Chapter 2
Rationale for Catalyst Characterization Kamil Klier
Downloaded by UNIV OF SYDNEY on March 20, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch002
Department of Chemistry, Lehigh University, Bethlehem, PA 18015
This volume provides a good cross section of current effort aiming at the design and development of heterogeneous catalysts for desired, mostly industrially impor tant, reactions and processes. The goal of virtually all catalysis research is to select, using ideas, hypotheses, previous empirical knowledge, and surface materials science, a catalyst that drives a desired reaction with minimum amount of side products economically, i.e., at high rates and with a long lifetime. What is sought in terms of scientific principles are "structure-function" relationships, selec tive utilization of different "reaction channels," and chemical paths that give high overall reaction rates. The word "structure" implies both the local geometric arrangement of atoms in and around that portion of the catalyst surface (or pore) which is called the active site and, equally if not more importantly, the chemical nature of the active site that is determined by the energy levels, distri bution, and symmetry of valence electrons, i.e., the "electronic structure." Further, the active sites co-exist with the rest of the surface and bulk of the heterogeneous catalyst, and there is keen interest in determining the relationship between the bulk, surface, and active site structure in any given catalytic system. Catalyst characterization thus has multiple tasks which must be linked together in order to understand why catalysts are active and selective. It has been recognized for some time that these tasks will be attacked successfully nei ther by a single experimental tool nor cheaply in terms of professional standards and instrumentation requirements in research departments that deal with catalyst development. However, the payoffs are substantial. First, catalyzed processes are the backbone of petroleum and chemical industry and amount annually to trillion dollar business worldwide. Second, the science is at the forefront of materials science: where microelectronic devices currently have functional elements of micrometer to just submicrometer size, catalysts used in industrial practice have particles of nanometer size and active centers of molecular, i.e., subnanometer, size. Third, the fundamental issues of chemical reactivity appear most prom inently in catalysis: how is the activation energy of a reaction lowered by bond ing of intermediates to the catalyst, what are the conditions for channeling the energy released in an exothermic step into an endoenergetic step, making thereby the latter possible, and how can an undesirable side reaction be stopped by pro viding a high barrier for it by a shape constraint or an increase of its activation energy. 0097-6156/89/0411-0012$06.00/0 o 1989 American Chemical Society
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF SYDNEY on March 20, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch002
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Rationale for Catalyst Characterization
One of the important areas of catalyst characterization is to provide informa tion on the bulk and surface properties in relation to the preparation methods, including precursors, calcined, prereduced, or otherwise pretreated catalysts. Other, no less important areas pertain to catalysts in their active working state and to post-mortem analysis of gradually or suddenly deactivated catalysts. There is no greater incentive for a rapid catalyst characterization than a 1,000-ton reac tor making a $400,000/day product going down due to a catastrophic catalyst deactivation. Although both the laboratory and industrial scale materials science of catalysts requires an integrated approach as already mentioned above, it is cus tomary to classify the characterization methods by their objects and experimental tools used. I will use the object classification and direct the introductory com ments to analysis, primarily elemental and molecular surface analysis, determina tion of geometric structure, approaches toward the determination of electronic structure, characterization by chemisorption and reaction studies, determination of pore structure, morphology, and texture, and, finally, the role of theory in interpreting the often complex characterization data as well as predicting reaction paths. Surface Analysis Commencing with surface analysis of catalysts, excellent tools are available, although few are used in a quantitative manner. Elemental surface analysis by Auger spectroscopy, XPS (ESCA), and SIMS is well established; in many cases the combination of core level XPS and Auger spectroscopy also gives information on the valence states of the elements present in the surface but often mono chromatic X-ray sources must be used in XPS that are provided in-house by crys tal monochromators (e.g., 1) or at synchrotron sites. Among the more advanced developments with increasing use in microelectronics surface materials science is the "small spot," or "imaging" E S C A (e.g., 2) that has now been pushed to some 20 fim lateral resolution still far from being useful in catalysis research for mapping the distribution of surface elements. Electron microscopy (EM), in particular in modern scanning transmission (STEM) instruments, has analytical resolution down to 1 nm, but within that lateral resolution, local bulk rather than surface analysis is provided. The semi quantitative analytical capabilities are impressive: 10" g (small spot E S C A or SIMS), or 5 x 10" g (analytical STEM) of matter. Molecular species are analyzed with great sensitivity by temperature pro grammed desorption (TPD) methods that employ detection by mass spectrometry, provided that the molecules desorb without decomposition in an accessible tem perature range. Less sensitive is IR spectroscopy, which, however, has the advan tage of analyzing molecular species and intermediates in the adsorbed state and is thus widely used for in situ analysis of adsorbates. In the experience of this writer, IR spectroscopy could be used in a quantitative manner upon careful cali bration and proper use of optics in scattering media, but this is not routinely done. Also, not routinely exploited is the overtone and combination spectral region of 4,000-10,000 cm" , which contains a wealth of information, particularly on hydrogen-containing adsorbates. Still lower sensitivity to adsorbed species is obtained in Raman spectroscopy, but its information content regarding the catalyst structure down to thin layers is significant. 14
