Environmental Catalysis - American Chemical Society

Chapter 1

NO Removal: An Overview x

John N. Armor

Downloaded by 186.216.247.190 on October 7, 2015 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch001

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NOx consists primarily of NO andNO2which are produced in all combustion processes by the oxidation of N2 and fuel bound nitrogen.NO2is linked to causing bronchitis, pneumonia, susceptibility to viral infection, and alterations to the immune system. It also contributes to acid rain, urban smog, and ozone (1). Figure 1 illustrates the various chemical transformations of NO in our atmosphere that lead to air pollution problems (2). Note that NO is the key starting point for all of the other oxides of nitrogen. NO is not only produced by the burning of fossil fuels, but also by lightning, microbial decomposition of proteins in the soil, and volcanic activity. Once produced, NO is rapidly oxidized by ozone, OH, orHO2radicals (3) to form the higher oxides of nitrogen, such as NO2, HNO2, and HO2NO2. Thus, if NO is prevented from entering the atmosphere, most of the downstream effects of NOx pollution can be eliminated. There are a number of commercial approaches to NOx removal which include absorptive, thermal, and catalytic. Since the 1960's a great deal of work was accomplished to control NOx emissions. For automotive exhausts, the current threeway catalyst uses anO2sensor to control the air/fuel ratio, which permits effective removal of NOx, hydrocarbons and CO. For exhaust gases where excessO2is present, these same automotive catalysts are not effective for removing NOx. TheseO2rich streams [such as in power plants and lean burn engines] represent major sources of NOx which must be treated. The thorough review in 1988 by Bosch and Janssen is an excellent source for further details (4). Alternatively one can try to minimize NOx formation with novel burner designs. Among the catalytic approaches to NOx emissionfrompower plants, SCR (Selective Catalytic Reduction) is growing in application. SCR uses a catalyst to facilitate reactions between NOx and NH3 in the presence of oxygen. There are a number of variants of this technology depending on the supplier. First generation plants were built in Japan and newer facilities in Germany and Austria. The Clean Air Act Amendments of 1990 will likely prompt more widespread use of SCR in the USA. Standards for NOx emission vary with the fuel and the type of utility.

0097-6156/94/0552-0002$08.00/0 © 1994 American Chemical Society

In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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ENVIRONMENTAL CATALYSIS

Beyond current SCR technology, the catalytic decomposition of NO to N2 and O2 is an attractive way to remove NO from exhaust streams. Until recentiy, numerous metals and their oxides were tried (5), but none had sufficient activity to be practical. It is generally presumed that the strongly bound product oxygen on the catalytic sites inhibits further decomposition of NO. The direct decomposition of NO to its elements is thermodynamically favorable to ~1000°C , but it had not been demonstrated in any substantial yield until recently by Iwamoto and co-workers (6). They found that a copper ion exchanged zeolite molecular sieve, Cu-ZSM-5, was very active for the NO decomposition reaction. Over the Cu-ZSM-5, catalyst sustained activities were obtained. This catalyst was so much more active than previously tested materials, that it has provoked a great deal of interest and study. This work is the focus of several papers presented at this Symposium. The reduction of NOx with hydrocarbons, instead of ammonia, in an oxidizing atmosphere is also a subject of intense research for mobile engine applications. By using hydrocarbons, the problems [e.g., ammonia slip, transportation of ammonia through residential areas and equipment corrosion] associated with the SCR process can be avoided. Currently, propane, propylene and ethylene are the most intensely studied hydrocarbons for the NOx reduction (7,8). In all these early reports, the presence of O2 was essential for the NOx reduction, demonstrating that hydrocarbons [present at low levels] readily found in most exhaust emissions can be effective for the selective reduction of NOx. A number of the papers within this Symposium also deal with this approach to removing NOx. Here the catalysts seem much more effective. Already state and local regulations, such as those in California or New Jersey, effectively result in the application of SCR to new boilers and furnaces. Near-term needs include improving the performance of the catalysts at lower operating temperatures [e.g., 150-300°C] to allow NOx control after other acid gas and particulate pollution controls, and improving the perfomance of the catalysts at higher operating temperatures [e.g., 1,450-2,000°C] to allow placement of the NOx control system at the discharge of gas turbines. The actual flow of papers within the Symposium covered all aspects of NOx removal, but for organization, this broad topic will be segregated by catalysts that operate in O2 rich or poor conditions. Robert McCabe was the chairperson of the topical area on "Mechanisms of NOx Removal" and provided the following summary of the papers presented at this meeting: "The papers dealing with lean NOx control by decomposition or selective hydrocarbon reduction, can best be described as a mechanistic free-for-all - as expected, given the relatively recent emergence of this area. Keynote lectures were delivered by W. K. Hall (Univ. of Pittsburgh) and M. Iwamoto (Hokkaido Univ.) Professor Hall, in addition to reviewing the general understanding of redox chemistry in metal-exchanged zeolites, also presented results of recent in-situ ESR experiments on Cu-ZSM-5 which showed that rates of selective NO reduction are highest when Cu is nominally in the +2 oxidation state. Hall also presented data suggesting the importance of NO2 as a reaction intermediate. Professor Iwamoto reviewed his pioneering work in NOx decomposition and selective reduction and presented recent IR spectroscopic and kinetic data suggesting that adsorbed isocyanate species are key intermediates in the selective reduction reaction. In contrast to the work of Hall and Iwamoto, A. P. Walker (Johnson Matthey Co.) used a temporal-



