Metal-support Interactions in CeO2 and SiO2 Supported Cobalt

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Surfaces, Interfaces, and Applications

Metal-support Interactions in CeO2 and SiO2 Supported Cobalt Catalysts: Effect of Support Morphology, Reducibility and Interfacial Configuration Zhongqi Liu, Junhao Li, Michael Buettner, Rajagopalan Ranganathan, Mruthunjaya Uddi, and Ruigang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02455 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Materials & Interfaces

Metal-support Interactions in CeO2 and SiO2 Supported Cobalt Catalysts: Effect of Support Morphology, Reducibility and Interfacial Configuration Zhongqi Liu,a Junhao Li,a Michael Buettner,b Rajagopalan V. Ranganathan,c Mruthunjaya Uddi,c and Ruigang Wang *a,b a Department

of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa,

AL 35487, USA. b

Center for Materials for Information Technology (MINT Center), The University of Alabama,

Tuscaloosa, AL 35487, USA c

Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487,

USA. Abstract With the increasing demand for highly efficient and durable catalysts, researchers have been doing extensive research to engineer the shape, size and even phase (e.g. hcp or fcc Co) of individual catalyst nanoparticles, as well as the interface structure between catalyst and support. In this work, cobalt oxides were deposited on ceria with rod-like morphology (CeO2NR), cube-like morphology (CeO2NC) and silica with sphere-like morphology (SiO2NS) via a precipitation-deposition method to investigate the effects of support morphology, surface defects, support reducibility, and the metal-support interactions on redox and catalytic properties. XRD, Raman, XPS, BET, H2-TPR, O2-TPD, CO-TPD, TEM and TPR/TPO cycling measurements have been mainly employed for catalysts characterization. Compared with CeO2NC and SiO2NS supports, as well as CeO2NC and SiO2NS supported cobalt catalysts, CeO2NR counterparts exhibited enhanced reducibility and CO

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oxidation performance at lower temperature. Both the apparent activation energy and CO conversion demonstrated the following catalytic activity order: 10 wt.% CoOx/CeO2NR > 10 wt.% CoOx/CeO2NC > 10 wt.% CoOx/SiO2NS. These results showed a strong support-dependent reducibility, CO oxidation and redox cycling activity/stability of the as-prepared catalysts. Moreover, the significantly enhanced catalytic CO oxidation of 10 wt.% CoOx/CeO2NR catalyst indicated the vital role of CeO2NR support with rich surface oxygen vacancies, superior oxygen storage capacity and mobility, and excellent adsorption/desorption behavior of CO and O2 species. Keywords: Cobalt-based catalyst; support effect; O2-TPD; CO-TPD 1. Introduction Oxide supported transition metal catalysts are widely used in heterogeneous catalysis such as in Fischer-Tropsch synthesis 1, ethanol steam reforming 2, removal of diesel soot particulates 3, and carbon monoxide oxidation 4, because of their outstanding catalytic performance with low cost compared to supported precious metal catalysts. Among various transition-metal catalysts, cobalt is of particular interest, and has shown great catalytic effectiveness due to the circular change of different valence states in Co/CoO/Co2O3/Co3O4, easy electron transfer, and rich surface vacancies in cobalt oxides. For example, many researchers have demonstrated that cobalt oxides are highly efficient electrocatalysts for oxygen evolution reaction (OER) in alkaline condition 5, and cobaltbased catalysts are also considered as an alternative catalyst replacing noble metals for selective hydrogenation of CO2 to ethanol 6. Apart from these, supported and unsupported cobalt oxides have been proved to be quite active for CO oxidation at low temperature

7-8.

Xie and coworkers

reported that nanorod-shaped cobalt oxide (Co3O4) even exhibited surprisingly high catalytic activity for CO oxidation at temperature as low as -77 °C 9.


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Meanwhile, it has been well recognized that catalyst supports can not only improve the dispersion and thermal stability of catalyst nanoclusters, but also interact with the catalysts (via solid state reaction, diffusion, and oxygen/electron transfer) during the sample preparation and under reaction conditions, leading to modulated catalytic activity and selectivity 10-12. A great deal of research has been devoted to synthesizing cobalt-based catalysts by depositing cobalt salts onto different types of irreducible or reducible oxide supports, such as Al2O3 13, SiO2 14, TiO2 15, ZrO2 16 or

