(I) Mediated Câ¡N Bond for Low Temperature CO Oxidation

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Energy, Environmental, and Catalysis Applications 2

Molecular O Activation over Cu (I) Mediated C#N Bond for Low Temperature CO Oxidation Siyu Hu, Wen Xiao, Weiwei Yang, Ji Yang, Yarong Fang, Juxia Xiong, Zhu Luo, Hongtao Deng, Yanbing Guo, Lizhi Zhang, and Jun Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02367 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Molecular O2 Activation over Cu (I) Mediated C≡N Bond for Low Temperature CO Oxidation

Siyu Hu†, Wen Xiao§, Weiwei Yang†, Ji Yang†, Yarong Fang†, Juxia Xiong†, Zhu Luo†, Hongtao Deng†, Yanbing Guo*,†, Lizhi Zhang†, Jun Ding§

†

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of

Environmental and Applied Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China §

Department of Materials Science and Engineering, National University of Singapore, 117575,

Singapore * To whom correspondence should be addressed. E-mail: [email protected] Phone: +86-17786507005

ACS Paragon Plus Environment

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ABSTRACT The activation of molecular oxygen (O2) is extremely crucial in heterogeneous oxidations for various industrial applications. Here, a charge-transfer complex CuTCNQ nanowires array (CuTCNQ NWs) grown on the copper foam was first reported to show CO catalytic oxidation activity at temperature below 200 °C with the activated O2 as an oxidant. The molecular O2 was energetically activated over the Cu (I) mediated C≡N bond with a lower energy of -1.167 eV, and preferentially reduced to •O2- through one-electron transfer during the activation process by density functional theory (DFT) calculations and Electron paramagnetic resonance (EPR). The theoretical calculations indicated that CO molecule was oxidized by the activated O2 on the CuTCNQ NWs surface via the Eley-Rideal (ER) mechanism, which had been further confirmed by in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS). These results indicated that the local C≡N bond electron state engineering could effectively improve the molecular O2 activation efficiency, which facilitate the low-temperature CO catalytic oxidation. The findings reported here enhance our understanding on molecular oxygen activation pathway over metal-organic nanocatalysts and provide a new avenue for rational design of novel low-cost, organic based heterogeneous catalysts. Keywords ：CuTCNQ NWs; Molecular O2 activation; CO oxidation; DFT calculations; in situ DRIFTS


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Introduction Oxidation reactions are essential steps involved in the chemical processes, which show enormous importance in various energy generation and environment remediation applications such as fuel cell, fine chemical production, catalytic combustion, exhaust emission control and waste water treatment1–5. With the increasing pressure on developing sustainable and environmentally friendly chemical processes, molecular oxygen (O2) has thus attracted everincreasing interests as an ideal green oxidant due to its ready existence, abundance and zero waste product6. The activation of molecular O2 on the surface of heterogeneous catalysts plays an indispensable role in oxidation reactions such as epoxidation7, CO8, hydrocarbon9, alcohol10 and glucose oxidations11. Among various heterogeneous catalysts, molecular activation over organic based nanocatalysts attracted a great attention for low temperature heterogeneous catalysis due to its low cost and excellent molecular tailorability12,13. Comparing with the inorganic catalysts, the stability of organic catalysts is poor due to its self-oxidation14. Developing an organic catalyst which can effectively activate the molecular oxygen without selfoxidation remains to be a great challenge. Interestingly, recent progresses have demonstrated that the large binding energy between the transition metal and π/π* states potentially could enhance the durability of organic nanocatalysts15,16. And the metal ion typically serve as the active site for reactant absorption and activation in such kind of heterogeneous catalysts17. Lately, density functional theory (DFT) computation has predicted that some organic compounds with triple bond (e.g., C ≡ N bond, C ≡ C bond)18,19 may be able to activate molecular oxygen in heterogeneous catalytic reactions. However, due to the high reactivity of triple bond, the molecular oxygen activation over triple bond based organic nanocatalysts has not been demonstrated. And the molecular O2 activation process and the active site is still unclear, which


