J. Phys. Chem. C 2008, 112, 9573–9578
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ARTICLES Formation of Hollow Spheres upon Oxidation of Supported Cobalt Nanoparticles P. A. Chernavskii,† G. V. Pankina,† V. I. Zaikovskii,‡ N. V. Peskov,§ and P. Afanasiev*,| Department of Chemistry, M. V. LomonosoV Moscow State UniVersity, Leninskie Gory, GSP-2, Moscow 119992, Russia., BoreskoV Institute of Catalysis, Russian Academy of Sciences, Pr. Akademika LaVrentieVa 5, NoVosibirsk, 630090, Russia, Department of Computational Mathematics and Cybernetics, M. V. LomonosoV Moscow State UniVersity, Leninskie Gory, GSP-2, Moscow 119992, Russia, and Institut de Recherche sur la Catalyse et EnVironnement, UniVersite de Lyon 1, 2, aV. A. Einstein, 69626 Villeurbanne, Cedex, France ReceiVed: August 31, 2007; ReVised Manuscript ReceiVed: March 31, 2008
Oxidation of cobalt nanoparticles supported on montmorillonite was studied using transmission electron microscopy and temperature programmed reduction of the oxide particles. It has been shown that hollow shells of cobalt oxide were formed in this process, consisting of the mixture of CoO and Co3O4 oxides. A linear relationship between the thickness of shells and the initial particles size was established which suggests that both diffusion of cobalt ions through the oxide layer and diffusion of the oxygen into cobalt occur, but particles sintering is negligible compared with the reverse diffusion of cobalt. The lower critical diameter of cobalt particles was estimated, above which formation of hollow shells occurred. The hollow shells formation is suggested to be the reason for cobalt redispersion upon oxidation-reduction cycling of supported heterogeneous catalysts. Introduction It is well-known that, during the high temperature oxidation of metals, voids can be formed at the metal-oxide phase boundary. They are generated by clusterisation of vacancies which appear due to the difference in diffusion coefficients of metal and oxygen through the growing oxide layer.1–3 Recently such a phenomenon was discovered for low-temperature oxidation of Co4 and Fe5 nanoparticles. This phenomenon was explained as follows: depending on the type of mobile defects predominating in the lattice, the growth of the oxide film can occur either at the metal-oxide interface (and then the transport of the oxygen ions inside the particle is dominating) or can it occur at the gas-solid interface (in which case the transport of metal ions toward the surface prevails). In the case of a significant difference in diffusion coefficients (Kirkendall’s effect), accumulation of vacancies at the metal-oxide interface can lead to the formation of cavities. In the work5 it was observed for the nanoparticles of iron that some critical size exists (8 nm) below which full oxidation of the particles at room temperature is possible, leading to the formation of hollow oxide spheres. For the particles larger than this critical size, only formation of voids at the metal-oxide interface was observed. Stability of nanoshells as a function of size was considered theoretically in work.6 Thermal stability of the spherical nanoshells was considered, and the Monte Carlo modeling of the oxidation kinetics was carried out. * To whom correspondence should be addressed. † Department of Chemistry, M. V. Lomonosov Moscow State University. ‡ Russian Academy of Sciences. § Department of Computational Mathematics and Cybernetics, M. V. Lomonosov Moscow State University. | Universite de Lyon.