1. ARMOR

NQ Removal: An Overview

5

analysis-of-products (TAP) reactor to show that NO can react with adsorbed hydrocarbon residue ("coke") on Cu-ZSM-5 catalysts. Walker stressed the importance of zeolite acidity in forming the carbonaceous species and indicated that catalyst acidity is a key parameter being employed by Johnson Matthey in developing other zeolite and non-zeolite catalyst formulations. Additional studies dealing with mechanistic aspects of lean NOx decomposition or selective reduction included presentations by G. Centi (Univ. of Bologna), showing reversible formation of mono- and dinitrosyl species on supported copper oxide catalysts, and presentations by H. W Jen (Ford Motor) and J. P. McWilliams (Mobil Oil Co.), both stressing the importance of hydrocarbon oxidation in the overall NOx reaction scheme. Other studies were directed at characterizing CuZSM-5 and related catalysts by various techniques including TPD-TGA of isopropyl amine (D. Parillo - U. of Pennsylvania), XPS (M. Shelef - Ford) and XRD plus highresolution TEM (K. Kharas - Allied Signal CO.). Another group of papers examined novel applications of lean NOx catalysts and/or novel modifications of the catalyst formulation. The Editor reported on a new catalytic process that efficiently reduces NOx with methane in the presence of excess O2 . This process, which is based on a cobalt-exchanged ZSM-5 catalyst, potentially provides an efficient low-temperature route to NOx emission control in natural gas fired power plants. R. Gopalakrishnan (Brigham Young Univ.) described Catalysts for Clean-Up of NOx, NH and CO from Nuclear Waste Processing. They looked at a variety of catalysts for NOx and CO removal and found that Cu-ZSM-5 was very good for ammonia SCR but not for CO oxidation. Pt/Al 0 was very good for ammonia and CO oxidation but not for NOx removal. They suggested using a two-stage catalyst: Cu-ZSM-5 followed by Pt/Al 0 in order to remove pollutants from dilute waste streams containing NOx, ammonia, and CO. M. Flytzani-Stephanopoulos (M.I.T.) reported on Cu-ZSM-5 catalysts prepared with a series of co-cations. Those prepared with rare-earth co-cations, in particular, showed improvements in low-temperature NO decomposition activity. It was clear from the wide range of papers presented, as well as the divergent mechanistic viewpoints, that NOx removal in O2 rich exhausts by selective hydrocarbon reduction is still in its infancy with respect to fundamental understanding and practical application. Clearly many materials are capable of promoting lean NOx catalysis, and the many mechanisms offered strongly suggest that there is more than one pathway by which NOx can be reacted with hydrocarbons in the presence of excess O2. By way of juxtaposition, the invited lecture by A. T. Bell (Univ. of California - Berkeley), which dealt with conventional NH -based SCR over both Pt/Al 0 and V 0 and ¥203/1102 catalysts, elegantly demonstrated the much greater extent of mechanistic understanding that has been garnered in the mature ammonia-based selective catalytic reduction process compared to the newer hydrocarbon-based processes." With the above as background to the problem of NOx emissions and treatment, this section of the book will focus on these new approaches to controlling NOx by catalytic decomposition or reduction. A majority of these presentations are presented as full papers within this book. Because of the current, intense interest in NOx removal and Iwamoto's work many of the papers focused on Cu-ZSM-5 for NO decomposition or reduction by hydrocarbons. The following section of mobile engine exhausts will revisit this topic for exhaust streams which do not contain excess O2. 2

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ENVIRONMENTAL CATALYSIS


References 1. Armor, J. N. Appl. Catal. B, 1992, 1, 221-256 2. Sloss, L.L.; Hjalmarsson, A-K.; Soud, H. N.; Campbell, L. M.;Stone, D. K.; Shareef, G. S.; Emmel, T.; Maibodi, M; Livengood, D.; Markussen, J. Nitrogen Oxides Control Technology Fact Book, Noyes Data Corp.; Park Ridge, New Jersey, 1992; p. 5. 3. Seinfeld, J. H. Science 1989, 243, 745. 4. Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369-532. 5. Hightower, J. W.; Van Leirsburg, D. A. In The Catalytic Chemistry of Nitrogen Oxides; Klimisch, R. L.; Larson, J. G., Eds.; Plenum Press: New York, 1975; p. 63. 6. Iwamoto, M. In Future Opportunities in Catalytic and Separation Technology; Misono, M.; Moro-oka, Y.; Kimura, S., Eds.; Elsevier: Amsterdam, 1990; p. 121 7. Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T. Appl. Catal.,1990, 64, L1. 8. Sato, S. S.; Hirabay, H.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Catal. Lett., 1992, 12, 193. RECEIVED

November 16, 1993


Environmental Catalysis - American Chemical Society

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