Enhanced activity for CO preferential oxidation via the CeO2-LaCoO3

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Kinetics, Catalysis, and Reaction Engineering

Enhanced activity for CO preferential oxidation via the CeO2-LaCoO3 interaction yuhao wang, Yane Zheng, hua wang, kongzhai Li, xing zhu, yonggang wei, yaming wang, lihong Jiang, and dong Tian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05727 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Industrial & Engineering Chemistry Research

Enhanced activity for CO preferential oxidation via the CeO2-LaCoO3 interaction Yuhao Wang b,c, Yane Zheng a,b,∗, Hua Wang b,c, Kongzhai Li b,c, Xing Zhu b,c Yonggang Wei b,c, Yaming Wang a, Lihong jiang a, Dong Tian b,c a Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China b State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China c Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China ∗Corresponding

author

E-mail addresses: [email protected] (Y. E. Zheng)

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract CeO2 nano particles are well dispersed on the wall surface of threedimensionally ordered macroporous (3DOM) LaCoO3, obtaining a highly CO conversion and O2 selectivity to CO2 in a wide temperature range for CO preferential oxidation (CO PROX). Among all the obtained samples, the CeO2(2.4)/LaCoO3 sample with appropriate loading amount of CeO2 exhibits the highest CO conversion and O2 selectivity. The CeO2(2.4)/LaCoO3 sample were active for the CO oxidation while inactive for the undesired H2 oxidation. In addition, the CeO2 (2.4)/LaCoO3 sample also showed high stability during the 100 h reaction experiments either in the activity or structure (macroporous frameworks) aspect. The In situ DRIFTs results suggest that the presence of H2 promoted the formation of OH species, which play an important role in the catalytic activity for CO PROX. Key words: 3DOM CeO2/LaCoO3, CO preferential oxidation, higher selectivity, interaction, OH species

1. Introduction The polymer electrolyte membrane fuel cell (PEMFC) has become to be regarded as one of the most promising candidates for utilizing hydrogen to produce heat and electricity, especially for electric vehicles or residential co-generation systems.

1-3

Hydrogen used in PEMFCs is generally produced by reforming streams of hydrocarbon or bioethanol, which still contain 0.5-2% CO. While the existence of CO could poison the anode of the PEMFC, therefore the allowed CO concentrations is limited to 10 ppm for Pt anodes. 4-6 The CO preferential oxidation (CO-PROX) reaction was recognized as the most effective method to remove CO in H2 rich gas for application in PEMFC. 1, 7, 8

The PROX reaction must reduce the concentration of CO and prevent the formation

of H2O (H2 oxidation). 9-11 The selection of a suitable catalyst owning high conversion and selectivity for CO PROX in the temperature range (80-250 ºC) is a key factor. 12, 13 This catalyst for CO PROX reaction must be active for the CO oxidation reaction and inactive for the H2 oxidation. Noble metal catalysts exhibited good catalytic performance for both CO 2


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oxidation and CO PROX.

12, 14, 15

However, the high cost of noble metal catalysts

limited its application. In addition, the shortcomings such as the rapid selectivity decrease of CO oxidation at high reaction temperatures need to be solved.

16-18

Many

reports have been reported to investigate low-cost catalysts, such as the transition metal Co. Co3O4 is one of the most promising transition metal oxide catalysts for CO oxidation. 19 LaCoO3 is also an attractive perovskite-type oxides for CO oxidation due to the abundant vacancies on the surface and high mobility of lattice oxygen in the bulk. 20

Liu et al.21 suggested that the activity of CO oxidation over the hollow spherical

LaCoO3 with porous structure can be attributed to the surface areas, oxygen vacancies concentrations and reducibility. A new perovskite composition, La0.8Sr0.2Co0.8Cu0.2O3 was also found to be well suited for CO oxidation. observed in the Co3O4/LaCoO3 system.

23

22

Similar phenomena were also

The enhanced CO catalytic oxidation over

Co3O4/LaCoO3 at lower temperature originates from the surface oxygen and Co3+ on the interface. CeO2 is a good promoter for Co-based catalysts due to the Ce-Co interaction, which could accelerate the dissociation of CO on the catalyst surface.24 Besides, CeO2 has gained considerable attention in numerous catalysis reaction due to its high oxygen storage capacity (OSC) ability. 25, 26 Alifanti et al. 27 prepared a series of LaCoO3/Ce1xZrxO2

materials, used as catalysts for total oxidation of VOC. These catalysts exhibited

a higher reactivity than the pure LaCoO3 and Ce1-xZrxO2, which can be attributed to the relatively high oxygen mobility between the interface of Ce1-xZrxO2 and LaCoO3. Our previous study showed that the CeO2/LaFeO3 oxygen carrier showed high conversion and stability during the successive Chemical-looping reforming of methane testing. 28 The presence of CeO2 induces abundant oxygen vacancies on the interface of mixed oxides, promoting the oxygen mobility and reducibility. The LaMnO3/CeO2 system showed high catalytic performance at low temperatures, due to the redox cycle in Ce4+Ce3+ and Mn4+-Mn3+. 29 Three-dimensionally ordered macroporous (3DOM) materials are usually applied in gas-solid catalysis reactions due to its well-defined macroporous structures to easily 3



transport and diffuse gas. Li et al.30 suggested that 3DOM LaCoO3 catalysts showed higher catalytic activities for CO oxidation than the nonporous ones. 3DOM catalysts can also be used as a catalyst support. Wei et al.

