11.2 SCIENTIFIC AND TECHNOLOGICAL ADVANCES IN THE PAST 10 YEARS.

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1 11.1 VISION Nanoparticles have traditionally played a critical role in the effectiveness of industrial catalysts [A. Bell et al., Science 299, 1688 (2003)], but recent advances in the control of nanoscale materials and the characterization and in situ probing of catalytic processes gives rise to a new vision for these pervasively important materials. As described in Section 11.3, a grand challenge and a vision for catalysis at the nanoscale is to control the composition and structure of catalytic materials over length scales from 1 nanometer to 1 micron to provide catalytic materials that accurately and efficiently control reaction pathways (Davis and Tilley 2003). By utilizing such control, we can anticipate the formation of nanostructured catalysts that can more efficiently and selectively convert lower grade chemicals into higher value hydrocarbons, that could provide more effective pollution abatement, and that could efficiently harness solar power and redirect that energy selectively into driving key chemical processes SCIENTIFIC AND TECHNOLOGICAL ADVANCES IN THE PAST 10 YEARS. A U.S. National Science Foundation (NSF) workshop, Future Directions in Catalysis: Structures that Function at the Nanoscale, was held at NSF headquarters in Arlington, VA (Davis and Tilley 2003). The organizers, Professor Mark Davis (California Institute of Technology) and Professor Don Tilley (University of California, Berkeley), assembled a distinguished group of 34 participants, primarily from U.S. academic institutions, government agencies, national laboratories, and major companies, to assess the state-of-the-art in the field and to provide visionary statements on the future directions of catalysis research. The workshop was organized around three working groups focused on (1) synthesis, (2) characterization, and (3) theoretical modeling of catalysts; these topics formed the basic framework of a subsequent assessment of the worldwide state-of-the-art and research trends in catalysis by nanostructured materials that was assembled in 2009 by a panel of 8 experts in the field, chaired by Professor Robert Davis (University of Virginia). Both studies (2003 and 2009) articulated the consensus view that synthesis of new nanoscopic catalysts will require fundamental understanding of molecular-scale selfassembly of complex, multicomponent, metastable systems and that newly developed tools of nanotechnology will likely play a key role in the synthesis of new catalytic structures. Moreover, both studies reported on the extensive use of nanoscale characterization methods in catalysis and the need for new in situ characterization methods that extend the limits of temperature, pressure, and spatial resolution to probe nanostructured catalysts in their working state. Finally, significant advances in theoretical descriptions of complex reactions and in models that span multiple time scales have been realized over the last decade, but additional improvements are required to develop the predictive capabilities of computation, especially in liquid phase systems. A grand challenge that emerged from the 2003 workshop is to control the composition and structure of catalytic materials over length scales from 1 nanometer to 1 micron to provide catalytic materials that accurately and efficiently control reaction pathways Page 1

