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Molybdenum Carbide Formation in Molybdenum-Doped Organic and Carbon Aerogels A. F. Pe´rez-Cadenas, F. J. Maldonado-Ho´dar, and C. Moreno-Castilla* Departamento de Quı´mica Inorga´ nica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Received July 21, 2005. In Final Form: August 30, 2005 A Mo-doped organic aerogel and its corresponding carbonized derivative at 1000 °C were obtained. Both samples were treated in a H2/Ar flow. After this treatment, a mixture of Mo(VI) and Mo2C was detected in both samples. The results obtained indicate that the presence of H2 in the gas flow is necessary to obtain the carbide phase, due to the formation of CH4 or even CHx species that reduce and carburize the molybdenum oxide phase. Carburization of the Mo-doped organic aerogel yielded better results compared with carburization of the Mo-doped carbon aerogel.
Introduction Carbon aerogels are one of the most promising new carbon forms due to the versatility of their surface properties. These materials are prepared from the carbonization of organic aerogels obtained from different organic reactants, but they are commonly prepared by polymerization of resorcinol (R) with formaldehyde (F) in aqueous (W) solutions catalyzed by Na2CO3.1 The characteristics of the gels formed depend on the R/F, R/W, and R/C reactant ratios and the solution pH. Supercritical drying methods prevent collapse of the porous texture of the gels, yielding aerogels. In previous papers we showed how the addition of soluble metal precursors to aqueous solutions leads to dispersal of metal particles into the highly porous organic matrix. These metal-doped carbon aerogels can have applications in catalysis.2-15 The presence of metal precursors in the organic solutions strongly influences polymerization,10 carbonization,5 and/ or activation processes,12 because metals present a different activity at each preparation step. Interactions between metal particles and the organic phase during carbonization of metal-doped organic aerogels can give * Corresponding author. Fax: 34-958-248526. E-mail: cmoreno@ ugr.es. (1) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221-3227. (2) Maldonado-Ho´dar, F. J.; Ferro-Garcı´a, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199-1205. (3) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Rodrı´guez-Castello´n, E. Appl. Catal., A 1999, 183, 345-356. (4) Miller, J. M.; Dunn, B. Langmuir 1999, 18, 799-806. (5) Maldonado-Ho´dar, F. J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y. Langmuir 2000, 16, 4367-4373. (6) Bekyarova, E.; Kaneko, K. Adv. Mater. 2000, 12, 1625-1628. (7) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Carrasco-Marı´n, F.; Rodrı´guez-Castello´n, E. Langmuir 2002, 18, 2295-2299. (8) Baumann, T. F.; Fox, G. A.; Satcher, J. H.; Yoshizawa, N.; Fu, R.; Dresselhaus, M. S. Langmuir 2002, 18, 7073-7076. (9) Maldonado-Ho´dar, F. J.; Pe´rez-Cadenas, A. F.; Moreno-Castilla, C. Carbon 2003, 41, 1291-1299. (10) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J.; Pe´rez-Cadenas, A. F. Langmuir 2003, 19, 5650-5655. (11) Baumann, T. F.; Satcher, J. H. Chem. Mater. 2003, 15, 37453747. (12) Maldonado-Ho´dar, F. J.; Moreno-Castilla, C.; Pe´rez-Cadenas, A. F. Microporous Mesoporous Mater. 2004, 29, 119-125. (13) Maldonado-Ho´dar, F. J.; Moreno-Castilla, C.; Pe´rez-Cadenas, A. F. Appl. Catal., B 2004, 54, 217-224. (14) Saquing, C. D.; Cheng, T. T.; Aindow, M.; Erkey, C. J. Phys. Chem. B 2004, 108, 7716-7722. (15) Moreno-Castilla, C.; Maldonado-Ho´dar, F. J. Carbon 2005, 43, 455-465.