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will be extremely beneficial for the design of low cost, high-activity organic based catalysts once accomplished. Herein, by selecting CO as a probe molecule, we firstly reported the low-temperature CO catalytic oxidation activity for a metal-organic complex of CuTCNQ nanowires array20 (CuTCNQ NWs). The defined CuTCNQ NWs grown on copper foam showed higher activity than Cu (I) oxides (such as Cu2O nanowires and Cu2O powder). Furthermore, through density functional theory (DFT) calculation, electron paramagnetic resonance (EPR) spectra and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) analysis, a new molecular O2 activation mechanism, in which molecular O2 was activated and converted into •O2- on Cu (I) mediated C≡N bond, was revealed for low-temperature CO oxidation over CuTCNQ NWs. The mechanism uncovered here will enhance our understanding of molecular oxygen activation on organic nanocatalysts in gas phase catalysis and pave a new avenue for novel low-cost, organic based heterogeneous catalysts design. Experimental Section Synthesis of CuTCNQ nanowires array. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) was obtained from Sigma-Aldrich. Foamy copper (99% purity) were successively cleaned with 2% dilute nitric acid, deionized water, ethanol in an ultrasonic bath and dried with nitrogen flow immediately for the following experiments. Then, 4mg TCNQ powder was loaded in a glass groove and the cleaned foamy copper was placed on top of the glass groove. Afterwards, the entire set-up was placed at the center of a quartz tube and inserted into a horizontal tube furnace. The furnace temperature and the reaction time was set to 250 °C and 10 min, respectively. After


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the reaction, both sides of foamy copper were observed to turn bule-black in color indicating the formation of CuTCNQ nanowires array (CuTCNQ NWs). Synthesis of Cu2O nanowires array. The Cu2O nanowires array (Cu2O NWs) were obtained directly by annealing the cleaned copper foam (according to above clean method) in a horizontal tube furnace for 4h at 600 °C. And the Cu2O powder (≥90%) was purchased (form Sinopharm Chemical Regent Co., Ltd.) and used without further treatment. Characterization. The field emission scanning electron microscopy (SEM) and energydepressive spectrum (EDS) were performed by using a JEOL JSM-6700F FE-SEM at an accelerating voltage of 10 kV. The transmission electron microscopy (TEM) image was recorded by a FEI Tecnai G2 F20 with an acceleration voltage of 200 kV. The IR spectra were measured on a NICOLET iS50 FT-IR spectrometer. X-ray photoelectron spectra (XPS) were collected by Thermo ESCALAB 250XI with Al Kα (hv =1486.6 eV) as the excitation source. And the binding energies were corrected by aligning C 1s to 284.8 eV. Electron paramagnetic resonance (EPR) spectra were detected on a Bruker EMX EPR Spectrometer (JES FA200) at room temperature. Catalytic activity evaluation. The catalytic activity of CO oxidation on CuTCNQ nanowires array, Cu2O nanowires array and Cu2O powder were evaluated at a quartz-tube plug flow reactor by using 100 mg catalyst in a mixed gas (1 vol% CO, 5 vol% O2 and 90 vol% nitrogen) with a gas hourly space velocity (GHSV) of 30,000 h-1. The catalyst was heated from 50 °C to 175 °C at a rate of 5 °C min-1 and then kept for 40 min until the catalytic reaction reached a steady state. The outlet gases were analyzed online by a gas chromatograph (GC-2030,


