Co3O4âCuCoO2 Nanomesh: An Interface-Enhanced Substrate that

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Energy, Environmental, and Catalysis Applications

Co3O4-CuCoO2 Nanomesh: An Interface-Enhanced Substrate that Simultaneously Promotes CO Adsorption and O2 Activation in H2 Purification Junfang Ding, Liping Li, Haorui Zheng, Ying Zuo, Xiyang Wang, Huixia Li, Shaoqing Chen, Dan Zhang, Xingliang Xu, and Guangshe Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19478 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Materials & Interfaces

Co3O4-CuCoO2 Nanomesh: An Interface-Enhanced Substrate that Simultaneously Promotes CO Adsorption and O2 Activation in H2 Purification Junfang Ding †, Liping Li †, Haorui Zheng †, Ying Zuo ‡, Xiyang Wang †, Huixia Li †, Shaoqing Chen§, Dan Zhang †, Xingliang Xu †, Guangshe Li* † †State

Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P.R. China ‡Scientific

Instrument Center, Shanxi University, Shanxi 030006, P.R.China

§Department

of Materials Science and Engineering, Southern University of Science and Technology,

Shenzhen 518055, P.R.China

KEYWORDS: Nanomesh, CO adsorption, O2 activation, Synergistic effect, CO-PROX ABSTRACT: Nanomaterials are widely used as redox-type reactions catalysts, while reactant adsorption and O2 activation are hardly to be promote simultaneously, restricting their applications in many important catalytic fields like preferential CO oxidation (CO-PROX) in H2-rich stream. In this work, an interface-enhanced Co3O4-CuCoO2 nanomesh was initially synthesized by a hydrothermal process using aluminum powder as a sacrificial agent. This nanomesh is systematically characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption, X-ray photoelectron spectra (XPS), UV−vis absorption spectra, Raman spectra, X-ray absorption near-edge (XANES) spectra, hydrogen 1

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temperature-programmed reduction (H2-TPR), and oxygen temperature-programmed desorption (O2-TPD). It is demonstrated that nanomesh possess high-density nanopores, enabling a large number of CO adsorption sites exposed to the surface. Meanwhile, electron transfer from O2− to Co3+/Co2+ and the weakened bonding strength of Co-O bond at surfaces promoted the oxygen activation and redox ability of Co3O4. When tested as a catalyst for CO-PROX, this nanomesh with an optimized pore structure and surface electronic structure, exhibits a strikingly high catalytic oxidation activity at low-temperature as well as a broader operation temperature window (i.e. CO conversion >99.0%, 100-200oC) in CO selective oxidation reaction. The present finding should be highly useful in promoting the quest for better CO-PROX catalysts, a hot topic for proton exchange membrane fuel cells and automotive vehicles.

1. INTRODUCTION Nanomaterials have been extensively studied in the past decades for the heterogeneous catalysis ﬁelds, especially in redox heterogeneous catalysis.1-4 It is well established that these redox reactions generally follow the Mars−Van Krevelen mechanism in terms of four primary steps: (i) reactant adsorption, (ii) formation of active oxygen, (iii) reactant reacting with active oxygen, and (iv) product desorption.5,

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Among all these steps, reactant adsorption and oxygen activation are two

crucial steps that govern the selectivity of product and reactants conversion. 7, 8 Although copious efforts have been tried to design an advanced catalyst, the problem is still not well solved in simultaneous adsorption of reactive gases and activation of oxygen species, which is seriously 2


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restricting the application of nanomaterials in many important catalytic fields.9, 10 Thus, it is urgent, but also highly challenging, to rationally design catalysts that could give effective adsorption of reactant and activation of oxygen species. Preferential CO oxidation in H2-rich stream (CO-PROX), a model system of redox-type reactions, is crucial for practical implementation of purifying H2 in proton exchange membrane fuel cells (PEMFCs).11-14 Effective adsorption of CO and activation of oxygen species could greatly improve the performance of CO-PROX reaction. In the past decades, many catalysts have been prepared, including those supported noble metal catalysts and transition metal oxides.10, 11, 14 Due to the low Co−O bond energy that could give a possible high capability to activate oxygen as well as strong CO adsorption, Co-based catalysts are recognized as the typical low cost and high activity catalysts for CO-PROX reaction.15-18 Therefore, considerable works have been done for preparing this kind of catalysts to effectively tune CO adsorption or O2 activation with an aim to broaden the operation temperature window or to achieve better oxidation activity, respectively. Indeed, more Co3+ active sites exposed on the surface of mesoporous Co-Ce oxides catalysts, achieved using a glycine-nitrate combustion approach, have been reported to yield an excellent CO-PROX performance.19 The insertion of Cu ions in Ce0.90Co0.10O2–δ can greatly enhance the reducibility and improve the activity, as reported by Friedrich group. And their analysis suggest that the insertion of Cu ions can change the active sites of H2 and CO oxidation over Ce0.90Co0.10O2 from same one to the independent.20 Further, as reported by Steven L. Suib et al., surface oxygen vacancy in mesoporous Co3O4 could promote lattice oxygen migration and activation to enhance the catalytic activity.21 Then, could the Co-based oxide nanostructure containing Cu, having the features of exposed more 3