CeO2 17, typically followed by oxidation treatment to form cobalt oxide(s) or further reduction

to form metallic cobalt or mixed valence cobalt oxides. Among numerous oxide support materials, CeO2 has been studied for decades and found great applications either as a catalyst promoter or as a reducible/active support because of its excellent oxygen storage capacity (OSC) 18-19. CeO2 can release and store lattice oxygen through a fast and reversible Ce4+/Ce3+ redox cycling (CeO2  CeO2-x + x/2O2) 20. Especially since 2005/2006, CeO2 with precisely controlled morphologies, such as nanorods (NR) 21, nanocubes (NC) 22, and nanoctahedra (NO) 23 have been fabricated via many wet chemical routes with or without surfactant addition. Since then, various shape and exposed crystal plane effects on the catalytic activity of CeO2 supported catalysts have been reported. Plenty of theoretical and experimental findings have demonstrated that controlling the morphology of support is crucial for optimizing the catalytic activity 24. Generally speaking, it was found that CeO2NR support favors a promoted low temperature activity attributed to the strong catalystsupport interactions. For example, ouyang et al. showed that Cu/CeO2NR catalyst can exhibit the highest methanol yield based on a systematic comparative study of support morphology effect 25. The Flytzani-Stephanopoulos group also reported a better water-gas shift (WGS) performance over Au/CeO2NR than Au/CeO2NC and Au/CeO2 nanopolyhedra

26.

Likewise, silica (SiO2) is often

considered to be another kind of outstanding catalyst support candidate by virtue of its large



surface area and controlled porosity, though it is assumed to be relatively inert or irreducible and exhibits weak metal-support interactions. It has been discovered that SiO2 surface typically plays an important role on the dispersion of supported metal or metal oxide, which can lead to the formation of relatively smaller catalyst particles on it, thus improving the catalytic performances 27.

Since the size effect is always accounted for another considerable impact on activity and

selectivity in heterogeneous catalysis as was reviewed by Che and Bennet 28, smaller SiO2 particles support is required so as to maximize catalyst dispersion and the number of exposed active sites. Moreover, many new approaches have been developed in order to control impregnations, metal nanostructures, as well as site homogeneity on SiO2-supported catalysts, driving SiO2-supported catalysts to get a new breath of life 29. Compared to CeO2, SiO2 has much lower surface energy and much higher oxygen vacancy formation energy, indicating unlikely oxygen exchange at the catalyst-oxide support interface. For this reason, reducible CeO2 and irreducible SiO2 were selected as two distinct supports to investigate the support reducibility effect on the catalyst-support interfacial structure and corresponding catalytic activity.

Compared to the unsupported cobalt catalysts, it has been well recognized that the thermal stability and catalytic activity of supported cobalt catalysts can be greatly enhanced. Catalytic reaction on such supported catalysts seems simple at the first glance, however, the underlying mechanism remains only preliminary, especially for clarifying the role of the structure at the interface between catalyst and support. Besides, there are numerous studies investigating the influence of precipitation agents, calcination temperature, metal dispersion, thermal stability and active species over supported cobalt catalysts

30-32,

but very limited research was conducted to

systematically examine CeO2 morphology effect on CeO2 supported cobalt catalysts and compare support reducibility effect between CeO2 and SiO2 supported cobalt catalysts. Particularly the support reducibility effect, which is related to oxygen and charge transfer at the catalyst-support


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interface, on the CoOx-support interfacial structure and catalytic activity has not been investigated. In this report, three different supports CeO2NR, CeO2NC, and SiO2NS were used to synthesize oxide supported cobalt catalysts via precipitation-deposition method, in order to understand the effects of support morphology, surface defects, support reducibility, as well as the metal-support interactions on their catalytic properties. 2. Experimental 2.1 Synthesis of support materials CeO2NR and NC supports were synthesized by a hydrothermal method as reported previously 33. Briefly, 8 mL of 6 M aqueous NaOH (VWR, 99%) was added dropwise to 88 mL of 0.1 M Ce(NO3)3·6H2O (Acros Organics, 99.5%) solution with quick stirring before the mixture was transferred into a 200 mL Teflon-lined autoclave. The hydrothermal reactions were carried out in a programmable box furnace at 90 °C for CeO2NR and 150 °C for CeO2NC both with a dwell time for 48 h, respectively. After the autoclave was cooled down to room temperature, the precipitates were collected, and then washed with deionized water and ethanol. The as-prepared powder samples were dried in a drying oven at 60 ℃ overnight. SiO2NS were synthesized via a modified Stöber method. 1.5 mL tetraethyl orthosilicate (TEOS, Acros Organics, 98%) was first dissolved in a mixture of 50 mL ethanol and 1 mL deionized water in a 100 mL beaker and heated to 50 ℃. Afterward, 1.7 mL ammonia solution (NH3·H2O, BDH, 28-30 vol.%) was added dropwise to the above solution under vigorous magnetic stirring and lasted for another 24 h at 50 ℃ to get complete hydrolysis. SiO2NS were finally obtained by placing the white solution in a drying oven at 60 ℃ overnight. 2.2 Deposition of cobalt oxide (CoOx)



