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CeO2 catalysts
Journal Pre-proof Effect of samarium and praseodymium additional on water gas shift performance of Co/CeO2 catalysts P. Tepamatr, N. Laosiripojana, T. Sesuk, S. Charojrochkul PII:
S1002-0721(19)30499-5
DOI:
https://doi.org/10.1016/j.jre.2019.12.003
Reference:
JRE 662
To appear in:
Journal of Rare Earths
Received Date: 12 July 2019 Revised Date:
5 November 2019
Accepted Date: 4 December 2019
Please cite this article as: Tepamatr P, Laosiripojana N, Sesuk T, Charojrochkul S, Effect of samarium and praseodymium additional on water gas shift performance of Co/CeO2 catalysts, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Effect of samarium and praseodymium additional on water gas shift performance of Co/CeO2 catalysts P. Tepamatra,∗, N. Laosiripojanab, T. Sesukc, S. Charojrochkulc a
Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumthani 12120, Thailand
b
The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand
c
National Metal and Materials Technology Center, NSTDA, Pathumthani 12120, Thailand
Abstract The performance of Co supported over ceria and doped ceria (by Sm and Pr) catalysts towards the water gas shift reaction were studied for the removal of CO from syngas to produce high purity hydrogen for a fuel cell application. It was found that 1%Co/Ce-5%Sm-O yields the highest catalytic performance towards this reaction compared to undoped-Sm and doped-Pr. An addition of Sm onto ceria support has revealed a small crystallite size with high surface area and well dispersed cobalt on ceria surface. Moreover, a presence of Sm has increased the reducibility of cobalt species and surface oxygen. The positive effect of Sm on increasing the WGS activity of Co/CeO2 is because Sm contributes to the reduction of Ce4+ to Ce3+ which gives rise to oxygen vacancies and facilitates the electron movement at the surface leading to an ease of surface reduction.
Keywords: water gas shift, hydrogen production, Co, ceria, Sm, rare earths
Foundation item: Project supported by Thammasat University under the TU New Research Scholar (5/2561) and the Synchrotron Light Research Institute (SLRI), Thailand. *Corresponding author. Tel.: 025644440 ext. 2401 E-mail address: [email protected] (P. Tepamatr).
1. Introduction
Hydrogen is a promising fuel of a fuel cell processor for stationary and mobile applications. Hydrogen energy is a sustainable fuel resource which can be generated from reforming of biomass and its products or from water electrolysis. The transportation and storage of hydrogen energy are easier than that of solar or wind energy. In addition, using hydrogen energy can reduce greenhouse gas. Hence, hydrogen energy has been widely studied to decrease a use of fossil fuels and the effect of air pollution. Hydrogen production has been explored in several methods such as borohydride hydrolysis, electrolysis of water and natural gas steam reforming. Among these methods, a steam reforming of natural gas is still the predominant and economical way for hydrogen generation [1-5]. The conventional technologies for hydrogen generation are steam reforming and partial oxidation of methane which produce a large amount of carbon monoxide as by-product. High purity hydrogen is required for utilization in fuel cells or gas turbines. Hydrogen is a clean fuel for the future and a promising alternative to fossil fuel. Fuel cells have been remarkably developed because of the enhancement of energy conversion efficiency and serious concerns in environmental impact from a combustion of fossil fuels. Fuel cells can be operated best on pure hydrogen. In a fuel cell system, O2 from air reacts with H2 to generate electrical energy without any pollution produced. The hydrogen purity grade requirement is dependent on the type of fuel cell. High temperature fuel cells can tolerate high concentrations of CO and CO2 in the hydrogen feed. On the other hand, the performance of low temperature fuel cells decrease when CO is contained in the H2 feed because CO can be adsorbed irreversibly on the surface of the electrode catalyst of fuel cell and then block the reaction sites for hydrogen oxidation. Hence, CO is a strong poison even at low CO concentrations for low temperature fuel cells such as a proton exchange membrane fuel cell (PEMFC). PEMFC is a potential candidate for the propulsion of vehicles and for dispersed power plants. This type of low temperature fuel cell needs H2 purity containing CO level of less than 10 ppm [6-8].
The increasing demand for hydrogen energy and progressively growing attention in fuel cell technology especially in carbon monoxide removal, water gas shift reaction becomes one of the key steps in changing of carbon monoxide into carbon dioxide and hydrogen through reaction with water. CO + H2O ↔ CO2 + H2
∆H = −41.2 kJ/mol
(1)
Water gas shift is a slight exothermic reaction. The reaction is favoured when the temperature is decreased due to thermodynamic limitations. However, its kinetics is limited at low temperatures. The water gas shift catalysts are not much active to reach equilibrium at low temperatures. Therefore, water gas shift reaction is industrially operated in two stages. A high temperature shift performed in the temperature of 350 – 450 ºC to achieve carbon monoxide in the range of 2 vol% – 3 vol% followed by a low temperature shift operated in the temperature of 200 – 300 ºC to obtain a very low amount of carbon monoxide to less than 0.3 vol%. Cu/ZnO/Al2O3 and Fe2O3/Cr2O3 catalysts are commonly used for low temperature and high temperature water gas shift, respectively [9,10]. The commercial Fe2O3/Cr2O3 catalyst is widely used in industrial applications. However, the researchers replace Cr with alternative metal such as Ni, Cu and Co because the carcinogenic nature and toxicity of Cr compounds. The literature survey presents that Co has been used in the water gas shift reaction as a sulfur resistant catalyst [11,12]. Recently, K/Co3O4 catalyst has been investigated for water gas shift reaction. It was found that an addition of K in Co3O4 increased the stability and activity of the catalyst [13]. In the present work, the effect of Sm and Pr on the water gas shift performance of Co/ceria catalyst has been studied. The influence of Sm and Pr on the structural and redox properties of Co/CeO2 catalysts has been intensively investigated. The results were supported by several catalyst characterization including X-ray diffraction, Raman spectroscopy, BET surface area, H2-chemisorption, H2temperature programmed reduction and X-ray absorption spectroscopy technique.
