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Benzene adsorption on Mo2C: A theoretical and experimental study
Applied Catalysis A: General 379 (2010) 54–60
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Benzene adsorption on Mo2 C: A theoretical and experimental study A.S. Rocha a , A.B. Rocha b , V. Teixeira da Silva a,∗ a b
NUCAT/PEQ/COPPE/UFRJ, Caixa Postal 68502, CEP 21941-972 Rio de Janeiro, RJ, Brazil Departamento de Físico-Química, Instituto de Química, UFRJ, Ilha do Fundão, CT, Bloco A, Rio de Janeiro, RJ, CEP 21949-900, Brazil
a r t i c l e
i n f o
Article history: Received 16 November 2009 Received in revised form 18 February 2010 Accepted 27 February 2010 Available online 6 March 2010 Keywords: Benzene hydrogenation Molybdenum carbide Adsorption DFT
a b s t r a c t Continuous gas-phase benzene hydrogenation at 363 K and atmospheric pressure was studied using bulk molybdenum carbide as a catalyst. It was observed that benzene conversion to cyclohexane was initially 100%, dropping to zero after four hours of reaction. After deactivation the catalyst was submitted to a heating program under helium flow from room temperature (RT) up to 1273 K. On-line mass spectrometry showed desorption of benzene and hydrogen suggesting that the deactivation occurred due to strong absorption of benzene on the molybdenum carbide surface. Density Functional Theory (DFT) calculations support this hypothesis showing the adsorption energy of benzene on the molybdenum carbide varies from −377 kJ mol−1 to −636 kJ mol−1 depending on the surface and on the crystal phase. Based on the experimental and theoretical results, a reaction scheme is proposed with the following steps: (i) after synthesis, the molybdenum carbide surface is completely covered by chemisorbed hydrogen; (ii) when the reaction begins, benzene molecules react with the chemisorbed hydrogen via an Eley-Riedeal mechanism, producing cyclohexane and vacant sites on the surface; (iii) other benzene molecules strongly and irreversibly adsorb to these vacant sites, poisoning the surface and thus leading to deactivation of the catalyst. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Transition metal carbides constitute an important and interesting class of materials for catalysis because, despite being conductors, they possess the physical and mechanical properties of ceramics. There are several reports of their activity in reactions typically catalyzed by noble metals such as hydrogenation, dehydrogenation, hydrogenolysis, isomerization, and also for hydrotreating reactions such as hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation [1–13]. Molybdenum carbide supported on acidic zeolites is a bifunctional catalyst that can be used for aromatization and dehydroaromatization reactions [14–18]. Previous works have reported the preparation of catalysts composed of carburized molybdenum supported on Y-zeolites with different acidities, which had activity for benzene hydrogenation at low temperature and pressure [19,20]. When nanoparticles of molybdenum carbide were dispersed on NaH-USY, high initial activity for benzene hydrogenation was observed, although rapid deactivation occurred, with conversion decreasing to nearly zero after 2 h of reaction [21]. Two other catalysts with carburized molybdenum supported on the same Yzeolite have not presented such deactivation behavior, but the
∗ Corresponding author. Tel.: +55 21 2562 8344; fax: +55 21 2562 8300. E-mail address: [email protected] (V.T. da Silva). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.02.032
active phases were clearly not of the carbide type, as seen by EXAFS and DFT calculations, which indicated the formation of an oxycarbidic species. Regarding the carbide species, the reasons for the fast deactivation have not been fully elucidated. The hydrogenation of benzene is known to be a structureinsensitive reaction [22] and the best catalyst for this reaction is Ru/Al2 O3 [23]. To confirm the noble metal behavior of molybdenum carbide, several authors have compared its activity either in bulk form [24,25] or supported on alumina [26,27] to that of Ru/Al2 O3 catalysts for the hydrogenation of benzene. In both cases it was found that the carbide had an initial turnover frequency (TOF) similar to or even higher than that of ruthenium. In those studies, the authors observed a complete deactivation of the carbide catalyst after several hours of reaction, a fact that was not explained or discussed. The main goal of this work was to elucidate the deactivation process that occurs during benzene hydrogenation when molybdenum carbide is used as the catalyst. The study included synthesis of bulk molybdenum carbide, measurements of its activity for benzene hydrogenation at low temperature and pressure, and DFT calculations of the interaction of benzene with the orthorhombic and hexagonal Mo2 C surfaces. A number of DFT calculations concerning the interaction of the Mo2 C (0 0 1) surface with small molecules have been reported [28–33], as well as a study of Mo2 C bulk and surface structure and stability [33]. To the best of our knowledge, there have been
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no previous theoretical studies of the interaction of benzene with Mo2 C. 2. Experimental 2.1. Preparation and characterization of the catalysts Molybdenum carbide was synthesized by carburization of molybdenum oxide (0.5 g, Aldrich) with a 20% (v/v) CH4 /H2 gas mixture (350 mL min−1 , Linde UP) using a temperature-program from room temperature (RT, 298 K) up to 923 K at a 10 K min−1 heating rate and holding the final temperature for 2 h. To evaluate the influence of the holding time at 923 K on the physical properties, two other samples were synthesized using 0.5 h and 1 h as the holding time, and then the samples were characterized by X-ray diffraction. Before characterization, the samples were passivated overnight at room temperature under a flow of a 0.5% (v/v) O2 /He gas mixture (50 mL min−1 , Linde UP). X-ray diffraction patterns were recorded on a Rigaku (Miniflex) diffractometer, with Cu K␣ radiation ( = 1.5418 Å, 30 kV and 15 mA) using the powder method. Specific surface area was determined from nitrogen adsorption isotherms at 77 K using the BET method, in an ASAP 2000 (Micromeritics) volumetric apparatus. Before the analyses, the materials were pre-treated under standard vacuum (6.7 × 10−6 MPa) at 423 K for 20 h. 2.2. Catalytic activity and Temperature-Programmed Desorption (TPD) measurements Benzene hydrogenation was carried out at 363 K and atmospheric pressure (0.1 MPa) using an 11.1% (mol/mol) benzene/hydrogen gas mixture which was obtained by flowing pure hydrogen (30 mL min−1 , Linde, UP) through a saturator filled with benzene (Riedel-deHaen) kept at 296 K by a circulating bath. The catalytic tests were performed immediately after in situ carburization using two different amounts of catalyst (0.375 g and 0.075 g) and reaction products were analyzed on-line by a Finnigan 9001 gas chromatograph equipped with a flame-ionization detector and a methyl siloxane capillary column (30.0 m × 250 m × 1.0 m). Under the employed conditions, cyclohexane was the only formed product detected by gas chromatography. Immediately after the catalytic measurements, the reactor was cooled down to room temperature under a flow of pure helium (50 mL min−1 , Linde UP), and after all gases (physisorbed + reactor dead volume) were removed, a temperature-program was applied, consisting of heating the reactor from RT to 1273 K at a heating rate of 10 K min−1 . The gas exiting the reactor was continuously introduced into a mass spectrometer (MKS-PPT) and the signals at m/z values of 2 (H2 ), 56 (cyclohexane) and 78 (benzene) were monitored and recorded. No other products were detected during the TPD experiments. 