TECHNOLOGY
New Catalysts Emerge from Work At Advanced Materials Center Work at Lawrence Berkeley Laboratory-based center includes range of studies for development of catalytic materials and applications Joseph Haggin, C&EN Chicago
With five years of operation now under its belt, the Center for Advanced Materials at Lawrence Berkeley Laboratory is starting to show results for some of its initial projects. And a considerable number of those involve catalysis and new catalysts. A Surface Science and Catalysis Workshop held early last month at LBL provided an overview of the center's projects in this area. Those projects indicate that the work is advancing catalysis along a broad front. One of the projects in the surface science and catalysis program is in the structural morphology of highsurface-area micropore catalysts. Directing the work is Alexis T. Bell, chemical engineering department chairman at the University of California, Berkeley, who lists three areas of interest: supported metal catalysts, metal oxide catalysts, and transition metal carbide and nitride catalysts. In supported metal catalyst research, three separate investigations are under way. One is the study of the reaction pathways involved in hydrogénation of carbon oxides, with the ultimate aim of making alcohols and hydrocarbons. Another deals with the effects of metal oxide promoters and supports on the activity and selectivity of metals. The third is a theoretical study concerned with predicting heats of adsorption of surface species and 34
December 19, 1988 C&EN
the activation energies for elementary processes. In metal oxide catalyst research, the major thrusts are concerned with determining the structure and chemistry of dispersed metal oxides, the molecular chemistry of zeolite synthesis, and the isomorphic substitution of elements in zeolites, the last of which is the newest effort and frequently referred to as the secondary chemistry of zeolites. The studies of transition metal carbides and nitrides deal with catalytic materials that are not yet commercial in any sense. Although this class of catalyst offers great promise, it also presents a number of problems. The first problem is making the catalysts and determining their structure. Most interest has focused on M02N and M02C. The first reaction studies with these materials deal with the adsorption and reaction of nitrogen-containing compounds on M02N. Bell emphasizes the importance of the direct interaction between the Center for Advanced Materials and industrial patrons. The work on prediction of heats of adsorption and activation energies, for example, is being conducted in collaboration with Eastman Kodak, and the work on zeolites in collaboration with W. R. Grace & Co. Other projects are being carried out with scientists from Chevron, and a new effort will shortly begin with scientists from Mobil. For illustrating the center's work in the area, Bell arbitrarily chose three of the projects to describe at the workshop. The first is application of the bond-order-conservation/ Morse potential approach (BOC/MP), a method for calculating heats of adsorption for atoms and molecular species in different environments and the activation energies for ele-
mentary processes. The second is characterization of dispersed vanadia on metal oxides. The third example is characterization of species present during the synthesis of zeolites and the further characterization of specific materials following isomorphic substitution of aluminum. The BOC/MP theory was developed a few years ago by Soviet émigré Evgeny Shustorovich, now a staff scientist at Eastman Kodak. Bell and Shustorovich teamed up to address the matter of carbon monoxide hydrogénation. This was considered a good target because of the amount of experimental information on these systems already published. The center has focused its study on the thermochemistry of carbon monoxide hydrogénation and C2 hydrocarbon decomposition on metals. Related studies focus on the effect of poisons and /or promoters on the adsorption processes in the hydrogénation of carbon monoxide. An application of the BOC/MP approach is deducing the thermal chemistry of a complete reaction. This has been done for the hydrogénation of carbon monoxide on a crystalline N i ( l l l ) surface, and Bell and his associates are looking for the energy profiles for two possible reaction paths. The first involves direct dissociation of molecularly adsorbed carbon monoxide to carbon and oxygen, and subsequent hydrogénation of the product to produce methane. The alternative path starts with the same materials but a different intermediate sequence. The idea is to develop the possible sequences and then, with the aid of experimental data on appropriate intermediates, to arrive at the operative sequence. With the aid of some of the newer forms of micrography, Bell has concluded that the BOC/MP approach
does, in fact, provide a useful theoretical framework for assessing the reaction energetics of carbon monoxide hydrogénation and dependence of the activation energy on the coverage for hydrogen desorption from selected catalysts. The second project Bell described involves vanadia, which dispersed on titania is a commercial catalyst used to oxidize xylene to phthalic anhydride and to control the emissions from stationary power plant engines. The activity and selectivity of these catalysts are known to be sensitive to how they are prepared, but this knowledge is rather empirical. Center researchers are trying to determine the effects of the preparative chemistry on surface structure and catalytic activity.
