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Comparative study of Co-rich and Ce-rich oxide nanocatalysts (CoxCe1−xOy) for low-temperature total oxidation of methanol
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Comparative study of Co-rich and Ce-rich oxide nanocatalysts (CoxCe1−xOy) for low-temperature total oxidation of methanol ⁎
R. Dziembaja,b, , A. Chojnackaa, Z. Piwowarskaa, M. Gajewskac, M. Świętosławskia, S. Góreckaa, M. Molendaa a
Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland State Higher Vocational School in Tarnow, Mickiewicza 8, 33-100 Tarnow, Poland c AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Mickiewicza 30, 30-059 Krakow, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: CoxCe1−xOy Solid solutions Co-rich nanocomposites methanol total oxidation Electron and X-ray diffraction XPS TPR
Two different nanoparticle oxide systems are obtained depended on Co:Ce atomic ratio (R) using the same reverse nanoemulsion method. The single-phase solid solutions (SPhSS) are formed up to the ratio 1:4 < R < 1:5. The higher Co concentration results in segregation of Co3O4 phase forming nanocomposite of 2 types of nanoparticles (SPhSS and Co3O4). These nanomaterials were characterized using X-ray and electron diffraction, transmission electron microscopy together with linescan EDX analysis of individual nanoparticles, high-resolution XPS, TPR of preoxidized or pre-reduced samples, and testing activity in methanol total oxidation. The catalytic activity increases with increase of Co concentration in SPhSS and is preserved after deposition on alumina support. The catalytic activity of pure Co3O4 nanoparticles is comparable with the Co0.15Ce0.8O2-y solution and is increasing with Ce doping in the Co-rich 2-phase nanoparticle systems. Such a coexistence seems to be responsible for the highest catalytic activity in methanol oxidation.
1. Introduction Methanol is one of the alternative fuels or additives to hydrocarbon (HC) fuels which reduces emission of COx and unburned HCs as well as improving anti-knock characteristics [1]. Methanol large-scale production from fossils and bio-materials and wastes is well developed providing low price fuel with low C:H ratio 1:4. Moreover, possible spread of methanol fuel cells in electric cars does not demand total and very expensive changes in fuel distribution nets and car fuel tanks as it would be in the case of hydrogen cells. Hence, development of effective, stable and not as expensive as noble metals catalysts for methanol incineration from the exhaust gases and other emitters seems to be necessary. Although aforementioned catalysts are the most active for catalytic combustion of volatile organic compounds, there are a few crucial drawbacks. The noble metals are really rare and expensive, sensitive for poisoning and require maintenance of nano-sized particles to achieve their high activity. This is not a trivial task due to possible hot-spots generation caused by high heat of the VOCs combustion and reinforced by thermal insulation of many catalytic supports. Nevertheless, various noble metals anchored on oxide support systems have been used in the VOCs incineration though not so often for methanol.
⁎
Many reports on electrochemical oxidation of methanol on Pt-CeO2 anodes in the direct methanol fuel cells were reviewed by Mori et al. [2]. The catalytic combustion of methanol has been rarely studied. Haruta et al. [3] reported that activity of Pt, Pd and Au catalysts strongly depends on reducibility of the oxide supports, like TiO2, αFe2O3 and MnOx. Pt deposited on TiO2 films is highly active in methanol combustion when the precursor is calcined up to 350 °C and loses its activity after increasing calcination temperature up to 550 °C. On the other hand, ceria-doped Pt-catalysts can partially preserve the high activity even after heat treatment in elevated temperatures [4]. Pd-Pt catalysts anchored on CeO2-Al2O3-TiO2 support were extremely active at as low temperature as 27 °C (35% methanol conversion) and reached 90% conversion level at 58 °C [5]. A similar high activity was achieved on Au/CeZrOx (100% conversion and 98% selectivity to CO2 at 80 °C) [6]. However, the authors did not test those catalysts at the peak-temperature achieved in catalytic systems of the petrol and Diesel engine cars. Also modified zeolites were used as catalysts for methanol combustion, eg. Y-type zeolites modified with Pd [7] or MCM-22 modified with Au and Cu [8]. The silver containing catalysts appeared less active, e.g. the complex Ag-Cu/γ-Al2O3 catalysts reached 100% of methanol combustion only around 280 °C [9]. Jabłońska et al. [9] detected by XPS and EPR presence of Ag2+ ions on the Ag2O/CuO
Corresponding author at: Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland. E-mail address: [email protected] (R. Dziembaj).
https://doi.org/10.1016/j.cattod.2018.03.042 Received 4 December 2017; Received in revised form 2 March 2018; Accepted 22 March 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Dziembaj, R., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.03.042
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interface which have been assumed to be the active centres for methanol combustion. Methanol oxidation on model cobalt oxides catalysts under pressure of 0.1–0.3 mbar was studied using in situ XPS, XAS and MS at ISISS beamline at BESSY in Berlin [10]. The surface of a single Co crystal was oxidized to CoO or Co3O4 and then the reactants adsorption and catalytic reaction were studied together with spectroscopic investigations. It was proved that the reaction mechanism proceeds through methoxy and formate intermediates, which can be further oxidized depending on the oxidation potential of CH3OH/O2 and Co oxidation state. Such a mechanism has been also demonstrated using density functional theory for partial methanol oxidation over vanadia/ceria catalysts [11] or using in situ spectroscopy (DRIFTS) combined with temperature programmed methods with hematite model catalyst. Bowker et al. observed conversion of methoxy to formate species on the surface of hematite catalyst at 200 °C [12]. At least since 20 years ceria materials modified by 3d metal oxides have been studied as potential substitutes of the noble metal catalysts for the VOC’s incineration [13,14]. The cobalt-cerium mixed oxides were used for total oxidation of many VOCs, e.g. ethyl acetate [15,16], benzene [17], toluene [16–19], xylenes [17], n-hexane [17,20], propane [21,22] and propene [23]. The catalytic combustion of methanol on cobalt based catalysts was relatively rarely studied. The only relevant papers are the one already mentioned above [10] and this paper, concerning CoxCe1−xOy and CoxCe1−xOy/Al2O3 nanocatalysts, which is a continuation of our previous studies on CuxCe1−xO2−y [24,25] and MnxCe1−yOy [26] nanoparticles. There are only few papers describing other 3d metal oxides used for low-temperature methanol combustion, for example Fe2O3 [12,27] and Cu, Mn and Ag oxides nanocatalysts [9]. The recent review entitled “Catalytic oxidation of volatile organic compounds (VOCs)” by Kamal [28] does not contain any information about total oxidation of methanol. In our previous papers concerning methanol, acetone and ethylene total oxidation, the formation of single-phase solid solution nanoparticles of CuxCe1−xO2−y up to somewhere between 0.15 < × < 0.20 was reported [24,25]. The catalytic activity in methanol total oxidation of these nanoparticles increased with x within the existence range of single-phase solid solution (SPhSS) providing 100% total oxidation at T100 = 240 °C and T50 = 210 °C for the most active Cu0.15Ce0.85O2−y. This catalytic activity grew in parallel with concentration of individually dispersed copper ions (measured by UV–vis diffuse reflectance spectroscopy [29]), electric conductivity and share of ionic conductivity (measured by impedance spectroscopy) what supports increase of oxygen mobility in the defected CuxCe1−xO2−y [25]. The analogous SPhSS nanoparticles were observed for MnxCe1−xO2−y within a slightly broader 3d metal concentration range up to somewhere between 0.20 < × < 0.25. On the other hand, for the Mn-rich system (0.75 ≤ × < 1) no single-phase solid solutions was observed [26]. It was a mesoporous composite material consisted of the separate single-phase nanoparticles of both oxides CeO2−y and Mn3O4, which was confirmed with XRD and TEM/EDS studies. The high catalytic activity in methanol total oxidation ensured achievement of T100 values at 240 °C and 280 °C for Mn0.25Ce0.75O2−y bare and embedded on α-alumina carrier, respectively. It was also found that similarly high activity of the nanocomposite two-phase materials was obtained for the catalyst with atomic ratio Mn:Ce = 3:1 [26]. The main reasons for undertaking current research were to confirm whether analogous SPhSS nanoparticles in the CoxCe1−xO2−y system are formed as well as a similar composite of two-separate-phase nanoparticles are formed when x is larger than 0.25. This oxide system was characterized in terms of crystal structure, morphology and chemical composition (XRD, N2-isotherms, TEM/EDS linescan), valence state (XPS), reducibility and oxygen storage capacity (H2-TPR and O2reoxidation) and catalytic activity in methanol combustion of the bare and α-Al2O3 supported catalysts.
Fig. 1. X-ray powder diffraction patterns for SPhSSs of Ce-rich nanoparticles (a), the same solutions embedded on α-Al2O3(b) and biphasic Co-rich samples (c).
2
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2. Experimental
Table 1 Surface areas, crystallite and grains sizes for obtained catalysts.
