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CeO2 catalysts: Effect of support calcination temperature on activity
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NO reduction by CO over CoOx/CeO2 catalysts: Effect of support calcination temperature on activity Shuhao Zhanga, Jaeha Leeb, Do Heui Kimb, Taejin Kima,* a b
Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, NY, 11794, USA School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul, 08826, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: NO reduction by CO Cobalt oxide Ceria Calcination temperature
CeO2 supported CoOx catalysts showed promising results in many NO reduction processes including NO reduction by CO. In this work, a series of CeO2 supports with different calcination temperatures (as received-800 °C calcination) were prepared, followed by the synthesis of a series of CoOx/CeO2 catalysts with a surface density of 2.5 Co/nm2 using the incipient wetness impregnation method. To understand the physicochemical properties of catalysts, various characterizations techniques, including BET, Raman spectroscopy, XRD, and H2-TPR, were performed. The results showed that as calcination temperature increased, physical properties of supporting materials were changed. Surface CoOx improved the reducibility of CeO2, but it was not affected by the support calcination temperatures confirmed by H2-TPR. The presence of CoOx was crucial to enhance the catalytic activity during the NO reduction by CO reaction, rather than physical properties (surface area, pore size, particle size) of the catalyst.
1. Introduction Transportation emissions are the leading contributors of greenhouse gases such as nitric oxide (NO). It is well known that NO can deplete ozone (NO + O3 → NO2 + O2) and form NO2, which is a source of atmospheric nitric acid that may lead to acid rains [1]. In order to tackle the air pollution issues, many approaches have been developed to catalytically remove NOx gases, including NO reduction by hydrogen, hydrocarbon, NO reduction by CO, and NH3-SCR (selective catalytic reduction) [2–7]. Among these approaches, NO reduction by CO (2NO + 2CO → N2 + 2CO2) has attracted great interest because it eliminates two harmful gases simultaneously [8,9]. Supported transition metal oxide catalysts have been extensively studied in the past decades and have shown promising results in NOx reduction reactions [10–14]. Cobalt oxide catalysts, which were applied in NO reduction by CO, water-gas shift, dry reforming, and ethanol steam reforming reaction [12,15–19], have received much attention because of their good catalytic activity and lower cost comparing to platinum group metals (PGMs) [20,21]. Deng et al. [22] studied different doped metals on CeO2 in NO reduction by CO reaction and found that CeCoOx was one of the best catalysts among tested metals (e.g. Co, Zr, Mn, Fe, etc.) in terms of NO conversion. CeO2 is an excellent catalyst supporting material due to its high thermal stability and oxygen storage capacity [7,23–27]. Lee et al. [28] reported that a very strong Pt-O-Ce bond was created in CeO2 ⁎
supported Pt catalysts. The Pt-O-Ce bond helped maintain Pt dispersion to resist Pt from sintering, which led to better catalytic performance. It has been reported that the calcination temperatures of bulk CeO2 or CeO2 supported catalysts affect the structure/surface properties as well as the chemical reaction activity [29–32]. Defect sites and oxygen vacancies are also believed to affect the properties of CeO2. Recently, Savereide et al. [33] reported that the CoOx/CeO2 catalyst with CeO2 nanorods as support performed better in NO + CO reaction comparing to CoOx/CeO2 catalyst with CeO2 nanoparticles or CeO2 nanocubes. The authors suggested that the CeO2 nanorod had a rougher surface and more oxygen vacancy defects which led to higher catalytic activity. Janoš et al. [34] studied the adsorption abilities of calcined CeO2 using Acid Orange 7 (AO7) and reported that the highest sorption efficiency of CeO2 was observed at calcination temperature 500 °C–600 °C, and the efficiency suddenly dropped at 700 °C or higher temperature. Wang et al. [35] explored the effect of calcination temperature on the NO oxidation of transition metal mixed oxide catalysts, such as Ce-Co-Ox, and reported that surface area, controlled by the calcination temperatures, was related to the activity. The authors found that the 400 °C calcined Ce-Co-Ox catalyst showed highest NO conversion among catalysts calcined between 300 °C and 600 °C. The aforementioned literatures’ results provided the importance of calcination temperature effect on the catalytic activity. In the present study, the NO reduction by CO reaction over a series
Corresponding Author. E-mail address: [email protected] (T. Kim).
https://doi.org/10.1016/j.mcat.2019.110703 Received 16 August 2019; Received in revised form 20 October 2019; Accepted 28 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shuhao Zhang, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110703
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2.4. Powder X-ray diffraction
of calcined bulk CeO2 catalysts as well as CoOx/CeO2 catalysts were investigated. In our previous work, it was observed that monolayer coverage of CoOx/CeO2 was between 2.3 and 2.7 Co/nm2 surface density, and the NO conversion followed the order: 2.3–31.3 Co/ nm2 > > 1.1 Co/nm2 > > 0.2 Co/nm2 [36]. To understand the effects of support calcination temperature (CeO2) with fixed Co surface density (2.5 Co/nm2) on the catalyst structure and catalytic activity, CeO2 was calcined at 400 °C∼800 °C and the CeO2 supported CoOx catalysts were prepared with the calcined CeO2. The physicochemical property and molecular structure of catalysts were explored by BET (N2 adsorption and desorption), Raman, XRD, and H2-TPR. Using these techniques, the relationships between support calcination temperature and surface properties of the CoOx/CeO2 catalysts, as well as the catalytic activities of the CoOx/CeO2 catalysts were studied.
To determine the crystalline phases in the catalysts, powder X-ray diffraction patterns were obtained with a Rigaku (mode 1 smartlab) diffractometer with Cu Kα radiation (λ = 0.1542 nm). The voltage and current of X-ray were 40 kV and 30 mA, respectively. The patterns were collected at a scanning step size of 0.02° at a rate of 2.5°/min and in a 2θ range from 5° to 90°. 2.5. Catalytic activity tests The gas phase reactions were carried out in a fixed bed quartz reactor (7 mm inner diameter, 9.6 mm outer diameter, and 9.6 in. long) packed with sieved catalyst powder and quartz wool. Catalyst sample weights used were ∼40 mg. Flow rates were measured by mass flow meters (FMA-1700A series, from Omega Engineering, Inc.) and temperature was monitored by a K-type thermocouple (Omega). Reaction products were identified and analyzed by GC (Thermo Scientific, TRACE 1300 model, TCD detector). Before the activity test, the catalysts were pre-treated at 400 °C under argon atmosphere for 1 h. For the NO reduction by CO reaction, 5% NO (10% with helium balance, Airgas) and 5% CO (10% with helium balance, Airgas) were used in the reactions. The total flow rate was 40 mL/min in all experiments and at least 25 min were allowed for the reaction conditions to stabilize at each reaction temperature before the data collection. The blank tests indicated that the conversions of NO and CO were negligible without any catalysts.
2. Experimental section 2.1. Catalyst synthesis The bulk CeO2 were calcined at different temperatures (400 °C–800 °C) before being used as supporting materials for the supported CoOx/CeO2 catalysts. The bulk CeO2 were held at desired calcination temperature for 6 h, then cooled down to ambient temperature and sieved through a 40-mesh sieve (Fisherbrand). The as received (or un-calcined) CeO2 catalyst was detonated as AR CeO2 and the calcined CeO2 catalysts were indicated with calcination temperature. For example: 400 °C calcined CeO2 was detonated as 400 CeO2. The supported cobalt oxide catalysts (CoOx) were synthesized with a fixed surface density of 2.5 Co/nm2. They were prepared by the incipient wetness impregnation of aqueous solution with various concentrations of cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ACS grade, 98.0∼102.0%, crystalline, Alfa Aesar) onto calcined CeO2 (Rhodia, HSA 5). After impregnation, the catalysts were dried at room temperature for 12 h. The dried catalysts were then transferred to a tube furnace (Lindberg/Blue Mini-Mite Tube Furnace, Model TF55030A-1) and further dried in air (Dry grade, from Airgas) at 120 °C (2 °C/min ramping rate) for 12 h. They were subsequently calcined at 400 °C (5 °C/min ramping rate) for 6 h. After the calcination process, the catalysts were cooled to room temperature and then sieved through a 40-mesh sieve (Fisherbrand). The CoOx supported by as-received CeO2 was detonated as Co-AR CeO2 and the CoOx supported by precalcined CeO2 was denoted as Co-x CeO2, where x is the calcination temperature of CeO2. For example, Co-600 CeO2 was prepared by the impregnation of CoOx (2.5 Co/nm2) on the 600 °C calcined CeO2.
