Document QkYNKLOb470wB7ZMmeY3rMbJ5
Synthesis and Characterization of Nano-
Sized Hydrodesulfurization Catalyst
Mouzhgan Anjom1, Irina Dovgani1, and Devinder Mahajan1,2,*
1Stony Brook University, Stony Brook, NY 11794 2 Brookhaven National Laboratory, Upton, NY 11973
*Corresponding author: dmahajan@bnl.gov
Introduction The effect of burning transportation fuels on the
environment is now gaining global attention. In addition to CO2, sulfur is also an issue. Sulfur removal is crucial because it also poisons catalysts during petroleum refining processes. The new regulations will likely limit S to low ppm (almost zero) levels in fuels.
We previously reported synthesis of unsupported nano-phase MoS2 catalysts using sonolysis (Mahajan et al., 2004). We are now further investigating this reaction. This work explores a reliable technique for nanoparticle synthesis. For characterization, in addition to X-ray Diffraction (XRD), the Computed Microtomography (CMT) technique is used, for the first time, to show 2-D and 3-D images of the material.
Experimental An experimental apparatus was an ultrasonic liquid
processor Model XL2020, from MISONIX, Inc., with a variable power output of up to 550 W, at a frequency of 20 kHz. The actual sonolysis took place at the probe tip of 0.5 inch diameter. The entire apparatus was placed in soundabsorber containment. The sonicator was programmed. The reaction vessel was a borosilicate glass four-neck flask that was made air-tight with a series of O-rings and standard ground-glass joints during sonication. The gas evolved during sonolysis was collected and metered in an inverted measuring cylinder in a water bath. The extent of reaction could be calculated from these data.
Hexacarbonylmolebdenum (MO(CO)6) (>99% purity), sulfur powder (99.9%) and hexadecane (99%, anhydrous) were purchased from Aldrich. N2 gas was obtained from Scotts Specialty Gases. All manipulations were carried out in a well-ventilated fume hood. Mo(CO)6 (10 mmol) and 25 (or 50) mmol S were added to hexadecane solvent (70 mL). The resulting off-white slurry was degassed with N2 for 15-20 minutes. The sonication was performed in the 4-second/1-second on/off mode. The gradual appearance of a black slurry in the reaction vessel was evident within minutes indicating metal carbonyl decomposition.
))) Mo(CO)6 + 2S MoS2 + 6CO
At STP, 60 mmols of CO accounts for 1.36 L. The CO evolution was recorded as a function of time until CO volume exceeded 1.23 L which corresponded to ~90% metal carbonyl decomposition.
The slurry work-up produced air-stable black particles of MoS2. The slurry was centrifuged and the upper hexadecane solvent layer was decanted to separate the product. The remaining black solid was washed and centrifuged three times with hexane (10 ml) to remove residual hexadecane. The collected solid was dried in vacuum and then stored in a gas-tight vial to avoid any sample oxidation. Samples of the isolated solid were used for characterization.
SEM images of MoS2 were obtained using Leo 1550 Field Emission SEM unit. For Computed Microtomography (CMT) analysis, at the Beamline X-2B, National Synchroton Light Source (NSLS), Brookhaven National Laboratory (BNL), an X-ray beam with energy 15 keV is produced with a bending magnet that was used for the CMT analysis. The 5X lens used in this analysis provided 4-micron resolution. A polypropylene syringe was filled with the sample from the Mo(CO)6/S ( 1/:2.5 molar ratio) and subjected to analysis. The 1200 images were taken with 4000 msec exposure for each image at every 1.5o increment from 0 to 180o. The output file, filename.prj contained all 1200 tomographic image slices, each composed of a rectangular array of reconstructed linear attenuation coefficient values and corresponding to a specific voxel of the sample. A reconstruction of 300 slices from the assembled file in a tomogram and their conversion into a stack of jpegs was performed using IDL tomography software. The vertical axis was optimized for each reconstruction to reduce artifacts in the images.
Results and Discussion Figure 1 shows a first-order plot for Mo(CO)6
decomposition in ~50 hours in the presence of S. Two runs were conducted with varying S/Mo molar ratio of 2.5/1 and 5/1 (Figure 1).
1. XRD Data Recently published XRD patterns of 150-200 nm
and ~100 nm sized MoS2 particles revealed a characteristic 002 peak at 2 = 14.5 to be broad (Pourabbas and Jamshidi, 2008). The peaks for the 100 nm sample were broader than those completely disordered ribbon-like stacking of S-Mo-S layers confirmed with TEM.
The XRD pattern of the product derived from sonication of Mo(CO)6 and S in 2.5/1 and 5/1 molar ratios are shown in Figures 2 and 3, respectively. A broad peak at 2 = 40o is very analogous to previously presented literature on nano-sized MoS2 especially by Xu et al. (1996). The broad peak at 2 = 40o may be a combination of peaks at 2 = 32.7oand 39.5o which are assigned to the (100), and (103)
Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 2009, 54 (1), 7
lattice planes of MoS2 respectively. The peak at 2 = 58o corresponding to (110) is also broad. The widened peaks confirm the absence of MoS2 crystallites with high defect densities. The XRD pattern of the product from the 5/1 ratio of S/Mo(CO)6 appears to have broadened effects of all peaks at 32.7o, 39.5o, and 58o.
