science/technology cles may become very mobile and practically behave as liquids. This occurs at a temperature roughly equal to half of the solid's melting point. And that's just what's happening with the little iron clusters. When the particles make contact with one another through matching crystallographic faces, they can coalesce the same way two water droplets can combine to form a larger droplet. The lessons learned from this model catalytic system are directly related to catalyst deactivation due to sintering, Baker pointed out. In that high-temperaed. It's unable to resolve lines spaced ture process, crystallites join to form largcloser than 4 A instead of its usual limit er crystals. As catalyst particles increase of 1.8 A, Baker explained. Nonetheless, in size, there is a loss of surface area and controlled atmosphere studies have giv- an associated decrease in catalytic activien microscopists a window through ty. Once the atmospheric conditions and which to observe catalysis events that step-by-step process that cause this type of deactivation are understood, remarked otherwise would not be seen. Electron microscopists often connect Baker, researchers should be in a better video cameras to a microscope's viewing position to prevent it and extend a cataarea. This enables them to record and re- lyst's lifetime. In another video scene, tiny pieces of play Angstrom-scale events in real time. In Dallas, Baker showed one such video an iron-nickel alloy sitting on a graphite that featured a number of graphite-based support showed their response to a mixed carbon monoxide-hydrogen atmocatalytic sequences. In one scene—recorded at 600 to sphere at 750 °C. The particles' most 700 °C and in the presence of carbon prominent feature was their regular, geomonoxide—50-A iron particles wan- metric shapes. Also noticeable was one dered across a graphite surface. Occa- particle that appeared to be jumping sionally two particles bumped into each awkwardly with one edge pinned to the other and merged—forming a single, support surface. "This is really quite remarkable," said larger particle. On supports that hold their catalysts Baker, referring to the particles' perfect rather weakly, commented Baker, parti- boxlike shapes. "Energetically, the preferred shape of a particle on a surface is a hemisphere," a dome. This is an example of how carbon monoxide and other gases reconstruct surfaces—that is, force surface atoms to realign themselves in particular orientations that otherwise they would not adopt. Surface reconstruction plays an important role in catalysis, noted Baker. When reactants make contact with rearranged catalyst surfaces, they exhibit chemical reactivity that differs from what would be observed if the catalyst were in a more relaxed Transmission electron micrograph (top) and a state. computer-drawn representation give a snapshot of a As an example, Baker carbon nanofiber growing from a nickel-copper particie pointed to hydrogenation (red). At high temperature, the metal catalytically of unsaturated aldehydes to decomposes ethylene allowing carbon atoms to diffuse form unsaturated alcohols. through some of its surfaces and reassemble at others The greater than 90% selecas a graphite fiber.
GETTING IN ON THE (RE)ACTION
Scientists adapt electron microscopy, NMR, and positron-emission methods to study heterogeneous catalysis in situ Mitch Jacoby C&EN Chicago
K
nowing only the before-reaction and after-reaction parts of a catalysis story just isn't enough to satisfy some researchers. In catalysis circles these days, the buzzword is "in situ": observing a catalyst while it's in positionthat is, while it's in a reactor and directing chemistry. Researchers are working to develop analytical methods that allow them to follow heterogeneous catalytic processes as they occur. At a symposium sponsored by the Division of Colloid & Surface Chemistry at the recent American Chemical Society national meeting in Dallas, scientists described many techniques that permit in situ probing of chemical systems. Some are based on familiar methods—such as electron microscopy and nuclear magnetic resonance (NMR) spectroscopy—while others rely on less common procedures, such as positron-emission imaging. R. Terry K. Baker, a professor of chemistry at Northeastern University, Boston, uses transmission electron microscopes (TEMs) in a "controlled atmosphere" mode to study properties of catalyst materials. Microscopists typically operate TEMs under high vacuum and at room temperature. But Baker and coworkers modify the instruments to permit their specimens to be studied while sitting in more reactor-like conditions— perhaps a billion times greater pressure than other microscopes and at temperatures up to 1,000 °C. "Ultimately, the electron microscope is a vacuum instrument," said Baker, acknowledging that the highest pressure currently possible with his method is about 5 torr. And even at that pressurefar greater than conventional microscopy but still only about 0.01 atm—"there's a price to pay in terms of resolution." At high pressure and temperature the microscope becomes somewhat nearsight-
