Selective Catalytic Reduction of Nitric Oxide by Propylene over Supported Platinum Catalysts
Michael D. Amiridis
Department of Chemical Engineering, University of South Carolina
Columbia, SC 29204, USA
Abstract
The selective catalytic reduction (SCR) of nitric oxide (NO) by propylene (C3H6) was studied over 1 wt. % Pt/Al2O3 and Pt/SiO2 catalysts. Detailed kinetic and in situ Fourier Transform Infrared (FTIR) spectroscopy experiments were conducted in an attempt to gain an understanding of the fundamental chemistry governing the SCR reaction. The kinetic studies indicate that the Pt/Al2O3 and Pt/SiO2 catalysts exhibit a similar overall activity. There is a presence of two kinetically distinct regimes above and below the temperature of maximum NO reduction; 285° C for Pt/SiO2 and 302° C for Pt/Al2O3. Both catalysts exhibited a zero order dependence on the NO concentration at low temperatures and a first order dependence at higher temperatures. The reaction rate exhibits a maximum with O2 concentration, consistent with the dual role of oxygen in activating propylene and competing with NO. These results suggest that the activation of the hydrocarbon by oxygen is the rate-determining step at low temperatures. Kinetic experiments carried out in the presence of H2O did not significantly affect the performance of these catalysts.
In situ FTIR studies over the Pt/Al2O3 catalyst at 250° C result in the formation of adsorbed carboxylate and nitrate species. These are believed to be ‘spectators’ that do not participate in the SCR process. Surface cyanides (-CN) and isocyanates (-NCO) are also observed. The cyanide group, in this case, is very stable and does not react with O2 and/or NOx. On the contrary, the isocyanate group is weakly held onto the surface, and reacts with O2, NO and NO2. Similar experiments with the Pt/SiO2 suggest that on this catalyst, the surface cyanide group is reactive towards O2, NO and NO2. Additionally, the presence of O2 is believed to promote the formation of the cyanide in this case. A reaction mechanism will be proposed based on these results.
Opportunities for Collaboration (M. Amiridis)
The environmental catalysis group at the University of South Carolina is eager to be engaged in international collaborations with colleagues who share similar research interests and have expertise and access to techniques which are complementary to our own. These efforts can also be designed to include 6-12 month exchange visits by graduate students, postdoctoral fellows and senior investigators. Our work focuses on the control of unwanted emissions (NOx and polychlorinated aromatics), the production of hydrogen for fuel cell applications and the synthesis of pharmaceutical compounds via heterogeneous catalytic routes. Short descriptions of the ongoing projects in these areas are given below. As far as experimental capabilities are concerned, we have access to and frequently employ in our studies kinetic and chemisorption measurements, in-situ FTIR spectroscopy, and structural characterization by X-Ray Diffraction and several Electron Microscopy techniques.
1. The Reduction of Nitric Oxide
Our efforts in this area focus on the selective catalytic reduction of NO by propylene over noble metal catalysts. We have characterized the kinetic behavior of various supported Pt catalysts and have correlated catalyst characteristics (such as the Pt loading and morphology and the nature of the support) with catalytic activity. We have also utilized in-situ FTIR spectroscopy extensively and have succeeded in identifying reaction intermediates. More recently, we have been focusing on Pt-Au bimetallic systems, since the presence of Au may improve the selectivity of Pt towards dinitrogen (which is the desired reduction product). Controlled catalyst preparation in this case is a prerequisite in order to obtain well-defined Pt-Au structures, which in turn are necessary for the development of a fundamental understanding of the surface chemistry taking place on both metals. For the preparation of such "tailor-made" catalysts we have been examining the use of bimetallic clusters with predefined structures and the problems associated with the "anchoring" of these clusters on high surface area inorganic supports.
2. The Catalytic Destruction of Polychlorinated Aromatics
Of particular interest to us is the control of dioxin and furan emissions from the flue gas of municipal waste incinerator units. The goal of our work in this area is the development of an understanding of the surface chemistry taking place during the oxidation of these compounds. Ortho-dichlorobenzene (o-DCB) is used as a model compound. We have kinetically characterized a number of transition metal oxides (e.g., Cr2O3, V2O5, Fe2O3, WO3, etc.) supported on Al2O3 and TiO2 and have outlined their similarities and differences. Furthermore, we have conducted in-situ FTIR studies and demonstrated that the oxidation of o-DCB proceeds through a similar mechanism in all cases. We are currently extending our work to perovskites with the general formula LaMO3, where M is a transition metal from the group of metals already examined. Our preliminary results in this area reveal significant differences in both the activity and selectivity of the transition metal sites in the two different environments (i.e., supported or incorporated in the perovskite framework). We are attempting to correlate these differences in catalytic behavior with changes in the electronic structure of the transition metal sites. If successful, this work could have far reaching implications for a number of oxidation reaction catalyzed by transition metal oxides.3. Hydrogen Production via the Direct Cracking of Hydrocarbons
The direct cracking of methane is an alternative route for the production of hydrogen (as opposed to steam reforming or partial oxidation) that eliminates the formation of carbon oxides. Hydrogen produced in this fashion can be directly used for the production of electricity in fuel cells. Recent studies have shown that in addition to the apparent environmental benefits (i.e., elimination of carbon oxide emissions), this approach is also economically attractive. We have demonstrated the feasibility of hydrogen production via the direct cracking of methane with Ni/SiO2 catalysts, which were found to maintain their activity for significant periods of time due to the formation of filamentous carbon. Although these catalysts eventually deactivate when the reactor is filled with filaments, they can be fully regenerated, and hence, participate in a cyclic process. We have also discovered a promoting effect by Cu, which we attributed to alloy formation that facilitates carbon diffusion and filament formation. We are currently investigating the activity of other Ni-containing alloys. We are also interested in unsupported metallic powders, as well as the development of methods for the separation and recovery of the filamentous carbon produced as a reaction by-product.
4. Heterogeneous Catalytic Routes for Synthesis of Pharmaceuticals
We have recently initiated some work in the development of new environmentally friendlier processes for the synthesis of fine chemicals and pharmaceuticals. This is a very fertile area for heterogeneous catalysis, since the homogeneous processes currently utilized for the synthesis of these compounds result to the generation of waste that frequently exceeds the amount of the desired product by a factor of 50 or more. Our first project in this area focuses on the heterogeneous catalytic synthesis of flavanone from benzaldehyde and acetophenone over Mg-based catalysts. We are at the early stages of this project and we are currently exploring the kinetics of the reaction.