The difference between the surface reactivity of amorphous silica in the gas and liquid phase due to material porosity

The confinement effect on surface reactivity of amorphous silica at the solid–gas and solid–liquid interface has been demonstrated based on the comparison between nonporous Spherosil XO75LS and four ordered porous silicas of the SBA-15 type, prepared with the use of block copolymer templates of different sizes at two temperatures of the hydrolysis–polycondensation stage in order to adjust the pore structure of the resulting materials. The surface density of acid sites at the solid–gas interface was determined by means of two-cycle ammonia adsorption at 373 K. The surface density of sites reactive towards H+/OH− ions in a 0.1 M aqueous solution of NaNO3 was inferred from the potentiometric titration of solid suspensions. The presence of micropores in the material, smaller pore ordering, and greater values of the BET energetic constant, indicating the increased surface heterogeneity, appeared advantageous to surface acidity in the gas phase. The surface reactivity of porous materials was significantly enhanced when passing from the solid–gas to solid–water interface, contrary to the behavior of Spherosil XO75LS.