MAY4, 1998 C&EN 41
science/technology tivity observed with one nickel-on-graphite catalyst is attributed to the way the metal sits on the support surface and the arrangement of the metal atoms in the crystallographic face seen by the organic reactants. Different catalyst preparation procedures lead to other surface arrangements and less selectivity. The awkwardness of the jumping particle also brings new information to catalysis. It indicates that certain faces of metallic, bimetallic, or other crystals preferentially adhere to support surfaces. By studying these catalyst-support interactions, researchers are learning to manipulate the particles so that the faces of choice are available for catalysis. The symposium's attendees also witnessed the growth of a nanometer-scale graphite fiber. The wormlike object grew in an ethylene atmosphere at 600 °C, Baker noted, because of a copper-nickel dehydrogenation catalyst that converts gaseous organic molecules to solid carbon. Scientists are interested in carbon nanofibers because of their potential for applications in catalysis and electronics. The nanofiber-growth mechanism starts with decomposition of ethylene on certain crystal faces of the catalyst particle, Baker explained. This process, known as dissociative adsorption, leaves elemental carbon on the catalyst surface. At high temperature, carbon atoms dissolve in the metal and diffuse through the particle, precipitating at other crystal faces. If the arrangement of metal atoms in these faces—the ones at which carbon atoms precipitate—meets certain crystallographic
Baker: controlled-atmosphere microscopy
requirements, then solid carbon accumulates at these faces in the form of graphite nanofibers. In addition to forming graphite, the Northeastern researcher presented data on destroying it. In some examples, metal catalysts converted graphite to carbon monoxide and carbon dioxide in the presence of oxygen. In others, graphite reacted with hydrogen to form methane. A detailed understanding of mechanisms that drive conversion of solid carbon to gaseous products may be applied to a number of different research problems. Catalyst rejuvenation, for example, often involves cleansing metal surfaces of an unwanted buildup of coke—a carbonaceous material that can ruin a catalyst's
Texas A&M in situ NMR group (from left): Wang, Feng Deng, Xu, Haw, Dewey Barich, and Song.
42 MAY 4, 1998 C&EN
performance. Steam gasification of coalformerly an area of active research—"may again raise its head," Baker said, because of the usefulness of hydrogen as a fuel. In that process, water and carbon react to form carbon monoxide and hydrogen. On the flip side, scientists use the lessons learned from mechanistic studies to turn off carbon reactions—for example, to prevent decomposition of valuable carbon composites. In a video clip that looked like a closeup of an (Ohio Art Co.'s) Etch-A-Sketch toy in action, 250-A-sized platinum dots quickly ate their way across a graphite surface. At 750 °C in an oxygen atmosphere, the metal catalyzes graphite oxidation, Baker explained, cutting random channels in the solid. The microscopist reported similar oxidation results for other metals. In the presence of hydrogen, however, many catalyst particles tend to cut grooves in graphite that follow specific crystallographic directions. The reasons for differences in channel orientation are not completely understood. Rather than cutting channels in graphite, some metals—including molybdenum, tungsten, and copper—catalyze the solid's oxidation by peeling off large carbon layers. This mode of attack, known as edge recession, occurs with metals that easily "wet" their supports, Baker said. Such materials maximize their contact with carbon by spreading out and forming thin films. "In the channeling mode, the only atoms that are oxidizing carbon are the ones at the catalyst particle's surface. The rest of the atoms are just going along for the ride," Baker said. If the object is to find a metal that can gasify large quantities of solid carbon, in situ studies like these indicate the catalyst should be one that spreads out—rather than balls up—and operates in the edge-recession mode. Sometimes the goal isn't to speed up a reaction—it's to prevent it. Researchers examining materials for the space shuttle's nose cone or other aerospace applications, for example, look for ways to protect carbon composites from oxygen attack. Though these lightweight materials are very strong, the high temperatures associated with reentry into Earth's atmosphere make them susceptible to oxidation. Controlled-atmosphere TEM studies show how boron and phosphorus additives help protect carbon-based materials. At 450 °C, oxides of boron interact
