- Email: [email protected]
Sonolysis induced decomposition of metal carbonyls: kinetics and product characterization
Ultrasonics Sonochemistry 11 (2004) 385–392 www.elsevier.com/locate/ultsonch
Sonolysis induced decomposition of metal carbonyls: kinetics and product characterization Devinder Mahajan a
a,b,*
, Elizabeth T. Papish
a,c
, Kaumudi Pandya
d
Department of Energy Sciences and Technology, Brookhaven National Laboratory, Building 815, Upton, NY 11973-5000, USA b Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794-2275, USA c Department of Chemistry, Salisbury University, Salisbury, MD 21801, USA d SFA, Advanced Technology Division, Largo, MD 20774, USA Received 20 January 2003; accepted 10 October 2003 Available online 5 December 2003
Abstract The decomposition kinetics of Fe(CO)5 and Mo(CO)6 induced by sonolysis in hexadecane solvent was studied as a function of temperature (303–343 K) under an inert atmosphere. The decomposition data, obtained over at least two half lives in most of the runs, yielded first-order rate constant (k) values with correlation co-efficient ðR2 Þ > 0:95. The products were characterized by various spectroscopic techniques. The transmission electron microscopy (TEM) yielded images from which the mean particle diameter (MPD) of 10 nm for Fe and <3 nm for Mo were estimated. The generation of amorphous Fe and semi-crystalline Mo particles was determined from line broadening and corresponding d-spacing values in the X-ray diffraction (XRD) spectra. The XAFS/XANES data were consistent with the production of Fe(0) metal but carbided Mo (Mo2 C). The one-step production of high-yield pyrophoric products demonstrated the applicability of sonolysis to effectively produce gram-quantity of zero-valent metals. 2003 Elsevier B.V. All rights reserved. Keywords: Nano synthesis; Sonolysis; Metal carbonyls; Decomposition kinetics
1. Introduction Nano technology is now of widespread interest with potential applications ranging from semi-conductors to direct coal liquefaction [1]. Our interest in nano metals stems from the availability of enhanced surface area that may enhance rates during catalytic reactions. An example of such an effect has been described for indirect methane conversion via slurry-phase Fischer–Fropsch (F–T) synthesis catalyzed by unsupported a-Fe2 O3 particles with mean particle diameter (MPD) of <100 nm [2]. Present methods to synthesize nano-sized metal particles include simple grinding that yields a wide range of irreproducible particle sizes and chemical va*
Corresponding author. Address: Department of Energy Sciences and Technology, Brookhaven National Laboratory, Building 815, Upton, NY 11973-5000, USA. Tel.: +1-631-344-4985; fax: +1-631-3447905. E-mail address: [email protected] (D. Mahajan). 1350-4177/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2003.10.009
por deposition (CVD) that yields better particle-size control [3]. Selective synthesis of nano-sized zero valent metals and metal carbides have been reported [4]. The use of acoustics initially developed by Suslick [5,6] is now being used extensively mainly to synthesize nano metal oxides whose applications range from coatings to magnetic fluids [7–9]. The sonolysis technique involves passing sound waves of fixed frequency through a slurry or solution of carefully selected metal complex precursors. In a solvent with vapor pressure of certain threshold, the alternating waves of expansion and compression cause cavities to form, grow and implode [10]. During this event, the attained temperature and pressure were calculated to be about 5200 K and 30 MPa, respectively with lifetime of 2 ls to <1 ns [11]. Due to extremely high cavity collapse rates (2 · 109 K s1 –1013 K s1 ), the sound energy translates into sonoluminescence [12]. But if a suitable metal complex is present, the energy can be harnessed to break metal– ligand bonds [5].
386
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
The use of metal carbonyl precursors to synthesize a mixture of metal clusters and nanometer metal particles [6], preparation of iron colloids [13] and molybdenum carbide [14] has been reported. In studies reported so far [5,7–9], sonolysis is typically carried out for about 3 h and the small amounts of synthesized products are separated from a large quantity of unreacted starting metal carbonyl and then purified. The scale-up issues in sonochemical synthesis have been identified [15]. Our interest is in developing the sonolysis technique for convenient synthesis of high-purity nano-sized metals for catalytic applications. We view the cavity produced during sonolysis as a transient high-pressure and hightemperature microvessel in which high-energy intermediates can be generated and subsequently stabilized by further complexation. Therefore, the emphasis of this study was to establish the effect of longer sonication periods on product quality and particle size and to evaluate the potential of producing quantitative yields of nano-sized zero valent metals that could be used without further purification. This focus led us to conduct kinetic and product characterization studies that are reported in this paper.
2. Experimental 2.1. Materials Hexacarbonylmolybdenum (98% pure) pentacarbonyliron (99.5%), hexadecane (99+%, anhydrous), hexanes (98.5+%, ACS reagent grade) were purchased from Aldrich Chemical Co. CO, H2 , CO2 , Ar, N2 gases were obtained from Scott specialty gases. Gases were passed through a Deoxo purifier for further oxygen removal before use. Since Mo(CO)6 is a toxic solid and Fe(CO)5 is toxic and flammable liquid, all manipulations were carried-out in a fume hood with appropriate precautions. 2.2. Sonolysis unit All experiments were done using an ultrasonic liquid processor Model XL2020, from MISONIX, Inc., with a variable power output of up to 550 W at a fixed frequency of 20 kHz. The unit was fitted with a 5-in. long 00 half wave extender tip with a probe tip of diameter 0.5 . The unit allowed precise control of power output, processing time and PULSAR cycle for cyclic intermittent operation to avoid heat build-up. 2.3. Synthesis of Fe and Mo nano particles For the work described herein, the following aspects need to be emphasized. (1) The reaction vessel, purchased from Ace Glass, Inc., was a borosilicate glass 4-
neck flask with walls tapered inward toward bottom that allowed maximum solution in the middle of the flask for adequate immersion of the sonication probe. (2) A series of O-rings and standard greased ground-glass joints ensured tight seals to maintain rigorous exclusion of air or gas leakage from the flask during sonication. (3) Any gas evolved during sonolysis was collected and analyzed and (4) The flask was immersed in a constant temperature (held within ±1 C) bath. Prior to sonication, the hexadecane solvent was thoroughly degassed with argon followed by the addition of metal carbonyl. In a typical run of the Fe System, a degassed yellow homogeneous solution of Fe(CO)5 (8–16 mmol) in 100 ml hexadecane was sonicated in the dark (Fe(CO)5 is light sensitive) at 100% intensity and 80% pulsed cycle settings. Gas evolution with concomitant appearance of black slurry in the reaction vessel was evident within minutes that were indicative of Fe(CO)5 decomposition. The decomposition of Fe(CO)5 was quantified by monitoring the gas evolution as a function of time. The collected gas was analyzed (see below Section 2.4) and the flask containing the black slurry was moved to an argon-filled glove box. The product work-up was as follows. The black slurry was centrifuged and the upper hexadecane solvent layer was decanted to separate the product. The remaining black solid was washed three times with hexane (3 · 10 ml) to remove any residual hexadecane. The resulting black solid was dried in vacuo and the dried solid was stored in a gas-tight vial in the glove box to avoid any sample oxidation. The yield of the dried black product varied between 71% and 87% for the completed runs. For example, the yield from sonolysis of 16 mmol Fe(CO)5 in hexadecane at 327 K was 0.78 g (87%). The Mo particles were prepared as follows. Mo(CO)6 was partial soluble in hexadecane and yielded a white slurry on addition of 4–8 mmol Mo(CO)6 in 100 ml hexadecane (1.05–2.10 wt.% slurry). In this case, a noteworthy observation was the appearance of a white solid along the walls of the flask above the solvent layer during sonolysis that was attributed to the sublimed Mo(CO)6 . In this case, the product work-up involved the same step sequence as shown for Fe(CO)5 . The yield of the black product varied between 51% and 93% for the completed runs. For example, the yield from sonolysis of 8 mmol Mo(CO)6 in hexadecane at 323 K was 0.66 g (86%). 2.4. Analytical The gases collected during sonolysis were analyzed on Gow-Mac Model 580 gas chromatographs: H2 on a 00 molecular sieve column (80 · 1/4 ) with N2 carrier gas; 00 00 CO on a molecular sieve column (9 · 1/8 ) under He; 00 0 CO2 on Carboxen-1000 (5 · 1/8 ) under He. The prepared black solid samples were subjected to spectroscopic analysis for characterization. The infrared
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
387
(IR) spectra were recorded on an ATI Mattson FTIR spectrophotometer. For the characterization of solids, a Phillips Vertical Goniometer and CuK radiation for X-ray powder diffraction (XRD) mea(k ¼ 1:54 A) surements was used. For the XRD sample preparation, a copper holey carbon grid was dipped in the hexanemetal particle slurry samples of Fe or Mo and the hexane were quickly evaporated leaving just the metal particles on the grid for analysis. The X-ray diffraction data were analyzed with a commercial Theta Software package available from Dapple Systems of Sunnyvale, California. The transmission electron microscopy (TEM) images were recorded on a JEOL 2000FX, 200 KV model to determine the particle sizes.
