Preparation, interconversion and characterization of ... - Springer Link

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Topics in Catalysis Vol. 39, Nos. 3–4, October 2006 ( 2006) DOI: 10.1007/s11244-006-0064-z

Preparation, interconversion and characterization of nanometer-sized molybdenum carbide catalysts Edwin L. Kuglera,*, Christopher H. Clarka, James H. Wrighta, Dady B. Dadyburjora, Jonathan C. Hansonb, Zhen Songb, Tanhong Caib, and Jan Hrbekb a Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506-6102, USA Department of Chemistry, Brookhaven National Laboratory, US Department of Energy, Upton, NY 11973-5000, USA

b

Nanometer-sized molybdenum carbide particles have been shown to have hydrogenation and other catalytic properties similar to those of the more-expensive noble metals. However, current preparation techniques for molybdenum carbide require the hightemperature reduction of unsupported molybdenum oxide in the presence of CH4 and H2. Although this method is effective, it yields particles of relatively large size, 11 nm. Different, simpler, synthesis procedures starting with ammonium heptamolybdate impregnated on carbon yield carbides of different stiochiometries. Particle sizes are in the 2–10 nm range, which are generally smaller than those previously obtained, and can be altered by changing the method of preparation. The focus of this work is to characterize the new procedures, specifically the bulk transitions of the new starting material to the different molybdenum carbides, using time-resolved X-ray diffraction, temperature-programmed reduction, and temperature-programmed desorption. KEY WORDS: Molybdenum carbide; time-resolved X-ray diffraction; temperature-programmed reduction; temperatureprogrammed desorption.

1. Introduction Molybdenum carbide catalysts have been shown to have many properties similar to those of noble metals [1]. It has been found that unsupported Mo2C has benzene hydrogenation activity similar to that of ruthenium [2,3]. Furthermore, the activities of these catalysts have been correlated to bulk particle size as measured by X-ray diffraction (XRD) line broadening [4]. Leclercq et al. formed unsupported Mo2C by reduction of MoO3 in a flowing CH4/H2 (20/80) mixture [1]. However, this method yields relatively large particles, with an average size of 11 nm [4]. Lee et al. have developed methods for the preparation of molybdenum carbides supported on alumina; see, for example, ref. [3]. These catalysts have been shown (by X-ray line broadening) to have a smaller particle size, 4 nm, and (by pulse CO chemisorption) to have a significant number of potentially reactive surface sites. Both the unsupported and alumina-supported catalysts have been shown to be active for reactions such as benzene hydrogenation, normally catalyzed by noble metals [2,3]. Dje´ga-Mariadassou et al. have shown [5] that molybdenum-on-carbon can be reduced to molybdenum carbide with the use of combinations of CH4, H2, and inert. These catalysts have a relatively large average particle size of 14 nm, as measured by X-ray line broadening. In this work, we show that molybdenum-on-carbon reduced in H2 at 700 C forms (hexagonal) Mo2C with 1–3 nm crystallites. Additionally, reduction of molyb* To whom correspondence should be addressed. E-mail: [email protected]

denum-on-carbon in flowing CO results in the formation of (cubic) MoC with an average size of 3 nm. Further, cubic MoC-on-carbon can be reduced at 675 C to hexagonal Mo2C in flowing H2.

2. Experimental Samples were prepared by impregnating activated carbon with an aqueous solution of ammonium heptamolybdate (AHM), followed by drying in air at 100 C. Both AHM and activated carbon were obtained from Sigma Aldrich. Multiple impregnations were used to achieve the desired loadings [6]. These AHM on activated carbon samples are referred to as ‘‘initial samples.’’ Temperature-programmed reduction (TPR) was performed in a Micromeritics Autochem 2910 using 50 mL/ min of 5% H2 in argon. The samples, typically 150 mg, were heated from room temperature to 900 C at 10 C/ min. The off-gas from the TPR was analyzed using a Thermo ONIX Prima db scanning sector mass spectrometer. Temperature-programmed desorption (TPD) was conducted in the same instrument using argon (UHP) at 50 mL/min. For the TPR and TPD studies, the initial samples had a loading of 13% molybdenum (metal weight basis). For the XRD studies, a higher loading, 21% molybdenum (metal weight basis), was used to obtain clearer diffraction patterns. The initial samples were heated to 500 C in flowing N2 and held for 2 h, decomposing the AHM precursor to MoO2 ex situ. These MoO2 on carbon materials are termed ‘‘nitrogen 1022-5528/06/1000–0257/0 2006 Springer Science+Business Media, Inc.