2. Experimental 2.1. Catalysts Preparation 2.1.1. Preparation of Support by Precipitation Method CeO2 and doped ceria supports were prepared by a precipitation method [14,15]. An
appropriate
amount
of
Ce(NO3)3·6H2O
(99%,
AlfaAesar),
M(NO)3·6H2O (M = Sm or Pr) and H2NCONH2 (98%, Aldrich) were dissolved in a minimal volume of deionized water. The mixture reactants were stirred and heated to obtain homogenous solution. Then ammonium hydroxide (99.99%, Sigma-Aldrich) was added dropwise at the rate of 1 mL/min to obtain the precipitate. The solution was further stirred and heated at 120 °C for 4 h to remove ammonia and age the support. The filtered sample was dried overnight at 110 °C in an oven and then calcined at 450 °C for 4 h. 2.1.2. Preparation of Catalysts Cobalt catalysts were prepared by impregnation method. An appropriate amount of Co(NO3)2·6H2O (98%, Sigma-Aldrich) was dissolved with deionized water. The salt solution was added to CeO2, 5%Pr doped CeO2 and 5%Sm doped CeO2 support. All samples were dried overnight at 110 °C and then calcined at 650 °C for 8 h. 2.2. Catalyst characterization 2.2.1. Standard characterization Quantachrome NOVA 1200e instrument was used to determine specific surface areas by N2 adsorption-desorption isotherms at 77.3 K. The specific surface areas of all samples were calculated from the BET procedure in the range of 0.05–0.3 of relative pressures of nitrogen. The samples (~0.20 g) were outgassed in vacuum at 300 °C for 6 h prior to the measurement.
The X-ray diffraction patterns presented in this study were recorded employing nickel-filtered Cu Kα radiation (λ = 0.15406 nm). The analyses were carried out at 0.02° per step and 0.5 s retention at each step over a 2θ range of 20 °– 90° with the current of 30 mA and 40 kV. The crystallite size of ceria is calculated using Scherrer’s equation. Dhkl =
0.9λ β cos θ
(2)
where Dhkl is the ceria crystallite size, λ is the wavelength of the used radiation (λ = 0.15406 nm), β is the full width at half maximum of the peak and θ is the incoming diffraction angle. 2.2.2. Raman spectroscopy The vibrational mode of all catalysts was observed using a Raman spectrometer and the vibrational signal was detected using a Charge Coupled Device detector. The Raman spectra of Co catalysts were analyzed using FTIR/FT-Raman (Perkin Elmer System 2000) and collected in the range of 200 – 1300 cm–1 with Ar ion laser irradiation. The samples were irradiated with an output power 10 mW and wavelength of 532 nm. 2.2.3. Temperature programmed reduction (TPR) H2-TPR was performed using a catalyst analyzer BELCAT-B. TPR experiments were performed under a flow of 5%H2/95%Ar mixture over 0.05 g of catalyst from room temperature to 1000 °C using a heating rate of 10°C/min. Prior to the TPR measurement, the catalysts were treated under high purity helium gas at 120 °C for 30 min. The amount of H2 uptake during the reduction was measured by thermal conductivity detector. The curves in H2-TPR profile correspond to H2 consumption. The sensitivity of the detector was calibrated by reducing a known weight of CuO.
2.2.4. Pulse injection method (chemisorption) Metal dispersion measurement in supported Co catalysts was operated by introducing 10%CO/He to the sample until chemisorption saturation and measuring the metal surface area of the sample from the total gas chemisorption. Metal dispersion can be calculated from the metal surface area and the weight of the catalyst. Approximately 50 mg of each sample was loaded into a quartz reactor. All catalysts were reduced under a flow of hydrogen at 400 °C for an hour. After cooling under a flow of helium, CO chemisorption pulse was performed under the flow of CO in a He carrier gas with the rate of 30 cm3/min at 50 °C. The flow of CO out from the reactor was monitored by a thermal conductivity detector. 2.2.5. X-ray absorption near edge structure (XANES) The oxidation states of metal in each sample were determined using X-ray absorption near edge structure on the beamline 8 of Synchrotron Light Research Institute in Nakhon Ratchasima, Thailand. The fluorescence mode was used for the investigation of Co K, Sm L3 and Pr L3 absorption edges and the transmission mode was operated for Ce L3 absorption edge determination with a Lytle detector. Synchrotron radiation is tunable by fixed-exit Double Crystal Monochromator equipped with InSb (111) and Ge (220) crystal for low and high energy ranges, respectively. The samples were placed over the Kapton window which was put on the sample frame. Ion chambers were filled with a mixture of argon and helium. These chambers were installed in front of and behind the sample to continuously detect the incident (I0) and transmitted (I1) X-rays beams. 2.3. Activity test Water gas shift activity measurements were carried out in the temperature range of 100 – 600 °C. Usually, 150 mg of catalyst was loaded into the stainless steel tube reactor which was sandwiched between two layers of quartz wool. The reactor was placed inside a tube furnace. Mass flow controllers (Albourg) were
used to control the flow rate of feed gas mixture of CO and N2. The amount of water vapor was controlled using a syringe pump. Preliminary experiments were conducted to determine suitable conditions from which internal and external mass transfer effects are not predominant. Considering the effect of external mass transfer, the total flow rate was kept constant at 100 ml/min in all testing. In addition, the catalyst particle size diameter was between 100 - 200 µm in all experiments. The feed gas consists of 5% CO, 10% H2O and balanced N2. The remaining reactant and product in the outlet gasses were analyzed by an on-line gas chromatography (Shimadzu GC-14B) equipped with thermal conductivity detector. A Unibeads C, 15 m × 0.53 mm stainless-steel packed column was utilized in the gas chromatography. The catalytic activities can be calculated by the following equation. %COconversion =
COin − COout × 100 COin
(3)
where COin and COout are the molar flow in the inlet and outlet feed gas, respectively. The reaction rates were obtained from separate experiments using 20 mg of catalysts and the feed gas mixture of 5% CO, 10% H2O and balanced N2 where the conversions of reactants were lower than 30%. The water gas shift rates can be calculated by the following equation RCO = (FCO × XCO)/Wcat
(4)
where RCO is the CO conversion rate (mol/(g·s)), FCO is the CO molar flow rate (mol/s), Wcat is the mass of the catalysts (g) and XCO is the CO conversion.