active Co3+ and surface oxygen vacancy characteristic together, achieve simultaneous CO adsorption and oxygen activation? Nanomesh with hierarchical interfaces provides us a possibility to achieve this challenging goal for CO-PROX reaction. Herein, a novel sacrificial agent-assisted chemical methodology was proposed to synthesize an interface-enhanced Co3O4-CuCoO2 nanomesh. The preparation mainly involves hydrothermal process and a subsequent calcination. The obtained nanomesh is proved to exhibit a large number of active sites Co3+ exposed on the surface and enrich defects, which are beneficial for CO adsorption, electronic transfer from O2− to Co3+/Co2+, and oxygen activation. When used for CO-PROX reaction, Co3O4-CuCoO2 nanomesh showed a strikingly high catalytic oxidation activity at low-temperatures as well as a broader operation temperature window (100-200oC). The present methodology of synthesizing an interface-enhanced nanomesh and the strategy of improving the catalytic performance are extremely important, which gives guidance to explore and optimize the advanced heterogeneous catalyst in the future. 2. EXPERIMENTAL SECTION 2.1. Materials Copper nitrate (Cu(NO3)2·3H2O, 98.0-102.0%), cobaltous nitrate (Co(NO3)2·6H2O, 99%) and potassium hydroxide (KOH, AR) were all purchased from Sinopharm Chemical Reagent Corp (P. R. China), and aluminum powder (Al, 99.95%) purchased from Aladdin. All chemicals and solvents were used without further purification.

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2.2. Synthesis of samples 2.2.1 Synthesis of Co3O4-CuCoO2 nanomesh All samples were synthesized using a hydrothermal method which is followed by a calcination. Typical synthetic procedures could be described as follows: 1.16 g of cobalt nitrate and 0.24 g of copper nitrate were dissolved in 30 ml of deionized water under magnetic stirring for 30 min. Meanwhile, 5.61 g of potassium hydroxide was dissolved in 40 ml of deionized water under magnetic stirring to form a homogeneous solution. Subsequently, KOH solution was dropwise added into the mixed solution of copper nitrate and cobalt nitrate at a speed of ~ 1 ml/min, and then the above suspension was stirred for 1 hour at room temperature. 0.2698 g of aluminum powder was added to the resulting mixture, which was then transferred to a Teflon-lined steel autoclave. After reaction at 200oC for 20 h, the obtained precipitates were separated by centrifugation, washed with deionized water several times, and then dried at 60oC overnight under vacuum. Finally, the powders were calcined at 200oC for 2 h in air at a heating rate of 2 oC·min-1. The obtained nanomesh was named as Co-Cu-Al-4-1-10 (4-1-10 represents the molar ratio of Co to Cu and Al). Regulating the molar ratio of Co, Cu and Al, we also obtained the samples Co-Cu-Al-3-1-10, Co-Cu-Al-5-1-10, Co-Cu-Al-4-1-5, Co-Cu-Al-4-1-15, and Co-Cu-Al-4-1-20. 2.2.2 Synthesis of Co3O4-CuO-CuCoO2 reference samples The preparation procedure of Co3O4-CuO-CuCoO2 samples is similar to Co3O4-CuCoO2 with the exception that no aluminum powder was involved. The obtained samples also underwent a similar calcination procedure, which were respectively named as Co-Cu-3-1, Co-Cu-4-1, and Co-Cu-5-1. 5



2.3. Characterizations Methods. Chemical compositions of the samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on an OPTIMA 3300DV instrument (Perkin Elmer). Powder X-ray diffraction (XRD) patterns were recorded in a Rigaku Miniflex apparatus equipped with Cu-Kα radiation (λ= 0.15418 nm) at 40 kV and 15 mA in the range of 2θ between 10o and 80o. KCl was used as internal standard for the correction of diffraction angle. The scanning electron microscopy images (SEM) were examined using a Hitachi SU8020 scanning electron microscope. Transmission electron microscopy images (TEM) were taken on a Tecnai G2S-TwinF20 apparatus. For the corresponding elemental mapping acquisition, the energy dispersive spectroscopy (EDS) was performed under TEM mode. Nitrogen adsorption-desorption measurements were carried out on an ASAP2020 instrument. Prior to the measurement, all samples have to be degassed at 200 oC under vacuum for 2 h. BET specific surface areas of the samples were obtained from desorption data. The X-ray photoelectron spectra (XPS) of the catalysts were obtained with an ESCALAB250 apparatus with a monochromatic Al Kα X-ray source. The charging shift was calibrated using C 1s photoemission line at 284.8 eV, and the smart mode was used as baseline in XPS data analysis. Raman data were collected on an INVIA Raman system under excitation of 532 nm. Fourier transform-infrared spectra (FT-IR) were collected using a Nicolet is50 FT-IR spectrometer. UV−vis absorption spectroscopy measurements for the samples were carried out on a Shimadzu U-4100 spectrometer, equipped with an integration sphere. The spectra were recorded in the wavelength

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range of 200-800 nm. Co L-edge and O K-edge XANES spectroscopy were measured at the Hefei Synchrotron Radiation Facility (SSRF). Hydrogen temperature-programmed reduction experiments (H2-TPR) were performed in a Micromeritics AutoChem II 2920 Apparatus equipped with Thermal Conductivity Detector (TCD). The as-prepared samples (30 mg) were heated from 30 to 800 oC in a 10% H2/Ar (30 mL·min−1) mixed gas at a heating rate of 10 oC·min-1. Prior to the measurement, the sample was treated at 200 oC for 1 h in an Ar stream to remove the contaminants. The temperature-programmed desorption of O2 (O2-TPD) was performed from 50 to 900 oC using the same apparatus. The procedures were described as follows: (i) The samples (about 30 mg) were pretreated in an inert gas flow (Ar) at 200 oC for 2 h to clean the surface before each test; (ii) catalysts were cooled to 50 °C with O2 adsorption for 1 h; (iii) purging with He for 1 h to remove any surface physically adsorbed gases and residual feed gas from the streams; and (iv) heating at a rate of 10 °C/min from 50 to 900 oC. The signal of O2 desorption from the samples was measured by a TCD. 2.4. Catalytic Activity Study. Catalytic activity of the samples was measured in a continuous flow reactor (8 mm outside diameter) at a flow rate of 50 mL/min under ambient pressure. The samples (50 mg) were grinded and sieved by a 180-mesh sieve and then mixed with 200 mg quartz sand in order to eliminate the external mass transfer. Subsequently, the mixed powders were enclosed between two quartz wool plugs and tested for CO oxidation in a fixed-bed (11 mm catalyst bed height). The reaction mixture contained 1% CO, 1.25% O2 and 50% H2 with He as balance gas. Samples were heated from 30 to 7



