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
It was confirmed from the change in the XRD profiles that CaO
was converted to CaCO3 and Ca(OH)2 after exposure to air for oneday. The set of diffraction peaks of CaO completely disappeared
in SiO2/CaO–5, as shown in Fig. 2(b). Catalysts of CaO covered
with 10 mol% silica or higher amounts showed a tolerance to H2O
and CO2 in air. The transformation of oxide into carbonate and
hydroxide was disturbed by the large amount of silica used for
the covering. In SiO2/CaO–20, the XRD profile of the exposed sample,
profile (h), was very similar to that of the fresh sample, profile
(g). The effect of transformation inhibition was increased with an
increase in the amount of silica.