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calcined samples’’ and have been studied previously [6]. Time-resolved X-ray diffraction measurements were made at Beamline X7B of the National Synchrotron Light Source, Brookhaven National Laboratory. For a description of experimental equipment at Beamline X7B, see for example, ref. 7. The XRD sample, weighing 2–3 mg, was loaded into a 1.09 mm OD (0.79 mm ID) sapphire capillary tube (St. Gobain), and held in place with small pieces of quartz wool. A 0.50 mm OD thermocouple with a stainless-steel sheath was inserted into the sapphire tube and placed in the sample bed. The tube was wrapped with a Kanthal heating element with a gap in the center for X-rays to pass through the sample. Monochromatic X-rays with 0.0922 nm wavelength were used for time-resolved diffraction measurements. Diffraction patterns were recorded at 5 or 6-min intervals, so that diffraction patterns are measured over 25 or 30 C intervals when the heating rate is 5 C/min. The final reduction temperature was limited by the heating element to about 825 C. Reduction was performed using 5% H2, 5% CH4, or 5% CO (balance helium) at flow rates of 15–20 mL/min. For scanning tunneling microscopy (STM), a drop of saturated AHM solution was added to the surface of HOPG graphite. The graphite surface had been roughened by neon-ion bombardment prior to adding the AHM solution [9]. After drying overnight at 20 C, the sample was reduced in a 12 mm quartz tube using 5% H2 (balance He) flowing at 100 mL/min. The sample was heated to 750 C at 10 C/min, held 10 min, and then cooled to room temperature. The sample was moved to the STM (Omicron) without passivation. The

sample, in a UHV preparation chamber, was heated to 580 C (by e-beam heating) and reduced with 2 · 10)6 mbar of H2 for 2 h. The STM apparatus and measurement procedure are discussed elsewhere [8,9].

3. Results and discussion The synthesis of Mo2C during the temperature-programmed reduction of AHM on carbon was first reported by Dje´ga-Mariadassou [5]. In the current study, the phases of molybdenum were determined during TPR using time-resolved X-ray diffraction. Figure 1 shows the in situ XRD patterns observed when the decomposed sample containing 21% Mo (metal weight basis) is reduced in 5% H2 from 25 to 835 C. X-ray diffraction patterns are separated by 30 C intervals. The diffraction pattern at ambient temperature (at the bottom of the figure) shows diffraction lines for MoO2 and SiO2, an impurity in the activated carbon support. Strong lines for MoO2 appear at 21.9, 31.1, 31.5 and 35.0 degrees 2-theta. The diffraction lines for the SiO2 impurity occur at 21.7, 25.1, 29.4 and 34.8 degrees, shifting to smaller angles as the sample is heated, but appearing in all the diffraction patterns since SiO2 does not react. Two extraneous peaks occur in scans 3 and 4 at 21.6 degrees 2-theta, caused by a diffraction spot from the sapphire capillary tube that was missed when working up raw data. At approximately 700 C, MoO2 rapidly reduces to hexagonal Mo2C. Figure 1 clearly shows that the transformation occurs without the oxide reducing to

Figure 1. In situ X-ray diffraction results from temperature-programmed reduction of MoO2 on activated carbon using 5% H2. Sample was heated from 25 to 835 C at 5 C/min. On the 3-d plot, X-axis is degrees 2-theta, Y-axis is scan number, and Z-axis is intensity. Scans on the Y-axis are separated by 30 C.

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Figure 2. Scanning tunneling micrograph of Mo2C particles formed ex situ on HOPG graphite. Each side of the micrograph is 25 nm.