31, 32

reported that both 3DOM Ce-

based and La-based support present a better thermal ability and improve the dispersion of active content. Based on the above discussions, the combination of LaCoO3 with CeO2 containing 3DOM structure may become a potential catalyst for CO-PROX to remove the CO in the H2 rich gas. Herein, we successfully prepared a series of 3DOM CeO2/LaCoO3 samples, and the CeO2 nanoparticles were uniformly dispersed on the 3DOM LaCoO3 support. These 3DOM CeO2/LaCoO3 catalysts were expected to own better reactivity than the pure LaCoO3 and CeO2 catalysts, and showed superior catalytic conversion, selectivity, and stability for CO PROX in H2-rich gases. We performed DRIFTS measurements during the reaction, focusing on the role of side or intermediate products under reaction conditions and on possible reaction-induced modifications of the CeO2 particles. The synergistic effect between LaCoO3 and CeO2 in CO-PROX gain insights into the LaCoO3-CeO2 interaction. Based on the discussions, the synergistic effect and OH species significantly improved the activity of the 3DOM CeO2/LaCoO3 catalysts.

2. Experimental 2.1. Catalyst preparation 2.1.1. Synthesis of 3DOM LaCoO3 supports The 3DOM LaCoO3 supports were synthesized via colloidal crystal template method according to previous procedures. 33, 34 2.1.2. Preparation of 3DOM xCeO2/LaCoO3 catalysts Appropriate amount of Ce (NO3)3·6H2O solutions were diluted with deionized water. A certain amount of LaCoO3 support was mixed with the Ce (NO3)3 solution under stirring for 8 h. The aqueous solution was firstly filtered and then washed with deionized water. Finally, the crude material was dried at 100 ºC for 24 h, and then calcined at 300, 450 and 600 ºC for 2 h, respectively, obtaining the 3DOM 4


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CeO2/LaCoO3 catalysts with different CeO2 sizes. The obtained CeO2 sizes of the CeO2/LaCoO3 catalysts were 1.6 nm (calcined at 300 ºC), 2.4 nm (calcined at 450 ºC) and 3.3 nm (calcined at 600 ºC), thus the acquired mixed oxides were denoted as CeO2(1.6)/LaCoO3, CeO2(2.4)/LaCoO3 and CeO2(3.3)/LaCoO3 catalysts, respectively. The actual loading amounts of CeO2 were 1.7% for the three samples. For comparison, the 3DOM CeO2/LaCoO3 catalysts with different CeO2 amount were prepared following the similar procedures. A certain amount of LaCoO3 support was mixed with the Ce(NO3)3 solution under stirring for 4 and 12 h, respectively. The dried precursor were calcined at 450 ºC for 2 h, obtaining the 3DOM CeO2/LaCoO3 catalysts with different CeO2 amount. The acquired samples with different CeO2 amount (0.9% and 2.8%) were referred to as CeO2 (0.9%)/LaCoO3 and CeO2 (2.8%)/LaCoO3. The obtained CeO2 sizes of the CeO2/LaCoO3 catalysts were about 2.4 nm.

2.2. Physical and chemical characterization The morphology, particle sizes and dispersion of the catalysts were analyzed by scanning electron micrographs (SEM, NOVA NANOSEM 450) and transmission electron microscopy (TEM, JEOL JEM-2100). X-ray powder diffraction (XRD) patterns of the catalysts were taken on a Rigaku diffractometer using Cu Ka radiation (wavelength: 1.54 Å). The CeO2 loading content was measured by Inductively coupled plasma-optical emission spectrometry (ICP-OES) analyses, performed on PerkinElmer Optima 3100 XL spectrometer. The surface area of the samples were performed on a NOVA instrument (Quantachrome, American) using Ar as carrier and N2 as adsorbent, determined by Brunauer-Emmett-Teller (BET) method. X-Ray photoelectron spectra (XPS) were acquired on a PHI 5000 Versaprobe II with a Al-Ka source. Binding energies were referenced to the C 1s peak at 284.8 eV. Hydrogen temperature-programmed reduction (H2-TPR) experiments were carried out on ChemBET Pulsar TPR (Quantachrome). After pretreatment, TPR was carried out by passing of 10% H2/Ar up to a final temperature 900 ºC. Oxygen Temperature programmed oxidation (O2-TPO) was analyzed with a mass spectrometer (QGA, Hiden 5



