Synthesis and Characterization of Carbon-Based Iron Catalysts Prepared by Ultrasonic Spray Pyrolysis

Carbon materials have been used in heterogeneous catalysis, both as direct catalysts and catalyst supports, for many years [1,2]. As a catalyst support, carbon provides desirable characteristics including chemical inertness, stability (in the absence of oxygen), mechanical resistance, and high surface area and desired porosity. Traditionally, carbon catalysts are prepared through a multistage process: (1) selection, preparation, and modification of a carbon support with desired physico-chemical properties; (2) loading of catalyst precursor with different methods including incipient-wetness, excess-solution, and ion-exchange impregnation or physical/chemical vapor deposition; and (3) conversion of the catalyst precursor to active catalyst species by physical (e.g., heating) and/or chemical (e.g., reduction) treatments. In this work, we considered a new approach for preparation of carbon catalysts through a continuous and possibly more efficient process. A novel, one-step synthesis method was used to simultaneously pyrolyze a carbon precursor, develop porosity, and disperse nano-sized metal catalyst particles onto the porous carbon support. An Ultrasonic Spray Pyrolysis (USP) system that has been previously used to prepare porous carbon powders was utilized [3]. An aqueous solution of sucrose, NaCl, and FeCl3 of selected stoichiometry was ultrasonically nebulized to produce a fine mist that was carried to a furnace with nitrogen flow. After solvent evaporation, sucrose and ferric chloride decomposed to form a carbon/NaCl/iron composite. A fine powder exiting the furnace was collected in water bubblers, where the NaCl crystals were dissolved, leaving behind a porous carbon structure supporting highly dispersed nano-sized iron particles. An experimental matrix was designed to investigate the impacts of pyrolysis temperature, aerosol flow rate, iron concentration, and iron-to-sucrose ratio on the carbon support's porosity and the iron catalyst's dispersion. Experiments were also conducted to determine the individual roles of NaCl, FeCl3, and solution pH on the product's yield and properties. Prepared catalysts were characterized for: surface area and pore size distribution (N2 adsorption); surface and bulk elemental composition (SEM-EDX, XPS, and ICP-MS); and carbon texture, support diameter, and catalyst particle size (SEM and TEM). Additional characterizations to identify carbon surface functionalities by spectroscopic methods and iron dispersion by CO chemisorption are planned. Tests to assess the catalytic activity of these materials for gas- and liquid-phase environmental applications, such as oxidation of NO in flue gas streams or destruction of water pollutants (e.g., trichloroethylene), are also planned. Characterization results show that carbon microspheres (0.5 ìm < dp < 3 ìm) impregnated with well dispersed, iron nanoparticles (10 nm < dp < 100 nm) can be produced using the USP method (e.g., Figure 1). The BET surface area of these materials varies, but rarely exceeds 500 m2/g. Control over surface area and iron particle size is established by varying the synthesis conditions (e.g., decomposition temperature, aerosol residence time). Results indicate that as the amount of iron in the precursor solution increases, both the surface area of the carbon product and the product yield increase. For example, at 600 oC and 0.5 LPM, increasing the precursor's ferric chloride concentration from 0.015 M to 0.12 M increases the product's BET surface area from 288 m2/g to 420 m2/g, while increasing the product yield by more than 300%. Elimination of iron from the same precursor solution reduces the product yield to a negligible amount for identical pyrolysis conditions. Experiments also show that, despite the presence of iron, when NaCl was not added to the precursor solution, the BET surface area was significantly reduced (e.g., from 390 m2/g to 12 m2/g) while the product yield changed only slightly. This illustrates the required role of NaCl as an in-situ template for porosity development. These observations demonstrate that the presence of iron and NaCl influences the formation of the carbon support and its resulting porous structure. References [1] Rodriguez-Reinoso F. Carbon, 1998, 36, 159-175. [2] Radovic LR, Rodriguez-Reinoso F., Chemistry and Physics of Carbon, Volume 25, Ed. Thrower PA., Marcel Dekker, New York, 1997, 243-358. [3] Skrabalak SE, Suslick KS. Journal of the American Chemical Society, 2006, 128, 12642-12643. Acknowledgements EPRI is acknowledged as the main funding source for this research and the Air and Waste Management Association is acknowledged for supplemental funding in the form of a student scholarship to John Atkinson. Analysis work including TEM and SEM was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which are partially supported by the U.S. Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471.