REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 154

chemical and physical and occurs simultaneously with the main reaction. Deactivation
can occur by a number of different mechanisms, commonly divided into four classes:
poisoning, coking or fouling, sintering, and phase transformation. Other mechanisms
of deactivation include masking and loss of the active elements via volatilization,
erosion, and attrition. Deactivation is inevitable, but it can be slowed and some of
its consequences can be avoided.
Poisoning : The loss of activity due to the strong chemisorption on the active sites
of impurities present in the feed stream (Forzatti and Lietti, 1999). A poison may act
simply by blocking an active site (geometric effect) or may alter the adsorptivity of
other species essentially by an electronic effect. The chemical nature of the active sites
may be modified, and these modified sites can no longer accelerate the reaction that
the catalyst was supposed to catalyze.
Coking : For catalytic reactions involving hydrocarbons (or even carbon oxides),
side reactions occur on the catalyst surface leading to the formation of carbonaceous
residues (usually referred to as coke or carbon), which tend to physically cover the
active surface. Sometimes, a distinction is made between coke and carbon, although
the difference is somewhat arbitrary — usually carbon is considered the product of CO

disproportionation:

2CO !

C+CO 2

ð Eq : 5 : 24 Þ

whereas coke is referred to the material originated by decomposition (cracking) or
condensation of hydrocarbons. Coke deposits may amount to 15 or even 20wt%
(Forzatti and Lietti, 1999) of the catalyst, and they may deactivate the catalyst either
by covering of the active sites or by pore blocking. The chemical nature of the car-
bonaceous deposits depends very much on how they are formed, the conditions of
temperature and pressure, the age of the catalyst and the chemical nature of the feed
and products formed.
Sintering : This usually refers to the loss of active surface via structural modification
of the catalyst. This is generally a thermally activated process and is physical in nature.
An extreme form of sintering occurring at high temperatures and leading to the trans-
formation of one crystalline phase into a different one is solid-state transformation.
Other Deactivation Mechanisms : Other mechanisms of deactivation include
masking or pore blockage, caused, e.g., by the physical deposit of substances on
the outer surface of the catalyst, thus rendering the active sites inaccessible to the
reactants.
Finally, loss of catalytic material due to attrition in moving or fluidized beds is a
serious source of deactivation since the catalyst is continuously abraded away.

CHAPTER SUMMARY AND STUDY GUIDE

The energetical, kinetic, and chemical equilibrium aspects of chemical reactions are
dealt with: the concept of enthalpy related to the heat of a chemical reaction is pre-
sented; the reaction rate, order of reaction, and elementary reactions are defined;