3. Results and Discussion
3.1. Catalysts characterization Figure 1 shows XRD patterns for supports and Co catalysts. X-ray diffraction patterns corresponding to CeO2 in a fluorite-type cubic crystal structure were found in every catalyst. All diffraction peaks are in good agreement with the standard data for ceria with peak intensity according to JCPDS No. 43-1002. For XRD patterns of Sm-doped ceria and Pr-doped ceria, other oxide reflection was not found indicating Sm and Pr incorporation into CeO2 lattice to form a solid solution [16,17]. In addition, XRD patterns of supports and Co catalysts show that incorporation of Pr3+ and Sm3+ ions into CeO2 shift the diffraction peak to lower angle side which corresponds to the lattice expansion. Table 1 presents the calculated lattice parameter of supports and Co catalysts. The increase of lattice parameters of Pr-doped ceria and Sm-doped ceria when compared with pure ceria (0.5404) related to the effective ionic radius of the dopant ion [18], as the ionic radii of Pr3+ (0.113 nm) and Sm3+ (0.108 nm) are larger than the ionic radius of Ce4+ (0.097 nm) [17]. The addition of Pr3+ and Sm3+ ions into ceria may enlarge the radius of cerium ion. In addition, oxygen vacancies are generated in the ceria lattice to maintain the charge equilibrium which is exhibited for further lattice expansion [19]. The average crystallite sizes of ceria and modified ceria, calculated from X-ray line broadening at 2θ ~ 28° using Scherrer’s equation, are compiled in Table 1 together with BET specific surface areas and average pore diameters. Doping CeO2 with samarium and praseodymium leads to a slight reduction of crystallite size. An addition of Co on ceria and modified ceria supports leads to larger crystallites with lower surface area. This result is suggested to be due to growth and aggregation of ceria crystallites after calcination at high temperature. Sm3+ and Pr3+ in 1%Co/Ce-5%M-O (M = Sm and Pr) stabilizes the support and prevent the support from sintering by maintaining small crystallite size and high surface area. The average pore diameters of Sm-doped and Pr-doped ceria in the presence of Co
catalysts are identical with the pore diameter of Co on pure ceria. Higher surface area of Co on doped ceria supports together with small crystallite size were observed. The result from % metal dispersion is also exhibited in Table 1. The metal dispersion data indicates that Sm assists in dispersing cobalt metal on a ceria support. Structural information of supported Co catalysts can be explained by Raman spectroscopy. The shape of Raman spectrum depends on the microstructure of ceria. As shown in Fig. 2, all samples exhibit a Raman peak at ∼460 cm–1 which can be assigned to the F2g mode of the cubic fluorite crystal structure of CeO2. This vibrational mode is the symmetrical stretching of the oxygen atoms around Ce ions [20]. This peak is very sensitive to any disorder in the oxygen sub-lattice which resulting from doping, thermal or grain size induced non-stoichiometric disorders [21]. Another broad peak at about 560 – 600 cm–1 was assigned to oxygen vacancies created in the ceria structure [22]. Sm and Pr addition to ceria exhibit broad weak peak at 598 and 569 cm–1, respectively. Therefore, it is confirmed that oxygen vacancies generation is facilitated by doping of Sm and Pr. Figure 3 shows the H2-TPR profiles of supports and Co catalysts. In pure ceria, the small reduction peaks appear at 500 °C and 850 °C which are attributed to the reduction of surface ceria and the bulk, respectively. The reduction peak of Ce5%Sm-O and Ce-5%Pr-O are similar to that of CeO2. For supported Co catalysts, there are three main reduction peaks corresponding to the reduction of cobalt oxide, surface ceria and the bulk. The reduction peak at low temperature of 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO is ascribed to the reduction of Co2+ interacting with ceria to Co. The result suggests that Co may exist in the form of Co-Ce-O solid solution or CoO which strongly interacts with CeO2. According to the literature [23,24], CoO or Co-Ce-O solid solutions is prominent in CoOx/CeO2 for the low content of cobalt in the materials, while Co3O4 is prominent in the materials for the high content of cobalt/Ce ratios. A stepwise
reduction process of Co3O4/ceria is Co3+ → Co2+ → Co. By comparing the reducibility of supported Co catalysts, it can be observed that the reduction of 1%Co/Ce5%SmO starts at a much lower temperature than the other catalysts, suggesting higher extent of dispersion and interaction of Co with Sm-doped ceria. The reduction peak of 1%Co/Ce5%SmO shifts to the lowest temperature among these catalysts. It is interesting to report that an addition of Sm into 1%Co/CeO2 significantly shifts the reduction temperature of Co species and surface ceria to lower temperature. Quantitative estimation of the H2 consumption of Co catalysts was carried out by measuring the area of H2-TPR curve. For this work, the reduction peaks of Co species were only considered. The results showed that 1%Co/Ce5%SmO has higher H2 consumption values than that of the other catalysts. 3.2. Water gas shift activity of Co catalysts Figure 4 presents the water gas shift activity of Co on ceria and doped ceria. Ceria and doped ceria supports are not very active for the WGS reaction. An addition of Co on various supports drastically increases the catalytic activity. It can be seen that 1%Co/Ce5%SmO is an active catalyst for WGS reaction. The activity of 1%Co/Ce5%SmO starts above 300 °C and increases rapidly to reach the maximum of 86% conversion at around 450 °C. The conversion above 450 °C reaches equilibrium and slightly decreases. A modification of ceria with Sm drastically increased the WGS activity of Co catalyst around 40% conversion when compared with 1%Co/CeO2. The activity of 1%Co/Ce5%PrO is very close to the activity of 1%Co/CeO2. According to the TPR results, the increased activity due to the reduction occurs at low temperature. In the regenerative mechanism, CeO2 plays an important role in transporting oxygen to CO via changes in oxidation state. H2-TPR of ceria support displayed that this process occurs at 500 °C. However, the reduction temperature of surface ceria drops to 437 °C when Co was
anchored onto ceria. Therefore, CeO2 in the vicinity of Co is easily reduced. Moreover, an addition of Sm to Co/CeO2 alter the reduction behaviour of Co catalyst. The cobalt-based catalyst is active for Fischer-Tropsch synthesis. Thus, CO2 and CH4 selectivity were studied for this work as shown in Fig.5. It presented that only
1%Co/ceria
produced
methane
at
lower
temperatures.