The series of the nanocrystalline Co-Ce-O system were the subject of study: CoxCe1−xOy and CoxCe1−xOy/Al2O3. The synthesis procedure related to the first two series contained from 0 to 20 at% of Co and from 0 to 20 at% of Ce, was described in a previous paper [30] concerning catalytic decomposition of N2O on these catalysts. In brief, solutions of cerium and cobalt salts (Ce(NO3)3·6H2O, Co(NO3)2·6H2O) as well as cyclohexane and Triton X-100 with n-hexanol (all purchased from POCh) were used in appropriate ratio to obtain the first nanoemulsion. The second nanoemulsion contained the precipitation agent TPAH (tetrapropylamonium hydroxide, Sigma Aldrich) instead of the metal salts. The third series was supported catalysts (CoxCe1−xOy/Al2O3) obtained by using appropriate amount of a commercial α-Al2O3 support (Sigma-Aldrich) into the reaction vessel during mixing both nanoemulsions. Such an embedding procedure was effective for nanometric MnxCe1−xO2−y/α-Al2O3 catalysts used in low-temperature combustion of methanol [26]. After precipitation and drying (90 °C) all of the obtained powders were preheated with a linear rate of 10 °C/min up to 500 °C and calcined at this temperature in the air flow for 3 h. Crystal structure of the catalysts was characterized by powder X-ray diffraction using a BRUKER D2 PHASER with CuKα radiation (0.154178 nm). The average size of CeO2 and Co3O4 crystallites (DXRD) was calculated using the Scherrer equation for (111), (200), (220), (311) or (311), (400), (511), and (440) reflections, respectively. In case of α-Al2O3 the reflections of (012), (104), (113) and (116) were taken for the calculations. The specific surface areas were evaluated from the N2 adsorptiondesorption isotherms (Micromeritics ASAP 2010). Each sample was preliminary outgassed at 350 °C for 12 h under vacuum (4·10−3 Pa). Then, N2-isotherms were measured up to pressure of 8·104 Pa. The values of specific surface areas were used to calculate the average grain size (DBET) assuming spherical shape of the grains and using crystallographic density of Co3O4 and CeO2, 6.11 and 7.215 g/cm3, accordingly. The total pore volume was determined at p/p0 = 0.98, and the pore size distribution was calculated by the BJH method. The XPS measurements were performed with a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyzer using the monochrome Al Kα radiation 1486.6 eV and an electron flood gun (FS40A-PS) to compensate charge on the surface during measurements. The analyzer operated in the constant energy mode at 100 eV with resolution of 0.1 eV. The composition of the surface was determined using areas and binding energies of Ce 3d, Co 2p and O 1 s photoelectron peaks after fitting of the high resolution spectra through the CasaXPS software.
Sample
S [m2/g]
DXRD [nm]
DBET [nm]
CeO2 Co3O4 Co0.1Ce0.9O2 10% Co0.1Ce0.9O2/α-Al2O3 15% Co0.1Ce0.9O2/α-Al2O3 20% Co0.1Ce0.9O2/α-Al2O3 Co0.15Ce0.85O2 10% Co0.15Ce0.85O2/α-Al2O3 15% Co0.15Ce0.85O2/α-Al2O3 20% Co0.15Ce0.85O2/α-Al2O3 Co0.2Ce0.8O2 10% Co0.2Ce0.8O2/α-Al2O3 15% Co0.2Ce0.8O2/α-Al2O3 20% Co0.2Ce0.8O2/α-Al2O3
131 51 76 22 28 34 93 23 23 27 43 27 28 38
8 21 6 3 5 7 5 5 6 5 10 4 6 4
11 32 11 8 7 7 9 7 10 10 20 5 7 6
α-Al2O3 (Sigma-Aldrich) mesh 100; SBET = 14 m2/g.
Transmission electron microscopy (TEM) investigations were carried out using a FEI Tecnai TF20 X-TWIN (FEG) microscope, equipped with an EDAX energy dispersive X-ray (EDX) detector, at an accelerating voltage of 200 kV. The samples were prepared by drop casting on carbon coated copper TEM grids. Prior to the cast onto the grids, the powder materials were dispersed in isopropanol using ultra-sonication for 5 min. The oxygen storage capacity (OSC) was characterized using temperature-programmed reduction (TPR) with H2. The TPR runs were carried out in a plug-flow reactor (from 60 to 500 °C) with a linear heating rate of 10 °C/min. The H2 uptake was monitored by a micro volume TCD (Valco). Two series of the experiments were performed. In the first series obtained material was preliminary oxidized by heating up to 500 °C in the stream of synthetic air (Ar/O2 = 4/1) using flow rate of 10 cm3/min. Immediately after described pre-oxidation tested catalyst was fast cooled to 60 °C and a TPR run was switched on with a constant flow rate (10 cm3/min) of 5 vol.% of H2 in Ar up to 500 °C and then cooled to 60 °C. Afterwards nanocatalyst was subjected to re-oxidation at a chosen temperature, and the next TPR run was carried out as before. The redox cycles were repeated three times. During the second series of TPR experiments the samples were outgassed by heating in the pure Ar stream preserving other conditions the same as in the first series of TPR measurements. Then, after cooling to 60 °C the H2-TPR was switch on with the same linear heating rate of 10 °C/min. The catalytic activity of the unsupported and supported catalysts in methanol combustion was studied in a fixed-bed plug-flow microreactor under atmospheric pressure. Each sample of the catalysts (100 mg) was preliminary outgassed in a flow of 5.00 vol.% of O2 in pure helium
Fig. 2. Comparison of BET isotherms on examples of the 10% Co0.2Ce0.8O2/α-Al2O3(a) and 20% Co0.2Ce0.8O2/α-Al2O3(b) catalysts. 3
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Fig. 3. The electron diffraction of Co0.95Ce0.05O2 (a) and Co0.15Ce0.85O2 (b) systems with corresponding bright field micrographs with marked regions used for diffraction studies (c,d).
Fig. 4. Bright field micrographs of Co0.95Ce0.05O2 (a) and Co0.15Ce0.85O2 (b) nanoparticles with EDS linscan analysis embedded on the pictures. 4
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Fig. 5. The Ce3d XPS spectra for CeO2 (a), Co0.15Ce0.85O2 (b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts. Table 2 The XPS spectra parameters of Ce3d for CeO2, Co0.15Ce0.85O2 and Co0.15Ce0.85O2/α-Al2O3. Sample
Final state
EB [eV]
CeO2
u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5 u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5 u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5
900.61 907.34 916.53 899.16 901.92 901.00 907.42 916.83 899.09 902.77 900.96 907.44 916.94 899.71 902.90
Co0.15Ce0.85O2
Co0.15Ce0.85O2/α-Al2O3
5
882.11 888.69 898.03 880.66 883.52 882.50 888.82 898.33 880.59 884.37 882.56 888.84 898.44 881.21 884.50
Area [a.u.]
Share [%.]
194872.9 280264.9 364624.4 14718.1 165613.9 368737.3 377145.5 470117.0 24609.5 164744.8 74178.9 75356.7 86719.7 2534.3 45215.4
18.9 27.4 36.1 1.4 16.1 26.0 26.8 33.8 1.7 11.6 25.9 26.5 30.9 0.9 15.8
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(99.9999%) at 450 °C for 1 h. The catalytic measurements were performed within the range from 55 to 500 °C with the linear temperature increase of 10 °C/min. During the catalytic studies the total feed flow was controlled at F = 20 cm3 min−1, corresponding to a W/F ratio of 0.30 g s cm−3 and GHSV = 29,000 h−1. The reactor feed contained sixfold excess of oxygen (0.50 vol.% of methanol and 4.50 vol.% of O2 in pure He) in relation to the stoichiometric ratio. The flow of all used gases were adjusted by electronic mass controllers (Brooks), and the methanol was introduced into the controlled gas stream in a temperature stabilized saturator. Concentration of the reactants were continuously monitored with a quadruple mass-spectrometer (PREVAC) connected on-line with the reactor outlet. Taking into account the same main mass lines of O2 and CH3OH, the line 31 (deprotonated methanol) has been adopted as a measure of methanol concentration based on the results of appropriate calibration of the mass spectrometer. To avoid any flow fluctuation the intensities of the 31 line were related to the intensity of helium line 4. The methanol fractional conversion α was calculated using Eq. (1). The observed intensities of CO2 mass line (44) change parallel to the values of α demonstrating high selectivity to the total oxidation of methanol. At the low temperatures and several % of methanol conversion, a trace of HCHO (line 30) was observed.
a=
( ) −( ) ∙100% ( ) I IHe 2
I IHe 1
I IHe 1
(1)
3. Results and discussion The XRD patterns of the unsupported Co-Ce-O catalysts with Co:Ce molar ratio below or equal to 1:6 (Fig. 1a) showed only reflexes from CeO2 fluorite-like structure which confirmed the formation of CoxCe1−xO2−y single-phase solid solution [30]. Similarly, as it was observed in case of Mn3O4 – CeO2-y system up to the atomic ratio Mn:Ce = 1:5 [26]. The most intense diffraction line of Co3O4 (311) at 2Ɵ = 37° was noticed only after reaching a ratio of Co:Ce = 1:4 (Co-Ce20 sample Fig. 1a). Bolivar-Diaz et al. [31] just reported the segregation of Co3O4 from the CoxCe1−xO2−y at Co:Ce = 2:3, although the traces of cobalt oxide could be noticed at smaller Co content, even at the ratio close to Co:Ce = 1:4, which is visible in XRD patterns published in [31]. In this paper there is no information on studies performed on samples with lower Co:Ce ratio than 1:4. Taking into account both of these results one may assume that separation of the Co3O4 phase from our SPhSSs begins when at least 2 Co atoms replace 2 of 12 Ce atoms contained in 3 elementary cells. The average crystallite diameters (DXRD) of this solution were between 5 and 6 nm; however, the sample Co0.2Ce0.8O2−y has twice bigger average diameter. This corresponds to the observed difference in DXRD of the comparative samples of the pure CeO2 and Co3O4 (obtained by the same preparative procedure) which were equal to 8 and 21 nm, respectively (Table 1). The XRD patterns of the supported Co-Ce-O catalysts (Fig. 1b) showed very intensive α-Al2O3 lines; however, the XRD pattern of CeO2 is the same as in the case of unsupported catalysts. This effect confirmed effective embedding of the SPhSS particles on the support grains. Nevertheless, the most intensive Co3O4 (311) reflex could not be observed due to the full cover by the very intensive Al2O3 (110) line at 2Ɵ = 37.7°. The calculated average nanocrystals size of the supported SPhSS particles was similar to the size of crystallites of related unsupported SPhSS particles (Table 1). The opposite results were obtained for the unsupported catalysts contained from 0 to 20 at% of Ce in the mixed oxide system (Fig. 1c). The most intense line of CeO2 (111) was observed at 2Ɵ = 28.5° even at the ratio of Ce:Co = 1:99 (Co-Ce-1 sample in Fig. 1c). All the main diffraction lines were already observed at the 1:20 ratio, therefore the formation of cerium solutions in Co3O4 phase should be excluded and consequently chemical formula Co3O4·nCeO2−y should be used to
Fig. 6. The O1 s XPS spectra for CeO2 (a), Co0.15Ce0.85O2 (b), 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts and pristine Al2O3 support (d).
