Alumina-Pillared Clay ... - ACS Publications

Aug 30, 2008 - UniVersity of Salamanca, Plza. de la Merced, E-37008 Salamanca, Spain. The aim of this work is to study the effect of the thermal treat...
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Ind. Eng. Chem. Res. 2008, 47, 7226–7235

MATERIALS AND INTERFACES Structure Evolution of Co/Alumina-Pillared Clay Catalysts under Thermal Treatment at Increasing Temperatures A. Gil,† R. Trujillano,‡ M. A. Vicente,‡ and S. A. Korili*,† Department of Applied Chemistry, Building Los Acebos, Public UniVersity of NaVarre, Campus of Arrosadia, E-31006 Pamplona, Spain, and Department of Inorganic Chemistry, Faculty of Chemical Sciences, UniVersity of Salamanca, Plza. de la Merced, E-37008 Salamanca, Spain

The aim of this work is to study the effect of the thermal treatment at 350 and 500 °C on the structure of a Co/alumina-pillared montmorillonite catalyst. For this purpose, the X-ray powder diffraction patterns of the solids, the nitrogen physisorption data at -196 °C, the micropore-size distributions, the UV-vis and NIR spectroscopies of the dried materials, the gravimetric and differential thermal analyses, and the temperatureprogrammed reduction analyses of the supported cobalt catalysts have been analyzed and compared. The impregnation with cobalt and the temperature has modified the textural properties of the alumina-pillared clay supports, giving rise to a loss of surface area and micropore volume in the final catalysts. Co3O4 is the only cobalt oxide species detected in all the supported catalysts. The oxidation of propene at low concentrations over selected cobalt oxide catalysts shows that the structural characteristics of the support influence on the catalytic performance. 1. Introduction In the last years, the synthesis of inorganic porous solids with a controlled pore structure has been of great interest because of the potential applications of these materials in catalysis, purification, and sorption-based processes.1 Pillared interlayered clays have been one of the more attractive groups of porous solids due to their controllable pore dimensions2 and catalytic properties, depending on the type of silicate layers and pillaring agents.3-6 The incorporation of metal ions, either at the stage of pillar formation or through a postpillaring treatment, has extended the field of application of pillared clays in several environmental processes such as deNOx reactions or oxidation of volatile organic compounds (VOCs) among others.3,5 The catalytic properties of these materials depend not only on the presence of catalytically active sites but also on the textural properties developed during synthesis. Since the 1980s, much interest has been focused on the surface characteristics of cobalt-supported catalysts, used in hydrocracking processes, Fischer-Tropsch synthesis, and hydrodesulfurization reactions. Several techniques, such as lowenergy ion scattering spectroscopy, photoacoustic spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, have been used in studies concerning the chemical nature of the cobalt species on the catalyst surface.7-12 On alumina-supported catalysts, two cobalt species can be formed after calcination: an easily reducible Co3O4 phase, observed for metal loadings higher than 2 wt %,7 and a surface spinel phase, CoAl2O4, composed by cobalt ions that have diffused into the support lattice. The calcination temperature and the cobalt loading affect the relative concentration of these two species in the alumina-supported catalysts. On silica-supported cobalt catalysts the usual cobalt * To whom correspondence should be addressed. Phone: +34-948168982. Fax: +34-948-169602. E-mail: [email protected]. † Public University of Navarre. ‡ University of Salamanca.

phase formed upon calcination is Co3O4.11 Under TPR analysis, the reduction of cobalt oxide proceeds through two stages, first Co3O4 is reduced to CoO, and then CoO is reduced to metallic cobalt. The formation of a surface spinel Co2SiO4 has also been reported.10,13 Pillared clays have also been used as supports for cobalt catalysts.14-22 The main use of these catalysts has been in the selective catalytic reduction of nitrogen oxides. The deNOx reaction was achieved over several catalysts such as precious metals, metal oxides, and metal ion-exchanged zeolites.23 Among the catalysts used in the deNOx reaction, those using ZSM-5 zeolite as a support have presented high performances and selectivities for NO decomposition in the presence of hydrocarbons. These catalysts are often deactivated by the deposition of residual carbon in the hydrocarbon combustion. Pillared clays can be considered as promising alternative supports because they have a larger pore size than the zeolites24 thus making pore blockage by carbon deposition more difficult. The destruction by complete oxidation of hazardous gaseous pollutants is other interesting application of pillared clay catalysts.22,25-30 Pollutants are usually present in air streams at low concentrations, requiring technologies highly efficient for the total oxidation avoiding the formation of harmful byproducts.31 In spite of the very interesting catalytic properties of cobalt oxides, to our best knowledge, there is only one previous paper dealing with cobalt oxides catalysts supported on pillared clays and applied in VOC abatement.22 Propene is used widely as an alkylation or polymer-gasoline feedstock for octane improvement. Large quantities are also used in plastics, as propylene, and in chemicals, e.g., cumene, glycerine, 2-propanol, and propylene oxide.32 Propene is considered as a highly reactive volatile organic compound because is involved in the formation of ground-level and tropospheric ozone and, therefore, in photochemical smog.33 Moreover, propene has anesthetic properties at high concentrations, can cause asphyxiation, and is a flammable gas under

10.1021/ie071320v CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7227 Table 1. Chemical Composition of the Dry Solids (wt %) sample

SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

GAmont [Co/(GAmont)350]350 [Co/(GAmont)500]500 GAmont-Al [Co/(GAmont-Al)350]350 [Co/(GAmont-Al)500]500

