The effect of the variation in the pH and the carrier porosity on the surface properties of CuO-SiO 2 catalysts

Abstract

The surface areas of four series of CuO-SiO 2 catalysts prepared at pH values larger than 10 and at pH 4.5 using mesoporous (59) and microporous (03) silica gels as supports were measured by nitrogen adsorption at 77 K. The surface area of the mesoporous silica gel was more strongly affected at pH 4.5, where the formation of micropores was favoured, than at pH values greater than 10, where the original mesoporosity was retained. The reverse behaviour was observed for gel (03). At high pH values the incorporation of about 1% Cu in either gel (59) or gel (03) affected the surface area. Increasing the copper content raised the surface area of gel (59) whereas that of gel (03) was decreased. A maximum in the surface area was observed at 400 °C for catalysts supported on silica (59) containing more than 1% Cu. The reverse behaviour was observed for such catalysts on silica (03). The location and the attachment of the copper, whether on the external surface or in the pore system, were significant parameters controlling the specific area. At low pH values the introduction of 1% Cu caused a decrease in the surface area of gel (59) and an increase in that of gel (03). A further increase in the copper content produced the same effect as that observed at high pH. Sintering was enhanced when microporous silica gel was the carrier. Pore structure analysis showed that catalysts supported on either carrier and containing about 1% Cu in the form of the ammine complex were predominantly microporous, whereas at higher copper concentrations both mesopores and micropores existed. Thermal treatment created mesoporosity in catalysts supported on carrier (03). The catalysts prepared at low pH values were predominantly microporous at temperatures up to 800 °C for carrier (03) and at temperatures below 800 °C for carrier (59).