rise to the partial graphitization of the organic matrix,5 while metals can be reduced or even carburized by the carbon matrix to an extent that depends on their nature.15 Thus, it has been shown that tungsten carbide can be formed when tungsten-doped carbon aerogels are pyrolyzed in flowing N2 at 1000 °C.3,9 This carbide, together with other transition metals, has been proposed as a new active catalyst for hydrotreating processes.16 Different studies have reported that molybdenum carbide exhibits the highest activity in these reactions.16 This carbide is usually formed by reaction between bulk or supported molydenum precursors and flowing hydrocarbon/H2 mixtures.17-19 However, it has been shown19 that hydrocarbons are not necessary in the reactant gas mixture if the support used is an activated carbon or a carbon black. Carbon aerogels have major advantages over other carbon materials for use in catalysis, as recently reported.15 The objective of this study was to explore the possibility of carburizing the molybdenum precursor in Mo-doped organic and carbon aerogels using the above method. Experimental Section The preparation of Mo-doped monolithic organic aerogels was reported elsewhere.3 Briefly, resorcinol (R) ammonium heptamolybdate and formaldehyde (F) were dissolved in water (W). The stoichiometric R/F and R/W molar ratios were 0.5 and 0.13, respectively. The amount of metal precursor added was calculated to be 1 wt % of the metal in the initial solution. The pH of this starting solution was 3.5. The mixture was stirred to obtain homogeneous solutions, which were cast into glass molds (25 cm length × 0.5 cm i.d.). After the curing period, the gel rods were cut in 5 mm pellets, exchanged with acetone, and supercritically dried with carbon dioxide to form the corresponding aerogels. The aerogel will be referred to as AMo. Pyrolysis of the AMo to obtain the corresponding Mo-doped carbon aerogel was carried out in N2 flow, 100 cm3/min, by heating to 1000 °C at a heating rate of 2 °C/min and soaking time of 5 h. This sample will be referred to in the text as AMo-1000. The exact metal content of the Mo-doped carbon aerogel was obtained by burning a portion in a thermobalance at 1000 °C under air flow up to constant weight. (16) Sayag, C.; Benkhaled, M.; Suppan, S.; Trawczynski, J.; Dje´gaMariadassou, G. Appl. Catal., A 2004, 275, 15-24. (17) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 348-356. (18) Ribeiro, R. H.; DallaBetta, R. A.; Boudart, M.; Baumgartner, J. E.; Iglesia, E. J. Catal. 1991, 130, 86-105. (19) Mordenti, D.; Brodzki, D.; Dje´ga-Mariadassou, G. J. Solid State Chem. 1998, 141, 114-120.
10.1021/la051969+ CCC: $30.25 © 2005 American Chemical Society Published on Web 10/04/2005
Molybdenum-Doped Organic Aerogels Characterization of the samples was carried out by CO2 adsorption at 0 °C, mercury porosimetry, thermogravimetric analysis coupled with infrared spectroscopy, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). The Dubinin-Radushkevich equation was applied to the CO2 adsorption isotherm, from which the micropore volume, W0, was obtained. This micropore volume was converted into an apparent surface area. The CO2 molar volume and molecular area used were 42.72 cm3/mol and 0.188 nm2, respectively.20,21 Mercury porosimetry was obtained up to a pressure of 4200 kg/cm2 using Quantachrome Autoscan 60 equipment. With this technique, the following parameters were obtained: volume of pores with a diameter between 3.7 and 50 nm, V2, referred to as mesopore volume (note the difference with the classic definition of the mesopore volume range as 2-50 nm);21 volume of pores with a diameter greater than 50 nm, or macropore volume, V3; and particle density, Fp. XRD patterns were recorded with a Phillips PW1710 diffractometer using Cu KR radiation. SEM experiments were carried out with a ZEISS DSM 950 (30 kV) microscope. HRTEM was performed with a Phillips CM-20 electron microscope. XPS measurements were made with an Escalab 200R system (VG Scientific Co.) equipped with a Mg KR X-ray source (hν ) 1253.6 eV) and a hemispherical electron analyzer. Prior to the analysis, the aerogels were evacuated at high vacuum and then introduced into the analysis chamber. A base pressure of 10-9 mbar was maintained during data acquisition. Survey and multiregion spectra were recorded at C1s, O1s, and Mo3d photoelectron peaks. Each spectral region of photoelectron interest was scanned several times to obtain good signal-to-noise ratios. The spectra obtained after background signal correction were fitted to Lorentzian and Gaussian curves in order to obtain the number of components, position of the peak, and peak areas. TPR was carried out in a tubular quartz reactor (5 mm i.d.) coupled to a TCD analyzer for monitoring H2 consumption. Samples (150 mg) were heated at 2 or 10 °C/min from room temperature to 800 °C in an Ar flow (30 cm3/min) containing different H2 concentrations (8, 12, and 16 vol %).
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Figure 1. SEM micrographs of samples: I, AMo; II, AMo1000.