Table 3 The XPS spectra parameters of O1 s for CeO2, Co0.15Ce0.85O2 and Co0.15Ce0.85O2/α-Al2O3. Sample CeO2 Co0.15Ce0.85O2 Co0.15Ce0.85O2/α-Al2O3
6
Final state 2−
O OH, etc. O2− OH, etc. O2− OH, etc.
EB [eV]
Area [a.u.]
Share [%.]
529.07 530.60 529.56 531.67 529.67 531.47
81577.8 74884.7 153239 32242.6 18503.9 281873
52.1 47.9 82.6 17.4 6.2 93.8
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Fig. 7. The Co2p XPS spectra for Co0.15Ce0.85O2 (a,b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c,d) catalysts and Co3O4 (e,f) reference.
Burroughs [34] as v and u related to 3d5/2 and 3d3/2. Components of the doublets differ by a constant value of 18.5 eV. Such a large number of doublets arises from different Ce4f occupancies in the final state [35,36] or/and from the coupled excitations of the O2p → metal 4f charge transfer [34,37]. The most characteristic for Ce(IV) is the peak labelled u’’’ (found in our case within 916.5 – 916.9 eV) as only one undisturbed by the other peaks. This signal is related to Ce(IV) with the 3d94f°O2p6 configuration and the doublets labelled as v0 – u0 and v’ – u’ are assigned to Ce(III). It was confirmed experimentally by Spanier et al. [38] and F. Zhang et al. [39] who showed drastic increase of these doublets intensity after reduction CeO2 possibly close to Ce2O3. Analysis of the our results leads to the conclusion that introduction of Co ions into ceria lattice causes only slight decrease of the Ce(III) concentration in the surface layers of the SPhSS, which undergoes further decrease after deposition of this SPhSS on α-alumina (Table 2). In Fig. 6a the O1 s spectrum of the nanocrystalline CeO2 is presented. The lower energy peak is ascribed to lattice oxygen ions while the higher energy peak to OH or to electrophilic oxygen species. Introduction of Co ions into ceria lattice strongly increases the share of O2− peak at 529.6 eV, although deposition of the SPhSS on α-Al2O3 (Fig. 6c) caused extreme increase of the other peak, most probably due to covering the alumina surface with OH species (Fig. 6d, Table 3). The XPS spectra of Co2p consist of one spin-orbit doublet, Co2p3/2 and Co2p1/2, differ by 15.0 eV and broad satellites at about 9 eV [40]. The reported BE values of Co2+ and Co3+ which are given in literature are not assigned uniquely as they depend strongly on non-stoichiometry of these oxides [41]. Nevertheless, characteristic feature of Co2+ is the higher binding energy of Co2+ than Co3+ within the Co2p3/2 peak and additionally higher intensity of the satellite. Taking into account
describe these composite nanomaterials. In the previous paper [30] the observed hysteresis loop of the N2adsorption-desorption isotherms measurements showed mesoporous character of the SPhSS materials (CoxCe1−xO2−y) and macroporous one for the composite materials (Co3O4·nCeO2−y). The analogous isotherms obtained for the supported SPhSSs (Fig. 2a,b) showed both macro- and mesoporous characters. The significantly smaller proportion of mesopores for the supported solutions is an effect of the excess of micrometric (0.155 mm) spheres and nanometric particles of the solid solution coexisting in the supported catalysts. The specific surface area of the supported catalysts (Table 1) is several times smaller than in case of the related unsupported catalysts which specific surface area is in the range from 40 to 90 m2/g [30]. Transmission electron microscopy studies with EDS linescan analysis are presented in Fig. 3 and Fig. 4. Results of electron diffraction of selected test areas which are shown in Fig. 3 confirm conclusions inferred from XRD analysis. The observed electron diffraction patterns for CoxCe1−xO2−y sample (Fig. 3b) clearly proves that it is the single-phase nanocrystalline (Fig.3d) material, isostructural to CeO2. All of the recognized diffraction rings are attributed to Fm-3 m lattice. The structural changes resulting from cobalt doping are too small to visibly influence presented electron diffraction pattern. This, together with EDS analysis (Fig. 4b), which confirms presence of both Ce and Co in the material’s nanocrystals, proves that obtained sample is the CoxCe1−xO2−y solid state solution. It can be observed (Fig. 3d) that CoxCe1−xO2−y catalysts consist of small, up to 15 nm in diameter, uniform nanoparticles. On the other hand, samples with Co concentration above 20 at% can be characterized as two phase system with bigger Co3O4 grains and smaller precipitates of CeO2 nanocrystals. It seems that Co3O4 nanoparticles grew much faster than CeO2. Electron diffraction (Fig. 3a) confirms two phases present in the sample and the EDS studies indisputably show different elemental composition of selected grains (namely Co3O4 and CeO2), which additionally support our conclusions coming from XRD analysis. X-ray photoelectron spectroscopy was used to recognize change in valence state of Ce ions after formation of the SPhSS and deposition particles of this solution on α-alumina carrier. The obtained results of Ce3d XPS after subtracting the background are presented in Fig. 5. These complex spectra consist of 5 doublets of Ce3d5/2 and Ce3d3/2 [32,33] and are labelled according to convention proposed by
Table 4 Atomic ratio based on XPS measurements. Sample
O2−/Onucl
Co2+/Co3+
Ce3+/Ce4+
Co3O4 CeO2 Co0.15Ce0.85O2 Co0.15Ce0.85O2/α-Al2O3 α-Al2O3
1.39 1.09 4.75 0.07 0*
0.62 – 0.58 1.5 –
– 0.21 0.15 0.20 –
* The O1 s peak in α-Al2O3 (531.6 eV) is symmetrical. 7
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Fig. 8. Comparison of the H2-TPR profiles for CeO2 (a,c,e) and Co3O4 (b,d,f) samples.
increase in binding energy of Co2p electrons was also observed by Biesinger et al. [41] after replacement of oxygen by OH ligands. Two series of the H2-TPR were carried out to characterize oxygen storage capacity in the catalysts; the first based on outgassed and preoxidized catalysts and the second only on outgassed catalysts. Two complex TPR peaks were detected during the first series with the nanocrystalline CeO2 sample. Within the low temperature peak (around 200 °C) two maxima were distinguished at 219 and 238 °C, and within the other peak (around 420 °C) also two at 411 and 428 °C (Fig. 8a). They were ascribed to reduction of the chemisorbed oxygen species and the lattice oxygen, respectively. After cooling in argon stream, and reoxidation up to 260 °C, the next TPR showed only a small change (Fig. 8c), but in the third TPR run preceded by re-oxidation up to 360 °C the peak of chemisorbed species clearly diminished (3-times) and the peak of surface lattice oxygen became a single peak (Fig. 8e). The same type of TPR plot but for the nanocrystalline Co3O4 consisted of only one, however very broad (about 180 °C) peak and there were no traces
relatively small intensity of the Co2p peaks and smaller amounts of cobalt ions (15 at%) in the SPhSS nanoparticles, and even smaller amounts in the supported particles we decided to base our analysis on only Co2p3/2 peaks. The raw Co2p spectra are presented in Fig. 7b, d and f, while the Co2p3/2 envelope divided for the Co2+ to Co3+ shares in Fig. 7a, c and e. The peak at 780.18 eV is ascribed to Co3+ and those at 781.89 eV for the unsupported Co0.15Ce0.85 catalyst while the appropriate binding energy measured are equal to 780.00 and 781.21 eV for the supported catalysts. Yang et al. [40] reported for Co3O4 the binding energies 779.9 and 781.2 eV for Co3+ and Co2+, respectively, while Biesinger et al. [41] 779.6 and 780.9 eV, so close to our results. The calculated atomic ratio of Co3+ to Co2+ in the unsupported catalyst was equal 5:3, instead of 6:3 in the Co3O4 stoichiometric. One may correlate this with corresponding reduction of Ce4+/Ce3+ ratio (Table 4). The observed inversion of the Co3+/Co2+ ratio after deposition SPhSS on α-Al2O3 may be related to the drastic increase of OH species on the surface of supported catalyst (Fig. 6d, Table 4). Such an 8
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Fig. 9. Comparison of the H2-TPR profiles for Co0.15Ce0.85O2 (a,c,e) and 20% Co0.15Ce0.85O2/α-Al2O3 (b,d,f) samples.
of the model cobalt catalyst providing increasing the number of active sites in the catalysts for carbon oxides hydrogenation [42,43]. Introduction of Co ions into the ceria lattice causes significant changes in the TPR profiles visible by comparing the curves marked a, c and e from Fig. 9 representing the results obtained on the most active SPhSS (Co0.15Ce0.85O2−y) with the respective curves in Fig. 8 obtained (under the same conditions) on nanoparticles of single oxides, CeO2 and Co3O4, i.e. with Fig. 8a–f, respectively. The changes in the chemisorbed species part of the TPR profiles are limited to disappearance of these species after re-oxidation at 360 °C (Fig. 9e), while the reduction of the lattice oxygen has changed significantly in comparison with the single oxides. The reduction of CeO2 in the first TPR runs begins at 390 °C while in case of Co3O4 at 220 °C with the maximum at 327 °C. The start temperature of the Co3O4 reduction is increased up to 270 °C and the maximum up to 367 °C after re-oxidation at 360 °C before the third TPR run (Fig. 8f).
of the chemisorbed oxygen reduction (Fig. 8b). This peak has been split into 4 components. Taking into account the total reduction of Co3O4 and stoichiometry of the two-step reduction, Co3O4 + H2 − > 3 CoO + H2O
(1)
3 CoO + 3 H2 − > 3 Co + 3 H2O
(2)
one may assume that the first component represents the first step of the reduction while the second is the sum of three other components. Moreover, total re-oxidation of Co3O4 after TPR runs needs higher temperature than in case of the nanocrystalline CeO2 as may be concluded from the difference between the TPR profiles after reoxidation up to 260 and 360 °C (Fig. 8a, c, e and b, d, f). It seems that the nanoparticles after reduction/oxidation/reduction process have core-shell morphology with reduced cobalt in the core and oxide phase in the shell. Such a model was used years before to explain increase of activity
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Fig. 10. Results of TPR runs performed with outgassed CeO2 (a), Co0.15Ce0.85O2 (b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts.