2.3. Theoretical calculations Calculations were done using periodic boundary conditions, a plane wave basis set and ultrasoft pseudopotentials [34]. The exchange and correlation functional developed by Perdew, Burke and Ernzerhof [35] was used. Occupation was treated by the cold smearing technique of Marzari et al. [36], with smearing parameter of 0.02 Ry. The kinetic energy cutoff was 30 Ry. All calculations were done with the Quantum Espresso suite of programs [37]. In order to attest to the quality of the pseudopotential, bulk calculations were done for orthorhombic and hexagonal Mo2 C in a 3 × 3 × 3 Monkhorst-Pack sampling. The calculated cell constants were a = 4.73 Å, b = 6.06 Å, and c = 5.25 Å for orthorhombic, which
Fig. 1. X-ray diffraction patterns of materials obtained by carburization of MoO3 at 923 K for 0.5 h (A), 1.0 h (B) and 2.0 h (C).
show good agreement with the experimental values of a = 4.729 Å, b = 6.028 Å, and c = 5.197 Å [38]. The same was true for hexagonal phase, a = b = 3.00 Å, c = 4.73 Å, which should be compared to experimental values a = b = 3.06 Å, c = 4.66 Å [39]. For surface calculations, a slab model was used, and a supercell was constructed by propagation of the original cell three times in the x-direction and two times in the y-direction with a vacuum layer of 17 Å set in the z-direction for orthorhombic (0 0 1) surface. For hexagonal phase the slab model was constructed by propagating the original cell 4 times in x, 3 in y and 2 in z, besides the vacuum layer. Calculations for surface were done only at the gamma point. 3. Results and discussion Molybdenum carbide can be synthesized by carburization of molybdenum oxide under flow of a mixture of methane and hydrogen at 923 K [1,40]. To investigate the time required to form crystalline phases at this temperature, X-ray measurements after carburization for 0.5 h, 1.0 h and 2.0 h, followed by passivation, were performed. The obtained patterns are presented in Fig. 1 which shows that for holding times of 0.5 h and 1.0 h the solid state transformation MoO3 → Mo2 C was not complete, as evidenced by the fact that peaks attributed to MoO2 at 26◦ , 37◦ and 53◦ are present in patterns (A) and (B) (JCPDS 32-0671). Moreover, the diffraction pattern of the material carburized for 0.5 h at 923 K also shows diffraction lines characteristic of MoO3 . On the other hand, the material carburized at 923 K for 2.0 h had only peaks associated with molybdenum carbide. However, using the diffraction pattern presented in Fig. 2C it was not possible to unequivocally associate these peaks to either the alpha (JCPDS 350787) or the beta (JCPDS 79-0744) phase because these two phases have very similar diffraction patterns. In fact, as pointed out by Pielaszek et al. [41], there is some confusion in the literature concerning the identification of these two phases, which according to the JCPDS data files are orthorhombic (alpha) and hexagonal (beta). For theoretical model purposes, both orthorhombic and hexagonal phases were explored. The specific surface area of the carbide synthesized at 923 K for 2 h and passivated at room temperature was 22 m2 g−1 , which is low when compared to values reported in the literature [1,8,42]. Such a small value can be related to the passivation step. In fact, while the majority of the specific surface area values reported in the literature have been obtained in situ immediately after carburization, the values reported in this work are for passivated
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Fig. 2. Benzene hydrogenation activity at 363 K with 0.375 g of molybdenum carbide.
samples. When measured after carburization and without exposure to the passivating gas mixture using methodology described elsewhere [2], molybdenum carbide presented a specific surface area of 94 m2 g−1 which is in agreement with the values found in the literature. It is worth mentioning that performing the passivation step at temperatures below 298 K did not lead to an increase in the specific surface area, resulting in values of 25 m2 g−1 and 27 m2 g−1 when the passivation was performed at 273 K and 185 K, respectively. These results indicate that the passivation step leads to a decrease in the specific surface area of the carbide probably due to the formation of an oxidized layer. This is consistent with XPS data, which clearly show that after passivation there is formation of an oxidic layer [43]. However, contrary to other studies in the literature [44,45] which report that passivated samples can be reactivated by a flow of pure hydrogen, the material studied in this work did not have any activity after attempted reactivation with hydrogen at 773 K. This difference may be related to the amount of pyrolytic carbon present in the samples. In fact, if carbide has some pyrolytic carbon on its surface after synthesis, then it is natural to assume that during the reactivation step this type of carbon is hydrogenated generating methane which recarburizes the surface. If it is assumed that when using the above conditions a very small amount of pyrolytic carbon is formed, then the oxidic layer is essentially reduced, forming a reduced molybdenum species, which does not have benzene hydrogenation activity. For this reason, catalytic evaluation was performed immediately after synthesis and without passivating or exposing the sample to the atmosphere. Fig. 2 shows the activity results expressed in terms of benzene conversion using 0.375 g of molybdenum carbide. From the figure, one can see that initially 100% conversion of benzene was achieved; however, after 30 min, the conversion began to decrease at an accelerated rate, such that after approximately 4 h of reaction the conversion was zero. Similar deactivation trends were also observed in other studies that have used molybdenum carbide as a catalyst for benzene hydrogenation [24–27]. The observed deactivation profile could be explained by coke deposition, by the presence of small amounts of oxygen – a leak in the system, oxygen contamination of the hydrogen flowed through the saturator or even dissolved oxygen in the benzene, which would promote oxidation of the carbide consequently inhibiting its activity – or by a strong adsorption of the reactant or product on the molybdenum carbide surface. After carefully checking each of the probable sources of oxygen contamination, this hypothesis was
Fig. 3. Desorption profiles of benzene (A), cyclohexane ×10 (B) and hydrogen (C) after benzene hydrogenation at 363 K.