Table 1 Effect of reaction parameters during Fe(CO)5 decomposition in hexadecane via sonolysisa
2.5. XAFS measurements
Fe(CO)5 decomposition during the sonolysis run. A slow CO evolution was still continuing when the reaction was quenched suggesting <100% decomposition. This is in agreement with the infrared data of the final solution in which small IR bands observed at 2020 and 1996 cm1 were assigned to the unreacted Fe(CO)5 [16]. These data suggest an excellent mass balance for decomposition of Fe(CO)5 to Fe metal particles. Gas chromatographic analysis of the collected gas (CO > 99%; CH4 ¼ 0%; CO2 < 0.2%) confirmed the absence of any side reactions. The CO evolution versus time data for the disappearance of Fe(CO)5 were plotted. Fig. 1 shows the data from one of these runs (Run 4 in Table 1) that were analyzed over three half-lives to obtain the first-order rate constant (k) of 2.1 · 106 s1 at 341 K. A similar analysis yielded first-order plots with excellent correlation coefficient (R2 > 0:95) for all other runs listed in Table 1. For Runs 1–4, the constant temperature bath settings were 278, 288, 303 and 323 K respectively during the sonolysis reaction. Table 1 lists measured temperature values inside the flask that were 18–28 C higher than the setting on the bath. It appears that the solvent used in each of these runs was unable to quickly dissipate the reaction heat that was generated at the ‘‘localized hot spot’’ during sonolysis. Therefore, the measured solution temperatures shown in Table 1 may represent values specific to the reaction conditions.
The XAFS measurements were carried out on beamline X-11A of the National Synchrotron Light Source, Brookhaven National Laboratory, using a Si(1 1 1) double crystal monochromator. The storage ring was operated at 2.8 GeV and the typical ring current was 200 mA. The monochromator was detuned appropriately to reject higher harmonic present in the monochromatic beam. The beam intensities were monitored by three gas flow ionization chambers. The gas mixture in the first chamber was selected to absorb about 10% of the incident beam and the gas mixture in the second and the third chambers was selected to absorb about 50% of the transmitted beam. For XAFS measurements special air-tight sample holders with Xray transparent Mylar Windows were used. The samples were prepared inside a glove box and the XFAS measurements were carried out within an hour of preparation. An adequate precaution was taken to avoid air exposure of the samples during transfer to the beamline. The Si(1 1 1) monochromator was calibrated using a 7.5 lm thick Fe foil and a 15 lm thick Mo foil. For XAFS measurements a sample was placed between the first and the second ion chambers and a reference foil (Fe or Mo) was placed between the second and the third ion chambers to monitor the monochromator calibration.
Run
Fe(CO)5 (M)
T (K)
Intensityb (%)
Fe yieldc (%)
k 106 (s1Þ
1 2 3 4 5 6
0.08 0.08 0.08 0.08 0.08 0.16
303 316 326 341 315 327
100 100 100 100 50 100
80 78 85 83 71 87
1.7 2.6 2.9 2.1 1.1 3.4
a
All runs were done under an argon atmosphere. At 100% intensity, the sonicator power was 550 W. c The yield is calculated from the collected CO via the stoichiometry Fe/CO ¼ 1/5. b
3. Results and discussion 3.1. Decomposition kinetics of carbonyls of Fe and Mo The Fe(CO)5 decomposition reaction data are shown in Table 1. By using the stoichiometry of 5 mol CO per mol of Fe(CO)5 , the %Fe yield column in Table 1 refers to values that were calculated by measuring the total CO evolution during sonication. The Fe yield varied from 71% to 87%. These values were not corrected for any Fe(CO)5 losses due to aerosoling or an incomplete
Fig. 1. A first-order plot for the Fe(CO)5 decomposition in hexadecane. Run conditions: 0.08 M Fe(CO)5 ; solvent: hexadecane; T ¼ 68 C; sonicator intensity: 100% (550 W); 80% pulsed cycle.
