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Reactivity of ceria, Gd- and Nb-doped ceria to methane
Applied Catalysis B: Environmental 36 (2002) 193–206
Reactivity of ceria, Gd- and Nb-doped ceria to methane E. Ram´ırez-Cabrera a,∗ , A. Atkinson a , D. Chadwick b a
b
Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Department of Chemical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Received 14 July 2001; received in revised form 20 August 2001; accepted 14 October 2001
Abstract The reactivity of methane with ceria, and ceria doped with either 10 cation% Gd, 1.4 cation% Nb, 5 cation% Nb has been investigated using temperature-programmed reaction (TPRx) and isothermal reaction at 900 ◦ C in 5% methane in argon. The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. All the cerias showed activity for conversion of methane to hydrogen and carbon monoxide with only small amounts of carbon deposition. Methane conversion is controlled by surface reaction with an activation energy of 165 ± 10 kJ mol−1 . Gd doping was found to have a relatively minor effect on the reaction rate, but reaction with Nb-doped ceria was significantly slower. XPS results suggested that this is due to segregation of Nb to the ceria surface on the samples calcined in air, however, the XPS results were more complex for the samples calcined in N2 /H2 mixture. Nb-doped ceria was capable of oxidising carbon deposits at lower temperatures than ceria or Gd-doped ceria. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cerium oxide; Carbon deposition; Carbon oxidation; Gadolinium oxide; Niobium oxide; Methane reforming
1. Introduction Cerium oxide is used as a catalyst in a wide variety of reactions involving the oxidation, or partial oxidation, of hydrocarbons (e.g. automotive catalysis). The reason for this is that the ceria lattice can contain a high concentration of highly mobile oxygen vacancies and can thus act as a local source or sink for oxygen involved in reactions taking place on the ceria surface or on other catalytic materials supported on the ceria. There is now increasing interest in using ceria in more reducing conditions, such as in methane reforming at the anodes of solid oxide fuel cells (SOFC), where the potential for deactivation through carbon deposition is much greater. ∗ Corresponding author. E-mail address: [email protected] (E. Ram´ırez-Cabrera).
The gas–solid reaction between CeO2 and CH4 produces [1] synthesis gas with a H2 /CO ratio of two without forming H2 O and CO2 , according to the following reaction: CeO2 + nCH4 = CeO2−n + nCO + 2nH2
(1)
It was also demonstrated [2,3] that contact of the reduced CeO2 with CO2 and steam produced CO and H2 , respectively, regenerating the CeO2 surface: CeO2−n + nCO2 = CeO2 + nCO
(2)
CeO2−n + nH2 O = CeO2 + nH2
(3)
The direct oxidation of CH4 with CeO2 in the absence of gaseous oxygen (Eq. (1)) and subsequent conversion of CO2 to CO (Eq. (2)) or of H2 O to H2 (Eq. (3)) by the reduced cerium oxide CeO2−n have been studied under atmospheric pressure in the temperature
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range 400–800 ◦ C [4]. Steam and CO can also interact via the shift reaction: H2 O + CO = H2 + CO2
(4)
Carbon deposition can occur under these conditions either by the Boudouard reaction, 2CO = CO2 + C
(5)
or by methane cracking CH4 = C + 2H2
(6)
Ceria is also noted for its ability to resist carbon deposition and to catalyse the combustion of carbon [5]. As a result, it has been investigated as an anode material (or a constituent of composite anodes) for SOFCs using fuels such as syngas, methanol and methane [6,7]. Recently, the successful use of ceria as a key constituent of the anode for a SOFC operating directly on “dry” methane has been reported [8]. Ceria is also of interest as a catalyst for indirect internal steam reforming of methane in SOFCs operating at about 900 ◦ C because conventional reforming catalysts are too active at these temperatures and the endothermic reaction induces dangerous thermal stresses in the ceramic components. In this paper, we report on the influence of Gd and Nb cation dopants on the direct conversion of methane to synthesis gas by CeO2 . In oxidising conditions, Gd increases the concentration of oxygen vacancies and Nb increases the concentration of mobile electronic carriers with respect to undoped ceria. Both these species are believed to be involved in hydrocarbon oxidation. Consequently, doping with Gd or Nb can be expected to have a significant influence on methane oxidation to synthesis gas. The materials were characterised by a range of physico-chemical techniques and the conversion of methane studied by temperature-programmed and isothermal reaction experiments. 2. Experimental 2.1. Material preparation Undoped ceria was prepared by precipitation of cerium hydroxide from aqueous cerium nitrate (Aldrich) solution using ammonium hydroxide to a final pH in the range 7–8. The precipitate was recovered,
washed with deionised water and then calcined for 1 h at 1000 ◦ C in air to convert to CeO2 . The calcination temperature was chosen to be slightly higher than the maximum temperature in the subsequent experiments. The samples of Nb-doped ceria were also synthesised using hydroxide co-precipitation [9,10] from aqueous cerium nitrate and niobium chloride (Aldrich), with a Nb level of 1.4% (expressed as cation fraction). 1.4% Nb was chosen to be at the most likely value of the solubility limit, and 5% cation fraction so as to be well in excess of the solubility limit. These doping levels are equivalent to 0.7 and 2.5% mole fraction of Nb2 O5 . One set of the Nb-doped samples was also finally calcined at 1000 ◦ C for 1 h in air. Another set of Nb-doped ceria was obtained by calcination at 1000 ◦ C in reducing conditions using forming gas (10% H2 in N2 ) and then furnace cooled in air. These samples are denoted “FG” in the following text. The level of Nb-doping in specimens prepared under reducing conditions was extended to 20 cation%. Gd-doped ceria with composition Ce0.9 Gd0.1 O2−x was supplied by Rhodia and was also calcined for 1 h at 1000 ◦ C in air, so that all samples had been exposed to the same final temperature before reaction with methane or hydrogen. 2.2. Characterisation The samples were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption. The XPS data were acquired on powder specimens (mounted on a stainless steel stub) using a VG ESCALAB MkII with Al K␣ radiation and a constant threshold pass energy of 50 eV. Binding energies were referenced using the C 1s peak at 284.8 eV. The base pressure during the acquisition was typically less than 1 × 10−8 mbar. The catalyst microstructure was characterised by transmission electron microscopy (TEM), using a JEOL JEM-100CX operating at 100 kV. The powder specimen was first dispersed in iso-propanol by grinding in an agate mortar and then applied by dipping to a 3 mm carbon grid supported on copper. The diffraction camera length of the microscope was calibrated using a fine grained polycrystalline gold specimen.
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The phase compositions of the calcined and aged samples were characterised by XRD using a Phillips PW1710 X-ray diffractometer with Cu K␣ radiation (λ = 1.54060 Å) using a step size (2θ ) of 0.020◦ and a count time of 10 s per step. The specific surface areas (SBET ) of the oxide powders were determined by the BET method using nitrogen as the adsorbate at its boiling point (77 K) in an ASAP 2000 (Micromeritics). 2.3. Chemical reactions Experiments were performed on powder samples having an initial mass of 300 ± 5 mg in a temperature-programmable quartz tube micro-flowreactor system operated at atmospheric pressure which has been described in detail elsewhere [11]. The exhaust gases were analysed using a quadrupole mass spectrometer (QMS) or, for temperature-programmed reduction (TPR), by using a thermal conductivity detector (TCD). The TCD was calibrated by reduction of a known mass of CuO to Cu. Samples were subjected to a consecutive chemical reaction sequence as described below at a standard flow rate 60 ml min−1 . TPR was first carried out (5% H2 /Ar, 10 ◦ C min−1 ) from ambient temperature to 1000 ◦ C. The reactor was then cooled, purged with argon and temperature-programmed reaction (TPRx) performed (5% CH4 /Ar, 25 ◦ C min−1 ). After cooling to room temperature and purging the system, temperature-programmed oxidation (TPO) was used to check for deposited carbon (10% O2 /He, 10 ◦ C min−1 ). The CO2 signal of the QMS was calibrated by injecting 4.7 ml of pure CO2 into the gas stream from an injection loop. After TPO the reactor was cooled rapidly to room temperature and purged with argon. The sample was then heated to 900 ◦ C at 25 ◦ C min−1 in 5% CH4 /Ar followed by exposure at constant temperature (900 ◦ C) for a period of 150 min. The CO signal of the QMS was calibrated by injecting 4.7 ml CO into the gas stream. Finally, after cooling and purging, TPO was again carried out to characterise carbon deposition. The TPR experiment was not performed for the specimens 1.4% Nb-doped CeO2 FG and Nb2 O5 . Spent catalysts, selected at various stages, were analysed by XPS.
