MgO Catalysts - The

Transmission electron microscope JEM 2010 with 200 kV acceleration voltage and 0.14 .... Mason (18) has shown that small metal particles (Pd, Pt, Au) ...
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J. Phys. Chem. C 2008, 112, 14776–14780

Valence State Study of Supported Ruthenium Ru/MgO Catalysts Yurii V. Larichev* BoreskoV Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika LaVrentieVa 5, NoVosibirsk, 630090, Russia ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: July 16, 2008

Ru/MgO catalysts have been studied by X-ray photoelectron spectroscopy, transmission electron microscopy, and X-ray diffraction. Significant part (up to 75%) of supported metal was found to exist in an X-ray amorphous state with the particle size smaller than 3 nm. The analysis of the spectra belonging to the core levels and the valence zone showed that the X-ray amorphous part of ruthenium may exist in the forms of both metal and oxide clusters and nanoparticles depending on the ruthenium precursor. TABLE 1: Texture Properties of Supports

1. Introduction Supported Ru is a very promising alternative to traditionally used Fe catalyst for ammonia synthesis.1-4 Ru supported on MgO is a convenient system for spectroscopic investigation judging by research in this field.5-8 The catalytic activity of Ru/MgO was studied earlier6-8 and associated with electron withdrawal from the MgO support to Ru leading to Ruδformation, which is favorable in nitrogen dissociative adsorption process. The latter is a bottleneck step in ammonia production. Some proofs for Ruδ- formation were given by the X-ray photoelectron spectroscopy (XPS) data.5,6,8 The Ru3d5/2 binding energy was found to be 279.5-279.8 eV, while for bulk Ru metal it was 280.2 eV. Such negative shift to lower binding energies, probably due to electron withdrawal to Ru and Ruδformation, was suggested. We have shown recently that differential charging affects on the binding energy of the Ru 3d5/2 core level measured for supported Ru. The account of the differential charging gives Ru3d5/2 binding energy of 280.5 eV.9 The corrected value is shifted positively relative to Ru bulk, instead of negatively as it was assumed for Ruδ-. Therefore, no evidence of such species were found, and Ru supported on MgO is partially oxidized rather than reduced. Ru/Al2O3 and Ru/TiO2 catalysts were investigated by Verykios et al.10 It was found that the valence state of Ru depends on the type of support. This finding opens a number of new questions. The first one is the problem of identification of all oxidative Ru forms present on MgO. This work is an attempt to address this problem using XPS, TEM, and XRD techniques. 2. Experimental Section 2.1. Sample Preparation. Three supported Ru/MgO catalysts, ruthenium black, and ruthenium oxide (RuO2) were studied. Two types of MgO with different surface areas were used as catalyst supports (see Table 1). The first one is a commercial MgO-R (KHIMPROM, Russia) sample with specific surface area of 22 m2/g. The second one with 200 m2/g surface area was prepared by precipitation from aqueous Mg(NO3)2 solution with KOH, followed by drying in air and calcination in dry air flow at 723 K for 2 h. * To whom correspondence should be addressed. E-mail: ylarichev@ gmail.com or [email protected].

support

S (m2/g)

Vpore (cm3/g)

Rpore (nm)

1. MgO-R 2. MgO

22 200

0.09 0.42

20.0 9.0

TABLE 2: Metal Concentrations and Nomenclature of Ru Samplesa sample

Ru (% wt)

precursor

Ru(Cl)/MgO Ru(Cl)/MgO-R Ru(AA)/MgO

5.0 4.4 5.0

Ru(OH)Cl3 Ru(OH)Cl3 Ru(acac)3

a Ru concentrations as determined by X-ray fluorescence technique.