65.14 62.41 62.76 60.32 58.11 57.31

24.98 24.34 24.42 32.94 32.21 33.09

2.66 2.62 2.68 2.54 2.35 2.41

0.09 0.06 0.06 0.08 0.07 0.07

5.22 5.82 5.75 3.83 3.76 3.83

1.32 1.08 1.08 0.01 0.01 0.01

0.25 0.11 0.10 0.03 0.04 0.02

0.14 0.10 0.11 0.10 0.12 0.11

0.18 0.17 0.17 0.15 0.15 0.15

normal atmospheric conditions.32 Due all these characteristics, there is an important interest in controlling the presence of propene in ambient air and gaseous emissions. In this work, which is a part of a systematic study on the preparation of metal catalysts supported on pillared clays, we report the detailed characterization of the cobalt species formed on an alumina-pillared montmorillonite. The catalysts were prepared by incipient wetness impregnation with cobalt solutions and calcined at several temperatures. The effect of the calcination temperature on the cobalt species was investigated. The catalytic performance on the oxidation of propene of selected catalysts was also presented. 2. Experimental Section 2.1. Raw Materials, Intercalation Process, and Catalyst Preparation. The natural clay mineral used in this work was a montmorillonite from Gador (Almerı´a, Spain). The raw material was washed with 0.5 M acetic acid solution before the extraction of the 2-µm fraction in order to facilitate its dispersion in water reducing the presence of carbonates. The particle fraction below 2 µm was obtained by careful aqueous decantation of the raw material. The parent clay was intercalated with aluminum polycations according to a procedure described previously.34 An aluminum polycation solution35,36 was prepared by slow addition of a solution of NaOH (Panreac, p.a.) to a solution of AlCl3 · 6H2O (Panreac, p.a.), under vigorous stirring, with an OH-/Al3+ mole ratio equal to 2.2. The resulting solution was left to age for 24 h at room temperature under constant agitation. The pH after aging was 4.1. The interlayered clay was obtained by addition of the aqueous solutions of hydroxy-aluminum to 8 g of the montmorillonite, previously suspended in water, at an Al/clay ratio of 5 mmol/g. The slurry was stirred for 24 h at room temperature and then centrifuged and washed by dialysis with distilled water until no chloride was present in the filter wash water. The resulting intercalated clay was dried in air at 50 °C for 16 h and then calcined at 350 and 500 °C for 4 h, at a heating rate of 5 °C/min, in order to obtain the alumina-pillared montmorillonite. The subsequent solids are designated below as (GAmont-Al)T, where GAmont refers to the Gador montmorillonite used in the intercalation process, Al indicates that aluminum polycations were used for intercalation, and T is the calcination temperature. Thus, for example, (GAmont-Al)500 corresponds to a Gador montmorillonite clay intercalated with aluminum polycations and calcined at a temperature of 500 °C. The supported cobalt oxide catalysts were prepared by incipient wetness impregnation of calcined solids. The required amount of an aqueous solution of Co(NO3)2 · 6H2O (Panreac, p.a.) was slowly added to the clays to give solids with a content of ∼3 wt % Co3O4. The solids were dried at 110 °C for 16 h and calcined at the same temperature as the support for 4 h. The subsequent solids are designated below as [Co/(GAmontAl)T]T, where Co indicates that cobalt was used as metal in the preparation of the supported catalysts, GAmont refers to the Gador montmorillonite used in the intercalation process, Al

Co3O4 3.26 2.87 3.18 2.99

indicates that aluminum polycations were used for intercalation, and T is the calcination temperature. Thus, for example, [Co/ (GAmont-Al)500]500 corresponds to a Gador montmorillonite clay intercalated with aluminum polycations and calcined at a temperature of 500 °C; then the solid was impregnated with a solution of cobalt, dried, and calcined at a temperature of 500 °C for 4 h. 2.2. Characterization Techniques. Physicochemical characterization included X-ray diffraction (XRD), chemical analysis, nitrogen adsorption, UV-vis and NIR spectroscopies, thermogravimetric (TG) and differential thermal analysis (DTA), and temperature-programmed reduction (TPR). XRD patterns of nonoriented powder samples were obtained on a Siemens D-500 diffractometer with Ni-filtered Cu KR radiation, at 40 kV and 30 mA. Elemental analysis of the solids was carried out by inductively coupled plasma optical emission spectroscopy at Activation Laboratories Ltd. (Ancaster, ON, Canada). Nitrogen (Air Liquide, 99.999%) adsorption experiments were performed at -196 °C on a static volumetric apparatus (Micromeritics ASAP 2010 adsorption analyzer). Adsorption data were collected in the relative pressure range between 0.00001 and 0.99. The nitrogen adsorption data were obtained using ∼0.2 g of sample and successive nitrogen doses of 4 cm3/g until a relative pressure of 0.01 was reached. Each point of the adsorption isotherm in this range was equilibrated for at least 2 h in order to characterize the smallest micropores. Then more nitrogen was added, and the volumes required to achieve a fixed set of relative pressures were measured. All samples were previously degassed at 200 °C for 24 h at a pressure lower than 6.67 Pa. The specific total surface area (SLang) was calculated using the Langmuir equation from adsorption data in the relative pressure range between 0.01 and 0.05, considering a nitrogen molecule cross-sectional area37 of 0.162 nm2. The total pore volume (Vp) of the solids was assessed from the amount of nitrogen adsorbed at a relative pressure of 0.99, assuming that the density of the nitrogen condensed in the pores is equal to that of liquid nitrogen at -196 °C (0.81 g/cm3).37 UV-vis and NIR spectroscopy studies were carried out on a Cary 5 spectrometer in the 200-2500 nm range fitted with a Cary 4-5 integrating sphere, operating in reflectance mode. Thermal analysis was performed on Perkin-Elmer analyzers, TGA7 and DTA7, for thermogravimetric and differential thermal analysis, respectively. All measurements were carried out at a heating rate of 10 °C/min under a flow of air (Air Liquide, 99.999%) of 20 cm3/min. Temperature-Programmed Reduction studies were carried out on a Micromeritics TPR/TPD 2900 instrument, from room temperature to 900 °C, at a heating rate of 10 °C/min, under a total flow of 60 cm3/min (5% H2 in Ar, Air Liquide). Water and other compounds that might be formed during the metal reduction and precursor decomposition were retained by a molecular sieve trap. Hydrogen consumption was measured by a thermal conductivity detector (TCD), and the calibration of the TCD signal was performed using Ag2O as external reference.

7228 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 1. Nitrogen adsorption data at -196 °C starting from low pressures of the GAmont samples series: (∇) GAmont, (O) (GAmont-Al), (0) (GAmont-Al)350, and (]) (GAmont-Al)500.

2.3. Catalytic Performance. The samples were tested as catalysts in the complete oxidation of propene, which was carried out on an automated bench-scale catalytic unit, model Microactivity Reference from PID Eng&Tech. The reactor was a tubular, fixed-bed, downflow one, having an internal diameter of 0.9 cm. About 0.1 g of catalyst was used in each experiment, mixed with 0.4 g of quartz in order to dilute the catalyst bed and prevent the creation of hot spots. The propene concentration in the feed stream was 0.5%, and the oxygen to hydrocarbon molar ratio was equal to 20 with the balance gas being He. The total gas flow in the feed was 150 cm3/min, thus giving high GHSV values (∼20 000 h-1). Prior to the catalytic tests, the catalysts were pretreated in situ by heating during 2 h at 150 °C in 100 cm3/min of air. The reactants and the reaction product streams were analyzed online using an Agilent 6890 gas chromatograph system equipped with two detectors. The analysis of the permanent gases was performed by separating them into a two-column system consisting of an HP-PLOT Q and an HP PLOT Molesieve 5A in a series-bypass configuration, connected to a TCD. An HP PLOT Alumina column connected to an FID was used for the hydrocarbon analysis. 3. Results and Discussion 3.1. Natural Clay Mineral, Alumina-Intercalated, and Pillared Supports. 3.1.1. X-ray Diffraction and Chemical Analysis. The basal spacing, d(001), of the pillared sample determined from the XRD pattern was 1.99 nm for (GAmontAl)500, clearly higher than 1.06, the basal spacing of the natural montmorillonite. This result is in accordance with what is reported in the literature38 and suggests that the pillaring process has been successfully accomplished. The chemical composition of the natural and the pillared clay is presented in Table 1. In the pillared samples, the exchangeable cations have been replaced by the Al-polycations, since the intercalation is in fact an ion exchange process. 3.1.2. Texture Evolution during Calcinations. The nitrogen adsorption data of the natural, intercalated, and pillared samples are shown in Figure 1. The adsorption isotherms of the natural clay is of type II in the Brunauer, Deming, Deming, and Teller (BDDT) classification.37 The adsorption isotherms at low relative pressures of the intercalated and pillared samples are