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3. Results 3.1. Physical characterisation XRPD patterns for the undoped and Nb-doped cerias after calcination for 1 h at 1000 ◦ C in either air or forming gas are shown in Fig. 1a and b, respectively. The data confirm ceria as the major crystalline phase, but there is clear evidence that fergusonite (CeNbO4 ) was present as a minor second phase [9] in some of the Nb-doped specimens. The four peaks seen for the 20% Nb-doped FG specimen in Fig. 1b that are not from the fluorite lattice can all be indexed as coming from fergusonite. Fergusonite was not detected in the 1.4% Nb cerias irrespective of the calcination atmosphere. Calcining Nb2 O5 under reducing conditions greatly reduced crystallinity (compare Fig. 1a and b). The Gd-doped ceria gave only the XRD of ceria. Selected area diffraction of a 5% Nb-doped ceria FG specimen in TEM showed only ceria and no crystallites of fergusonite in spite of the XRD evidence. Fig. 2 shows a TEM image of the 5% Nb-doped CeO2 sample calcined in forming gas in which the 10 nm crystallites are clearly visible. The crystallite size was estimated to be approximately 10 nm from XRD line broadening. In contrast, TEM examination showed that the crystallite size in the undoped ceria was approximately 100 nm. No evidence of carbon deposition could be found by TEM examination of these materials after exposure to methane at high temperature. The materials prepared by calcination in forming gas were found to have higher surface areas. The influence of Nb doping level on the surface area of the materials prepared in forming gas atmospheres is presented in Fig. 3. The surface area reaches a maximum at approximately 1–5 cation% Nb. Surface areas for 5% Nb-doped ceria proved to be rather variable. The surface areas of the catalysts are summarised in Table 1 from which it can be seen that the high surface area of the 5% Nb-doped CeO2 specimen calcined in forming gas is consistent with its small crystallite size (the equivalent sphere size calculated from the surface area is approximately 30 nm for this specimen). 3.2. XPS Fig. 4a and b present the changes in the Ce 3d spectra before and after isothermal exposure to CH4 ,
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Fig. 1. XRPD diffractograms for catalysts calcined for 1 h at 1000 ◦ C; (a) in air and (b) in forming gas.
Fig. 2. Transmission electron micrograph of 5% Nb-doped CeO2 FG catalyst before exposure to methane.
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Fig. 3. Specific surface area of Nb-doped ceria calcined at 1000 ◦ C for 1 h in forming gas (FG) as a function of cation fraction of Nb.
respectively (following the above sequence, or independent treatment). The six-line spectrum of undoped CeO2 (Fig. 4a) is characteristic of the Ce4+ core level [12] with the lines denoted by a, a1 , a2 and b, b1 , b2 for the 3d5/2 and 3d3/2 electrons, respectively. The spectrum from CeF3 is included for comparison and demonstrates that detecting minor proportions of Ce3+ mixed with the Ce4+ is difficult because the main characteristic of the Ce3+ spectrum is the absence of the b2 peak. The addition of gadolinium or niobium broadens the lines; even with only a small amount of niobium (1.4%). After isothermal exposure to CH4 (Fig. 4b) the Ce lines tend to become sharper (ex-
Table 1 BET specific surface area (SSA) and turnover frequencies (N) for the reaction with methane at 900 ◦ C for different catalysts Specimen
Specific surface area (m2 g−1 )
CeO2 2.6 Ce0.9 Gd0.1 O1.95 a 7.1 Ce0.986 Nb0.014 O2.007 3.7 Ce0.986 Nb0.014 O2.007 25.3 (FG) 1.1 Ce0.95 Nb0.05 O2.025 Ce0.95 Nb0.05 O2.025 (FG) 28.6 1.4 Nb2 O5 a
Specimen issued from Rhodia.
N (molecules per site s−1 × 10−3 ) dry CH4 30.0 20.0 5.5 8.0 16.7 2.2 6.8
cept for the 1.4 and 5% Nb-doped CeO2 specimens calcined in forming gas) indicating a more uniform chemical environment. These observations are similar to those previously published for nano-crystalline Cex La1−x O2−y [13]. Fig. 5a and b show the Nb 3d emission spectra for the same samples before and after isothermal exposure to CH4 treatments, respectively. The Nb 3d emission spectrum for Nb2 O5 (prepared by precipitation and calcination in air) was taken as a reference. It shows a peak and a shoulder corresponding to the spin doublet 3d5/2 and 3d3/2 with binding energies of 208.4 and 210.0, respectively, which are in agreement with literature data [14]. As with the cerium spectra, the lines are sharper after exposure to methane (again except for the specimens calcined in forming gas) and the doublet more clearly resolved. This might also be enhanced by reduction of some Nb5+ to Nb4+ , which is reported to cause a slight increase in separation [14]. In Fig. 5b is also included the spectrum of Ce 4p from the undoped CeO2 specimen after exposure to CH4 . This shows that there is significant overlap of the Ce 4p and Nb 3d peaks, which complicates detailed interpretation. However, several trends in variation in the Nb spectra are clear. Firstly, there is no evidence for Nb in the surface of the 1.4% Nb FG sample. Secondly, that all the Nb-doped cerias show that the surface concentration of Nb decreases on exposure to methane at 900 ◦ C.
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Fig. 4. Ce 3d XPS spectra for ceria catalysts (a) as-prepared and with a CF3 reference, and (b) after exposure to methane for 150 min at 900 ◦ C.
The effect is most dramatic for the FG samples— the 1.4% has no visible Nb prior to methane exposure so that no change can be observed. This effect is borne out by the surface Nb/Ce or Gd/Ce atomic ratios are given in Table 2 (measured peak areas were converted to atomic ratios using the atomic sensitivity factors: Ce = 10; Gd = 3; Nb = 2.4 [15]; a correction has been applied for overlap of the Nb 3d and Ce 4p peaks). In general, Nb and Gd are segregated to the surface of the ceria. However, as pointed out above, the 1.4% Nb-doped CeO2 specimen calcined in forming gas showed no segregation of Nb to the surface, which could indicate a higher solubility limit in reducing conditions. The Nb/Ce ratio is significantly reduced on exposure to methane. The binding energies (BE) measured for the different elements are summarised in Table 2. They show little variation with either composition or exposure to methane. The ratio of the area of the b2 line to the total area of the Ce 3d spectrum can be used to assess
how much Ce3+ is present. This ratio should be approximately 14% for all cerium as Ce4+ [16,17]. The measured ratios are presented in Table 2. Values were observed to cover a wide range (8–13%), apparently indicating some Ce3+ content. The results in Table 2 indicate that this ratio is lower in the Nb-doped specimens, which suggests a possible complication due to electronic interactions between the Nb and Ce generating Ce3+ , bearing in mind that the spectrum of fergusonite is unknown. The values for the ratio of the b2 peak to total area show no obvious trend after exposure to methane. Thus, any changes in surface oxidation states of Ce were probably below the relatively insensitive, detection limit, and may be in any case affected by the transfer to the spectrometer, which was done under air. The one exception is the 5% Nb FG sample, which perhaps shows a significant reduction in the ratio on exposure to methane, consistent with the observed strong dissolution of the Nb in the ceria under reaction conditions.
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Fig. 5. Nb 3d XPS spectra for ceria catalysts (a) as-prepared and with a Nb2 O5 reference, and (b) after exposure to methane for 150 min at 900 ◦ C. Table 2 XPS binding energy (eV), surface atomic ratios and Ce reduction parameter for ceria catalysts as prepared and after isothermal exposure to methane Catalyst
Ce 3d5/2
Ce 4d5/2
CeO2 As prepared After methane
882.3 882.7
107.8 108.8
– –
Ce0.9 Gd0.1 O1.95 As prepared After methane
882.4 882.2
107.9 108.6
– –
Ce0.986 Nb0.014 O2.007 As prepared After methane
882.2 883.0
109.1 109.6
207.4 207.7
– –
Ce0.986 Nb0.014 O2.007 (FG) As prepared 882.4 After methane 881.9
110.3 109.9
– –
– –
Ce0.95 Nb0.05 O2.025 As prepared After methane
882.3 883.1
110.5 109.3
208.0 207.4
– –
0.54 0.24
9 10
Ce0.95 Nb0.05 O2.025 (FG) As prepared 882.3 After methane 882.3
109.7 109.5
207.1 –
– –
0.48 ∼0
10 8
Nb 3d5/2
Gd
Nb/Ce or Gd/Ce – –
1185.8 1186.2
Ce b2 /total ratio (±1%)
– –
12 13
0.48 0.33
12 12
0.41 0.21
8 9
∼0 ∼0
11 10
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Fig. 6. Temperature-programmed reduction of ceria catalysts in 5% H2 –95% Ar. The baselines have been shifted for clarity.
3.3. Chemical reactions The TPR profiles (Fig. 6) typically exhibit more than one reduction peak [18]. The lower temperature minor peak (at approximately 550 ◦ C) is believed to correspond to reduction of Ce4+ to Ce3+ on the ceria surface and the higher temperature peak (at approximately 850 ◦ C) is due to creation of bulk oxygen vacancies by similar reduction of bulk Ce4+ : ¨ O + 2e OxO = 21 O2 + V
(7)
The relatively large low temperature reduction peak for the 5% Nb-doped CeO2 (FG) specimen is consistent with its high surface area, but the precise interpretation is probably more complex. TPRx in methane/argon to 1000 ◦ C gave similar results for all the specimens. (The TPRx results are similar to the results obtained during heating to the isothermal reaction temperature, which are shown in the first 35 min period of Figs. 7 and 8). Reaction commenced at approximately 750 ◦ C and methane was oxidised by the ceria to CO and H2 in accordance
Fig. 7. Mass spectrometer signal trends during TPRx/isothermal reaction between Gd-doped ceria and 5% CH4 –95% Ar. During the first 35 min the specimen was heated to 900 ◦ C at 25 ◦ C min−1 as indicated by the temperature profile shown as a broken line (data have not been corrected for MS sensitivities).
