Heterogeneous phase reaction

The cathode reaction involves reduction of silver oxide to metallic silver [7440-22-4J. The reaction is a two-phase, heterogeneous reaction producing a substantially constant voltage during discharge. Some manganese dioxide may be added to the cathode, as in the case of mercury oxide cells. 

Our interest is in solution kinetics, so we will concern ourselves only with homogeneous reactions, which take place in a single phase. Heterogeneous reactions take place, at least in part, at interfaces between phases.) Further, we will mainly work with closed systems, those in which matter is neither gained nor lost during the period of observation. 

In such an apparatus, a chemical reaction takes place with a conversion of compound A into the products B and C. Typically, a sharp pulse of component A is fed into the column. During the passage through the column, compound A is converted into the products B and C and the amount of component A decreases. Because of their different retention times, the products B and C are concomitantly separated from each other and component A. Due to the removal of the products from the reaction zone, chemical equilibrium is never reached and the reaction will ideally proceed until the total conversion of the compound A. The reaction may take place in the stationary and/or the mobile phase. Heterogeneous reactions maybe either catalyzed by the packed adsorbent or by an additional catalyst, which is mixed with the adsorbent. 

Exchange of components between phases (heterogeneous reactions)... 

Gas-phase heterogeneous reactions have been carried out in a closed circulating system evacuable to high vacuum (Fig. 13.1). For heterogeneous photocatalytic reactions, a quartz reaction cell with flat bottom (R. C. in Fig. 13.1) has been used. When Pt/Ti02 powder is spread over the bottom of the cell followed by evacuation (this is referred to as dry state) and illuminated in the presence of gas-phase water, neither H2 nor 02 is observed. The same is true for... 

Homogeneous reactions are those reactions that occur within a single phase (physical state), especially a liquid or gaseous phase. Heterogeneous reactions are those reactions that take place at least in part at the interface between two phases, such as solid and liquid or liquid and gas, etc. The discussion and problems in this chapter will address homogeneous reactions unless otherwise stated. Please note that rate means that there is a time element involved which is usually expressed in seconds (s), minutes (min), or hours (h). 

Reactions that otherwise would be carried out in more than one phase (heterogeneous reactions) can be transformed to homogeneous ones, with the aid of supercritical fluids, where interphase transport limitations are eliminated. This is realized due to enhanced solubilities of the supercritical fluids. Typical examples are reactions in water (supercritical water can solubilize organic compounds), homogeneous catalytic reactions, reactions of organometallic compounds. Homogenizing one compound more than the other may also affect relative rates in complex reactions and enhance the selectivity. 

Heterogeneous reactions—Reactions that involve species (reactants or products) that exist in more than one phase. Heterogeneous reactions are categorized further as ... 

Reactive transport equations In the formulation proposed by Guimaraes (2002), several dissolved chemical species (solutes) are transported in the water phase through the porous medium and, simultaneously, they react among themselves (homogeneous reactions) and, also, with species present in the solid phase (heterogeneous reactions). The homogeneous reactions include aqueous complex formation, acid/base and oxidation/reduction. The relevant heterogeneous reaction for the case presented in this paper refers to dissolution and precipitation of minerals. 

A palladium catalyst supported on carbon is used in a three-phase heterogeneous reaction system. The reaction takes place at 75°C (170 F) and 500 psig. The overall yield is 98.8%. 

Heterogeneous reactions. Components of water or air pollution are usually in the fluid phase. Hence we may write equations such as equations 6.2, 6.6, 6.8, and 6.12 for the fluid. The fluid may have non-permeable boundaries (the reactor walls) and permeable boundaries (entrances and exits of the system as well as catalytic surfaces where mass fluxes must be equal to the superficial reaction rates). Usually, these reaction rates are modeled as pseudo-homogeneous and, moreover, concentration measurements are almost always made in the fluid phase. Heterogeneous reactions are the result of a process that occurs at phase interfaces. This means that for the differential equation written for the fluid phase, heterogeneous reactions (surface reactions, for example) are just boundary conditions. The problem is very simple to formulate at steady state and at the boundary of an active surface, the normal mass or molar fluxes must be made equal to the heterogeneous, superficial reaction rate. Then,... 

Homogeneous Reactants (and catalysts) are mixed and remain a homogeneous single phase Heterogeneous Reactions are performed with a heterogenized/immobilized catalyst... 

---