388
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
Since the data of Runs 1–4 are statistically scattered, no Arrhenius plot was generated and the Eact value could not be extracted from these data. The varied temperature range of 303–341 K that represent small temperature perturbations (3–7%) of the reaction medium, compared to the extremely high transient temperature (>5000 K) within the ‘‘hot spots or bubbles’’ generated during sonolysis [11], are expected to have a minimal effect on rate. The effect of intensity as it directly relates to the energy input is evident with k decreasing almost linearly (2.6 · 106 s1 in Run 2 versus 1.1 · 106 s1 in Run 5). Doubling the Fe(CO)5 concentration, k was essentially constant (Run 6 versus Run 3) confirming the validity of the first-order analysis. Suslick et al. initially reported the sonolysis of Fe(CO)5 in decalin to yield a mixture of dodecacarbonyl triiron and iron metal with a first-order rate constant, k ¼ 13:4 104 s1 at 21 C [6]. To explain the observed mixture of products, a mechanism was put forward that initially involved CO dissociation followed by secondary reactions with Fe(CO)5 to generate Fe3 (CO)12 and Fe via the intermediacy of Fe(CO)3 , Fe(CO)4 /Fe2 (CO)8 species (Eqs. (1) and (2)): FeðCOÞ5 ) FeðCOÞ5n þ nCO FeðCOÞ5n ) Fex ðCOÞy þ Fe
ðn ¼ 1–5Þ
ð1Þ
ðx ¼ 1–3; y ¼ 3–12Þ: ð2Þ
A sonolysis study involving the effect of the nature of hydrocarbon solvent on rate has been reported for the Fe(CO)5 system for which the rate constants were measured to be 8.0, 5.5, 5.1 and 0.7 · 102 min1 at 25 C in decalin, decane, nonane and octane solvents respectively [6]. The hexadecane solvent used in the present study falls between decalin and decane with respect to solvent vapor pressure. It is, therefore, reasonable to assume that in decalin and hexadecane solvents, rates should be within ±30%. Our measured k values in Table 1 are approximately three-orders of magnitude lower than those reported in Ref. [6]. A more likely explanation relates to the nature of the measured k values. While conducting the present study, a complete Fe(CO)5 decomposition was targeted to establish the formation of Fe metal particles as the sole product (mol CO@5 · mol Fe(CO)5 ). Thus, we conclude that the k values in Table 1 represent either the rate determining step or the composite rate constant for the production of Fe derived from the rate law: Rate ¼ k½FeðCOÞ5
ð3Þ
The exclusive formation of Fe metal in our study (see later discussion) has been confirmed via the IR data of the final solutions from Runs 1–6 that showed the absence of Fe3 (CO)12 or other potential iron-carbonyl intermediates. In comparison, the k values reported in Ref. [6] were derived from a study in which no infor-
mation was provided on the extent of the Fe(CO)5 decomposition reaction. It is inferred that the larger k values in Ref. [6]: (1) refer to rate constants corresponding to the summation of different reaction products and (2) are based on the first 2–3 h of the Fe(CO)5 decomposition reaction that may be overestimated. Our study reveals that the intermediate Fe-carbonyl fragments formed during reaction 1 fast dissociate to Fe metal particles because no such intermediates were observed in the IR spectra. Therefore, the Fe(CO)5 decomposition values reported in Table 1 are reliable. A kinetic study was also conducted with Mo(CO)6 to generate nanometer particles of Mo. The Mo(CO)6 offwhite powder was only slightly soluble in hexadecane and resulted in a slurry. The Mo yields were calculated from the actual CO collected via the stoichiometry Mo/ CO ¼ 1/6. For all six runs, the data were plotted to fit the first-order dependence and the k values, derived from the slopes, are listed in Table 2. In the temperature dependence study (Runs 1–4), the k values varied by as much as a factor of four but not in any particular order. This may be attributed to an increase in solubility as the temperature was increased from 305 to 341 K. Other reaction parameter variation runs showed close to a linear dependence on intensity (Run 2 versus Runs 5). Doubling the Mo concentration, k was essentially unchanged as expected for a first-order reaction (Run 3 versus Run 6). A specific characteristic of the Mo(CO)6 sonolysis runs was the varying degree of deposition of a white solid along the walls of the reaction flask. The white sublined crystalline solid was confirmed to be pure Mo(CO)6 (heat of sublimation ¼ 73.6 kJ mol1 ) by the characteristic Mo–CO band at 1980 cm1 in the IR spectrum. The sublimation phenomenon decreases the solution concentration of Mo(CO)6 that may contribute to a reduction in the decomposition rate. As with Fe(CO)5 , the absence of any unreacted Mo(CO)6 in the final black solid generated in the Mo(CO)6 /hexadecane system was established via infrared. The exclusive formation of Mo particles was inferred. Mechanistically, as Table 2 Effect of reaction parameters during Mo(CO)6 decomposition in hexadecane via sonolysisa Run
Mo(CO)6 (wt.%)
T (K)
Intensityb (%)
Mo yieldc (%)
k 107 (s1 )
1 2 3 4 5 6
1.06 1.06 1.06 1.06 1.06 2.12
305 313 325 341 313 323
100 100 100 100 50 100
51 83 93 65 45 86
1.6 4.6 5.8 3.1 1.7 5.0
a
All runs were done under an argon atmosphere. At 100% intensity, the sonicator power was 550 W. c The yield was calculated from the collected CO via the stoichiometry Mo/CO ¼ 1/6. b
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
discussed for the Fe(CO)5 /hexadecane system, the kinetic data are consistent with an initial slow CO dissociation from Mo(CO)6 followed by fast dissociation of CO from intermediate species to generate Mo metal. The k values in Table 2 thus are consistent with a rate equation that is similar to Eq. (3) in which Mo(CO)6 replaces Fe(CO)5 . A comparison of the decomposition data in Tables 1 and 2 shows a general trend of a slower Mo(CO)6 decomposition when compared to the corresponding Fe(CO)5 run. Specifically, the k values were higher by at least a factor of five for Fe(CO)5 . There are several plausible explanations for this trend. First, unlike the liquid Fe(CO)5 and hexadecane that are miscible, Mo(CO)6 is only slightly soluble in the solvent. Second, in agreement with the data in Ref. [6], the rate difference is also attributed to the higher strength of the Mo–CO bond (150.3 kJ mol1 ) in Mo(CO)6 compared to the Fe– CO bond (116.0 kJ mol1 ) in Fe(CO)5 [17]. The products isolated from both the Fe(CO)5 and Mo(CO)6 sonolysis runs were characterized. As discussed in the Section 2, the pyrophoric nature of these
Fig. 2. TEM data for Fe (top) and Mo (bottom) particle samples obtained by sonolysis of Fe(CO)5 and Mo(CO)6 respectively. The samples were prepared under run conditions shown in Fig. 1.
389
Fig. 3. XRD spectra of prepared (a) Fe and (b) Mo particle samples. The arrows in (b) indicate characteristic peaks expected for Mo(0).
black solids dictated that these materials be handled with extreme care in the glove box. Fig. 2 (top) shows the electron micrograph and microdiffraction pattern for the Fe particles produced by sonolysis. The sample contains particles of diameters 10 nm size with several distinct diffraction rings. The TEM micrograph and the corresponding XRD spectrum (Fig. 3a) are consistent with an amorphous nature of this material. These results match closely with those reported in the literature in which the initially produced amorphous Fe powder was subsequently heated at 350 C for 6 h under N2 to induce crystallite formation that was identified as a-Fe metal by XRD [11]. We did not subject the synthesized Fe sample to such heat treatment because our emphasis was on the synthesis of an amorphous, zerovalent material. It is to be noted that the Fe nano-sized particles (MPD10 nm) produced by sonolysis are smaller than those prepared by the thermal decomposition of Fe(CO)5 (MPD30 nm) reported earlier by our group [18]. A similar analysis of a sample of Mo particles was also conducted. The Mo sample shows much smaller particles of size less than 3nm that are discrete compared to those observed with the Fe sample and a corresponding much broader diffraction ring pattern (Fig. 2 (bottom)). Consistent with the TEM data, the XRD measurement displayed several diffuse diffraction peaks that were further analyzed (Fig. 3b). The five peaks of the Mo sample with d-spacing of 3.20, 2.49, 2.22, 2.14 were identified. The relevant known Mo and 1.54 A compounds show intense signature peak as follows: for Mo metal, [0 2 1] at 3.26 A for [1 1 0] at 2.225 A for Mo2 C. Given the MoO3 and [1 1 1] at 2.38 A broadness of the peaks in Fig. 3b, the phases of the prepared Mo material could be described as Mo metal with some molybdenum oxide impurity. An Mo-based
390
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
material prepared by sonolysis of Mo(CO)6 in hexadecane at 90 C has been previously reported as molybdenum carbide (Mo2 C) [14]. The carbide structure was based on the d spacing from the XRD pattern. values of 2.39, 1.49 and 1.27 A A closer look at the literature method of sample preparation prior to XRD revealed that the initial sample prepared by sonolysis was first heated at 100 C under vacuum followed by another heat treatment at 450 C under He for 12 h [14]. Such heat treatments could potentially carbide Mo with residual solvent serving as the source of carbon. In the present study, no such thermal treatment was given to the samples whose spectra are shown in Figs. 2 and 3. Therefore, the XAFS spectra were measured to further confirm the structure of both black nano-sized Fe and Mo products.
Fig. 5. Derivative of XANES spectra of Fe foil ( ) and prepared Fe sample (–).