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Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF SYDNEY on March 20, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch002
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CHARACTERIZATION AND CATALYST D E V E L O P M E N T
Surface science that is carried out under clean, ultrahigh-vacuum conditions utilizes with advantage the high resolution electron energy loss spectroscopy (HR EELS) that excels with its high sensitivity, but this technique does not match the energy resolution of IR or Raman spectroscopies. For adsorbates on real catalysts, therefore, the long time used IR spectroscopy, nowadays practiced almost exclusively in its Fourier transform mode (FTIR), is still the method of choice. Selective chemisorption methods have been used with success for the deter mination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quanti tative piece of information about the catalyst surface. Structural Analysis Structural analysis of catalysts relies on diffraction and selected spectroscopic methods. The fascination with zeolites, their shape selectivity, controlled acidity, and ion exchange behavior, no doubt arises from our knowledge of their struc ture. A combination of X-ray crystallography and knowledge of chemical compo sition and pore size allowed many zeolite and related phosphoaluminate struc tures to be resolved even where these materials are not available as single crys tals. Magic angle spinning solid state NMR made its impact in determining the order of aluminum and silicon, location of cations, and structures of intracavital sorption complexes. However, the most reliable structures are still obtained by diffraction on single crystals; a sufficient crystallite size for X-ray crystallography is 0.1 mm on edge. To obtain a first overall picture of a catalyst structure, X-ray powder diffrac tion is perhaps the most commonly used method: crystalline phases are readily identified and particle sizes determined with some degree of accuracy. A difficult subject is that of "X-ray amorphous" materials, i.e., those not showing discernible powder pattern. Often the "X-ray amorphous" materials show good crystallinity, albeit of small particles, upon examination by electron diffraction in the electron microscope. The reason for this is that electron microscopy permits examination, in several modern instruments, of very small individual particles by convergent beam diffraction (CBD) or microdiffraction techniques on spots as small as 2 nm, thereby enhancing the signal-to-noise ratio of the diffracted beams and their observability. Even if the particles are "amorphous," electron scattering can in principle be analyzed for atomic positions by computer and optical simulation of the scattering patterns based on geometric models, comparison with the observed patterns, and refinement. Since multiple scattering phenomena play a nonnegligible role, the crystallography in the electron microscope (I am referring to methodology developed primarily by A. Klug and applied with success to biologi cal molecules) is a sophisticated science; however, potential payoffs for catalysis have yet to materialize. A "direct" observation of individual atoms is achieved in atomic resolution transmission electron microscopes (AR-TEM), in the scanning tunneling micro scope (STM), and in the atomic force microscope (AFM) (3, 4). While A R - T E M are large machines with very high voltage (6 x 10 to 10 volts) applied to an electron-transparent small object, S T M and A F M are small devices with ultrasen sitive tip positioning mechanics that is suited for flat or near-flat objects and will 5
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Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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initially likely make inroads in studies of model, rather than real microdispersed, catalysts. A method that yields local structural information is E X A F S which utilizes the scattering of photoelectrons emitted from specific atomic core levels for determining the interatomic distances and counting the near neighbors (but does not determine the directions between the emitting atom and its neighbors). In a related development, the feasibility of determining local structures in E S C A has been demonstrated utilizing both photoelectron diffraction and the so-called forward-directed-focusing (FDF). The F D F utilizes photoelectrons emit ted from subsurface atoms as a spherical wave that is refocused by the positive potential of neighboring surface atoms, giving rise to angular peaks whose posi tions are independent of the incident X-ray wavelength and uniquely determine the relative positions of the surface and subsurface atoms (5, 6). As imaging ESCA makes further advances, it is expected that F D F will be used for structure determination in the imaged areas. The low energy electron diffraction (LEED) crystallography