The active component precursor, either Ru(OH)Cl3 or Ru(acac)3, was supported by incipient wetness impregnation from the acetone solution. The precursor concentration was 25 mg/ mL. The catalyst samples were reduced in pure hydrogen flow in a glass tube reactor at 723 K for 6 h. The sample preparation method is described in detail elsewhere.11 The ruthenium concentrations in the samples determined by X-ray fluorescence technique are reported in Table 2. Ru content in the samples was determined with the accuracy of 0.1 wt % Ru. Ruthenium black was obtained by Ru(OH)Cl3 reduction with formalin in aqueous alkali solution at 353 K.12 After washing and drying, the sample was additionally reduced by heating in hydrogen flow at 623 K. Ruthenium oxide sample was obtained from ruthenium hydroxide precipitated from Ru(OH)Cl3 solution with NaOH by dehydrating at 423 K. According to the XRD data, the obtained material has the RuO2 (database JCPDS 40-1290) structure. 2.2. Sample Characterization. Supported samples were studied by transmission electron microscopy (TEM) to obtain information on their particles sizes. Transmission electron microscope JEM 2010 with 200 kV acceleration voltage and 0.14 nm resolution was used in the studies. X-ray diffraction (XRD) was performed using a HZG-4 X-ray diffractometer with Cu KR radiation and a graphite monochromator. The data were collected for 5 s per step with step size 0.05 ° within the range 20° e 2θ e 130°. The parameter of the MgO cell (cubic structure, a) was calculated using {4.2.0.} and {4.2.2.} reflections. The parameters of the Ru cell (hexagonal structure a, c) were calculated using {1.0.0.}, {1.0.2.} and {1.1.0.} reflections. The particle sizes of the Ru crystallites and

10.1021/jp803742g CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

Valence State Study of Supported Ruthenium Ru/MgO Catalysts

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14777

Figure 1. Representative TEM micrographs of the Ru/MgO catalysts: Ru(Cl)/MgO (A), Ru(AA)/MgO (B), and Ru(Cl)/MgO-R (C).

MgO crystallites were calculated by the Scherrer equation: ) Kλ/(b - b0) cos θ; where λ is wavelength of the X-rays, θ is half of the scattering angle; b and b0 are the observed and the instrumental half-width of the peak on the 2θ scale in radians; K ) 1.0. N2 adsorption-desorption isotherms were performed at 77 K using a Micromeritics ASAP 2400 instrument. The pore size distribution was calculated from the nitrogen adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method. The electronic properties of the samples were studied by XPS. The spectra were registered using VG ESCALAB HP electron spectrometer with nonmonochromatic Al KR irradiation (hν) 1486.6 eV, 200 W). The binding energy scale of the spectrometer was calibrated using positions of the core levels Au4f7/2 (84.0 eV) and Cu2p3/2 (932.6 eV). Ru/MgO samples reduced with hydrogen were pressed into a Ni grid and placed into the spectrometer pretreatment chamber. During this procedure, the samples were exposed to air for no longer than 5 min. In the pretreatment chamber, the samples were additionally treated with hydrogen under static conditions at 623 K and 0.1 MPa pressure for 1 h. Then, they were evacuated to 10-5 Pa and transferred to the analytical chamber to register the spectra. The Mg2s line from support with binding energy 88.1 eV did not change during the experiments. For this reason, it was used to calibrate the photoelectron lines. The charging energy was determined as the difference between the measured binding energy and the tabulated value. The photoelectron lines of other elements are shifted by this value. 3. Results 3.1. TEM Measurements. TEM images of samples (Figure 1) are used to build particle size distribution histograms of supported Ru metal (Figure 2) and determine average sizes of ruthenium particles (Figure 2, Table 3). Despite high resolution of the electron microscope (0.14 nm), the minimum observable size of the metal particles is limited to 1.5 nm due to the low contrast of the small metal particles at the MgO background. The obtained data (Table 3) show that the size of Ru particles is smaller in Ru(AA)/MgO sample whereas the use of Ru(OH)Cl3 as the precursor leads to the formation of larger Ru particles. Ru(acac)3 precursor leads to catalysts with high metal dispersion.13,14 3.2. XPS Measurements. Figure 3 presents Ru3d spectra of the samples. The spectra consist of the Ru 3d5/2 and Ru 3d3/2 peaks resulting from the spin-orbital splitting. The Ru 3d3/2 peak overlaps with C 1s peak (∼285 eV) from the traces of carbonaceous impurities (CHx), which were present on the catalyst surface or slowly accumulated in the spectrometer

Figure 2. Particle size distributions for Ru(AA)/MgO, Ru(Cl)/MgO and Ru(Cl)/MgO-R. ( ) Σdi/N, ) Σidi 3/Σidi2, where N is the total number of particles measured in the TEM images, i is the summation index).

