J. Phys. Chem. B 1997, 101, 6429-6435
6429
Preparation and Characterization of a Novel Class of Catalysts: Ultradispersed Molybdenum Oxynitrides Supported on NaEMT and HEMT Zeolite Thierry Be´ cue, Jean-Marie Manoli, Claude Potvin, and Ge´ rald Dje´ ga-Mariadassou* Laboratoire de Re´ actiVite´ de Surface, URA 1106, Casier 178, UniVersite´ P. & M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
Michel Delamar ITODYS, URA 34, UniVersite´ D. Diderot, 1 Rue Guy de la Brosse, 75005 Paris Cedex 05, France ReceiVed: July 24, 1996; In Final Form: May 28, 1997X
Decomposition under partial pressure of ammonia of Mo(CO)6/EMT and subsequent nitridation under flowing ammonia were used to prepare EMT zeolite-encaged molybdenum nitride clusters. The HRTEM and EDS measurements indicate that the final molybdenum phases were homogeneously dispersed, the final clusters’ diameter being less than 10 Å. X-ray diffraction (XRD) patterns and specific surface area measurements showed that the EMT zeolite crystallinity was not affected by ammonia treatments. Ammonia decomposition during the preparation step and X-ray photoelectron spectroscopy (XPS) data allowed us to assess the nitridic nature of the clusters for a nitridation temperature as low as 723 K.
1. Introduction Transition metal carbides and nitrides are catalytically active in several reactions.1-7 Since many of these reactions are typically catalyzed by noble metals, the potential for using carbides and nitrides in these processes provides a strong development of fundamental research on the structure and reactivity of these new intriguing materials. Thus, a flurry of activity research on the synthesis and the catalytic behavior of these compounds has occurred over the last two decades, and one of the major aims of these works has been to prepare nitrides and carbides with high specific surface areas.3,8-12 Due in part to the difficulty of preparing high surface area bulk materials, heterogeneous catalysts are typically prepared on high surface area supports. In this way, Lee et al. have reported a high catalytic activity in benzene hydrogenation for alumina-supported molybdenum carbide catalysts.13 Nagai et al. have also underlined that molybdenum nitrides supported on alumina were extremely active in hydrodesulfurgation (HDS) of dibenzothiophene14 and in hydrodenitrogenation (HDN) of carbazole,15 while Colling and Thompson pointed out the influence of the size of molybdenum nitride particles supported on alumina in the HDN of pyridine.16 These results have encouraged the investigation of supported molybdenum nitrides. Molybdenum nitrides prepared by nitridation of Mo(CO)6 deposited by chemical vapor on the EMT zeolite will be the focus of this paper. The EMT zeolite is a hexagonal polytype of the Y cubic faujasite which was recently synthesized in a pure form.17 This zeolite exhibits a more open structure18 and a high Si/Al ratio, leading to a stronger acidity than the Y zeolite.19 Two series of catalysts were prepared depending whether the molybdenum phase is supported on the sodium-form or on the ammoniumform zeolite. This paper deals with their preparation and physicochemical characterization. The catalytic properties of these compounds for the n-heptane conversion will be discussed in a following paper. 2. Experimental Section 2.1. Catalyst Preparation. Using a slightly modified procedure based on that of Delprato et al.,17 the NaEMT zeolite * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 15, 1997.
