functionalized surfaces on porous silica by atomic layer ... - Aaltodoc

Figure 7. Surface species on and inside a silica particle after Zhuravlev 102 .
As the heat treatment is continued, the silanol groups begin to desorb and dehydroxylation
occurs. First, the vicinal hydroxyls react and form isolated silanols and siloxane bridges on
the surface. Most of these types of silanols react within a quite narrow temperature region of
200-400 °C. 1,104 However, the dehydroxylation reaction of vicinal silanols may begin already
at 110 °C and continue up to 600°C. 1
The decrease in the number of strongly and weakly hydrogen-bonded hydroxyl groups on the
silica surface during heat treatment was clearly observed in the region 3700-3500 cm -1 in the
DRIFT spectra of heat-treated silica samples (Figure 8). The intensity of bands for strongly
hydrogen-bonded and weakly H-bonded (internal) OH groups distinctly decreases when the
pretreatment temperature is raised from 200 to 600 °C. 1,38,94,108 Isolated silanols are seen as a
sharp and strong peak at 3748-3740 cm -1 in the DRIFT spectra of all silica samples regardless
of pretreatment temperature. In IR spectroscopy, the isolated and geminal silanols are called
free silanols and the hydrogen-bonded silanols bridged or perturbated silanols. 1 When the
silica is heated up to higher temperatures, isolated silanols slowly start to react and the water
molecules desorb from the surface. Vicinal and isolated silanols partly react in the same
temperature region. At temperatures higher than 600 °C the silica surface is mainly covered
with isolated silanols. Isolated silanols react with one other to form stable siloxane bridges:
(≡Si-OH) + (≡Si-OH) → (≡Si-O-Si≡) + H2O ↑. (1)
The nature of the heat-treatment (isothermal or dynamic) and the ambient pressure strongly
affect the dehydroxylation reaction. Also the size and number of pores on the silica affect the
desorption of water from the surface.
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