2 (Davis and Tilley 2003). Although great strides have been made in fulfilling this challenge, significant hurdles remain. Synthesis of Nanostructured Catalysts The need to control chemical reactivity at the nanometer length scale is paramount in catalysis research and engineering. Zeolites are archetypical high surface area crystalline solids with nanometer-size pores and cavities that allow for rational manipulation of catalytic components and diffusion paths of reagents and products. For example, research in the 1990 s artfully demonstrated the exquisite control of metal particle sizes in zeolite pores for hydrocarbon reactions relevant to fuels production. In particular, Pt clusters composed of only a few atoms can be readily synthesized by chemical methods in the nanometer-sized channels of zeolite L to form a catalyst that is especially active and selective for the formation of aromatic molecules from linear alkanes. However, a common problem with nanoporous catalysts is the severe diffusional resistance encountered by reactants and products that have molecular dimensions similar to those of the pores. If side products or carbonaceous deposits block the entrance to these pores, the entire interior regions of the pores can be rendered inaccessible for catalysis. A strategy employed by a variety of labs around the globe to overcome these inherent diffusional limitations has been to synthesize microporous zeolites (i.e. zeolites with nanometer-sized pores) with an interconnected array of mesopores (i.e. pores of much larger dimension) to improve accessibility of the internal regions of the material to reactants and products. Multiple synthetic strategies for synthesizing micro-mesoporous zeolites have evolved recently to include: 1. the use of novel organic structure directing agents that contain quaternary ammonium ions to template the original zeolite synthesis as well as alkyl chains to promote aggregation of the structure directors into micellar units (Choi et al.,2006, Nature Materials, 5: ) 2. the use of elemental carbon (in the form of carbon black, fibers, nanotubes, etc.) as a template for mesopores in the zeolite synthesis gel that can be eventually removed by complete oxidation at elevated temperature, (Christensen et al., 2005, J. Am. Chem. Soc. 127: ) 3. the use of pre-formed zeolite nanoparticles as structural building blocks to form larger crystals in the presence of a surfactant (Li et al., 2005, J. Mater. Chem. 15: ). In the above mentioned synthetic methods, the resulting materials contain both micropores and mesopores within a single solid particle, which greatly facilitates transport of reactants and products to the interior regions where the active sites are located. Another approach to overcome diffusional limitations is to synthesize new zeolite structures with larger pore systems, thus avoiding the creation of very small pore dimensions altogether (Corma et al., 2006, Nature, 443: ). Although this approach is appealing, there are significant challenges in the generalized synthesis of large pore materials as well as their stability under catalytic reaction conditions. Recent advances in the synthesis of metals and metal oxides with specific crystal facets such as Pd nanoparticles that expose the selective {100} plane for hydrogenation of dienes (Berhault et al., 2007, Appl. Catal. A 327:32-43) and CeO 2 nanoparticles that exposes the active {100} plane for CO oxidation (Aneggi et al., 2005, J. Catal. 234:88) point toward a future in catalyst preparation that will enable practitioners to rationally Page 2

3 prepare catalysts with specific surface structures that are required for desired structuresensitive catalytic reactions. Characterization of Nanostructured Catalysts The rapid developments in in-situ spectroscopic tools and atomic resolution electron microscopy over the last decade have revolutionized the understanding of catalyst structures at the nanoscale. Modern catalysis laboratories utilize routinely characterization methods such as adsorption, X-ray diffraction, scanning and transmission electron microscopy, and electron spectroscopies (XPS and Auger), as well as electron paramagnetic, nuclear magnetic resonance, ultraviolet/visible, Raman, and infrared spectroscopies. A major development over the last decade has been the emphasis on characterizing catalysts in their working state, not simply prior to or after catalytic reaction. The realization that catalysts are dynamic solids that can restructure under reaction conditions has motivated the use of characterization tools that allow for catalyst evaluation in-situ. Thus, new sample cells have been devised to interrogate materials by all of the photon-based spectroscopies as well as by EPR and NMR spectroscopy. A major recent development in characterization instrumentation involves the adaptation of electron-based methods such as X-ray photoelectron spectroscopy and electron microscopy to conditions that more closely approach catalytic reaction conditions. For example, the high photon flux provided by a synchrotron radiation source as well as differential pumping of the reaction chamber allows acquisition of XPS data on catalytic surfaces up to millibar reaction pressures. Likewise, new sample cells for transmission electron microscopy allow for the direct observation of reactive environments on metal nanoparticles at elevated temperature and mbar reaction pressure (Hansen et al., 2002, Science, 295: ). The introduction of aberration correctors onto high resolution electron microscopes has recently enabled unprecedented resolution of features on catalyst samples so that observation of single metal atoms on supports is now possible. Synchrotron radiation methods have continued to play a major role in characterization of catalysts since hard X-rays can be used to probe solids at high temperatures and pressures or in the liquid phase without the need for the catalyst to be crystalline. Therefore, recent improvements in energy resolution, spatial resolution and temporal resolution of synchrotron-based characterization methods have been implemented quickly by the catalysis community worldwide. It is now possible to simultaneously acquire small angle and wide angle X-ray scattering from catalysts while recording the X-ray absorption spectrum, all in a time-resolved manner. Indeed, some beamlines can acquire an EXAFS spectrum in less than 0.1 sec, which is on the scale of a catalytic turnover event. Although commercial IR and Raman microscopes have been available for some time and are used by some catalysis laboratories, the implementation of spatially-resolved X-ray absorption spectroscopy has taken longer to develop. Recent work in the Netherlands has achieved spatially-resolved near edge spectra on a single 600 nm iron oxide particle used for the Fischer-Tropsch synthesis reaction. The rapid development of instruments and methods that allow for the evaluation of catalyst structures at the nanoscale while in their working state has had a profound Page 3