Catalytic reactor uses fast cooling to quench reaction Heating element (
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A pulse-quench reactor delivers reactants to a heated catalyst (injector valves) then rapidly quenches a reaction (less than one second) by cooling the catalyst to room temperature with frigid gas using high-speed valves. NMR methods re veal the identity of intermediates "fossilized" on the catalyst. strongly with certain crystallographic faces of graphite and coat the solidforming an oxygen barrier. The protec tion lasts up to about 815 °C, Baker not ed. Above that temperature, the oxide's wetting properties change and oxygen sneaks in through graphite's "armchair" faces. Switching to phosphorus additives, Baker and coworkers found that these ma terials completely shield graphite from oxygen's adverse effects up to 830 °C. But even at higher temperatures, as oxy gen begins causing carbon gasification, phosphorus' grip on the armchair face leaves that face many more times pro tected than graphite's "zigzag" face. The researchers conclude that a mixture of these additives will protect carbon-based materials up to 1,000 °C. NMR spectroscopy is another tool that has been adapted to in situ heteroge neous-catalysis studies. Using magicangle spinning, a procedure for probing solid specimens, researchers often ana lyze powdered-catalyst samples while holding the catalyst under conditions that in some regards simulate a reactor's environment. In some experiments, chemists heat reactants and a catalyst in a sealed rotor (a spinning sample holder) and record
spectra at successively higher tempera tures. However, due to differences in re action timescales—short for industrial reactors and long for conventional exper iments—researchers often run experi ments 150 °C cooler than commercial re actors. To get around this difference, some researchers have tried rapid heat ing experiments using radio-frequency sources or lasers. Still others have rede signed rotors to permit gas flow during data collection—again to better mimic reactor conditions. Not satisfied with shortcomings of these and other procedures, James F. Haw, professor of chemistry at Texas A&M University, College Station, has de signed a highly controllable laboratory reactor that allows NMR analysis of cata lyst samples that have been subjected to standard reactor conditions. "We insist on using reaction condi tions that are identical to those used in conventional benchtop flow reactors— which in turn model commercial, catalyt ic reactors," said Haw. "We use actual re actor temperatures and flowing reagents, and we limit our reaction contact times [between reactants and catalyst] to the timescale commonly found in commer cial reactors: from several tenths of a sec ond to several tens of seconds."
Haw stressed that the timescale issue is particularly important. Other in situ NMR experiments—especially 13C stud ies—require data collection times of several minutes or longer. Those exper iments may even last up to several hours, he added. The large discrepancy in timescales between commercial reactors and some experiments may cause re searchers to miss catalytically important species. Haw's research group uses a "pulsequench" reactor fitted with fast-acting, computer-controlled valves to overcome the timescale and flow problems. As an example of how the reactor may be used, Haw said a pulse of 13C-labeled reactant can be injected onto a preheated catalyst sample and allowed to react for a predetermined length of time—say, one second. Then the reaction may be rapid ly quenched by flowing cryogenically cooled nitrogen gas at a high-flow rate over the catalyst particles. The catalyst temperature drops 150 °C in just 170 milliseconds, Haw claimed. And within one second, the catalyst can be cooled to 25 °C. "The idea is you have a reaction that might be fast at 400 °C but quite a bit slower or negligible at 200 °C," ex plained Haw. "By the time the reactor hits room temperature, it's certainly negligible." By dropping the temperature so quickly, the Texas A&M chemists essen tially "fossilize" species present on the catalyst at one second or any other time into the reaction. The researchers then transfer the room-temperature catalyst— in an inert atmosphere—to an NMR sam ple rotor. During the reactor run, the group watches for volatile products with gas chromatography or gas chromatography-mass spectrometry. Haw presented results from a recent study of acetone chemistry on an acidic zeolite, HZSM-5 [Angew. Chem. Int. Ed., 37, 948 (1998)]. At 350 °C, acetone un dergoes an aldol condensation and is converted to a number of hydrocarbons. Working with chemistry graduate stu dents Patrick W. Goguen, Timothy W. Skloss, Weiguo Song, and Zhike Wang and postdoctoral associate Teng Xu, Haw observed that the NMR signature of spe cies such as the 1,3-dimethylcyclopentenyl carbenium ion appears after just two seconds of reaction. Though acetone reactions on acidic zeolite have been examined previously, the lower temperatures used in earlier in situ NMR studies have prevented this staMAY 4, 1998 C&EN 43
