Roles of Surface-Active Oxygen Species on 3DOM Cobalt-Based

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Roles of Surface Active Oxygen Species on 3DOM Cobalt-Based Spinel Catalysts MxCo3-xO4 (M=Zn and Ni) for NOx-Assisted Soot Oxidation Minjie Zhao, Jianlin Deng, Jian Liu, Yongheng Li, Jixing Liu, Zhichen Duan, Jing Xiong, Zhen Zhao, Yuechang Wei, Weiyu Song, and Yuanqing Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01995 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Roles of Surface Active Oxygen Species on 3DOM Cobalt-Based Spinel Catalysts MxCo3-xO4 (M=Zn and Ni) for NOx-Assisted Soot Oxidation

Minjie Zhao, Jianlin Deng, Jian Liu*, Yongheng Li, Jixing Liu, Zhichen Duan, Jing Xiong, Zhen Zhao, Yuechang Wei, Weiyu Song, Yuanqing Sun

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

1

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ABSTRACT: Co3O4 is a well-known catalyst in the oxidation reaction. In such a catalyst, the geometric and electronic structures of tetrahedrally coordinated Co2+ and octahedrally coordinated Co3+ can be regulated by directional metal ion substitution strategy, accompanied by the modification of catalytic activity. Herein, normal and inverse cobalt-based spinel catalysts MxCo3-xO4 (M=Zn and Ni) with three-dimensionally ordered macroporous (3DOM) structure were successfully fabricated through the carboxy-modified colloidal crystal templating (CMCCT) method. The relationship between the dopant and activity during NOxassisted soot oxidation was systematically studied by means of XPS, H2-TPR, soot-TPR, isothermal anaerobic titrations, NO-TPO, soot-TPO and so on. The well-defined 3DOM structure for MxCo3-xO4 catalysts can improve the contact efficiency of soot and catalysts. 3DOM NiCo2O4 exhibits high catalytic activity for soot oxidation under loose contact mode between soot and catalyst. For instance, its T50 and TOF values are 379 oC and 1.36 × 10-3 s-1, respectively. The doping of Ni to Co3O4 will induce the structural distortion, improve the density of oxygen vacancies and enhance lattice oxygen mobility. It leads to more surface active oxygen species. A vacancy-mediated pathway of NO oxidation on spinel catalyst is proposed according to the experimental results of in situ DRIFT spectra, in situ Raman spectra and the theoretical knowledge of coordination chemistry of metal-NO. The catalytic performance of soot oxidation is highly related to the capacity of a catalyst in oxidizing NO to NO2. Therefore, indirect NO2-assisted mechanism is proposed for soot oxidation under NO/O2/N2 atmosphere. KEYWORDS: Three-dimensionally ordered macroporous structure, Cobalt-based oxides, Spinel, Surface active oxygen species, NO oxidation, Soot oxidation 2


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1. INTRODUCTION Soot particles, as one of the unsolicited byproducts emitted from the diesel engine exhaust, cause severe problem to human health and environmental protection.1-3 A sustained effort has been made to achieve the maximum elimination of soot particles, hitherto, the continuously regenerating particulate trap (CRT) is thought as one of the most efficient after-treatment techniques.4-6 In CRT reactor, platinum (Pt) containing catalysts are located in the washcoat of the filter to accelerate the oxidation of NO to NO2. The generated NO2 can promote soot oxidation at exhaust gas temperatures (200-500 oC) due to that it is a better oxidant than O2.7 However, Pt is very rare and expensive. Thus, it is necessary to find cheap and effective catalysts for NO and soot oxidation at low temperature. Numerous catalytic materials such as supported noble metal nanoparticles,8-10 transition metal oxides (TMOs),11 perovskites12-14 and ceria-based oxides15,

16

have been studied for

several years. Among the TMOs, Co3O4 exhibits excellent activity for oxidation reactions owing to its strong redox ability.17-19 Co3O4 has a spinel structure with Fd3m symmetry, containing Co3+ on octahedral coordination sites (Co3 + Oh) and Co2+ on tetrahedral coordination sites (Co2 + Td). By tailoring cobalt oxide with a secondary element to form a binary oxide one can modulate the cation properties and ameliorate the density of oxygen vacancies on the oxides.20 For the doping effect on the structure and reactivity of Co3O4, two factors ought to be considered: one is the modified electronic structure of Co2 + Td and Co3 + Oh, which are related to the adsorption and desorption properties of gaseous reactants. The other one is the bond strength of Co2 + Td-O-Co3 + Oh, which is related to the degree of difficulty in oxygen vacancy formation. Lou et al. confirmed that the doping of In cations to Co3O4 could modify the active 3


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sites electronic structure and weaken the bond strength of Co-O simultaneously. It downshifts the d-band center of Co3O4 and promotes the formation of oxygen vacancies, resulting in the enhancement of the redox ability.21 Several binary cobalt oxides have been studied for the oxidation of soot, such as K-Co3O4, CeO2-Co3O4, Bi2O3-Co3O4, MnCo2O4 and PdCo2O4,22-27 etc. The results demonstrated that lattice oxygen mobility and O2 activation ability can be promoted through rational substitution of Co atoms. It may be helpful for NOx-assisted soot oxidation due to the enhancement of NOx storage capacity and NO oxidation ability. Though numerous researches focus on the role of NO2 in accelerating soot oxidation, the nature of NO oxidation on binary cobalt oxides has not been reported so far. Besides, the relationship between NO-to-NO2 oxidation capacity and the catalytic performance of soot oxidation has seldom been discussed. Thus, it is significant to elucidate the intrinsic roles of active sites on binary cobalt oxides in catalytic NO and soot oxidation. As is well known, the catalytic soot oxidation is a typical heterogeneous catalytic reaction occurring at a triple-phase boundary of solid (soot)-solid (catalyst)-gas (O2, NO, NO2). Considering the loose contact condition of soot particles and diesel filter in the actual aftertreatment environment, it is important to improve the contact efficiency between soot particles and catalyst for excellent catalytic performance on such solid-solid-gas reaction. 3DOM catalysts with interconnected macroporous structure (> 50 nm) provide the feasibility of soot particles passing through the interior of catalysts.9,

28

Thus, the number of contact points

between soot and 3DOM catalysts can be improved dramatically during the reaction. Meantime the catalytic performance of soot oxidation is promoted significantly.8, 9, 12, 27, 29 Herein, the well-defined 3DOM structure of MxCo3-xO4 (M=Zn and Ni) catalysts were 4


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successfully fabricated via the carboxy-modified colloidal crystal templating (CMCCT) method. The normal and inverse cobalt-based spinel catalysts for NOx-assisted soot oxidation were separately studied. Moreover, NO oxidation pathway on cobalt-based spinel catalysts and soot oxidation mechanism were systematically investigated in this work.

2. EXPERIMENTAL SECTION

2.1. Catalyst Preparation. Well-arrayed carboxy-modified polymethyl methacrylate (cPMMA) microspheres with a mean diameter of ca. 350 nm were prepared according to our previous works.12 An interconnected solid skeleton with a 3DOM structure was initially fabricated by infiltrating the metal precursors (cobalt nitrate, zinc nitrate and nickel nitrate) within the interstitial voids of the well-ordered c-PMMA microspheres, followed by thermal removal of the polymer microsphere templates, which is called the CMCCT method (Figure 1). Detailed procedures for synthesizing 3DOM MxCo3-xO4 catalysts are described in the Supporting Information. 2.2. Catalyst Characterization. X-ray powder diffraction (XRD) patterns were collected in Shimadzu XRD 6000 with Cu Kα radiation (λ=0.15406 nm). Raman spectra were recorded on a Renishaw inVia Reflex Raman spectrometer. A He−Gd laser with 532 nm excitation wavelength was used as the exciting source. Specific surface area of the samples were measured by N2 adsorption-desorption isotherms at -196 oC using Micromeritics TriStar-II 3020 equipment by Brunauer-Emmett-Teller (BET) method. Scanning electron microscope (SEM) images were obtained on ZEISS GeminiSEM 300. Transmission electron microscopy 5