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240 oC and were stabilized at each temperature plateau under the same reaction conditions for 10 min to reach the equilibrium. The products and reactants were analyzed with an on-line GC-2014C gas chromatograph equipped with a thermal conductivity. The CO conversion (XCO), O2 conversion (Xo2) and CO2 selectivity (Sco2) were calculated according to the following equations13: XCO (%) = ([CO]in - [CO]out) / [CO]in × 100%

(1)

Xo2 (%) = ([O2]in - [O2]out) / [O2]in × 100%

(2)

Sco2 (%) = XCO / (2.5×Xo2)

(3)

Where [CO]in and [O2]in are the corresponding GC response peak area values of the inlet gas of CO and O2 at room temperature, respectively. [CO]out and [O2]out correspond to GC response peak area values of CO and O2 after the reactor, respectively. The stability of the samples was tested in reaction mixture (1% CO, 1.25% O2 and 50% H2 with He as balance gas) at 100 °C for 50 h. Turnover frequencies (TOFs) of these catalysts were performed under CO-PROX conditions (1 % CO, 1.25 % O2 and 50 % H2 in He balance) at a flow rate of 50 ml/min at a specific temperature. Turnover frequencies (TOFs) were calculated via follow formula: TOFs (s-1) = (XCO × 1% ×V × Mcat)/(22.4×60×1000×mcat)

(4)

where XCO is the conversion rate of CO (%); V represents the total flow rate (ml/min); Mcat (g/mol), average relative molecular weight of samples, was calculated by XRD data refinement; mcat (g) is 8


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the mass of the catalysts.

3. RESULTS AND DISSCUSION 3.1. Formation of Co3O4-CuCoO2 nanomesh

Figure 1. Preparation Scheme of Co3O4-CuCoO2 nanomesh by a hydrothermal process followed with calcination using aluminum powder as a sacrificial agent. Co3O4-CuCoO2 nanomesh were derived by sacrificial agent-assisted chemical route, as schematically elucidated in Figure 1. Firstly, cobalt and copper hydroxides were generated when dropwise adding KOH into the mixed solution of cobalt nitrate and copper nitrate. Introducing Al powder in the reaction system gave rise to an intermediate of hydrotalcite-like compounds 9



containing aluminum, as indicated by the appearance of characteristic XRD peak at two theta of 11.6 degrees in Figure S1. With prolonging the reaction time in a strong alkali hydrothermal conditions, hydrotalcite-like compounds gradually disappeared (Figure. S2), indicating that amphoteric Al in hydrotalcite-like compounds was etched during the hydrothermal process. Thus, a nanomesh skeleton with a high-density nanopore was formed (Figure S3), which, upon annealing at 200 °C for 2 h in air, was transformed to Co3O4-CuCoO2 nanomesh with hierarchical interfaces.

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Figure 2. (a) XRD patterns of the samples Co-Cu-4-1 and Co-Cu-Al-4-1-10, when compared to the standard data for CuO, Co3O4 and CuCoO2. SEM images and particle size distribution diagram of the samples (b) Co-Cu-4-1 and (c) Co-Cu-Al-4-1-10. (d) TEM and (e) HRTEM images of the sample Co-Cu-4-1. Inset is the FFT pattern of the rectangle region. (f) Corresponding elemental mapping analysis in the area of (d). (g) TEM and (h) HRTEM images the samples Co-Cu-Al-4-1-10. Inset is the FFT pattern of the rectangle region, and (i) corresponding elemental mapping analysis in (g). Figure 2a displays XRD patterns of the samples Co-Cu-4-1 and Co-Cu-Al-4-1-10. For both samples, the diffraction peaks at two theta of 19.0o, 31.3o, 36.8o, 38.5o, 44.8o, 59.4o and 65.2o were assigned to the crystal planes (111), (220), (311), (222), (400), (511) and (440) for spinel Co3O4 phase (JCPDS Card No. 43-1003), respectively. When comparing to those of the samples in Figure 2a, the diffraction peak width of Co3O4 at two theta of 36.8o for Co-Cu-Al-4-1-10 sample increased obviously, indicating that the particles of Co3O4 in this sample is smaller than that in Co-Cu-4-1 samples. Two diffraction peaks of CuCoO2 located at two theta of 38.2o and 42.4o could be distinguished for all samples. Besides this, a diffraction peak of CuO in a monoclinic structure at two theta of 35.6º is also observed for the sample Co-Cu-4-1, while this diffraction peak is absent for sample Co-Cu-Al-4-1-10. There are two possible reasons for this observation: (i) when amphoteric Al components were etched, the dissolution of copper species would occur, which accounts for a decrease in the content of copper oxide; and (ii) the etching of the amphoteric Al components and the dissolution of copper species may lead to a nanomesh skeleton. The subsequent Ostwald ripening process could be presented, as confined by the highly anisotropic 2D structure, in 11



