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J. Phys. Chem. B 2001, 105, 7178-7181
Insertion Direction of Hydrogen in Protonation of r-MoO3 H. C. Zeng,* W. K. Ng, L. H. Cheong, F. Xie, and R. Xu Department of Chemical and EnVironmental Engineering, Faculty of Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: January 17, 2001
In this work, we report an experimental investigation on hydrogen insertion mechanism into layered solid R-MoO3 by creating different traveling paths for hydrogen during the protonation reaction with H2S at 152155 °C. Our results show that hydrogen preferentially enters the layered host along 〈001〉 direction, although an entire interlayer space is open to this guest species.
Introduction The inclusion/intercalation chemistry has entered an entirely new era since the 1992s discovery of periodic mesopourous materials,1 which lead to a significant advance in tailor-making nanostructured materials and hybrid nanocomposites.2 Concerning the first steps of insertion chemistry, the entrance point of atomic or molecular guests into mesoporous host materials such as MCM41 and its derivatives can be well comprehended because any insertion processes must be started in the pore openings of channels.2 However, for the layered host materials, the entrance picture becomes less clearer because a twodimensional (2D) interlayer space (or interlamellar gap) is fully open in 360°.3-5 Furthermore, chemical entities such as protons may enter the hosts from a direction perpendicular to the 2D basal layers due to small atomic size and the presence of intralayer interstitials. It has been known that large molecules or even proteins (ca. 10-100 Å) can be intercalated between the basal layers with certain prediction for guest orientations.6 However, most of these studies are focused only on final intercalation structures and properties, and information on dynamical insertion is relatively lacking due to lack of suitable methods. Here we report a novel experimental method to determine hydrogen insertion direction in orthorhombic molybdenum trioxide (R-MoO3). Using controlled single-crystal growth and post growth chemical etching, we can select and engineer crystal structures with different traveling paths for guest species, which allows a direct differentiation of insertion modes (directions) of guest species in layered compounds with various ex-situ analytical techniques. Our experimental results show that the hydrogen enters the R-MoO3 preferentially along 〈001〉 direction, although the interlayer space of this layered solid is open in all directions. Experimental Section Single crystals of the orthorhombic MoO3 with smooth or stepped morphology were prepared with a high-temperature flux at the mole ratio of MoO3:Na2MoO4 ) 85:15.7 The grown single crystals were harvested by using a 1.0 M nitric acid solution to dissolve the growth solvent Na2MoO4. Typically for protonation experiments, a high-quality R-MoO3 (transparent and flawless) single crystal with a size of 10 mm (in [001]) × 3 mm (in [100]) * To whom correspondence should be addressed. Tel: +65 874 2896. Fax: +65 779 1936. E-mail:
[email protected].
Figure 1. Experimental setup and crystal orientation of the sample in the R-MoO3 to HxMoO3 reaction (input gas H2S:H2 ) 5 mol %:95 mol %, 20-30 mL/min, 1 atm). L is the propagation distance of the R-MoO3-HxMoO3 boundary.
× 0.5 mm (in [010]) was selected and cut from a large crystal with the aid of an optical microscope (Olympus BH-2). Chemical etching of the R-MoO3 single crystals was carried out at room temperature for 90 s with a 0.1 M NaOH aqueous solution (50 mL).8 The H + MoO3 reaction was carried out in a glass reactor at 152-155 °C using a H2S stream (5 mol % H2S + 95 mol % H2) at atmospheric pressure (Figure 1). The temperature of reactor was monitored with a K-type thermocouple. A small flow rate of 20-30 mL/min (controlled by a mass-flow control system, Brooks 5950) in the input stream (versus the large reactor volume ) 250 mL) was to ensure that there was no concentration gradient of H2S across the reactor. The off-gas was introduced to a ZnSO4 solution to remove unreacted H2S before it was vented into the atmosphere. Crystallographic information on samples was investigated with X-ray diffraction (XRD; Shimadzu XRD-6000, Cu KR radiation, λ ) 1.5418 Å). The surface topography of the samples was examined with an atomic force microscope (AFM; DI NanoScope MultiMode) in tapping mode to minimize the damage of the stepped surfaces by the single-crystal silicon probe. The typical scan size in this work was set as 8 µm × 8 µm. On average, AFM images from four sampling locations per sample were recorded to ensure a well representation of the surface topography. The surface analysis of reacted and unreacted samples was made with the X-ray photonelectron spectroscopy (XPS; AXIS-Hsi, Kratos Analytical) method using a monochromatized Al KR X-ray source (hν ) 1486.6 eV). The XPS spectra of all studied elements were measured with a constant analyzer-pass-energy of 20.0 eV. All binding energies (BE) were referenced to the C 1s peak (BE ) 284.7 eV) arising from adventitious carbon. Prior to peak deconvolution, X-ray satellites and inelastic background (Shirley-type) were subtracted for all spectra.