ruled out; thus, the hypothesis associated with the strong adsorption of reactant/product remains the most plausible. On the other hand, the hypothesis of deactivation by coke can also be discarded because the reaction temperature is low to have this kind of phenomenon occurring. To verify the assumption of strong adsorption, another experiment was performed using identical conditions to those employed in the experiment depicted in Fig. 1, and, as expected, the same deactivation profile was obtained. At the end of the experiment, the gas flowing through the reactor was switched from the hydrogen/benzene gas mixture to pure He, the system was cleaned for approximately 30 min, that is, until all of the signals monitored in the mass spectrometer were within their threshold values, and the temperature was then raised from RT to 1273 K at a heating rate of 10 K min−1 . The obtained profiles for each of the ions followed by mass spectroscopy (hydrogen, cyclohexane and benzene) are shown in Fig. 3. The assumption that deactivation is due to a strong adsorption of the product can be discarded because the amount of cyclohexane desorbing from the catalyst surface was quite small when compared to the amounts of benzene and hydrogen (it should be noted that the signal for m/z = 58 was multiplied by a factor of 10). The assumption that the low signal detected for cyclohexane is not related to cyclohexane desorption but is instead related to a reaction occurring between the desorbing benzene and hydrogen cannot be excluded. If the assumption that deactivation is due to strong product adsorption is false, then the observed deactivation can only be explained by the strong adsorption of the benzene reactant. In fact, Fig. 3 shows a desorption peak for benzene that has a maximum and a well-defined shoulder located at 400 K and 507 K, respectively. These temperatures are higher than that employed in the catalytic evaluation thus indicating that strong adsorption of benzene occurred and can in fact be responsible for the observed deactivation. In addition to the benzene and cyclohexane desorption peaks, Fig. 3 also shows the evolution of hydrogen which has three maxima at 380 K, 507 K and 758 K and occurs over a broad temperature range, from 323 K up to 1000 K. The source of the evolved hydrogen can be (i) desorption of chemisorbed hydrogen and/or hydrogen absorbed within the bulk of the carbide during the carburization process and/or, (ii) decomposition of very strongly adsorbed benzene, with the formation of superficial species with structure
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Fig. 4. Hydrogen desorption profile after carburization of molybdenum oxide.
similar to coke (Cx Hy ), which would decompose during the TPD experiment. To investigate whether any hydrogen was released from the carbide during heating after the carburization and cleaning of the surface at room temperature with helium, another experiment was performed in two steps: first, carbide was synthesized as previously described, including cleaning the surface with helium at room temperature, and; second, the system was heated at a rate of 10 K min−1 from RT up to 873 K under a flow of pure helium. As can be seen in Fig. 4, even making a cleaning procedure at room temperature with helium after the carbide synthesis, some hydrogen desorbs upon heating presenting a maximum at 380 K thus accounting for the first of the hydrogen maxima seen in Fig. 3. The other maxima at 507 K and 758 K in the hydrogen desorption profile could be related to the dehydrogenation of the Cx Hy species strongly bonded to the surface. Taking into account the results presented in Fig. 4, i.e., that after the carburization step carbide has hydrogen chemisorbed on its surface (Fig. 5A), it is possible to propose a reaction scheme as shown in Fig. 5. According to this scheme, after synthesis, the catalyst surface is covered by hydrogen (Fig. 5A) which reacts with benzene via an Eley-Riedeal mechanism producing cyclohexane, and some of the sites on the surface become vacant (Fig. 5B). Other molecules of benzene then strongly and irreversibly adsorb to these sites, poisoning the surface (Fig. 5C). In addition to explaining the deactivation, this scheme accounts for the desorption profiles seen in Fig. 3: part of the benzene and hydrogen that desorb could react and form the small amounts of cyclohexane; the most strongly adsorbed molecules of benzene would remain on the surface and when the temperature is sufficiently high in the TPD experiment,
Fig. 5. Proposed scheme for benzene hydrogenation on the molybdenum carbide surface. Fresh carbide (A), beginning of the reaction (B), and partially used carbide (C).
these molecules would decompose forming Cx Hy species with evolution of hydrogen. If the proposed scheme is correct, the adsorption energy of benzene on the molybdenum carbide surface has to be very high, and a reliable method of obtaining this information is to perform quantum mechanical calculations. In view of that, DFT calculation of the interaction of benzene with orthorhombic Mo2 C (0 0 1) surface was done. The slab model shown in Fig. 6 was used, in which the atom positions for the pure Mo2 C (0 0 1) surface were fully relaxed, while the volume of the cell was kept fixed. The same was done for adsorbed benzene, as shown in Fig. 7, where isolated benzene was calculated in a cubic box of 10 Å on a side. The adsorption energy was defined as Eads = Ebenz/slab − Ebenz − Eslab . where Eads is the calculated adsorption energy, Eads/slab is the energy of the adsorbed system (benzene + surface), Ebenz is the energy of benzene in gas phase and Eslab is the surface energy. With this definition, a negative Eads corresponds to a stable adsorption of benzene on the carbide surface. The calculated adsorption energy was −636 kJ mol−1 , a value that indicates that benzene has a very strong interaction with the
Fig. 6. Model slab for the Mo2 C (3 × 2) surface, constructed from the original orthorhombic cell: top view (A) and side view (B).