One important matter in the work is to characterize the surface area of the dispersed materials. A useful technique has proven to be oxygen uptake on the surface. The investigators find that, unlike with claims of past experiments, the optimal temperature for oxygen adsorption is about 641 K. Furthermore, they find that oxygen uptake at this temperature can be used to determine the dispersion of V2O5/M0X. The chemical reactions of the vanadyl groups on the surface can be monitored by Raman spectroscopy. The third center project described focuses on zeolites, which are well known as important catalysts for many reactions and industrial processes. The way that the zeolite performs depends on the size and shape of the cavities and channels and on the strength and distribution of acid sites over the internal and external surfaces. Work that Bell and his associates are doing in the center's project is aimed at developing a fundamental understanding of zeolite synthesis. Special attention is being given to the role of precursors in determining the final crystal structure, Bell says. Zeolite synthesis is thought to oc-
Scanning tunneling micrographs of graphite in ultrahigh vacuum (left) and graphite overlayer on Pt(lll) surface demonstrate use of advanced instrumentation for catalytic studies
cur in a sequence. During aging, silica sol dissolves to produce monomeric and oligomeric silicates. The dissolved silicates react with aluminate anions to form an aluminosilicate gel. During the synthesis, most of the aluminum is tied up in the gel, while the excess of silicon stays in solution as silicate anions. The aluminosilicate gel slowly dissolves, releasing into solution aluminosilicate anions, which can slowly react to form the zeolite. A principal tool in the center's work is high-resolution 29Si nuclear magnetic resonance spectroscopy (NMR), Bell says. It is used for the identification of the precursor structures, which evidently determine the final zeolitic structure. The principal structures observed when tetraethyl ammonium ions, tetrapropyl ammonium ions, and tetrabutyl ammonium ions were used as reactants included monomer, dimer, linear trimer, cyclic trimer, and double threering anions. According to Bell, the spectrum of precursors that appeared when tetramethyl ammonium ions were used was distinctive in that it was dominated by double four-ring anions, the secondary building unit for zeolite-A. Further control of the structure can be achieved by manipulating the solvent composition of the synthesis medium, he says. Methanol focuses dissolved silica into double four-ring anions. Dimethyl sulfoxide leads to the selective formation of double four-ring and double fivering anions. The double five-ring anion is of unusual interest because it can serve as the secondary building block for the ZSM-5 silicate zeolite, which has gained wide acceptance in numerous modern processes, including the Mobil MTG (methanol-to-gasoline) process. NMR spectroscopy is also used at the center to study adsorbed species on a potentially useful catalyst, molybdenum nitride (M02N), which has the virtue of being able to remove the aromatic forms of sulfur and nitrogen from hydrocarbon materials in the manufacture of liquid fuels. This would have the practical value of avoiding catalyst poisoning, but it would have great potential geopolitical importance as well. The work is under the direction of December 19, 1988 C&EN
35
Technology
Center works to address materials problems The Center for Advanced Materials atit Lawrence Berkeley Laboratory enterss its sixth year with accomplishments in n hand but an indistinct funding future.). Government support, which has droppedd precipitously in the past three years,i, may stabilize at the present level. But it center officials believe that the new ie Bush Administration's commitment to0 basic research remains unclear. Industry has picked up some of the e slack through the close collaborationη now developing with the center. Of>f particular interest to chemists aree projects in the surface science and d catalysis program, but other programss are also of fundamental importance. Center director Robert 0 . Ritchiee told the Surface Science and Cataly-'sis Workshop last month that many of>f the problems of U.S. industry can be e attributed to inadequate materials.>. Speeding up development of new and d improved materials, he says, will re-)quire more funds from more sources,>, a new and closer liaison among govfernment, industry, and academe, and d the training of more students in the e new materials science of the 1980s. The same plea has been made byy other educators and industrialists in η recent years. One response to earlier)x pleas came from LBL, which in 1983 3 established the center to marshal the e basic materials research capabilitiess