3.2. Fe sample The normalized XANES spectra and the corresponding derivative spectra of an Fe foil and Fe particles obtained from sonication of Fe(CO)5 are shown in Fig. 4. The shoulder around 7112 eV in the XANES spectra is due to an excitation of 1s electron to the empty d band orbital below the vacuum level. The first inflection point in the derivative spectra of Fe foil was assigned an energy value of 7112 eV (Fig. 5). The intensity of this peak is lower for the Fe(0) sample as compared to that for the Fe foil. For the Fe(0) sample, the edge position and hence the oxidation state, obtained form the first inflection point, is the same as that for the Fe foil indicating that the Fe is in zero valence state. However, the main edge is shifted by about 3 eV indicating that a fraction of Fe is in a higher oxidation state. In order to determine the local structure of Fe, EXAFS analysis was carried out. Fig. 6 shows the radial structure functions (RSF) for the prepared Fe sample The absence of any with a wide peak around 2 A.
Fig. 6. Radial structure function (RSF) of prepared Fe sample.
structure beyond this peak shows that the sample is highly amorphous and that there is no order beyond 3 Further EXAFS analysis shows that this peak corA. responds to two or possibly 3 shells and that the major contribution is from a low-Z scatterer such as C or O. This peak was further analyzed using Fe–C (Ferrocene reference compound) and Fe–Fe (Fe foil, FEFF calculations) interactions as models. These data show that on average each Fe is surrounded by three carbon or oxy and 1 Fe at 2.57(0.03) A. The gen atoms at 2.05(0.01) A data imply that the prepared Fe product has a major contribution from Fe(0) metal and an oxide impurity but some Fe may be carbided. Further modeling as well as XANES analysis is in progress. 3.3. Mo sample
Fig. 4. Normalized XANES spectra of Fe foil ( ) and prepared Fe sample (–).
The normalized XANES spectra for Mo(0) sample, Mo foil and Mo carbide are shown in Fig. 7. The edge position for Mo foil and Mo Carbide is 20 000 eV. The
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
391
gesting that the prepared Mo sample is nano-sized and mostly carbided. The XRD and EXAFS/XANES data of both synthesized Fe and Mo products in Figs. 3–8 are consistent with the presence of reduced Fe and Mo species with minor contribution (<5%) from oxidized species. We, therefore, conclude that it is possible to prepare highpurity Fe and Mo materials by sonolysis method.
4. Conclusions
Fig. 7. Normalized XANES spectra of prepared Mo sample (–), Mo foil ( ) and Mo2 C (–).
edge position for the prepared Mo(0) sample is shifted by +3 eV. The RSFs for the Mo sample and Mo carbide are shown in Fig. 8. The sample shows a broad peak indicating that the Mo product is not fully around 1.8 A crystalline. These data are consistent with the broad XRD bands in Fig. 3 that were attributed to semicrystalline Mo product. Preliminary EXAFS analysis shows significant contribution to the first peak from the with some Mo-O interaction. Mo–C interaction (2.10 A) for A previous study reported XAFS parameters (2.00 A Mo–C; 2.82 A for Mo–Mo; 4.10 A for Mo–Mo) of a Mo/ZSM-5 material [19]. These distances were shorter and the calculated coordination numbers were much lower than the structural parameters deduced from XAFS for bulk b-Mo2 C that show three C atoms at 2.09 and 12 Mo atoms at 2.96 A [20]. The Mo in the Mo/ A ZSM-5 material was shown to contain a highly dispersed Mo2 C phase within the zeolite cavity [19]. The XAFS and XANES spectra in Figs. 7 and 8 closely match those reported for the Mo/ZSM-5 material sug-
Sonolysis is a versatile technique for one-step generation of nano-sized Fe and Mo from the corresponding metal carbonyls. The first-order decomposition kinetic rate constants follow the order: Fe(CO)5 > Mo(CO)6 . The XRD data together with the XAFS/XANES data establish the amorphous nature of Fe and semicrystalline Mo and show that Fe(0) metal and carbided Mo (Mo2 C) are produced under sonolysis conditions. The spectroscopic data show that longer sonolysis periods are not detrimental to the production of nano metal particles and zero-valent nano metals can be conveniently produced in high purity (>90%) and in gram quantities. The TEM data show that the produced Mo2 C particles (<3 nm) are smaller than the Fe particles (10 nm). Nano Fe particles produced by the sonolysis technique are smaller compared to those generated via thermal decomposition of Fe(CO)5 (<30 nm) [18]. The sonolysis method is presently being explored as a pathway to synthesize polyethylene glycol complexed nano metals for application as nano coatings.
Acknowledgements ETP thanks the BNL Office of Educational Program for selection through the Science and Engineering Research Semester (SERS) Program. The authors are grateful to Mr. Robert L. Sabatini for TEM analysis. KP thanks the Office of Naval Research for support of the X 11A beamline at the National Synchrotron Light Source (NSLS) at BNL. This work was supported by the U.S. Department of Energy under contract no. DE-AC02-98CH10886.
References
Fig. 8. RSFs of Mo2 C ( ) and prepared Mo sample (–).
[1] D.B. Dadyburjor, A.H. Stiller, C.D. Stinespring, A. Chadha, D. Tian, S.B. Martin Jr., S. Agarwal, in: W.R. Moser (Ed.), Advanced Catalysts and Nanostructured Materials, Modern Synthetic Methods, Academie Press, New York, 1996, Chapter 22. [2] D. Mahajan, A. Kobayashi, N. Gupta, J. Chem. Soc. Chem. Commun. (1994) 795; H. Itoh, E. Kikuchi, Appl. Catal. 67 (1990) 1.
392
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
[3] P. Serp, R. Fuerer, R. Morancho, P. Kalck, J.-C. Daran, J. Vaisserman, J. Organomet. Chem. 498 (1995) 41. [4] J. Zeng, M.J. Hampden-Smith, Chem. Mater. 4 (1992) 968; H. B€ onnemann, W. Brijoux, T. Joussen, Angew. Chem. Int. Ed. Engl. 29 (3) (1990) 273. [5] K.S. Suslick, T. Hyeon, M. Fang, A.A. Cichowlas, Chapter 8 in Ref. [1]. [6] K.S. Suslick, J.W. Goodale, P.F. Schubert, H.H. Wang, J. Am. Chem. Soc. 105 (1983) 5781. [7] K.V.P.M. Shafi, A. Ulman, X. Yan, N.-L. Yang, C. Estournes, H. White, M. Rafailovich, Langmuir 17 (2001) 5093. [8] K.V.P.M. Shafi, S. Wizel, T. Prozorov, A. Gadenken, Thin Solid Films 318 (1998) 38. [9] G. Kataby, R. Prozorov, A. Gadenken, Nanostruct. Mater. 12 (1999) 421. [10] K.S. Suslick, MRS Bull. (1995) 29.
[11] K.S. Suslick, S.-B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414. [12] K.S. Suslick, E.B. Flint, M.W. Grinstaff, K.A. Kemper, J Phys. Chem. 97 (1993) 3098. [13] K.S. Suslick, M. Fang, T. Hyeon, J. Am. Chem. Soc. 118 (1996) 11960. [14] T. Hyeon, M. Fang, K.S. Suslick, J. Am. Chem. Soc. 118 (1996) 5492. [15] T.J. Mason, Ultrason. Sonochem. 7 (2000) 145. [16] R.B. King, C.C. Frazier, R.M. Hanes, A.D. King Jr, J. Am. Chem. Soc. 100 (1978) 2925. [17] F.A. Cotton, A.K. Fischer, G. Wilkinson, J. Am. Chem. Soc. 81 (1959) 800. [18] D. Mahajan, P. Vijayaraghavan, Fuel 78 (1999) 93. [19] J.-Z. Zhang, M.A. Long, R.S. Howe, Catal. Today 44 (1998) 293. [20] J.S. Lee, L. Volpe, F.H. Ribeiro, M. Boudart, J. Catal. 112 (1988) 44.