2.6. H2-TPR To study the reducibility of various CeO2 and CoOx/CeO2 samples, H2-TPR was performed in a BET-CAT-BASIC (BEL Japan Inc.) with a thermal conductivity detector (TCD). The catalysts were pre-oxidized at 400 °C for 1 h under the air flow before the analysis, the catalysts were then cooled to −90 °C with the cryo-apparatus. Afterwards, 5% H2/Ar was used for analysis. After stabilizing the TCD signal in H2 flow for 30 min at the starting temperature, temperature was increased to 900 °C with a heating rate of 10 °C/min. Finally, the catalyst temperature was maintained at 900 °C for 15 min. TPR profiles are quantified by integrating peak areas. The TCD signal was calibrated by flowing a known amount of H2 and measuring its TCD peak area. 3. Results 3.1. Physical properties of CeO2 and CoOx/CeO2
2.2. Specific surface area and pore volume measurement The BET specific surface area (SSA) and pore structure of the CeO2 and CoOx/CeO2 were investigated by the N2 adsorption-desorption isotherm. As shown in Fig. 1 (a), as received and calcined CeO2 samples exhibited a typical type IV isotherm curve, implying the existence of mesoporous structures [37–39]. However, the different starting point of capillary condensation steps and different hysteresis loops indicated that the pore structures were varied with different support calcination temperatures. The CoOx/CeO2 samples showed similar isotherm curves as bulk CeO2 (Fig. 1(b)). This result provided that the mesoporous CeO2 structure was preserved after adding CoOx (2.5 Co/nm2). The specific surface area (SSA), pore size, and pore volume of CeO2 and 2.5 Co/nm2 CoOx/CeO2 samples were shown in Fig. 2, Table S1, and Table S2. The SSA of bulk CeO2 decreased with increased calcination temperatures (Fig. 2(a)), while pore diameters showed reversed trends (Fig. 1(b)). Up to 500 °C, both SSA and pore diameter were not changed much compared to AR CeO2. However, the values changed sharply at and over 600 °C. For instance, the 800 CeO2 sample (44 m2/g and 14.8 nm) showed ∼80% SSA decrease and ∼5 times increase in pore diameter comparing to AR CeO2 (233 m2/g and 2.8 nm). The results indicated that the pore structures were collapsed, or small pores
The specific surface area and pore volume of the series of calcined CeO2 and CoOx/CeO2 catalysts were calculated from N2 adsorption/ desorption isotherms using the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method, respectively. Micromeritics ASAP 2010 device was used at the liquid N2 temperature of −196 °C for analysis. Before the analysis, 0.1 g of catalyst was pretreated in a vacuum at 300 °C for 4 h to remove impurities. 2.3. Raman spectroscopy Molecular structure and bonding vibrations of calcined CeO2 and CoOx/CeO2 catalysts were determined via Raman spectroscopy. Raman spectra were recorded with visible (514 nm, He-Ne laser) excitation (Renishaw, inVia Raman microscope) under ambient atmosphere. The scattered photons were directed into a single monochromator and focused onto an air-cooled charge-coupled device. The Raman shift was calibrated with a built-in silicon standard catalyst. The spectral acquisition times were 10 scans accumulated with 10 s/scan and under a 5X objective. 2
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Fig. 1. N2 adsorption (filled symbols) and desorption (hollow symbols) isotherms of (a) CeO2 samples, and (b) 2.5 Co/nm2 CoOx/CeO2 samples.
calcination temperatures, the average particle size of CeO2 was dependent on temperatures. As shown in Fig. 3(c), with increasing calcination temperature, the average crystallite size of CeO2 increased from 5.3 nm (AR CeO2) to 19.2 nm (800 CeO2). This result was very similar to the trend of average pore diameter as shown in Fig. 1(b). XRD patterns of 2.5 Co/nm2 CoOx/CeO2 catalysts were shown in Fig. 3(b), and the average size of CeO2 were presented in Fig. 3(c) and Table S2. XRD peaks related to CoOx (e.g. 36.6° and 65.2° 2θ) were not detected in CoOx/CeO2 samples (Fig. 3(b)). This indicates that Co is finely dispersed on CeO2 surface. In addition, solid solutions of Co and CeO2 were not formed since the XRD pattern of CeO2 did not change after Co loading (Fig. S2). Besides, the crystallite size of CeO2 was not affected by Co loading (Fig. 3(c)). In short conclusion, the crystal structures of CeO2 remained without changes after the Co loading.
were blocked with increased calcination temperature, especially > 600 °C. Although both SSA and pore diameter changed drastically with different calcination temperatures, pore volumes stayed relatively unchanged (0.16∼0.19 cm3/g) as shown in Table S1. By adding the CoOx, it was observed that the SSA of Co-AR CeO2∼Co-600 CeO2 were lower than those of their bulk CeO2 counterparts, while the SSA of Co-700 CeO2 and Co-800 CeO2 were similar to their bulk CeO2 counterparts, 700 CeO2 and 800 CeO2 (Fig. 1(a)). It could also be observed that the pore size distribution and average pore diameter of the CeO2 were not changed by adding CoOx (Fig. 1(b) and Fig. S1). 3.2. Powder X-ray diffraction (XRD) To understand the crystalline structures and estimate the average particle sizes of the CeO2 and CoOx/CeO2 catalysts, powder X-ray Diffraction (XRD) technique was used. Fig. 3 showed the XRD patterns of CeO2 and CoOx/CeO2 catalysts with different support calcination temperatures (400 °C–800 °C). As shown in Fig. 3(a), the characteristic patterns of cubic fluorite CeO2 structure were found in all AR and calcined samples. The peaks at 28.6°, 33.1°, 47.6°, 56.4°, 59.1°, 69.5°, 76.8°, 79.5° and 88.6° 2θ could be assigned to (111), (200), (220), (311), (222), (400), (331), (420) and (422) of the cubic lattice of CeO2, respectively [40–42]. With increasing calcination temperatures, the peak intensity of CeO2 increased and the shape of peak sharpened. The crystallite size of CeO2 was calculated using Scherrer’s equation (Eq. 1) and the results were shown in Fig. 3(c) and Table S1.
D=
0.9λ Bcosθ
3.3. Raman spectroscopy In addition to XRD, Raman spectroscopic technique was employed for investigating the molecular structure of CeO2 and CoOx/CeO2 samples. Fig. 4(a) showed the Raman spectra of AR and x CeO2 (x = 400 °C–800 °C) samples. The band at ∼460 cm−1 was assigned to the symmetrical stretching of the Ce-O bond in F2g mode of the fluorite structure, confirming with the XRD results (Fig. 3(a)) [43,44]. The ∼600 cm−1 band could be assigned to oxygen vacancies and defectinduced (D) vibrations [45,46]. The intensity of ∼460 cm−1 band increased with increasing temperature, while the ∼600 cm−1 band intensity decreased with increasing temperature (inset of Fig. 4(a)). As shown in Fig. 4(c), the intensity ratio of defect vibrations (I600) and F2g vibrations (I460) clearly explained that defect sites were linearly decreased with increasing calcination temperatures. Fig. 4(b) displayed the Raman spectra of the 2.5 Co/nm2 CoOx/CeO2 catalysts. The main CeO2 band (460 cm−1) was present in the spectra of CoOx/CeO2 catalysts. The strong band at ∼690 cm−1 corresponded to
(1)
where λ is the X-ray wavelength, B is the full width at half maximum (FWHM) of the CeO2 (111) peak, and θ is the Bragg’s angle of the peak. Although the crystalline phase of CeO2 was not affected by
Fig. 2. (a) BET surface area and (b) Average pore diameter of calcined CeO2 catalysts and 2.5 Co/nm2 CoOx/CeO2 catalysts. 3
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Fig. 3. (a) XRD patterns of CeO2 catalysts, (b) XRD patterns of 2.5 Co/nm2 CoOx/CeO2 catalysts, and (c) CeO2 crystallite size calculated from XRD patterns.
the A1g vibration mode of the crystalline Co3O4 [47]. In addition to the ∼690 cm−1 band, several weak bands were also observed at ∼484 cm−1, ∼520 cm−1, and ∼618 cm−1, which could be assigned to
the Co3O4 Eg, F2g, and F2g vibration modes, respectively [48]. It should be noted that CoOx species was not detected using the XRD (Fig. 3(b)). This result proved that Raman spectroscopy was very sensitive for
Fig. 4. (a) Raman Spectra of CeO2 catalysts, (b) Raman Spectra of 2.5 Co/nm2 CoOx/CeO2 catalysts, and (c) Peak intensity ratio of ∼600 cm−1 band and ∼460 cm−1 band. 4
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Fig. 5. H2-TPR profiles of (a) calcined CeO2 catalysts, (b) 2.5 Co/nm2 CoOx/CeO2 catalysts, (c) 500 CeO2 and Co-500 CeO2 catalyst, and (d) 800 CeO2 and Co-800 CeO2 catalyst.