Whereas
1%Co/Ce5%SmO and 1%Co/Ce5%PrO are highly selectivity towards the water gas shift reaction throughout the studied temperature. The Arrhenius plot of Co on ceria and doped ceria are presented in Fig.6. These plots present that the CO conversion rate of 1%Co/Ce5%SmO is higher than the rate of other catalysts. Table 2 summarizes the rate of water gas shift reaction at 450 °C and activation energy that are obtained from the slope of the Arrhenius plot. 1%Co/Ce5%SmO gives the highest WGS rate and the lowest of the activation energy. It is interesting to study the role of Sm in enhancing the water gas shift performance of Co on ceria. From the TPR results, 1%Co/Ce5%SmO catalyst can be reduced at lower temperature than of 1%Co/CeO2. Since electron transfer among the involved species occur in the reduction process, the X-ray absorption spectroscopy (XAS) technique was employed to explain the electron movement within the catalyst. XAS spectra consist of two main features of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES technique provides information on oxidation states and valence shell electron population. In this study, XANES was practiced to determine the oxidation states of metals in the catalyst. Figure 7(a) presents XANES spectra of Ce L3 absorption edges for 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO as compared to CeO2 and Ce(NO3)3·6H2O. XANES spectrum of Ce(NO3)3·6H2O exhibits a single white line at 5725.1 eV which was assigned to Ce in the trivalent state [25,26]. A double white line is usually observed for ceria. These two peaks were ascribed to the
transition of Ce 2p to mixed valence state of ground state electron configuration Ce 4f0 and another one with 4f1 [26]. The plot of XANES spectra of Co K absorption edges for 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO as compared to Co(II,III) oxide, Co(NO3)2.6H2O and Co0 are displayed in Fig. 7(b). Linear combination fit was used to quantify elemental species in an unknown sample, interpreting that the total mass absorption of an edge element in the unknown is a linear sum of mass absorptions of standards. Linear combination fit result of 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO is shown in Figure 8 which Co(II,III) oxide and Co(NO3)2·6H2O are standards for this plot. The result indicates that the oxidation states of cobalt in 1%Co/Ce5%SmO compound are higher than the oxidation states of cobalt in 1%Co/Ce5%PrO and 1%Co/CeO2. It can be concluded that the electron density was transferred from Co2+ to Ce4+ leading to an increase in oxidation state of Co2+ and lowering of oxidation state of Ce4+. The oxidation state of cobalt in 1%Co/Ce5%SmO was the highest among supported Co catalysts (Figure 9). This result suggests that cobalt also donates some electrons to Sm leading to a further increase in its oxidation state. From these results, a conclusion can be drawn that cobalt donates its electron to both cerium and samarium. Therefore, Sm helps Co in reducing Ce4+ and giving rise to more oxygen vacancies which facilitates the electron movement at the surface leading to an increase in the water gas shift rates. 4. Conclusions The influence of Sm and Pr on the WGS activity of Co/CeO2 was studied and the results show that the WGS performance of 1%Co/Ce5%SmO was much higher than the WGS activity of 1%Co/CeO2 and 1%Co/Ce5%PrO. 1%Co/Ce5%SmO yields the highest catalytic activity with the maximum of 86% conversion at about 450 °C. An addition of Sm into Co/CeO2 increases the surface area, reduces crystallite size and assists a better dispersion of Co on the ceria surface. The major role of Sm in enhancing the WGS activity of Co/CeO2 may be the ability to
promote the reduction of Co and the ceria surface which leads to lowering of reduction temperature, increased oxygen vacancies and increased surface redox property. The reduction of catalyst surface becomes easier since an electron density transfer between Co, Sm and Ce occur. XANES technique was used to explain electron movement within the catalysts. Linear combination fit indicates that Sm assists Co in reducing CeO2 and giving rise to more Ce2O3 at the surface of the ceria support. The presence of Ce3+ at the ceria surface gives rise to oxygen vacancies which facilitate the redox process at the surface and these effects conduct to an enhance in the WGS rates.
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Graphical Abstracts:
Cobalt donates its electron to both cerium and samarium. Therefore, Sm helps Co in reducing Ce4+ and giving rise to more oxygen vacancies which facilitates the electron movement at the surface leading to an increase in the water gas shift rates.
Fig.1 XRD patterns of supports and Co catalysts. (1) CeO2; (2) Ce5%SmO; (3) Ce5%PrO; (4) 1%Co/CeO2; (5) 1%Co/Ce5%SmO; (6) 1%Co/Ce5%PrO.
Fig.2 Raman spectra of Co catalysts and supports.
Fig.3 H2-TPR profiles of Co catalysts and insert of the figure corresponding to H2TPR profiles of supports.
Fig.4 WGS catalytic activity of Co catalyst.
Fig. 5 Selectivity to CO2 and CH4 as a function of temperature over Co catalysts.
Fig. 6. Arrhenius plot of reaction rate over Co catalysts.
Fig. 7. XANES spectra of Ce L3 absorption edge (a) and XANES spectra of the Co K absorption edge (b).
Fig. 8 Linear combination fit results of Co compounds.
Fig. 9 Relationship between the rate at 450 °C and the oxidation state of Co.
Table 1 Physicochemical properties of supported Co catalysts. Catalysts
SBETa (m2/g)
RBJH (nm)
CeO2
70.7
2.2
12.9
0.5404
1.63
Ce5%SmO
69.9
2.2
11.2
0.5419
1.79
Ce5%PrO
70.9
2.1
7.3
0.5421
1.77
1%Co/CeO2
33.2
2.2
14.8
0.5411
13.3
1%Co/Ce5%SmO 42.0
2.2
13.8
0.5421
22.5
1%Co/Ce5%PrO
2.2
9.2
0.5416
18.4
47.9
Crystalliteb size (nm)
Latticeb %dispersionc parameter(nm)
a
Estimated from N2 adsorption at -196 oC ; RBJH - BJH adsorption average pore diameter
b
Calculated from the (111) crystallographic plan; c Estimated from CO-chemisorption.
Table 2 The rate of reaction at 450° C and activation energies of Co catalysts. Sample
H2 consumptiona mmol/g
Rate at 450°C (µmol/(g·s))
Ea (kJ/mol)
1%Co/CeO2
0.20
46.4
76 ± 0.7
1%Co/Ce5%SmO
0.66
72.8
68 ± 0.6
1%Co/Ce5%PrO
0.52
53.2
74 ± 0.4
a
Estimated from H2-TPR.
Highlights •
The effects of Sm and Pr on the catalytic activity of Co/ceria were studied.
•
Doping of Sm onto Co/CeO2 increases the surface area and reduces crystallite size.
•
Sm prevents the support from sintering and helps in dispersing Co metal on ceria support.
•
Addition of Sm in Co/ceria gives rise to oxygen vacancies which leading to easier surface reduction.