Sonolysis induced decomposition of metal carbonyls: kinetics and product characterization Devinder Mahajan a
a,b,*
, Elizabeth T. Papish
a,c
, Kaumudi Pandya
d
Department of Energy Sciences and Technology, Brookhaven National Laboratory, Building 815, Upton, NY 11973-5000, USA b Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794-2275, USA c Department of Chemistry, Salisbury University, Salisbury, MD 21801, USA d SFA, Advanced Technology Division, Largo, MD 20774, USA Received 20 January 2003; accepted 10 October 2003 Available online 5 December 2003
Abstract The decomposition kinetics of Fe(CO)5 and Mo(CO)6 induced by sonolysis in hexadecane solvent was studied as a function of temperature (303–343 K) under an inert atmosphere. The decomposition data, obtained over at least two half lives in most of the runs, yielded first-order rate constant (k) values with correlation co-efficient ðR2 Þ > 0:95. The products were characterized by various spectroscopic techniques. The transmission electron microscopy (TEM) yielded images from which the mean particle diameter (MPD) of 10 nm for Fe and <3 nm for Mo were estimated. The generation of amorphous Fe and semi-crystalline Mo particles was determined from line broadening and corresponding d-spacing values in the X-ray diffraction (XRD) spectra. The XAFS/XANES data were consistent with the production of Fe(0) metal but carbided Mo (Mo2 C). The one-step production of high-yield pyrophoric products demonstrated the applicability of sonolysis to effectively produce gram-quantity of zero-valent metals. 2003 Elsevier B.V. All rights reserved. Keywords: Nano synthesis; Sonolysis; Metal carbonyls; Decomposition kinetics
1. Introduction Nano technology is now of widespread interest with potential applications ranging from semi-conductors to direct coal liquefaction [1]. Our interest in nano metals stems from the availability of enhanced surface area that may enhance rates during catalytic reactions. An example of such an effect has been described for indirect methane conversion via slurry-phase Fischer–Fropsch (F–T) synthesis catalyzed by unsupported a-Fe2 O3 particles with mean particle diameter (MPD) of <100 nm [2]. Present methods to synthesize nano-sized metal particles include simple grinding that yields a wide range of irreproducible particle sizes and chemical va*
Corresponding author. Address: Department of Energy Sciences and Technology, Brookhaven National Laboratory, Building 815, Upton, NY 11973-5000, USA. Tel.: +1-631-344-4985; fax: +1-631-3447905. E-mail address: [email protected] (D. Mahajan). 1350-4177/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2003.10.009
por deposition (CVD) that yields better particle-size control [3]. Selective synthesis of nano-sized zero valent metals and metal carbides have been reported [4]. The use of acoustics initially developed by Suslick [5,6] is now being used extensively mainly to synthesize nano metal oxides whose applications range from coatings to magnetic fluids [7–9]. The sonolysis technique involves passing sound waves of fixed frequency through a slurry or solution of carefully selected metal complex precursors. In a solvent with vapor pressure of certain threshold, the alternating waves of expansion and compression cause cavities to form, grow and implode [10]. During this event, the attained temperature and pressure were calculated to be about 5200 K and 30 MPa, respectively with lifetime of 2 ls to <1 ns [11]. Due to extremely high cavity collapse rates (2 · 109 K s1 –1013 K s1 ), the sound energy translates into sonoluminescence [12]. But if a suitable metal complex is present, the energy can be harnessed to break metal– ligand bonds [5].
386
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
The use of metal carbonyl precursors to synthesize a mixture of metal clusters and nanometer metal particles [6], preparation of iron colloids [13] and molybdenum carbide [14] has been reported. In studies reported so far [5,7–9], sonolysis is typically carried out for about 3 h and the small amounts of synthesized products are separated from a large quantity of unreacted starting metal carbonyl and then purified. The scale-up issues in sonochemical synthesis have been identified [15]. Our interest is in developing the sonolysis technique for convenient synthesis of high-purity nano-sized metals for catalytic applications. We view the cavity produced during sonolysis as a transient high-pressure and hightemperature microvessel in which high-energy intermediates can be generated and subsequently stabilized by further complexation. Therefore, the emphasis of this study was to establish the effect of longer sonication periods on product quality and particle size and to evaluate the potential of producing quantitative yields of nano-sized zero valent metals that could be used without further purification. This focus led us to conduct kinetic and product characterization studies that are reported in this paper.
2. Experimental 2.1. Materials Hexacarbonylmolybdenum (98% pure) pentacarbonyliron (99.5%), hexadecane (99+%, anhydrous), hexanes (98.5+%, ACS reagent grade) were purchased from Aldrich Chemical Co. CO, H2 , CO2 , Ar, N2 gases were obtained from Scott specialty gases. Gases were passed through a Deoxo purifier for further oxygen removal before use. Since Mo(CO)6 is a toxic solid and Fe(CO)5 is toxic and flammable liquid, all manipulations were carried-out in a fume hood with appropriate precautions. 2.2. Sonolysis unit All experiments were done using an ultrasonic liquid processor Model XL2020, from MISONIX, Inc., with a variable power output of up to 550 W at a fixed frequency of 20 kHz. The unit was fitted with a 5-in. long 00 half wave extender tip with a probe tip of diameter 0.5 . The unit allowed precise control of power output, processing time and PULSAR cycle for cyclic intermittent operation to avoid heat build-up. 2.3. Synthesis of Fe and Mo nano particles For the work described herein, the following aspects need to be emphasized. (1) The reaction vessel, purchased from Ace Glass, Inc., was a borosilicate glass 4-
neck flask with walls tapered inward toward bottom that allowed maximum solution in the middle of the flask for adequate immersion of the sonication probe. (2) A series of O-rings and standard greased ground-glass joints ensured tight seals to maintain rigorous exclusion of air or gas leakage from the flask during sonication. (3) Any gas evolved during sonolysis was collected and analyzed and (4) The flask was immersed in a constant temperature (held within ±1 C) bath. Prior to sonication, the hexadecane solvent was thoroughly degassed with argon followed by the addition of metal carbonyl. In a typical run of the Fe System, a degassed yellow homogeneous solution of Fe(CO)5 (8–16 mmol) in 100 ml hexadecane was sonicated in the dark (Fe(CO)5 is light sensitive) at 100% intensity and 80% pulsed cycle settings. Gas evolution with concomitant appearance of black slurry in the reaction vessel was evident within minutes that were indicative of Fe(CO)5 decomposition. The decomposition of Fe(CO)5 was quantified by monitoring the gas evolution as a function of time. The collected gas was analyzed (see below Section 2.4) and the flask containing the black slurry was moved to an argon-filled glove box. The product work-up was as follows. The black slurry was centrifuged and the upper hexadecane solvent layer was decanted to separate the product. The remaining black solid was washed three times with hexane (3 · 10 ml) to remove any residual hexadecane. The resulting black solid was dried in vacuo and the dried solid was stored in a gas-tight vial in the glove box to avoid any sample oxidation. The yield of the dried black product varied between 71% and 87% for the completed runs. For example, the yield from sonolysis of 16 mmol Fe(CO)5 in hexadecane at 327 K was 0.78 g (87%). The Mo particles were prepared as follows. Mo(CO)6 was partial soluble in hexadecane and yielded a white slurry on addition of 4–8 mmol Mo(CO)6 in 100 ml hexadecane (1.05–2.10 wt.% slurry). In this case, a noteworthy observation was the appearance of a white solid along the walls of the flask above the solvent layer during sonolysis that was attributed to the sublimed Mo(CO)6 . In this case, the product work-up involved the same step sequence as shown for Fe(CO)5 . The yield of the black product varied between 51% and 93% for the completed runs. For example, the yield from sonolysis of 8 mmol Mo(CO)6 in hexadecane at 323 K was 0.66 g (86%). 2.4. Analytical The gases collected during sonolysis were analyzed on Gow-Mac Model 580 gas chromatographs: H2 on a 00 molecular sieve column (80 · 1/4 ) with N2 carrier gas; 00 00 CO on a molecular sieve column (9 · 1/8 ) under He; 00 0 CO2 on Carboxen-1000 (5 · 1/8 ) under He. The prepared black solid samples were subjected to spectroscopic analysis for characterization. The infrared
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
387
(IR) spectra were recorded on an ATI Mattson FTIR spectrophotometer. For the characterization of solids, a Phillips Vertical Goniometer and CuK radiation for X-ray powder diffraction (XRD) mea(k ¼ 1:54 A) surements was used. For the XRD sample preparation, a copper holey carbon grid was dipped in the hexanemetal particle slurry samples of Fe or Mo and the hexane were quickly evaporated leaving just the metal particles on the grid for analysis. The X-ray diffraction data were analyzed with a commercial Theta Software package available from Dapple Systems of Sunnyvale, California. The transmission electron microscopy (TEM) images were recorded on a JEOL 2000FX, 200 KV model to determine the particle sizes.
