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JOHN T. YATES, JR. Surface Chemistry Section National Bureau of Standards
CATALYSIS From all indications, we are on the threshold of a significant surge of effort to understand heterogeneous chemical processes from a fundamental point of view. New insights into the structural chemistry of surfaces have developed recently, and rapid advances are likely to occur in the near future. Within the past few years, a new arsenal of research tools and techniques has become available to surface scientists. Surface chemistry today is being strongly influenced by many new kinds of experiments performed by both chemists and physicists on atomically clean single crystal surfaces, as well as on surfaces of practical importance such as high-area catalytic surfaces (1). As a result of these experiments, certain theorists currently are focusing their attention on adsorbed layers and on heterogeneous catalysis—that is, catalysis at an interface. The outcome of this joint effort between chemists and physicists, experimentalists and theorists, will be a new level of understanding of the electronic nature of solid surfaces and adsorbed layers—a level comparable to that currently existing in structural inorganic and organic chemistry. The influence on heterogeneous catalysis could be profound. Much catalytic work has been carried out in the past using the measurement of product yields and kinetics for different catalyst conditions—BET surface area, catalyst support, catalyst composition, temperature, pressure, contact time, and the like. This approach has been very successful and many significant advances using these procedures will continue to be made. But experimental work of this type usually cannot be expected to yield direct information about intermediate species at the atomic level. And reasoning about surface structures and reaction mechanisms at particular sites is usually indirect or nonexistent because of the structural complexity of
Insights from new technique and theory Chemists and physicists are approaching a more basic understanding of catalysis using new research techniques for studying kinetics of surface processes, spectroscopy of surface species, and lattice structure of surface layers
the adsorbent. By contrast, many of the new research methods being applied in surface studies can yield detailed structural information for adsorbed species on definite crystal planes. At present, the field of heterogeneous catalysis is virtually untouched by most of the advances that have been made in fundamental surface studies. In part, this is related to the historical success of empirical methodology in improving practical catalysts. In addition, atomically clean single crystals may bear little direct relation to the small, highly contaminated particles used in practice in catalysis. Despite these factors, however, there are reasons to believe that research on welldefined surfaces is of fundamental importance in heterogeneous catalysis: • Many methods of surface analysis are used in common by "clean surface" and by catalysis research workers. These modern techniques form a link between the two communities. • Simplification of experimental systems is a common theme in the history of science, and new understanding often comes from the reduction of a complex problem to its essential elements. Research on clean, well-defined surfaces represents a limit of simplification for surface processes. The insights and ideas obtained from modern surface studies on model systems are conceptually transferable to catalytic science. In addition, modern surface research now is capable of directly testing traditional surface-catalysis concepts, such as activated chemisorption, crystallographic specificity of chemisorption and catalysis, the "active" site hypothesis, and induced heterogeneity in chemisorption. • It is only a matter of time until the "clean surface" workers will advance to studies of heterogeneous catalysis at high pressure on single crystals. Already there are examples pointing in this direction, notably studies of August 26, 1974 C&EN
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Capsule descriptions of some select ed examples of the new research meth ods useful mainly for studying metal surfaces may serve to illustrate the po tential value of these new methods to gaining fundamental knowledge about surface science with application to het erogeneous catalysis. Among them are Theorists James W. Davenport (left), Paul Soven, and Nikhiles Kar (standing) surface kinetics methods for studying are members of a group formed at the University of Pennsylvania by Dr. J. Rob adsorption kinetics and desorption ki ert Schrieffer (second from left) to undertake the study of surfaces from a theo netics on single crystals, as well as sur retical standpoint. Dr. Theodore L. Einstein is also a member of the group face spectroscopic methods involving vibrational spectroscopy and electronic the dehydrocyclization of η-heptane to topic is "Adsorption-Desorption, Sur spectroscopy. In addition, electron dif toluene over platinum crystals (2). face Diffusion, and Reaction Kinetics." fraction by surfaces is discussed brief Also, there have been studies of ammo There is also a second symposium, ly, as are certain theoretical advances. nia decomposition on tungsten single jointly sponsored by the division with Since heterogeneous catalysis neces crystals at high pressures (3), with ki the divisions of Petroleum Chemistry sarily must involve elementary mole netic measurements correlated