The nitrogen adsorption-desorption isotherms were determined
at its boiling point (77 K) using a gas adsorption-desorption
instrument (Nova 1200, Quantachrome). Prior to the measurements,
the samples were degassed under flowing of nitrogen gas
at 300 ◦C for 3 h. The specific surface area of the materials was
calculated using the multi-point Brunauer-Emmett-Teller (BET)
method at P/P0 < 0.3. The total pore volume was evaluated from
the adsorption-desorption profiles. The average pore diameter and
pore size distribution were estimated from the desorption branch
using the Barrett-Joyner-Halenda (BJH) method. The BJH method
is the most widely used method for the calculation of pore size
distribution over the mesopore range