Synthesis, Characterization, and Thermochemical Redox Performance

Article pubs.acs.org/JPCC

Synthesis, Characterization, and Thermochemical Redox Performance of Hf4+, Zr4+, and Sc3+ Doped Ceria for Splitting CO2 Jonathan R. Scheffe,*,† Roger Jacot,‡ Greta R. Patzke,*,‡ and Aldo Steinfeld†,§ †

Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland § Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland ‡

S Supporting Information *

ABSTRACT: We present results on the thermochemical redox performance and analytical characterization of Hf4+, Zr4+, and Sc3+ doped ceria solutions synthesized via a sol−gel technique, all of which have recently been shown to be promising for splitting CO2. Dopant concentrations ranging from 5 to 15 mol % have been investigated and thermally cycled at reduction temperatures of 1773 K and oxidation temperatures ranging from 873 to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr doped materials is substantially higher than those of pure ceria and Sc doped ceria and increases with dopant concentration. Overall, 10 mol % Hf doped ceria results in the largest CO yields per mole of oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based on measured mass changes during oxidation. However, these yields were largely influenced by their rate of reoxidation, not necessarily thermodynamic limitations, as equilibrium was not achieved for either Hf or Zr doped samples after 45 min exposure to CO2 at all oxidation temperatures. Additionally, sample preparation and grain size strongly affected the oxidation rates and subsequent yields, resulting in slightly decreasing yields as the samples were cycled up to 10 times. X-ray diffraction, Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX have been applied to characterize the elemental, crystalline, and morphological attributes before and after redox reactions.

■

INTRODUCTION Ceria and ceria-based materials have gained considerable interest as reactive intermediates in solar thermochemical redox cycles. These cycles are capable of producing renewable fuels and fuel precursors (H2/CO) at elevated temperatures according to the following redox reactions. High-temperature endothermic reduction T , pO

2

CeO2 − δi ⎯⎯⎯⎯→ CeO2 − δf +

δf − δi O2 (g ) 2

reduction temperature (T) and oxygen partial pressure (pO2), as described by Panlener et al.1 Following thermal reduction, the reduced ceria is cooled and reacted with either H2O or CO2, or a mixture of the two, to produce H2 (eq 2a) and/or CO (eq 2b), while ceria is reoxidized to its initial oxygen stoichiometry. The reoxidation potential, or free energy of oxidation with H2O or CO2 (ΔGrxn), is dependent on its deviation from stoichiometry (↑δ, ↓ΔGrxn). It is most thermodynamically favorable (↓ΔGrxn) below 1200 K and increases as temperature is decreased.2 Ceria thermochemical redox cycles were first investigated in 2006 by Abanades et al.,3 who studied the stoichiometric reduction of CeO2 to Ce2O3 in a radiatively heated particle based reactor. The reduction was performed near 2273 K, resulting in extensive sublimation. More recently Chueh et al.4 and Furler et al.5,6 have studied the nonstoichiometric reduction of ceria at more moderate temperatures in a cavity based, batch-type solar reactor. Temperatures were generally lower than 1823 K, resulting in lower yields than those obtained by Abanades et al., but the material stability was superior. Furler et al.6 recently demonstrated mean solar-to-fuel

(1)

Low-temperature exothermic oxidation CeO2 − δf + (δf − δi)H 2O(g ) ΔGrxn

⎯⎯⎯⎯→ CeO2 − δi + (δf − δi)H 2(g )

(2a)

CeO2 − δf + (δf − δi)CO2 (g ) ΔGrxn

⎯⎯⎯⎯→ CeO2 − δi + (δf − δi)CO(g )

(2b)

In the first endothermic reaction (eq 1), ceria is reduced at elevated temperatures (generally greater than 1673 K) using concentrated solar energy as process heat. The amount of oxygen released is dictated by the change from its initial degree of nonstoichiometry (δi) to its degree of nonstoichiometry after thermal reduction (δf), which is highly dependent on the © 2013 American Chemical Society

