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
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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
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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
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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