which the thermodynamically unstable Cu species starts to dissolve and the as-dissolved Cu species could be successively redeposited onto the nanomesh, bonding to Co and O elements to form CuCoO2, as demonstrated by XRD data refinement in Figure S4 and Table S1.22 Table S2 shows the average bond lengths of Co-O bonds for the samples, obtained from XRD refinement. It is seen that the average bond lengths of Co-O bond for sample Co-Cu-Al increased distinctly when comparing to that for the sample Co-Cu. The formation of nanomesh structure is thus indicated to weaken the bond energy of Co−O bond and promote the oxygen activation. In addition, no diffraction peaks of Al species were detected for Co-Cu-Al samples. In order to accurately evaluate the content of Al in these samples, ICP was measured. As shown in Table 1, mass fraction of Al in sample Co-Cu-Al was only about 0.3%. For Co-Cu samples, as synthesized using the similar procedure to those of Co3O4-CuCoO2 with the exception that no aluminum powder was added, about 0.1% aluminum was also detected. This means that the samples reported in this work contained traces of aluminum. In combination with ICP results, it is reasonable that Al species were almost completely etched. Morphologies of the samples were characterized via SEM. As shown in Figures 2b, 2c and S5, Co-Cu samples are mainly composed of irregular polyhedron (with an average side-length of 400 nm) and small particles (with a particle size of about 100 nm). As the molar amount of cobalt increases, those small particles gradually disappeared, while other particles tend to become homogeneous as represented by a narrowed particle size distribution. Comparatively, Co-Cu-Al samples did not show irregular polyhedron and small particles upon addition of Al powder, but uniform hexagonal nanomesh with a thickness of approximately 20 nm and a side length of about 150 nm. The latter one is in accordance with an increase in the diffraction peak width of Co3O4 at 12


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two theta of 36.8o. Since the size of nanopores is too small to be identified by SEM, in the following, TEM and HRTEM were used to provide the detailed information about the crystal structure and pore size distribution of all samples. As seen in Figure 2d, sample Co-Cu-4-1 exhibits irregular polyhedron and small particles, in agreement with the observations by SEM. A HRTEM image of the region I in Figure 2e indicates two sets of clear crystalline lattice with the same interplanar spacing of 2.9 Å and interface angle of 60o, which could be assigned to the crystal planes (022) and (220) of Co3O4. Fourier transform pattern in inset of Figure 2e displays a hexagonal symmetry diffraction spots, typical of the crystal orientation of [111]. Thus, the as-obtained Co-Cu samples showed the exposed facet of {111} plane. Besides this, HRTEM images of the regions II in Figure 2e showed that the lattice spacing of 2.3 Å is associated with the (111) crystal plane of CuO. After addition of aluminum powder to form Co-Cu-Al-4-1-10 sample, hexagonal nanosheets with hierarchical interfaces were observed (Figure 2g). Strikingly, abundant nanopores with a high density and uniform distribution in these nanosheets were also revealed. N2 adsorption-desorption isotherms were applied to survey the specific surface area as well as the corresponding Barrett-Joyner-Halenda (BJH) pore size distributions of the samples. As demonstrated in Figure S6, Co-Cu samples exhibited the type-III isotherm, characteristics of nonporous nature. While Co-Cu-Al samples showed a type-IV isotherm, which is followed by a representative H3 hysteresis loop, suggesting the presence of a slit-shaped porous architecture in these samples, as reported in other systems.23 Accordingly, surface areas were greatly enhanced from 34.1 m2 g-1 for Co-Cu-3-1 to 68.1 m2 g-1 for Co-Cu-Al-3-1-10, from 46.1 m2 g-1 for Co-Cu-4-1 to 72.0 m2 g-1 for Co-Cu-Al-4-1-10, and from 57.1 m2 g-1 for Co-Cu-5-1 to 80.1 m2 g-1 13



for Co-Cu-Al-5-1-10. Meanwhile, BJH pore volume and BJH pore size also increased (see more details in Table 1). Similar to the sample Co-Cu-4-1, the sample Co-Cu-Al-4-1-10 also showed the exposed facet {111} of Co3O4, since HRTEM image and Fourier transform pattern in Figure 2h for Co-Cu-Al-4-1-10 are the same as those for Co-Cu-4-1 in region I of Figure 2e. Even so, CuO lattice spacing cannot be clearly seen for sample Co-Cu-Al-4-1-10. From TEM, HRTEM images and their corresponding FFT patterns data analyses of the samples Co-Cu-4-1 and Co-Cu-Al-4-1-10, one can see that addition of aluminum powder in the reactants could only change the morphologies (such as from irregular polyhedron and small particles to hexagonal nanomesh), but not the exposed facet. Therefore, exposed facet is not a major factor in affecting its CO-PROX performance as described latter for the samples Co-Cu and Co-Cu-Al. Same phenomena have been found in TEM and HRTEM images for samples Co-Cu-3-1, Co-Cu-5-1, Co-Cu-Al-3-1-10, and Co-Cu-5-1-10 (Figures S7, S8). Figures 2f and 2i depict TEM-EDX mapping of the samples Co-Cu-4-1 and Co-Cu-Al-4-1-10, and those for other four samples Co-Cu-3-1, Co-Cu-Al-3-1-10, Co-Cu-5-1 and Co-Cu-Al-5-1-10 are given in Figure S9 and Figure S10. For Co-Cu samples, cobalt, copper and oxygen are homogeneously distributed. Upon addition of aluminum, as shown for Co-Cu-Al samples, cobalt, copper and oxygen are homogeneously distributed across the whole nanomesh, while signal of Al is negligible, implying a high efficiency of alkaline etching in the removal of amphoteric Al component. Based on the above analyses, Co-Cu-Al nanomesh with hierarchical interfaces is confirmed with no aluminum remained. Table 1. Textural properties and catalytic performance of the samples 14


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Samples

Mass fraction of Al (%)a

Surface area (m2 g-1)

pore width (nm)

pore volume (cm3 g-1)

I572/I668b (%)

Co3+ (%)

Oads (%)

T50% (oC)

Temperature window (oC)

Co-Cu-3-1 Co-Cu-4-1 Co-Cu-5-1 Co-Cu-Al-3-1-10 Co-Cu-Al-4-1-10 Co-Cu-Al-5-1-10

0.1 0.3 0.1 0.5 0.3 0.3

34.1 46.1 51.7 68.1 72.0 80.1

4.98 4.16 4.74 7.65 6.14 6.99

0.035 0.034 0.067 0.112 0.121 0.142

1.05 1.07 1.16 1.82 1.48 1.62

58.9 59.2 60.7 66.9 73.2 69.8

34.1 31.4 35.4 46.6 48.4 47.1

106 110 103 82 68 76

------------------------120-200 100-200 120-200

a

bI

Mass fraction of Al determined by ICP-AES.

