Structure dependence and reaction mechanism of CO oxidation: A model study on macroporous CeO2 and CeO2-ZrO2 catalysts

Journal of Catalysis 344 (2016) 365–377

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Structure dependence and reaction mechanism of CO oxidation: A model study on macroporous CeO2 and CeO2-ZrO2 catalysts Yane Zheng a,b, Kongzhai Li a,b,⇑, Hua Wang a, Yuhao Wang a,b, Dong Tian a,b, Yonggang Wei a,b, Xing Zhu a,b, Chunhua Zeng a, Yongming Luo c a b c

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China Faculty of Environmental Engineering, Kunming University of Science and Technology, Kunming 650093, China

a r t i c l e

i n f o

Article history: Received 1 July 2016 Revised 24 September 2016 Accepted 12 October 2016

Keywords: 3DOM CeO2-ZrO2 CO oxidation Oxygen vacancy In situ DRIFTs Reaction mechanism

a b s t r a c t Three-dimensionally ordered macroporous (3DOM) CeO2 and CeO2-ZrO2 oxides with different particle sizes, oxygen vacancy concentrations, oxygen mobility and preferentially exposed surface planes were synthesized, which, as model catalysts, give a new approach for understanding the structural dependence and reaction mechanism of CO oxidation over CeO2-based catalysts. The prepared 3DOM CeO2 and CeO2ZrO2 catalysts exhibited much higher catalytic activity for CO oxidation than the nonporous samples. Although the textural and reducible features of catalysts could affect the catalytic performance, the preferentially exposed surface plane is crucial for determining the catalytic activity. The {1 1 0} plane of CeO2 could create more active sites for CO adsorption, resulting in relatively high activity for CO oxidation. Higher concentration of oxygen vacancy would also enhance the reactivity for CO oxidation. LangmuirHinshelwood mechanism should be the crucial reaction pathway for CO oxidation over the 3DOM CeO2-based catalysts, although the Mars-van Krevelen mechanism cannot be ignored. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction Catalytic oxidation of carbon monoxide (CO), one of the most important prototype reactions in heterogeneous catalysis, has attracted considerable attention due to its extensive application in environmental and energy areas (such as stationary and vehicle exhaust control, air purification and CO removal in H2-rich feed) [1]. Among various heterogeneous catalysts involved, supported metals and reducible oxides show prominent properties. Currently, noble metals, especially Au and platinum group metals, with the presence of supports present high activity for CO oxidation at low temperature [2]. Metallic oxides used as supports can be classified as inert or active according to their redox properties. Thus, Al2O3 and SiO2 fit within the inert supports [3,4], while reducible transition metal oxides such as, MnO2, Fe2O3, Co3O4 and CeO2 have to be considered as active supports [5–8]. Liu et al. [6] prepared Ptgroup metals on both the inert supports and reducible supports, such as Al2O3- or Fe2O3-supported Pt or Pd. It was found that Pt or Pd on Fe2O3 can facilitate Fe3+ reduction at relatively low tem⇑ Corresponding author at: State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China. E-mail addresses: [email protected], [email protected] (K. Li). http://dx.doi.org/10.1016/j.jcat.2016.10.008 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

peratures, which leads to the production of large numbers of oxygen vacancies. The partly reduced FeOx is involved in the CO oxidation, acting as an oxygen donor. On the other hand, the interface sites between Al2O3 and Pt/Pd are insignificant. Ceria-based supports as reducible oxides owned higher redox capacity, and the high dispersion of Pt on ceria-based oxides plays an important role for its superior catalytic activity [9]. The interaction between Pt and ceria-based oxides can inhibit the Pt growth and sintering at high temperature, which is absent on the Pt/Al2O3 or Pt/ZrO2 catalyst. For the Au on inert supports system, such as Au/Al2O3 [3], the noble metals are the only active sites, which determined the activity for CO oxidation. As for the Au-reducible supports system, the support plays an important role in the catalytic process, particularly for CeO2 and other transition metal oxides [10–14]. CeO2 was proposed to enhance the dispersion and stability of metal components which behave as active sites [12–14]. More importantly, since CO oxidation usually takes place at the metalsupport interface, the ‘‘active” support CeO2 is also involved in the reaction owing to the abundant active oxygen species present on the surface [15]. Kim et al. [8] reported that, for Au/CeO2 catalysts, the chemical modification on the CeO2 support is promising for the optimization of oxidation catalysis.

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As for the independent reducible oxide catalysts, Venkataswamy et al. [5] suggested that the mixed reducible CeO2-MnOx oxides showed remarkable enhancement on the activity for CO oxidation at lower temperatures, which can be attributed to a highly dispersed state of Mn2+/Mn3+ in the ceria matrix, facile redox behaviour, and a synergistic Mn-Ce interaction. Luo et al. [7] found that in Co3O4-CeO2 catalyst, Co3O4 crystallites are considered to be encapsulated by nanosized CeO2, with only a small fraction of Co ions exposing on the surface and strongly interacting with CeO2. They proposed that the CO oxidation over Co3O4-CeO2 should take place preferentially at the interface of Co3O4-CeO2 instead of the surface of Co3O4. Experimental results also reveal that CeO2 and CeO2-ZrO2 oxides show prominent catalytic activity for CO oxidation [16–18]. CeO2-ZrO2 mixed oxides with better oxygen storage capacity have been considered as an outstanding material in three-way catalysts (TWCs), which aim at the simultaneous purification of CO, HC and NOx in automotive exhaust. The introduction of Zr could retard the sintering of oxide particles and improve the thermal stability, which are required for highly active TWCs. Currently, the interaction between CO and CeO2 during the reaction has emerged as one of the most important issues to understand the catalytic mechanism. The reaction models include rapid adsorption of CO molecules on low-coordination surface noble metal sites and its subsequent oxidation at the perimeter of metal nano-particles where oxygen is activated [19–21]. For the independent ceria based catalysts, CO oxidation is believed to proceed via Mars-van Krevelen mechanism over ceria [22–24]. It involves a complex series of redox reactions: the removal of surface oxygen by CO with formation of oxygen vacancies and consequent annihilation of vacancies by gas-phase oxygen. Since the oxidation of reduced cerium oxides is very fast, the reduction process, i.e., removal of surface oxygen, is the rate determination step. Moreover, it is also found that CO interaction with ceria is structure dependent and oxygen vacancy formation energy is sensitive to the exposing crystal planes of CeO2 nano particles [24–27]. The activity of CeO2 can be improved through predominantly exposing their {1 0 0} and {1 1 0} planes [24,25,27]. Further investigation suggests that the interaction of CeO2 with CO also involves the formation of carbonate-like complexes on the {1 0 0} and {1 1 0} surfaces [28]. The carbonate species is competitive to CO2 formation and desorption, although CO could directly react with the surface active oxygen on CeO2 [29]. This is similar with the characteristics of CO oxidation over noble metal catalysts. Despite the intensive studies of CO oxidation on ceria based catalysts, there is still a lack of clear recognition as to the relationship among different factors (i.e., CO adsorption, formation/concentration of oxygen vacancies, oxygen mobility and the crystal planes of CeO2 exposed) in the catalytic process. Strategy for investigating the catalytic mechanism usually involves preparing a series of catalysts with variable characteristics (e.g., particle size, morphology, externally-exposed facets, or defect concentration) and observing their catalytic behaviours using in situ instruments. Three-dimensionally ordered macroporous (3DOM) materials, which possess periodic features with uniform large pore size (>50 nm), are proposed to benefit the catalytic activity resulting from the more abundant active sites on the accessibly macroporous wall surface [30]. The effect of catalyst features on the catalytic behaviour may be magnified over the 3DOM catalysts, which would make the processes able to respond to nuances in its physicochemical properties. It is therefore reasonable to believe that the 3DOM CeO2-based catalysts would be ideal candidates to thoroughly investigate the relationship between their textural/reducible properties and catalytic activity. In the present work, two series of 3DOM CeO2 and CeO2-ZrO2 solid solutions were prepared, which present different characteristics in CeO2 particle size, oxygen vacancy concentration, reducibil-

