SCIENCE & TECHNOLOGY ALLOY CAT Theoretical studies predict that some alloys consisting of a host metal (large blue spheres) such as Pt or Pd and a subsurface solute (red) such as Ta or W will make excellent hydrogenation catalysts because of their unique ability to dissociate H2 (small spheres) readily and bind atomic H weakly.
CATALYSIS BY THE NUMBERS Advanced computational methods are revealing mechanistic details and guiding catalyst design M I T C H J A C O B Y , C & E N CHICAGO
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research in action, and the picture will show scientists working with reagents, benchtop reactors, gas chromatographs, and other analytical instruments. Until recently those elements would have filled the frame. But that picture leaves out a key component ofmodern catalysis research: computation. The goals of computational investigations in heterogeneous catalysis can be stat-
ed simply "Our aim is to understand what makes a particular surface a good catalyst for a given reaction," says Jens K. Norskov, a physics professor at Technical University of Denmark, Lyngby "Of course, that has been the aim of catalysis research for a long time," he notes. "But I think we may finally be getting there for the simplest systems." According to Mark A. Barteau, a professor of chemical engineering at the University of Delaware, Newark, powerful
theoretical methods developed in the past decade have shown promise—not just for identifying catalytic intermediates and reaction pathways that are accessible to experiments, but for providing quantitative predictions about elementary processes that, in large part, are inaccessible to experiments. The advances in computational techniques have armed theoreticians with detailed information about the energetics, structures, transition states, and other key features of catalytic reaction mechanisms and enabled them to formulate new concepts and develop chemical models. The models provide a basis for understanding catalytic behavior and for predicting the types of materials and conditions that catalyze reactions efficiently and selectively The predictions, in turn, lead to the design and development of new catalysts. The alternative to rational design is the conventional approach to discovering new catalysts—trial and error. That approach has been around for quite some time. A famous example dating back to the early 1900s comes from the work ofAlwin Mittasch and coworkers at BASF in Germany Searching for an ammonia-synthesis catalyst, the researchers conducted some 6,500 experiments involving 2,500 candidate materials and eventually discovered an effective iron-based catalyst. NO DOUBT, the time required to carry out the multitude of experiments could have been cut back using modern high-throughput synthesis and testing methods—had they been available in Mittasch's day But today's theoretical methods for screening would-be catalysts may have shortened the search even more markedly by zeroing in on just a small handful of promising-looking catalysts, possibly including some materials that were never studied. That kind ofthinking led Norskov, Claus J. H. Jacobsen, and their coworkers at Dan-
Advances have armed theoreticians with detailed information about catalytic reaction mechanisms and enabled them to formulate new concepts and develop chemical models. HTTP://WWW.CEN-ONLINE.ORG
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SCIENCE & TECHNOLOGY Combining cobalt, which binds nitrogen riodic table computationally, Norskov's reish catalyst manufacturer HaldorTbpsoe to search group teamed up with Peter Strasstoo weakly with molybdenum, which binds use computational techniques to search the er, W Henry Weinberg, and coworkers at the gas too tightly should result in an alloy periodic table for materials with just the SymyxTechnologies, Santa Clara, Calif, to right characteristics for converting nitrocatalyst with a nitrogen binding energy that compare theoretical and experimental gen and hydrogen to ammonia. In the study, corresponds roughly to the maximum of the high-throughput methods for screening which was conducted a few years ago, the volcano plot. 