0.1 0.01
1:2.5 Molar ratio Mo(CO)6 and S 1:5 Molar ratio of Mo(CO)6 and S
ln[Mo(CO)6]
0.001
0.0001 0
600
1200
1800
2400
3000
Time(min)
Fi
gure 1. A first-order plot for sonolysis-assisted Mo(CO)6 decomposition with sulfur in hexadecane at T ~50oC, using
pulsed sonication.
2. Scanning Electron Microscopy (SEM) and EDAX
Elemental Analysis Figure 4 shows the SEM image, at 100 KX
magnification, of MoS2 that was obtained from Mo(CO)6 and S in a 1:2.5 molar ratio. The product appears porous with dark and light areas representing heavy and light portions of the elements, respectively. The cluster-like nature of the MoS2 is also obvious. The particles are not distinguishable, possibly due to their small size.
The EDAX elemental analysis showed a strong peak between 2.0-2.5, corresponding to S and Mo peaks (Figure 5). The peaks at 0.2, 0.3, 0.5 corresponding to Mo, C, and O, respectively indicate the presence of residual CO, unreacted Mo(CO)6 and an oxidized product.
Figure 4. SEM of the product from the Mo(CO)6 and S (1/2.5 molar ratio) reaction.
Figure 2. XRD pattern of MoS2 particles derived from sonication of Mo(CO)6 and S in a 1:2.5 molar ratio
Figure 3. XRD pattern of MoS2 particles derived from sonication of Mo(CO)6 and S in a 1:5 molar ratio
Figure 5. EDAX analysis of the product from the Mo(CO)6 and S (1/2.5 molar ratio) reaction.
Figure 6 shows the SEM image of the MoS2, obtained from the Mo(CO)6/S (1/5 molar ratio) reaction, at 100 KX magnification. The image shows the same pattern of cluster-like porous material with dark and light areas.
The EDAX pattern (Figure 7), however, is somewhat different. There are noticeably smaller peaks for C and O, compared to the previous sample. This could indicate that increasing the concentration of S enhances the conversion of Mo(CO)6.
Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem. 2009, 54 (1), 8
Figure 6. SEM of the product from the Mo(CO)6 and S (1/5 molar ratio) reaction.
Figure 8. An X-ray CMT image of a sample from the Mo(CO)6/S (1/2.5 molar ratio) reaction.
Figure 7. EDAX analysis of product from the Mo(CO)6/S (1/5 molar ratio) reaction.
3. X-ray Computed Microtomography (CMT) Analysis Figure 8 shows an X-ray CMT image of the sample
taken with 4000 msec exposure. All 1200 angular images were reconstructed to get horizontal cross-section shown in Figure 9. A 3-D volume (not shown here) was created from the stack of images using a volume rendering software, Drishti (Limye, 2006). The 2-D image shows an uneven size of the product. The absence of histogram peak corresponding to air within sample may be due to nano-sized particles resulting into even smaller air pathways to get detected in the X-ray beam. The CMT 2-D and 3-D images confirmed that the synthesized particles of MoS2 are indeed nano-sized.
Figure 9. Reconstructed 2-D cross-section (7mm diameter) of sample from Figure 8.
6. Conclusions The sonolysis product of Mo(CO)6 and S in both 1/2.5 and 1/5 molar ratios, exhibited cluster-like aggregates without distinguishable nano-particles. The EDAX analysis of both products confirmed the presence of Mo, S, C and O in variable concentrations. Increasing the sulfur concentration enhanced Mo(CO)6 conversion. The XRD pattern of the product showed a broad peak at 2 = 40o which may be a combination of peaks at 2 = 32.7oand 39.5o, which are assigned to the (100) and (103) lattice planes of MoS2, respectively. Moreover, another broad peak at 2 = 58o corresponding to (110) and the CMT 2-D and 3-D images confirmed the synthesis of nano-sized particles and the absence of MoS2 crystallites.
Acknowledgements This work was performed at Brookhaven National Laboratory under contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. ID thanks the Summer Undergraduate Laboratory Internship (SULI) program, administered by the Office of Science, U.S. Department of Energy, for summer fellowship. We also thank Mr. Prasad Kerkar for the CMT analysis of the samples.
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References 1. Li, Q., Newberg, J. et. al., Polycrystalline Molybdenum Disulfide (2H-MoS2) Nano- and Microribbons by Electrochemical/Chemical Synthesis, Materials Research Bulletin, 43 (2008) pp. 2427-2433 2. Limaye, A., Drishti Volume Exploration and Presentation Tool, Poster Presentation, Vis 2006, Baltimore, USA. 3. Mahajan, D., Ch. L. Marshall, Sono synthesis and characterization of nano-phase molybdenum-based materials for catalytic hydrodesulphurization, Applied Catalysis A: General, 258, pp. 83-91, 2004. 4. OSHA Technical Manual, Chapter 2, Petroleum Refining Processes, 2003 5. Pourabbas and Jamshidi, et. al. Preparation of MoS2 nanoparticles by a modified hydrothermal method and the photo-catalytic activity of MoS2/TiO2 hybrids in photo-oxidation of phenol, Chemical Engineering Journal, 138 (2008) pp. 55-62 6. Xu, T. et al., Study on the Structure of SurfaceModified MoS2 Nanoparticles, Material Research Bulletin, 31, 4, pp. 345-349, 1996.
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