2004 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, 2004 through February 28, 2005
Project Amount: $350,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
The 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 platinum (Pt)-group metals for the reduction of NOx.
Two types of samples were prepared by laser ablation . The first one was pure tungsten carbide (WC), in which a pure WC target was used for ablation. The second type of sample was prepared from pure W nanoparticles and was subsequently carburized in a CH4 atmosphere at 1,000° C for 1 hour. The X-ray diffraction (XRD) patterns for both pure WC and carburized W were indexed with the diffraction lines of WC in the powder diffraction diagram reported by the International Center for Diffraction Data . The XRD of the carburized sample shows W reflections in addition to the WC reflections, indicating that the samples are not completely carburized. The particle size was determined from the XRD data using the Scherer’s formula (Cullity and Stock, 2001). The particle size of the as- prepared WC was found to be around 60 nm. The particle size of the carburized sample was a little larger, about 100 nm, perhaps due to sintering during carburization.
Both as- prepared and carburized WC particles were tested for catalytic activity for NOx reduction. Samples of equal weight were loaded in a reactor. Five DeNOx cycles were run consecutively for both as-prepared and carburized samples to check the stability of the catalyst. For the as-prepared WC, the onset of activity was around 350° C, and reached a conversion efficiency of over 80 percent at temperatures above 600° C. For the carburized sample a slightly higher onset of activity temperature, approximately 400° C, was observed. The activity through the first cycle also was different than that for the subsequent cycles. In the first cycle the onset of activity was at around 600° C. This is probably caused by incomplete carburization, which may result in some extra carbon depositing on the particles during carburization which was then removed in the first cycle. The subsequent results, however, are repeatable, and the conversion efficiency also reaches over 80 percent at temperatures 550° C and above
To investigate the structural stability of the samples after the DeNOx process, XRD and X-ray photoelectron spectroscopy (XPS) analyses were carried out on samples before and after the DeNOx cycle(s). For XRD structural investigation, the samples were kept in a He plus NO atmosphere at three different temperatures for 2 hours. The gas flow rate was fixed for all the samples. In both the as-prepared and carburized sample cases, the formation of oxide was noticed. The intensity of the oxide peaks increased with temperature.
XPS analyses were carried out on the as-prepared WC samples, b oth as virgin samples and after five cycles of DeNOx, to investigate the nature of the oxide. The oxide peak in the virgin sample is just surface oxide caused by atmospheric exposure because the XRD analysis of the as-prepared WC shows only WC reflections. The XPS spectra of the sample after 5 cycles of DeNOx show a higher intensity of the oxide peak. Both the XPS and XRD confirm the oxidation of the catalyst. However, some pure WC phase remains, which explains why the catalyst remained active. In a real automobile exhaust, there is also CO and hydrocarbons that can serve to replenish the C in WC and keep it from oxidizing which our preliminary experiments confirm. This work is continuing during our extension period, in addition to stabilizing the carbide phase with intentional addition of the oxide phase.
We will: (1) conduct DeNOx catalysis experiments with the addition of hydrocarbons and CO to the feed gas, and (2) expand the catalysis studies to include oxide and nitride stabilized WC.
Cullity D, Stock SR. Elements of X-Ray Diffraction. (Prentice Hall, New Jersey, 2001).