'That's exacdy what was found materials. The study focused on identifyteam examined properties of molybdenum, experimentally" the team notes. Guided by ing new carbon monoxide-tolerant iron, ruthenium, and other metals and o alloy catalysts for fuel-cell anodes. looked for trends in the catalytic per2 On the experimental side, parallel formance of those elements. jg synthesis and testing methods were By plotting the calculated turnover t used to prepare a collection of alloys frequency (a measure of the rate of J consisting of platinum, ruthenium, catalytic activity) as a function of ni> and a third metal—cobalt, nickel, or trogen adsorption energy on the sur£ tungsten—and evaluate their catfaces of the various metals, the team S alytic properties. Computational found a familiar-looking result: a soprocedures were employed to calcucalled volcano curve. As its name imlate adsorption energies, activation plies, the curve, which is common in barriers, and catalytic activities of catalysis research, rises, plateaus, then the same types of alloys. falls. The implication is that metals that bind nitrogen too weakly or too The comparative study shows that strongly are poor catalysts. The ideal experimental and theoretical apcatalyst possesses an optimum bindproaches to materials screening reveal ing energy that maximizes catalytic similar trends in the relationship beactivity This concept is referred to as tween electrocatalytic activity and alSabatier's principle, in recognition of loy composition. In addition, both the 1912 chemistry Nobel Laureate, types of investigation point to similar Paul Sabatier. three-component alloys as candidates for improved fuel-cell anode catalysts The calculations indicate that, [J. Phys. Chem. B, 107,11013 (2003)}. among the metals examined, ruthe- NO FUELING AROUND Adding ruthenium Each approach has strengths. For nium and osmium would be the most (yellow) to platinum (blue-green) prevents CO example, computation can be used effective ammonia-synthesis cata- poisoning of fuel-cell anodes in the presence of to analyze thermodynamic and kilysts. Third-ranked on the list is iron, water by providing adjacent surface sites for netic parameters that are inaccessiwhich is much less expensive than the forming hydroxyl species that oxidize CO to C02 ble to experiment or cannot be meaother two metals. The predictions are (H = white, 0 = red, C = green). sured easily. And high-throughput backed up by experimental results, laboratory measurements provide inforthe researchers point out, but the theoinsights derived from the theoretical results, mation about the properties of test specretical study goes a step further and sugthe Danish scientists prepared Co-Mo catimens under real experimental conditions. gests a way to design a novel catalyst that alysts and found the alloys to be more active Norskov and coworkers emphasize that can outperform the pure metals. for ammonia synthesis than pure cobalt, the screening approaches are complemolybdenum, ruthenium, or iron. mentary and that the real power of highAccording to the researchers, the alloy throughput techniques can be tapped by is more active than its constituents because combining theoretical and experimental the properties of the mixed material fall screening methods. between the properties of the individual elements. It seems counterintuitive. But o the group determined through additional ANOTHER TYPE of reaction being scruticalculations that the interaction energy benized by computational chemistry methO tween an adsorbed molecule and a multiods is ethylene epoxidation. The Univercomponent surface can be approximated sity of Delaware's Barteau studies the by interpolating between the interaction process using theoretical and experimenenergies of the components. Interpolattal techniques to elucidate details of the ing places Co-Mo near the maximum of reaction mechanism and design catalysts the volcano plot. that are more selective than the silverbased materials used commercially Similarly, Fe-Ru and Fe-Co alloy catalysts and the nitrides of N i 2 M o 3 and A couple ofyears ago, Barteau's research C03M03 were predicted theoretically and group identified oxametallacycles—species IN TRANSITION Quantum confirmed experimentally to outperform with an - O - C - C - structure attached at calculations show that six-palladiumtheir components. The group contends both ends to a metal surface—as key interatom (blue) ensembles are required to mediates in the epoxidation reaction. The that the finding is rather general and apsupport the transition-state structure species were identified on the basis of surplies throughout the periodic table [J.Am. for the vinyl + acetate coupling Chem. Soc, 123,8404 (2001)]. face science measurements combined with quantum mechanical calculations. reaction (H = white, C = gray, 0 = red). In another example of perusing the pe26