CeO2- and SiO2-supported cobalt oxide catalyst samples were fabricated through a precipitation-deposition method. In a typical procedure, the as-prepared CeO2NR, CeO2NC or SiO2NS powders (0.9 g) were first suspended in 100 mL deionized water under magnetic stirring. Then, 0.4938g Co(NO3)2·6H2O (Acros Organics, 99%) was dissolved in the suspension above. During the process, 0.5 M aqueous ammonium hydroxide (NH3·H2O, BDH, 28-30 vol.%) was added dropwise into the mixture until the pH achieves ~9. After further aged at 80 ℃ for 4 h, the precipitates were filtered and washed with deionized water and ethanol. The as-prepared catalyst powders were kept in a drying oven at 80 ℃ overnight and calcined in a box furnace at 400 ℃ for 5 h. In this work, the cobalt content is based on metal loading in weight percentage (Co/(Co + CeO2 or SiO2)]wt × 100%), and CeO2NR supported cobalt oxide was denoted as 10 wt.% CoOx/CeO2NR, while CeO2NC and SiO2NS supported cobalt oxides were denoted as 10 wt.% CoOx/CeO2NC and 10 wt.% CoOx/SiO2NS, respectively. All resultant powders were further reduced in a tube furnace under 5 vol.% H2/95 vol.% He atmosphere at 400 ℃ for 5 h, and denoted as 10 wt.% Co/CeO2NRR.T., 10 wt.% Co/CeO2NC-R.T. and 10 wt.% Co/SiO2NS-R.T. 2.3 Catalyst characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Philips X’Pert MPD with Cu K radiation source (: 1.5405 Å) operating at 45 kV and 40 mA. The data was collected in a 2θ range from 10° to 90°, with a scanning step of 0.005°/s. The recorded patterns were analyzed using JADE 6.0 software to determine peak positions and average crystallite sizes through Scherer's formula. Raman spectra were acquired using a Horiba LabRAM HR 800 Raman spectrometer (equipped with the 100 LWD objective, NA=0.60) in the spectral window from 100 to 1200 cm-1. Diode-Pumped Solid-State (DPSS) laser system (Laser Quantum MPC6000) tuned at λ=532 nm


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was used for excitation. 1% filtering optics, 50 s of exposure time and 10 accumulation numbers were used for the measurements of all the samples. All Raman spectra were calibrated using a silicon single crystal wafer as the reference (520.7 cm-1) and analyzed to obtain the elemental coordination environment and concentration of defect sites (i.e., oxygen vacancy). The morphologies and sizes of different powder samples were examined with a transmission electron microscope (TEM, FEI Tecnai F20) operated at 200 kV. All of the investigated powder samples were sonicated in ethanol for 10 min and then the liquid suspension was dropped onto an ultrathin (2 nm) carbon-coated Cu grid (from Ted Pella) with a pipette. X-ray photoelectron spectroscopy (XPS) was carried out by a Kratos Axis Ultra DLD spectrometer with monochromatic Al Kα radiation under ultra-high vacuum (UHV) conditions, which had a base pressure of < 8x10−10 Torr. The photoelectron emission spectra were recorded using an Al-Kα (hν = 1486.6 eV). The carbonaceous C 1s line (284.4 eV) was used as an internal standard to calibrate the binding energies. The spectra were analyzed using CasaXPS software. The following relative sensitivity factors (RSF) were used to determine the relative concentration of the atoms: Ce 3d (RSF = 8.808), Co 2p (RSF = 3.59), O 1s (RSF = 0.78) and C 1s (RSF = 0.278). Single point BET surface area analysis of the powder catalysts was determined by N2 adsorption/desorption isotherms at liquid nitrogen temperature (-196 oC). Hydrogen temperature programmed reduction (H2-TPR) was performed using a Micrometrics AutoChemTM II 2920 with the temperature rising from ambient temperature to 900 °C with a heating rate of 10 oC/min. The gas mixture of 5 vol.% H2 and 95 vol.% He passed through the different samples (~100 mg) with a flow rate of 50 mL/min. Commercial cuprous oxide (Cu2O) was used for H2-TPR calibration in order to calculate the quantitative H2 consumption at elevated temperatures. Temperature-



