Catalyst and catalyst poisoning

Catalyst is a substance that increases the rate of reaction without being consumed in the process. A catalyst, therefore, does not appear in the overall stoichiometry of the reaction it catalyzes, but it appears in at least one of the elementary reactions in the mechanism for the catalyzed reaction. The SI-derived unit for measuring the catalytic activity of a catalyst is the katal, which is quantified in moles per second. The productivity of a catalyst can be described by the turn over number (or TON). TON is the number of moles of substrate that a mole of catalyst can convert before becoming inactivated. An ideal catalyst would have an infinite turnover number in this sense because it wouldn't ever be consumed, but in actual practice one often sees turnover numbers that go from 100 up to 40 million in some cases. Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semi-metallic elements, boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports. Catalysts are selective. That is the catalyst doesn't just speed up all reactions, but only a very particular reaction.

Mechanism of working

Generally, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway - or mechanism - with lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst usually reacts to form an intermediate, which then regenerates the original catalyst in a process. 

Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst, X and Y are reactants, and Z is the product of the reaction of X and Y:

 X + C → XC------------- (1)

Y + XC → XYC---------- (2)

XYC → CZ-------------- (3)                                                                                  

CZ → C + Z------------ (4)

Although the catalyst is consumed by reaction 1, it is subsequently produced by reaction 4. As a catalyst is regenerated in a reaction, often only small amounts are needed to increase the rate of the reaction. In practice, however, catalysts are sometimes consumed in secondary processes.

The catalyst does often appear in the rate equation. For example, if the rate-determining step in the above reaction scheme is the first step

X + C → XC, the catalyzed reaction will be second order with rate equation v = kcat[X][C], which is proportional to the catalyst concentration [C]. However [C] remains constant during the reaction so that the catalyzed reaction is pseudo-first order: v = kobs[X], where kobs = kcat[C].

Catalysts reduce activation energy [Ea.] for a chemical reaction

The catalyzed pathway in the above image has a lower Ea, consequently, catalysts provide more molecular collisions needed to reach the transition state. It may be observed in the above image that the net change in energy that results from the reaction (the difference between the energy of the reactants and the energy of the products) is not affected by the presence of a catalyst. Nevertheless, because of its lower Ea, the reaction rate of a catalyzed reaction is faster than the reaction rate of the uncatalyzed reaction at the same temperature. Because a catalyst decreases the height of the energy barrier its presence increases the reaction rates of both the forward and the reverse reactions by the same amount.

Types of Catalysts

Catalysts can either be in the same phase as the chemical reactants or in a distinct phase.

Catalysts in the same phase are called homogeneous catalysts, while those in different phases are called heterogeneous catalysts. For example, if we have Pt metal as a catalyst for the reaction of hydrogen gas and ethene gas, then the Pt is a heterogeneous catalyst. 

Catalyst components

A solid catalyst consists of mainly three components:

- Catalytic agent

- Support /carrier

-Promoters and Inhibitors

Promoters:

Promoters are generally defined as substances added during the preparation of catalysts that improve the activity or selectivity or stabilize the catalytic agents. The promoter is present in a small amount and by itself has little or no activity

Inhibitors:

Inhibitors act opposite to promoters. When added in small amounts, these can reduce catalyst activity, selectivity, or stability. The inhibitor is particularly useful for reducing the activity of a catalyst for undesirable side reactions. In the oxidation of ethylene, ethylene dichloride is added to inhibit CO2 formation thus acting as an inhibitor.

Modes of reaction

The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid-base reactions, oxidation-reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts, the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures.

Typical example

Fluid catalytic cracking [FCC]

Fluid catalytic cracking (FCC) is one of the most important conversion processes used in petroleum refineries. In short, the feedstock to FCC has an initial boiling point of 340 degc or higher at atmospheric pressure and an average molecular weight ranging from about 200 to 600 or higher. In the FCC process, the feedstock is heated to a high temperature and moderate pressure and brought into contact with a hot, powdered catalyst. The catalyst breaks the long-chain molecules of the high-boiling hydrocarbon liquids into much shorter molecules, which are collected as a vapor.

Catalyst poisoning

Catalyst poisoning refers to the partial or total deactivation of a catalyst by a chemical compound.

Mechanism of catalyst poisoning

Poisoning process

Poisoning often involves compounds that chemically bond to a catalyst's active sites. Poisoning decreases the number of active sites, and the average distance that a reactant molecule must diffuse through the pore structure before undergoing reaction increases as a result, poisoned sites can no longer accelerate the reaction with which the catalyst was supposed to catalyze. When the poisoning reaction rate is slow relative to the rate of diffusion, the poison will be evenly distributed throughout the catalyst and will result in homogeneous poisoning of the catalyst. Conversely, if the reaction rate is fast compared to the rate of diffusion, a poisoned shell will form on the exterior layers of the catalyst, a situation known as "pore-mouth" poisoning, and the rate of catalytic reaction may become limited by the rate of diffusion through the inactive shell

Credit: Google