of type I in the same classification. The nitrogen adsorption at relative pressures lower than 0.1 shows the most important differences among the isotherms of the samples at various stages of the synthesis procedure. The textural properties are presented in Table 2. As expected, the intercalation with aluminum polycations caused a notable increase of the specific surface area and the total and microporous specific pore volume. These properties, together with the basal spacing and chemical composition (see section 3.1.1), confirm that the natural clay used in this work has been successfully intercalated and pillared. Gador montmorillonite had a relatively high total specific surface area and pore volume. This may be due to the fact that the raw clay was washed with a 0.5 M acetic acid solution before the extraction of the 2-µm fraction in order to facilitate its dispersion in water. This step, although not aggressive for the clay because of the very mild acid conditions used, may lead to the opening of the interlayer space and an increase of the surface area and the pore volume of the clay. After thermal treatment at 500 °C, the specific surface area decreased by 7%. The aluminum intercalated clays of this work exhibit remarkably high specific surface areas, reaching values as high as 388 m2/g, a value considerably higher than those of the natural clay. Upon calcination, a significant surface area loss of 19-36% took place, which affected mainly the micropores; see Table 2. A more detailed description of the microporous and mesoporous regions of the clays has been obtained from the pore size distributions. The micropore size distributions are shown in Figure 2. The micropore volumes, calculated according to the method proposed by Gil and Grange,39 and the maximums of the micropore size distributions are summarized in Table 2. The microporous volume was calculated also by means of the Dubinin-Astakhov (DA) method40 by the application of the DA equation in the relative pressures range between 0.01 and 0.05. The microporous volume obtained for the intercalated sample is 2.6 times higher than the specific micropore volume of the parent clay. When the intercalated sample is calcined, the specific micropore volume decreased, in a similar way to the specific surface area. The mesoporous region was characterized using the Barrett-Joyner-Halenda (BJH) method.41 The Halsey equation41 was used to calculate the statistical thickness of adsorbed nitrogen, considering a nitrogen molecule crosssectional area37 of 0.162 nm2. The comparison of the mesopore size distributions of the natural and the intercalated clays is presented in Figure 3. The cumulative mesopore volumes (Vp) for pores in the range of 1.5-50 nm, as determined by the BJH analysis, are given also in Table 2. The intercalation process and the thermal treatment do not influence the mesopore size range. A decrease of the specific external surface area and the mesoporous volume when the intercalation process is observed. The n exponent of the DA equation is related to the degree of heterogeneity of the material.42 The intercalation with aluminum polycations caused an increase of the surface area and the microporous pore volume with respect to the values showed by the natural clay; see Table 2. The value of the exponent of the DA equation increases also. These facts together with the changes in the external surface areas and mesoporous volumes indicate that the structural properties exhibited by the alumina-pillared clay are related to the microporous framework developed during the intercalation and pillaring processes. The characteristic energy, E, varies inversely with x2, according to the relationship proposed by Dubinin,43 x being the half-width for slitlike micropores. This is in accordance with the increase of E from 16.9 to 18.2 kJ/mol found for GAmont upon pillaring.

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7229 Table 2. Textural Properties Derived from Nitrogen Adsorption Studies at -196 °C Horvath-Kawazoe sample (GAmont) (GAmont)350 (GAmont)500 (GAmont-Al) (GAmont-Al)350 (GAmont-Al)500

SLanga(m2/g) 140 (Ki)275) 138 (K)264) 130 (K)279) 388 (K)481) 316 (K)501) 249 (K)487)

Dubinin-Astakhov

Sextb (m2/g)

Vpc (cm3/g)

Vµp(HK)d (cm3/g)

dpHKe (nm)

Vµp(DA)d (cm3/g)

Ef (kJ/mol)

ng

Vph (cm3/g)

55 57 62 29 31 33

0.180 0.183 0.191 0.216 0.190 0.168

0.059 0.057 0.054 0.144 0.118 0.093

0.58;0.81 0.60;0.83 0.61;0.85 0.52;0.65 0.53;0.65 0.53;0.67

0.054 0.054 0.050 0.139 0.113 0.089

16.9 17.6 17.6 18.2 18.1 18.3

2.1 1.9 2.0 3.1 3.2 3.1

0.113 0.117 0.127 0.057 0.060 0.063

a Specific surface area from Langmuir equation over the pressure interval 0.01 < p/p° < 0.05. b Specific external surface areas obtained from the t-plot method. c Specific total pore volumes at a relative pressure of 0.99. d Specific micropore volumes derived from the Horvath-Kawazoe (HK) and the Dubinin-Astakhov (DA) methods. e Maximums of the HK micropore size distributions. f Characteristic energy from DA equation. g Exponent of the DA equation. h Specific mesopore volumes from the Barrett-Joyner-Halenda (BJH) method for pores in the 1.5-50 nm range. i Langmuir K values, characteristic of the intensity of the adsorbate-adsorbent interactions.

Figure 2. Micropore size distributions derived from the Horvath-Kawazoe model: (b) (GAmont), (O) (GAmont-Al), (0) (GAmont-Al)350, and (]) (GAmont-Al)500.

Figure 4. Nitrogen adsorption data at -196 °C starting from low pressures of the GAmont samples series: (∇) GAmont, (O) (GAmont-Al), (0) (GAmont-Al)350, (]) (GAmont-Al)500. (s) Results from the DFT; symbols, experimental data.

Figure3.MesoporesizedistributionsderivedfromtheBarrett-Joyner-Halenda method: (O) (GAmont-Al)500 and (b) (GAmont)500.