programmed reduction and temperature-programmed oxidation (TPR/TPO) cycling experiments, including five TPR and four TPO, were carried out from room temperature to 350 °C under 5 vol.% H2/95 vol.% Ar or 5 vol.% O2/95 vol.% He atmosphere, respectively. Between TPR and TPO or TPO and TPR, the system was flushed with 5 vol.% O2/95 vol.% He or 5 vol.% H2/95 vol.% Ar for 1 h, respectively. Carbon monoxide and oxygen temperature-programmed desorption (CO-TPD and O2TPD) were conducted using the same instrument (Micrometrics AutoChemTM II 2920) as H2-TPR to investigate the interaction of CO or O2 with the catalyst/support surface. The powder sample in quartz U-tube was heated from room temperature to 400 °C under a helium stream (flow rate: 50 mL/min) to remove residual moisture. After the sample was cooled to room temperature, 10 vol.% CO - 90 vol.% He (for CO-TPD) or 5 vol.% O2 - 95 vol.% He (for O2-TPD) mixture gas was flowed at 50 mL/min through the sample for 60 min. Finally, the sample was heated up to 800 °C again under helium gas and the desorption behavior of CO and O2 can be monitored by a TCD detector at elevated temperatures. The oxygen storage capacity of the samples was performed on the same AutoChemTM II 2920 instrument. Generally, ca. 30 mg sample was pretreated from room temperature to 550 °C under a 10 vol.% H2/90 vol.% Ar flow (50 mL/min) and kept for 30 min. Then it was cooled down to 200 °C with He (50 mL/min) flow, and purged until the reactor baseline was established. Subsequently, 0.48 mL of 5 vol.% O2/95 vol.% He was injected into the reactor at 200 ℃ every 1.5 min in the He carrier gas (50 mL/min) until the saturated adsorption of O2 was reached. The oxygen storage capacity complete (OSCC) was determined by total consumption during the O2 pulse. 2.4 Catalytic evaluation


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The catalytic activity measurement of the catalysts for CO conversion was carried out in a downward and fixed-bed plugged glass tubular reactor system, loaded with ca. 30 mg of catalyst mixed with quartz wool. No pretreatment was applied before the catalytic activity test. The reaction gas mixture (1 vol.% CO, 20 vol.% O2 and 79 vol.% He) was fed over the catalyst at a flow rate of 38 mL/min corresponding to a Weight Hour Space Velocity (WHSV) value of 76,000 mL h-1 cat .

1g

The CO and CO2 concentration in the reactor effluent were analyzed by an online gas

chromatograph (SRI multiple gas analyzer GC). The CO conversion was calculated as follows: 𝐶𝑂 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) =

[𝐶𝑂]𝑖𝑛𝑙𝑒𝑡 ― [𝐶𝑂]𝑜𝑢𝑡𝑙𝑒𝑡 [𝐶𝑂]𝑖𝑛𝑙𝑒𝑡

× 100%

The apparent activation energies were determined using Arrhenius plot based on differential CO conversion below than 15% to minimize the effect of reactant concentration. Time on stream studies for oxidation of CO were conducted in the same conditions maintaining at a constant reaction temperature of 165 ℃ continuously for 24 h. 3. Results and discussion 3.1 Structural characterization of the catalysts The crystal structures of pure supports and supported cobalt catalysts were confirmed by powder XRD. In Fig. 1, the diffraction peaks at 2θ of 28.6°, 33.2°, 47.6°, 56.4°, 59.2°, 69.6°, 76.9°, 79.3° and 88.6°, which can be found in the XRD patterns of pure CeO2 support and CeO2 supported cobalt catalysts, correspond to a cubic CeO2 phase with typical fluorite structure (JCPDS #340394). After calcination at 400 ℃, the diffraction peaks at 2θ = 31.3°, 37.0°, 59.4° and 65.4° are consistent with the characteristic peaks of Co3O4 phase (JCPDS #42-1467) shown in Fig. 1B. After reduction treatment at 400 ℃, no diffraction peaks corresponding to metallic Co0 or cobalt oxides can be observed in Fig. 1C. Several possible explanations for this observation of missing cobaltrelated phases are: a) the reduced cobalt species are highly dispersed or scattered on CeO2 supports



