mesoporous CoCeOx mixed oxides for propane combustion

Applied Catalysis A, General 572 (2019) 61–70

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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata

Correlation between the physicochemical properties and catalytic performances of micro/mesoporous CoCeOx mixed oxides for propane combustion Xiang Li , Xinghua Li, Xiaolan Zeng, Tianle Zhu ⁎

T

⁎

School of Space and Environment, Beihang University, Beijing, 100191, PR China

ARTICLE INFO

ABSTRACT

Keywords: Mixed metal oxide Propane combustion Micro-mesoporous Oxygen vacancies In situ DRIFTS

A series of micro-mesoporous CoCeOx catalysts have been successfully prepared using a double template combining sol-gel method. The catalytic performance of novel CoCeOx catalysts are investigated and compared with pure Co3O4 and CeO2 catalysts for total oxidation of propane. It is found that the Co1Ce1 catalyst shows the highest catalytic activity (T50 = 217 °C) as well as a good reaction stability and water tolerance among the five catalysts. The Co3O4, CeO2 and CoCeOx catalysts are characterized using XRD, BET, Raman, XPS, H2-TPR, O2TPD, HRTEM, HAADF-STEM and in situ DRIFTS. The results demonstrate that the larger BET surface area, smaller grain size, stronger reducibility and more active oxygen species of the Co1Ce1 catalyst are responsible for its outstanding catalytic performance among the five catalysts. Moreover, the synergistic effect between Co and Ce over CoCeOx catalysts are probably relevant to the formation of CoxCe1−xO2−σ solid solution. In addition, on the basis of the results of XPS, kinetic analysis and in situ DRIFTS, the surface Co3+ ions and active oxygen species are regarded as the major active sites of the CoCeOx catalysts for total oxidation of propane, and then a scheme of reaction model based on Langmuir-Hinshelwood mechanism is suggested at last. It can be expected that the micro-mesoporous CoCoOx catalysts are promising materials for VOCs removal, and the results in this research may also provide some new insights into the catalyst design and mechanism exploration for VOCs catalytic oxidation.

1. Introduction

higher energy and stronger active oxygen species to participate in reaction. It remains a great challenge to seek a suitable catalyst to achieve high conversion of propane at relative low temperatures. Supported noble metals (Pd, Pt and Rh) exhibit excellent low-temperature efficiency for VOCs (toluene, formaldehyde, methane etc.) combustion. However, the high costs and easily sintering and aggregation restrict their extensive application for VOCs control [3,4]. Nowadays, a series of transition-metal-oxides catalysts have been developed and shown good catalytic performance for VOCs combustion. Bai et al. found that 3D-MnO2 with abundant surface-adsorbed oxygen species exhibited good catalytic properties for ethanol oxidation [5]. He et al. synthesized mesoporous CuCeOx catalysts via a simple self-precipitation protocol for toluene combustion. The T90 of toluene conversion over Cu0.3Ce0.7Ox reached 212 °C with the GHSV of 36,000 h−1 [6]. Among the various metal oxide catalysts, Co3O4 shows excellent catalytic performance for oxidation of CO and total oxidation of hydrocarbons [7–9]. In the Co3O4 spinel structure, one-eighth of the

Volatile organic compounds (VOCs) emissions from stationary and mobile sources often results in serious environmental problems like photochemical smog, ozone depletion, haze weather and greenhouse effect [1,2]. As one of light alkane VOCs, propane is commonly used as industrial chemicals and motor vehicle fuels. On the one hand, oxidative dehydrogenation of propane (ODHP) represents an economic and alternative process to produce propene, which is an important raw material in the modern chemical industry. On the other hand, as one of the principal components in liquefied petroleum gas (LPG), compressed natural gas (CNG) and liquefied natural gas (LNG), propane emission from the automotive exhaust source has been concerned in many countries. So its emission control is currently one of the most urgent and compelling problems for improvement of environment. Recently, catalytic oxidation is the common technology for both dehydrogenation of propane to propene and total combustion of propane to CO2. Compared with partial oxidation, propane total oxidation usually needs a

⁎

Corresponding authors. E-mail addresses: [email protected], [email protected] (X. Li), [email protected] (T. Zhu).

https://doi.org/10.1016/j.apcata.2018.12.026 Received 24 September 2018; Received in revised form 24 November 2018; Accepted 22 December 2018 Available online 24 December 2018 0926-860X/ © 2018 Elsevier B.V. All rights reserved.

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available tetrahedral sites are occupied by Co2+, and half of octahedral sites are occupied by Co3+. Co3+ sites with low CoeO bond energy and high capability to activate oxygen has been considered as the major active sites for CO oxidation over Co3O4 [10]. In addition, several paper found that the oxidation activity could also be influenced by morphology and particle size of Co3O4. Xie et al. prepared Co3O4 nanorods with a diameter of 10–20 nm and mainly exposed (110) planes, and found they could catalyse CO oxidation to CO2 at temperature as low as −77 °C [11]. Benjamin et al. reported that bulk cobalt oxide with small crystallite size could catalysis the complete conversion of C3H8 to CO2 down to 250 °C, but subjected to deactivation with time-on-stream [12]. Furthermore, abundant oxygen vacancies on catalysts surface are also believed as a precondition of high activity and stability for VOCs catalytic oxidation [13,14]. Therefore, introducing another metal element into Co3O4 is an common way to increase the amount of oxygen detects and improve catalytic performance [15–17]. Recently, CeO2 has been commonly used as main active ingredient or catalytic promotor for VOCs oxidation because of its non-toxic, high oxygen storage capacity (OSC) and remarkable oxygen mobility [6,18,19]. There are two potential advantages for incorporation CeO2 into Co3O4 catalyst for the enhancement of oxygen vacancies amount. On the one hand, the bond length of CeeO in cubic fluorite CeO2 (2.343 Å) is longer than that of Co2+–O (1.897 Å) and Co3+–O (1.971 Å) in Co3O4 spinel. So the oxygen vacancy on CeO2 may form more easily than Co3O4. On the other hand, Ce cation with a larger radius can distort the structure and generate CeeOeCo units, which can lead to chemical state change and new oxygen vacancy formation [20]. Therefore, the design of CeeCo composite catalyst is a good way for enhancing activity and stability of propane combustion. However, the product selectivity of propane oxidation should be further considered. Besides propane combustion, CeO2 has also been chosen as a support for the oxidative dehydrogenation of propane reaction [21,22]. Although the two reactions both need break the CeH bond of the propane further quantity, higher oxidation capacity and larger contact area of active oxygen species are essential for total oxidation than ODHP [23]. In short, it is expected that highly active Co-Ce catalyst should be metastable nanocrystalline oxide with plentiful cavity construction, high surface area, small grain size and remarkable redox property. As is well known, preparation method can greatly affect the structure and properties of the target catalysts. Recently, a general method for the synthesis of mesoporous materials with thermally stable, crystalline and monomodal pore size had been developed by Suib et al. [24]. Preparation of such materials involved the use of inverse micelles, sol-gel chemistry, NO3− decomposition and calcination control. Various mesoporous metal oxides (Mn2O3, Co3O4, CuO etc.) with tunable porosity and crystallinity crystalline had been successfully synthesized with nitrates as precursors, P123 as the template and n-butanol as solvent [25–27]. So it is also a promising method for mixed traditional metal oxides preparation. On the other hand, hierarchical porous materials has attracted widespread interest in the potential application of catalysis, gas separations, energy storage because of highly porous, large surface area and abundant accessible space. Several researchers have found that dual-templating method is a feasible strategy for hierarchical material synthesis [28]. In general, anionic surfactants like CTAB, TTAB and SDS have been widely used for the synthesis of microporous materials, while nonionic surfactant like Pluronic P123 is beneficial to the formation of mesoporous materials [29,30]. Therefore, One way to get the hierarchical porous mixed oxides is adding anionic surfactants and multi-precursors to Suib’s micellar system. Herein, micro-mesoporous CoCeOx catalysts are developed based on the modified Suib’s method by addition of SDS as a co-template. The synthesized CuCeOx oxides presents excellent catalytic activity, stability and water tolerance for total oxidation of propane. The physicochemical parameters, surface area, crystal structure, reducibility, and surface oxygen concentration of prepared catalysts are well correlated