Such formation of hollow objects due to the preferential diffusion of the reactant from the material bulk toward its exterior seems to be a common phenomenon, extendable on various classes of materials and chemical processes. Thus, colloidal hollow nanoparticles of metals phosphides have been prepared solvothermally,7,8 aluminum hollow nitride has been prepared by high temperature heating of aluminum powder in ammonia,9 whereas molybdenum sulfide hollow spheres were obtained under very soft conditions in zwater-acetone solution obviously due to outward sulfur diffusion from amorphous MoSx particles.10 Despite the extreme variability of conditions, a common mechanism leads to the hollow structures in all of the cited cases, related to the nanoscale Kirkendall effect. The formation of nanotubes and hollow spheres due to the Kirkendall effect has been recently generalized taking into account surface diffusion11 and reviewed.12 Heterogeneous catalysts often contain nanoparticles of an active metal (Co, Fe, and Pd) smeared over a high surface area support which can be oxide like alumina, a clay, a zeolite, and so on. Morphology evolution of the supported active phase during activation/regeneration cycles plays a crucial role for the catalysts performance. To our knowledge, no Kirkendall effect related morphology patterns have yet been reported in the supported catalysts. The present work deals with the formation and geometric parameters of hollow cobalt oxide nanoshells produced upon oxidation of cobalt supported on the Montmorillonite clay. Experimental Section Cobalt supported on Montmorillonite was prepared using impregnation of the support by aqueous cobalt nitrate. Montmorillonite grade F-160 with specific surface area 270 m2/g and
10.1021/jp077007o CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
9574 J. Phys. Chem. C, Vol. 112, No. 26, 2008 mean pore size 6.6 nm was applied. After the impregnation, the solid was dried at 100 °C under air flow for 3 h. The dried sample was obtained with 20% Co weight loading. It is further designated as Co/M. The dried specimen was further reduced under hydrogen flow in a micro reactor which was simultaneously applied as a cell of vibration magnetometer.13 As a result the curve of magnetization as a function of reduction degree was obtained in situ. Reduction of Co/M was carried out upon linear increase of temperature up to 700 °C with a heating rate of 0.47 °/s (temperature programmed reduction, TPR), and then the specimen was kept at 700 °C in hydrogen until the steady value of magnetization was reached, which signified that the reduction was complete at this value of temperature. The reduced sample was then cooled in H2 to 200 °C and then the hydrogen flow was replaced by Ar one in order to remove adsorbed hydrogen. Then the solid was further cooled to room temperature in argon flow. Oxidation of cobalt nanoparticles was carried out in the temperature programmed regime (TPO), with a heating rate of 0.47 °/s in the 1% O2/He flow. The deepness of oxidation was monitored using the magnetization decrease. After reducing the magnetization to zero, the solid was cooled to room temperature in argon flow. The oxidized specimen as obtained was studied by transmission electron microscopy on a JEM-2010 device at a resolution 0.19 nm. The sample was supported on a standard carbon film covered copper grid sample holder. The sample Co/M was also studied in 5% H2 + Ar flow by the temperature programmed reduction (TPR) with the simultaneous registration of magnetization changes and detection of hydrogen consumption. This combination of methods allows additional information about the reduction stages sequence to be obtained.7 To check whether the mass transport (sintering) occur between the particles of cobalt oxide, nonsupported Co3O4was prepared by decomposition of cobalt nitrate (Aldrich) in dry argon flow, at 300 °C (heating rate 2 °C min). The powders were then thermally treated in flowing air at various temperatures for 12 h. The specific surface area (low temperature nitrogen adsorption using Micrometrics ASAP 2000 device) and XRD particles size of the resulting cobalt oxide were then compared to observe sintering. XRD measurements were carried out on a Bruker device using Cu KR emission.
Chernavskii et al.
Figure 1. TEM micrograph of the 20% Co/M specimen after oxidation upon linear heating to 200 °C.
Results and Discussion The materials studied in this work are heterogeneous catalysts destined to Fischer-Tropsch reaction (or other processes including cobalt). The choice of these systems was motivated by catalysis-related reasons. Discussing of their catalytic properties is out of the scope of this work and will be reported elsewhere. Montmorillonite was chosen as an advantageous support for metals including cobalt.14,15 Cobalt particle sizes in the reduced specimen Co/M ranged from 3 to more than 20 nm. Such a significant polydispersity might be interesting to follow the rules relating to the geometrical parameters of the shells obtained from their oxidation. In Figures 1 and 2 are presented the micrographs of the 20% Co/M specimen after reduction by hydrogen up to 700 °C followed by oxidation to 200 °C (both reduction and oxidation were carried out in a temperature programmed regime). Spherical hollow structures of cobalt oxide of different size can be clearly seen in these figures. As far as our TEM observation allows us to infer, only spherical shells are present without any significant deformations. We do not see as well any evidence of the formation of necks between spheres. At higher magnifica-
Figure 2. TEM micrograph of one particle after oxidation. Insert: digital diffraction of a cobalt oxide shell. Numbers are Miller indexes of the planes corresponding to the spots.
tion, one can see that cobalt oxide shells are not regular but contain a considerable amount of cracks. Their walls can be better represented as polycrystalline mosaic blocks. Two consecutive reactions may occur upon the oxidation of metallic cobalt by oxygen: 2Co + O2 ) 2CoO (I) and 6CoO + O2 ⇒ 2Co3O4 (II), whereas reduction precedes them according to reverse reactions Co3O4 + H2 ⇒ 3CoO + H2O (III) and CoO + H2 ⇒ Co + H2O (IV). To clarify which oxide is formed in our experiments, the initial, calcined, and reduced-oxidized solids were studied by TPR technique under flow 5% H2 + Ar. The corresponding oxides can be distinguished by comparing the amounts of consumed hydrogen and by analyzing the steps of reduction observed in TPR simultaneously with magnetization change. Indeed, reaction III occurs without magnetization
Formation of Hollow Spheres
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9575
Figure 3. TPR pattern and magnetization vs temperature for the initial (oxidized) Co/M specimen.