(CeO2NR and CeO2NC); b) during reduction, the cobalt species are redispersed to smaller clusters from Co3O4 particles; c) the cobalt species diffused into CeO2 lattice to from Co3O4-CeO2 solid solution. For pure SiO2 support and SiO2 supported cobalt catalysts, a broad peak typical of amorphous SiO2 phase at 2θ = 23° was observed from the XRD patterns (Fig. 1A, 1B, and 1C). Surprisingly, in the XRD pattern of SiO2 supported catalyst samples after oxidation or reduction treatment, no CoOx oxides or metallic Co0 peaks could be found, but two new broad peaks appeared at ca. 2θ = 33.5° and 59.2° after cobalt loading. In literature, Trujillano et al. reported that a poorly crystallized Co (II) phyllosilicate with talc structure was synthesized by the deposition-precipitation method, and their XRD pattern was quite similar to our results 34. Also, Barbier et al. explored the “ammonia method” for preparing Co/SiO2 catalysts and provided an insight into the chemistry concerning the formation of two-dimensional phyllosilicates from the hexammine cobalt nitrate through analogy of [Ni (H2O)6−n(NH3)n]2+ (n 10 wt.% CoOx/CeO2NR > 10 wt.% Co/CeO2NC-R.T. > 10 wt.% CoOx/CeO2NC > CeO2NR > CeO2NC. These result not only confirmed the higher reducibility of CeO2NR compared with CeO2NC, but also demonstrated that the cobalt deposition and reduction treatment could further enhance the oxygen storage capacity of CeO2 support. Fig. 5(E) depicts the quantitative analysis of hydrogen consumed (below 500 ℃) during the H2-TPR experiments which is also summarized in Table 3. Obviously, all CeO2NR related samples show the highest H2 consumption values, in other words the largest amount of surface or lattice oxygen provided (released) during the H2-TPR experiment. This demonstrates that the support effect is a critical factor for the enhanced low temperature performance of catalyst via possible interfacial oxygen exchange. 3.3 Recycling stability of the catalysts In order to further evaluate the thermal recycling stability of supported cobalt oxide species, a series of reduction or redox cycling experiments including five consecutive H2-TPR runs or H2-TPR/O2-TPO cycling experiment with five H2-TPR runs and four O2-TPO runs in between



were conducted. During the consecutive H2-TPR cycling experiments, between the five TPR runs, the sample in the U-tube system was kept with helium gas flow during the cooling process to remove any residual oxygen in the U-tube. In the TPR-TPO cycling experiments, 5 vol.% O2/95 vol.% He or 5 vol.% H2/95 vol.% Ar was employed to flush the system for 1 h to ensure that residual gases from the previous run can be fully removed. For both cycling tests, the temperature was chosen up to 350 ℃ to avoid catalyst thermal deactivation. For CeO2NR supported CoOx sample, shown in Fig. 6(A), with no oxidation treatment (O2-TPO) in between, the hydrogen consumption peaks in the second TPR cycling experiments dramatically decreased, and the third to fifth TPR cycling experiments show only one small peak at ~150℃. In comparison, there is almost no reduction peak can be observed from the TPR profiles (Fig. 6(B)) for the SiO2NS supported CoOx sample after the second TPR cycling experiment. One possible explanation for the observable small reduction peak at ~150℃ might be due to the migration of lattice oxygen supplied by CeO2NR support, as the presence of CoOx can facilitate the mobility of oxygen species from CeO2

62.

It is also noticed that this small reduction peak continuously shifted to lower

temperature with the increment of cycling time, indicating that the interaction between cobalt species and CeO2NR support got strengthened due to the redox cycling treatment. For comparison, Fig. 6(C) depicts the TPR profiles of 10 wt.% CoOx/CeO2NR catalyst during TPR-TPO cycling test, the “shoulder peak” mentioned above (Fig. 5C) at 171 ℃ which is from the reduction of smaller size supported cobalt oxide clusters was observed after the first TPR run. And based on the peak assignment above, it can be rationally proposed that two kinds of CoOx species coexist in the CeO2 supported samples, one is large Co3O4 particle which undergoes α→β reduction (Co3O4 → CoO → Co), while the other is the direct reduction from small CoOx clusters to metallic Co (Fig. 5F). Besides, the α and β reduction peaks corresponding to Co3+ to Co2+ and Co2+ to metallic