with their catalytic performance. Moreover, the potential active sites and reaction scheme for propane combustion are also proposed at last. The results in this work indicate that CoCeOx catalysts are efficient and promising porous materials for alkanes removal. What’s more, some new insights into the catalyst design and mechanism exploration may also provide referrals for VOCs catalytic oxidation. 2. Experimental 2.1. Catalysts preparation Micro-mesoporous CoCeOx catalysts were prepared by the modification of the Suib’s method for sodiumdodecylsulfate (SDS) introduction [24]. Typically, 2 g P123 (Sigma-Aldrich, Mn ˜ 5800), 0.1 g SDS (Beijing reagent, > 98.5%) and the desired amount of Co (NO3)2·6H2O (Shanghai reagent, > 98.5%) and Ce(NO3)6·4H2O (SigmaAldrich, 99.999%) with the total inorganic source (0.01 mol) were dissolved in the n-butanol firstly under magnetic stirring. Then 2 mL HCl (> 36%) was added dropwise the above solution with vigorous stirring for 1 h. The obtained clear gel was transferred to an oven at 120 °C for 4 h. The obtained powder was further washed and centrifuged by ethanol and DI water, then collected and dried in a vacuum oven. The dried samples were calcined by a multi-step heating in air with a heating rate of 2 °C min−1, which is firstly heated to 150 °C for 12 h, then heated to 250 °C for 4 h, and finally heated to 350 °C for 3 h. The CoCeOx catalysts with the 4:1, 1:1 and 1:4 M ratios of Co to Ce were labeled with Co4Ce1, Co1Ce1and Co1Ce4, respectively. The single Co3O4 and CeO2 catalysts were also synthesized with the same method for comparison. 2.2. Catalyst Characterization The XRD patterns were obtained by Rigaku-D/max-2200/PC between 25° and 75° at a step length of 5° min−1 with a Cu Kα radiation (λ = 0.154056 nm). The crystalline sizes of catalysts were estimated by the Scherrer formula as shown below:

D=

K Bcos

(1)

Where K is a constant “0.89” ; B is half-width of X-ray diffraction; θ is diffraction angle. The crystalline sizes of CeO2 and Co3O4 were calculated based on the (1 1 1) and (3 1 1) planes, respectively. The N2 adsorption-desorption isotherm, BET surface area, t-plot area, pore size and pore volume of the catalysts were measured at 77 K with a Micromeritics- ASAP2020 instrument. Before the test, each sample was pretreated at 350 °C for 3 h under vacuum. The Field Scanning Electron Microscope (FSEM) images were recorded on a Hitachi S-4800 apparatus at a voltage of 15 kV. Transmission electron microscopy (TEM) images were collected on a Talos f200x instrument at a voltage of 200 kV. The as-prepared samples were firstly treated by ultrasonic dispersion in ethanol and then dried on a carbon film supported by a copper grid. Laser Raman spectra (LRS) were obtained by a Renishaw-InVia Reflex microscope equipped with an in situ reaction cell and a temperature controller for in-situ experiment. The 532 nm line of the laser beam was chosen for excitation under different atmosphere in this study. The dehydration Raman was carried out in an insitu cell. Before the test, the sample was pretreated by He (100 mL min−1) at 150 °C for 0.5 h, and then cooled down to room temperature for further test. H2 temperature programmed reduction (H2-TPR) and O2 temperature programmed desorption (O2-TPD) were operated on a Micromeritics ChemiSorb-2920 TPx apparatus with the same pretreatment. Briefly, 50 mg samples with 40–60 mesh were pretreated at 300 °C for 60 min under Helium in the U-shape quartz reactor. For H2TPR, the samples were firstly cooled down to 50 °C, and then reacted with H2 from 50 °C to 900 °C with a heating rate of 10 °C min−1 under 62

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the atmosphere of 5% H2/N2 (50 mL min−1). For O2-TPD, after cooling with He, the samples were treated by a 5% O2/N2 gas flow (50 mL min−1) for O2 adsorption and saturation. Finally, the samples were heated and recorded from 50 °C to 450 °C under a He gas flow (50 mL min−1) with a heating rate of 10 °C min−1. X-ray photoelectron spectroscopy (XPS) patterns were investigated using an VG ScientificESCALab220i-XL electron spectrometer at 300 W Al Kα radiations. All the binding energies were calibrated by the C 1s signal at a binding energy of 284.8 eV. In situ DRIFTS spectra were obtained on a Thermo Fisher ScientificNicolet 6700 instrument equipped with an MCT/A detector and an ZnSe in situ cell. The spectra were collected by accumulating 32 scans with a resolution of 4 cm−1. Prior to each test, the sample was pretreated by He (100 mL min−1) at 300 °C for 1 h, and then cooled down to room temperature for further reaction.