572/I668

were calculate by IR.

3.2. Chemical states and redox behavior of the samples

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Figure 3. (a) XPS survey scans, (b) FT-IR spectra, XPS spectra of (c) Co 2p and (d) O 1s for all samples. The composition and chemical states of the as-synthesized samples were further investigated by XPS spectra, and the corresponding data were summarized in Table 1. Shown in Figure 3a are the XPS survey spectra of the Co-Cu and Co-Cu-Al samples, in which no obvious difference is observed for these samples, implying the same elements (Co, Cu and O) existent in these samples. Co 2p spectrum in Figure 3c consists of two main peaks of 2p1/2 and 2p3/2 with a separation of about 15.1 eV and a pair of weak satellite doublet peaks, which is in accordance with the mixed-valence, typical of Co3O4.24, 25 The characteristic peaks at 779.6 and 794.7 eV are identified as Co3+ species, whereas the other two peaks at binding energies of 781.3 and 796.4 eV correspond to Co2+ species.25-27 Besides, weak satellite peaks at about 790 and 805 eV, characteristic of Co3O4, are arising from the unpaired electrons existed in Co2+ valence orbital. 25, 28 Further, from Figure 3c, all samples possess a pair of weak satellite peaks, implying cobalt species are mainly in the state of Co3+ species. The concentration of surface Co3+ in the samples is calculated based on the area of XPS fitting results (Tables S3 and S4). As listed in Table 1, one can see that addition of aluminum powder in the starting materials obviously increased the surface content of Co3+, which is also confirmed by FT-IR spectra (Figure 3b). The vibrational bands at 572 and 668 cm-1 in FT-IR spectra are associated with the component Co3O4 in a spinel structure: the former one is related to the OB3 vibration in spinel lattice (B denotes Co3+ in an octahedral site), while the latter one is

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corresponding to ABO3 vibration (A denotes Co2+ in a tetrahedral site).29, 30 The content of Co3+ was estimated by the intensity ratio of a band at 572 cm-1 to that at 668 cm-1 (I572/I668). As shown in Table 1, the ratio of I572/I668 for Co-Cu-Al samples is much larger than that of Co-Cu samples, which is in accordance with the above XPS fitting results. Further data comparison of the content of Co3+ at surfaces of these samples indicates that the ratio follows such a sequence, Co-Cu-Al-4-1-10 > Co-Cu-Al-5-1-10 > Co-Cu-Al-3-1-10 > Co-Cu-5-1 > Co-Cu-4-1 > Co-Cu-3-1. Alternatively, when referring to Co3+ content by IR results, the ratio sequence slightly changed: Co-Cu-Al-3-1-10 > Co-Cu-Al-5-1-10 > Co-Cu-Al-4-1-10 > Co-Cu-5-1 > Co-Cu-4-1 > Co-Cu-3-1. This observation should be associated with a surface enrichment effect of Co3+ species for Co-Cu-Al-4-1-10. It is well known that Co3+ could be the primary active sites for CO adsorption and oxidation. Thus, it is expected that the catalytic activity of Co-Cu-Al samples with hierarchical interfaces could be greatly enhanced by exposing more active species Co3+ on the surface, especially for the Co-Cu-Al-4-1-10 sample. Figure 3d shows O 1s XPS spectra of the samples. The curve fitting in O 1s region for all samples showed two component peaks centered at 529.6 and 531.2 eV, which are corresponding to the lattice Co-O (Olatt) and the surface adsorbed oxygen(Oads), respectively.25, 31 Surface adsorbed oxygen could be activated easily, and usually considered as the main active oxygen species, playing a predominant role in determining the catalytic activity of oxidation reactions.31, 32 When compared with Co-Cu samples, Co-Cu-Al samples with hierarchical interfaces possess a relatively high content of Oads/(Oads+Olatt). In other words, Co-Cu-Al samples with nanomesh structure could promote the formation of more surface oxygen defects and expose more easily activated oxygen 17



(Oads).

Figure 4. (a) Co L-edge, (b) O K-edge XANES spectra of Co-Cu-Al-4-1-10 sample (blue curve), Co-Cu-4-1 sample (red curve), and commercial Co3O4 (black curve); (c) Crystal structure models of Co3O4, CuCoO2; and (d) Splitting of Co 3d orbitals in octahedral and tetrahedral crystal fields. To further probe the electronic structures of Co-Cu-4-1 and Co-Cu-Al-4-1-10 samples, X-ray absorption near-edge (XANES) spectra were performed. Co L-edge XANES spectra normalized 18


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from 768 to 798 eV are presented in Figure 4a. There are two main peaks of L3 and L2 edges that are respectively associated with the electronic transition from Co 2p3/2 and Co 2p1/2 to Co 3d projected unoccupied states. 33, 34 In L3 region, the peaks A at about 775 eV is assigned to Co 3d unoccupied t2g(β) orbit in CoO6 octahedra, which derives from the existence of middle spin and high spin of Co3+ ions (Figures 4c and 4d).35,