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TETWORTHY) equipped with a flame ionization detector (FID). Then the CO conversion was calculated based on the change of CO concentration on the inlet and outlet of catalytic reactor. Theoretical calculations. Our calculations were performed using density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Packages (VASP)19. The exchangecorrelation energy function of the Perdew-Burke-Ernzerhof (PBE) is described within the generalized gradient approximation (GGA). The kinetic energy cutoff for plane-wave basis set in 450 eV, and all atoms were allowed to relax until the force and energy are less than 2×10-2 eV Å1 and 10-4 eV, respectively. The primitive cell of CuTCNQ (Figure S6) was used as a model system. To make sure that the calculations is appropriate, we first optimized the configurations of CuTCNQ. The optimized cell parameter of CuTCNQ is a=b=15.93 Å, c=3.88 Å. Furthermore, the unit cells were constructed on the basis of (2×2×1) supercells of CuTCNQ, where the vacuum space was specified to be ~15 Å. And the Brillouin zone integration was performed with 3×3×1 k-points sampling according to the Monkhorst-Pack scheme21. DRIFTS experiment. DRIFTS experiments were carried out using a NICOLET iS50 FTIR spectrometer fitted with an MCT detector. The DRIFTS cell (Harrick) was fitted with ZnS2 windows and a heating cartridge that allowed samples to be heated. The CuTCNQ NWs and Cu2O powder were heated at 120 °C with a rate of 10 °C min-1, which were successively purged in flow of 1% CO/N2, pure N2 and 1% CO /5% O2/N2, respectively. The total flow is 50 mL/min and the catalysts are exposed to each gas mixture at 120°C for 20 min. Results and discussion Catalyst Characterization. A large-scale organic charge-transfer complex of CuTCNQ nanowires array grown on the copper foam was synthesized by a simple vapor-solid-phase


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reaction following previous literatures23. In brief, the TCNQ powder was loaded in a glass groove and placed at the center of a quartz tube inserted in a horizontal tube furnace. The cleaned copper foam (1.5 × 0.7 cm2) was placed on top of the glass groove immediately and the reaction was performed at 250 °C for 10 min. As depicted in Figure 1A, the shinning copper foam has completely transformed into blue-black in color after reaction. The morphologies and microstructures of the as-obtained products were investigated by scanning electron microscope (SEM). Figure 1B shows that the final copper foam still maintains an interconnected porous framework, which is beneficial to gas transport and absorption. Under the high-magnification SEM observation (Figure 1C), the large-scale, uniform and flexible CuTCNQ nanowires are found to grow on copper foam with length more than 10µm. Furthermore, Figure 1D clearly displays the transmission electron microscopy (TEM) image of a typical CuTCNQ nanowire with the diameter of 400-500 nm. In addition, the bonding structure and elementary composition of CuTCNQ nanowires were studied systemically with Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), respectively. The FTIR spectra of pristine TCNQ (black curve) and CuTCNQ nanowires (red curve) are characterized in Figure 1E. The pure TCNQ molecule exhibits four characteristic principle vibration modes at 862 cm-1 (C=CH bending), 1357 cm-1 (C=C wing stretching), 1542 cm-1 (C=C ring stretching), and 2234 cm-1 (C≡N stretching). Resulting from the charge transfer between Cu and TCNQ0 molecule, the spectrum of CuTCNQ NWs has changed obviously. It can be seen that the C=CH stretching vibration mode shifts to 826 cm-1 and the C≡N stretching band shifts to 2198 cm-1, which indicates the presence of TCNQ-. Moreover, the C=C ring stretching mode in TCNQ at 1542 cm-1 splits into two peaks at 1507 cm-1 and 1572 cm-1, which are assigned to the TCNQ anion radicals24. Also, the C=C wing stretching exhibits a slight shift to


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1354 cm-1. The C, N and Cu signals detected from these nanowires in EDS analysis suggest that the observed structures are CuTCNQ (Figure S1)25. The XPS spectrum (Figure 1F) shows binding energies (BE) of Cu 2p1/2 and 2p3/2 peaks at 931.7 and 951.8 eV, which exhibits no evidence of satellites attributed to Cu2+, suggesting that the products are essentially Cu+

