Langmuir 1999, 15, 4927-4929
Microcalorimetric Study of H2 Adsorption on Molybdenum Nitride Catalysts A. Guerrero-Ruiz Departamento de Quı´mica Inorga´ nica y Te´ cnica, UNED, Senda del Rey s/n, 28040 Madrid, Spain Q. Xin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, CAS, Dalian 116023, PR China Y. J. Zhang, A. Maroto-Valiente, and I. Rodriguez-Ramos* Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain Received November 17, 1998. In Final Form: April 7, 1999
Introduction Catalytic properties of molybdenum nitrides have been widely investigated in recent years. These studies have revealed that γ-Mo2N is an excellent catalyst for CO hydrogenation,1 ethane hydrogenolysis,2 NO reduction,3 and in particular for hydrotreating reactions such as hydrodenitrogenation 4-12 or hydrodesulfurization. 6,9,13-18 In many of these reactions, the interaction between hydrogen and γ-Mo2N surface plays a significant role. Previous publications dealing with the temperatureprogrammed desorption and volumetric adsorption of hydrogen have shown that hydrogen in the form of dissociated species is adsorbed on the surface of γ-Mo2N.19-22 To further understand the catalytic processes in where H2 and the γ-Mo2N surface are involved, complementary information on the energy of hydrogen adsorption * Corresponding author. Tel.: 34 915854765. Fax: 34 915854760. E-mail:
[email protected]. (1) Ranhotra, G. S.; Haddix, G. W.; Bell. A. T.; Reimer, J. A. J. Catal. 1987, 108, 24. (2) Ranhotra, G. S.; Bell, A. T.; Reimer, J. A. J. Catal. 1987, 108, 40. (3) Tsuchimoto, K.; Suzuki, M.; Yamaki, N. Nippon Kagaku Kaishi 1979, 10, 1420. (4) Schlatter, J. C.; Oyama, S. T.; Metcalf, J. E.; Lambert, J. M., Jr. Ind. Eng. Chem. Res. 1988, 27, 1648. (5) Choi, J. G.; Brenner, J. R.; Colling, C. W.; Demczyk, B. C.; Dunning, J. L.; Thompson, L., T. Catal. Today 1992, 15, 201. (6) Sajkowski, D. J.; Oyama, S. T. Appl. Catal. A 1996, 134, 339. (7) Nagai, M.; Miyao, T. Catal. Lett. 1992, 15, 105. (8) Lee, K. S.; Abe, H.; Reimer, J. A.; Bell, A. T. J. Catal. 1993, 139, 34. (9) Abe, H.; Bell, A. T. Catal. Lett. 1993, 18, 1. (10) Colling, C. W.; Thompson, L. T. J. Catal. 1994, 146, 193. (11) Ozkan, U. S.; Zhang, L.; Clark, P. A. J. Catal. 1997, 172, 294. (12) Li, Y.; Zhang, Y.; Raval, R.; Li, C.; Zhai, R.; Xin, Q. Catal. Lett. 1997, 48, 239. (13) Markel, E. J.; Van Zee, J. W. J. Catal. 1990, 126, 643. (14) Zhang, Y.; Wei, Z.; Yan, W.; Ying, P.; Xin, Q. Catal. Today 1996, 30, 135. (15) Nagai, M.; Miyao, T.; Tuboi, T. Catal. Lett. 1993, 18, 9. (16) Aegerter, P. A.; Quigley, W. W. C.; Simpson, G. J.; Ziegler, D. D.; Logan J. W.; McCrea, K. R.; Glazier, S.; Bussell, M. E. J. Catal. 1996, 164, 109. (17) McCrea, K. R.; Logan, J. W.; Tarbuck, T. L.; Heiser, J. L.; Bussell, M. E. J. Catal. 1997, 171, 255. (18) Zhang, Y.; Li, Y.; Raval, R.; Li, C.; Zhai, R.; Xin, Q. J. Mol. Catal. A: Chem. 1998, 132, 241. (19) Wei, Z.; Xin, Q.; Grange, P.; Delmon, B. J. Catal. 1997, 168, 176. (20) Zhang, Y.; Li Y.; Li, C.; Xin, Q. Stud. Surf. Sci. Catal. 1997, 112, 457. (21) Li, X. S.; Chen, Y. X.; Zhang, Y.; Ji C. X.; Xin, Q. React. Kinet. Catal. Lett. 1996, 58, 391. (22) Li, X. S.; Zhang, Y.; Xin, Q.; Ji, C. X.; Miao, Y. F.; Wang, L. React. Kinet. Catal. Lett. 1996, 57, 177.
