2005 Progress Report: Nanostructured Catalytic Materials for NOx Reduction Using Combinatorial MethodologiesEPA Grant Number: R830896
Title: Nanostructured Catalytic Materials for NOx Reduction Using Combinatorial Methodologies
Investigators: Senkan, Selim M.
Institution: University of California - Los Angeles
EPA Project Officer: Lasat, Mitch
Project Period: June 1, 2003 through May 31, 2007
Project Period Covered by this Report: June 1, 2004 through May 31, 2005
Project Amount: $356,000
RFA: Environmental Futures Research in Nanoscale Science Engineering and Technology (2002) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Nanotechnology , Safer Chemicals
The objective of this research is to integrate the new advances made in nanostructured materials with combinatorial methodologies for the discovery and optimization of novel catalytic materials for the reduction of NOx emissions under fuel lean combustion conditions. Although the existing three-way catalysts allow for the effective control of NOx, CO, and hydrocarbon emissions in current gasoline engines that operate under stoichiometric conditions, they are ineffective in the presence of excess oxygen encountered in lean-burn engine exhausts. Therefore, the development of a new generation of catalysts that will allow NOx control in oxygen-rich environments is urgently needed.
Recent developments in automotive engineering have made possible the production of more fuel efficient–up to 25 percent–lean burning gasoline engines. However, the lack of appropriate catalytic technology to reduce NOx emissions under lean-burn conditions impedes the commercialization of such engines (Farrauto and Heck, 1999). In principle, NOx reduction could be achieved by either decomposition of NOx directly into N2 and O2 or by selective catalytic reduction (SCR) using hydrocarbons (HC), which also are present in the exhaust gases. The direct decomposition of NOx is thermodynamically feasible below 900°C (ΔG0f= -86 kJ/mol), and thus represents the best option for lean exhaust treatment (Bhattacharya and Das, 1999). However, because of the lack of progress in this approach, efforts were directed towards developing SCR-based technologies for NOx reduction. These efforts were spurred by the discoveries that NO can selectively be reduced over Cu/ZSM-5 (Held, et al., 1990; Sato, et al., 1991) and Pt-Al2O3 (Hamada, et al., 1991; Obuchi, et al., 1993) by HCs such as C2H4, C3H6, and C3H8. Nevertheless, both of these materials subsequently were shown to possess considerable operational problems. For example, although Cu/ZSM-5 initially has high activity and selectivity, its activity severely decreases when exposed to steam, which is an inevitable combustion product. Platinum supported on Al2O3 is stable when exposed to steam, but its activity is restricted to a narrow temperature window, and also suffers from substantial N2O production, which is a pollutant (Obuchi, et al., 1993).
The emergence of combinatorial or high-throughput experimentation methods and tools now offers fresh new hope to many pressing problems in catalytic science and technology (Senkan, 2001), including lean burn NOx reduction catalysis. In order to exploit this new methodology, we have developed array channel microreactors to rapidly screen libraries of catalytic materials (Senkan, et al., 1999), together with optical (Senkan, 1998; Senkan, et al., 2003) and mass spectrometric (MS) (Senkan, et al., 1999) detection techniques. Previously, we reported on the feasibility of using array microreactors and MS as a rapid screening tool for NOx reduction catalysis research (Krantz, et al., 2000). Most recently, we developed a high-throughput pulsed laser ablation (HT-PLA) system for the rapid synthesis of uniformly sized metal nanoparticles for catalytic applications (Senkan, et al., 2006).
To date, more than 1,000 distinct catalytic materials were prepared using either impregnation or PLA. In the former case, catalysts were prepared by individually impregnating aqueous salt solutions of 42 elements from the periodic table into five support materials at five different metal loadings. The powders of support materials (γ-Al2O3, CeO2, SiO2, TiO2, Y-ZrO2) were acquired from commercial vendors. These materials were chosen because of their hydrothermal stability, availability, and ability to be pelletized without a binder. The powders then were formed into 1 x 4 mm cylindrical pellets by a commercial press using a custom designed die and punch set. Forty-six metal salt solutions were created for 42 elements from the periodic table. For K, In, Sn, and La, multiple precursors were used. The metal salts were chosen so as to have adequate solubility, availability, and low toxicity. The salts had to be soluble enough to allow for high metal loadings on the pellets using microliter volumes of solution. Radioactive and toxic metals were disregarded from consideration for safety reasons. Predetermined quantities of stock solutions were introduced into the wells of a 96-chamber well-plate using a computer-controlled liquid dispensation system (Cartesian Technologies, Inc., Irvine, CA). Five different concentrations of precursor solutions were used to prepare a library of catalytic materials, with metal loadings being in the range 0.001 percent to 25 percent, depending on the specific element and the support involved. This range of metal loading should create a good diversity of catalytic materials from well dispersed small metal/metal-oxide ensembles at low loading to large clusters at high loadings. After the stock solutions were dispensed, they were diluted, if necessary, to give 50 μL total solution on the well plate. The pellets of support materials then were placed into the wells to affect impregnation. The pellets were soaked in the solutions for 72 hours under an atmosphere of 100 percent relative humidity. Finally, the pellets were dried at 50°C for 12 hrs, 80°C for 12 hours, 120°C for 2 hours, and calcined in air at 600°C for 4 hours. In some cases, the supports were dissolved by the solutions used or were overloaded with the metal within a single impregnation step; these materials were discarded. Consequently, for some metal-support combinations, less than five distinct materials were tested. The reaction screening of these catalytic materials was summarized in previous progress reports and in Krantz, et al. (2004).