also shifted to lower temperature (centered at < 500 °C), indicating that surface CoOx improved the reducibility of CeO2. It was also observed that when Co was loaded on low surface area CeO2, such as 800 CeO2, the promotional effect of surface CeO2 reduction by Co became more significant. It should be noted that CoOx reduction peak positions were very similar for all CoOx/CeO2 samples indicating that the surface CoOx species over a series of CoOx/CeO2 samples had a similar reducibility. For a clearer comparison, the H2-TPR profiles of 500 CeO2, Co-500 CeO2, 800 CeO2, and Co-800 CeO2 were shown in Fig. 5(c) and (d). While the reduction temperature of bulk ceria (or lattice oxygen reduction) was not affected by the addition of Co, the change of reduction temperature of surface CeO2 was clear: 550 °C (500 CeO2) → 460 °C (Co-500 CeO2), 550 °C (800 CeO2) → 400 °C (Co-800 CeO2). The amount of H2 consumed by CoOx/CeO2 samples and CeO2 was calculated from H2-TPR curves and compared in Table S3. The addition of Co increased H2 consumption due to the presence of surface binding O (Osurf) in Co species. As shown in Table S3, the calculated ratio of Osurf (mmol/g) and Co loading (mmol/g) was similar (O/Co ∼ 1.2). This result was expected because the tested samples were contained the similar Co surface density.
detecting small or nano-crystalline structures. As shown in Fig. 4(c), CoOx/CeO2 samples’ I600/I460 ratios decreased as support calcination temperature increased, corresponding to decreased amount in defect sites. Comparing the peak intensity ratio of CeO2 and CoOx/CeO2 catalysts, the I600/I460 ratio of CoOx/CeO2 is higher than that of CeO2 when support calcination temperature is ≤ 600 °C, indicating that there were more defect sites in CoOx/CeO2 catalysts than CeO2 catalysts [49]. 3.4. H2 temperature programmed reduction (TPR) H2-TPR was employed for investigating the reduction characteristics of CeO2 and CoOx/CeO2 catalysts. As shown in Fig. 5 (a), for all of the calcined CeO2 catalysts, the shape of H2-TPR profiles was very similar and exhibited reduction peaks at ∼560 °C and ∼860 °C, corresponding to the reduction of surface and lattice oxygen of CeO2, respectively [50–52]. The reduction temperature of all CeO2 samples did not change, but as the calcination temperature increased, the corresponding H2 consumption measured on the H2-TPR curve decreased (Table S3). For example, the H2 consumed for CeO2 surface reduction for 500 CeO2 was 0.51 mmol/g, while only 0.08 mmol/g H2 was consumed for 800 CeO2 surface reduction (Table S3). This result could be explained with the previous findings in physical properties of CeO2 that, as calcination temperature increased, the specific surface area decreased, leading to less reducible surface oxygen. The H2-TPR profiles of the CoOx/CeO2 catalysts were shown in Fig. 5(b). In contrast to bulk CeO2, CoOx/CeO2 catalysts showed several reduction peaks at < 400 °C corresponding to the reduction of Co3O4 to CoO (< 350 °C) [53,54] and CoO to Co (350–400 °C) [53,55,56]. With the addition of CoOx, the reduction peak of surface oxygen of CeO2 was
3.5. Gas-phase catalytic activity of CeO2 and CoOx/CeO2 catalysts for NO reduction by CO The relationship between calcination temperature of CeO2 and gasphase catalytic activity of CeO2 and CoOx/CeO2 for the NO reduction by CO was investigated. Fig. 6 illustrated the NO reduction by CO performance of the bulk CeO2 and CoOx/CeO2 catalysts as a function of reaction temperature. In the case of bulk CeO2, as the reaction 5
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Fig. 6. Gas-phase catalytic activity results for NO reduction by CO oxidation over calcined CeO2. (a) NO conversion of calcined CeO2, (b) CO conversion of calcined CeO2, (c) NO conversion of CoOx/CeO2 catalysts, and (d) CO conversion of CoOx/CeO2 catalysts. Reaction conditions: 20 mL/min (5%) NO, 20 mL/min (5%) CO, ∼40 mg of catalyst was used.
temperature; (2) CeO2 showed very low activity for the NO reduction by CO, for instance, no activity up to 250 °C and < 50% NO and CO conversion up to 350 °C; (3) The addition of 2.5 Co/nm2 increased NO and CO conversion especially at lower reaction temperature. For instance, > 80% NO conversion (except for Co-800 CeO2) at 250 °C; (4) Gas-phase catalytic activity was not affected much by SSA and average particle size differences.
temperature increased, both NO and CO conversion increased monotonically (Fig. 6 (a) and (b)). However, the bulk CeO2 catalysts showed no or very low NO and CO conversions when the reaction temperature was < 300 °C. Overall, the NO and CO conversion for tested samples were quite similar although AR CeO2 and 600 CeO2 catalysts showed slightly higher NO and CO conversion comparing to 800 CeO2 catalyst. It should be noted that the SSA of 800 CeO2 was 4∼5 times lower than that of AR CeO2 and 600 CeO2. (Table S1) Also particle size of 800 CeO2 was 3∼4 times larger than that of AR CeO2 and 600 CeO2. This result indicated that SSA and particle size of CeO2 did not much affect the gasphase catalytic activity in the NO reduction by CO reaction much. The NO + CO reaction activity results of the CoOx/CeO2 catalysts were shown in Fig. 6(c) and (d). Comparing to the bulk CeO2 catalysts, the CoOx/CeO2 catalysts showed much higher catalytic performances during the NO reduction by CO reaction, especially < 300 °C, confirming that surface CoOx played a key role for the gas-phase catalytic activity. The NO and CO conversion started at 150 °C, which was 100 °C lower than that of bulk CeO2, and > 90% conversion of NO and CO was achieved at 250∼300 °C and 350∼450 °C, respectively. Unlike the results for bulk CeO2 catalysts, NO conversion was higher than CO conversion at ≥ 200 °C, and the difference became smaller as reaction temperature increased. The calcination temperature effects showed that at < 300 °C the NO and CO conversions followed the order Co-AR CeO2∼Co-600 CeO2 > Co-700 CeO2 > Co-800 CeO2. At and over 300 °C, the NO conversion was not affected by calcination temperature, while CO conversion was slightly affected by calcination temperature. Similar to the bulk CeO2, the differences in NO and CO conversions were minor compared to the SSA and average particle size differences (Table S2). Based on the observed results, it can be concluded that (1) The NO and CO conversion increased with increasing reaction
4. Discussion 4.1. Support calcination temperature effect on physicochemical properties The molecular structure (and physical property) of supporting materials and supported catalysts could be directly affected by support calcination temperature. Consequently, changed structure of catalysts (bulk or supported) could affect the catalytic activity. With varied support calcination temperatures (400∼800 °C), physicochemical properties of bulk CeO2 and CoOx/CeO2 were investigated using various techniques, such as BET, XRD, Raman, and H2-TPR. Comparing all results obtained for the series of CeO2 samples (AR ∼ 800 CeO2), it was observed that both SSA and particle size of samples were directly related to the calcination temperature. From both XRD (Fig. 3(a)) and Raman spectra (Fig. 4(a)), the CeO2 pattern sharpened with increasing calcination temperature, indicating an increased crystallite size of CeO2. The increase in crystallite size could be caused by sintering effects or agglomeration under high calcination temperatures. Correspondingly, a decrease in specific surface area and increase in average pore diameter were also noticed (Fig. 2) due to the blocking of small pores, formation of larger crystalline structures, or collapsing of pore structures. Because peaks and bands at XRD and Raman spectra of the 6
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because the lower Co wt% was loaded on high temperature treated CeO2 to achieve the same Co surface density (2.5 Co/nm2). Therefore, it can be concluded that, although the Co wt% varied, the reducibility of the CoOx/CeO2 catalysts were very similar because of the fixed Co surface density (2.5 Co/nm2).
prepared samples were not shifted, it was expected that the molecular structure of CeO2 remained the same, although defect sites were decreased with increasing calcination temperatures (Fig. 4(a)).With decreasing SSA and increasing particle size, less surface and bulk oxygen in CeO2 was exposed, resulting in less amount of H2 consumption observed in high temperature calcined CeO2 (Fig. 5). However, the reduction peak temperatures did not shift much even with the drastic change in SSA and particle size at different calcination temperatures. Thus, it can be deduced that the reducibility of CeO2 was not affected by changing of SSA and particle size. The SSA and pore diameters of 2.5 Co/nm2 CoOx/CeO2 catalysts showed similar trends as observed with bulk CeO2: decreasing SSA, increasing pore diameter, and similar pore volume with increasing support calcination temperatures. However, the SSA differences (%) between CeO2 and CoOx/CeO2 varied with different support calcination temperatures. As shown in Table S2, the SSA difference (%) was ∼18.8% between AR CeO2 and Co-AR CeO2, while ∼2.2% change was observed between 800 CeO2 and Co-800 CeO2. It can be deduced that AR CeO2 and low temperature calcined CeO2 (e.g. 400–600 CeO2) contained small pore structure (< 4 nm) which facilitated CoOx dispersion. In the case of high temperature (> 600 °C) calcined CeO2, small pores were already blocked during the calcination procedures and surface CoOx was located on larger pores (> 8 nm) inside the CeO2 mesopore structure. It is worthwhile to re-emphasize that, to achieve the same Co surface density (2.5 Co/nm2), the Co wt% changed (e.g., 5.4 wt% for Co-AR CeO2 and 1.0 wt% for Co-800 CeO2) based on the surface area of calcined CeO2. The corresponding Co wt% were calculated using Eq. (2) shown below and the values were included in Table S2.