Mo0 as an intermediate step. The carbide is present as nanometer-sized particles with an average size of 3 nm, determined by peak widths. This is confirmed by the STM image shown in figure 2 where most particles are measured as 1–2 nm. The STM sample is Mo2C on HOPG graphite reduced ex situ with 5% H2 at 750 C. Figure 3 shows the TPR-MS pattern for H2 consumption as well as CO and CO2 evolution when AHM on carbon (13% Mo) is reduced with 5% H2 in argon. As expected, ammonia (not shown) is formed in significant quantities below 250 C through the decomposition of the AHM. The hydrogen consumption occurs from 400 to 700 C, however no substantial change in XRD pattern is observed in figure 1. The molybdenum

particles being reduced must be either amorphous or too small to be detected by XRD. The transition of the MoO2 to Mo2C around 700 C is not marked by a consumption of H2. Rather, a large amount of CO is evolved at approximately 700 C. The reduction of MoO2 to Mo2C is immediate, as seen in figure 1. Further, figure 3 indicates that very little CO2 is formed. Since neither H2 nor CO is reducing MoO2, Mo2C must form by a carbothermal reduction using carbon from the activated-carbon support. Figure 4 shows the in situ XRD patterns recorded when the MoO2 on carbon sample is heated from ambient temperature to 825 C in helium only. The MoO2 diffraction lines become narrower and more intense as the sample is heated, indicating particle growth. This sample is reduced from MoO2 to Mo2C without going through Mo0, but at a temperature of 800 C. X-ray line broadening indicates that the average particle size of MoO2 just before the transition is 20 nm. The average particle size of Mo2C is 9 nm at 825 C. Larger particles of Mo2C are formed from the carbothermal reduction in inert gas because the precursor compound is larger when the transition occurs. The corresponding TPD-MS spectrum for an AHM on carbon sample is shown in figure 5. As in the reduction with 5% H2, NH3 is evolved as a product of AHM decomposition. There is clearly an evolution of CO at high temperature, 850 C. This is higher than the reduction temperature of MoO2 to Mo2C observed by in situ XRD. However, TPD and TPR measurements record furnace temperature rather than sample temperature. Like the TPR in H2 shown in figure 3, there is little CO2 evolved in the transition between MoO2 and Mo2C. Carbon from the support must be responsible for reducing MoO2 to Mo2C. Carbon monoxide is evolved during the transition but cannot be a reducing agent

6

Concentration (mole %)

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H2

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CO

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CO2

0 0

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Temperature (οC) Figure 3. TPR-MS spectra of AHM on activated carbon sample reduced in 5% H2 at 10 C/min. Temperature range is 25 to 900 C.

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Figure 4. In situ X-ray diffraction results for MoO2 on activated carbon heated in flowing helium at 5 C/min. Temperature range is 25–825 C. Scans on the Y-axis are separated by 25 C.

unless it is oxidized to CO2. Hydrogen is evolved at high temperature. This may come from water impurities reacting with carbon or be hydrogen released from the activated carbon support. Figure 6 shows the in situ XRD patterns when the MoO2 on carbon sample is heated from ambient temperature to 800 C in 5% CH4. As in the case with heating in inert gas, the MoO2 diffraction lines grow in intensity as the temperature in raised. The transition to

Mo2C occurs between 700 and 725 C. The MoO2 particles grow to 16 nm before forming 8 nm Mo2C. When MoO2 on activated carbon is reduced in 5% CO the product is cubic MoC. Figure 7 shows diffraction patterns measured from 400 to 725 C. The major transition in the XRD patterns occurs at approximately 600 C. The MoO2 diffraction lines disappear and the MoC lines appear at 21.6, 25.0 and 35.6 degrees 2-theta. Six lines from the SiO2 impurity appear in all of the

5.0 4.5

Concentration (mol%)

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CO

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H2

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CO2

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Temperature (οC) Figure 5. TPD-MS spectra of AHM on activated carbon sample heated in flowing argon from 25 to 900 C at 10 C/min. and then held at 900 C for 1 h.