Results and Discussion Characterization of the Mo-Doped Organic and Carbon Aerogels. The organic AMo aerogel was opaque and brown colored. Its surface morphology obtained by SEM is shown in Figure 1, indicating that this sample was formed by small rounded primary particles whose interconnection yielded a highly macroporous network. In organic aerogels, the size of these particles depends on the synthesis conditions.22 In the case of metal-doped organic aerogels, the particle size depends on the nature of the metal and the solution pH. The surface of AMo1000 was very similar to that of AMo except for its slightly smaller primary particle size. Textural characteristics of AMo and AMo-1000 are compiled in Table 1. Pore size distributions (PSD) obtained by mercury porosimetry are shown in Figure 2. The results obtained indicate that AMo is a highly macroporous material but has no mesoporosity. Carbonization of this sample to obtain AMo-1000 did not affect the macropore volume or the PSD. However, carbonization produced a large increase in micropore volume and consequently in surface area. AMo underwent a weight loss (WL) of around 56% after carbonization at 1000 °C to produce AMo-1000. The carbonization process was followed by thermogravimetry (TG-DTG), and the profile is shown in Figure 3. Results (20) Ehrburger-Dolle, F.; Achard, P.; Berthon-Fabry, S.; Bley, F.; Carrasco-Marı´n, F.; Djurado, D.; Faire´n-Jime´nez, D.; Moreno-Castilla, C.; Morfin, I. Carbon 2005, 43, 3010-3013. (21) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1998. (22) Al-Muhtaseb, S. A.; Ritter, J. A. Adv. Mater. 2003, 15, 101-114.
Figure 2. Pore size distributions from mercury porosimetry: AMo, light line; AMo-1000, heavy line. Table 1. Textural Characteristics of the Organic and Carbon Aerogels sample
weight loss %
Fp g/cm3
SCO2 m2/g
W0 cm3/g
V2 cm3/g
V3 cm3/g
AMo AMo-1000
56
0.52 0.53
254 686
0.10 0.26
0.00 0.00
1.39 1.30
obtained in previous studies,2,23 indicated that the weight loss (typically around 15%) below 250 °C is related to the solvent and residual organic desorption. Carbonization occurs at higher temperatures, and the DTG profile showed two typical minima at 450 and 650 °C that correspond to the breakage of C-O and C-H, respectively.23 The position of these peaks depended on the heating rate, as expected. (23) Lin, C.; Ritter, J. A. Carbon 1997, 35, 1271-1278.
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Pe´ rez-Cadenas et al. Table 2. Binding Energy Values (BE) of the Mo3d5/2 Level and Surface (MXPS) and Total (MTotal) Metal Contents aerogel AMo AMo-1000 carburized AMo-1000 carburized AMo
Figure 3. TG-DTG profile of AMo heated in Ar flow (100 cm3/ min) at 2 °C/min.
Figure 4. CO, CH4, and CO2 evolution profiles from A and AMo during carbonization. A (9) AMo (O).
The evolution of CO, CH4, CO2, and water during carbonization of AMo and blank samples was followed by infrared spectroscopy. Water was continuously evolved above a temperature of around 200 °C due to decomposition of -OH groups from the aerogel structure. The CO, CH4, and CO2 evolution profiles were plotted against temperature, as depicted in Figure 4. These profiles were compared with those obtained from a blank organic aerogel, A, prepared with the same ingredients but without the Mo precursor. Results for this blank sample were published elsewhere.12 According to the polymerization mechanism of resorcinol with formaldehyde,22 the surface oxygen functionalities of the aerogel should essentially be of phenol, alcohol, and ether type. Therefore, the CO evolution comes from the breakage of methylene ether structures and from phenol and alcohol functionalities, and the CH4 evolution is from methylene (-CH2-) cross-links between aromatic rings.24 The CO2 evolution likely comes from the reaction of CO with other oxygen functionalities or even from the disproportionation of CO. This disproportionation can be catalyzed by some of the metal-doped organic aerogels. The CO2 evolution may also derive from reaction of the (24) Kuhn, J.; Brandt, R.; Mehling, H.; Petricevic, R.; Fricke, J. J. Non-Cryst. Solids 1998, 225, 58-63.