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Meanwhile, the N 1s orbital appears with a single feature at 398.1 eV (Figure S2), indicating the presence of only one type of TCNQ in the materials27. Combining the above SEM, TEM, FTIR, EDS and XPS results, we can conclude that CuTCNQ nanowires array has been uniformly grown on copper foam. CO catalytic performance and reaction kinetics. To verify the low temperature catalytic activity of CuTCNQ NWs, the corresponding CO catalytic performance of the CuTCNQ NWs, Cu2O NWs (Figure S3) and Cu2O powder was investigated. Since Cu2O was reported to have better CO catalytic activity at low temperature in Cu based catalysts28, Cu2O NWs and Cu2O powder were selected as control samples. Beyond our expectation, as shown in Figure 2A, the CO oxidation rate on CuTCNQ NWs is significantly higher than those over Cu2O NWs and Cu2O powder (see the detailed calculations in the supporting information). At temperature below 100 °C, CO2 has been detected, which indicates that CO and O2 have begun to react with a low oxidation rate over CuTCNQ NWs. While the other two catalysts hardly show any reaction activity. Furthermore, the CO oxidation rate on CuTCNQ NWs is 2.2 × 10-3 mol.g-1.s-1, which is about 22 times higher than those over Cu2O NWs and Cu2O powder at 150 °C. Even more compelling is that the corresponding Arrhenius plots shown in Figure 2B, the apparent activation energy over CuTCNQ NWs (18.4 kJ mol-1) is much lower than that over Cu2O NWs (33.1 kJ mol-1). The reaction orders of CO and O2 over the CuTCNQ NWs were 0.57 and 0.85 (Figure 2C, 2D), respectively (detailed calculations in the supporting information). The reaction order of


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O2 is near to first-order over the CuTCNQ NWs, which is consistent with the Eley–Rideal (ER) mechanism29. Additionally, the reaction order of CO is smaller than O2, leading to very strong dependencies of the reaction rate on O2 concentration. Since the activation of O2 molecule is the rate-limiting step, this also demonstrates that there may be much more active oxygen species on the surface of CuTCNQ NWs catalyst at the reaction condition30. The spectra of CO conversion versus temperature over CuTCNQ NWs, Cu2O NWs and Cu2O powder (Figure S5) clearly display that CuTCNQ NWs has higher catalytic activity. These results demonstrate that CuTCNQ NWs hold great potential for CO catalytic oxidation at temperature below 150 °C. Besides, the XPS spectra of CuTCNQ NWs showed the pure presence of Cu+ with no Cu2+ peak before and after CO catalytic activity test, which suggested that CuTCNQ NWs is quite stable during the CO oxidation process (as shown in Figure S8). It’s worth noting that the N 1s orbital of CuTCNQ NWs appears with a new broader peak at 398.1 eV owing to electron transfer from Cu to TCNQ, which results in the change of chemical state of N atoms24. Besides, in nearinfrared region (Figure 1E), a peak of conjugate CN group at 2198 cm-1 in CuTCNQ NWs shows obvious red-shift of 36 wavenumbers comparing with TCNQ molecule. The shift demonstrates the decrease of bond strength because of the π-π stacking and the enlarged conjugated system of the CuTCNQ molecule31. The strong π-π stacking may enhance the charge-carrier concentration and mobility of CuTCNQ molecule, which is favorable to the excitation of ground state O2 to highly reactive oxygen species (ROS)32. Therefore, it is reasonable to infer that the molecular O2 may be activated either on Cu (I) ion or C≡N bond of CuTCNQ molecule. DFT calculation for CO oxidation reaction mechanism. To better understand the possible reaction mechanism and catalytic kinetics, we modeled the CO oxidation on CuTCNQ surface by using density functional theory (DFT). For the calculation, we constructed and