Analytical). After the H2-TPR experiments, the TPO experiments was performed in 10%O2/Ar up to 900 ºC. The CO-TPR experiments was carried out on the CATLAB instruments (Hiden Analytical). The pretreated sample was exposed to the reduction gas compositions (10%CO/Ar), heated to 600 ºC. CO-and O2-temperature programmed desorption (CO-TPD and O2-TPD) were also conducted using the CATLAB microreactor system. After a cleaning pretreatment, the sample was passed through 10%CO/Ar or 10%O2/Ar for 1h at RT. After removing CO or O2 in gas phase by Ar purge for 30 min, the pretreated sample was heated up to 600ºC. In Situ DRIFTS measurements were executed on a FTIR spectrometer (vertex 70, Bruker) using a in situ DRIFTS reaction cell (HC-900, Pike Technologies). The catalyst powders were pressed into the DRIFTS cell equipped with BaF2 windows. To remove the possible residual surface species prior to testing, each sample was heated at 300 ºC for 1 h in Ar. In CO PROX experiments, the pretreated sample was carried out in a mixture gas (1.67%CO+1.67%O2+50%H2) balanced with Ar and then ramped up to 200 ºC. The spectra were recorded as a function of the temperature. In CO oxidation experiments, the pretreated sample was exposed to the reaction mixture 1.67%CO/1.67%O2/Ar up to 200 ºC for 30 min. After that, the 50%H2/Ar was injected for 15 min at 200 ºC.

2.3. Catalytic Activity The CO PROX reaction in the H2-rich gasses was carried out in a reactor consisting of a quartz tube. Before testing, the catalyst was pretreated in pure Ar at 300 ºC for 1 h. After pretreated, the gas mixture (1.67%CO, 1.67%O2, 50%H2 in balance Ar) was allowed to pass through the reactor at RT, heated up to 300 ºC. CO PROX experiments were also conducted to test the effects of weight hourly space velocity (WHSV), and the WHSV were 27000, 36000 and 45000 cm3 g (cat)-1 h-1, respectively. The compositions of gaseous products were measured with a gas chromatograph (Agilent 7890A GC). The activities of the catalysts were evaluated on the basis of CO conversion. 6


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The selectivity of CO PROX was defined as the fraction of O2 consumption used for the CO oxidation to CO2 over the total O2 consumption. The equations are as follows: COconversion (%) =

[CO]in ― [CO]out [CO]in

O2 selectivity to CO2 (%) =

× 100

[CO]in ― [CO]out 2([O2]in ― [O2]out)

(1)

× 100

(2)

where COin and O2in represent the concentration of CO and O2 at the inlet, respectively, COout and O2out are the concentration of CO and O2 at the outlet, respectively.

3. Results The morphology and structures of 3DOM LaCoO3 and CeO2/LaCoO3 were investigated using SEM and TEM technologies, as shown in Figure. 1. Figure.1 A1-A2 shows the SEM images of 3DOM LaCoO3 support. The SEM images reveal that 3DOM LaCoO3 support possesses a well-defined interconnected macroporous structure with a pore size of ∼200 nm. The HRTEM images (insert) showed that the lattice spacing (d value) of the LaCoO3 phase is about 0.271-0.272 nm, which can be attributed to the (110) crystal plane of the LaCoO3. The XRD patterns of the LaCoO3 support are presented in Figure 1A3. It can be observed that the sample possess a perovskite LaCoO3 structure (JCPDS PDF 48-0123), and no diffraction peaks assignable to La or Co oxides appeared. The TEM images shows that CeO2 particles with different sizes were highly dispersed on the surface of 3DOM LaCoO3 support. The particle size distribution showed that the average CeO2 sizes were 1.6, 2.4 and 3.3 nm, respectively. In addition, the lattice fringes of CeO2 with a width of 1.9 Å indexed to (111) plane 35 can be found in all the samples.

7



Figure. 1. Morphological and macrostructural characterizations of the 3DOM LaCoO3 and CeO2/LaCoO3. (A1, A2) SEM images of 3DOM LaCoO3 (insert image: the HRTEM image of LaCoO3 ). (A3) XRD patterns of 3DOM LaCoO3. (B1-B3) TEM images of CeO2(1.6)/LaCoO3. (C1-C3) TEM images of CeO2(2.4)/LaCoO3. (D1-D3) TEM images of CeO2(3.3)/LaCoO3.

The catalytic activity of the samples was evaluated in the CO-PROX reaction. The CO conversion of the samples were shown in Figure. 2A. The CO conversion of the pure LaCoO3 catalyst reached to 90% at 209 ºC, and then kept stable. For the series of CeO2/LaCoO3 samples, the CO conversion increased with the temperature and the 8


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samples with smaller CeO2 particles displayed better catalytic activities. Both CeO2(1.6)/LaCoO3 and CeO2(2.4)/LaCoO3 could realize 90% CO conversion at 150 ºC. However, when it reached to 200 ºC, the CO conversion of the catalysts decreased, especially the CeO2(1.6)/LaCoO3. When the temperature reached to 250 ºC, the CO conversion of the CeO2(1.6)/LaCoO3 catalyst decreased obviously due to the competitive combustion of hydrogen.