TABLE 3: Mean Size of Ru Particles Measured by TEM and XRD (nm) samples





Ru(Cl)/MgO-R Ru(Cl)/MgO Ru(AA)/MgO Ru black RuO2

7.2 3.5 2.2 33.0

30.0 10.0 5.5 45.0 13.0

chamber. Taking into account the effects of the final state (differential charging effect9,15), Ru 3d5/2 binding energies in all Ru/MgO samples were found to be about 280.5 eV (Table 4). Such binding energies usually correspond to finely dispersed ruthenium metal.16 The value of the Ru 3d5/2 binding energy observed for bulk ruthenium metal in the ruthenium black sample is slightly lower 280.2 eV (Table 4) and agrees with the earlier obtained data.17 Ru 3d5/2 binding energies in RuO2 and Ru(OH)Cl3 are significantly higher than in Ru0 (Table 4). Mason18 has shown that small metal particles (Pd, Pt, Au) on the carbonaceous support obtained via thermal deposition in vacuum possesses higher binding energies than bulk metals.1 The typical binding energy increases by 1 eV. Only small (no

14778 J. Phys. Chem. C, Vol. 112, No. 38, 2008

Larichev

Figure 3. Ru3d core level spectra of Ru(AA)/MgO, Ru(Cl)/MgO, and Ru(Cl)/MgO-R.

TABLE 4: Values of Binding Energy Ru3d5/2 and FWHM (Full Width at Half Maximum) for Ruthenium Samples Samples

Ebin(Ru3d5/2), eV

FWHM, eV

Ru(Cl)/MgO Ru(Cl)/MgO-R Ru(AA)/MgO Ru black RuO2 Ru(OH)Cl3

280.5 280.4 280.6 280.2 281.4 282.0

3.0 2.3 3.6 1.3 2.7 2.3

more than 10 atoms) clusters of metal are affected this way. For example, the Pd/SiO2 catalyst with average size of Pd particle of 1.5 nm has the Pd3d5/2 binding energy approximately equal to that of bulk Pd.19 According to TEM measurements, Ru particles in our samples are much larger than 1-2 nm, so such effects can be neglected. Ru3d spectra of Ru/MgO samples (Figure 3) are relatively wide with half-width about 3 eV. This may indicate the presence of additional ruthenium valence states in spectrum. This could be RuO2 or Ru3+ and Ru4+ ions dissolved in the MgO lattice.20 However, the Ru3d spectra are fairly symmetric and can be described well by a single peak. So, it is impossible to make an unambiguous conclusion on the presence or absence of additional ruthenium states different from Ru0 based only on these data. Some information on the formation of oxidized ruthenium can be obtained from the analysis of the O1s spectra. The O1s XPS spectra of samples MgO, Ru(AA)/MgO, Ru(Cl)/MgO, and Ru(Cl)/MgO-R are presented in Figure 4. The spectra look as an asymmetric line that can be deconvolved into two components related to the different chemical states of oxygen. The main component at Ebin ) 529.8 eV belongs to O2- ions in the MgO lattice. A less intense component observed in the O1s spectra at Ebin ) 532.3 eV can be attributed to OH groups of the MgO surface.21 In the spectra of Ru(Cl)/MgO and Ru(Cl)/ MgO-R, this component is shifted to lower binding energies by approximately 0.5 eV in comparison with the spectrum of pure MgO. Meanwhile, this shift is not observed for Ru(AA)/ MgO sample (Figure 4). The shift of the O1s lines to lower energy values can be interpreted as the formation of Ru-O bonds, because typical O1s bonding energies in RuO2 are 530.0 and 531.7 eV (Figure 4).22 Figure 5 presents the difference valence band spectra of Ru(Cl)/MgO catalyst and the spectrum of bulk Ru for comparison. The difference spectra were obtained by subtracting the spectra of the corresponding MgO supports from the spectra of Ru/MgO samples. Two bands with different intensities are

Figure 4. O1s core level spectra of MgO support, Ru(AA)/MgO, Ru(Cl)/MgO, and Ru(Cl)/MgO-R. A RuO2 spectrum is shown on inset.

Figure 5. Valence band spectra of Ru(Cl)/MgO, MgO, difference spectra between Ru(Cl)/MgO and MgO, and spectra of Ru bulk.

Figure 6. Difference valence band spectra for supported systems (left): 1-Ru(Cl)/MgO-R,2-Ru(Cl)/MgO, 3-Ru(AA)/MgO. Valence band spectra for Ru compounds (right): 4-Ru black,5-RuO2, 6-Ru(OH)Cl3. Oval shows field corresponding to Ru4d5s and O2p valence bands.

observed in the difference spectra at 4.1 and 7.1 eV in addition to the wide band of ruthenium metal (Figure 6). These bands are not observed in the difference spectrum of Ru(AA)/MgO. Note that the difference spectra of Ru(Cl)/MgO and Ru(Cl)/ MgO-R samples qualitatively resemble the valence band spectra of Ru(OH)Cl3 and RuO2.

