The XRD profiles of SiO2/CaO–n and SiO2/CaO–5 + 10 calcined at
773 K are shown in Fig. 2. The XRD profiles of samples heat-treated
and samples allowed to stand in air for one day after heat treatment
were compared. These are denoted fresh and exposed, respectively.
The XRD profile of SiO2/CaO–5 + 10 used for the reaction of diacetone
alcohol with 5 wt% water is also shown.
The set of strong diffraction peaks observed in heat-treated fresh
samples was assigned to CaO. Small peaks from CaCO3 were also
observed. Diffraction peaks attributable to a compound containing
both Ca and Si, such as CaSiO3, were not observed. It is suspected
that Si exists on the surface of the CaO as a SiO2 monolayer.