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Methods for Preparation of Catalytic Materials

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Methods for Preparation of Catalytic Materials Chem. Rev. 1995, 95,477-51 0 477 Methods for Preparation of Catalytic Materials James A. Schwarz* Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244- 1190 Cristian Contescu and Adriana Contescu Institut...
Methods for Preparation of Catalytic Materials
Chem. Rev. 1995, 95,477-51 0 477 Methods for Preparation of Catalytic Materials James A. Schwarz* Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244- 1190 Cristian Contescu and Adriana Contescu Institute of Physical Chemistty, Romanian Academy, Spl. lndependentei 202, Bucharest 77208, Romania Received May 2, 1994 (Revised Manuscript Received October 31, 1994) Contents I. Introduction II. Three-Dimensional Chemistry A. Liquid-Liquid Blending 1. Precipitation 2. Coprecipitation 3. Complexation 4. Gelation 5. Crystallization 8. Solid-Solid Blending C. Liquid-Solid Blending A. Epitaxial Metallic Films B. Unsupported Bulk Metals C. Amorphous Alloys D. Colloidal Metals IV. Two-Dimensional Chemistry A. Mounting Dissolved Precursors from Aqueous Phase 1. Impregnation 2. Homogeneous Deposition-Precipitation 3. ton Exchange 4. Colloidal Events: Electrostatic Adsorption 5. Coordinative Events: Grafting by 6. Molecular Events: Formation of Chemical B. Mounting Dissolved Precursors from Organic Media C. Mounting Precursors from the Vapor Phase D. Mounting Precursors from the Solid Phase E. Mounting Preformed Active Phases 111. Solid Transformations Hydroxyl Interactions Compounds V. The Next Dimension VI. Acknowledgments VII. References 477 480 480 481 482 483 484 486 489 490 490 491 491 492 492 493 494 495 496 498 499 500 501 502 503 503 504 505 506 506 I. Introduction Catalytic materials exist in various forms and their preparation involves different protocols with a mul- titude of possible preparation schemes, many times larger than the number of known catalysts. More- over, preparation of any catalyst involves a sequence of several complex processes, many of them not completely understood. As a result, subtle changes in the preparative details may result in dramatic alteration in the properties of the final catalyst. Our objective in this review is to provide the various 0009-2665/95/0795-0477$15.50/0 preparative procedures available to create catalytic materials. To accomplish this objective, we sought the most recent literature. Our review, therefore, focuses on research reported mainly in the past five years. The goal of a catalyst manufacturer is to produce and reproduce a commercial product which can be used as a stable, active, and selective catalyst. To achieve this goal, the best preparative solution is sought which results in sufficiently high surface area, good porosity, and suitable mechanical strength. The first of these, surface area, is an essential require- ment in that reactants should be accessible to a maximum number of active sites. The properties of a good catalyst for industrial use may be divided, at least for the purpose of easy classification, into two categories: (1) properties which determine directly catalytic activity and selectivity, here such factors as bulk and surface chemical composition, local micro- structure, and phase composition are important; and (2) properties which ensure their successful imple- mentation in the catalytic process, here thermal and mechanical stability, porosity, shape, and dimension of catalyst particles enter. The requirements which are fundamental for catalyst performance generally require a compromise in order to produce a material which meets the contradictory demands imposed by industrial processes. An acceptable solution is typi- cally ascertained by a trial-and-error route. Catalytic materials become catalysts when they are used in industrial pr0cesses.l A way this can be realized occurs when the variety of methods used to prepare catalytic materials are viewed in relation to their successful implementation in commercial applica- tions. In our attempt to develop the elements for a scientific basis for catalyst preparation, we return to the fundamental ‘blending“ and “mounting” proce- dures used to prepare catalytic materials. Figure 1 is a simplified diagram which summarizes the tra- ditional methods used for the preparation of hetero- geneous catalysts. The vertical ordering takes into account the fact that the final catalyst is a solid phase with new properties which have to be acquired and stabilized during the preparation process while the horizontal delineation depicts the various methods for “blending“ and “mounting” to produce the cata- lytic material. A noticeable discontinuity does de- velop here, however, because some preparative pro- cedures can fit into both cases. 