The nitrogen adsorption isotherms obtained by the DFT method2 have been compared with the experimental data and summarized in Figure 4. From these isotherm models and the experimental isotherm data, the integral equation proposed by Stoeckli,44 which represents the overall adsorption isotherm, can be inverted by a regularization method45 to yield the microporeand mesopore-size distribution of pillared clays. These distributions are presented in Figure 5. The distributions only show a peak at 1.18 nm for all the samples. The discrepancies appearing between the maximums of the distribution curves obtained by means of the DFT and the corresponding values derived from the HK method may be due to the properties of the adsorbentadsorbate system used by the DFT-based model (homotactic graphite).46 The distance between the clay layers and the distance between the intercalating species are characteristic of the microporous structure of the pillared clays. These distances can be modified during the calcination step of the synthesis procedure, where phenomena such as dehydration and sintering of the pillars may

take place.2,47 Gil and Montes48 investigated the influence of the calcination step on the texture of an alumina-pillared montmorillonite and found that the microporous properties increased with temperature for calcination temperatures up to 400 °C. Calcination temperatures above this value resulted in a degradation of the pillared clays microstructure and, consequently, in a loss of the micropore volume. The evolution of other textural properties as the specific external surface area and the specific total pore volume also indicated that the mesoporous structure is not changed by the thermal treatment. This trend was also observed by other authors,47,49-56 relating to the dehydroxylation of the initial pillaring species and a progressive clay and pillar degradation. 3.2. Cobalt Alumina-Pillared Clay Catalysts. 3.2.1. X-ray Diffraction. The XRD patterns of the cobaltcontaining samples are presented in Figure 6. The diffraction peaks at 31.3°, 36.8°, 44.9°, 59.4°, and 65.4° correspond to Co3O4. No peaks from other cobalt species were detected for any of the catalyst samples. 3.2.2. Textural Properties. The nitrogen adsorption data of the cobalt alumina-pillared catalysts is shown in Figure 7. The textural properties are more explicitly given in Table 3. When comparing the nitrogen adsorption of the pillared clays and the one of the supported cobalt catalysts, it can be noticed that the micropores as well as the mesopores of the supports seem to be affected by the presence of cobalt oxide. In fact, the catalysts adsorb less nitrogen than the pillared clays, in the whole pressure range studied. From the results presented in Tables 2 and 3, it

7230 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 5. Pore-size distributions derived from the density functional theory.

Figure 7. Cobalt catalysts: nitrogen adsorption data at -196 °C starting from low pressures. (O) [Co/(GAmont-Al)350]350, (0) [Co/(GAmontAl)500]500.

Figure 6. XRD patterns of the GAmont series of samples: (b) Co3O4. (A) (a) [Co/(GAmont)350]350, (b) [Co/(GAmont)500]500. (B) (a) [Co/(GAmontAl)350]350, (b) [Co/(GAmont-Al)500]500.

is evident that the presence of cobalt oxide also produces important modifications in the specific surface area and specific pore volume values. The loss of the specific total surface area ranges from 17 to 24%, and the loss of the specific total pore

volume is between 19 and 32%. A more detailed analysis of these results may be made by taking into account that an indirect measurement of the specific mesoporous volumes is obtained by subtracting the micropore volumes, e.g., the ones derived from the HK model, from the total pore volumes. This way it can be seen that the mesopores and micropores of the GAmont samples series are similarly affected by the presence of the cobalt species. The micropore size distributions of the supports and of the cobalt oxide catalysts calcined at 500 °C are presented in Figure 8. The distributions of two samples are bimodal, showing two maximums, at 0.54 and at 0.67 nm. Interestingly, the whole range of micropore diameters of the supports is almost uniformly affected by the presence of cobalt, and the differences observed

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7231 Table 3. Textural Properties of the Co Catalysts, Derived from Nitrogen Adsorption Studies at -196 °C Horvath-Kawazoe sample [Co/(GAmont)350]350 [Co/(GAmont)500]500 [Co/(GAmont-Al)350]350 [Co/(GAmont-Al)500]500

Dubinin-Astakhov

SLanga (m2/g) Sextb (m2/g) Vpc (cm3/g) Vµp(HK)d (cm3/g) dpHKe (nm) Vµp(DA)d (cm3/g) Ef (kJ/mol)

ng

Vph (cm3/g)

114(K)274) 105(K)261) 226(K)431) 190(K)479)

1.9 1.8 3.0 3.0

0.093 0.105 0.026 0.042

45 49 12 21

0.147 0.155 0.129 0.125

0.047 0.042 0.083 0.071

0.62;0.82 0.64 0.54;0.67 0.54;0.66

0.045 0.041 0.081 0.069

17.6 18.0 18.1 18.2

a Specific surface area from Langmuir equation over the pressure interval 0.01 < p/p° < 0.05. b Specific external surface areas obtained from the t-plot method. c Specific total pore volumes at a relative pressure of 0.99. d Specific micropore volumes derived from the HK and the DA methods. e Maximums of the HK micropore size distributions. f Characteristic energy from DA equation. g Exponent of the DA equation. h Specific mesopore volumes from the BJH method for pores in the 1.5-50 nm range. i Langmuir K values, characteristic of the intensity of the adsorbate-adsorbent interactions.

Figure 8. Micropore size distributions derived from Horvath-Kawazoe model: (O) (GAmont-Al)500, (b) [Co/(GAmont-Al)500]500.

Figure 9. NIR spectra of [Co/(GAmont-Al)500] and [Co/(GAmontAl)500]240.

between the supports and the catalysts concern only the pore volume. It seems that the preparation procedure resulted in a good distribution of cobalt oxide throughout the pillared clays surface, even reaching their inner porous network. This result is also confirmed if the n exponents of the DA equation obtained for the supports (see Table 2) and the cobalt catalysts (see Table 3) are compared. 3.2.3. Uv-Vis-NIR Spectroscopy. The impregnation of the precursor on the surface of the supports, and their further transformation by heating at 500 °C, were studied by means of spectroscopic techniques. NIR spectrum of the solids treated at 110 and 240 °C, presented in Figure 9, show the bands characteristic of structural hydroxyl groups of the clay supports. Thus, the overtone of their stretching mode is observed as a band at 1390 nm, while the combination band of water (ν + δ) appears at 1907 nm. The combination of structural OHs appears as a wide band at 2210 nm.57 No bands due to nitrate groups are detected in this region. The spectroscopic study of the solids was completed by means of UV-vis spectroscopy; the spectra of the materials are shown in Figure 10. The spectrum of the impregnated dried solid is

Figure 10. UV-vis spectra of [Co/(GAmont-Al)500] and [Co/(GAmontAl)500]240.

