Modified Ceria-Zirconia Fluorite-Like Catalysts for the Combustion of Methane

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Journal of Natural Gas Chemistry 15(2006)149-163 SCIENCE PRESS

Article

Modified Ceria- Zirconia Fluor it e-Like Catalysts for the Combustion of Methane

Tatiana Kuznetsova* , Vladislav Sadykov, Lubsan Batuev, Ella Moroz, Elena Burgina, Vladimir Rogov, Vladimir Kriventsov, Dmitrii Kochubey

Boreskov Institute of Catalysis SB RAS, Pr. Lavrentieva, 5, Novosibirsk, 630090, Russia

[Manuscript received March 14, 2006; revised April 24, 20061

Abstract: For dispersed ceria-airconia-based solid solutions prepared via the polymerized complex method and annealed at 700 "C, effects of bulk doping by Ca, Mn, Co, Bi or Nb cations and surface modification by Mn and Pt on their structural features, surface/bulk oxygen reactivity and catalytic activity in methane combustion are considered. With up to 20 mol% doping, a structural type of homogeneous solid solutions of anion-deficient fluorite with disordered anion vacancies is formed. Doping by transition metal cations or Pt increases the mobility and reactivity of the surface/bulk oxygen. A broad variation in specific rates of methane combustion for the studied systems was observed, suggesting structural sensitivity of this reaction. In general, there is no universal relationship between the oxygen mobility, the reactivity and the catalytic activity in methane combustion, which is explained by the factor of specific methane activation on surface active sites. For the Pt-promoted samples, Pt efficiency in methane activation depends on the Pt-support interaction, and the most favorable ones being mixed Pt/MnO, and Pt/NbO, clusters on the surface of the supports that exhibit high lattice oxygen mobilities. Key words: Ce-Zr-0; Ca; Mn; Co; Bi; Nb; structural features; oxygen reactivity; oxygen mobility; methane combustion

1. Introduction

Total oxidation processes are widely used for VOC abatements in industry, and combustion of natural gas in boilers, turbines, heaters etc [l]. Catalytic combustion allows of decreasing the NO, emissions as compared with thermal combustion due to the lower oper- ating temperatures and the ability to sustain combustion with ultra-lean fuel mixtures. A lot of catalytic systems have been tested and reviewed for methane combustion [2-91. They include noble metal catalysts supported on different refractory oxides, bulk and supported transition metal oxides, complex oxides (perovskites, doped fluorites, hexaaluminates etc.). Among them the fluorite-like systems based upon ceria and zirconia and modified by transition metals

are rather attractive as catalysts for methane combustion, being claimed to have a comparable activity with alumina-supported noble metals [3-91. In those studies, Mn, Co and Cu (+Ag) are considered as efficient dopants for zirconia and ceria to enhance their activities in methane combustion. This is assigned to the increase of the lattice oxygen mobility and the reactivity due to doping.

Ceria-zirconia-based systems are known for their high oxygen mobility and storage capacity, which ex- ceed those of pure ceria and zirconia oxides. This makes them attractive as components of catalysts for different redox processes, in particular for three-way car exhaust clean-up, or for partial methane oxidation into syngas or hydrogen by the lattice oxygen [lo-131. Defects generated due to incorporation of

* Corresponding author. Tel: +7 383 3308764; Fax: +7 383 3308056; E-mail: tgkuznQcatalysis.nsk.su

150 Tatiana Kuznetsova et al./ Journal of Natural Gas Chemistry Vol. 15 No. 3 2006

smaller Zr4+ cations (ionic radius 0.84 A) into the fluorite lattice of CeOz (Ce4+ ionic radius 0.97 A) can increase the oxygen reactivity and mobility in the lattice [10,12,14]. Though even pure ceria-zirconia solid solutions are active by themselves in the redox reac- tions, promotion by transition metal cations or noble metals can sharply enhance their activities in CO and hydrocarbons oxidation and, in particular, in methane combustion [4,5.15,16]. Addition of transition metals cations (hlri, Cu or Co) strongly affects the redox be- haviors of the catalysts, facilitating a low-temperature reduction of Ce4+ [16.17]. 111 the case of supported noble metals, methane oxidation is considered to pro- ceed at the ceria-zirconialmetal interface via the interaction of dissociated CH, species with the lattice oxygen of the supports [4]. According to the theoret- ical analysis by Frost [18], electron transfer between the metal and the oxide decreases the activation bar- ricr for oxygen vacancy formation in the vicinity of the supported metal clusters. This facilitates the surface oxygen mobility which could be of importance as well for the activity increase [4]. However, as was shown in our previous paper [19], for Pt-supported Ce-Zr-La-0 solid solutions, the enhanced lattice oxygen mobility was associated with a lower activity in methane combustion due to the stabilization of the oxidized Pt6+ species which is less efficicnt for methane activation. Hence, even though in the majority of publications, in which only the role of the oxygen mobility in methane combustion activity is considered, other factors such as specific methane activation on surface sites and the related structural sensitivity of this reaction have to be considered as well.

The present work was undertaken to clucidate the effect of the real structure and the chemical com-

position of ceria-zirconia based systems on t'he 1110- bility and reactivity of t,he surface/bulk oxygen, i ts

well as thcir specificity in methane activation. Basi- cally two fluorite-like systems, namely, CeZr (I) and CeZrCa (11) were selected. To control the lattice oxygen mobility and reactivity, t,hese systems were doped by transition metal cations differing by their charges, sizes and redox properties. Thus, system I was doped by Mn or Nb, while system I1 was doped by Mn: Co, and Bi. For system 11, along with bulk doping by hln cations, surface modification by supported Mn cations was carried out as well, while for all samples the effects of surface modification by P t (0.2 wt%) were considered. The struct'ural features of the sa111- ples were studied by XRD, IR, EXAFS, UV-Vis spectroscopy and were compared for oxygen mobility, reactivity (estimated by 0 2 TPD, H2 TPR and CH4 TPR) and catalytic activity in methane combustion. To obtain highly dispersed single phase samples of Ce- Zr-0-based solid solutions with zirconia contents up to 50 mol%, they were prepared by the polymerized complex method (PCM) [11-13].