Received: May 22, 2013 Revised: September 8, 2013 Published: October 16, 2013 24104

dx.doi.org/10.1021/jp4050572 | J. Phys. Chem. C 2013, 117, 24104−24114

The Journal of Physical Chemistry C

Article

investigated dopant concentrations of 10% in ceria at reduction and oxidation temperatures of 1773 and 773 K, respectively. We have synthesized 5, 10, and 15% of each of the aforementioned dopants in ceria via a sol−gel method and cycled these materials in a TGA at a reduction temperature of 1773 K and oxidation temperatures ranging from 873 to 1073 K. These materials, primarily 10% Hf doped ceria and pure ceria, have been analyzed via powder X-ray diffraction (XRD) together with Raman, FT-IR, and UV/vis spectroscopy, as well as SEM-EDX (scanning electron microscopy with energy dispersive spectroscopy) to characterize the resulting elemental composition, crystalline phase, and microstructural morphology before and after redox reactions. Finally, selected materials are subjected to thermal cycling, and insights into rate limiting mechanisms during the oxidation reaction with CO2 are elucidated from a thermodynamic and kinetic perspective.

energy conversion efficiencies of the CO2-splitting cycle, defined as the ratio of the calorific value of the fuel produced to the solar energy and other energy source input, of 1.73%, which to date is the highest reported efficiency for the reduction of CO2 to CO with a solar-driven device.6 Doping ceria with rare earth and transition metals has proven to be an effective method to increase the reduction and oxidation yields at a given reduction temperature and pO2 compared to those of pure ceria. Most notably, zirconium7−11 and chromium12 doped ceria have exhibited remarkably lower thermal reduction temperatures than pure ceria. Although reduction extents were increased, oxidation rates were often slower compared to pure ceria. It is not clear if the slower rates are simply due to sintering, leading to larger diffusion barriers and a lower specific surface area, or less favorable kinetic and thermodynamic driving forces for oxidation. Le Gal et al.8 have recently shown that the addition of lanthanum to Zr doped ceria results in increased stability and leads to greater oxidation extents. Additionally, Fe, Ni, Mn, and Cu based dopants have been investigated with various degrees of success.13 Chueh et al.14 experimentally investigated Sm doped ceria for the production of syngas and methane, but the high-temperature thermal reduction was not analyzed. Recently Meng et al. experimentally investigated several dopants for H2 production including Zr, Hf, Sc, Ca, Sr, and Y and concluded that only the addition of Zr, Hf, and Sc based dopants resulted in larger H2 yields compared to pure CeO2.10 Ca, Sr, and Y all had a negligible or negative effect on the H2 yields, which is in good agreement with the thermodynamic analysis of Scheffe et al.15 Of all of the aforementioned dopants, the introduction of isovalent Hf4+ or Zr4+ guest cations into the CeO2 host lattice is a particularly promising strategy to improve fuel yields.10,16 HfxCe1−xO2 solid solutions have been considerably less explored than their ZrxCe1−xO2 analogues, and although both cations display practically identical ionic radii in 7- and 8-fold coordination,17 notable differences in transition points, mechanical properties, and other phenomena are still under investigation and remain to be fully understood.18,19 Interestingly, Hf doping of CeO2 has been found to generate more active CO oxidation catalysts than Zr doping that was ascribed to higher oxygen vacancy contents and enhanced mobility in HfxCe1−xO2.20 These results are in line with resistance measurements on M0.1Ce0.9O2 (M = Zr, Hf) samples by Izu et al. showing lower resistance for the Hf doped mixed oxides.21 Izu et al. first linked such different mobility and conductivity properties of Zr and Hf doped CeO2 to subtle structural differences, such as the slight lattice distortions in pseudocubic Hf0.1Ce0.9O2 that are absent in the cubic Zr-containing analogue.21,22 The improved solar fuel yields of M0.1Ce0.9O2 (M = Hf, Zr, Sc) compared to CeO2 reported by Meng et al.10 were mainly explained through increasing electrostatic parameters of the dopant which were considered to outweigh bulk conductivity contributions in Hf0.1Ce0.9O2. However, this hypothesis is not fully in line with the above-mentioned preceding studies20 and does not account for differences between M0.1Ce0.9O2 (M = Hf, Zr) materials containing dopants with equal radii either. This calls for further investigations into the M0.1Ce0.9O2 (M = Hf, Zr) solid solutions. In this study, we thus systematically compare the influence of Hf4+ and Zr4+ doping on reduction and oxidation extents to Sc doped and pure ceria as references. Meng et al.10 only