of Co3O4 can also happen when the Co content is larger than Ce in the CoCeOx catalysts. Furthermore, the crystallite sizes were estimated by Scherrer equation on the basis of CeO2 (1 1 1) or Co3O4 (3 1 1) plane as listed in Table 1. It can be seen that the crystallite sizes of the Co1Ce1 catalyst calculated by CeO2 and Co3O4 plane are smaller than other samples. The smaller size of the Co1Ce1 catalyst indicates that CoxCe1−xO2−σ solid solution may form easily when the concentration of ceria is close to cobalt in the CoCeOx catalyst. Fig. S1 shows the N2 adsorption/desorption isotherms and pore size distributions of CeO2, Co3O4 and CoCeOx catalysts. All the catalysts exhibit type-IV adsorption isotherms with hysteresis loops, indicating a regular mesopore structure [31]. Barrett–Joyner–Halenda (BJH) pore size distribution calculated from the desorption curves (inset in Fig. S1), implies that pore diameters increase and their distributions broaden when the Co/Ce ratio rises. According to the previous report by Steven et al., this kind of mesoporous materials were formed by the synergistic effect from inverse micelles formed by P123 and nitrate ion decomposition on the control of the sol-gel process [24]. On this basis, with the addition of a small amount of SDS, the hierarchical (micro/mesopore) materials were obtained in our study. As listed in Table 1, the tplot area contributed by micropore decreases as the Ce proportion drops (46.2–2.4 m2/g). This result demonstrates that CeO2 are prone to form micropores under the action of SDS. Among the three CoCeOx catalysts, Co1Ce1 has the largest BET surface area (130.9 m2/g) that is primarily provided by mesopores. It means that cobalt oxide seems in favor of mesoporous frameworks formation by the results of pore distribution curves and t-plot areas. Therefore, the formation of hierarchical pore structure for CoCeOx is not only related to surfactants (P123 and SDS) and nitrate ion, but also element type and molar ratio. As shown in the SEM images (Fig. S2), a wool-ball like structure with a large size (> 100 nm) is formed by secondary particle aggregation for CeO2 catalyst. Furthermore, the Co4Ce1 and Co1Ce4 catalysts show more severe particle aggregation without specific shape. By contrast, the Co1Ce1 catalysts is composed of monodispersed spheroid nanoparticle (˜30 nm) with plenty of interparticle voids. Although the particle size of Co3O4 catalyst looks smaller than Co1Ce1, the tightly packed nanoparticles leads to limited porosity. The results above suggest that the Co1Ce1 is beneficial for the improvement of the nanoparticle dispersion and porosity of single cobalt or cerium oxide. To further identify the structural details of the Co1Ce1 catalyst, HRTEM, HAADF-STEM and elemental mapping images were collected and shown in Fig. 2. The Co1Ce1 catalyst exhibits mesoporous characteristics with the pore size of 3–5 nm derived from uniform particles accumulation. These particles are homogeneous and composed of polycrystalline pore walls and channels. According to the HRTEM results shown in Fig. 2b, the grain size of the Co1Ce1 catalyst is about 6–8 nm, which is close to but slightly smaller than the results from Scherrer equation. As mentioned above in the XRD results section, both the diffraction peaks of the cubic CeO2 and Co3O4 crystal phase are present for the Co1Ce1 catalyst. There are surface lattice spacings of 0.23, 0.27 and 0.284 nm for the (2 2 2, Co3O4), (2 0 0, CeO2) and (2 2 0, Co3O4) crystal planes, respectively, suggesting mixed phases exist. Furthermore, to further investigate the elemental distribution of Ce, Co and O in the catalyst, the elemental mapping analyses were recorded by STEM-EDS mode. It can be seen that Co (green), O (red) and Ce (blue) are uniformed distributed in the whole district for the Co1Ce1 catalyst, indicating the high dispersion of Co and Ce. On account of the high magnification STEM image in the selected area (Fig. 2d), nanoparticles of the Co1Ce1 catalyst shows ball-like morphology with an average diameter of ˜ 6 nm, much smaller than that prepared by other methods like coprecipitation, supercritical CO2, hard template method, etc. [17,32–34]. Fig. 3a shows the dehydration Raman spectra of the CeO2, Co3O4 and CoCeOx catalysts recorded from 300 to 1000 cm−1. For CeO2, the intense signal centered at around 451 cm−1 corresponds to the firstorder-allowed, triply degenerate F2g vibration mode of octahedral local

2.3. Evaluation of catalytic performance The propane catalytic oxidation activity tests were tested using a fixed-bed quartz tube reactor (Inner diameter = 6 mm) with 100 mg of the catalyst (40–60 mesh). The activity measurements were performed in the temperature range of 150–350 °C. The feed gas mixture was consist of 0.6% C3H8, 20% O2, 5% H2O (if used) and N2 as the balance gas. The total flow rate was maintained at 100 mL min−1, corresponding to a gas hourly space velocity (GHSV) of 60,000 mL (g h)−1. The reaction products were monitored and analyzed online by an Shimadzu-2014C gas chromatograph with TCD and FID detectors for CO2 and VOCs detection. The C3H8 conversion, CO2 selectivity and TOF for catalysts were calculated with reference to the following equation:

C3 H8

conversion =

CO2 selectivity =

TOF =

[C3 H8]inlet [C3 H8]outlet × 100% [C3 H8]inlet

[CO2 ]outlet × 100% 3 × ([C3 H8]inlet [C3 H8]outlet )

[C3 H8]inlet × XC3H8 × V [Co3 +]

(2) (3) (4)

Where [C3H8]inlet and [C3H8]outlet were the concentrations of gaseous C3H8 in the inlet and outlet, respectively; [CO2]outlet was the concentration of gaseous CO2 in the outlet. [Co3+] was the concentration of Co3+ over the catalysts obtained by the results of H2-TPR. In addition, V represented the total flow rate (mL s−1), S was the BET surface area of catalyst (m2/g) and XC3H8 was the C3H8 conversion at different temperatures. 3. Results and discussions 3.1. Textural characteristics The XRD patterns of the CoCeOx catalysts with varied Co/Ce ratios are displayed in Fig. 1. Pure CeO2 and Co1Ce4 catalysts only show a weak cubic fluorite structure (PDF-JCPDS 43-1002). No visible diffraction peaks of Co3O4 can be observed on Co1Ce4 catalyst but slight peak shift, which signifies the high dispersion of CoOx species in CeO2. It is considered that the Co ions can enter the ceria lattice because the radius of the Ce4+ cation with octahedral coordination (r = 1.01 Å) is larger than that of the Co2+ cation (0.79 Å). Based on calculation of the CeO2 lattice parameters, it is found that the lattice parameters of Co1Ce4 (0.5382 nm) are slightly less than that of CeO2 (0.5408 nm), which proves that CoxCe1−xO2−σ solid solution exists. As for the Co1Ce1 catalyst, some small diffraction peaks attributed to cubic phase Co3O4 (PDF-ICDD 42-1407) appears, indicating some pure CoOx species exist except for solid solutions. When the Co/Ce molar ratio reaches to 4, the CeO2 phase disappears with the characteristic peaks of Co3O4 left. These results mean that the replacement of Co by Ce in the framework 63

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Fig. 1. The XRD patterns of CeO2, Co3O4 and CoCeOx catalysts. Table 1 Structural parameters and surface element concentration of the catalysts. Sample

CeO2 Co1Ce4 Co1Ce1 Co4Ce1 Co3O4

SBET (m2/g)

161.9 104.3 130.9 83.3 64.3

St-plot (m2/g)

Dcrys.(nm)

SMicro

SMeso

CeO2

46.2 20.8 10.1 6.9 2.4

115.7 83.5 120.8 76.4 62

8.4 8 7.6 — —

(111)

Vp (cm3/g)

Dp (nm) Co3O4 — — 9.1 12.1 15

(311)

3.1 5.3 7.3 10.5 16.8

0.11 0.08 0.20 0.28 0.02

Conc.xps (at. %)