4 impact on the field over the past decade. As catalyst research problems expand into new technology areas such as the conversion biomass into liquid products, there will be a growing need for characterization methods that allow for interrogation of solid structures in liquid environments. Thus, the important role of national-level synchrotron radiation sources in catalysis research will likely grow in the coming decade. Theory and Simulation in Catalysis Computational catalysis has reached the stage over the last decade in which it provides a necessary complement to experimental research in the field. Advances in computer processor speeds, large scale implementation of parallel architectures, and development of theoretical methods allow for complex simulations of catalytic reactions on solid surfaces that often match well to experimental results. As noted in the 2009 WTEC panel report, theory and simulation are now able to predict structures and properties of well-defined model catalysts, simulate spectra provided by a variety of experimental tools, elucidate catalytic reaction paths, and begin guiding the search for new catalytic materials and compositions. Indeed, the computational tools available today can provide structural information such as bond lengths and bond angles of surface adsorbates to within about nm and 2 degrees, respectively, and spectroscopic features of the adsorbates within a few percent of experiment. Moreover, adsorption bond energies, activation barriers and overall reaction energies are routinely calculated within a couple of days at an accuracy of less than 20 kj mol -1. Although the energies determined computationally are beyond chemical accuracy for predicting absolute reaction rates, computational catalysis can be used to discriminate between competing reaction paths and predict overall trends involving catalyst composition and reaction conditions. While theory and simulation of catalytic reactions has experienced significant success over the past decade, there are still several areas for improvement. The accuracy of the methods is still an issue for reacting systems. In addition, there needs to be a better connection between the simulation of adsorbates on surfaces and their rate of transformation to products. Thus, more work is needed to link ab initio methods used to describe the structure and energetics associated with adsorbed species to the atomistic simulations needed to describe their diffusion and reaction on the surface. Theoretical methods also do not handle very well the description of photoexcitation events that would occur during photocatalysis, an area that is likely to become more important in the next decade as researchers search for better ways to utilize solar energy. Although there has been rapid progress in computational catalysis regarding the description of catalytic reactions at the molecular level, there has been relatively little progress on the use of theory and simulation to guide catalyst synthesis. Catalyst preparation often involves the self-assembly of meta-stable structures that are guided by weak interactions with molecules within the system. Thus, catalyst synthesis presents a serious challenge to the computational community since weak forces are not welldescribed by current theoretical methods and non-equilibrium structures are difficult to find computationally. Page 4