36

Peak B at 777 eV is ascribed to Co 3d eg(α) orbit in CoO6

octahedra and Co 3d t2g(β) orbit in CoO4 tetrahedra.35, 36 Peak C centered at 779 eV can be assigned to Co 3d eg(β) orbit in CoO6 octahedra. While in L2 region, there only exists peak D that is recognized as the transition from 2p1/2 to Co 3d eg(α) orbit in CoO6 octahedra and Co 3d t2g(β) orbit in CoO4 tetrahedra.35, 36 As seen in Figure 4d, the energy of eg(α) orbit is lower than that of t2g(β) orbit. Thus, if there are more eg(α) orbit in CoO6 octahedra (Co3+), a shift to lower energies would be accompanied. Carefully comparing the Co L-edge XANES spectra of Co-Cu-Al-4-1-10 with Co-Cu-4-1, one can find two striking changes: (i) the positions of peaks B and D for Co-Cu-Al samples obviously shifted towards a lower energy; and (ii) the relative intensity of peak A dramatically increased in Co-Cu-Al samples (Figure S11). These observations suggest that Co-Cu-Al-4-1-10 sample exhibits a higher concentration of Co3+ and smaller particle size relative to those of Co-Cu-4-1, in accordance with the above XRD and XPS results.37 O XANES spectrum normalized from 520 to 570 eV is presented in Figure 4b. For these samples, the O K-edge energy region can be mainly divided into two regimes. The first regime is the low-energy range extending from 525 to 535 eV and it corresponds to the transition of O 1s to the unoccupied O 2p states hybridized with transition metal (TM) 3d states.38 The second regime is defined as the high-energy range that covers the energies from 535 to 550 eV, which is owing to the 19



transition of O 1s to unoccupied states of O p character hybridized with the TM 4sp band.38 In commercial Co3O4 sample, three strong multiple peaks were observed in this region. Peak “a” at 530 eV is regarded as the covalence of Co 3d t2g(β), eg(α) orbit in CoO6 octahedra, Co 3d t2g(β) orbit in CoO4 tetrahedra with O 2p; peak “b” near to peak “a” is considered as the result of hybridization between Co 3d eg(β) orbit in CoO6 octahedra and O 2p; while peak “d” in high-energy region is usually attributed to the covalence of Co or Cu 4sp with O 2p. Comparing to commercial Co3O4, peak “a” for Co-Cu-Al-4-1-10 and Co-Cu-4-1 samples shifted toward higher energies, which is mainly because introduction of Cu+ ions could induce the transition of Co2+ to Co3+, thus enhancing the covalence of Co-O bond.39 Besides this, the relative intensity of peak “a” for Co-Cu-4-1 is higher than those for other samples, and this is because of the overlapping of Co 3d-O 2p and Cu 3d-O 2p that yield more Cu2+ ions (CuO) in Co-Cu-4-1 sample, as confirmed by XRD data analyses.39 Furthermore, a new peak “c” appeared at higher energies, which could be the consequence of the hybridization between Cu+ 3d and O 2p in Co-Cu-Al-4-1-10 sample. All these observations are indicative of the presence of more Cu+ ions and empty levels involving O 2p and Co 3d orbitals closer to Fermi level in the sample, favorable for the absorption and activation of oxygen.40

20


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Figure 5. (a) Raman spectra of the samples using 532 nm excitation and (b) UV−vis spectra of the samples. The spectrum of commercial Co3O4 is also given for comparison. The local crystal structures and coordination environment of the samples were further examined by Raman spectroscopy. It is well documented that for Co3O4, there are five characteristic Raman peaks in the range of 100-800 cm-1: The set of bands at 192, 524, and 618 cm-1 could be attributed to F2g mode, and the band centered at 192 cm-1 is related to F2g(3) mode of tetrahedral sites (CoO4).41, 42 A band at 485 cm-1 is associated with Eg symmetry, and a strong band at 686 cm-1 corresponds to the characteristics of octahedral CoO6 sites, matching well A1g mode.41,

43

As shown in Figure 5a,

Co-Cu samples almost showed the same characteristic peaks to bulk Co3O4, in which five peaks centered at 186, 483, 524, 618, and 685 cm-1 correspond to the modes F2g(3), Eg, F2g(2), F2g(1) and A1g, respectively. Whereas, for Co-Cu-Al samples, Raman bands are broadened and obviously shifted towards lower wavenumbers when compared to bulk Co3O4 in Co-Cu samples, which is mainly attributed to the phonon confinement effect, as concluded in the previous studies for 21



atomically-thick porous Co3O4 nanosheets.42, 44 Thus, Co-Cu-Al samples are demonstrated to be thin nanosheets with abundant pores, in good agreement with SEM and TEM observations (Figure 2). UV−vis spectra of the samples are presented in Figure 5b. Two absorption bands located at about 410 and 700 nm can be observed for all samples, which can be respectively associated with the ligand-to-metal charge transfer from O2− to Co2+ and O2− to Co3+.45 For Co-Cu samples, the UV−vis spectra are almost the same as those for commercial Co3O4, while for Co-Cu-Al samples, the absorption edge shifted towards higher wavelengths relative to that of commercial Co3O4. This phenomenon implies that the nanomesh structure of Co-Cu-Al samples significantly decreased the band gap and promoted the electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).46 It is generally accepted that in the redox reaction, the electron transfer from HOMO to LUMO and the break of weaker metal−oxygen bonds are accompanied.6,

47

Thus, the red shift observed in absorption edge indicates the formation of

nanomesh structure, making the extraction of O easier from the surface of Co3O4 and promoting the activation of oxygen. What's more, a high density of negative charge would be probably yielded on the surfaces of Co-Cu-Al samples, which may form the electrophilic oxygen species and assist the reaction via LH mechanism.

