SCIENCE/TECHNOLOGY
Kinetics Plays Growing Role in Modeling Chemical Systems Approach shows promise in studies of ozone depletion, tropospheric air pollution, high-energy chemistry, and other complex phenomena Stu Borman, C&EN Washington
Kinetics, the study of reaction rates and mechanisms, is playing an increasingly important role in modeling of a wide range of complex chemical phenomena. Areas benefiting from greater kinetics emphasis include ozone depletion, tropospheric air pollution, plasma processing of electronic devices, combustion and oxidation processes, and high-energy chemistry. "Kinetics is used in almost all ap-
plied areas of chemistry," says John T. Herron, director of the chemical kinetics data center at the National Institute of Standards & Technology, Gaithersburg, Md. "For example, in environmental science, it goes into all the models that predict how much ozone we're going to deplete in the stratosphere or how much we're going to form in the troposphere. Kinetics is often the deciding factor." NIST plays a central role in the kinetics community because its kinetics data center provides databases that are widely used for modeling complex chemical systems and because the institute sponsors the only conference series in North America devoted to all aspects of applied kinetics. The latest meeting in this series, the 2nd International Conference on Chemical Ki-
Herron: kinetics is used in almost all applied areas of chemistry
Kinetics research involves three levels of abstraction Chemical kinetics embraces three distinct levels of understanding or abstraction—qualitative description, thermochemical kinetics, and reaction dynamics—according to Dudley R. Herschbach, professor of chemistry at Harvard University. Herschbach shared the 1986 Nobel Prize in Chemistry, for research in reaction dynamics, with Yuan T. Lee of the University of California, Berkeley, and John C. Polanyi of the University of Toronto. At the qualitative description level, says Herschbach, the emphasis is on substances: What products are obtained from what reagents under what conditions? At the second level, the aim is to identify elementary steps and transient intermediates in reaction mechanisms and to evaluate thermochemical parameters in the Arrhenius equation,
which describes the temperature dependence of rate constants. According to Herschbach, thermochemical kinetics research is of "enormous practical value because one can establish rate parameters for individual steps and then use those to make predictions for other reaction systems." The third level, molecular reaction dynamics, is "more incisive but seemingly less practical," says Herschbach. Rate constants average events in which reactants in different initial states form products in a range of final states. Reaction dynamics, which is primarily an academic discipline, goes beyond this averaging concept to study collisions between molecules in preselected states, often using the molecular beam techniques that Herschbach, Lee, and Polanyi pioneered. "A molecule in a given electronic
or vibrational state may undergo a different reaction than one in a neighboring state," explains A. Welford Castleman Jr., professor of chemistry at Pennsylvania State University. 'The dynamicist tries to ascertain how each one of these energy levels leads to a different rate constant or a different product. The classical kineticist, on the other hand, studies the overall rate of a whole collection of states in a thermal distribution. There's a little bit of a different philosophy, although everyone's interested in the same ultimate goal." In fact, there is a strong kinship between the academic and applied kinetics communities. Herschbach says, "Both these cultures are reinforcing each other's efforts to a much greater degree than people tend to think."
November 6, 1989 C&EN
25
Science/Technology
Kinetics database lists and plots parameters from Arrhenius equation OH + CO F9 to graph A(k) cm3, Reference T/K molecule, s Ea/R Order Review CO + OH — C02 + H 1 71BRA/BEL 1300-1900 6.97e-13 503 2 2 71IZO/KIS 1400-2200 1.50e-12 503 2 3 72DIX 1050 (3.98±0.20)e-13 2 4 72DIX 298-1330 5.13e-13 2 370 ± 100 5 72STU/NIK 298 1.35e-13 2 6 73DAY/THO 1050 (3.98±0.20)e-13 2 7 73GAR/MAL 1200-2500 6.64e-12 2 4026 8 73PEE/MAH 1600-1900 2.26e-12 2 2768 ± 996 9 73SMI/ZEL 300 1.45e-13 2 10 73WES/DEH 298 1.33e-13 2 11 74DAV/FIS 220-373 2 (2.15±0.19)e-13 81 ± 4 1 12 74HOW/EVE 296 2 (1.56±0.20)e-13 13 74TRA/ROS2 300 2 1.25e-13 14 75BIO/LAZ 1350-1750 7.80e-13 2 15 75GOR/MUL 298 2 (1.51 ±0.02)e-13 16 75STE/ZEL 300 2 (1.55 ±0.19)e-13' 17 75VAN/PEE 400 2 1.33e-13 18 75VAN/PEE 1000-1800 3.85e-12 2 2868 19 76ATK/PER2 2 299 (1.54±0.15)e-13 PgUp/PgDn other choices, End to exit Page 1/3 Choice? 1-48
Log k
CO + OH — - C0 2 + H
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One screen in set of three (left) on elementary (single-step) reaction, CO + OH - * C 0 2 + H, from a National Institute of Stan dards & Technology kinetics database. The database contains values for variables from the Arrhenius equation, k = ATnexp(—Ea/RT), where k is the rate constant, both A (the Arrhenius preexponential factor) and Ea (the Arrhenius activation en ergy) are constants characteristic of the reaction, Τ is temperature (kelvin), n is a constant (value often not given) that corrects for the temperature dependence of A, and R is the gas constant. Columns of data are, from left to right, record number; code for literature reference; temperature point or range where reaction was studied; either A (when temperature range is given) or k (single temperature given), both expressed in units of cm 3 molecule -1 sec - 1 ; n; Ea/R; molecularity of reaction; and notation for review articles. Associated Arrhenius plot (right), showing all rate constants determined for same reaction as a function of temperature, is useful for revealing discrepancies in experimental results.