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(TEM) and high-resolution transmission electron microscopy (HRTEM) images were got on JEOL JEM 2100 electron microscope. X-ray photoelectron spectra (XPS) were conducted on a PerkinElmer PHI-1600 ESCA spectrometer. Al Kα X-ray source (hν=1253.6 eV) was used as the X-ray source and C 1s at 284.8 eV was used as an internal binding energy standard. Detailed procedures such as H2 temperature-programmed reduction (H2-TPR), soot-TPR, in situ diffuse reflection infrared Fourier transform spectroscopy (in situ DRIFTS) experiments, in situ Raman spectroscopy experiments were summarized in the Supporting Information. 2.3. DFT Calculations. Detailed computational methods and models were summarized in the Supporting Information. 2.4. Catalytic Activity Evaluation and Kinetic Studies. 2.4.1. Soot-TPO reactions. Catalytic activities of 3DOM MxCo3-xO4 catalysts were investigated by the temperatureprogrammed oxidation (TPO) reactions on a continuous-flow fixed-bed quartz tubular microreactor (i.d.=10.0 mm). Before every catalytic activity test, 100 mg of catalyst and 10 mg of soot (Printex-U, Degussa, diameter of 25 nm) were gently mixed with a spatula to realize “loose contact” condition.29 Subsequently, the mixture was placed into the reactor and fixed by quartz wool to avoid the axial diffusion. The sample was then subjected to 1000 ppm NO/5% O2/N2 under atmospheric pressure with a total flow rate of 300 mL min-1, and the gas hourly space velocity (GHSV) was about 60,000 h-1. The concentrations of the outlet gases, including NO, NO2, CO2 and CO were monitored by an online infrared spectroscopy (Nicolet IS50 FTIR equipped with a 2.4 m gas cell) and obtained by OMNIC software. Prior to each catalytic 6


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activity evaluation, all the samples were firstly swept with 300 mL min-1 N2 for 1h to collect the background spectrum. Then, the tube furnace was raised to 150 oC with a heating rate of 10 oC min-1 and purged with 1000 ppm NO/5% O2/N2 for 30 min to achieve a steady state. After that the reactor was heated from 150 oC to 650 oC at a heating rate of 2 oC min-1. The temperatures of soot conversion at 10%, 50% and 90% points are denoted as T10, T50 and T90, respectively. These values are taken as indices of catalytic activity and can be a horizontal comparison of soot oxidation activity. The selectivity of CO2 (SCO2) in the outlet gases was defined as the proportion of CO2 concentration (CCO2) in the total COx concentrations (CO and CO2), i.e., SCO2 = CCO2/(CCO + CCO2). SmCO2 was defined as SCO2 at the maximum CCO2 value in soot-TPO test. To investigate the catalytic stability of the optimum catalyst, six consecutive cycles of sootTPO experiments were performed. 2.4.2. NO-TPO reactions. NO-TPO experiments were carried out in the same fixed-bed quartz reactor using 100 mg of samples. The pretreatment steps were consistent with soot-TPO reaction. After half an hour of pretreatment, the reactor was ramped from 150 to 500 oC with a heating rate of 2 oC min-1 under 1000 ppm NO/5% O2/N2 atmosphere (300 mL min-1). The concentrations of the outlet gases (NO, NO2) were recorded and analyzed by Nicolet IS50 FT-IR. The following equation was used to calculate the conversion percentage: NO to NO2 conversion (%) =

[NO2]out [NOx]in

2.4.3. Isothermal reactions. 7


× 100 %

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The turnover frequency (TOF) value is taken as an index of the intrinsic activity of catalyst for NOx-assisted soot oxidation, which was defined as the ratio of reaction rate (R) to the amounts of active redox sites.30, 31 The reaction rate of soot oxidation was determined by an isothermal reaction at 300 oC in the kinetic regime because the conversion of soot was less than 10% and nearly constant over time. During the isothermal reaction process, the loosely mixed soot-catalyst was heated to 300 oC in 1000 ppm NO/5% O2/N2 atmosphere with a total flow rate of 300 mL min-1. The R values were calculated by the slopes of soot conversion rate with time, which are reflected on the concentration of CO2 per unit time. The amount and density of active redox sites for catalysts were calculated by isothermal anaerobic titrations. During this process, soot was considered as the probe molecule. When the generated CO2 concentration became stable, O2 was immediately cut off from reactant flow. The transient decay of CO2 concentration from the steady state was on-line recorded and analyzed using Nicolet IS50 FT-IR. In order to avoid the influence of the residual oxygen in pipeline, the relatively short distance between the detector and samples (about 1m) and high gas flow rate (300 mL min-1) were used. The amount of active redox sites (O* amount) and the density of active redox sites (O* density) available to soot oxidation can be calculated by integrating the diminishing rate of CO2 formation over time: O * amount (AO * , mol/g) = 2P0V × A × 10 -6 RT ∙ m O * density (DO * , nm -2) = O * amount × 6.02 × 1023 S × 1018 Where P0 is the barometric pressure (Pa); V is the volumetric flow rate of gases through the reactor (m3/s); A is the intergrated area of CO2 concentration as a function of time (s); R is the ideal gas constant; T is the room temperature (K); m is the mass of the catalyst (g); and S 8


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is the BET specific surface area of the catalyst (m2/g). The absence of mass transfer resistance and heat transfer limitation for NOx-assisted soot oxidation under reaction conditions was verfied by Weisz-Prater criterion (CWP) and Mears’ criterion (CM). In the present work, the diameter of catalyst was about 2.2 × 10-4 m, the gas flow rate was 300 mL min -1, and the kinetic calculation was obtained at low soot conversion ( 0.5) when the temperature was higher than 500 oC. It is consistent with XRD results of 3DOM NixCo3-xO4 materials.35 The existence of cubic NiO crystal phase can be observed on 3DOM NiCo2O4 and Ni1.5Co1.5O4 from the wide-angle XRD patterns (Figure 3A). The average crystallite sizes (D) of 3DOM MxCo3-xO4 catalysts were calculated by the 13


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Debye-Scherrer equation: D(hkl) = 0.89λ/(βcosθ), and the intensity of the most prominent peak (the (311) crystal plane) was used as the reference. From Table 2, it can be clearly observed that the average crystallite size of 3DOM Co3O4 is 23.5 nm, while the binary oxides of 3DOM Zn0.5Co2.5O4 and NixCo3-xO4 are 26.2 nm and 12.2-17.5 nm, respectively. It can be concluded that Ni substitution suppresses the growth of cobalt-based crystallites, and the specific surface area will increase with smaller crystallite size. 3.2.2. Raman spectra. XRD analysis suggests the cubic spinel structure with space group Fd3m for 3DOM MxCo3-xO4 catalysts. The simulated structures of unit cells of Co3O4 and NiCo2O4 are shown in Figure 1. The cubic spinel Co3O4 possesses octahedrally coordinated Co3 + Oh and tetrahedrally coordinated Co2 + Td. For NiCo2O4, the introducing of Ni will occupy octahedral coordination sites while Co is distributed over both octahedral and tetrahedral coordination sites. As shown in Raman spectra of crystalline 3DOM Co3O4 (Figure 3C), the factor group analysis predicts five Raman-active A1g, F2g(3), F2g(2), Eg and F2g(1) bands.21 The band assigned to A1g symmetry at 666 cm-1 is characteristically ascribed to the octahedral coordination sites (CoO6), and the band assigned to F2g(1) symmetry at 188 cm-1 is characteristically ascribed to the tetrahedral coordination sites (CoO4). Raman bands with medium strength located at 466 and 510 cm-1 represent Eg and F2g(2) symmetry, and the weak band located at 605 cm-1 represents F2g(3) symmetry.21, 27 Compared with 3DOM Co3O4, Raman peak of CoO6 (666 cm1)

significantly shift to lower frequencies for 3DOM NixCo3-xO4 catalysts while it is nearly

unchanged for 3DOM Zn0.5Co2.5O4. This further indicates that the doped Ni ions mainly replace octahedrally coordinated Co3 + Oh, accompanied by a significant effect on the symmetry of 14


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CoO6. Though Raman curves for these Zn and Ni doped samples are similar with Co3O4, the peaks shift to lower frequencies and become broader, which are due to the change in the original coordination environment of tetrahedrally and octahedrally coordinated cations. It indicates that the doping of Zn and Ni to Co3O4 can cause the lattice distortion or residual stress of spinel structure.24 Therefore, it can be concluded that the doping of Zn and Ni (especially for Ni substitution) will cause structural defects and lattice distortion, which are beneficial for the formation of oxygen vacancies. Besides, a broad band centered at 1055 cm-1 for 3DOM Ni1.5Co1.5O4 can be observed, which is assigned to the longitudinal optical phonon mode of NiO.36, 37 3.2.3. Results of SEM and (HR)TEM. The morphology and macroporous structure of the catalysts were observed by SEM and (HR)TEM. Figure 4 shows SEM images of 3DOM MxCo3-xO4 catalysts obtained by the CMCCT method. It can be observed that more than 98% of the synthesized macroporous materials display a high-quality of 3DOM structure with a high degree of perpendicular arrangement to the direction of the first layer. The macroporous materials containing skeletons surrounding uniform close-packed periodic voids with average diameter of 270 ± 10 nm, which correspond to shrinkage of 20-25% compared with the initial size (350 nm) of c-PMMA microspheres (Figure S1). Moreover, the wall thicknesses observed from SEM images are 30 ± 5 nm. The next layer is highly visible and the voids are interconnected through the open windows, ca. 120 ± 5 nm in diameter. Figure 4 shows TEM and HRTEM images of 3DOM MxCo3-xO4 catalysts. As a typical inverse opal structure, 3DOM crystalline compounds are classified into skeleton structure.38 15