4927
as well as the strength distribution of adsorption sites for these catalysts is needed. The microcalorimetry technique is a powerful and unique technique for this purpose. However, no application of this technique has been described for the hydrogen adsorption on γ-Mo2N.23 In the present study, we report the differential heats of H2 adsorption on various in situ prepared molybdenum nitride catalysts exhibiting specific surface areas in a wide range. Experimental Section Three samples with different surface areas were prepared in situ by temperature-programmed nitridation of MoO3 powder with NH324 in the same quartz tubular cell where calorimetric measurements are obtained. A temperature-programmed nitridation procedure with two linear heating segments was applied for the synthesis of γ-Mo2N samples. The heating steps of this preparation method were (i) from room temperature to 573 K at 5 K/min and (ii) from 573 to 973 K at a rate between 0.5 and 10 K/min (by varying the heating rate solids with various specific surface areas are obtained). Finally, the temperature of 973 K was maintained for 1 h. The NH3 flow rate of 100 mL/min was employed in all synthesis and the weight of treated MoO3 was closed to 0.5 g. After preparation, the samples were outgassed in the cell at 673 K for 15 h under high vacuum. Microcalorimetric measurements of H2 adsorption were performed at 330 K using a SETARAM C80 II Tian-Calvet heatflow calorimeter connected to glass vacuum adsorption chamber. Hydrogen was used as a probe molecule and pulses of approximately 2 × 1017 molecules were introduced into the system to titrate the surface of molybdenum nitride. This experimental system was described in detail elsewhere.25 Samples pretreated as described above were cooled to the adsorption temperature in a vacuum. Specific surface areas of the samples were evaluated by applying the BET method to the nitrogen adsorption isotherms measured at 77 K without exposure of the samples to air. These are reported in Table 1. In the nomenclature of the samples, the surface area is included. The crystalline phase structures of samples after exposure to air at room temperature were identified using a SEIFERT XRD3000P X-ray diffractometer operated at 40 kV and 40 mA.
Results and Discussion Figure 1 shows the volumetric isotherms of hydrogen adsorption on the Mo nitride catalysts with different surface areas. The shape of the adsorption isotherms is in general similar to that of a classical isotherm of Langmuir chemisorption type. Samples of Mo nitride with different surface areas differ considerably in their capacities for hydrogen adsorption. The monolayer uptake was determined by extrapolating the lineal part of the isotherm fitted curve to zero pressure. The capacity of hydrogen adsorption was 80 µmol/g for the MoN-90 catalyst with SBET ) 90 m2/g, 16 µmol/g for the MoN-17 catalyst with SBET ) 17 m2/g, and 7 µmol/g for the MoN-8 catalyst with SBET ) 8 m2/g. These results suggest that the amount of hydrogen uptake is nearly directly proportional to the surface area of the catalyst, and consequently that the larger the surface area of the Mo nitride sample the higher the number of active centers per unit weight that the sample possesses for hydrogen adsorption. This observation is consistent with our previous H2-TPD results.26 Thus, (23) Cardona-Martinez, N.; Dumesic, J. A. Adv. Catal. 1992, 38, 149. (24) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 332. (25) Bachiller-Baeza, B.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Langmuir 1998, 14, 3556. (26) Zhang, Y. J.; Xin, Q.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Appl. Catal. A 1999, 180, 237.
10.1021/la9816095 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999
4928 Langmuir, Vol. 15, No. 14, 1999
Notes
Table 1. Physical Properties and Crystalline Composition of Catalysts catalyst
heating rate second step (K/min)
SBET (m2/g)
crystalline composition
MoN-90 MoN-17 MoN-8 MoN-2
0.5 1 3 10
90 17 8 2
γ-Mo2N γ-Mo2N, MoO2 γ-Mo2N, MoO2 MoO2
Figure 2. Differential heats of H2 adsorption as a function of H2 coverage for the molybdenum nitride catalysts: (b) MoN-8, (2) MoN-17, (9) MoN-90.