The TPR profile of the lattice oxygen reduction in the most active SPhSS includes two clearly separated but complex reduction peaks with maxima at 325 and 430 °C (Fig. 9a), most probably the second peak may be related mainly to reduction of Ce4+. It is more visible in the second TPR run, showing larger increase of the ceria reduction degree. The sharp increase of the reduction rate (with peak at 367 °C) during the third TPR after pre-oxidation at 360 °C can be explained by catalytic effect of Co° appearance in the reduced nanomaterial (Fig. 9e). The TPR profile of the sample containing 20 wt% of the most active SPhSS embedded on α-Al2O3 turned out, how one could expect, more complex. The total TPR curve consists of 6 components, with beginning at 200 °C and ending at 425 °C, providing even higher extent of the reduction as it was in case of unsupported SPhSS. There is a low-temperature peak at 218 °C which is moved to 210 °C in the second and third TPR runs. This peak should be related consequently to reduction of chemisorbed oxygen and OH covering α-Al2O3 surface as it was observed before during XPS measurements of O1 s spectrum. The sharp peak close to the end of the reduction, especially in the case of the supported catalysts, can be explained by autocatalytic effect of the Co° species appearing in this stage of the reduction. The second type of the TPR runs performed after outgassed procedure up to 500 °C in Ar stream showed meaningful lowering of the low temperature peak, so the amount of chemisorbed oxygen on the nanometric CeO2 has been considerably diminished; nevertheless advance of the lattice oxygen removal has been preserved or even increased
Fig. 11. Comparison of catalytic activity for CoxCe1−xOy (Ce-rich system (a) and Co-rich system and nanoparticles with pristine Co3O4 and CeO2 (c)), CoxCe1−xOy/Al2O3 (b).
(please note the differences between the scales in Figs. 8, 9 and 10). The increase be related to creation of the oxygen vacancies by outgassing procedure of CeO2 what makes oxygen transport in this oxide easier as reported by Lawrence et al. [44]. Similar increase of the oxygen mobility in the CuxCe1−xO2−y single-phase solid solution, owing to increase in concentration of oxygen defects resulting from introduction of Cu ions into ceria lattice, was stated based on increase of electric conductivity and share of ionic conductivity measured by impedance spectroscopy [25]. The results of catalytic combustion of methanol over CoxCe1−xO2−y are presented in Fig. 11a and compared with results of homogeneous and catalytic combustion over the CeO2 nanoparticles. All of the obtained materials were very selective towards total oxidation, traces of formaldehyde were observed only at very low conversion. Already 1 at 10
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phase separation, namely Co0.15Ce0.85O2−y. Perhaps suggested by Schlogel et al. [10], Sauer [11] and Gibbson [12] two steps mechanism of total methanol oxidation proceeds on two different active sites which being close together accelerate the combustion. Such active assembles may by created as well on the surface of SPhSS or at the phase boundaries between the nanoparticles of these different phases in nanocomposite catalysts. Obviously, the role of the oxygen vacancies, chemisorbed species and ions with easy variable valence cannot be neglected in complete explanation of the mechanism of methanol total oxidation.
Table 5 T50 and T100 parameters for obtained catalysts and other selected examples. Sample
T50 [°C]
T100 [°C]
CeO2 Co0.01Ce0.99O2 Co0.05Ce0.95O2 Co0.1Ce0.9O2 Co0.15Ce0.85O2 Co0.2Ce0.8O2 10% Co0.1Ce0.9O2/α-Al2O3 10% Co0.15Ce0.85O2/α-Al2O3 10% Co0.2Ce0.8O2/α-Al2O3 15% Co0.1Ce0.9O2/α-Al2O3 15% Co0.15Ce0.85O2/α-Al2O3 15% Co0.2Ce0.8O2/α-Al2O3 20% Co0.1Ce0.9O2/α-Al2O3 20% Co0.15Ce0.85O2/α-Al2O3 20% Co0.2Ce0.8O2/α-Al2O3 Co0.95Ce0.05O2 Co0.99Ce0.01O2 Co3O4 * Cu0.15Ce0.85O2 [24] Cu0.15Ce0.85O2 [25] Mn0.25Ce0.75O2 [26] Mn0.75Ce0.25O2 [26]
445 285 265 242 222 228 278 272 269 267 270 254 266 255 260 161 177 200 – – – –
600 437 413 438 363 374 400 390 400 375 390 375 400 370 375 225 256 375 375 275 245 335
4. Conclusions Electron and X-ray diffractions confirmed formation of nanoparticles of the SPhSS CoxCe1−xO2−y up to Co/Ce atomic ratio = 1:6. At higher Co concentration the samples obtained by the same precipitation from nanoemulsions and further calcination, are composed of separated nanocrystallites of Co3O4 and CeO2, or rather Co3O4 and SPhSS of Co ions in CeO2−y phase, forming nanocomposite. Formation of analogous SPhSS has been also observed for CuxCe1−xO2−y and MnxCe1−xO2−y and in spite differences between the charge of these 3d metals ions the boundary limits of the solid solution are located between 0.15 < × < 0.25, though the related values of x increases from Cu to Mn. Then the limit x is increasing together with higher charge of these ions what may be related to lower stress evolved in the crystallites by the strange ions with charge closer to charge of Ce ions. Co introduced to ceria lattice generates oxygen vacancies what increase mobility of the oxygen ions making reduction easier during the TPR. In consequence advance of the ceria reduction is increased and observed at lower temperature. The catalytic activity increases with x values in the SPhSS but after appearance of the segregated Co3O4 crystallites, the activity is stabilize or slightly diminish. On the other hand, the activity of Co3O4 nanoparticles obtained in the same procedure is high, comparable with the best SPhSS (Co0.15Ce0.85O2−y), though nanocomposite of Co3O4 and CeO2 (or rather SPhSS of Co ions in CeO2−y phase) contained as low amounts as Ce as 1 or 5 at%, which results in further increase of activity. One may suppose that special assembles formed as well in the SPhSS as at the phase boundaries between the nanoparticles of these different phases are responsible for the observed phenomena. The catalytic activity of SPhSS in methanol total oxidation are generally preserved after embedding on alumina support taking into account the SPhSS loading on the support being 4–10 times smaller than in case of the bare SPhSS catalysts.
* Obtained by CeO2 impregnation.
% of Co introduced to CeO2 lattice extremely increased catalytic activity and this activity increased further up to x = 0.143. The catalyst with x = 0.214, thus with trace of separate Co3O4 phase, showed slightly lower activity. Such maxima of catalytic activity were observed before in the CuxCe1−xO2−y [24,25] and MnxCe1−xO2−y [26] systems and were related to appearance of the separate CuO or Mn3O4 phases. The results obtained for the supported catalysts contained 10, 15 and 20 wt% of the active SPhSS with various Co content are presented in Fig.11b. All these catalysts were active but relation between Co content and catalyst loading on alumina was not so clear, probably due to high OH species concentration on supported catalyst what is consistent with XPS results, showing drastic increase of Co2+ share in the Co2p spectra (Fig. 7b). Such a shift of the binding energy of Co2p electrons was observed by Biesinger after replacement of oxygen by OH ligands [41]. It is possible that less hydrophilic catalyst carrier support would be more appropriate to distinguish the activity of supported catalysts. The choice of α-Al2O3 evolved from intention to preserve the same support as was used before in our lab, which in turn was consequence of the Salsona et al. [21] results that this type of alumina support (from several tested) provided the best catalytic performance in total oxidation of propane. On the other hand, the catalytic experiments were performed every time with the same mass of the catalyst what caused that amounts of active phases in the supported catalysts was from 5 to 10 times lower than in case of the appropriate bare SPhSS. Taking into account these differences in amounts of the active phase one may suggest that the high activity of the supported catalysts were generally preserved. The temperature necessary to achieve 100% and 50% combustion of methanol (T100, T50) on obtained catalysts, together with several other examples to compare, are gathered in Table 5. The Co3O4 nanoparticles appeared nearly as active as the best SPhSS bare catalyst, Co0.15Ce0.85O2−y. However, the samples obtained by the same procedure but with majority of Co content in the nanoemuslsion and only 1 or 5 at% Ce in relation to Co, showed even higher catalytic activity. Then, there is a clear synergy observed in such nanocomposite system consisted of larger nanoparticles of Co3O4 and smaller (Table 1) nanoparticles of CeO2−y or more probably CoxCeO2−y (Fig. 11c). Hence the catalytic activity in the Co3O4-CeO2−y cannot be simple related to amount of the Co contents in the catalysts as the highest performance is related to coexistence of both nanoparticles of Co3O4 and SPhSS or only nanoparticles of SPhSS with the highest contents of Co without Co3O4
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Comparative study of Co-rich and Ce-rich oxide nanocatalysts (CoxCe1−xOy) for low-temperature total oxidation of methanol ⁎
R. Dziembaja,b, , A. Chojnackaa, Z. Piwowarskaa, M. Gajewskac, M. Świętosławskia, S. Góreckaa, M. Molendaa a
Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland State Higher Vocational School in Tarnow, Mickiewicza 8, 33-100 Tarnow, Poland c AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Mickiewicza 30, 30-059 Krakow, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: CoxCe1−xOy Solid solutions Co-rich nanocomposites methanol total oxidation Electron and X-ray diffraction XPS TPR
Two different nanoparticle oxide systems are obtained depended on Co:Ce atomic ratio (R) using the same reverse nanoemulsion method. The single-phase solid solutions (SPhSS) are formed up to the ratio 1:4 < R < 1:5. The higher Co concentration results in segregation of Co3O4 phase forming nanocomposite of 2 types of nanoparticles (SPhSS and Co3O4). These nanomaterials were characterized using X-ray and electron diffraction, transmission electron microscopy together with linescan EDX analysis of individual nanoparticles, high-resolution XPS, TPR of preoxidized or pre-reduced samples, and testing activity in methanol total oxidation. The catalytic activity increases with increase of Co concentration in SPhSS and is preserved after deposition on alumina support. The catalytic activity of pure Co3O4 nanoparticles is comparable with the Co0.15Ce0.8O2-y solution and is increasing with Ce doping in the Co-rich 2-phase nanoparticle systems. Such a coexistence seems to be responsible for the highest catalytic activity in methanol oxidation.