typical of Co2+ cations in a hexacoordinated octahedral environment, so this cation may be this way by coordination after impregnation, coordinated by hydroxyl groups in the surface of the support, from the clay layers or from the pillars, by nitrate groups from the precursor, and by water molecules. The spectrum of the calcined solid shows a charge transference band in the UV region, with two maximums at about 210 and 260 nm, and very small peaks at about 515 and 630 nm in the visible region. These effects may help to clarify the evolution of the cobalt species, and their oxidation state, although generally no conclusive results can be obtained. The bands have been ascribed to 4T1 g(F) f 4T1 g(P) and 4T1 g(F) f 4A2 g transitions in CoII in octahedral environment;58 the most common oxide-like compound in which CoII species are found in this environment is the inverse spinel Co3O4. It may be taken into account that a precursor of CoII has been used for impregnation, that this cation can be oxidized to CoIII under calcination, and that the spinel is a very stable oxide. However, no bands of tetrahedral or octahedral CoIII species, also present in the inverse Co3O4 spinel, were detected in the solid calcined at 500 °C, probably due to the intense black color of this solid. 3.2.4. Thermogravimetric Analyses. Thermal analyses of the impregnated solids, presented in Figure 11, are very similar to those of the supports, as can be expected for the low amount of precursor used in the impregnation. Thus, the endothermic effect at ∼110 °C is due to the loss of physisorbed water from the supports and is followed by a slow loss of structural water, both from the clay and, in the pillared solids, also from the pillars, observed in the thermogravimetric curves as a small and continuous slope up to 800 °C. No effects due to the removal of nitrate groups can be observed, although an intense weight loss effect is detected for the [Co/(GAmont)500] sample close to 600 °C (see Figure 11, upper curve), for which we have not a clear explanation. This effect may in part be due to the clay itself, because it also appears, although a much less intense, for [Co/(GAmont-Al)500] sample (Figure 11, lower curve), the difference between the two supports is only pillaring and this process may not cause the disappearance of any effect attribut-

7232 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 11. Thermal changes of [Co/(GAmont)500] and [Co/(GAmontAl)500] in air atmosphere.

Figure 12. TPR patterns of cobalt catalysts: (a) [Co/(Gamont-Al)500]110, (b) [Co/(Gamont-Al)500]240, and (c) [Co/(Gamont-Al)500]500.

able to the clay. This effect is not expected to be due to the decomposition of nitrate anions, since such a decomposition is expected to happen at a lower temperature, although the dynamic conditions used in thermal analyses, with a heating rate of 10 °C/min, may make this effect to appear at higher temperature. In any case, the observed weight loss can be only explained by the removal of hydroxyl constitutional groups. No other effects are attributable to possible phase changes of the cobalt species, which is expected because of their small amount in the solids. The thermal changes at high temperature, above 800 °C, are due to the phase change in the clays; for pure clays, the transformation of montmorillonite to a mixture of mullite and silica has been reported, and similar transformations are observed when this clay is pillared with Al-species. The only effect produced by pillaring is a higher Al content in the solids, and Co may also participate in these transformations giving rise to oxide or silicate phases actives at high temperature, as CoAl2O4 or Co2SiO4, involving reactions with Al from the clay layers or from the pillars and with Si from the clay layers, respectively. However, significant differences are observed in the form of these effects when the natural or the pillared clays are used as supports. In the first case, the effects are very close to those reported for the thermal curves or the natural clays, while when using pillared clays as supports the endothermal dehydroxylation effect is less intense and the exothermal effect due to phase change practically disappears. This suggests that in the presence of Co the phase changes to form mullite and silica do not happen immediately after dehydroxylation, this presence delaying the phase change, an effect already reported for clay supports impregnated with other transition metals.21,53 The differences may be related to the reaction of Co with the pillars in the pillared clays or with the clay’s own layers in the solids based on natural clays, in this last case inhibiting the usual high-phase transformations. A Co-Al spinel was undoubtedly identified by X-ray diffraction after calcination at 1000 °C. It is remarkable that it was not the expected CoAl2O4 spinel, but Co2AlO4, that is CoIICoIIIAlO4. 3.2.5. Temperature-Programmed Reduction. In order to investigate the reducibility of the cobalt oxide supported catalysts, (GAmont-Al)500 impregnated with cobalt(II) nitrate was treated in air at increasing temperatures, 110, 240, and 500 °C, for 4 h. The TPR curves of these samples are presented in Figure 12. No significant reduction processes take place in the case of the pillared clay support. The contribution of the reduction of the structural Fe3+ cations of the clays at ∼650 °C is rather negligible. The reduction of the supported cobalt oxides starts at ∼200 °C, and two peaks can be observed for all the catalysts as the temperature increases. The first peak of the samples treated at low temperatures, 110 °C, is always larger than the one observed when the solids are treated at higher

temperature. A shift in this first TPR peak to higher temperatures is observed for all the catalysts as the temperature of treatment increases. Several authors8,9,59,60 have described the reduction of supported cobalt oxides. Arnoldy and Moulijn8 in a detailed study discussed the cobalt species present in alumina catalysts prepared from impregnation with cobalt(II) nitrate solutions. These authors indicated that the peaks correspond to distinct cobalt species in various oxidation states and different chemical environments. In accordance with the data reported by these authors, Co3O4 is reduced first at low temperature in one step to CoO and in a second step to Co metallic. However, the two reduction steps may not always be observed as separate peaks in TPR analyses.8,61-64 This sequence is confirmed by reducing bulk cobalt oxide.8,65-68 It has also been found that often, due to interactions between Co3O4 and support materials such as silica or alumina, TPR of supported Co3O4 can also show a separation of the two reduction steps.63,65 We have found for the cobalt oxide catalysts supported on pillared clays two reduction peaks, which are in good accordance with the results reported by various authors for aluminasupported cobalt oxide catalysts.8,9 The peak maximums depend on the calcination temperature and shifts to higher values at higher calcination temperatures. The reduction peak ∼270 °C is very big for the sample treated at 110 °C, strongly decreases in intensity and shifts about 70-100 °C, when the solids calcined at higher temperatures are considered. It has been suggested that this first peak is due to the decomposition of residual cobalt nitrate to Co3O4,8,61,66 and the comparison of the profile of the solids calcined at several temperatures confirms this hypothesis. This peak is larger in samples calcined at higher temperatures, because of the removal of nitrate anions upon calcination. Besides, this peak appears at higher temperatures, because of the reduced accessibility of the remaining nitrate anions. The fact that this peak may be observed in the solids calcined at 500 °C agrees with the presence of small amounts of nitrate anions even after calcination at this temperature. After the elimination of cobalt anions, the next reduction peak corresponds to the reduction of Co3O4 to CoO and Co metal. The reduction peak at 700-800 °C was attributed to the reduction of cobalt strongly interacting with the support,61,67 which can only be reduced at higher temperatures. This compound is usually described as an amorphous surface cobalt aluminate or silicate.7,9,59,62,68,69 The calculated H2/Co ratios, also included in Figure 10, are in accordance with this suggestion, because the values are not in accordance with the expected theoretical behavior, the reduction of Co3O4 to CoO, 1.36, or to Co, 5.44. 3.2.6. Catalytic Oxidation of Propene. There are several reports concerning the catalytic oxidation of propene and various