Sample S. D . =
wt % Co atoms × 6.02 × 1023 100 mol g ] M . W of Co [= 58.9332 mol 2 m wt % Co
Support S. A [
g
] × (1 −
100
) × 1018
4.2. Support calcination temperature effect on catalytic activity in NO + CO reaction As discussed above, the calcination temperature of CeO2 led to changes in molecular structures in both bulk CeO2 and supported CoOx/ CeO2 catalysts. Consequently, changed structure of catalysts (bulk or supported) could affect the catalytic activity. As shown in Fig. 6, it is clear that, starting from 200 °C, the CoOx/CeO2 catalysts showed much higher NO conversion than bulk CeO2, confirming the critical role of surface CoOx in enhancing the catalytic activity. It has been proposed that the NO reduction by CO reaction happens in the following reaction steps [57]
2NO + CO → N2 O + CO2
(3)
N2 O + CO → N2 + CO2
(4)
According to previous studies [9,58], NO adsorption temperature is much lower than that of CO adsorption for supported transition metal oxide catalysts. This explained the difference in NO conversion and CO conversion under the same reaction temperature (especially 150 °C ∼ 350 °C for CoOx/CeO2 catalysts in Fig. 6 (c) and (d)), where NO conversion was higher than CO conversion. However, from Fig. 6 (a) and (b), it can be noticed that, the NO and CO conversion over bulk CeO2 catalysts were very similar and occurred at > 300 °C. This result can be explained that NO adsorption over bulk CeO2 was initiated at higher temperature (> 250 °C), while N2O formation and decomposition by CO occurred at > 300 °C. In the case of CoOx/CeO2 catalysts, the surface CoOx species facilitated N2O decomposition at lower temperature comparing to bulk CeO2, thus freeing up many adsorption sites for other NO molecules, resulting in much higher NO conversion starting from 200 °C. Also, since the surface CoOx enabled N2O decomposition at lower temperature, consequently, the N2 selectivity (Fig. 7) of the CoOx/CeO2 catalysts were higher than that of bulk CeO2 catalysts, especially for reaction temperature < 400 °C. Moreover, as shown in Fig. 6, NO and CO conversion increased after adding the CoOx, possibly due to the increased defect sites/oxygen vacancies (Fig. 4(c)), improved reducibility of CeO2 (Fig. 5) and intrinsically higher activity of CoOx. It has been reported that defected
(2)
2
The XRD patterns of 2.5 Co/nm CoOx/CeO2 provided that surface CoOx was fully dispersed or crystallite size of CoOx was too small to be detected (Fig. 3(b)), which was well matched to the literature results. Recently, Peck et al [10] and S. Zhang et al [36] investigated the monolayer coverage ranges of the CeO2 supported CoOx catalysts using by XRD, Raman, and XPS techniques. The authors reported that monolayer coverage of CoOx/CeO2 should be 2.58 Co/nm2 or 2.3 Co/ nm2 ∼ 2.7 Co/nm2. The Raman spectroscopy results (Fig. 4 (b)), however, provided the evidence of Co3O4 crystalline structure (∼690 cm−1 Raman shift) indicating the inhomogeneity of catalyst surface. Due to the limitation of XRD, it can be also expected that 2.5 Co/nm2 CoOx/CeO2 samples contained < 5 nm Co3O4 nanocrystalline structure under the monolayer coverage. In addition to the detection of Co3O4, Raman spectroscopy results showed that the defect sites of CeO2 increased by adding the surface CoOx. (Fig. 4(c)). The reducibility of CoOx/CeO2 also improved comparing to bulk CeO2. The addition of CoOx species could facilitate the redox cycle of CeO2 (Ce3+ ↔ Ce4+), leading to the reduction temperature of surface oxygen of CeO2 shifting to lower temperature (Fig. 5). Moreover, Fig. 4(c) showed that the defect concentration in ceria decreased as the support calcination temperature increased. It could be hypothesized that, when Co was loaded on CeO2 with less defect sites, larger amount of gaseous O2 can chemisorb on Co to form larger crystalline structures of CoOx. Upon reduction, the CoOx species was reduced to metallic Co first, then the exposed Co metal on the surface of CeO2 with less defect sites would promote H2-spillover to facilitate the surface reduction, resulting in surface CeO2 reduction temperature shifting to even lower temperatures. Also, as shown in Fig. 5(b), the reduction temperature of CoOx (< 300 °C) did not shift, suggesting that the reducibility of surface CoOx species was not affected by the inhomogeneity of catalyst surface. However, the amount of H2 consumed to reduced CoOx (< 300 °C) decreased as the support calcination temperature increased (Fig. 5b),
Fig. 7. Comparison between bulk CeO2 and CoOx/CeO2 catalysts in N2 selectivity under different reaction temperatures. 7
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rates were performed to ensure that the reaction conditions were free of external diffusion effects [61,62]. As shown in Fig. S3, catalytic activity was not changed with different total flow rates, such as 20 ml/min, 40 ml/min, and 60 ml/min. Based on the results, it can be deduced that the rate data will be free from external mass transfer effect under the experimental condition used in this study. 5. Conclusions In this work, a series of CeO2 samples, calcined at different temperature (400 °C∼800 °C), were synthesized, as well as CeO2 (SSA: 44∼233 m2/g) supported CoOx (1.0∼5.4 Co wt%) catalysts. The synthesized catalysts were then tested for the NO reduction by CO reaction. As the support calcination temperature increased (especially at ≥700 °C), physical properties (SSA, pore diameter, and particle size) changed significantly comparing to as-received (AR) CeO2. No evidence of surface CoOx was observed by XRD, while nanocrystalline structure of surface CoOx species and CeO2 defect sites were identified by Raman. H2-TPR showed that the reducibility of surface CeO2 was improved by adding CoOx, while the reduction temperature of bulk CeO2 was not affected. It was clearly observed that CoOx/CeO2 catalysts had higher gas-phase catalytic activity than bulk CeO2 catalysts in the NO reduction by CO reaction. Although it was clear that physical properties of synthesized catalysts were affected by support calcination temperatures, the defect site ratio (I600/I460 by Raman) to NO conversion and the molar rate of NO and CO during the NO reduction by CO at 200 °C over the studies CoOx/CeO2 catalysts (2.5 Co/nm2 and CeO2 calcined at 400 °C∼800 °C) provided that the catalytic activity was mainly controlled by the surface CoOx, while physical properties of the support material had little impacts on the catalytic activity.
Fig. 8. Comparison of NO conversion between 800 CeO2 and Co-800 CeO2.
Declaration of Competing Interest None. Acknowledgments S. Zhang and T. Kim would like to thank the Advanced Energy Research and Technology Center (AERTC) at the Stony Brook University for providing us with the facilities. The authors would also like to thank Anson Law and Ming Hu for their assistance in performing Raman experiments. This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A5A1009592).
Fig. 9. Catalytic activity of NO and CO at 200 °C for CeO2 and CoOx/CeO2 catalysts.
CeO2 sites improved the catalytic activity for the NO reduction by CO and CO oxidation reaction [8,59,60]. As shown in Fig. 8, however, although the catalytic activity (NO conversion) of Co-800 CeO2 (T20 = 210 °C and T50 = 238 °C) was much higher than that of 800 CeO2 (T20 = 355 °C and T50 = 411 °C), the I600/I460 ratio was very similar for the 800 CeO2 (I600/I460 = 0.006) and Co-800 CeO2 (I600/ I460 = 0.009). Granted that the effect of CeO2 defect sites (or oxygen vacancy) on catalytic activity cannot be completely neglected, the observed results indicated that the NO and CO conversion were not much related to the change of defect sites under the experimental conditions. In order to further understand the impacts of support calcination temperature on the inherent activity of CoOx/CeO2 catalysts, the catalytic activity of NO and CO (moles of NO and CO converted/unit time/ catalyst surface area) at 200 °C were calculated. As shown in Fig. 9, the catalytic activity of both NO and CO did not change much with the support calcination temperature difference, while the catalytic activity for the bulk CeO2 were neglectable. By combining the physicochemical and activity results, it can be concluded that the catalytic activity of the reaction was mainly controlled by the surface density of cobalt, but not by the physical properties of the catalysts (SSA, pore diameters, crystallite size). It was also found that the catalytic activity of NO was about twice as that of CO at 200 °C, which corresponded well with the N2O formation step (Eq. (3)), where NO and CO reacted at a 2:1 ratio. Gas phase catalytic activity tests with fixed GHSV and various total gas flow
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NO reduction by CO over CoOx/CeO2 catalysts: Effect of support calcination temperature on activity Shuhao Zhanga, Jaeha Leeb, Do Heui Kimb, Taejin Kima,* a b
Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, NY, 11794, USA School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul, 08826, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: NO reduction by CO Cobalt oxide Ceria Calcination temperature
CeO2 supported CoOx catalysts showed promising results in many NO reduction processes including NO reduction by CO. In this work, a series of CeO2 supports with different calcination temperatures (as received-800 °C calcination) were prepared, followed by the synthesis of a series of CoOx/CeO2 catalysts with a surface density of 2.5 Co/nm2 using the incipient wetness impregnation method. To understand the physicochemical properties of catalysts, various characterizations techniques, including BET, Raman spectroscopy, XRD, and H2-TPR, were performed. The results showed that as calcination temperature increased, physical properties of supporting materials were changed. Surface CoOx improved the reducibility of CeO2, but it was not affected by the support calcination temperatures confirmed by H2-TPR. The presence of CoOx was crucial to enhance the catalytic activity during the NO reduction by CO reaction, rather than physical properties (surface area, pore size, particle size) of the catalyst.
