Heterogeneous reactions, definition

The yield of a gas-solid heterogeneous reaction depends not on the total time that molecules spend in the reactor but on the time that they spend on the catalyst surface. The contact time distribution provides a standardized measure of times spent in the absorbed state. A functional definition is provided by the following equation applicable to a first-order, heterogeneous reaction in an isothermal reactor ... 

Equation 2.63 is valid for any homogeneous or heterogeneous reaction. The only difference is in the definition of activities. For a species in a perfect gas-phase mixture a = pi/p°, where pi is the partial pressure of species i andp° is the standard pressure (1 bar). For a real gas-phase mixture a =f/p°, where is the fugacity of i. The fugacity concept was developed for the same reason as the activity to extend to real gases the formalism used to describe perfect gas mixtures. In the low total pressure limit (p -> 0), fi = pi. 

Overall, while the combinations of substrate effects, ambient NOz levels, and other gas-particle phenomena preclude a definitive answer, the formation of significant amounts of nitroarenes in heterogeneous particle-phase N02-PAH, atmospheric reactions seems unlikely, e.g., much slower than photooxidation or ozonolysis. This conclusion also applies to heterogeneous reactions of N205 with particle-bound PAHs on diesel and wood soot (Kamens and co-workers, 1990 see also Pitts et al., 1985c, 1985d, 1985e). 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

The surface composition, usually represented by site fractions Z, must adjust itself to be consistent with the local gas-phase composition, temperature, and the heterogeneous reaction mechanism. When the surface composition is represented by site fractions, the definition requires that... 

Riesenfeld and Bohnholtzer and Riesenfeld and Schumacher used ozone concentrated by liquefaction and distillation. From their kinetic measurements they conclude that a reaction of the second order and one of the first order take place simultaneously at quite low pressures, 6-60 mm. Hg the first order reaction predominates. The velocity constants of the second order reaction are not influenced by the total pressure, while those of the first order reaction appear to be inversely proportional to the total pressure. The figures given show that the first order reaction at the lower pressures is considerably influenced by the surface, and is quite probably a heterogeneous reaction, though the authors themselves do not consider this to be definitely shown. The decomposition appears to be rather sensitive to catalysts such as dust particles. 

We adopt this kinetic definition of reaction order without reference to the actual number of molecules involved in each act of chemical transformation. In homogeneous reactions the kinetically determined order is equal to the number of molecules participating in the actual change of which the rate is being measured. In heterogeneous reactions this equality is not necessarily preserved. It will be convenient to call the order inferred from the effect of pressure on the time of half-change the apparent order, and to refer to the number of molecules involved as the true order of the reaction. We have now to consider the relation of the true and the apparent order in various cases. ... 

As the nitration of cellulose is a heterogeneous reaction involving irregular esterification of some or all OH groups, the attributing of definite formula to products of nitration is not necessarily valid. The N content of any particular NC largely defines its soly other props, and consequently its industrial application. A summary of the types of NC s used in industry is as follows(Refs 43 59)... 

It is quite ambitious for a scientist to describe a natural phenomenon in terms of a specific reaction. The situation in the atmospheric environment is however more complicated as a variety of reactions are occurring simultaneously and a certain species may take part in different reactions affecting the relative equilibria. Most data are coming from laboratory work and experimental conditions are definitely different from the ones observed in the troposphere. As an example the mechanism of oxidation of sulphur dioxide, in gas phase is usually reported occur to a large extent through free radicals. If the presence of humidity and of particulated matter is considered, specifically in the lower part of the troposphere, definitely also heterogeneous reactions play a very important role. I feel that experiments carried on in the atmosphere yield more consistent results to elucidate the chemistry of the atmospheric environment. 

For heterogeneous reactions involving fluid and solid phases, the areal rate is a good choice. However, the catalysts (solid phase) can have the same surface area but different concentrations of active sites (atomic configuration on the catalyst capable of catalyzing the reaction). Thus, a definition of the rate based on the number of active sites appears to be the best choice. The turnover frequency or rate of turnover is the number of times the catalytic cycle is completed (or tumed-over) per catalytic site (active site) per time for a reaction at a given temperature, pressure, reactant ratio, and extent of reaction. Thus, the turnover frequency is ... 

Other important definitions include those of homogeneous, heterogeneous, and irreversible reactions. A homogeneous reaction occurs within a single phase, as in a stream that is, + HCOj = H2CO3. Heterogeneous reactions occur between phases, as gas-water, water-mineral, or, rarely, gas-mineral, as illustrated by the following reactions... 

This chapter covers the second fundamental concept used in chemical reaction engineering—chemical kinetics. The kinetic relationships used in the analysis and design of chemical reactors are derived and discussed. In Section 3.1, we discuss the various definitions of the species formation rates. In Section 3.2, we define the rates of chemical reactions and discuss how they relate to the formation (or depletion) rates of individual species. In Section 3.3, we discuss the rate expression that provides the relationship between the reaction rate, the temperature, and species concentrations. Without going into the theory of chemical kinetics, we review the common forms of the rate expressions for homogeneous and heterogeneous reactions. In the last section, we introduce and define a measure of die reaction rate—the characteristic reaction time. In Chapter 4 we use the characteristic reaction time to reduce the reactor design equations to dimensionless forms. 

Furthermore, for a large ratio of bulk metal ion concentration to the interfacial aqueous extractant concentration, the reaction zone is located vary close to the liquid-liquid interlace, and the extraction rate per unit area of interface becomes independent of the aqueous-film mass transfer coefficient as well as the system volume. Under these conditions a truly heterogeneous reaction and a homogeneous process are indistinguishable in the sarise that the rate of ench will be a definite fonction of reactant concentrations in the vicinity of the interface. In eithar case, tha foncdonal form of the kinetics aral its parameters must be determined experimentally,... 

It would be very desirable to use the a constants, which have been carefully determined for homogeneous reactions in solution, for the reactions of solid catalysts also. This would overcome the difficult task of finding suitable catalytic transformation as a standard for the definition of special catalytic a values such a process must exhibit high reproducibility which is difficult to attain with heterogeneous catalytic reactions. However, the application of the liquid-phase a values requires the same blend of interaction mechanisms to operate in both homogeneous and heterogeneous reactions. This a strict condition which might not be always met. 

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