2. Experimental

Following an earlier described procedure [ 131. doped fluorites were prepared by PCM mixing of all components as water-soluble salts (nitrates of Ce (111), Ca, Mn (11). Co (11), Bi, zirconium oxychloride and solution of Nb hydroxide in thc oxalic acid), eth- ylene glycol and citric acid, followed by heating at 100-150 "C. with final air calcination at 700 "C for 2 h. Chemical compositions and designations of the fluorites sample are given in Table 1. The sample of 5wt% MnlCeZrCa was prepared by incipient wetness

Table 1. Lattice parameter (LP)/particle size (PS) of CeZr based solid solutionsa and amounts of removed oxygen during Hz TPR runs

Amounts of removed oxygen x104 (mol & / g )

Chemical composition Designation ( L p ) b l p s (4 Ceo.5 Zro.5 CeZr 5.280/45 7.9

Cen.sZro.4Cao.1 CeZrCal 5.314 (5.297)/70 -

Ce0.6Zro.zCa0.z CeZrCa2 5.358 (5.354)/85 8.7 Ceo.sZro.zCao.iMno.2 CeZr CaMn 5.350/95 12.3

Ce0.5Zro.zCao.iBio.z CeZrCaBi 5.351/95 1 5 . 3 5 wt.% Mn/Ceo.GZro.zCao.z Mn/CeZrCa 5.358/95 11.5

Ceo.sZro.zCao.1 Coaz CeZrCaCo 5.346/85 17.6

Ceo.3Zro.3 Mn0.s CeZrMn 5.259/80 14.5 Ceo.sZro.-?Nbo. 1 CeZrNb 5.309/60 11.7

a For CeZrCaMn-traces of Mn,OY oxide; for CeZrCaCo-traces of Co304 with particle size 100 A; for CeZrCaBi- traces of Bi407; for CeZrMn-Mn304 with particles size 140 A; for CeZrNGtraces of NbzOj and N b 0 2 ;

for CeZrCa fluorites lattice parameter of the binary composition Ce-Zr-0 are shown in parenthesis

Journal of Natural Gas Chemistry Vol. 15 No. 3 2006 151

impregnation of a Ceo.sZro.zCao.2 support with a Mn (2+) nitrate solution, followed by calcinations at 600 "C for 4 h. Pt (0.2 wt%) was supported by incipient wetness impregnation from aqueous solutions of H2PtCl6, followed by drying and calcination at 700 "C for 1 h.

X-ray phase analysis of the samples was carried out using a HZG-4C diffractometer (Cu K , radiation and a flat monochromator) in the range of 26, angles equal to 1-70°. The unit cell parameters of the modified fluorites were determined from the position of the (311) diffraction peak.

IR spectra of the samples in the 220-4000 cm-I range were registered using a BOMEM M 102 spectrometer.

EXAFS spectra of the Ce-Ls and Zr-K edges were obtained at the EXAFS Station of Siberian Syn- chrotron Radiation Center. The storage ring VEPP-3 with the electron beam energy of 2 GeV and the average stored current of 90 mA were used as the source of radiation. The X-ray energy was monitored with a channel cut Si (111) monochromator. All the spectra were recorded under the transmission mode using two ionization chambers as detectors. Harmonic re- jection was performed by using a gold mirror. The EXAFS spectra were treated using the standard pro- cedures [20]. The radial distribution function (RDF) was calculated from the EXAFS spectra in k3x(lc) as modulus of Fourier transform at wave number inter- vals of 3.5-10.4 k1 for Ce-LS and 3.0 to 15.0 k1 for Zr L3 spectra edges, respectively. Curve fitting procedure with EXCURV92 code [21] was employed to determine the distances and coordination numbers. It was realized for k 3 x ( k ) in similar wave number in- tervals after preliminary Fourier filtering using the known XRD data for the bulk compounds.

UV-Vis spectra were recorded using a Shimadzu 8300 spectrometer equipped with a diffuse scatter- ing DRS 8000 cell. The spectra were recorded in the range of 10000-60000 cm-' with 4 cm-l resolution, with number of scans equal to 50.

For samples pretreated in 0 2 at 500 "C, 0 2 desorption into the He stream (flow rate 2.4 L/h), reduction by Hz (10% H2 in Ar, flow rate 10 L/h), reduction by CH4 (1% CH4 in He, flow rate 10 L/h) and CH4 oxidation (1% CH4 and 3% 0 2 in He, 0.1 g catalysts, flow rate 10 L/h) were studied in a temperature- programmed mode with a heating rate of 10°/min in the range of 20-900 "C using microcatalytic installa- tions equipped with TCD and IR detectors. The de-

gree of sample reduction was expressed in monolayers (1 m~no laye r= lO~~ atom/m2). The rates of CH4 catalytic oxidation were calculated for differential (less than 20%) conversions, and apparent activation energies were estimated from their temperature depen- dencies.

3. Results and discussion

3.1. Phase composition and structural features

3.1.1. XRD

Samples prepared by PCM are highly dispersed and are mainly homogeneous solid solutions based upon the fluorite structure of ceria (Table l) , while traces of simple oxides of Mn, Co, Bi or Nb were detected as well. In the Ce-Zr-0 system prepared by PCM, the lattice parameter decreases linearly with the xircoriia content from 5.411 A for pure ceria to 5.280 A for the sample containing 50 mol% ZrOz (Ta- ble 1) [13]. For the doped fluorites, the effect of the dopant cation radius on the lattice parameter depends on the Ce/Zr ratio. In the binary Ce-Ca-0 system, with the Ca content up to 20 mol%, the lattice parameter increases for about 0.004 A at the addition of every 10 mol% of CaO due to a bigger ionic radius of Ca [22]. For the CeZrCa samples the difference in lattice parameters is smaller for samples with a higher Ce/Zr ratio (Table 1) [13]. For the CeZrCal sample, the lattice parameter is bigger in spite of a smaller Ca content. For the sample with a high content of Zr, it could be explained by the appearance of Ce3+ cations and the pronounced distortion of the fluorite structure, along with anion vacancies formation caused by the addition of Ca. A trace amount (<1%) of Ce3+ cations was detected earlier by the Faraday method for the CeZrCa2 sample [13], which agrees with the UV-Vis data (see below).

CeZrCa samples doped with Mn, Co, or Bi have approximately equal lattice parameters in spite of their differences in the radii of the dopant cations (Table 1). Probably, for complex compositions, rearrangement of the fluorite-like structure masks any effects of the radius of doping cations. For the sample of Mn ( 5 wt%) supported on CeZrCa, MnO, oxides are not detected by XRD due to their high disper- sion on the surface, and the lattice parameter of the support does not change at all.