Ratioxps

Co

Ce

O

Co3+/Co2+

Ce4+/Ce3+

Oads/Olatt

— 4 19.1 28.6 35.8

23.4 21.2 10.5 3.3 —

76.6 74.8 70.4 68.1 64.2

— 1.2 1.8 1.1 1.2

4.6 4.3 2 2.7 —

0.8 0.9 1.2 1.1 0.8

Previous reports proved that the more percentages of Ce3+ were, the greater the likelihood of charge imbalance, oxygen vacancies and unsaturated chemical bonds formed over the catalyst surface. So the Ce3+ is facilitate for the catalytic oxidation reaction [41,42]. As listed in Table 1, the ratio of Ce4+/Ce3+ decrease at first and then increase as the CoOx content increases. The minimum ratio are obtained for the Co1Ce1 catalyst, indicating that more CoOx introduction can effectively reduce the surface Ce4+ to Ce3+ and generate charge imbalance in CeO2 phase. As shown in Fig. 4b, the asymmetrical Co 2p3/2 XPS spectrum of the Co3O4 and CoCeOx catalysts can all be divided into two components at binding energies of 779.9 and 781.4 eV. These two peaks can be assigned to surface Co3+ (the blue area) and Co2+ species (the orange area), respectively [3]. The surface molar ratio of Co3+/Co2+ increased as follows: Co4Ce1 (1.06) < Co1Ce4 (1.16) < Co3O4 (1.24) < Co1Ce1 (1.8). The result clearly suggests that there is presence of strong interaction between Ce and Co on the surface of Co1Ce1. The deconvoluted O1s XPS spectra for the catalysts are given in Fig. 4c. As can be seen, all the five samples show two distinct sub-bands. The band located at around 530 eV can be corresponded to lattice oxygen O2− (labeled as “Olatt’’). Whereas the binding energies at 531.3–531.8 eV are belonging to surface labile oxygen like defect-oxide O− or hydroxyl OH− (designated as “Oads’’) [43,44]. It is obviously shown that the Olatt peak position shifts towards higher wavenumber from CeO2 (529.5 eV) to Co3O4 (530 eV) with the increase of CoOx concentration. It can also be interpreted as an evidence that Co can penetrate into the CeO2 lattice to form a CoxCe1−xO2−σ solid solution. Furthermore, as is well known, the surface chemisorbed labile oxygen (Oads) is more active than lattice oxygen due to its higher mobility, and thus it always plays a key role in catalytic oxidation reactions. The relative concentration ratios of Oads/ Olatt for each catalyst is provided in Table 1, and follow the sequence: Co3O4 /CeO2 (0.79) < Co1Ce4 (0.91) < Co4Ce1 (1.19) < Co1Ce1 (1.29). The above experimental results reveal that the most abundant surface Ce3+, Co3+ and Oads species are exhibited for the Co1Ce1

symmetry around CeO2 lattice [35,36]. A weak broad band around 600 cm−1 is also found, which can be attributed to the Frenkel-type oxygen vacancy (FeOV) from Ce3+ in CeO2 [37]. This type of O-vacancies is originated from partial oxygen anions transforming from original tetrahedral sites to the octahedral sites, leading to the lattice oxygen turning into interstitial oxygen. When the CoOx is gradually doped in CeO2 with the increase of Co/Ce ratio, the F2g vibration and FeOV signals weaken. Meantime several characteristic peaks belonging to Co3O4 starts to appear [17]. For the Co1Ce1 catalyst, three new peaks at 469, 511, 609 and 677 cm−1 occur, which are depicted as Eg, F2g’, F2g’’ and A1g, indicating a spinel structure for Co3O4 [38]. Eg and F2g vibration modes are linked to the octahedral (Co3+–O) and tetrahedral (Co2+–O) sites respectively [14]. Furthermore, A1g vibration (677 cm−1) can be seen as a characteristic of the sub-lattice (octahedral/tetrahedral) in which the highest valence cations are primarily located [38]. As shown in the Fig. 3b, the peak shifts towards low wavenumber direction at 469 and 677 cm−1 corresponding to highlycharged Co ions are observed, while the peak at 511 cm−1 connected with Co2+ is almost unchanged with the increase of temperature from 50 to 350 °C under the atmosphere of 10%O2/Ar. This results show that octahedral-coordination Co3+ in the Co1Ce1 catalyst with lattice distortion are more unstable and active than Co2+ with tetrahedral-coordination at the reaction temperature [39]. 3.2. Surface chemical states analysis and reduction properties In order to investigate the information of surface element concentration and valence over the CeO2, Co3O4 and CoCeOx catalysts, XPS spectra are recorded and shown in Fig. 4. Fig. 4a presents the deconvoluted Ce3d XPS results of the CeO2 and CoCeOx catalysts. As can be seen from figure, the peaks with black dotted line and purple fill can be ascribed to surface Ce3+ atoms corresponding to the 3d104f1 initial electronic configuration. Whereas the bands with blue curves can be attributed to surface Ce4+ species with 3d104f0 electronic state [40]. 64

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Fig. 2. HRTEM (a, b) and HAADF-STEM (c, d) images of Co1Ce1. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

located at around 298 °C corresponds to the reduction of Co3+ to Co2+; while the broad peak at 403 °C with a shoulder at 347 °C represents Co2+ conversion to metallic Co [26,46]. For the CoCeOx catalysts, the peak centers of Co3+ → Co2+ → Co0 and Ce4+ → Ce3+ are all shifted to lower temperature. So both the incorporation of Co into the cubic CeO2 lattice and Ce into the spinel Co3O4 lattice will lead to better redox properties. Moreover, multi-peaks gaussian fitting were used to further investigate the relationship between structure and H2 consumption amount for the CoCeOx catalysts. Combined with the results of XRD, the

catalyst. It can be seen as an important qualification for providing abundant oxygen deficiency and remarkable reducibility. H2-TPR was used to further investigate the reducibility of CeO2, Co3O4 and CoCeOx catalysts, and the results are presented in Fig. 5. In the TPR curve of CeO2, three reduction peaks are detected at 312, 419 and 708 °C. The first two peaks can be attributed to reduction of the surface CeO2 to Ce2O3, while the 708 °C peak is due to the reduction process of CeO2 in the bulk of catalyst [45]. On the other hand, two reduction peaks could be observed in the Co3O4 catalyst. The one

Fig. 3. Dehydration Raman spectra of CeO2, Co3O4 and CoCeOx (a) and in situ Raman spectra of Co1Ce1 at 50–350 °C under 10%O2/Ar with dark green vertical lines as reference of peak shifts (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 65

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Fig. 4. Ce3d (a), Co2p (b) and O1 s (c) XPS spectra of CeO2, Co3O4 and CoCeOx catalysts.

