Nitrogen adsorption isotherms at −196 ◦C were determined in
a Quantachrom NOVA 4200e apparatus. In a typical experiment,
around 100 mg of sample was used and degasification was carried
out for 3 h at 160 ◦C. The specific surface area (SBET) was
calculated by the Brunauer–Emmett–Teller (BET) equation [35],
the total pore volume (Vp) was determined at P/P0 = 0.98, the
mesopore size distribution and the average mesopore width (L)
by the Barrett–Joyner–Halenda (BJH) method [36], and micropore
volume (Vmicro) and the external area (Sexternal) were determined
by the t-method using an appropriate standard isotherm