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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
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Author’s name Pannipa Tepamatr Navadol Laosiripojana
Sumittra charojrochkul Thanathon Sesuk
Affiliation Department of Chemistry, Faculty of Science and Technology, Thammasat University, Thailand The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Thailand National Metal and Materials Technology Center, NSTDA, Thailand National Metal and Materials Technology Center, NSTDA, Thailand
S1002-0721(19)30499-5
DOI:
https://doi.org/10.1016/j.jre.2019.12.003
Reference:
JRE 662
To appear in:
Journal of Rare Earths
Received Date: 12 July 2019 Revised Date:
5 November 2019
Accepted Date: 4 December 2019
Please cite this article as: Tepamatr P, Laosiripojana N, Sesuk T, Charojrochkul S, Effect of samarium and praseodymium additional on water gas shift performance of Co/CeO2 catalysts, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Effect of samarium and praseodymium additional on water gas shift performance of Co/CeO2 catalysts P. Tepamatra,∗, N. Laosiripojanab, T. Sesukc, S. Charojrochkulc a
Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumthani 12120, Thailand
b
The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand
c
National Metal and Materials Technology Center, NSTDA, Pathumthani 12120, Thailand
Abstract The performance of Co supported over ceria and doped ceria (by Sm and Pr) catalysts towards the water gas shift reaction were studied for the removal of CO from syngas to produce high purity hydrogen for a fuel cell application. It was found that 1%Co/Ce-5%Sm-O yields the highest catalytic performance towards this reaction compared to undoped-Sm and doped-Pr. An addition of Sm onto ceria support has revealed a small crystallite size with high surface area and well dispersed cobalt on ceria surface. Moreover, a presence of Sm has increased the reducibility of cobalt species and surface oxygen. The positive effect of Sm on increasing the WGS activity of Co/CeO2 is because Sm contributes to the reduction of Ce4+ to Ce3+ which gives rise to oxygen vacancies and facilitates the electron movement at the surface leading to an ease of surface reduction.
Keywords: water gas shift, hydrogen production, Co, ceria, Sm, rare earths
Foundation item: Project supported by Thammasat University under the TU New Research Scholar (5/2561) and the Synchrotron Light Research Institute (SLRI), Thailand. *Corresponding author. Tel.: 025644440 ext. 2401 E-mail address: [email protected] (P. Tepamatr).
1. Introduction
Hydrogen is a promising fuel of a fuel cell processor for stationary and mobile applications. Hydrogen energy is a sustainable fuel resource which can be generated from reforming of biomass and its products or from water electrolysis. The transportation and storage of hydrogen energy are easier than that of solar or wind energy. In addition, using hydrogen energy can reduce greenhouse gas. Hence, hydrogen energy has been widely studied to decrease a use of fossil fuels and the effect of air pollution. Hydrogen production has been explored in several methods such as borohydride hydrolysis, electrolysis of water and natural gas steam reforming. Among these methods, a steam reforming of natural gas is still the predominant and economical way for hydrogen generation [1-5]. The conventional technologies for hydrogen generation are steam reforming and partial oxidation of methane which produce a large amount of carbon monoxide as by-product. High purity hydrogen is required for utilization in fuel cells or gas turbines. Hydrogen is a clean fuel for the future and a promising alternative to fossil fuel. Fuel cells have been remarkably developed because of the enhancement of energy conversion efficiency and serious concerns in environmental impact from a combustion of fossil fuels. Fuel cells can be operated best on pure hydrogen. In a fuel cell system, O2 from air reacts with H2 to generate electrical energy without any pollution produced. The hydrogen purity grade requirement is dependent on the type of fuel cell. High temperature fuel cells can tolerate high concentrations of CO and CO2 in the hydrogen feed. On the other hand, the performance of low temperature fuel cells decrease when CO is contained in the H2 feed because CO can be adsorbed irreversibly on the surface of the electrode catalyst of fuel cell and then block the reaction sites for hydrogen oxidation. Hence, CO is a strong poison even at low CO concentrations for low temperature fuel cells such as a proton exchange membrane fuel cell (PEMFC). PEMFC is a potential candidate for the propulsion of vehicles and for dispersed power plants. This type of low temperature fuel cell needs H2 purity containing CO level of less than 10 ppm [6-8].
The increasing demand for hydrogen energy and progressively growing attention in fuel cell technology especially in carbon monoxide removal, water gas shift reaction becomes one of the key steps in changing of carbon monoxide into carbon dioxide and hydrogen through reaction with water. CO + H2O ↔ CO2 + H2
∆H = −41.2 kJ/mol
(1)
Water gas shift is a slight exothermic reaction. The reaction is favoured when the temperature is decreased due to thermodynamic limitations. However, its kinetics is limited at low temperatures. The water gas shift catalysts are not much active to reach equilibrium at low temperatures. Therefore, water gas shift reaction is industrially operated in two stages. A high temperature shift performed in the temperature of 350 – 450 ºC to achieve carbon monoxide in the range of 2 vol% – 3 vol% followed by a low temperature shift operated in the temperature of 200 – 300 ºC to obtain a very low amount of carbon monoxide to less than 0.3 vol%. Cu/ZnO/Al2O3 and Fe2O3/Cr2O3 catalysts are commonly used for low temperature and high temperature water gas shift, respectively [9,10]. The commercial Fe2O3/Cr2O3 catalyst is widely used in industrial applications. However, the researchers replace Cr with alternative metal such as Ni, Cu and Co because the carcinogenic nature and toxicity of Cr compounds. The literature survey presents that Co has been used in the water gas shift reaction as a sulfur resistant catalyst [11,12]. Recently, K/Co3O4 catalyst has been investigated for water gas shift reaction. It was found that an addition of K in Co3O4 increased the stability and activity of the catalyst [13]. In the present work, the effect of Sm and Pr on the water gas shift performance of Co/ceria catalyst has been studied. The influence of Sm and Pr on the structural and redox properties of Co/CeO2 catalysts has been intensively investigated. The results were supported by several catalyst characterization including X-ray diffraction, Raman spectroscopy, BET surface area, H2-chemisorption, H2temperature programmed reduction and X-ray absorption spectroscopy technique.
