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2007, 111, 14035-14039 Published on Web 09/01/2007
Low-Temperature Redox Properties of Nanocrystalline Cerium (IV) Oxides Revealed by in Situ XANES Deena R. Modeshia,† Christopher S. Wright,‡ Julia L. Payne,† Gopinathan Sankar,‡ Steven G. Fiddy,§ and Richard I. Walton*,† Department of Chemistry, The UniVersity of Warwick, Gibbet Hill Road, CoVentry, CV4 7AL United Kingdom, Materials Chemistry Centre, Department of Chemistry, UniVersity College London, 20 Gower Street, London, W1CH 0AJ United Kingdom, and STFC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD United Kingdom ReceiVed: July 11, 2007; In Final Form: August 19, 2007
Time-resolved, in situ XANES studies of some cerium titanate pyrochlores and a fluorite-type bismuthcerium oxide under reducing and oxidizing gas atmospheres reveal that reduction of cerium occurs at temperature below 500 °C in each of the materials. Cerium oxidation state during these treatments is quantified by deconvolution of the Ce LIII-edge XANES signal using fitted Lorentzians and comparison to data from reference materials of known average oxidation state. In the case of the cerium titanate pyrochlore, complementary XANES experiments at the Ti K-edge show that average titanium oxidation state is largely unaffected on reduction of the sample, although small changes in titanium local environment are apparent that are most likely due to thermal expansion effects as well as an expansion of the structure due to the reduction of cerium. Reoxidation of the pyrochlore is extremely facile, occurring even under hydrogen, which may be attributed to an increase in crystallite size and loss of oxide-rich surfaces, which is backed up by TEM observations.
Cerium dioxide is well-known for considerable practical applications in solid electrolytes for solid-oxide fuel cells1 and in catalysis,2 such as three-way catalytic converters for automotive exhaust depollution,3 and in water-gas shift catalysis for the purification and production of hydrogen.4 The origin of these properties is an oxygen storage capacity that arises from the ease of reversible reduction of Ce(IV) to Ce(III) coupled with the open fluorite lattice that permits ready oxide-ion migration.5 Although in real catalytic applications CeO2 is in fact used as a catalyst support in combination with a precious metal or other active component, it is important in the future development of new active and selective catalysts to optimize the properties of the support. For example, in the water gas shift reaction, the conversion of carbon monoxide and water to hydrogen and carbon dioxide, the forward reaction is exothermic and catalytic systems that operate at low temperatures must be sought to improve activities, particularly for small-scale purification of hydrogen for low-temperature fuel cells.4 One of the most widely studied ways of doing this is to introduce dopant metals into the fluorite lattice that results in either an increase in oxide-ion vacancies (if the doping is aliovalent, Bi3+ or Ln3+ (Ln ) lanthanide), for example)6 or results in some lattice strain and distortion because of ionic radius mismatch, which aids oxide ion migration (if the doping is isovalent, Zr4+, for example).7,8 These mixed-metal oxides are usually prepared by coprecipi* To whom correspondence should be addressed. E-mail: r.i.walton@ warwick.ac.uk. † The University of Warwick. ‡ University College London. § STFC Daresbury Laboratory.