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Fig. 8. Comparison of CO signal trends for different ceria specimens during TPRx/isothermal reaction with 5% CH4 –95% Ar. During the first 35 min the specimen was heated to 900 ◦ C at 25 ◦ C min−1 as indicated by the temperature profile (broken line). The baselines have been shifted for clarity.
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with Eq. (1). Only small quantities of CO2 and H2 O were produced at 750–950 ◦ C. There was no carbon deposition during TPRx detectable by subsequent temperature-programmed oxidation. The lower limit of carbon dioxide that could be detected by TPO was estimated to be 1 × 10−6 mol g−1 . Methane oxidation and C formation were investigated during TPRx followed by isothermal reaction at 900 ◦ C. The results for Gd-doped ceria are given in Fig. 7. The maximum observed methane conversion rate was 9 × 10−5 mol min−1 g−1 and the H2 /CO ratio was 2.14, compared with the expected value of 2 from reaction (1). The yields of H2 and CO together with the H2 /CO ratio for all the samples are given in Table 3. Overall, the H2 /CO ratio tends to be greater than 2, but the measurements for the Nb-doped samples are subject to larger errors because of their slower reaction rates. CO production during isothermal reaction with methane at 900 ◦ C tends to show two maxima (Fig. 8), but there are large differences between the cerias. In particular, reaction is significantly slower in the low surface area specimens containing 1.4 and 5% niobium, and oxygen is still being removed from these specimens even after 150 min of exposure. The total quantity of CO produced for each specimen is given in Table 3 together with the molar change in oxygen content of the ceria (n in Eq. (1)). Similar experiments were also carried out with a pure Nb2 O5 specimen and the results are illustrated in Fig. 8 and also summarised in Table 3. They indicate that niobia does oxidise methane, but at a lower intrinsic rate than does ceria.
Table 3 Total production of H2 and CO and change in stoichiometry during TPRx and isothermal reaction with methane at 900 ◦ C; also given is the amount of CO2 produced during subsequent TPOa Specimen
CeO2 Ce0.9 Gd0.1 O1.95 Ce0.986 Nb0.014 O2.007 Ce0.986 Nb0.014 O2.007 (FG) Ce0.95 Nb0.05 O2.025 Ce0.95 Nb0.05 O2.025 (FG) Nb2 O5 a
TPR H2 /Ar
TPRx + isothermal reaction with methane
Change in stoichimetry, n
H2 yield (mol g−1 × 10−3 )
CO yield (mol g−1 × 10−3 )
H2 /CO ratio
Change in stoichiometry, n
CO2 yield (mol g−1 × 10−3 )
CO2 yield (mol m2 × 10−5 )
0.2 0.24 0.27 – 0.21 0.19 –
4.60 5.33 2.63 4.70 4.43 2.90 1.01
2.16 2.49 1.66 2.50 2.15 1.26 0.5
2.13 2.14 1.58 1.90 2.06 2.30 2.01
0.37 0.40 0.28 0.43 0.37 0.22 0.13
0.04 0.18 0.21 0.06 0.16 0.13 0.008
1.39 2.56 5.63 0.24 14.1 0.46 0.56
Moles of CO/moles of cations.
TPO
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Fig. 9. Comparison of CO2 signals for different ceria specimens during TPO after isothermal reaction with 5% CH4 –95% Ar at 900 ◦ C. The baselines have been shifted for clarity.
There was no detectable hydrogen in the gas phase from methane cracking (Eq. (6)) even after removal of oxygen from the ceria was complete (Fig. 7). This was also true for the doped cerias. However, subsequent TPO experiments (Fig. 9) revealed that some carbon deposition had occurred and so we assume that the H2 production rate was below the limit of detection in our experimental arrangement. The total quantities of CO produced, and the amount of C deduced from the CO2 evolved in TPO, per unit area of ceria are given
in Table 3. When the deposited carbon is taken into account, the ratio H2 /(CO+C) is 2.09 for the undoped ceria and 2.00 for the Gd-doped ceria; the samples for which the data are most accurate. The gaseous oxygen depletion during TPO for the Nb2 O5 (Fig. 10) specimen shows two peaks, while the CO2 signal presents a shoulder and a peak, which appear at higher temperature than that of the O2 signal depletion. Thus, the oxygen depletion in this case is not associated with carbon consumption and could be indicating re-oxidation of
Fig. 10. Temperature-programmed oxidation for Nb2 O5 in 10% O2 –90% He after isothermal reaction with 5% CH4 –95% Ar at 900 ◦ C.
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the partially reduced oxide. However, this should be even more evident for the ceria specimens (which have larger values of n) and so cannot be accounted for at present. The specimen 1.4% Nb-doped CeO2 calcined in forming gas showed two TPO peaks, which indicates two forms of deposited carbon. 4. Discussion The doping of ceria by Gd3+ is known to generate oxygen vacancies: ¨ O + 3Ox Gd2 O3 = 2Gd Ce + V (8) O whereas doping with Nb5+ fills oxygen vacancies and/or creates electronic carriers: ¨ O = 2Nb˙ Ce + 5Ox Nb2 O5 + V (9) O Nb2 O5 = 2Nb˙ Ce + 4OxO + 21 O2 + 2e
(10)
Electronic carriers and oxygen vacancies are also favoured under reducing conditions from Eq. (7). The oxygen thermodynamic activity is not known exactly in the present experiments since the water vapour level was not measured. A computer program, HSC Chemistry, was applied to calculate the oxygen activity, using its in-built thermodynamic database. Assuming a water vapour partial pressure of 10−4 atm then at 900 ◦ C PO2 is estimated to have been approximately 2 × 10−20 atm in the TPR experiments (5% H2 /100 ppm H2 O) and 7×10−26 atm in the isothermal reaction experiments (5% CH4 /100 ppm H2 O). Under these strongly reducing conditions, reduction of Ce4+ (Eq. (7)) is known to dominate, even in doped ceria, and all the materials have an oxygen deficit (with respect to their nominal composition in equilibrium with air) of approximately 0.15 at PO2 = 10−19 atm and 900 ◦ C [19]. The values of n observed after reaction with methane are larger than observed after TPR in hydrogen (Table 3). This is consistent with the lower oxygen activity in reaction with methane. Furthermore, the phase boundaries Nb2 O5 /Nb2 O4 and Nb2 O4 /NbO are at oxygen pressures of approximately 10−20 and 10−24 atm at 900 ◦ C and so we would anticipate that some of the Nb5+ might also be reduced. However, the XPS examination of specimens after exposure to methane shows that the extent of any such reduction is small, or was reversed by oxidation during transfer from the reactor to the XPS chamber.