netics, was held this past summer atNIST. Although NIST's databases all originally started out as text files, several have now been made avail able for use on personal computers. For example, NIST's chemical ki netics database, containing thou sands of elementary (single-step) rate constants of gas-phase reactions, is currently available for personal computers. An ion energetics data base lists ionization energies and heats of formation of gas-phase ions, in addition to bibliographic data. And a mass spectral database con taining electron impact spectra for some 50,000 compounds "is also of interest to kineticists because mass spectrometry is one of the major ways to detect species in gas-phase reactions/' says Stephen E. Stein, group leader of data programs in NIST's chemical kinetics division. Several other computerized kinet ics databases are currently under 26 November 6, 1989 C&EN
development at NIST, including files on plasma processing of electronics components, explosives and propellants research, and high-temperature chemistry. A goal for the future is to make it easier for scientists to estimate rate constants not yet determined ex perimentally. "Thousands of rate constants are needed for modeling practical systems," says Stein, "and it's inconceivable that they could be measured in our lifetime. How ever, patterns and trends in rate constant values could be put in a rigorous form on computer. Based on the type of reaction involved, a program would estimate the rate constant and the error limit. This is a major direction we are moving in right now because we would like to be able to fill in the gaps where there are missing data." Another challenge that NIST's chemical kinetics division has set for itself is to find better ways of
searching and displaying kinetics data. According to Stein, future plans are to "tie the databases to gether into a structure-based sys tem. You would draw a molecule on a screen and the system would compute properties from that struc ture—heats of formation, entropies, and boiling points, for example. This would unify our databases, make them more sensitive to structure, and make it easier to communicate with other databases." Among many research areas that depend on accurate kinetics data, one that has received considerable public attention in the past few years is stratospheric ozone depletion. "Ki netics is absolutely critical in stud ies of ozone depletion," says chem ist Margaret A. Tolbert of SRI Inter national. "To understand what's going on, modelers have to put in formation on wind dynamics and air currents into a model. But along with that they need hundreds of
chemical reactions in their models to predict all the trace species—one of which is ozone—in the atmo sphere. For their models to make any sense at all, they need to know t h e rate for each one of these processes." According to Tolbert, heteroge neous reactions—reactions of gas eous components at liquid or solid surfaces—are thought to play a cen tral role in the chemistry of the stratosphere. For example, hetero geneous reactions on polar strato spheric cloud surfaces have been implicated in the chemical mecha nisms thought to be responsible for seasonal loss of ozone in the Ant arctic. The surfaces available for stratospheric heterogeneous pro cesses include ice, ice-nitric acid par ticles, and water-sulfuric acid par ticles. "Specific reactions of chlorine ni trate (CIONO2) and dinitrogen pentoxide (N2O5) on ice are important in the Antarctic for two reasons," says Tolbert. "One is that they can provide a source of active chlorine, and ozone depletion is caused by catalytic cycles involving chlorine. The other is that they can provide a sink for odd nitrogen [nitrogen radicals]. For example, in the reac tion of N2O5 on ice, the product, nitric acid, stays in the ice. So what you're doing is removing nitrogen from the gas phase, a factor that's important in explaining the ozone loss." Nitrogen species react with and remove the active forms of chlorine that catalyze ozone de struction. "These two phenomena are the result of heterogeneous chemistry, and they can't really be explained without that," Tolbert says. "We and others are looking at these re actions in the lab to try to charac terize them—exactly how fast do they go, can they go rapidly enough to explain the ozone hole, what are the products, and what are the mechanisms? Kinetics is especially important when you're trying to predict what's going to happen in the future. If you have the kinetics right, you have much more confi dence in predicting what is going to happen in 10 years or a hundred years." Kinetics also provides a scientific
understand the troposphere remain diffuse. "How many active tropo spheric kinetics groups are there in U.S. university chemistry depart ments?" he asks. "The answer is zilch. It falls between the cracks, so to speak. Most of the research is performed by government research groups or institutes attached to universities." However, Niki contends that the complexity of tropospheric chemis try has held back the field as well. "It's a much richer chemical system than the stratosphere," he says. "Technically it's more challenging and tedious to study." However, he emphasizes that considerable prog ress is now being made in under standing tropospheric interactions.