Table 1 Effect of reaction parameters during Fe(CO)5 decomposition in hexadecane via sonolysisa
2.5. XAFS measurements
Fe(CO)5 decomposition during the sonolysis run. A slow CO evolution was still continuing when the reaction was quenched suggesting <100% decomposition. This is in agreement with the infrared data of the final solution in which small IR bands observed at 2020 and 1996 cm1 were assigned to the unreacted Fe(CO)5 [16]. These data suggest an excellent mass balance for decomposition of Fe(CO)5 to Fe metal particles. Gas chromatographic analysis of the collected gas (CO > 99%; CH4 ¼ 0%; CO2 < 0.2%) confirmed the absence of any side reactions. The CO evolution versus time data for the disappearance of Fe(CO)5 were plotted. Fig. 1 shows the data from one of these runs (Run 4 in Table 1) that were analyzed over three half-lives to obtain the first-order rate constant (k) of 2.1 · 106 s1 at 341 K. A similar analysis yielded first-order plots with excellent correlation coefficient (R2 > 0:95) for all other runs listed in Table 1. For Runs 1–4, the constant temperature bath settings were 278, 288, 303 and 323 K respectively during the sonolysis reaction. Table 1 lists measured temperature values inside the flask that were 18–28 C higher than the setting on the bath. It appears that the solvent used in each of these runs was unable to quickly dissipate the reaction heat that was generated at the ‘‘localized hot spot’’ during sonolysis. Therefore, the measured solution temperatures shown in Table 1 may represent values specific to the reaction conditions.
The XAFS measurements were carried out on beamline X-11A of the National Synchrotron Light Source, Brookhaven National Laboratory, using a Si(1 1 1) double crystal monochromator. The storage ring was operated at 2.8 GeV and the typical ring current was 200 mA. The monochromator was detuned appropriately to reject higher harmonic present in the monochromatic beam. The beam intensities were monitored by three gas flow ionization chambers. The gas mixture in the first chamber was selected to absorb about 10% of the incident beam and the gas mixture in the second and the third chambers was selected to absorb about 50% of the transmitted beam. For XAFS measurements special air-tight sample holders with Xray transparent Mylar Windows were used. The samples were prepared inside a glove box and the XFAS measurements were carried out within an hour of preparation. An adequate precaution was taken to avoid air exposure of the samples during transfer to the beamline. The Si(1 1 1) monochromator was calibrated using a 7.5 lm thick Fe foil and a 15 lm thick Mo foil. For XAFS measurements a sample was placed between the first and the second ion chambers and a reference foil (Fe or Mo) was placed between the second and the third ion chambers to monitor the monochromator calibration.
Run
Fe(CO)5 (M)
T (K)
Intensityb (%)
Fe yieldc (%)
k 106 (s1Þ
1 2 3 4 5 6
0.08 0.08 0.08 0.08 0.08 0.16
303 316 326 341 315 327
100 100 100 100 50 100
80 78 85 83 71 87
1.7 2.6 2.9 2.1 1.1 3.4
a
All runs were done under an argon atmosphere. At 100% intensity, the sonicator power was 550 W. c The yield is calculated from the collected CO via the stoichiometry Fe/CO ¼ 1/5. b
3. Results and discussion 3.1. Decomposition kinetics of carbonyls of Fe and Mo The Fe(CO)5 decomposition reaction data are shown in Table 1. By using the stoichiometry of 5 mol CO per mol of Fe(CO)5 , the %Fe yield column in Table 1 refers to values that were calculated by measuring the total CO evolution during sonication. The Fe yield varied from 71% to 87%. These values were not corrected for any Fe(CO)5 losses due to aerosoling or an incomplete
Fig. 1. A first-order plot for the Fe(CO)5 decomposition in hexadecane. Run conditions: 0.08 M Fe(CO)5 ; solvent: hexadecane; T ¼ 68 C; sonicator intensity: 100% (550 W); 80% pulsed cycle.
388
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
Since the data of Runs 1–4 are statistically scattered, no Arrhenius plot was generated and the Eact value could not be extracted from these data. The varied temperature range of 303–341 K that represent small temperature perturbations (3–7%) of the reaction medium, compared to the extremely high transient temperature (>5000 K) within the ‘‘hot spots or bubbles’’ generated during sonolysis [11], are expected to have a minimal effect on rate. The effect of intensity as it directly relates to the energy input is evident with k decreasing almost linearly (2.6 · 106 s1 in Run 2 versus 1.1 · 106 s1 in Run 5). Doubling the Fe(CO)5 concentration, k was essentially constant (Run 6 versus Run 3) confirming the validity of the first-order analysis. Suslick et al. initially reported the sonolysis of Fe(CO)5 in decalin to yield a mixture of dodecacarbonyl triiron and iron metal with a first-order rate constant, k ¼ 13:4 104 s1 at 21 C [6]. To explain the observed mixture of products, a mechanism was put forward that initially involved CO dissociation followed by secondary reactions with Fe(CO)5 to generate Fe3 (CO)12 and Fe via the intermediacy of Fe(CO)3 , Fe(CO)4 /Fe2 (CO)8 species (Eqs. (1) and (2)): FeðCOÞ5 ) FeðCOÞ5n þ nCO FeðCOÞ5n ) Fex ðCOÞy þ Fe
ðn ¼ 1–5Þ
ð1Þ
ðx ¼ 1–3; y ¼ 3–12Þ: ð2Þ
A sonolysis study involving the effect of the nature of hydrocarbon solvent on rate has been reported for the Fe(CO)5 system for which the rate constants were measured to be 8.0, 5.5, 5.1 and 0.7 · 102 min1 at 25 C in decalin, decane, nonane and octane solvents respectively [6]. The hexadecane solvent used in the present study falls between decalin and decane with respect to solvent vapor pressure. It is, therefore, reasonable to assume that in decalin and hexadecane solvents, rates should be within ±30%. Our measured k values in Table 1 are approximately three-orders of magnitude lower than those reported in Ref. [6]. A more likely explanation relates to the nature of the measured k values. While conducting the present study, a complete Fe(CO)5 decomposition was targeted to establish the formation of Fe metal particles as the sole product (mol CO@5 · mol Fe(CO)5 ). Thus, we conclude that the k values in Table 1 represent either the rate determining step or the composite rate constant for the production of Fe derived from the rate law: Rate ¼ k½FeðCOÞ5
ð3Þ
The exclusive formation of Fe metal in our study (see later discussion) has been confirmed via the IR data of the final solutions from Runs 1–6 that showed the absence of Fe3 (CO)12 or other potential iron-carbonyl intermediates. In comparison, the k values reported in Ref. [6] were derived from a study in which no infor-
