Interactions between Molybdenum and Activated Carbons on the Preparation of Activated Carbon-Supported Molybdenum Catalysts

Interactions between Molybdenum and Activated Carbons on the Preparation of Activated Carbon-Supported Molybdenum Catalysts

JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO. 202, 155–166 (1998) CS985461 Interactions between Molybdenum and Activated Carbons on the Prep...

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JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.

202, 155–166 (1998)

CS985461

Interactions between Molybdenum and Activated Carbons on the Preparation of Activated Carbon-Supported Molybdenum Catalysts G. de la Puente,† A. Centeno,* A. Gil,‡ and P. Grange* ,1 *Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Universite´ Catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium; †Instituto Nacional del Carbo´n, CSIC, Apdo. 73, 33080 Oviedo, Spain; and ‡Departamento de QuıB mica Aplicada, Universidad Pu´blica de Navarra, Campus de ArrosadıB a s/n, 31006 Pamplona, Spain Received September 30, 1997; accepted February 9, 1998

The influence of the porous texture and surface chemistry of an activated carbon on the preparation of activated carbonsupported molybdenum catalysts was studied. Activated carbons with various degrees of oxidation were used as supports. Supports and catalysts were characterized by nitrogen adsorption at 77 K, temperature-programmed desorption ( TPD ) , Fourier transform infrared spectroscopy ( FTIR ) , and X-ray photoelectron spectroscopy ( XPS ) . The impregnation with molybdenum produced a reduction of the textural properties. TPD results indicated that there was a transformation of CO2- and CO-evolving oxygen functional groups on the activated carbon as a consequence of the impregnation of molybdenum and that acidic groups were acting as chemical anchorage centers. FTIR and XPS analyses confirmed that interactions were established between molybdenum species and carbonyl and ether surface functional groups. These results indicate that the molybdenum was well distributed and reached the inner porous network of the carbon support. q 1998 Academic Press Key Words: activated carbon oxidation; activated carbon-supported molybdenum catalysts; textural properties; surface characterization; surface oxygen groups.

INTRODUCTION

This work is part of a program aiming at the preparation of activated carbon-based catalysts for upgrading bio-oils obtained from biomass pyrolysis. In order to meet the required product quality improvement by catalytic hydrotreatment, new metal sulfide catalysts with higher activities, better selectivity to desired products, and greater resistance to deactivation are needed. Hydrodeoxygenation activity of model compounds representative of the composition of biooils on traditional CoMo- and NiMo-sulfided catalysts has 1 To whom correspondence should be addressed. Telephone: /32 10 473648. Fax: /32 10 473649. E-mail: [email protected]

been reported in previous studies ( 1, 2 ) . Because of their instability, some of the corresponding molecules are suspected to lead to coking reactions. This tendency to coke formation could render the low-temperature stabilization of bio-oils more difficult and limit the lifetime of the catalysts. In this respect, the use of carbon as a support is of particular interest, since it is has been shown that it has lower coking propensity than alumina supports and should thus maintain its activity over extended periods of operation ( 3 ) . Furthermore, the use of carbon-supported sulfide catalysts for the conversion of residua would make it possible to easily recover the transition metal valuables from the spent catalysts by burning off the carbon carrier. While on the commercially applied alumina support the exposed surface hydroxyl groups serve as adsorption sites for the molybdate ions in a condensation reaction (4), a more complex situation is encountered in the case of activated carbons due to the nature of the carbon surface. The frequent activity discrepancies reported for the catalysts supported on what are supposed to be similar carbons are difficult to predict and understand (5). The origin of these discrepancies seems to lie in the chemical properties of the carbon surface, which are influenced by the nature and quantity of surface oxygen functional groups (6, 7). For example, in iron and ruthenium (6), molybdenum (8–12), nickel (11), palladium (13), and platinum (7, 14–16) supported on carbon, modification of the carbon surface resulted in a significant change of the loading capacity and catalytic properties. The present paper focuses on the interaction of the carbon surface with molybdate ions. Activated carbons with various degrees of oxidation were used as supports in the preparation of activated carbon-supported molybdenum catalysts. We want this work to be a contribution to the unravelling of the role that the texture and carbon surface oxygen functionality might play with respect to the dispersion of the catalyst

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0021-9797/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.