underpinning for studies of tropospheric chemistry, an area that some believe has been neglected relative to stratospheric chemistry. "What we're ignoring," says Herron, "is the fact that control strategies for the troposphere—the city ozone problem, in effect—are going to be incredibly expensive. The fact is, there isn't much science going on in tropospheric chemistry. For ex ample, people are talking about clean fuel using m e t h a n o l a n d ethanol, but what are the conse quences?" Hiromi Niki, professor of chem istry at York University, Toronto, believes that the level of interest in tropospheric chemistry has increased in recent years but that efforts to
Tropospheric reactions of hydrocarbons Peroxynitrates
R0 2 N0 2 hv H20 Organic free radicals
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Highlighted compounds are representative reaction products from tropospheric degradation of natural and anthropogenic hydrocarbons. Many are environmen tal pollutants, such as peroxyacetyl nitrate (PAN), a potent lachrymator. Al though a general picture of this complex system is beginning to emerge, additional kinetics data are needed—including rate constants and mechanisms for various steps—to help establish which of many competing reaction chan nels are operative. Source: Hiromi Niki, York University, Canada
November 6, 1989 C&EN
27
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Science/Technology In the troposphere, a large vari ety of volatile organic and inorgan ic compounds from both natural and anthropogenic sources under go chemical transformations induced by solar ultraviolet radiation or by reactive photochemical products. A common focus of tropospheric re search involves the chemical trans formations of hydrocarbons and ox ides of nitrogen (NO x ), leading to oxidant formation. These reactions are extremely complicated because they tend to involve many possible reaction pathways. In addition, the large organic free radicals and reac tion intermediates that take part in the reactions are currently difficult to detect and identify. "At p r e s e n t it is difficult to uniquely characterize these reaction mixtures," says Niki. "The compo nents are highly labile, and many are present at extremely low con centrations. If we manage to see spectroscopic signals from an un identified compound, then it's just a matter of time before we can identify a n d quantify the com pound, but it's an extremely chal lenging task." The first step in this process of identification, he says, "is to sim plify systems so we are not dealing with hundreds of free radicals"—for example, by using photochemical techniques to generate key free rad icals in a selective manner. "We characterize expected products by FTIR [Fourier transform infrared spectrometry] and systematically build up the complexity of the chemical system. It's an interesting detective story w e ' r e t r y i n g to solve." For example, peroxyacetyl nitrate, an oxidant and potent lachrymator that forms in the air of cities all over the world, "was first detected some 30 years ago by infrared spec troscopy, but it stayed compound V for 10 of those years," says Niki. "It was eventually identified by us ing complementary tools. Many ox idants are chemically reactive by def inition and are therefore difficult to sample and gas-handle for chem ical analysis." The complex mechanisms lead ing to smog formation in cities need to be studied much more carefully, according to Niki. "How well do 30
November 6, 1989 C&EN
Niki: tropospheric interactions we know the rate constants and mechanism of each of these steps under prevailing atmospheric con ditions?" he asks. "More importantly, which reaction channels are opera tive? What are the species coming out and how are the products being fed back? These are very important issues that need to be addressed." To answer questions such as these, "you have to know the chemical kinetics," he says. "Kinetics is the key to modeling." Another key issue in tropospher ic chemistry is the problem of ur ban photochemical smog. Accord ing to William L. Chameides, pro fessor of geophysical sciences at Georgia Institute of Technology, summertime ozone levels in U.S. cities have failed to decrease signif icantly despite a decade of costly efforts to control ozone precursor emissions. "Over 100 cities in the U.S. still are not able to comply with the National Ambient Air Quality Standard for ozone," he says, "and it is estimated that over 120 million Americans are exposed to unhealthful levels of ozone each year." Chameides believes that one rea son for the failure to reduce ozone pollution in the U.S. is that the strat egy adopted to control ozone fails to account for natural emissions of hydrocarbons from trees and other vegetation. Ozone is produced in
