Adsorption term

All of these relations are brought together in the fundamental form (kinetic term)(driving force) adsorption term... 

Reaction Special condition Basic rate equation Driving force Adsorption term... 

When an inert substance I is adsorbed, the term Kpi is to be added to the adsorption term. 

Adsorption rate of substance A is controlling in each case. When an inert substance I is adsorbed, the term K pi is to be added to the adsorption term. SOURCE From Walas, Reaction Kinetics for Chemical Engineers, McGraw HiU, 1959 Butterworths, 1989. 

Adsorption Term When all reactants, products, and inerts are adsorbed without dissociation such that adsorption equilibrium prevails, the adsorption term is given by... 

The exponent n on the adsorption term is equal to the number of surface sites participating in the reaction, whether they hold adsorbed... 

Fig. 10). With the completion of the structure transition, the current should drop to zero, which is indeed the case except for peak B, where a slight leak current is seen (ascribed to the side reaction Cu++ I c > Cu+). According to the theory by Bewick, Fleischmann and Thirsk (BFT) the transients can be used to distinguish between instantaneous and progressive nucleation [45], A corresponding analysis revealed that the falling part of the transients agrees well with the model for instantaneous nucleation, while the rising part shows a systematic deviation. This was explained by the existence of surface defects on a real electrode in contrast to the ideal case of a defect-free surface assumed in the theoretical model. By including an adsorption term in the BFT theory to account for Cu deposition at defects, the experimentally obtained transients could indeed be reproduced very well [44], We shall return to the important role of surface defects in metal deposition later (sec. 3.2).

Since r0 is known to vary linearly with the pressure, the adsorption terms must be large in comparison with unity, so that... 

Equation 17.102 was derived on the assumption that concentration and thermal waves propagated at the same velocity. Amundson et al.<4y> showed that it was possible for the temperatures generated in the bed to propagate as a pure thermal wave leading the concentration wave. A simplified criterion for this to occur can be obtained from equations 17.75 and 17.101. Since there is no adsorption term associated with a pure thermal wave, and if changes within the bed voids are small, then ... 

An attempt has been made to summarize the available literature for comparison of adsorption constants and forms of the equations used. Table XV presents a number of parameters reported by different authors for several model compounds on CoMo/A1203 in the temperature range 235-350°C (5,33,104,122,123,125-127). The data presented include the adsorption equilibrium constants at the temperatures employed in the studies and the exponential term (n) of the denominator function of the 0 parameter that was used in the calculation. The numbers shown in parentheses, relating to the value of n, indicate that the hydrogen adsorption term (Xh[H2]) is expressed as the square root of this product in the denominator. Data are presented for both the direct sulfur extraction site (cr) and the hydrogenation site (t). 

An inhibiting effect of the reaction product (acrolein) is reported for several catalysts, and usually accounted for by an adsorption term in the denominator, leading to equations of the form... 

It should be noted that equation 3.81 contains a driving force term in the numerator. This is the driving force tending to drive the chemical reaction towards the equilibrium state. The collection of terms in the denominator is usually referred to as the adsorption term, since terms such as KBPB represent the retarding effect of the adsorption of species B on the rate of disappearance of A. New experimental techniques enable the constants KB etc. to be determined separately during the course of a chemical reaction139 and hence, if it were found that the adsorption of... 

This equation also contains a driving force term and an adsorption term. A similar equation may be derived for the case of an Eley-Rideal mechanism and its form is interpreted in Table 3.3. 

The expreuion for the rate of reaction in terms of partial pressures is proportional to the driving force divided by the adsorption term. 

To derive the corresponding kinetic expressions for a bimolecular-unimolecular reversible reaction proceeding via an Eley-Rideal mechanism (adsorbed A reacts with gaseous or physically adsorbed B), the term K Pt should be omitted from the adsorption term. When the surface reaction controls the rate the adsorption term is not squared and the term KgKg is omitted. 

If two products are formed (P and Q) the equations are modified by (a) multiplying the second term of the driving force by Pq, and (b) adding KqPq within the bracket of the adsorption term. 

If A or B dissociates during the process of chemisorption, both the driving force and the adsorption term should be modified (see for example cases l(v) and l(vi) above). For a full discussion of such situations see Hougen and Watson(<0 ... 

This equation can be interpreted as indicating a surface reaction being rate-controlling (although the controlling step could be adsorption), but all the species involved being only weakly adsorbed i.e. the adsorption terms KeP , KSPS, K PH, which would otherwise appear in the denominator, are all much less than unity. 

This indicates that, for these catalysts, the adsorption terms KEPE and KSPS are significant compared with unity. 

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