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precursor, its accessibility, and its stability during thermal treatment in catalyst activation. EXPERIMENTAL

1. Catalyst Preparation The supports used in the preparation were a commercial granular microporous gas-activated carbon based on coconut shell (Merck-9631) (sample K0) and a series of samples of this carbon subjected to an oxidative treatment with HNO3 (Janssen Chimica) at various temperatures (298, 333, and 363 K and reflux at 383 K) during 3 h. Details of the activated carbon treatments are given elsewhere (17). These supports will be referred to as N298, N333, N363, and NR, respectively. Since the main concern of this study is to evaluate the interaction of the carbon surface with molybdenum, all the samples were prepared in such a way that the support loading was kept constant. Activated carbon supported molybdenum catalysts were prepared by slowly adding to the supports the required amount of an aqueous solution of ammonium heptamolybdate tetrahydrate (Merck, p.a.) in order to obtain catalysts with a nominal composition of 15 wt% MoO3 . The samples were subsequently dried for 16 h at 373 K in a stream of nitrogen (Air Liquide, 99.995%). These samples will be referred to in Mo/support notation (Mo/K0, Mo/ N298, Mo/N333, Mo/N363, and Mo/NR, respectively). Heat treatments were performed by placing the dried catalyst samples in a tubular furnace and heating them for 1 h in a stream of helium (Air Liquide, 99.995%). When the samples were treated under inert atmosphere at 673 K, the temperature used during the activation process (1–3), ‘‘-673’’ is added to the notation of the catalysts (Mo/K0673, Mo/N298-673, Mo/N333-673, Mo/N363-673, and Mo/NR-673, respectively). 2. Characterization The textural characterization of the activated carbons was carried out by nitrogen adsorption at 77 K using a static volumetric apparatus (Micromeritics ASAP 2000M adsorption analyzer). The adsorption isotherms were obtained in the relative pressure range of 10 06 õ p/p 7 õ 0.99. To obtain an adequate characterization of the micropore region, sufficient data points at low pressures are needed, and this requires the addition of constant small nitrogen volumes. In this work, nitrogen adsorption data were obtained using 0.1 g of sample and successive doses of nitrogen of 5 cm3 /g until p/p 7 Å 0.04 was reached. Further nitrogen was added and the volumes required to achieve a fixed set of p/p 7 were measured. Previous to analysis, activated carbons were degassed at 473 K during 10 h (p õ 10 03 mmHg). Specific total surface areas (ALang ) were calculated using the Lang-

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muir equation (0.01 ° p/p 7 ° 0.05 interval) for monolayers. Specific external surface areas (Aext ) were obtained from the t-method (18) and the specific total pore volumes (Vp ) were estimated from nitrogen uptake at p/p 7 Å 0.95. The Dubinin–Radushkevich equation (19) was used to calculate the micropore volume (VDR ) and the characteristic energy (E). The apparatus used for the temperature-programmed desorption (TPD) experiments consists of a U-shaped flow reactor coupled to a gas chromatograph and a mass spectrometer (GC–MS, Hewlett-Packard G1800A GCD System) for the analysis of the gases evolved during thermolysis. In these experiments, 0.2 g of sample were heated at 10 K/min up to 1073 K in a stream of 60 cm3 /g of helium (Air Liquide, 99.995%). FTIR spectra were recorded using a Bruker FT 88 spectrometer. The samples were diluted in KBr to a 0.1% carbon/ (carbon / KBr) content. Corrections were made with respect to the mass of carbonaceous material in the pellets. The spectrum of the diluent itself was obtained and subtracted from those of the mixtures. X-ray photoelectron spectroscopy (XPS) analyses were carried out with a Surface Science Instruments SSX-100, model 206, with a monochromatic AlKa source (1486.6 eV), operating at 10 kV and 15 mA. The spectrometer was interfaced to a Hewlett-Packard 9000/310 computer for data acquisition and treatment. The powdered samples were pressed on stainless steel capsules which were mounted on top of the specimen holder. The samples were degassed under a minimum vacuum of 5 1 10 07 mmHg previous to their introduction in the analysis chamber. During analysis, the pressure did not exceed 5 1 10 08 mmHg. The C 1s, O 1s, and Mo 3d lines were investigated. The binding energy of C 1s line was found to be 284.6 { 0.2 eV in all the cases. This line was checked at the beginning and end of the analysis of each catalyst and no modification in its position could be found. A nonlinear, Shirley-type (20) baseline and an iterative least-squares fitting algorithm were used to deconvolute the peaks, the curves being taken as 85% Gaussian and 15% Lorenzian. Surface atomic concentration ratios were calculated as the ratio of the corresponding peak areas, corrected with theoretical sensitivity factors based on Scofield’s photoionization cross sections (21). RESULTS AND DISCUSSION

1. Textural Properties The primary role of a catalyst support is to provide a structural framework for the dispersed phase. In this respect, activated carbons with high surface area and significant pore volume are ideally suited as catalyst supports. The activated carbon surface atoms are made up of defects, dislocations or discontinuities in the carbon layers, functional groups, and