Figure 4. TPR pattern and magnetization vs temperature for the Co/M specimen after the oxidation-reduction treatment.
change, whereas in reaction IV, metallic cobalt appears, possessing ferromagnetic properties. Moreover, it is well-known that the only product of cobalt nitrate decomposition is Co3O4 oxide, either in the pure16 or supported state.17 The last oxide decomposes giving CoO only above 900 °C; therefore, in the initial sample of the oxide obtained from the decomposition of nitrate, only Co3O4 is present. This sample can be used as a reference to compare it with the reduced-oxidized counterpart, in which by contrast the presence of both Co3O4 and CoO is possible. While the peaks temperatures are not specific and may depend on the sample morphology, comparison of the amounts of hydrogen consumed with and without magnetization change can be reliably used to estimate the ratio of two oxides. Simultaneously detected hydrogen consumption and magnetization curves are presented in Figure 3 for the specimen heated under Ar flow for 1 h at 400 °C (i.e., that one which contains only Co3O4). The maximum of hydrogen consumption rate in the range 200-300 °C was not accompanied by a magnetization change and therefore corresponds to reaction III. At T ) 350 °C magnetization growth occurred due to the beginning of reaction IV. Hydrogen consumption was not monotonous in this region of temperatures, presumably because of mass transfer within the support pores.18 The saturated magnetization Ms (or rather the value of magnetization at maximum device field 6KOe) of reduced solid was 10.1-10.3 emu/g, corresponding to 50.5-51.5 emu per gram of loaded cobalt, as measured in the range 600-700 °C. This value of magnetization is roughly two times lower than that of bulk Co in the same temperature range (166 emu/g at RT and 110-120 emu/g in the range 600-700 °C).19 Such a difference may be attributed to the small particles size and their interaction with the support. Indeed, cobalt nanoparticles show superparamagnetism as the particle size decreases below 7-8 nm. The magnetization of the superparamagnetic part of cobalt particles obeys Curie’s law, whereas the saturation magnetization of ferromagnetic ones also decreases with temperature according to Weiss theory. Note also that in a polydisperse small particles ensemble the relative part of particles becoming supermaramagnetic changes (increases) as temperature increases. This results in a rather complex behavior of total Ms value versus temperature, analysis of which is out of scope of this paper. Similar decrease of Ms was earlier observed for small particles of cobalt.20 After the end of reduction, the specimen was oxidized 1% O2 + He by means of linear heating from room temperature to 250 °C. Then the sample was again reduced in TPR regime in the 5% H2 + Ar flow. The second reduction TPR pattern is shown in Figure 4. From the results represented in Figure 4 it
follows again that the hydrogen consumption peak in the range 200-300 °C is due to Co3O4 oxide (reduction occurs according to reaction III). However in the second TPR cycle the relative area of this first peak decreased, whereas that of the reaction IV increased, corresponding to the presence of some CoO. From the comparison of peaks areas, the amount of CoO was estimated to be in the range from 26% to 33%. As a conclusion of the TPR experiment, the hollow spherical oxide shells observed in the micrographs consist mostly of Co3O4, and some CoO, with the weight ratio CoO/ Co3O4 ∼ 1/3. Digital diffraction of the shells structure observed by HREM confirms these conclusions. Only the spots of cubic Co3O4 phase were found with distances 4.66, 2.85, and 2.44 Å, corresponding respectively to the (111), (220), and (311) planes of the Co3O4 cubic phase (43-1003 PDF card). The oxides identification by powder XRD was carried out. Unfortunately, the XRD patterns of two cobalt oxides considerably overlap and the most intense peaks almost coincide, which somewhat hinders clear identification of phases. There are only few nonoverlapping intense peaks in the XRD patterns of the two oxides, namely that of 2.848 Å for Co3O4 phase ((220) plane, JCPDS 01-76-1802) and 2.1315 and 1.498 Å for CoO ((200) and (112) planes, respectively, JCPDS 01-070-2857). Other peaks are overlapping, or their intensities are too small. However, clear evidence of the presence of both phases was obtained from the powder XRD. In the Co/M sample, we found the 2.85 Å (31.37°) peak which is seen only in Co3O4 as well as 1.489 Å reflection, characteristic only for CoO. It follows from the ensemble TPR and diffraction results that both Co3O4 and CoO phases are present in the Co/M sample, being poorly crystallized. Any quantitative XRD