■

EXPERIMENTAL METHODS Synthetic Technique. 5, 10, and 15 mol % of Hf4+, Zr4+, and Sc3+ doped ceria were synthesized by sol−gel methods according to ref 10, but with a lower concentration of citric acid, followed by calcination at 1373 K for 24 h instead of 6 h at 1273 K (ref 10). The syntheses were carried out with Ce(NO3)3·6H2O (99%), ZrO(NO3)2·6.3H2O (99%), HfCl4 (99.9%), and dry citric acid in aqueous solution. The ratio of citric acid to the total metal cations was 1.5:1. A description of desired dopant concentrations, actual concentrations as determined by induced coupled plasma-atomic emission spectroscopy (ICP-AES) analysis, and associated sample identifications is provided in Table 1. For the sake of brevity, sample names are abbreviated with XM (X = initial dopant concentration, M = chemical symbol of dopant cation). Table 1. Target and Actual Material Compositions with Associated Sample Names sample name

target composition

actual composition (mol %)

5Hf 10Hf 15Hf 5Zr 10Zr 15Zr 5Sc 10Sc 15Sc

Ce0.95Hf0.05O2 Ce0.9Hf0.1O2 Ce0.85Hf0.15O2 Ce0.95Zr0.05O2 Ce0.9Zr0.1O2 Ce0.85Zr0.15O2 Ce0.95Sc0.05O2 Ce0.9Sc0.1O2 Ce0.85Sc0.15O2

Ce:Hf = 95.0: 5.0 Ce:Hf = 90.1: 9.9 Ce:Hf = 85.0: 15.0 Ce:Zr = 95.1: 4.9 Ce:Zr = 90.2: 9.8 Ce:Zr = 85.2: 14.7

Characterization. Powder X-ray diffraction (PXRD) was performed on a STOE STADI P diffractometer in transmission mode with Cu−Kα radiation, Ge-monochromator, and a position sensitive detector. Raman spectra were obtained with a Renishaw Ramascope 1000 equipped with a green laser (524.5 nm) from Spectra Physics with 50 mW capacity. Scanning electron microscopy (SEM) was conducted on a LEO 1530 (FEG) SEM (1.8 kV), and powder samples were dispersed in ethanol and deposited on silicon wafers. Energy dispersive X-ray spectroscopy (EDX) mappings were recorded on a JEOL SEM (JSM-6060) equipped with a BRUKER AXS 133 eV X FLASH detector 4010 (Supporting Information) and on a Zeiss Supra 50VP equipped with a EDAX detector at 30 kV and a resolution of 256 × 200 pixels per frame and a magnitude of 20.000 (Figure 4). Surface areas were characterized with the Brunauer−Emmett−Teller (BET) 24105



Article

method on a Quadrasorb SA 6 in N2 adsorption mode. Samples (ca. 300 mg) were degassed at 423 K for 43 h in vacuo prior to nitrogen adsorption measurements with a Quantachrome FLO VAC degasser. UV/vis measurements were performed in diffuse reflectance mode with a standard Teflon background (labsphere certified reflectance standard USRS-99-020; AS-01159060; Serial number: OD81E-4813). FT-IR spectroscopy was performed on a Bruker Vertex 70 spectrometer equipped with a Platinum ATR. Thermal Cycling. Redox experiments were carried out in a thermogravimetric analyzer (TGA, Netzsch 409 CD). Approximately 500 mg samples were uniaxially cold pressed and placed on an Al2O3 crucible within the electrically heated TGA. Reduction was performed by heating at 20 K min−1 from room temperature to 1773 K in an argon atmosphere (pO2 ∼ 10−4 atm, total gas flow rate = 250 sccm). Oxidation was performed isothermally (pCO2 = 0.4 atm in Ar, total gas flow rate = 250 sccm) after cooling at 20 K min−1 to temperatures between 873 and 1073 K. Thermal cycling experiments were carried out at a reduction temperature of 1773 K and oxidation temperature of 1073 K under the same atmospheric conditions described above. The sample size of the reactant was varied by pulverizing the pressed powder with a mortar and pestle.