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Application of the law of mass action to Eq. (10) leads to the prediction that the solubility limit of −1/8 Nb2 O5 in ceria should be proportional to PO2 and should increase under reducing conditions. Hence the Nb-doped specimens prepared under forming gas atmospheres are expected to have more Nb in solid solution than those prepared in air. The Nb/Ce ratios determined by XPS are consistent with this—the 1.4% Nb sample showing no evidence of Nb segregation. This might be related also to the observation that the specimens calcined in forming gas have much greater specific surface areas than those calcined in air. The isothermal reaction results for CO production and change in oxygen stoichiometry (Table 3) confirm that oxygen loss, n, was not strongly dependent on doping. The quantities are lower on average for the Nb-doped samples, except for the specimen 1.4% Nb-doped CeO2 (FG) which had all the Nb in solid solution after calcination and a much greater surface area. The air-calcined, Nb-doped specimens reacted significantly more slowly than the undoped or Gd-doped specimens and reaction was incomplete even after 180 min (Fig. 8). The measured value of the oxygen diffusion coefficient in ceria [20] is sufficiently high that in the powder specimens used here the overall reaction rate will be controlled by the surface reaction and not by diffusion of oxygen from the bulk of the solid particles to their surfaces. Therefore, turnover frequencies were estimated for the rate of reaction with methane at 900 ◦ C on the basis that the reaction was surface controlled and that all surface sites accessible by nitrogen adsorption (area per molecule 16.2 × 10−20 m2 [21]) were active. For Gd-doped ceria and for the specimen 1.4% Nb-doped CeO2 calcined in forming gas the reaction rate was estimated from the maximum depletion in the methane signal. However, this was not sufficiently sensitive for the less reactive Nb-doped specimens and in their case the reaction rate was calculated from the maximum in the CO signal. The same procedure using the CO signal was also applied for the undoped ceria. The estimated turnover frequencies are given in Table 1. The inhibiting effect of Nb is evident in the air-calcined specimens, which show a high degree of Nb segregation, consistent with the low value determined for Nb2 O5 (the value of the turnover frequency obtained for Nb2 O5 is very uncertain because of its low surface area and low reaction rate). The catalysts
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prepared by calcination in forming gas, where Nb is in solid solution under reaction conditions, show a strong decrease in activity as the Nb concentration increases. This could be associated with the apparent increase in Ce3+ in the 5% Nb case noted above. The turnover frequencies for all the oxides are very much lower than those estimated for Ni/MgO steam reforming catalysts under similar conditions. The latter have been reported for the temperature range 500–600 ◦ C [22] and were extrapolated by fitting to the Arrhenius equation: −103(kJ mol−1 ) −6 −1 N = 9.2 × 10 (s ) exp (11) RT to give an estimated turnover frequency of approximately 240 molecules per site s−1 at 900 ◦ C. The turnover frequency can also be compared with the measured rate of isotopic exchange between ceria and oxygen-containing gases. The isotopic surface exchange rate constant k is related to the surface site turnover frequency by: NA N =k As (12) Vm where NA is Avogadro’s number, Vm is the molar volume of the solid and As is the area of a surface site. The measured values of k at 900 ◦ C are [23] 3 × 10−5 cm s−1 for reduced ceria in CO/CO2 and 1 × 10−5 cm s−1 for Ce0.8 Y0.2 O1.9 in oxygen. The corresponding turnover frequencies are 840 and 280
atoms of oxygen per site s−1 . These are several orders of magnitude greater than the turnover frequencies for methane reaction given in Table 1, which indicates that reaction with methane is much slower than with oxygen, CO or CO2 . This difference in rate of reaction is also reflected in the activation energies. We estimated the activation energy for methane oxidation from the TPRx experiments by assuming that the reaction rate is proportional to the hydrogen signal intensity. This is plotted in Arrhenius form in Fig. 11 for Gd-doped ceria and the corresponding activation energy is 176 ± 10 kJ mol−1 . A similar analysis of the CO signal gave 155 ± 10 kJ mol−1 . The average value taken from the H2 and CO signals results in 165 ± 10 kJ mol−1 and is in good agreement with the value 160 kJ mol−1 reported by Otsuka et al. [4] for undoped ceria and is significantly greater than reported [23] for isotopic exchange with oxygen (140 kJ mol−1 ) or CO/CO2 (120 kJ mol−1 ). The slower reaction rate on the surfaces of the Nb-doped powders is therefore due to their different composition. The XPS analyses show that in the air-calcined Nb-doped specimens approximately one-third of the surface cation sites are occupied by Nb. Since niobia reacts more slowly with methane than does ceria under these conditions it is concluded that the segregation of Nb to the ceria surface is inhibiting reaction with methane. In contrast, in the Nb-doped FG specimens, which show no detectable Nb segregation, the lower rate would appear to be
Fig. 11. Arrhenius plot of the hydrogen signal intensity (assumed to be proportional to the reaction rate) for TPRx of Gd-doped ceria with methane.
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mainly due to the effect of bulk doping (e.g. by reducing the concentration of oxygen vacancies according to Eq. (9)). The XPS results also indicate surface enrichment of Gd to similar levels as Nb. However, the Gd appears to have a much smaller (if any) inhibiting effect on methane oxidation than does Nb. The products of reaction with methane confirm the dominance of Eq. (1) until the ceria is exhausted of reducible oxygen. Carbon deposition by methane cracking (Eq. (6)) occurs only very slowly since hydrogen production was not detected after oxygen exhaustion. On an equivalent surface area basis, the 1.4 and 5% Nb-doped CeO2 (FG) specimens have the lowest carbon deposition (Table 3). Using a value of 0.026 nm2 for the area occupied by a carbon atom in a surface monolayer of the basal plane in graphite, corresponding to 6.4 × 10−5 mol m−2 , the quantities of carbon deposited range from approximately 0.04 monolayers on the 1.4% Nb-doped CeO2 (FG), (0.07 monolayers on the 5% Nb (FG) specimen) to 0.9 monolayers on the 1.4% Nb specimen. These low values explain why deposited carbon was not detectable either by XPS or TEM. The temperature at which deposited carbon is combusted during TPO is lowest for the Nb-doped specimens and highest for the Gd-doped specimens, although it is even higher for the Nb2 O5 sample. This probably reflects the greater electronic carrier concentration in the Nb-doped cerias under oxidising conditions, which are available for ionising adsorbed oxygen species that subsequently react with the carbon. Methane reaction and carbon deposition over the cerias are very different from the behaviour reported previously for a perovskite La0.8 Ca0.2 CrO3 , another potential SOFC anode material, by Baker and Metcalfe [24]. They found that over the chromite, initial TPRx of methane produced mainly CO2 and H2 O (at up to approximately 600 ◦ C) followed by Eq. (1), which reached a maximum rate at approximately 850 ◦ C. The slower rate at higher temperatures is in marked contrast to the cerias and probably reflects the easier supply of bulk oxygen vacancies in the cerias. However, the most striking difference is in relation to carbon deposition and combustion. Although the chromite is more resistant to carbon deposition than a Ni-zirconia cermet [25] the cerias are more resistant than the chromite. Furthermore, the temperature at
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which deposited carbon is combusted is lower for the Nb-doped cerias (approximately 230 ◦ C) than for the chromite (approximately 400 ◦ C).
5. Conclusions Doped and undoped cerias exhibit significant reactivity with methane to produce syngas. Gd-doping has little effect on the reaction rate, but Nb-doping slows down the rate considerably. XPS has shown that there is strong segregation of Nb to the ceria surface. Calcination of Nb-doped ceria under reducing conditions leads to significantly lower crystallite size, higher specific surface area and greater Nb solubility. The reaction rate is controlled by surface reaction with methane and has an activation energy of 165 ± 10 kJ mol−1 . Carbon deposition by methane cracking at 900 ◦ C is extremely slow on all cerias. The propensity for carbon deposition is much lower than for Ni-based catalysts and even lower than for Ca-doped lanthanum chromite. Carbon deposition is least on high surface area Nb-doped ceria. The Nb-doped ceria is the most effective for removing deposited carbon in subsequent oxidation, probably because of its higher electronic conductivity under oxidising conditions.
Acknowledgements The authors are grateful for financial support from the UK Engineering and Physical Sciences Research Council under Grant GR/L 81970. They are also indebted to Dr. K. Senkiw for his assistance in the X-ray photoelectron spectroscopy analysis. E. Ram´ırez-Cabrera wishes to thank the Consejo Nacional de Ciencia y Tecnolog´ıa (CONACyT), Mexico, for the award of a study scholarship. References [1] K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517. [2] K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493. [3] K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193. [4] K. Otsuka, E. Sunada, T. Ushiyama, I. Yamanaka, in: M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholtz, M.S. Scurrell (Eds.), Proceedings of the 4th International Natural
206
[5] [6] [7]
[8] [9] [10] [11] [12] [13] [14]
E. Ram´ırez-Cabrera et al. / Applied Catalysis B: Environmental 36 (2002) 193–206 Gas Conversion Symposium, Kruger Park, South Africa, Studies in Surface Science and Catalysis, Vol. 107, Elsevier, Amsterdam, 1997, p. 531. B.T. Kilbourn, Cerium: A Guide to its Role in Chemical Technology, Molycorp Inc., White Plains, USA, 1992. B.C.H. Steele, P.H. Middleton, R.A. Rudkin, Solid State Ionics 28–30 (1990) 388. O.A. Marina, C. Bagger, S. Primdahl, M. Mogensen, in: P. Stevens (Ed.), Proceedings of 3rd European SOFC Forum (Nantes), U. Bossell, Oberrohrdorf, Switzerland, 1998, p. 427. E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. I.K. Naik, T.Y. Tien, J. Electrochem. Soc. 126 (1979) 562. M.R. De Guire, M.J. Shingler, E. Dincer, Solid State Ionics 52 (1992) 155. D. Chadwick, P.J.R. O’Malley, J. Chem. Soc., Faraday Trans. 83 (1987) 2227. F. Le Normand, J. El Fallah, L. Hilaire, P. Légaré, A. Kotani, J.C. Parlebas, Solid State Commun. 71 (1989) 885. M.L. Trudeau, A. Tschöpe, J.Y. Ying, Surf. Interface Anal. 23 (1995) 219–226. M.K. Bahi, J. Phys. Chem. Solids. 36 (1975) 485.
[15] D. Briggs, M.P. Seah, Practical surface analysis, in: Auger and X-ray Photoelectron Spectroscopy, 2nd Edition, Vol. 1, Wiley, New York, 1990. [16] P.J. Schmitz, R.K. Usmen, C.R. Peters, G.W. Graham, R.W. McCabe, Appl. Surf. Sci. 72 (1993) 181–187. [17] J.Z. Shyu, W.H. Weber, H.S. Gandhi, J. Phys. Chem. 92 (1988) 4964–4970. [18] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [19] B.C.H. Steele, K.M. Hori, S. Uchino, Solid State Ionics 135 (2000) 445–450. [20] B.C.H. Steele, J.M. Floyd, Proc. Br. Ceram. Soc. 19 (1971) 55. [21] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corporation, USA, 1997. [22] J.R. Rostrup-Nielsen, J.H. Bak-Hansen, J. Catal. 144 (1993) 38. [23] J.M. Floyd, Indian Journal of Technology 11 (1973) 589. [24] R.T. Baker, I.S. Metcalfe, Appl. Catal. A: Gen. 126 (1995) 297. [25] R.T. Baker, I.S. Metcalfe, Ind. Eng. Chem. Res. 34 (1995) 1558.