S1089-5647(96)02249-3 CCC: $14.00
(Si/Al ) 3.8, SBET ) 670 m2‚g-1) was synthesized in a pure form. The partially NH4-exchanged EMT zeolite (denoted as NH4EMT) was obtained by ion exchange at room temperature, using 5 g of NaEMT suspended in 50 mL of ammonium acetate solution (1 mol‚L-1). The experiment was repeated three times leading to a degree of ion exchange of ca. 75% for NH4+. The protonic form is generated by a thermal treatment at 773 K under flowing hydrogen (720 mL‚h-1). To avoid any further oxidation of encaged molybdenum phases by Brønsted acidic sites and a subsequent collapse of the EMT framework, the zeolite was evacuated in high vacuum for 2 h at 423 K. By such a treatment, water was thus removed from Na- and NH4EMT porosity without deammoniation. The vapor phase deposition of Mo(CO)6 was then carried out according to the procedure described by Gallezot et al.20 Without exposure to the air, the quartz reactor containing the dehydrated EMT zeolite was connected via a grease-free stopcock to a cell containing a suitable amount (5 wt % Mo) of Mo(CO)6, which was first outgassed by repeated freeze pumpings. Nitridation of the Mo-loaded samples was carried out by linearly increasing the sample temperature (60 K‚h-1) in the tubular quartz reactor under partial pressure (300 Torr) or flowing ammonia (6 L‚h-1). The reactor was cooled down to room temperature under flowing ammonia, and then the sample was flushed with argon for 30 min. It will be checked further on that a subsequent total or partial decomposition of the Mo(CO)6 complex before heating in flowing ammonia is necessary to avoid a substantial loss of the molybdenum. This decarbonylation under partial pressure of NH3 will be followed by infrared spectroscopy. In addition, two stopcocks can isolate the reactor when necessary to avoid any contact of the sample with the air. Bulk molybdenum nitrides were synthesized as references for XPS measurements. A γ-Mo2N type nitride (JCPDS 251366, SBET ) 120 m2‚g-1) was prepared from 1 g of ammonium heptamolybdate using the protocol described by Volpe and Boudart.9 A pure molybdenum nitride (crystallographic type δ-MoN, JCPDS 25-1367, SBET ) 5 m2‚g-1) could be prepared by heating (60 K‚h-1) 1 g of MoCl3 (not exposed to air) under flowing © 1997 American Chemical Society
6430 J. Phys. Chem. B, Vol. 101, No. 33, 1997 ammonia (10 L‚h-1) up to 973 K. From 573 to 773 K, a white precipitate identified as NH4Cl was formed at the outlet of the reactor. It was made sure that the same experiment carried out on Mo(CO)6 led to a nitride. A decarbonylation of the complex was performed under ammonia (300 Torr) at 423 K, to avoid sublimation, before nitriding with flowing ammonia (5 L‚h-1) at atmospheric pressure. At this stage, a pale yellow complex, Mo(CO)3(NH3)3, was obtained with the same color and IR frequencies as mentioned by Barlow and Holywell21 (ν(CO) at 1880 and 1730 cm-1). The ultimate phase obtained after nitridation was identified as mainly δ-MoN, with some of the compound only partially reacted. 2.2. Procedures and Characterization Techniques. FTIR Absorption-Transmission Spectroscopy. The infrared study was carried out on self-supported wafers (ca. 15 mg, pressed under a pressure of 103 kg‚cm-2, 18 mm in diameter). The disc, supported on a quartz holder, was placed into an infrared cell equipped with CaF2 windows and could be heated either in vacuum or under static or flowing ammonia. The FTIR spectra were recorded at room temperature using a Bruker IFS 66 V spectrometer, with a resolution of 4 cm-1 (number of scans 30). XRD Measurements. A Siemens D500 automatic diffractometer with a Cu KR monochromatized radiation source was used for the X-ray diffraction (XRD) patterns of the various solid phases. The degree of X-ray crystallinity of NH4EMT and of the loaded zeolites was estimated from the intensity of all reflections in the range 2θ ) 14.5-29.3° 22 and compared with those of the calcined NaEMT zeolite. For catalysts containing molybdenum phases, the measured crystallinity was corrected23 to assess the crystallinity of the EMT support. BET Measurements. Specific surface areas were determined by physisorption of nitrogen at 77.3 K using a Quantasorb Jr. dynamic sorption system, linked to a thermal conductivity detector (TCD). The specific surface area was measured by the three-point BET method using nitrogen adsorption at different partial pressures. The standard pretreatment consisted in heating the samples in nitrogen at 373 K for 1 h and at 673 K for 3 h. Temperature-Programmed Reaction (TPR). A gas chromatograph (GC) using a thermal conductivity detector (TCD) and an automatic injection valve was used to analyze the gas phase composition at the outlet of the preparation reactor. Nitrogen, hydrogen, and ammonia were separated on a 1 m, 1/8 in. Porapak Q column, at 350 K with helium as the carrier gas. A two-pen recorder was used to follow the general features of the nitridation versus the temperature. HRTEM and STEM. HRTEM studies of the catalysts were performed on a Jeol apparatus (JEM 100 CXII) equipped