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The results from nitrogen physisorption data at -196 °C indicate that the intercalation with aluminum polycations causes a notable increase of the surface areas and the total and microporous pore volumes with respect to the natural clay. After calcination, the progressive degradation of the pillars can explain the loss of textural properties observed among the solids. The presence of cobalt species produces important modifications in the surface area and pore volumes. The micropores and mesopores of the pillared clays results were similarly affected by the presence of cobalt oxide. Results from spectroscopic, thermal, and reduction techniques indicate the partial oxidation of CoII impregnating cations to CoIII and the formation of various species depending on the nature of the support. The reactions transforming under calcination the impregnating species to oxidic phases in the final catalysts seems to be influenced by the nature of the clay support. The inverse spinel Co3O4 is presented in the final catalysts, obtained by calcination at 500 °C, while if calcination is done at higher temperatures, amorphous surface cobalt aluminate CoIICoIIIAlO4 or cobalt silicate are formed. Acknowledgment Figure 13. Conversion of propene over [Co/(GAmont)350]350 (b), [Co/ (GAmont-Al)350]350 (9), (GAmont)350 (O), and (GAmont-Al)350 (0).

types of catalysts have been used. Several authors70-73 have explored the electrochemical promotion to improve Pt catalytic performance for propene oxidation using various supports as solid electrolytes. Considering metal oxides as catalysts, the propene oxidation has also been investigated.74-77 Recently, we have studied the oxidation of propene using chromium-saponite pillared clay catalysts.78 The results found indicated that the catalytic performance of all the catalysts evaluated is very similar independently of the method used for the incorporation of the chromium. The propene conversion over [Co/(GAmont)350]350 and [Co/ (GAmont-Al)350]350 is presented in Figure 13. For comparison reasons, the conversion achieved over the clays used as supports, (GAmont)350 and (GAmont-Al)350, respectively, is also presented. The samples were heated up to the reaction temperature at a rate of 5 °C/min, in reactant flow. Special attention was given to the oven control so that the temperature was stabilized fast and then remained constant during the reaction at all temperatures studied. The points included in Figure 13, correspond to steady state, which was usually reached after 30-45 min of reaction. The main product of the reaction, as expected, was carbon dioxide. Depending on the sample and the conversion, there could also be very small quantities of carbon monoxide. The temperatures required for propene oxidation are lower over the cobalt oxide catalysts supported on Gador montmorillonite than on the corresponding pillared clay. T50, the temperature at which the conversion of propene reaches 50%, are 365 and 412 °C, respectively. The same behavior is obtained when the supports are evaluated as catalysts. In a previous work,27 we have observed a similar influence of the clay characteristics in the catalytic performance, indicating that the support nature and the catalyst acid-base properties are important factors conditioning the oxidation of propene on cobalt oxide catalysts. 4. Summary and Conclusions As summary of the results presented in this work, the temperature and the cobalt content produce a loss of the textural properties of the clay-based supports used for the preparation of Co/clay-supported catalysts.

This work was supported by the Spanish Ministry of Education and Science (MEC) and the European Regional Development Fund (FEDER) (MAT2003-01255 and MAT200766439-C02). S.A.K. and R.T. acknowledge financial support by MEC through the Ramo´n-y-Cajal program. Literature Cited (1) Moser, W. R. AdVanced Catalysts and Nanostructured Materials. Modern Synthetic Methods; Academic Press: New York, 1996. (2) Gil, A.; Korili, S. A.; Vicente, M. A. Recent advances in the control and characterization of the porous structure of pillared clay catalysts. Catal. ReV. Sci. Eng. 2008, 50, 153. (3) Gil, A.; Gandı´a, L. M.; Vicente, M. A. Recent advances in the synthesis and catalytic application of pillared clays. Catal. ReV. Sci. Eng. 2000, 42, 145. (4) Nikalje, M. A.; Phukan, P.; Sudalai, A. Recent advances in claycatalyzed organic transformations. Org. Prep. Proc. Int. 2000, 32, 3. (5) Ding, Z.; Kloprogge, J. T.; Frost, R. L.; Lu, G. Q.; Zhu, H. Y. Porous clays and pillared clays-based catalysts. Part 2: a review of the catalytic and molecular sieve applications. J. Porous Mater. 2001, 8, 273. (6) Varma, R. S. Clay and clay-supported reagents in organic synthesis. Tetrahedron 2002, 58, 1235. (7) Chin, R. L.; Hercules, D. M. Surface spectroscopic characterization of cobalt-alumina catalysts. J. Phys. Chem. 1982, 86, 360. (8) Arnoldy, P.; Moulijn, J. A. Temperature-programmed reduction of CoO/Al2O3 catalysts. J. Catal. 1985, 93, 38. (9) Okamoto, Y.; Adachi, T.; Nagata, K.; Odawara, M.; Imanaka, T. Effects of starting cobalt salt upon the cobalt-alumina interactions and hydrodesulfurization activity of CoO/Al2O3. Appl. Catal. 1991, 73, 249. (10) Okamoto, Y.; Nagata, K.; Adachi, T.; Imanaka, T.; Inamura, K.; Takyu, T. Preparation and characterization of highly dispersed cobalt oxide and sulfide catalysts. J. Phys. Chem. 1991, 95, 310. (11) Riva, R.; Miessner, H.; Vitali, R.; del Piero, G. Metal-support interaction in Co/SiO2 and Co/TiO2. Appl. Catal., A 2000, 196, 111. (12) Bourikas, K.; Kordulis, C.; Vakros, J.; Lycourghiotis, A. Adsorption of cobalt species on the interfaces, which is developed between aqueous solution and metal oxides used for the preparation of supported catalysts: a critical review. AdV. Colloid Interface Sci. 2004, 110, 97. (13) van’t Blik, H. F. J.; Konningsberg, D. C.; Prins, R. Characterization of supported cobalt and cobalt-rhodium catalysts: III. TemperatureProgrammed Reduction (TPR), oxidation (TPO), and EXAFS of Co-Rh/ SiO2. J. Catal. 1986, 97, 210. (14) Yang, R. T.; Tharappiwattananon, N.; Long, R. Q. Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen. Appl. Catal., B 1998, 19, 289. (15) Konin, G. A.; Il’ichev, A. N.; Matyshak, V. A.; Khomenko, T. I.; Korchak, V. N.; Sadykov, V. A.; Doronin, V. P.; Bunina, R. V.; Alikina,V; Kuznetsova, T. G.; Paukshtis, E. A.; Fenelonov, V. B.; Zaikovskii, V. I.; Ivanova, A. S.; Beloshapkin, S. A.; Rosovskii, A.Ya.; Tretyakov, V. F.;