BE (eV) 232.2 232.2 (55.0%) 228.7 (45.0%) 232.3 (71.6%) 228.0 (28.4%) 231.9 (60.2%) 228.2 (39.8%)
% % MXPS MTotal 2.9 3.2 3.0 3.1
1.2 2.7 3.4 3.0
organic matrix with the decomposition products of the metallic salts used. The CO and CH4 evolution started at 200 °C, whereas in the blank sample it started at 400 °C. In these evolution profiles, the maximum was reached at around 700 °C except in the CO evolution from AMo, when it was reached at around 900 °C. The CO2 evolution profile of AMo showed two maxima at 600 and 900 °C. However, the blank sample showed one maximum in the temperature range studied, centered at around 550 °C. These results indicate that Mo influenced CO and CO2 evolution from the aerogels. An additional source of carbon oxides is the possible reduction of molybdenum (VI) oxide by the carbon matrix; according to the Ellingham diagram, this reduction occurs at 650700 °C, and the DTG profile showed an intense peak at this temperature range. XPS of aerogels was carried out, recording the C1s and the Mo3d core-level spectra. XPS patterns corresponding to the Mo3d region are depicted in Figure 5. The peak at a binding energy (BE) of 284.9 eV, obtained for the C1s signal, was chosen as the reference value for the deconvolution of all the spectra. The spectrum of Mo3d region for AMo (Figure 5A) showed a Mo3d5/2 peak at 232.2 eV, which is characteristic of Mo(VI).25-27 Thus, the metal oxidation state did not change during preparation of the Mo-doped organic aerogel. Total and surface (obtained by XPS) metal contents are shown in Table 2. The metalcontaining phase in AMo was more concentrated on the external surface of the pellet, indicating a surface segregation of the metal. After carbonization at 1000 °C, sample AMo-1000 (Figure 5B) showed a mixture of 55% Mo(VI) and 45% Mo(IV), with Mo3d5/2 peaks at BEs of 232.2 and 228.7 eV,25-28 respectively. In addition, Mo atomic surface concentration (Table 2) did not change during pyrolysis and was similar to the total metal content, indicative of a good dispersion of the metal oxide phase. XRD patterns of both AMo and AMo-1000 samples showed no diffraction peaks, indicating either that these were amorphous materials or that they have a particle size smaller than 4 nm, the lower detection limit in X-ray diffraction. Particle size of the metallic phase in AMo-1000 was studied by HRTEM. A selected micrograph is shown in Figure 6A, as an example. Particle size ranged from 4 to 14 nm, indicating that the molybdenum oxide phase has a poor crystallinity. Heat Treatments of Mo-Doped Organic and Carbon Aerogels in H2/Ar Flows. TPR of AMo-1000 was studied by heating the sample in an 8 vol % H2/Ar flow at two different heating rates, 10 and 2 °C/min, and the results are depicted in Figure 7. The TPR profile at a heating rate of 10 °C/min showed only one peak centered (25) http://srdata.nist.gov/xps/. (26) Chen, H. Y.; Chen, L.; Lu, Y.; Hong, Q.; Chua, H. C.; Tang, S. B.; Lin, J. Catal. Today 2004, 96, 161-164. (27) De la Puerta, G.; Centeno, A.; Gil, A.; Grange, P. J. Colloid Interface Sci. 1998, 202, 155-166. (28) Manoli, J. M.; Da Costa, P.; Brun, P.; Brinat, M.; Mauge´, F.; Potvin, C. J. Catal. 2004, 221, 365-377.
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Figure 5. Curve-fitted Mo3d core-level spectra for samples AMo and AMo-1000 before (A and B) and after (C and D) treatment in a 16 vol % H2/Ar flow.
at 500 °C, with small shoulders at around 390 and 735 °C, whereas at a heating rate of 2 °C/min the hydrogen consumption took place in two well-differentiated steps. Thus, the first peak appeared at around 430 °C and the second at 700 °C. The effect on TPR profiles of the H2 concentration in the H2/Ar flow is shown in Figure 8. Hydrogen consumption at the first peak remained invariable, whereas it progressively increased at the second peak with an increased H2 concentration in the H2/Ar flow. Samples were analyzed by XRD after the TPR experiments. The results are shown in Figure 9. It is noteworthy that no peaks corresponding to any molybdenum oxide were detected. The organic matrix induced the formation of two broad bands located at 23° and 44°, indicating that molybdenum does not favor carbon matrix graphitization. The only peak observed after a TPR experiment appeared at 39.6°, which corresponds to the more intense diffraction peak of hexagonal molybdenum carbide, β-Mo2C.16,19 The intensity of this peak slightly increased with higher hydrogen concentration in the H2/Ar flow. These results indicate that molybdenum carbide was formed during the TPR experiments. Mordenti et al.19 and Sayag et al.16 also reported the formation of this carbide during TPR runs when using an activated carbon and a carbon black, respectively, as supports of the Mo precursor. They proposed that the first peak in the TPR corresponded to reduction of MoO3 to MoO2 and the second peak to carburization of the latter oxide according to reaction 1.