2. Experimental 2.1. Catalysts Preparation 2.1.1. Preparation of Support by Precipitation Method CeO2 and doped ceria supports were prepared by a precipitation method [14,15]. An
appropriate
amount
of
Ce(NO3)3·6H2O
(99%,
AlfaAesar),
M(NO)3·6H2O (M = Sm or Pr) and H2NCONH2 (98%, Aldrich) were dissolved in a minimal volume of deionized water. The mixture reactants were stirred and heated to obtain homogenous solution. Then ammonium hydroxide (99.99%, Sigma-Aldrich) was added dropwise at the rate of 1 mL/min to obtain the precipitate. The solution was further stirred and heated at 120 °C for 4 h to remove ammonia and age the support. The filtered sample was dried overnight at 110 °C in an oven and then calcined at 450 °C for 4 h. 2.1.2. Preparation of Catalysts Cobalt catalysts were prepared by impregnation method. An appropriate amount of Co(NO3)2·6H2O (98%, Sigma-Aldrich) was dissolved with deionized water. The salt solution was added to CeO2, 5%Pr doped CeO2 and 5%Sm doped CeO2 support. All samples were dried overnight at 110 °C and then calcined at 650 °C for 8 h. 2.2. Catalyst characterization 2.2.1. Standard characterization Quantachrome NOVA 1200e instrument was used to determine specific surface areas by N2 adsorption-desorption isotherms at 77.3 K. The specific surface areas of all samples were calculated from the BET procedure in the range of 0.05–0.3 of relative pressures of nitrogen. The samples (~0.20 g) were outgassed in vacuum at 300 °C for 6 h prior to the measurement.
The X-ray diffraction patterns presented in this study were recorded employing nickel-filtered Cu Kα radiation (λ = 0.15406 nm). The analyses were carried out at 0.02° per step and 0.5 s retention at each step over a 2θ range of 20 °– 90° with the current of 30 mA and 40 kV. The crystallite size of ceria is calculated using Scherrer’s equation. Dhkl =
0.9λ β cos θ
(2)
where Dhkl is the ceria crystallite size, λ is the wavelength of the used radiation (λ = 0.15406 nm), β is the full width at half maximum of the peak and θ is the incoming diffraction angle. 2.2.2. Raman spectroscopy The vibrational mode of all catalysts was observed using a Raman spectrometer and the vibrational signal was detected using a Charge Coupled Device detector. The Raman spectra of Co catalysts were analyzed using FTIR/FT-Raman (Perkin Elmer System 2000) and collected in the range of 200 – 1300 cm–1 with Ar ion laser irradiation. The samples were irradiated with an output power 10 mW and wavelength of 532 nm. 2.2.3. Temperature programmed reduction (TPR) H2-TPR was performed using a catalyst analyzer BELCAT-B. TPR experiments were performed under a flow of 5%H2/95%Ar mixture over 0.05 g of catalyst from room temperature to 1000 °C using a heating rate of 10°C/min. Prior to the TPR measurement, the catalysts were treated under high purity helium gas at 120 °C for 30 min. The amount of H2 uptake during the reduction was measured by thermal conductivity detector. The curves in H2-TPR profile correspond to H2 consumption. The sensitivity of the detector was calibrated by reducing a known weight of CuO.
2.2.4. Pulse injection method (chemisorption) Metal dispersion measurement in supported Co catalysts was operated by introducing 10%CO/He to the sample until chemisorption saturation and measuring the metal surface area of the sample from the total gas chemisorption. Metal dispersion can be calculated from the metal surface area and the weight of the catalyst. Approximately 50 mg of each sample was loaded into a quartz reactor. All catalysts were reduced under a flow of hydrogen at 400 °C for an hour. After cooling under a flow of helium, CO chemisorption pulse was performed under the flow of CO in a He carrier gas with the rate of 30 cm3/min at 50 °C. The flow of CO out from the reactor was monitored by a thermal conductivity detector. 2.2.5. X-ray absorption near edge structure (XANES) The oxidation states of metal in each sample were determined using X-ray absorption near edge structure on the beamline 8 of Synchrotron Light Research Institute in Nakhon Ratchasima, Thailand. The fluorescence mode was used for the investigation of Co K, Sm L3 and Pr L3 absorption edges and the transmission mode was operated for Ce L3 absorption edge determination with a Lytle detector. Synchrotron radiation is tunable by fixed-exit Double Crystal Monochromator equipped with InSb (111) and Ge (220) crystal for low and high energy ranges, respectively. The samples were placed over the Kapton window which was put on the sample frame. Ion chambers were filled with a mixture of argon and helium. These chambers were installed in front of and behind the sample to continuously detect the incident (I0) and transmitted (I1) X-rays beams. 2.3. Activity test Water gas shift activity measurements were carried out in the temperature range of 100 – 600 °C. Usually, 150 mg of catalyst was loaded into the stainless steel tube reactor which was sandwiched between two layers of quartz wool. The reactor was placed inside a tube furnace. Mass flow controllers (Albourg) were
used to control the flow rate of feed gas mixture of CO and N2. The amount of water vapor was controlled using a syringe pump. Preliminary experiments were conducted to determine suitable conditions from which internal and external mass transfer effects are not predominant. Considering the effect of external mass transfer, the total flow rate was kept constant at 100 ml/min in all testing. In addition, the catalyst particle size diameter was between 100 - 200 µm in all experiments. The feed gas consists of 5% CO, 10% H2O and balanced N2. The remaining reactant and product in the outlet gasses were analyzed by an on-line gas chromatography (Shimadzu GC-14B) equipped with thermal conductivity detector. A Unibeads C, 15 m × 0.53 mm stainless-steel packed column was utilized in the gas chromatography. The catalytic activities can be calculated by the following equation. %COconversion =
COin − COout × 100 COin
(3)
where COin and COout are the molar flow in the inlet and outlet feed gas, respectively. The reaction rates were obtained from separate experiments using 20 mg of catalysts and the feed gas mixture of 5% CO, 10% H2O and balanced N2 where the conversions of reactants were lower than 30%. The water gas shift rates can be calculated by the following equation RCO = (FCO × XCO)/Wcat
(4)
where RCO is the CO conversion rate (mol/(g·s)), FCO is the CO molar flow rate (mol/s), Wcat is the mass of the catalysts (g) and XCO is the CO conversion.