For the CeZrMe (Me=Mn, Nb) series (Table l), the lattice parameter decreases with Mn addition due

152 Tatiana Kuznetsova et al./ Journal of Natural Gas Chemistry VoJ. 15 No. 3 2006

to the small size of the cation. This proves the formation of a solid solution, and, hence, the generation of anion vacancies to meet the charge imbalance due to the substitution of Ce4+/Zr4+ by Me3+. A weak effect of Mn on the lattice parameter suggests segregation of a part of the Mn cations as microinclusions or even particles of Mn,O, phases (Table 1). For tk CeZrNb samples, the lattice parameter increases despite a smaller (0.69 f i ) ionic radius of the Nb5+ cation. This suggests that the compensation of charge excess in the cation sublattice caused by the incorporation of Nb5+ is achieved by the generation of Ce3+ cations with a larger (1.03 f i ) ionic radius than that of Ce4+ cation (0.92 a). 3.1.2. IR

IR spectra of the doped samples are shown in Fig- ure 1. The IR spectra of the CeZrCaMe and CeZrMe series are similar to those of the Ce-Zr-Ca-0 and Ce- Zr-0 samples without doping, respectively. Only for the two samples of CeZrCaCo and CeZrMn, addi- tional bands of a small intensity appear (Figure 1, spectra 4 and 7), which belong to simple oxides of Co and Mn, respectively. This agrees with the XRD data for these samples (Table 1).

1// .. (1)

I , , , , I , , , / I , / , , I , , , , / , , , , 1 , 1 1 1 1 , , 1 ,

200 300 400 500 600 700 800 900 1000

Wavenumber (cm-' ) Figure 1. IR spectra of doped fluorites: CeZr

(l), CeZrCa2 (2), CeZrCaMn (3), CeZr- CaCo (4), CeZrCaBi ( 5 ) , Mn/CeZrCa (6), CeZrMn (7) and CeZrNb (8)

3.1.3. EXAFS

To analyze in detail the effect of the dopant cations on the local structure of fluorite-like solid solutions based upon ceria or ceri&zirconia, EXAFS has

been applied. Radial distribution function curves describing the local environments of Ce and Zr for the studied fluorites are shown in Figure 2. while respec- tive Me-0 distances and coordination numbers are given in Table 2. For pure ceria prepared by PCM [13], a peak at -2.3 A corresponding to the first Me- 0 distance (Figure 2(a)) is strongly asymmetric. As contrary to reference [23], two Ce-0 distances are revealed (Table 2). Distortions of the oxygen polyhedra are probably caused by residual lattice hydroxyls. Ce- Ce distance is 3.84-3.86 A, and the coordination number (CN) is equal to 8.5, which is smaller than that of the ideal fluorite structure (CN=12) [23]. For the Ce-0 coordination sphere. the asymmetry (difference between two Ce-0 distances) is somewhat increased by the incorporation of Zr cations (Figure 2(a), Table 2), while the declining of the Ce-O peak intensity suggests some disordering of the oxygen sublattice, which is reflected in the decrease of the effective coordination numbers. It could not be excluded that in ceria- zirconia solid solutions. some anion vacancies will appear in the coordination sphere of the Ce cations [14]. Incorporation of Zr strongly decreases the intensity of the Ce-Me peak and also splits it. This phenomenon is due to the overall structure disordering and the appearance of chemically different cations in the cation sublattice. The existence of several (at least two) Ce- Me (Ce, Zr) distances in the Ce-Zr solid solution can be suggested. Note that peaks corresponding to these distances remain within the range of Ce-Ce distances in the initial ceria structure (Figure 2(a)). At bigger distances, some Me-Me peaks corresponding to the fluorite-like structure remain to be seen, though they are decreased in intensity and shifted. This agrees with the XRD data of preserving a cubic fluorite-like structure in the studied CeZr samples. More detailed analysis of these coordination spheres is complicating due to the limited range of wave-vectors (3-10 k') available for the L3-edge spectra of Ce. As was shown by Nagai et al. [23], this is due to the lack of EXAFS signals for heavy elements in the high-k part of absorption spectra. More precise information about the Ce-cation distances can be obtained using measure- ments on the Ce k-edge XAFS spectrum [23] not available at the EXAFS facilities a t disposal. Nevertheless, some conclusions for the modified fluorites could be made. Addition of different cations (Ca, Bi, Mn or Nb) into the CeZr solid solution does not change the radial distribution function curves describing the Ce local arrangements (Figure 2(a)). Ce-0 (Table 2) and Ce-Me distances fall within the same ranges as for


ceria-zirconia fluoritelike structure, which confirm the XRD data on preservation of this type of structure in doped samples. The contribution of a shorter Ce-0 distance becomes bigger with the addition of cations having a large radius like Ca or Bi, which suggests the strengthening of the corresponding bonds. The asymmetry of Ce-0 distances grows with increasing of the Ca content in the samples (CeZrCal and CeZrCa2 samples). Distortions of the fluorite structure are also

0 2 4 6 8 10

Distance (A)

reflected in the variation of the intensities in the set of Ce-Ce distances with dopant addition (Figure 2(a)). These changes are more visible for Bi and Mn dopants incorporated into the bulk of the particles. All these data are in favor of the presence of isolating doping cations in the second (Ce-Me) coordination sphere of the cerium cations, however, a part of the dopant can exist as segregated microinclusions/oxidic clusters of the doping oxide within the fluorite-like matrix.

Zr-K edge

Figure 2. Radial distribution function (RDF) curves describing Ce(a), Zr(b) local arrangements for arrangements of the studied samples: CeZrCa2(1), CeZrCaBi(2), CeZrCaMn(S), Mn/CeZrCaZ(l), CeZrCal(5), CeZrNb(G), CeZr(7), CeOz(8)

Table 2. EXAFS parameters of CeO2 and CeZr based fluorites

Sample Distance Me-0 R, A CN Reference CeO2 Ce-0 2.24 3.0 This work

Ce-0 2.39 5.7

CeO2 Ce-0 2.343 8 ~ 3 1 CeZr ce-0 2.20 2.8 This work

ce-0 2.40 4.7 Z r - 0 2.10 4.1

CeZr Z r - 0 Ce-0

2.26 2.6

2.30 8.0 ~ 3 1 Z r - 0 2.19 6.0

Zr-0 2.37 2.0 CeZrMe (Me=Ca, Mn, Bi, Nb) ce-0 2.20-2.25 2.8-3.4 This work

ce-0 2.38-2.41 3.8-4.2 Zr-0 2.10-2.13 3.3-4.1 Zr-0 2.23-2.28 3.0-4.0

ZrOz (Tetragonal) Zr-0 Zr-0

2.10 4.0 ~ 5 1 2.33 4.0

ZrOz (Stabilized pseudocubic) Zr-0 2.15 7.0 (251 ZrOz (Stabilized pseudocubic) Z r - 0 2.22 8.0 ~ 3 1