Valence State Study of Supported Ruthenium Ru/MgO Catalysts

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14779

TABLE 5: Values of Binding Energy Cl2p3/2 and Chlorine Content in Samples samples

Ebin(Cl2p3/2), eV

Cl/Mg

Ru(Cl)/MgO Ru(Cl)/MgO-R Ru(AA)/MgO MgO MgO-R

198.5 198.3 198.4 198.4 198.4

0.033 0.106 0.003 0.003 0.007

TABLE 6: Lattice Parameters and Mean Sizes of MgO Particles Measured by XRD (nm) samples

a (MgO)

D (MgO)

MgO MgO-R Ru(Cl)/MgO Ru(AA)/MgO Ru(Cl)/MgO-R

0.4224 0.4213 0.4212 0.4224 0.4213

6.0 50.0 22.0 7.0 65.0

The observed Cl2p3/2 signal indicates a small amount of chloride ions present as an impurity (Table 5). The highest concentration of chloride ions is in Ru(Cl)/MgO-R, while the lowest one in Ru(A A)/MgO. All Cl2p3/2 binding energies of the studied samples correspond to those of the initial supports without Ru (Table 5). Therefore the MgO support is responsible for chloride ions impurities in Ru(AA)/MgO. 3.3. XRD Measurements. Reflections attributed to ruthenium metal (marked with an asterisk) and MgO support (marked with a double asterisk) are observed in the X-ray diffraction pattern (Figure 7). The Ru black and RuO2 X-ray diffraction patterns are shown on the inset of Figure 7. The average sizes of the Ru particles were estimated (Table 3). The lattice parameters and mean sizes of MgO particles are shown in Table 6. The MgO lattice parameter of the Ru(Cl)/MgO sample is smaller than in the other samples and in pure MgO powder. The mean size of MgO particles is also increased in this case. The lattice parameters of supported ruthenium match those obtained for the ruthenium black within the experimental error (for supported ruthenium a ) 0.270(4) nm, c ) 0.428(2) nm; for bulk ruthenium a ) 0.2707(2) nm, c ) 0.4280(3) nm). No reflections attributed to ruthenium oxide are observed. To determine whether all deposited ruthenium exists in the form of Ru particles with the measured particle size, a calibration was performed using mechanical mixtures of Ru black and MgO containing 2, 4, and 6% Ru (Figure 8).23 The ratios of the integral intensities of {1.0.0.} Ru reflection and {1.1.1.} MgO reflection were measured for different ruthenium concentration. The application of this calibration to Ru(Cl)/MgO showed that the observed diffraction accounts for 40% of the supported metal whereas 60% of the metal remains in an invisible X-ray amorphous state. For Ru(Cl)/MgO-R diffraction, 76% of the supported metal is observed with the remaining 24% being X-ray amorphous. For Ru(AA)/MgO most of the supported metal (75%) is in an X-ray amorphous state with the observed diffraction accounting for only 25%. Small crystalline Ru particles (with sizes less than 3 nm) together with amorphous particles and oxidized forms of Ru are X-ray invisible. 4. Discussion Comparison of the valence band spectra of different ruthenium compounds (Figure 6) suggests that the range 4-8 eV is predominantly related to Ru4d5s and O2p bands. This conclusion agrees with the data reported by Sven et al. 24 who studied the valence band spectra of oxidized ruthenium. So, additional peaks observed in the difference spectra (Figure 5) at 4.1 and

Figure 7. X-ray diffraction patterns for MgO and Ru(Cl)/MgO. A Ru black and RuO2 X-ray diffraction patterns are shown on inset. (Asterisk corresponding to Ru reflections, double asterisk corresponding to MgO reflections).

Figure 8. Straight line shows Ru content in mechanical mixtures of Ru black and MgO. Corresponding dots show part of crystalline Ru in samples ((O), Ru(Cl)/MgO-R, (4), Ru(Cl)/MgO, (]), Ru(AA)/MgO).