■

RESULTS Redox Experiments: Initial Screening. Hf and Zr doped samples proved to be reduced more substantially than Sc doped and pure ceria at 1773 K. Figure 1a shows one complete redox cycle with 10Sc, 10Hf, 10Zr and pure ceria. As the temperature is increased to 1773 K, 10Hf and 10Zr both undergo reduction at lower temperatures than 10Sc and ceria, as indicated by their earlier onset of mass loss (∼1023 K). On the other hand, the masses of 10Sc and ceria are stable until temperatures near 1273 K, and lose mass as the temperature is increased further. The total mass loss after the isotherm at 1773 K for 10Hf and 10Zr is roughly 0.6% compared to 0.4% for 10Sc and ceria. However, none of the samples achieved their thermodynamic equilibrium, as indicated by the still decreasing mass loss after 30 min at 1773 K. The mass increases slightly for each sample as the temperature is decreased due to a small amount of residual oxygen in the gas stream. Following the decrease in temperature to 1073 K, the samples are subjected to 40 vol % CO2 and the resulting mass increases due to oxidation are seen in Figure 1b. The 10Zr oxidizes noticeably slower than all other samples, and even after 45 min the oxidation is not completed. In fact, the mass gain was only slightly more than half of the mass loss during thermal reduction. Both 10Sc and ceria oxidize very rapidly, and oxidation proceeds stoichiometrically when compared to the mass loss observed during reduction. The 10Hf also initially oxidizes nearly as rapidly as 10Sc and ceria, but slows substantially for longer times. Similarly to 10Zr, oxidation does not proceed to completion in the amount of time allotted, but the oxidation extent is still substantially greater than 10Sc and ceria (∼41%). The samples were further cycled twice at lower oxidation temperatures, and as the temperature was decreased the oxidation rates of 10Zr and 10Hf both decrease. This can be observed in Figure 1b for the oxidation at 973 and 873 K in addition to the cycle at 1073 K in Figure 1a. The decrease in rates is especially pronounced for 10Zr. It is not clear whether the decrease in rates is due to sintering, resulting in decreased surface area and increased diffusion length scales,

Figure 1. (a) The initial redox cycle for 10 mol % dopant compositions and pure ceria reduced at 1773 K (pO2 = 10−4 atm) and oxidized at 1073 K (pCO2 = 0.4 atm) and (b) subsequent cycles oxidized at 973 and 873 K (pCO2 = 0.4 atm) and reduced at 1773 K (pO2 = 10−4 atm).

or slower kinetics as the temperature is decreased. It is known that the oxidation thermodynamics (ΔGrxn) become more favorable as the temperature is lowered for pure ceria (and presumably the various dopants considered) and therefore this effect is not due to thermodynamic constraints.15 It should be noted that the slow increase in mass after long oxidation times may be partly due to residual oxygen in the system. The measured pO2 at the outlet of the reactor is 2.4 × 10−4 atm. Meng et al.10 observed larger oxidation yields with 10 mol % Hf doped ceria compared to pure ceria (an average of 30% over a total of nine cycles) when oxidizing at only 773 K. While we would presumably obtain lower yields than ceria at such a low oxidation temperatures based on an extrapolation from Figure 1b, this difference may be explained in terms of variations of individual grain and sample sizes and specific surface areas. A direct comparison between our materials and theirs is not possible because such detailed information is not discussed by the aforementioned authors. Their yields did decrease by ∼20% over nine cycles, whereas for pure ceria their yields were stable. The reason for this behavior is presumably not due to sintering because the reactions were conducted until equilibrium was achieved. We observed a decrease in BET surface area of 10Hf after the three cycles from 1.41 to 0.66 m2 g−1, which could possibly influence the reaction rate and affect the subsequent yields provided the surface area is rate limiting. Similar behavior was observed by Rudisill et al. when increasing the accessible surface area of pure ceria for oxidation with CO2.23 However, 24106