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Co are retained during the reduction and re-oxidized processes. The β peak starts to show larger H2 consumption than the α peak since the second TPR run, which benefits from reduction-oxidationreduction (ROR) treatments that affect the metal-support interaction 63. Besides, the peak at around 113 ℃ belonging to the reduction of surface oxygen species adsorbed on oxygen vacancies on CeO2NR also was observed during the cycling experiments but continuously moved to lower temperature range. This phenomenon can be explained by the role of Co in facilitating great accessibility of oxygen from the CeO2NR lattice and was also reported by several previous researchers 60, 64. Similarly, with the TPR cycling result, the TPR/TPO cycling test on 10 wt.% CoOx/SiO2NS sample still shows unnoticeable hydrogen consumption even with reoxidation treatment. These results suggest a good thermal stability and cycling performance of the 10 wt.% CoOx/CeO2NR nanocatalysts in a low-temperature domain. 3.4 Catalyst activity test for CO Oxidation As a typical probe reaction, CO oxidation was carried out to assess the catalytic properties of CeO2 and SiO2 supported cobalt oxide catalysts. As is shown in Fig. 7(A), the activity of different supported CoOx catalysts follows this sequence: 10 wt.% CoOx/CeO2NR > 10 wt.% CoOx/CeO2NC > 10 wt.%CoOx/SiO2NS. It is clear that the reducible CeO2 supported cobalt oxides display lower CO conversion temperature and higher conversion efficiency compared with irreducible SiO2-supported cobalt oxide. Meanwhile, CO conversion profile also reveals the much lower half-conversion temperature (T50) of 10 wt.% CoOx/CeO2NR, compared to 10 wt.% CoOx/CeO2NC (T50 = 118 ℃ vs 143 ℃). The apparent activation energies (Fig. 7(C)) for 10 wt.% CoOx/CeO2NR, 10 wt.% CoOx/CeO2NC and 10 wt.% CoOx/SiO2NS are 51.1 kJ/mol, 53.0 kJ/mol and 73.9 kJ/mol, respectively. These results are consistent with the activation energy for the CO to CO2 oxidation reaction reported previously over 10 wt.% Co/ZrO2 catalyst as 54 kJ/mol 65. The



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stability test at 165 ℃ for the CO oxidation reaction was also performed and illustrated in Fig. 7(B), showing that the CO conversion for all the measured catalysts were nearly unchanged (93% conversion rate for 10 wt.% CoOx/CeO2NR and 89% conversion rate for 10 wt.% CoOx/CeO2NC). The surface and/or interfacial structures of the supported CoOx catalysts (Fig.9) play a vital role in the catalytic CO oxidation reaction. It was reported that CO species are adsorbed on the active sites of the supported catalysts, then the CO species migrate to the interface between the active components and support and react with the adsorbed oxygen or lattice oxygen on the surface 66.

For reducible CeO2NR and CeO2NC supported catalysts, the interaction between Co3+/Co2+ and

Ce3+/Ce4+ redox couples can supply the lattice oxygen and generate oxygen vacancies, thus giving greatly enhancement to the CO oxidation activity in comparison with the irreducible SiO2NS. Besides, due to the various surface defects (i.e. voids, vacancies) of CeO2NR

67,

O2 from the

environment can be easily trapped on oxygen vacancies and then transformed to either active oxygen species or lattice oxygen, which can be responsible for the higher activity of CeO2NR supported CoOx catalysts. 3.5 O2 and CO temperature-programmed desorption (TPD) O2-TPD was carried out to investigate the mobility of the active oxygen species, and the results are shown in Fig. 8(A). Generally speaking, there are three main characteristics peaks in the O2-TPD profiles: α (550 °C), corresponding to physically/chemically adsorbed oxygen O2, dissociated oxygen O2−/O− at the vacancy sites and bulk lattice oxygen, respectively 68. It is obvious that 10 wt.% CoOx/CeO2NR exhibits a stronger α peak, following by 10 wt.% CoOx/CeO2NC and then 10 wt.% CoOx/SiO2NS. This result suggests the larger amount of physically/chemically adsorbed O2 on CeO2NR supported CoOx sample. Moreover, the desorption amount of dissociated oxygen species at the vacancy sites is also found


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to be the largest and at lower temperature over the 10 wt.% CoOx/CeO2NR, which can reflect more oxygen vacancies on CeO2NR supported CoOx and is concordant with the observed results and discussion above. Since the first two oxygen desorption peaks (α and β) are more crucial for the CO oxidation reaction at the low temperature range (Fig. 7) according to Mars-van Krevelen mechanism, these results can provide clear evidence of the promotion effect of CeO2NR support. In other words, the enhanced low-temperature CO oxidation activity of 10 wt.% CoOx/CeO2NR should be ascribed to CeO2NR support effect as it is well-known that it possesses rich amount of oxygen vacancies and can generate more chemically adsorbed O2−/O− species. These results can also be correlated quite well to O 1s spectra analysis in the XPS results. As shown in Fig. S1, the integrated peak area of both peak α and peak β show the same trend with 10 wt.% CoOx/CeO2NR > 10 wt.% CoOx/CeO2NC > 10 wt.% CoOx/SiO2NS, in comparison to the oxygen species analysis regarding to the Oc and Ov peaks from XPS data (Fig. 3D-F and Table 2). CO-TPD measurements were also conducted to understand the CO adsorption behavior, mainly the number of the surface adsorption sites and the strength of CO chemisorption. Moreover, CO-TPD can provide supplemental information about the activity of surface oxygen species that involve in oxidizing the adsorbed CO to CO2 8, because typically, CO desorbs as CO2 during the heating process produced by the reaction of adsorbed CO with the most reactive surface oxygen species under the inert atmosphere. Fig. 8(B) illustrates the CO-TPD profiles of CeO2 and SiO2 supported CoOx catalysts. The peak region I (below 150 ℃) is assigned to the desorption of CO2 transferred from the oxidation of CO on surface oxygen and surface bicarbonate, and the peak region II from 150 ℃ to 600 ℃ can be assigned to CO2 by the conversion of surface and interface carbonates. The most intense CO2 desorption at temperatures higher than 600 °C (Peak III) is ascribed to the thermal decomposition of the carbonate species 69-70. It is obvious that the relative