with a top entry device and operating at 100 kV. Ultramicrotome cuts (80-100 nm) of the samples were prepared for such studies. EDS analysis (STEM mode) were obtained with another JEM 100 CXII using a LINK AN 10000 system connected to a silicon-lithium diode detector and a multichannel analyzer. The EDS analyses were obtained either from relatively large domains of the samples (150 × 200 nm2 to 400 × 533 nm2) or from smaller domains (fixed STEM beam analysis area of 710 nm2). X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were measured using a Surface Science Instrument SSX 100 spectrometer with a monochromated Al KR X-ray source (hν ) 1486.6 eV), a hemispherical electron analyzer and a multichannel detector. After nitriding, the nonpassivated samples were outgassed in high vacuum at 673 K to remove the chemisorbed ammonia molecules and kept in a sealed reactor which was opened in a glovebox (dry argon) connected to the
Be´cue et al. XPS spectrometer. The materials were then transferred onto the sample holder and introduced into the spectrometer chamber without exposure to the air. Samples were then outgassed in the pretreatment chamber at 10-5 Torr prior transferring into the analysis chamber. The vacuum applied during the measurements was typically less than 5 × 10-9 Torr, and data were accumulated in a separate region for acquisition times ranging from 5 to 10 s per spectral point. Surface charging was compensated and made homogeneous by flooding the sample surface with low-energy electrons (5-10 ev). The C1s peak of the contamination carbon (binding energy (BE) ) 284.6 eV) was taken as reference in calculating the binding energies and accounting for the charging effects. Experimental peaks were decomposed into components (90% Gaussian, 10% Lorentzian) using a nonlinear least-squares fitting algorithm and a Shirley baseline. To reconstruct the levels of molybdenum, some parameter constraints were imposed. The intensity ratio for the Mo3d5/2-Mo3d3/2 doublet should be I(3d5/2)/I(3d3/2) ) 3/2 and I(3p3/2)/I(3p1/2) ) 2 for the Mo3p3/2-Mo3p1/2 doublet; a splitting energy of 3.2 eV was used for the Mo3d5/2-Mo3d3/2 doublet and 17.5 eV for the Mo3p3/2-Mo3p1/2 doublet. For all the elements, atomic concentrations were estimated upon comparisons of the integrated peak intensities normalized by the atomic sensivity factors.24 Binding energies were reproducible to within (0.2 eV. 3. Results 3.1. Catalysts Preparation. The elemental chemical analysis of the nitrided compounds indicated a substantial loss of molybdenum (50% of the Mo introduced) when the Mo(CO)6loaded zeolites were directly heated under flowing ammonia. Therefore, before the treatment under flowing NH3, a previous partial decomposition of the Mo(CO)6 molecules deposited on the zeolite was undertaken to reduce or eliminate their volatility. This decarbonylation was studied by in-situ FTIR spectroscopy under a partial pressure of ammonia to maintain the Mo(CO)6 complex in a reductive and nitriding atmosphere. The domain of carbonyl stretching (2150-1650 cm-1) showed changes due to adsorption and decarbonylation of Mo(CO)6. Figure 1 shows the series of spectra obtained after exposure of NaEMT and NH4EMT to Mo(CO)6 vapor for increasing periods of time and different thermal treatments under ammonia. Fifteen minutes after introduction of Mo(CO)6 over the zeolite (15 s of exposure duration) at 298 K, the bands corresponding to Mo(CO)6 were no longer observed in the gas phase of the infrared cell, showing that the encagement of Mo(CO)6 was complete. The adsorption of Mo(CO)6 was evidenced by the lowering of the molecular symmetry (spectrum not shown). In good agreement with several literature data,20,25-28 the stretching bands showed that the structure of Mo(CO)6 retained its integrity with only some distortions in symmetry. After 48 h (spectrum b), new bands appeared at 1903, 1762, and 1708 cm-1 in the case of the NaEMT zeolite but not for the NH4EMT one. These bands are characteristic of subcarbonyl species in Cs symmetry (Mo(CO)3-NaEMT) and show a stronger interaction of Mo(CO)6 in NaEMT than in NH4EMT, as reported for adsorption of metal carbonyl on zeolite.28,29 When ammonia was introduced into the infrared cell (spectrum c), the spectra were simplified and showed (i) two sharp bands at 2002 and 1891 cm-1 on NaEMT and (ii) only one sharp band at 2002 cm-1 on NH4EMT. The strong band at 2002 cm-1 belongs to an IR active mode of free Mo(CO)6 molecules with no interaction with the zeolite walls. No carbonyl stretching band was detected in the gas phase of the cell; therefore, the free Mo(CO)6 molecules
Molybdenum Oxynitrides on NaEMT and HEMT Zeolite
Figure 1. Transmission infrared spectra of Mo(CO)6/EMT decomposed under ammonia: (a) EMT zeolite dehydrated for 2 h at 423 K under 5 × 10-5 Torr before exposure to Mo(CO)6; (b) EMT (a) exposed for 15 s at room temperature to Mo(CO)6 vapors, spectrum 48 h after exposure; (c) sample b under NH3 (1 atm); (d) sample c heated at 423 K under NH3 (100 Torr); (e) sample d heated at 423 K under flowing NH3 (1 atm).
