Oxidation and Reduction Reaction Kinetics of Mixed Cerium Zirconium

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Oxidation and Reduction Reaction Kinetics of Mixed Cerium Zirconium Oxides Brendan Bulfin, Friedemann Call, Josua Vieten, Martin Roeb, Christian Sattler, and Igor Shvets J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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The Journal of Physical Chemistry

Oxidation and Reduction Reaction Kinetics of Mixed Cerium Zirconium Oxides B. Bulfin,∗,† F. Call,† J. Vieten,† M. Roeb,† C. Sattler,† and I. V. Shvets‡ Institute of Solar Research, German Aerospace Center (DLR), 51147 K¨ oln, Germany, and School of Physics, Trinity College Dublin, Dublin 2, Ireland E-mail: [email protected] Phone: +49 (0)2203 6014130. Fax: 0

∗

To whom correspondence should be addressed Institute of Solar Research, German Aerospace Center (DLR), 51147 K¨ oln, Germany ‡ School of Physics, Trinity College Dublin, Dublin 2, Ireland †

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Abstract The oxidation and reduction of mixed cerium zirconium oxides, Ce1−x Zrx O2 , with x varied in the range 0 to 0.3 in steps of 0.05, in a low pressure oxygen atmosphere were experimentally investigated. The experiments utilised a novel method developed by the authors, in which the samples are reduced and oxidised in a sealed vacuum chamber by irradiating them with a Xenon arc lamp. The changes in pressure due to the release or consumption of oxygen are used to quantify the extent and rates of the reactions. The system can achieve heating rates in excess of 100 ◦ C s−1 ; which is hundreds of times faster than is possible with a thermo-balance. In addition, the gas phase transport is very rapid as it is driven by pressure gradients. The combined high heating rate and fast gas phase transport offers unique conditions in which to measure the actual kinetics of the reactions. The oxidation kinetics showed a strong dependence on the amount of Zr4+ in the samples, where the rate of oxidation decreased with increasing Zr4+ concentration. The effect of adding 2.5 % Sm3+ or La3+ to the Ce0.85 Zr0.15 O2 was also investigated with the hope of improving the kinetics. The addition of La3+ to Ce0.85 Zr0.15 O2 was seen to offer an increase in the reaction rates, although the oxidation kinetics were still slower than that of pure CeO2 . Finally, the reduction kinetic data is presented, but even with the optimal conditions the reduction reaction still appeared to be limited by the heating rate and thus it was concluded that the reduction kinetics were too rapid to quantify by this method.

Introduction Cerium dioxide has some rather unique and useful redox properties. At high temperatures it is an oxygen ion conductor and is also noted for its oxygen storage and redox properties. 1–3 This makes it a good material for applications in catalysis and solid oxide fuel cells. 4–7 In particular, it has found far-reaching applications in catalysis where it is used in automotive catalysts, oxidation catalysts and reforming catalysts.

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Remarkably ceria in the fluorite phase remains stable over a large range of oxygen stoichiometry, 8,9 with up to 17 % oxygen removal theoretically possible without a phase change. 10,11 This allows ceria to essentially breath oxygen with the equilibrium stoichiometry depending on the temperature and oxygen partial pressure. 12 The system can be considered as an equilibrium reaction, δ CeO2 ⇀ ↽ CeO2−δ + O2 2

(1)

where the non stoichiometry is a function of temperature and oxygen partial pressure, δ(PO2 , T ). 10,12 This property has lead to ceria being proposed for use in thermochemical fuel production cycles. 13,14 Fuel production using the ceria redox cycle has already been demonstrated. 15 Ceria which has been reduced at high temperature can be used to produce fuel by splitting H2 O and CO2 producing H2 and CO. 16,17 Syn-gas can be refined into denser liquid fuels using the Fischer-Tropsch process. 18,19 This method of fuel production has in recent years been extensively investigated both theoretically 10,20–25 and experimentally. 26–30 Marxer et al. used cerium dioxide to perform over 200 cycles in a lab scaled reactor, producing 700 standard litres of syn-gas, which was then processed via the Fischer-Tropsch process to produce Kerosene. 31 By replacing some of the cerium cations in the fluorite crystal structure with other metals, the redox properties of ceria can be modified. The addition of ions which have the same valence, but lower ionic radius than ceria, such as Zr4+ and Hf4+ , can increase the reducibility. 32–40 This is very important as the reduction is the energy intensive step implying benefits for the overall efficiency of the cycles. Ceria has a very accommodating lattice for Zr4+ , allowing up to 40 % of the cerium ions to be replaced and still maintaining the fluorite structure. 41 Replacing some Ce4+ with metals which have a lower valence, such as Sm3+ , introduces oxygen vacancies into the fluorite structure, which increases the oxygen ion mobility. 42,43 It is possible that the increased oxygen ion mobility may improve the reaction kinetics, but it 3