ity, oxygen mobility and preferential exposed crystal planes of CeO2. Investigations on the physicochemical properties of these catalysts were performed, which were associated with their catalytic activity for CO oxidation. Particularly, the nature of the interaction of CO with 3DOM catalysts using in situ infrared technology was also studied. Based on the experimental data, the possible reaction mechanisms are discussed in detail. The results showed that the crystal plane of CeO2 preferentially exposed and oxygen vacancy concentration are the critical factors for catalytic CO oxidation over 3DOM CeO2-based oxide catalysts. These results will bring new insights into the determinant factors and reaction mechanisms.

2. Experimental 2.1. Catalyst preparation The 3DOM CeO2 and CeO2-ZrO2 solid solutions were fabricated via PMMA-templating route. The precursor solutions of catalysts were obtained by mixing stoichiometric amounts of Ce(NO3)3
6H2O and ZrOCl2
8H2O with deionized water at room temperature (RT). The atomic ratio of Ce:Zr was 8:2. And then citric acid was added into the precursor and dissolved at 60 °C for 1 h under stirring. After that, dried PMMA colloidal crystal templates were added in the solutions for 4 h. The precursors were then dried at 60 °C for 24 h. Finally, the CeO2 and CeO2-ZrO2 samples were obtained by calcination at 350 °C for 2 h with a ramp rate of 2 °C/min. The obtained powders were then heated at 2 °C/min up to 450 °C with a dwell time of 2 h. These as-synthesized CeO2 and CeO2-ZrO2 samples were abbreviated accordingly as C450 and CZ450, respectively. Both the C450 and CZ450 samples were further calcined at 600 and 800 °C in air for 2 h, respectively, labelled as C600, C800, CZ600 and CZ800. The nonporous CeO2-ZrO2 sample was prepared via a coprecipitation method. The stoichiometric amounts of Ce(NO3)3
6H2O and ZrOCl2
8H2O were dissolved in deionized water under stirring at RT. The atomic ratio of Ce:Zr was also 8:2. The hydroxides were precipitated by adding drops of ammonia solution. The mixtures were stirred for one hour, when the pH increased to 8.0. The precipitates were dried at 60 °C for 24 h, and then calcined at 350 °C for 2 h with a ramp rate of 2 °C/min. The obtained solids were then heated at 2 °C/min up to 450 °C with a dwell time of 2 h. The as-synthesized nonporous CeO2-ZrO2 sample was labelled as N-CZ450. Similarly with the 3DOM samples, the N-CZ450 was also calcined at 600 and 800 °C in air for 2 h, respectively, named as NCZ600 and N-CZ800.

2.2. Physical and chemical characterizations Specific surface area of samples was performed on a Quantachrome Autosorb-iQ instrument. The data were calculated according to the BET method by the N2 adsorption isotherm at liquid N2 temperature (77 K). X-ray powder diffraction (XRD) patterns of the catalysts were measured by a Rigaku diffractometer using Cu Ka radiation (k = 0.15406 nm). The operating voltage and current were 40 kV and 200 mA. The scanning rate is 5°/min. Raman spectra of the catalysts were recorded at RT in a Renishaw Invia Raman imaging microscope. The exciting wavelength was 514.5 nm from an Ar ion laser with a powder of ca. 10 mW on the catalysts. The morphology of the catalysts was observed by a scanning electron microscopy (SEM) by a Hitachi S4800 instrument with accelerating voltages of 3 kV.

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The morphology of the catalysts was observed by transmission electron microscopy (TEM) characterization, using a JEOL-2100 microscope operating at 200 kV. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5000 Versaprobe II system using a monochromatic Al-Ka X-rays source. Spectra were recorded at RT in vacuum (<107 Pa). The C 1s signal at 284.8 eV was used for calibration of the XP-signals. Temperature-programmed reduction (TPR) and temperatureprogrammed desorption (TPD) profiles were obtained from RT to 900 °C. Experimental data were collected from Quantachrome Instruments. The amount of catalyst either for TPR or for TPD testing was 100 mg using a heating rate 10 °C/min in all cases. After a standard cleaning pretreatment, TPR was carried out in a flow of 10% H2/Ar (25 mL/min) up to 900 °C. For TPD testing, the pretreated sample was exposed to 5% O2/N2 (25 mL/min) at 100 °C for 30 min and then purged with N2 for 30 min. Then the sample was heated up to 900 °C using a heating rate of 10 °C/min in all cases. CO-TPR over different catalysts (100 mg) was also conducted on the CATLAB catalyst characterization system (produced by Hiden Analytical Co., England). Before each of the experiments, the sample was pretreated in flowing 10% O2/Ar (25 mL/min) at 400 °C for 1 h, cooled down to RT and then switched to argon purging for 30 min. After that, the pretreated sample was exposed to 1%CO/ Ar (25 mL/min) at RT for 30 min and then ramped (10 °C/min) up to 600 °C. The gas was analysed by an online mass spectrometer (MS). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was used to evaluate the adsorbed species on the catalyst under reaction conditions. Infrared spectra were recorded by an FTIR spectrometer (vertex 70, Bruker, Germany) equipped with a liquid N2 cooled Mercury-Cadmium-Telluride (MCT) detector. A Pike Technologies HC-900 DRIFTS cell with nominal cell volume of 6 cm3 was used. Scans were collected from 4000 to 1000 cm1 at a resolution of 4 cm1. Catalyst samples were placed in a DRIFTS cell equipped with SeZn windows. Before each of the following in situ DRIFTS experiments, the sample was pretreated in the DRIFTS cell in flowing 10%O2/Ar (25 mL/min) at 400 °C for 1 h and then cooled to RT before switching to Ar. In CO adsorption experiments, the pretreated sample was purged with Ar at RT before switching to 1%CO/Ar (25 mL/min) flow for 30 min. For the In situ DRIFTS CO-TPR experiments, the pretreated sample was exposed to 1%CO/Ar (25 mL/min) at RT for 30 min and then purged with argon at RT for 30 min. Then the sample was heated (10 °C/min) up to 600 °C in flowing argon (50 mL/min).