remained in the EMT cages. The strong absorption at 1891 cm-1 occurring only with the NaEMT zeolite is of particular interest and is probably due to Mo(CO)3L3 (L is a ligand). Since ammonia is a σ-donor ligand, we can imagine its participation in a substitution reaction of the CO ligands to form Mo(CO)3(NH3)3. Indeed, ammonia does not form any retro-coordination bond and does not behave as a π-acceptor ligand; thus, it cannot contribute to the removal of the excess electron density from molybdenum, and the trisubstituted derivatives represent the highest degree of substitution encountered.30 Therefore, the stretching band at 1891 cm-1 can be considered as characteristic of the Mo(CO)3(NH3)3 molecule behaving as a free species and the second band at about 1750 cm-1 corresponding to C3V symmetry being masked by the absorption bands of NH3 vapor. After having outgassed the infrared cell under high vacuum at room temperature (spectrum not shown), the same bands as for spectrum b are retrieved which correspond to the adsorbed form of Mo(CO)6 for NH4EMT (C2V symmetry) and to the adsorbed forms of Mo(CO)6 and Mo(CO)3(NH3)3 (C2V or Cs symmetry) for NaEMT. To make sure the leaving CO ligands have been substituted by ammine ligands, the same experiments were carried out with a high loading of molybdenum. The pellet changed in color from brown (characteristic of Mo(CO)3 species) to white after exposure to ammonia, which persisted after outgassing (high vacuum). This color of the pellet agrees with
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6431 the assumption of Mo(CO)3(NH3)3 (pale yellow)21 diluted in the white zeolite EMT. When the samples were heated under partial pressure of ammonia at 423 K (spectrum d), the bands corresponding to Mo(CO)6 totally disappeared and only the bands corresponding to Mo(CO)3(NH3)3 in C2V or lower symmetry on NaEMT and in C3V symmetry on NH4EMT remained. Once again, the interactions were stronger when molybdenum carbonyl molecules were adsorbed on NaEMT rather than on NH4EMT. Furthermore, the pellets became brownish, indicating that some of the molybdenum complexes were totally decarbonylated. The extent of decarbonylation under flowing ammonia was significant at 423 K (spectrum e) and complete at 523 K. As the subcarbonyl complex stabilized on the zeolite framework (Mo(CO)3(NH3)3) was probably less volatile, the following procedure of nitridation was established for our supported compounds. After the vapor deposition step, the Mo(CO)6loaded zeolite was heated (1 K‚min-1) under a partial pressure of ammonia (300 Torr) from room temperature to 423 K. After 1 h at 423 K, flowing ammonia (6 L‚h-1) was introduced through the quartz reactor and the temperature kept at 423 K for 1 h. Then, the temperature was linearly increased from 423 to 973 K (1 K‚min-1) and held for 4 h at 973 K. It was previously confirmed by XRD that a nitride (δ-MoN) was obtained by such a treatment on bulk Mo(CO)6. After cooling the reactor down to 298 K, it was purged with helium to remove ammonia. No passivation of the supported samples was necessary as the samples were not pyrophoric when exposed to the air. The samples will be noted as MoN-T/zEMT, where “MoN” represents the molybdenum nitride obtained whatever the nitride phase, T the temperature in K of the ammonia treatment, and z the cation (Na+ or NH4+) involved in the EMT support. 3.2. Catalysts Characterization. 3.2.1. Chemical Analysis, BET XRD, HRTEM, and STEM-EDS. Characteristics of the various compounds obtained after the final step of nitridation and exposure to air are summarized in Table 1 and compared to those of the zeolites. All XRD peaks appearing in the patterns of the initial zeolite support were also present in the patterns of the loaded EMT catalysts. No XRD peak corresponding to metallic molybdenum or molybdenum nitride, carbide, or oxide was observed. No noticeable decrease of crystallinity was induced after the nitridation treatment. The HRTEM micrographs on thin cuts did not reveal any damage of the zeolite framework, fringes of the EMT lattice being readily visible (Figure 2). From the micrographs it is clear that the molybdenum phase particles (diameter equal or smaller than 10 Å) were well dispersed inside the zeolite pores and did not segregate into domains. Even after a collapse of the zeolite framework in the electron beam, it was impossible to get some good electron diffraction patterns to determine the nature of those molybdenum phases. 