3. Results and Discussion
3.1. Catalysts characterization Figure 1 shows XRD patterns for supports and Co catalysts. X-ray diffraction patterns corresponding to CeO2 in a fluorite-type cubic crystal structure were found in every catalyst. All diffraction peaks are in good agreement with the standard data for ceria with peak intensity according to JCPDS No. 43-1002. For XRD patterns of Sm-doped ceria and Pr-doped ceria, other oxide reflection was not found indicating Sm and Pr incorporation into CeO2 lattice to form a solid solution [16,17]. In addition, XRD patterns of supports and Co catalysts show that incorporation of Pr3+ and Sm3+ ions into CeO2 shift the diffraction peak to lower angle side which corresponds to the lattice expansion. Table 1 presents the calculated lattice parameter of supports and Co catalysts. The increase of lattice parameters of Pr-doped ceria and Sm-doped ceria when compared with pure ceria (0.5404) related to the effective ionic radius of the dopant ion [18], as the ionic radii of Pr3+ (0.113 nm) and Sm3+ (0.108 nm) are larger than the ionic radius of Ce4+ (0.097 nm) [17]. The addition of Pr3+ and Sm3+ ions into ceria may enlarge the radius of cerium ion. In addition, oxygen vacancies are generated in the ceria lattice to maintain the charge equilibrium which is exhibited for further lattice expansion [19]. The average crystallite sizes of ceria and modified ceria, calculated from X-ray line broadening at 2θ ~ 28° using Scherrer’s equation, are compiled in Table 1 together with BET specific surface areas and average pore diameters. Doping CeO2 with samarium and praseodymium leads to a slight reduction of crystallite size. An addition of Co on ceria and modified ceria supports leads to larger crystallites with lower surface area. This result is suggested to be due to growth and aggregation of ceria crystallites after calcination at high temperature. Sm3+ and Pr3+ in 1%Co/Ce-5%M-O (M = Sm and Pr) stabilizes the support and prevent the support from sintering by maintaining small crystallite size and high surface area. The average pore diameters of Sm-doped and Pr-doped ceria in the presence of Co
catalysts are identical with the pore diameter of Co on pure ceria. Higher surface area of Co on doped ceria supports together with small crystallite size were observed. The result from % metal dispersion is also exhibited in Table 1. The metal dispersion data indicates that Sm assists in dispersing cobalt metal on a ceria support. Structural information of supported Co catalysts can be explained by Raman spectroscopy. The shape of Raman spectrum depends on the microstructure of ceria. As shown in Fig. 2, all samples exhibit a Raman peak at ∼460 cm–1 which can be assigned to the F2g mode of the cubic fluorite crystal structure of CeO2. This vibrational mode is the symmetrical stretching of the oxygen atoms around Ce ions [20]. This peak is very sensitive to any disorder in the oxygen sub-lattice which resulting from doping, thermal or grain size induced non-stoichiometric disorders [21]. Another broad peak at about 560 – 600 cm–1 was assigned to oxygen vacancies created in the ceria structure [22]. Sm and Pr addition to ceria exhibit broad weak peak at 598 and 569 cm–1, respectively. Therefore, it is confirmed that oxygen vacancies generation is facilitated by doping of Sm and Pr. Figure 3 shows the H2-TPR profiles of supports and Co catalysts. In pure ceria, the small reduction peaks appear at 500 °C and 850 °C which are attributed to the reduction of surface ceria and the bulk, respectively. The reduction peak of Ce5%Sm-O and Ce-5%Pr-O are similar to that of CeO2. For supported Co catalysts, there are three main reduction peaks corresponding to the reduction of cobalt oxide, surface ceria and the bulk. The reduction peak at low temperature of 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO is ascribed to the reduction of Co2+ interacting with ceria to Co. The result suggests that Co may exist in the form of Co-Ce-O solid solution or CoO which strongly interacts with CeO2. According to the literature [23,24], CoO or Co-Ce-O solid solutions is prominent in CoOx/CeO2 for the low content of cobalt in the materials, while Co3O4 is prominent in the materials for the high content of cobalt/Ce ratios. A stepwise
reduction process of Co3O4/ceria is Co3+ → Co2+ → Co. By comparing the reducibility of supported Co catalysts, it can be observed that the reduction of 1%Co/Ce5%SmO starts at a much lower temperature than the other catalysts, suggesting higher extent of dispersion and interaction of Co with Sm-doped ceria. The reduction peak of 1%Co/Ce5%SmO shifts to the lowest temperature among these catalysts. It is interesting to report that an addition of Sm into 1%Co/CeO2 significantly shifts the reduction temperature of Co species and surface ceria to lower temperature. Quantitative estimation of the H2 consumption of Co catalysts was carried out by measuring the area of H2-TPR curve. For this work, the reduction peaks of Co species were only considered. The results showed that 1%Co/Ce5%SmO has higher H2 consumption values than that of the other catalysts. 3.2. Water gas shift activity of Co catalysts Figure 4 presents the water gas shift activity of Co on ceria and doped ceria. Ceria and doped ceria supports are not very active for the WGS reaction. An addition of Co on various supports drastically increases the catalytic activity. It can be seen that 1%Co/Ce5%SmO is an active catalyst for WGS reaction. The activity of 1%Co/Ce5%SmO starts above 300 °C and increases rapidly to reach the maximum of 86% conversion at around 450 °C. The conversion above 450 °C reaches equilibrium and slightly decreases. A modification of ceria with Sm drastically increased the WGS activity of Co catalyst around 40% conversion when compared with 1%Co/CeO2. The activity of 1%Co/Ce5%PrO is very close to the activity of 1%Co/CeO2. According to the TPR results, the increased activity due to the reduction occurs at low temperature. In the regenerative mechanism, CeO2 plays an important role in transporting oxygen to CO via changes in oxidation state. H2-TPR of ceria support displayed that this process occurs at 500 °C. However, the reduction temperature of surface ceria drops to 437 °C when Co was
anchored onto ceria. Therefore, CeO2 in the vicinity of Co is easily reduced. Moreover, an addition of Sm to Co/CeO2 alter the reduction behaviour of Co catalyst. The cobalt-based catalyst is active for Fischer-Tropsch synthesis. Thus, CO2 and CH4 selectivity were studied for this work as shown in Fig.5. It presented that only
1%Co/ceria
produced
methane
at
lower
temperatures.