areas/intensities of the Peaks I and II follow the order: 10 wt.% CoOx/CeO2NR > 10 wt.% CoOx/CeO2NC > 10 wt.% CoOx/SiO2NS, while no significant difference was observed on the desorption temperature. This trend gives the evidence that 10 wt.% CoOx/CeO2NR catalyst possesses more adsorption sites for CO, and further confirms the observation from the XPS spectra that the presence of more Co3+ on 10 wt.% CoOx/CeO2NR catalyst surface is beneficial for adsorption and catalytic oxidation of CO. Overall, both O2-TPD and CO-TPD results demonstrated that the support or the metalsupport interface plays a critical role in adsorbing O2, providing surface oxygen to react with the adsorbed CO and altering the desorption property of CO2 from the catalyst surface. Among the catalysts discussed, reducible CeO2 supports achieved much better performance than irreducible SiO2 support due to the interaction between Co3+/Co2+ and Ce3+/Ce4+ redox couples present at the interface. And especially, the CoOx/CeO2NR catalyst showed a significantly enhanced CO oxidation performance, which correlates well with the larger amount of surface defects (i.e. oxygen vacancies) in contrast to CeO2NC counterparts. 4. Conclusion In this work, the support effect for CeO2 and SiO2 supported cobalt oxide catalysts on CO oxidation reaction was investigated by CO conversion test and various characterization techniques. All the as-discussed catalysts showed excellent low-temperature catalytic activity for CO oxidation. The apparent activation energy of CO oxidation reaction demonstrated that the catalytic performances follow the sequence: 10 wt.% CoOx/CeO2NR > 10 wt.% CoOx/CeO2NC > 10wt. % CoOx/SiO2NS. The metal-support interaction was also demonstrated to play a key role for the dispersion, reducibility, thermal stability and catalytic performance of supported cobalt oxide(s). By comparison, CeO2NR support with various defects exposed on its surface, excellent storage


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capacity and high oxygen mobility is responsible for the superior reducibility and catalytic CO oxidation in CeO2NR supported cobalt oxide catalysts. Notably, TPR/TPO cycling measurement results have proved the low-temperature activity and long-duration stability of the 10 wt.% CoOx/CeO2NR sample. Besides, it was found out that the interaction between cobalt oxide species and SiO2NS leads to the formation of cobalt phyllosilicate and thus shows the high temperature H2-consumption peak at 715 oC. These results not only provide insights into the support morphology and reducibility effects of supported cobalt catalysts, but also offer a systematic understanding of the structure-function relation of different supported cobalt catalysts.

Acknowledgements This work is supported by the National Science Foundation (NSF CHE-1657943) and the American Chemical Society Petroleum Research Fund (#52323). The use of TEM facilities at Central Analytical Facility (CAF) of the University of Alabama is gratefully acknowledged. The authors also thank Yu Cheng (University of Virginia) for stimulating discussions.



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Fig. 1 XRD patterns of CeO2 and SiO2 supports (A), CeO2 and SiO2 supported 10 wt.% CoOx (B) and CeO2 and SiO2 supported 10 wt.% CoOx after reduction treatment (C).



Fig. 2 Raman spectra of CeO2 and SiO2 supports (A), CeO2 and SiO2 supported 10 wt.% CoOx (B) and CeO2 and SiO2 supported 10 wt.% CoOx after reduction treatment (C).


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Fig. 3 XPS spectra of Ce 3d (A and B), Si 2p (C), O 1s (D to F), and Co 2p (G to I) of CeO2 and SiO2 supported 10 wt.% CoOx catalysts, and all curve fitting were performed based on a Shirleytype background.



Fig. 4 TEM images of (A) CeO2NR, (B) CeO2NC, (C) SiO2NS, (D) CeO2NR-supported CoOx, (E) CeO2NC-supported CoOx, and (F) SiO2NS supported CoOx; HRTEM images (G-I) and EDS spectra (J-L) of CeO2NR, CeO2NC and SiO2NS supported CoOx catalysts, respectively.


