Among the prepared catalysts, SiO2/CaO–5 + 10 had the highest
tolerance toward H2O and CO2. A small change was observed in the
XRD profile of SiO2/CaO–5 + 10 when it was exposed to air for one
day, profile (j). The intensity of the XRD peaks of SiO2/CaO–5 + 10
was greatly decreased when it was used for diacetone alcohol
decomposition with 5 wt% of H2O. A large part of the CaO should
be converted into hydroxide. However, the intensities of peaks
assigned to Ca(OH)2 were much smaller than expected from the
degree of decrease of the CaO peaks. It is assumed that the crystal
size of Ca(OH)2 was too small to produce diffracted X-rays. The
growth of Ca(OH)2 crystals by hydration of CaO might be strongly
restricted by covering with a silica layer.