Whereas
1%Co/Ce5%SmO and 1%Co/Ce5%PrO are highly selectivity towards the water gas shift reaction throughout the studied temperature. The Arrhenius plot of Co on ceria and doped ceria are presented in Fig.6. These plots present that the CO conversion rate of 1%Co/Ce5%SmO is higher than the rate of other catalysts. Table 2 summarizes the rate of water gas shift reaction at 450 °C and activation energy that are obtained from the slope of the Arrhenius plot. 1%Co/Ce5%SmO gives the highest WGS rate and the lowest of the activation energy. It is interesting to study the role of Sm in enhancing the water gas shift performance of Co on ceria. From the TPR results, 1%Co/Ce5%SmO catalyst can be reduced at lower temperature than of 1%Co/CeO2. Since electron transfer among the involved species occur in the reduction process, the X-ray absorption spectroscopy (XAS) technique was employed to explain the electron movement within the catalyst. XAS spectra consist of two main features of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES technique provides information on oxidation states and valence shell electron population. In this study, XANES was practiced to determine the oxidation states of metals in the catalyst. Figure 7(a) presents XANES spectra of Ce L3 absorption edges for 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO as compared to CeO2 and Ce(NO3)3·6H2O. XANES spectrum of Ce(NO3)3·6H2O exhibits a single white line at 5725.1 eV which was assigned to Ce in the trivalent state [25,26]. A double white line is usually observed for ceria. These two peaks were ascribed to the
transition of Ce 2p to mixed valence state of ground state electron configuration Ce 4f0 and another one with 4f1 [26]. The plot of XANES spectra of Co K absorption edges for 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO as compared to Co(II,III) oxide, Co(NO3)2.6H2O and Co0 are displayed in Fig. 7(b). Linear combination fit was used to quantify elemental species in an unknown sample, interpreting that the total mass absorption of an edge element in the unknown is a linear sum of mass absorptions of standards. Linear combination fit result of 1%Co/CeO2, 1%Co/Ce5%SmO and 1%Co/Ce5%PrO is shown in Figure 8 which Co(II,III) oxide and Co(NO3)2·6H2O are standards for this plot. The result indicates that the oxidation states of cobalt in 1%Co/Ce5%SmO compound are higher than the oxidation states of cobalt in 1%Co/Ce5%PrO and 1%Co/CeO2. It can be concluded that the electron density was transferred from Co2+ to Ce4+ leading to an increase in oxidation state of Co2+ and lowering of oxidation state of Ce4+. The oxidation state of cobalt in 1%Co/Ce5%SmO was the highest among supported Co catalysts (Figure 9). This result suggests that cobalt also donates some electrons to Sm leading to a further increase in its oxidation state. From these results, a conclusion can be drawn that cobalt donates its electron to both cerium and samarium. Therefore, Sm helps Co in reducing Ce4+ and giving rise to more oxygen vacancies which facilitates the electron movement at the surface leading to an increase in the water gas shift rates. 4. Conclusions The influence of Sm and Pr on the WGS activity of Co/CeO2 was studied and the results show that the WGS performance of 1%Co/Ce5%SmO was much higher than the WGS activity of 1%Co/CeO2 and 1%Co/Ce5%PrO. 1%Co/Ce5%SmO yields the highest catalytic activity with the maximum of 86% conversion at about 450 °C. An addition of Sm into Co/CeO2 increases the surface area, reduces crystallite size and assists a better dispersion of Co on the ceria surface. The major role of Sm in enhancing the WGS activity of Co/CeO2 may be the ability to
promote the reduction of Co and the ceria surface which leads to lowering of reduction temperature, increased oxygen vacancies and increased surface redox property. The reduction of catalyst surface becomes easier since an electron density transfer between Co, Sm and Ce occur. XANES technique was used to explain electron movement within the catalysts. Linear combination fit indicates that Sm assists Co in reducing CeO2 and giving rise to more Ce2O3 at the surface of the ceria support. The presence of Ce3+ at the ceria surface gives rise to oxygen vacancies which facilitate the redox process at the surface and these effects conduct to an enhance in the WGS rates.
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Graphical Abstracts:
Cobalt donates its electron to both cerium and samarium. Therefore, Sm helps Co in reducing Ce4+ and giving rise to more oxygen vacancies which facilitates the electron movement at the surface leading to an increase in the water gas shift rates.
Fig.1 XRD patterns of supports and Co catalysts. (1) CeO2; (2) Ce5%SmO; (3) Ce5%PrO; (4) 1%Co/CeO2; (5) 1%Co/Ce5%SmO; (6) 1%Co/Ce5%PrO.
Fig.2 Raman spectra of Co catalysts and supports.
Fig.3 H2-TPR profiles of Co catalysts and insert of the figure corresponding to H2TPR profiles of supports.
Fig.4 WGS catalytic activity of Co catalyst.
Fig. 5 Selectivity to CO2 and CH4 as a function of temperature over Co catalysts.
Fig. 6. Arrhenius plot of reaction rate over Co catalysts.
Fig. 7. XANES spectra of Ce L3 absorption edge (a) and XANES spectra of the Co K absorption edge (b).
Fig. 8 Linear combination fit results of Co compounds.
Fig. 9 Relationship between the rate at 450 °C and the oxidation state of Co.
Table 1 Physicochemical properties of supported Co catalysts. Catalysts
SBETa (m2/g)
RBJH (nm)
CeO2
70.7
2.2
12.9
0.5404
1.63
Ce5%SmO
69.9
2.2
11.2
0.5419
1.79
Ce5%PrO
70.9
2.1
7.3
0.5421
1.77
1%Co/CeO2
33.2
2.2
14.8
0.5411
13.3
1%Co/Ce5%SmO 42.0
2.2
13.8
0.5421
22.5
1%Co/Ce5%PrO
2.2
9.2
0.5416
18.4
47.9
Crystalliteb size (nm)
Latticeb %dispersionc parameter(nm)
a
Estimated from N2 adsorption at -196 oC ; RBJH - BJH adsorption average pore diameter
b
Calculated from the (111) crystallographic plan; c Estimated from CO-chemisorption.
Table 2 The rate of reaction at 450° C and activation energies of Co catalysts. Sample
H2 consumptiona mmol/g
Rate at 450°C (µmol/(g·s))
Ea (kJ/mol)
1%Co/CeO2
0.20
46.4
76 ± 0.7
1%Co/Ce5%SmO
0.66
72.8
68 ± 0.6
1%Co/Ce5%PrO
0.52
53.2
74 ± 0.4
a
Estimated from H2-TPR.
Highlights •
The effects of Sm and Pr on the catalytic activity of Co/ceria were studied.
•
Doping of Sm onto Co/CeO2 increases the surface area and reduces crystallite size.
•
Sm prevents the support from sintering and helps in dispersing Co metal on ceria support.
•
Addition of Sm in Co/ceria gives rise to oxygen vacancies which leading to easier surface reduction.
Conflict of Interest and Authorship Conformation Form Please check the following as appropriate:
o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o
This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
o
The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
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The following authors have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript:
Author’s name Pannipa Tepamatr Navadol Laosiripojana
Sumittra charojrochkul Thanathon Sesuk
Affiliation Department of Chemistry, Faculty of Science and Technology, Thammasat University, Thailand The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Thailand National Metal and Materials Technology Center, NSTDA, Thailand National Metal and Materials Technology Center, NSTDA, Thailand