7234 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Ross, J. R. H.; Breen, J. P. Cu, Co, Ag-containing pillared clays as catalysts for the selective reduction of NOx by hydrocarbons in an excess of oxygen. Top. Catal. 2001, 16/17, 193. (16) Sapag, K.; Mendioroz, S. Synthesis and characterization of micromesoporous solids: pillared clays. Colloid Surf. A 2001, 187-188, 141. (17) Sapag, K.; Rojas, S.; Lo´pez Granados, M.; Fierro, J. L. G.; Mendioroz, S. CO hydrogenation with Co catalyst supported on porous media. J. Mol. Catal. A 2001, 167, 81. (18) Choy, J.-H.; Jung, H.; Han, Y.-S.; Yoon, J.-B.; Shul, Y.-G.; Kim, H.-J. New CoO-SiO2-sol pillared clays as catalysts for NOx conversion. Chem. Mater. 2002, 14, 3823. (19) Chmielarz, L.; Kustrowski, P.; Zbroja, M.; Rafalska-Lasocha, A.; Dudek, B.; Dziembaj, R. SCR of NO by NH3 on alumina or titania-pillared montmorillonite various modified with Cu or Co. Part I. General characterization and catalysts screening. Appl. Catal., B 2003, 45, 103. (20) Gonza´lez, E.; Moronta, A. The dehydrogenation of ethylbenzene to styrene catalyzed by a natural and an Al-pillared clays impregnated with cobalt compounds: a comparative study. Appl. Catal., A 2004, 267, 99. ´ lvarez, A. C.; (21) Vicente, M. A.; Belver, C.; Trujillano, R.; Rives, V.; A Lambert, J.-F.; Korili, S. A.; Gandı´a, L. M.; Gil, A. Preparation and characterization of Mn- and Co-supported catalysts derived from Al-pillared clays and Mn- and Co-complexes. Appl. Catal., A 2004, 267, 47. (22) Li, J. J.; Mu, Z.; Xu, X. Y.; Tian, H.; Duan, M. H.; Li, L. D.; Hao, Z. P.; Qiao, S. Z.; Lu, G. Q. A new and generic preparation method of mesoporous clay composites containing dispersed metal oxide nanoparticles. Mesoporous Mater. In press. (23) Shelef, M. Selective catalytic reduction of NOx with N-free reductants. Chem. ReV. 1995, 95, 209. (24) Gil, A.; Massinon, A.; Grange, P. Analysis and comparison of the microporosity in Al-, Zr-, and Ti-pillared Clays. Microporous Mater. 1995, 4, 369. (25) Gil, A.; Vicente, M. A.; Lambert, J.-F.; Gandı´a, L. M. Platinum catalysts supported on Al-pillared clays. Application on the catalytic combustion of acetone and methyl-ethyl-ketone. Catal. Today 2001, 68, 41. (26) Gandı´a, L. M.; Vicente, M. A.; Gil, A. Complete oxidation of acetone over manganese oxide catalysts supported on alumina- and zirconiapillared clays. Appl. Catal., B 2002, 38, 295. (27) Gil, A.; Vicente, M. A.; Korili, S. A. Effect of the nature and structure of pillared clays in the catalytic behaviour of supported manganese oxide. Catal. Today 2006, 112, 117. (28) Jinjun, L.; Zheng, J.; Zhengping, H.; Xiuyan, X.; Yahui, Z. Pillared laponite clays-supported palladium catalysts for the complete oxidation of benzene. J. Mol. Catal. A 2005, 225, 173. (29) Zuo, S.; Zhou, R. Al-pillared clays supported rare earths and palladium catalysts for deep oxidation of low concentration of benzene. Appl. Surf. Sci. 2006, 253, 2508. (30) Oliveira, L. C. A.; Lago, R. M.; Fabris, J. D.; Sapag, K. Catalytic oxidation of aromatic VOCs with Cr or Pd-impregnated Al-pillared bentonite: byproduct formation and deactivation studies. Appl. Clay Sci. 2008, 39, 218. (31) Spivey, J. J. Catalysis; The Royal Society of Chemistry: Cambridge, 1989; Vol. 8. (32) Schoenberg, M. R. ; Blieszner, J. W.; Papadopoulos, Ch. G. Propylene. In Kirk-Othmer Encyclopedia of Chemical Technology; Mark, H. F., Othmer, D. F., Overberger, Ch. G., Seaborg, G. T., Eds.; John Wiley & Sons: New York, 1982; Vol. 19, p 228. (33) Murphy, C. F.; Allen, D. T. Hydrocarbon emissions from industrial release events in the Houston-Galveston area and their impact on ozone formation. Atmos. EnViron. 2005, 39, 3785. (34) Lahav, N.; Shani, U.; Shabtai, J. Cross-linked smectites. 1. Synthesis and properties of hydroxy-aluminum-montmorillonite. Clays Clay Miner. 1978, 26, 107. (35) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. Studies of hydrolyzed aluminum chloride solutions. 1. Nature of aluminum species and composition of aqueous solutions. J. Phys. Chem. 1980, 84, 2933. (36) Bradley, S. M.; Kydd, R. A.; Yamdagni, R. Comparison of the hydrolyses of gallium(III) and aluminium(III) solutions by nuclear magnetic resonance spectroscopy. J. Chem. Soc., Dalton Trans. 1990, 2653. (37) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1991. (38) Gil, A.; Montes, M. Analysis of the microporosity in pillared clays. Langmuir 1994, 10, 291. (39) Gil, A.; Grange, P. Comparison of the microporous properties of an alumina pillared montmorillonite and an activated carbon from nitrogen adsorption at 77 K. Langmuir 1997, 13, 4483. (40) Gil, A.; Grange, P. Application of the Dubinin-Radushkevich and Dubinin-Astakhov equations in the characterization of microporous solids. Colloid Surf. A 1996, 113, 39.