10.1021/jp010177v CCC: $20.00 © 2001 American Chemical Society Published on Web 07/06/2001
Protonation of a-MoO3
J. Phys. Chem. B, Vol. 105, No. 30, 2001 7179
Figure 2. Basic principle of the path differentiation method: bulk insertion (HA) and surface insertion (H B) of hydrogen in an etched R-MoO3 single crystal. Grey arrows indicate the two opposite insertion directions along the 〈001〉 direction; the enlarged part shows the side view of the crystal observed from the [100] direction.
TABLE 1: Propagation Distances (L, in mm) of the Boundary Line for r-MoO3 Samples Reacted at 155 °C (in H2S + H2, 30 mL/min) for Various Total Reaction Times (also see Figure 1) time (h)
0
0.5
1.0
2.0
3.0
sample 1a sample 2a sample 3a sample 4b
0 0 0 0
1.6 1.9 1.6 2.0
1.9 2.1 1.9 2.5
2.2 2.3 2.1 4.6
2.2 5.0
a Samples 1-3 were as-grown high-quality crystals (sample 2 with an additional chemical etching). b Sample 4 was an imperfect crystal (not fully transparent plus chemical etching). Samples 1 and 2 and samples 3 and 4 were reacted in pair in the same reactor, respectively.
Results and Discussion Figure 1 depicts the experimental setup used for the hydrogen insertion reaction. A long retention time of input stream (8.3 or 12.5 min) used was to minimize its concentration gradient across the reactor. The insertion reaction started from the both end of a single crystal, which was indicated by the color change from lemon yellow (R-MoO3) to light blue (HxMoO3). The boundary of the two distinct color zones, which is perpendicular to 〈001〉 directions, was clearly defined and was moving toward unreacted MoO3 side. The propagation rate, nonetheless, depends on the crystal perfection of R-MoO3. For high-quality (transparent and flawless) single crystals, L became almost constant after an initial rapid propagation, although for imperfect crystals the propagation rate was much faster, as reported in Table 1. There was no appreciable increase in propagation rate with chemical etching (sample 2 vs sample 1), as this modification was only on external surfaces (see AFM results later). The XRD analysis for the blue-colored samples reveals that the HxMoO3 belong to a C-centered orthorhombic cell with lattice parameters of a ) 3.88 Å, b ) 14.05 Å, and c ) 3.73 Å, identical to the reported data for the hydrogen molybdenum bronze phase HxMoO3 with x ) 0.23-0.40.9-11 When H2S was absent from the input stream, there was no color change in the R-MoO3 and thus no formation of HxMoO3, since the reduction of MoO3 with H2 takes place only at >350 °C.12 It is therefore concluded that the hydrogen in the R-MoO3 protonation comes from catalytic dissociation of H2S. The directional boundary propagation in the above macroscopic crystals (Table 1) reveals unambiguously that the dissociation of H2S occurred on the {001} surfaces (two ends of the crystals). Furthermore, by breaking the crystals along 〈001〉 directions, it is clearly seen that the blue color of the two ends is in bulk phase, which suggests that the dissociated hydrogen enters the interlayer space preferentially along 〈001〉.