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can also reduce the overall oxygen yield. 36,37,44,45 The reaction kinetics are also very important for fuel production cycles, as they contribute to the amount of time it takes to perform a full reduction and oxidation cycle with ceria. This cycle time and the yield determine the quantities of ceria required for a given output power. 22 Kinetic studies of the Cerium dioxide redox system typically take the form of relaxation experiments. The system is displaced from equilibrium and the return to equilibrium is measured by either changes in mass, 34,46 gas stream composition 17,47 or changes in ionic conductivity. 48 Another common experimental method is to study oxygen isotope exchange under equilibrium conditions. 49,50 This later method however does not offer insight into the case of non-equilibrium conditions in which the vacancy concentration offers an additional driving force for reactions. In relaxation experiments, it can be very difficult to determine the rate limiting factor. In the case of mass change relaxation experiments using a thermo gravimetric analyser (TGA), in most cases the ceria is approximately in equilibrium throughout the experiments due to the restricted rates at which the temperature and atmosphere can be changed. 32,37,51 This means the measured reaction kinetics simply depend on the rate of change of the conditions (Temperature and gas atmosphere) and not on the material’s intrinsic kinetic properties. This was well demonstrated by Ackermann et al., 52 where they found that for a relatively large sample (a dense cylinder with a diameter of 0.6 cm, a height of 0.35 cm and a mass of 715 mg) the ceria was approximately in equilibrium during their TGA relaxation experiments. By increasing the size of the sample by a factor of five to around 4000 mg, they then found that the ceria oxidation and reduction kinetics in an oxygen atmosphere could be measured under non equilibrium conditions. The results and analysis of this large sample suggest that both reduction and oxidation were diffusion controlled in the temperature range 1673 - 1823 K. Another recent TGA relaxation study by Knoblauch et al., 53 suggests that for smaller

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samples of the order of 0.1 cm thickness, that the oxidation and reduction kinetics in an oxygen atmosphere could be surface controlled. This is in agreement with the work of Bulfin et al., 10 where the oxidation kinetics of porous ceria samples also appeared to be surface reaction controlled. Some previous studies have touched upon the effect that Zr4+ has on the oxidation kinetics of ceria samples, 32,47,54 where the oxidising gas used was either H2 O or CO2 . For the most part the results showed that the addition of Zr4+ decreased the oxidation kinetics. However, these results can be difficult to interpret from a purely kinetic point of view. The addition of Zr4+ to the ceria lattice increases the reducibility which also reduces both the enthalpy and the entropy of formation of the oxide. 39,55–57 This decrease in enthalpy and entropy of reaction also has a negative impact on the H2 O and CO2 splitting thermodynamics. 58 Therefore, the reduced kinetics could simply be a result of the lower thermodynamic driving force of the oxidation reaction. In this study the redox reaction kinetics in an oxygen atmosphere of Ce1−x Zrx O2 are studied. The concentration of Zr4+ in the ceria lattice x, is varied and the changes in kinetics are measured. A novel experimental technique, which is described in the next section, is utilized which offers very optimal conditions for equilibrium relaxation experiments. This combined with the fact that oxidation is performed in an oxygen atmosphere, removes uncertainties regarding the rate limiting properties of the apparatus and potential thermodynamic restrictions on the reactions. Finally, the addition of a small amount of Sm3+ or La3+ into the oxide was also investigated.