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The hierarchical macroporous frameworks exhibit highly periodic arrays of uniform macropores. The macropores have a dimension of 270 ± 10 nm, and the voids are interconnected through the open windows (ca.120 ± 5 nm) with the wall thicknesses of macroporous skeletons about 30 ± 5 nm, which are in accordance with SEM results. The interconnected macroporous architecture with high degree of perpendicular arrangement provides the feasibility of soot particles passing through the interior of catalyst, which can improve the contact efficiency between soot particles and catalyst with the help of airflow in the reaction. Besides, HRTEM images show (311), (111), (220) and (400) surface terminations with d-spacing being consistent with that of the cubic Co3O4 (Figure 4d, h, and l). 3.2.4. Pore structure and surface area. N2 adsorption-desorption isotherms and pore-size distributions profiles by BJH measurement of 3DOM MxCo3-xO4 catalysts are presented in Figure S8, and the pore parameters and BET surface areas are summarized in Table 2. In the relative pressure (p/p0) range of 0.9-1.0, all of these catalysts show type Ⅱ N2 adsorption-desorption isotherm with a type H3 hysteresis loop. It is important to point out that a near-linear middle section of each isotherms is observable under the low pressure portion (p/p0=0-0.4), which is ascribed to the unrestricted mono-multilayer adsorption. Figure S8B represents the BJH pore size distributions scattering from 4 to 80 nm of these catalysts, which indicates the existence of macroporous structure. As can be seen from Table 2, SBET of 3DOM Co3O4 is 26.8 m2/g, and 3DOM Zn0.5Co2.5O4 possesses smaller specific surface area (21.9 m2/g) while 3DOM NixCo3-xO4 catalysts possess larger specific surface area (27.6-30.6 m2/g). It may be due to that Zn substitution promotes the growth of cobalt-based crystallites while Ni substitution suppresses 16


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it, which have been confirmed by XRD results. The catalyst with larger specific area is beneficial for the enhancement of contact efficiency between gaseous reactants and catalyst. 3.2.5. Results of XPS. It is well known that the catalytic performance is greatly dependent on the surficial properties of catalysts. XPS is employed to obtain the surficial chemical composition information of these 3DOM catalysts. Figure 5 illustrates Zn 2p, Ni 2p, Co 2p and O 1s XPS spectra of 3DOM MxCo3-xO4 catalysts, and Table 3 summarizes the surface element compositions. Figure 5A shows Zn 2p spectra of 3DOM Zn0.5Co2.5O4, and two peaks centered at 1020.9 and 1044 eV (Zn 2p3/2 and Zn 2p1/2), indicating that Zn ions exist as 2+ state in 3DOM Zn0.5Co2.5O4. It is observed from Figure 5B that Ni 2p3/2 spectra are asymmetrical and two characteristic peaks can be observed, which are attributed to Ni3+ (856 eV), Ni2+ (854.4 eV) and the satellite peak of Ni 2p3/2 (861.5 eV), respectively.39,

40

As shown in Table 3, Ni species ratio of

Ni3+/(Ni2++Ni3+) is 0.75, indicating that nearly three-quarter of Ni species are Ni3+. It has been demonstrated that not only octahedrally coordinated Co3 + Oh acts as the active site but also octahedrally coordinated Co2 + Oh exhibits highly catalytic activity for VOC oxidation.41 Co2 + Oh species can be easily oxidized to the active Co3 + Oh species by the gasphase oxygen molecules, resulting in more surface active oxygen species.42 Thus, it is important to determine the surface element compositions and Con+ species of these normal and inverse spinel catalysts. From Figure 5C, it can be seen that Co 2p3/2 can be divided into four sets of components: one peak at BE=779.9 eV is assigned to the surface Co3+ and the other one at BE=781.4 eV is assigned to the surface Co2+ species and the other two peaks are assigned to the shake-up satellite. As illustrated in Table 3, the surface Co3+/(Co2++Co3+) ratio follows the 17


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order: Zn0.5Co2.5O4 (0.60) > Co3O4 (0.54) > Ni0.5Co2.5O4 (0.48). Considering the definition of normal and inverse spinel structure (AB2O4 and B(AB)O4), Zn0.5Co2.5O4 can be described as (Zn0.5Co0.5)Co2O4, and Ni0.5Co2.5O4 can be described as Co(Ni0.5Co1.5)O4. The detailed coordination environments of Co, Zn and Ni can be found in Table 4. Combined with XPS data, it is revealed that Ni substitution will decrease the oxidation state of Co3+, indicating that the presence of Ni3+ in Ni0.5Co2.5O4 leads to excess electrons in lattice oxygen and thus lattice oxygen tends to attract fewer electrons from adjacent Co cations to achieve balance. The oxygen vacancy plays an important role in catalytic oxidation reactions due to that the oxygen vacancy in a catalyst surface may accelerate the adsorption and dissociation of oxygen molecules,43 i.e. the higher the surface oxygen vacancy density, the more the surface chemisorbed oxygen species. They are beneficial for the oxidation of NO and soot. By using the curve-fitting method, the asymmetrical O 1s spectra of every cobalt-based spinel catalysts can be fitting into three components at BE=530.1, 531.6 and 532.7 eV (Figure 5D), attributable to lattice oxygen (Olatt, O2-) and chemisorbed active oxygen (Oads, e.g. O2- , O22 - or O - ) species, respectively.44 The peak area of O 1s spectra is integrated to calculate the relative contents of three kinds of oxygen species in Table 3. With the incorporation of transition metals Zn and Ni to cobalt oxide, the relative percentage of the peak of O&-(O2- , O22 - or O - )/Ototal follows the order: Ni0.5Co2.5O4 (0.52) > Co3O4 (0.46) > Zn0.5Co2.5O4 (0.43). As octahedrally coordinated Co3 + Oh is partially replaced by Nin + Oh, the ratio of Oads/Ototal increases, implying that Nin + Oh plays an important role in activating O2 molecules compared to Co3 + Oh. 3.2.6. Results of H2-TPR and soot-TPR. The reducibility of a solid oxide catalyst is highly related to its catalytic performance. As 18


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shown in Figure 6A, H2-TPR experiments were performed to assess the redox properties of 3DOM MxCo3-xO4 catalysts. Table S6 lists the quantitative analysis results of these catalysts. There are a shoulder peak and a major peak centered at ca. 362 and 445 oC for 3DOM Co3O4, which is ascribed to the reduction of Co3+ to Co2+ and Co2+ to Co0, respectively. After half of Co2 + Td is replaced by Zn2 + Td, there are two reduction peaks centered at 407 and 506 oC for 3DOM Zn0.5Co2.5O4 catalyst. The bare ZnO does not exhibit any reduction peaks in the temperature range of 30-900 oC.45 The first peak at 407 oC is attributed to the reduction of Co3+ to Co2+, and the second one at 506 oC is related to the reduction of Co2+ to Co0. For 3DOM NixCo3-xO4 catalysts, the major peaks located at ca. 393, 358 and 363 oC, respectively. The equation (1) and (2) illustrate the variation process of 3DOM NiCo2O4 under H2 atmosphere. 1

Ⅲ Ⅱ Ⅲ NiⅡ 1 - xNix Co2 - yCoy O4 + 2(x + y)H2 1

→ NiⅡCoⅡ 2 O4 - (x + y) 2 + 2(x + y)H2O (1) NiⅡCoⅡ 2 O4 - (x + y) 2 + (4 –

x + y 2

)H2 → Ni0 + 2Co0 + (4 –

x + y 2

)H2O (2)

It is noteworthy that the reduction peaks of 3DOM Zn0.5Co2.5O4 shift to higher temperature in comparison with 3DOM Co3O4, while the major reduction peaks shift to lower temperature for 3DOM NixCo3-xO4. Considering the reduction process is related to the relaxation of M-O (M=metal) bond,21 the results of H2-TPR reveal that the doping of Ni to Co3O4 will significantly accelerate the extraction of lattice oxygen species and weaken the bond strength of Co-O. Thus, Ni substitution promotes the formation of structural defects in Co3O4. After 19