1. Introduction Transportation emissions are the leading contributors of greenhouse gases such as nitric oxide (NO). It is well known that NO can deplete ozone (NO + O3 → NO2 + O2) and form NO2, which is a source of atmospheric nitric acid that may lead to acid rains [1]. In order to tackle the air pollution issues, many approaches have been developed to catalytically remove NOx gases, including NO reduction by hydrogen, hydrocarbon, NO reduction by CO, and NH3-SCR (selective catalytic reduction) [2–7]. Among these approaches, NO reduction by CO (2NO + 2CO → N2 + 2CO2) has attracted great interest because it eliminates two harmful gases simultaneously [8,9]. Supported transition metal oxide catalysts have been extensively studied in the past decades and have shown promising results in NOx reduction reactions [10–14]. Cobalt oxide catalysts, which were applied in NO reduction by CO, water-gas shift, dry reforming, and ethanol steam reforming reaction [12,15–19], have received much attention because of their good catalytic activity and lower cost comparing to platinum group metals (PGMs) [20,21]. Deng et al. [22] studied different doped metals on CeO2 in NO reduction by CO reaction and found that CeCoOx was one of the best catalysts among tested metals (e.g. Co, Zr, Mn, Fe, etc.) in terms of NO conversion. CeO2 is an excellent catalyst supporting material due to its high thermal stability and oxygen storage capacity [7,23–27]. Lee et al. [28] reported that a very strong Pt-O-Ce bond was created in CeO2 ⁎
supported Pt catalysts. The Pt-O-Ce bond helped maintain Pt dispersion to resist Pt from sintering, which led to better catalytic performance. It has been reported that the calcination temperatures of bulk CeO2 or CeO2 supported catalysts affect the structure/surface properties as well as the chemical reaction activity [29–32]. Defect sites and oxygen vacancies are also believed to affect the properties of CeO2. Recently, Savereide et al. [33] reported that the CoOx/CeO2 catalyst with CeO2 nanorods as support performed better in NO + CO reaction comparing to CoOx/CeO2 catalyst with CeO2 nanoparticles or CeO2 nanocubes. The authors suggested that the CeO2 nanorod had a rougher surface and more oxygen vacancy defects which led to higher catalytic activity. Janoš et al. [34] studied the adsorption abilities of calcined CeO2 using Acid Orange 7 (AO7) and reported that the highest sorption efficiency of CeO2 was observed at calcination temperature 500 °C–600 °C, and the efficiency suddenly dropped at 700 °C or higher temperature. Wang et al. [35] explored the effect of calcination temperature on the NO oxidation of transition metal mixed oxide catalysts, such as Ce-Co-Ox, and reported that surface area, controlled by the calcination temperatures, was related to the activity. The authors found that the 400 °C calcined Ce-Co-Ox catalyst showed highest NO conversion among catalysts calcined between 300 °C and 600 °C. The aforementioned literatures’ results provided the importance of calcination temperature effect on the catalytic activity. In the present study, the NO reduction by CO reaction over a series
Corresponding Author. E-mail address: [email protected] (T. Kim).
https://doi.org/10.1016/j.mcat.2019.110703 Received 16 August 2019; Received in revised form 20 October 2019; Accepted 28 October 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Shuhao Zhang, et al., Molecular Catalysis, https://doi.org/10.1016/j.mcat.2019.110703
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2.4. Powder X-ray diffraction
of calcined bulk CeO2 catalysts as well as CoOx/CeO2 catalysts were investigated. In our previous work, it was observed that monolayer coverage of CoOx/CeO2 was between 2.3 and 2.7 Co/nm2 surface density, and the NO conversion followed the order: 2.3–31.3 Co/ nm2 > > 1.1 Co/nm2 > > 0.2 Co/nm2 [36]. To understand the effects of support calcination temperature (CeO2) with fixed Co surface density (2.5 Co/nm2) on the catalyst structure and catalytic activity, CeO2 was calcined at 400 °C∼800 °C and the CeO2 supported CoOx catalysts were prepared with the calcined CeO2. The physicochemical property and molecular structure of catalysts were explored by BET (N2 adsorption and desorption), Raman, XRD, and H2-TPR. Using these techniques, the relationships between support calcination temperature and surface properties of the CoOx/CeO2 catalysts, as well as the catalytic activities of the CoOx/CeO2 catalysts were studied.
To determine the crystalline phases in the catalysts, powder X-ray diffraction patterns were obtained with a Rigaku (mode 1 smartlab) diffractometer with Cu Kα radiation (λ = 0.1542 nm). The voltage and current of X-ray were 40 kV and 30 mA, respectively. The patterns were collected at a scanning step size of 0.02° at a rate of 2.5°/min and in a 2θ range from 5° to 90°. 2.5. Catalytic activity tests The gas phase reactions were carried out in a fixed bed quartz reactor (7 mm inner diameter, 9.6 mm outer diameter, and 9.6 in. long) packed with sieved catalyst powder and quartz wool. Catalyst sample weights used were ∼40 mg. Flow rates were measured by mass flow meters (FMA-1700A series, from Omega Engineering, Inc.) and temperature was monitored by a K-type thermocouple (Omega). Reaction products were identified and analyzed by GC (Thermo Scientific, TRACE 1300 model, TCD detector). Before the activity test, the catalysts were pre-treated at 400 °C under argon atmosphere for 1 h. For the NO reduction by CO reaction, 5% NO (10% with helium balance, Airgas) and 5% CO (10% with helium balance, Airgas) were used in the reactions. The total flow rate was 40 mL/min in all experiments and at least 25 min were allowed for the reaction conditions to stabilize at each reaction temperature before the data collection. The blank tests indicated that the conversions of NO and CO were negligible without any catalysts.
2. Experimental section 2.1. Catalyst synthesis The bulk CeO2 were calcined at different temperatures (400 °C–800 °C) before being used as supporting materials for the supported CoOx/CeO2 catalysts. The bulk CeO2 were held at desired calcination temperature for 6 h, then cooled down to ambient temperature and sieved through a 40-mesh sieve (Fisherbrand). The as received (or un-calcined) CeO2 catalyst was detonated as AR CeO2 and the calcined CeO2 catalysts were indicated with calcination temperature. For example: 400 °C calcined CeO2 was detonated as 400 CeO2. The supported cobalt oxide catalysts (CoOx) were synthesized with a fixed surface density of 2.5 Co/nm2. They were prepared by the incipient wetness impregnation of aqueous solution with various concentrations of cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ACS grade, 98.0∼102.0%, crystalline, Alfa Aesar) onto calcined CeO2 (Rhodia, HSA 5). After impregnation, the catalysts were dried at room temperature for 12 h. The dried catalysts were then transferred to a tube furnace (Lindberg/Blue Mini-Mite Tube Furnace, Model TF55030A-1) and further dried in air (Dry grade, from Airgas) at 120 °C (2 °C/min ramping rate) for 12 h. They were subsequently calcined at 400 °C (5 °C/min ramping rate) for 6 h. After the calcination process, the catalysts were cooled to room temperature and then sieved through a 40-mesh sieve (Fisherbrand). The CoOx supported by as-received CeO2 was detonated as Co-AR CeO2 and the CoOx supported by precalcined CeO2 was denoted as Co-x CeO2, where x is the calcination temperature of CeO2. For example, Co-600 CeO2 was prepared by the impregnation of CoOx (2.5 Co/nm2) on the 600 °C calcined CeO2.