Fig. 6. O2-TPD curves of the CeO2, Co3O4 and CoCeOx catalysts.

lattice oxygen from bulk to the surface, which results in an improvement of the catalytic performance [17]. Herein, O2-TPD was carried out with the profiles shown in Fig. 6. Generally speaking, physically adsorbed oxygen species should be completely excluded after the pretreatment stage. Consequently, the oxygen species of the metal oxide catalysts are desorbed from the easiest to the hardest following the sequence: oxygen molecule anion (O2−) > oxygen anion (O−) > lattice oxygen (O2−). Additionally, as bulk lattice oxygen from Co3O4 and CeO2 is hardly extracted at below 450 °C, the peaks below 400 °C is ascribed to the desorption of O2− and O−. These are usually considered as active oxygen species involved in oxidation reaction [7]. As shown in the figure, pure Co3O4 catalyst shows clearly two types of active oxygen. The low temperature peak at 108 °C can be attributed to O2−, while the peak centered at 289 °C with a shoulder at 350 °C is assigned to O-. Unlike the Co3O4 catalyst, the low temperature peak intensity of the CeO2 catalyst is obviously larger than the high temperature peak. This suggests that O2− species are more easily formed than O− on the CeO2 surface. For the CoCeOx catalysts, as the value of Ce/Co goes up, the number of O2− species increases at first and then decreases; meanwhile a reverse trend is shown for O− species. As listed in Table S1, the Co1Ce1 catalyst has the maximum number of O2− species with 0.21 mmol/g but few O− species in the selected scope. So the original O− active oxygen provided by CoOx should be transformed into O2− for the CoCeOx catalysts. This phenomenon probably

Fig. 5. H2-TPR curves of the CeO2, Co3O4 and CoCeOx catalysts.

reduction ways of surface CeO2 can be divided into amorphous and cubic fluorite structure. According to the integral areas of surface CeO2 species listed in Table S1, the H2 consumption by two types of surface Ce4+ reaches to the maximum for the Ce1Co1 catalyst. At the same time, the amount of H2 reduction attributed to bulk CeO2 almost disappears. Therefore, it is believed that the reducibility is greatly promoted by the interaction of CoOx and CeO2 in the form of CoxCe1−xO2−σ solid solution for Co1Ce1. In addition, since the reduction peak of Co3+ to Co2+ also moves towards lower temperature, the connection of Co3+–O–Ce4+ in the solid solution may also reduce the redox potential of Co species. It can also act as a bridge for O transfer between Co and Ce, which makes for the effective redox cycle during oxidation reaction [6]. It is believed that oxygen vacancies can facilitate the diffusion of 66

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Fig. 7. Propane conversion (a) and product selectivity as a function of temperature over CeO2 (b), Co1Ce4 (c) Co1Ce1 (d), Co4Ce1 (e) and Co3O4 (f). Reaction conditions: 0.1 g catalyst, 6000 ppm propane, 20% O2, balance He, total flow rate = 100 mL/min, and GHSV = 60,000 mL/(g h).

attributes to the better oxygen transferability in CoxCe1−xO2−σ solid solution, because its lattice defect and oxygen vacancy are easily generated. The O2− species with lower desorption temperatures and stronger intensities in the Co1Ce1 catalyst is beneficial for oxygen adsorbed and activated on the catalyst surface, thus presenting better catalytic ability.

rC3H 8 = Aexp

Ea PC3H8 PO2 RT

(5)

The following equation can be obtained after taking the logarithm of Eq. (5).

lnr = lnA + lnPC3H8 + lnPO2

3.3. Catalytic activity, kinetics and stability

Ea RT

(6)

Accordingly, the obtained values of α and β represent constant reaction orders of propane and oxygen, respectively. The calculated orders, α = 0.06, 0.24, 0.38 and 0.69; while β = 0.34, 0.28, 0.28, 0.37 for 200, 210, 220 and 230 °C, respectively. Consequently, it is presented that the logarithmic reaction rates gradually increase with an increase in Pprop. At the same time, the order of α accelerates significantly with the raising temperature. On the other hand, an increase in logarithmic reaction rate is also observed with an increase in PO2, but the order of β only shows a little changed, suggesting that it may not be affected by temperature. Xu et al. held that the positive reaction orders with respect to reactants indicated catalytic combustion proceeded primarily through the Langmuir–Hinshelwood mechanism through the interaction between the adsorbates on the surface [4,55]. However, several authors believed that alkane oxidation on metal oxides took place via a Mars-Van-Krevelen mechanism involved the participation of a lattice oxygen by a redox cycle [53,56]. Nevertheless, there seems no correlation between the surface content of the lattice oxygen (Olatt) and the reaction rate of the CoCeOx catalysts can be found in our study. This result indicates that the fast migration of oxygen ions through the lattice of cobalt oxide may not be prominent in the Co-Ce binary system. Therefore, it is recommended that the total oxidation of propane over the CoCeOx catalysts may follow the Langmuir-Hinshelwood mechanism. Fig. S4 gives the Arrhenius plots over the CeO2, Co3O4 and CoCeOx catalysts for propane combustion reaction. A linear relationship between logarithmic reaction rates and the reciprocal of temperature was obtained for all the catalysts. According to the slopes of the Arrhenius plots, the apparent activation energies (Ea) are calculated and summarized in Table S1. The results manifests that the Ea of the CoCeOx catalysts follows the order: Co1Ce1 (65.4 kJ/mol) < Co1Ce4 (84.5 kJ/ mol) < Co4Ce1 (102.8 kJ/mol). The lowest Ea value means that the propane oxidation is more easier to take place for Co1Ce1, which is in accord with its highest catalytic efficiency among the CoCeOx catalysts. Lifetime experiment have been carried out over Co1Ce1 to

The catalytic activities for propane total oxidation over the CeO2, Co3O4 and CoCeOx catalysts were performed in a fixed-bed flow reaction and presented in Fig. 7a. The reaction rate, T10, T50 and T90 and TOF (225 °C) are listed in Table S1. Blank experiments showed no significant activity even at 350 °C, demonstrating that no homogeneous reactions happened under the adopted temperature window. It can be seen that the complete conversion of propane over the Co1Ce1, Co4Ce1 and Co3O4 catalysts can be achieved at above 275 °C under the GHSV of 60,000 mL/(g h). The TOF for Co-containing catalysts decreases as follows: Co1Ce1 (5.84 × 10−4 s−1) > Co1Ce4 (2.42 × 10−4 s−1) > Co4Ce1 (7.1 × 10−5 s−1) > Co3O4 (2.6 × 10−5 s−1). What is more, the light-off temperature (T10) of the Co1Ce1 catalyst is lower than 200 °C, much better than conventional Pt based catalysts [47]. This phenomenon further confirms that the synthetic route in this study is an effective method to prepare the high active binary CoCex catalyst. The product selectivities over five catalysts are presented in Fig. 7. It is found that pure Co3O4 and CeO2 catalysts show three major byproducts, i.e. CO, CO2 and propane. While CO cannot be seen at the whole temperature range for CoCeOx catalysts, indicating that propane incomplete oxidation into CO has been inhibited. This result is probably due to the abundant surface active oxygen. Furthermore, unlike Co1Ce4 and Co4Ce1, the propane has been entirely oxidized to carbon dioxides at the lower temperature of 225 °C for the Co1Ce1 catalyst. Finally, the catalytic performance of the Co1Ce1 catalyst is compared with that of the catalysts reported in the references. As listed in Table 2, the catalytic activity of the Co1Ce1 catalyst is fairly efficient among that of the similar catalysts reported for total oxidation of propane. Fig. S3 shows the dependence of the propane oxidation rate on the partial pressure of propane (Pprop) ranging from 0.1–0.6 kPa and oxygen (PO2) from 5 to 40 kPa at 200, 210, 220 and 230 °C. Since a dependence of the reaction rate on the products of CO2 and H2O can be ignored at low propane conversion, the kinetic expression of propane oxidation is described as below: 67