Article

the BET surface area of pristine ceria underwent a decrease in the same order of magnitude from 1.37 to 0.39 m2 g−1 that was not accompanied by a comparable temperature dependence of the reaction rate. This behavior is not indicative of a kinetically limited reaction where the rate is strongly correlated with the temperature. Diffusion of reactant species through interstitial pores might thus be a possible limiting step for pure ceria. Another possibility is that the oxidation kinetics of pure ceria becomes more favorable at lower temperatures than 1073 K because the equilibrium driving force of the reverse reaction is less pronounced. This would result in faster rates per unit surface area at 873 K but would not be easily observable because of the decrease in surface area due to sintering. Diffusion of oxygen through the bulk oxides of either material is not expected to be rate limiting at these temperatures because they are both known to be rapid ionic conductors.2,24 A more detailed kinetic analysis of both materials is necessary to provide any more insight into the hypotheses presented but is out of the scope of the current work. For example, using dense samples with defined geometries would help distinguish surface from bulk rate limiting effects. The results of 10Zr are consistent with the observations of Abanades et al.16 who obtained larger reduction extents for Zr doped ceria compared to pure ceria but diminishing yields at lower oxidation temperatures and cycle numbers. Meng et al.10 also observed larger reduction extents for 10 mol % Zr doped ceria compared to pure ceria but did not discuss the oxidation extents. The relatively low oxidation yields of 10Zr compared to the reduction extent can certainly be attributed partly to the unfavorable thermodynamics compared to pure ceria, at least for 19 mol % Zr doped ceria,25 meaning the reaction is less selective to carbon dioxide compared to pure ceria and would require excess amounts to oxidize completely. Indeed, Hao et al.26 have observed H2 yields up to 80% higher for 20 mol % Zr doped ceria compared to pure ceria at a reduction temperature of 1573 K, but because of the excess water that is required to oxidize it to this extent the predicted solar-to-fuel energy conversion efficiencies are still lower compared to pure ceria. The reduction extents for Zr and Hf doped materials increase with dopant concentration. Figure 2a shows the change in mass percent during the initial reduction step as a function of dopant concentration for all material compositions studied. As seen, the mass changes nearly linearly with concentration for Zr and Hf. In fact, the reduction extent of 15Zr is nearly 100% greater than pure ceria. To our knowledge, nonstoichiometry data has not been presented for differing concentrations of Hf in ceria, but these results agree qualitatively well with data from Abanades et al.7 who investigated the reduction extents of ZrO2−CeO2 solutions as a function of ZrO2 concentration. They observed increasing yields up to concentrations as high as 50 mol-% (further concentrations were not investigated and pO2 was not reported), at which point the yields were about four times greater than pure ceria at 1773 K. We did not observe any effect on the reduction extent by doping with Sc, contrary to the results of Meng et al.10 The oxidation yields for all Hf concentrations were greater than pure ceria after the first oxidation at 1073 K, as seen in Figure 2b, and they reach a maximum for 10Hf. 5Zr also oxidizes to a greater extent compared to pure ceria, but for higher concentrations of Zr the yields decrease, ultimately to values much lower than ceria. Sc concentration had little or no effect on the observed oxidation yields. In general, these trends

Figure 2. (a) Relative mass changes during the initial reduction performed at 1773 K and pO2 of 10−4 atm as a function of dopant concentration and (b) relative mass changes during the subsequent oxidation at 1073 K and pCO2 of 0.4 atm after 45 min.

were similar at lower oxidation temperatures, namely, a diminishing rate of return of the yields with respect to the dopant concentration. For example, at 973 K the oxidation yields of Zr doped ceria were decreasing with any increase in dopant concentration and only 5Hf had larger yields than pure ceria. There were no dopant concentrations that resulted in larger yields than pure ceria at oxidation temperatures of 873 K. Characterization of CeO2 and 10Hf. On the basis of the positive results of 10Hf discussed in the preceding section, we have focused further analytical and thermochemical characterization on this material in comparison with pure ceria. SEM images of pristine ceria and 10Hf before and after TGA show some sintering effects due to particle growth after TGA analysis. Following uniaxial pressing and calcination at 1373 K, pristine ceria exhibits particle/grain diameters in the range of 250−500 nm (Figure 3a). These smaller particles/grains appear to be slightly sintered into larger clusters on the order of 1−3 μm. Following redox cycles in the TGA, the smaller ceria particles are no longer evident and only larger grains with ∼1− 3 μm diameters are observed (Figure 3b). The 10Hf appears to be even more strongly sintered after TGA analysis (Figure 3d), and the grain boundaries appear to be completely fused. Nevertheless, BET measurements indicate that 10Hf has a 24107



