All industrial catalysts must have the following properties :
- It should adsorb one or more reactants sufficiently strongly.
- The adsorption must not be strong to prevent desorption of products.
For example :
Silver (Ag) — Doesn’t have strong adsorption properties.
Tungsten — Adsorbs very strongly so desorption will not occur. So, Ag and W are not considered as good catalyst but Pt and Ni fulfills both the conditions to become good catalyst.
The two most important considerations for a solid to become good catalyst are–
- High activity.
- Long term stability or durability.
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We know Chemistry is the study of how atoms, molecules react in the microscopic level. On the other hand, the word ‘Quantum‘ came from ‘Quanta‘. Actually radiation is considered to be consists of particles, each such particle is called ‘Quanta‘. For light radiation, it is called ‘Photon‘.
Well, we now know what each of the words means separately. But what meaning comes out when we join both of the words? So Quantum Chemistry is that branch of chemistry which deals every physical phenomena assuming the dual nature of matter. Basically it contains pure mathematics like Calculous, Integration, Summation, Product and many other operations.
It is interesting in the fact that we can prove and mathematically understand every physical phenomena. In some Universities, Quantum Chemistry is taught independently. But in most of the Universities, it is taught as a chapter of physical chemistry.
There is a lot more things in the universe to uncover with the help of ‘Quantum Chemistry‘.
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Langmuir-Hinshelwood has proposed a mechanism for adsorption of gases on the surface of solid. The surface reaction is assumed to occur in the following steps:
1. Diffusion of the reactants on the surface of the solid adsorbent.
2. Adsorption of the reactants at the surface.
3. Chemical reaction at the surface.
4. Desorption of the products from the surface.
5. Diffusion of the products from the surface of the adsorbent.
Step 1 & 5 are generally very fast and step 2 & 4 are generally faster than 3. Thus step 3 is the rate determining step in the surface reaction, i.e., chemisorption of the reactants on the surface of the solid adsorbent.
Two feature of this type of surface reaction:
1. Chemisorption plays a very important role in such type of adsorption.
2. The reaction rate per unit surface area is proportional to the fraction of total surface covered with adsorbed gas molecule.
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Lindlar’s catalyst is Palladium metal partially poisoned with Lead (Pb) to make it chemoselective to certain compounds.
Palladium is a powerful reducing agent. So it is less selective and reduces an wide number of organic species. Suppose you want to reduce ethyne to ethane, then Palladium is a great hydrogenation agent to do the work. It will first convert your ethyne to ethene and then ethene to ethane almost instantly. But what if you want the ethene??? This is the idea behind the evolution of Lindlar’s catalyst.
Imagine the surface of Palladium metal with large number of active sites where the organic species binds with it in order to get reduced. What Lindlar has done is that he deactivated some of the active sites by Lead (Pb) therefore made the Palladium metal surface partially poisoned. Now the reduction of ethyne to ethane can be stopped at the ethene level because the ethene produced won’t get the active sites on the palladium surface to further bind and convert to ethane.
Preparation of Lindlars catalyst:
It is prepared by precipitating PdCl2 on CaCO3 and then mixed with Pb(OAc)2. On heating the mixture it becomes Pd-CaCo3-PbO which is Lindlar’s catalyst.
This is a review of Lindlar’s catalyst- A hydrogenation agent.
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