154 Tatiana Kuznetsova et al./ Journal of Natural Gas Chemistry VoI. 1.5 No. 3 2006'

For the Zr-0 coordination sphere, at least two distances are observed in the CeZr sample (Figure 2(b), Table 2). As far as variation of the bond length is concerned, the asymmetry of the Zr-0 coordination sphere is comparable to that of the Ce-0 sphere. How- ever! while for the former bigger coordination numbers correspond to a shorter distance, the reverse is tr$e for the latter. For doped samples, at least two Zr- Q distances are observed as well. However, changes in Zr-Me distances are more pronounced than for Ce- Me sphere perhaps due to preferential location of the doping cations in the vicinity of smaller Zr cations. Addition of Nb in the CeZr sample enhances disorder in the cation sublattice (intensity of the Zr-Me peaks decreases). However, addition of Ca increases the intensity of these peaks (more stronger for the compo- nent corresponding to a shortened Zr-Me distance) , while further intensity increase is observed due to the doping of the CeZrCa system with Bi or Mn cations. These effects suggest some ordering within the coordination spheres of Zr cations, probably caused by the generation of anion vacancies leading to an ordered rearrangement of the oxygen polyhedra. Indeed, similar effect,s were observed for Ce-Me peaks (Figure 2(a)), suggesting some ordering within the next-neighbor coordination spheres of both Zr and Ca cations. Nev- ertheless, new shorter Zr-Mc distances which could correspond to doping cations in the interstitial positions have not been detected.

In general! for the ceria-zirconia samples studied here, the structural parameters of the Zr-0 coordi- nat'ion sphere agree rather well with those previously described for cubic or tetragonal t"-phase of Ce-Zr- 0 mixed oxides [23,24]. In these phases, the oxygen shell is divided into two subshells, and the ex- ternal one with a longer distance is strongly relaxed so that coordination number is decreased from 4 to 2.6. This asymmetry of the Zr-0 shell is preserved even in the low-surface area n-phase, as obtained by a ceria-zirconia sample reduced into the pyrochlore phase with CO at 1200 "C, followed by air oxidation at 500 "C [23]. In our case, the Zr-0 distances differ from those revealed for partially stabilized tetragonal or cubic ZrOa and Ce-Zr-0 phases [23-25] (Ta- ble 2). Furthermore, in the CeZr phase of the high- surface area sample studied here, the Ce-0 coordination sphere is also distorted, and the integral Ce-0 coordination number is slightly decreased, while for the low-surface area sample it is symmetric with CN=8 (Table 2) [23,24]. In general, positions of the peaks corresponding to the first Zr-Me and Ce-Me coordi-

nation spheres appear to be rather close to each other, namely, the appearance of long Zr-0 and Ce-0 distances will weaken these bonds, which is thought to be able to explain the higher lattice oxygen mobility of the CeZr phase prepared by the PChl rout'e (as stated below); as compared with the n-phase [26].

3.1.4. UV-Vis

Generation of point defects (oxygen vacancies, Ce3+ cations, or F and V centers such as those revealed for piire zirconia [28,29]) and rearrangement of coordination polyhedra caused by aliovalent doping of ccria-zirconia could be probed by UV-Vis Spec- troscopy [13, 271. Diffuse reflectance spectra of doped fluorites are shown in Figure 3. In a previous paper [13], absorption in the visible range observed for Ce-Zr-0 samples was assigned to these point, defect's. Doping of ceria-zirconia by Ca2+ increases the absorption in the visible (13,000-17,000 cni-l) range (Figure 3 inset, spectrum 2), which implies! as ex- pected, generation of anion vacancies. In addition, a shift of the absorption edge from -24,000 cn1-l to -26,000 cm-l and some decreases of the intensities of charge transfer bands (CTB) at -28,000 cm-' and -40,000 cm-' were revealed. This suggests some strengthening of the Cc-O/Zr-0 bonds along with rearrangement of the coordination polyhcdra and changing of their symmetry, and, thus, affecting the intensity of the oxygen-cation (Cc4+, Zr4+) charge transfer band (CTB). For a Bi-doped CeZrCa sample, absorption in the visible range remains at the same level as for the CeZrCa, while absorption edge was red-shifted to 24,000 cm-', and the intensity of CTB increases (Figure 3, inset, spectrum 5). This suggests a weakening of the Me-0 bonds and an enhanced disordering of t,he fluorite-like oxide structure due to the incorporation of big Bi3+ cations. For the CeZrNb sample, absorption in the visible range (12;OOO-20,000 cm-l) is also quite high (Figure 3 inset, spectra 1 and 8), apparently caused by the formation of Ce3++Nb5+ pairs to compensate the charge imbalance in the cation sublattice, though anion vacancies could be generated as well. In the UV range! the peak intensity does not change due to a doping with Nb, but the high-frequency part is somcwhat decreased and the absorption edge red-shift suggests some weakening of the Ce-O/Zr-0 bonds due to overall lattice expansion (see above). Similar changes in the UV-Vis spectra were revealed earlier by Brayner et al. [30] for Ce-Nb-0 samples and explained by the generation of defects in the fluorite-like latt,ice.


........