Figure. 2. (A) CO conversion and (B) O2 selectivity to CO2 over (a) LaCoO3, (b)CeO2 (1.6)/LaCoO3, (c) CeO2 (2.4)/LaCoO3 and (d) CeO2 (3.3)/LaCoO3 during PROX. Reaction conditions: 1.67 vol% CO, 1.67 vol% O2, 50 vol% H2, and balance Ar. WHSV (weight hourly space velocity): 36,000 cm3 g(cat)-1 h-1. (C) CO2 and (D) H2O trend during temperature-programmed PROX reaction over (a) LaCoO3, (b) CeO2(1.6)/LaCoO3, (c) CeO2(2.4)/LaCoO3 and (d) CeO2(3.3)/LaCoO3 in the presence of 1.67% CO, 1.67 % O2, 50% H2, and balance Ar. WHSV (weight hourly space velocity): 36,000 cm3 g(cat)-1 h-1.

Figure. 2B shows the selectivity of O2 to CO2 during the PROX reaction. The maximum of 100% O2 selectivity is achieved for all the samples at lower temperature (< 205 ºC). With the increasing of the reaction temperatures, the O2 selectivity to CO2 9



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decreased sharply. At 300 ºC, the O2 selectivity to CO2 for the CeO2 (2.4)/LaCoO3 catalyst dropped to 90%, while only 82%, 81% and 80.5% O2 selectivity are obtained at the same temperature for CeO2(2.4)/LaCoO3, CeO2(3.3)/LaCoO3 and LaCoO3, respectively. The CeO2(2.4)/LaCoO3 catalyst with an appropriate CeO2 particle size exhibited the highest catalytic activity in both CO conversion and O2 selectivity to CO2. To examine the possible side reactions during the PROX reaction over the CeO2 /LaCoO3 catalyst, a temperature-programmed reaction experiment was conducted. As shown in Figure. 2C-D, only two products were observed during the course of the experiment: CO2 and H2O. By following the product signals, the process can be divided into three distinct regions. First, the CO2 signals increased in a temperature range of 50200 ºC, which is attributed to the oxidation of CO. The amounts of produced CO2 over the series of CeO2/LaCoO3 catalysts were higher than the pure LaCoO3. In the second stage, the H2O was produced in a temperature range of 200-260 ºC due to the conversion of H2 to H2O. In this stage, smaller amounts of H2O were produced over the pure LaCoO3 than the loaded CeO2/LaCoO3 catalysts. The amounts of CO2 over the LaCoO3 and

CeO2(3.3)/LaCoO3

kept

stable,

while

the

produced

CO2

over

the

CeO2(1.6)/LaCoO3 and CeO2(2.4)/LaCoO3 decreased slowly. When the temperatures higher than 260 ºC, the H2O signals over the series of CeO2/LaCoO3 catalysts increased sharply, especially CeO2(3.3)/LaCoO3, while there was no obvious change over the pure LaCoO3. The above results indicated that compared with the series of CeO2/LaCoO3 catalysts, the LaCoO3 is inactive for both CO and H2 oxidation. Figure. 3 showed the H2-TPR (A1) and cumulative H2 uptake (A2) of the CeO2/LaCoO3 catalysts. For pure LaCoO3, two major reduction peaks signed as β and γ are detected in the H2-TPR profile: a low temperature range from 330 to 525 ºC and a high temperature peak in a range of 525-710 ºC. The first reduction peak in the low temperature region (denoted β) is suggested to be caused by the reduction of surface Co3+ to Co2+, and the second one in the high-temperature range (denoted γ) was caused by the reduction of Co2+ to Co0. 30, 36, 37 In addition, the γ peak (located at ~630 ºC) can also be attributed to the reduction of surface CeO2 38. CeO2 (2.4)/LaCoO3 showed a 10


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lower peak temperature, indicative of relatively high oxygen mobility. The total amount of H2 consumption over LaCoO3 was 0.59 mol/g, as shown in Figure. 3A2. After loading of CeO2 particles, the two reduction peaks of the LaCoO3 catalyst shifted towards the lower temperature. This phenomenon revealed that CeO2/LaCoO3 catalysts exhibited better reducibility than the pure LaCoO3 sample. For the series of CeO2/LaCoO3 catalysts, the two reduction peaks of CeO2(2.4)/LaCoO3 showed a lower temperature. However, the CeO2(1.6)/LaCoO3 showed a reduction peaks below 300 ºC (labelled as α), which can be correlated to the reduction of the chemisorbed oxygen species. 30, 39 The cumulative H2 uptake of the three CeO2/LaCoO3 catalysts followed the order: CeO2(2.4)/LaCoO3 (0.67 mol/g) > CeO2(1.6)/LaCoO3 (0.63 mol/g) > CeO2(3.3) /LaCoO3 (0.62 mol/g).

Figure. 3. H2-TPR profiles (A1) and cumulative H2 uptake (A2) over (a) LaCoO3, (b) CeO2 (1.6)/LaCoO3, (c) CeO2(2.4)/LaCoO3 and (d) CeO2(3.3)/LaCoO3. CO, CO2 (B1) and H2 (B2) evolution during CO-TPR over (a) LaCoO3, (b) CeO2(1.6)/LaCoO3, (c) CeO2 (2.4)/LaCoO3 and (d) CeO2 (3.3)/LaCoO3. (C) CO2 evolution during CO-TPD process over (a) LaCoO3, (b) CeO2 (1.6)/LaCoO3, (c) CeO2 (2.4)/LaCoO3 and (d) CeO2 (3.3)/LaCoO3.