to simi and Physical Chemistry, on "The Na cule-surface collision steps, it is of lar measurements under low-pressure ture of the Adsorbate-Catalyst Bond." fundamental importance to investigate conditions. Whether research on atomically the efficiency of the primary act of • Theoretical calculations on small clean single crystal surfaces using mod chemisorption, as well as the rates of clusters of metal atoms containing ad ern physical techniques should be en subsequent surface processes. The sorbed species are likely to add new in couraged depends upon the goals antic most accurate adsorption kinetics work sights into the special nature of small ipated. Although the more convention has been carried out using molecular metal particles as catalysts (4). Fur al catalytic research on actual catalysts beams of known flux incident on single thermore, the complexes studied may may satisfy short-term goals and pro crystal substrates that are atomically bear a relationship to homogeneous vide stepwise catalyst improvement, it clean—that is, having less than a few catalysts such as the polynuclear tran is probable that major scientific ad per cent of a monolayer of atoms of im sition metal carbonyls. Links between vances involving novel concepts also purity on their surfaces. By measuring homogeneous and heterogeneous catal will come from new fundamental the flux of reflected molecules, the ysis have been sought for years. knowledge gained in the modern ap variation of sticking coefficient with Institutional evidence for the open proach using model adsorption sys adsorbate coverage is determined di ing of communication lines between tems. rectly. Since the absolute saturation those doing fundamental research on For example, someday it may be coverage (atoms per square centimeter) well-defined surfaces and the catalysis possible from fundamental knowledge of the monolayer can be measured di community in general comes from the about the properties of solids to tailor- rectly on the single crystal surface number of national scientific organiza make a catalyst cheaply to function ef using molecular beams, a correlation of tions and meetings that have had such ficiently and selectively, as is now done adsorbate surface density with sub communication as one of their goals. for devices in the semiconductor field. strate surface atom density also can be Included are the Surface Science Divi It also may be possible to minimize the made. sion of the American Vacuum Society, problems of catalyst poisoning by a For example, for oxygen incident on which meets annually each October, similar method, or by the use of anti the (100) crystal plane of tungsten, the and the Physical Electronics Confer dotes added either to a feedstock or in initial sticking coefficient is near unity, ence, which meets annually. In March termittently to the catalyst. and it is found that the oxygen mono1973, there was a meeting at Stanford University sponsored by the National Science Foundation-supported Materi als Research Laboratories, entitled at Massachusetts Institute of Technol "Workshop on Catalysts as Materials," ogy, receiving a Ph.D. in chemistry in as well as a meeting in October 1973 of 1960. the Solid State Sciences Committee of Dr. Yates recently spent one year as the National Academy of Sciences at a senior visiting fellow at the Universi Ford and General Motors research lab ty of East Anglia, Norwich, England, oratories. where he continued his research on surface chemistry. Last year, he re More recently, in June this year, the ceived a Silver Medal Award from the National Science Foundation spon U.S. Department of Commerce for his sored a workshop at Rice University research work carried out jointly with entitled "Needs for Fundamental Re search in Catalysis as Related to the Dr. John T. Yates, Jr., is α staff Dr. Theodore E. Madey at NBS. Energy Problem." A final report with member of the physical chemistry divi The author of more than 40 research recommendations for important funda sion of the National Bureau of Stan papers and reviews, Dr. Yates has been mental research needs soon will be dards. He has been at NBS since 1963, an active member of the surface available from NSF. at which time he was selected as a Na science community. He is currently postdoctoral program chairman for the American Additionally, the Division of Colloid tional Research Council and Surface Chemistry of the Ameri research associate in the NBS surface Chemical Society symposium series on can Chemical Society, recognizing the chemistry section. Prior to that he was "Molecular Processes at Solid Surfac trend, is sponsoring a new symposium assistant professor of chemistry at An- es," as well as chairman-elect of the surface science division of the Ameri series at each national ACS meeting, tioch College. under the general title "Molecular Pro Following receipt in 1956 of a B.S. in can Vacuum Society. He is also an cesses at Solid Surfaces." At the ACS chemistry from Juniata College, Hun avid amateur astronomer and astroAtlantic City meeting next month, the tingdon, Pa., he did his graduate work photographer. 20
C&EN August 26, 1974
Thermal desorption detects chemisorbed binding states
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Temperature, °K. Thermal desorption of carbon monoxide from clean W(100) single crystal indicates multiple binding states, designated α and β. The curves above result from desorption, following adsorp tion at 300° K. to various initial coverages, the area under each curve being proportional to the initial coverage. As the initial coverage of CO increases, new binding states with lower desorption temperatures appear. Source: Reference 7