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Table 2 Comparison of catalytic performance for propane combustion over various catalysts. Sample

Reaction conditions (ratio of O2/C3H8)

GHSV (mL g−1 h−1)

Selectivity to CO2a (%)

T50a

Reference

Co1Ce1 0.99%Pd/CeO2 NiCeOx-4 NiCe Co3O4 Co3O4-AC nano-RuOx/TiO2 Ordered Co3O4 Au/Co3O4 Co3O4/SiO2 Co3O4/ZSM-5 MnxCo3-xO4

33.3 5 10 26.3 40 33.3 21 26.3 210 80 6.7 33.3

60,000 60,000 30,000 42,800 30,000 240,000 60,000 12,000 40,000 30,000 30,000 12,000

100 — > 99 94.3 — — 100 100 — — — —

217 268 ～280 255 230 235 260 205 255 280 235 310

This study [19] [22] [48] [34] [13] [49] [50] [51] [52] [53] [54]

a

At the propane conversion of 50%.

investigate the catalytic stability of propane combustion within 48 h of on stream reaction at 220 and 250 °C. As shown in Fig. S5a, there is hardly significant activity loss during the 48-h long-periodic experiment at 220 and 250 °C. Because water is one of the main components in the VOCs exhaust emission, the catalytic efficiency in the presence of H2O should be considered. As shown in Fig. S5b, when 5% H2O was introduced into the reactor at 250 °C, the catalytic activity for the Co1Ce1 catalyst decreased slightly but remained higher than 83% with a GHSV of 60,000 mL/(g h) during the 24-h activity test. When the water was shut down, the conversion of propane had almost recovered to original value. All the results above suggest that the Co1Ce1 catalyst has excellent catalytic stability, water tolerance and promising application prospect.

20%O2/N2 after pretreated by a flow of C3H8 for 1 h and N2 purging for 20 min at 200 °C. As illustrated in the figure, when catalyst is treated with C3H8, adsorbed methyl functional groups with the corresponding peaks at 2969 (νas (CeH)), 1470 (δas (CH3)) and 1367 cm−1 (δs (CH3)) form on CWT surface [60]. Almost no O-containing group can be seen. If the CoCeOx catalyst mainly follows the Mars-van Krevelen mechanism, there should be enough lattice oxygen of Co3O4 to oxidize the immediate species. While for LH mechanism, it is reasonable that limited oxygen vacancies cannot be fully filled by gas phase oxygen thereby inhibiting the activity. After switching the gas to O2/N2, these peaks decrease gradually with several new bands attributed to νas (OeH) at 3730 cm−1, νs (C]O) at 1705 cm−1 and νas (COO) at 1581 cm−1 appearance. These results indicate that H2O and the reaction intermediate of carbonate occurs on the catalyst surface. Additionally, the adsorption band at around 2340 cm−1 assigned to gaseous CO2 emerges, suggesting the total oxidation of propane happens. Because the surface carbonate occurrence is independent on the O2 in the reaction gas at lower temperature, carbonates are speculated to be produced from the reaction between adsorbed propane and surface active oxygen. So it is believed that the propane oxidation probably follows the Langmuir-Hinshelwood mechanism where surface active oxygen acts as one of the major reaction active sites over the Co1Ce1 catalyst.

3.4. Operando/in situ DRIFTS In situ DRIFTS experiments were also employed to explore the C3H8 adsorption and reaction behavior on the Co1Ce1 catalyst. The C3H8 adsorption on the catalyst at 50–250 °C under the N2 atmosphere is shown in Fig. 8a. The bands at 2969 cm−1 can be assigned to the νas (CeH) mode of propane adsorbed on the catalyst [57]. Whereas the band at 1461 and 1354 cm−1 can be attributed to δas (CH3) and δs (CH3), respectively [58]. When the temperature is below 100 °C, some uncoordinated CO32− at around 1420 cm−1 can be detected, hinting that the adsorbed propane is oxidized by surface oxygen species on the catalyst. As the temperature increases to 250 °C, these bands associated with hydrocarbon gradually weakens and two bands at 1650 and 1461 cm−1 belonging to v (C]C) and δas (CH3) appear [59]. Therefore, the oxidative dehydrogenation of propane to form alkene is easier to takes place than total oxidation with the catalyst’s oxygen under the anaerobic atmosphere. Fig. 8b shows the DRIFTS spectra of the Co1Ce1 catalyst in a flow of

3.5. Analysis of the structure-activity relationship Among the three kinds of the CoCeOx catalyst, the Co1Ce1 catalyst shows the best catalytic activity, CO2 selectivity and excellent stability for propane combustion. It is known that the crystalline phases and the surface redox characteristics are two critical factors for high activity, selectivity and stability [61]. Compared with the results of XRD and Raman, peaks shifts of the CoCeOx catalyst are found for both the Co-

Fig. 8. In situ DRIFTS of C3H8 adsorption at 50–250 °C (a) and catalytic propane oxidation reactions with pre-adsorbed C3H8 at 200 °C (b) over the Co1Ce1 catalyst. 68

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Fig. 9. The surface element contents of the CoCeOx catalysts as a function of the surface reaction rate (a) and plausible reaction model (b).

rich and Ce-rich conditions, which suggests some Co and Ce ions can enter individually into the cubic CeO2 and Co3O4 spinel lattice. The substitution of a Ce4+ ion by Co in the CeO2 network or a Co3+ ion by Ce in the Co3O4 network can lead to a deficient electron and the formation of CoxCe1−xO2−σ solid solution, which is beneficial to the oxygen vacancy generation. Moreover, the smallest crystallite size and the largest mesoporous surface area of Co1Ce1 catalyst also make for its highest catalytic performance. The existence of a synergistic effect of Co3O4 and CeO2 in the CoCeOx catalysts not only reflects in bulk structure but also in surface chemical states. As is well known, surface Ce3+ ions may generate charge imbalance and unsaturated chemical bonds over the catalyst, which can cause the formation of oxygen vacancies and surface labile oxygen. The XPS results confirm that the Co1Ce1 catalyst possesses more active oxygen species to participate in the reaction of propane total oxidation compared with the other catalysts. This conclusion is also supported by the results of H2-TPR and O2-TPD. Moreover, in order to further identify the active sites of the CoCeOx catalysts for propane combustion, the correlation between surface element contents and surface reaction rate is investigated in this study. As shown in Fig. 9a, there is no obvious correlation between reaction rate and the concentration of single Co3+, Ce4+ and Oads. But a good line correlation can be established between the reaction rate and sum of the surface Co3+ and Ce3+ or Oads concentrations. As explained before, the Ce3+ concentration reflects the oxygen vacancy sites to a certain degree, which can be occupied by gaseous oxygen adsorption to form the active oxygen species (Oads). For this reason, surface Co3+ and Oads species can be regarded as the two major active sites for propane combustion. Similarly, synergistic effect among multiple active sites (Co3+-O2c) is also found and deeply studied in catalytic combustion of methane over Co3O4 in Hu’s research [62]. Hence, a great deal of Co doping in CeO2 can form the solid solution and enhance the Co3+ amount in CeO2 cubic fluorite phase. Meantime a plenty of oxygen vacancies can simultaneously be obtained. This structure change is responsible for the activity improvement from Co1Ce4 to Co1Ce1. However, since oxygen vacancies are primarily originated from the structure characteristics of CeO2, the quantity of these vacancies is insufficient after superfluous Co introduction like the Co4Ce1 catalyst. Consequently, it can be a reasonable explanation for the better performance of the Co1Ce1 catalyst than Co1Ce4 and Co4Ce1. The experiment of in situ DRIFTS spectra reveals that the Co1Ce1 catalyst possesses a superior ability to break the CeH bond of propane and generate CO2 under the oxygen-containing condition. While under the N2 atmosphere, some C]C bonds are formed for partial oxidation of propane at below 100 °C. Therefore, gaseous oxygen species and appropriate temperature are indispensable for total oxidation of propane. In view of the results of DRIFTS and kinetics, it is considered that propane combustion on the CoCeOx catalysts probably follows the Langmuir–Hinshelwood mechanism. A proposed four-stage cycle