Figure. 3B shows CO2 profiles evolved during CO-TPR of the samples. All the 11



samples exhibited three major reduction peaks with the increase of the temperature: a low temperature range from 220 to 350 ºC, a middle temperature range from 350 to 425 ºC, and a high temperature peak in a range of 425-600 ºC. Unlike the reduction in H2 atmosphere, the reduction peaks of the samples during CO-TPR located at a lower temperature than that in the H2-TPR process. This phenomenon indicated that the samples was ready to react with CO to produce CO2. The CeO2(1.6)/LaCoO3 sample showed a weak shoulder peaks at 186 ºC, which can be attributed to the reduction of the chemisorbed oxygen species. And the oxygen species on the surface of CeO2 is more reactive for promoting CO oxidation. However, except the major product CO2, another gas product H2, is also detected over all the samples (see Figure. 3B2). The H2 is formed due to water-gas shift reactions 15, 40, 41,

as the following reactions:

CO + OH→1/2H2 + CO2

(3)

CO + 2OH→H2 +CO2+OL

(4)

The major peaks (~429 ºC) of the LaCoO3 sample was higher than that of the other three CeO2/LaCoO3 samples. The CeO2(2.4)/LaCoO3 sample showed a weaker shoulder peak at 306 ºC, which was not observed on the other two CeO2/LaCoO3 samples. It can be deduced that the CeO2(2.4)/LaCoO3 sample own a higher activity for water-gas shift reactions. Figure. 3C shows the CO2 evolution during CO-TPD process over the CeO2/LaCoO3 catalysts. There was essentially no CO desorption observed from all the samples during TPD process, and CO2 is the major product, due to the thermal decomposition of surface carbonate species. There are two major temperature range for the desorption of CO2: a low temperature in the range of 50-170 ºC and a high temperature range from 170 to 600 ºC, indicating the existence of both weakly and strongly held carbonate species on these two surfaces.

42

The CeO2(2.4)/LaCoO3 sample showed a much stronger CO2

peak in the high temperature range than the other samples, suggesting that there was a stronger interaction between CeO2(2.4)/LaCoO3 and CO. 12


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To investigate the desorption of the oxygen species (such as carbonate species, OH species……), the O2 temperature-programmed desorption (O2-TPD) in helium was conducted over the CeO2/LaCoO3 samples. The evolution of both CO2 and H2O were detected by online mass spectra, but there was no O2 desorption observed from the samples during O2-TPD process (see Figure. S2). CO2 was the major product, due to the thermal decomposition of adsorbed carbonate species on the surface of the catalysts. As compared in Figure. 4A, the intensity of CO2 decreased in the following order: CeO2 (1.6)/LaCoO3 > CeO2(2.4)/LaCoO3> CeO2(3.3)/LaCoO3> LaCoO3. It indicates that CeO2 (1.6)/LaCoO3 own larger amount of surface carbonate species. The H2O product is mainly originated from the thermal decomposition of the OH species. It can be seen from the Figure. 4B, little amount of H2O were detected on the CeO2(3.3)/LaCoO3 and LaCoO3 samples. However, there were more OH species on the surface of the CeO2 (1.6)/LaCoO3 and CeO2(2.4)/LaCoO3. The experiments of H2-TPD were also performed, and the results were shown in Figure. S2. Nearly no H2 was detected among the four samples. A

B The Intensity of H2O

The Intensity of CO2

O2-TPD

a b c d

d c b a

200

300 400 500 Temperature (oC)

H2-TPR

  

C b

4th 3th 2th   

a

4th 3th 2th

200

400 600 Temperature (oC)

100

600

D b The Intensity of O2

100

TCD signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

200

300 400 500 Temperature (oC)

600

O2-TPO 4th 3th 2th

242

a

349

200

400 600 Temperature (oC)

4th 3th 2th

800

Figure. 4. CO2 (A) and H2O (B) evolution during O2-TPD process over (a) LaCoO3, (b) CeO2(1.6)/LaCoO3, (c) CeO2 (2.4)/LaCoO3 and (d) CeO2 (3.3)/LaCoO3. The H2-TPR (C) and O213



TPO (D) recycled profiles over LaCoO3 (a) and CeO2 (2.4)/LaCoO3 (b).