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Fig. 7. Model of benzene/orthorhombic (0 0 1) Mo2 C one-side adsorption: top view (A) and side view (B).
Fig. 8. Model of benzene/orthorhombic (0 0 1) Mo2 C two-side adsorption: top view (A) and side view (B).
Mo2 C surface. Furthermore, the surface significantly relaxes in the presence of the benzene molecule. One could argue that adsorption on a single side of the slab could generate spurious dipole–dipole interactions among cells, but this could in principle be minimized by making both sides available for adsorption. To determine the extent of this effect, calculations with adsorption on both sides of the slab were performed, as shown in Fig. 8. The adsorption energy calculated for this model was −526 kJ mol−1 , which, though smaller in absolute value than the former value, is still very high and is able to explain the observed deactivation. Bearing in mind the fact that the actual surface was not determined – in fact it is probably polycrystalline – other possibilities for adsorption were tested. First, the adsorption on (0 1 0) orthorhombic and on (0 0 0 1) hexagonal surfaces were calculated for both sides of the planes being the results presented in Table 1. As can be seen (0 0 0 1) hexagonal and (0 0 1) orthorhombic adsorption energies have similar values, i.e., −495 kJ mol−1 and −526 kJ mol−1 , respectively. For orthorhombic (0 1 0) the adsorption energy has a smaller (−377 kJ mol−1 ) but this value is still a considerable high value. The slab model for hexagonal adsorption is shown in Fig. 9.
Table 1 Calculated values for adsorption energy (kJ mol−1 ) of benzene on Mo2 C surfaces. Orthorhombic (0 0 1)
Orthorhombic (0 1 0)
Hexagonal (0 0 0 1)
−526
−377
−495
Adsorption is done on both sides of the slab. The values are referred to gas-phase molecules and bare metal surface.
The adsorption of benzene on noble metal surfaces, which could be compared to the adsorption on molybdenum carbide, is a favorable process that depends on the nature of the metal. Benzene can adopt several structures on Pt, Pd and Rh, for example, with distinct adsorption energies. The exposed surface is another important parameter. Morin et al. have investigated the symmetry of benzene adsorption on the Pt (1 1 1) surface [46] and it was found that the bridge sites created the most stable complex, with energies in the range of −0.80 eV to −1.27 eV (−77.18 kJ mol−1 to −122.5 kJ mol−1 ) depending on the method used. In another work, the authors compared the adsorption on the (1 1 1) surfaces of Pt, Pd and Rh metals using first principles density functional periodic calcula-
Fig. 9. Model of benzene/hexagonal (0 0 0 1) Mo2 C two-side adsorption: top view (A) and side view (B).
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orate this conclusion, DFT calculations within periodic boundary conditions and using a plane basis set for the energy of adsorption of benzene on the Mo2 C several surfaces were performed. The values of energy adsorption obtained by DFT were very high, which is consistent with the proposed deactivation mechanism. In summary, the steps towards deactivation are as follows: first, benzene reacts with adsorbed hydrogen via an Eley-Rideal mechanism; then, as the reaction proceeds, benzene molecules reach surface sites that are free of hydrogen and irreversibly bind to them, covering the surface in such a way that it becomes inaccessible to other hydrogen molecules disabling the hydrogenation process. Acknowledgements The authors acknowledge Conselho Nacional de Desenvolvimento e Pesquisa (CNPq), FAPERJ and PETROBRAS for financial support. Fig. 10. Benzene hydrogenation activity at 363 K with 0.075 g of molybdenum carbide.
tions [47], and the obtained results showed that benzene molecules adsorb predominantly in a bridge position on Pt (1 1 1), while on Pd (1 1 1) and Rh (1 1 1), the molecules can adsorb to the bridge and hollow positions. The values obtained for bridge adsorption were −86.6 kJ mol−1 for Pt (1 1 1), −114.8 kJ mol−1 for Pd (1 1 1) and −147.6 kJ mol−1 for Rh (1 1 1). The authors also showed that benzene adsorbs to Ni (1 1 1) with an energy of −96.5 kJ mol−1 . Therefore, the adsorption on all studied surfaces is very favorable. The strong interaction of benzene with the molybdenum carbide surfaces is in perfect agreement with the experimental results and can be directly related to the deactivation process, as previously mentioned. The obtained energy value for the carbide surface is at least several times greater than the values on noble metals, and it is widely known that noble-metal-based catalysts are not poisoned by benzene, in contrast to molybdenum carbide, thus corroborating the proposed interpretation for the deactivation process. One additional experiment was performed to further validate the reaction scheme, with the following reasoning: if the hydrogenation of benzene using molybdenum carbide proceeds via an Eley-Rideal mechanism, and if part of the hydrogen chemisorbed on the surface is removed before the reactions begins, then one would expect a lower value of initial conversion than that reported in Fig. 2, i.e., a value lower than 100%. There are two obvious ways of reducing the amount of chemisorbed hydrogen available to react with benzene: (i) heating the system to a certain temperature or (ii) reducing the amount of catalyst. Because the control of the amount of desorbed hydrogen by heating would not be sufficiently accurate, option (ii) was selected and the weight of molybdenum carbide used for reaction was 0.075 g instead of the previously used of 0.350 g. Decreasing the catalyst weight from 0.350 g to 0.075 g led to a decrease in benzene conversion from 100% to 44% (Fig. 10) thus providing more evidence that the reaction does occur via an Eley-Rideal mechanism when molybdenum carbide is used as the catalyst. 4. Conclusions Molybdenum carbide activity for benzene hydrogenation at 363 K and 0.1 MPa was evaluated, and it was observed that under these conditions, complete and irreversible deactivation of the catalyst occurs after several hours. Considering this result for the activity, as well as the result of TPD performed after the reaction, it can be concluded that the deactivation occurs due to a strong interaction between benzene and the carbide surface. To corrob-
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Benzene adsorption on Mo2 C: A theoretical and experimental study A.S. Rocha a , A.B. Rocha b , V. Teixeira da Silva a,∗ a b
NUCAT/PEQ/COPPE/UFRJ, Caixa Postal 68502, CEP 21941-972 Rio de Janeiro, RJ, Brazil Departamento de Físico-Química, Instituto de Química, UFRJ, Ilha do Fundão, CT, Bloco A, Rio de Janeiro, RJ, CEP 21949-900, Brazil
a r t i c l e
i n f o