Reactivity of ceria, Gd- and Nb-doped ceria to methane E. Ram´ırez-Cabrera a,∗ , A. Atkinson a , D. Chadwick b a
b
Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Department of Chemical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Received 14 July 2001; received in revised form 20 August 2001; accepted 14 October 2001
Abstract The reactivity of methane with ceria, and ceria doped with either 10 cation% Gd, 1.4 cation% Nb, 5 cation% Nb has been investigated using temperature-programmed reaction (TPRx) and isothermal reaction at 900 ◦ C in 5% methane in argon. The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. All the cerias showed activity for conversion of methane to hydrogen and carbon monoxide with only small amounts of carbon deposition. Methane conversion is controlled by surface reaction with an activation energy of 165 ± 10 kJ mol−1 . Gd doping was found to have a relatively minor effect on the reaction rate, but reaction with Nb-doped ceria was significantly slower. XPS results suggested that this is due to segregation of Nb to the ceria surface on the samples calcined in air, however, the XPS results were more complex for the samples calcined in N2 /H2 mixture. Nb-doped ceria was capable of oxidising carbon deposits at lower temperatures than ceria or Gd-doped ceria. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Cerium oxide; Carbon deposition; Carbon oxidation; Gadolinium oxide; Niobium oxide; Methane reforming
1. Introduction Cerium oxide is used as a catalyst in a wide variety of reactions involving the oxidation, or partial oxidation, of hydrocarbons (e.g. automotive catalysis). The reason for this is that the ceria lattice can contain a high concentration of highly mobile oxygen vacancies and can thus act as a local source or sink for oxygen involved in reactions taking place on the ceria surface or on other catalytic materials supported on the ceria. There is now increasing interest in using ceria in more reducing conditions, such as in methane reforming at the anodes of solid oxide fuel cells (SOFC), where the potential for deactivation through carbon deposition is much greater. ∗ Corresponding author. E-mail address: [email protected] (E. Ram´ırez-Cabrera).
The gas–solid reaction between CeO2 and CH4 produces [1] synthesis gas with a H2 /CO ratio of two without forming H2 O and CO2 , according to the following reaction: CeO2 + nCH4 = CeO2−n + nCO + 2nH2
(1)
It was also demonstrated [2,3] that contact of the reduced CeO2 with CO2 and steam produced CO and H2 , respectively, regenerating the CeO2 surface: CeO2−n + nCO2 = CeO2 + nCO
(2)
CeO2−n + nH2 O = CeO2 + nH2
(3)
The direct oxidation of CH4 with CeO2 in the absence of gaseous oxygen (Eq. (1)) and subsequent conversion of CO2 to CO (Eq. (2)) or of H2 O to H2 (Eq. (3)) by the reduced cerium oxide CeO2−n have been studied under atmospheric pressure in the temperature
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range 400–800 ◦ C [4]. Steam and CO can also interact via the shift reaction: H2 O + CO = H2 + CO2
(4)
Carbon deposition can occur under these conditions either by the Boudouard reaction, 2CO = CO2 + C
(5)
or by methane cracking CH4 = C + 2H2
(6)
Ceria is also noted for its ability to resist carbon deposition and to catalyse the combustion of carbon [5]. As a result, it has been investigated as an anode material (or a constituent of composite anodes) for SOFCs using fuels such as syngas, methanol and methane [6,7]. Recently, the successful use of ceria as a key constituent of the anode for a SOFC operating directly on “dry” methane has been reported [8]. Ceria is also of interest as a catalyst for indirect internal steam reforming of methane in SOFCs operating at about 900 ◦ C because conventional reforming catalysts are too active at these temperatures and the endothermic reaction induces dangerous thermal stresses in the ceramic components. In this paper, we report on the influence of Gd and Nb cation dopants on the direct conversion of methane to synthesis gas by CeO2 . In oxidising conditions, Gd increases the concentration of oxygen vacancies and Nb increases the concentration of mobile electronic carriers with respect to undoped ceria. Both these species are believed to be involved in hydrocarbon oxidation. Consequently, doping with Gd or Nb can be expected to have a significant influence on methane oxidation to synthesis gas. The materials were characterised by a range of physico-chemical techniques and the conversion of methane studied by temperature-programmed and isothermal reaction experiments. 2. Experimental 2.1. Material preparation Undoped ceria was prepared by precipitation of cerium hydroxide from aqueous cerium nitrate (Aldrich) solution using ammonium hydroxide to a final pH in the range 7–8. The precipitate was recovered,
washed with deionised water and then calcined for 1 h at 1000 ◦ C in air to convert to CeO2 . The calcination temperature was chosen to be slightly higher than the maximum temperature in the subsequent experiments. The samples of Nb-doped ceria were also synthesised using hydroxide co-precipitation [9,10] from aqueous cerium nitrate and niobium chloride (Aldrich), with a Nb level of 1.4% (expressed as cation fraction). 1.4% Nb was chosen to be at the most likely value of the solubility limit, and 5% cation fraction so as to be well in excess of the solubility limit. These doping levels are equivalent to 0.7 and 2.5% mole fraction of Nb2 O5 . One set of the Nb-doped samples was also finally calcined at 1000 ◦ C for 1 h in air. Another set of Nb-doped ceria was obtained by calcination at 1000 ◦ C in reducing conditions using forming gas (10% H2 in N2 ) and then furnace cooled in air. These samples are denoted “FG” in the following text. The level of Nb-doping in specimens prepared under reducing conditions was extended to 20 cation%. Gd-doped ceria with composition Ce0.9 Gd0.1 O2−x was supplied by Rhodia and was also calcined for 1 h at 1000 ◦ C in air, so that all samples had been exposed to the same final temperature before reaction with methane or hydrogen. 2.2. Characterisation The samples were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption. The XPS data were acquired on powder specimens (mounted on a stainless steel stub) using a VG ESCALAB MkII with Al K␣ radiation and a constant threshold pass energy of 50 eV. Binding energies were referenced using the C 1s peak at 284.8 eV. The base pressure during the acquisition was typically less than 1 × 10−8 mbar. The catalyst microstructure was characterised by transmission electron microscopy (TEM), using a JEOL JEM-100CX operating at 100 kV. The powder specimen was first dispersed in iso-propanol by grinding in an agate mortar and then applied by dipping to a 3 mm carbon grid supported on copper. The diffraction camera length of the microscope was calibrated using a fine grained polycrystalline gold specimen.
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The phase compositions of the calcined and aged samples were characterised by XRD using a Phillips PW1710 X-ray diffractometer with Cu K␣ radiation (λ = 1.54060 Å) using a step size (2θ ) of 0.020◦ and a count time of 10 s per step. The specific surface areas (SBET ) of the oxide powders were determined by the BET method using nitrogen as the adsorbate at its boiling point (77 K) in an ASAP 2000 (Micromeritics). 2.3. Chemical reactions Experiments were performed on powder samples having an initial mass of 300 ± 5 mg in a temperature-programmable quartz tube micro-flowreactor system operated at atmospheric pressure which has been described in detail elsewhere [11]. The exhaust gases were analysed using a quadrupole mass spectrometer (QMS) or, for temperature-programmed reduction (TPR), by using a thermal conductivity detector (TCD). The TCD was calibrated by reduction of a known mass of CuO to Cu. Samples were subjected to a consecutive chemical reaction sequence as described below at a standard flow rate 60 ml min−1 . TPR was first carried out (5% H2 /Ar, 10 ◦ C min−1 ) from ambient temperature to 1000 ◦ C. The reactor was then cooled, purged with argon and temperature-programmed reaction (TPRx) performed (5% CH4 /Ar, 25 ◦ C min−1 ). After cooling to room temperature and purging the system, temperature-programmed oxidation (TPO) was used to check for deposited carbon (10% O2 /He, 10 ◦ C min−1 ). The CO2 signal of the QMS was calibrated by injecting 4.7 ml of pure CO2 into the gas stream from an injection loop. After TPO the reactor was cooled rapidly to room temperature and purged with argon. The sample was then heated to 900 ◦ C at 25 ◦ C min−1 in 5% CH4 /Ar followed by exposure at constant temperature (900 ◦ C) for a period of 150 min. The CO signal of the QMS was calibrated by injecting 4.7 ml CO into the gas stream. Finally, after cooling and purging, TPO was again carried out to characterise carbon deposition. The TPR experiment was not performed for the specimens 1.4% Nb-doped CeO2 FG and Nb2 O5 . Spent catalysts, selected at various stages, were analysed by XPS.