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optimized the geometric structure of CuTCNQ as a model system. As shown in Figure 3A, the Cu atom is coordinated to four nitrogen atoms in a highly distorted tetrahedral environment as evidenced by the N-Cu-N angles of 92° and 142°. And the quinoid rings of the TCNQ units are engaged in interplanar stacking33,34. In order to make sure that the computational approach is appropriate, we optimized the configurations of CuTCNQ by Vienna ab initio simulation packages (VASP) in Figure 3B. The optimized cell parameter of CuTCNQ is a=b=15.93 Å, c=3.88 Å (the primitive cell of CuTCNQ is depicted in Figure S6), the mean C=C bond length in rings and wings is 1.38 Å and 1.46 Å, respectively. While, the C≡N bond length, N-Cu bond length and C-H bond length is 1.17 Å, 1.95Å and 1.08 Å. It has been reported that CO molecules generally reacted with oxygen via the Eley-Rideal (ER) mechanism over metal-organic complexes35,36. According to the widely accepted ER mechanism37,38, the CO oxidation on the CuTCNQ may occur through the following reactions: O2(ad) + CO(gas) → O(ad) + O(ad) + CO(gas) → O(ad) + CO2(gas). It means that the CO(gas) directly reacts with the one adsorbed O(ad) atom after the dissociation of adsorbed O2(ad), then proceeding with CO(gas) + O(ad) → CO2(gas). Typically, oxygen adsorbs on metal center in most metal-organic complexes39,40. While recent density functional theory (DFT) computation has predicted that the triple bond may be responsible for activation of the molecular oxygen12. We have firstly studied the adsorption energy of CO and O2 on the CuTCNQ surface to explore the reaction mechanism of CO oxidation over CuTCNQ step by step. Figure 3C demonstrates the optimized configurations at various steps of CO oxidation on the surface of CuTCNQ and the minimum energy pathway (MEP) in ER mechanism. The most stable bare surface of CuTCNQ and the pre-adsorbed O2 molecule is selected as the initial state (Figure 3C, IS). Then the O2 molecule starts to approach the bare surface (Figure 3C, state ⅰ), the adsorption energy (Ead) of O2 near the CuTCNQ is -


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0.285 eV. It’s worth noting that the O2 molecule is activated between C≡N bond (C0≡N0) rather than Cu (I) ion, which is very different from the Cu center activation mechanism reported on majority metal-organic complex. Meanwhile, a new C0-O2 bond as well as N0-O1 bond forms with the energy of -1.167 eV (Figure 3C, state ⅱ), suggesting that the activation of O2 molecule on the CuTCNQ surface is energetically favorable. Along the reaction pathway, CO(gas) molecule approaches the activated O2(ads) to reach the state ⅲ, and this step needs the adsorption energy of - 2.945 eV. Then, the reaction can proceed following two paths starting from state ⅲ. In one path, with the CO continuing to approach, the state

formed with generating the first

CO2 molecule away from the CuTCNQ surface by combination with O1 binding the N0 atom but the O2 (activated oxygen) strongly adsorbed on the C0 atom (Ead = - 0.013 eV). As the reaction continuing, another CO molecule combined with O2 to produce the second CO2 molecule (state ⅴ) with the adsorption energy of - 3.277 eV. Finally, the system recovered to its original state and to reach the final state (FS) with the energy of 0.075 eV and was ready for the next reaction cycle. In contrast, another possible reaction path involved in the process may be that the first CO2 molecule was generated from C0 atom and the O1 remained on N0 atom (state ⅵ) with the adsorption energy of 0.981 eV. After that, the second CO2 molecule gradually deviated from CuTCNQ surface (state ⅶ) with the energy of - 4.206 eV. In the end, the system reached the same FS with the energy of 0.01 eV. In short, the CO molecule and O2 preferentially reacted through the first path from state

due to the lower energy. During the CO oxidation over

CuTCNQ NWs via ER mechanism, the activation of O2 molecule was the rate-limiting step. It should be noted that the O2 molecule was spontaneously activated between C0≡N0 bond with a slightly lower energy of -1.167 eV. In addition, the state

was more thermally favorable and

had a modest adsorption energy. Meanwhile, the C≡N bond was elongated from 1.17 to 1.45 Å