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science/technology ble class of carbenium ion from being observed. The strength of the pulsequench method, Haw pointed out, is that it allows carbenium ions—or other species that play a role in industrial reaction mechanisms—to be observed. The Texas A&M team also studies conversion of methanol to olefins or gasolinetype products. This area of chemistry is important in production of synthetic fuels, Haw said, noting, for example, that New Zealand at one time made much of its gasoline this way. Haw added that olefins are important feedstocks for many commercial chemical processes. In one set of experiments using the pulse-quench reactor, Haw's group injected 13C-labeled methanol onto freshly activated samples of HZSM-5 at 370 °C and allowed the reaction to proceed for a period of 0.2 second to 16 seconds and then quenched the reaction. The chemists concluded that under the conditions used, an induction period of four seconds is required before the catalyst can convert much methanol to hydrocarbons. Among the products observed, the researchers reported C2 to C4 olefins in the gas phase, and alkanes and substituted cyclopentenyl carbenium ions on the catalyst surfaces [/. Am. Chem. Soc, 120,2650(1998)]. In a set of follow-up experiments, the team injected one of several substances in a first pulse, allowed it to react with the zeolite for 10 seconds, and then delivered a second pulse of the labeled methanol. To discover what effect the first chemical might have on the reaction pathway, Haw and coworkers quenched the methanol reaction before the foursecond induction period ended. When the first pulse delivered nitrogen, difluorobenzene, or heptane to the catalyst, the subsequent methanol reaction produced essentially no hydrocarbons in two seconds'of reaction. However, when olefins, olefin precursors, or even unlabeled methanol was injected in the first pulse, two seconds was more than enough time for the 13 C-labeled methanol in the second pulse to be turned into an assortment of hydrocarbons. Those observations point toward an olefin-coated-catalyst mechanism "People have suspected for a long time that there might be two kinds of carbonaceous deposits that are important to catalytic processes," Haw said. "Bad coke—which deactivates a catalyst by blocking pores or other reactive sites—and good coke—which may have some poorly understood but important 46
MAY 4, 1998 C&EN
Jonkers: positron emission for catalysis
mechanistic role in heterogeneous catalysis. What we have here is evidence for good coke forming in the zeolite, and we believe this good coke—or carbon p o o l might be the locus of carbon-carbon bondforming reactions." In Kolboe's carbon-pool mechanism, Haw explained, methanol or dimethyl ether alkylates a carbonaceous species of unknown stoichiometry—perhaps a carbenium ion. The species eliminates an olefin and the catalytic cycle starts again. Though Haw was tempted—because of NMR results—to identify carbeniumions as the key players in the mechanism, first-principles calculations dissuaded him from doing so. Working with John B. Nicholas, a theoretical chemist at Pacific Northwest National Laboratory, Richland, Wash., Haw determined that the intermediates in question are even more reactive than cyclopentenyl cations. "We concluded that at high temperatures cyclopentenyl cations are in equilibrium with polyenes or less stable carbenium ions," said Haw. "It's possible that during the quench these carbon-pool species rearrange to give more stable cations. The cyclopentenyl cations may be the fossils of the actual species that are present at higher temperatures." As a final example of the importance of using standard reactor conditions in in situ studies, Haw presented results from another recent study in which his group detected pentamethylbenzenium ions in the cavities of HZSM-5 [/. Am. Chem. Soc, 120, 4025 (1998)]. In that investigation, the chemists reacted benzene or toluene with methanol at 300 °C for four
