Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey’s research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

Catalysis Volume 10

A Review of Recent Literature

By James J. Spivey, Sanjay K. Agarwal

The Royal Society of Chemistry

Copyright © 1993 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-614-7

Contents

Chapter 1 Toward Supported Oxide Catalysts via Solid-Solid Wetting By Helmut Knözinger and Edmund Taglauer, 1,
Chapter 2 Model Catalyst Studies of Supported Metal Sintering and Redispersion Kinetics By Calvin H. Bartholemew, 141,
Chapter 3 Techniques for Measuring Zeolite Acidity By George Marcelin, 83,
Chapter 4 Applications of Raman Spectroscopy to Heterogeneous Catalysis By Israel E. Wachs and Franklin D. Hardcastle, 102,
Chapter 5 Oxidative Coupling of Methane By Zbigniew Kalenik and Eduardo E. Wolf, 154,

CHAPTER 1

Toward Supported Oxide Catalysts via Solid-Solid Wetting

BY HELMUT KNOZINGER AND EDMUND TAGLAUER

1 Introduction

Supported oxides of transition metals, particularly of groups Vb (V), Vlb (Cr, Mo, W), and Vllb (Re) are widely used as catalysts for various reactions. These so-called “monolayer-type” catalysts are formed when one metal-oxide phase is dispersed on the surface of a second metal-oxide support. Typical catalyst supports in industrial applications are transition aluminas, silica, and titania. Alumina-supported molybdenum and tungsten-based catalyst precursors are extensively used in the petroleum industry in hydrotreating processes. Consequently, numerous studies have been carried out to analyze their function in hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodemetalization (HDM) of petroleum and coal products. The oxidation of hydrocarbons, carbon monoxide hydrogenation and the water gas shift reaction are also catalyzed by supported molybdena and tungsta. TiO2-supported vanadium, molybdenum, and tungsten oxide catalysts were found to be highly active for the selective catalytic reduction (SCR) of NOx with NH3. The vanadium oxide/TiO2 system is also widely used for selective catalytic oxidations of hydrocarbons. Supported Re2O7 effectively catalyzes the metathesis reaction and chromia-based catalysts are active for polymerizations (SiO2 supported) or redox reactions (Al2O3 supported).

Typically, this class of catalysts is prepared by impregnation of the support from an aqueous solution containing a suitable precursor compound or (less frequently) by gas-phase chemisorption of a volatile metal compound (e.g., Mo(CO)6) on a carrier. When catalysts are prepared by impregnation on an industrial scale, large volumes of solutions must be handled and eventually large volumes of wastewater must be disposed. As a consequence, there might be an interest to synthesize catalysts via alternative routes that would not require impregnation and precipitation steps. Solid-state reactions provide a significant potential in this context, since reactions between two (or more) solids necessarily must involve the interfaces between them. Several processes can occur when an active solid component undergoes reactive interactions with another solid, the support. The active component may (1) retain its chemical identity, the support simply acting as a dispersing agent, (2) dissolve in the support matrix to form a solid solution, or (3) form new surface and/or stoichiometric bulk compounds. Haber has strongly emphasized the role which surfaces and interfaces play in the reactivity of solids. In powder mixtures, depending on the relative rates of nucleation and nuclei growth on one hand and surface migration or gas-phase transport on the other hand, two principal routes for the reaction progress can be envisaged. If the nucleation and nuclei growth rates are much higher than migration rates, a solid-state reaction can only occur at intergranular contacts and will lead to the formation of a bulk compound (Route I). If, in contrast, the migration of one mobile component across the surface of another less mobile component is very fast, grains of the latter will be encapsulated by a thin layer of the former, so that the entire surface becomes the reaction interface (Route II). A schematic representation of the propagation of the reaction interface via routes I and II is given in Figure 1. Several examples of solid-state reactions proceeding via route II have been reported in the literature. If the rate of formation of a bulk compound across the reaction interface is negligibly small, the process may come to a close once the surface layer has formed.

The migration of one solid over the surface of another solid is frequently described as surface diffusion of constituents of the lattice in a concentration gradient. Haber and coworkers suggested the wetting of one solid by a second solid under the action of forces of surface tension as an alternative mechanism.

