Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 467

of LHHW rate expressions can be as accurate as power-law forms even for nonideal
surfaces. This is due largely to the fact that the power-law rate forms are approximate
and only valid in narrow ranges of concentrations (intermediate). When wide range of
concentrations are examined, LHHW rate expressions are not showing larger deviations
because of their better agreement at low surface coverages either than Temkin or Freund-
lich isotherms.
To obtain rate expressions valid at low coverages as well as at high surface coverages,
nonideal isotherms must be employed without Freundlich and/or Temkin approximation.
ExLan isotherms consider the distribution of available adsorption sites to be exponentially
dependent on the interaction energy between adsorbate molecules and adsorbent. When
ExLan isotherm is employed to describe nonideal surfaces, some of the reaction rate expres-
sions are shown in Table 9.5 . These relations are valid in wide ranges of concentrations and
thus have similar quality as LHHW expressions for ideal surfaces.
Similar to the ExLan isotherm, UniLan isotherms consider that the nonideality on the
adsorbent surface gives rise to linear distribution of available sites with the interaction
energy between the adsorbate molecules and adsorbent. UniLan isotherms can also be
employed to describe nonideal surfaces, some of the reaction rate expressions are shown
in Table 9.6 .
The multilayer adsorption can also be applied to describe reactions on nonideal surfaces as
steric interactions and interaction potential differences can be effectively modeled by multi-
layers. Table 9.7 show some examples of the rate expressions for nonideal surface reactions.
Rate expressions for noncatalytic reactions involving solids can be derived in the same
manner as LHHW. The participation of the surface active centers in actual reactions can affect
the site balance, either active center renewal or depletion occurs.
For catalytic surfaces, the activity can decrease with increase duration of service. The
surface activity decline can be treated the same way as the concentration of a reacting compo-
nent in the mixture.

Further Reading

Butt, J.B., Petersen, E.E., 1989. Activation, Deactivation and Poisoning of Catalysts , Academic Press, Inc. San Diego, CA.
Fogler, H.S., 1999. Elements of Chemical Reaction Engineering , (3rd ed.). Prentice Hall PTR, Inc., Upper Saddle
River, NJ.
Liu, S., 2011. “A sustainable woody biomass biorefinery”, Journal of Biotechnology Advances .
Masel, R.I., 1996. Principles of Adsorption and Reaction on Solid Surfaces , Wiley & Sons, New York.
Saterfield, C.N., 1991. Heterogeneous Catalysis in Industrial Practices , (2nd ed.). McGraw-Hill, New York.
Smil, V., 2001. Enriching the Earth: Frith Haber, Carl Bosch and the Transformation of World Food Production , MIT Press.
Somojai, G.A., 1994. Introduction to surface chemistry and catalysis , John Wiley & Sons.
White, M.G., 1990. Heterogeneous Catalysis , Prentice Hall, Upper Saddle River, NJ.

PROBLEMS

9.1. One way of modeling the multilayer adsorption is to employ the available site
distribution function. Assume that there are n þ 1 layers, with equal fraction of