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edge atoms with a high tendency to chemisorb other elements (22). The characterization of the porous texture of the activated carbon supports and catalysts with nitrogen adsorption at 77 K can be used as a first approximation to analyze the localization of molybdenum species dispersed on the carbon matrix. The achievement of a high degree of dispersion of the supported catalysts is one of the major requirements of all preparation techniques. It can be easily conceived that the textural properties of the support may exert a definite effect on the accessibility of the metal precursor to the adsorption sites and hence, on the final dispersion of the catalyst. In the same way, the high surface area provided by small pores may stabilize highly dispersed particles by forming physical barriers that suppress surface migration and the resulting agglomeration during the activation process. A comparison between the nitrogen adsorption isotherms of the supports at 77 K, after treatment with HNO3 , is shown in Fig. 1. The comparison of the nitrogen adsorption isotherms of the catalysts and their corresponding supports is presented in Fig. 2. All the adsorption isotherms are of type I in the Brunauer, Deming, Deming, and Teller (BDDT) classification (23). The textural properties are more explicitly given in Table 1, where the specific surface areas, ALang and Aext , and specific pore volumes, Vp and VDR , of all the samples are summarized. The Langmuir C values, characteristic of the intensity of the adsorbate–adsorbent interactions, are also reported. The Dubinin–Radushkevich plots of the samples are the characteristic curves describing the micropore filling (24). The linear portions of these plots (4 °

FIG. 1. Nitrogen adsorption isotherms at very low pressures for the activated carbons treated with HNO3 . ( l ) Sample K0, ( s ) sample N298, ( h ) sample N333, ( L ) sample N363, and ( n ) sample NR.

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log 2 (p 7 /p) ° 10) have been used to obtain the micropore volume, VDR , and characteristic energy, E. This last parameter is the characteristic free energy of adsorption for a given system. When the results obtained from nitrogen adsorption analyses of Merck activated carbon (sample K0) and of its acidtreated derivatives (samples N298, N333, N363, and NR) are compared, it is observed that the treatment with HNO3 (17) produced a continuous loss of the textural properties of the activated carbon supports as the treatment temperature increases (see Fig. 1), mainly at 363 K and reflux conditions. The treatment at reflux temperature (sample NR) caused the destruction of a great part of the pores as a consequence of the loss of the walls between neighboring pores, giving an almost flat isotherm. As can be seen in Table 1, the oxidation with HNO3 at 298 K (sample N298) produces a 10% loss of specific surface area and micropore volume, ALang and VDR , and this loss increases up to 89% when the activated carbon is treated in reflux conditions (sample NR). A slight increase of the characteristic energy (E) is shown, samples K0 and N298, from 25.9 to 27.2 kJ/mol. This value is practically constant when the oxidation treatment temperature is up to 363 K, samples N333 and N363. The oxidation treatment at reflux conditions causes an important decrease of this parameter, from Ç27.5 to 22.0 kJ/mol. According to the relationship proposed by Dubinin (25), E varies inversely with x 2 , x being the half-width of slit-like micropores. Therefore, an increase of E can be explained by a decrease of the mean micropore diameter, as can be expected from the presence of oxygen surface groups. The characteristic energies obtained for samples N363 and NR must be explained by the loss of the textural properties (microporosity) in the sample NR. When the nitrogen adsorption isotherms of the supports and molybdenum catalysts are compared, it can be noticed that micropores, supermicropores ( 26 ) , and mesopores are affected by the presence of molybdenum ( see Fig. 2 ) . No difference between the nitrogen adsorption isotherms of the supports K0, N298, and N333 and that of their respective catalysts was observed for relative pressures lower than 10 05 . This could indicate that the blockage of the fine micropore did not take place or that there is a possible creation of pores in this pressure range. The nitrogen adsorption isotherms of the supports N363 and NR present differences in all the pressure ranges with respect to the adsorption isotherms of their respective catalysts. The presence of molybdenum also produces important modifications in the specific surface areas and specific pore volumes when compared with its respective supports ( see Table 1 ) . The catalysts Mo / K0, Mo / N298, and Mo / N333 show a 21 – 31% loss of the textural properties. This textural loss increases to 76% when the activated carbon treated in reflux conditions is impregnated with a solution

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FIG. 2. Nitrogen adsorption isotherms at very low pressures for the activated carbons. ( l ) Activated carbon supports and ( s ) activated carbonsupported molybdenum catalysts.