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quantifying the reduction peaks, the actual total H2 consumption in the moderate and lowtemperature region (< 550 oC) are 13.17, 12.03, 14.05, 14.22 and 13.88 mmol/g for 3DOM Co3O4, Zn0.5Co2.5O4, Ni0.5Co2.5O4, NiCo2O4 and Ni1.5Co1.5O4, respectively. Theoretical H2 consumption amounts are quiet close while such tiny differences in the actual total H2 consumption amounts are related to the formation of oxygen vacancies in cobalt-based spinel solid solution (Table S6). The temperature range of 3DOM MxCo3-xO4 catalysts reduction are close to that of soot oxidation. Former temperature range (200-550 oC) is the extraction process of oxygen by H2 from catalyst surface to bulk, while latter temperature range (200-500 oC) is the oxidation-reduction reaction processes on catalyst surface. As shown in Figure 7A, there is a good correlation with the total H2 consumption amounts and T50 values, indicating that the catalytic activities for soot oxidation on cobalt-based spinel catalysts are strongly dependent on the redox properties of composite metal oxides. It can be concluded that 3DOM NiCo2O4 possesses superior redox performance at low temperature. Surface active oxygen species are particularly important for oxygen-involved oxidation reactions. Soot-TPR experiments were performed in a highly pure N2 (99.99%) atmosphere for these 3DOM MxCo3-xO4 catalysts, and the results are illustrated in Figure 6B. In the process of soot-TPR, soot can only be consumed by surface active oxygen species on the catalyst due to that there is no gaseous oxygen. As shown in Figure 6B, the profiles have been divided into three temperature intervals, corresponding to O2- or O22 - (350-550 oC), O - (550-750 oC) and bulk lattice O2 - (˃ 690 oC), respectively.26, 46, 47 In consideration of the temperature range of soot oxidation on these cobalt-based spinel catalysts, surface chemisorbed oxygen species (O2- , O22 - or O - ) ought to be the main active oxygen species for soot oxidation. Generally, 20


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surface chemisorbed oxygen species are usually formed through the adsorption of gaseous O2 on surface oxygen vacancies. By comparing the profiles in Figure 6B, it is clear that Ni substituted catalysts possess more surface chemisorbed oxygen species, especially for 3DOM NiCo2O4, indicating that the introducing of Ni to Co3O4 lattice matrix promotes the formation of oxygen vacancies. It should be mentioned that the peaks of lattice oxygen shift to lower temperature with the doping of Ni to Co3O4, indicating the enhancement of mobility for lattice oxygen and exchange capacity between lattice oxygen and gas-phase oxygen. Thus, it will provide more active oxygen species on catalyst surface, which is in accord with the result of XPS analysis. 3.3. Isothermal anaerobic titration. Soot oxidation is a solid-solid-gas reaction occurring at the three-phase boundary of catalystsoot-O2, NO, NO2. It is important to determine the amount of available active redox sites on catalyst surface for soot oxidation. Different probe methods are derived from the characteristics of supported and bulk metal oxide catalysts, which are used to calculate the density of active redox sites. For supported metal oxide catalysts, the metal sites are default to the active redox sites and they can be quantified by H2 or CO chemisorption.1, 48 For bulk metal oxide catalysts, O2 chemisorption on H2 prereduction catalyst can be used as a redox sites probe. It involves reducing the catalyst surface with H2 and then re-oxidizing to determine the number of redox sites by the amount of O2 adsorbed.49 However, neither of these methods can be a universal method for solid-solid reactions such as soot oxidation owning to that soot is loosely mixed with catalyst and only part of active redox sites acts on the oxidation of soot. Besides, using the actual reactant as the probe molecule is quite important to accurate estimate the number of 21


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active redox sites. As for soot oxidation, isothermal anaerobic titrations at kinetic regime is an effective method to accurate identification and quantification of the density of active redox sites on both supported and bulk catalysts.12, 26, 27, 28, 30, 31 This technique using soot particles as the probe molecules. Except for the absence of oxygen in the gas phase, all experimental conditions are consistent with the actual reactions. The principle of specific quantitative analysis is summarized in the Supporting Information. In this technique, the active redox sites are also supposed as surface active oxygen sites. Herein, the amount and density of surface active oxygen sites on 3DOM MxCo3-xO4 catalysts for soot oxidation were quantified by isothermal anaerobic titrations at 300 oC. The absence of mass transfer resistance and heat transfer limitation for NOx-assisted soot oxidation under reaction conditions have already been checked. As shown in Figure 7B, Figure S9 and Table 1, the amount of surface active oxygen sites follows the order: 3DOM NiCo2O4 (32.93 × 10-5 mol g-1) > Ni1.5Co1.5O4 (28.29 × 10-5 mol g-1) > Ni0.5Co2.5O4 (25.22 × 10-5 mol g-1) > Co3O4 (20.08 × 10-5 mol g-1) > Zn0.5Co2.5O4 (15.56 × 10-5 mol g-1). The doping of Ni to Co3O4 can significantly improve the amount and density of active oxygen sites on catalyst surface, it is derived not only from the increase of specific surface area, but also from structural distortion caused by Ni doping, which improve the density of oxygen vacancies on 3DOM NixCo3-xO4. The results of isothermal anaerobic titrations are well in agreement with the results of XPS, soot-TPR and catalytic activities of soot oxidation. 3.4. NO-to-NO2 oxidation. The evaluation of NO-to-NO2 oxidation capacity on 3DOM MxCo3-xO4 catalysts were performed by means of TPO experiments. Figure 8A and B illustrate the profiles of NO2 percentage as a function of temperature obtained from the tests of NO-TPO and soot-TPO. The 22


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ACS Catalysis

equilibrium conversion of NO to temperature is also presented in dashed line as a reference. Generally speaking, the outcome of the test looks like an asymmetric bell curve. When the temperature is high enough (roughly above 200 oC) that cobalt-based spinel catalysts are able to catalyze the oxidation of NO to NO2, and NO2 is a more effective oxidant in facilitating soot oxidation compared to O2 and NO. Qualitatively, all of these cobalt-based spinel catalysts possess similar trends of NO conversion. The maximum NO conversion is obtained at intermediate temperature and NO conversion decreases in a thermodynamically controlled regime owning to the exothermicity of the reaction. NO2 profiles presented in Figure 8A show that the capacity of NO-to-NO2 oxidation is significantly promoted with the doping of Ni to Co3O4, and is slightly decreased with the doping of Zn to Co3O4. From Table S7, it can be observed that the maximum NO2 level follows the order: NiCo2O4 (69.6 %) > Ni1.5Co1.5O4 (64.8 %) > Ni0.5Co2.5O4 (63.3 %) > Co3O4 (60.3 %) > Zn0.5Co2.5O4 (55.9 %). Over the past several years, a number of studies have reported the effect of surface active oxygen species on the oxidation of NO.20, 50-54 The results revealed that the higher lattice oxygen mobility and oxygen vacancies density would result in the higher NO oxidation capacity. Figure 8B illustrates NO2 concentration profiles during soot-TPO processes, From Table S7, it can be observed that the maximum NO2 level follows the order: NiCo2O4 (59.4 %) > Ni1.5Co1.5O4 (53.3 %) > Ni0.5Co2.5O4 (46.1 %) > Co3O4 (38.5 %) > Zn0.5Co2.5O4 (30.9 %). The order of maximum NO2 concentration in NO-TPO and soot-TPO are in consistent with the catalytic activity of soot -TPO. It is an essential prerequisite of excellent catalytic activity of soot oxidation for improving NO oxidation ability. 3.5. In situ DRIFT spectra of NOx adsorption and desorption. 23


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The DRIFT spectra of adsorbed species on 3DOM MxCo3-xO4 arising from contact of NO + O2 were recorded to gain insight into the types of stored NOx species. Figure 9 shows in situ time-resolved DRIFT spectra of the catalysts under NO + O2 atmosphere at room temperature and 350 oC, respectively. In the case of 3DOM Co3O4, these bands are attributed to nitro compound (1322 cm-1), monodentate nitrites (1458 cm-1), chelating and bridging nitro compound (1240 and 1520 cm-1), monodentate nitrates (1251 or 1268 cm-1), bidentate nitrates (1036, 1287 and 1547 cm-1), free ionic nitrate species (1380 cm-1) and adsorbed NO2 molecules (1628 cm-1).55-61 Table S8 summarizes the band assignments, positions and structures of these NOx species. As time goes on, nitro compound forms firstly, then chelating and bridging nitro compounds gradually produce and become stronger. There is a band at 1458 cm-1, which is ascribed to ν(N=O) vibration of monodentate nitrites. It can be observed that the adsorption of gaseous NO2 (1628 cm-1) and monodentate nitrates (1268 cm-1) are much weaker at room temperature. In general, the main adsorbed NOx species on 3DOM Co3O4 are nitrite species at low temperature. It is reported that the strength of NO adsorption and NO stretching frequency are closely related to the electronic structure of metal.62 The kind of adsorbed NOx species on 3DOM Zn0.5Co2.5O4 and Ni0.5Co2.5O4 are similar to Co3O4, but the band positions for Zn and Ni substituted catalysts have changed slightly, indicating the modification of electronic structure for Co3O4. For NO + O2 adsorption at 350 oC, it is very soon for the adsorption of NOx to reach the saturation point. The adsorbed NOx species at high temperature mainly exist as nitrate species. A major band is located at about 1380 cm-1, arising from the formation of free ionic nitrate species (NO3- ), which has a D3h symmetry and possesses one IR active mode (ν3, 1380 cm-1) and one IR inactive mode (ν1, 1050 cm-1).56, 60 As for surface nitrate species 24