analysis was not possible because of the mentioned overlap and poor quality of XRD patterns which demonstrate low intensity broad lines (not shown). As with the shells thickness and void size, the results of statistical treatment of transmission electron microscopy (TEM) data are presented in Figure 5. In this figure, the thickness of the spherical shell δ is represented as a function of the cavity diameter d. Linear regression shows the existence of correlation between these two values, with the relation ship δ ) 2.05 + 0.27d (Figure 5). The value δ ) 2.05 corresponds formally to some critical diameter of the initial Co particle, below which the spherical voids do not form anymore in the oxidation products. It follows from the stoechiometry of reactions I and II that
nCo ) nCoO
1 nCo ) nCo3O4 3
(1)
where ni are molar amounts of Co, CoO, and Co3O4 in the initial
9576 J. Phys. Chem. C, Vol. 112, No. 26, 2008
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δ)
Figure 5. Relationship between the oxide shell thickness δ and diameter of the spherical void d. The line corresponds to eq 4 with ε ) 2.66 and C ) 0.35.
particle and oxidized hollow shell, respectively. A spherical Co particle of d0 diameter contains nCo moles
nCo )
πFCo 3 d 6MCo 0
(2)
where F is density and M is atomic mass of cobalt. Similarly, for the oxide Co3O4
nCo3O4 )
πFCo3O4 6MCo3O4
(D3 - d3)
(3)
where D is the external diameter of the particle after complete oxidation and d is the diameter of the spherical void formed at the place of the initial Co particle. Let us assume now that the diameter of the spherical void and the diameter of the initial particle are related as d ) d0 ε, where ε is a parameter. If ε ) 0, the growth of the oxide layer must occur only due to the diffusion of metal ions toward the oxide-gas interface. In this case, the thickness of the shell should be perfectly proportional to the initial size of particles as obviously follows from the mass conservation law. The proportionality coefficient must be ((1 + 1/C)1/3 - 1)/2, where C is the ratio of molar volume of the initial metal to that of the product. In the case of cobalt oxidation this gives the value of 0.203 ((0.001) depending on the type of oxide formed, i.e. the thickness of the shell should represent approximately one-fifth of the diameter of initial particles. From the linear regression between the observed geometrical parameters (Figure 5) we see that, although the points are significantly scattered, a clear positive correlation exists with the proportionality coefficient close to the theoretical value. However, its slope is somewhat higher than can be expected from the hypothesis of purely unidirectional diffusion. At the same time, the dependence obviously does not pass through zero, namely the void size becomes zero at some nonzero shell thickness. Unfortunately, this correlation can in no case be further traced to smaller sizes, because of the observation difficulty but also because we approach the limit below which the particles can not be anymore considered as continuous solid bodies and must be treated as clusters of some definite number of atoms. If ε > 0, the growth of oxide layer occurs due to both metal ions and vacancies diffusion to the oxide-gas interface and/or due to the diffusion of oxygen ions in the opposite direction. In this case shell thickness is not anymore linear function of d but a polynomial. From eqs 1-3, one can derive the relation between the oxide shell thickness and the diameter of spherical void
[( d +2 ε ) C1 + ( 2d ) ] 3
3 1/3
-
d 2
(4)
where C ) V(Co)/V(CoO) ) 0.55 for the CoO and C) V(Co)/ 3V(Co3O4) ) 0.56 for the Co3O4 formation (here V is the molar volume of Co and its oxides, respectively). As the analysis shows, the oxide layer contained ∼33% CoO and ∼67% Co3O4. Note that the values of C are not very different for the two oxides, and from this point of view, the exact composition of shells is not very important. More important is that the density of bulk oxide might be different from the density of the same oxide in a layer. Therefore, a precise value of C is difficult to obtain, and we varied it together with ε as a fitting parameter in the equation 4. This way we obtained the values ε ) 2.66 and C ) 0.35, with the standard error weakly depending on C. It follows from our data that the diameter of the spherical cavity is always smaller than the diameter of the initial Co particle. Apparently at the initial stage of oxidation several outer cobalt layers are oxidized topotactically by means of oxygen diffusion inside the particle. Then, pulled by the Mott’s potential, the cobalt ions move through the oxide layer and form the oxide at the solid-gas interfaces. The initial oxidation steps for small