1. Introduction Methanol is one of the alternative fuels or additives to hydrocarbon (HC) fuels which reduces emission of COx and unburned HCs as well as improving anti-knock characteristics [1]. Methanol large-scale production from fossils and bio-materials and wastes is well developed providing low price fuel with low C:H ratio 1:4. Moreover, possible spread of methanol fuel cells in electric cars does not demand total and very expensive changes in fuel distribution nets and car fuel tanks as it would be in the case of hydrogen cells. Hence, development of effective, stable and not as expensive as noble metals catalysts for methanol incineration from the exhaust gases and other emitters seems to be necessary. Although aforementioned catalysts are the most active for catalytic combustion of volatile organic compounds, there are a few crucial drawbacks. The noble metals are really rare and expensive, sensitive for poisoning and require maintenance of nano-sized particles to achieve their high activity. This is not a trivial task due to possible hot-spots generation caused by high heat of the VOCs combustion and reinforced by thermal insulation of many catalytic supports. Nevertheless, various noble metals anchored on oxide support systems have been used in the VOCs incineration though not so often for methanol.
⁎
Many reports on electrochemical oxidation of methanol on Pt-CeO2 anodes in the direct methanol fuel cells were reviewed by Mori et al. [2]. The catalytic combustion of methanol has been rarely studied. Haruta et al. [3] reported that activity of Pt, Pd and Au catalysts strongly depends on reducibility of the oxide supports, like TiO2, αFe2O3 and MnOx. Pt deposited on TiO2 films is highly active in methanol combustion when the precursor is calcined up to 350 °C and loses its activity after increasing calcination temperature up to 550 °C. On the other hand, ceria-doped Pt-catalysts can partially preserve the high activity even after heat treatment in elevated temperatures [4]. Pd-Pt catalysts anchored on CeO2-Al2O3-TiO2 support were extremely active at as low temperature as 27 °C (35% methanol conversion) and reached 90% conversion level at 58 °C [5]. A similar high activity was achieved on Au/CeZrOx (100% conversion and 98% selectivity to CO2 at 80 °C) [6]. However, the authors did not test those catalysts at the peak-temperature achieved in catalytic systems of the petrol and Diesel engine cars. Also modified zeolites were used as catalysts for methanol combustion, eg. Y-type zeolites modified with Pd [7] or MCM-22 modified with Au and Cu [8]. The silver containing catalysts appeared less active, e.g. the complex Ag-Cu/γ-Al2O3 catalysts reached 100% of methanol combustion only around 280 °C [9]. Jabłońska et al. [9] detected by XPS and EPR presence of Ag2+ ions on the Ag2O/CuO
Corresponding author at: Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland. E-mail address: [email protected] (R. Dziembaj).
https://doi.org/10.1016/j.cattod.2018.03.042 Received 4 December 2017; Received in revised form 2 March 2018; Accepted 22 March 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Dziembaj, R., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.03.042
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interface which have been assumed to be the active centres for methanol combustion. Methanol oxidation on model cobalt oxides catalysts under pressure of 0.1–0.3 mbar was studied using in situ XPS, XAS and MS at ISISS beamline at BESSY in Berlin [10]. The surface of a single Co crystal was oxidized to CoO or Co3O4 and then the reactants adsorption and catalytic reaction were studied together with spectroscopic investigations. It was proved that the reaction mechanism proceeds through methoxy and formate intermediates, which can be further oxidized depending on the oxidation potential of CH3OH/O2 and Co oxidation state. Such a mechanism has been also demonstrated using density functional theory for partial methanol oxidation over vanadia/ceria catalysts [11] or using in situ spectroscopy (DRIFTS) combined with temperature programmed methods with hematite model catalyst. Bowker et al. observed conversion of methoxy to formate species on the surface of hematite catalyst at 200 °C [12]. At least since 20 years ceria materials modified by 3d metal oxides have been studied as potential substitutes of the noble metal catalysts for the VOC’s incineration [13,14]. The cobalt-cerium mixed oxides were used for total oxidation of many VOCs, e.g. ethyl acetate [15,16], benzene [17], toluene [16–19], xylenes [17], n-hexane [17,20], propane [21,22] and propene [23]. The catalytic combustion of methanol on cobalt based catalysts was relatively rarely studied. The only relevant papers are the one already mentioned above [10] and this paper, concerning CoxCe1−xOy and CoxCe1−xOy/Al2O3 nanocatalysts, which is a continuation of our previous studies on CuxCe1−xO2−y [24,25] and MnxCe1−yOy [26] nanoparticles. There are only few papers describing other 3d metal oxides used for low-temperature methanol combustion, for example Fe2O3 [12,27] and Cu, Mn and Ag oxides nanocatalysts [9]. The recent review entitled “Catalytic oxidation of volatile organic compounds (VOCs)” by Kamal [28] does not contain any information about total oxidation of methanol. In our previous papers concerning methanol, acetone and ethylene total oxidation, the formation of single-phase solid solution nanoparticles of CuxCe1−xO2−y up to somewhere between 0.15 < × < 0.20 was reported [24,25]. The catalytic activity in methanol total oxidation of these nanoparticles increased with x within the existence range of single-phase solid solution (SPhSS) providing 100% total oxidation at T100 = 240 °C and T50 = 210 °C for the most active Cu0.15Ce0.85O2−y. This catalytic activity grew in parallel with concentration of individually dispersed copper ions (measured by UV–vis diffuse reflectance spectroscopy [29]), electric conductivity and share of ionic conductivity (measured by impedance spectroscopy) what supports increase of oxygen mobility in the defected CuxCe1−xO2−y [25]. The analogous SPhSS nanoparticles were observed for MnxCe1−xO2−y within a slightly broader 3d metal concentration range up to somewhere between 0.20 < × < 0.25. On the other hand, for the Mn-rich system (0.75 ≤ × < 1) no single-phase solid solutions was observed [26]. It was a mesoporous composite material consisted of the separate single-phase nanoparticles of both oxides CeO2−y and Mn3O4, which was confirmed with XRD and TEM/EDS studies. The high catalytic activity in methanol total oxidation ensured achievement of T100 values at 240 °C and 280 °C for Mn0.25Ce0.75O2−y bare and embedded on α-alumina carrier, respectively. It was also found that similarly high activity of the nanocomposite two-phase materials was obtained for the catalyst with atomic ratio Mn:Ce = 3:1 [26]. The main reasons for undertaking current research were to confirm whether analogous SPhSS nanoparticles in the CoxCe1−xO2−y system are formed as well as a similar composite of two-separate-phase nanoparticles are formed when x is larger than 0.25. This oxide system was characterized in terms of crystal structure, morphology and chemical composition (XRD, N2-isotherms, TEM/EDS linescan), valence state (XPS), reducibility and oxygen storage capacity (H2-TPR and O2reoxidation) and catalytic activity in methanol combustion of the bare and α-Al2O3 supported catalysts.
Fig. 1. X-ray powder diffraction patterns for SPhSSs of Ce-rich nanoparticles (a), the same solutions embedded on α-Al2O3(b) and biphasic Co-rich samples (c).
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2. Experimental
Table 1 Surface areas, crystallite and grains sizes for obtained catalysts.