To confirm this result, we further investigated the protonation of R-MoO3 with the microscopic path differentiation. As illustrated in Figure 2, chemical etching creates rectangular etch pits along the [100] direction (see later AFM results), which provides two types of R-MoO3 on the surface region. The first type (A, bottom of etch pits) is essentially the same as an R-MoO3 crystal with smooth (010) termination, while the second type (B, convex region among etch pits) has many chemically created {001} planes/steps on the walls of the etch pits. From this new surface microstructure, it is understood that the formation rate of HxMoO3 in regions A and B would be the same if hydrogen enters the R-MoO3 along [010] direction, since both A and B are vicinal to the walls of etch pits where the hydrogen is generated. However, significant variations in hydrogen insertion rate would be observed if hydrogen enters the crystal from 〈001〉 directions because the traveling path of hydrogen (HB) to B regions is much shorter than that (HA) to A regions. In Figure 3, two AFM images are displayed to elucidate the differentiation of hydrogen insertion modes in R-MoO3 using the surface microscopic PDM. In both samples, the rectangular etch pits can be clearly seen (with darker color); the singular double-layer steps at a height of 0.69 nm can be resolved in cross-sectional analysis.8c Our AFM study shows the formation of elongated HxMoO3 (light strips) islands protruding from the R-MoO3 base;9 the growing islands along 〈203〉 are stretching throughout the surface, regardless of etched or nonetched morphology. However, the width of an HxMoO3 island formed in regions B is much greater than that of the same island formed in regions A (bottoms of etch pits). Our cross-sectional analysis reveals that the island width in regions B is about 0.5-3.6 times greater than that in regions A (2 h, Figure 3a). In fact, some parts of the islands are even discontinued at the bottoms of etch pits. These observations indicate unambiguously that the protonation reaction is controlled by the 〈001〉 insertion but not the 〈010〉 insertion. In good agreement with this result, longer reaction leads to higher density and larger size of the islands (3h, Figure 3b). More importantly, the width difference of HxMoO3 islands over the A and B regions is reduced. The prolonged protonation actually is smoothing out the path difference of hydrogen travelling in the interlayer space, as described in Figure 2. It has been known that the hydrogen atoms of HxMoO3 (x ) 0.36) are attached to the bridging oxygen atoms.10 This fact may help to explain why hydrogen insertion does not take place along 〈100〉 directions due to lack of bridging oxygen atoms on the {100} planes of R-MoO3 (Figure 2). The formation of HxMoO3 from H2S and R-MoO3 is a redox reaction, which can be
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Figure 4. XPS spectra of Mo 3d: (a) as-grown R-MoO3 single crystal after etching, (b) R-MoO3 single crystal with H2 (20 mL/min) gas at 152 °C for 1 h, (c) unetched R-MoO3 single crystal after reacted with H2S + H2 (20 mL/min) at 152 °C for 2 h, and (d) etched R-MoO3 single crystal after reacted with H2S + H2 (20 mL/min) at 152 °C for 2 h.
described below:
x x MoO3 + H2S ) HxMoO3 + S 2 2
Figure 3. (a) AFM image of an etched R-MoO3 after 2 h reaction in H2S + H2 (20 mL/min) at 152 °C; (b) AFM image of an etched R-MoO3 after 3 h reaction in H2S + H2 (20 mL/min) at 152 °C; elongated HxMoO3 islands are along 〈203〉 directions.
(1)
Although the formal charge of molybdenum on the {100} is higher than that on the {001} (+1.5 vs +0.779),13 the ability of hydrogen bonding formation with the bridging oxygen may be crucial for some initial elementary steps (such as adsorption) in the molecular dissociation of H2S on {001} surfaces. It has been known that the molybdenum atoms on the (010) plane are the least reactive.12 Nevertheless, a recent density functional calculation reveals that hydrogen is most strongly adsorbed over the terminal oxygen on the (010) plane, followed by the bridging oxygen atoms.5 Even if atomic hydrogen can diffuse easily on a smooth (010) surface from the other surfaces/steps, the formation of surface layer of O-H is likely to hinder hydrogen from insertion in the already densest (010) surface. Our experimental evidences in Table 1 and Figure 3 indeed strongly indicate that the [010] insertion is not favorable for hydrogen. Further confirmation of the above findings was made with the XPS method. As compared in Figure 4, the binding energy of Mo 3d5/2 in the H2-treated R-MoO3 sample (232.5 eV, (b)) is identical to that of clean R-MoO3 (232.6 eV, (a)),14 confirming that there is no chemical reaction between H2 and R-MoO3 at the studied temperature.12 Reduction of R-MoO3 can be observed in all samples reacted in H2S + H2 stream. The new component