7.1 eV can be attributed to RuO2 particles having the particle size ranging from 0.6-0.7 nm (ca. 100 atoms) to 1.5-2.0 nm (ca. 1000 atoms). At sizes smaller than 0.6 nm, an oxide particle does not have a wide band structure and possesses only a set of narrow lines that will not be identified due to the relatively low spectrometer sensitivity. Larger oxide particles starting from 2 nm can be identified by TEM and XRD. On the basis of these data, the presence of RuO2 particles is expected for Ru(Cl)/MgO and Ru(Cl)/MgO-R samples. Since no reflections attributed to ruthenium oxide are observed by XRD. The calibration measurements using mixtures of Ru black and MgO powders were performed to estimate the amount of the X-ray invisible part of Ru. A significant part of the supported Ru is found in the X-ray amorphous state. TEM application to supported catalysts does not allow for distinguishing objects smaller than 1.0-1.5 nm due low contrast. This makes TEM and XRD straightforward techniques inapplicable for an exploration of the supported Ru nanoparticles below 1.5 nm in size. It seems unlikely for Ru to penetrate in the support and form spinel-like phases, because lattice parameter changes significantly in that case. For example, when the NixMg1-xO phase is formed, the lattice parameter decreases from 0.42123 nm down

14780 J. Phys. Chem. C, Vol. 112, No. 38, 2008 to 0.41801 nm.25 The differences occurring in our samples are in ranges typical for variation of the concentrations of crystal defects and surface OH- groups.26 The differences in the MgO support calcination temperatures lead to different concentrations of the MgO defects. Thus, the MgO lattice parameter decrease in Ru(Cl)/MgO from 0.4224 to 0.4212 nm is most probably due to the sintering of the support (Table 6). Summarizing all the above, we see that significant part of supported Ru is hidden from XRD and TEM observation. This part is not dissolved in the MgO support and according to XPS it looks like some oxidized form of Ru, such as RuO2. Despite the high amount of X-ray amorphous part in Ru(AA)/MgO (75%), according to XPS it does not contain oxidized Ru. The reason for the presence of finely dispersed RuO2 in Ru(Cl)/MgO and Ru(Cl)/MgO-R and its lack in Ru(AA)/MgO can be explained as follows. Ru(OH)Cl3 is partially hydrolyzed by the traces of water in acetone on the MgO surface to form Ru(OH)4 that is later decomposed to RuO2. Meanwhile Ru(acac)3 is relatively stable under these conditions and not subjected to such decomposition process. The stability of RuO2 nanoparticles during the sample reduction in hydrogen can be explained by the presence of an epitaxial interaction between the RuO2 and MgO layers. According to ref 27, such epitaxial interaction indeed exists in the RuO2/MgO system between {1.1.0.} RuO2 and {1.0.0.} MgO layers. Thus, an epitaxy between RuO2 and MgO stabilizes RuO2 nanoparticles during treatment of the sample in hydrogen at 723 K. So, in Ru(Cl)/MgO and Ru(Cl)/MgO-R some part of supported Ru is present in the oxidized state, whereas in Ru(AA)/MgO such state appears to be unlikely. 5. Conclusions Ru/MgO catalysts are studied by XPS, TEM, and XRD. A significant part of supported metal is found to exist in an X-ray amorphous state with the particle size smaller than 3 nm. For Ru(Cl)/MgO the X-ray amorphous part is 60%, for Ru(AA)/ MgO it is 75%, and for Ru(Cl)/MgO-R it is only 24%. Independent of the precursor, there is no evidence found for the penetration of the supported metal ions in the support to form spinel-like phases. The analysis of the XPS spectra belonging to the core levels and the valence zone shows that the X-ray amorphous part of ruthenium may exist in the forms of both metal and oxide clusters and nanoparticles depending on the ruthenium precursor. In Ru(Cl)/MgO and Ru(Cl)/MgO-R prepared from ruthenium chloride, supported ruthenium exists after reduction in hydrogen both in the form of relatively large (larger than 2 nm) Ru0 particles and relatively small RuO2 nanoparticles with the sizes ranging from 0.6 to 1.5-2.0 nm. According to ref 27 an epitaxy between RuO2 and MgO layers is the reason for RuO2 nanoparticles to be stabilized. For Ru(AA)/MgO sample prepared from Ru(acac)3, all ruthenium is supported in the form of Ru metal nanoparticles.