CO conversion%

80

60

3. Results 3.1. Catalytic activity Fig. 1 exhibits the CO oxidation activity over 3DOM CeO2 and CeO2-ZrO2 and nonporous CeO2-ZrO2 samples. It is convenient to compare the catalytic activities of the samples by adopting the reaction temperatures T10, T50, and T90 (corresponding to CO conversion of 10%, 50% and 90%, respectively), as summarized in Table 1. For the series of 3DOM CeO2 samples, C450 is the most active, followed by C600 and C800. For example, the temperature for T50 is 300 °C over C450, 340 °C over C600, and 366 °C over C800. For the series of 3DOM CeO2-ZrO2 samples, CZ450 shows a very low catalytic activity, while a drastic increase in the activity occurs over the CeO2-ZrO2 sample preheated at 600 °C (CZ600 sample). Further increasing the ageing temperature to 800 °C would reduce the activity. It can be seen that a T50 is achieved at 255 °C for CZ600, while only 4% and 13% CO conversion is obtained at the same temperature for CZ450 and CZ800 samples, respectively. The macroporous CeO2-ZrO2 samples present totally different phenomenon compared with the 3DOM CeO2-ZrO2 samples, as shown in Fig. 1B. Their activity for CO oxidation linearly decreases with increasing the calcination temperature. The N-CZ450 catalyst oxidizes 50% of CO at a temperature of 345 °C, while the T50 values for N-CZ600 and N-CZ800 catalysts are 370 and 420 °C, respectively. This is a usual case for the CeO2-ZrO2 catalyst, because calcination at high temperature may result in sintering of the catalyst, reducing the catalytic activity. It should be stressed that the CZ450 sample exhibits the lowest catalytic activity, while the CZ600 sample shows the best catalytic performance among the six samples. This suggests that some evolution of the structural and/or surface properties of the material

100

A T90

80

T50 C450 C600 C800 CZ450 CZ600 CZ800

40

20

Catalytic activity evaluation was performed in a plug-flow, temperature-controlled microreactor system. The sample (100 mg), loaded into the quartz tube with inner diameter of 6 mm, was pretreated in flowing 10%O2/Ar (25 mL/min) at 400 °C for 1 h. And then, it was cooled down to RT in O2/Ar atmosphere and switched to argon purging for 30 min thereafter. After that, the gas mixture (1% of CO, 10 vol% O2 balance Ar) was introduced into the reactor with a flow of 50 mL/min at RT, and the system was ramped up to 600 °C and cooled back to RT at a rate of 10 °C/min. The outlet gaseous products were analysed by a gas chromatograph (Agilent 7890A GC System, produced by Agilent Co.), which is equipped with HP-Plot 5A and HP-Plot-Q column.

T10

CO conversion%

100

2.3. Catalytic activity testing

60

B T90

T50

40

20

N-CZ450 N-CZ600 N-CZ800

T10

0

0 200

300

400

500

Temperature (oC)

600

200

300

400

500

600

Temperature (oC)

Fig. 1. CO conversion as a function of reaction temperature over macroporous (A) and nonporous (B) samples.

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Table 1 Catalytic activities, activation energies (Ea) and BET specific surface areas (SBET) of different samples. Samples

T10 (°C)

T50 (°C)

T90 (°C)

Ea (kJ/mol)

SBET (m2/g)

C450 C600 C800 CZ450 CZ600 CZ800 N-CZ450 N-CZ600 N-CZ800

240 260 300 308 200 225 270 295 325

300 340 366 445 255 310 345 370 420

355 425 473 534 343 394 445 545 600

62 92 112 137 57 83 73 75 94

42 40 38 50 46 43 87 70 50

-4

-4

C450 C600 C800 CZ450 CZ600 CZ800

A -5

CZ450 CZ600

-5

CZ800

-6

ln k

ln k

-6

B

-7

-7

-8

-8

-9 1.4

1.6

1.8

2.0

2.2

2.4

-9 1.5

1.6

1.7

1.8

1.9

2.0

1000/T (K -1)

1000/T (K -1)

Fig. 2. Arrhenius plots for CO oxidation over macroporous (A) and nonporous (B) catalysts under the conditions of CO concentration = 1 vol%, CO/O2 molar ratio = 1/10, and SV = 30,000 mL/(g h).

CeO2-ZrO2 samples. In addition, the stability of the CZ600 catalyst is examined at 300 °C with a CO conversion at ca. 80%, and the results shown in Fig. S1. As can be seen, the CZ600 catalyst maintained its activity during 100 h testing. 3.2. Structural properties of catalysts

(400)

(331) (420)

CZ600

(311) (222)

(200)

(220)

(111)

X-ray diffraction patterns of the different 3DOM CeO2 and CeO2ZrO2 catalysts are shown in Fig. 3. In the case of the bare CeO2, all the diffraction peaks can be indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0), (3 1 1) and (4 2 0) crystal faces, corresponding to a facecentred cubic (fcc) fluorite structure of CeO2 [37]. No ZrO2 crystal phase was detected in the XRD profiles of all the CeO2-ZrO2 samples, which indicates the formation of solid solutions. For

Intensity (a.u.)

may occur when increasing the calcination temperature from 450 to 600 °C. On the other hand, it is an exceptional phenomenon that the CeO2-ZrO2 solid solutions (i.e., CZ450 sample) possess much lower catalytic activity than single CeO2 (i.e., C450 sample), because CeO2-ZrO2 solid solutions prepared in the same conditions usually have smaller particle size and a higher oxygen vacancy concentration which prove to be beneficial to improving catalytic activity. Further investigation of the difference among the above different samples may lead to a better understanding of the CO oxidation mechanism (involving the key factor to determine the activity and the reaction path) over CeO2-based catalysts. The apparent activation energy (Ea) over different samples was also obtained based on the catalytic activity. It is reported that the reaction order was related to the concentration of CO and O2. When the CO concentration is relatively low, the CO should be first-order [31,32]. Jia et al. [33] also claimed that the oxidation of CO obeys a reaction mechanism of first order towards CO concentration. Some works suppose that the oxidation of CO in the presence of excess oxygen (CO/O2 molar ratio P1/2) would obey a first-order reaction [34–36]. In the present work, the amount of O2 is excess (CO/O2 molar ratio = 1/10). Therefore, it is reasonable to propose that the oxidation of CO should obey a first-order reaction with respect to CO concentration(c). r = kc = (A exp(Ea/ RT))c, where r, k, A, and Ea are the reaction rate (mol/s), rate constant (s1), pre-exponential factor, and apparent activation energy (kJ/mol), respectively. The Arrhenius plots for CO oxidation over the macroporous and nonporous samples are shown in Fig. 2, and their apparent activation energies are summarized in Table 1. It can be observed that the Ea value for CO oxidation decreased in the sequence CZ450 (137 kJ/mol) > C800 (112 kJ/mol) > N-CZ800 (94 kJ/mol) > C600 (92 kJ/mol) > CZ800 (83 kJ/mol) > N-CZ600 (75 kJ/mol) > N-CZ450 (73 kJ/mol) > C450 (62 kJ/mol) > CZ600 (57 kJ/mol). This suggests that CO oxidation proceeds more readily over the macroporous