mation was provided on the extent of the Fe(CO)5 decomposition reaction. It is inferred that the larger k values in Ref. [6]: (1) refer to rate constants corresponding to the summation of different reaction products and (2) are based on the first 2–3 h of the Fe(CO)5 decomposition reaction that may be overestimated. Our study reveals that the intermediate Fe-carbonyl fragments formed during reaction 1 fast dissociate to Fe metal particles because no such intermediates were observed in the IR spectra. Therefore, the Fe(CO)5 decomposition values reported in Table 1 are reliable. A kinetic study was also conducted with Mo(CO)6 to generate nanometer particles of Mo. The Mo(CO)6 offwhite powder was only slightly soluble in hexadecane and resulted in a slurry. The Mo yields were calculated from the actual CO collected via the stoichiometry Mo/ CO ¼ 1/6. For all six runs, the data were plotted to fit the first-order dependence and the k values, derived from the slopes, are listed in Table 2. In the temperature dependence study (Runs 1–4), the k values varied by as much as a factor of four but not in any particular order. This may be attributed to an increase in solubility as the temperature was increased from 305 to 341 K. Other reaction parameter variation runs showed close to a linear dependence on intensity (Run 2 versus Runs 5). Doubling the Mo concentration, k was essentially unchanged as expected for a first-order reaction (Run 3 versus Run 6). A specific characteristic of the Mo(CO)6 sonolysis runs was the varying degree of deposition of a white solid along the walls of the reaction flask. The white sublined crystalline solid was confirmed to be pure Mo(CO)6 (heat of sublimation ¼ 73.6 kJ mol1 ) by the characteristic Mo–CO band at 1980 cm1 in the IR spectrum. The sublimation phenomenon decreases the solution concentration of Mo(CO)6 that may contribute to a reduction in the decomposition rate. As with Fe(CO)5 , the absence of any unreacted Mo(CO)6 in the final black solid generated in the Mo(CO)6 /hexadecane system was established via infrared. The exclusive formation of Mo particles was inferred. Mechanistically, as Table 2 Effect of reaction parameters during Mo(CO)6 decomposition in hexadecane via sonolysisa Run
Mo(CO)6 (wt.%)
T (K)
Intensityb (%)
Mo yieldc (%)
k 107 (s1 )
1 2 3 4 5 6
1.06 1.06 1.06 1.06 1.06 2.12
305 313 325 341 313 323
100 100 100 100 50 100
51 83 93 65 45 86
1.6 4.6 5.8 3.1 1.7 5.0
a
All runs were done under an argon atmosphere. At 100% intensity, the sonicator power was 550 W. c The yield was calculated from the collected CO via the stoichiometry Mo/CO ¼ 1/6. b
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
discussed for the Fe(CO)5 /hexadecane system, the kinetic data are consistent with an initial slow CO dissociation from Mo(CO)6 followed by fast dissociation of CO from intermediate species to generate Mo metal. The k values in Table 2 thus are consistent with a rate equation that is similar to Eq. (3) in which Mo(CO)6 replaces Fe(CO)5 . A comparison of the decomposition data in Tables 1 and 2 shows a general trend of a slower Mo(CO)6 decomposition when compared to the corresponding Fe(CO)5 run. Specifically, the k values were higher by at least a factor of five for Fe(CO)5 . There are several plausible explanations for this trend. First, unlike the liquid Fe(CO)5 and hexadecane that are miscible, Mo(CO)6 is only slightly soluble in the solvent. Second, in agreement with the data in Ref. [6], the rate difference is also attributed to the higher strength of the Mo–CO bond (150.3 kJ mol1 ) in Mo(CO)6 compared to the Fe– CO bond (116.0 kJ mol1 ) in Fe(CO)5 [17]. The products isolated from both the Fe(CO)5 and Mo(CO)6 sonolysis runs were characterized. As discussed in the Section 2, the pyrophoric nature of these
Fig. 2. TEM data for Fe (top) and Mo (bottom) particle samples obtained by sonolysis of Fe(CO)5 and Mo(CO)6 respectively. The samples were prepared under run conditions shown in Fig. 1.
389
Fig. 3. XRD spectra of prepared (a) Fe and (b) Mo particle samples. The arrows in (b) indicate characteristic peaks expected for Mo(0).
black solids dictated that these materials be handled with extreme care in the glove box. Fig. 2 (top) shows the electron micrograph and microdiffraction pattern for the Fe particles produced by sonolysis. The sample contains particles of diameters 10 nm size with several distinct diffraction rings. The TEM micrograph and the corresponding XRD spectrum (Fig. 3a) are consistent with an amorphous nature of this material. These results match closely with those reported in the literature in which the initially produced amorphous Fe powder was subsequently heated at 350 C for 6 h under N2 to induce crystallite formation that was identified as a-Fe metal by XRD [11]. We did not subject the synthesized Fe sample to such heat treatment because our emphasis was on the synthesis of an amorphous, zerovalent material. It is to be noted that the Fe nano-sized particles (MPD10 nm) produced by sonolysis are smaller than those prepared by the thermal decomposition of Fe(CO)5 (MPD30 nm) reported earlier by our group [18]. A similar analysis of a sample of Mo particles was also conducted. The Mo sample shows much smaller particles of size less than 3nm that are discrete compared to those observed with the Fe sample and a corresponding much broader diffraction ring pattern (Fig. 2 (bottom)). Consistent with the TEM data, the XRD measurement displayed several diffuse diffraction peaks that were further analyzed (Fig. 3b). The five peaks of the Mo sample with d-spacing of 3.20, 2.49, 2.22, 2.14 were identified. The relevant known Mo and 1.54 A compounds show intense signature peak as follows: for Mo metal, [0 2 1] at 3.26 A for [1 1 0] at 2.225 A for Mo2 C. Given the MoO3 and [1 1 1] at 2.38 A broadness of the peaks in Fig. 3b, the phases of the prepared Mo material could be described as Mo metal with some molybdenum oxide impurity. An Mo-based
390
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
material prepared by sonolysis of Mo(CO)6 in hexadecane at 90 C has been previously reported as molybdenum carbide (Mo2 C) [14]. The carbide structure was based on the d spacing from the XRD pattern. values of 2.39, 1.49 and 1.27 A A closer look at the literature method of sample preparation prior to XRD revealed that the initial sample prepared by sonolysis was first heated at 100 C under vacuum followed by another heat treatment at 450 C under He for 12 h [14]. Such heat treatments could potentially carbide Mo with residual solvent serving as the source of carbon. In the present study, no such thermal treatment was given to the samples whose spectra are shown in Figs. 2 and 3. Therefore, the XAFS spectra were measured to further confirm the structure of both black nano-sized Fe and Mo products.
Fig. 5. Derivative of XANES spectra of Fe foil ( ) and prepared Fe sample (–).
3.2. Fe sample The normalized XANES spectra and the corresponding derivative spectra of an Fe foil and Fe particles obtained from sonication of Fe(CO)5 are shown in Fig. 4. The shoulder around 7112 eV in the XANES spectra is due to an excitation of 1s electron to the empty d band orbital below the vacuum level. The first inflection point in the derivative spectra of Fe foil was assigned an energy value of 7112 eV (Fig. 5). The intensity of this peak is lower for the Fe(0) sample as compared to that for the Fe foil. For the Fe(0) sample, the edge position and hence the oxidation state, obtained form the first inflection point, is the same as that for the Fe foil indicating that the Fe is in zero valence state. However, the main edge is shifted by about 3 eV indicating that a fraction of Fe is in a higher oxidation state. In order to determine the local structure of Fe, EXAFS analysis was carried out. Fig. 6 shows the radial structure functions (RSF) for the prepared Fe sample The absence of any with a wide peak around 2 A.