10.1021/jp075410p CCC: $37.00
tation to yield an amorphous precursor which is then fired to form a polycrystalline oxide. We have recently been exploring hydrothermal chemistry for the one step crystallization of nanocrystalline mixed-metal oxides of cerium with some control of metal oxidation state,9,10 and in this letter, we demonstrate how in situ XANES allows their redox activities to be followed in real time, showing their extremely facile reduction and oxidation. In situ XANES spectroscopy allows real materials to be studied under reactive gas atmospheres and therefore offers significant advantages over other commonly used methods for examining metal oxidation states in such complex oxides, such as XPS spectroscopy, which is surface sensitive and requires vacuum conditions for data collection. Although XANES has previously been used to examine the oxidation state changes in ceria-based catalysts, including some early in situ studies on the effect of precious metal activation,11,12 the majority of these studies were ex situ studies on quenched samples.13-15 The materials we have studied are a (1) cerium titanate pyrochlore (Na1/3Ce2/3)2Ti2O7, whose synthesis and preliminary structural characterization some of us have recently reported elsewhere,10 (2) a tin-doped analogue, (Na1/3Ce2/3)2Ti1.1Sn0.9O7 prepared by hydrothermal reaction of CeCl3‚7H2O,TiF3 and Sn(OCH2CH3)4 in aqueous sodium hydroxide solution (2 M) in the presence of 50 equiv of H2O2 per cerium at 240 °C for 5 h, and (3) a bismuth-cerium oxide of composition Ce1-xBixO2-x/2 with x ) 0.5, prepared by a new hydrothermal route from sodium bismuthate and cerium (III) chloride in aqueous 2 M sodium hydroxide at 200 °C for 1.5 h. The latter material shows © 2007 American Chemical Society
14036 J. Phys. Chem. C, Vol. 111, No. 38, 2007 a powder pattern that may be indexed on a face-centered cubic unit cell, with a ) 5.48(1) Å, larger than expected for CeO2 and consistent with an oxide-ion deficient fluorite lattice, similar to that seen for room-temperature stabilized cubic δ-Bi2O3.16 The first two solids have cubic pyrochlore structures with a ) 10.12(1) Å and a ) 10.19(1) Å respectively, with the tin-doped sample having a larger unit cell volume, expected with the slightly larger radius of Sn4+ compared to Ti4+. For each of the materials, the width of Bragg peaks in the powder diffraction experiments indicates small particle size, typically 10-20 nm by Scherrer analysis, consistent with TEM observations (vide infra). XANES experiments were performed on Station 7.1 of the Daresbury SRS at the Ce LIII and Ti K edges using a Si(111) monochromator, the second crystal of which allows sagittal focusing of the X-ray beam. Harmonic rejection was set to 50% for all experiments, by detuning the second crystal to 50% of the maximum X-ray intensity. For the in situ XANES experiments, a specially designed quartz reaction cell was used in which the sample is held in a pelletized form on a quartz holder, and desired temperature provided by Kanthal heater wires surrounding the sample (maximum sample temperature ∼550 °C). This assembly sat within a quartz chamber in which a gas of choice was passed across the sample and vented through an exit. Temperature was measured and controlled by a thermocouple passed down the sample holder close to the surface of the pellet. The chamber has three Kapton windows which can each be used to make simultaneous spectroscopic measurements, although in our experiments since the samples are sufficiently concentrated in the absorbing element only transmission XANES data were recorded. Samples were mixed with fumed silica or boron nitride to achieve dilution and pressed into 20 mm diameter pellets (around 20 mg of sample and 40 mg of diluent were used to give a pellet of ∼1 mm thick) which were mounted in the reaction cell which was then flushed with reducing gas for 60 min, before commencing heating which was remotely controlled using a Eurotherm electronic controller. XANES data were collected with a total data acquisition time of 10 min per spectrum with data measured also from a CeO2 or Ti foil reference placed between the second (It) and a third (Im) ion chamber. The data were normalized using the programs EXCALIB and EXBROOK.17 Figure 1a shows a three-dimensional representation of Ce LIIIedge XANES data recorded during two cycles of reduction (using 10% H2 in N2) and oxidation (using dry air) of the pyrochlore. This representation of the data clearly shows dramatic changes in the near-edge region of the absorption spectrum. The temperature scale of Figure 1a shows maximum and minimum sample temperatures during cycling and switching of gas mixtures, which are also indicated. Figure 1b shows an individual XANES spectrum recorded from the sample in its most reduced state (at 550 °C). The near-edge features of the XANES spectrum of CeO2 have been described by a number of workers and the consensus of opinion is that the distinctive double peak seen for Ce4+ in oxides is due to a higher energy transition from core 2p to valence 5d-like states with no occupancy of 4f levels in final or initial states (labeled A in Figure 1b) and a lower energy transition (labeled B in Figure 1b) that is due to a transition to states in which the 4f levels are occupied.12,18,19 As we have previously reported, the XANES spectrum of (Na1/3Ce2/3)2Ti2O7 closely resembles that of CeO2, suggesting a similarity of local cerium structure in the materials.10 Upon complete reduction of Ce(IV) to Ce(III) the feature A disappears and a single peak, B0, shifter to lower energy than B is oberved.12,18,19 Thus, the Ce LIII -edge XANES may be
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Figure 1. (a) Three-dimensional surface plot of the Ce LIII edge XANES data measured during two cycles of reduction-oxidation of the pyrochlore (Na1/3Ce2/3)2Ti2O7. (b) individual Ce LIII edge XANES spectrum the pyrochlore in its most reduced state with fitted Lorentzians to determine oxidation cerium oxidation state (see text).