60

70

80

CZ450

C600 C450 10

20

30

40

50

2 Theta (deg.) Fig. 3. XRD patterns of different 3DOM CeO2 and CeO2-ZrO2 samples.

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Y. Zheng et al. / Journal of Catalysis 344 (2016) 365–377 Table 2 Average crystal parameters of different 3DOM CeO2 and CeO2-ZrO2 samples. Samples

Phase

Crystallite size (nm) CeO2

Lattice constant (nm) CeO2

C450 C600 CZ450 CZ600

Cubic Cubic Cubic Cubic

15.2 23.5 5.9 7.8

0.5401 0.5410 0.5365 0.5347

CeO2, the diffraction peaks become sharper and stronger after thermal treatment at 600 °C. This is a consequence of the better arrangement of the atoms into the framework of pure CeO2 and the decreased number of lattice defects [38]. However, for the CeO2-ZrO2, these changes are less remarkable, indicating a good thermal stability. The average crystal parameters of the samples are listed in Table 2. It is obvious that both the two CeO2-ZrO2 samples show much lower lattice constants for CeO2 compared with pure CeO2. This lattice shrinkage should result from the smaller Zr4+ ion entering into the fluorite-type lattice of CeO2. It is also noted that the lattice constant for CeO2-ZrO2 samples (CZ450 vs CZ600) further decreases when increasing the calcination temperature from 450 to 600 °C, suggesting greater formation of CeO2-ZrO2 solid solu-

tions. This explanation should be that some isolated Zr species may exist in an amorphous state on the sample heated at low temperatures, which could be incorporated into CeO2 lattice after thermal treatment at relatively high temperatures. Raman spectra of the 3DOM CeO2 and CeO2-ZrO2 samples are shown in Fig. 4. The pure CeO2 sample exhibits a strong peak centred at 462 cm1 corresponding to F2g Raman active mode in metal oxides with fluorite-like structure [39]. This peak broadens and slightly shifts to 470 cm1 for the CeO2-ZrO2 samples. It has been reported that the broadening of the ceria main line is related either to a decrease of ceria crystallite size or to the presence of defects such as oxygen vacancies [37]. For the CeO2-ZrO2 samples, a broad band centred at 601 cm1 can be observed. This band could be linked to oxygen vacancies due to the substitution of Zr4+ into the ceria lattice [40]. The appearance of a very weak band at 300 cm1 could be related to the displacement of the oxygen atoms from their ideal fluorite lattice positions [41]. To make a quantitative analysis of the concentration of oxygen vacancies, the I601/I462 ratios were calculated from the Raman spectra, as shown in Fig. 4B. The I601/I462 ratio is only 0.04 and 0.02 for the C450 and C600 respectively, while it is 0.24 for the CZ600 sample. Fig. 5 shows the SEM images of the 3DOM CeO2 and CeO2-ZrO2 samples. It can be seen that the C450 and CZ450 samples have 0.25

A

601

B

CZ600 CZ450

I 601 /I 462

Intensity (a.u.)

0.20

462

0.15 0.10

C600

0.05

C450 0.00 200

300

400

500

600

700 -1

Raman shift (cm )

800

900

C450

C600

CZ450

CZ600

Samples

Fig. 4. Raman spectra (A) and the I601/I462 ratios calculated from Raman spectra (B) of different 3DOM CeO2 and CeO2-ZrO2 samples.

Fig. 5. SEM images of 3DOM C450 (a1), CZ450 (b1), C600 (a2) and CZ600 (b2) samples.

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open, periodic and interconnected three-dimensionally (3D) frameworks. The pore sizes estimated from the SEM images are in the range of 100–150 nm. When the samples were further preheated at 600 °C, there is no great difference in the morphologies and textural structure among the two samples (C600 and CZ600). It indicates that the 3DOM ceria-based materials have higher thermal and structural stability, which are required for the further development of durable, highly active and versatile catalysts for CO oxidation. Fig. 6 shows the TEM and HRTEM images of different 3DOM CeO2 and CeO2-ZrO2 samples. The macroporous structure with overlapped pores can be clearly observed in the TEM images. The

pore sizes of macroporous samples are 100 ± 20 nm, and the voids are interconnected through open windows which agrees with the observed by SEM images. The CeO2 particle size of C450 is in the range of 4–10 nm with a narrow distribution (Fig. 6-a2) and a mean diameter of 6.0 nm by statistical analysis of more than 300 CeO2 particles. The 3DOM C600, CZ450 and CZ600 samples reveal a mean particle diameter of 8.0, 5.1 and 5.9 nm, respectively (see Fig. 6-b2, c2, d2). In addition, the macroporous walls are composed of closely packed particles with limited microporous or mesoporous. The lattice fringes with a width of 3.1 Å and 1.9 Å indexed as {1 1 1} and {1 1 0} planes of CeO2, respectively, can be found in all the samples (see Fig. 6-a3, b3, c3, d3). To identify the percentage of

Fig. 6. TEM and HRTEM images of 3DOM C450 (a1-3), C600 (b1-3), CZ450 (c1-3) and CZ600 (d1-3).

Y. Zheng et al. / Journal of Catalysis 344 (2016) 365–377

Volume Adsorbed(cm3/g)

the exposed {1 1 0} plane, the ratio of {1 1 0}/{1 1 0 + 1 1 1} was obtained by statistical analysis of more than 300 CeO2 particles for each sample. Parts of the TEM images used for statistical analysis were plotted in Fig. S2. The ratio of {1 1 0}/{1 1 0 + 1 1 1} for C450, C600, CZ450 and CZ600 samples is 23%, 20%, 8% and 45%, respectively. This suggests that there are more {1 1 0} planes on the surface of CZ600 sample. Fig. 7 shows the N2 adsorption-desorption isotherms of different 3DOM CeO2 and CeO2-ZrO2 samples. The CZ450 sample displays a type II isotherm and a type H2 hysteresis loop in the relative pressure (P/P0) range of 0.2–0.85, which could be linked to capillary condensation taking place in mesopores, indicating that textural mesopores existed within the skeletons. However, some of the mesopores are lost after calcination at higher temperatures, which can be explained by grain growth. The two 3DOM CeO2 samples presented a nonporous model, indicating that there is almost no mesopores existing within the macroporous walls. Compared with the CeO2 samples (surface area = 40–42 m2/g), the macroporous CeO2-ZrO2 samples showed a slightly higher surface area (46–50 m2/g). XPS was performed to determine the chemical state of Ce and O species on the surface of different samples. The oxidation states of Ce were analysed by fitting the curves of Ce 3d XPS bands, as shown in Fig. 8. Table 3 presents the relative percentages of the

0.0

C450 C600 CZ450 CZ600

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0) Fig. 7. Nitrogen adsorption-desorption isotherms of different 3DOM CeO2 and CeO2-ZrO2 samples.

