Nanotechnology and the Environment - American Chemical Society

Chapter 36

Nanocatalysts for Environmental Technology

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 9, 2014 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch036

Sarah C. Larsen Department of Chemistry, University of Iowa, Iowa City, IA 52242

Introduction Typical industrial heterogeneous catalysts are inorganic solids that consist of metal, metal oxide or metal sulfide nanometer-sized particles dispersed on high surface area supports, such as alumina.(1-4) These materials can be used to catalyze reactions such as hydrocarbon conversion reactions, partial oxidation reactions, and hydrodesulfurization reactions, to name just a few. Heterogeneous catalysts based on supported metal nanoparticles date back to the 1920's and therefore represent the earliest successful commercial applications of nanotechnology.(2) Many things have changed since the early 20 century, notably the scientific tools, such as electron and probe microscopies, that are currently available to investigate the fundamental properties and reactivity of nanometer-sized particles. Using these tools, scientists can now investigate, manipulate and control the surface reactivity of nanocatalyst materials on the atomic scale. Nanostructured materials have important applications in environmental technology as environmental catalysts.(5-8) Environmental catalysis refers to catalytic technologies that address environmental issues, such as waste treatment and remediation, pollution prevention and the development of sustainable chemical processes.(7,8) One of the earliest success stories illustrating the use of a nanocatalyst for environmental technology is the catalytic converter that has been present in the exhaust manifold of automobiles since the early 1970's.(1,9) The catalyst in a catalytic converter consists of porous alumina which contains nanometer-sized particles of platinum, rhodium, ceria and zirconia. The role of th

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In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

269 the platinum nanoparticles is to oxidize hydrocarbons and carbon monoxide and the function of the rhodium nanoparticles is to reduce NO . The ceria and zirconia components enhance the oxidation capabilities of the catalytic converter. The catalytic converter is an example of an environmental technology designed to reduce harmful emissions in a waste treatment process. Another category of environmental technologies is pollution prevention in which the goal is to completely eliminate waste production at the source. Applications of nanocatalyts to 1) waste treatment and remediation and 2) pollution prevention and waste minimization are discussed in more detail in the next two sections.


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Waste Treatment and Remediation

Nanocatalyst materials have many important applications in waste treatment and remediation. Waste treatment and remediation includes applications such as air quality (indoor and outdoor), soil and water remediation, as well as emission controls for NOx and volatile organic compounds (VOC's). The goal of a nanocatalyst for these applications is to achieve high conversions and high selectivities so that the toxic pollutants are converted into more environmentally benign compounds. Several advantages can be realized using nanostructured catalyst materials for waste treatment and remediation processes.^ Nanostructured catalyst materials exhibit different catalytic activities and selectivities based on their size, shape, composition and surface properties. By varying these structural and compositional parameters, nanocatalysts with specific properties can be designed for applications in waste treatment and remediation. In this section, the recent work of Klabunde and coworkers on the use of oxide nanoparticles for the destructive adsorption of chemical toxins and toxic industrial chemicals is described. Klabunde and coworkers have successfully used nanoparticles of oxides, such as MgO, and mixed metal oxides to destructively adsorb toxic industial chemicals// 0) In other applications described in this chapter, nanostructured photocatalyst materials, such as T i 0 , have been used to degrade organic contaminants from polluted water and air. Current research by Shah and coworkers, focuses on increasing the photocatalytic efficiency in the visible range through the use of dopants, thereby changing the composition of the nanostructured materials//1) 2


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Pollution Prevention and Waste Minimization Pollution prevention is defined as "source reduction and practices that efficiently use raw materials, energy, water or other resources to reduce or eliminate the creation of waste.''(12) The concept of pollution prevention is at the heart of green chemistry. Specifically, green chemistry is concerned with the design of chemical products and processes that reduce or eliminate the production of toxic chemicals//3) The holy grail in catalysis research is to achieve 100% selectivity in a catalytic reaction which is compatible with the green chemistry perspective.. In the 12 principles of green chemistry, Anastas states that "catalytic reagents should be as selective as possible'//.?,) By achieving 100% selectivity, the production of by-products which must be utilized or disposed will be avoided and the goal of pollution prevention will be achieved. Nanocatalyst materials such as metal nanoclusters, nanocrystalline metal oxides, carbides or sulfides and nanostructured aluminosilicates are promising catalysts for environmental technologies designed to fulfill the goal of pollution prevention. For example, supported nanometer-sized clusters of gold exhibit very different catalytic reactivities relative to bulk gold. Supported gold nanocluster catalysts are selective catalysts for CO oxidation and other partial oxidation reactions//Similarly, supported palladium nanoclusters have been investigated for CO and NO oxidation reactions as described by Ozensoy and coworkers in this section. Son and coworkers describe the use of nanometersized octahedral molecular sieves for applications to the environmentally friendly selective oxidation of hydrocarbons. Grassian and coworkers report the use and advantages of nanocrytalline zeolites as environmental catalysts. Future development of nanocatalyst materials with the goal of pollution prevention and waste minimization will lead to the implementation of sustainable processes and products.

Summary Nanocatalyst materials hold significant promise for future applications in environmental technology. Potential nanocatalyst materials include metal nanoclusters, nanocrystalline metal oxides, carbides or sulfides, ceramics or composites, and nanostructured aluminosilicates. These nanoscale materials have many unique properties and, using the new tools of nanoscience, can be tailored for specific applications in order to achieve increased selectivity and yield for a catalytic process. Some of the advantages that may be achieved by improved nanocatalyst materials include increased energy efficiency and conversion, reductions in chemical waste, more effective waste remediation and


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the development of sustainable processes and products. The environmental benefits of nanocatalysts include cleaner air and water and, ultimately, a sustainable future.

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Chemistry for the Environment; Anastas, P. T., Williamson, T. C., Eds.; American Chemical Society, 1996, pp 1-17. 14. Choudhary, T. V.; Goodman, D. W. Topics in Catal. 2002, 21, 25-34.


Nanotechnology and the Environment - American Chemical Society

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