Experimental Techniques Oxidation and Reduction experiments The investigation of the oxidation and reduction reactions was performed using a custom built system. An apparatus was constructed by the authors which allows oxides to be 5



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◦

C. The changes in sample temperature, gas temperature, pressure and gas composition are

all recorded once per second. At the end of the reduction the system is again pumped to remove the released oxygen. The chamber is sealed again and the sample is allowed to cool for 40 min reaching approximately 30 - 40 ◦ C. This is to ensure that the sample is cool enough to prevent the oxidation reaction beginning immediately on the introduction of oxygen into the system. A re-oxidation cycle is then conducted by first flushing the chamber with oxygen to remove other gases, refilling with oxygen and finally pumping down to a pressure of 18 Pa. The sample is then heated using the Xenon lamp, where the input power is reduced by a filter and the shutter again uncovers the beam. The changes in temperature, pressure and gas composition are again recorded. The background pressure increase due to heating of the chamber without a sample present (release of surface absorbed gases) were accounted for by running oxidation and reduction cycles with no sample present. In the case of the re-oxidation cycles, the power incident into the chamber is relatively moderate and the increase in pressure due to heating alone was negligible. Figure 2 shows a photograph of the experimental apparatus. The system has two chambers, the main chamber where the sample is placed and the mass spectrometer chamber which is pumped to lower pressure and receives a small leak of gas from the main chamber. The pressure in the main chamber is monitored using the capacitance manometer. In addition the oxygen supply and the type B thermocouple feed-through can be seen. The system has a number of novelties when compared to other experimental methods used for the analysis of the ceria redox cycle. For example, reduction and oxidation using a thermo-balance is usually restricted to temperature increase rates of the order 20 - 50 K min−1 , 32,51 where as the system described here can achieve temperature increase rates in excess of 100 K s−1 ; which is hundreds of times faster. Another example of the unique nature of the system is the gas phase transport properties.

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In this work, both oxidation and reduction take place at less than 20 Pa, as opposed to atmospheric pressure 105 Pa, resulting in much faster diffusion rates. More importantly, oxygen released during reduction or absorbed during oxidation accounts for a significant amount of the total gas in the chamber. This means that gas phase transport is also driven by pressure gradients generated when oxygen is released or absorbed by the sample. Pressure gradients can disperse the gases at the speed of sound. For these reasons it is very unlikely that gas phase transport will in any way restrict the kinetics measured in this work. In other analysis techniques, such as using a thermo-balance, as previously discussed, gas phase transport and heating rates are likely to be the dominant factors affecting the kinetics.

Sample preparation A wet chemical method was used to produce the mixed oxides. Desired amounts of Ce3+ nitrate hexahydrate (99.9 % purity, Merck), Zr4+ oxynitrate hexahydrate (99.99 % purity, Sigma Aldrich) and hexahydrates of Sm3+ or La3+ nitrate (99.9 % purity, Alfa Aesar) were dissolved in deionised water using a reaction vessel made of quartz. Citric acid (99 % purity, Merck) dissolved in deionised water was added to the nitrates in a molar ratio of 1:2 (cations:citric acid). Water was then evaporated off at 95 ◦ C on a heating plate under continuous stirring, which yielded a yellow coloured gel. Heating this gel to 200 ◦ C for 20 minutes resulted in a swollen foam of very low density. This foam was then slowly heated to 500 ◦ C, during which auto combustion took place leaving a fine oxide powder in the reaction vessel. Subsequent calcination in a muffle furnace at 800 ◦ C for 1 hour under air ensured the removal of remaining carbonaceous species. Further calcination at 1400 ◦ C for 1 h in a Pt crucible completed the synthesis route. For each composition two batches were synthesised to guarantee reproducibility. The same method was used to produce mixed ceria oxides with Zr, Sm and La in previous work by the authors. 37,51 These publications together with the supporting information, contain characterisation data (XRD and EDX) for the synthesised samples. 9



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Figure 3: Scanning electron microscope (SEM) images of select samples showing the porous structure. The synthesised oxide powders were then mixed with 60 % by volume pore former (Poly Ethylene Glycol) and pressed into pellets using a 5 mm diameter pellet die. These pellets were then annealed at 1000 ◦ C to remove the pore former, followed by annealing at 1650 ◦ C for two hours to induce sintering. The final porosity of the samples was determined by mass volume measurements (see supporting information) and were in the range 48-55%. From the SEM scans shown in figure 3 the porous structure of the samples can be seen. All the samples were seen to have a similar structure in terms of grain size and porosity. The mass of the synthesised pellets was in the range 34 - 42 mg, and with a diameter of 5 mm, the entire samples fit into the hotspot of the lamp. A selection of the produced pellets were ground up for further XRD analysis to verify the peak positions were still consistent with the corresponding Zr4+ concentrations. These scans can be seen in the supporting information, where the peak widths for each sample were very similar suggesting again that the grain size is relatively uniform throughout the samples. Also found in the supporting information are EDX scans which confirm the stoichiometric

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compositions of the pellets.

