Heterogeneous Catalysis Features, Practice Problems and FAQ

Catalyst use has greatly expanded recently across a variety of industries, particularly those in the chemical and petroleum sectors. As intended, these industrial catalysts increase yield and accelerate the production process. This is the reason why the petroleum industry is expanding despite the fact that it produces non-renewable fuel and has a finite amount of it.

What exactly are industrial catalysts then?

Industrial catalysts, which are generally heterogeneous in nature can be defined as the catalysts that accelerate reactions and enhance product selectivity in industries like chemical, petroleum, and agrochemical ones.

In this article, we will get to know more about heterogeneous catalysts, their mechanism of action, and their importance.

TABLE OF CONTENTS

Heterogeneous Catalysis
Heterogeneous Catalysis – Examples
Heterogeneous Catalysis – Mechanism
Heterogeneous Catalysis – Important features
Heterogeneous Catalysis – Zeolite
Practice Problems
Frequently Asked Questions – FAQ

Heterogeneous Catalysis

Heterogeneous catalysis, as used in chemistry, is the catalysis in which the phases of the catalyst and the reactants or products are different.

In contrast, in homogeneous catalysis, the catalyst, reactants, and products are all present in the same phase. Phase can differentiate between components that are solid, liquid, or gas, as well as immiscible mixes (like oil and water), or any location where an interface is present.

Typically, solid-phase catalysts and gas-phase reactants are used in heterogeneous catalysis.

At the catalyst surface, in this instance, a cycle of molecule adsorption, reaction, and desorption takes place. The rate (kinetics) of a reaction is influenced by thermodynamics, mass transfer, and heat transmission.

Because it permits more rapid, large-scale production and the synthesis of selective products, heterogeneous catalysis is crucial.

Heterogeneous catalysis is vital to the chemical and energy sectors.

Heterogeneous Catalysis – Examples

Below are a few typical instances of reactions involving heterogeneous catalysis (reactions in which the physical states of the reactants and catalysts differ).

Process	Catalyst
Haber’s Process: In industry, ammonia is manufactured by Haber’s process. $N_{2} (g) + 3 H_{2} (g) \to 2 N H_{3} (g)$	Finely divided iron is used as the catalyst with molybdenum as the promoter. The necessary conditions are 200 bar pressure and 723 – 773 K.
Contact Process: Sulphuric acid is manufactured in the industry by the contact process. $2 S O_{2} (g) + O_{2} (g) \to 2 S O_{3} (g)$ $S O_{3} (g) + H_{2} S O_{4} (a q) \to H_{2} S_{2} O_{7} (a q)$ $H_{2} S_{2} O_{7} (l) + H_{2} O (l) \to 2 H_{2} S O_{4} (a q)$	The temperature needed for this procedure is 673 – 723 K, and the catalyst can be vanadium pentoxide (V2O5) or platinized asbestos.
Ostwald’s Process: Ostwald's method is used in the industry to produce nitric acid. $4 N H_{3} (g) + 5 O_{2} (g) \to 4 N O (g) + 6 H_{2} O (g)$ $2 N O (g) + O_{2} (g) \to 2 N O_{2} (g)$ $4 N O_{2} (g) + 2 H_{2} O (l) + O_{2} (g) \to 4 H N O_{3} (a q)$	The reaction temperature required for this technique is 573 K, and the catalyst used is platinum gauze.
Decomposition of H₂O₂: When H₂O₂ is exposed to impurities or catalysts like metallic surfaces, it decomposes quickly. ${2 H}_{2} O_{2} (l) \to H_{2} O (l) + O_{2} (g)$	In an exothermic reaction, hydrogen peroxide also breaks down into water and oxygen when a metal oxide-like solid MnO₂catalyst is present.

Heterogeneous Catalysis – Mechanism

Here are a few basic points which must be kept in mind to understand heterogeneous catalysis mechanism.

Reactants diffuse from the bulk fluid phase to adsorb to the catalyst surface in heterogeneous catalysis.

Reactant molecules must move across the surface to an active site since the adsorption site is not always an active catalyst site. Reactant molecules will react at the active site to create product molecules by using a more energy-comfortable route through catalytic intermediates.

The product molecules then diffuse away after desorbing from the surface. The catalyst itself is unharmed and available to mediate other reactions. The observed response rate is also affected by transport phenomena like mass and heat transfer.

Few steps are mentioned below to understand the mechanism of heterogeneous catalysis which is known as modern adsorption theory.

Step 1: The reactants diffuse onto the catalyst's surface. The reactants approaching the catalyst bed get diffused to the surface of the catalyst.

Step 2: Adsorption of reactant molecules onto the catalyst's surface.

Step 3: Chemical reaction takes place on the catalyst's surface and results in the creation of an intermediate.

Step 4: Desorption of reaction products from the catalyst surface frees up the surface for other reactions to take place.

Step 5: The reaction product diffused away from the catalyst's surface.

Heterogeneous Catalysis – Important Features

Because these catalysts permit more rapid, large-scale production and the synthesis of selective products, heterogeneous catalysis is crucial.

Since 90% of all chemical reactions are thought to be catalysed, catalysis has a significant economic influence, accounting for 30% to 40% of the world's gross domestic product.

Heterogeneous catalysts aid in the synthesis of 90% of compounds (by volume).

