Q22.46P - Chemistry: Molecular Nature Of Matter And Change

Question

(a) What are the components of the reaction mixture following the water-gas shift reaction? 

(b) Explain how zeolites (Gallery,) are used to purify the ${\mathbf{\text{H}}}_{\mathbf{\text{2}}}$ formed

Step-by-Step Solution

Answer

a) Water and carbon monoxide are heated at ${400}^0\mathrm{C}$ over an iron or cobalt oxide catalyst in the water-gas shift reaction mixture. The mixture is then passed into liquid water, where ${\text{CO}}_{\text{2}}$ is dissolved, or through $\text{CaO}$ , where ${\text{CaCO}}_{\text{3}}$ is formed.

b) Zeolites with varying size cavities are used to further purify the ${\text{98\% H}}_{\text{2}}$ gas. Small molecules of ${\text{H}}_{\text{2}}$ flow through, but larger molecules are held in the zeolite cavities. 

Step 1: Concept Introduction

Carbon monoxide and water vapour react to generate carbon dioxide and hydrogen in the water-gas shift reaction:

${\mathbf{\text{CO + H}}}_{\mathbf{\text{2}}}\mathbf{\text{O}}\mathbf{\rightleftharpoons }{\mathbf{\text{CO}}}_{\mathbf{\text{2}}}\mathbf{\text{ + H}}$

Zeolites are crystalline solids comprised of silicon, aluminium, and oxygen that have cavities and channels inside where cations, water, and/or tiny molecules can live. They're also known as molecular sieves.

Step 2: Water Gas Shift Reaction

a) The water-gas shift reaction is a reaction that produces when the product mixture of water and carbon monoxide is heated to ${400}^0\mathrm{C}$ over an iron or cobalt oxide catalyst.

${\text{H}}_{\text{2}}\text{O(g) + CO(g)}\rightleftharpoons {\text{CO}}_{\text{2}}{\text{(g) + H}}_{\text{2}}\text{(g)}$

This reaction is exothermic and therefore can be recycled.

The mixture is passed through liquid water, where ${\text{CO}}_{\text{2}}$ is dissolved, or through $\text{CaO}$, generating ${\text{CaCO}}_{\text{3}}$, to purify the ${\text{H}}_{\text{2}}$ gas.

Step 3: Role of Zeolites in Purifying Hydrogen Gas

b) Zeolites are utilised to purify the ${\text{98\% H}}_{\text{2}}$ gas further. Zeolites are synthetic aluminosilicates made up of small polyhedrons with voids of varying size cavities.

The smaller molecules of ${\text{H}}_{\text{2}}$ flow through, but the larger molecules are held in the zeolite cavities. A high purity $\text{99.9\% }$ hydrogen gas can be obtained in this manner.