associate staff scientist Jeffrey A. Reimer, an assistant professor of chemical engineering at UC Berkeley. Molybdenum nitride has already been shown to be effective as a catalyst for removing nitrogen from aromatic systems. It is a face-centered cubic solid with variable stoichiometry. Reimer says that the amount of nitrogen can be adjusted within prescribed limits, and the substance is available in both single crystals and bulk forms. M02N functions rather like a bad metal in most respects, he says, but it can be prepared with very high surface areas of between 100 and 150 square meters per gram. One of the bad things about it is its extreme sensitivity to oxygen; it oxidizes exothermally. So far, Reimer's group has concentrated on the study of model ad36
December 19, 1988 C&EN
of LBL and the University of California, Berkeley. Its purpose is to attack materials problems facing U.S. industry and to train materials scientists. Though there is significant and growing support from industry, primary support for the center comes from the Départm e n t of Energy. In addition t o s u r f a c e s c i e n c e a n d catalysis, t h e center is a c t i v e in s u perconductivity, electronic materials, structural materials, and p o l y m e r s and
composites research. Pervading all of these interconnected areas of research is the development of new instruments and analytical techniques. At present, the center has a scientific staff of about 50, aided by more than 100 graduate students and the considerable facilities of LBL. The basic research and development that the center pursues is done in close concert with government and industry. Through workshops and individual consultations, the center tries to identify the areas that need the most attention and which it is uniquely capable of addressing. Most of the people who do the research are the center's staff, who hold regular appointments at the University of California. There are also postdoctoral a p p o i n t m e n t s , a n d t h e r e is a n increasing n u m b e r of industry scientists w h o c o m e t o LBL for periods up to a year or t w o , depending o n
sorption systems with NMR. The three adsorbates of interest have been hydrogen, ammonia, and acetonitrile. The group's present task is to investigate the structure and mobility of the adsorbates on the catalyst surface. Hydrogen adsorbs on the surface in patches. Ammo nia adsorbs first molecularly and then appears to convert to other forms. Most recently, Reimer's group has been looking at acetonitrile, which adsorbs with the nitrogen end down on the surface. The equip ment in which this work is being done* has been designed such that all the features of the microreactor on the lab bench are available in side the NMR apparatus. The adsorption of hydrogen caused some confusion among the research ers for a while, because the amount
the jnterest of their
P a r e n t companies
and the needs of the project. Other universities have established research institutes that are similar to the Center for Advanced Materials. Some are more successful than others, depending on the way that research contracts are structured. LBL believes that it has something to offer by virtue of research funding that can be applied to secure maximum value to the patron in place of a fixed procedure dictated only by the needs of the university. Director of the program in surface science and catalysis is UC Berkeley chemistry professor Gabor A. Somorjai, who speculates that the recent drop in federal funding of basic research may be characteristic of the immediate future. "The original funding levels enjoyed by [the center]," he says, "were reasonable, but perhaps not very realistic." In any event, the center is now trying to interest industry in funding and in otherwise participating in a wide variety of R&D in modern materials science. It seems safe to say that modern materials science has been largely equated with high-tech chemistry. This is certainly a shot in the arm for chemistry in general, but it also challenges a well-established discipline to undergo change at a rate that at times can be uncomfortable, sometimes extremely so.