reaction model is provided as shown in Fig. 9b. The first stage of the model is propane adsorption on the CoOx surface and oxygen vacancy being filled by gas phase oxygen; the second is reactive intermediate species generation with the reduction of Co3+ to Co2+; the third is the oxidation of intermediate species to carbon dioxide and water; the last is the oxidation of Co2+ to Co3+ by surface active oxygen. Finally, the left oxygen vacancies are participate into the next reaction cycle. Compared with other CoCeOx catalysts, the best reducibility and largest amount of surface active oxygen species of Co1Ce1 are responsible for its optimum catalytic performance for propane combustion. 4. Conclusions A novel micro/mesoporous hybrid CoCeOx catalysts were prepared using a double template combining sol-gel method and showed a good performance for total oxidation of propane. The optimal molar ratio of Co/Ce is 1, and the activity of propane combustion follows the sequence: Co1Ce1 > Co4Ce1 > Co3O4 > Co1Ce4 > CeO2. Besides, the Co1Ce1 catalyst presents good CO2 selectivity, reaction stability and water resistance for total oxidation of propane. The excellent performance of Co1Ce1 is in connection with the existence of interaction between Co and Ce and the formation of CoxCe1−xO2−σ solid solution. This change leads to a large surface area, small grain size, high concentration of oxygen vacancies and improved reducibility. Furthermore, a certain correlation has been established between the surface reaction rate and the sum of the surface Co3+ and Oads concentrations. Hence, the surface Co3+ ions and active oxygen species are considered as the major active sites of the CoCeOx catalysts. Based on the in situ DRIFTS and kinetics analysis, it is suggested that total oxidation of propane over the CoCeOx catalysts follows a four stage LH route. Additionally, the synthetic method in this study may open up new possibilities in the preparation and application of multicomponent hierarchical pore materials. Competing financial interest The authors declare no competing financial interest. Acknowledgements This work was financially supported by the National Key R&D Program of China (2016YFC0209203) and the Natural Science Foundation of Beijing (8182033). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2018.12.026. 69

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References

[31] K.S.W. Sing, Pure Appl. Chem. 57 (1985) 603–619. [32] X. Zhang, J. Wang, B. Tan, Z. Li, Y. Cui, G. He, Catal. Commun. 98 (2017) 5–8. [33] Y. Wang, X. Hu, K. Zheng, H. Zhang, Y. Zhao, React. Kinet. Mech. Cat. 123 (2018) 707–721. [34] R.P. Marin, S.A. Kondrat, R.K. Pinnell, T.E. Davies, S. Golunski, J.K. Bartley, G.J. Hutchings, S.H. Taylor, Appl. Catal. B: Environ. 140–141 (2013) 671–679. [35] X. Yao, Y. Xiong, W. Zou, L. Zhang, S. Wu, X. Dong, F. Gao, Y. Deng, C. Tang, Z. Chen, L. Dong, Y. Chen, Appl. Catal. B: Environ. 144 (2014) 152–165. [36] B. Beck, M. Harth, N.G. Hamilton, C. Carrero, J.J. Uhlrich, A. Trunschke, S. Shaikhutdinov, H. Schubert, H.-J. Freund, R. Schloegl, J. Sauer, R. Schomaecker, J. Catal. 296 (2012) 120–131. [37] X. Lin, S. Li, H. He, Z. Wu, J. Wu, L. Chen, D. Ye, M. Fu, Appl. Catal. B: Environ. 223 (2018) 91–102. [38] I. Lopes, N. El Hassan, H. Guerba, G. Wallez, A. Davidson, Chem. Mater. 18 (2006) 5826–5828. [39] H. Liang, L. Zhengcao, Z. Zhengjun, Nanotechnology 19 (2008) 155606. [40] S. Zhang, Q. Zhong, Y. Shen, L. Zhu, J. Ding, J. Colloid Interfae Sci. 448 (2015) 417–426. [41] T. Boningari, P.R. Ettireddy, A. Somogyvari, Y. Liu, A. Vorontsov, C.A. McDonald, P.G. Smirniotis, J. Catal. 325 (2015) 145–155. [42] T. Zhang, R. Qu, W. Su, J. Li, Appl. Catal. B: Environ. 176–177 (2015) 338–346. [43] T. Gu, Y. Liu, X. Weng, H. Wang, Z. Wu, Catal. Commun. 12 (2010) 310–313. [44] Z. Wu, Z. Sheng, Y. Liu, H. Wang, J. Mo, J. Hazard. Mater. 185 (2011) 1053–1058. [45] K. Krishna, A. Bueno-López, M. Makkee, J.A. Moulijn, Appl. Catal. B: Environ. 75 (2007) 189–200. [46] Z. Fei, S. He, L. Li, W. Ji, C.-T. Au, Chem. Commun. 48 (2012) 853–855. [47] M.S. Avila, C.I. Vignatti, C.R. Apesteguia, T.F. Garetto, Chem. Eng. J. 241 (2014) 52–59. [48] B. Solsona, T. Garcia, E. Aylón, A.M. Dejoz, I. Vázquez, S. Agouram, T.E. Davies, S.H. Taylor, Chem. Eng. J. 175 (2011) 271–278. [49] D.P. Debecker, B. Farin, E.M. Gaigneaux, C. Sanchez, C. Sassoye, Appl. Catal. A: Gen. 481 (2014) 11–18. [50] T. Garcia, S. Agouram, J.F. Sánchez-Royo, R. Murillo, A.M. Mastral, A. Aranda, I. Vázquez, A. Dejoz, B. Solsona, Appl. Catal. A: Gen. 386 (2010) 16–27. [51] B. Solsona, M. Pérez-Cabero, I. Vázquez, A. Dejoz, T. García, J. Álvarez-Rodríguez, J. El-Haskouri, D. Beltrán, P. Amorós, Chem. Eng. J. 187 (2012) 391–400. [52] M.S.L. Aparicio, I.D. Lick, React. Kinet. Mech. Cat. 119 (2016) 469–479. [53] Z. Zhu, G. Lu, Z. Zhang, Y. Guo, Y. Guo, Y. Wang, ACS Catal. 3 (2013) 1154–1164. [54] W. Tang, Z. Ren, X. Lu, S. Wang, Y. Guo, S. Hoang, S. Du, P.-X. Gao, ChemCatChem 9 (2017) 4112–4119. [55] J. Xu, Y.-Q. Deng, X.-M. Zhang, Y. Luo, W. Mao, X.-J. Yang, L. Ouyang, P. Tian, Y.F. Han, ACS Catal. 4 (2014) 4106–4115. [56] B. Solsona, T. García, R. Sanchis, M.D. Soriano, M. Moreno, E. Rodríguez-Castellón, S. Agouram, A. Dejoz, J.M. López Nieto, Chem. Eng. J. 290 (2016) 273–281. [57] M.A. Hasan, M.I. Zaki, L. Pasupulety, J. Phys. Chem. B 106 (2002) 12747–12756. [58] T.E. Hoost, K.A. Laframboise, K. Otto, Appl. Catal. B: Environ. 7 (1995) 79–93. [59] S. Swislocki, K. Stoewe, W.F. Maier, J. Catal. 316 (2014) 219–230. [60] D.R. Sellick, A. Aranda, T. García, J.M. López, B. Solsona, A.M. Mastral, D.J. Morgan, A.F. Carley, S.H. Taylor, Appl. Catal. B: Environ. 132–133 (2013) 98–106. [61] B. Puertolas, A. Smith, I. Vazquez, A. Dejoz, A. Moragues, T. Garcia, B. Solsona, Chem. Eng. J. 229 (2013) 547–558. [62] W. Hu, J. Lan, Y. Guo, X.-M. Cao, P. Hu, ACS Catal. 6 (2016) 5508–5519.