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TABLE 1 Specific Surface Areas, Specific Pore Volumes, and E Values for the Samples Indicated ALanga,b

Sample K0 Mo/K0 N298 Mo/N298 N333 Mo/N333 N363 Mo/N363 NR Mo/NR

1242 880 1116 772 1075 785 842 453 129 34

(Cf (C (C (C (C (C (C (C (C (C

Å Å Å Å Å Å Å Å Å Å

333) 332) 365) 377) 391) 349) 358) 354) 269) 173)

Vpc

Aexta

VDRd

Ee

0.589 0.433 0.525 0.369 0.498 0.381 0.397 0.227 0.070 0.022

70 62 67 52 59 54 44 33 12 6

0.413 0.293 0.374 0.261 0.358 0.263 0.278 0.152 0.042 0.010

25.9 25.6 27.2 27.0 27.4 26.8 27.5 25.8 22.0 25.4

a

Specific surface area in m2/g. 0.01 ° p/p7 ° 0.05 interval of p/p7. c Specific total pore volume at p/p7 Å 0.95. d Specific pore volume in cm3/g. e Characteristic energy in kJ/mol. f Langmuir C value, characteristic of the intensity of the adsorbate–adsorbent interactions. b

of molybdenum. In the same way, the decrease of the characteristic energy must be explained by an increase of the mean micropore diameter. Nevertheless, the presence of molybdenum in the sample NR produces an increase of characteristic energy, from 22.0 to 25.4 kJ /mol, related with an important blockage of the porosity. Zdrazil (27) reported that the extent of micropore blockage by the catalyst precursor depends on the catalyst preparation method. Afzal (28) used a Merck activated carbon impregnated with varying amounts of Ni, Cu, Zn, and Cd precursors and concluded that progressive closure of the pores occurs when metals are supported on the surface of activated carbons. The dried temperature of the activated carbon catalysts is also a factor to take into account (29). In the present work, the impregnation with molybdenum affected the porosity properties of the impregnated catalysts in relation to the corresponding supports (see Fig. 2 and Table 1), who have undergone a clear reduction of pore volume in all cases. This loss of porosity is observed in the overall pressure range. Therefore, no restriction of the accessibility of nitrogen due to the blocking of micropores as a consequence of the fixation of molybdenum at the entrance of pores was observed. A comparison between the adsorption potential distributions of the supports and catalysts is presented in Fig. 3. These distributions have been obtained by the Dubinin–Astakhov (DA) formalism (30) and have been evaluated in terms of the condensation approximation method (31). The characteristic energy (E), the micropore volume (V0 ) and the value of n exponent are needed to applicate this formalism. The DA equation in terms of these parameters can be expressed as

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F S DG

V A Å exp – V0 E

n

,

[1]

where A is the potential adsorption (A Å RT ln p 7 /p) and u the relative adsorption. The adsorption potential distribution X ( A) can be expressed in terms of these parameters as X ( A) Å 0

d u (A) dA m

Å nA n01

∑ fi iÅ1

F S DG

1 A exp – n Ei Ei

n

,

[2]

where fi ( fi Å V0i /V0 ) is the fraction of adsorption sites located in the micropores with an adsorption characteristic energy Ei . The optimum value of the exponent n for the DA equation was calculated for linear regression and it was selected as the value of n that gives the smallest standard error of the y intercept (log V0 ). The linear regressions have been applied in the range of relative pressures of 2 1 10 06 ° p/p 7 ° 0.2. The characteristic energy and the micropore volume have been obtained by applying the DA equation to the DA plots (log V versus log n (p 7 /p)) in the region of ultramicropores and micropores, corresponding with the linear portions that it is possible to apply the DA equation. In order to discuss the physical interpretation of the distributions, it is necessary to consider that, in all samples, Amp @ Aext (considering Amp Å ALang 0 Aext ), and it can therefore be assumed that the observed energetic heterogeneity is mainly due to microporosity and not to surface heterogene-

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FIG. 3. Adsorption potential distributions for the activated carbons. ( l ) Activated carbon supports and ( s ) activated carbon-supported molybdenum catalysts.

ity. The adsorption potential distributions of the samples NR and Mo/NR have not been included because of the loss of specific surface areas of these samples. Modifications in the adsorption potential distributions can be seen when the catalysts are compared with the supports. The modifications mainly affect the width of the adsorption potential distributions. The distributions of all catalysts are broader than the respective distributions for the supports. The adsorption potentials related with the maxima of the catalyst distributions are lower than those related to the supports. Stoeckli et al. (32) observed that the distributions become broader as the value of the n exponent decreases. The distributions behavior and the values of n exponent obtained in this work are in agreement with this observation. These values are 5.1, 6.3, 6.3, 2.8, and 1.8 for the supports and 4.1, 3.9, 3.7, 2.4, and 1.0 for the catalysts, respectively. These results must be carefully interpreted because the activated carbon treatments and the impregnation with molybde-