seconds and then quenched the reaction. Benzenium ions are believed to be key players in the mechanisms of economically important reactions such as toluene disproportionation. According to the Texas researchers, the essential requirement for detecting these reaction intermediates is the use of flow conditions. Previous attempts to catch the elusive ions failed, Haw claimed, because they used conventional sealed sample holders. In a flow reactor, water—which is produced during the reaction—diffuses out of the catalyst and is removed by a carrier gas. In a sealed container, water sticks around and prevents benzenium ions from forming. "By using standard reaction conditions and then quenching so rapidly, [ordinarily] transient reaction intermediates exist—not for just a few milliseconds, but for hours or longer—giving you plenty of time to examine them with NMR or other methods," commented Eric J. Munson, an assistant professor of chemistry at the University of Minnesota, Minneapolis. Munson uses solid-state NMR methods to study catalytic processes. A problem with some of the earlier methods, Munson added, is that too many products and reactants were present in the catalyst. The zeolite pores were full and many side reactions were occurring. Now, investigators can use very low concentrations of reactants and stop a reaction when they want to without worrying about contamination from unwanted reactions. Another technique used to study catalytic reactions as they occur is based on positron emission tomography. Positronemission techniques are as unfamiliar to modern chemical analysis as NMR spectroscopy is familiar. But that may change thanks to a number of researchers who have adapted a medical procedure used to image working human hearts and brains as a chemical method used to study running catalytic reactors. Gert G. Jonkers, a nuclear-measurements and radioisotope-applications specialist at Shell Research & Technology Center, Amsterdam, explained the basis of the imaging method. Some radioactive nuclei undergo transformation to more stable nuclei by changing their neutronto-proton ratio, he said. Carbon-11, for example, converts one of its protons to a neutron—forming n B—and emits a positron in the process. During its short lifetime, a positron wanders around—bouncing off nearby
molecules and losing kinetic energy. Af ter just a short stroll—on the order of 5 mm in many materials—a low-energy positron collides with an electron (its an timatter nemesis) and the two annihilate each other, leaving behind a pair of y-ray photons. Fortunately for catalysis and other ar eas of chemistry, positron-emitting nu clei are known for carbon, oxygen, and nitrogen, Jonkers said. By delivering isotopically labeled molecules to a catalytic reactor and detecting the telltale, metalpiercing y photons, investigators can map the concentrations of reactants, in termediates, and products throughout the reactor in time and space. Demonstrating the method's potential as an in situ probe, Jonkers presented re sults from a study on treatment of syn thetic exhaust gas (a mixture of carbon monoxide, carbon dioxide, and oxygen) over a ceria-promoted platinum-on-yalumina catalyst. Because of very high de tection sensitivity, Jonkers stressed that only femtomole quantities of positronemitting compounds need to be injected onto the catalyst bed to follow their paths as they undergo reaction. In a series of elementary steps, l ^-la beled carbon monoxide adsorbs on the catalyst and is oxidized to carbon diox ide. Labeled C0 2 molecules desorb from the catalyst surface and exit the reactor in the gas phase. Since the radiation de tectors do not provide molecule-specific information, Jonkers pointed out that he and his coworkers analyze gas products leaving the reactor with chromatograph ic methods. These ex situ detectors can be used to determine if labeled CO, for example, exits the reactor before being oxidized. "Once you've measured the concen tration [of labeled species] as a function of position and time, you construct a model based on the elementary steps of the reaction mechanism and try to simu late the data," Jonkers said. The idea is, if the simulation reproduces the measured data well, as it does for the n CO experi ment, the model probably describes the mechanism accurately. But it doesn't always work the first time. As a case in point, Jonkers ac knowledged being quite surprised when he and his coworkers compared the in teractions of l ^-labeled carbon dioxide to 150-labeled carbon dioxide (15OCO). The researchers found that n C 0 2 passes through the reactor quickly—on the order of seconds—whereas the 1 5 0 analog leaves its mark on the reactor for several
minutes. Based on control experiments, the group ultimately attributed the differ ences to the presence of ceria (Ce02). "We overlooked one important reac tion that occurs on the catalyst bed," said Jonkers, "one between C0 2 and ceria." Labeled C0 2 molecules can dissociate on the ceria surface and transfer their labels to ceria—forming Ce0 4 complexes, he explained. "Eventually carbon asks for its oxygen atom back. But not having a good memory, cerium oxide gives back an unlabeled oxygen." Because of this
oxygen exchange, 1 5 0 remains on the catalyst bed for a long time. The correct ed model leads to a better simulation. Bruce G. Anderson uses positron emis sion methods to probe hydroisomerization of n-hexane over a platinum-containing acidic zeolite (H-mordenite) while the re action proceeds at elevated temperatures. Anderson, a postdoctoral researcher, works with professors Rutger A. van Santen in the chemical engineering depart ment, Martien J. A. de Voigt in the applied physics department, and other scientists at