2.6. H2-TPR To study the reducibility of various CeO2 and CoOx/CeO2 samples, H2-TPR was performed in a BET-CAT-BASIC (BEL Japan Inc.) with a thermal conductivity detector (TCD). The catalysts were pre-oxidized at 400 °C for 1 h under the air flow before the analysis, the catalysts were then cooled to −90 °C with the cryo-apparatus. Afterwards, 5% H2/Ar was used for analysis. After stabilizing the TCD signal in H2 flow for 30 min at the starting temperature, temperature was increased to 900 °C with a heating rate of 10 °C/min. Finally, the catalyst temperature was maintained at 900 °C for 15 min. TPR profiles are quantified by integrating peak areas. The TCD signal was calibrated by flowing a known amount of H2 and measuring its TCD peak area. 3. Results 3.1. Physical properties of CeO2 and CoOx/CeO2
2.2. Specific surface area and pore volume measurement The BET specific surface area (SSA) and pore structure of the CeO2 and CoOx/CeO2 were investigated by the N2 adsorption-desorption isotherm. As shown in Fig. 1 (a), as received and calcined CeO2 samples exhibited a typical type IV isotherm curve, implying the existence of mesoporous structures [37–39]. However, the different starting point of capillary condensation steps and different hysteresis loops indicated that the pore structures were varied with different support calcination temperatures. The CoOx/CeO2 samples showed similar isotherm curves as bulk CeO2 (Fig. 1(b)). This result provided that the mesoporous CeO2 structure was preserved after adding CoOx (2.5 Co/nm2). The specific surface area (SSA), pore size, and pore volume of CeO2 and 2.5 Co/nm2 CoOx/CeO2 samples were shown in Fig. 2, Table S1, and Table S2. The SSA of bulk CeO2 decreased with increased calcination temperatures (Fig. 2(a)), while pore diameters showed reversed trends (Fig. 1(b)). Up to 500 °C, both SSA and pore diameter were not changed much compared to AR CeO2. However, the values changed sharply at and over 600 °C. For instance, the 800 CeO2 sample (44 m2/g and 14.8 nm) showed ∼80% SSA decrease and ∼5 times increase in pore diameter comparing to AR CeO2 (233 m2/g and 2.8 nm). The results indicated that the pore structures were collapsed, or small pores
The specific surface area and pore volume of the series of calcined CeO2 and CoOx/CeO2 catalysts were calculated from N2 adsorption/ desorption isotherms using the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method, respectively. Micromeritics ASAP 2010 device was used at the liquid N2 temperature of −196 °C for analysis. Before the analysis, 0.1 g of catalyst was pretreated in a vacuum at 300 °C for 4 h to remove impurities. 2.3. Raman spectroscopy Molecular structure and bonding vibrations of calcined CeO2 and CoOx/CeO2 catalysts were determined via Raman spectroscopy. Raman spectra were recorded with visible (514 nm, He-Ne laser) excitation (Renishaw, inVia Raman microscope) under ambient atmosphere. The scattered photons were directed into a single monochromator and focused onto an air-cooled charge-coupled device. The Raman shift was calibrated with a built-in silicon standard catalyst. The spectral acquisition times were 10 scans accumulated with 10 s/scan and under a 5X objective. 2
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Fig. 1. N2 adsorption (filled symbols) and desorption (hollow symbols) isotherms of (a) CeO2 samples, and (b) 2.5 Co/nm2 CoOx/CeO2 samples.
calcination temperatures, the average particle size of CeO2 was dependent on temperatures. As shown in Fig. 3(c), with increasing calcination temperature, the average crystallite size of CeO2 increased from 5.3 nm (AR CeO2) to 19.2 nm (800 CeO2). This result was very similar to the trend of average pore diameter as shown in Fig. 1(b). XRD patterns of 2.5 Co/nm2 CoOx/CeO2 catalysts were shown in Fig. 3(b), and the average size of CeO2 were presented in Fig. 3(c) and Table S2. XRD peaks related to CoOx (e.g. 36.6° and 65.2° 2θ) were not detected in CoOx/CeO2 samples (Fig. 3(b)). This indicates that Co is finely dispersed on CeO2 surface. In addition, solid solutions of Co and CeO2 were not formed since the XRD pattern of CeO2 did not change after Co loading (Fig. S2). Besides, the crystallite size of CeO2 was not affected by Co loading (Fig. 3(c)). In short conclusion, the crystal structures of CeO2 remained without changes after the Co loading.
were blocked with increased calcination temperature, especially > 600 °C. Although both SSA and pore diameter changed drastically with different calcination temperatures, pore volumes stayed relatively unchanged (0.16∼0.19 cm3/g) as shown in Table S1. By adding the CoOx, it was observed that the SSA of Co-AR CeO2∼Co-600 CeO2 were lower than those of their bulk CeO2 counterparts, while the SSA of Co-700 CeO2 and Co-800 CeO2 were similar to their bulk CeO2 counterparts, 700 CeO2 and 800 CeO2 (Fig. 1(a)). It could also be observed that the pore size distribution and average pore diameter of the CeO2 were not changed by adding CoOx (Fig. 1(b) and Fig. S1). 3.2. Powder X-ray diffraction (XRD) To understand the crystalline structures and estimate the average particle sizes of the CeO2 and CoOx/CeO2 catalysts, powder X-ray Diffraction (XRD) technique was used. Fig. 3 showed the XRD patterns of CeO2 and CoOx/CeO2 catalysts with different support calcination temperatures (400 °C–800 °C). As shown in Fig. 3(a), the characteristic patterns of cubic fluorite CeO2 structure were found in all AR and calcined samples. The peaks at 28.6°, 33.1°, 47.6°, 56.4°, 59.1°, 69.5°, 76.8°, 79.5° and 88.6° 2θ could be assigned to (111), (200), (220), (311), (222), (400), (331), (420) and (422) of the cubic lattice of CeO2, respectively [40–42]. With increasing calcination temperatures, the peak intensity of CeO2 increased and the shape of peak sharpened. The crystallite size of CeO2 was calculated using Scherrer’s equation (Eq. 1) and the results were shown in Fig. 3(c) and Table S1.
D=
0.9λ Bcosθ
3.3. Raman spectroscopy In addition to XRD, Raman spectroscopic technique was employed for investigating the molecular structure of CeO2 and CoOx/CeO2 samples. Fig. 4(a) showed the Raman spectra of AR and x CeO2 (x = 400 °C–800 °C) samples. The band at ∼460 cm−1 was assigned to the symmetrical stretching of the Ce-O bond in F2g mode of the fluorite structure, confirming with the XRD results (Fig. 3(a)) [43,44]. The ∼600 cm−1 band could be assigned to oxygen vacancies and defectinduced (D) vibrations [45,46]. The intensity of ∼460 cm−1 band increased with increasing temperature, while the ∼600 cm−1 band intensity decreased with increasing temperature (inset of Fig. 4(a)). As shown in Fig. 4(c), the intensity ratio of defect vibrations (I600) and F2g vibrations (I460) clearly explained that defect sites were linearly decreased with increasing calcination temperatures. Fig. 4(b) displayed the Raman spectra of the 2.5 Co/nm2 CoOx/CeO2 catalysts. The main CeO2 band (460 cm−1) was present in the spectra of CoOx/CeO2 catalysts. The strong band at ∼690 cm−1 corresponded to
(1)
where λ is the X-ray wavelength, B is the full width at half maximum (FWHM) of the CeO2 (111) peak, and θ is the Bragg’s angle of the peak. Although the crystalline phase of CeO2 was not affected by
Fig. 2. (a) BET surface area and (b) Average pore diameter of calcined CeO2 catalysts and 2.5 Co/nm2 CoOx/CeO2 catalysts. 3
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Fig. 3. (a) XRD patterns of CeO2 catalysts, (b) XRD patterns of 2.5 Co/nm2 CoOx/CeO2 catalysts, and (c) CeO2 crystallite size calculated from XRD patterns.
the A1g vibration mode of the crystalline Co3O4 [47]. In addition to the ∼690 cm−1 band, several weak bands were also observed at ∼484 cm−1, ∼520 cm−1, and ∼618 cm−1, which could be assigned to
the Co3O4 Eg, F2g, and F2g vibration modes, respectively [48]. It should be noted that CoOx species was not detected using the XRD (Fig. 3(b)). This result proved that Raman spectroscopy was very sensitive for
Fig. 4. (a) Raman Spectra of CeO2 catalysts, (b) Raman Spectra of 2.5 Co/nm2 CoOx/CeO2 catalysts, and (c) Peak intensity ratio of ∼600 cm−1 band and ∼460 cm−1 band. 4
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Fig. 5. H2-TPR profiles of (a) calcined CeO2 catalysts, (b) 2.5 Co/nm2 CoOx/CeO2 catalysts, (c) 500 CeO2 and Co-500 CeO2 catalyst, and (d) 800 CeO2 and Co-800 CeO2 catalyst.