is also a method where multiple scattering is not negligible (although it is often neglected), and the resolved structures of single crystal surfaces have been summarized in a pack age available for microcomputers (7). Among spectroscopic tools applied to structural analysis, FTIR is routinely used for both bulk structure and identifica tion of adsorbates. Laser Raman spectroscopy is a less widely used but neverthe less an outstanding tool for resolving structures of oxides; in the laser Raman microprobe, where the laser source light is passed through an optical microscope, a lateral resolution of the order of 1 /im is obtained. A particularly successful characterization of titanias, vanadias, molybdenas, rhenias, hydroxycarbonates, and other inorganic catalysts and precursors has been made by laser Raman spectros copy. Electronic Structure Electronic structure of catalysts and their active sites is the subject of a number of experimental and theoretical investigations. In fact, all experiments that probe into the electronic structure are critically dependent on theoretical interpretation based on quantum mechanics. Among the electron spectroscopies, XANES (or NEXAFS) deals with core-level-to-excited valence level transitions, and a good theory is required for the understanding of the upper states, the core levels being simpler. Currently the scattered wave X-a (SWXa) method originated by Slater is most widely used for interpreting the XANES spectra (8). UPS and valence band XPS deal with electron transitions from highest occupied levels to vacuum and probe the energies, densities of states, and symmetries of valence bands of solids including catalysts. Optical transitions usually occur between the highest occupied and lower unoccupied levels, such as the d-d excitations in transition metal oxides and aluminosilicates (9), valence band-to-conduction band transitions in oxides and sulfides, and intraband transitions in metals. EPR and magnetic moment measurements determine the spin and orbital momentum in catalytic compounds and have been used to resolve the ground states of sorption and catalytic centers and their association with molecules. Spin-polarized XPS resulted in discovering surface ferromagnetism in otherwise diamagnetic metals such as chromium. Reactor Studies In all of the catalyst characterization techniques, reactor studies have a central
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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CHARACTERIZATION AND CATALYST D E V E L O P M E N T
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role in that they determine whether a catalyst or a class of catalysts is worthy of a detailed structural and surface analysis. The results of characterization in turn provide feedback for improved preparation of the catalysts. Examination of a catalyst by E M often reveals which preparation methods ensure the desired fine and uniform dispersion of metal particles on the support, surface analysis unrav els the presence of impurities that may accumulate from both the bulk of the catalyst and the gas or liquid phase, and chemisorption methods determine whether all metal particles seen in the electron microscope are accessible to reactants. Pore structure analysis that is usually obtained by sorption methods allows engineers to estimate whether the catalyst works in kinetic or mass transfer lim ited regime. Hardness and attrition resistance tests are useful predictors of the durability of a catalyst in fluid beds. Summary It is evident from the above discussion that catalyst characterization is an activity important for scientific understanding, design, and troubleshooting of catalyzed processes. There is no universal recipe as to which characterization methods are more expedient than others. In the opinion of the writer, we will see continued good use of diffraction methods and electron microscopy, surface analysis, IR spectroscopy, and chemisorption methods, increased use of combined E M and E S C A analyses for determining the dopant dispersion, increased use of MASNMR and Raman spectroscopies for understanding of solid state chemistry of catalysts, and perhaps an increased use of methods that probe into the electronic structure of catalysts, including theory. Science of catalysts has much to learn from materials science of metals, alloys, ceramic materials, and semiconducting materials. In turn, because catalytic science is practiced on a molecular nanostructure and surface submonolayer scale, it is one that is at the cutting edge of materials science in general and will no doubt have its impact on the technology of new, catalytic and non-catalytic materials. This symposium volume demonstrates that the field is well and alive and that progress toward a scientific catalyst design is substantial. Literature Cited 1. Gelius, U., Asplund, L.; Basilier, E.; Hedman, S.; Helenelund, E.; Siegbahn, K. Nucl. Instr. Methods Phys. Res. 1984, B1, 85. 2. Gurker, N.; Ebel, M . F.; Ebel, H.; Mantler, M.; Hedrich, H.; Schon, P. Surf. Interface Anal. 1987, 10, 242. 3. Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. 4. Hansma, P. K.; Elings, V. B.; Marti, O.; Bracker, C. E . Science 1988, 242, 209. 5. Kono, S.; Goldberg, S. M.; Hall, N. F. T.; Fadley, C. S. Phys. Rev. 1980, B22, 6085. 6. Egelhoff, W. F. Phys. Rev. 1984, B30, 1052. 7. McLaren, J. M . ; Pendry, J. B.; Rous, P. J.; Saldin, D. K.; Somorjai, G . A.; Van Hoev, M . A. ; Vvendensky, D. D. Surface Crystallographic Information Service, A Handbook of Surface Structures; Kluwer Academic: Norwell, M A , 1987. 8. Horsley, J. A. MO Calculations by SWXα, a course and text given at Lehigh University, 1987. 9. Klier, K. Langmuir 1988, 4, 13. R E C E I V E D April 27, 1989
Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