195
3. Results 3.1. Physical characterisation XRPD patterns for the undoped and Nb-doped cerias after calcination for 1 h at 1000 ◦ C in either air or forming gas are shown in Fig. 1a and b, respectively. The data confirm ceria as the major crystalline phase, but there is clear evidence that fergusonite (CeNbO4 ) was present as a minor second phase [9] in some of the Nb-doped specimens. The four peaks seen for the 20% Nb-doped FG specimen in Fig. 1b that are not from the fluorite lattice can all be indexed as coming from fergusonite. Fergusonite was not detected in the 1.4% Nb cerias irrespective of the calcination atmosphere. Calcining Nb2 O5 under reducing conditions greatly reduced crystallinity (compare Fig. 1a and b). The Gd-doped ceria gave only the XRD of ceria. Selected area diffraction of a 5% Nb-doped ceria FG specimen in TEM showed only ceria and no crystallites of fergusonite in spite of the XRD evidence. Fig. 2 shows a TEM image of the 5% Nb-doped CeO2 sample calcined in forming gas in which the 10 nm crystallites are clearly visible. The crystallite size was estimated to be approximately 10 nm from XRD line broadening. In contrast, TEM examination showed that the crystallite size in the undoped ceria was approximately 100 nm. No evidence of carbon deposition could be found by TEM examination of these materials after exposure to methane at high temperature. The materials prepared by calcination in forming gas were found to have higher surface areas. The influence of Nb doping level on the surface area of the materials prepared in forming gas atmospheres is presented in Fig. 3. The surface area reaches a maximum at approximately 1–5 cation% Nb. Surface areas for 5% Nb-doped ceria proved to be rather variable. The surface areas of the catalysts are summarised in Table 1 from which it can be seen that the high surface area of the 5% Nb-doped CeO2 specimen calcined in forming gas is consistent with its small crystallite size (the equivalent sphere size calculated from the surface area is approximately 30 nm for this specimen). 3.2. XPS Fig. 4a and b present the changes in the Ce 3d spectra before and after isothermal exposure to CH4 ,
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Fig. 1. XRPD diffractograms for catalysts calcined for 1 h at 1000 ◦ C; (a) in air and (b) in forming gas.
Fig. 2. Transmission electron micrograph of 5% Nb-doped CeO2 FG catalyst before exposure to methane.
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Fig. 3. Specific surface area of Nb-doped ceria calcined at 1000 ◦ C for 1 h in forming gas (FG) as a function of cation fraction of Nb.
respectively (following the above sequence, or independent treatment). The six-line spectrum of undoped CeO2 (Fig. 4a) is characteristic of the Ce4+ core level [12] with the lines denoted by a, a1 , a2 and b, b1 , b2 for the 3d5/2 and 3d3/2 electrons, respectively. The spectrum from CeF3 is included for comparison and demonstrates that detecting minor proportions of Ce3+ mixed with the Ce4+ is difficult because the main characteristic of the Ce3+ spectrum is the absence of the b2 peak. The addition of gadolinium or niobium broadens the lines; even with only a small amount of niobium (1.4%). After isothermal exposure to CH4 (Fig. 4b) the Ce lines tend to become sharper (ex-
Table 1 BET specific surface area (SSA) and turnover frequencies (N) for the reaction with methane at 900 ◦ C for different catalysts Specimen
Specific surface area (m2 g−1 )
CeO2 2.6 Ce0.9 Gd0.1 O1.95 a 7.1 Ce0.986 Nb0.014 O2.007 3.7 Ce0.986 Nb0.014 O2.007 25.3 (FG) 1.1 Ce0.95 Nb0.05 O2.025 Ce0.95 Nb0.05 O2.025 (FG) 28.6 1.4 Nb2 O5 a
Specimen issued from Rhodia.
N (molecules per site s−1 × 10−3 ) dry CH4 30.0 20.0 5.5 8.0 16.7 2.2 6.8
cept for the 1.4 and 5% Nb-doped CeO2 specimens calcined in forming gas) indicating a more uniform chemical environment. These observations are similar to those previously published for nano-crystalline Cex La1−x O2−y [13]. Fig. 5a and b show the Nb 3d emission spectra for the same samples before and after isothermal exposure to CH4 treatments, respectively. The Nb 3d emission spectrum for Nb2 O5 (prepared by precipitation and calcination in air) was taken as a reference. It shows a peak and a shoulder corresponding to the spin doublet 3d5/2 and 3d3/2 with binding energies of 208.4 and 210.0, respectively, which are in agreement with literature data [14]. As with the cerium spectra, the lines are sharper after exposure to methane (again except for the specimens calcined in forming gas) and the doublet more clearly resolved. This might also be enhanced by reduction of some Nb5+ to Nb4+ , which is reported to cause a slight increase in separation [14]. In Fig. 5b is also included the spectrum of Ce 4p from the undoped CeO2 specimen after exposure to CH4 . This shows that there is significant overlap of the Ce 4p and Nb 3d peaks, which complicates detailed interpretation. However, several trends in variation in the Nb spectra are clear. Firstly, there is no evidence for Nb in the surface of the 1.4% Nb FG sample. Secondly, that all the Nb-doped cerias show that the surface concentration of Nb decreases on exposure to methane at 900 ◦ C.
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Fig. 4. Ce 3d XPS spectra for ceria catalysts (a) as-prepared and with a CF3 reference, and (b) after exposure to methane for 150 min at 900 ◦ C.
The effect is most dramatic for the FG samples— the 1.4% has no visible Nb prior to methane exposure so that no change can be observed. This effect is borne out by the surface Nb/Ce or Gd/Ce atomic ratios are given in Table 2 (measured peak areas were converted to atomic ratios using the atomic sensitivity factors: Ce = 10; Gd = 3; Nb = 2.4 [15]; a correction has been applied for overlap of the Nb 3d and Ce 4p peaks). In general, Nb and Gd are segregated to the surface of the ceria. However, as pointed out above, the 1.4% Nb-doped CeO2 specimen calcined in forming gas showed no segregation of Nb to the surface, which could indicate a higher solubility limit in reducing conditions. The Nb/Ce ratio is significantly reduced on exposure to methane. The binding energies (BE) measured for the different elements are summarised in Table 2. They show little variation with either composition or exposure to methane. The ratio of the area of the b2 line to the total area of the Ce 3d spectrum can be used to assess
how much Ce3+ is present. This ratio should be approximately 14% for all cerium as Ce4+ [16,17]. The measured ratios are presented in Table 2. Values were observed to cover a wide range (8–13%), apparently indicating some Ce3+ content. The results in Table 2 indicate that this ratio is lower in the Nb-doped specimens, which suggests a possible complication due to electronic interactions between the Nb and Ce generating Ce3+ , bearing in mind that the spectrum of fergusonite is unknown. The values for the ratio of the b2 peak to total area show no obvious trend after exposure to methane. Thus, any changes in surface oxidation states of Ce were probably below the relatively insensitive, detection limit, and may be in any case affected by the transfer to the spectrometer, which was done under air. The one exception is the 5% Nb FG sample, which perhaps shows a significant reduction in the ratio on exposure to methane, consistent with the observed strong dissolution of the Nb in the ceria under reaction conditions.
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199
Fig. 5. Nb 3d XPS spectra for ceria catalysts (a) as-prepared and with a Nb2 O5 reference, and (b) after exposure to methane for 150 min at 900 ◦ C. Table 2 XPS binding energy (eV), surface atomic ratios and Ce reduction parameter for ceria catalysts as prepared and after isothermal exposure to methane Catalyst
Ce 3d5/2
Ce 4d5/2
CeO2 As prepared After methane
882.3 882.7
107.8 108.8
– –
Ce0.9 Gd0.1 O1.95 As prepared After methane
882.4 882.2
107.9 108.6
– –
Ce0.986 Nb0.014 O2.007 As prepared After methane
882.2 883.0
109.1 109.6
207.4 207.7
– –
Ce0.986 Nb0.014 O2.007 (FG) As prepared 882.4 After methane 881.9
110.3 109.9
– –
– –
Ce0.95 Nb0.05 O2.025 As prepared After methane
882.3 883.1
110.5 109.3
208.0 207.4
– –
0.54 0.24
9 10
Ce0.95 Nb0.05 O2.025 (FG) As prepared 882.3 After methane 882.3
109.7 109.5
207.1 –
– –
0.48 ∼0
10 8
Nb 3d5/2
Gd
Nb/Ce or Gd/Ce – –
1185.8 1186.2
Ce b2 /total ratio (±1%)
– –
12 13
0.48 0.33
12 12
0.41 0.21
8 9
∼0 ∼0
11 10
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Fig. 6. Temperature-programmed reduction of ceria catalysts in 5% H2 –95% Ar. The baselines have been shifted for clarity.
3.3. Chemical reactions The TPR profiles (Fig. 6) typically exhibit more than one reduction peak [18]. The lower temperature minor peak (at approximately 550 ◦ C) is believed to correspond to reduction of Ce4+ to Ce3+ on the ceria surface and the higher temperature peak (at approximately 850 ◦ C) is due to creation of bulk oxygen vacancies by similar reduction of bulk Ce4+ : ¨ O + 2e OxO = 21 O2 + V
(7)
The relatively large low temperature reduction peak for the 5% Nb-doped CeO2 (FG) specimen is consistent with its high surface area, but the precise interpretation is probably more complex. TPRx in methane/argon to 1000 ◦ C gave similar results for all the specimens. (The TPRx results are similar to the results obtained during heating to the isothermal reaction temperature, which are shown in the first 35 min period of Figs. 7 and 8). Reaction commenced at approximately 750 ◦ C and methane was oxidised by the ceria to CO and H2 in accordance
Fig. 7. Mass spectrometer signal trends during TPRx/isothermal reaction between Gd-doped ceria and 5% CH4 –95% Ar. During the first 35 min the specimen was heated to 900 ◦ C at 25 ◦ C min−1 as indicated by the temperature profile shown as a broken line (data have not been corrected for MS sensitivities).