It is tempting to take advantage of these phenomena known from solid-state chemistry in the preparation of supported oxide catalysts, although this has been realized in practice only in exceptional cases. The increasing interest in this area is in fact documented by a recent review by Xie and Tang on spontaneous spreading, which covers the literature up to 1987. In the present review we are reporting on wetting and spreading phenomena in systems of particular interest for catalyst preparation, where mixtures of oxides will play a central role.

2 Theoretical Considerations

2.1 Thermodynamics of Wetting and Spreading. – The thermodynamics of wetting of a solid by a liquid is well established and discussed in detail in relevant textbooks. The same principles can be applied in the phenomenological treatment of the wetting of one solid by another solid, a phenomenon that also plays a major role in the redispersion of supported particles on the surface of an oxide carrier (e.g., supported catalysts). Sintering and redispersion in supported metal catalysts have been discussed by Ruckenstein in several papers and excellent review articles.

Redispersion of particles on the surface of a carrier is a phenomenon that has much in common with the spreading of one solid component over the surface of a second solid in the course of solid- state reactions as discussed in the introduction. In this case, grains of both components are contacting each other in powder mixtures and the spreading will be initiated from the contact zones. This same situation is apparent when supported catalysts are to be prepared by spreading from powder mixtures containing the support and the precursor of the final supported active phase, where the active phase is formed by spreading of the precursor. It is therefore important to define the conditions under which solid-solid wetting and spreading can be expected to occur. A schematic representation of wetting and spreading is shown in Figure 2.

The overall change in interfacial-free energy ΔF is given by Equation (1):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where γij denotes the specific surface-free energy between phases i and j, ΔA the change in surface/interface area, and subscripts a, s, and g denote active phase, support and gas phases, respectively. For wetting of the support by the active phase to occur, the interfacial-free energy change must be negative (ΔF<0), hence, the condition

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

or

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

if /ΔAa/ = /ΔAas/ = /ΔAs/ must be fulfilled. Hence, for predictions to be made of whether or not solid-solid wetting can principally occur in a given system, the specific surface and interface-free energies must be known for the experimental temperature and environmental conditions applied. Surface-free energies of pure binary oxides have been compiled by Overbury et al. The available data are typically measured near the melting point of the material and the temperature coefficients of the y-values are not known in most cases. Surface-free energies will also vary with the nature and composition of the gas phase in an unknown form. Therefore, the tabulated values can only be used for order-of-magnitude considerations.

Interface-free energies Γas are practically always unknown. γas is given by Equation (4):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where Uint is the interaction energy between the two oxides per unit area and Ustrain is an energy term brought about by a possible mismatch of the lattice parameters of the two oxides in contact. The interaction energy Uint can be considered as the adhesion energy; it may, however, also contain contributions from “chemical” interactions in the interface.

Surface-free energies of several oxides, which bear relevance in the present context as either supports or active oxides, are summarized in Table 1 together with their bulk melting points Tmelt and Tammann temperatures TTam ≈ 0.5 Tmelt·

2.2 Dynamics of Spreading. – As mentioned in the introduction, the spreading of a solid on the surface of another solid has been described as surface diffusion of constituents of the lattice of the mobile solid in a concentration gradient. Haber et al. argued that in oxide systems surface diffusion should be slow in the temperature ranges frequently encountered in the solid-state synthesis of oxide systems due to the typically high values of the lattice energies of oxides. The concept of solid-solid wetting was therefore introduced. However, even if the surface-free energy gradient is responsible for the wetting phenomenon to occur, migration of one component over the surface of the second oxide requires mobility of the constituents of the lattice.