also shifted to lower temperature (centered at < 500 °C), indicating that surface CoOx improved the reducibility of CeO2. It was also observed that when Co was loaded on low surface area CeO2, such as 800 CeO2, the promotional effect of surface CeO2 reduction by Co became more significant. It should be noted that CoOx reduction peak positions were very similar for all CoOx/CeO2 samples indicating that the surface CoOx species over a series of CoOx/CeO2 samples had a similar reducibility. For a clearer comparison, the H2-TPR profiles of 500 CeO2, Co-500 CeO2, 800 CeO2, and Co-800 CeO2 were shown in Fig. 5(c) and (d). While the reduction temperature of bulk ceria (or lattice oxygen reduction) was not affected by the addition of Co, the change of reduction temperature of surface CeO2 was clear: 550 °C (500 CeO2) → 460 °C (Co-500 CeO2), 550 °C (800 CeO2) → 400 °C (Co-800 CeO2). The amount of H2 consumed by CoOx/CeO2 samples and CeO2 was calculated from H2-TPR curves and compared in Table S3. The addition of Co increased H2 consumption due to the presence of surface binding O (Osurf) in Co species. As shown in Table S3, the calculated ratio of Osurf (mmol/g) and Co loading (mmol/g) was similar (O/Co ∼ 1.2). This result was expected because the tested samples were contained the similar Co surface density.
detecting small or nano-crystalline structures. As shown in Fig. 4(c), CoOx/CeO2 samples’ I600/I460 ratios decreased as support calcination temperature increased, corresponding to decreased amount in defect sites. Comparing the peak intensity ratio of CeO2 and CoOx/CeO2 catalysts, the I600/I460 ratio of CoOx/CeO2 is higher than that of CeO2 when support calcination temperature is ≤ 600 °C, indicating that there were more defect sites in CoOx/CeO2 catalysts than CeO2 catalysts [49]. 3.4. H2 temperature programmed reduction (TPR) H2-TPR was employed for investigating the reduction characteristics of CeO2 and CoOx/CeO2 catalysts. As shown in Fig. 5 (a), for all of the calcined CeO2 catalysts, the shape of H2-TPR profiles was very similar and exhibited reduction peaks at ∼560 °C and ∼860 °C, corresponding to the reduction of surface and lattice oxygen of CeO2, respectively [50–52]. The reduction temperature of all CeO2 samples did not change, but as the calcination temperature increased, the corresponding H2 consumption measured on the H2-TPR curve decreased (Table S3). For example, the H2 consumed for CeO2 surface reduction for 500 CeO2 was 0.51 mmol/g, while only 0.08 mmol/g H2 was consumed for 800 CeO2 surface reduction (Table S3). This result could be explained with the previous findings in physical properties of CeO2 that, as calcination temperature increased, the specific surface area decreased, leading to less reducible surface oxygen. The H2-TPR profiles of the CoOx/CeO2 catalysts were shown in Fig. 5(b). In contrast to bulk CeO2, CoOx/CeO2 catalysts showed several reduction peaks at < 400 °C corresponding to the reduction of Co3O4 to CoO (< 350 °C) [53,54] and CoO to Co (350–400 °C) [53,55,56]. With the addition of CoOx, the reduction peak of surface oxygen of CeO2 was
3.5. Gas-phase catalytic activity of CeO2 and CoOx/CeO2 catalysts for NO reduction by CO The relationship between calcination temperature of CeO2 and gasphase catalytic activity of CeO2 and CoOx/CeO2 for the NO reduction by CO was investigated. Fig. 6 illustrated the NO reduction by CO performance of the bulk CeO2 and CoOx/CeO2 catalysts as a function of reaction temperature. In the case of bulk CeO2, as the reaction 5
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Fig. 6. Gas-phase catalytic activity results for NO reduction by CO oxidation over calcined CeO2. (a) NO conversion of calcined CeO2, (b) CO conversion of calcined CeO2, (c) NO conversion of CoOx/CeO2 catalysts, and (d) CO conversion of CoOx/CeO2 catalysts. Reaction conditions: 20 mL/min (5%) NO, 20 mL/min (5%) CO, ∼40 mg of catalyst was used.
temperature; (2) CeO2 showed very low activity for the NO reduction by CO, for instance, no activity up to 250 °C and < 50% NO and CO conversion up to 350 °C; (3) The addition of 2.5 Co/nm2 increased NO and CO conversion especially at lower reaction temperature. For instance, > 80% NO conversion (except for Co-800 CeO2) at 250 °C; (4) Gas-phase catalytic activity was not affected much by SSA and average particle size differences.
temperature increased, both NO and CO conversion increased monotonically (Fig. 6 (a) and (b)). However, the bulk CeO2 catalysts showed no or very low NO and CO conversions when the reaction temperature was < 300 °C. Overall, the NO and CO conversion for tested samples were quite similar although AR CeO2 and 600 CeO2 catalysts showed slightly higher NO and CO conversion comparing to 800 CeO2 catalyst. It should be noted that the SSA of 800 CeO2 was 4∼5 times lower than that of AR CeO2 and 600 CeO2. (Table S1) Also particle size of 800 CeO2 was 3∼4 times larger than that of AR CeO2 and 600 CeO2. This result indicated that SSA and particle size of CeO2 did not much affect the gasphase catalytic activity in the NO reduction by CO reaction much. The NO + CO reaction activity results of the CoOx/CeO2 catalysts were shown in Fig. 6(c) and (d). Comparing to the bulk CeO2 catalysts, the CoOx/CeO2 catalysts showed much higher catalytic performances during the NO reduction by CO reaction, especially < 300 °C, confirming that surface CoOx played a key role for the gas-phase catalytic activity. The NO and CO conversion started at 150 °C, which was 100 °C lower than that of bulk CeO2, and > 90% conversion of NO and CO was achieved at 250∼300 °C and 350∼450 °C, respectively. Unlike the results for bulk CeO2 catalysts, NO conversion was higher than CO conversion at ≥ 200 °C, and the difference became smaller as reaction temperature increased. The calcination temperature effects showed that at < 300 °C the NO and CO conversions followed the order Co-AR CeO2∼Co-600 CeO2 > Co-700 CeO2 > Co-800 CeO2. At and over 300 °C, the NO conversion was not affected by calcination temperature, while CO conversion was slightly affected by calcination temperature. Similar to the bulk CeO2, the differences in NO and CO conversions were minor compared to the SSA and average particle size differences (Table S2). Based on the observed results, it can be concluded that (1) The NO and CO conversion increased with increasing reaction
4. Discussion 4.1. Support calcination temperature effect on physicochemical properties The molecular structure (and physical property) of supporting materials and supported catalysts could be directly affected by support calcination temperature. Consequently, changed structure of catalysts (bulk or supported) could affect the catalytic activity. With varied support calcination temperatures (400∼800 °C), physicochemical properties of bulk CeO2 and CoOx/CeO2 were investigated using various techniques, such as BET, XRD, Raman, and H2-TPR. Comparing all results obtained for the series of CeO2 samples (AR ∼ 800 CeO2), it was observed that both SSA and particle size of samples were directly related to the calcination temperature. From both XRD (Fig. 3(a)) and Raman spectra (Fig. 4(a)), the CeO2 pattern sharpened with increasing calcination temperature, indicating an increased crystallite size of CeO2. The increase in crystallite size could be caused by sintering effects or agglomeration under high calcination temperatures. Correspondingly, a decrease in specific surface area and increase in average pore diameter were also noticed (Fig. 2) due to the blocking of small pores, formation of larger crystalline structures, or collapsing of pore structures. Because peaks and bands at XRD and Raman spectra of the 6
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because the lower Co wt% was loaded on high temperature treated CeO2 to achieve the same Co surface density (2.5 Co/nm2). Therefore, it can be concluded that, although the Co wt% varied, the reducibility of the CoOx/CeO2 catalysts were very similar because of the fixed Co surface density (2.5 Co/nm2).
prepared samples were not shifted, it was expected that the molecular structure of CeO2 remained the same, although defect sites were decreased with increasing calcination temperatures (Fig. 4(a)).With decreasing SSA and increasing particle size, less surface and bulk oxygen in CeO2 was exposed, resulting in less amount of H2 consumption observed in high temperature calcined CeO2 (Fig. 5). However, the reduction peak temperatures did not shift much even with the drastic change in SSA and particle size at different calcination temperatures. Thus, it can be deduced that the reducibility of CeO2 was not affected by changing of SSA and particle size. The SSA and pore diameters of 2.5 Co/nm2 CoOx/CeO2 catalysts showed similar trends as observed with bulk CeO2: decreasing SSA, increasing pore diameter, and similar pore volume with increasing support calcination temperatures. However, the SSA differences (%) between CeO2 and CoOx/CeO2 varied with different support calcination temperatures. As shown in Table S2, the SSA difference (%) was ∼18.8% between AR CeO2 and Co-AR CeO2, while ∼2.2% change was observed between 800 CeO2 and Co-800 CeO2. It can be deduced that AR CeO2 and low temperature calcined CeO2 (e.g. 400–600 CeO2) contained small pore structure (< 4 nm) which facilitated CoOx dispersion. In the case of high temperature (> 600 °C) calcined CeO2, small pores were already blocked during the calcination procedures and surface CoOx was located on larger pores (> 8 nm) inside the CeO2 mesopore structure. It is worthwhile to re-emphasize that, to achieve the same Co surface density (2.5 Co/nm2), the Co wt% changed (e.g., 5.4 wt% for Co-AR CeO2 and 1.0 wt% for Co-800 CeO2) based on the surface area of calcined CeO2. The corresponding Co wt% were calculated using Eq. (2) shown below and the values were included in Table S2.