371

cerium species calculated by the area ratios of the Ce3+/(Ce + Zr) and two oxygen species. The peaks labelled as V correspond to Ce 3d5/2 contributions, and those of labelled as U represents the Ce 3d3/2 contributions. The peaks labelled as V, V0 0 and V000 are assigned to Ce(IV) final states, and the peak V0 is denoted as Ce (III) final states. The same assignment can be applied to the U structures, which correspond to the Ce 3d3/2: the line U0 is assigned to Ce(III) final state coordinated to oxygen vacancies and the other lines indicative of the presence of Ce4+ ions [42–44]. The quantitative analysis (see Table 3) shows that the atom ratio of Ce/Zr for CZ450 is lower than that for the CZ600 sample (2.34 vs. 3.25), indicating that the CZ450 sample is enriched with ZrO2 on the surface. Since CeO2 is the active component for CO oxidation, this can be used to explain the relatively lower activity of CZ450 sample. In addition, it is also found that the average ratios of Ce3+/(Ce + Zr) decreased (see Table 3) in the order of CZ600 > C450 > C600 > CZ450 (which ranges from 0.13 to 0.18), which is completely consistent with the order of the catalytic activity over different samples. This indicates that Ce3+ concentration plays a very important role for determining the catalytic property. To further confirm the importance of the oxygen mobility, oxygen storage capacity (OSC) measurements were also performed by the successive CO pulse mode (see Fig. S3). The quantitative data on the TOSC for different samples are shown in Table 3. The TOSC values decrease in the order of CZ600 > C450 > C600 > CZ450 which is completely agreement with the order of catalytic activity. The O 1s spectra of pure CeO2 samples (C450 and C600) can be fitted with three peaks at ca. 529.6 eV (main band, labelled as OIII), 531.1 eV (labelled as OII) and 532.2 eV (labelled as OI), respectively. It is generally accepted that the 529.6 eV peak is characteristic of the lattice oxygen (O2) in cerium oxide with the peak at 531.1 and 532.2 eV attributed to surface oxygen (Os) and/or surface hydroxyl species OHs, respectively [45]. However, the O 1s spectra of CeO2-ZrO2 samples (CZ450 and CZ600) can only be fitted with two peaks (OII and OIII), as indicated by the disappearance of the surface hydroxyl species (OI). It is noted that the lattice oxygen (OIII) is the most abundant species for each sample. Since surface oxygen species usually play a very important role in the catalytic process, the surface-Oads.(OI + OII)/bulk-Olatt.(OIII) molar ratios are calculated based on the XPS data. As shown in Table 3, the ratio decreases in the order of C450 > CZ600 > C600 > CZ450. However, by ignoring the effect of surface hydroxyl species (OI), the sequence of the Oads./Olatt. ratios among the four samples would be as follows: CZ600 > C450 > CZ450 > C600.

Fig. 8. Ce 3d and O 1s XPS patterns of different 3DOM CeO2 and CeO2-ZrO2 samples.

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Table 3 XPS-derived characteristics for different 3DOM CeO2 and CeO2-ZrO2 samples.

a

Samples

Ce3+

Ce4+

Ce3+/Ce4+

Atom ratio of Ce/Zr

Ce3+/(Ce+Zr)

OI

OII

OIII

(OI + OII)/OIII

TOSCa mmol/g

C450 C600 CZ450 CZ600

17 15 19 24

83 85 81 76

0.20 0.18 0.23 0.32

– – 2.34 3.25

0.17 0.15 0.13 0.18

10 7 – –

19 13 16 22

71 80 84 78

0.41 0.25 0.19 0.28

1.01 0.93 0.77 1.12

TOSC is determined by summarizing OSC of 20 CO pulses at T = 400 °C.

Fig. 9. H2-TPR profiles (A) and cumulative H2 uptake (B) and initial H2 consumption rate (C) of different 3DOM CeO2 and CeO2-ZrO2 samples.

3.3. Reducibility and oxygen mobility

O2-TPD CZ600 CZ450

TCDsignal (a.u.)

Fig. 9A shows the H2-TPR profiles of the four samples, and Fig. 9B exhibits the cumulative hydrogen uptake associated with the phases present in the samples as a function of reduction temperature. For pure CeO2, two reduction peaks are detected in the H2-TPR profile: a low temperature peak at 580 °C and a high temperature peak in a range higher than 800 °C corresponding to the reduction of surface and bulk ceria, respectively. The CeO2-ZrO2 samples only show one obvious reduction peak, centred at about 656 °C, and the intensity of this peak is much higher than either of the two peaks over the pure CeO2 sample. This suggests that oxygen located within the subsurface or bulk of CeO2-ZrO2 samples could migrate to the surface immediately as surface oxygen is consumed, indicative of relatively high lattice oxygen mobility [38]. The total hydrogen consumption of CeO2-ZrO2 samples (123 mmol/g for CZ450 and 134 mmol/g for CZ600) is much higher than that of CeO2 (68 mmol/g for C450 and 42 mmol/g for C600). It is generally accepted that the low-temperature reducibility of a catalyst can be conveniently evaluated using the initial H2 consumption rate where less than 25% oxygen in the sample is removed for the first reduction peak [46]. As shown in Fig. 9C, CZ600 presents the highest H2 consumption rate among the four samples. To further investigate the adsorption and activation of oxygen on CeO2 and CeO2-ZrO2, O2-TPD measurements were carried out, as shown in Fig. 10. The O2 desorption peaks of the pure CeO2 samples (C450 and C600) can be divided into three parts in the temperature ranges of 80–250 °C (low-temperature), 250–500 °C (middle-temperature), and 500–900 °C (high-temperature), respectively. The low-temperature peak is assigned to the desorption of physically adsorbed oxygen (O2), and the middletemperature peak can be associated with the consumption of chemisorbed surface-active oxygen species [47]. The high temperature must be related to the release of lattice oxygen. For the CeO2-ZrO2 samples, the peaks at temperatures lower than 300 °C are very weak and only one main peak in the range of 300–900 °C is observed. This is similar to the phenomena observed in the TPR

C600 C450

100

200

300

400

500

600

700

800

900

Temperature (οC) Fig. 10. O2-TPD profiles of different 3DOM CeO2 and CeO2-ZrO2 samples.

profiles, which indicates that there was no clear distinction between the desorption of surface and bulk oxygen. Because the oxygen in the subsurface can quickly migrate to the surface as the surface oxygen is released indicates a relatively high lattice oxygen mobility [38]. Fig. 11A shows the CO2 profiles evolved during CO-TPR over different 3DOM CeO2 and CeO2-ZrO2 samples. Both the two CeO2 samples exhibit three weak reduction peaks with the increase of the temperature. For the two CeO2-ZrO2 samples, only a strong band is observed in the range of 240–600 °C. The production temperature of CO2 in the CO-TPR is in general alignment with the trend of the catalytic activity (Fig. 1A), which is an indication that the lattice oxygen participates in CO oxidation on 3DOM CeO2 and CeO2-ZrO2 samples. H2, as a byproduct, is also observed during CO-TPR over the four samples, as shown in Fig. 11B. This is similar to the observation by Wu et al. [24]. It is proposed that the forma-

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373

Fig. 11. CO2 (A) and H2 (B) evolution during CO-TPR over different 3DOM CeO2 and CeO2-ZrO2 samples. (C) In situ DRIFT spectra collected during CO-TPR of C450 at different temperature.