Article

Figure 3. SEM images of CeO2 and 10Hf before (a,c) and after (b,d) TGA analysis.

larger specific BET surface area than pristine ceria after TGA cycling (0.66 m2 g−1 compared to 0.39 m2 g−1). Although these surface areas are above the detection limit27 (cf. Experimental Methods), differences among such low values should not be overestimated. Thus, the amount of adsorbed nitrogen during BET analysis does not seem to depend on the apparent sintered surface area and might be influenced by gas diffusion processes instead. Compositional stability of 10Zr (Figure 4a) and 10Hf (Figure 4b) during CO2-splitting cycles was checked with EDX mapping after TGA cycling and both images show homogeneous distribution of hafnium or zirconium, thus pointing to stable solid solutions. PXRD patterns of doped ceria with the optimized compositions 10Sc, 10Zr, and 10Hf show that all samples maintain the cubic fluorite structure of pristine ceria (S.G. Fm3m ̅ ; PDF 00-034-0394, cf. Supporting Information Figure S1). Absence of Sc2O3, ZrO2, and HfO2 peaks indicates that the dopants are incorporated into the ceria lattice. Additionally, PXRD patterns of the aforementioned materials show a shift of the (111) reflection toward higher angles (Figure 5) with 10Sc exhibiting the smallest peak shift. Although incorporation of Sc into the ceria host lattice seems to be less favorable than Zr and Hf based on the observed peak shifts, EDX mapping (Supporting Information Figure S2) of 10Sc display an equally homogeneous dopant distribution. The degree of ceria doping was determined from Rietveld refinements (Figure 6 and Table 2, for details cf. Supporting Information). The smaller cell volume of 10Hf compared to pristine ceria arises from the smaller effective ionic radius of Hf4+ (0.83 Å) in comparison with Ce4+ (0.97 Å).17 This observation is in line with a study of Ramesh et al.28 that correlates an increase of cell volume in doped ceria with dopants exhibiting a larger effective ionic radius than Ce4+. Rietveld refinement of 10Hf shows increased lattice constants after TGA while the lattice constants of pure ceria after TGA are slightly decreased (Table 2). As the reoxidation of 10Hf was less favorable than for pure ceria during redox cycling, the increase in lattice constants might be related to residual oxygen vacancies.

Figure 4. (a) EDX mapping of 10Zr (yellow = Ce, blue = Zr) and of (b) 10Hf (blue = Ce, yellow = Hf) after TGA.