.... . . . . . . ,. '.. 16

14

12

i2 10 Y,

8

6

4

2

0 10000 20000 30000 40000 moon

Wavenumber (cm-' )

Figure 3. UV-Vis spectra of doped fluorites: CeZr (l), CeZrCa2 (2), CeZrCaMn (3), CeZr- CaCo (4), CeZrCaBi ( 5 ) , Mn/CeZrCa (6), CeZrMn (7) and CeZrNb (8)

For Mn and Co-doped samples, pronounced absorption in the visible range is due to the incorporation of the doping cations or their oxidic clusters into the fluorite-like structure. As judged by the dark color of all these samples (between black and dark brown, depending on the chemical composition) which is typical for bulk oxides of Co and Mn (C02O3, Co304, Mn304, Mn2O3, and MnOz), the clustering degree of these dopants is rather high, especially in the surface layer. For the Mn-doped samples, a struc- tureless absorption in the visible region falls into the range of d-d transitions for isolated octahedrally co- ordinated Mnn+ cations as well as their simple oxides [31,32]. A stronger absorption for samples doped by Mn in the bulk, as compared with Mn-supported sample (spectra 3, 7 and 6, respectively), suggests a different state of Mn cations, which shows apparently a higher clustering degree on the surface for the latter sample. Similarly, a stronger absorption for a Co- doped sample with two clearly developed maxima at -14,000 and 19,000-28,000 cm-I (Figure 3, spectrum 4) [33] implies a higher uniformity of the distribution of Co cations within the lattice as compared with that of Mn cations. Since a band at -14,000 cm-I is typical of Co2+ cations in the tetrahedral coordination [31] (such as that in C0304) which could not be realized in the ideal fluorite-like structure with %fold oxygen environment, this suggests that either oxy-

gen sublattice in the vicinity of the incorporated Co cations is strongly relaxed or these cations are mainly segregated within the domain boundaries.

3.2. tivity

Surface/bulk oxygen mobility and reac-

3.2.1. 0 2 TPD

Temperature dependence of the rates of 0 2 desorption in the temperature-programmed mode and the amounts of desorbed oxygen from CeZrCaMe and CeZrMe fluorites are shown in Figure 4 and Table 3.

20

15

h h

v . - B - ; 10 - - - lo

5

0 1 " 1 * " " 1 " I c 0 300 600 900

Temperature ("c)

Figure 4. Temperature dependence of the rate of oxygen evolution from the doped fluorites: CeZr (I), CeZrCa2 (2), CeZr- CaMn (3), CeZrCaCo (4), CeZrCaBi ( 5 ) , Mn/CeZrCa (6), CeZrMn (7) and CeZrNb (8)

Analogous to the analysis of 0 2 TPD spectra for modified lanthanum manganites [34], complex TPD desorption profiles were deconvoluted into Gaussian components, and the ascending parts of these peaks were used for the estimation of E d . Five forms of desorbed oxygen were thus distinguished, denoted as cr ( ~ 4 0 kJ/mol), (60-80 kJ/mol), pz (100-130 kJ/mol), y1 (230-290 kJ/mol) and 72 (around 400 kJ/mol) (Table 3). As for the lanthanum manganite [34], these forms can be assigned to the desorption of oxygen from various surface sites (low-and middle- temperature forms) followed by oxygen diffusion to the surface from the near-surface layers and the bulk of the particles.


Table 3. Amounts of different forms of oxygen desorbed from doped fluorites during Op TPD run with E d 5 40 kJ/mol (a form), 60-80 kJ/mol (pi form), 100-130 kJ/mol ( p p form), 230-290 kJ/mol

(71 form) and about 400 kJ/mol ( 7 2 form) in monolayer coverage (N)

Sample Amounts of different forms of desorbed oxygen (N)

cy Ri a2 71 7 2 Total amounts ~ ~ ~ CeZr 0.5 0.7 1.2

1.3

1.7

- 0.6 1.4 1.2

2.0

1.8

2.0

1.1

1.2

1.6

0.6 1.2

1.7

2.8

Pt/CeZrMn 0.6 2.1 0.5 3.2 2.3 Pt/CeZrNb 1.0 1.0 0.3

~ ~ - CeZrCa2 0.4 0.9 - - CeZr CaMn 0.3 0.9 0.5

CeZr CaCo 0.8

CeZr CaBi 0.8 0.4

- -

- - -

- - 1.4

CeZrMn 0.6 1.0 0.2

CeZrNb 1.3

Pt/CeZr 0.2 0.6 0.3

- Mn/CeZrCa 0.6 - -

- 0.7 - - ~ -

- - - Pt/CeZrCa 0.3 0.9 - - Pt/CeZrCaMn 0.2 0.7 0.7

Pt/CeZrCaCo 0.6

Pt/CeZrCaBi 0.1 1.0 0.6

Pt/Mn/CeZrCa 0.6 1.8 0.4

- - -

- - ~ -

~ ~

~ -

For CeZr and CeZrCa samples, the 0 2 TPD curves (Figure 4) are rather flat, whilst the total amounts of the desorbed oxygen are close (Table 3). Doping of CeZr by Ca decreases the low-temperature oxygen desorption, probably due to Ca segregation on the surface, as was revealed by SIMS [ll]. The increase of high-temperature oxygen desorption can be explained by the facilitation of the bulk diffusion due to generation of anion vacancies (Figure 4). For the CeZr sample, the amount of desorbed oxygen with Ed 5 40 kJ/mol (a form) is equal to 0.5 monolayer (Table 3 ) . This amount of oxygen could not be assigned exclusively to the surface defects of the par- ent fluorite structure, which could provide only a few percent of the monolayer coverage. Hence, this form should be assigned to oxygen dissolution within the disordered domain boundaries of nanocrystalline ceria-zirconia support [35]. Next, the thermally stable ,!31 form with E d 60-80 kJ/mol could be assigned to the oxygen supplied from the bulk to the surface, and hence, Ed can be used to characterize the bulk oxygen diffusion. These data agree rather well with the E d

estimated by other methods [22, 361. Doping by Ca affects the ratio of a and 01 forms by increasing the share of the latter one due to the generation of anion vacancies.

Doping of the CeZrCa and CeZr samples with Mn or supporting Mn on the surface of the CeZrCa sample can facilitate oxygen desorption, especially in the low-

temperature region (Figure 4 and Table 3). The total amount of desorbed oxygen from the Mn-containing fluorites varies in the range of 1.7 2.0 monolayers and exceeds the amounts desorbed from pure CeZr and CeZrCa supports (Table 3 ) . For the impregnated Mn/CeZrCa sample, the most intense desorption of 0 2 moves to higher (600-900 "C) temperatures, which can be explained by a higher clustering degree of the Mn cations on the surface. This range is close to the decomposition temperatures of bulk Mn oxides (Figure 5(a)), which agrees with the known data on phase stability of these oxides in air [37], as well as on oxygen desorption from Mn-doped zirconia [8]. Low-temperature (200-600 "C) desorption of 0 2 for the Mn-containing samples can be assigned to the removal of the capping oxygen forms bound with isolated surface Mn cations as well as to fast oxygen migration along domain boundaries enriched by Mn cations. The addition of Mn facilitates diffusion of bulk oxygen as well, which is reflected in the appearance of a new high-temperature Pz form with Ed 100-130 kJ/mol. This form is more pronounced for impregnated samples.