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Fig. 8. Comparison of CO signal trends for different ceria specimens during TPRx/isothermal reaction with 5% CH4 –95% Ar. During the first 35 min the specimen was heated to 900 ◦ C at 25 ◦ C min−1 as indicated by the temperature profile (broken line). The baselines have been shifted for clarity.
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with Eq. (1). Only small quantities of CO2 and H2 O were produced at 750–950 ◦ C. There was no carbon deposition during TPRx detectable by subsequent temperature-programmed oxidation. The lower limit of carbon dioxide that could be detected by TPO was estimated to be 1 × 10−6 mol g−1 . Methane oxidation and C formation were investigated during TPRx followed by isothermal reaction at 900 ◦ C. The results for Gd-doped ceria are given in Fig. 7. The maximum observed methane conversion rate was 9 × 10−5 mol min−1 g−1 and the H2 /CO ratio was 2.14, compared with the expected value of 2 from reaction (1). The yields of H2 and CO together with the H2 /CO ratio for all the samples are given in Table 3. Overall, the H2 /CO ratio tends to be greater than 2, but the measurements for the Nb-doped samples are subject to larger errors because of their slower reaction rates. CO production during isothermal reaction with methane at 900 ◦ C tends to show two maxima (Fig. 8), but there are large differences between the cerias. In particular, reaction is significantly slower in the low surface area specimens containing 1.4 and 5% niobium, and oxygen is still being removed from these specimens even after 150 min of exposure. The total quantity of CO produced for each specimen is given in Table 3 together with the molar change in oxygen content of the ceria (n in Eq. (1)). Similar experiments were also carried out with a pure Nb2 O5 specimen and the results are illustrated in Fig. 8 and also summarised in Table 3. They indicate that niobia does oxidise methane, but at a lower intrinsic rate than does ceria.
Table 3 Total production of H2 and CO and change in stoichiometry during TPRx and isothermal reaction with methane at 900 ◦ C; also given is the amount of CO2 produced during subsequent TPOa Specimen
CeO2 Ce0.9 Gd0.1 O1.95 Ce0.986 Nb0.014 O2.007 Ce0.986 Nb0.014 O2.007 (FG) Ce0.95 Nb0.05 O2.025 Ce0.95 Nb0.05 O2.025 (FG) Nb2 O5 a
TPR H2 /Ar
TPRx + isothermal reaction with methane
Change in stoichimetry, n
H2 yield (mol g−1 × 10−3 )
CO yield (mol g−1 × 10−3 )
H2 /CO ratio
Change in stoichiometry, n
CO2 yield (mol g−1 × 10−3 )
CO2 yield (mol m2 × 10−5 )
0.2 0.24 0.27 – 0.21 0.19 –
4.60 5.33 2.63 4.70 4.43 2.90 1.01
2.16 2.49 1.66 2.50 2.15 1.26 0.5
2.13 2.14 1.58 1.90 2.06 2.30 2.01
0.37 0.40 0.28 0.43 0.37 0.22 0.13
0.04 0.18 0.21 0.06 0.16 0.13 0.008
1.39 2.56 5.63 0.24 14.1 0.46 0.56
Moles of CO/moles of cations.
TPO
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Fig. 9. Comparison of CO2 signals for different ceria specimens during TPO after isothermal reaction with 5% CH4 –95% Ar at 900 ◦ C. The baselines have been shifted for clarity.
There was no detectable hydrogen in the gas phase from methane cracking (Eq. (6)) even after removal of oxygen from the ceria was complete (Fig. 7). This was also true for the doped cerias. However, subsequent TPO experiments (Fig. 9) revealed that some carbon deposition had occurred and so we assume that the H2 production rate was below the limit of detection in our experimental arrangement. The total quantities of CO produced, and the amount of C deduced from the CO2 evolved in TPO, per unit area of ceria are given
in Table 3. When the deposited carbon is taken into account, the ratio H2 /(CO+C) is 2.09 for the undoped ceria and 2.00 for the Gd-doped ceria; the samples for which the data are most accurate. The gaseous oxygen depletion during TPO for the Nb2 O5 (Fig. 10) specimen shows two peaks, while the CO2 signal presents a shoulder and a peak, which appear at higher temperature than that of the O2 signal depletion. Thus, the oxygen depletion in this case is not associated with carbon consumption and could be indicating re-oxidation of
Fig. 10. Temperature-programmed oxidation for Nb2 O5 in 10% O2 –90% He after isothermal reaction with 5% CH4 –95% Ar at 900 ◦ C.
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the partially reduced oxide. However, this should be even more evident for the ceria specimens (which have larger values of n) and so cannot be accounted for at present. The specimen 1.4% Nb-doped CeO2 calcined in forming gas showed two TPO peaks, which indicates two forms of deposited carbon. 4. Discussion The doping of ceria by Gd3+ is known to generate oxygen vacancies: ¨ O + 3Ox Gd2 O3 = 2Gd Ce + V (8) O whereas doping with Nb5+ fills oxygen vacancies and/or creates electronic carriers: ¨ O = 2Nb˙ Ce + 5Ox Nb2 O5 + V (9) O Nb2 O5 = 2Nb˙ Ce + 4OxO + 21 O2 + 2e
(10)
Electronic carriers and oxygen vacancies are also favoured under reducing conditions from Eq. (7). The oxygen thermodynamic activity is not known exactly in the present experiments since the water vapour level was not measured. A computer program, HSC Chemistry, was applied to calculate the oxygen activity, using its in-built thermodynamic database. Assuming a water vapour partial pressure of 10−4 atm then at 900 ◦ C PO2 is estimated to have been approximately 2 × 10−20 atm in the TPR experiments (5% H2 /100 ppm H2 O) and 7×10−26 atm in the isothermal reaction experiments (5% CH4 /100 ppm H2 O). Under these strongly reducing conditions, reduction of Ce4+ (Eq. (7)) is known to dominate, even in doped ceria, and all the materials have an oxygen deficit (with respect to their nominal composition in equilibrium with air) of approximately 0.15 at PO2 = 10−19 atm and 900 ◦ C [19]. The values of n observed after reaction with methane are larger than observed after TPR in hydrogen (Table 3). This is consistent with the lower oxygen activity in reaction with methane. Furthermore, the phase boundaries Nb2 O5 /Nb2 O4 and Nb2 O4 /NbO are at oxygen pressures of approximately 10−20 and 10−24 atm at 900 ◦ C and so we would anticipate that some of the Nb5+ might also be reduced. However, the XPS examination of specimens after exposure to methane shows that the extent of any such reduction is small, or was reversed by oxidation during transfer from the reactor to the XPS chamber.