0 1995 American Chemical Society 470 Chemical Reviews, 1995, VoI. 95, No. 3 Jim Schwarz was born in the early 194Os, experienced the 'Sputnik" era of the 1950% and finally grew up in California in the 1960s where he received his Ph.D. from Stanford University. After a series of post doctoral positions at Cambridge University, U.C. Berkeley, and (back again) Stanford, he established his industrial credentials at Chevron Research and then Exxon. in 1979. he was appointed Associate Professor of Chemical Engineering and Materials Science at Syracuse University. In a timely fashion, his title was changed to Professor; his scientific interests focus on phenomena occurring at interfaces. He fills his daily life with a balance between pursuits of the mind and the body. Cristian Contescu was born in Galati and raised in Tulcea, Romania, two cnies on the border of the Danube. AHer he received a B.A. in Physical Chemistry from the University of Bucharest (1971), he joined the Institute of Physical Chemistry in Bucharest and received a Ph.D. from the Polyiechnical Institute in Bucharest (1979). His interest has always focused on the study of interfaces but has shined from phenomena at the gas-solid intelface studied by field emission microscopy to the problems of catalyst preparation and phenomena at the solid-liquid interface. In 1992, he joined Professor Schwarz's group at Syracuse University as a visiting research associate. He enjoys classical music, literature, and fine arts. In Figure 1, two preparation routes define the extremes of traditional procedures used in catalyst preparation: precipitation (with the variant of co- precipitation) and impregnation (with such variants as ion exchange, deposition, and grafting). In the precipitation route, a new solid phase is obtained by the "blending of proper reagents (precipitating agents) from a liquid medium; the resulting precipitate is transformed in subsequent preparation stages into the active catalyst. During these transformations, both the mechanical properties of the catalyst and those intrinsically related to the catalysts' perfor- mance have to be considered simultaneously. In contrast, in the impregnation route, a solid phase preformed in a separate process is used as a support, and the catalytically active material is "mounted" and stabilized on it. In this way, a t least a part of the J ~ . . ._.._ __ - - _ ( , ~ Adriana Contescu was born in Argetoaia, Romania, and raised in Oltenia, a rich wunty in southern Romania. She received her B.A. in Inorganic Chemistry from Bucharest University (1971) and a Ph.D. from the Polytechnical Institute in Bucharest (1984). AHer joining the Institute of Physical Chemistry in Bucharest in 1979, her research concentrated on the chemistry of polynuclear inorganic complexes and nonconventional routes for the preparation of mixed oxides. This review paper is a result of her 1993 visit to Syracuse University. Her spare time pursuits include needlework, gardening, and playing with her dog. mechanical properties of the final catalyst'are con- trolled by the preexisting support, and the prepara- tion process is basically focused on the introduction of the catalytic compound(s). Between these two extremes there lies methods which are best charac- terized as solid transformation. Here physical and chemical processes are used to reconstruct a solid into a form that meets the demands imposed by the processes in which they will be used. To establish guidelines for the development of a scientific basis for catalyst preparation is perhaps a very ambitious goal. We would be required first to answer the following rhetorical questions: *What are the properties which determine the performance of a catalytic material? How can these properties be introduced, devel- oped, and/or improved during preparation? The answer to these questions involves a compre- hensive discussion of the theories of catalysis, which is beyond the scope of our review. We will attempt, instead, to provide a rationale for each reader to answer these questions on the basis of hisher own interests. We start our discussion by describing the fundamental steps in producing bulk catalysts and/ or catalyst supports. The fundamental processes involved are those derived from traditional three- dimensional chemistry. The topic areas will include single-component and multicomponent metal oxides. Unsupported metallic catalysts are formed by trans- formations involving physical or chemical processes, and the preparation methods for this class of materi- als will be discussed next. Our attention will then turn to the preparation of supported catalytic materi- als. The main topics to be discussed will be those related to the interaction between the support and the active phase when they are put together to generate the catalyst. In this approach, we exploit the virtually unexplored field of surface, or two- dimensional, physical chemistry. The materials con- sidered include dispersed metals and alloys and composite oxides. We recognize that this organization might seem arbitrary