Doping of the CeZrCa sample with Bi increases the share of the low-temperature cr form in comparison with the PI form (Figure 4, Table 3). Dop- ing with Nb increases the total amount of desorbed oxygen (Table 3). Along with a large (1.3 monolayers) contribution of easily removed ( E d 40 kJ/mol)

Journal of Natural Gas C7hemistr.y Vol. 15 No. 3 2006 157

14

12

10 ?-

2 v

.- 8 8 : *

oxygen, desorption of stronger-bound oxygen with E d 2 230 kJ/mol takes place as well, apparently cor- relating with the generation of lattice defects-anion vacancies and Ce3+ cations (see above).

For the Co-doped sample a sharp peak of 0 2 des-

f (b) 315

-

-

405

580

i 1 - : mo3

Isotherm - 1 hour 0 I , I I I I 1 8 I -

0 300 600 900 900 Temperature (C)

Figure 5. 0 2 TPD (a) and Ha TPR spectra (b) of manganese oxides

0 200 400 600 800 Temperature ('C)

The effect of Pt (0.2 wt%) supporting on the 0 2

TPD spectra depends on the kind of the dopant (Ta- ble 3). For the majority of the samples, formation of oxidized forms (like Pt6 [19]) will shift 0 2 desorption peaks to higher temperatures, thus decrease the total amount of desorbed oxygen and redistribute the desorption forms. For these samples, the amount of easily removed oxygen with Ed M 40 kJ/mol decreases as well (Table 3). The positive effect of Pt supporting on the total amount of desorbed oxygen is revealed for Mn/CeZrCa, CeZrMn and CeZrNb samples.

40 A

351 30 I \

0 200 400 600 800 Temperature ('C)

3.2.2. Ha TPR

H2 TPR curves of CeZrCaMe (Me=Mn, Co and Bi), Mn/CeZrCa and CeZrMe (Me=Mn, Nb) samples are shown in Figure 6(a) and Figure 6(b), respectively, and the total amount of oxygen removed within a TPR run is given in Table 1. For pure CeZr and CeZrCa samples, oxygen is removed due to the reduction of the Ce4+ cations to the Ce3+ state, and the reduction degree corresponds to 20% and 17%, respectively. Doping with Ca shifts somewhat the

0 200 400 600 800 Temperature ('C)

Figure 6. Ha TPR of doped fluorites: CeZrCa2 (l), CeZrCaMn (2), CeZrCaCo (3), CeZrCaBi (4), Mn/CeZrCa ( 6 ) , CeZr (6) , CeZrMn (7) and CeZrNb (8)

158 Tatiana Knznetsova et al./ Journal of Natural Gas Chemistry Vol. 1 5 No. 3 2006

reduction peaks to higher temperatures, suggesting some strengthening of the Ce-0 bonds. This agrees with the decreasing the amount of the most reactive and weakly bound oxygen (Table 3) as well as the amount of oxygen removed in a TPR run related to one Ce cation (average Ce reduction degree, Table 1). All these features could be explained by the strengthening of the Ce-0 bonds, as revealed by EXAFS (see above).

The first series of doped fluorites CeZrCaMe (hIe=iLln, Co and Bi) is characterized by an increased amount of removed oxygen and the appearance of low-temperature TPR peaks (Figure 6(a). Table 1). For comparison. typical TPR data for the reduction of pure manganese oxides are shown in Figure 5(b). Reduction of MnOa (specific surface area- SSA, 10 rn2/g) proceeds in two consecutive steps at 300 "C (Mn4++hfn3+) and 400 "C (Mn3+ +Mn2+). Peaks of hln2O3 or hZn304 (SSA 0.1 m2/g), corresponding to reduction to MnO, are situated near 500 "C. For these highly disperscd MnOz and Mnz03 oxides, TPR peaks are situated at temperatures lower than those reported earlier for low-surface-area samples [37]. For Mn-doped fluorites, TPR peaks already appear at 160-250 "C. suggesting the existence of small and highly reactive MnO, clusters on their surfaces. Disappearance of the high-temperature (-600 "C) peak of bulk Ce4+ after reduction suggests that, similar to the Pt-supported ceria-zirconia samples [lo], reduced surface clusters of Mn cations arc able to activate hydrogen efficiently, while lattice oxygen diffusion is not the rate-controlling stage. As judged by the reduction balance assuming the same degree of Ce4+ reduction in the TPR run, such as the CeZrCa sample and the oxidized Mn-doped samples, around 60% of the Mn cations are in the Mn4+ state. For the surface-supported Mn-containing sample. the positions of the TPR peaks are close to those revealed for the CeZrCaMn sample, though the relative intensities of the low-temperature peaks are somewhat decreased. This implies a lower reactivity of the surface MnO, clusters with the impregnated samples, perhaps due to their bigger sizes.

For the cobalt-doped sample. low-temperature pcaks of Hz consumption arc observed as well. As- suming a constant level of Ce4+ reduction achieved in the TPR run for doped samples, the amount of removed oxygen corresponds to nearly a 100% content of Co3+ in the sample. Reduction of bulk cobalt oxide C0304+~ by Hz to Coo gives one peak situated at -350 "C, while for the complex oxide

Lac003 two peaks are observed at 350-400 "C and 600 "C (data not shown for brevity). Hence! for the Co-containing sample, low and medium-temperature reduction peaks can be assigned to the reduction of' clustered and isolated surface Co cations to the Co2' or Coo state. Pronounced reduction plateau between 450 and 600 "C could correspond to the reduction of' bulk Ce4+ cations. A high temperature (-650 "C) peak absent for pure ceria-xirconia could be explained by the reduction of Co2+ cations in the bulk to the metal state, which is apparently hampered due to the stabilization of thesc cations by fluorite-like matrix, thus preventing the formation of the Coo nuclei.

Doping with Bi allows of reducing such a sample into one TPR peak, with a maximum at -300 "C. As- suming a fixed reduction degree of the Ce4+ cations in the TPR runs for all samples of a basic composition (see above), an amount of oxygen removal corresponding to 100% reduction of all Bi3+ cations t,o the Bi" state is present in this sample. The difference between this sample and the reduction behavior of the Co-doped sample is explained by a high volatil- ity of metallic Bi, thus helping its fast migration from the bulk of the particles to their surface and following evaporation into the ga.s phase.