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(M-NO3), it has a C2v symmetry and its ν3 mode splits into two bands as νas(NO2) and νs(NO2), while the ν1 mode becomes IR active. A minor band at 1251 cm-1 is attributable to monodentate nitrates. The bands at about 1547, 1287 and 1036 cm-1 are assigned to νas(NO2), νs(NO2) and ν1 mode of bidentate nitrates. In addition, the adsorption of gaseous NO2 is clearly observed at high temperature, which is generally supposed to generate from the decomposition of surface nitrite/nitrate species and free ionic nitrate species.56 Table S8 shows the simulated chemical structure of M-NO2 and M-NO3. As for surface nitrite species, M-NO2 is coordinated via its O atom to metal, coordinated via its N atom to metal or bound simultaneously via an O and its N atom. As for surface nitrate species, M-NO3 is only coordinated via its O atom to metal. It can be formed by the interaction of NO2 (or M-NO2) to the basic sites (coordinatively unsaturated oxygen anions) or disproportionation of N2O4.63 Figure 10 shows in situ DRIFT spectra of NOx desorption on 3DOM MxCo3-xO4 catalysts at different temperatures. The sample was pretreated at 350 oC under NO + O2 atmosphere for NOx to reach adsorption saturation and then cooled to 100 oC. In comparison with the DRIFT spectra in Figure 9, the bands are assigned to monodentate nitrites (1458 cm-1), nitro compound (1318 cm-1), monodentate nitrates (1247 and 1266 cm-1), bidentate nitrates (1043 and 1540 cm-1) and free ionic nitrate species (1385 cm-1). As the temperature increases, monodentate nitrites and nitro compound are gradually vanished, and the main NOx stored on the catalyst surface are bidentate nitrates and free ionic nitrate species. Several studies have focused on the energetics of M-NO2-NO3 conversion and NO2 desorption, and the formation of gaseous NO2 is supposed as the rate-determining step, which is highly related to the oxide electronic structure.52,

53, 64

From Figure 10, one can observe the stored NOx species on 3DOM 25


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Ni0.5Co2.5O4 are almost disappeared after 400 oC while it still exist on 3DOM Co3O4 and Zn0.5Co2.5O4. At lower temperature the complete desorption of the stored NOx species indicates the weaker adsorption strength of NOx on the catalyst, which is induced by the introducing of Ni to Co3O4. This is further proved that nickel substitution can modify the electronic structure of Co3O4, which is beneficial for the formation of NO2 and it has been proved by the results of NO-TPO. 3.6. In situ Raman spectra. In situ Raman spectroscopy is an intriguing tool for monitoring and studying surface intermediates during the reaction process, and it is well known that surface active oxygen species play a critical role in oxygen-involved oxidation reactions. Thus, 3DOM NiCo2O4 with optimum catalytic behavior was selected as research subject, undergoing different gaseous reactants (O2 and NO/O2) from 50 to 350 oC. The real-time detection of intermediates on catalyst surface and original vibration bands of catalyst can give us effective information to understand the influencing factors of catalytic activity. Figure 11 shows Raman spectra of NiCo2O4 under NO/O2 and O2 atmosphere, respectively. As shown in Figure 11, one Raman band at about 805 cm-1 is assigned to O-O stretching of adsorbed peroxide species (O22 - ) and the other one at about 1050 cm-1 is attributed to O-O stretching of adsorbed superoxide species (O2- ).65-68 It can be observed that the intensity of the band O2- is slightly stronger in NO/O2 than in O2 at low temperature (< 150 oC). In Figure 11A, the nearly unchanged intensity of surface active oxygen species during NO-TPO may be ascribed to that NO oxidation on NiCo2O4 not only consumes surface active oxygen species, but also generates some surface oxygen vacancies in the meantime, and gas-phase oxygen can 26


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ACS Catalysis

be activated to form new surface active oxygen species. Moreover, Raman band at 666 cm-1 corresponding to A1g symmetry of CoO6 shifts to low wavenumber and becomes weaker with the increasing of reaction temperature. While Raman band at 188 cm-1 corresponding to F2g symmetry of CoO4 is nearly unchanged, indicating that the cation in octahedral coordination sites play a critical role in NO oxidation. In Figure 11B, the intensities of the bands O22 - and O2- are slightly enhanced, which is owing to lattice oxygen mobility increases with the increasing of temperature.

4. DISCUSSIONS

4.1. Surface active oxygen species and oxygen vacancy. Oxygen vacancy can be divided into bulk oxygen vacancy and surface oxygen vacancy, and bulk oxygen vacancy can migrate from bulk to surface while surface oxygen vacancy is generally supposed as surface oxygen defect site.69 O2 molecules can be activated by surface oxygen defect sites to form surface chemisorbed oxygen species, which is also called surface active oxygen species. In addition, the formed bulk oxygen vacancy can be replenished by the migration of adjacent bulk lattice oxygen, creating a new oxygen vacancy. Thus, there is a transport pathway for oxygen from gas-phase O2 to bulk oxygen vacancy through O2→O2- → O22 - →2O - →2O2 - .70, 71 An oxide possesses high mobility of lattice oxygen indicates its high exchange capacity between surface oxygen defect sites and gas-phase oxygen. Surface oxygen defect sites act as oxygen carriers for an oxide, which are proposed to participate in many catalytic reactions via a Mars-van Krevelen mechanism as 2-M-VO-M- + 27


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O2 → 2-M-O-M- and R + - M-O-M- → R-O +-M-VO-M-, where R is the reductant. As NO has one more electron than CO, the coordination chemistry of metal-NO is in parallel development with metal-CO and have high degree of similarity.72 Zhou et al. reported that Co3+ acted as the active sites for CO adsorption on (110) plane of Cu substituted Co3O4 nanowire and the doping of Cu had negligible effect on CO adsorption energies, but it was more favorable to form oxygen vacancy in the bonding of Co3+-O-Cu2+ than in Co3+-O-Co2+.73 Lou et al. reported that the doping of In to Co3O4 could elongate the bond length and weaken the bond strength of Co-O, which promoted the formation of oxygen vacancy. Most importantly, introducing appropriate amount of In to octahedral coordination sites could tune the adsorption strength of CO and O2 activation simultaneously.21 Figure 12A vividly shows the role of surface oxygen defect site in Co3O4 for the formation of surface active oxygen species. Every tetrahedrally coordinated Co2 + Td and octahedrally coordinated Co3 + Oh is linked by an oxygen as Co2 + Td -O - Co3 + Oh. Doping appropriate heteroatom through directional metal ion substitution strategy can efficiently improve the density of oxygen vacancy in Co3O4. Herein, Zn ion and Ni ion were selected as the doping component to substitute tetrahedrally coordinated Co2 + Td and octahedrally coordinated Co3 + Oh in Co3O4 due to the similar ionic radii of Zn and Ni to Co. The electronic structure of Co is modified with the cation occupation and concentration changed in normal and inverse cobalt-based oxides. As NO approaches to catalyst surface, it can be efficiently oxidized to NO2 due to strong oxidation ability of Co3O4 and large amounts of surface active oxygen species. The produced NO2 can improve the catalytic performance of soot oxidation significantly. In order to further understand the influence of Zn and Ni substitution on Co3O4, the 28