cobalt particles are still not completely clear and represent a subject of investigation.21 Taking account of the possible mixed composition of the oxide does not change significantly the results of ε and C parameters estimation. From the obtained ε value, it follows that the void diameter d ) 0 corresponds to the particle diameter d0 ) 2.66 nm. This value is not far from the parameter of linear regression (2.05 nm, Figure 5). We suppose this to be an estimate of the critical particle size above which oxidation occurs with the formation of spherical cavity. The existence of such critical value must have kinetics-related physical nature since in any case hollow shells are thermodynamically unstable objects and their shrinking is always favorable being only a question of time. However at appropriate conditions, this time might be long enough to consider the shells as stable. The estimate of collapse time of a shell made in ref 6 predicts that at a given temperature the collapse time should be proportional to the third power of the final solid particle radius (which for CoO shells is related to initial shell radius by a proportionality coefficient 0.92). Therefore, the collapse time of smaller shells will decrease very rapidly, so that they just do not form. Probably this is the reason of the observed critical size, above which the particles do not shrink significantly, but below which they rapidly collapse. Note that the distribution of thickness and void diameter observed in this work suggests that no significant collapse of shells occurred. Any considerable extent of collapse would lead to a strictly different void size- shell thickness distribution. Indeed, if collapse would occur, cobalt oxide shell exterior radius should just slightly decrease while the void size drops to zero. This relationship is shown in Figure 6a, where the change of radius and void size during collapse of an imaginary cobalt oxide shell is represented. In this figure, the variations of geometrical parameters are represented for a model shell with the initial void size 12 nm, but of course these geometrical relationships can be averaged and/or scaled to any size because they depend only on relative, not on absolute, values. In other words, when collapse occurs the voids shrink and the walls thicken much more rapidly than the particles size decreases. Therefore, for any advanced degree of collapse one should observe a negative correlation between void size and shell thickness, as shown in Figure 6b. Such a correlation is quite contrary to the observed data.
Formation of Hollow Spheres
Figure 6. (a) Evolution of the exterior (R exterior) and interior (R interior) radii of a model cobalt oxide shell upon diffusion induced collapse. (b) Evolution of shell thickness vs void diameter during collapse of a model cobalt oxide shell.
Another point of concern is the interparticle sintering with formation of necks and further mass transport, which could corrupt the size distributions observed. Indeed, as seen in Figure 1, the particles considerably overlap and some contact can not be excluded. However the matter transport seems negligibly weak at the temperatures of oxidation experiment (250 °C). It is known that both Co3O4 and CoO oxides are refractory compounds. Upon heating, Co3O4 decomposes without melting to CoO, whereas the last oxide has very high melting point (1935 °C).22 Moreover, the presence of surface cobalt oxide was reported to hinder cobalt powders sintering.23 To check whether any sintering and interparticle mass transport occurs for small particles of cobalt oxide at 250 °C, cobalt oxide powder with a particles size comparable to that of Co/M was prepared by gentle decomposition of cobalt nitrate at 250 °C under argon flow. This oxide sample had a specific surface area of 32 m2/g and showed an XRD pattern perfectly corresponding to the Co3O4 oxide (JCPDS 01-080-1535) with the line width leading to a particle size 19.7(5) nm according to the Scherrer equation. After heating in air overnight at 250 °C or even 300 °C, the specific surface area changed to 30.3 and 31.1 m2/g, respectively, which within the range of experimental error corresponds to a fairly constant value. XRD particles size was also constant, namely 20.2(5) nm for the sample heated in air at 250 °C and 19.9 (5) nm for that heated at 300 °C. Some sintering was observed only above 400 °C (17 m2/g for specific surface area and 24.1 (5) nm for XRD particle size) and continued above this temperature, for the solid heated at 500 °C showing a specific surface area of 15 m2/g and a mean particle size of 30.4 (5) nm. In brief, the sintering of cobalt oxide particles was negligible at the oxidation experiment condition. Therefore, upon oxidation the formation of hollow shells is driven by chemical potential difference and when completed, the morphology of particles stays stable. Note that sintering was studied in pure cobalt oxide in which the