For the second series fluorites CeZrMe (Me=Mn and Nb) (Figure 6(b), low tcmperature Hz TPR peaks appear only for the CeZrrvln sample. Reduc- tion starts only at temperatures exceeding 150 "C; with the strongest peak of Hz consumption situated at 400 "C and a shoulder at 300 "C. This correlates with a lower (-30%) fraction of Mn4+ in this sample. Apparently, a t a higher (40 at.%) Mn content of this sample, the clustering degree of the Mn cations is higher, which increases the size of the clusters, weak- ens their interaction with the fluorite-like matrix and, hence, decreases the content of the Mn4+ and the reactivity of the clusters.

Addition of Nb to the CeZr sample does not affect the position of the main peak of bulk Ce4+ reduction, apparently due to the inability of the Nb cations to activate hydrogen. A high temperature (-600 "C) TPR peak absent for the ceria-zirconia sample can be assigned t,o the reduction of Nb5+ to Nb3+. Indeed, the amount of removed oxygen corresponds to -100% reduction of Kb5+ to Nb"? provided that the degree of Ce4+ reduction is the same as for the CeZr sample.

Hence, the doping of ceria-zirconia based fluorites by transition metal cations allows of increasing the reactivity of the surface and the bulk oxygen of these


0.8

samples with respect to hydrogen. Pronounced stabilization of Mn and Co cations in their highest oxidation states can be derived by analysis of the total hydrogen consumption after reduction of the samples. This could be explained by a high (1.61 eV) redox PO- tential of the Ce4+/Ce3+ couple [4].

Nb, Ce or Zr cations, and/or oxygen forms bound with the latter cations. Indeed, for Mn-containing samples, methane oxidation starts at much lower temperatures, generating mainly COz (and water) as the combustion products. After consumption of the most reactive oxygen, CO appears at 900 "C as the result of interaction of activated CH, fragments with strongly bound surface oxygen species recuperated due to oxy- 3.2.3. CH4 TPR

(a)

900 -

CH4 TPR experiments have been carried out for Mn and Nb-doped fluorites, including those promoted by Pt, to clarify the reasons for their different properties in methane combustion (see below). Temper- ature dependences of CH4 conversion and products (COz and CO) concentrations during CH4 TPR runs are shown in Figures 7 and 8. Methane is apparently much easier to be activated on Mn cations than on

h 5 0 0 . ii

1.0, I

0.8

0.6

0.4

0.2

0

gen diffusion from the bulk. Meanwhile, at 900 "C, the rate of methane transformation exceeds the rate of CO formation, thus evidencing methane pyrolysis on reduced Mn cations, with coke deposition on the surface. For the Nb-doped sample, methane pyrolysis is practically absent, thus evidencing a low ability of reduced Nb3+ cations to activate methane, as has already been reported for Ce-Nb-0 samples [39].

0 300 600 900 900 Temperature ('C )

0 300 600 900 900 Temperature ("C)

Figure 7. Conversion of CH4 (1) and concentrations of COz (2) and CO (3) during CH4 TPR (1% CHI in He) for CeZrCaMn (a) and 0.2 wt% Pt/CeZrCaMn (b) fluorites

0 0

$

. i

o t 0 300 600 900 900

Temperature ("C)

1 .o

0.8 h

- 5 0.4 0 z $ 0.2

0

0 300 600 900 900 Temperature ("C)

Figure 8. Conversion of CH4 (1) and concentrations of COz (2) and CO (3) during CH4 TPR (1% CHI in He) for CeZrNb (a) and 0.2 wt% Pt/CeZrNb (b) fluorites

160 Tatima Kuznetsova et al./ Journal of Natural Gas Chemistry Vol. 15 No. 3 2006

Supporting of Pt facilitates activation of methane, and shifts CH4 TPR peaks to lower temperatures. The strongest effect is observed for Pt/CeZrNb. For the Mn-doped sample, only COz evolution is observed in the first peak, thus suggesting a fast combustion of the activated methane fragments by highly reactive surface/near-surface oxygen. For this sample, supporting of Pt shifts CO formation to lower temperatures due to a higher efficiency of methane activation. This is also reflected in a high rate of methane pyrolysis in the isothermal part of the run that increases with the lattice oxygen consumption.

For the Nb-doped sample, Pt supporting can increase medium-temperature reactivity of the surface oxygen forms with respect to methane combustion and syngas generation. In the high-temperature range, TPR features are similar to those observed for Mn-containing samples, which are apparently con- trolled by the ability of Pt to activate methane and the mobility of the lattice oxygen.

3.3. Catalytic activity in methane combustion

Temperature dependence of methane conversion

I00

80

- 5 6 g 60 .-

2 U 40

20

0

200 300 400 500 600 700 800 900 Temperature ('C)

on doped fluorites, including those promoted by Pt, is shown in Figure 9. Temperatures for 50% and 90% methane conversion are given in Table 4. The activities of the doped samples strongly depend upon their composition and the types of doping cations. Among them, the highest activity is demonstrated by Mn and Co-doped Ce-Zr fluorites: a t a high (lo5 h-' ) space velocity, the temperature for 50% methane' conversion is 520-560 "C. For these samples, the apparent activation energy of methane combustion is the lowest and equals to 50-80 kJ/mol. This seems to correlate with the presence of reactive clusters of transition metal cations on the surface of these samples, which are able to retain rather reactive (presumably, weakly or mod- erately bound) oxygen forms. Indeed, the specific rate of methane combustion increases with the bulk content of Mn in the fluorite-like matrix (Figure lo), and, hence, with the surface concentration of Mn. Even the higher specific activity of the sample having Mn supported on the surface (Figure 10) agrees with this suggestion. The ability of Mn cations fixing on the surface of those supports for activating methane (see above CH4 TPR data) could also be responsible for this high activity of the Mn-containing systems.