Experimental Results Figure 4 shows some typical unprocessed data retrieved from the system for both an oxidation and a reduction cycle. The data shown is the pressure in the chamber and the temperature of the sample. These particular results are for the Ce0.95 Zr0.05 O2 sample. The pressure measurements combined with the known volume of the system and the temperature of the gas, allows the change in the number of moles of gas to be determined. In this case the pressure is quite low and the ideal gas equation ∆P V , RTgas

∆nO2 =

(3)

can be used to calculate the number of moles of oxygen ∆nO2 absorbed or released by the sample. The number of moles of oxide material is

nCe1−x Zrx O2 =

msample MCe1−x Zrx O2

,

(4)

where msample is the sample mass and MCe1−x Zrx O2 is the molar mass of the oxide. Using this combined with the change in moles of oxygen in the system ∆nO2 , the unit-less oxygen vacancy concentration δ can be calculated as

δ=

2∆nO2 nCe1−x Zrx O2

.

(5)

Ideally the kinetics should be determined by a function of the sample temperature Tsample , the oxygen partial pressure PO2 , and oxygen vacancy concentration δ, dδ = f (δ, PO2 ). dt

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(6)



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Figure 6 shows the kinetic data for a number of samples. For each sample the results from four different oxidation cycles are plotted together. The noise from the raw data is amplified due to taking the derivative, but on average the difference in the kinetics of the samples is significant. These plots show the oxidation reaction in the range of 2 % - 80 % completion, where 100 % corresponds to full re-oxidation (δ = 0). For the later points of the reaction (> 80 % completion) the temperature is in any case approximately constant, and little can be illustrated by including more data. The initial stages of the reaction for each Zr concentration (the data below the thick dashed line) have a relatively linear dependence. In addition, the main difference between the samples at this early stage would appear to be their linear intercept with the y-axis which corresponds to the frequency factor ln(Aox ). The part of the reaction which is below the dashed line is only in the range 0 - 15 % of complete oxidation. The reactions appear to have two temperature regimes and could be analysed in this way. However the deviation from the linear dependence, seen below the dashed lines, seems to correlate more with the extent of the reaction. This would suggest that deviation could also be due to diffusion becoming more important at later stages of the reaction. To include diffusion in the analysis, the particles can be assumed spherical for simplicity, which gives the spherically symmetric diffusion equation ∂δ(T, t, r) 1 ∂ ∂δ(T, t, r) = 2 D(δ, T )r2 , ∂t r ∂r ∂r

(9)

where the solution only depends on the radial position. The boundary condition at the surface of a spherical particle of radius rp can now be set as the previous reaction rate given in equation 7, ∂δrp = −δrp POn2 Aox exp ∂t

! −Eox , RT

(10)

where δrp is the boundary vacancy concentration. Again here it is assumed that for the 15




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Figure 7 shows the linear fits of the kinetic data taken from the section of figure 6 which is below the dashed line (approximately in the range 0 - 15 % of complete oxidation). The slope gives the activation energy Eox [kJ mol−1 ] and with analysis of the y-intercept values, Aox can be calculated. These extracted values are given in table 1. The activation energy remained relatively constant. The main difference between the samples from these fits is the frequency factors Aox [s−1 Pa−n ]. Note, the units of Pa−n are required to balance the units with the partial pressure term in the rate equation. Table 1: Slope and intercept data of each linear fit giving Eox and Aox . Sample CeO2 Ce0.9 Zr0.1 O2 Ce0.8 Zr0.2 O2 Ce0.7 Zr0.3 O2

