A simple FTIR technique for estimating the surface area of silica powders and films

Composite Membranes for High Temperature Direct Methanol Fuel Cells

“…Therefore, the difference in water diffusion coefficient between SiO 2 -and Al 2 O 3 -based membranes is probably not related to morphology but, as discussed above, it is more likely due to the different level of interaction of the particles with hydronium ions. These aspects and the larger water retention on SiO 2 [28] make the composite SiO 2 -Nafion membrane more proton conducting and better performing than the Al 2 O 3 -based membrane, as previously observed.…”

“…The surface characteristics of most of the selected oxides are known from the literature since they find wide application as catalyst supports in heterogeneous catalytic reactions. The silica surface is mainly characterized by the presence of silanol (-Si-OH) and siloxane (-Si-O-Si) groups, the first having higher acidic strength [28]. Similar surface characteristics are present on alumina [29]; the ZrO 2 surface is characterized by surface defects, often saturated by strongly bonded hydrogen forming OH species [30].…”

“…Phosphotungstic acid crystallizes in its solid form with 36 water molecules [27,31,32] at room temperature (H 3 PW 12 O 40 Á36 H 2 O); thus, it possess the strongest water retention characteristics since water is present in the crystallographic structure. Amorphous silica, such as Aerosil or Cabosil, is widely used in catalysis; its surface functionalities have been investigated by infrared spectroscopy [28]. These functional groups are mainly silanol groups which may be isolated, geminal or connected by hydrogen bonding.…”

“…FT-IR investigations [28] have shown that Cabosil is characterized by a large content of isolated silanol groups, both on the surface and in the bulk, compared to other amorphous silica materials. Silica dehydration is known to occur through the following steps [28]: (i) evaporation of water formed in the silica pores by capillary condensation occurs below 140 8C in the presence of a dry nitrogen stream (this effect should be less significant for well dispersed spherical silica particles); (ii) desorption of physically adsorbed water occurs at 140-150 8C leaving a large content of silanol groups on the surface; (iii) the silanol groups desorb in the temperature range 150 to 800 8C depending on their characteristics (e.g., isolated, geminal) and environment, forming siloxane groups and releasing water. At 800 8C, the surface of silica is almost totally covered by siloxane groups showing hydrophobic properties and basic surface characteristics as opposed to the hydrophilic characteristics of silica covered by silanol groups (acidic surface).…”

“…The surface characteristics depend, for both oxides, on the concentration and acid-base strength of the functional groups which are influenced by the chemical environment and may be modified by surface reactions as well as thermal treatments. A large concentration of acidic groups as well as high acidic strength (larger polarization of the OH dipoles) produce a significant amount of physically adsorbed water in both silica and alumina [28]. OH groups bonded to silicon are more acidic than those bonded to the less electronegative Al.…”