CHAPTER 1: Introduction to Catalysis 1.1 Definition
1.4 Principles and key concepts .1 Steps in heterogeneous catalysis
1.4.3 Selectivity, activity and stability
Three important properties of any catalyst are the activity, selectivity and the stability of the catalyst. Ideally a catalyst should be optimized on the basis of each of these three variables.
In reality, this is usually not possible and certain trade-offs have to be made. Often these variables have an inversely proportional relationship. Typically activity increases with temperature, at the same time the stability of the catalyst decreases with temperature.
Selectivity can go up or down depending on the product and reaction. Therefore choices have to be made and these choices depend on the demands of any particular process or reaction for which that catalyst will be utilized.
1.4.3.1 Selectivity
Most industrial catalytic processes place most emphasis on selectivity. Selectivity is defined as the ratio of the moles of a particular (usually the desired) product formed to the moles of all products formed or to the moles of reactant converted. It is a measure of the extent to which a catalyst accelerates the reaction to form one or more specified products relative to others. Selectivity varies with temperature, pressure, conversion of reactants, feed composition and with different catalysts. Completely different products can be obtained depending on the catalyst used (Scheme 1) [3]:
Methanization Methanol synthesis
Fischer-Tropsch Glycol synthesis
Scheme 1: Products from synthesis gas over different catalysts
When comparing selectivities of different catalysts, temperature and conversion or space velocity must be kept constant [3]. Conversion is defined as the fraction of the feed or some component of the feed that is converted to products. Thus percent conversion is:
Moles of feed that react x 100 Moles of feed introduced
The volume of gas (or liquid) passing through a specified volume of catalyst per unit time is known as the space velocity:
Flow rate (ml / hr) Volume of catalyst (ml) Ni
Cu/Cr/Zn oxide Fe, Co
Rh cluster
CH4 + H20 CH3OH
CnH2n + H20 CH2OHCH2OH
The ratio of the number of moles of a specific product formed to the total number of moles of reactant into a reactor is called the yield of that product. Whilst selectivity calculations only considers the amount of reactant consumed; yield is calculated on the basis of the total reactant into the reactor. It is the product of selectivity and conversion:
Yield = Selectivity x Conversion
It follows that at total conversion, the selectivity of a product will be equal to its yield.
1.4.3.2 Activity
The activity of a catalyst is the rate at which it causes a reaction to proceed towards equilibrium. The following activity measurements can be used for activity comparison [3]:
- Conversion under constant reaction conditions - Space velocity at constant conversions
- Space-time yield
- Temperature required for a specific conversion
The quantity space-time yield is the amount of product made in a reactor per unit time and per unit of reactor volume. The performance of reactors of different size and construction can be compared in this way. When measuring the temperature required for a specific conversion, the best catalyst is the one that gives the desired conversion at a lower temperature. This method can sometimes result in misinterpretations as the kinetics of a reaction sometimes change at higher temperatures [3,28]. It is frequently used to carry out deactivation measurements on catalysts in pilot plants [3].
1.4.3.3 Stability
The activity or the lifetime of a catalyst is dependent on its mechanical, physical and chemical stability under operating conditions. Any process that decreases the activity of a catalytic surface is called deactivation. Mechanical deactivation results from physical breakage or attrition of catalyst particles. Physical deactivation can occur through sintering or agglomeration of metal crystallites in supported metal catalysts. Sintering causes loss of surface area due to growth of particles and this process is irreversible. Physical loss of catalytically active ingredients also results in loss of activity. Here, metals in the catalyst vaporize slowly as a result of high temperatures and oxidizing atmospheres. An example of this is the Ostwald process (oxidation of ammonia) [31].
Chemical deactivation occurs through poisoning or fouling. A catalyst poison is an impurity present in the feed stream that gradually reduces catalytic activity. Poisons are usually strongly chemisorbed on active sites and this results in the restricted access of reactant molecules to the surface. Poisons may also exhibit an electronic effect in which bond strength of an adsorbed reactant or product molecule is altered [32]. Sulphur is a well-known poison. Fouling is the loss of catalytic activity due to the formation of coke on the surface of the catalyst. Coke refers to fine carbonaceous deposits produced by organic reactions. The mechanisms of coke formation are complex and not well understood. Burning off the coke in air can sometimes regenerate a coked catalyst. This regeneration process can be built into a process and the heat produced by regenerating spent catalysts can be utilized for an endothermic reaction within that process.
General regeneration methods include: oxidation, reduction, exposure to high temperatures to decompose or desorb poisons and chemical treatment such as treatment of the catalyst with dilute acid. If the deactivated catalyst cannot be regenerated, it has to be disposed in an environmentally friendly manner. Sometimes it can be economical to recover components from the catalyst. This is especially the case if the catalyst contains precious metals [31].