Fig. 12. In situ DRIFT spectra of CO adsorption on different 3DOM CeO2 and CeO2-ZrO2 samples at RT.

tion of H2 is mainly due to the reaction between adsorbed surface OH groups and CO via water-gas shift type reactions [48]:

CO þ OH ! 1=2H2 þ CO2

ð1Þ

CO þ 2OH ! H2 þ CO2 þ OL

ð2Þ

To investigate the evolution of surface OH groups during COTPR of the samples up to 600 °C, the in situ DRIFT spectra of C450 were employed, as shown in Fig. 11C. It can be seen that, bands characteristic of formate (2842 cm1) derived from reaction between CO and surface OH groups start to appear at 300 °C. Formate species is an intermediate in water-gas shift type reactions. With the temperature increase, the formate decomposes to H2 and CO2. It should be noted that the intensity of H2 peaks over pure CeO2 is much higher than that over CeO2-ZrO2 samples, indicating more

OH groups on the surface of CeO2. On the other hand, the 3DOM CeO2-ZrO2 samples (CZ450 and CZ600) produce much more CO2 than the pure CeO2 in the CO-TPR testing. This suggests that the CO2 formation over the 3DOM CeO2-ZrO2 samples should be due to direct CO reaction with reactive lattice oxygen instead of the water-gas shift reaction between CO and hydroxyl groups. This must be associated with relatively high reducibility and oxygen mobility of the 3DOM CeO2-ZrO2 samples. 3.4. Interaction with CO DRIFT spectroscopy technology was used to investigate CO adsorption and oxidation behaviour on these 3DOM CeO2 and CeO2-ZrO2 samples. Fig. 12 shows the in situ DRIFT spectra from CO adsorption on different samples at RT as a function of time after switching in the CO feed. Based on the investigations of CO and CO2

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Table 4 Assignments of in situ DRIFT spectra observed in the CO adsorption at room temperature over the 3DOM CeO2 and CeO2-ZrO2 samples. Species

v(CO3)

Bicarbonate Unidentate carbonate Bidentate carbonate Bridged carbonate

1420, 1420, 1036, 1036,

1626 1360 1270, 1560 1310, 1695

adsorption on ceria [24,48–51], CO interacts strongly with the surface oxygen and forms a variety of carbonate species in the range from 800 to 1800 cm1, and the assignments of the observed vibrational bands are summarized in Table 4. Bicarbonate species, unidentate carbonate species, bidentate carbonate species, bridged carbonate species are formed upon the interaction of CO with the surface oxygen of CeO2. Similar carbonate species are also observed on the surface of the two CeO2-ZrO2 samples, while the corresponding peaks are stronger for the CZ600 sample. It indicates that CZ600 possesses more CO active sites [10]. In addition to the carbonate species (wave numbers in the range from 800 to 1800 cm1), the bands at 2000–2200 cm1 were also distinguished. Bands in the 2000–2200 cm1 region can be associated with linear or bridge-bonded CO interacting with sites on the surfaces. A bridged carbonyl species will adsorb at lower wave numbers than a linear carbonyl species [52]. The vibrational peaks at ca. 2172 and 2117 cm1 are ascribed to CO adsorbed on the Ce sites [53]. The bands with a wave number lower than 2100 cm1 are often detected on noble metal catalysts (Au or Pt catalysts), which can be assigned to negatively charged metal clusters or metal atoms [54]. The presence of the band at 2007 cm1 should be part of the wave number at 2117 cm1 shifted to the lower wave number. The shift of CO vibrational frequency is determined by the d and p-back bonds orbitals [55–57]. When only a d bond is formed, the stability of the carbonyls increases with the effective positive charge of the cations, and, correspondingly, the CO vibrational frequency is

p(CO3)

d(OH)

v(OH)

864 – 864 –

1216 – – –

3619 – – –

shifted towards higher frequencies. However, the p-back bond is formed at the expense of electron transfer from the d electrons of the metal cation to the 2p⁄ antibonding orbitals of the CO molecule [58,59]. As a result, the metalAC bond becomes stronger (i.e. the carbonyls are more stable), which reflects in a red shift of the CAO stretching vibrations [57]. Therefore, the lower band at 2007 cm1 can be attributed to the enhanced CeACO bond, indicating a stronger interaction between CeO2 and CO. It can be seen in Fig. 12 that the CZ600 sample shows relatively high intensity of peaks in the ranges of both 800–1800 cm1 and 2007 cm1, which indicates that this sample could strongly interact with the CO molecules. Fig. 13 shows the in situ DRIFT spectra obtained as the samples are heated in the reactant mixture. Three major types of carbonate species were detected over the samples: bicarbonate species (864, 1440, 1626 and 3619 cm1), unidentate carbonate species (1440– 1360 cm1), and bidentate carbonate species (864, 1036, 1260– 1290 and 1563 cm1). The bicarbonate species (major peak at 1626 cm1) gradually disappeared, while the bidentate carbonate (major peak at 1523–1563 cm1) species began to increase to a great extent when the reaction temperature increased to 200 °C. On the other hand, gaseous CO2 peaks (2293 and 2356 cm1) started to appear at 200 °C and kept increasing in the intensity with temperature. This is consistent with the increasing of CO conversion (see Fig. 1A). These phenomena can be summarized as follows: bicarbonate species first transformed into bidentate carbonate species (below 200 °C), followed by the decomposition

Fig. 13. In situ DRIFT spectra of CO oxidation on different 3DOM CeO2 and CeO2-ZrO2 samples.

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40

80

30

60

20

40

10

20

0 C450

C600

CZ450

CZ600

Samples

100

100 CO conversion at 673 K Crystallite Size

B

surface areas

80

80

60

60

40

40

20

20

Relative intensity (a.u.)

A

CO conversion / %

CO conversion at 673 K ratio of (110) oxygen vacancy

Relative intensity (a.u.)

CO conversion / %

100

120

50

120

0 C450

C600

CZ450

CZ600

Samples

Fig. 14. The relationship between the catalytic activities of CO oxidation and physicochemical properties of different catalysts (the relative intensity is calculated from that amplify the same multiple on the original data).

of bidentate carbonate species into gaseous CO2 with increasing temperature. The band intensity of carbonate species over CeO2 samples, especially C600, is much higher than that over CeO2ZrO2 samples in the range of 200–400 °C. It is reported that the accumulation of carbonate species could cover the CO active sites, leading to a reversible deactivation [10,60]. Therefore the C600 and C450 samples show lower activity than CZ600.