the troposphere from chemical re actions of hydrocarbons and NO x in the presence of sunlight. Over the past 10 years, ozone abatement efforts have focused on reducing the amount of hydrocarbons emit ted from automobiles and factories. Very little has been done to control ΝΟχ emissions. However, Chameides and col leagues at Georgia Tech have found that, in some cities, natural emis sions of hydrocarbons from trees are as large as anthropogenic hy drocarbon emissions and that, like man-made hydrocarbons, hydrocar bons from natural sources react with ΝΟχ to form ozone. He believes that reductions in man-made hydrocar bon emissions have not been effec tive because of the presence of these natural hydrocarbons. According to Chameides, innovative strategies in volving controls on NO x emissions from cars and factories will be needed to effectively control ozone in regions where natural hydrocar bon emissions are significant. However, he points out that "to infer from our results that trees cause the problem or that trees are polluters is erroneous. The hydro carbons from trees by themselves do not pose a health threat and, in the absence of man-made nitrogen
Tolbert: ozone depletion models
oxides, do not result in production of ozone. Our studies suggest that the real culprit is emission of nitrogen oxides and that those are the emissions we should be controlling." Kinetics is also playing a growing role in studies of oxidation and combustion chemistry. "In a combustion system, the chemical extent of reaction defines how much energy is released," says Frederick L. Dryer, associate dean of academic affairs and professor of mechanical and aerospace engineering at Princeton University. This energy release changes the temperature and couples with the local chemistry, he says, a n d this e n e r g y c o u p l i n g makes it a complicated problem to deal with. However, considerable progress is being made in a number of areas of combustion chemistry. For example, says Dryer, "Today one can model the burning of a liquid droplet in a quiescent environment with complete detailed chemistry in the gas phase. In fact, it is also possible to model the interaction of that gasphase chemistry with the surface of a solid particle in a time-dependent fashion. That would not have been possible five years ago." New applications for kinetics are also appearing in studies of plasma chemistry, particularly in plasma processing of electronic devices. For example, interest has been growing in kinetic modeling of chemical vapor deposition (CVD) and plasma etching of electronic components. "People are just beginning to get serious about modeling these kinds of processes," says Herron, "but there's a real shortage of basic data, and there aren't any databases yet that plasma modelers can use. We've started trying to develop a database here at NIST for plasma chemistry, but it's a real uphill battle." In the manufacture of integrated circuits, CVD is increasingly being used to deposit thin films of semiconductors, conductors, and insulators on substrates. A series of deposition, lithography, and etching techniques are used to make patterns that determine where transistors and other components will appear on the chips. "Because integrated circuit devices are getting smaller, there are ef-
forts to try to make them under milder and more controllable conditions," says research staff member Joseph M. Jasinski of IBM's T. J. Watson Research Center in Yorktown Heights, N.Y. "But you don't want to have to do this totally empirically, which is what's been done up to now. Right now, it's easier to take a precursor you think will deposit a film you want, stick it into a glow discharge or other kind of CVD reactor, turn the knobs and dials, make a bunch of runs, and see which film looks like what you want." However, he says, "Some of us hope that won't always be necessary. We'd like to have predictive models, similar to combustion models, where you put in fundamental, first-principles chemistry and physics: rate constants for relevant gasphase chemical reactions, sticking coefficients for radical species, and other parameters. Then you turn the crank and predict things like film growth rates and other materials properties. That's what you'd like to have, but nobody has that for any CVD system that I'm aware of yet." To model the plasma-induced growth of silicon-containing thin films, it is important to understand which silicon radicals are generated in the plasma and how quickly they are removed by gas-phase reactions, because these processes ultimately determine the flux of reactive species to the film growth surface. For example, radicals such as H, SiH, SiH 2/ and SiH 3 are generated in a silane (SiH4) plasma. "What I'd like to be able to explain," says Jasinski, "is why I have the amounts I think I have and what importance each of those species has to some growth process. Is the S1H2 radical going to live a long time in the gas phase so that it can diffuse to the surface and stick and start to grow film, or is it going to run into another molecule, react with it, and turn into something else?" For the past four years, Jasinski and colleagues have been using state-of-the-art chemical kinetics techniques—typically laser techniques—to generate silicon hydride radicals under controlled conditions. "We generate them under well-defined kinetic conditions in a low-
Chemical vapor deposition of silicon from silane siH 4
1
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