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science/technology Eindhoven University of Technology, Eindhoven, the Netherlands. After flowing an unlabeled stream of hexane and hydrogen through a reactor for several hours, the group delivered a 1 ^-labeled pulse of 7-z-hexane via a hexane-hydrogen stream to a catalyst bed heated to 240 °C. Following isomerization, labeled molecules—including nhexane and its isomers—exited the reactor in a relatively narrow time window, Anderson stated. But when the researcher repeated the experiment on a catalyst freshly reduced in hydrogen, he observed very different results. This time-labeled hexane reacted rapidly near the reactor's entrance, Anderson claimed. Some labeled molecules underwent hydrogenolysis and were converted to lighter species such as ethane or propane. These molecules exited the reactor quickly, while others were turned into a carbonaceous layer that had to be coaxed off the surface—even at high temperature. Lesson learned: Running the hydroisomerization reaction until it reaches a steady state forms a passivating carbon layer on platinum. This form of good coke—needed for industrial isomerization
reactions—steers the catalyst away from cracking hydrocarbons, which is what uncoated platinum would prefer to do. Commercially, hydroisomerization is run in a large excess of hydrogen to avoid catalyst deactivation, noted Anderson. To better understand the mechanism responsible for deactivation—to know how to avoid it—the Eindhoven researchers conducted an experiment in which they deliberately deprived their catalyst of hydrogen. First, the Eindhoven team ran the hydroisomerization reaction until a passivating carbon layer covered the catalyst. Next, the researchers switched off the hexane gas flow and replaced hydrogen with helium. Finally, the group delivered a pulse of * ^-labeled n-hexane into the reactor and collected data. "Now you see very little coming out of the reactor quickly. Most of the pulse reacts at the front of the bed and just sits there indefinitely," asserted Anderson. "By laying down [a film of] coke on platinum, you suppress the hydrogenolysis reactions seen with the reduced catalyst and only observe dehydrogenation." Anderson explained that the fraction that remains on the catalyst bed may be
Positron-emission profiles chart the concentration of isotopically labeled compounds while traveling through a running reactor in time (vertical axis) and space (horizontal axis). Color coding indicates concentration or radioactivity level (yellow indicates higher concentration, blue lower). In hydrogen (left), an unpassivated catalyst converts part of a pulse of 1:LC-labeled hexane into lightweight molecules that exit the reactor quickly (horizontally, to the right) and part of the pulse into a carbon layer that remains in the first 20 mm of the catalyst bed (vertical blue band). In helium (right), a pretreated catalyst does not convert hexane to lighter molecules. The horizontal band does not extend completely across the axis, indicating that labeled species did not exit the reactor.
48 MAY 4, 1998 C&EN
Anderson: in situ hydroisomerization
desorbed by heating in hydrogen and analyzed outside the reactor chromatographically. In the carbon-coated catalyst experiment, the group used this procedure to identify the "stuck" species as hexene. According to Anderson, these results are consistent with a hydrogen-deficient version of the classical bifunctional mechanism. In that description, hexane adsorbs on platinum sites and dehydrogenates to hexene. Hexene isomerizes over zeolite acid sites and would desorb as hexane isomers if hydrogen was available. In a helium atmosphere, isoalkenes remain stuck on the catalyst. "I think it's a very important technique," remarked University of California, Berkeley, chemistry professor Gabor A. Somorjai, one of the symposium's organizers. "Positron emission gives a concentration profile as a function of time of reactants and products moving through a reactor. It can be used to see how close the performance of a real reactor comes to engineering models. That information should be extremely helpful for designing more efficient and more uniform reactors." Referring to the way catalyst studies have been traditionally carried out, Somorjai stressed that before-reaction and after-reaction experiments might miss important features in a catalytic cycle. Catalyst properties such as surface structure and composition often change during a reaction and "not using in situ methods to examine catalytic processes," he said, "is like studying a life with access only to the prenatal and postmortem states."^