4. Discussion 4.1. Dominant factors for CO oxidation over CeO2-based catalysts It is generally accepted that the importance of ceria in catalysis originates from its extraordinary efficiency for reversible oxygen release via the rapid formation and elimination of oxygen vacancies [61]. The presence of oxygen vacancies provides relative freedom for the movement of lattice oxygen and thus increases the mobility of oxygen [62]. O2 can adsorb on the vacancies, which could reduce surface Gibbs energy, balance surface charge, and form surface active oxygen species [11]. This would promote oxygen reactivity to CO in the catalytic process [15]. The results in the present work reveal a similar phenomenon. The XRD, Raman spectra and XPS results (see Figs. 3, 4 and 8 and Tables 2 and 3) indicate that more oxygen vacancies are detected on the surface of CZ600 compared with the other samples, resulting in relatively high reducibility and oxygen mobility (observed by H2-TPR and O2-TPD, as shown in Figs. 9 and 10, respectively). The CZ600 shows much higher catalytic activity for CO oxidation than the other three samples (see Fig. 1). This suggests that both the oxygen vacancies and oxygen mobility are important for catalytic CO oxidation. The textural properties such as the crystallite size and specific surface area also influence the catalytic performance for CO oxidation. Fan et al. [18] reported that the higher specific surface area and smaller crystallite size of the catalyst can enhance the catalytic performance for CO oxidation. The CZ450 is also an exception in this respect. It can be seen in Table 1 that the specific surface areas of 3DOM CZ450 are higher than or equal to those of the other 3DOM CeO2 or CeO2-ZrO2 samples. In addition, it also possesses a smaller crystallite size of CeO2, as shown in Table 2. However, the CZ450 sample shows the worst catalytic performance for CO oxidation among all the samples. Surface structure dependence of CeO2 catalysts for CO oxidation is also confirmed by intensive experimental and theoretical studies [63–66]. To get a greater understanding of the nature of different CeO2 surface planes in the catalytic process, Wu et al. [24] systematically investigated CO oxidation over a series of ceria nanocrystals with defined surface planes (nanoshapes) including rods ({1 1 0} + {1 0 0}), cubes ({1 0 0}), and octahedra ({1 1 1}). The results

showed that the TOF (converted CO per surface O per second) of CO oxidation follows the order of {1 1 0} > {1 0 0}  {1 1 1}. The TEM images show that all the 3DOM CeO2 and CeO2-ZrO2 samples expose both {1 1 0} and {1 1 1} surfaces (see Fig. 6). The statistical analysis based on the HRTEM images suggests that the ratios of {1 1 0}/{1 1 0 + 1 1 1} decrease in the order of CZ600 > C450 > C600 > CZ450 (see Figs. 6 and S2). This order is consistent with the sequence of catalytic activity (see Fig. 1). The relationship between the catalytic activities (CO conversion) for CO oxidation and physicochemical properties of different catalysts is summarized in Fig. 14. It can be seen that, for pure CeO2 samples (C450 and C600), the increase in particle size and the decrease in oxygen vacancy concentration would reduce the catalytic activity. We also note that this influence occurs with the precondition that there are no changes of the preferentially exposed surface plane over the two pure CeO2 samples (ratios of {1 1 0}/ {1 1 0 + 1 1 1} for C450 and C600 are 23% and 20%, respectively, as shown in Fig. 6). For the two CeO2-ZrO2 samples (CZ450 and CZ600), the sample heated at 600 °C (CZ600 sample) showed more abundant oxygen vacancy and higher catalytic activity. This can be ascribed to the relatively higher ratios of {1 1 0}/{1 1 0 + 1 1 1} over the CZ600 sample. CZ600 also revealed lower activation energies for CO oxidation (see Table 1), which should be linked to the binding energies of CO or O (O2) with the exposed surface plane. Chen et al. [29] suggested that the barrier for the CO to react with a surface O at CeO2 {1 1 0} is 0.25 eV, while it is 0.61 eV for the CO to react with a surface O at CeO2 {1 1 1}. In other words, the O at CeO2 {1 1 0} is more active compared to that at CeO2 {1 1 1}. The fact that the {1 1 0} surface is more reactive for promoting CO oxidation can also be seen from the overall CO oxidation energy, which was calculated to be exothermic by 1.1 eV at the {1 1 0} surface and only 0.4 eV at the {1 1 1} surface (with respect to the noninteracting CO-CeO2 systems). Nolan and Watson [28] reported that the interaction with CO molecule on the CeO2 {1 1 1} surface leads to a weak adsorption, with an adsorption energy of 6 kcal/mol. However, for the {1 1 0} surfaces, the adsorption energy is much higher (45 kcal/mol), indicating a relatively strong interaction between adsorbed CO and CeO2 {1 1 0} surface. Fig. 14A also showed that the ratios of {1 1 0}/{1 1 0 + 1 1 1} and oxygen vacancy concentration over different samples followed the same trend as in the values of CO conversion for CO oxidation. This phenomenon demonstrates that the preferentially exposed surface plane and oxygen vacancy concentration are crucial factor in determining catalytic activity. 4.2. CO adsorption and reaction mechanism As observed in the CO-TPR process (Fig. 11), surface water-gas shift reaction between CO and surface OH groups was detected.

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Scheme 1. Reaction mechanism model of CO oxidation over 3DOM CeO2-ZrO2 catalysts.

The results of in situ DRIFTS during CO-TPR (Fig. 11C) show the presence of formate species, which is indentified as an intermediate from CO to carbonate [48]. Jardim et al. [53] also reported that the presence of surface OH groups on the catalyst could enhance the catalytic activity for CO oxidation. In our results, there are abundant OH groups on the surface of 3DOM CeO2 sample (Figs. 8 and 11), while low catalytic activity for CO oxidation observed over the CZ600 sample. By contrast, the CZ600 shows the best catalytic performance, although limited OH groups can be detected on the surface of CeO2-ZrO2 samples. In addition, Fig. S5 shows the curves of CO conversion over the C450 catalyst both in the heating and cooling processes in CO oxidation. As seen, there are no obvious differences between the heating and cooling processes. Since the fresh catalyst has abundant OH groups, these phenomena indicate that the surface OH groups should not be a crucial factor in determining the catalytic activity. The results of in situ DRIFTS during CO adsorption (Fig. 12) show the presence of two types of CO species: adsorbed CO on surface Ce sites (the band at ca. 2007 cm1) and CO chemically bonded to surface oxygen forming carbonate species. The former species were often detected on the supported noble metal catalysts, indicating a strong interaction between CO and catalysts [54]. The high intensity of the band at 2007 cm1 on the CZ600 sample suggests strong interaction between CO and CZ600. This can be related to the fact that there are more {1 1 0} surfaces exposed on this sample, because Ce is more active on {1 1 0} surface than on {1 1 1} [67]. On the other hand, the Raman (Fig. 4) and XPS (Fig. 8) results indicate that the CZ600 sample also possesses abundant oxygen vacancies, which can adsorb O2 molecules. In this case, a Langmuir-Hinshelwood (L-H) mechanism should occur over the CZ600 catalyst. According to this mechanism, the CO molecules firstly adsorb on the catalyst surface and react with the adsorbed oxygen thereafter, followed by the formed CO2 desorbing into the gas phases (see Scheme 1). It should be noted that large amounts of carbonate species were detected on the C600 sample during the CO catalytic oxidation (see Fig. 13), indicating the accumulation of carbonate species. The accumulation should be related to the relatively low concentration of oxygen vacancy on this sample, because less O2 can be adsorbed on the vacancies to form active oxygen to remove the adsorbed CO species. This results in relatively lower catalytic activity of this sample.