3.2.2. Ammonia Decomposition during Preparation: TPR Experiments. The NH3 decomposition was carried out on the Mo(CO)6/NaEMT and Mo(CO)6/NH4EMT samples using nitriding parameters mentioned earlier. Plots show the TCD signal and temperature versus time of run (Figure 3) and can be compared with those obtained for the zeolite alone or during the nitridation of the ammonium heptamolybdate. The small decrease of the ammonia peaks occurring at 890 K on NaEMT and NH4EMT (not shown in Figure 3), respectively, corresponds to the thermal decomposition of ammonia.31 On the supported samples, the NH3 decomposition was similar to that on ammonium heptamolybdate that begins at about 650 K (beginning of the nitridation) and is complete at 973 K under
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TABLE 1: Characteristics of the Catalysts NaEMT theoretical Mo loading chemical analysisa EDS analysisb 2
SBET/m ‚g XRD crystallinityc -1
Na20 Al20Si76O192 Si/Al ) 3.7 (1) 670 100
MoN-973/NaEMT Mo/Al ) 0.17 (MoN)3.4Na20 Al20Si76O192 Mo/Al ) 0.16 (2) Si/Al ) 3.5 (3) 603 100
NH4EMT (NH4)15Na5 Al20Si76O192 Si/Al ) 3.7 (1) 645 106
MoN-973/NH4EMT Mo/Al ) 0.17 (MoN)3.2(NH4)15 Na5Al20Si76O192 Mo/Al ) 0.17 (6) Si/Al ) 3.6 (1) 590 105
a Elemental analysis performed by the Service Central d’Analyse du CNRS (Vernaison, France) and expressed by unit cell. b Numbers into parentheses represent the standard deviation over 7 analyses for each sample. c Crystallinity corrected from atomic absorption by the Mo phase for the supported materials.
Figure 2. HRTEM micrograph of a microtome cut on MoN-973/NH4EMT.
our conditions; the plot profile is typical of a self-catalytic decomposition. For both molybdenum-loaded zeolites, some nitrogen release was detected at 523 K. When these experiments were repeated on supported samples already heated at 773 K under ammonia, no N2 peak at 523 K was observed, but NH3 decomposition at 700 K was pointed out with the same reaction ratio as before. 3.2.3. XPS Measurements. Figure 4 depicts the XPS spectra of the Mo3d, Mo3p, and N1s levels for bulk δ-MoN and γ-Mo2N type oxynitrides. There is an overlap of the N1s peak with the Mo3p3/2 signals. Therefore, the spectrum at the Mo3d levels was first deconvoluted, and the Mo3p experimental peaks were then reconstructed preserving the same intensity ratio between the different Mo species peaks. To complete the experimental envelope, the nitrogen component (N1s peak) was then introduced. For the MoN-T/NH4EMT samples, the presence of remaining ammonium zeolite groups precluded an accurate analysis of the nitrogen signal. The XPS spectra of MoN-973/NaEMT, MoN-723/NaEMT, and MoN-523/NaEMT are shown in Figure 5. The XPS data for all the supported catalysts are summarized in Table 2 and can be compared with those of the bulk oxynitrides and some reference compounds (Table 3). 4. Discussion 4.1. Presence and Dispersion of the Molybdenum Phase. The elemental chemical analysis, which permits a global composition determination, agreed with the numerous EDS analyses on different zeolite domains and confirm that the quantity of initially deposited molybdenum was preserved and totally included in the zeolite pores. Hence, the vapor phase deposition appears to be a very efficient quantitative method of preparation. Furthermore, in order to prevent a loss of molybdenum, a preliminary partial decomposition of the Mo(CO)6 complex inside the EMT pores was shown to retain the molybdenum phase even at 973 K under a high flow of ammonia
Figure 3. Changes in NH3, N2, and H2 chromatographic peaks as a function of time and temperature during an ammonia treatment (6 L‚h-1) of the compounds: (I) NaEMT (300 mg), (II) Mo(CO)6/NaEMT (500 mg), (III) (NH4)6Mo7O24‚4H2O (1 g). Temperature versus time schedules used are noted β1 (60 K‚h-1) or β2 (35 K‚h-1).