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num introduced changes not only in the porous structure but also in the chemistry of the surface, as is considered in previous section. Nevertheless, if in order to discuss the physical interpretation of the distributions this approach assumes that the observed energetic heterogeneity is mainly due to microporosity and not to surface heterogeneity, the terms heterogeneous–homogeneous must be related here with the shape and size of pores of the microporous system. A homogeneous system is related with a shape or size of pores, the maximum of the distribution is related with the size of the pore. A heterogeneous system is related with various shapes or sizes of pores. An extension of the distribution is related with a heterogeneous system. In this way, the behavior observed in the adsorption potential distributions of the supports is related with a homogenation of the microporous system and the same can be considered for the catalysts. As can be seen in Fig. 3, as the temperature of oxidation treatment increases, the microporous system becomes

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more heterogeneous to the loss of the structure in sample CR. Similar behavior can be observed when molybdenum is present in the activated carbons. These results allow us to conclude that the impregnation of the supports proceeded from adequate diffusion conditions and, thus, the molybdenum was well distributed and reached the inner porous network of the carbon supports. A lower sintering rate is expected in these porous systems, due to the fact that the walls of the pores can act as barriers that hinder the mobility of the molybdenum precursor during the activation process. In this respect, Ehrburger et al. (33, 34) observed that the extent of gasification of graphitized carbon black influence in the dispersion of platinum. The authors indicated that the gasification increases the heterogeneity of the carbon surface which raised the potential energy barrier for platinum diffusion. Linares-Solano et al. (35) also shown a relation between the dispersion of platinum supported on graphitized carbon black with the burn-off of the support. Other authors (7, 36) indicated that both dispersion and resistence to sintering of platinum on activated carbon catalysts can be a function of the number of surface oxygen groups present on the activated carbon surface. In order to check the influence of the functional surface groups in the dispersion of molybdenum, an analysis of the carbon surface groups is needed. 2. Temperature-Programmed Desorption One way to analyze the effect of the various types of oxygen surface groups is to carry out temperature-programmed desorption (TPD) analysis of the supports and resulting catalysts, the samples being heated in an inert atmosphere. This kind of experiment can give information about the nature of the species present on the carbon surface and the metal support interaction (15). In these analyses, the CO2 , CO, and H2O evolution was continuously recorded. In general, CO2 appears at lower temperatures than CO and it has been attributed to the decomposition of carboxylic, anhydride (37), and/or lactone (38) groups, acidic groups where C is bonded to two oxygen atoms. CO desorption takes place at higher temperatures, from the decomposition of various types of surface oxygen groups (i.e., phenol, carbonyl, quinone, ether, and pyrene groups), weakly acidic, neutral, and basic groups where C is bonded to an oxygen atom (15, 39). It is observed that the intensity of the CO2 peaks is larger than those evolved as CO, which suggests that an important part of the surface groups are of an acidic nature. The amounts of CO2 and CO released of the oxidized activated carbons with respect the parent activated carbon (K0) are summarized in Fig. 4. As can be seen from this figure, the amounts of CO2 and CO desorbed increases with increasing treatment temperature, showing a temperature dependence of the HNO3 treatment on the total amount of oxygen complexes. The CO2 desorbed is much larger than

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FIG. 4. Relative amounts of CO2 and CO released by TPD with respect to sample K0 for the activated carbon supports.

CO which suggest that the majority of the surface groups are of an acidic nature. These groups are created on the activated carbon surface during HNO3 treatment and/or formed during heating rearrangements. The comparison between the amount of groups released as CO2 with the temperature of the activated carbon supports and activated carbonsupported molybdenum catalysts are shown in Fig. 5. The effect of the impregnation of molybdenum on the surface chemistry of activated carbons is presented in this figure. From this figure, the TPD spectra of the catalysts differed significantly from those obtained for the supports. The main differences being the following: (i) CO2 evolution at low temperatures, up to 900 K, was much lower for the impregnated catalysts than for the supports. (ii) An important CO2 release was found at higher temperatures in the impregnated catalyst, mainly for the samples Mo/K0 and Mo/N298. At 1000 K, CO2 evolution was almost finished for the supports, while the rate of desorption for the impregnated catalysts presented a maximum at about this temperature. Similar differences are observed from the comparison between the amount of groups released as CO with the temperature of the activated carbon supports and activated carbon-supported molybdenum catalysts. TPD spectra of the catalysts compared to that of supports indicated that the amount of impregnated molybdenum affected the decomposition of CO2- and CO-evolving oxygen complexes. MartıB n-GulloB n et al. (9) have found that the groups evolving as CO2 do not change much in respect to the support. The groups evolving as CO2 on TPD experi-