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Barteau and coworkers Suljo Linic and Jerome T. Jankowiak determined that oxametallacycles can go on to form the desired product, ethylene oxide, or the intermediates can form acetaldehyde, which leads to unwanted combustion products. Unfortunately the two processes have similar activation barriers, which compromises selectivity So the Delaware researchers employed computational screening methods to search for a catalyst that would boost selectivity in the epoxidation reaction relative to traditional single-metal silver catalysts. They focused on identifying bimetallic alloys of silver that accentuate the difference in activation barriers for the desired and undesired reactions. The theoretical results indicate that a bimetallic surface with roughly 25% copper atoms should do the trick. Indeed, the predictions were verified experimentally in tests of alumina-supported Cu-Ag catalysts. The group reports that some of the bimetallic catalysts are 1.5 times more selective than pure silver catalysts under 00
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surface is more complex than the simple model used in the simulations. They suggest that more detailed information can be deduced by combining surface spectroscopy with additional calculations. Meanwhile, at the University ofVirginia, Charlottesville, Matthew Neurock, a pro28
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fessor of chemical engineering and chemloy surface is energetically favorable [Elecistry, also applies computational methods trochim.Acta, 48,3759 (2003)}. Earlier this year, James A. Dumesic, a to investigate complex catalytic reaction professor of chemical engineering at the environments. For example, Neurock and University of Wisconsin, Madison, recoworkers studied vinyl acetate synthesis ported that gold nanotubes can mediate on palladium surfaces. The reaction involves a large number of elementary steps and competing pathways that lead to unwanted by-products. The calculations reveal a wide range of mechanistic information. For example, activation of ethylene to form surface vinyl species requires access to a surface site with a rninimum of four palladium atoms available for bonding. And the transition state for the coupling reaction between vinyl and acetate species requires a sixatom ensemble. By feeding quantum mechanically derived reaction energies and rate constants into a Monte Carlo simulation, Neurock was able to model the reaction and predict the effect DOWN THE TUBE Calculating an optimized that adjusting partial pres- configuration for water adsorbed in a gold nanotube sures, temperatures, and (as shown) is a first step toward understanding the other reaction conditions nanotube's enhanced CO-oxidation activity in the would have on reaction yield presence of water. and selectivity [J. CataL, 216, 73 (2003)}. In addition, he showed room-temperature oxidation of carbon that certain arrangements of gold and palmonoxide with oxygen. Dumesic also obladium atoms could improve catalytic acserved that the reaction is enhanced in the tivity and selectivity thereby providing a presence of water [Angew. Chem. Int. Ed., path toward atomic-scale catalyst design. 43,1140(2004)}. Another complex reaction environment To gain insight into the oxidation mechthat has given up some of anism, Neurock studied the gold nanotube its secrets to theoretical system using theoretical methods. He finds probing by Neurock and that in the presence ofwater, it is energetcoworkers is the anode of ically feasible for CO to be oxidized by hybimetallic fuel cells. Platdroxyl intermediates or by water via a hetinum is often alloyed with erolytic path that forms carbon dioxide, another metal such as protons, and electrons. ruthenium to control the The picture of catalysis research conadverse effect of CO poitinues to change as new tools and techsoning by oxidizing the conniques become available to scientists. taminant to C 0 2 . BimetalWhereas in decades past, basing a catallic catalysts can be very ysis research program entirely on comeffective, but researchers putation was unimaginable, nowadays are uncertain of the oxidaexcluding t h e o r y entirely is equally tion mechanism. unimaginable. Neurock and graduate 'Are we on the verge of catalysis by design?" Barteau asks. If "by design" means student Sanket Desai found that the ruthestarting strictly from quantum chemistry, nium component of the alloy together with the presence of liquid water at the anode, then catalysis is not there yet, he says. But with continued advances in computationprovide favorable conditions for formaal methods and the benefit of ever-imtion of surface hydroxyl species. The team's proving experimental techniques, the field results also show that subsequent oxidacontinues to move closer to that ideal. • tion of CO by hydroxyl species on the alHTTP://WWW.CEN-ONLINE.ORG