and that the reader might equally propose Methods for Preparation of Catalytic Materials Chemical Reviews, 1995, Vol. 95, No. 3 479 n & /) ..... * ;. .. . .. .... . .....' . :::.: . .:*... ::.. G ul 480 Chemical Reviews, 1995, Vol. 95, No. 3 a different classification scheme. The difference between supported and unsupported (or bulk) cata- lysts is not always apparent. Even bulk catalysts or supports, usually thought to have uniform chemical composition, may present a multiphase structure that might be the result of either doping, promoting, surface or bulk segregation, or even the effect of the reaction environment on the catalyst. We prefer the above organization in that, from one point of view, there are only two main routes for the preparation of almost all catalysts. These can be divided into the two categories: methods in which the catalytically active phase is generated as a new solid phase by either precipitation or a decomposition reaction, and methods in which the active phase is introduced and fixed onto a preexisting solid by a process which is intrinsically dependent on the surface of the support. Schwarz et al. Il. Three-Dimensional Chemistry Recent years have witnessed marked progress in the preparation of stable catalytic materials, many of them with potential applications as catalysts. This success has been achieved by either the selection of a suitable support or by choosing a proper method of preparation, or by a combination of both approaches. The simplest kinds of catalysts, from a structural point of view, are single phase catalysts, such as bulk metals and alloys, bulk oxides, sulfides, carbides, borides, and nitrides. These materials are, more or less, uniform solids at the molecular level that exhibit catalytic properties on their external surface. There- fore, these materials are preferably used in a physical form which allows for a maximum development of contact of the surface of the material with its environment. To this end, preparation methods are selected which avoid excessive heat treatments which would result in the system acquiring a more stable lower surface energy state at the expense of its active surface. Bulk oxide catalysts, either single metal or multi- metal, used in industrial processes are usually in the form of powders, pellets, or tablets, with either amorphous or polycrystalline structure. The most common method used for preparation of bulk oxide catalysts is the (co)precipitation of a precursor phase, followed by thermal transformation that leads to the oxidic phase. The ceramic method involving grinding and firing mixtures of oxides is not very convenient for preparation of oxide catalyst because of the high temperatures needed. Thus, the trend in the devel- opment of preparation methods has witnessed efforts to eliminate the high-temperature treatments of the coprecipitated materials (such as calcination of mix- tures of hydroxides and decomposition to oxides) which affect the solid state reactions that produce the intimately mixed oxide phase that acts as a catalyst. Several alternative preparation routes that enable a better mixing of the components have been proposed. A method of continuous homogeneous precipitation was developed, wherein the precipitat- ing agent (hydroxyl ions in the classical coprecipita- tion method) is slowly and continuously generated in the synthesis medium by a controlled hydrolysis process (such as hydrolysis of urea). The advantage of slow precipitation is a more efficient mixing of the components in the precipitated product. The sol- gel method, although related to the coprecipitation method, provides better control of the texture of the resulting catalyst and ensures an increased unifor- mity of the product. The method consists in forma- tion of a colloidal dispersion of the metal constituents, usually by hydrolysis of metal alkoxides. The col- loidal solution is then subjected to gelation by either changing the pH, the temperature, or the electrolyte. The resulting gel is then heat treated to remove the solvent. Decomposition of coordination compounds, including polynuclear compounds, is another pre- parative route that starts from a precursor where the metallic elements are intimately mixed at the mo- lecular or at the atomic level. Among the metal complBxes that can be decomposed at relatively low temperatures are oxalates, formates, citrates, and carbonyls. Bulk sulfide catalysts and mixed sulfide catalysts are prepared most commonly by either direct sulfi- dation (i.e., reaction with hydrogen sulfide) of oxides, halides, or other metal salts. The direct method may require the use of high temperatures. A second variant is the decomposition of a sulfur-containing precursor, such as a thiosalt, which is obtained by low-temperature precipitation. A type of low-tem- perature coprecipitation is homogeneous sulfide pre- cipitation, wherein the mixing of the metal salts is made before any addition of the