layer corresponds to one Ο atom per surface tungsten atom (5). For hydro gen on W(100), about half of the inci dent H2 is bound to the clean surface during a single collision (6). Study of the thermal desorption of chemisorbed molecules is one of the easiest methods for detecting various modes of bonding in the adsorbed layer. When a single crystal containing an adsorbed gas layer is heated slowly such that the temperature rises uni formly, the gas is often desorbed in several discrete stages at characteristic temperatures. These multiple binding states, which are commonly given arbi trary Greek letter designations, may originate from at least three effects. One of these effects is adsorption on different symmetry sites on the crystal. A second involves the influence of in teractional effects between adsorbate species. In this case, at least two gen eral types of interactions can be postu lated. The first involves direct adsorbate-adsorbate interaction between species adsorbed on sites whose chemisorptive properties are invariant with coverage. The second, termed induced heterogeneity, is envisioned to occur when adsorption of an atom or mole cule on a particular site causes the chemisorptive properties of neighboring sites to change significantly. In both cases, "surface phase transitions" are possible in the adsorbed layer as sur face coverage and temperature are changed in a thermal desorption exper iment, and the desorption of multiple binding states may then occur. Still a third effect possibly leading to multi ple binding states in the chemisorbed layer is substrate lattice distortion and restructuring of the adsorbent surface as a result of chemisorption. A good example of multiple binding states is seen in the adsorption of car bon monoxide by tungsten single crys tals at room temperature. Various Re
sorption states, exhibiting desorption activation energies ranging from about 90 kcal. per mole to about 15 kcal. per mole, have been observed. The chemi sorbed states are liberated by slowly raising the crystal temperature and by simultaneously measuring the CO pressure evolution in an ultrahigh-vacuum chamber that is pumped contin uously. The desorption states are di vided into groups designated «- and 0-CO (Figure 1). From auxiliary reflection-infrared spectroscopic measurements (8), it has been shown that the α-CO state origi nates from "carbonyl" type CO ligands—much like those found in W(CO)e, for example. The molecular nature of α-CO had been proposed pre viously from experiments in which no isotopic mixing of a- 1 2 C 1 8 0 and a13QI6Q w a s found upon desorption (9). By contrast, the β-CO states involve much more strongly bound CO species, which have experienced extensive weakening of the C = 0 bond, or per haps even dissociation, and extensive isotopic mixing occurs upon desorption of these states (9). The α-CO carbonyl species form only at the last stages of the filling of the monolayer, presum ably when sites capable of binding the more strongly held 0-CO species are nearing saturation. This has been de termined directly by comparison of the infrared spectral development with flash desorption state distributions (8). The reasons for a variety of chemi sorbed CO states aren't well under stood at present. Presumably, though, they reflect a combination of the three effects involved in producing multiple binding states, as described above. In addition to adsorption-desorption studies of simple molecules, the study of catalytic processes on single crystals can be undertaken with the thermalprogramed desorption method. One system studied (10) is the catalytic de composition of formaldehyde on W(100) and W ( l l l ) . These two crystal planes differ significantly in the pack ing symmetry and surface density of the tungsten atoms. It has been found on both crystal planes that at low coverages of H2CO (about one half monolayer) the desorp tion products are exclusively H2 and CO, and the behavior of the system is similar to that obtained when H2 and CO are coadsorbed. At H2CO coverages of more than one half monolayer, new and more complex adsorbed species originating from parent H2CO mole cules are formed, and new desorption products—methane, carbon dioxide, and undecomposed formaldehyde—are detected upon thermal desorption. Methane liberation takes place in two main stages on both crystals—kinetically unlike the desorption of pure
methane from either crystal surface (Figure 2). What is taking place is a complex re active process that occurs as the crys tal temperature rises. The basic chem istry, product desorption kinetics, and work function behavior seem to be only slightly dependent on crystal plane. In contrast, for H2 desorption from pure hydrogen layers (11), a large difference between the same two planes is ob served in H2 desorption kinetics (Fig ure 3). It is of interest to note that mixtures of H 2 and CO on W(100) do not yield methane (12). Thus, in the catalytic decomposition of H2CO on tungsten, the structure of intermediate species is probably domi-
Desorption indicates catalytic reaction
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Temperature, °K. Methane is detected on thermal desorption from W(100) and W(111), following adsorption of Η CO at about 100° K. Thermal desorption curves, representing increasing initial cover ages of Η CO, are unlike that for desorption of pure methane (dashed curve). Comparison of desorption from W(100) and W(111) indicates that crystal plane structure plays only a minor role in determining the kinetics of the catalytic decomposition of H.CO on tungsten, with structures of intermediate species probably dominating. Source: Reference 10