200 300 400 500 600 700 800 900

Temperature ("C) Figure 9. Temperature dependence of methane conversion on doped fluorites CeZr (l), CeZrCa2 (2), CeZr-

CaMn (3), CeZrCaCo (4), CeZrCaBi ( 5 ) , Mn/CeZrCa (6), CeZrMn (7) and CeZrNb (8); Initial samples (a), samples after supporting of 0.2 wt% Pt (b)

(Reaction condition: tests in a mixture of 1% CH4-3% 0 2 at 100000 h-l)

Doping of ceria-zirconia based fluorites with Nb or Bi strongly decreases their specific activities in methane combustion (Table 4, Figure 10). This is not a trivial result, since both of the dopants would increase the surface coverage by the weakly bound oxygen forms (Table 3), and the lattice oxygen mobil-

ity is traditionally thought to bc responsible for the combustion of methane catalyzed by ceria-containing complex fluorites [40]. Moreover. doping with Bi can strongly incrcasc the reducibility of the sample by hydrogen (Figure 6). On the other hand, doping with Nb will decrease the intensity of the Hz TPR peak at

161 Journal of Natural Gas Chemistry Vol. 15 No. 3 2006

-500 "C, which is typical for a ceria-zirconia solid solution, and will shift the reduction process to higher (-620-650 "C) temperatures. These results suggest that surface Nb cations can hamper the ability of the native surface sites of the ceria-zirconia support (presumably, Lewis acid sites -coordinatively unsaturated Zr4+ cations) for activating methane. Though the Bi cations can efficiently activate hydrogen (see above), their ability to activate methane is rather weak, which is reflected in a poor medium-temperature reducibility of the Bi-containing fluorite-like complex oxides [41,42]. All these results imply that the efficiency of the surface sites in methane activation is a more important factor for the methane combustion activity of the studied complex oxide catalysts than the bonding strength of the surface oxygen forms, as well as their coverage or lattice oxygen mobility.

Table 4. Specific surface area (SSA), temperatures of the 60% and 90% methane conversions and

apparent activation energy of doped fluorites in methane combustion

SsA Temperature of Apparent --. . Sample CH4 conversion ('C) activation

(m2/g) 50% 90% energy (kJ/mol) CeZr 65 630 730 100

CeZrCa2 44 600 680 90

CeZrCaMn 124 530 640 80

CeZrCaCo 62 530 610 70

CeZrCaBi 29 860 >900 110

Mn/CeZrCa 51 530 660 50

CeZrMn 35 560 650 80

CeZrNb 73 700 770 110

Pt/CeZr 54 580 700 100

Pt/CeZrCa 35 610 680 70

Pt/CeZrCaMn 74 480 550 70

Pt/CeZrCaCo 62 510 620 60

Pt/CeZrCaBi 21 800 >900 70

Pt/Mn/CeZrCa 39 500 590 60

Pt/CeZrMn 31 520 590 60

Pt/CeZrNb 42 500 630 60

For all samples but Pt/CeZrCa, supporting with Pt decreases the temperature of the 50% methane conversion (Table 4) and increases the specific catalytic activity (Figure 10). The most striking effect is observed for the Nb-doped sample, which correlates with its enhanced ability to activate methane after P t supporting (see above CH4 TPR data). Appar- ently, the same factor is responsible for the increase of the performance of the Ce-Zr and the Co-doped samples, since in these cases, due to the P t supporting, surface coverage by reactive surface forms

declines (Table 3). For the Mn-doped samples, the strongest activation effect was also observed when Pt supporting decreased the surface coverage due to rela- tively easily desorbed oxygen forms (CeZrCaMn support). Hence, P t supporting can increase the performance of fluorite-like complex oxides, mainly due to a more efficient activation of methane. Declining of the apparent activation energies of methane combustion over Pt-promoted samples agrees with the conclusion about the participation of Pt in methane activation.

15

h h

2 E 10 v . 3

3

v a - '2 5

n

Figure 10. Rates of methane conversion at 500 'C on initial and Pt supported doped fluorites

However, in general, supported P t by itself could not provide a high activity for methane combustion when combined with Ce cations on the surface of these complex fluorites. Moreover, some cations such as Bi, or in a lesser extent Ca, are able to suppress the combustion activity of supported Pt, apparently via specific chemical interaction. Since Ca cations on the surface of fluorite-like supports could stabi- lize the oxidic forms of P t , which is known to be much less efficient for C-H bond rupture as compared with Pto clusters [43], the negative effect of Bi on the ability of Pt to activate methane, as reported earlier [42], requires further studies. Similarly, the synergism in methane activation by Mn cations and supported P t , as well as the tremendous positive effect of highly charged Nb5+ cations on the performance of supported Pt, suggest their strong chemical interaction, which deserves to be elucidated further by spectroscopic methods. Stabilization of highly charged Mn4+ cations on the surface of ceria-zirconia supports could play some role as well. These results are of importance not only for the designing of catalysts for the combustion of methane and other hydro-


carbons, but also for their selective oxidation into syngas by molecular or lattice oxygen of complex oxide systems, including those supported onto the surface of oxygen-conducting membranes.

4. Conclusions

For dispersed ceria-zirconia based solid solutions prepared via the polymerized complex method arid annealed at 700 "C, an anion-deficient fluorite structural type with disordered anion vacancies is preserved under doping of up to 20 mol% of Ca, Mn, Co, Bi or Nb cations. Doping with transition metal cations or Pt increases the mobility and reactivity of the surface/bulk oxygen in these complex oxide systems. For the studied samples, a broad variation of the specific rates of methane combustion was observed, suggesting a structural sensitivity of this reaction. In general, no straightforward rela- tionships between the activity of methane combustion and the characteristics of the surface and lattice oxygen (bonding strength, surface coverage, lattice oxygen mobility) were revealed. This is explained by the important role played by the specific chemical interaction between the surface sites and the methane molecules even in an excess of oxygen in the feed. Pt interaction with Mn or Nb cations on the surface of the doped ceria-zirconia supports would provide the highest activity for methane combustion. Since in steady-state methane combustion, the variation of lattice oxygen mobility and reactivity of the ceria- zirconia supports caused by bulk doping is apparently of no direct relevance, the revealed pronounced effects of doping by the studied cations on the real structure of the support and the oxygen transport properties could be used in the design of catalysts for syngas generation in membrane reactors or under periodi- cal reduction-reoxidation mode [ l l ] . Moreover, for methane selective oxidation into syngas catalyzed by Pt' supported on fluorite-like complex oxides at short contact times, even under steady-state conditions, lattice oxygen mobility and reactivity could play an important role in preventing carbon build-up on the surface through removing of the CH, fragments formed due to methane dissociation on the metal part,icles

[la].

Acknowledgements

This work is supported in part by the ISTC 3234 and RFBR-CNRS 05-03-34761 projects.

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Modified Ceria-Zirconia Fluorite-Like Catalysts for the Combustion of Methane

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