On the other hand, the CO-TPR result (see Fig. 11A) reveals that, in the absence of O2, the 3DOM CeO2 or CeO2-ZrO2 can be reduced by CO during the similar temperature region with the catalytic CO oxidation. This indicates that a Mars-van Krevelen (M-K) mechanism based on the redox property of CeO2 or CeO2-ZrO2 catalysts also cannot be ignored. In a Mars-van Krevelen process, CO reacts with lattice oxygen of ceria to form CO2 and leave an oxygen vacancy, and then the replenishing of the vacancy by gas-phase oxygen completes the cycle (see Scheme 1). However, it should not be crucial compared with the L-H mechanism, because there is no obvious relationship between the reducibility and the catalytic activity among the different samples.

5. Conclusions A series of 3DOM CeO2 and CeO2-ZrO2 catalysts with varying particle sizes, oxygen vacancy concentrations, oxygen mobility and preferentially exposed surface planes were synthesized by a colloidal crystal template method. Their physicochemical properties were associated with the catalytic performances to identify the most crucial factor in determining catalytic activity. The interactions of catalysts with CO and reaction mechanism were further investigated in detail. The prepared 3DOM CeO2 and CeO2-ZrO2 materials were highly ordered and interconnected with each other by small pore windows, and the good macroscopic order still retained even after ageing at 800 °C, indicating a high thermal stability of the macroporous structure. Such 3DOM framework is beneficial for CO oxidation. One of the most important phenomena is that CZ450 sample, which possesses higher surface area and smaller particle size compared with the pure CeO2, showed much poorer catalytic activity for CO oxidation. By contrast, the CZ600 sample presented the highest catalytic activity among all the catalysts due to the highest ratio of {1 1 0}/{1 1 0 + 1 1 1} and oxygen vacancy concentration. The ratios of {1 1 0}/{1 1 0 + 1 1 1} and oxygen vacancy concentration over different samples follow the same trend as the CO conversion. These results suggest that the preferentially exposed surface plane and oxygen vacancy concentration are more crucial for determining the catalytic activity of CeO2based catalysts when compared with the other parameters (e.g., crystallite size and surface area).

Y. Zheng et al. / Journal of Catalysis 344 (2016) 365–377

Langmuir-Hinshelwood mechanism should be the crucial reaction pathway for CO oxidation over the 3DOM CeO2-based catalysts, although the Mars-van Krevelen mechanism cannot be ignored. It is also observed that the two CeO2 samples possess large amounts of carbonate species, while they show a low catalytic activity for CO oxidation. This can be attributed to the accumulation of carbonate species on the samples which may cover the CO active sites, leading to a reversible deactivation.

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The authors acknowledge the fruitful discussions with Prof. Robert (Bob) Farrauto, Columbia University. This work was supported by the National Natural Science Foundation of China (Nos. 51604137, 51374004 and 51204083), the Candidate Talents Training Fund of Yunnan Province (Nos. 2012HB009, 2014HB006), an Applied Basic Research Program of Yunnan Province (No. 2014FB123), a School-Enterprise Cooperation Project from Jinchuan Corporation (No. Jinchuan 201115) and a Talents Training Program of Kunming University of Science and Technology (No. KKZ3201352038). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2016.10.008. References [1] S. Najafishirtari, P. Guardia, A. Scarpellini, M. Prato, S. Marras, L. Manna, M. Colombo, J. Catal. 338 (2016) 115. [2] Y. Xu, L. Chen, X.C. Wang, W.T. Yao, Q. Zhang, Nanoscale 7 (2015) 10559. [3] K. Ding, A. Gulec, A.M. Johnson, N.M. Schweitzer, G.D. Stucky, L.D. Marks, P.C. Stair, Science 385 (2015) 189. [4] C.K. Costello, J. Guzman, J.H. Yang, Y.M. Wang, M.C. Kung, B.C. Gates, H.H. Kung, J. Phys. Chem. B 108 (2004) 12529. [5] P. Venkataswamy, K.N. Rao, D. Jampaiah, B.M. Reddy, Appl. Catal. B 162 (2015) 122. [6] L.Q. Liu, F. Zhou, L.G. Wang, X.J. Qi, F. Shi, Y.Q. Deng, J. Catal. 274 (2010) 1. [7] J.Y. Luo, M. Meng, X. Li, X.G. Li, Y.Q. Zha, T.D. Hu, Y.N. Xie, J. Zhang, J. Catal. 254 (2008) 310. [8] H.Y. Kim, G. Henkelman, J. Phys. Chem. Lett. 3 (2012) 2194. [9] M.Q. Shen, L.F. Lv, J.Q. Wang, J.X. Zhu, Y. Huang, J. Wang, Chem. Eng. J. 255 (2014) 40. [10] S. Zhang, X.S. Li, B.B. Chen, X.B. Zhu, C. Shi, A.M. Zhu, ACS Catal. 4 (2014) 3481. [11] L. Meng, A.P. Jia, J.Q. Lu, L.F. Luo, W.X. Huang, M.F. Luo, J. Phys. Chem. C 115 (2011) 19789. [12] P. Bera, A. Gayen, M.S. Hegde, N.P. Lalla, L. Spadaro, F. Frusteri, F. Arena, J. Phys. Chem. B 107 (2003) 6122. [13] Y. Nagai, T. Hirabayashi, K. Dohmae, N. Takagi, T. Minami, H. Shinjoh, S. Matsumoto, J. Catal. 242 (2006) 103. [14] U. Oran, D. Uner, Appl. Catal. B 54 (2004) 183. [15] C. Bozo, N. Guilhaume, J.M. Herrmann, J. Catal. 203 (2001) 393. [16] Z. Wang, Q. Wang, Y.C. Liao, G.L. Shen, X.Z. Gong, N. Han, H.D. Liu, Y.F. Chen, Chem. Phys. Chem. 12 (2011) 2763. [17] H.J. Wu, L.D. Wang, Catal. Commun. 12 (2011) 1374. [18] J. Fan, D. Weng, X.D. Wu, X.D. Wu, R. Ran, J. Catal. 258 (2008) 177. [19] G.C. Bond, D.T. Thompson, Gold Bull. 33 (2000) 41. [20] D.R. Schryer, B.T. Upchurch, B.D. Sidney, G.B. Hoflund, R.K. Herz, J. Catal. 130 (1991) 314. [21] M. Sheintuch, J. Schmidt, Y. Lecthman, G. Yahav, Appl. Catal. 49 (1989) 55. [22] A. Trovarelli, Catal. Rev. - Sci. Eng. 38 (1996) 439. [23] G.S. Zafiris, R.J. Gorte, J. Catal. 143 (1993) 86.

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