Porous solid catalytic phase

A heterogeneous catalyst is a catalyst present in a phase different from that of the reactants. The most common heterogeneous catalysts are finely divided or porous solids used in gas-phase or liquid-phase reactions. They are finely divided or porous so that they will provide a large surface area for the elementary reactions that provide the catalytic pathway. One example is the iron catalyst used in the... 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

Heterogeneous catalytic reactions, by their nature, involve a separate phase of catalyst, embedded in a phase of reacting species Therefore, the chemical transformation relies on a number of physical transport processes which may have a strong influence on the rate of the overall process and which may introduce an additional dependence on the operating conditions In the industrially important situation that the catalyst is a porous solid and the reactants form either a gaseous or a liquid phase, the following seven steps can be observed (Fig 1)... 

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

During recent years, studies of a number of hydrocarbon transformations catalyzed by porous solid oxides containing a transition metal, notably platinum, have evolved some concrete examples and demonstrations of truly polystep catalytic reactions. Specifically, these reactions have been shown to be performed by catalysts which contain geometrically separate and different catalyst components, each of which catalyzes separate steps. The chemical intermediates exist as true compounds, although often at undetected concentrations. The term true is used in this context to characterize the intermediate as a normal chemical species, existing independently of, and desorbed from, the catalyst phase, and subject to ordinary physical laws of diffusion. 

Those inunobilization procedures generally yielding supported solid-phase catalysts (SSPCs) have already been described in the preceding sections this section deals with catalytically active species (e. g., Wilkinson s or Vaska s complex) that are dissolved in liquids (therefore the catalyst is exactly the same as in homogeneous catalysis) or are even liquids themselves supported on porous solids. The principal structure of such an SLPC and the principal difference from an SAPC (Supported Aqueous-Phase Catalyst) is shown in Figure 2. 

In reality, a typical catalyst pellet will be a porous solid that may be quite complicated or even irregular in shape with a large number of catalytic reaction sites distributed throughout. However, to simplify the problem for present purposes, the catalyst pellet will be approximated as being spherical in shape. Furthermore, we will assume that the catalyst pellet is uniform in constitution. Thus we assume that it can be characterized by an effective reaction-rate constant kef that has the same value at every point inside the pellet. In addition, we assume that the transport of reactant within the pellet can be modeled as pure diffusion with a spatially uniform effective diffusivity To Author simplify the problem, we assume that the transport of product out of the pellet is decoupled from the transport of reactant into the pellet. Finally, the concentration of reactant in the bulk-phase fluid (usually... 

This review concentrates on characterization of the actual pore space, and transport within the pore space, using NMR methods. Thus, it neglects several areas. For instance the adsorption properties of a porous solid are of considerable interest in the fields of gas separation and catalysis. Adsorption and catalytic properties depend critically on the surface chemistry of the solid phase, and NMR studies of this topic have been widely reviewed elsewhere. " We shall, however, consider NMR studies of adsorbates when they specifically give information on the pore space. 

Gas-solid reactions between a fluid and a solid are important in a number of applications such as coal gasification, metallic ore processing, and catalyst regeneration. They are related in many aspects to the gas-solid catalytic reactions we have treated in developing the concepts of catalytic effectiveness, but differ in the very important aspect that the solid itself (in the form of a porous matrix) is one of the reactants. Since the solid phase itself is involved in reaction, often conditions of diffusion/ reaction change with time of reaction and the overall process is an unsteady-state one. As with effectiveness factors, many variants on a theme can be envisioned, i.e., is the reaction fast or slow, does the particle porosity (hence D ff) change with reaction, are boundary layer transport effects of importance, etc. We will present in some detail the developments of Wen concerning these questions [C.Y. Wen, Ind. Eng. Chem., 60, 34 (1968) H. Ishida and C.Y. Wen, Amer. Inst. Chem. Eng. J., 14, 311 (1968)]. 

Manipulate the multicomponent thermal energy balance in the gas-phase boundary layer that surrounds each catalytic pellet. Estimate the external resistance to heat transfer by evaluating all fluxes at the gas/porous-solid interface, invoking continuity of the normal component of intrapellet mass flux for each component at the interface, and introducing mass and heat transfer coefficients to calculate interfacial fluxes. 

From the start, we should be clear on a number of points. First, the treatment which follows is applicable to heterogeneous catalytic reactions carried out on porous solid catalysts. Secondly, the results are applicable to both gas and liquid phase reactions, at any pressure. Our main outlook, however, will be slanted toward gas phase reactions at moderate pressures. Thirdly, this treatment by no means purports to prove that catalyst surface contained in small pores has an intrinsic chemical behavior different from plane surfaces. In fact, throughout this treat-... 

This is a general situation When reactants, gaseous or liquid, have to diffuse from one phase to another phase which may be a liquid or a porous solid with which these reactants react or in which they react catalyticaUy, this effed of penetration will be more or less marked depending on the relative rates of reaction and diffusion. Since many solid catalysts consist of porous grains or pellets with a large internal surface area, this phenomenon will be of importance in all catalytic processes using such solids. 

Another way in which catalysis can be achieved using solid materials is to use them as supports for other catalysts such as metal salts. In this way a catalytic species is held immobile on the solid phase. This means that with amorphous solids a highly dispersed layer of catalyst is created and with lamella or porous solids the catalyst is held in a restricted space. The combination of the catalyst and the restriction of molecular movement brought about by the solid can give powerful control over reactive species. 

In catalytic three-phase reactors, a gas phase, a liquid phase, and a solid catalyst phase coexist. Some of the reactants and/or products are in the gas phase under the prevailing conditions (temperature and pressure). The gas components diffuse through the gas-liquid interface, dissolve in the liquid, diffuse through the liquid film to the liquid bulk phase, and diffuse through the liquid film around the catalyst particle to the catalyst surface, where the chemical reaction takes place (Figure 6.1). If catalyst particles are porous, a chemical reaction and diffusion take place simultaneously in the catalyst pores. The product molecules are transported in the opposite direction. 

The first one is a general methodology developed by Abu-Reziq et al [26,27] for the conversion of fully hydrophobic catalytic reactions - the catalyst, the substrate, and the product, are all hydrophobic - into a catalytic reaction that is carried out in water, eliminating the need for organic solvents. The method is based on a three-phase system composed of an emulsion (oil in water of the substrate and product molecule) and a solid (the catalysts), and was termed the EST (emulsion/solid transport) process. The idea (Figure 31.11) relies on the transport of hydrophobic substrates to an entrapped catalyst, and the transport of the resulting product from the catalyst porous solid back into the bulk. Specifically, the catalyst is entrapped inside a hydrophobically modified porous sol-gel matrix the hydrophobic substrate for that catalyst is emulsified in water in the presence of a suitable surfactant and the powdered catalytic sol-gel material is dispersed in that emulsion. Upon contact of the surfactant with the hydrophobic interface of the sol-gel matrix, it reorients and spills the substrate into the pores... 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

The main focus of the following considerations is on catalysis using inorganic materials. Similar considerations come into play for catalysis with molecular compounds as catalytic components of course, issues related to diffusion in porous systems are not applicable there as molecular catalysts, unless bound or attached to a solid material or contained in a polymeric entity, lack a porous system which could restrict mass transport to the active center. It is evident that the basic considerations for mass transport-related phenomena are also valid for liquid and liquid-gas-phase catalysis with inorganic materials. 

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

---