Iron-based nanocatalyst

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2011

A more effective and efficient process can impregnate iron nanoparticles onto carbon supports.

KEY CONCEPTS
• A technique known as ultrasonic spray pyrolysis is used in a continuous process to impregnate iron nanoparticles into porous carbon spheres.
• Most of the impregnated iron nanoparticles are fewer than 20 nanometers in diameter.
• The iron-impregnated microsphere efficiently reduces aqueous hexavalent chromium and may be useful in multipollutant control applications, including removing No_x from gas streams. 

Interest in the development of new nanocatalysts continues as researchers see the value of working with nanoparticles. Catalysts impact the lubricant industry in a wide range of applications from the processing of base oils to reducing automotive emissions.

In a previous TLT article, a catalyst based on the placement of tungsten oxide nanoparticles on zirconia is used to accelerate the isomerization of n-pentane to isopentane (1). this process is needed to boost the octane rating of gasoline. Peak catalyst performance was obtained by placing 5 +/- 0.5 tungsten sites per square nanometer on the surface of the zirconia.

Another catalyst that has been used in a number of applications is iron. Mark Rood, Ivan Racheff professor of civil and environmental engineering at the University of Illinois in Champaign, Ill., says, “Iron is attractive as a catalyst because of its low cost and ready availability. It has been studied as a Fischer-Tropsch catalyst for oxygen reduction in fuel cells and as an important component of environmental adsorbents for carbon monoxide or arsenic.”

Porous carbon is an effective support for catalysts such as iron. John Atkinson, graduate student at the University of Illinois, says, “A big advantage for porous carbon as a catalyst support is that it works well under harsh conditions because of its resistance to both highly acidic and alkaline environments. Carbon is also readily available, and the porosity and surface chemistry of the support can be tailored for specific applications.”

Rood and Atkinson believe that incorporation of iron nanoparticles onto a porous carbon support can improve catalyst activity. Such a nanomaterial can boost catalyst performance by increasing iron’s available surface area.

But the current procedures for impregnating metals into carbon materials require multiple-step processes and can be costly and energy inefficient. Atkinson says, “The traditional method involves obtaining the carbon from an organic material such as biomass or coal. High-temperature carbonization isolates the carbon followed by physical or chemical activation to develop the porous structure. The catalyst precursor is then impregnated into the carbon using excess solution, incipient wetness, ion exchange or chemical vapor deposition techniques. Finally, a high-temperature reduction or pyrolysis step is needed to form the metal nanoparticles.”

A more effective and efficient process is needed to impregnate iron nanoparticles onto carbon supports. Such a process has now been developed.

ULTRASONIC SPRAY PYROLYSIS
Rood, Atkinson and their fellow researchers from the chemistry department and the Illinois state Geological survey at the University of Illinois have developed a continuous process to impregnate iron nanoparticles into porous carbon spheres. Rood says, “We prepared the carbon-iron material using ultrasonic spray pyrolysis (USP). This process involves the creation of a fine mist that facilitates the carbonization of an organic precursor, development of porosity, dispersion of iron in carbon spheres and activation of the iron nanoparticles.”

These researchers used sucrose as the carbon precursor and sourced iron from either ferric chloride or ferric nitrate. The corresponding sodium salts were used in conjunction with the iron salts to aid in carbon porosity development.

During USP, the precursors are dissolved in water and the resulting solutions are ultrasonically nebulized. The produced mist is entrained in a carrier gas (either argon or nitrogen) stream and transported into a quartz tube reactor that is preheated between 500 C and 900 C.

The pyrolysis process takes less than seven seconds and iron-carbon products are collected in deionized water bubblers. The iron-impregnated carbon spheres are isolated from the water using filtration or centrifugation.

Atkinson says, “The spherical carbon supports have diameters between one and three micrometers, and the impregnated iron nanoparticles ranged from four to 90 nanometers in diameter with most being less than 20 nanometers.”

Transmission electron microscopy (TEM) was used to characterize the iron-based nanocatalysts. Rood says, “We used TEM to see inside the carbon spheres and determine how well the iron nanoparticles are dispersed.”

Scanning electron microscopy was also used to evaluate the surface morphology of the microspheres. The researchers found that the external texture of the carbon spheres prepared from chloride salts was different than those prepared from nitrate salts. Atkinson explains, “The material prepared from chloride salts has a rougher surface. This phenomenon is most likely due to a faster precipitation rate for sodium chloride compared to sodium nitrate when the water evaporates during processing, leading to an increase in surface heterogeneities.”

Atkinson emphasizes that the iron-impregnated carbon microspheres exhibit relatively high surface areas, reaching as high as 800 square meters per gram for both of the salts. Figure 3 shows an iron-impregnated carbon microsphere prepared from chloride salts.


Figure 3. The surface texture of this iron-impregnated carbon microsphere has a rougher surface because it was prepared from chloride salts as compared to nitrate salts. (Courtesy of the University of Illinois)

The mechanism for preparation of the iron-impregnated microspheres is comparable for both chloride and nitrate salts but depends on the temperature during thermal processing.

Atkinson says, “We propose that the ideal iron-carbon formation temperature is above 700 C because fresh carbon atoms isolated during pyrolysis of sucrose can be gasified by steam to develop more porosity than what is seen at lower temperatures. During this process, the hydrogen and carbon monoxide produced may help to reduce impregnated iron oxides to magnetite (Fe₃O₄).”

The efficacy of the iron-impregnated carbon microspheres was evaluated by reducing aqueous hexavalent chromium. During a 48-hour test, the concentration of chromium (VI) dropped by over 80% compared to a decrease of less than 10% when exposed to carbon microspheres not containing the iron nanoparticles.

The researchers have incorporated 35% iron by weight into the microspheres. Characterization of the microspheres shows that the iron is well dispersed. Rood adds, “This combination of high loading and good dispersion of the nanocatalyst is very unique and highly desirable for air quality applications.”

Future work will involve exploring USP as a technique to make metal-impregnated carbon microspheres from other metals such as cobalt, copper, nickel and zinc. The researchers envision using iron-impregnated carbon microspheres in multipollutant control applications such as simultaneous removal of dioxin, mercury and NO_x from gas streams.

Additional information on this research can be found in a recent article (2) or by contacting rood at mrood@illinois.edu. 

REFERENCES
1. Canter, N. (2010), “Nanocatalyst for Refinery Use,” TLT, 66 (12), pp. 12–13.
2. Atkinson, J., Fortunato, M., Dastgheib, S., Abadi, M., Rood, M. and Suslick, K. (2011), “Synthesis and Characterization of Iron-Impregnated Porous Carbon Spheres Prepared by Ultrasonic Spray Pyrolysis,” Carbon, 49 (2), pp. 587–598.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.