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due to the charge transfer from the CuTCNQ monolayers to the O2-2π* orbitals. And the process of CO oxidation on the CuTCNQ NWs needs a lower energy (-7.613 eV). Consequently, DFT modeling suggests that the O2 molecule has been effectively activated over Cu (Ι) mediated C≡N bond rather than on Cu (I), which rendered the CuTCNQ NWs to show a high catalytic activity of CO oxidation at low temperatures. Molecular O2 activation mechanism. To further understand the catalytic active oxygen specie and electron-transfer pathway of molecular O2 activation on C≡N bond of CuTCNQ molecule, the charge density difference and Bader charge were employed to trace the electron behaviors. The exchange and transfer of electrons mainly took place among the activated O2, Cu atom, C0 atom and N0 atom. As shown in Figure 4A, the Cu atom depletes 0.7 electrons and C0 atom depletes 0.8 electrons, while N0 atom accumulate 1.3 electrons. Thus, these electrons delocalized around C≡N bond and then transferred to the O2 π* orbital to generate a new electron configuration of O2 (accumulated 1.4 electrons). The electron transfer accumulated charges on the oxygen atoms, which is in accord with the calculated one-electron transfer in Bader charge (Table S1). Meanwhile, the electron-transfer pathway of O2 dissociated on TCNQ surface was also employed as comparison. It’s known that there are C≡N bonds with different length in the TCNQ molecule, where the C1≡N1 bond length is 1.25 Å, the C2≡N2 bond length is 0.91 Å (Figure S7a, S7b). The configurations of each state and minimum energy profiles for O2 dissociation on TCNQ are demonstrated in Figure S7c. The molecular O2 dissociated on C2≡N2 bond of TCNQ needs a higher dissociation energy of 25.5 eV, which indicates the dissociation of O2 molecule on C2 ≡ N2 bond of TCNQ surface is extremely unfavorable. Thus we only calculated the charge density difference of O2-dissociated on C1≡N1 bond in TCNQ molecule (Figure 4B). Despite the molecular O2 could dissociate on C1 ≡ N1 bond of TCNQ, the


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adsorption O atoms got 0.8 e- Bader charge, which is less than the activated O2 on CuTCNQ (1.4 electrons) from Bader charge analysis. Therefore, Bader charge and charge density confirmed that the molecular O2 was energetically much easier to activate on CuTCNQ surface and a new electron configuration of O2 formed through one-electron transfer between C≡N bond and O2. Moreover, the reactive oxygen species (ROS) generated on catalysts surface were monitored to support the calculation results by using electron paramagnetic resonance (EPR) techniques (Figure 4C). An obvious signal was observed in the CuTCNQ NWs on exposure to air, which is likely to be the surface superoxide radical anion •O2- exhibiting the g factor at 2.00241. This finding verified that the surface of CuCTNQ preferred to reduce O2 to •O2-, which was involved in the subsequent CO oxidation. A relatively weak EPR signal of •O2- at g = 2.001 was also observed on TCNQ powder, which may be ascribed that the O2 gets fewer electrons and reduces to fewer superoxide species during dissociation process, as indicated by the charge density difference of configurations (Figure 4B). Intriguingly, the EPR signal intensity of CuTCNQ NWs was much stronger than that of TCNQ powder, which confirmed that the amount of •O2generated on CuTCNQ NWs was much greater than that generated on TCNQ powder. It can be inferred that the electrons of •O2- 2π* antibonding orbital transfer to the empty 2π* antibonding orbital of gaseous CO, facilitating the CO oxidation reaction42. For Cu2O NWs, a g factor at 2.070 corresponding to the Cu2+ ions suggests that the Cu foam was partially oxidized to copper (II) species during the annealing process, which is also in consistent with the XPS results43. And no apparent EPR signals were detected on Cu2O powder sample due to the silent Cu+. Thus, comparing with the easy sublimation of TCNQ powder below 200°C, the DFT calculation and EPR results demonstrated that Cu atom enhanced the charge density and the thermal stability of