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FIG. 5. CO2 release profiles for the activated carbon supports (solid lines) and activated carbon-supported molybdenum catalysts (dotted lines) upon heat treatment in inert atmosphere.

ments may also play an important role in the preparation of activated carbon-supported catalysts by increasing the hydrophobicity of the carbon and facilitating the access of the aqueous solution to the internal pore structure (7). MartıB n-GulloB n et al. (9) also observed an increase of the molybdenum adsorbed on the activated carbon surface as the oxygen surface groups increased. These authors indicated that there is a possible relation between the molybdenum adsorbed and the groups evolved as CO2 . The results presented in Fig. 5 indicate that there is a transformation of CO2evolving oxygen functional groups on the activated carbon as a consequence of the impregnation of molybdenum and that interactions between these oxygen groups and molybdenum

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species were established. This could indicate that the acidic groups are acting as chemical anchorage centers, as reported for example by Vissers et al. (12) who concluded that the oxygen surface groups are responsible for the strength and extent of the interaction between the molybdenum and carbon surface, the dispersion increasing with the number of surface groups. The oxygen surface groups have also been described as being responsible for the increase of palladium (13) and platinum (7, 14–16, 36) dispersion. In hydrotreating catalysis, once the precursor is deposited on the support, the system is submitted to an activation process (1–3). At the temperatures used, some of the surface groups of the carrier are not stable. If the molybdenum

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species are attached to these groups, the molybdenum mobility can be favored. It is not sufficient to create adsorption sites on the support surface, these sites must also be accessible to the metal precursor and preserved during catalyst activation. It is observed (see Fig. 5) that the quantity of CO2 desorbed during heating up to 673 K (temperature generally used in the activation process) is notably higher for the supports than for the impregnated catalysts. It is not observed difference in the CO desorption of both samples series because this process takes place at higher temperatures than 673 K. This behavior indicates that, for the impregnated catalysts, a large amount of CO2-type complexes remain on the surface, whereas in the case of the supports alone these complexes are unstable at 673 K. From these results it is concluded that molybdenum has a noticeable effect on the groups present on the carbon surface, probably because the interaction of metal species with the oxygen functional groups stabilizes their decomposition. This fact suggests that carboxylic groups, responsible for CO2 evolution in this range of temperatures, participate in the interaction between the molybdenum species and the support. The complexes formed by interaction between molybdenum species and oxygen on active carbon surfaces are not desorbed upon treatment up to 673 K, the temperature used in the activation process, and do not promote molybdenum mobility. 3. FTIR Spectroscopy To determine the changes in the functionality of the carbon surface oxygen in the impregnated catalysts compared to the supports, the FTIR spectra recorded for the catalysts were compared with those of the unloaded supports. These FTIR spectra are summarized in Fig. 6. The FTIR spectra of the activated carbon supports indicated that there is an increase of the band between 1000 and 1300 cm01 which is attributed to C{O

single bonds, such as those in ethers, phenols and hydroxyl groups. A new peak centered at about 1720 cm01 appears for the spectrum of the sample N333. It has been assigned to stretching vibration of carboxyl groups on the edges of the layer planes (40) or conjugated carbonyl groups (C|O in carboxylic acid and lactone groups). The intensity of this band is stronger in the spectrum of the sample NR than in the spectrum of the sample N333. No band in the range 1740–1880 cm01 is observed, indicating that anhydrides are not formed, or are formed in very low quantity, after the nitric acid oxidation of this activated carbon. When the 1720 cm01 band forms, an increase in the absorption at 1580 cm01 is observed that is related with the presence of the oxygen double bond, conjugated with the carbon basal planes (40). Derbyshire et al. (8) indicated that heptamolybdate was associated to substituents of the aromatic ring or to conjugated double bonds in the surface of the carbon. Other authors (12) attributed the interaction of molybdenum species with carbon to the oxygen surface groups, although no great differences in spectral characteristics before and after impregnation could be noticed. The attempts to unravel the role of the oxygen functionality were based on the supposition that, if a chemical reaction takes place between the molybdate ions and a specific oxygen group, a change in the spectral characteristics of the impregnated activated carbons compared to the unloaded ones will become apparent. The obtained spectra are plotted in Fig. 6. The ordinate scale is arbitrary, although all curves have been plotted in the same range of absorbance units. The differences between the spectral characteristics of the catalysts and those of the supports indicated that variations in the carbon surface functionality occurred upon impregnation with molybdate ions. The following features can be discerned: (i) There is a decrease in the intensity of the peak centred at about 1720 cm01 , assigned to stretching vibration of carboxyl groups on the edges of the layer planes (40) or to conjugated carbonyl groups. This indicates that carboxyl and conjugated carbonyl groups participate in the interaction between the precursor and the support by forming oxygen complexes with the molybdenum species. This corroborates the above discussed results of TPD analyses, which indicate that the acidic groups act as chemical anchorage centers. A possible mechanism of interaction between molybdenum anions and the acidic carbon surface functional groups can be considered as O

O O

Mo –

O

H O–

O

O C FIG. 6. FTIR spectra for the activated carbon-supported molybdenum catalysts compared to those of corresponding activated carbon supports.