The series of the nanocrystalline Co-Ce-O system were the subject of study: CoxCe1−xOy and CoxCe1−xOy/Al2O3. The synthesis procedure related to the first two series contained from 0 to 20 at% of Co and from 0 to 20 at% of Ce, was described in a previous paper [30] concerning catalytic decomposition of N2O on these catalysts. In brief, solutions of cerium and cobalt salts (Ce(NO3)3·6H2O, Co(NO3)2·6H2O) as well as cyclohexane and Triton X-100 with n-hexanol (all purchased from POCh) were used in appropriate ratio to obtain the first nanoemulsion. The second nanoemulsion contained the precipitation agent TPAH (tetrapropylamonium hydroxide, Sigma Aldrich) instead of the metal salts. The third series was supported catalysts (CoxCe1−xOy/Al2O3) obtained by using appropriate amount of a commercial α-Al2O3 support (Sigma-Aldrich) into the reaction vessel during mixing both nanoemulsions. Such an embedding procedure was effective for nanometric MnxCe1−xO2−y/α-Al2O3 catalysts used in low-temperature combustion of methanol [26]. After precipitation and drying (90 °C) all of the obtained powders were preheated with a linear rate of 10 °C/min up to 500 °C and calcined at this temperature in the air flow for 3 h. Crystal structure of the catalysts was characterized by powder X-ray diffraction using a BRUKER D2 PHASER with CuKα radiation (0.154178 nm). The average size of CeO2 and Co3O4 crystallites (DXRD) was calculated using the Scherrer equation for (111), (200), (220), (311) or (311), (400), (511), and (440) reflections, respectively. In case of α-Al2O3 the reflections of (012), (104), (113) and (116) were taken for the calculations. The specific surface areas were evaluated from the N2 adsorptiondesorption isotherms (Micromeritics ASAP 2010). Each sample was preliminary outgassed at 350 °C for 12 h under vacuum (4·10−3 Pa). Then, N2-isotherms were measured up to pressure of 8·104 Pa. The values of specific surface areas were used to calculate the average grain size (DBET) assuming spherical shape of the grains and using crystallographic density of Co3O4 and CeO2, 6.11 and 7.215 g/cm3, accordingly. The total pore volume was determined at p/p0 = 0.98, and the pore size distribution was calculated by the BJH method. The XPS measurements were performed with a Prevac photoelectron spectrometer equipped with a hemispherical VG SCIENTA R3000 analyzer using the monochrome Al Kα radiation 1486.6 eV and an electron flood gun (FS40A-PS) to compensate charge on the surface during measurements. The analyzer operated in the constant energy mode at 100 eV with resolution of 0.1 eV. The composition of the surface was determined using areas and binding energies of Ce 3d, Co 2p and O 1 s photoelectron peaks after fitting of the high resolution spectra through the CasaXPS software.
Sample
S [m2/g]
DXRD [nm]
DBET [nm]
CeO2 Co3O4 Co0.1Ce0.9O2 10% Co0.1Ce0.9O2/α-Al2O3 15% Co0.1Ce0.9O2/α-Al2O3 20% Co0.1Ce0.9O2/α-Al2O3 Co0.15Ce0.85O2 10% Co0.15Ce0.85O2/α-Al2O3 15% Co0.15Ce0.85O2/α-Al2O3 20% Co0.15Ce0.85O2/α-Al2O3 Co0.2Ce0.8O2 10% Co0.2Ce0.8O2/α-Al2O3 15% Co0.2Ce0.8O2/α-Al2O3 20% Co0.2Ce0.8O2/α-Al2O3
131 51 76 22 28 34 93 23 23 27 43 27 28 38
8 21 6 3 5 7 5 5 6 5 10 4 6 4
11 32 11 8 7 7 9 7 10 10 20 5 7 6
α-Al2O3 (Sigma-Aldrich) mesh 100; SBET = 14 m2/g.
Transmission electron microscopy (TEM) investigations were carried out using a FEI Tecnai TF20 X-TWIN (FEG) microscope, equipped with an EDAX energy dispersive X-ray (EDX) detector, at an accelerating voltage of 200 kV. The samples were prepared by drop casting on carbon coated copper TEM grids. Prior to the cast onto the grids, the powder materials were dispersed in isopropanol using ultra-sonication for 5 min. The oxygen storage capacity (OSC) was characterized using temperature-programmed reduction (TPR) with H2. The TPR runs were carried out in a plug-flow reactor (from 60 to 500 °C) with a linear heating rate of 10 °C/min. The H2 uptake was monitored by a micro volume TCD (Valco). Two series of the experiments were performed. In the first series obtained material was preliminary oxidized by heating up to 500 °C in the stream of synthetic air (Ar/O2 = 4/1) using flow rate of 10 cm3/min. Immediately after described pre-oxidation tested catalyst was fast cooled to 60 °C and a TPR run was switched on with a constant flow rate (10 cm3/min) of 5 vol.% of H2 in Ar up to 500 °C and then cooled to 60 °C. Afterwards nanocatalyst was subjected to re-oxidation at a chosen temperature, and the next TPR run was carried out as before. The redox cycles were repeated three times. During the second series of TPR experiments the samples were outgassed by heating in the pure Ar stream preserving other conditions the same as in the first series of TPR measurements. Then, after cooling to 60 °C the H2-TPR was switch on with the same linear heating rate of 10 °C/min. The catalytic activity of the unsupported and supported catalysts in methanol combustion was studied in a fixed-bed plug-flow microreactor under atmospheric pressure. Each sample of the catalysts (100 mg) was preliminary outgassed in a flow of 5.00 vol.% of O2 in pure helium
Fig. 2. Comparison of BET isotherms on examples of the 10% Co0.2Ce0.8O2/α-Al2O3(a) and 20% Co0.2Ce0.8O2/α-Al2O3(b) catalysts. 3
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Fig. 3. The electron diffraction of Co0.95Ce0.05O2 (a) and Co0.15Ce0.85O2 (b) systems with corresponding bright field micrographs with marked regions used for diffraction studies (c,d).
Fig. 4. Bright field micrographs of Co0.95Ce0.05O2 (a) and Co0.15Ce0.85O2 (b) nanoparticles with EDS linscan analysis embedded on the pictures. 4
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Fig. 5. The Ce3d XPS spectra for CeO2 (a), Co0.15Ce0.85O2 (b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts. Table 2 The XPS spectra parameters of Ce3d for CeO2, Co0.15Ce0.85O2 and Co0.15Ce0.85O2/α-Al2O3. Sample
Final state
EB [eV]
CeO2
u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5 u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5 u/v Ce(IV): 3d94f2O2p4 u’’/v’’ Ce(IV): 3d94f1O2p5 u’’’/v’’’ Ce(IV): 3d94f°O2p6 u0/v0 Ce(III): 3d94f1O2p6 u’/v’ Ce(III): 3d94f2O2p5
900.61 907.34 916.53 899.16 901.92 901.00 907.42 916.83 899.09 902.77 900.96 907.44 916.94 899.71 902.90
Co0.15Ce0.85O2
Co0.15Ce0.85O2/α-Al2O3
5
882.11 888.69 898.03 880.66 883.52 882.50 888.82 898.33 880.59 884.37 882.56 888.84 898.44 881.21 884.50
Area [a.u.]
Share [%.]
194872.9 280264.9 364624.4 14718.1 165613.9 368737.3 377145.5 470117.0 24609.5 164744.8 74178.9 75356.7 86719.7 2534.3 45215.4
18.9 27.4 36.1 1.4 16.1 26.0 26.8 33.8 1.7 11.6 25.9 26.5 30.9 0.9 15.8
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(99.9999%) at 450 °C for 1 h. The catalytic measurements were performed within the range from 55 to 500 °C with the linear temperature increase of 10 °C/min. During the catalytic studies the total feed flow was controlled at F = 20 cm3 min−1, corresponding to a W/F ratio of 0.30 g s cm−3 and GHSV = 29,000 h−1. The reactor feed contained sixfold excess of oxygen (0.50 vol.% of methanol and 4.50 vol.% of O2 in pure He) in relation to the stoichiometric ratio. The flow of all used gases were adjusted by electronic mass controllers (Brooks), and the methanol was introduced into the controlled gas stream in a temperature stabilized saturator. Concentration of the reactants were continuously monitored with a quadruple mass-spectrometer (PREVAC) connected on-line with the reactor outlet. Taking into account the same main mass lines of O2 and CH3OH, the line 31 (deprotonated methanol) has been adopted as a measure of methanol concentration based on the results of appropriate calibration of the mass spectrometer. To avoid any flow fluctuation the intensities of the 31 line were related to the intensity of helium line 4. The methanol fractional conversion α was calculated using Eq. (1). The observed intensities of CO2 mass line (44) change parallel to the values of α demonstrating high selectivity to the total oxidation of methanol. At the low temperatures and several % of methanol conversion, a trace of HCHO (line 30) was observed.
a=
( ) −( ) ∙100% ( ) I IHe 2
I IHe 1
I IHe 1
(1)
3. Results and discussion The XRD patterns of the unsupported Co-Ce-O catalysts with Co:Ce molar ratio below or equal to 1:6 (Fig. 1a) showed only reflexes from CeO2 fluorite-like structure which confirmed the formation of CoxCe1−xO2−y single-phase solid solution [30]. Similarly, as it was observed in case of Mn3O4 – CeO2-y system up to the atomic ratio Mn:Ce = 1:5 [26]. The most intense diffraction line of Co3O4 (311) at 2Ɵ = 37° was noticed only after reaching a ratio of Co:Ce = 1:4 (Co-Ce20 sample Fig. 1a). Bolivar-Diaz et al. [31] just reported the segregation of Co3O4 from the CoxCe1−xO2−y at Co:Ce = 2:3, although the traces of cobalt oxide could be noticed at smaller Co content, even at the ratio close to Co:Ce = 1:4, which is visible in XRD patterns published in [31]. In this paper there is no information on studies performed on samples with lower Co:Ce ratio than 1:4. Taking into account both of these results one may assume that separation of the Co3O4 phase from our SPhSSs begins when at least 2 Co atoms replace 2 of 12 Ce atoms contained in 3 elementary cells. The average crystallite diameters (DXRD) of this solution were between 5 and 6 nm; however, the sample Co0.2Ce0.8O2−y has twice bigger average diameter. This corresponds to the observed difference in DXRD of the comparative samples of the pure CeO2 and Co3O4 (obtained by the same preparative procedure) which were equal to 8 and 21 nm, respectively (Table 1). The XRD patterns of the supported Co-Ce-O catalysts (Fig. 1b) showed very intensive α-Al2O3 lines; however, the XRD pattern of CeO2 is the same as in the case of unsupported catalysts. This effect confirmed effective embedding of the SPhSS particles on the support grains. Nevertheless, the most intensive Co3O4 (311) reflex could not be observed due to the full cover by the very intensive Al2O3 (110) line at 2Ɵ = 37.7°. The calculated average nanocrystals size of the supported SPhSS particles was similar to the size of crystallites of related unsupported SPhSS particles (Table 1). The opposite results were obtained for the unsupported catalysts contained from 0 to 20 at% of Ce in the mixed oxide system (Fig. 1c). The most intense line of CeO2 (111) was observed at 2Ɵ = 28.5° even at the ratio of Ce:Co = 1:99 (Co-Ce-1 sample in Fig. 1c). All the main diffraction lines were already observed at the 1:20 ratio, therefore the formation of cerium solutions in Co3O4 phase should be excluded and consequently chemical formula Co3O4·nCeO2−y should be used to
Fig. 6. The O1 s XPS spectra for CeO2 (a), Co0.15Ce0.85O2 (b), 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts and pristine Al2O3 support (d).