Article history: Received 16 November 2009 Received in revised form 18 February 2010 Accepted 27 February 2010 Available online 6 March 2010 Keywords: Benzene hydrogenation Molybdenum carbide Adsorption DFT
a b s t r a c t Continuous gas-phase benzene hydrogenation at 363 K and atmospheric pressure was studied using bulk molybdenum carbide as a catalyst. It was observed that benzene conversion to cyclohexane was initially 100%, dropping to zero after four hours of reaction. After deactivation the catalyst was submitted to a heating program under helium flow from room temperature (RT) up to 1273 K. On-line mass spectrometry showed desorption of benzene and hydrogen suggesting that the deactivation occurred due to strong absorption of benzene on the molybdenum carbide surface. Density Functional Theory (DFT) calculations support this hypothesis showing the adsorption energy of benzene on the molybdenum carbide varies from −377 kJ mol−1 to −636 kJ mol−1 depending on the surface and on the crystal phase. Based on the experimental and theoretical results, a reaction scheme is proposed with the following steps: (i) after synthesis, the molybdenum carbide surface is completely covered by chemisorbed hydrogen; (ii) when the reaction begins, benzene molecules react with the chemisorbed hydrogen via an Eley-Riedeal mechanism, producing cyclohexane and vacant sites on the surface; (iii) other benzene molecules strongly and irreversibly adsorb to these vacant sites, poisoning the surface and thus leading to deactivation of the catalyst. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Transition metal carbides constitute an important and interesting class of materials for catalysis because, despite being conductors, they possess the physical and mechanical properties of ceramics. There are several reports of their activity in reactions typically catalyzed by noble metals such as hydrogenation, dehydrogenation, hydrogenolysis, isomerization, and also for hydrotreating reactions such as hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation [1–13]. Molybdenum carbide supported on acidic zeolites is a bifunctional catalyst that can be used for aromatization and dehydroaromatization reactions [14–18]. Previous works have reported the preparation of catalysts composed of carburized molybdenum supported on Y-zeolites with different acidities, which had activity for benzene hydrogenation at low temperature and pressure [19,20]. When nanoparticles of molybdenum carbide were dispersed on NaH-USY, high initial activity for benzene hydrogenation was observed, although rapid deactivation occurred, with conversion decreasing to nearly zero after 2 h of reaction [21]. Two other catalysts with carburized molybdenum supported on the same Yzeolite have not presented such deactivation behavior, but the
∗ Corresponding author. Tel.: +55 21 2562 8344; fax: +55 21 2562 8300. E-mail address: [email protected] (V.T. da Silva). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.02.032
active phases were clearly not of the carbide type, as seen by EXAFS and DFT calculations, which indicated the formation of an oxycarbidic species. Regarding the carbide species, the reasons for the fast deactivation have not been fully elucidated. The hydrogenation of benzene is known to be a structureinsensitive reaction [22] and the best catalyst for this reaction is Ru/Al2 O3 [23]. To confirm the noble metal behavior of molybdenum carbide, several authors have compared its activity either in bulk form [24,25] or supported on alumina [26,27] to that of Ru/Al2 O3 catalysts for the hydrogenation of benzene. In both cases it was found that the carbide had an initial turnover frequency (TOF) similar to or even higher than that of ruthenium. In those studies, the authors observed a complete deactivation of the carbide catalyst after several hours of reaction, a fact that was not explained or discussed. The main goal of this work was to elucidate the deactivation process that occurs during benzene hydrogenation when molybdenum carbide is used as the catalyst. The study included synthesis of bulk molybdenum carbide, measurements of its activity for benzene hydrogenation at low temperature and pressure, and DFT calculations of the interaction of benzene with the orthorhombic and hexagonal Mo2 C surfaces. A number of DFT calculations concerning the interaction of the Mo2 C (0 0 1) surface with small molecules have been reported [28–33], as well as a study of Mo2 C bulk and surface structure and stability [33]. To the best of our knowledge, there have been
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no previous theoretical studies of the interaction of benzene with Mo2 C. 2. Experimental 2.1. Preparation and characterization of the catalysts Molybdenum carbide was synthesized by carburization of molybdenum oxide (0.5 g, Aldrich) with a 20% (v/v) CH4 /H2 gas mixture (350 mL min−1 , Linde UP) using a temperature-program from room temperature (RT, 298 K) up to 923 K at a 10 K min−1 heating rate and holding the final temperature for 2 h. To evaluate the influence of the holding time at 923 K on the physical properties, two other samples were synthesized using 0.5 h and 1 h as the holding time, and then the samples were characterized by X-ray diffraction. Before characterization, the samples were passivated overnight at room temperature under a flow of a 0.5% (v/v) O2 /He gas mixture (50 mL min−1 , Linde UP). X-ray diffraction patterns were recorded on a Rigaku (Miniflex) diffractometer, with Cu K␣ radiation ( = 1.5418 Å, 30 kV and 15 mA) using the powder method. Specific surface area was determined from nitrogen adsorption isotherms at 77 K using the BET method, in an ASAP 2000 (Micromeritics) volumetric apparatus. Before the analyses, the materials were pre-treated under standard vacuum (6.7 × 10−6 MPa) at 423 K for 20 h. 2.2. Catalytic activity and Temperature-Programmed Desorption (TPD) measurements Benzene hydrogenation was carried out at 363 K and atmospheric pressure (0.1 MPa) using an 11.1% (mol/mol) benzene/hydrogen gas mixture which was obtained by flowing pure hydrogen (30 mL min−1 , Linde, UP) through a saturator filled with benzene (Riedel-deHaen) kept at 296 K by a circulating bath. The catalytic tests were performed immediately after in situ carburization using two different amounts of catalyst (0.375 g and 0.075 g) and reaction products were analyzed on-line by a Finnigan 9001 gas chromatograph equipped with a flame-ionization detector and a methyl siloxane capillary column (30.0 m × 250 m × 1.0 m). Under the employed conditions, cyclohexane was the only formed product detected by gas chromatography. Immediately after the catalytic measurements, the reactor was cooled down to room temperature under a flow of pure helium (50 mL min−1 , Linde UP), and after all gases (physisorbed + reactor dead volume) were removed, a temperature-program was applied, consisting of heating the reactor from RT to 1273 K at a heating rate of 10 K min−1 . The gas exiting the reactor was continuously introduced into a mass spectrometer (MKS-PPT) and the signals at m/z values of 2 (H2 ), 56 (cyclohexane) and 78 (benzene) were monitored and recorded. No other products were detected during the TPD experiments. 