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ACS Catalysis

formation energy of surface oxygen vacancies on Co3O4, Ni0.5Co2.5O4 and Zn0.5Co2.5O4 surface were investigated by DFT calculations. As shown in Figure S3A, five-coordinated Co3+ cation and two types of lattice oxygen are exclusively exposed on Co3O4 (100) surface. One type is binding with three Co3+ (denoted as Oa), another type is binding with two Co3+ and one Co2+ (denoted as Ob). The calculated oxygen vacancy formation energies for Oa and Ob are 1.52 and 2.05 eV, respectively. It is quite unfavorable that a 3-cordinated Co will be formed when a vacancy is formed at Ob site. Likewise, the value of Oa vacancy formation energy on Zn0.5Co1.5O4 and Ni0.5Co2.5O4 surface are 2.06 and 1.09 eV, respectively. The corresponding configurations are shown in Figure S3E and Figure S3F. As for oxygen vacancy on Ni0.5Co2.5O4 surface, a “split vacancy” is formed due to the strong local relaxation effect, leading to the nearest oxygen moves to the middle between two metal cations.74 The excess electrons associated with the vacancy cause the original Ni fractional valence between +2 and +3 to form a stable Ni2+ species. This may be the reason that the doping of Ni results in a decrease of oxygen vacancy formation energy. In a word, H2-TPR and soot-TPR reveal that Ni substitution can weaken the bond strength of Co-O, which is beneficial for the improvement of the density of oxygen vacancy and the mobility of lattice oxygen simultaneously. XPS and isothermal anaerobic titration experiments are further proved that there are abundant surface active oxygen species on nickel-substituted cobalt oxides. DFT calculations provide a new perspective to understand surface oxygen vacancy formation energy, which follows the order: Zn2+-O-Co3+ > Co2+-O-Co3+ > Co2+-O-Ni3+ on (100) plane of MxCo3-xO4 oxides. 4.2. NO2 generation process on 3DOM MxCo3-xO4 catalysts. The coordination chemistry might give us some inspiration on NO oxidation reaction. NO 29


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is known as a stable free radical, and as shown in the molecular orbital diagram (Figure 12B), an unpaired electron in this molecule locates in a 2π (also known as π*) molecular orbital, which is extraordinarily easy to release.72 Owing to its long-pair electron on 5σ orbital, NO can bind to metals in both terminal and bridging modes to form metal nitrosyl complexes. Normally, the chemical bond between NO and transition metals involves the attachment of N atom to the metal as M-N-O.72, 75 According to the stereochemistry of terminal mode, NO can exhibit two complexes character as nitrosonium ion (NO+, is isoelectronic with CO) and nitroxide ion (NO-, is isoelectronic with O2), and the detailed information are listed in the Table S9. In the former kind of M-N-O, the metal atom will accept one electron transferred from NO, thereby reducing its oxidation state by one, followed by coordination of the resulting NO+ to metal atom with bond angle nearly 180o (linear angle). In the latter kind of M-N-O, the metal atom will donate one electron to NO, thereby increasing its oxidation state by one, followed by coordination of the resulting NO- to metal atom with bond angle in the range of 120-140o (bent angle).76 Obviously, the chemical properties of NO+ is electrophilic while NO- is nucleophilic. As a catalyst possesses higher oxidation state of metal atoms and larger amount of surface active oxygen species (e.g. O2- , O22 - , or O - ), it will be easy to form M-NO2 and M-NO3 when NO molecule approaches to catalyst surface due to that NO+ is very susceptible to surface active oxygen species to act as the reactive intermediate. Wang et al. and Fang et al. have proposed the formation of NO+ on Mn-based oxides in NH3-SCR of NOx.77, 78 Furthermore, what vitally important is that Jason et al. and Ferenc et al. have proved the existence of NO+ coordinated at framework sites in the zeolite pores (Si-O-(NO+)-Al) plays a direct role in catalytic NO oxidation,79, 80 and Wang et al. have also proved the generation of NO+ on SrCO3-BiOI core30


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shell heterojunction for photocatalytic NO purification.81 On the other hand, a catalyst with lower oxidation state of metal atoms will be inclined to the formation of NO- under high temperature for directly reduction of NO to N2 and O2.82 In situ time-resolved DRIFT spectra of NOx adsorption on 3DOM MxCo3-xO4 catalysts further give the evidences of Co-NO2 when the catalysts under exposure of NO + O2 at room temperature. With the increasing of temperature, O2 can be further activated to form surface active oxygen species. Three possible pathways are present in Figure 12C, D and E for molecular O2 adsorption to form O2- , O22 and O - on the surface of Co3O4. From path 1, the process of molecular O2 adsorption at one oxygen defect site do not break O-O bonds while NO can adsorb at the nearby Co atom and react with one of the oxygen atoms in the adsorbed O2 to form NO2, leaving the remaining oxygen atom of O2 to fill the oxygen defect site. From path 2, O2 dissociative adsorption to fill two adjacent oxygen defect sites on Co3O4 surface, which can promote the formation of O on the oxygen defect sites. From path 3, molecular O2 can bind with two adjacent Co atoms and then dissociate to form two extra-lattice oxygen on Co atoms. The adsorbed O - can act as the candidate sites for NO or NO2 coordination to form Co-NO2 or Co-NO3. Besides, the bonding of NO to a metal is belonging to σ-π system in ligand field theory.72 As mentioned earlier, NO+ is isoelectronic with CO. Electrons on 5σ orbital of NO+ can transfer to metal 3d orbital while NO+ π* orbital receives back electrons from metal dπ orbital. In this work, the electronic structure of Co is regulated by Zn and Ni substitution, which play an important role in modification of the back-donation level of dπ orbital electrons into π* antibonding orbital of NO, and it is embodied in the binding strength of NO chemisorption on active site.61 In situ DRIFT spectra of NOx desorption on 3DOM MxCo3-xO4 catalysts reveal that Ni substitution 31


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can decrease the energy of NO2 desorption, which is rate-determining step in NO oxidation reaction. Based on the results of XPS, H2-TPR, soot-TPR, isothermal anaerobic titrations, NO-TPO, in situ DRIFT spectra and in situ Raman spectra for 3DOM MxCo3-xO4 catalysts, and combined with DFT calculations, we conclude that surface active oxygen species and the cation in octahedral coordination sites are responsible for superior NO-to-NO2 oxidation capacity of 3DOM NixCo3-xO4 catalysts. A vacancy-mediated pathway, similar to a Mars-van Krevelen mechanism can be proposed to elucidate the detailed process of NO oxidation on 3DOM NiCo2O4: Firstly, Ovacancy (VO) near Co2+ or Ni2+ species will pick up an electron from Co2+ or Ni2+ to form O2- , accompanied by the oxidation of Co2+ to Co3+ or Ni2+ to Ni3+: (Ni/Co)2 + + VO + O2 → (Ni/Co)3 + + O2O2- ⇌ O22 - ⇌ 2O - , denoted as O * Then, an electron transfers from NO to (Ni/Co)3+ and the coordination process is accompanied by σ-π bonding: (Ni/Co)3 + + NO → (Ni/Co)2 + - (NO) + While NO+ reacts with nearby lattice oxygen or surface active oxygen species: (Ni/Co)2 + - (NO) + + Olatt → (Ni/Co)2 + - NO2 + VO (Ni/Co)2 + - (NO) + + O * → (Ni/Co)2 + - NO2 (Ni/Co)2 + - (NO) + + 2O * → (Ni/Co)2 + - NO3* Then, the stored M-NO2 and M-NO3 decompose to gaseous NO2: (Ni/Co)2 + - NO2 → (Ni/Co)2 + + VO + NO2(g) 32


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(Ni/Co)2 + - NO3* → (Ni/Co)2 + + O * + NO2(g) Finally, the gas-phase O2 molecule participates the reaction to complete the cycle: (Ni/Co)2 + + VO + O2 → (Ni/Co)3 + + O2It is worth mentioning that the oxidation ability tendency of Co2 + Td toward Co3 + T𝑑 and Co2 + Oh toward Co3 + Oh have been explained by ligand field theory.41,

83

The results

demonstrate that Co3 + Oh is energetically substantially favored over Co2 + Oh in a moderate ligand field and provides a higher driving force to oxidize Co2 + Oh, while for tetrahedral coordination that the ligand field stabilization of Co3 + Td over Co2 + Td is relative small, resulting in the oxidation of Co2 + Td to Co3 + Td being relatively difficult. Combined with NO oxidation pathway described above, it can be concluded that octahedral coordination sites play a bigger role in O2 activation and NO oxidation than tetrahedral coordination sites do. It is in accord with the result that 3DOM Zn0.5Co2.5O4 possesses less amount of surface active oxygen species when half of tetrahedrally coordinated Co2 + Td is replaced by catalytically inactive Zn2 + Td in Co3O4, and 3DOM NixCo3-xO4 catalysts possess larger amount of surface active oxygen species when octahedrally coordinated Co3 + Oh is partially replaced by Nin + Oh. XPS spectra not only reveal that Ni substitution increases the amount of surface active oxygen species, but also decreases the oxidation state of Co3+. The oxidation state of dopant (Ni) mainly exists as Ni3 + Oh, which will give a preferential adsorption of NO to form the reactive intermediate (NO+), and the existence of Ni2 + Oh and Co2 + Oh in octahedral coordination sites indicate the formation of the structural defects and oxygen vacancies. The reduction peaks of H2-TPR for 3DOM NixCo3-xO4 catalysts shift to lower temperature. It reveals that Ni substitution will weaken the bond strength of Co-O. This is further proved by the results of 33