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Fig. 5 H2-TPR profiles of Co3O4(A), CeO2 and SiO2 supports (B), CeO2 and SiO2 supported 10 wt.% CoOx (C), CeO2 and SiO2 supported 10 wt.% CoOx after reduction treatment (D), integrated H2 consumption (up to 500 °C) (E), and schematic illustration showing structural changes of CoOx on CeO2NR support during the reduction treatment (F).


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Fig. 6 H2-TPR cycling profiles for (A) five consecutive H2-TPR runs for 10 wt.% CoOx/CeO2NR, (B) five consecutive H2-TPR runs for 10 wt.% CoOx/SiO2NS, (C) five H2-TPR runs with a TPO in between for 10 wt.% CoOx/CeO2NR and (D) five H2-TPR runs with a TPO in between for 10 wt.% CoOx/SiO2NS.



Fig. 7 (A) CO conversion; (B) CO conversion at a constant temperature of 165 ℃ (1 vol.% CO/20 vol.% O2/79 vol.% He, space velocity: 76,680 mL h-1 gcat-1); (C) Arrhenius plots for CO oxidation over 10 wt.% CoOx/CeO2NR, 10 wt.% CoOx/CeO2NC and 10 wt.% CoOx/SiO2NS.


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(A) O2-TPD

(B) CO-TPD

Fig. 8 O2-TPD profiles of CeO2 and SiO2 supported 10 wt.% CoOx (A) and CO-TPD profiles of CeO2 and SiO2 supported 10 wt.% CoOx (B).



Fig. 9 Proposed structural models for interfacial configuration of CeO2NR, CeO2NC and SiO2 NS supported cobalt oxide catalysts.


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Table 1. BET surface area, crystallite size, lattice constant, the concentration of defect sites and oxygen storage capacity. Samples

SBET (m2/g)

Crystallite size of supporta (nm)

a(Å)

A(D)/A(F2g)

OSCC (µmol O2 gcat.-1)

CeO2NR

112.5

4.9

5.4162

0.086

108

CeO2NC

48.7

23.8

5.4109

0.021

33

SiO2NS

123.2

-

-

-

-

10% CoOx/CeO2NR

103.0

6.2

5.3954

0.155

498

10% CoOx/CeO2NC

46.1

22.5

5.4094

0.025

226

10% CoOx/SiO2NS

162.6

-

-

-

-

10% Co/CeO2NR-R.T.

103.3

6.2

5.3945

0.161

555

10% Co/CeO2NC-R.T.

46.6

21.9

5.4113

0.076

238

10% Co/SiO2 NS-R.T.

164.8

-

-

-

-

aEstimated

based on X-ray line broadening of the CeO2 (111) reflection plane by Scherrer

equation.



Page 46 of 48

Table 2. Surface composition and oxidation states of Ce (3d), Co (2p) and O (1s) species in CeO2 and SiO2 supported CoOx catalysts derived from XPS analysis.

Sample

Co/Ce or Co/Si

Co3+/(Co2+ + Co3+) (%)

Ce3+/(Ce3+ + Ce4+) (%)

OC

OV

OL

10% CoOx/CeO2NR

0.30

39.2

18.2

14.8

35.1

50.1

10% CoOx/CeO2NC

0.16

25.1

15.9

5.5

30.1

64.4

10% CoOx/SiO2NS

0.28

-

-

-

-


O species (%)

Page 47 of 48 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


Table 3. Reduction temperatures (TR) and H2 consumption determined by H2-TPR. Sample

Mass (mg)

TR (℃)

H2 consumptiona (µmol/g)

Co3O4

101.1

307, 401

17,755.7

CeO2NR

99.8

488, 817

500.9

CeO2NC

100.1

495, 811

90.5

SiO2NS

98.6

-

0

10% CoOx/CeO2NR

100.5

113, 171, 249, 315, 496, 773

2742.6

10% CoOx/CeO2NC

100.1

159, 245, 306, 484, 811

1471.5

10% CoOx/SiO2NS

99.6

219, 715

38.9

10% Co/CeO2NR-R.T.

101.2

193, 293, 490, 776

1811.0

10% Co/CeO2NC-R.T.

101.5

222, 263, 473, 807

817.3

10% Co/SiO2NS-R.T.

102.4

625

25.6

aCalculated

between 30 and 500 ℃.



Graphic abstract:


Page 48 of 48

Metal-support Interactions in CeO2 and SiO2 Supported Cobalt

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