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activated carbon

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O Mo

3 H+

O

O C

activated carbon

+ 2 H¤O.

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(ii) A new band appears with a maximum at 3125 cm01 and a shoulder at 3010 cm01 in the impregnated catalysts, assigned to the C{H stretch vibration in aromatic carbon. However, unloaded supports did not exhibit any significant adsorption in this region. (iii) The appearance of a band at about 900 cm01 is ascribed to C{H vibrations in aromatic carbons, which is consistent with the observed 3200–3100 cm01 band. The appearance of these bands after the impregnation of molybdenum may be attributed to the formation of C{H bonds in aromatic rings as a consequence of the opening of ether bridges between them. This opening could be promoted by the interaction of molybdenum species with the ether groups on the activated carbon surface. A possible mechanism of interaction between molybdenum anions and the ether-like carbon surface functional groups can be considered as O

O

O

Mo –

O

O

O Mo

O–

H

O

2 H+

O– + 2 H¤O.

4. X-ray Photoelectron Spectroscopy The surface composition of the activated carbon supports and activated carbon-supported molybdenum catalysts evaluated by XPS is summarized in Table 2. The relative amount of molybdenum on catalyst surfaces increased with the amount of oxygen functional groups on the supports. XPS data confirmed that interactions between metal and oxygen functional groups were developed and favored the dispersion of molybdenum. The C 1s signal of all the samples consists in a major graphitic peak at 284.5 eV and satellites at higher binding TABLE 2 Quantitative XPS Analysis of the Activated Carbon Supports and Activated Carbon-Supported Molybdenum Catalysts Sample

C (at%)

O (at%)

K0 Mo/K0 N298 Mo/N298 N333 Mo/N333 N363 Mo/N363 NR Mo/NR

95.37 89.78 90.26 87.37 88.11 85.22 80.36 78.65 75.99 70.36

4.32 8.96 8.36 11.35 10.59 13.50 18.13 19.91 22.14 28.15

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Mo (at%)

1.26 1.28 1.28 1.44 1.49

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energies corresponding to functionalized C. The shifts from the main peak range from 1.7 to 4 eV have been related with carbon atoms singly bonded to oxygen and carbon atoms in carboxyl groups or esters (41), respectively. The XPS spectra of the activated carbon supports confirm that the intensity of the peak at 284.5 eV increases with the increasing treatment temperature in HNO3 , indicating that some of the carbon atoms were functionalized by means of the creation of carbon–oxygen bonds. The increase in the C 1s satellite at around 288.5 eV is especially marked, indicating the formation of a great quantity of functional groups with C|O double bonds, in agreement with FTIR results. It can be observed that the peak of the activated carbon supports at 288.5 eV, assigned to C|O double bonds, became much less important after impregnation with molybdenum. An illustration of these differences is given in Fig. 7, where C 1s spectra of the supports N363 and NR (solid lines) are compared to those of the impregnated catalysts Mo/N363 and Mo/NR (dotted lines). These results, in agreement with those of FTIR analyses, confirmed that interactions are established between the molybdenum species and C|O containing surface functional groups on the activated carbon upon impregnation. These groups were transformed and did not show the corresponding peaks in the spectra of the activated carbon catalysts. The Mo 3d5 / 2 –Mo 3d3 / 2 were fitted so that each peak had the same Gaussian line shape. The relative area ratios of spin–orbit doublet peaks are given by the ratio of their respective degeneracies (2 j / 1) (42). Therefore, for the Mo 3d5 / 2 –Mo 3d3 / 2 doublet the intensity ratio should be I(3d5 / 2 )/I(3d3 / 2 ) Å 32. A splitting energy of 3.2 eV was used for the Mo 3d5 / 2 –Mo 3d3 / 2 doublet. The quantity of molybdenum present on the catalyst surfaces was examined by XPS before and after treatment in inert atmosphere (helium) at 673 K, temperature used in the activation process of hydrotreating catalysts (1–3). As mentioned above, the effect of the heat treatment in the activation stage has to be considered because, in spite of the thermal stability at 673 K of most of the oxygen complexes contained in the supports, the more acidic CO2-type complexes are desorbed at this temperature. Moreover, it must be kept in mind that this is a dynamic system and some transformation or rearrangement of oxygen groups can happen and favor the molybdenum mobility. Before heating, the spectra of the activated carbon catalysts showed the presence of two well-resolved spectral lines at 232.3 and 235.5 eV assigned to the Mo 3d5 / 2 and Mo 3d3 / 2 binding energies, respectively, and related to Mo(VI) (43). After treatment in helium at 673 K, the binding energies in the molybdenum spectra showed almost no changes, indicating that the molybdenum remained in the same oxidation state. The surface composition of catalysts obtained upon treatment in inert atmosphere at 673 K is summarised in Table