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9577 particles are in intimate contact, whereas in the supported system, the interaction between oxide particles should be even weaker. It is worth emphasizing than the walls of the observed cobalt oxide shells are irregular with many cracks and they consist of polycrystalline cobalt oxide. Further reduction of this oxide would not lead to the metallic shells but certainly break it into many smaller cobalt pieces; that is, it would lead to the redispersion of cobalt. Any other chemical or thermal treatment will also easily destroy these shells. For example, we observed rapid decomposition of shells to smaller pieces under the electron beam of the transmission microscope (this was very pronounced at high magnifications when the beam was condensed on a small area of sample). Note as well that the thickness of the cobalt oxide shell is by definition smaller than the diameter of the initial cobalt particle. The redispersion of Co particles induced by reduction-oxidation treatment was reported previously for Co/SiO224,25 and Co/C26 catalysts. This phenomenon was ascribed earlier to cracking of the faulted oxidized particles upon reduction in hydrogen. In the present work, we provide a straightforward explanation of cobalt redispersion in supported catalysts by the spacing of cobalt species due to formation of shells. The redispersion phenomena are known for cobalt27 and other metals such as palladium,28,29 iridium,30 and rhodium.31 These phenomena play considerable role in the evolution of catalytic materials during their regeneration. Conclusion In this work, we first report on the formation of hollow spheres on a supported heterogeneous catalyst. We suppose that the phenomena similar to that observed here may lead to redispersion of metal phase in various supported metal catalysts, including nickel, cobalt, iron, and some noble metals. At the same time the straightforward method of improving morphology of the supported phase follows from these results. The novel point is that oxidation should be carried out at a low enough temperature in order to allow formation of hollow particles. This should be followed by their subsequent low-temperature reduction. Future work will aim on other metals and on the thorough morphological and catalytic characterization of metal phase after reduction of hollow shells. On the other hand, this works shows that that, during low temperature oxidation, the morphology of the hollow shells depends mostly on the initial particles size, though some correction should be made for backward diffusion occurring probably at the initial stages of oxidation. Acknowledgment. This work has been carried out with the financial support of the Russian Fundamental Research Foundation (Grant No. 06-03-32500 a`). References and Notes (1) Atkinson, A. ReV. Mod. Phys. 1985, 57, 437. (2) Kofstad, P. High Temperature Corrosion, Elsevier Applied Science: London, 1988. (3) Rojas, T. C.; Greneche, J. M.; Conde, A.; Fernandez, A. J. Mater. Sci. 2004, 39, 4877. (4) Yin, Y.; Riou, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (5) Wang, C. M.; Baer, D. R.; Thomas, L. E.; Amonette, J. E.; Antony, J.; Qiang, Y.; Duscher, G. J. Appl. Phys. 2005, 98, 094308. (6) Gusak, A. M.; Zaporozhets, T. V.; Tu, K. N.; Gosele, U. Phil. Mag. 2005, 85, 4445. (7) Henkes, A. E.; Vasquez, Y.; Schaak, R. E. J. Am. Chem. Soc. 2007, 129, 1896. (8) Chiang, R. K.; Chiang, R. T. Inorg. Chem. 2007, 46, 369.
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Chernavskii et al. (21) Chernavskii, P. A.; Pankina, G. V.; Chernavskii, A. P.; Peskov, N. V.; Afanasiev, P.; Perov, N. S.; Tennov, V. A. J. Phys. Chem. C 2007, 111, 5576. (22) Lide, D. R., Ed.; Handbook of Chemistry & Physics; CRC Press: Boca Raton, FL, 2008. (23) Sakka, Y. J. Less Common Met. 1991, 168, 277. (24) Potoczna-Petru, D.; Keˆpin˜ski, L. Catal. Lett. 1991, 9, 355. (25) Potoczna-Petru, D.; Jabłlon´ski, J. M.; Okal, J.; Krajczyk, L. Appl. Catal., A 1998, 175, 113. (26) Potoczna-Petru, D.; Krajczyk, L. J. Mater. Sci. Lett. 1995, 14, 1294. (27) de la Pen˜a O’Shea, V. A.; Homs, N.; Fierro, J. L. G.; Ramı´rez de la Piscina, P. Catal. Today 2006, 114, 422. (28) Datye, A. K.; Bravo, J.; Nelson, T. R.; Atanasova, P.; Lyubovsky, M.; Pfefferle, L. Appl. Catal., A 2000, 198, 179. (29) Dacquin, J. P.; Dujardin, C.; Granger, P. J. Catal. 2008, 253, 37. (30) Fung, S. C. Ind. Eng. Chem. Res. 2003, 42, 1551. (31) Dictor, R.; Roberts, S. J. Phys. Chem. 1989, 93, 5846.
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