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C ≡ N bond, which then transferred the electrons to O2, leading to an easy activation of molecular O2 on CuTCNQ NWs surface. In situ DRIFTS study of CO oxidation. In order to further verify the CO oxidation process on CuTCNQ NWs, the in situ DRIFTS experiments over CuTCNQ NWs and Cu2O powder (a sequent flow of (a) 1% CO/N2, (b) pure N2 and (c) 1% CO/5% O2/N2 for 20 min for each condition at 120 °C) are displayed in Figure 5. As seen in Figure 5A and 5B, the peaks appearing at 2359 cm-1 can be ignored, which could be resulted from the exposed CO2 in environement44. In Figure 5A, the peaks at 2201 cm-1 can be attributed to the C≡N bond45 of CuTCNQ in accordance with the FTIR spectrum (Figure 2E). And no peaks of CO adsorption were observed on the CuTCNQ NWs surface46,47. Besides, the adsorption energy of O2 adsorption is much lower than the CO, indicating that O2 is preferentially adsorbed on the C≡N bond of CuTCNQ (as seen in Figure S9). These results suggest that there is no adsorbed CO on the CuTCNQ NWs surface during the process. In the meantime, two characteristic peaks at 1503 cm-1 and 1352 cm-1 are detected, which can be attributed to the carbonate and carboxylate species48. Given the lower energy of O2 molecule activation between C≡N bond ( -1.167 eV) as well as the detected •O2- in air at room temperature via EPR analysis, it can be postulated that the O2 molecule could be activated on the CuTCNQ NWs surface in ambient atmosphere (Figure 5C). The gaseous CO directly reacted with •O2- to form the carbonate-likes intermediates (Figure 5D). This reaction process is in accordance with the DFT calculations and EPR spectra analysis. Furthermore, the peaks of carbonate species are observed as CO in flow, confirming that the gaseous CO reacted with adsorbed O2 rapidly over CuTCNQ NWs, which supports the ER mechanism predicted by DFT calculations. In the case of Cu2O powder (Figure 5B), two peaks appearing at 2170 cm-1 and 2117 cm-1 could be assigned to the gaseous CO, and no peak for


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adsorbed CO is observed37, which reveals there is no interaction between CO and O2 on Cu2O powder during the process. As a consequence, the CuTCNQ NWs exhibited better catalytic activity for CO oxidation than Cu2O powder, as shown in Figure S5. In short, in situ DRIFTS spectra over CuTCNQ NWs and Cu2O powder indicates that the gaseous CO directly reacts with activated oxygen over CuTCNQ NWs, which further proved the proposed ER mechanism from DFT calculations in Figure 3C. Conclusions In summary, we first reported that a charge-transfer complex CuTCNQ nanowires array grown on copper foam exhibited a high CO catalytic oxidation activity at temperatures below 200 °C. DFT calculations and EPR spectra demonstrated that the molecular O2 was energetically activated over the Cu (I) mediated C≡N bond and preferentially reduced to superoxide radical anion •O2- through one-electron transfer during the activation process on CuTCNQ surface. And the CO molecule was energetically oxidized by the activated •O2- via the Eley–Rideal (ER) mechanism. The in situ DRIFTS data further confirmed that the CO molecule can directly react with the activated oxygen atom to form carbonate intermediates via ER mechanism. This work revealed that the local C≡N bond electron state engineering could effectively improve the molecular O2 activation efficiency, which could facilitate the low-temperature oxidation. The new mechanism of molecular oxygen activation described here can provide a new avenue for rational design of novel low cost, organic based metal heterogeneous catalysts for low temperature air pollution control.

