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Title: Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts

Abstract

The development of new catalytic materials is still dominated by trial and error methods, even though the experimental and theoretical bases for their characterization have improved dramatically in recent years. Although it has been successful, the empirical development of catalytic materials is time consuming and expensive with no guarantee of success. We have been exploring computationally complex but experimentally simple systems to establish a 'catalysis by design' protocol that combines the power of theory and experiment. We hope to translate the fundamental insights directly into a complete catalyst system that is technologically relevant. The essential component of this approach is that the catalysts are iteratively examined by both theoretical and experimental methods. This approach involves state-of-the-art first principle density functional theory calculations, experimental design of catalyst sites, and sub-Angstrom resolution imaging with an aberration-corrected electron microscope to characterize the microstructure. We are employing this approach to understand the catalytic sites in oxidation and lean NOx catalysts. The model material for the oxidation catalyst system is Pt/Al2O3 and that for lean NOx catalysis is Ag/Al2O3. We present our initial results on theoretical and experimental studies of the oxidation and reactivity of catalyst clusters towards O, CO, and NOx. Our theoretical studiesmore » indicate that the reaction energetics are strongly dependent on the size of the clusters as well as the extent of oxidation of the clusters. We speculate that the energetics of CO and NO oxidation may be more favorable on the oxidized clusters than on the pure Pt clusters because of the weakened adsorption of the reactants. Experimentally, we have synthesized supported catalysts that contain small metal particles that mimic the theoretical models. We have also synthesized various supported catalysts with larger metal particles. We have studied CO oxidations on these catalysts and the results (including microstructural changes in Pt particles) will also be presented. In addition, we will summarize the results of our study of microstructural changes in supported catalysts, especially Ag/Al2O3, exposed to simulated lean NOx exhaust via the ORNL ex situ reactor in order to determine the impact of operating conditions on the catalyst.« less

Authors:
 [1];  [1];  [1];  [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
930912
DOE Contract Number:
AC05-00OR22725
Resource Type:
Conference
Resource Relation:
Journal Volume: 1; Conference: Society of Automotive Engineers Congress, Detroit, MI, USA, 20070416, 20070419
Country of Publication:
United States
Language:
English