Diffusion in a concentration gradient can be described by the model of independent particle diffusion as schematically represented in Figure 3(A), where the arrows indicate the time-averaged particle displacements along the concentration gradient. In this case the individual particle must be separated from the mobile phase and it must overcome an activation energy in each elementary jump on the support surface. As a rule of thumb, the Tammann temperature TTam ≈ 0.5 Tmelt,bulk (in K) is considered to be sufficient to make atoms or ions of the bulk of a solid sufficiently mobile for bulk-to-surface migrations, while the Huttig temperature (approximately one-third of the bulk melting temperature) is enough to make the species already located on the surface adequately mobile to undergo agglomeration or sintering. Ruckenstein has demonstrated that the enhanced mobility can be associated with the two-dimensional melting of the surface of a solid particle, i.e., with the occurrence of a “liquid-like” behavior of the surface layer. A theory of two-dimensional melting has been advanced by Kosterlitz and Thouless which is based on the dislocation pairs model of melting. The two-dimensional melting temperature is given by Equation (5):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where m is the atomic mass, k the Boltzmann constant, h Planck’s constant, a the lattice parameter and ΘD the Debye temperature. It turns out that Tmelt,2-dim as given by equation (5) is proportional to the bulk melting temperature as obtained by Lindemann with the proportionality constant being close to 0,5, the value used in the definition of the Tammann temperature. Values of the 3-dimensional melting temperatures and Tammann temperatures of several binary oxides are summarized in Table 1.

The mechanism of spreading in powder mixtures may thus be described as migration of species from a “liquid-like” surface layer of one solid across contact boundaries between grains onto the surface of the support, where they may be immobilized again if the interaction energy Uint is sufficiently high. A thin film (possibly a single atomic or molecular layer) may thus extend from the contact boundary onto the support surface. Further transport of the active oxide material can then be envisaged to occur via migration of active phase species over the film surface toward the leading edge of the film where they would ultimately be trapped again on the support surface. This process may be described by the unrolling-carpet mechanism which is schematically represented in Figure 3(B). Depending on the individual properties of the interacting oxide materials, formation of thick films (several molecular layers) or islands (particles) with finite contact angle may also occur.

Baker observed mobilization of small particles of several metals and metal oxides on graphite at a temperature (so-called “mobility temperature”) that was identical with the Tammann temperature. Thus, in systems exhibiting relatively weak interactions with the support surface, particle mobility may be induced at this temperature which might ultimately lead to agglomeration rather than spreading.

2.3 Mixing of Powders. – Intimate mixing of the two powder components is required so as to obtain a homogeneous product. Therefore, grinding or milling is usually applied to the powder mixture prior to thermal treatments. Often these processes are not well controlled when powder mixtures are prepared for solid-state synthesis of bulk products or supported catalysts, although they must be expected to influence the reactivity of the powders very significantly. Grinding will certainly influence the grain sizes and grain-size distributions and thus the rates of spreading. Also the two-dimensional melting temperature should be dependent on the grain size. In addition, several phenomena occur in the very complex grinding mechanisms that must influence the spreading and reactivity behavior of powder mixtures. During grinding, several particles are simultaneously and repeatedly subjected to stress application in the grinding zone. With each stress application, several fractures may occur in each particle. Cracks will be initiated and will propagate; flaw interaction in a particle, secondary breakage, and interaction of particles with each other will occur. The physical and chemical interaction between particles and the grinding environment and the transport of material through the grinding zone will also affect the nature of the product obtained. Occasionally material transport between chemically distinct particles may already lead to spreading and wetting during the grinding or milling procedure. Even solid-state reactions in bulk phases can be induced by mechanical activation of solid materials and several tribochemical processes have found technological applications. Angelov and Bonchev have reported on the formation of a Cu-rich surface layer on Co304 by mechanically treating a powder mixture of CuO and Co3O4 in a friction grinder. These reactions are believed to occur due to strong local temperature increases which may lead to melting of microscopic zones within contact regions.

Although the discussion in this section is qualitative, it must be concluded that the mixing and grinding procedures in preparation of powder mixtures for catalyst synthesis via solid/solid wetting and spreading undoubtedly play an important role and must be carried out carefully and under controlled and reproducible conditions.

3 Experimental Evidence for Spreading

3.1 Alumina-supported Systems. – Alumina-supported systems are by far the most studied in relation to solid/solid wetting and spreading with several group Vb and VIb oxides being used as mobile phases. Among these molybdenum oxide (MoO3) has found particular interest. The spreading behavior of MoO3 on the surface of transition aluminas will therefore be reviewed in some detail, followed by a discussion of other mobile phases including V2O5, CrO2, and WO3, and several salts of interest for catalyst preparation.

(Continues…)Excerpted from Catalysis Volume 10 by James J. Spivey, Sanjay K. Agarwal. Copyright © 1993 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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