How catalytic chemical reactions are explained in a layman language?

by  Maryam Hussain

What is a catalyst?

A catalyst is a material that increases the rate and selectivity of a chemical reaction. Whenever a catalytic reaction occurs, cyclic regeneration of the catalyst is required to maintain the catalytic activity. On the basis of the physical phases of reactants and the catalysts, there are two types of catalytic reactions and here only one of those will be described. The type of catalytic reaction in an automobile’s catalytic converter is called heterogeneous catalysis.

How do the catalytic reactions occur?

In this process, reactants (CO, O₂, NOx, HC, etc.) adsorb onto the surface of a solid catalyst and are activated by chemical interaction with the catalyst surface, and are then rapidly and selectively transformed to adsorbed products, which desorb from the catalyst surface and go away to the environment through the exhaust pipe.

Once the product is desorbed from the surface of the catalyst, the catalyst returns to its original state until additional reactants adsorb and repeat the catalytic cycle. These catalytic chemical interactions provide a chemical shortcut in which reactants are converted to products more rapidly and at much friendly and milder conditions than if no surface interactions of catalytic reactions occurred.

How to ensure that only CO₂, N₂ and H₂O are produced as the desired product?

Apart from increasing the rate of reactions, catalysts have another important property of influencing the selectivity of chemical reactions. Influencing the selectivity of chemical reactions means that a number of completely different products can be obtained from the same starting materials, the reactants, by using different catalytic systems.

Because of this very important feature, industrial plants obtain the targeted and desired products and get rid of or control the amounts of undesired products or impurities without installing additional processing units.

When are the environment pollutants, CO, produced by an internal combustion engine? How CO is get rid of?

A catalytic converter is the heart of an automobile’s emissions control system. An internal combustion engine in automobiles combusts gasoline (HC: hydrocarbons)) /air mixture to generate heat and this is converted to mechanical work by the same engine.

Whenever the combustion process is less than 100% efficient then some undesirable by-products of combustion are produced which principally include carbon monoxide (CO) and un-combusted hydrocarbons (HC) are produced in addition to carbon dioxide, CO₂, and water, H₂O.

Hydrocarbon fuel    +    O₂    ⇀    CO₂    +   H₂O    +    CO    +     Un-combusted hydrocarbons    +     Heat Energy

The flow rates of auto exhausts are at such a high level that there is insufficient time for the produced carbon monoxide (CO) and un-combusted hydrocarbons (HC) to react further in the gas phase with available O₂ to be converted to carbon dioxide, CO₂, and water, H₂O before CO and HC exit the exhaust pipe.

However, installing an appropriate catalyst bed in the exhaust manifold, to allow the flow of exhaust gases through & over the catalyst bed, allows these catalytic combination reactions to take place within the limited duration of residence time and at the moderate reaction conditions of the catalytic converter.

Thus the catalytic converter provides a rapid alternative chemical path for the destruction of environmental pollutants before these reach the environment.

When are the environmental pollutants, (NOx), produced by an internal combustion engine? How NOx are get rid of?

The next consequence of combustion processes is the generation of nitrogen oxides (NOx) which are nitric oxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O). These nitrogen oxides (NOx) take birth from either the oxidation of nitrogen compounds in the fuel or the oxidation of nitrogen in the combustion air at temperatures in excess of about 1400°C.

These undesirable environmental pollutants are converted catalytically to N₂ in a reducing environment, which is present in the catalytic converter of automotive exhausts, by reduction with carbon monoxide (CO) to carbon dioxide (CO₂).

When & why to replace a catalytic converter?

A car’s catalytic converter, fixed bed reactor, contains a catalyst made of precious transition metals like platinum and rhodium. The temperature of the converter is increased from about 280-300°C to about 480-500°C because of the placement of the converter closer to the exhaust manifold in order to achieve effective catalytic reaction conditions. Greenhouse gases in the exhaust, that are, carbon monoxide, nitric oxide, and unburned fuel are converted to harmless carbon dioxide, nitrogen, and water as they pass through the converter.

If you smell rotten egg smell around your car, while the engine is ON, then the cause of this smell is the catalytic converter.  At this moment, chemical reactions occur in the converter and convert any Sulphur in the fuel into hydrogen Sulphide, and this is what the rotten egg smell is. So, this smell will only be felt when the catalytic converter is actually working well.

But the cars are not designed to be smelly all the time, and a very frequent smell of rotten eggs indicates that there is something wrong with the air-fuel ratio of the engine. So, take it to a mechanic who has emissions testing and a computerized checkup system for your engine. If the disturbed air-fuel ratio of an engine is fixed, the problem of the rotten egg smell should also be rectification.

To reach a logical solution about your cars’ engine, simply connect the dots: frequent rotten egg smell AND air fuel ratio of an engine is OK AND engine emissions are higher than normal range THEN it is time for replacement of the catalytic converter of your car.

When the catalytic converter gets overloaded with gas, because it can’t process the gas fast enough, then a rock of carbon forms in the converter. This carbon buildup restricts exhaust flow and dramatically reduces power. If this continues, then a time comes when the car fails to start because you did not maintain the fuel system according to factory standards.

How to devise an SOP to refuel an automobile?

Prerequisites

1). Have a car.

2). Have a valid credit card or money in your pocket.

3). Car fuel gauge reached a set threshold.

4). Fuel is available at gas station

5). There is no traffic jam on the route.

Precautions:

1). In the periphery of the fuel pouring nozzle, Do not ignite the flame.

2).  In the periphery of the fuel pouring nozzle, Do not use a Cellular phone.

Action steps:

1). Park the car beside the pump

2). Pull hand brake

3). Turn off the engine

4). Set the fuel volume totalizer to zero.

5). Open the Fuel tank door.

6). Rotate and open the lid.

7). Place the fuel pouring nozzle in the opening of the fuel tank.

8). Start the fuel filling.

9). Stop the fuel pump when the required volume appears on the volume totalizer; in auto mode, it stops automatically when the preset volume is filled.

10). Rotate and close the fuel tank lid.

11). Close the Fuel tank door.

12). Close the fuel tank door.

13). Pay the bill

14). Get the bill receipt.

15). Start the car

16). Confirm the filled volume on the fuel gauge.

17). Release the hand-break

18). Have a safe drive again.