24108



Article

pristine ceria are calculated as 113.6 nm before and 113.5 nm after TGA, and for 10Hf 114.3 nm before and 113.3 nm after TGA, respectively. These particle sizes are practically identical within an estimated net deviation of experimental and methodological errors around 5−10%.31 The absence of crystallite size differences indicates that neither doping nor short-term TGA treatment exert a significant influence. Therefore, the effect of calcination post-treatment was investigated in more detail, and peak widths are narrowed upon higher calcination temperature for both pristine ceria (Supporting Information Figure S3) and 10HfO2 (Figure 7).32 Crystal growth rates (estimated from the slopes in Figure 8) for pristine ceria are about 0.14 nm K−1 between 873 and 1073 K, 0.11 nm K−1 (1073−1273 K) and 0.07 nm K−1 (1273−1473 K), compared to values of 0.005 nm K−1 (873−1073 K), 0.06 nm K−1 (1073−1273 K), and 0.16 nm K−1 (1273−1473 K) for 10Hf. Consequently, 10Hf crystallites grow slower than those of pristine ceria at calcination temperatures up to 1273 K, whereas they grow faster at higher calcination temperatures. Figure 8 also shows that the crystal growth rate for pristine ceria decreases slightly with increasing calcination temperature while 10Hf displays increasing crystal growth rate with higher calcination temperature. Faster crystallite growth in 10Hf above 1273 K indicates that this sample sinters faster as well, which is in line with SEM investigations after TGA (Figure 3d). Onset of 10Hf crystallite growth is observed at higher temperatures and with higher growth rates compared to pure ceria. This points to a more sharply defined sintering interval and higher crystallite stability at lower temperatures. The strongly temperature-dependent sintering of Hf doped ceria is in line with a study on 20 mol-% Nd doped ceria which displays a closely related curve.32 Raman spectroscopy was furthermore employed as a versatile analytical tool to characterize metal−oxygen bonds as well as lattice defects.33 Raman spectra of pristine ceria and 10Hf are compared in Figure 9. For ideal crystalline ceria only one peak at 464 cm−1 is predicted,33 which arises from the triply degenerate F2g Raman active mode in the fluorite structure. This F2g peak originates from the symmetric breathing mode of the oxygen atoms which are located around Ce4+ ions. If they are replaced by vacancies or other cations (such as Hf4+), the F2g mode becomes asymmetric and a weak shoulder at ca. 570 cm−1 arises with increasing intensity upon higher doping levels.32 The 10Hf exhibits this shoulder around 570 cm−1 (Figure 9), which arises from oxygen vacancies that are stabilized through dopant-induced lattice distortions. The intensity of the shoulder peak at 570 cm−1 decreases with growing crystallite size, which might eventually lead to its complete disappearance.34 All other peaks arise from crystal defects or oxygen vacancies.33 The low intensity peak at 1122 cm−1 is absent in 20% Nd doped ceria,32 but present in Pd doped ceria with a Pd content ≥6%,34 and it might arise from resonance effects which permit this forbidden Raman mode. As the characteristic shoulder peak around 570 cm−1 is absent in pristine ceria (Figure 9), the small peak at 1122 cm−1 is probably not caused by oxygen vacancies. However, this peak increases after TGA or thermal treatment, respectively, indicating a strong dependence on particle size and/or crystallinity. The 10Hf furthermore exhibits a rather weak peak at 280 cm−1, which appears to be unaffected by TGA treatment (Figure 9b). This low intensity peak is a characteristic indicator for the presence of a tetragonal phase,22 which is induced by

Figure 5. (111) reflection of CeO2 in comparison with 10Sc, 10Zr, and 10Hf.

Figure 6. Rietveld refinement of 10Hf (a) before and (b) after TGA.

Crystallite sizes were calculated from the (111), (200), (220) and (311) peaks using the Scherrer equation29 shown below D hkl =

Kλ β cos θ hkl

(3)

with K = 0.94 for cubic crystals, λ (X-ray wavelength) = 1.5051 Å, β = fwhm (full width at half-maximum), θhkl = angle at selected peak maximum. The average crystallite sizes for 30

24109



Article

Table 2. Rietveld Refinement Results for 10Hf and CeO2 and Particle Sizes (Calculated from Scherrer Equation) a/Å V/Å3 Rp Rwp Rexp GOF size/nm

10Hf

10Hf after TGA

CeO2

CeO2 after TGA

CeO2 (ICSD 180851)

5.3831(1) 155.997(8) 5.71 6.90 3.04 2.27 54

5.3901(2) 156.592(8) 4.59 5.61 3.03 1.85 56

5.4117(1) 158.497(8) 6.18 7.38 2.90 2.54 68

5.4109(1) 158.418(5) 4.90 5.93 2.93 2.02 72

5.420(2) 159.22(18)

Figure 7. PXRD patterns of 10Hf after calcination (4 h) at 873, 1073, 1273, and 1473 K.

Figure 9. (a) Raman spectra of 10Hf compared to pure CeO2; (b) comparison of Raman spectra of doped pristine ceria samples in the 200−400 cm−1 region. Figure 8. Crystallite sizes after calcination (4 h) at 873, 1073, 1273, and 1473 K of pure ceria (top) and 10Hf (bottom). The solid lines are least-squares fits of a second order polynomial to the data and are intended for visual guidance only.

of pure CeO2, 10Hf, and 10Zr with UV/vis and FT-IR spectroscopy (Supporting Information Figures S4 and S5). UV/vis spectra of all three samples (Supporting Information Figure S4) clearly show their closely related structural features, thus confirming the presence of homogeneous solid solutions as shown in the PXRD patterns (Figure 7). Pure CeO2 displays two characteristic bands at 278 and 349 nm which can be assigned to Ce4+ ← O2‑ charge transfer and interband transitions.35 These bands undergo a slight blue shift (6 nm) in 10Hf and 10Zr that might be due to different coordinations of surface cations in the doped samples.35 After TGA all samples show a red shift of 13 nm for pure CeO2, 39 nm for 10Hf, and 33 nm for 10Zr. Furthermore, absence of both the characteristic shoulder at 265 nm arising from Ce3+ ← O2− charge transfer and the peak for Ce3+ → Ce4+ interactions at

