Question

Which of the following substances absorb ${H_2}$ gas more strongly?
A. Platinum black
B. Nickel powder
C. Activated charcoal
D. Silica gel

Answer 1

Surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules retain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

Complete step-by-step answer: We should remember that the molecular species which accumulates at the surface is called adsorbate and the substance on the surface of which adsorption occurs is called adsorbent.
We have already discussed that Adsorption is a surface phenomenon. Solids in finely divided state act as good adsorbent due to large surface area, so charcoal, silica gel, nickel powder, platinum black, alumina gel etc. in finely divided state acts as good adsorbent.
The question asks us which of the given adsorbent adsorbs hydrogen gas more strongly.
Hydrogen is most strongly adsorbed by platinum black. Platinum black is a fine powder of platinum with good catalytic properties.
Platinum black undergoes adsorption strongly as the platinized platinum has a true surface area much higher than the geometrical surface area of the electrode and therefore, exhibits action superior to that of shiny platinum.
The process of adsorption of hydrogen over the platinum black is called occlusion. This process is a multi-step process and involves major processes such as diffusion, desorption, occurrence of chemical reaction etc.
But it doesn’t mean that the other three options are bad adsorbent. They too are strong adsorbents but the fact is that hydrogen is most preferably adsorbed by the platinum black.

Note: It should be noted that adsorption is of two major types. Physisorption or physical adsorption and chemisorption or chemical adsorption. Occlusion is a type of physisorption. As physical adsorption is non-specific therefore every gas will get adsorbed at the surface of any solid to some extent.