Fig. 6. Radial structure function (RSF) of prepared Fe sample.
structure beyond this peak shows that the sample is highly amorphous and that there is no order beyond 3 Further EXAFS analysis shows that this peak corA. responds to two or possibly 3 shells and that the major contribution is from a low-Z scatterer such as C or O. This peak was further analyzed using Fe–C (Ferrocene reference compound) and Fe–Fe (Fe foil, FEFF calculations) interactions as models. These data show that on average each Fe is surrounded by three carbon or oxy and 1 Fe at 2.57(0.03) A. The gen atoms at 2.05(0.01) A data imply that the prepared Fe product has a major contribution from Fe(0) metal and an oxide impurity but some Fe may be carbided. Further modeling as well as XANES analysis is in progress. 3.3. Mo sample
Fig. 4. Normalized XANES spectra of Fe foil ( ) and prepared Fe sample (–).
The normalized XANES spectra for Mo(0) sample, Mo foil and Mo carbide are shown in Fig. 7. The edge position for Mo foil and Mo Carbide is 20 000 eV. The
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
391
gesting that the prepared Mo sample is nano-sized and mostly carbided. The XRD and EXAFS/XANES data of both synthesized Fe and Mo products in Figs. 3–8 are consistent with the presence of reduced Fe and Mo species with minor contribution (<5%) from oxidized species. We, therefore, conclude that it is possible to prepare highpurity Fe and Mo materials by sonolysis method.
4. Conclusions
Fig. 7. Normalized XANES spectra of prepared Mo sample (–), Mo foil ( ) and Mo2 C (–).
edge position for the prepared Mo(0) sample is shifted by +3 eV. The RSFs for the Mo sample and Mo carbide are shown in Fig. 8. The sample shows a broad peak indicating that the Mo product is not fully around 1.8 A crystalline. These data are consistent with the broad XRD bands in Fig. 3 that were attributed to semicrystalline Mo product. Preliminary EXAFS analysis shows significant contribution to the first peak from the with some Mo-O interaction. Mo–C interaction (2.10 A) for A previous study reported XAFS parameters (2.00 A Mo–C; 2.82 A for Mo–Mo; 4.10 A for Mo–Mo) of a Mo/ZSM-5 material [19]. These distances were shorter and the calculated coordination numbers were much lower than the structural parameters deduced from XAFS for bulk b-Mo2 C that show three C atoms at 2.09 and 12 Mo atoms at 2.96 A [20]. The Mo in the Mo/ A ZSM-5 material was shown to contain a highly dispersed Mo2 C phase within the zeolite cavity [19]. The XAFS and XANES spectra in Figs. 7 and 8 closely match those reported for the Mo/ZSM-5 material sug-
Sonolysis is a versatile technique for one-step generation of nano-sized Fe and Mo from the corresponding metal carbonyls. The first-order decomposition kinetic rate constants follow the order: Fe(CO)5 > Mo(CO)6 . The XRD data together with the XAFS/XANES data establish the amorphous nature of Fe and semicrystalline Mo and show that Fe(0) metal and carbided Mo (Mo2 C) are produced under sonolysis conditions. The spectroscopic data show that longer sonolysis periods are not detrimental to the production of nano metal particles and zero-valent nano metals can be conveniently produced in high purity (>90%) and in gram quantities. The TEM data show that the produced Mo2 C particles (<3 nm) are smaller than the Fe particles (10 nm). Nano Fe particles produced by the sonolysis technique are smaller compared to those generated via thermal decomposition of Fe(CO)5 (<30 nm) [18]. The sonolysis method is presently being explored as a pathway to synthesize polyethylene glycol complexed nano metals for application as nano coatings.
Acknowledgements ETP thanks the BNL Office of Educational Program for selection through the Science and Engineering Research Semester (SERS) Program. The authors are grateful to Mr. Robert L. Sabatini for TEM analysis. KP thanks the Office of Naval Research for support of the X 11A beamline at the National Synchrotron Light Source (NSLS) at BNL. This work was supported by the U.S. Department of Energy under contract no. DE-AC02-98CH10886.
References
Fig. 8. RSFs of Mo2 C ( ) and prepared Mo sample (–).
[1] D.B. Dadyburjor, A.H. Stiller, C.D. Stinespring, A. Chadha, D. Tian, S.B. Martin Jr., S. Agarwal, in: W.R. Moser (Ed.), Advanced Catalysts and Nanostructured Materials, Modern Synthetic Methods, Academie Press, New York, 1996, Chapter 22. [2] D. Mahajan, A. Kobayashi, N. Gupta, J. Chem. Soc. Chem. Commun. (1994) 795; H. Itoh, E. Kikuchi, Appl. Catal. 67 (1990) 1.
392
D. Mahajan et al. / Ultrasonics Sonochemistry 11 (2004) 385–392
[3] P. Serp, R. Fuerer, R. Morancho, P. Kalck, J.-C. Daran, J. Vaisserman, J. Organomet. Chem. 498 (1995) 41. [4] J. Zeng, M.J. Hampden-Smith, Chem. Mater. 4 (1992) 968; H. B€ onnemann, W. Brijoux, T. Joussen, Angew. Chem. Int. Ed. Engl. 29 (3) (1990) 273. [5] K.S. Suslick, T. Hyeon, M. Fang, A.A. Cichowlas, Chapter 8 in Ref. [1]. [6] K.S. Suslick, J.W. Goodale, P.F. Schubert, H.H. Wang, J. Am. Chem. Soc. 105 (1983) 5781. [7] K.V.P.M. Shafi, A. Ulman, X. Yan, N.-L. Yang, C. Estournes, H. White, M. Rafailovich, Langmuir 17 (2001) 5093. [8] K.V.P.M. Shafi, S. Wizel, T. Prozorov, A. Gadenken, Thin Solid Films 318 (1998) 38. [9] G. Kataby, R. Prozorov, A. Gadenken, Nanostruct. Mater. 12 (1999) 421. [10] K.S. Suslick, MRS Bull. (1995) 29.
[11] K.S. Suslick, S.-B. Choe, A.A. Cichowlas, M.W. Grinstaff, Nature 353 (1991) 414. [12] K.S. Suslick, E.B. Flint, M.W. Grinstaff, K.A. Kemper, J Phys. Chem. 97 (1993) 3098. [13] K.S. Suslick, M. Fang, T. Hyeon, J. Am. Chem. Soc. 118 (1996) 11960. [14] T. Hyeon, M. Fang, K.S. Suslick, J. Am. Chem. Soc. 118 (1996) 5492. [15] T.J. Mason, Ultrason. Sonochem. 7 (2000) 145. [16] R.B. King, C.C. Frazier, R.M. Hanes, A.D. King Jr, J. Am. Chem. Soc. 100 (1978) 2925. [17] F.A. Cotton, A.K. Fischer, G. Wilkinson, J. Am. Chem. Soc. 81 (1959) 800. [18] D. Mahajan, P. Vijayaraghavan, Fuel 78 (1999) 93. [19] J.-Z. Zhang, M.A. Long, R.S. Howe, Catal. Today 44 (1998) 293. [20] J.S. Lee, L. Volpe, F.H. Ribeiro, M. Boudart, J. Catal. 112 (1988) 44.