(6 L‚h-1). Moreover, the distribution of molybdenum was homogeneous throughout the EMT zeolite, as could be seen with the low standard deviations of the EDS measurements and the satisfactory correlation between XPS composition and chemical analysis. The absence of molybdenum enrichment of
Molybdenum Oxynitrides on NaEMT and HEMT Zeolite
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6433
Figure 5. Experimental and reconstructed XPS spectra for the Mo(CO)6/NaEMT samples heated under NH3 at 973 K, (a) Mo3d level and (a′) Mo3p and N1s levels; at 723 K, (b) Mo3d level and (b′) Mo3p and N1s levels; and at 523 K, (c) Mo3d level and (c′) Mo3p and N1s levels.
Figure 4. Superposition of the experimental XPS spectra and the spectra after deconvolution for nonpassivated bulk oxynitrides at the Mo3d level for (a) γ-Mo2N and (b) δ-MoN and at the Mo3p and N1s levels for (a′) γ-Mo2N and (b′) δ-MoN.
zeolite surface together with total molybdenum content points toward the internal confinement of the molybdenum. Furthermore, the HRTEM micrographs and the absence of XRD peaks corresponding to a molybdenum phase clearly showed that an ultradispersed molybdenum phase was obtained (particle diameter smaller than 10 Å). Finally, the crystallinity of the support was totally preserved (corrected crystallinity grade deduced from XRD), and consequently there was little or no decrease of the EMT specific surface area. 4.2. Identification of a Molybdenum Nitride Supported Phase. Ammonia Decomposition. For many years, ammonia decomposition has been widely studied, and it is now well established that NH3 decomposition on transition metals or transition metal oxides is accompanied by a nitridation of the metal or of the oxides. Moreover, the nitrides thus formed decompose ammonia catalytically. In the 1960s, K. Tamaru39 noticed that NH3 decomposition on metallic tungsten begins at 550 K and is followed by a nitridation of tungsten, molecular nitrogen desorption beginning at 773 K. The total reaction has been considered as a nitride formation followed by its decomposition, involving an atomic nitrogen chemisorption followed by its desorption. Tsuchiya and Ozaki40 have confirmed the nitridation of both MoO3 and metallic molybdenum on several
layers;40 the so-formed surface was active for NH3 decomposition and the number of active sites increased with nitridation. Thus, if ammonia decomposition occurs on our samples, it is reasonable to assume that a nitride phase was formed first. For the supported samples (Mo(CO)6/NaEMT and Mo(CO)6/ NH4EMT), ammonia decomposition started at 700 K; it is due to the presence of a molybdenum phase, as it was not detected on the zeolites alone. This reaction is also catalyzed by the Mo phase, the decomposition being already complete at 973 K in our conditions. Furthermore, the ammonia decomposition with a (nitrogen + hydrogen) release started at 700 K and presented exactly the same profile as that occurring on the ammonium heptamolybdate, which led to a nitride (γ-Mo2N). Thus, since the molybdenum phase particles have a diameter less than 10 Å (less than four metallic atoms layers), a molybdenum nitride phase was undoubtedly formed on our supported samples at a