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Application of the law of mass action to Eq. (10) leads to the prediction that the solubility limit of −1/8 Nb2 O5 in ceria should be proportional to PO2 and should increase under reducing conditions. Hence the Nb-doped specimens prepared under forming gas atmospheres are expected to have more Nb in solid solution than those prepared in air. The Nb/Ce ratios determined by XPS are consistent with this—the 1.4% Nb sample showing no evidence of Nb segregation. This might be related also to the observation that the specimens calcined in forming gas have much greater specific surface areas than those calcined in air. The isothermal reaction results for CO production and change in oxygen stoichiometry (Table 3) confirm that oxygen loss, n, was not strongly dependent on doping. The quantities are lower on average for the Nb-doped samples, except for the specimen 1.4% Nb-doped CeO2 (FG) which had all the Nb in solid solution after calcination and a much greater surface area. The air-calcined, Nb-doped specimens reacted significantly more slowly than the undoped or Gd-doped specimens and reaction was incomplete even after 180 min (Fig. 8). The measured value of the oxygen diffusion coefficient in ceria [20] is sufficiently high that in the powder specimens used here the overall reaction rate will be controlled by the surface reaction and not by diffusion of oxygen from the bulk of the solid particles to their surfaces. Therefore, turnover frequencies were estimated for the rate of reaction with methane at 900 ◦ C on the basis that the reaction was surface controlled and that all surface sites accessible by nitrogen adsorption (area per molecule 16.2 × 10−20 m2 [21]) were active. For Gd-doped ceria and for the specimen 1.4% Nb-doped CeO2 calcined in forming gas the reaction rate was estimated from the maximum depletion in the methane signal. However, this was not sufficiently sensitive for the less reactive Nb-doped specimens and in their case the reaction rate was calculated from the maximum in the CO signal. The same procedure using the CO signal was also applied for the undoped ceria. The estimated turnover frequencies are given in Table 1. The inhibiting effect of Nb is evident in the air-calcined specimens, which show a high degree of Nb segregation, consistent with the low value determined for Nb2 O5 (the value of the turnover frequency obtained for Nb2 O5 is very uncertain because of its low surface area and low reaction rate). The catalysts
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prepared by calcination in forming gas, where Nb is in solid solution under reaction conditions, show a strong decrease in activity as the Nb concentration increases. This could be associated with the apparent increase in Ce3+ in the 5% Nb case noted above. The turnover frequencies for all the oxides are very much lower than those estimated for Ni/MgO steam reforming catalysts under similar conditions. The latter have been reported for the temperature range 500–600 ◦ C [22] and were extrapolated by fitting to the Arrhenius equation: −103(kJ mol−1 ) −6 −1 N = 9.2 × 10 (s ) exp (11) RT to give an estimated turnover frequency of approximately 240 molecules per site s−1 at 900 ◦ C. The turnover frequency can also be compared with the measured rate of isotopic exchange between ceria and oxygen-containing gases. The isotopic surface exchange rate constant k is related to the surface site turnover frequency by: NA N =k As (12) Vm where NA is Avogadro’s number, Vm is the molar volume of the solid and As is the area of a surface site. The measured values of k at 900 ◦ C are [23] 3 × 10−5 cm s−1 for reduced ceria in CO/CO2 and 1 × 10−5 cm s−1 for Ce0.8 Y0.2 O1.9 in oxygen. The corresponding turnover frequencies are 840 and 280
atoms of oxygen per site s−1 . These are several orders of magnitude greater than the turnover frequencies for methane reaction given in Table 1, which indicates that reaction with methane is much slower than with oxygen, CO or CO2 . This difference in rate of reaction is also reflected in the activation energies. We estimated the activation energy for methane oxidation from the TPRx experiments by assuming that the reaction rate is proportional to the hydrogen signal intensity. This is plotted in Arrhenius form in Fig. 11 for Gd-doped ceria and the corresponding activation energy is 176 ± 10 kJ mol−1 . A similar analysis of the CO signal gave 155 ± 10 kJ mol−1 . The average value taken from the H2 and CO signals results in 165 ± 10 kJ mol−1 and is in good agreement with the value 160 kJ mol−1 reported by Otsuka et al. [4] for undoped ceria and is significantly greater than reported [23] for isotopic exchange with oxygen (140 kJ mol−1 ) or CO/CO2 (120 kJ mol−1 ). The slower reaction rate on the surfaces of the Nb-doped powders is therefore due to their different composition. The XPS analyses show that in the air-calcined Nb-doped specimens approximately one-third of the surface cation sites are occupied by Nb. Since niobia reacts more slowly with methane than does ceria under these conditions it is concluded that the segregation of Nb to the ceria surface is inhibiting reaction with methane. In contrast, in the Nb-doped FG specimens, which show no detectable Nb segregation, the lower rate would appear to be
Fig. 11. Arrhenius plot of the hydrogen signal intensity (assumed to be proportional to the reaction rate) for TPRx of Gd-doped ceria with methane.
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mainly due to the effect of bulk doping (e.g. by reducing the concentration of oxygen vacancies according to Eq. (9)). The XPS results also indicate surface enrichment of Gd to similar levels as Nb. However, the Gd appears to have a much smaller (if any) inhibiting effect on methane oxidation than does Nb. The products of reaction with methane confirm the dominance of Eq. (1) until the ceria is exhausted of reducible oxygen. Carbon deposition by methane cracking (Eq. (6)) occurs only very slowly since hydrogen production was not detected after oxygen exhaustion. On an equivalent surface area basis, the 1.4 and 5% Nb-doped CeO2 (FG) specimens have the lowest carbon deposition (Table 3). Using a value of 0.026 nm2 for the area occupied by a carbon atom in a surface monolayer of the basal plane in graphite, corresponding to 6.4 × 10−5 mol m−2 , the quantities of carbon deposited range from approximately 0.04 monolayers on the 1.4% Nb-doped CeO2 (FG), (0.07 monolayers on the 5% Nb (FG) specimen) to 0.9 monolayers on the 1.4% Nb specimen. These low values explain why deposited carbon was not detectable either by XPS or TEM. The temperature at which deposited carbon is combusted during TPO is lowest for the Nb-doped specimens and highest for the Gd-doped specimens, although it is even higher for the Nb2 O5 sample. This probably reflects the greater electronic carrier concentration in the Nb-doped cerias under oxidising conditions, which are available for ionising adsorbed oxygen species that subsequently react with the carbon. Methane reaction and carbon deposition over the cerias are very different from the behaviour reported previously for a perovskite La0.8 Ca0.2 CrO3 , another potential SOFC anode material, by Baker and Metcalfe [24]. They found that over the chromite, initial TPRx of methane produced mainly CO2 and H2 O (at up to approximately 600 ◦ C) followed by Eq. (1), which reached a maximum rate at approximately 850 ◦ C. The slower rate at higher temperatures is in marked contrast to the cerias and probably reflects the easier supply of bulk oxygen vacancies in the cerias. However, the most striking difference is in relation to carbon deposition and combustion. Although the chromite is more resistant to carbon deposition than a Ni-zirconia cermet [25] the cerias are more resistant than the chromite. Furthermore, the temperature at
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which deposited carbon is combusted is lower for the Nb-doped cerias (approximately 230 ◦ C) than for the chromite (approximately 400 ◦ C).
5. Conclusions Doped and undoped cerias exhibit significant reactivity with methane to produce syngas. Gd-doping has little effect on the reaction rate, but Nb-doping slows down the rate considerably. XPS has shown that there is strong segregation of Nb to the ceria surface. Calcination of Nb-doped ceria under reducing conditions leads to significantly lower crystallite size, higher specific surface area and greater Nb solubility. The reaction rate is controlled by surface reaction with methane and has an activation energy of 165 ± 10 kJ mol−1 . Carbon deposition by methane cracking at 900 ◦ C is extremely slow on all cerias. The propensity for carbon deposition is much lower than for Ni-based catalysts and even lower than for Ca-doped lanthanum chromite. Carbon deposition is least on high surface area Nb-doped ceria. The Nb-doped ceria is the most effective for removing deposited carbon in subsequent oxidation, probably because of its higher electronic conductivity under oxidising conditions.
Acknowledgements The authors are grateful for financial support from the UK Engineering and Physical Sciences Research Council under Grant GR/L 81970. They are also indebted to Dr. K. Senkiw for his assistance in the X-ray photoelectron spectroscopy analysis. E. Ram´ırez-Cabrera wishes to thank the Consejo Nacional de Ciencia y Tecnolog´ıa (CONACyT), Mexico, for the award of a study scholarship. References [1] K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517. [2] K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493. [3] K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193. [4] K. Otsuka, E. Sunada, T. Ushiyama, I. Yamanaka, in: M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholtz, M.S. Scurrell (Eds.), Proceedings of the 4th International Natural
206
[5] [6] [7]
[8] [9] [10] [11] [12] [13] [14]
E. Ram´ırez-Cabrera et al. / Applied Catalysis B: Environmental 36 (2002) 193–206 Gas Conversion Symposium, Kruger Park, South Africa, Studies in Surface Science and Catalysis, Vol. 107, Elsevier, Amsterdam, 1997, p. 531. B.T. Kilbourn, Cerium: A Guide to its Role in Chemical Technology, Molycorp Inc., White Plains, USA, 1992. B.C.H. Steele, P.H. Middleton, R.A. Rudkin, Solid State Ionics 28–30 (1990) 388. O.A. Marina, C. Bagger, S. Primdahl, M. Mogensen, in: P. Stevens (Ed.), Proceedings of 3rd European SOFC Forum (Nantes), U. Bossell, Oberrohrdorf, Switzerland, 1998, p. 427. E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. I.K. Naik, T.Y. Tien, J. Electrochem. Soc. 126 (1979) 562. M.R. De Guire, M.J. Shingler, E. Dincer, Solid State Ionics 52 (1992) 155. D. Chadwick, P.J.R. O’Malley, J. Chem. Soc., Faraday Trans. 83 (1987) 2227. F. Le Normand, J. El Fallah, L. Hilaire, P. Légaré, A. Kotani, J.C. Parlebas, Solid State Commun. 71 (1989) 885. M.L. Trudeau, A. Tschöpe, J.Y. Ying, Surf. Interface Anal. 23 (1995) 219–226. M.K. Bahi, J. Phys. Chem. Solids. 36 (1975) 485.
[15] D. Briggs, M.P. Seah, Practical surface analysis, in: Auger and X-ray Photoelectron Spectroscopy, 2nd Edition, Vol. 1, Wiley, New York, 1990. [16] P.J. Schmitz, R.K. Usmen, C.R. Peters, G.W. Graham, R.W. McCabe, Appl. Surf. Sci. 72 (1993) 181–187. [17] J.Z. Shyu, W.H. Weber, H.S. Gandhi, J. Phys. Chem. 92 (1988) 4964–4970. [18] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [19] B.C.H. Steele, K.M. Hori, S. Uchino, Solid State Ionics 135 (2000) 445–450. [20] B.C.H. Steele, J.M. Floyd, Proc. Br. Ceram. Soc. 19 (1971) 55. [21] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corporation, USA, 1997. [22] J.R. Rostrup-Nielsen, J.H. Bak-Hansen, J. Catal. 144 (1993) 38. [23] J.M. Floyd, Indian Journal of Technology 11 (1973) 589. [24] R.T. Baker, I.S. Metcalfe, Appl. Catal. A: Gen. 126 (1995) 297. [25] R.T. Baker, I.S. Metcalfe, Ind. Eng. Chem. Res. 34 (1995) 1558.