used as distinctive fingerprint of cerium oxidation state by virtue of both the edge position and the form of the near-edge region, and in the case where mixtures of Ce4+ and Ce3+ are present the XANES is a sum of the two contributing signals. In order to quantify the cerium oxidation state, we have used an empirical peak fitting of the near-edge region in order to deconvolute the contributions from each of the cerium oxidation states. This proved to be a much more reliable approach to extracting a value of average oxidation state than other possibilities, including measuring the edge shift, and in fact a similar method has been used previously by others, for example, by Takahashi et al., who quantified the oxidation state of cerium in a number of mineral specimens,20 and by El Fallah et al., who studied Rh/CeO2 catalysts.11 We used a series of fitted Lorenztians to determine the relative areas of the Ce(IV) double features (A and B) and the Ce(III) white line (B0) and included an additional Lorenztian to simulate the underlying edge step, as indicated in Figure 1b. Then, by measuring XANES data from a set of physical mixtures of known amounts of CeCl3‚ 7H2O and CeO2 it proved possible to convert the ratio of peak intensities of Ce(IV) and Ce(III) features to a value for average Ce oxidation state. Note that the references for Ce(IV) and Ce(III) must be chosen carefully. Eight-coordinate Ce(IV) in CeO2 provides a sensible reference material for local cerium environment in each of the materials in their as-made form, since in both the pyrochlore and fluorite type oxides cerium is found in an eight-coordinate site, but for Ce(III) the XANES signal, in particular the intensity of the feature B0, is very sensitive to the metal coordination number. Since we expect the coordination
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Figure 2. (a) Average cerium oxidation state during reduction-oxidation cycles performed on the pyrochlore (Na1/3Ce2/3)2Ti2O7 and Ti K edge data XANES data measured during (b) heating and (c) cooling of the pyrochlore sample under H2/N2.
numberofceriumtobelowereduponreductionof(Na1/3Ce2/3)2Ti2O7, the crystalline material CeCl3‚7H2O was selected as a reference for the reduced state: this contains Ce(III) coordinated directly to seven oxygens of neighboring water molecules and the chloride ions lie at longer distances, beyond the first coordination shell of cerium.21 Figure 2 shows plots of average oxidation of cerium obtained from the in situ XANES data corresponding to reaction of the pyrochlore (Na1/3Ce2/3)2Ti2O7with 10% H2 in N2 followed by air then a second dose of 10% H2 in N2. Several important observations may be made from these data. First, the onset of reduction is at just less than 500 °C: this was found to be reproducible over several experimental runs at the SRS and is much lower than seen for high surface ceria prepared by hydroxide precipitation, whose reduction we could not observe using the same experimental setup. It should be noted that the data collection time is relatively large compared to the heating ramp we have used (due to the step-scan mode necessary with the sagittally focused X-ray beam) so the real temperature of reduction onset is in fact could to be up to 50 °C lower than we actually observed. Second, reoxidation of the sample commences even while cooling under the H2-N2 and the use of the oxidizing gas appears redundant: this is true for both the first and second cycles of the experiment although for the second cycle this occurs after a longer period of cooling (and indeed reduction continues for some time after heating is stopped), suggesting that some change in crystal form has occurred (see below). This result was again found to be highly
reproducible and the possibility of contamination of the reducing gas mixture with air was ruled out. A third observation is that the second reduction of cerium takes place at approximately the same temperature as the first. This was at first somewhat surprising as our earlier temperature programmed reduction results showed that the same material reproducibly activated after a first redox cycle, with subsequent reduction occurring at less than 200 °C,10 but it should be noted that the TPR experiment involved heating the sample to 900 °C, whereas the in situ XANES cell is limited by a maximum operating temperature of 550 °C. It is apparent that the higher temperature must be required in order to “activate” the sample and a similar result has been reported for ceria-zirconia specimens.7,8 One possible explanation for the differences seen between TPR and XANES is that the former experiment uses powdered samples and the latter a pelletized form so diffusion limitation effect might slow the kinetics of reduction we have seen in the current work; however, we saw no effect of the choice of diluent nor of the pressure used to fabricate the pellets and the relatively long data collection will probably avoid diffusion limitation effects. In order to further understand the apparently unusual result of reoxidation of the cerium in (Na1/3Ce2/3)2Ti2O7 while cooling under a hydrogen flow, we performed some further experiments. First, in situ Ti K edge XANES data were measured under identical conditions: these are shown in Figure 2, panels b and c. The changes in these spectra are much more subtle than those shown by the Ce LIII edge spectra and indicate that any changes
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Figure 4. Average Ce oxidation determined during heating and cooling cycles under H2-N2 for (a) Ce1-xBixO2-x/2 (x ) 0.5) and (b) (Na1/3Ce2/3)2Ti1.1Sn0.9O7.