Sample S. D . =
wt % Co atoms × 6.02 × 1023 100 mol g ] M . W of Co [= 58.9332 mol 2 m wt % Co
Support S. A [
g
] × (1 −
100
) × 1018
4.2. Support calcination temperature effect on catalytic activity in NO + CO reaction As discussed above, the calcination temperature of CeO2 led to changes in molecular structures in both bulk CeO2 and supported CoOx/ CeO2 catalysts. Consequently, changed structure of catalysts (bulk or supported) could affect the catalytic activity. As shown in Fig. 6, it is clear that, starting from 200 °C, the CoOx/CeO2 catalysts showed much higher NO conversion than bulk CeO2, confirming the critical role of surface CoOx in enhancing the catalytic activity. It has been proposed that the NO reduction by CO reaction happens in the following reaction steps [57]
2NO + CO → N2 O + CO2
(3)
N2 O + CO → N2 + CO2
(4)
According to previous studies [9,58], NO adsorption temperature is much lower than that of CO adsorption for supported transition metal oxide catalysts. This explained the difference in NO conversion and CO conversion under the same reaction temperature (especially 150 °C ∼ 350 °C for CoOx/CeO2 catalysts in Fig. 6 (c) and (d)), where NO conversion was higher than CO conversion. However, from Fig. 6 (a) and (b), it can be noticed that, the NO and CO conversion over bulk CeO2 catalysts were very similar and occurred at > 300 °C. This result can be explained that NO adsorption over bulk CeO2 was initiated at higher temperature (> 250 °C), while N2O formation and decomposition by CO occurred at > 300 °C. In the case of CoOx/CeO2 catalysts, the surface CoOx species facilitated N2O decomposition at lower temperature comparing to bulk CeO2, thus freeing up many adsorption sites for other NO molecules, resulting in much higher NO conversion starting from 200 °C. Also, since the surface CoOx enabled N2O decomposition at lower temperature, consequently, the N2 selectivity (Fig. 7) of the CoOx/CeO2 catalysts were higher than that of bulk CeO2 catalysts, especially for reaction temperature < 400 °C. Moreover, as shown in Fig. 6, NO and CO conversion increased after adding the CoOx, possibly due to the increased defect sites/oxygen vacancies (Fig. 4(c)), improved reducibility of CeO2 (Fig. 5) and intrinsically higher activity of CoOx. It has been reported that defected
(2)
2
The XRD patterns of 2.5 Co/nm CoOx/CeO2 provided that surface CoOx was fully dispersed or crystallite size of CoOx was too small to be detected (Fig. 3(b)), which was well matched to the literature results. Recently, Peck et al [10] and S. Zhang et al [36] investigated the monolayer coverage ranges of the CeO2 supported CoOx catalysts using by XRD, Raman, and XPS techniques. The authors reported that monolayer coverage of CoOx/CeO2 should be 2.58 Co/nm2 or 2.3 Co/ nm2 ∼ 2.7 Co/nm2. The Raman spectroscopy results (Fig. 4 (b)), however, provided the evidence of Co3O4 crystalline structure (∼690 cm−1 Raman shift) indicating the inhomogeneity of catalyst surface. Due to the limitation of XRD, it can be also expected that 2.5 Co/nm2 CoOx/CeO2 samples contained < 5 nm Co3O4 nanocrystalline structure under the monolayer coverage. In addition to the detection of Co3O4, Raman spectroscopy results showed that the defect sites of CeO2 increased by adding the surface CoOx. (Fig. 4(c)). The reducibility of CoOx/CeO2 also improved comparing to bulk CeO2. The addition of CoOx species could facilitate the redox cycle of CeO2 (Ce3+ ↔ Ce4+), leading to the reduction temperature of surface oxygen of CeO2 shifting to lower temperature (Fig. 5). Moreover, Fig. 4(c) showed that the defect concentration in ceria decreased as the support calcination temperature increased. It could be hypothesized that, when Co was loaded on CeO2 with less defect sites, larger amount of gaseous O2 can chemisorb on Co to form larger crystalline structures of CoOx. Upon reduction, the CoOx species was reduced to metallic Co first, then the exposed Co metal on the surface of CeO2 with less defect sites would promote H2-spillover to facilitate the surface reduction, resulting in surface CeO2 reduction temperature shifting to even lower temperatures. Also, as shown in Fig. 5(b), the reduction temperature of CoOx (< 300 °C) did not shift, suggesting that the reducibility of surface CoOx species was not affected by the inhomogeneity of catalyst surface. However, the amount of H2 consumed to reduced CoOx (< 300 °C) decreased as the support calcination temperature increased (Fig. 5b),
Fig. 7. Comparison between bulk CeO2 and CoOx/CeO2 catalysts in N2 selectivity under different reaction temperatures. 7
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rates were performed to ensure that the reaction conditions were free of external diffusion effects [61,62]. As shown in Fig. S3, catalytic activity was not changed with different total flow rates, such as 20 ml/min, 40 ml/min, and 60 ml/min. Based on the results, it can be deduced that the rate data will be free from external mass transfer effect under the experimental condition used in this study. 5. Conclusions In this work, a series of CeO2 samples, calcined at different temperature (400 °C∼800 °C), were synthesized, as well as CeO2 (SSA: 44∼233 m2/g) supported CoOx (1.0∼5.4 Co wt%) catalysts. The synthesized catalysts were then tested for the NO reduction by CO reaction. As the support calcination temperature increased (especially at ≥700 °C), physical properties (SSA, pore diameter, and particle size) changed significantly comparing to as-received (AR) CeO2. No evidence of surface CoOx was observed by XRD, while nanocrystalline structure of surface CoOx species and CeO2 defect sites were identified by Raman. H2-TPR showed that the reducibility of surface CeO2 was improved by adding CoOx, while the reduction temperature of bulk CeO2 was not affected. It was clearly observed that CoOx/CeO2 catalysts had higher gas-phase catalytic activity than bulk CeO2 catalysts in the NO reduction by CO reaction. Although it was clear that physical properties of synthesized catalysts were affected by support calcination temperatures, the defect site ratio (I600/I460 by Raman) to NO conversion and the molar rate of NO and CO during the NO reduction by CO at 200 °C over the studies CoOx/CeO2 catalysts (2.5 Co/nm2 and CeO2 calcined at 400 °C∼800 °C) provided that the catalytic activity was mainly controlled by the surface CoOx, while physical properties of the support material had little impacts on the catalytic activity.
Fig. 8. Comparison of NO conversion between 800 CeO2 and Co-800 CeO2.
Declaration of Competing Interest None. Acknowledgments S. Zhang and T. Kim would like to thank the Advanced Energy Research and Technology Center (AERTC) at the Stony Brook University for providing us with the facilities. The authors would also like to thank Anson Law and Ming Hu for their assistance in performing Raman experiments. This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A5A1009592).
Fig. 9. Catalytic activity of NO and CO at 200 °C for CeO2 and CoOx/CeO2 catalysts.
CeO2 sites improved the catalytic activity for the NO reduction by CO and CO oxidation reaction [8,59,60]. As shown in Fig. 8, however, although the catalytic activity (NO conversion) of Co-800 CeO2 (T20 = 210 °C and T50 = 238 °C) was much higher than that of 800 CeO2 (T20 = 355 °C and T50 = 411 °C), the I600/I460 ratio was very similar for the 800 CeO2 (I600/I460 = 0.006) and Co-800 CeO2 (I600/ I460 = 0.009). Granted that the effect of CeO2 defect sites (or oxygen vacancy) on catalytic activity cannot be completely neglected, the observed results indicated that the NO and CO conversion were not much related to the change of defect sites under the experimental conditions. In order to further understand the impacts of support calcination temperature on the inherent activity of CoOx/CeO2 catalysts, the catalytic activity of NO and CO (moles of NO and CO converted/unit time/ catalyst surface area) at 200 °C were calculated. As shown in Fig. 9, the catalytic activity of both NO and CO did not change much with the support calcination temperature difference, while the catalytic activity for the bulk CeO2 were neglectable. By combining the physicochemical and activity results, it can be concluded that the catalytic activity of the reaction was mainly controlled by the surface density of cobalt, but not by the physical properties of the catalysts (SSA, pore diameters, crystallite size). It was also found that the catalytic activity of NO was about twice as that of CO at 200 °C, which corresponded well with the N2O formation step (Eq. (3)), where NO and CO reacted at a 2:1 ratio. Gas phase catalytic activity tests with fixed GHSV and various total gas flow
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