22


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Figure 6. (a) H2-TPR and (b) O2-TPD profiles of the given samples. Relevant data for commercial Co3O4 are also given for comparison. Hydrogen temperature programmed reduction (H2-TPR) was performed to investigate the reducibility of the samples. For comparison, the relevant data for commercial Co3O4 were also given, as shown in Figure 6a. Commercial Co3O4 sample gave a broad peak centered at 390 oC, in accordance with the previous report by Arnoldy and Moulijin.48 It is commonly believed that the reduction of Co3O4 follows a two-step process via Co3+ →Co2+ →Co0.49, 50 Spadaro et al. and Luo et al. found that the reduction behavior of Co3O4 is size-dependent.51, 52 Co3O4 of large particles often possesses a single step via Co3+ →Co0, while Co3O4 of nanoparticles usually undergoes a two-step process.51, 52 Thus, commercial Co3O4 is reduced directly to metallic Co owing to its large particles. Similarly, there is only one reduction peak of Co3O4 in Co-Cu samples and Co-Cu-Al samples, indicating a single step reduction of Co3O4. For Co-Cu samples, a distinct peak at about 300 oC is 23



detected. Upon addition of aluminum powder in the starting materials to form Co-Cu-Al samples, the reduction peak showed a down shift to 240 oC. It appears that nanomesh structure with hierarchical interfaces of Co-Cu-Al samples is more favorable to accelerate the extraction of surface lattice oxygen species, most likely due to the weakened Co−O bond at surface that promoted the reduction of Co3O4.6 Besides, for Co-Cu samples, two bands centered around 200 oC are detected, which could be assigned to the reduction of different CuO species: The first one at low temperature is associated with the most easily reducible CuO species that are highly dispersed on to the surface with a strong interaction with samples. The other one is assigned to the reduction of isolated CuO that weakly interacts with samples or two- or three-dimensional copper clusters at large sizes.53 While for Co-Cu-Al samples, both peaks became weaker in intensity than those for Co-Cu samples, which is probably due to the fact that a majority of Cu2+ is converted to Cu+. This has been verified by the reduction peak of Cu+ at 360 oC in H2-TPR profiles of Co-Cu-Al samples. Surface defects, lattice oxygen mobility, and oxygen desorption behaviors of the samples were further investigated by O2-TPD, as presented in Figure 6b. In general, the surface adsorbed oxygen species undergo the following transformation process with electron gain: O2 (ads) → O2− (ads) → O− (ads) → O2− (ads/lattice).21 O2 (ads) is associated with the physically adsorbed oxygen, which can be removed by purging argon prior to the data analyses. Surface adsorbed peroxy species O2− (ads) and surface adsorbed monatomic species O− (ads), related to the surface defects, are easier to desorb in the temperature ranges of 150−250 oC and 280−340 oC, respectively.54, 55 O2− (ads/lattice) is the surface or bulk lattice oxygen and is difficult to be extracted unless the temperature is above 350oC.54, 56 For commercial Co3O4, as shown in Figure 6b, there is a large intense peak about 800oC 24


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owing to the desorption of bulk lattice oxygen. For Co-Cu samples, two diverse oxygen species are existed. One is surface adsorbed monatomic species O− (ads) located at 280oC and the majority is lattice oxygen centered at 780oC. In contrast, for Co-Cu-Al samples with hierarchical interfaces, the majority is oxygen adsorbed species (O2− and O−), considered as surface-active oxygen species, easily desorbed at about 250oC, and the other is lattice oxygen and it is hard to be activated. This result is in accordance with the XPS and UV-vis spectra analyses that the nanomesh structure in Co-Cu-Al samples is beneficial to the production of more electophilic oxygen specie, and thus promoted the activation of oxygen. Moreover, compared with Co-Cu-Al-3-1-10 and Co-Cu-Al-4-1-10, the area of lattice oxygen in Co-Cu-Al-5-1-10 has increased significantly, which is unfavorable for CO-PROX.

3.3. Catalytic performance of the samples

25



Figure 7. Catalytic performance of the samples for CO-PROX reaction. (a) CO conversion, (b) CO2 selectivity ([CO]in=1%, [O2]in=1.25%, [H2]in=50%, He balance; T=30–240oC, Flow rate=50 ml/min, atmospheric pressure), and (c) Temperature window in this work and cobalt-based catalysts reported in literature. Catalytic performances of Co-Cu and Co-Cu-Al samples for reduction of CO with excess H2 in the temperature range of 30-200oC were tested. Figure 7a gives the conversion of CO on various catalysts as a function of reaction temperature. In order to compare the activity data clearly, the 26


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temperature for 50% (T50%) and 100% (T100%) of CO conversion are listed in Table 1. For Co-Cu samples, it did not show any detectable catalytic activity until the temperature is above 50oC. The T50% values of Co-Cu-3-1, Co-Cu-4-1 and Co-Cu-5-1 is 106oC, 110oC and 103oC, respectively. And there is no temperature window exits in Co-Cu samples, in other words, the Co-Cu samples cannot completely remove CO in the evaluated temperature range. However, after the aluminum powder was added in the preparation process, the catalytic activities increases dramatically, implying that the nanomesh structure with hierarchical interfaces is beneficial to CO oxidation. What is the reason for the improved catalytic performance? Three primary factors should be taken into account: (i) Co-Cu-Al nanomesh, with hierarchical interfaces, possess a large specific surface area (Figure S6 and Table. 1), which is conducive to the exposure of more active sites Co3+ (Figure 3 and Table. 1), thus improving the catalytic performance of CO-PROX; (ii) Compared with the Co-Cu samples, Co-Cu-Al have lower reduction temperature (Figure 6a), so its catalytic performance at low temperature are greatly improved; and (iii) The nanomesh structure with hierarchical interfaces is beneficial to the activation of oxygen (Figure 6b), and O2 activation is an important factor for CO oxidation of cobalt based catalyst. The enhanced ability for providing active oxygen species in Co-Cu-Al samples would promote the surface reaction, which account for its outstanding catalytic performance. Among these Co-Cu-Al catalysts, Co-Cu-Al-4-1-10 was the best catalyst and completely oxidized CO at temperatures as low as 100 °C. Besides, the Co-Cu-Al-4-1-10 showed much higher TOFs at given temperatures (30, 50, 70, 80 and 100 oC) than that of the other samples, as shown in Figure S12. Furthermore, when comparing to the traditional Co-based and Cu-based catalysts reported in literatures (Figure 7c and Table S5), the Co-Cu-Al-4-1-10 catalyst exhibits a 27