In PLA, nanoparticles of a number of transition metals were deposited on the same support materials described above. In Figure 1 the HT-PLA system developed is shown. The setup consists of a rotatable target holder that houses multiple targets (24 cylindrical targets with 1.25 cm diameters in the current design). The target under laser illumination is continuously spun at a rate of about 5-10 rpm for the better utilization of the target material. A pulsed laser beam (Lambda Physik Compex 100 Excimer Laser, 300 mJ/pulse, 30 ns pulse duration) is focused to a spot of about 0.1 cm diameter, off axis on the surface of the target, arriving at an angle of about 45 degrees. Nanoparticles created in the ablation plume then are collected either on support pellets (0.4 cm diameter by 0.1 cm thick cylinders), on standard transmission electron microscope (TEM) grids, or single crystal silicon wafers as shown in Figure 1. Consequently, it is possible to readily characterize the nanoparticles before reaction screening. In the current design, a rotatable holder is used to house up to 30 support pellets and/or TEM grids. A mask prevents cross-contamination of the sites. The distance between the target and collection site also can be adjusted. This assembly is placed inside a gas tight (vacuum) chamber to control the ambient gas conditions during PLA (Ar in the present experiments). The laser beam is introduced into the chamber through a UV transparent fused silica window. The size and composition of the nanoparticles formed in PLA are primarily determined by the laser power density (i.e. fluence, energy of the photons, and pulse duration), the nature and density of the ambient gas in the ablation chamber (Senkan, et al., 2006), and the distance from the target surface. Other parameters, such as the light absorbance, heat capacity, enthalpy of vaporization, boiling point, and thermal conductivity of the target also influence the properties of the nanoparticles. The HT-PLA system reported here enables the systematic exploration of all of these variables within a single experiment, and allows the rapid determination of conditions under which the synthesis of nanoparticles suitable for specific catalytic applications can be accomplished.
An actual picture of a rhodium (Rh) ablation plume also is shown in Figure 1. It is particularly important to note that, in PLA, nanoparticles are deposited on the external surfaces of the support materials, without penetration into their pores. Consequently, catalyst screening is accomplished in the absence of pore diffusion limitations. In other words, catalytic materials prepared by PLA can be evaluated under identical intrinsic reaction rate limited (i.e., transport free) conditions, thus their rank order can be obtained unambiguously. This is another advantage of PLA compared to other catalyst preparation methods.
In Figure 2, TEM images of Rh nanoparticles collected on carbon film are presented as a function of distance from the target and the number of pulses. As evident from this composite image, nanoparticle diameters systematically increased with increasing number of pulses as a consequence of the landing of new nanoparticles on or adjacent to pre-existing ones followed by their fusion to create a single spherical particle. Although the particle size distribution is narrow at low laser pulses, this distribution broadens with increasing number of pulses due to the random nature of the particle encounter process on the support surface, as indicated in the histograms embedded into each TEM image in Figure 2. Also seen from Figure 2 is the systematic decrease in particle size with increasing distance from the target at a fixed number of laser pulses. In this case, the particle size distribution remains nearly the same, although the number and density of particles decrease with increasing distance from the target due to the spherical expansion of the plume. Currently, research is in progress for the reaction screening of catalytic materials prepared by HT-PLA.
Figure 1. A Sketch of the High Throughput Pulsed Laser Ablation (HT-PLA) System That Enables the Rapid Preparation of a Large Number of Supported Multi-Metallic Nano-Clusters. (A) Target, (B) Rotatable (selectable) target holder, (C) Pulsed laser beam, (D) Ablation plume, (E) Catalyst support or TEM grid containing nanoparticles, (F) Catalyst support or TEM grid holder, (G) Mask. All of the components are placed in a gas tight (vacuum) chamber for the control of pressure and the nature of the ambient gas. A multitude of different targets (24 shown) can be ablated sequentially, and the nanoparticles created can be deposited directly on support pellets or TEM grids. The pellets can then be used for catalytic screening. A mask is used to prevent cross-contamination of sites. The distance between the target and support pellets also can be adjusted. The pulsed laser beam enters the chamber through a fused silica window and irradiates the target at a 45° angle. The target under illumination is continuously spun at 5-10 rpm for the better utilization of the target material. Also shown is a rhodium metal ablation plume showing the target and schematic collection of nanoparticles on the external surface of support pellets, TEM grids, or silicon wafers, by placing them inside the plume.
Figure 2. Transmission Electron Microscope (TEM) Images of Rh Nanoparticles on Carbon Film. Images from left to right correspond to increasing number of laser pulses at the same plume location. Images from top to bottom correspond to increasing distance from the target.
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Senkan S, Kahn M, Duan S, Ly A, Leidholm C. Catalysis Today 2006;117(1-3):291-296.
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Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 5 publications||5 publications in selected types||All 5 journal articles|
||Duan S, Senkan S. Catalytic conversion of ethanol to hydrogen using combinatorial methods. Industrial & Engineering Chemistry Research 2005;44(16):6381-6386.||
||Krantz K, Senkan S. Systematic evaluation of monometallic catalytic materials for lean-burn NOx reduction using combinatorial methods. Catalysis Today 2004;98(3):413-421.||
||Senkan S, Kahn M, Duan S, Ly A, Leidholm C. High-throughput metal nanoparticle catalysis by pulsed laser ablation. Catalysis Today 2006;117(1-3):291-296.||