It can be seen from the H2-TPR recycled profiles (Figure. 4C) that after 4 redox cycles, the reduction peaks of the LaCoO3 sample shifted to higher temperature, while the reduction peaks of the CeO2(2.4)/LaCoO3 sample shifted to lower temperature. During the recycled O2-TPO process, the samples showed a big oxygen consumption peak located at 349 and 242 ºC over LaCoO3 and CeO2(2.4)/LaCoO3, respectively. This phenomenon suggest that the reduced CeO2(2.4)/LaCoO3 samples can be rapidly restored by O2 at lower temperatures. The introduction of the CeO2 promote the renewability of the pure LaCoO3 in the oxidation atmosphere. No obvious change was observed on the two samples during the recycled O2-TPO process, indicating a good thermal stability. The results of the H2-TPR& O2-TPO redox cycles revealed that the CeO2(2.4)/LaCoO3 sample owned better reducibility and oxygen mobility than the pure LaCoO3. XPS was performed to determine the chemical state of O and Co species on the surface of different samples, the results were shown in Figure. 5. The O 1s spectrum of the LaCoO3 could be fitted with two primary features at ca. 531.3 eV (labelled as OI) and ca. 528.7 eV (labelled as OII), respectively. It is generally accepted that the peak at 531.3 and 528.7 eV can be attributed to the adsorbed oxygen (Oads) and lattice oxygen (Olatt).30, 43 The surface-Oads (OI)/bulk-Olatt(OII) molar ratios are calculated based on the XPS data. As shown in Table 1, the ratio decreases in the order of CeO2 (1.6)/LaCoO3 > CeO2(2.4)/LaCoO3> CeO2(3.3)/LaCoO3> LaCoO3. This results indicated that the introduction of CeO2 enhanced the amount of adsorbed oxygen species, which can be attributed to the synergistic effect between LaCoO3 and CeO2. This phenomenon is consistent with the results of O2-TPD, suggesting that more oxygen species adsorbed on the surface of CeO2/LaCoO3, especially the samples with smaller CeO2 particles sizes.

14


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Figure. 5. O 1s and Co 2p XPS patterns of the different samples: (a) LaCoO3, (b) CeO2 (1.6)/LaCoO3, (c) CeO2 (2.4)/LaCoO3 and (d) CeO2 (3.3)/LaCoO3.

The Co 2p spectrum shows two peaks assigned to the doublet 2p3/2 and 2p1/2, at 781 and 795.6 eV, respectively. The asymmetrical Co 2p3/2 signal at ca. 781 eV can be decomposed into two components at 780.0 and 781.8 eV, ascribable to the surface Co3+ and Co2+ species. 23 The intensity of the Co3+ peak was largely enhanced after the Ce doping. Table 1 presents the relative percentages of the Co3+ species calculated by the area ratios of the Co3+/Co2+. Among all the samples, the CeO2 (2.4)/LaCoO3 exhibits the highest ratio of Co3+, which plays a very important role for determining the catalytic property. Table 1. Surface chemical composition of the LaCoO3 and CeO2/LaCoO3 obtained by XPS. Samples

Surface element composition Oads

Olatt

Oads /Olatt

Co3+

Co2+

Co3+/Co2

LaCoO3

0.550

0.450

1.22

0.625

0.375

1.67

CeO2(1.6)/LaCoO3

0.650

0.350

1.86

0.580

0.420

1.38

CeO2(2.4)/LaCoO3

0.638

0.362

1.76

0.651

0.349

1.87

CeO2(3.3)/LaCoO3

0.561

0.439

1.28

0.552

0.448

1.23

15



Figure. 6. Morphological and macrostructural characterizations of the 3DOM CeO2/LaCoO3. (A1A3) TEM images of CeO2 (0.9%)/LaCoO3. (B1-B3) TEM images of CeO2 (2.8%)/LaCoO3.

Figure. 6 shows the TEM images of the CeO2/LaCoO3 samples with different CeO2 content. For these two samples, the average size of the CeO2 nanoparticles was in the range of 2.2-2.5 nm, which was similar to that of the CeO2(2.4)/LaCoO3. The TEM images of CeO2(0.9%)/LaCoO3 (Figure. 6A1-A3) show that CeO2 nanoparticles were highly dispersed on the 3DOM LaCoO3 surface. While for the CeO2 (2.8%)/LaCoO3 sample, some CeO2 particles agglomerated (Figure. 6B1-B3). The reduction properties of the CeO2/LaCoO3 samples with different CeO2 content were also evaluated using H2-TPR, and the results are shown in Figure. 7A. All the samples showed two major reduction peaks at ~450 and ~635 ºC. No noticeable increase in the intensity of the two peaks was observed when the CeO2 content increased from 0.8% to 1.7%. However, when the CeO2 content reached to 2.8%, all the reduction peaks shifted towards the higher temperature and the intensity of the peaks decreased. Among the three samples, CeO2(2.4)/LaCoO3 with the appropriate CeO2 content (2.1%) exhibited the highest reducibility.

16


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Figure.7. H2-TPR profiles (A1) and cumulative H2 uptake (A2) over (a) CeO2 (0.9%)/LaCoO3, (b) CeO2 (2.4)/LaCoO3 and (c) CeO2 (2.8%)/LaCoO3. CO, CO2 (B1) and H2 (B2) evolution during COTPR over (a) CeO2 (0.9%)/LaCoO3, (b) CeO2 (2.4)/LaCoO3 and (c) CeO2 (2.8%)/LaCoO3.