adsorbed was only about 10% of the expected value. However, they now believe that the hydrogen molecule arrives at the surface and dissoci ates. The atoms then adsorb but are much closer than anticipated. The distance is about 3 nm apart. The adsorbing hydrogen molecules dissociatively adsorb at defect sites where there are nitrogen-deficient patches in the surface, he says. In the case of ammonia, Reimer and his group used proton NMR to follow the fate of the ammonia mol ecule. Ammonia takeup at the sur face was much greater than that for hydrogen. At room temperature, the ammonia migrates about the sur face rather freely, but at low tem peratures the molecules remain close together. The work with acetonitrile is still
sketchy, but some data have been gathered using 13C labels. At room temperature, the acetonitrile adsorbs on the surface with the N-terminus attached to the surface. However, the entire molecule appears to re main mobile. Reimer says that, hav ing demonstrated the facility of the apparatus and the technique with the model compounds, he is now ready to move on to "more realistic molecules/' One area of technology that is likely to resume its former impor tance is coal gasification. During the present oil glut, coal conversion has been put on the back burner, but researchers expect this to change— the only question is when. One ba sic research project still being funded by the U.S. Department of Energy is at the Center for Advanced Mate rials under the direction of Heinz Heinemann. He and his coworkers are seeking to achieve the gasifica tion of chars and coal with steam at temperatures below 1000 K. Today, temperatures greatly in excess of 1000 Κ are necessary. Heinemann is also trying to understand the mechanism of catalytic gasification. In recent months, this project has led to the development of new cat alysts that permit gasification at low temperatures and, equally impor tant, permit the production of hy drogen and carbon dioxide rather than synthesis gas (hydrogen and carbon monoxide). This is impor tant in gasification because it re moves the constraints inherent in including the water-gas shift reac tion in the reaction scheme. The work so far indicates that a good gasification catalyst for Heinemann's purposes must be able to react with the surface oxycarbon and also be able to dissociate water. The known catalysts that can do these things have some rather significant draw backs. Potassium-nickel catalysts, for instance, quickly deactivate, primar ily because of interaction with the char and catalysts. Nickel oxide by itself is too costly and not very active. Heinemann has been looking at materials that are cheaper than nick el and iron catalysts, and in the past few months has found that mix tures of alkalies—for example, potas sium-calcium mixtures—have much the same properties as the potassium-
Precursor structures may determine zeolite structure n=1
n=5
n=2
n=3
n=7
n=8
n=6
n = 10or12
η = number of Si atoms Various intermediate structures have been detected in the synthesis of zeo lites and are considered precursors in the formation of the final zeolitic struc ture. They were determined at the Law rence Berkeley Laboratory with the aid of 29 Si NMR spectroscopy. The evi dence suggests that the nucleation and growth of zeolites occur through the assembly of these precursors, which can be regulated to some degree by the reagents used in the growth medium.
nickel catalysts. Several researchers have tried these materials individu ally, Heinemann notes, but appar ently nobody has tried mixtures of the two. The mixtures had been used, though, in the investigation of coal swelling some time ago. Mixed feelings are often expressed by researchers about the use of mod el systems of adsorbed species in the basic research into catalysis. Practical industrialists sometimes want to avoid model systems in fa vor of "practical catalysts" already in use. The counterargument stresses the need to reduce the complexity of "practical catalysts" to a reason
able level. There is a growing like lihood that the catalytic twains are approaching each other, if they haven't already met. University of California, Berke ley, chemistry professor Gabor A. Somorjai, director of the surface sci ence and catalysis program at the Center for Advanced Materials, be lieves that there is no real conflict at all. In his work, surface science in general and the surface chemical bond in particular are both studies on model systems. There are two basic reasons for carrying out studies using model systems, Somorjai says. One is the tremendous scope of instrumental techniques that can be used on ex ternal surfaces of model compounds. The other is that, because of re searchers' ability to characterize ex ternal surfaces well, conclusions can be extended, within reasonable lim its, to internal surfaces for the same reactions. The single-crystal surface in this case provides excellent guid ance in the sense of an upper per formance boundary for all more complex catalysts. Somorjai stresses that this is certainly the case for ammonia synthesis, and he believes that the same is true for other cata lytic reactions. During the past 20 years, Somorjai says, detailed knowledge of transi tion metal catalysis has been ac quired, and researchers are now moving on to oxides, which are dif ficult to characterize because of their often variable stoichiometry. A clean oxide surface is also difficult to at tain. It usually requires a very long time to thoroughly clean a singlecrystal catalyst, and a new technique is needed. Somorjai believes that an adaptation of an old technique used in industry may be an answer. In this technique, layers of metal are alternately vaporized and oxi dized in place. With controlled heating, the layers can be well or dered and the oxidation state can be controlled by regulating the par tial pressure of the oxygen. Somorjai notes that, by this technique, 10 or more monolayers have been built up, permitting some important stud ies with model catalysts. The meth odology can be applied to other sys tems as well, and other techniques are also in use. December 19, 1988 C&EN