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165

FIG. 7. C 1s spectra for the activated carbon supports (solid lines) and activated carbon-supported molybdenum catalysts (dotted lines).

3. The quantitative XPS analyses of treated catalysts indicated that the relative amount of molybdenum on the catalyst surface slightly decreased in the case of the less oxidized supports (samples K0, N298) compared to the activated carbon catalysts (see Table 2). However, when more oxidized supports with a large quantity of CO2-evolving functional groups are considered, the relative amount of molybdenum increases. These results can indicate that some transformation or rearrangement of the molybdenum complexes happened during the heat treatments and affected the dispersion of the molybdenum on the more oxidized supports. This effect seemed to be avoided in more oxidized supports, where the oxygenated groups on the carbon surface can act as chemical anchorage centers. As mentioned above, TPD spectra showed that CO2 evolution at low temperatures, up to 900 K, is much lower for the catalysts than for the supports, indicating that there is a transformation of CO2-evolving oxygen functional groups on the activated carbon as a consequence of the impregnation of molybdenum. The groups that are bonded to the molybdenum species are thermally more

stable than those on the carbon surface of the unloaded supports, and they are desorbed only at temperatures above 673 K. For that the dispersion of the molybdenum during the heat treatment in the activation process is mainly not affected. The sintering of molybdenum may also be hampered by the ruggedly shaped carbon surfaces (corresponding to large surface areas) due to the physical barrier of the carbon surface on a microscopic scale. In the more oxidized samples, in particular the NR sample whose micropore network has been destroyed by the treatment in nitric acid at reflux temperature, creating a large amount of oxygen complexes, the molybdenum can only be fixed to the surface by means of interaction with the created oxygen functional groups. These results show that an interaction was developed between the molybdenum and C|O containing functional groups on the activated carbon surface. These groups were thus stabilized by the established interactions and were not desorbed during the heat treatments at the temperatures used in the activation process, avoiding the mobility of the metal complexes.

TABLE 3 Quantitative XPS Analysis of the Activated Carbon-Supported Molybdenum Catalysts Obtained upon Treatment in Inert Atmosphere at 673 K

CONCLUSIONS

Sample

C (at%)

O (at%)

Mo (at%)

Mo/K0-673 Mo/N298-673 Mo/N333-673 Mo/N363-673 Mo/NR-673

90.59 88.56 86.65 81.58 74.29

8.19 10.18 12.02 16.94 23.93

1.22 1.26 1.33 1.48 1.78

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The textural characterization indicated that the impregnation of the activated carbons with various degrees of oxidation proceeded with adequate diffusion conditions. The molybdenum precursor was well distributed, and it reached the inner porous network of the activated carbon support, which can act as a barrier that hinders the mobility of the molybdenum species. TPD spectra of the catalysts indicated that the amount of impregnated molybdenum affected the decomposition of

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DE LA PUENTE ET AL.

oxygen complexes and modified the evolution of CO2- and CO-type oxygen complexes. FTIR and XPS analysis confirmed that interactions were established between the molybdenum species and C|O containing surface functional groups on the activated carbon upon impregnation. The complexes formed by these interactions were not desorbed upon treatment up to 673 K, the temperature used in the activation process, and did not promote the molybdenum mobility. Interactions of molybdenum species with ether groups on the active carbon surface were also developed and could promote the opening of ether bridges between aromatic rings. Quantitative XPS analysis showed that the relative amount of molybdenum on catalyst surfaces increased with the amount of oxygen functional groups on the supports. These results confirmed that interactions between metal and oxygen functional groups were developed and favored the molybdenum dispersion. ACKNOWLEDGMENT G.d.l.P. thanks the Fundacio´n para el Fomento en Asturias de la Investigacio´n CientıB fica Aplicada y la TecnologıB a (FICYT) for her postdoctoral fellowship.

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