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현 택 환

서울대학교 응용화학부

Novel Sonochemical Synthesis of Nanostructured Catalysts

Taeghwan Hyeon

School of Chemical Engineering

Seoul National University

Introduction

The sonochemical decomposition of volatile organometallic compounds produces high surface area solids that consist of agglomerates of nanometer clusters.  For iron pentacarbonyl and tricarbonylnitrosylcobalt, nanostructured metals and alloys are formed; for molybdenum hexacarbonyl, the metal carbide is produced.  These sonochemically produced nanostructured solids are active heterogeneous catalysts for hydrocarbon reforming.

Sonochemistry arises from acoustic cavitation; the formation, growth, and implosive collapse of bubbles within a liquid.  The collapsing bubbles generate localized hot spots with transient temperatures of ~5000 K, pressures of ~1800 atm, and cooling rates in excess of 1010 K/s. Using these extreme conditions, we have produced a variety of nanostructured metals, alloys, and carbides and examined their catalytic activity.

The sonochemical synthesis of nanostructured materials is also extremely versatile: various forms of nanophase materials (powders, colloids, supported catalysts) can be generated simply by changing the reaction medium.2  I will present the sonochemical synthesis and heterogeneous catalytic studies of nanostructured Fe-Co alloys and nanostructured Mo2C.

Experimental

For the synthesis of nanostructured Fe, Co, and Fe-Co alloy materials, solutions containing various relative concentrations of Fe(CO)5 and Co(CO)3(NO) were irradiated in dry decane at 0□C with a high-intensity ultrasonic probe for 3 hours under argon. The resulting black powders were filtered and washed with dry pentane in the glove box.

Nanostructured molybdenum carbide catalyst was prepared by ultrasonic irradiation of a slurry of molybdenum hexacarbonyl (1 g Mo(CO)6  in 50 mL of hexadecane) at 90□C for 3 hours under argon.

Results and discussion

Synthesis and catalytic studies of nanostructured molybdenum carbide.

Recently, molybdenum and tungsten carbides have been explored as heterogeneous catalysts because of the similarity in activity that these carbides share with platinum group metals. The preparation of interstitial carbides of molybdenum and tungsten with high surface areas, however, is very difficult.  We have developed a simple sonochemical synthesis of nanophase molybdenum carbide from the ultrasonic irradiation of molybdenum hexacarbonyl.3 XRD after heating at 450 oC under helium revealed that the material is a fcc molybdenum carbide(Mo2C). The synthesis of Mo2C is particularly prone to substantial oxygen contamination.  Since the presence of oxygen could poison the catalytic activity, it was removed before catalysis by heating in a flowing 1:1 CH4/H2 mixture at 500□C for 10 h.  After this carburization, the elemental analysis results showed that the sample had a stoichiometry of Mo2C1.02 without other substantial contamination.  The material remains a porous aggregate with particle size of 3 nm; the BET surface area was 130 m2/g.

The catalytic activity and selectivity of the sonochemically produced molybdenum carbide was examined for cyclohexane dehydrogenation and hydrogenolysis reactions.   Two kinds of products were formed during the cyclohexane reaction:  benzene from the dehydrogenation reaction and methane from the hydrogenolysis.  To compare the catalytic properties, commercial ultrafine powders of platinum and ruthenium were also used under identical conditions.  Figure on right side shows the catalytic activity as a function of temperature for the Mo2C sample pretreated under CH4/H2 at 500 oC compared to commercial ultrafine platinum powder. At all reaction temperatures examined, benzene was the only product formed for both samples and their activities were comparable:  no hydrogenolysis products were detected.  In contrast, only hydrogenolysis, mostly to methane, occurred with commercial  ruthenium powder. The analogy has often been made that Mo2C is similar to Ru whereas W2C behaves like Pt. These results demonstrate, however, that for dehydrogenation of alkanes, sonochemically prepared nanostructured molybdenum carbide has a selectivity similar to Pt rather than to Ru.

Synthesis and catalytic studies of nanostructured Fe-Co alloys.

Sonochemical decomposition of decane solution caontaing mixtures of  Fe(CO)5 and Co(CO)3(NO) produced nanostructured Fe-Co alloys.4  The composition of the Fe-Co alloys can be controlled simply by changing the ratio of solution concentrations of the precursors; alloy compositions ranging from pure Fe to pure Co are readily obtained. The EDX results show that the alloys are homogeneous on a nanometer scale and were consistent with bulk elemental analysis.  The original amorphous powder crystallized after thermal treatment under hydrogen at 400 oC.  All the alloys crystallize in the bcc structure, which is consistent with the known Fe-Co equilibrium phase diagram.  Elemental analysis results show that nearly pure metal and alloys are produced.  SEM at high magnification indicates that these materials are porous aggregates of small clusters of 10-20 nm particles.

Catalytic studies of the sonochemically prepared Fe-Co alloys were made for cyclohexane dehydrogenation and hydrogenolysis reactions.  The catalytic activity as a function of temperature is shown in Figure 2. The catalytic selectivity as a function of temperature is shown in Figure 3. The catalytic properties of the sonochemically prepared Fe, Co and Fe-Co alloys in the cyclohexane reaction exhibit interesting trends.  First, they are all active catalysts for cyclohexane conversion:  pure Co has the highest activity, pure Fe has the lowest activity, and Fe-Co alloys have intermediate activity between pure Fe and pure Co.  Second, Fe-Co alloys generate much more dehydrogenation product (benzene) than pure Fe or Co.  Third, the 1:1 Fe/Co alloy has both much higher dehydrogenation selectivities(almost 100%) than the other alloys or pure metals.

References

1.  Suslick, K.S. Science 1990, 247, 1439.

2.  Suslick, K. S.; Hyeon, T.; Fang, M. J. Am. Chem. Soc. 1996, 118, 11960.

3.  Hyeon, T.; Fang, M.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5492.

4.  Suslick, K. S.; Hyeon, T.; Fang, M. Chem. Mater. 1996, 8, 2172.