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soot-TPR that the doping of Ni to Co3O4 will promote the formation of oxygen vacancies and enhance lattice oxygen mobility. Thus, the oxidation capacity of NO to NO2 will be significantly promoted with the doping of Ni to Co3O4. 4.3. Mechanism of 3DOM MxCo3-xO4 catalysts for NOx-assisted soot oxidation. All of these normal and inverse 3DOM MxCo3-xO4 catalysts exhibit high catalytic activity for soot oxidation in NO/O2/N2 atmosphere, which confirms the crucial role of NO2 in initiating soot oxidation. From Table 1 and Table S7, it is to be observed that T10 of 3DOM MxCo3-xO4 catalysts (308-356 oC) are close to the temperature at maximum NO-to-NO2 conversion (about 320-352 oC), indicating that soot will be ignited under the maximum rate of NO2 production. Bensaid et al. have suggested that NOx-assisted catalytic soot oxidation was highly controlled by the presence of NO2 in the temperature range of 250-400 oC, and oxygen would start to dominate the reaction above 500 oC.15 It has been proposed that NO2 can attack soot on the surface actively, and thus the carbon-oxygen compounds, which is called “surface-oxygenated complexes” (SOCs), will gradually formed, followed by the decomposition of SOCs to CO and CO2. The generated SOCs are more reactive than the pure soot, and when SOCs exist on the outer layer of soot and accumulate to high concentration, soot will be ignited.1, 3, 84 As illustrated in Figure 13 (A-E), NO2 concentration profiles during NO-TPO and soot-TPO (NO-TPO in the presence of soot) are plotted. The shadow area reflects the disappearance of NO2 owing to NO2 participation in soot oxidation, and the disappearance of NO2 starts at about 200 oC while ends at about 470 oC. The consumption of NO2 intensifies with the increasing of temperature, creating a discrepancy in integral areas between two experiments. It is commonly believed that the shadow area is closely associated with the catalytic activity of soot oxidation, 34


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the higher the shadow area, the better the catalytic activity.14, 47, 85 However, 3DOM MxCo3xO4

catalysts exhibit completely different laws of change for the catalytic performance of soot

oxidation. This different situation may be related to the utilization efficiency of NO2 for these catalysts. Liu et al. have investigated the utilization efficiency of NO2 on soot oxidation for Pt/H-ZSM5 and sulfated Pt/Al2O3 catalysts, and the results revealed that not only NO2 can be effectively transferred onto the surface of soot to initiate the creation of SOCs, but also NO2 can be re-adsorbed on the catalyst surface. Thus, there is a competitive adsorption of NO2 on the surface of soot and catalyst, and the higher amount of NO2 attack soot surface can result in more NO2-soot reactions, accompanied by the higher utilization efficiency of NO2.1, 84 The utilization efficiency of NO2 for 3DOM MxCo3-xO4 catalysts are summarized in Table S10. The calculated NO2 efficiency of 3DOM Co3O4 and NiCo2O4 is 1.5 and 13.1 at 350 oC, respectively. It is significantly promoted with the doping of Ni to Co3O4 lattice matrix. Considering the thermodynamic equilibrium of NO-to-NO2 oxidation reactions have already achieved for these catalysts at the temperature of 50% soot conversion (T50, 379-423 oC). The only difference is the oxidation rate of NO at a constant temperature, like 350 oC. As shown in Figure 1, soot particles can enter and pass through the inner pores of these 3DOM catalysts. Thus, the generated NO2 can transfer to the surface of soot at the first time during the reaction. Therefore, the higher rate of NO2 products, the more NO2 molecules attack soot per unit time. It creates larger amount of SOCs accumulated on the outer layer of soot, which is reflected in lower T10 and T50 values and higher reaction rate of soot oxidation. Figure 13F shows two linear relationships between T50 values and maximum NO2 levels in NO-TPO and soot-TPO tests for 3DOM MxCo3-xO4 catalysts. Besides, NO oxidation rate is much faster than NO2 reduction rate 35


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by soot.29 One can identify the utilization efficiency of NO2 in catalytic soot oxidation by NO2 level in the presence of soot. In a word, NO2 level in the absence of soot simply reflects the capacity of a catalyst in storing nitrate species and producing NO2, while NO2 level in the presence of soot reflects the utilization efficiency of NO2 in catalytic soot oxidation. A clear linear relationship between NO2 production level and soot oxidation is further giving the evidence that 3DOM MxCo3-xO4 catalysts for catalytic soot oxidation mainly occurs through indirect NO2-assisted mechanism. Figure 14 and Figure S10 show the stability of 3DOM NiCo2O4 during six consecutive cycles of NOx-assisted soot oxidation tests. Brief comparison of the data from first cycle to sixth cycle shows that there is a slight deactivation occurs with the number of repetitions. In order to trace the source of why the reaction deactivated and reinforce the evidence of how NO2 affect the reaction, the data of NO2 concentration during six consecutive soot-TPO reactions are plotted. As shown in Figure 14B, a visible decrease of NO2 concentration is observed during six consecutive cycles in the temperature range of 200-400 oC, which is resulted from gradually growth of NiCo2O4 crystallites (Figure S11A) and slightly damage of 3DOM structure (Figure S11B) during the reactions. This further confirms the assumption that NO2 level in the presence of soot directly reflects the utilization efficiency of NO2 for 3DOM MxCo3-xO4 catalysts. Based on the above results and discussions, Figure 15 vividly displays the pathway of soot oxidation over 3DOM NiCo2O4 catalyst. On the one hand, soot particles can freely pass through the interior of 3DOM NiCo2O4 catalyst, and collide with catalyst surface with the help of reactant gas flow. On the other hand, octahedral coordination sites play a bigger role in O2 36


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activation and NO oxidation than tetrahedral coordination sites do on cobalt-based spinel catalysts. As NO and O2 molecules approach to Con + Oh or Nin + Oh on 3DOM NiCo2O4 catalyst surface in perpendicular and tilted configuration, respectively. M-NO2, M-NO3 and NO2 (g) species will be gradually formed. The gaseous NO2 promotes the formation of SOCs on soot surface and accelerates soot oxidation ultimately. Hence, indirect NO2-assisted mechanism is the dominant reaction pathway for 3DOM MxCo3-xO4 catalysts in catalytic soot oxidation.

5. CONCLUSIONS 3DOM MxCo3-xO4 catalysts were successfully prepared by using the CMCCT method. The doping effect on the structure and reactivity of Co3O4 for NOx-assisted soot oxidation have been systematically studied. Modulating the electronic structure of active sites and engineering oxygen vacancies in cobalt oxide are two effective ways to maximize the redox performance of Co3O4. The highly ordered macropores and interconnected small pore windows can facilitate the transportation of gaseous reactants (O2, NO, NO2) and soot in the reaction process, thus enhancing the contact efficiency of soot-catalyst. It is an essential prerequisite of excellent catalytic activity of soot oxidation for improving NO oxidation ability. For oxygen-involved oxidation reaction as NO oxidation, active sites electronic structure on Co3O4 can be tailored at the atomic level by engineering binary metal oxide with high oxidation state in the octahedral coordination sites. It is not only beneficial for O2 activation, but also preferential for the formation of reactive intermediate as nitrosonium ion (NO+) when NO molecule approaches to the catalyst surface. The introducing of Ni to Co3O4 lattice matrix induces structural distortion, 37


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improve the density of oxygen vacancies and enhance lattice oxygen mobility. The energy of rate-determining step in NO oxidation reaction is decreased for NixCo3-xO4, thus promoting NO2 formation. NO2 plays a crucial role in NOx-assisted soot oxidation reaction. 3DOM NiCo2O4 possesses superior catalytic activity for soot oxidation: i.e., its TOF value is 1.36 × 10-3 s-1, apparent activation energy E is 89.7 kJ mol-1 and T10, T50, T90 values are 308, 379 and 423 oC, respectively. The linear relationships between T50 values and maximum NO2 levels in NO-TPO and soot-TPO tests for 3DOM MxCo3-xO4 catalysts have been obtained. This is further corroborated indirect NO2-assisted mechanism for 3DOM MxCo3-xO4 catalysts in catalytic soot oxidation.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: http://dx.doi.org/10.1021/acscatal.0000000. Detailed synthesis processes of c-PMMA microspheres, 3DOM MxCo3-xO4 catalysts and experimental procedures as H2-TPR, soot-TPR, in situ DRIFTS and in situ Raman. Computational methods and models of DFT calculations. Explanation of kinetic measurements and calculations. Additional experimental data (Figure S1-S11 and Table S1-S10).