Table 3 The XPS spectra parameters of O1 s for CeO2, Co0.15Ce0.85O2 and Co0.15Ce0.85O2/α-Al2O3. Sample CeO2 Co0.15Ce0.85O2 Co0.15Ce0.85O2/α-Al2O3
6
Final state 2−
O OH, etc. O2− OH, etc. O2− OH, etc.
EB [eV]
Area [a.u.]
Share [%.]
529.07 530.60 529.56 531.67 529.67 531.47
81577.8 74884.7 153239 32242.6 18503.9 281873
52.1 47.9 82.6 17.4 6.2 93.8
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Fig. 7. The Co2p XPS spectra for Co0.15Ce0.85O2 (a,b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c,d) catalysts and Co3O4 (e,f) reference.
Burroughs [34] as v and u related to 3d5/2 and 3d3/2. Components of the doublets differ by a constant value of 18.5 eV. Such a large number of doublets arises from different Ce4f occupancies in the final state [35,36] or/and from the coupled excitations of the O2p → metal 4f charge transfer [34,37]. The most characteristic for Ce(IV) is the peak labelled u’’’ (found in our case within 916.5 – 916.9 eV) as only one undisturbed by the other peaks. This signal is related to Ce(IV) with the 3d94f°O2p6 configuration and the doublets labelled as v0 – u0 and v’ – u’ are assigned to Ce(III). It was confirmed experimentally by Spanier et al. [38] and F. Zhang et al. [39] who showed drastic increase of these doublets intensity after reduction CeO2 possibly close to Ce2O3. Analysis of the our results leads to the conclusion that introduction of Co ions into ceria lattice causes only slight decrease of the Ce(III) concentration in the surface layers of the SPhSS, which undergoes further decrease after deposition of this SPhSS on α-alumina (Table 2). In Fig. 6a the O1 s spectrum of the nanocrystalline CeO2 is presented. The lower energy peak is ascribed to lattice oxygen ions while the higher energy peak to OH or to electrophilic oxygen species. Introduction of Co ions into ceria lattice strongly increases the share of O2− peak at 529.6 eV, although deposition of the SPhSS on α-Al2O3 (Fig. 6c) caused extreme increase of the other peak, most probably due to covering the alumina surface with OH species (Fig. 6d, Table 3). The XPS spectra of Co2p consist of one spin-orbit doublet, Co2p3/2 and Co2p1/2, differ by 15.0 eV and broad satellites at about 9 eV [40]. The reported BE values of Co2+ and Co3+ which are given in literature are not assigned uniquely as they depend strongly on non-stoichiometry of these oxides [41]. Nevertheless, characteristic feature of Co2+ is the higher binding energy of Co2+ than Co3+ within the Co2p3/2 peak and additionally higher intensity of the satellite. Taking into account
describe these composite nanomaterials. In the previous paper [30] the observed hysteresis loop of the N2adsorption-desorption isotherms measurements showed mesoporous character of the SPhSS materials (CoxCe1−xO2−y) and macroporous one for the composite materials (Co3O4·nCeO2−y). The analogous isotherms obtained for the supported SPhSSs (Fig. 2a,b) showed both macro- and mesoporous characters. The significantly smaller proportion of mesopores for the supported solutions is an effect of the excess of micrometric (0.155 mm) spheres and nanometric particles of the solid solution coexisting in the supported catalysts. The specific surface area of the supported catalysts (Table 1) is several times smaller than in case of the related unsupported catalysts which specific surface area is in the range from 40 to 90 m2/g [30]. Transmission electron microscopy studies with EDS linescan analysis are presented in Fig. 3 and Fig. 4. Results of electron diffraction of selected test areas which are shown in Fig. 3 confirm conclusions inferred from XRD analysis. The observed electron diffraction patterns for CoxCe1−xO2−y sample (Fig. 3b) clearly proves that it is the single-phase nanocrystalline (Fig.3d) material, isostructural to CeO2. All of the recognized diffraction rings are attributed to Fm-3 m lattice. The structural changes resulting from cobalt doping are too small to visibly influence presented electron diffraction pattern. This, together with EDS analysis (Fig. 4b), which confirms presence of both Ce and Co in the material’s nanocrystals, proves that obtained sample is the CoxCe1−xO2−y solid state solution. It can be observed (Fig. 3d) that CoxCe1−xO2−y catalysts consist of small, up to 15 nm in diameter, uniform nanoparticles. On the other hand, samples with Co concentration above 20 at% can be characterized as two phase system with bigger Co3O4 grains and smaller precipitates of CeO2 nanocrystals. It seems that Co3O4 nanoparticles grew much faster than CeO2. Electron diffraction (Fig. 3a) confirms two phases present in the sample and the EDS studies indisputably show different elemental composition of selected grains (namely Co3O4 and CeO2), which additionally support our conclusions coming from XRD analysis. X-ray photoelectron spectroscopy was used to recognize change in valence state of Ce ions after formation of the SPhSS and deposition particles of this solution on α-alumina carrier. The obtained results of Ce3d XPS after subtracting the background are presented in Fig. 5. These complex spectra consist of 5 doublets of Ce3d5/2 and Ce3d3/2 [32,33] and are labelled according to convention proposed by
Table 4 Atomic ratio based on XPS measurements. Sample
O2−/Onucl
Co2+/Co3+
Ce3+/Ce4+
Co3O4 CeO2 Co0.15Ce0.85O2 Co0.15Ce0.85O2/α-Al2O3 α-Al2O3
1.39 1.09 4.75 0.07 0*
0.62 – 0.58 1.5 –
– 0.21 0.15 0.20 –
* The O1 s peak in α-Al2O3 (531.6 eV) is symmetrical. 7
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Fig. 8. Comparison of the H2-TPR profiles for CeO2 (a,c,e) and Co3O4 (b,d,f) samples.
increase in binding energy of Co2p electrons was also observed by Biesinger et al. [41] after replacement of oxygen by OH ligands. Two series of the H2-TPR were carried out to characterize oxygen storage capacity in the catalysts; the first based on outgassed and preoxidized catalysts and the second only on outgassed catalysts. Two complex TPR peaks were detected during the first series with the nanocrystalline CeO2 sample. Within the low temperature peak (around 200 °C) two maxima were distinguished at 219 and 238 °C, and within the other peak (around 420 °C) also two at 411 and 428 °C (Fig. 8a). They were ascribed to reduction of the chemisorbed oxygen species and the lattice oxygen, respectively. After cooling in argon stream, and reoxidation up to 260 °C, the next TPR showed only a small change (Fig. 8c), but in the third TPR run preceded by re-oxidation up to 360 °C the peak of chemisorbed species clearly diminished (3-times) and the peak of surface lattice oxygen became a single peak (Fig. 8e). The same type of TPR plot but for the nanocrystalline Co3O4 consisted of only one, however very broad (about 180 °C) peak and there were no traces
relatively small intensity of the Co2p peaks and smaller amounts of cobalt ions (15 at%) in the SPhSS nanoparticles, and even smaller amounts in the supported particles we decided to base our analysis on only Co2p3/2 peaks. The raw Co2p spectra are presented in Fig. 7b, d and f, while the Co2p3/2 envelope divided for the Co2+ to Co3+ shares in Fig. 7a, c and e. The peak at 780.18 eV is ascribed to Co3+ and those at 781.89 eV for the unsupported Co0.15Ce0.85 catalyst while the appropriate binding energy measured are equal to 780.00 and 781.21 eV for the supported catalysts. Yang et al. [40] reported for Co3O4 the binding energies 779.9 and 781.2 eV for Co3+ and Co2+, respectively, while Biesinger et al. [41] 779.6 and 780.9 eV, so close to our results. The calculated atomic ratio of Co3+ to Co2+ in the unsupported catalyst was equal 5:3, instead of 6:3 in the Co3O4 stoichiometric. One may correlate this with corresponding reduction of Ce4+/Ce3+ ratio (Table 4). The observed inversion of the Co3+/Co2+ ratio after deposition SPhSS on α-Al2O3 may be related to the drastic increase of OH species on the surface of supported catalyst (Fig. 6d, Table 4). Such an 8
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Fig. 9. Comparison of the H2-TPR profiles for Co0.15Ce0.85O2 (a,c,e) and 20% Co0.15Ce0.85O2/α-Al2O3 (b,d,f) samples.
of the model cobalt catalyst providing increasing the number of active sites in the catalysts for carbon oxides hydrogenation [42,43]. Introduction of Co ions into the ceria lattice causes significant changes in the TPR profiles visible by comparing the curves marked a, c and e from Fig. 9 representing the results obtained on the most active SPhSS (Co0.15Ce0.85O2−y) with the respective curves in Fig. 8 obtained (under the same conditions) on nanoparticles of single oxides, CeO2 and Co3O4, i.e. with Fig. 8a–f, respectively. The changes in the chemisorbed species part of the TPR profiles are limited to disappearance of these species after re-oxidation at 360 °C (Fig. 9e), while the reduction of the lattice oxygen has changed significantly in comparison with the single oxides. The reduction of CeO2 in the first TPR runs begins at 390 °C while in case of Co3O4 at 220 °C with the maximum at 327 °C. The start temperature of the Co3O4 reduction is increased up to 270 °C and the maximum up to 367 °C after re-oxidation at 360 °C before the third TPR run (Fig. 8f).
of the chemisorbed oxygen reduction (Fig. 8b). This peak has been split into 4 components. Taking into account the total reduction of Co3O4 and stoichiometry of the two-step reduction, Co3O4 + H2 − > 3 CoO + H2O
(1)
3 CoO + 3 H2 − > 3 Co + 3 H2O
(2)
one may assume that the first component represents the first step of the reduction while the second is the sum of three other components. Moreover, total re-oxidation of Co3O4 after TPR runs needs higher temperature than in case of the nanocrystalline CeO2 as may be concluded from the difference between the TPR profiles after reoxidation up to 260 and 360 °C (Fig. 8a, c, e and b, d, f). It seems that the nanoparticles after reduction/oxidation/reduction process have core-shell morphology with reduced cobalt in the core and oxide phase in the shell. Such a model was used years before to explain increase of activity
9
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Fig. 10. Results of TPR runs performed with outgassed CeO2 (a), Co0.15Ce0.85O2 (b) and 20% Co0.15Ce0.85O2/α-Al2O3 (c) catalysts.
