Reasons of low formaldehyde adsorption capacity on activated carbon: Multi-scale simulation of dynamic interaction between pore size and functional groups

Exploring the dynamic mechanism of pore structure and surface functional groups on formaldehyde adsorption is the key to understanding the root cause of the low adsorption capacity of activated carbon on formaldehyde. This paper thoroughly investigated the pore volume change and subsequent adsorption capacity of micropores with different sizes under different adsorption processes by combining multi-scale simulation methods. The interaction between formaldehyde molecules in the pores was dissected. The influence law and mechanism of carboxyl groups on formaldehyde adsorption were explored. The results indicate that the narrow effective adsorption pore size range in activated carbon is one of the fundamental reasons for low formaldehyde adsorption capacity. Micropores with a pore size of "4-7 Å"that can hold only one formaldehyde play an leading role in the adsorption of formaldehyde. Although larger micropores can accommodate multiple formaldehyde molecules simultaneously, there is a strong interaction between the adsorbed formaldehyde molecules, leading to deformation and even desorption of the adsorbed formaldehyde molecules. The rapid decline in the adsorption capacity of functional groups is another reason for the low adsorption capacity of activated carbon for formaldehyde. Although the carboxyl with the best adsorption effect will generate hydrogen bonds with formaldehyde in the early stage of adsorption, it can only be adsorbed by weaker van der Waals force in the later stage. The carboxyl group will generate double-layer formaldehyde adsorption, and the adsorbed formaldehyde tends to be arranged perpendicular to each other.