanoparticles grew bigger from 1.22 to ∼20 nm during the measurement process ( Supporting Information Figure S21a). HRTEM imaging showed their lattice spacing of 0.32 nm ( Supporting Information Figure S21b), corresponding to the (110) plane of RuO2. After performance testing, Ru diffraction peaks in Ru NPs/TiO2 disappeared, and new peaks of RuO2 (JCPDS no. 40-1290) emerged (Figure 4e). In fresh Ru NPs/TiO2 sample, nanoparticles mainly existed in the formation of metallic Ru with surface RuO2 (Figure 4f and Supporting Information Figure S22). In the recycled Ru NPs/TiO2 sample, the peaks assigned to Ru0 disappeared, and two peaks assigned to Ru4+ remained.47,48 The aforementioned results proved that Ru nanoparticles would be converted into bigger RuO2 particles and become inactivated during testing, while the single-atomic structure efficiently inhibited the aggregation of Ru, leading to outstanding durability enhancement.

    Figure 4

    Figure 4 | (a) Time-dependent NO yield over different samples at 250 °C. (b) HADDF-STEM and TEM (inset) images of used Ru SAs/TiO2. (c) Ru K-edge XANES spectra and (d) Fourier transforms of EXAFS data of fresh and used Ru SAs/TiO2. (e) XRD patterns of used Ru NPs/TiO2 and used Ru SAs/TiO2. (f) XPS spectra of fresh and used Ru NPs/TiO2.

    Conclusion

    Ru SAs/TiO2 were successfully synthesized and adopted as the thermally assisted catalysts for solar-driven NO synthesis from air with outstanding performance (NO yield rate: 192 μmol g−1 h−1; AQE: 0.77%). 15N isotope-labeling experiments verified that NO originated from N2 oxidation. In situ NAP-XPS, in situ DRIFTS, and DFT calculations unveiled the possible reaction pathway and suggested the breaking of the O–O bond to produce the (N–O)2–Ru intermediate as the rate-determining step. Moreover, it was proven that the single-atomic structure could inhibit the aggregation of Ru, leading to outstanding stability. This work may pave a new avenue for rationally designing and constructing efficient catalysts toward solar-driven air-to-NO synthesis.

    Supporting Information

    Supporting Information is available and includes experiment details, characterization methods, and additional data.

    Conflict of Interest

    There is no conflict of interest to report.

    Acknowledgments

    This work was financially supported by the National Natural Science Foundation of China (nos. 22071173 and 21871206) and the Natural Science Foundation of Tianjin City (no. 17JCQNJC03200).

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