37
Technology Whatever the merits of singlecrystal catalytic systems, they have been essential in the recent successes of basic research. Somorjai notes that during the past decade, his group has found that the surface chemical bond has three properties. It is "clusterlike"; there is thermal activation of the bond breaking involved; and there is coadsorption involved. With respect to the "clusterlike" character of the surface chemical bond, most of the data have been obtained by studying small organic molecules on the surface. In every case where he determined a structure on a surface, Somorjai says, he always found an organometallic equivalent for that particular structure. This is a multinuclear organometallic cluster, which is affected by at least the first layer beneath the surface itself. There must be at least four metal atoms involved. Somorjai's group has studied numerous surfaces and adsorbates, including benzene and substituted benzenes. In the case of the benzene molecule, the van der Waals envelope remains essentially unchanged when adsorbed on crystalline P d ( l l l ) . In the gas phase, the carbon-carbon bond distances are 1.4 A and remain about this value on the P d ( l l l ) . However, on rhodium or platinum surfaces, the bond distance is expanded in a way that depends on the symmetry of the site. Thus, the bond distances alternate between long and short. This distortion results from the site symmetry and the nature of the electron donation. The ring radius expands and the metal-carbon bond distances between the surface and the adsorbate shortens. Somorjai notes that this type of distortion is also seen in organometallic multinuclear clusters, and the analogy of the clusterlike nature of the metal surface bond is extendable to aromatic molecules. In the case of ethylene on crystalline R h ( l l l ) at low temperatures (77 K), the molecule lies down on the catalyst surface along the length of the molecule. As temperature is raised to 310 K, ethylidyne groups are formed, with one carbon being bonded to the surface. At 450 K, still other bonds and species are 38
December 19, 1988 C&EN
formed. These events occur in sequence, and the sites where these occur are always present. Instrumentation improvements pervade all facets of the work at the California center. One example is the direct observation of molecular adsorbates with the scanning tunneling microscope (STM). A research group under Miquel B. Salmeron has developed an STM that operates in air, vacuum, and liquid environments to allow the imaging of conductive surfaces on the atomic scale. It is being used to study catalytic surfaces as well as the structure of adsorbates. This is a significant improvement over the crystallographic a p p r o a c h — w h i c h was limited, for example, to high vacuums or to periodic structures. Of special interest is the imaging of the surface of molybdenum and rhenium covered with a monoatomic layer of sulfur in both vacuum and air. Another new surface spectroscopic tool used at the center is infraredvisible sum-frequency generation (SFG), which is highly surfacespecific and applicable to all types of interfaces accessible to light. It has the potential to monitor in-situ dynamics, surface reactions, and intermediate surface species with picosecond time resolution. Two major problems with nonlinear optical techniques have been the tendency of materials to absorb significant fractions of incident optical pulses, and metals' and semiconductors' usually large susceptibilities, which generate nonresonant signals interfering with that of the monolayer. Both problems have been overcome with the new technique. •
Program to seek ways to cut waste generation A new program involving the chemical engineering and environmental studies departments at the University of California, Davis, will take an integrated approach to finding ways that small and medium-sized businesses can economically reduce the amount of hazardous wastes they generate. The three-year, $450,000 project
Schwartz: emphasis on smaller firms will focus on businesses that generate heavy metals and organic solvents, according to Robert L. Powell, an associate professor of chemical engineering. The project will look initially at the metal finishing, semiconductor, and printed circuit board industries. The goal of the research will be to develop in-plant modifications, through process design and automation, that will reduce the amount of hazardous waste entering the environment. The researchers hope to discover generic solutions that are suitable for small and medium-sized firms that can be applied across industries that share common problems. The project will be funded by the University of California Toxic Substances Research & Teaching Program, which is based at Davis. The major objective of the project is close integration of the technical analyses with economic and policy analyses, says Seymour I. Schwartz, a Davis professor of environmental studies. The three chemical engineers and two environmental studies faculty members involved in the program will work to avoid "slicing the project down the middle," Schwartz says. The emphasis of the project is small and medium-sized firms because such companies often cannot afford staff or consulting services
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