Citation Formats

Narula, Chaitanya Kumar, DeBusk, Melanie Moses, Xu, Ye, Blom, Douglas Allen, Allard Jr, Lawrence Frederick, and Shelton Jr, William Allison. Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts. United States: N. p., 2007. Web. doi:10.4271/2007-01-1018.
Narula, Chaitanya Kumar, DeBusk, Melanie Moses, Xu, Ye, Blom, Douglas Allen, Allard Jr, Lawrence Frederick, & Shelton Jr, William Allison. Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts. United States. doi:10.4271/2007-01-1018.
Narula, Chaitanya Kumar, DeBusk, Melanie Moses, Xu, Ye, Blom, Douglas Allen, Allard Jr, Lawrence Frederick, and Shelton Jr, William Allison. Mon . "Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts". United States. doi:10.4271/2007-01-1018.
@article{osti_930912,
title = {Catalysis by Design - Theoretical and Experimental Studies of Model Catalysts},
author = {Narula, Chaitanya Kumar and DeBusk, Melanie Moses and Xu, Ye and Blom, Douglas Allen and Allard Jr, Lawrence Frederick and Shelton Jr, William Allison},
abstractNote = {The development of new catalytic materials is still dominated by trial and error methods, even though the experimental and theoretical bases for their characterization have improved dramatically in recent years. Although it has been successful, the empirical development of catalytic materials is time consuming and expensive with no guarantee of success. We have been exploring computationally complex but experimentally simple systems to establish a 'catalysis by design' protocol that combines the power of theory and experiment. We hope to translate the fundamental insights directly into a complete catalyst system that is technologically relevant. The essential component of this approach is that the catalysts are iteratively examined by both theoretical and experimental methods. This approach involves state-of-the-art first principle density functional theory calculations, experimental design of catalyst sites, and sub-Angstrom resolution imaging with an aberration-corrected electron microscope to characterize the microstructure. We are employing this approach to understand the catalytic sites in oxidation and lean NOx catalysts. The model material for the oxidation catalyst system is Pt/Al2O3 and that for lean NOx catalysis is Ag/Al2O3. We present our initial results on theoretical and experimental studies of the oxidation and reactivity of catalyst clusters towards O, CO, and NOx. Our theoretical studies indicate that the reaction energetics are strongly dependent on the size of the clusters as well as the extent of oxidation of the clusters. We speculate that the energetics of CO and NO oxidation may be more favorable on the oxidized clusters than on the pure Pt clusters because of the weakened adsorption of the reactants. Experimentally, we have synthesized supported catalysts that contain small metal particles that mimic the theoretical models. We have also synthesized various supported catalysts with larger metal particles. We have studied CO oxidations on these catalysts and the results (including microstructural changes in Pt particles) will also be presented. In addition, we will summarize the results of our study of microstructural changes in supported catalysts, especially Ag/Al2O3, exposed to simulated lean NOx exhaust via the ORNL ex situ reactor in order to determine the impact of operating conditions on the catalyst.},
doi = {10.4271/2007-01-1018},
journal = {},
number = ,
volume = 1,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Conference:
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  • Traditionally, knowledge in heterogeneous catalysis has come through empirical research. Nowadays, there is a clear interest to change this since millions of dollars in products are generated every year in the chemical and petrochemical industries through catalytic processes. To obtain a fundamental knowledge of the factors that determine the activity of heterogeneous catalysts is a challenge for modern science since many of these systems are very complex in nature. In principle, when a molecule adsorbs on the surface of a heterogeneous catalyst, it can interact with a large number of bonding sites. It is known that the chemical properties ofmore » these bonding sites depend strongly on the chemical environment around them. Thus, there can be big variations in chemical reactivity when going from one region to another in the surface of a heterogeneous catalyst. A main objective is to understand how the structural and electronic properties of a surface affect the energetics for adsorption processes and the paths for dissociation and chemical reactions. In recent years, advances in instrumentation and experimental procedures have allowed a large series of detailed works on the surface chemistry of heterogeneous catalysts. In many cases, these experimental studies have shown interesting and unique phenomena. Theory is needed to unravel the basic interactions behind these phenomena and to provide a general framework for the interpretation of experimental results. Ideally, theoretical calculations based on density-functional theory have evolved to the point that one should be able to predict patterns in the activity of catalytic surfaces. As in the case of experimental techniques, no single theoretical approach is able to address the large diversity of phenomena occurring on a catalyst. Catalytic surfaces are usually modeled using either a finite cluster or a two-dimensionally periodic slab. Many articles have been published comparing the results of these two approaches. An important advantage of the cluster approach is that one can use the whole spectrum of quantum-chemical methods developed for small molecules with relatively minor modifications. On the other hand, the numerical effort involved in cluster calculations increases rather quickly with the size of the cluster. This problem does not exist when using slab models. Due to the explicit incorporation of the periodicity of the crystal lattice through the Bloch theorem, the actual dimension of a slab calculation depends only on the size of the unit cell. In practical terms, the slab approach is mainly useful for investigating the behavior of adsorbates at medium and high coverages. Very large unit cells are required at the limit of low to zero coverage, or when examining the properties and chemical behavior of isolated defect sites in a surface. In these cases, from a computational viewpoint, the cluster approach can be much more cost effective than the slab approach. Slab and cluster calculations can be performed at different levels of sophistication: semi-empirical methods, simple ab initio Hartree-Fock, ab initio post-Hartree-Fock (CI, MP2, etc), and density functional theory. Density-functional (DF) based calculations frequently give adsorption geometries with a high degree of accuracy and predict reliable trends for the energetics of adsorption reactions. This article provides a review of recent theoretical studies that deal with the behavior of novel catalysts used for hydrodesulfurization (HDS) reactions and the production of hydrogen (i.e. catalytic processes employed in the generation of clean fuels). These studies involve a strong coupling of theory and experiment. A significant fraction of the review is focused on the importance of size-effects and correlations between the electronic and chemical properties of catalytic materials. The article begins with a discussion of results for the desulfurization of thiophene on metal carbides and phosphides, systems which have the potential to become the next generation of industrial HDS catalysts. Then, systematic studies concerned with the hydrogen-evolution reaction (HER) on extended surfaces, organometallic complexes and enzymes are presented. Finally, the reasons for the high catalytic activity of Au-CeO{sub 2} and Cu-CeO{sub 2} in the production of hydrogen through the water-gas shift reaction (CO + H{sub 2}O {yields} H{sub 2} + CO{sub 2}) are analyzed. It is shown that theoretical methods are very valuable tools for helping in the rational design of heterogeneous catalysts.« less