Question

In the manufacturing of sulphuric acid in presence of platinum catalyst which metal impurity acts as catalytic poison?
a.) As
b.) Cr
c.) Cu
d.) V

Answer 1

Hint : The metals which are reactive in nature can act as negative catalysts and reduce the activity of catalysts used in the reaction or process.
The process of manufacturing of sulphuric acid in huge concentrations for commercial and industrial purposes is known as the contact process. The contact process uses Platinum as a catalyst but Platinum reacts with the arsenic (an impurity) which is present in the feedstock for Sulphur.

Complete step by step solution: Due to the reactivity of platinum with arsenic, the catalytic activity of platinum gets reduced. Here, arsenic acts as a catalytic poison and the process of manufacturing of sulphuric acid gets interrupted. To avoid this inconvenience, nowadays, vanadium oxide is preferred over platinum for use in the contact process.

The process of formation of sulphuric acid in presence of vanadium oxide occurs in three stages-

Sulphur oxide production (IV)
Reaction of oxygen with Sulphur (IV) oxide to form Sulphur (VI) oxide
Sulphur Dioxide then get absorbed in sulphuric acid (97%) and forms sulphuric acid (100%)
The catalyst which is used here enables the reactants to reach equilibrium quickly and fastens the process. Arsenic acts as a catalytic poison if Platinum is present as a catalyst. In the presence of platinum, the efficiency of the process of sulphuric acid manufacturing gets reduced due to which more energy will be consumed in the reaction. So, it is advisable to get rid of the arsenic impurities in the sulphur feedstock.

Note : Besides arsenic, if chromium is present in the sulphur feedstock, it can also act as a catalytic poison. This is because chromium is a highly reactive metal and as a result, acts as a negative catalyst. Negative catalyst reduces the activity or the efficiency of the catalyst.