strikingly high catalytic oxidation activity at low-temperature coupled with a broader operation temperature window.57-64 Figure 7b shows CO2 selectivity of the catalysts, CO2 selectivity of all samples remained more than 80% from 30 to 100oC. When the reaction temperature reached 120oC, competitive oxidation reaction of H2 and CO occurs, and O2 selectivity for CO steeply decreases. To accurately evaluate the catalytic performance (from CO conversion and CO2 selectivity) of the as-prepared samples, the relationship between CO conversion and CO2 selectivity is presented in Figure S13. Co-Cu-Al-4-1-10 sample shows a relatively higher performance than others, no matter from the viewpoint of CO conversion or CO2 selectivity. It all benefits from its excellent structure and property, such as nanomesh morphology, large surface area, low reduction temperature, and excellent ability to activate oxygen. In addition, the stability of a catalyst is a crucial factor for its catalytic application. We further tested the stability of Co-Cu-Al-4-1-10 catalysts by operating at constant temperature (100 oC) for a relatively prolonged time (50 h) in H2-rich streams (1% CO, 1.25% O2 and 50% H2 with He as balance gas). As shown in Figure S14, Co-Cu-Al-4-1-10 sample shows excellent reaction stability: the CO conversions drop only about 5% during the 50 h test. What’s more, its CO2 selectivity can be reached 75%, which is beneficial for their practical application. Furthermore, H2-rich gas is primarily produced by steam reforming of hydrocarbons and the subsequent water gas shift reactions (WGS), which typically consists of a certain amount of CO2 and H2O. Thus, the study on catalytic performance of catalysts for CO-PROX reaction with CO2 and H2O in feed gas is of great 28


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significance and it presents in Figure S15. For Co-Cu-Al-4-1-10 sample, the existence of CO2 has a negligible effect on catalytic activity, and a little negative effect of H2O on catalytic activity. Even in the presence of both CO2 and H2O, the 100% CO conversion can still be maintained at a wide temperature range of 120-200oC over the Co-Cu-Al-4-1-10 sample. The window is much wider than the previously reported.61, 62 Moreover, in order to investigate the effect of aluminum content on the catalytic performances, we fixed the molar ratio of Co and Cu to 4: 1, and adjusted the molar ratio of aluminum to synthesize Co-Cu-Al-4-1-5, Co-Cu-Al-4-1-10, Co-Cu-Al-4-1-15, and Co-Cu-Al-4-1-20 samples. As shown in Figure S16, there is no distinct difference in the XRD patterns of all samples, implying these samples consist of Co3O4 and CuCoO2 phases. Whereas, the morphology of the sample is closely related to the content of aluminum. If the aluminum content is insufficient, some small particles cannot be completely ruined and still existed, as shown in the SEM image of Co-Cu-Al-4-1-5 (Figure S17). Besides, as predicted, the specific surface area of Co-Cu-Al series samples increases with an increase of aluminum content (Figure S18 and Table S6). However, different from the order of specific surface area in the Co-Cu-Al samples, Co-Cu-Al-4-1-10 catalyst exhibits much higher activity than that of other catalysts (Figure S19). This is due to the fact that the CO conversion is not only related to the specific surface area, but also to the activation of oxygen (Figure S20), etc. To sum up, Co-Cu-Al-4-1-10 exhibits a broad temperature window because of its optimal structure and ratio, which can promote CO adsorption and O2 activation simultaneously. 4. CONCLUSION

29



For heterogeneous catalysis over the nanomaterial, the reactant adsorption and O2 activation should be considered if the excellent catalytic activity was desired. Here, we have provided a novel strategy to design an interface-enhanced Co3O4-CuCoO2 nanomesh tested as a catalyst for CO-PROX. This nanomesh catalysts possess a large specific surface area, which is favorable to the exposure of more active sites Co3+, thus improving CO adsorption and catalytic performance of CO-PROX. Furthermore, the geometric changes and defects weakened the bond strength of Co−O, which promoted the activation of O2 and enhanced the redox ability, and thus the surface reaction became much easier. Among Co-Cu-Al samples, the Co-Cu-Al-4-1-10 sample exhibits the best catalytic performance, which could realize 100% CO conversion at 100oC, showing a broad operation window up to 200oC. The present methodology of synthesizing nanomesh and the strategy of improving the catalytic performance is extremely important, which may show great potential in preparing other advanced transition metal oxide catalysts for heterogeneous catalytic reaction.

■ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns, the data refinements and the Co-O bond lengths, TEM image of nanomesh skeleton, SEM images and nanoparticle size distribution diagram, N2 adsorption–desorption isotherms and pore size distributions, TEM/HRTEM images, TOF of the samples, Selectivity to CO2 as a function of CO conversion for samples, Reaction stability, CO conversion and CO2 selectivity in the presence of CO2 and H2O, XRD, SEM, N2 adsorption–desorption isotherms, pore size distributions, BET 30


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specific surface area, BJH pore volumeand, H2-TPR, O2-TPD profiles, CO conversion and CO2 selectivity of Co-Cu-Al-4-1-5, Co-Cu-Al-4-1-10, Co-Cu-Al-4-1-15, and Co-Cu-Al-4-1-20.

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS

This work is financially supported by NSFC (Grants 21871106, 21771075, 21671077, and 21571176). The authors are grateful to Hefei Synchrotron Radiation Facility (SSRF) for their technical assistance.

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