2.3. Theoretical calculations Calculations were done using periodic boundary conditions, a plane wave basis set and ultrasoft pseudopotentials [34]. The exchange and correlation functional developed by Perdew, Burke and Ernzerhof [35] was used. Occupation was treated by the cold smearing technique of Marzari et al. [36], with smearing parameter of 0.02 Ry. The kinetic energy cutoff was 30 Ry. All calculations were done with the Quantum Espresso suite of programs [37]. In order to attest to the quality of the pseudopotential, bulk calculations were done for orthorhombic and hexagonal Mo2 C in a 3 × 3 × 3 Monkhorst-Pack sampling. The calculated cell constants were a = 4.73 Å, b = 6.06 Å, and c = 5.25 Å for orthorhombic, which
Fig. 1. X-ray diffraction patterns of materials obtained by carburization of MoO3 at 923 K for 0.5 h (A), 1.0 h (B) and 2.0 h (C).
show good agreement with the experimental values of a = 4.729 Å, b = 6.028 Å, and c = 5.197 Å [38]. The same was true for hexagonal phase, a = b = 3.00 Å, c = 4.73 Å, which should be compared to experimental values a = b = 3.06 Å, c = 4.66 Å [39]. For surface calculations, a slab model was used, and a supercell was constructed by propagation of the original cell three times in the x-direction and two times in the y-direction with a vacuum layer of 17 Å set in the z-direction for orthorhombic (0 0 1) surface. For hexagonal phase the slab model was constructed by propagating the original cell 4 times in x, 3 in y and 2 in z, besides the vacuum layer. Calculations for surface were done only at the gamma point. 3. Results and discussion Molybdenum carbide can be synthesized by carburization of molybdenum oxide under flow of a mixture of methane and hydrogen at 923 K [1,40]. To investigate the time required to form crystalline phases at this temperature, X-ray measurements after carburization for 0.5 h, 1.0 h and 2.0 h, followed by passivation, were performed. The obtained patterns are presented in Fig. 1 which shows that for holding times of 0.5 h and 1.0 h the solid state transformation MoO3 → Mo2 C was not complete, as evidenced by the fact that peaks attributed to MoO2 at 26◦ , 37◦ and 53◦ are present in patterns (A) and (B) (JCPDS 32-0671). Moreover, the diffraction pattern of the material carburized for 0.5 h at 923 K also shows diffraction lines characteristic of MoO3 . On the other hand, the material carburized at 923 K for 2.0 h had only peaks associated with molybdenum carbide. However, using the diffraction pattern presented in Fig. 2C it was not possible to unequivocally associate these peaks to either the alpha (JCPDS 350787) or the beta (JCPDS 79-0744) phase because these two phases have very similar diffraction patterns. In fact, as pointed out by Pielaszek et al. [41], there is some confusion in the literature concerning the identification of these two phases, which according to the JCPDS data files are orthorhombic (alpha) and hexagonal (beta). For theoretical model purposes, both orthorhombic and hexagonal phases were explored. The specific surface area of the carbide synthesized at 923 K for 2 h and passivated at room temperature was 22 m2 g−1 , which is low when compared to values reported in the literature [1,8,42]. Such a small value can be related to the passivation step. In fact, while the majority of the specific surface area values reported in the literature have been obtained in situ immediately after carburization, the values reported in this work are for passivated
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Fig. 2. Benzene hydrogenation activity at 363 K with 0.375 g of molybdenum carbide.
samples. When measured after carburization and without exposure to the passivating gas mixture using methodology described elsewhere [2], molybdenum carbide presented a specific surface area of 94 m2 g−1 which is in agreement with the values found in the literature. It is worth mentioning that performing the passivation step at temperatures below 298 K did not lead to an increase in the specific surface area, resulting in values of 25 m2 g−1 and 27 m2 g−1 when the passivation was performed at 273 K and 185 K, respectively. These results indicate that the passivation step leads to a decrease in the specific surface area of the carbide probably due to the formation of an oxidized layer. This is consistent with XPS data, which clearly show that after passivation there is formation of an oxidic layer [43]. However, contrary to other studies in the literature [44,45] which report that passivated samples can be reactivated by a flow of pure hydrogen, the material studied in this work did not have any activity after attempted reactivation with hydrogen at 773 K. This difference may be related to the amount of pyrolytic carbon present in the samples. In fact, if carbide has some pyrolytic carbon on its surface after synthesis, then it is natural to assume that during the reactivation step this type of carbon is hydrogenated generating methane which recarburizes the surface. If it is assumed that when using the above conditions a very small amount of pyrolytic carbon is formed, then the oxidic layer is essentially reduced, forming a reduced molybdenum species, which does not have benzene hydrogenation activity. For this reason, catalytic evaluation was performed immediately after synthesis and without passivating or exposing the sample to the atmosphere. Fig. 2 shows the activity results expressed in terms of benzene conversion using 0.375 g of molybdenum carbide. From the figure, one can see that initially 100% conversion of benzene was achieved; however, after 30 min, the conversion began to decrease at an accelerated rate, such that after approximately 4 h of reaction the conversion was zero. Similar deactivation trends were also observed in other studies that have used molybdenum carbide as a catalyst for benzene hydrogenation [24–27]. The observed deactivation profile could be explained by coke deposition, by the presence of small amounts of oxygen – a leak in the system, oxygen contamination of the hydrogen flowed through the saturator or even dissolved oxygen in the benzene, which would promote oxidation of the carbide consequently inhibiting its activity – or by a strong adsorption of the reactant or product on the molybdenum carbide surface. After carefully checking each of the probable sources of oxygen contamination, this hypothesis was
Fig. 3. Desorption profiles of benzene (A), cyclohexane ×10 (B) and hydrogen (C) after benzene hydrogenation at 363 K.