Hf4+ doping into the ceria host lattice. However, the Hf-related shift of the oxygen positions along the c direction (ratio c/a = 1 ± 0.00004) falls below the PXRD detection limits and can only be observed spectroscopically.22 The 10Zr does not exhibit any peaks in the 280 cm−1 region before or after thermal treatment (Figure 9b) so that its cubic structure remains undistorted as reported in preceding studies.21 Given that spectroscopic investigations of doped CeO2 are more sensitive toward subtle structural changes than PXRD techniques, we furthermore investigated as-synthesized samples 24110



Article

670 nm35 clearly indicate that none of the as-synthesized samples contains significant amounts of Ce3+. After TGA measurements, the characteristic shoulder at 265 nm arising from Ce3+ ← O2− charge transfer might overlap with the Ce4+ ← O2− charge transfer band at 278 nm. UV/vis spectra of 10Hf and 10Zr after TGA display a high absorption over the whole wavelength range that is in line with the dark gray color of the doped samples after TGA, pointing to a certain extent of reduction. However, the characteristic band for reduced ceria at 670 nm is apparently concealed by the strong absorption of both samples in the visible range. As the spectrum of pure CeO2 does not display the 670 nm absorption of Ce3+, no significant reduction after TGA cycling occurred. The band gaps of as-synthesized 10Hf and 10Zr determined from UV/vis spectra (2.9 eV) differ only slightly from pure ceria (3.1 eV), and all values are in the preparation-dependent range from 2.7 to 3.4 eV that has been reported for CeO2 samples.36 FT-IR spectra of the pristine sample series (Supporting Information Figure S5a) do not display substantial differences between pure ceria and 10Hf or 10Zr either. Characteristic peaks of ceria appear around 730 and 1025 cm−1 and they represent a fundamental ν(CeO) mode and a first overtone of another vibration at 530 cm−1.37 After TGA, the characteristic peaks at 1025 cm−1 are lost in all samples and the peak at 730 cm−1 can only be observed in pure CeO2. Note that the ν(CeO) vibration appears to be more intense for CeO2 than for 10Hf and 10Zr before TGA, respectively. The dark color of 10Hf and 10Zr after TGA suggests the presence of Ce3+, but its characteristic band at 2127 cm−1 or broader peaks in the 2500 cm−1 region37 cannot be observed before or after TGA (Supporting Information Figure S5). This information complements the UV/vis spectra, which suffer from concealment of Ce3+ bands. Furthermore, the samples were investigated for the presence of carbonate-based species before and after TGA. Neither the characteristic IR-bands of bridged carbonate at 1132, 1219, 1396, and 1728 cm−138 nor main peak for bidentate carbonate at 1454 cm−1 were detected.38 The two possible IR bands of inorganic carboxylate at 1310 and 1510 cm−1 were not observed either38 so that no indications were found for the presence of carbonate or related species before or after thermochemical cycling. Thermochemical Cycling of CeO2 and 10Hf. We have observed a strong dependence between the initial size of the pressed pellet on the initial reduction rate and extent and subsequent oxidation reaction rates of 10Hf. Three different sample dimensions of 10Hf were subjected to 10 redox cycles operating at 1773 K (pO2 = 10−4 atm) during reduction and 1073 K during oxidation (pCO2 = 0.4 atm). Results were compared to those of pristine ceria. The three 10Hf sample sizes investigated were (1) Sample A, a single pressed cylindrical pill with a diameter of 7 mm, length of 3 mm, and mass of 579 mg (ρ = 5.03 g cm−2); (2) Sample B, smaller pieces of the aforementioned sized pill pulverized with a mortar and pestle (∼0.5 mm in diameter, total mass = 212 mg); and (3) Sample C, fine particles (diameter

Synthesis, Characterization, and Thermochemical Redox Performance

Recommend Documents