Larichev Acknowledgment. The author is grateful to B.L. Moroz for assistance in the synthesis of the samples, to V.I. Zaikovskii and E.M. Moroz for assistance in the investigation of the samples by TEM and XRD, and S.E. Malykhin for his interest and stimulating discussions. References and Notes (1) Czuppon, T. A.; Knez, S. A.; Schneider, R. V.; Worobets, G. Chem. Eng. 1993, 100, 19. (2) Kowalczyk, Z.; Jodzis, S.; Rarog, W.; Zielinski, J.; Pielaszek, J. Appl. Catal., A 1998, 173, 153. (3) Aika, K.; Niwa, Y. In Studies in Surface Science and Catalysis; Hattori, H., Otsuka, K., Eds.; Elsevier Science: Tokyo, 1999; Vol. 121, p 327. (4) Bielawa, H.; Hinrichsen, O.; Birkner, A.; Muhler, M. Angew. Chem., Int. Ed. 2001, 40, 1061. (5) Muhler, M.; Rosowski, F.; Hinrichsen, O.; Hornung, A.; Ertl, G. In Studies in Surface Science and Catalysis; Hightower, J. W., Delgass, W. N., Iglesia, E., Bell, A. T., Eds.; Elsevier: Berlin, 1996; Vol. 101, p 317. (6) Aika, K.; Takano, T.; Murata, S. J. Catal. 1992, 136, 126. (7) Kubota, J.; Aika, K. J. Phys. Chem. 1994, 98, 11293. (8) Aika, K.; Ohya, A.; Ozaki, A.; Inoue, Y.; Yasumori, Y. J. Catal. 1985, 92, 305. (9) Larichev, Y. V.; Moroz, B. L.; Zaikovskii, V. I.; Yunusov, S. M.; Kalyuzhnaya, E. S.; Shur, V. B.; Bukhtiyarov, V. I. J. Phys. Chem. C 2007, 111, 9427. (10) Elmasides, C.; Kondarides, D. I.; Gru1nert, W.; Verykios, X. E. J. Phys. Chem. B 1999, 103, 5227. (11) Larichev, Y. V.; Moroz, B. L.; Moroz, E. M.; Zaykovskii, V. I.; Yunusov, S. M.; Kaluzhnaja, E. S.; Shur, V. B.; Bukhtiyarov, V. I. Kinet. Catal. 2005, 46 (6), 891. (12) Kobayashi, M.; Shirasaki, T. J. Catal. 1973, 28, 289. (13) Neri, G.; Mercadante, L.; Donato, A.; Visco, A. M.; Galvagno, S. Catal. Lett. 1994, 29, 379. (14) Murata, S.; Aika, K. J. Catal. 1992, 136, 110. (15) Larichev, Y. V.; Moroz, B. L.; Prosvirin, I. P.; Bukhtiyarov, V. I.; Likholobov, V. A. Chem. Sustainable DeV. 2003, 11, 155. (16) Cattania, M. G.; Parmigiani, F.; Ragani, V. Surf. Sci. 1989, 211/ 212, 1097. (17) Briggs, D.; Seah, M. P. Practical Surface Analysis; Wiley: Chichester, England, 1992. (18) Mason, M. G. Phys. ReV. B 1983, 27 (2), 748. (19) Bertolini, J. C.; Delichere, P.; Khanra, B. C.; Massardier, J.; Noupa, C.; Tardy, B. Catal. Lett. 1990, 6, 215. (20) Ketchik, S. V.; Plyasova, L. M.; Chigrina, V. A.; Minyukova, T. P.; Yurieva, T. M. React. Kinet. Catal. Lett. 1980, 14 (2), 135. (21) Nefedov, V. I.; Gati, D.; Dzhurinskii, B. F.; Serguhin, N. P.; Salyn, Y. V. Russ. J. Inorg. Chem. 1975, 20, 2307. (22) Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. J. Catal. 1997, 172, 336. (23) Al-Jaroudi, S. S.; Ul-Hamid, A.; Mohammed, A. I.; Saner, S. Powder Technol. 2007, 175, 115. (24) Sven, J. Y.; Adnot, A.; Kaliaguine, S. Appl. Surf. Sci. 1991, 51, 47. (25) Kuzminy, A.; Mironovaz, N. J. Phys.: Condens. Matter 1998, 10, 7937. (26) Wang, J. A.; Novaro, O.; Bokhimi, X.; Lopez, T.; Gomez, R.; Navarrete, J.; Llanos, M. E.; Lopez-Salinas, E. J. Phys. Chem. B. 1997, 101, 7448. (27) Gao, Y.; Bai, G.; Liang, Y.; Dunham, G. C.; Chambers, S. A. J. Mater. Res. 1997, 12 (7), 1844.

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