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Crystal plane is a factor η some desorption processes
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Temperature, K. Crystal plane exerts a large influence on the kinetics of desorption of hydrogen from tung sten—unlike the action shown in elimination of methane from adsorbed H.CO. 7-H. states are from adsorbed molecular H_. species, as shown by absence of hydrogen-deuterium exchange, whereas β-Η. states are completely exchanged. Source: Reference 11
nant in determining kinetic and work function behavior, with the difference in substrate crystal structure playing only a minor role. The classic problem of the decompo sition of formic acid by nickel also has been studied recently using thermal desorption methods. Dr. Robert J. Madix and coworkers at Stanford Uni versity used a Ni(110) crystal, initially prepared so as to contain less than 10% of a monolayer of carbon impurity as deduced by Auger spectroscopy (13). For adsorption of HCOOD, the desorp tion products are D2O, CO, CO2, and H2. This distribution of isotopic prod ucts implies that " C — H " hydrogens yield H2, whereas "O—H" hydrogens yield H2O upon catalytic decomposi tion. In the last stages of thermal de sorption at constant temperature, the H2 and CO2 desorption rate accelerates rapidly (the decomposition becomes "explosive"), presumably because of autocatalytic effects related to the ap pearance of empty sites in islands of adsorption complexes. These islands may be composed of chemisorbed species such as HCO(ads.) and HCOO(ads.). These examples of thermal desorp tion spectroscopy from single crystals 22
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illustrate well the power of the thermal desorption method for seeing the com plexity that exists in the chemisorbed layer. Unfortunately, little or no struc tural information is obtainable using the method, and the chemisorbed layer is destroyed in the course of the experi ment. As a result, various forms of sur face spectroscopy must be employed in combination with thermal desorption techniques in order to relate surface ki netics to particular adsorbed species. Vibrational spectroscopy based on in frared transmission and electron energy loss is being applied in this way, as are various forms of electronic spectrosco py· Infrared transmission studies on sup ported catalytic metal surfaces were first carried out by Dr. A. Terenin in the Soviet Union and later by Dr. Rob ert P. Eischens at Texaco in the mid1950's. The method has been highly successful in distinguishing adsorbed states on catalysts (14). Recently, the transmission infrared method has been applied successfully to the study of catalytic intermediates under reactive conditions. As an example, the interaction of CO and NO has been studied on supported catalytic ruthenium, rhodium, palladi um, iridium, and platinum surfaces (15). In each case, an infrared stretch ing frequency at about 2260 c m . - 1 has been detected and has been assigned to an isocyanate intermediate, M-NCO. The observation of this intermediate species in the surface reaction suggests that it may be of importance in the mechanism for ammonia production during catalytic reduction of nitrogen oxides in automobile exhaust gases. Transmission spectroscopy through powdered catalysts is of high sensitivi ty because of the high adsorption area exposed to the infrared beam in such samples. However, for fundamental
studies on initially clean single crystal samples, it is important to be able to work with much lower adsorption areas—of the order of 1 sq. cm. In 1966, Dr. Robert G. Greenler of the University of Wisconsin, Milwaukee, showed theoretically that infrared spectral sensitivity could be enhanced by a thousandfold or more by choosing the correct infrared beam incidence angle (in the range of 80° to 88°) on a reflective metal surface (16). Soon in frared spectra for CO chemisorbed on initially clean films or single crystals of copper, silver, and gold were obtained by Dr. John Pritchard and his students at Queen Mary College, London (17). Recently, Dr. John T. Yates, Jr., and his coworkers at the University of East Anglia, Norwich, England, combined reflection infrared studies with thermal desorption studies of α-CO on a tung sten ribbon (Figure 4). For α-CO on tungsten, the optically observed vibra tional frequency of the CO group is at about 2100 c m . - 1 This frequency is very similar to that observed for a number of transition-metal carbonyl compounds containing linear (sp-hybridized) CO groups, implying that a-CO is bound to a single surface tungsten atom—schematically W — C = 0 rather thanW> c = = 0 Using the infrared technique, it is therefore now possible to study vibra tional spectra for certain species ad sorbed on single crystals and to corre late these measurements directly with similar measurements on supported catalysts, as is now being done by Dr. Pritchard in London. There is thus a possible experimental link between ca talysis studies and work on well-de fined atomically clean crystals. Electron spectroscopy is another technique that has been applied to studying the vibrations of adsorbed
Apparatus combines reflection IR, thermal desorption capabilities Tungsten support leads for heating current. MgO window
To IR spectromete
"ungsten ribbon for spectroscopic