The TPR profile of the lattice oxygen reduction in the most active SPhSS includes two clearly separated but complex reduction peaks with maxima at 325 and 430 °C (Fig. 9a), most probably the second peak may be related mainly to reduction of Ce4+. It is more visible in the second TPR run, showing larger increase of the ceria reduction degree. The sharp increase of the reduction rate (with peak at 367 °C) during the third TPR after pre-oxidation at 360 °C can be explained by catalytic effect of Co° appearance in the reduced nanomaterial (Fig. 9e). The TPR profile of the sample containing 20 wt% of the most active SPhSS embedded on α-Al2O3 turned out, how one could expect, more complex. The total TPR curve consists of 6 components, with beginning at 200 °C and ending at 425 °C, providing even higher extent of the reduction as it was in case of unsupported SPhSS. There is a low-temperature peak at 218 °C which is moved to 210 °C in the second and third TPR runs. This peak should be related consequently to reduction of chemisorbed oxygen and OH covering α-Al2O3 surface as it was observed before during XPS measurements of O1 s spectrum. The sharp peak close to the end of the reduction, especially in the case of the supported catalysts, can be explained by autocatalytic effect of the Co° species appearing in this stage of the reduction. The second type of the TPR runs performed after outgassed procedure up to 500 °C in Ar stream showed meaningful lowering of the low temperature peak, so the amount of chemisorbed oxygen on the nanometric CeO2 has been considerably diminished; nevertheless advance of the lattice oxygen removal has been preserved or even increased
Fig. 11. Comparison of catalytic activity for CoxCe1−xOy (Ce-rich system (a) and Co-rich system and nanoparticles with pristine Co3O4 and CeO2 (c)), CoxCe1−xOy/Al2O3 (b).
(please note the differences between the scales in Figs. 8, 9 and 10). The increase be related to creation of the oxygen vacancies by outgassing procedure of CeO2 what makes oxygen transport in this oxide easier as reported by Lawrence et al. [44]. Similar increase of the oxygen mobility in the CuxCe1−xO2−y single-phase solid solution, owing to increase in concentration of oxygen defects resulting from introduction of Cu ions into ceria lattice, was stated based on increase of electric conductivity and share of ionic conductivity measured by impedance spectroscopy [25]. The results of catalytic combustion of methanol over CoxCe1−xO2−y are presented in Fig. 11a and compared with results of homogeneous and catalytic combustion over the CeO2 nanoparticles. All of the obtained materials were very selective towards total oxidation, traces of formaldehyde were observed only at very low conversion. Already 1 at 10
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phase separation, namely Co0.15Ce0.85O2−y. Perhaps suggested by Schlogel et al. [10], Sauer [11] and Gibbson [12] two steps mechanism of total methanol oxidation proceeds on two different active sites which being close together accelerate the combustion. Such active assembles may by created as well on the surface of SPhSS or at the phase boundaries between the nanoparticles of these different phases in nanocomposite catalysts. Obviously, the role of the oxygen vacancies, chemisorbed species and ions with easy variable valence cannot be neglected in complete explanation of the mechanism of methanol total oxidation.
Table 5 T50 and T100 parameters for obtained catalysts and other selected examples. Sample
T50 [°C]
T100 [°C]
CeO2 Co0.01Ce0.99O2 Co0.05Ce0.95O2 Co0.1Ce0.9O2 Co0.15Ce0.85O2 Co0.2Ce0.8O2 10% Co0.1Ce0.9O2/α-Al2O3 10% Co0.15Ce0.85O2/α-Al2O3 10% Co0.2Ce0.8O2/α-Al2O3 15% Co0.1Ce0.9O2/α-Al2O3 15% Co0.15Ce0.85O2/α-Al2O3 15% Co0.2Ce0.8O2/α-Al2O3 20% Co0.1Ce0.9O2/α-Al2O3 20% Co0.15Ce0.85O2/α-Al2O3 20% Co0.2Ce0.8O2/α-Al2O3 Co0.95Ce0.05O2 Co0.99Ce0.01O2 Co3O4 * Cu0.15Ce0.85O2 [24] Cu0.15Ce0.85O2 [25] Mn0.25Ce0.75O2 [26] Mn0.75Ce0.25O2 [26]
445 285 265 242 222 228 278 272 269 267 270 254 266 255 260 161 177 200 – – – –
600 437 413 438 363 374 400 390 400 375 390 375 400 370 375 225 256 375 375 275 245 335
4. Conclusions Electron and X-ray diffractions confirmed formation of nanoparticles of the SPhSS CoxCe1−xO2−y up to Co/Ce atomic ratio = 1:6. At higher Co concentration the samples obtained by the same precipitation from nanoemulsions and further calcination, are composed of separated nanocrystallites of Co3O4 and CeO2, or rather Co3O4 and SPhSS of Co ions in CeO2−y phase, forming nanocomposite. Formation of analogous SPhSS has been also observed for CuxCe1−xO2−y and MnxCe1−xO2−y and in spite differences between the charge of these 3d metals ions the boundary limits of the solid solution are located between 0.15 < × < 0.25, though the related values of x increases from Cu to Mn. Then the limit x is increasing together with higher charge of these ions what may be related to lower stress evolved in the crystallites by the strange ions with charge closer to charge of Ce ions. Co introduced to ceria lattice generates oxygen vacancies what increase mobility of the oxygen ions making reduction easier during the TPR. In consequence advance of the ceria reduction is increased and observed at lower temperature. The catalytic activity increases with x values in the SPhSS but after appearance of the segregated Co3O4 crystallites, the activity is stabilize or slightly diminish. On the other hand, the activity of Co3O4 nanoparticles obtained in the same procedure is high, comparable with the best SPhSS (Co0.15Ce0.85O2−y), though nanocomposite of Co3O4 and CeO2 (or rather SPhSS of Co ions in CeO2−y phase) contained as low amounts as Ce as 1 or 5 at%, which results in further increase of activity. One may suppose that special assembles formed as well in the SPhSS as at the phase boundaries between the nanoparticles of these different phases are responsible for the observed phenomena. The catalytic activity of SPhSS in methanol total oxidation are generally preserved after embedding on alumina support taking into account the SPhSS loading on the support being 4–10 times smaller than in case of the bare SPhSS catalysts.
* Obtained by CeO2 impregnation.
% of Co introduced to CeO2 lattice extremely increased catalytic activity and this activity increased further up to x = 0.143. The catalyst with x = 0.214, thus with trace of separate Co3O4 phase, showed slightly lower activity. Such maxima of catalytic activity were observed before in the CuxCe1−xO2−y [24,25] and MnxCe1−xO2−y [26] systems and were related to appearance of the separate CuO or Mn3O4 phases. The results obtained for the supported catalysts contained 10, 15 and 20 wt% of the active SPhSS with various Co content are presented in Fig.11b. All these catalysts were active but relation between Co content and catalyst loading on alumina was not so clear, probably due to high OH species concentration on supported catalyst what is consistent with XPS results, showing drastic increase of Co2+ share in the Co2p spectra (Fig. 7b). Such a shift of the binding energy of Co2p electrons was observed by Biesinger after replacement of oxygen by OH ligands [41]. It is possible that less hydrophilic catalyst carrier support would be more appropriate to distinguish the activity of supported catalysts. The choice of α-Al2O3 evolved from intention to preserve the same support as was used before in our lab, which in turn was consequence of the Salsona et al. [21] results that this type of alumina support (from several tested) provided the best catalytic performance in total oxidation of propane. On the other hand, the catalytic experiments were performed every time with the same mass of the catalyst what caused that amounts of active phases in the supported catalysts was from 5 to 10 times lower than in case of the appropriate bare SPhSS. Taking into account these differences in amounts of the active phase one may suggest that the high activity of the supported catalysts were generally preserved. The temperature necessary to achieve 100% and 50% combustion of methanol (T100, T50) on obtained catalysts, together with several other examples to compare, are gathered in Table 5. The Co3O4 nanoparticles appeared nearly as active as the best SPhSS bare catalyst, Co0.15Ce0.85O2−y. However, the samples obtained by the same procedure but with majority of Co content in the nanoemuslsion and only 1 or 5 at% Ce in relation to Co, showed even higher catalytic activity. Then, there is a clear synergy observed in such nanocomposite system consisted of larger nanoparticles of Co3O4 and smaller (Table 1) nanoparticles of CeO2−y or more probably CoxCeO2−y (Fig. 11c). Hence the catalytic activity in the Co3O4-CeO2−y cannot be simple related to amount of the Co contents in the catalysts as the highest performance is related to coexistence of both nanoparticles of Co3O4 and SPhSS or only nanoparticles of SPhSS with the highest contents of Co without Co3O4
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