Figure 1. Volumetric isotherms of H2 adsorption at 330 K on molybdenum nitride catalysts with different surface areas: (b) MoN-8, (2) MoN-17, (9) MoN-90.
assuming that the hydrogen molecules adsorb dissociatively on the surface of γ-Mo2N, a hydrogen number density close to 10.8 × 1017 atoms of hydrogen per square meter of γ-Mo2N can be estimated for all the samples. Using the usual approximation that the surface of γ-Mo2N consists of equal proportions of the main low-index planes, a Mo number density of 1.09 × 1019 Mo/m2 is expected for γ-Mo2N. Therefore, an uptake of 10.8 × 1017 H/m2 indicates that only ∼10% of the surface Mo atoms are active sites for adsorption of atomic hydrogen. Hadix et al. reported similar surface coverage for hydrogen (∼10%) on high surface area Mo nitrides.27 They conclude that hydrogen adsorbs in the form of rafts of strongly bound hydrogen atoms on nitrogen-deficient patches of Mo on the surface. The differential heats of hydrogen adsorption as a function of the hydrogen coverage (determined as the ratio between the adsorbed amount at a given point and the monolayer uptake) for various Mo nitride catalysts are shown in Figure 2. Comparison of the curves reveals that the energy distribution of surface hydrogen adsorption sites is somewhat controlled by the surface area of the molybdenum nitride. It can be clearly observed that the MoN-8 sample exhibits the highest initial differential heat of hydrogen adsorption (142 kJ/mol). The initial differential heat of hydrogen adsorption for the MoN-17 catalyst with medium surface area (122 kJ/mol) is quite similar to that of the highest surface area MoN-90 catalyst (120 kJ/mol). For these two latter catalysts, MoN-90 and MoN-17, the curves of differential heats of adsorption of (27) Haddix, G. W.; Reimer, J. A.; Bell, A. T. J. Catal. 1987, 108, 50.
hydrogen present a large plateau which extends up to H2 coverages of 0.8 before the evolved heat drastically falls in the physisorption field. This constant value over a wide range of surface coverage indicates that an appreciable part of the nitride surface acts homogeneously for the chemisorption of H2. In contrast, the Mo2N-8 catalyst displays a wide adsorption heterogeneity concerning H2. For the latter sample, the curve of differential heats of hydrogen adsorption is s-shaped. The shape of the curves of the differential heats of adsorption of the probe molecule as a function of coverage reveals the energetic distribution of the surface adsorption sites and this latter relied on the crystalline phases and surface nature of the samples. Finally, the hydrogen adsorption microcalorimetric experiments performed over the MoN-2 sample showed the absence of hydrogen chemisorption on this catalyst. The composition of the crystalline phases of the different catalysts determined from XRD patterns shown in Figure 3 are summarized in Table 1. The XRD pattern (Figure 3a) of the MoN-90 catalyst shows the structure of pure γ-Mo2N phase. However, the patterns from MoN-17 and MoN-8catalysts reveal two-phase constituents: γ-Mo2N and MoO2 (Figure 3b and Figure 3c). For the MoN-2 catalyst with the lowest surface area only the MoO2 phase is detected by XRD (Figure 3d). It has been reported24,28,29 that the properties (surface areas, phase constituents) of materials prepared by the temperature-programmed reduction of MoO3 with NH3 appear to be strongly dependent on the conditions (heating rates, space velocities) employed during the process. The production of catalysts with high surface area requires the use of slow heating rates during the early stages of the reaction and high space velocities. Jaggers et al.28 suggested that the solid-state reaction of MoO3 with NH3 proceeds through two parallel reaction pathways: MoO3 f MoOxN1-x f γ-Mo2N and MoO3 f MoO2 f γ-Mo2N + δ-Mo2N. Formation of oxynitride, MoOxN1-x, resulted in a large increase (28) Jaggers, C. H., Michaels, J. N., Stacy, A. M. Chem. Mater. 1990, 2, 150. (29) Zhang, Y. J.; Xin, Q., Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Mater. Res. Bull. 1999, 34, 145.