AUTHOR INFORMATION Corresponding Author *E-mail for J.L.: [email protected] 38


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (U1662103, 21673290), Beijing Natural Science Foundation (2182060).

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Table 1. Catalytic Activity, Densities of Surface Active Oxygen Species (O*), Reaction Rates, and TOF Values of 3DOM MxCo3-xO4 Catalysts for NOx-Assisted Soot Oxidation under Loose Contact Mode Soot oxidation activity Soot oxidation at 300 oC T10a T50a T90a SCO2m Soot Oxygen amount Density of Rw (mol s-1 g1 -7 d conversion (mol g-1cat × 10-5)b oxygen (nm-2)c (oC) (oC) (oC) (%) cat × 10 ) Co3O4 340 412 457 100 4.8 % 20.08 4.51 2.22 Zn0.5Co2.5O4 356 423 459 100 2.6 % 16.56 4.55 1.79 Ni0.5Co2.5O4 331 402 452 100 5.7 % 25.22 5.50 2.88 NiCo2O4 308 379 423 100 8.7 % 32.93 6.54 4.47 Ni1.5Co1.5O4 322 392 435 100 6.5 % 28.29 5.57 3.39 NiO 402 462 498 98 ― ― ― ― ZnO 486 575 615 58 ― ― ― ― Pure soot 493 592 623 55 ― ― ― ― a Reaction conditions: 1000 ppm NO/5% O /N , 300 mL min-1. 2 2 b The amount of surface active oxygen species determined by isothermal anaerobic titration experiments at 300 oC. c The density of surface active oxygen species. d R and R are the differential rates normalized by catalyst weights and surface areas, measured at 300 oC. w s e TOF = R /A 23 w O* or (Rs/DO*) × 6.02 × 10 . Catalysts (3DOM)

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Rs (mol s-1 m-2 × 10-9)d 8.28 8.17 10.43 14.75 11.08 ― ― ―

TOF (s-1 × 10-3)e 1.11 1.08 1.14 1.36 1.20 ― ― ―

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Table 2. Structural Parameters of 3DOM MxCo3-xO4 Catalysts Catalysts Surface area Pore volume Average pore Lattice Crystallite 2 a 3 a b c (3DOM) (m /g) (cm /g) diameter (nm) constant (Å) size (nm)d Co3O4 26.8 0.09 270-281 8.0839 23.5 Zn0.5Co2.5O4 21.9 0.08 265-277 8.0947 26.2 Ni0.5Co2.5O4 27.6 0.10 265-275 8.0959 17.5 NiCo2O4 30.3 0.09 8.0963 15.4 ― Ni1.5Co1.5O4 30.6 0.09 8.1017 12.2 ― a Obtained from the BET results. b Estimated according to the SEM images. c Calculated through the XRD patterns by the Scherrer equation. d Determined by the Scherrer equation using the FWHM based on Co O (311) crystal face. 3 4 Table 3. Surface Element Concentrations of Ni, Co, O and the Relative Concentration Ratios on Catalyst Surface Derived from XPS Analyses Catalyst (3DOM)

O species (%) O2-

O22 -

O2-

Ra

Ni species (%)

Co species (%)

(Ni3+/Nin+)b

(Co3+/Con+)c

Co3O4

18.7 26.8 54.5 0.46

―

0.54

Zn0.5Co2.5O4

15.8 27.6 56.6 0.43

―

0.60

Ni0.5Co2.5O4

12.4 39.5 48.1 0.52

0.75

0.48

O species ratio of the (O2- +O22 - )/Ototal b The Ni species ratio of the Ni3+/(Ni2++Ni3+) c The Co species ratio of the Co3+/(Co2++Co3+) a The

Table 4. Coordination Environments of Cobalt-Based Spinel Catalysts a Coordinationb Tetrahedral Octahedral 2+ Co3O4 Co Co3+ Zn0.5Co2.5O4 Zn2+, Co2+ Co3+ Ni0.5Co2.5O4 Co2+, Co3+ Ni2+, Co3+ a In all the materials, the oxygen sublattice is fcc. The cations are located in the voids of the oxygen sublattices. b The oxidation state of cations presented in this table is in an ideal coordination environment. Sample

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Figure 1. Schematic illustration of 3DOM cobalt-based spinel catalysts.

Figure 2. (A) Conversion percentage of soot oxidation and (B) soot conversion amounts as a function of time at 300 oC over 3DOM MxCo3-xO4 catalysts. Reaction conditions: loose contact mode of soot and catalyst, mass ratio of soot/catalyst = 0.1, GHSV = 60,000 h-1, 1000 ppm of NO, 5% O2 in N2.

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Figure 3. (A) XRD patterns, (B) evolution of the XRD highest peak and (C) Raman spectra of 3DOM MxCo3-xO4 catalysts: (a) Co3O4, (b) Zn0.5Co2.5O4, (c) Ni0.5Co2.5O4, (d) NiCo2O4 and (e) Ni1.5Co1.5O4.

Figure 4. SEM, TEM and HRTEM images of 3DOM MxCo3-xO4 catalysts: (a-d) Co3O4, (e-h) Zn0.5Co2.5O4 and (i-l) Ni0.5Co2.5O4. 53


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Figure 5. (A) Zn 2p, (B) Ni 2p, (C) Co 2p and (D) O 1s XPS spectra of 3DOM MxCo3-xO4 catalysts: (a) Co3O4, (b) Zn0.5Co2.5O4 and (c) Ni0.5Co2.5O4.

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Figure 6. (A) H2-TPR and (B) soot-TPR profiles of 3DOM MxCo3-xO4 catalysts: (a) Co3O4, (b) Zn0.5Co2.5O4, (c) Ni0.5Co2.5O4, (d) NiCo2O4 and (e) Ni1.5Co1.5O4.

Figure 7. (A) The relationships between T50 values and total H2 consumption amounts and (B) active oxygen amounts quantified at 300 oC for 3DOM MxCo3-xO4 catalysts: (a) Co3O4, (b) Zn0.5Co2.5O4, (c) Ni0.5Co2.5O4, (d) NiCo2O4 and (e) Ni1.5Co1.5O4.

Figure 8. (A) NO2 concentration curves of NO-TPO and (B) soot-TPO over 3DOM MxCo3xO4

catalysts.

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Figure 9. In situ time-resolved DRIFT spectra of NO adsorption on (A1, A2) 3DOM Co3O4, (B1, B2) 3DOM Zn0.5Co2.5O4 and (C1, C2) 3DOM Ni0.5Co2.5O4 at 30 oC (left) and 350 oC (right) in the mixed gas of 1000 ppm NO/5% O2/N2.

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Figure 10. In situ DRIFT spectra of NOx desorption on (A) 3DOM Co3O4, (B) 3DOM Zn0.5Co2.5O4 and (C) 3DOM Ni0.5Co2.5O4 as a function of temperature after the catalysts were exposed to a flow of 1000 ppm NO/5% O2/N2 for 60 min at 350 oC.

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Figure 11. In situ Raman spectra of 3DOM NiCo2O4 catalyst with different gaseous reactants from 50 oC to 350 oC: (A) 1000 ppm NO/5% O2/Ar; (B) 5% O2/Ar.

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Figure 12. (A) Schematic illustration of surface oxygen defect site in Co3O4; (B) Molecular orbital diagram of NO molecule; (C), (D) and (E) Three possible pathways of O2 activation on Co3O4 surface.

Figure 13. (A, B, C, D, E) NO2 concentration as a function of temperature during NO-TPO and soot-TPO catalytic tests over 3DOM MxCo3-xO4 catalysts; (F) The linear relationships 59


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between T50 values and maximum NO2 level in NO-TPO (cyan) and soot-TPO (pink) for 3DOM MxCo3-xO4 catalysts: (a) Co3O4, (b) Zn0.5Co2.5O4, (c) Ni0.5Co2.5O4, (d) NiCo2O4 and (e) Ni1.5Co1.5O4.

Figure 14. (A) Stability test of 3DOM NiCo2O4 for NOx-assisted soot oxidation (The lighter the color, the larger the value of T10/T50/T90) and (B) NO2 concentrations during six consecutive soot-TPO reactions of 3DOM NiCo2O4 catalyst.

Figure 15. Schematic diagram of 3DOM NiCo2O4 catalyst for NOx-assisted soot oxidation.

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Roles of Surface-Active Oxygen Species on 3DOM Cobalt-Based

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