Eox [kJ mol−1 ] 29 ± 2.5 26 ± 2 30 ± 2.2 30 ± 2.5

Aox [s−1 Pa−n ] 7±5 1.4 ± 0.7 0.96 ± 0.4 0.6 ± 0.3

R2 0.84 0.89 0.93 0.84

For a surface reaction a change in frequency factor can be physically interpreted as a change in the number of surface reaction sites. This could mean that the overall surface area is reduced or that the density of surface reaction sites are reduced. From the SEM scans, the XRD peak widths and the porosity measurements (supporting information) the morphology of the samples in terms of grain size and porosity is relatively uniform. Therefore, the large decrease in the frequency factor cannot be due to a reduction in sample surface area. A simple physical interpretation of the decrease in frequency factor with increasing Zr4+ , is that the zirconium ions are blocking surface reaction sites. This is supported by the fact that zirconium should remain in the Zr4+ state, and at the surface it will reduce the number of sites in which a vacancy can be formed. In a previous kinetic analysis conducted by the authors for pure ceria, data in the range of 0 - 40 % completion showed a linear dependence when plotted in the same way. 10 The values for pure ceria were found to be Eox = 36 ± 4 kJ mol−1 and Aox = 11 ± 5 s−1 Pa−n . 10 These values are relatively close and indeed we would expect a higher frequency factor as the samples in that study were annealed at lower temperatures (larger surface area). This 17



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might also be the reason why the data showed a linear dependence over a smaller range in this work. The higher annealing temperature would reduce the surface area and increase the role of diffusion. In addition the samples in this work have a lower porosity which could mean that trapped volumes of material increase the role of diffusion in the reaction. It is difficult to compare our results to other methods of relaxation experiments. As demonstrated by Ackermann et al., 52 it can be difficult to examine ceria under non equilibrium conditions, thus many studies incorrectly attribute the kinetic limitations of the apparatus to the kinetics of the material. However, an interesting study by Otsuka et al., 13 in which pure ceria was oxidised also at reduced pressures (10−4 Pa), but using H2 O as the oxidiser, found an activation energy of 33.4 kJ mol−1 for oxidation at temperatures in the range 453 - 673 K. This falls into the range of temperatures that was fit linearly here. Below 453 K they observed a lower activation energy of 18 kJ mol−1 . Another very interesting study was conducted by Ban and Notwick, 11 in which large single crystals of ceria were oxidized at close to ambient temperatures (< 350 K). The ceria was initially reduced to a large non stoichiometry of approximately CeO1.8 , and then reoxidized at low temperatures in 1 bar of oxygen. Incredibly, temperatures as low as 293 K were enough to measure a very slow rate of re-oxidation. The single crystal samples were 1 mm thick and full re-oxidation of a sample at 315 K took twelve hours. They measured an activation energy of 65 kJ mol−1 , but suggested that the reaction at these temperatures was neither surface nor diffusion controlled, rather it was in the intermediate region between the two.

Addition of Sm3+ and La3+ As mentioned earlier, the addition of metals with lower valence than Ce4+ such as Sm3+ or La3+ , introduces oxygen vacancies into the fluorite structure, which can increase the oxygen ion mobility. 42,43 This increase in mobility could also have a positive effect on the reaction kinetics. To investigate this, samples with both Sm3+ and La3+ were also synthesised. 18


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Discussion Overall the results offer useful insight into the oxidation and reduction kinetics for ceria doped with Zr4+ and Sm3+ or La3+ . For the reduction reaction the results show that the kinetics at high temperature are very rapid, which is a good sign for thermochemical redox cycles using ceria. In most previous experiments for ceria the slow reduction kinetics, 37,45 are likely restricted by both the heating rate and the gas phase transport properties. The benefits of the fast reduction kinetics shown here could be very significant for thermochemical redox cycles. It reduces the overall time required to cycle ceria, which in turn reduces the amount of ceria required per kW of energy conversion to fuel. Given that the reduction reaction also consumes a lot of heat, the fast kinetics would result in a very large rate of energy consumption. This means higher input powers can be achieved without overheating of the material. If concentrated solar power is used as the heat input, faster kinetics means that a higher solar concentration ratio C, can be used for a given reduction tempera 4 σTrd ture Trd . Thus the relative losses ≈ can be reduced, improving the efficiency. C × 1 kW The rapid kinetics seen here were achievable due to the rapid heating rates and the low system pressure, suggesting that reactor designs which aim to use vacuum pumps to reduce the oxygen partial pressure, 61 should have benefits over those which only use a sweep gas. For oxidation, the kinetics decreased with increasing Zr4+ concentration. For thermochemical fuel production cycles this means there is likely an optimal balance between improving the yield and decreasing the kinetics. The addition of trivalent lanthanides must also be considered. In this study an additional 2.5% La3+ added to Ce0.85 Zr0.15 O2 was shown to improve the oxidation kinetics. The addition of small quantities of La3+ and Sm3+ to Ce1−x Zrx O2 has been shown in two separate studies to improve the stability of the oxides when they are subjected to multiple oxidation and reduction cycles. 33,37 Thus materials of the form Ce1−x−y Zrx Lay O2 could offer very favourable redox activity, redox kinetics and thermal stability for use in thermochemical fuel production cycles. It is interesting that a difference was observed between the oxidation rates for Ce0.85 Zr0.15 O2 22