5 Areas of Application The key applications stimulating most of the catalysis research worldwide are related to energy and the environment (WTEC Panel Report, 2009). Conversion of nonpetroleum feedstocks such as coal, natural gas, and biomass to energy and chemicals was a high priority in nearly all of the countries visited. China, in particular, has a major emphasis on energy applications, especially those involving the conversion of coal to liquid fuels. Significant activities in photocatalysis, hydrogen generation, and fuel cells are carried out in many locations. There is a general recognition that energy carriers and chemicals should be produced, and ultimately used, with as little impact on the environment as possible; catalytic solutions are thus being pursued in this framework of environmental sustainability. Catalytic production of ultra-low sulfur fuels, use of renewable carbon sources and sunlight, conversion of the greenhouse gas CO 2 to useful products, highly selective oxidation of hydrocarbons, and catalytic after-treatment of waste streams are all being pursued vigorously around the globe. A growing area of interest is the catalytic transformation of various plant sources to energy-relevant compounds such as bio-oil (a carbonaceous liquid that can be blended into a refinery stream), biodiesel fuel, hydrogen, alcohols, and so forth. Summary Statement As documented in the 2009 WTEC panel report, catalysis by nanostructured materials is an active area of research around the globe. Its rate of growth appears to be increasing faster than that of all science according to a detailed bibliometric analysis, presumably because of significant concerns regarding future energy security and environmental sustainability. Western Europe currently holds a dominant position in the world in terms of research paper output, but the rapid growth of research in Asia over the previous decade could challenge that position in the near future. Indeed, the overall investment levels in catalysis research in Western Europe and Asia appear to be significantly greater than that of the United States over the last decade. A bright spot in the analysis confirms that recent U.S. publications are the highest-cited in the world, which suggests that research funds in the United States are distributed effectively to the highest-quality laboratories GOALS FOR THE NEXT 5-10 YEARS: BARRIERS AND SOLUTIONS (Erik Scher) 11.4 SCIENTIFIC AND TECHNOLOGICAL INFRASTRUCTURE In recent years, substantial progress has been made in the synthesis of new, nanoscopic catalysts. A critical contributor to that progress lies with advances in instrumentation that makes possible monitoring of catalysts in their working state. This allows researchers to close the loop between catalyst structure and function. Further progress requires a better fundamental understanding of the molecular-scale selfassembly of complex, multicomponent, metastable systems. This in turn will depend on critical advances in theory and simulation, as well as the development and accessibility of new in situ characterization methods that will provide the requisite spatial, energy and time resolution, allowing the study of catalysts in their working state. Page 5

6 In particular, instrumentation must be developed that will allow probing of nanostructured catalysts under the appropriate conditions of pressure and temperature, while maintaining the highest spatial resolution possible. High resolution microscopy in which dynamic reshaping of metal nanocrystals can be studied in situ, would provide important insights into the mechanisms of catalytic action in nanostructures. Means of carrying out high-throughput synthesis and screening of nanostructure catalysts would allow faster convergence to optimal structures and compositions. As stated in Section 11.3, synchrotrons have played a major role in catalyst characterization, since the hard X-rays generated can probe solids at high temperatures and pressures (catalytic working conditions ), or in the liquid phase, without the requirement that the catalyst be crystalline. It is important that large-scale, complex and expensive facilities such as synchrotrons be made broadly available to the community of researchers in nanocatalysis. Computational techniques in catalysis have shown major advances, but further improvements are needed to enhance the accuracy of computational methods, particularly for reacting systems, and to develop predictive capabilities. Substantial progress is required in the use of theory and simulation to guide catalyst synthesis. Catalyst preparation often involves the self-assembly of meta-stable structures that are guided by weak interactions with molecules within the system. Thus, catalyst synthesis presents a serious challenge to the computational community since weak forces are not well-described by current theoretical methods and non-equilibrium structures are difficult to find computationally. (reproducing Mark s words in section 11.3) R&D INVESTMENT AND IMPLEMENTATION STRATEGIES (Mark, Davis, Peter Cummings) 11.6 CONCLUSIONS & PRIORITIES (Evelyn) 11.7 CASE STUDIES (Mark Davis, Bob Davis) Page 6

APPLICATIONS: CATALYSIS BY NANOSTRUCTURED MATERIALS

341 CHAPTER 10 APPLICATIONS: CATALYSIS BY NANOSTRUCTURED MATERIALS Evelyn L. Hu, S. Mark Davis, Robert Davis, Erik Scher 34 10.1 VISION FOR THE NEXT DECADE Changes in the Vision over the Last Ten Years