[1] I. Riipinen, T. Yli-Juuti, J.R. Pierce, T. Petaja, D.R. Worsnop, M. Kulmala, N.M. Donahue, Nat. Geosci. 5 (2012) 453–458. [2] A. Guenther, C.N. Hewitt, D. Erickson, R. Fall, C. Geron, T. Graedel, P. Harley, L. Klinger, M. Lerdau, W.A. McKay, T. Pierce, B. Scholes, R. Steinbrecher, R. Tallamraju, J. Taylor, P. Zimmerman, J. Geophys. Res. Atmos. 100 (1995) 8873–8892. [3] S. Zhao, F. Hu, J. Li, ACS Catal. 6 (2016) 3433–3441. [4] J. Xu, L. Ouyang, W. Mao, X.-J. Yang, X.-C. Xu, J.-J. Su, T.-Z. Zhuang, H. Li, Y.F. Han, ACS Catal. 2 (2012) 261–269. [5] B. Bai, J. Li, J. Hao, Appl. Catal. B: Environ. 164 (2015) 241–250. [6] C. He, Y. Yu, L. Yue, N. Qiao, J. Li, Q. Shen, W. Yu, J. Chen, Z. Hao, Appl. Catal. B: Environ. 147 (2014) 156–166. [7] B. Bai, H. Arandiyan, J. Li, Appl. Catal. B: Environ. 142 (2013) 677–683. [8] B. Bai, J. Li, ACS Catal. 4 (2014) 2753–2762. [9] Z. Ren, Z. Wu, W. Song, W. Xiao, Y. Guo, J. Ding, S.L. Suib, P.-X. Gao, Appl. Catal. B: Environ. 180 (2016) 150–160. [10] Y. Lou, J. Ma, X. Cao, L. Wang, Q. Dai, Z. Zhao, Y. Cai, W. Zhan, Y. Guo, P. Hu, G. Lu, Y. Guo, ACS Catal. 4 (2014) 4143–4152. [11] X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Nature 458 (2009) 746. [12] B. Solsona, T.E. Davies, T. Garcia, I. Vazquez, A. Dejoz, S.H. Taylor, Appl. Catal. B: Environ. 84 (2008) 176–184. [13] W. Tang, W. Xiao, S. Wang, Z. Ren, J. Ding, P.-X. Gao, Appl. Catal. B: Environ. 226 (2018) 585–595. [14] Q. Liu, L.-C. Wang, M. Chen, Y. Cao, H.-Y. He, K.-N. Fan, J. Catal. 263 (2009) 104–113. [15] B. Faure, P. Alphonse, Appl. Catal. B: Environ. 180 (2016) 715–725. [16] J. Chen, C. Li, S. Li, P. Lu, L. Gao, X. Du, Y. Yi, Chem. Eng. J. 338 (2018) 358–368. [17] C. Wang, C. Zhang, W. Hua, Y. Guo, G. Lu, S. Gil, A. Giroir-Fendler, Chem. Eng. J. 315 (2017) 392–402. [18] F. Hu, Y. Peng, J. Chen, S. Liu, H. Song, J. Li, Appl. Catal. B: Environ. 240 (2019) 329–336. [19] Z. Hu, X. Liu, D. Meng, Y. Guo, Y. Guo, G. Lu, ACS Catal. 6 (2016) 2265–2279. [20] Y. Lou, X.M. Cao, J. Lan, L. Wang, Q. Dai, Y. Guo, J. Ma, Z. Zhao, Y. Guo, P. Hu, G. Lu, Chem. Commun. 50 (2014) 6835–6838. [21] Y. Liu, L. Luo, Y. Gao, W. Huang, Appl. Catal. B: Environ. 197 (2016) 214–221. [22] Z. Hu, S. Qiu, Y. You, Y. Guo, Y.L. Guo, L. Wang, W.C. Zhan, G.Z. Lu, Appl. Catal. B: Environ. 225 (2018) 110–120. [23] M. Fattahi, M. Kazemeini, F. Khorasheh, A. Rashidi, Chem. Eng. J. 250 (2014) 14–24. [24] A.S. Poyraz, C.H. Kuo, S. Biswas, C.K. King’ondu, S.L. Suib, Nat. Commun. 4 (2013). [25] C.-H. Kuo, I.M. Mosa, A.S. Poyraz, S. Biswas, A.M. El-Sawy, W. Song, Z. Luo, S.Y. Chen, J.F. Rusling, J. He, S.L. Suib, ACS Catal. 5 (2015) 1693–1699. [26] W. Song, A.S. Poyraz, Y. Meng, Z. Ren, S.-Y. Chen, S.L. Suib, Chem. Mater. 26 (2014) 4629–4639. [27] S. Biswas, B. Dutta, K. Mullick, C.-H. Kuo, A.S. Poyraz, S.L. Suib, ACS Catal. 5 (2015) 4394–4403. [28] L. Meng, X. Zhu, W. Wannapakdee, R. Pestman, M.G. Goesten, L. Gao, A.J.F. van Hoof, E.J.M. Hensen, J. Catal. 361 (2018) 135–142. [29] A. Roucher, M. Emo, F. Vibert, M.-J. Stebe, V. Schmitt, F. Jonas, R. Backov, J.L. Blin, J. Colloid Interface Sci. 533 (2019) 385–400. [30] X. Wu, H. Zhang, K. Huang, Y. Zeng, Z. Zhu, Bioelectrochemistry. 120 (2018) 150–156.

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