ruled out; thus, the hypothesis associated with the strong adsorption of reactant/product remains the most plausible. On the other hand, the hypothesis of deactivation by coke can also be discarded because the reaction temperature is low to have this kind of phenomenon occurring. To verify the assumption of strong adsorption, another experiment was performed using identical conditions to those employed in the experiment depicted in Fig. 1, and, as expected, the same deactivation profile was obtained. At the end of the experiment, the gas flowing through the reactor was switched from the hydrogen/benzene gas mixture to pure He, the system was cleaned for approximately 30 min, that is, until all of the signals monitored in the mass spectrometer were within their threshold values, and the temperature was then raised from RT to 1273 K at a heating rate of 10 K min−1 . The obtained profiles for each of the ions followed by mass spectroscopy (hydrogen, cyclohexane and benzene) are shown in Fig. 3. The assumption that deactivation is due to a strong adsorption of the product can be discarded because the amount of cyclohexane desorbing from the catalyst surface was quite small when compared to the amounts of benzene and hydrogen (it should be noted that the signal for m/z = 58 was multiplied by a factor of 10). The assumption that the low signal detected for cyclohexane is not related to cyclohexane desorption but is instead related to a reaction occurring between the desorbing benzene and hydrogen cannot be excluded. If the assumption that deactivation is due to strong product adsorption is false, then the observed deactivation can only be explained by the strong adsorption of the benzene reactant. In fact, Fig. 3 shows a desorption peak for benzene that has a maximum and a well-defined shoulder located at 400 K and 507 K, respectively. These temperatures are higher than that employed in the catalytic evaluation thus indicating that strong adsorption of benzene occurred and can in fact be responsible for the observed deactivation. In addition to the benzene and cyclohexane desorption peaks, Fig. 3 also shows the evolution of hydrogen which has three maxima at 380 K, 507 K and 758 K and occurs over a broad temperature range, from 323 K up to 1000 K. The source of the evolved hydrogen can be (i) desorption of chemisorbed hydrogen and/or hydrogen absorbed within the bulk of the carbide during the carburization process and/or, (ii) decomposition of very strongly adsorbed benzene, with the formation of superficial species with structure
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Fig. 4. Hydrogen desorption profile after carburization of molybdenum oxide.
similar to coke (Cx Hy ), which would decompose during the TPD experiment. To investigate whether any hydrogen was released from the carbide during heating after the carburization and cleaning of the surface at room temperature with helium, another experiment was performed in two steps: first, carbide was synthesized as previously described, including cleaning the surface with helium at room temperature, and; second, the system was heated at a rate of 10 K min−1 from RT up to 873 K under a flow of pure helium. As can be seen in Fig. 4, even making a cleaning procedure at room temperature with helium after the carbide synthesis, some hydrogen desorbs upon heating presenting a maximum at 380 K thus accounting for the first of the hydrogen maxima seen in Fig. 3. The other maxima at 507 K and 758 K in the hydrogen desorption profile could be related to the dehydrogenation of the Cx Hy species strongly bonded to the surface. Taking into account the results presented in Fig. 4, i.e., that after the carburization step carbide has hydrogen chemisorbed on its surface (Fig. 5A), it is possible to propose a reaction scheme as shown in Fig. 5. According to this scheme, after synthesis, the catalyst surface is covered by hydrogen (Fig. 5A) which reacts with benzene via an Eley-Riedeal mechanism producing cyclohexane, and some of the sites on the surface become vacant (Fig. 5B). Other molecules of benzene then strongly and irreversibly adsorb to these sites, poisoning the surface (Fig. 5C). In addition to explaining the deactivation, this scheme accounts for the desorption profiles seen in Fig. 3: part of the benzene and hydrogen that desorb could react and form the small amounts of cyclohexane; the most strongly adsorbed molecules of benzene would remain on the surface and when the temperature is sufficiently high in the TPD experiment,
Fig. 5. Proposed scheme for benzene hydrogenation on the molybdenum carbide surface. Fresh carbide (A), beginning of the reaction (B), and partially used carbide (C).
these molecules would decompose forming Cx Hy species with evolution of hydrogen. If the proposed scheme is correct, the adsorption energy of benzene on the molybdenum carbide surface has to be very high, and a reliable method of obtaining this information is to perform quantum mechanical calculations. In view of that, DFT calculation of the interaction of benzene with orthorhombic Mo2 C (0 0 1) surface was done. The slab model shown in Fig. 6 was used, in which the atom positions for the pure Mo2 C (0 0 1) surface were fully relaxed, while the volume of the cell was kept fixed. The same was done for adsorbed benzene, as shown in Fig. 7, where isolated benzene was calculated in a cubic box of 10 Å on a side. The adsorption energy was defined as Eads = Ebenz/slab − Ebenz − Eslab . where Eads is the calculated adsorption energy, Eads/slab is the energy of the adsorbed system (benzene + surface), Ebenz is the energy of benzene in gas phase and Eslab is the surface energy. With this definition, a negative Eads corresponds to a stable adsorption of benzene on the carbide surface. The calculated adsorption energy was −636 kJ mol−1 , a value that indicates that benzene has a very strong interaction with the
Fig. 6. Model slab for the Mo2 C (3 × 2) surface, constructed from the original orthorhombic cell: top view (A) and side view (B).