Protonation of a-MoO3 at 231.3 eV (in panels c and d) can be assigned to Mo 3d5/2 in HxMoO3, which is smaller than that of MoVI but much greater than those of MoIV, noting that the binding energies of Mo 3d5/2 for MoO2 and MoS2 are 229.3 and 229.0 eV, respectively.12,15 In excellent agreement with the AFM results (Figure 3), the significant increase in hydrogen insertion is also shown in the Mo 3d for the etched R-MoO3. On the basis of an atomic ratio analysis for the Mo 3d spectra, it is known that 16.6% of R-MoO3 on the etched surface region has been converted to HxMoO3 after 2 h reaction, compared to only 7.1% on the unetched surface (Note that as-grown surface steps are also present on “perfect” crystals7). Accordingly, the sulfur content (in atomic ratio S/Mo) is 0.27 on the etched sample surface while only 0.09 on the unetched one. The binding energy of S 2p (163.3-163.7 eV) for the sulfur produced is indeed close to its elemental form,15 as described in eq 1. By creating different traveling paths, we have shown in this work that hydrogen enters the layered host R-MoO3 preferentially in the 〈001〉 direction, although an entire interlayer space is open to this guest species (Figures 1 to 3 and Table 1). One important point in this work is that we can simultaneously investigate two different types of surface regions under exactly the same reaction conditions (Figure 3), which eliminates experimental uncertainties that might occur if they were investigated individually. For example, even though there might be a concentration gradient within the reactor, the H2S + H2 influx above two neighboring A and B regions should be considered identical because the sampling regions are only in submicrometer scale. Similarly, reaction temperature of the two neighboring regions should be the same, as the temperature gradient at the submicron level is virtually zero. Conclusion In summary, using controlled single-crystal growth and post growth chemical-etching, we can prepare different traveling paths for hydrogen to determine its insertion direction in layered compound orthorhombic molybdenum trioxide (R-MoO3). As the formed HxMoO3 islands are self-registered and the concentration and temperature gradients for the formation of these HxMoO3 islands are virtually zero among the compared crystal areas, a direct differentiation of insertion modes of the guest species in the layered compounds can be made with various ex-situ analytical techniques. On the basis of experimental results of this work, it is concluded that hydrogen preferentially
J. Phys. Chem. B, Vol. 105, No. 30, 2001 7181 enters the layered host along the 〈001〉 direction, although an entire interlayer space is open to this guest species. Acknowledgment. This work was supported by a grant for encouragement of fundamental research from the Ministry of Education, Singapore. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) (a) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950-2963 and references therein. (b) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem. Int. Ed. 1999, 38, 56-77 and references therein. (c) Thomas, J. M. Angew. Chem. Int. Ed. 1999, 38, 3588-3628 and references therein. (3) (a) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173301 and references therein. (b) Rives, V.; Ulibarri, M. A. Coordin. Chem. ReV. 1999, 181, 61-120 and references therein. (c) Zeng, H. C.; Sheu, C. W.; Hia, H. C. Chem. Mater. 1998, 10, 974-979. (d) Xu, Z. P.; Zeng, H. C. Chem. Mater. 1999, 11, 67-74. (e) Chellam, U.; Xu, Z. P.; Zeng, H. C. Chem. Mater. 2000, 12, 650-658. (4) (a) Eda, K. J. Mater. Chem. 1992, 2, 533-538. (b) Eda, K. J. Solid State Chem. 1989, 83, 292-303. (c) Wang, J.; Rose, K. C.; Lieber, C. M. J. Phys. Chem. B 1999, 103, 8405-8409. (5) Chen, M.; Waghmare, U. V.; Friend, C. M.; Kaxiras, E. J. Chem. Phys. 1998, 109, 6854-6860. (6) (a) Bissessur, R.; Degroot, D. C.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis, M. G. J. Chem. Soc. Chem. Commun. 1993, 8, 687-689. (b) Kerr, T. A.; Leroux, F.; Nazar, L. F. Chem. Mater. 1998, 10, 25882591. (c) Fogg, A. M.; Rohl, A. L.; Parkinson, G. M.; O’Hare, D. Chem. Mater. 1999, 11, 1194-1200. (d) Kumar, C. V.; Chaudhari, A. J. Am. Chem. Soc. 2000, 122, 830-837. (e) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 10206-10214. (f) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2001, 105, 1743-1749. (7) Balakumar, S.; Zeng, H. C. J. Cryst. Growth 1998, 194, 195-202. (8) (a) Zeng, H. C. J. Cryst. Growth 1998, 186, 393-402. (b) Zeng, H. C. Inorg. Chem. 1998, 37, 1967-1973. (c) Hsu, Z. Y.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 11891-11898. (9) (a) Smith, R. L.; Rohrer, G. S. J. Solid State Chem. 1996, 124, 104-115. (b) Smith, R. L.; Rohrer, G. S. J. Catal. 1998, 173, 219-228. (c) Smith, R. L.; Rohrer, G. S. J. Catal. 1998, 180, 270-278. (10) Dickens, P. G.; Crouch-Baker, S.; Weller, M. T. Solid State Ionics 1986, 18 & 19, 89-97. (11) Sotani, N.; Shimada, I.; Suzuki, T.; Eda, K.; Kunitomo, M. Solid State Ionics 1998, 113-115, 377-385. (12) Spevack, P. A.; McIntyre, N. S. J. Phys. Chem. 1992, 96, 90299035. (13) (a) Haber, J. In Molybdenum: An Outline of Its Chemistry and Uses; Braithwaite, E. R., Haber, J., Eds.; Elsevier Science: Amsterdam, 1994; Chapter 10, p 479. (b) Haber, J.; Lalik, E. Catalysis Today 1997, 33, 119-137. (14) Gu¨nther, S.; Marsi, M.; Kolmakov, A.; Kiskinova, M.; Noeske, M.; Taglauer, E.; Mestl, G.; Schubert, U. A.; Knozinger, H. J. Phys. Chem. B 1997, 101, 10004-10011. (15) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation: MN, 1992.