2MoO2 + CH4 + 2H2 f Mo2C + 4H2O
(1)
The present results show that the carburization step is highly dependent on the H2 concentration in the gas phase
(Figure 8), likely because of the formation of methane, which acts as carburizing agent. Figure 5D depicts the XPS of the Mo3d region of sample AMo-1000 after the TPR run with 16 vol % H2/Ar flow, and Table 2 shows the BE values and percentage of each species. These results show that two Mo species were present after the TPR run, one at a BE of 232.2 eV due to Mo(VI) and the other at a BE of 228.0 eV, which can be attributed to the presence of molybdenum carbide.25,26,28 Both surface and total metal contents were similar, indicating a homogeneous metal distribution. Comparison between these results and those obtained in AMo-1000 shows that the formation of this carbide needs the presence of H2 in the gas phase. This gas cannot reduce Mo(IV) in the temperature range at which it is reduced and carburized to Mo2C. Hence, the importance of H2 is that it can form and mobilize CH4 or CHx species, which reduce and carburize Mo(IV). An HRTEM micrograph of AMo1000 after TPR is shown in Figure 6, as an example. The particle size of the oxide-carbide phases ranged from 4 to 58 nm. Therefore, the carburizing treatment of AMo-1000 widened the particle size range of this sample. The AMo organic aerogel was also heated in a 16 vol % H2/Ar flow from room temperature to 800 °C at 2 °C/min, with a soak time at this temperature of 1 h. No TPR was recorded to protect the TC detector. After this treatment, the sample was analyzed by XRD, XPS, and HRTEM. The XRD pattern showed no diffraction peak that corresponded to either molybdenum oxide or carbide. However, XPS results (Figure 5C and Table 2) showed the presence of Mo(VI) and Mo2C in this sample. The proportion of the carbide phase in this sample was greater than that obtained in the carburized AMo-1000 sample. This might be due to the formation of more carburizing species during
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Figure 7. Influence of heating rate on the TPR profile of AMo1000 heated in a 8 vol % H2/Ar flow at 100 cm3/min (light line, 10 °C/min; heavy line, 2 °C/min).
Figure 8. Effect of H2 concentration on the TPR profile. A, 8 vol %; B, 12 vol %; C, 16 vol %.
Figure 6. HRTEM micrographs of AMo-1000 before (A) and after (B) treatment in a 16 vol % H2/Ar flow. (C) AMo after treatment in a 16 vol % H2/Ar flow.
pyrolysis of the organic matrix of AMo compared with that of AMo-1000. Both surface and total metal contents were similar, indicating a homogeneous metal distribution. The nondetection of the carbide phase by XRD may be caused by either the lower crystallinity or the higher dispersion of this sample compared with that of AMo1000 treated under the same conditions. An HRTEM micrograph of this sample is shown in Figure 6C, as an example. In this case, the particle size ranged from 2 to 16 nm. Thus, this sample showed a smaller particle size and a narrower particle size distribution compared with those of the carburized AMo-1000 sample. These results are important, because better carbide dispersion can be obtained by directly carburizing a Mo-doped organic aerogel than by carburizing a Mo-doped carbon aerogel. This can be attributed to the fact that the latter sample was previously obtained by pyrolyzing the former at 1000 °C.
Figure 9. XRD diffraction patterns of AMo-1000 after treatments in H2/Ar flow of different H2 concentrations: A, 8 vol %; B, 12 vol %; C, 16 vol %.
Conclusions A Mo-doped organic aerogel was prepared by sol-gel procedure from the polymerization reaction of a resorcinol-formaldehyde mixture containing ammonium heptamolybdate. After heating this aerogel to 1000 °C in an inert atmosphere, it was transformed into a Mo-doped carbon aerogel, and Mo(VI) was partially reduced to Mo(IV) by the organic matrix. The metal oxide phase had low crystallinity. When the Mo-doped carbon aerogel obtained at 1000 °C was treated in a H2/Ar flow up to 800 °C, the Mo precursor was partially carburized, giving rise to β-Mo2C,
Molybdenum-Doped Organic Aerogels
which was detected by both XRD and XPS. Therefore, carburization needs the presence of H2, apparently required to form the carburizing agents. Formation of the carbide depended on the H2 concentration in the gas phase. Likewise, when the Mo-doped organic aerogel was treated in a H2/Ar flow at 800 °C, the Mo precursor was partially carburized. In this case, the carbide phase was detected by XPS but not by XRD. The XPS results showed that the proportion of the carbide phase in this sample was higher than in the carburized AMo-1000 sample. In
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addition, the carbide phase had a better dispersion in the carburized Mo-doped organic aerogel than in the carburized Mo-doped carbon aerogel. In conclusion, carburization of Mo-doped organic aerogel yields better results than carburization of Mo-doped carbon aerogel. Acknowledgment. The authors are grateful to MCYT and FEDER, Project MAT2001-2874, for financial support. LA051969+