(41) Barrett, G. P.; Joyner, L. G.; Halenda, R. H. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 1951, 73, 373. (42) Gil, A.; Gandı´a, L. M. Microstructure and quantitative estimation of the micropore-size distribution of an alumina-pillared clay from nitrogen adsorption at 77 K and carbon dioxide adsorption at 273 K. Chem. Eng. Sci. 2003, 58, 3059. (43) Dubinin, M. M. Generalization of the theory of volume filling of micropores to nonhomogeneous microporous structures. Carbon 1985, 23, 373. (44) Stoeckli, H. F. A generalization of the Dubinin-Radushkevich equation for the filling of heterogeneous micropore systems. J. Colloid Interface Sci. 1977, 59, 184. (45) Von Szombathely, M.; Brauer, M.; Jaroniec, M. The solution of the adsorption integral equations by means of the regularization method. J. Comput. Chem. 1992, 13, 17. (46) Olivier, J. P.; Conklin, W. B.; Szombathely, M. V. Determination of pore size distributions from the density functional theory. Stud. Surf. Sci. Catal. 1994, 87, 81. (47) Ge, Z.; Li, D.; Pinnavaia, T. J. Preparation of alumina-pillared montmorillonite with high thermal stability, regular microporosity and Lewis/Bro¨nsted acidity. Microporous Mater. 1994, 3, 165. (48) Gil, A.; Montes, M. Effect of thermal treatment on microporous accessibility in aluminium pillared plays. J. Mater. Chem. 1994, 4, 1491. (49) Yamanaka, S.; Hattori, M. Iron oxide pillared clay. Catal. Today 1988, 2, 261. (50) Sterte, J. P.; Otterstedt, J.-E.; Thulin, H.; Massoth, F. E. Characterization of alumina-montmorillonite complexes. Appl. Catal. 1988, 38, 119. (51) Matsuda, B.; Asanuma, M.; Kikuchi, E. Effect of high-temperature treatment on the activity of montmorillonite pillared by alumina in the conversion of 1,2,4-trimethylbenzene. Appl. Catal. 1988, 38, 289. (52) Pesquera, C.; Gonza´lez, F.; Benito, I.; Mendioroz, S.; Pajares, J. A. Synthesis and characterization of pillared montmorillonite catalysts. Appl. Catal. 1991, 69, 97. (53) Vicente, M. A.; Ban˜ares-Mun˜oz, M. A.; Gandı´a, L. M.; Gil, A. On the structural changes of a saponite intercalated with various polycations upon thermal treatments. Appl. Catal., A 2001, 217, 193. (54) Ohtsuka, K. Preparation and properties of two-dimensional microporous pillared interlayered solids. Chem. Mater. 1997, 9, 2039. (55) Hutson, N. D.; Hoekstra, M. J.; Yang, R. T. Control of microporosity of Al2O3-pillared clays: effect of pH, calcination temperature and clay cation exchange capacity. Microporous Mesoporous Mater. 1999, 28, 447. (56) Cheng, L. S.; Yang, R. T. Tailoring micropore dimensions in pillared clays for enhanced gas adsorption. Microporous Mater. 1997, 8, 177. (57) Vicente, M. A.; Lambert, J.-F. Al-pillared saponites. Part 4. Pillaring with a new Al13 oligomer containing organic ligands. Phys. Chem. Chem. Phys. 1999, 1, 1633. (58) Lever, A. B. P. Inorganic Electronic Spectroscopy; Elsevier: Amsterdam, 1984. (59) Jongsomjit, B.; Paupranot, P.; Goodwin, J. G., Jr. Co-support compound formation in alumina-supported cobalt catalysts. J. Catal. 2001, 204, 98. (60) Boot, L. A.; Kerkhoffs, M. H. J. V.; van der Linden, B. Th.; van Dillen, A. J.; Geus, J. W.; van Buren, F. R. Preparation, characterization and catalytic testing of cobalt oxide and manganese oxide catalysts supported on zirconia. Appl. Catal., A 1996, 137, 69. (61) Hilmen, A. M.; Schauke, D.; Holmen, A. TPR study of the mechanism of rhenium promotion of alumina-supported cobalt FischerTropsch catalysts. Catal. Lett. 1996, 38, 143. (62) Wang, W.; Chen, Y. Influence of metal loading on the reducibility and hydrogenation activity of cobalt/alumina catalysts. Appl. Catal., A 1991, 77, 223. (63) Lapidus, A.; Krylova, A.; Kazanshii, V.; Borovknov, Z.; Ratnousky, J.; Zukal, A.; Jan, M. C. Hydrocarbon synthesis from carbon monoxide and hydrogen on impregnated cobalt catalysts. Part. I. Physico-chemical properties of 10% cobalt/alumina and 10% cobalt/silica. Appl. Catal., A 1991, 73, 65. (64) Seaton, B. A.; Hughes, A. E.; Turney, T. W. An XPS and TPR study of the reduction of promoted cobalt-kieselguhr Fischer-Tropsch catalysts. J. Catal. 1986, 97, 390. (65) Reuel, R. C.; Bartholomew, C. H. The stoichiometries of H2 and CO adsorptions on cobalt: effects of support and preparation. J. Catal. 1984, 85, 63. (66) Kogelbaner, A.; Goorwin, J. G., Jr.; Oukaci, R. Ruthenium promotion of Co/Al2O3 Fischer-Tropsch catalysts. J. Catal. 1996, 160, 125.

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7235 (67) van’t Blik, H. F. J.; Prins, R. Characterization of supported cobalt and cobalt-rhodium catalysts. J. Catal. 1986, 97, 188. (68) Lee, W. H.; Bartholomew, C. H. Multiple reaction states in CO hydrogenation on alumina-supported cobalt catalysts. J. Catal. 1989, 120, 256. (69) Puskas, I.; Fleisch, T. H.; Hall, J. B.; Meyers, B. L.; Roginski, R. T. Metal-support interactions in precipitated, magnesium-promoted cobalt-silica catalysts. J. Catal. 1992, 134, 615. (70) Kaloyannis, A.; Vayenas, C. G. Non-faradaic electrochemical modification of catalytic activity. J. Catal. 1999, 182, 37. (71) Vernoux, P.; Gaillard, F.; Bultel, L.; Siebert, E.; Primet, M. Electrochemical promotion of propane and propene oxidation on Pt/YSZ. J. Catal. 2002, 208, 412. (72) Vernoux, P.; Gaillard, F.; Lopez, C.; Siebert, E. In-situ electrochemical control of the catalytic activity of platinum for the propene oxidation. Solid State Ionics 2004, 175, 609. (73) De Lucas-Consuegra, A.; Dorado, F.; Valverde, J. L.; Karoum, R.; Vernoux, P. Low-temperature propene combustion over Pt/K-βAl2O3 electrochemical catalyst: characterization, catalytic activity measurements, and investigation of the NEMCA effect. J. Catal. 2007, 251, 474. (74) Chien, Ch.-Ch.; Chuang, W.-P.; Huang, T. J. Effect of heattreatment conditions on Cu-Cr/g-alumina catalyst for carbon monoxide and propene oxidation. Appl. Catal., A 1995, 131, 73.

(75) Baldi, M.; Sanchez Escribano, V.; Gallardo Amores, J. M.; Milella, F.; Busca, G. Characterization of manganese and iron oxides as combustio´n catalysts for propane and propene. Appl. Catal., B 1998, 17, L175. (76) Rida, K.; Benabbas, A.; Bouremmad, F.; Pen˜a, M. A.; Sastre, E.; Martı´nez-Arias, A. Effect of calcination temperature on the structural characteristics and catalytic activity for propene combustion of sol-gel derived lanthanum chromite perovskite. Appl. Catal., A 2007, 327, 173. (77) Lamonier, J.-F.; Labaki, M.; Wyrwalski, F.; Siffert, S.; Aboukaı¨s, A. Thermal behaviour and catalytic properties towards propene combustion of zirconia modified by different first row transition metals. J. Anal. Appl. Pyrolysis 2008, 81, 20. (78) Mata, G.; Trujillano, R.; Vicente, M. A.; Belver, C.; Ferna´ndezGarcı´a, M.; Korili, S. A.; Gil, A. Chromium-saponite clay catalysts: preparation, characterization and catalytic performance in propene oxidation. Appl. Catal., A 2007, 327, 1.

ReceiVed for reView October 1, 2007 ReVised manuscript receiVed June 6, 2008 Accepted July 6, 2008 IE071320V