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Fig. 7. Model of benzene/orthorhombic (0 0 1) Mo2 C one-side adsorption: top view (A) and side view (B).
Fig. 8. Model of benzene/orthorhombic (0 0 1) Mo2 C two-side adsorption: top view (A) and side view (B).
Mo2 C surface. Furthermore, the surface significantly relaxes in the presence of the benzene molecule. One could argue that adsorption on a single side of the slab could generate spurious dipole–dipole interactions among cells, but this could in principle be minimized by making both sides available for adsorption. To determine the extent of this effect, calculations with adsorption on both sides of the slab were performed, as shown in Fig. 8. The adsorption energy calculated for this model was −526 kJ mol−1 , which, though smaller in absolute value than the former value, is still very high and is able to explain the observed deactivation. Bearing in mind the fact that the actual surface was not determined – in fact it is probably polycrystalline – other possibilities for adsorption were tested. First, the adsorption on (0 1 0) orthorhombic and on (0 0 0 1) hexagonal surfaces were calculated for both sides of the planes being the results presented in Table 1. As can be seen (0 0 0 1) hexagonal and (0 0 1) orthorhombic adsorption energies have similar values, i.e., −495 kJ mol−1 and −526 kJ mol−1 , respectively. For orthorhombic (0 1 0) the adsorption energy has a smaller (−377 kJ mol−1 ) but this value is still a considerable high value. The slab model for hexagonal adsorption is shown in Fig. 9.
Table 1 Calculated values for adsorption energy (kJ mol−1 ) of benzene on Mo2 C surfaces. Orthorhombic (0 0 1)
Orthorhombic (0 1 0)
Hexagonal (0 0 0 1)
−526
−377
−495
Adsorption is done on both sides of the slab. The values are referred to gas-phase molecules and bare metal surface.
The adsorption of benzene on noble metal surfaces, which could be compared to the adsorption on molybdenum carbide, is a favorable process that depends on the nature of the metal. Benzene can adopt several structures on Pt, Pd and Rh, for example, with distinct adsorption energies. The exposed surface is another important parameter. Morin et al. have investigated the symmetry of benzene adsorption on the Pt (1 1 1) surface [46] and it was found that the bridge sites created the most stable complex, with energies in the range of −0.80 eV to −1.27 eV (−77.18 kJ mol−1 to −122.5 kJ mol−1 ) depending on the method used. In another work, the authors compared the adsorption on the (1 1 1) surfaces of Pt, Pd and Rh metals using first principles density functional periodic calcula-
Fig. 9. Model of benzene/hexagonal (0 0 0 1) Mo2 C two-side adsorption: top view (A) and side view (B).
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orate this conclusion, DFT calculations within periodic boundary conditions and using a plane basis set for the energy of adsorption of benzene on the Mo2 C several surfaces were performed. The values of energy adsorption obtained by DFT were very high, which is consistent with the proposed deactivation mechanism. In summary, the steps towards deactivation are as follows: first, benzene reacts with adsorbed hydrogen via an Eley-Rideal mechanism; then, as the reaction proceeds, benzene molecules reach surface sites that are free of hydrogen and irreversibly bind to them, covering the surface in such a way that it becomes inaccessible to other hydrogen molecules disabling the hydrogenation process. Acknowledgements The authors acknowledge Conselho Nacional de Desenvolvimento e Pesquisa (CNPq), FAPERJ and PETROBRAS for financial support. Fig. 10. Benzene hydrogenation activity at 363 K with 0.075 g of molybdenum carbide.
tions [47], and the obtained results showed that benzene molecules adsorb predominantly in a bridge position on Pt (1 1 1), while on Pd (1 1 1) and Rh (1 1 1), the molecules can adsorb to the bridge and hollow positions. The values obtained for bridge adsorption were −86.6 kJ mol−1 for Pt (1 1 1), −114.8 kJ mol−1 for Pd (1 1 1) and −147.6 kJ mol−1 for Rh (1 1 1). The authors also showed that benzene adsorbs to Ni (1 1 1) with an energy of −96.5 kJ mol−1 . Therefore, the adsorption on all studied surfaces is very favorable. The strong interaction of benzene with the molybdenum carbide surfaces is in perfect agreement with the experimental results and can be directly related to the deactivation process, as previously mentioned. The obtained energy value for the carbide surface is at least several times greater than the values on noble metals, and it is widely known that noble-metal-based catalysts are not poisoned by benzene, in contrast to molybdenum carbide, thus corroborating the proposed interpretation for the deactivation process. One additional experiment was performed to further validate the reaction scheme, with the following reasoning: if the hydrogenation of benzene using molybdenum carbide proceeds via an Eley-Rideal mechanism, and if part of the hydrogen chemisorbed on the surface is removed before the reactions begins, then one would expect a lower value of initial conversion than that reported in Fig. 2, i.e., a value lower than 100%. There are two obvious ways of reducing the amount of chemisorbed hydrogen available to react with benzene: (i) heating the system to a certain temperature or (ii) reducing the amount of catalyst. Because the control of the amount of desorbed hydrogen by heating would not be sufficiently accurate, option (ii) was selected and the weight of molybdenum carbide used for reaction was 0.075 g instead of the previously used of 0.350 g. Decreasing the catalyst weight from 0.350 g to 0.075 g led to a decrease in benzene conversion from 100% to 44% (Fig. 10) thus providing more evidence that the reaction does occur via an Eley-Rideal mechanism when molybdenum carbide is used as the catalyst. 4. Conclusions Molybdenum carbide activity for benzene hydrogenation at 363 K and 0.1 MPa was evaluated, and it was observed that under these conditions, complete and irreversible deactivation of the catalyst occurs after several hours. Considering this result for the activity, as well as the result of TPD performed after the reaction, it can be concluded that the deactivation occurs due to a strong interaction between benzene and the carbide surface. To corrob-
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