Figure 3. Transmission electron microscopy images of (a) as-made (Na1/3Ce2/3)2Ti2O7 and (b) material after heating to 550 °C under H2N2.
in the titanium local environment upon exposure to reducing or oxidizing gases are very small. The edge shift of the Ti K edge is very sensitive to small changes in average titanium oxidation state, as has previously demonstrated, with shifts of up to 6 eV expected on reduction of Ti(IV) to Ti(III),22 so the new data indicate little change in titanium oxidation state under reaction conditions (a maximum shift in edge position of 0.5 eV is seen), implying that the average oxidation state of titanium remains largely as +4. There are also small changes in the intensity of the preedge feature of the titanium K edge, and for Ti(IV) this might be attributed to a slight adjustment of titanium local structure: coordination number and distribution of nearest neighbors affect the intensity of this formally spin forbidden preedge 1s-3d transition,23 which in this case will be brought about by both thermal expansion and the additional lattice expansion upon reduction of Ce(IV) to Ce(III). As with the edge shift, however, the observed changes in the preedge of the Ti K edge XANES are very small compared to the changes observed in the Ce LIII edge. It is clear that it is cerium and not titanium that is reduced. (Na1/3Ce2/3)2Ti2O7 possesses a cubic pyrochlore structure in which two types of oxide ions sites are present: the 48f bridging Ce and Ti sites and the 8a bridging pairs of Ce atoms. Thus we can deduce that it is the 8a oxygen of the pyrochlore that is reversibly removed on
reduction, resulting in only modification of the local structure of cerium. Thus titanium remains largely as Ti(IV) on heating under hydrogen flow and is also not reduced on cooling, ruling out any internal oxide ion migration causing redistribution of charge: such a model cannot be the cause of reoxidation of Ce on cooling. The most likely explanation for the reoxidation of Ce(III) in the absence of an oxygen atmosphere even after mild heating is provided by transmission electron microscopy, Figure 3. For a sample of the pyrochlore phase heated in a tube furnace under the same gas flow conditions as in the in situ experiments to 550 °C, we observe an increase in particle size from the asmade form of the material. Thus we can propose that the loss of oxide-rich surfaces of the ∼10 nm particles as they anneal to form larger particles (>20 nm), even on mild heating, provides an excess of oxide ions which can then diffuse into the solid reoxidising the cerium (III) to cerium (IV). The fact that only around 50% of the cerium is reduced and that the pyrochlore structure also contains 48f oxide ions, bound to titanium, suggests this mechanism is reasonable: excess oxide ions from either type of site close to the surface could conceivably be incorporated into the vacant 8a sites in the bulk as the particles anneal with a loss of surface vs bulk structure. We have also made some preliminary studies of other active cerium(IV) oxides. A bismuth-doped ceria, which is expected to possess oxide ions vacancies and also lattice distortion owing to the presence of Bi3+, which has a larger radius than Ce4+ and also a requirement for a distorted coordination environment, shows a lower temperature reduction (