temperature of about 700 K. Indeed, Shindo et al.41 have shown that several nitride layers were formed when ammonia decomposed on a tungsten foil. Moreover, for the reverse reaction (ammonia synthesis from N2 + H2) Oyama and Boudart42 have shown that the rates of ammonia synthesis reaction at the steady state were similar for different catalysts such as MoOxCy, Mo2C, and metallic molybdenum. This reaction being structure sensitive, they have deduced that the surface of these samples had similar composition and structure, which, at the steady state, were (i) one nitride layer with a MoN stoichiometry on MoOxCy, Mo2C and (ii) a threelayer nitridation (one MoN layer and two Mo2N sublayers) on Mo. In our case it was also possible to detect a nitrogen release at a lower temperature (about 523 K) on both supported catalysts during the preparation, but it was shown that this phenomenon was not extended, the irreversible adsorption of the nitrogen atoms preventing any self-catalytic reaction. Matsushita and Hansen43 have observed a similar adsorption of nitrogen atoms at 500 K and a desorption above 720 K when they studied the
6434 J. Phys. Chem. B, Vol. 101, No. 33, 1997
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TABLE 2: X-ray Photoelectron Spectroscopy Data for Mo(CO)6/NaEMT and Mo(CO)6/NH4EMT (5% wt Mo) Heated under Flowing Ammonia at Different Temperatures and Not Exposed to Air; The Binding Energies Are Corrected from the Charge Shift binding energies (eV) and concentrations (%) Mo3d5/2 Mo
Moδ+ Mo-N
MoIV
MoN-973 /NaEMT
227.8 (14)a
229.0 (37)
230.8 (49)
MoN-723 /NaEMT
227.9 (37)a
228.8 (36)
MoN-523 /NaEMT
227.7 (19)a
229.1 (42)
230.5 (39)
229.3 (34)a
230.6 (28) 230.1 (40)a 230.8 (30)
sample
MoN-973 /NH4EMT MoN-723 /NH4EMT MoN-523 /NH4EMT a
229.6 (20)a
MoV
MoVI
231.5 (27)
232.5 (38) 230.9 (17) 232.3 (50)
233.7 (43)
N1s N-Mo
Si2p
Al2p
O 1s
C1s C-C
chemical composition XPS
chemical analysis
396.8
103
74.2
532.2
284.6
Si/Al ) 3.9 Mo/Al ) 0.17
396.6
102.8
74.2
532.0
284.6
399.9
103.6
74.8
532.2
284.6
399.7 nmb 401.8 nm 402.5 nm
103.3
75.1
532.6
284.6
103.1
74.6
532.1
284.6
103
75.1
532.0
284.6
Si/Al ) 3.9 Mo/Al ) 0.11 Mo/N ) 1.1 Si/Al ) 3.9 Mo/Al ) 0.19 Mo/N ) 1.0 Si/Al ) 4.0 Mo/Al ) 0.16 Mo/N ) 1.1 Si/Al ) 4.1 Mo/Al ) 0.10 Si/Al ) 3.5 Mo/Al ) 0.10 Si/Al ) 4.0 Mo/Al ) 0.11
Si/Al ) 3.7 Mo/Al ) 0.16 Si/Al ) 3.7 Mo/Al ) 0.16 Si/Al ) 3.9 Mo/Al ) 0.16 Si/Al ) 3.7 Mo/Al ) 0.11 Si/Al ) 3.7 Mo/Al ) 0.13
Area percent. b nm: not measured because of remaining ammonium groups of the NH4EMT zeolite.
TABLE 3: Binding Energies (in eV) for Related Systems
sample
Mo0 metal
Moδ+ Mo-N Mo-C
MoIV
MoV
MoVI
230.8 MoCl5 231.7 (15) 231.7 (24)
232.4 MoO3
228.8 (56)b 229.1 (43)b 228.7
229.1 MoO2a 229.7 (29) 229.9 (33) 229.9
227.7 bulk Mo2N bulk MoN bulk MoN or Mo2N bulk Mo2N bulk Mo2C Mo species /NaY zeolite Mo species /NaY zeolite Mo species /NaY zeolite a
227.8 227.9 228.5 Mo(CO)6a 227.8 Mo(CO)6a 227.8 Mo(CO)6a
228.6 228.4
229.9 230.0 229.9 MoO2
229.2 231.7
N1s N-Mo
284.4
32
284.6
this work
397.8
284.6
this work
284.8
33, 34
284.8 284.5 285
35 36 28
285
37
284.6
38
232.6 232.9 232.5 233.0 MoO3 233.7 233.6
ref
Cgrap.
397.2 WN 397.8
397.5
Compound used for the measurement. b Area percent.