Notes
Figure 3. XRD profiles of molybdenum nitride catalysts with different specific surface areas: (a) Mo2N-90, (b) Mo2N-17, (c) Mo2N-8, and (d) Mo2N-2.
in surface area due to the pseudomorphic nature of the reaction in which nitrogen replaces oxygen in the MoO3 lattice with minimal disruption of molybdenum atoms. In contrast, the formation of MoO2 led to lower surface materials. The results of Table 1 support the latter suggestions. The correct estimation of the proportion at which the γ-Mo2N and MoO2 phases are present in the MoN-17 and MoN-8 catalysts is difficult because the (111) plane of γ-Mo2N and the (-211) plane of MoO2 occurred in serious overlapping. Moreover, the ratio of the intensities of the (200) and (111) diffraction peaks of γ-Mo2N varies with the surface area of the nitride materials;29 at is, the highest surface area materials have a I(200)/I(111) ratio much greater than 0.5, the value expected for randomly distributed γ-Mo2N crystallites of uniform dimensions.30 However, as a first approximation by following the intensity change of the (200) plane from γ-Mo2N phase in Figure 3a to 3c, it can be inferred that the amount of γ-Mo2N decreases when the BET surface area of catalysts declines. As reported above, the MoN-2 sample constituted by pure MoO2 does not chemisorb hydrogen in the microcalorimetric experiments. This finding suggests that for those catalysts involving MoO2 crystalline phase as one of their constituents, the molecules of hydrogen are only adsorbed on the crystalline phase of γ-Mo2N. Moreover, considering that all samples adsorb hydrogen with the same relative extent (similar hydrogen number density), we can deduce that the MoN-17 and MoN-8 samples with (30) McClune, W. F., Ed. Powder Diffraction File; Alphabetical Index Inorganic Materials. International Centre for Diffraction Data; Swarthmore: PA, 1991.
Langmuir, Vol. 15, No. 14, 1999 4929
two crystalline phases are formed by particles having a core of MoO2 superficially covered with γ-Mo2N, this latter phase being the only one exposed to the hydrogen. Therefore, a reasonable relationship between the differential heats of H2 adsorption on each catalyst and its surface crystalline structure can be performed. The XRD patterns for the two samples with higher surface area, MoN-90 and MoN-17, show a very high I(200)/I(111) ratio. These results suggest that the γ-Mo2N crystallites in these materials were platelike extended out in the {100} direction. This type of texturing, which is characteristic of high surface area Mo nitrides,13,24 is a consequence of the pseudomorphic nitridation of the MoO3 particles into γ-Mo2N, while that the MoN-8 sample displays the expected random distribution of the γ-Mo2N crystallites. These differences in texturing between MoN-8 sample and MoN-90 and MoN-17 samples are reflected in the shape of the differential heat curves of hydrogen adsorption. The homogeneous energetic distribution of surface sites in MoN-90 and MoN-17 samples is a consequence of the preferential exposition of the (200) plane in these catalysts. However, the MoN-8 sample with its γ-Mo2N crystallites randomly distributed presents a heterogeneous energetic distribution of surface sites. The higher initial differential heat of H2 adsorption observed over this latter sample (Figure 2) is assigned to hydrogen species adsorbed on surface sites over the (111) plane, which is in larger fraction in this low surface area sample. These conclusions are consistent with previous H2-TPD studies26 where the area of the high-temperature desorption peak, associated with species adsorbed on high-energy surface sites, was found to be inversely proportional to the surface area of the catalyst. This fact indicated that the low surface area Mo nitrides have a relatively larger contribution of H2 adsorbed on high-energy surface sites. In this sense, Choi et al. 5 reported that the γ-Mo2N catalysts possess at least two types of surface sites for hydrogen adsorption whose relative contribution is a function of the surface area of the sample. Conclusions Microcalorimetric measurements of H2 adsorption on γ-Mo2N catalysts with various specific surface areas are reported. Samples with high and medium surface areas (90 and 17 m2‚g-1) present a homogeneous energetic distribution of surface sites concerning hydrogen adsorption which corresponds to the preferential orientation of their γ-Mo2N crystallites. Molybdenum nitride with low surface area has its γ-Mo2N crystallites randomly oriented and as a consequence, this sample displays a heterogeneous energetic distribution of surface sites. The higher initial differential heat of hydrogen adsorption observed for the low surface area Mo nitride is attributed to species adsorbed on surface sites associated to the (111) plane; the (111) is in larger proportion in samples with low surface area. Acknowledgment. This work was supported by a joint project of the CSIC (Consejo Superior de Investigaciones Cientificas) and the CAS (Chinese Academy of Sciences). Y.J.Z. thanks the CSIC for a postdoctoral fellowship. LA9816095