precipitant. Recently, a new genre of single phase catalysts has emerged in which the entire solid rather than just the external surface is involved in catalysis. The new materials are crystalline solids which contain active sites uniformly distributed throughout their bulk at the intracrystalline level. This family of uniform heterogeneous catalysts, generally referred to as molecular sieves, includes microporous zeolites, alu- minum phosphates, with metal- and silicon-substi- tuted analogs, layered compounds such as clays and their pillared variants, layered oxides with perovskite structures, and heteropolyacids with a liquid-like behavior. The possibilities for preparation of materi- als in this class are vast since they exploit the virtually unlimited number of ways to link together atomic units in a crystalline or polymeric structure. Their methods of preparation consist of a combina- tion of chemical (precipitation, leaching) and physical (supercritical crystallization) procedures. The common features of all the preparation meth- ods summarized above for bulk catalytic materials is the use of traditional methods and techniques from preparative chemistry, such as precipitation, hy- drolysis, and thermal decomposition. The chemistry involved during these preparation steps does not differ much from that taught in classical handbooks of analytical or inorganic chemistry. These processes involve mixing of solutions, blending of solids, pre- cipitation, filtration, drying, calcination, granulation, tableting, and extrusion. In other words, the chem- istry involved is three dimensional with the meaning that it is isotropic with respect to the container in which it is done. A. Liquid-Liquid Blending The method of precipitation is the best known and most widely used procedure for synthesis of both Methods for Preparation of Catalytic Materials monometallic and multimetallic oxides. Precipitation results in a new solid phase (precipitate) that is formed discontinuously (i.e., with phase separation) from a homogeneous liquid solution. A variety of procedures, such as addition of bases or acids, addi- tion of complex-forming agents, and changes of tem- perature and solvents, might be used to form a precipitate. The term coprecipitation is usually reserved for preparation of multicomponent precipitates, which often are the precursors of binary or multimetallic oxidic catalysts. The same term is sometimes im- properly used for precipitation processes which are conducted in the presence of suspended solids. Depending on the particular application, the newly formed solid phase may be further subjected to various treatments, such as aging and hydrothermal transformation, washing, filtration, drying, grinding, tableting, impregnation, mixing, and calcination. During all these preparative steps, physicochemical transformations occur which can profoundly affect the structure and composition of the catalyst surface and even its bulk composition. If the adage the catalyst “remembers” how it was prepared, even after being subjected to various heat treatments at el- evated temperatures is valid, then any cause-and- effect correlations that can eventually be made between the precipitation procedures and the final characteristics of the catalyst becomes significant. 1. Precipitation A scientific approach to the preparation of catalysts by precipitation routes was introduced by Mar~ i l ly .~ ,~ The formation of the precipitate from a homogeneous jiquid phase may occur as a result of physical transformations (change of temperature or of solvent, solvent evaporation) but most often is determined by chemical processes (addition of bases or acids, use of complex forming agents). In almost all cases, the formation of a new solid phase in a liquid medium results from two elementary processes which occur simultaneously or sequentially: (1) nucleation, i.e., formation of the smallest elementary particles of the new phase which are stable under the precipitation conditions; and (2) growth or agglomeration of the particles. Marcilly2a stressed the importance of supersatu- ration, among other factors such as pH, temperature, nature of reagents, presence of impurities, and method of precipitation in determining the morphol- ogy, the texture and the structure of the precipitates. For example, under conditions of high supersatura- tion, the rate of nucleation of solid particles is much higher than the rate of crystal growth and leads to the formation of numerous but very small particles. Under the condition when the critical nucleation size is very small, only a metastable and poorly organized phase can develop; this may further change to a more stable phase during the hydrothermal treatment of the precipitates. Obtaining high supersaturation conditions is a difficult task in practice because of the natural evolution of the system toward a decrease of super- saturation by nucleation
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