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with added Sm3+ and La3+ . One might expect the two cations to have a very similar effect when added to the ceria lattice. The difference may be a result of different distributions of the cations within the samples. It could also be a result of the different electronegativity which has been shown for other cations to correlate with the oxidation of CO over ceria. 62 In short, without a more detailed knowledge of the samples (for example are the added lanthanides uniformly distributed) it is difficult to say what exactly leads to the difference in kinetics.

Conclusions Porous pellets of mixed cerium zirconium oxides were investigated in a novel experimental system to determine the effects of Zr4+ on the kinetics of oxidation and reduction. The oxides were of the form Ce1−x Zrx O2 , with x varied in the range 0 - 0.3 in steps of 0.05. The results from oxidation reactions showed that the rate of oxidation decreased with increasing Zr4+ concentration. From linear fits of the initial stage of each reaction this reduction was due to a decrease in the frequency factor. This could be most easily explained by Zr4+ at the surface blocking some of the available reaction sites. Although Zr4+ has been shown to improve the overall yield, the effect on the kinetics may also impact the practicality of the cycle. When selecting the optimal mixed oxide, a compromise must be made between the best options for both thermodynamics and kinetics. As an attempt to improve this, samples with 2.5 % of the Ce4+ replaced by either Sm3+ and La3+ (to improve oxygen ion mobility) in the Ce0.85 Zr0.15 O2 were also investigated. Interestingly the samples with added La3+ appeared to show an improvement in oxidation kinetics when compared to Ce0.85 Zr0.15 O2 , but the reaction rates were still slower than pure ceria. Finally, the reduction reaction was also investigated, but the temperature ramp rates appeared to be the rate determining factor, despite being extremely rapid (> 100 ◦ C s−1 ).

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Thus, no difference in reduction kinetics between the samples was measurable. From this kinetic work it would seem that in most cases (reactions at atmospheric pressure) both the heating rate and the gas phase transport properties will play a very important role in the kinetics.

Supporting Information The supporting information contains XRD and EDX scans of the samples, as well as results from a quantitative analysis of the EDX scans. Also included is a table of the void space of each sample as obtained from density measurements.

Acknowledgements This work has received funding from SFI-12/IA/1264, the International Graduate Research Programme in Micro- & Nano Engineering, an IRCSET graduate research education programme, and the Helmholtz Association VH-VI-509. It was conducted in association with the Cleaner Energy Lab Trinity College Dublin.

References (1) Trovarelli, A. Structural and Oxygen Storage/Release Properties of CeO2 -Based Solid Solutions. Comments Inorg. Chem. 1999, 20, 263–284. (2) Imagawa, H.; Suda, A.; Yamamura, K.; Sun, S. Monodisperse CeO2 Nanoparticles and Their Oxygen Storage and Release Properties. J. Phys. Chem. C 2011, 115, 1740–1745. (3) Subbarao, E.; Maiti, H. Solid Electrolytes with Oxygen Ion Conduction. Solid State Ionics 1984, 11, 317–338. (4) Wilson, E. L.; Grau-Crespo, R.; Pang, C. L.; Cabailh, G.; Chen, Q.; Purton, J. A.;

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Figure 11: TOC image

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Oxidation and Reduction Reaction Kinetics of Mixed Cerium Zirconium

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