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Figure 6. In situ X-ray diffraction results for MoO2 on activated carbon heated in 5% CH4 at 5 C/min. Temperature range is 25–825 C. Scans on the Y-axis are separated by 25 C.

patterns. Again, the transformation occurs directly, without first reducing to Mo0. Line-broadening measurements indicate that the average particle size of MoC is around 3 nm. Interestingly, MoC can be reduced to Mo2C with 5% H2 at 675 C. Figure 8 shows results from the in situ XRD experiment. In this figure the MoC sample, prepared as in figure 6, was heated in 5% H2 from 25 to 825 C at a rate of 10 C/min. In this experiment, diffraction patterns are separated by 50 C intervals. Here cubic MoC loses carbon to become hexagonal Mo2C. The data obtained in this study indicates that active carbon is the reducing agent for the MoO2 to Mo2C

Figure 7. In situ X-ray diffraction results for MoO2 on activated carbon heated in 5% CO at 5 C/min. Temperature range is 25– 825 C. Scans on the Y-axis are separated by 25 C.

transition. Reduction in helium occurs when the carbon support becomes sufficiently active to reduce MoO2. There is very little hydrogen, methane or CO present prior to the onset of reduction in inert gas. The observed oxidation product is CO. In helium at 800 C the activatedcarbon support expels oxygen impurities as CO and produces an active carbon surface that reacts with MoO2. Carbon monoxide is the preponderant oxidation product since thermodynamics favor CO in preference to CO2 above 700 C. Reduction with 5% CH4 occurs at 700–725 C. This temperature range has been used to convert unsupported MoO3 to Mo2C using H2/CH4 mixtures [1,4]. Apparently CH4 decomposition to H2 and C becomes rapid at 700 C, providing active carbon for carbide formation. For both 5% methane and inert gas alone, the Mo2C particles are large since MoO2 sinters prior to carbide formation. Reduction with 5% H2 produces the smallest Mo2C particles as the phase transition producing Mo2C occurs just below 700 C. Hydrogen consumption begins about 400 C and reaches a maximum near 500 C. Hydrogen consumption has some effect on the rolling background of the XRD pattern but does not produce new diffraction lines. The MoO2 diffraction lines become stronger, but do not grow as they do in 5% CH4. Certainly, the hydrogen is reacting somewhere. Perhaps it is reducing the carbon support. Perhaps it is reducing very small molybdenum oxide particles to carbide or oxycarbide. Additional research is needed to investigate the behavior of particles too small to be detected by XRD.

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Figure 8. In situ X-ray diffraction results for reduction in 5% H2 of MoC-on-carbon prepared as in figure 7. The sample was heated from 25 to 825 C at 10 C/min. Scans on the Y-axis are separated by 50 C.

Reaction with 5% CO produces a phase transition at 600 C when MoO2 is reduced to cubic MoC. This carbide is different from the one observed in other experiments. Carbon monoxide disproportionation produces active carbon and CO2. As before, the active carbide reduces MoO2. However, in this case cubic MoC forms. The formation of the monocarbide must be driven by the concentration of active carbon since MoC contains 50 mol% carbon whereas Mo2C only contains 33 mol% carbon. 4. Conclusions Nanometer-size molybdenum carbide particles can be prepared on a carbon support by heating AHM in inert or reducing environments. Reduction in hydrogen produces the smallest Mo2C particles, observed to be 1– 3 nm. Hydrogen consumption begins below 500 C but MoO2 reduction is not observed by XRD until 700 C. Reaction with methane occurs at 700–725 C as MoO2 diffraction lines disappear as Mo2C forms, producing carbides with an average size of 8 nm. When MoO2 on carbon is heated in inert gas, the transition to Mo2C occurs about 800 C, producing 9 nm carbides. In all cases, the carburization source is the carbon support. Furthermore, the reduction temperature and annealing time allow the size of hexagonal Mo2C to be tuned. In contrast, reduction of molybdenum-on-carbon in CO leads to the formation of cubic MoC particles, with an average size of 3 nm. Further reduction of cubic MoC in H2 changes the carbide structure to hexagonal Mo2C, as was observed for reaction in H2, CH4 and inert gas.

Acknowledgments We acknowledge financial support from the U.S. Department of Energy under Cooperative Agreement DE-AC22-99FT40540 with the Consortium for Fossil Fuel Science (CFFS). We acknowledge Brookhaven National Laboratory for travel support and General User time allocations at the National Synchrotron Light Source (time-resolved diffraction at Beamline X7B) and the Center for Functional Nanomaterials (STM). We appreciate the help of Todd Gardner and his associates at the National Energy Technology Laboratory for conducting the TPR-MS experiments.

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