AUTHOR INFORMATION


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Corresponding Author * Phone: +86-17786507005; e-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful for the financial support from the Recruitment Program of Global Young Experts start-up funds, The Program of Introducing Talents of Discipline to Universities of China (111 program, B17019) and the National Natural Science Foundation of China (21777051).

ASSOCIATED CONTENT Supporting Information. Theoretical calculation and reaction kinetics calculation details; EDS spectrum of CuTCNQ NWs; XPS spectra of N 1s in CuTCNQ NWs, Cu 2p in CuTCNQ NWs, Cu2O NWs, Cu2O powder; TCNQ powder; SEM images of Cu2O NWs; CO conversion versus temperature over CuTCNQ NWs, Cu2O NWs and Cu2O powder; primitive cell of CuTCNQ; structure and optimized configurations of TCNQ; configurations of each state and minimum energy profiles for O2 dissociation on TCNQ; XPS spectra of Cu 2p in CuTCNQ NWs before and after CO catalytic activity test; Calculated energy profiles for CO adsorption on CuTCNQ ; Bader charges (Q) of Cu, C, N atoms and O2.


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ABBREVIATIONS TCNQ, 7,7,8,8-Tetracyanoquinodimethane; CuTCNQ NWs, CuTCNQ nanowires array; CO, carbon monoxide; CO2, carbon dioxide; Cu2O NWs, Cu2O nanowires array; IS, initial state; FS, final state; ROS, reactive oxygen species.

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Figure Captions


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Figure 1. (A) Photograph of pure copper foam and CuTCNQ NWs integrated copper foam. (B) Low magnification and (C) high magnification SEM images of CuTCNQ NWs grown on copper foam. (D) TEM image of a typical single CuTCNQ NW. (E) FTIR spectra of CuTCNQ NWs and TCNQ powder. (F) XPS Cu 2p spectrum of CuTCNQ NWs.

Figure 2. (A) The reaction rate of CO oxidation versus temperature over CuTCNQ NWs, Cu2O NWs and Cu2O powder. (B) The corresponding Arrhenius plots and apparent activation energy of the CuTCNQ NWs and Cu2O NWs. Reaction rates of CO conversion as a function of (C) CO and (D) O2 concentration over CuTCNQ NWs at 125 °C. CO and O2 concentrations range are 2000 ppm to 10000 ppm and 10000ppm to 50000 ppm, respectively.


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Figure 3. (A) The molecular structure of CuTCNQ. (B) Optimized configurations of CuTCNQ. (C) The proposed reaction pathway for CO oxidation on the CuTCNQ catalyst. The inset shows the minimum energy profiles for CO oxidation over the CuTCNQ along reaction pathway. On the basis of state ⅲ, two reaction paths are denoted with red and blue dotted lines, respectively.


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Figure 4. The charge density difference of (A) O2-activated on CuTCNQ surface and (B) O2dissociated on TCNQ surface, where electron accumulation and depletion are represented by yellow (∆ρ=+5×10-3 e·bohr-3) and cyan (∆ρ=-5×10-3 e·bohr-3); +1.4e means acceptance of 1.4 electrons, while – 0.7e means loss of 0.7 electrons. The transfer of Bader charge on interested atoms are indicated in the brackets. (C) EPR signals detected in different samples at room temperature.


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Figure 5. In situ DRIFTS of CO oxidation over CuTCNQ NWs (A) and Cu2O powder (B) in a sequent flow of 5% O2/N2, pure N2 and 1%CO/5%O2/N2 for 20 min for each condition at 120 °C. (C) The configurations of O2 molecule adsorption and activation on the CuTCNQ surface. (D) The intermediate state ⅲ of CO oxidation over CuTCNQ by DFT calculations.


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(I) Mediated Câ¡N Bond for Low Temperature CO Oxidation

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