Heterogeneous catalysis is vital to the chemical and energy sectors. As an illustration, the Haber-Bosch process uses metal-based catalysts to create ammonia, a crucial component of fertiliser, of which 144 million tonnes were produced in 2016.

Heterogeneous Catalysis – Zeolite

Natural or artificial microporous aluminosilicates with the formula $M_{x / n} [(A l O_{2})_{x} (S i O_{2})_{y}] . z H_{2} O$ are known as Zeolite.

Where $M = N a^{+}, K^{+}, C a^{2 +}, e t c, .$

n = Valency of metal cation

z = Water of crystallisation

A catalytic reaction that is affected by the size of the molecules in the reactant and product as well as the pore structure of the catalyst is referred to as shape-selective catalysis. Zeolite means "Boiling Stone" in Greek. It received its name from the water that got stuck in the catalyst's pores. For instance, a zeolite catalyst called ZSM-5 (zeolite sieve molecular porosity 5) transforms alcohol into gasoline or hydrocarbons in the petrochemical industry. In the crevices, the alcohol dehydrates and produces hydrocarbons.

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Some essential points about zeolites:

1. These are aluminosilicates with the general formula Mx/n [(AlO2)x (SiO2)y]. zH2O.
2. These are three-dimensional silicate networks in which some silicon atoms have been replaced by aluminium atoms.
3. Zeolites are shape-selective catalysts with honeycomb-like structures.

4. A specific size of the molecule can enter and exit the active sections of zeolites' pores. These have molecular-sized cavities and are permeable.
5. These have molecular-sized cavities and are permeable. An intricate system of tubes and chambers makes up a zeolite's interior structure. Zeolites have a huge surface area as a result.

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Practice Problems

1. The mentioned reaction is an example of which type of catalysis?

a. Homogeneous catalysis
b. Mixed catalysis
c. Heterogeneous catalysis
d. Demoivre catalysis

Answer: C

Solution: In the given reaction, the reactants and products are in the gaseous phase whilst the catalyst is in the solid phase. Since the phases of the reactants, products and catalyst are different, the mentioned reaction is an example of heterogeneous catalysis.

So, option C is the correct answer.

2. Which theory is more appropriate for heterogeneous catalysis?

a. Intermediate
b. Absorption
c. Paratoid
d. Nucleate

Answer: B

Solution: Because there are other molecules present in a heterogeneous complex, the primary component divides into smaller pieces before disintegrating. Absorption theory is the name given to this hypothesis. Reactants diffuse from the bulk fluid phase to adsorb to the catalyst surface in heterogeneous catalysis.

So, option B is the correct answer.

3. Read the below-mentioned steps.

a. Adsorption of the reactant molecules on the catalyst surface.
b. Diffusion of reactants to the catalyst's surface.
c. Diffusion of products from the catalyst's surface.
d. Desorption of reaction products from the catalyst surface, making the surface available for further reaction.

1. Chemical reaction on the catalyst's surface that results in the formation of an intermediate.

Arrange the following steps in the correct order.

a. ii-i-v-iv-iii
b. i-ii-iii-iv-v
c. ii-iii-i-iv-v
d. v-iii-ii-i-iv

Answer: A

Solution: The Modern Adsopion theory has five important steps in its mechanism. The correct order of the given steps is mentioned below.

1. Diffusion of reactant to the catalyst's surface.
2. Adsorption of reactant molecules on the catalyst surface.
3. Chemical reaction on the catalyst's surface results in the formation of an intermediate.
4. Desorption of reaction products from the catalyst surface, making the surface available for further reaction.
5. Diffusion of product from the catalyst's surface

So, option A is the correct answer.

4. __________ is the catalyst used in the contact process for the production of sulphuric acid.

a. P₂O₅
b. V₂O₅
c. Cl₂O₅
d. V₂O₃

Answer: B

Solution: Sulphuric acid is manufactured in the industry by the contact process. The temperature needed for this procedure is 673–723 K, and the catalyst can be vanadium pentoxide (V2O5) or platinized asbestos.

$2 S O_{2} (g) + O_{2} (g) \to 2 S O_{3} (g)$

$S O_{3} (g) + H_{2} S O_{4} (a q) \to H_{2} S_{2} O_{7} (a q)$

$H_{2} S_{2} O_{7} (l) + H_{2} O (l) \to 2 H_{2} S O_{4} (a q)$

So, option B is the correct answer.

Frequently Asked Questions – FAQ

1. Surface catalysis is one of the names given to heterogeneous catalysis, why?
Answer: Heterogeneous catalysis, in which the catalyst is in a different phase, is a key component of many significant industrial processes. The rate-limiting step often takes place at the solid surface since the catalyst is typically solid and the reactants are gases. As a result, surface catalysis is another name for heterogeneous catalysis.

2. What are the conditions required for a catalyst to act as an efficient heterogeneous catalyst?
Answer: Strong enough to allow the reactant molecules to react, but not so strongly that the product cannot desorb, is required for something to function as a suitable heterogeneous catalyst.

3. What distinguishes homogenous catalysis from heterogeneous catalysis?
Answer: There are two basic categories of catalysts: heterogeneous and homogeneous. The catalyst and the reactants are in various phases during heterogeneous catalysis. The catalyst and reactants are in the same phase during a homogeneous catalysis.

4. Can we consider enzymes as heterogeneous catalyst or not?
Answer: Since enzymes must be in the same state as the reactants in order to mix well and hasten the breakdown of the biomolecule, they are homogeneous catalysts.