NH3 decomposition on a molybdenum filament. Consequently the temperature could be too low to permit the desorption of the nitrogen atoms adsorbed in the bulk of the encaged particles, leading to a nitride phase but preventing a self-catalytic decomposition of ammonia. XPS Analysis. The results on δ-MoN and γ-Mo2N associated with published data allow us to identify a nitride phase which is characterized in Table 3 by a Mo3d5/2 peak at about 228.7 eV (Moδ+, 0 < δ < 4) and a low binding energy for the N1s signal (about 397.5 eV). On the supported samples, the XPS spectrum deconvolutions led to a substantial contribution of molybdenum (Moδ+) engaged in a nitride phase (between 20 and 42%). Generally no Mo(VI)+ was detected, for MoN-T/ NaEMT (nonacidic support) the Mo species consisted of metallic Mo, Moδ+, and Mo(IV)+, while for MoN-T/NH4EMT the Mo species consisted of Moδ+, Mo(IV)+, and Mo(V)+ states. One can assume that these Mo species belong to an oxynitride phase since the same oxidation states are observed for nonpassivated bulk MoN or Mo2N. For MoN-T/NaEMT, the XPS data fully agree with the NH3 decomposition process, showing that a nitride phase is formed at 723 K. Indeed, nitrogen atoms engaged in a nitride are clearly evidenced for MoN-723/NaEMT and MoN-973/NaEMT with N1s binding energies at 396.8 and 396.6 eV, respectively. For MoN-523/NaEMT, the N1s peak
at about 400 eV probably corresponds to a strongly chemisorbed ammonia (or imide group) species on the metallic molybdenum atoms. So, the nitrogen release at 523 K during the ammonia decomposition could be linked to the reduction by ammonia of partially oxidized molybdenum (oxidation caused by water traces remaining in the zeolite). However, no confirmation of a nitride phase formation at 523 K was detected by XPS. By analogy, the nitride formation should also be conceivable on the Moloaded NH4EMT at a temperature of 723 K. 5. Conclusion This paper shows that, using the chemical vapor deposition method, it is possible to introduce a known amount of molybdenum in zeolite pores and maintain it even after a thermal treatment under flowing ammonia at 973 K. A decomposition in a NH3 partial pressure of the supported molybdenum hexacarbonyl is necessary prior to the nitridation step under flowing NH3. The molybdenum phases are found to be very well dispersed in the NaEMT or NH4EMT zeolites throughout the process. The Mo/Al ratio is quite homogeneous in the zeolite crystals, and the diameter of the so-formed particles of nitrides is less than 10 Å. Even a thermal treatment at high temperature (973 K) under the corrosive ammonia atmosphere
Molybdenum Oxynitrides on NaEMT and HEMT Zeolite does not affect the crystallinity of the support (the EMT zeolite). Thus, the catalysts so prepared that way preserve a high specific surface area. The supported Mo phase formed at 523 K under ammonia could not be clearly identified as a nitride and probably consisted of small metallic molybdenum particles that strongly chemisorbed ammonia. However, a molybdenum nitride phase can be formed at very low temperatures in comparison to those usually required to obtain bulk molybdenum nitride (973 K). This low nitridation temperature of the molybdenum supported on EMT corresponds to the temperature at which the catalytic ammonia decomposition starts (723 K); this is due to the ultradispersion of the molybdenum phase, which permits a rapid and total nitridation of the metal as soon as the ammonia decomposes. As will be reported in a following paper, all these catalysts showed very interesting and promising behavior when they were tested in the reaction with n-heptane. Acknowledgment. The authors wish to express their gratitude to M. Leclerc for his assistance in acquiring XPS measurements. P. Beaunier’s and. M. Lavergne’s help for STEM-EDX and TEM measurements was greatly appreciated. References and Notes (1) Boudart, M.; Levy, R. Science 1973, 181, 547. (2) Oyama, S. T.; Haller, G. L. Catal. Special. Period. Rep. 1981, 5, 333. (3) Leclercq, L. In Surface Properties and Catalysis by Non-Metals; Bonnelle, J. P., et al., Eds.; 1983; p 433. (4) Oyama, S. T.; Schlatter, J. C.; Metcalfe, J. E., III; Lambert, J. M., Jr. Ind. Eng. Chem. Res. 1988, 27, 1639. (5) Abe, H.; Bell, A. T. Catal. Lett. 1993, 18, 1. (6) Dje´ga-Mariadassou, G.; Boudart, M.; Bugli, G.; Sayag, C. Catal. Lett. 1995, 31, 411. (7) Kim, H. S.; Sayag, C.; Bugli, G.; Dje´ga-Mariadassou, G.; Boudart, M. Mater. Res. Sci. Symp. Proc. Ser. 1995, 368, 3. (8) Volpe, L.; Oyama, S. T.; Boudart, M. In Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 147. (9) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 332. (10) Volpe, L.; Boudart, M. Catal. ReV.-Sci. Eng. 1985, 27 (4), 515. (11) Jaggers, C. H.; Michaels, J. N.; Stacy, A. M. Chem. Mater. 1990, 2, 150. (12) Wise, R. S.; Markel, E. J. J. Catal. 1994, 145, 344. (13) Lee, J. S.; Yeom, M. H.; Park, K. Y.; Nam, I.-S.; Chung, J. S.; Kim, Y. G.; Moon, S. H. J. Catal. 1991, 128, 126. (14) Nagai, M.; Myia, T. Catal. Lett. 1992, 15, 105.
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