Figure. 7B shows CO2 profiles evolved during CO-TPR of the samples. All the samples exhibited three major reduction peaks with the increase of the temperature: a low temperature (located at ~306 ºC), a middle temperature (located at ~379 ºC) and a high temperature (located at ~556 ºC) peak. With the increase of the CeO2 content, the peaks shifted towards higher temperature. While the CeO2(2.4)/LaCoO3 sample possessed the highest peak area. As for the H2, the intensity of the peaks at middle temperatures (~400ºC) was strongly weakened by the increase of the CeO2 content, while the high temperature peak (~580 ºC) was enhanced. Only CeO2 (2.4)/LaCoO3 sample exhibited a lower temperature peak at 306 ºC.

17



Figure. 8. (A1) CO conversion over (a) CeO2 (0.9%)/LaCoO3, (b) CeO2 (2.4)/LaCoO3 and (c) CeO2 (2.8%)/LaCoO3 during PROX. Reaction conditions: 1.67 vol% CO, 1.67 vol% O2, 50 vol% H2, and balance Ar. WHSV (weight hourly space velocity): 36,000 cm3 g (cat)-1 h-1. (A2) Effect of WHSV on the catalytic activity of the CeO2 (2.4)/LaCoO3 during PROX: (d) WHSV =27,000 cm3 g (cat)-1 h-1, (e) WHSV =36,000 cm3 g (cat)-1 h-1, (f) WHSV =45,000 cm3 g (cat)-1 h-1. (B) CO conversions and O2 selectivity to CO2 as a function of reaction time over the CeO2 (2.4)/LaCoO3 catalyst during PROX. Reaction conditions: 1.67 vol% CO, 1.67 vol% O2, 50 vol% H2, and balance Ar. WHSV (weight hourly space velocity): 36,000 cm3 g(cat)-1 h-1, and reaction temperature = 150 ºC. (C) Comparison of XRD patterns over fresh and cycled CeO2 (2.4)/LaCoO3 after 100 h treatment. (D) SEM images of the CeO2 (2.4)/LaCoO3 after 100 h treatment.

Figure. 8A1 shows the effect of CeO2 content on CO conversion during the steadystate PROX reaction. The CeO2(0.9%)/LaCoO3 with a lower CeO2 loading achieved the maximum CO conversion at 150 ºC. With the increasing of the CeO2 loading amount, the CeO2(2.4)/LaCoO3 with 1.7% CeO2 loading content, gained a better activity. However, when the CeO2 content reached to 2.8%, the catalytic activity decreased. It can be attributed that the agglomerated CeO2 particles reduced the reactivity. To investigate the effect of SV, we examined the catalytic activities of the bestperforming CeO2 (2.4)/LaCoO3 sample at different WHSV values, as shown in Figure. 18


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8A2. Apparently, the CO conversion of CeO2 (2.4)/LaCoO3 decreased at elevated SV values. In order to evaluate the catalytic stability, 100 h reaction experiments over CeO2 (2.4)/LaCoO3 for PROX were conducted, as shown in Figure. 8B. It is apparent that there was a slight loss in CO conversions within 100 h of reaction, while there is little loss in the O2 selectivity to CO2. This phenomenon indicated that the CeO2(2.4)/LaCoO3 sample was catalytically durable. The effect of the cycling treatment on the structural stability and reducibility of the catalyst was also investigated. Figure. 8C shows the XRD patterns of fresh and cycled CeO2(2.4)/LaCoO3 sample. No obvious evolution on the structure and new phase of the cycled CeO2(2.4)/LaCoO3 was observed after the long term cycling at 150 ºC. However, the crystallite sizes increased a little after the cycled reaction (see Table S1). It can be seen from the SEM image (Figure. 8D), the cycled treatment did not induce significant changes in the 3DOM architecture of the CeO2(2.4)/LaCoO3, indicating a good thermal stability. This result is consistent with the results in the H2-TPR & O2-TPO redox cycles (Figure. 4C-D).

4. Discussions 4.1. The higher selectivity Noble metal catalysts exhibited good catalytic performance for both CO oxidation and CO PROX. However, the noble metal catalysts are also active for the H2 oxidation (produce H2O). Denkwitz et al.18 proposed that the selectivity for CO oxidation over Au/TiO2 catalysts decreases sharply with increasing temperature due to the increasing tendency for H2 oxidation. Similar phenomenon was also observed on the Pt-Au/CeO2 catalysts during CO PROX. 44 The 3DOM CeO2/LaCoO3 catalysts were active for the CO oxidation reaction, meanwhile inactive for the undesired H2 oxidation. The CO oxidation over CeO2 (2.4)/LaCoO3 sample reached to 90% at 150 ºC, while the H2 oxidation started at 205 ºC, which is much higher than the T90 of CO oxidation ( see Figure. 2A-B). The major product CO2 and H2O during the CO PROX can also be detected by the mass spectrum (see Figure. 2C-D). It can be seen that when the 19



temperature is lower than 205 ºC, nearly no H2O produced. However, the CO2 have already reached to the maximum at 200 ºC. It can be seen from the H2-TPR profiles (see Figure. 3A) that the H2 began to be oxidized at 330 ºC, and the two cumulative H2 uptake peaks located at ~430 and 640 ºC. As for the CO-TPR process, CO began to be oxidized at 200 ºC, and the three major reduction peaks located at ~306, ~379 and 556 ºC (see Figure. 3B). This phenomenon indicated that the CO were more active over 3DOM CeO2/LaCoO3 catalysts than that of H2. Although the adsorbed oxygen could enhance the reducibility (observed by H2-TPR and CO-TPR, as shown in Figure. 3AB, respectively), too much adsorbed oxygen over CeO2(1.6)/LaCoO3 accelerated the H2 oxidation, reducing the O2 selectivity to CO2 (see Figure. 2B). However, CeO2(2.4)/LaCoO3 with the appropriate amounts of adsorbed oxygen, exhibiting the highest catalytic performance in both CO conversion and O2 selectivity to CO2.

