2002 Progress Report: Synthesis, Characterization, and Catalytic Studies of Transition Metal Carbide Nanoparticles as Environmental NanocatalystsEPA Grant Number: R829624
Title: Synthesis, Characterization, and Catalytic Studies of Transition Metal Carbide Nanoparticles as Environmental Nanocatalysts
Investigators: Shah, S. Ismat , Chen, Jingguang G.
Institution: University of Delaware
EPA Project Officer: Hahn, Intaek
Project Period: March 1, 2002 through February 28, 2006 (Extended to February 28, 2007)
Project Period Covered by this Report: March 1, 2002 through February 28, 2003
Project Amount: $350,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
The main objective of this research project is to explore the possibility of using alternative catalytic materials, transition metal carbides, and oxycarbides (defined as oxygen-modified carbides) to replace Pt-group metals for the reduction of NOx.
Modeling. We have developed a model based on the structural and thermodynamic considerations that proves it is only in the nanoscale regime that the relative surface area of the catalytically more useful open-structured surfaces almost exponentially increase with a decrease in the particle size. Thermodynamic considerations require the surface energy to be at a minimum, which can be accomplished by using the lowest energy surfaces. These surfaces are closed-packed surfaces due to the decrease in the total surface area of the particles. The lower energy surface condition leads to the formation of an octahedron shape, as shown in Figure 1. Lower surface area condition modifies this shape by truncating the vortices and forming a cuboctahedron (see Figure 2a). This also increases the surface area of the less closely packed (100) surfaces. The p and d dimensions of Figure 2b are determined by how close the particles are to the equilibrium shape. If a metastable equilibrium can be maintained, as is possible during various synthesis processes, the relative surface area of the less closely packed surface will increase rapidly. This increase is most pronounced in the nanoscale regime, as in Figure 3. It is important to note that this model is independent of the crystal structure of the material of the nanoparticles.
Synthesis of Tungsten-Carbon (WCx) Nanoparticles. As a first attempt to synthesize WCx nanoparticles, we used planar magnetron sputtering and chemical vapor infiltration (CVI). Both yielded satisfactory results albeit the amount of material synthesized by sputtering was limited. The samples were analyzed by x-ray diffraction for the crystal structure, transmission electron microscope (TEM) and scanning electron microscope (SEM) for size determination, Rutherford Backscattering Spectrometry for composition analysis, and x-ray photoelectron spectrometer for the oxidation state analysis. Figure 4 shows a TEM micrograph of particles that were synthesized by reactive sputtering and an SEM micrograph of the particles prepared by CVI. With the two techniques, particles in the range of 5-50 nm can be prepared.
Figure 1. The Octahedron of (111) Surfaces.
Figure 2a. The Cuboctahedron of (111) and (100) Surfaces.
Figure 2b. The Open (100) Surface.
Figure 3. The Relative Area R of the (100) Surfaces as a Function of the p and d.
Figure 4. (Left) 2-5 nm WCx (Right) 20-50 nm WCx Nanoparticles.
Catalytic Activity of WCx Nanoparticles. To confirm that the WCx nanoparticles are catalytically active, we tested and compared the dehydrogenation and hydrogenation activities between nano-WCx and supported nanoparticles of Pt. Another objective is to determine the feasibility of using nano-WCx as less expensive, more selective catalysts to replace Pt-group metals for the removal of aromatics molecules. The comparison clearly indicates that nano-WC and nano-Pt demonstrate different product selectivity in terms of hydrogenation and dehydrogenation. We are exploring the hydrogenation of cyclohexene and benzene on nano-WCx and nano-Pt at different hydrogen to hydrocarbon ratios. We hope to identify reaction conditions (temperature, pressure, hydrogen/hydrocarbon ratios, etc.) that would efficiently utilize the unique catalytic activities of nano-WCx. Such conditions will be adopted to further investigate the removal of the aromatics pollutants by selective hydrogenation using nano-WCx and to remove nitrogen oxides using nano-WCx.
Future activities of this research project include: (1) employing reactive gas condensation with an inductive evaporator for the synthesis of WCx nanoparticles; (2) characterizing the structure of WCx to determine the exact composition and structure of the nanoparticles; (3) studying the addition of oxygen towards the stability of the WCx nanoparticles De-NOx catalysis experiments; and (4) expanding the catalysis studies to include the oxidation of hydrocarbons.