E Solid Catalyzed Reactions

The first experimental investigation in a solid catalyzed reaction system was reported by Hugo (1968). He observed periodic fluctuations in the exothermic decomposition of N20 on a CuO catalyst. 

Experimenting with CO oxidation on a platinum screen (catalytic wire) under isothermic conditions Hugo (1972) reported observing undamped oscillations. Although these oscillations may be explained on the basis of a complicated reaction mechanism involving concentrations of the activated molecules, this is not elaborated in Hugo s paper. 

Beusch et al. reported their experimental findings and exhibited the oscillations in C02 concentration as in Fig. III. 13. 

Based on the work of Beusch et al. Eigenberger (1976) repbrted a kinetic mechanism to explain the reaction as  

Carbon monoxide oxidation by pure oxygen on a porous catalyst of the type CuO on A12Os was studied in a laboratory differential recycle reactor. Under certain experimental conditions sustained oscillations of the catalyst bed temperature and CO concentrations in the reactor described in article I of Eckert et al. were observed and reported in article II of the series. 

---

In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. As solids - unless they are porous - are commonly impenetrable, catalytic reactions occur at the surface. To use the often expensive materials (e.g. platinum) in an economical way, catalysts are usually nanometer-sized particles, supported on an inert, porous structure (see Fig. 1.4). Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry and we will discuss many applications of heterogeneous catalysis throughout this book. 

A special type of fluid-solid catalyzed reaction is obtained when either the reaction rate is so fast that the reactants become completely exhausted at the external catalyst surface (i.e. at very high reaction temperatures) or when the catalyst is nonporous. Then, pore diffusion and effective heat conduction inside the pellet need not be considered. Thus, the problem is reduced to a treatment of the coupled interphase heat and mass transport. 

For the particular case of an irreversible gas solid catalyzed reaction with no accompanying volume change, the mass (mole) balance for a species A in the interstitial gas phase moving through the emulsion phase is frequently simplified assuming axially constant transport parameters (i.e. and kbe) [141, 142, 58] ... 

Theoretical studies for the B-Z system and the solid catalyzed reactions are included in Sections C.2 and E.10, respectively. 

The solid phase could be a reactant, product, or catalyst. In general the decision on the choice of the particle size rests on an analysis of the extra-and intra-particle transport processes and chemical reaction. For solid-catalyzed reactions, an important consideration in the choice of the particle size is the desire to utilize the catalyst particle most effectively. This would require choosing a particle size such that the generalized Thiele modulus < gen, representing the ratio of characteristic intraparticle diffusion and reaction times, has a value smaller than 0.4 see Fig. 13. Such an effectiveness factor-Thiele modulus analysis may suggest particle sizes too small for use in packed bed operation. The choice is then either to consider fluidized bed operation, or to used shaped catalysts (e.g., spoked wheels, grooved cylinders, star-shaped extrudates, four-leafed clover, etc.). Another commonly used procedure for overcoming the problem of diffu-sional limitations is to have nonuniform distribution of active components (e.g., precious metals) within the catalyst particle. 

There are two fundamental types of experimental reactors for measuring solid-catalyzed reaction rates, integral and differential. The integral reactor consists essentially of a tube of diameter less than 3 cm filled with, say, IF g of catalyst. Each run comprises steady-state operation at a given feed rate, and based on several such runs, a plot of the conversion X/ versus IF/F o is prepared. Differentiation of this curve gives the rate at any given (i.e., concentration) as... 

Orcutt et. al. (157) seem to have been first to notice that fluidized bed performance for first-order solids catalyzed reactions can be given by the analogy to the segregated flow model for homogeneous reactions provided an appropriate contact time density function, i.e. probability density of sojourn times in the reaction environment is defined. They showed (using somewhat different symbols) that ... 

Homogeneous reactions occur in the fluid phase, and the volume available for reaction is sV. Solid-catalyzed reactions occur on the catalyst surface, and area available for the reaction is Vpca where V is the total reactor volume (i.e., gas plus catalyst), is the average density of catalyst in the reactor (i.e., mass of catalyst per total reactor volume), and is the surface area per mass of catalyst. The pseudohomogeneous reaction rate calculated using Equation (10.37) is multiplied by eF to get the rate of formation of component A in moles per time. The equivalent heterogeneous rate is based on the catalyst surface area and is multiplied by Vpc flc to obtain the rate of formation of component A in moles per time. Setting the two rates equal gives... 

Other factors have been identified as rate controlling in other types of solid—solid interaction, and some of these are described in subsequent sections. These include, for example, the decomposition of a solid catalyzed by a (different) solid and rate processes in which one reactant is volatilized, e.g. reaction of carbon (-> C02) with a solid oxidizing agent. 

If the catalyst is in the same phase as the reactant (e.g., dissolved metals catalyzing transformation of dissolved organic substances), the catalysis is called homogeneous. When the catalytic process is determined by a catalyst in a different phase than the reactant (e.g., solid metal oxides catalyzing transformation of dissolved organic or inorganic substances), the catalysis is called heterogeneous. In this case, the catalyzed reaction steps occur very close to the solid surface the reactions may be between the molecules adsorbed on the catalyst surface or may involve the top-most atomic layer of the catalyst. 

In general, zero-order reactions are those whose rates are determined by some factor other than the concentration of the reacting materials, e.g., the intensity of radiation within the vat for photochemical reactions, or the surface available in certain solid catalyzed gas reactions. It is important, then, to define the rate of zero-order reactions so that this other factor is included and properly accounted for. 

Tppts is a white solid which is slightly air sensitive. Deaerated solutions of tppts are stable if stored under inert gas atmosphere. The 31P NMR (161.8 MHz, D20) exhibits a singlet at 8 — 5.1 ppm. Detailed NMR data can be found in reference 19. Tppts is widely used as a ligand for metal-catalyzed reactions in water (e.g., hydrogenation, hydroformylation, carbonylation, and Heck reactions).1... 

Oxides Vanadium monoxide VO, gray solid vanadium trioxide V2O3, black solid vanadium dioxide VO2, dark blue solid vanadium pentoxide V2O5, orange to red solid. The last is the most important oxide formed by the ignition in an of vanadium sulfide, 01 othei oxide, 01 vanadium used as a catalyzer, e.g., the reaction SO2 gas plus oxygen of air to form sulfur tnoxide. and the oxidation of naphthalene by air to form phthalie anhydride. 

---