4.2. Dominant factors for CO PROX over LaCoO3-based catalysts The textural properties, such as specific surface area could influence the catalytic performance for CO PROX. It is reported that the higher specific surface area can enhance the catalytic performance for CO PROX.12,45 The specific surface area could also enhance the activities for other catalytic reaction, such as CO oxidation and selective catalytic reduction of NOx, through improving the dispersion of the active components.46-48 Figure. S3 showed that the surface area of CeO2-LaCoO3 samples decreased in the order: CeO2(1.6)/LaCoO3> CeO2(2.4)/LaCoO3> CeO2(3.3)/LaCoO3> LaCoO3. However, the surface area of the CeO2-LaCoO3 catalyst was in a narrow scope, ranged from 10 to 20 m2/g. The catalyst with higher surface area exhibited a higher reactivity. In addition, both the particle size and the loading amount of CeO2 have effect on the catalytic activity of CO PROX (see Figs. 2 and 8A). The addition of CeO2 oxides to perovskite-type could modify the concentration of oxygen vacancies, which play an important role on the catalytic performances. Soykal et al. 49 have investigated the effect of support particle size over Co/CeO2 catalysts. It revealed that the particle size of CeO2 play an important role on the catalytic performances due to the oxygen vacancies.

4.3 The interaction between CeO2 and LaCoO3 20


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The oxygen vacancies were induced by the interaction of the CeO2 and LaCoO3. Alifanti et al.50 prepared a series of LaCoO3/Ce1-xZrxO2 catalysts, which exhibited high activity for the total oxidation of VOC. The improved activity can be attributed to oxygen mobility (or oxygen vacancies). The results in the present work reveal a similar phenomenon. The O2-TPD results (see Figure. 4) showed that more adsorbed oxygen species existed on the surface of CeO2/LaCoO3, especially the samples with smaller particle size of CeO2. The XPS results (see Figure. 5) is consistent with the O2-TPD results, and it indicates that more oxygen vacancies are detected on the surface of CeO2/LaCoO3 compared with that of pure LaCoO3 sample, resulting in relatively high reducibility and oxygen mobility (observed by H2-TPR and CO-TPR, as shown in Figs. 3A-B, 4C and 7A-B, respectively). Liu et al.23 have investigated the enhanced CO catalytic oxidation over Co3O4/LaCoO3 at lower temperature. It revealed that the enhanced activity originates from the surface oxygen vacancies of LaCoO3/Co3O4 and Co3+ in the Co3O4 nanoparticles. As the content of Co3+ species located on the surface of spinel Co3O4 increased, the capability of LaCoO3 for CO oxidation was enhanced. Similar phenomenon was observed on the CoOx/CeO2 catalysts. 12 It also revealed that the Co3+ sites are the active sites for oxidation of carbon monoxide. The XPS results showed that CeO2(2.4)/LaCoO3 owned higher concentration of Co3+ (see Table1), exhibiting better catalytic performances. The Co3+ sites were formed on the surface of the catalyst. There are difference among the CeO2/LaCoO3 samples on the amount of Co3+ sites, mainly due to the interaction of CeO2 and LaCoO3. The existence of Co3+ promoted the formation of oxygen defect and the oxygen transfer ability. In addition, the CeO2/LaCoO3 samples exhibited better reducibility than the pure LaCoO3. The interaction modifies the surface properties enhancing the reactivity of CO PROX.

4.4. The reaction mechanism As observed in the CO-TPR process (see Figs. 3B and 7B), surface water-gas shift reaction between CO and surface OH groups was detected. It can be seen that the H2 over CeO2/LaCoO3 is produced at a lower temperature compared with the pure LaCoO3. Jardim et al.

51

reported that the presence of surface OH groups on the catalyst could 21



enhance the catalytic activity for CO oxidation. Lin et al.52 also demonstrated that the CO and OH adsorbed on the surface of catalyst need a lower activation energy than that of CO and O, and the OH groups can react with CO directly. We also proved the routes of CO with OH species to CO2 from DRIFTS spectra, as shown in Figure. 9. Figure. 9A shows the DRIFTS spectra obtained during PROX as the samples are heated in the reactant mixture. The bands of OH groups at ~3745 and 3225 cm-1 52, 53 decreased with the increase of the temperature (

Enhanced activity for CO preferential oxidation via the CeO2-LaCoO3

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