Related surface and heat of immersion characterization of some alumina samples

Abstract

MnCl2- and CuCl2-loaded aluminas were thermally treatedin vacuo in the temperature range 20–500 °C. The water loss, specific surface area from nitrogen adsorption measurements and the integral heat of immersion in water were determined for all the samples. The high-pressure hysteresis loop displayed in the adsorption isotherms, together with the limited increase in adsorption values near saturation indicate the dominance of mesopores in the tested samples, as also detected by theVa-t method of isotherm analysis. Changes in the slope ofVa-t plots suggests that the mesopores rather belong to narrow-ranged mesopore sizes, which are thus considered responsible for adsorption in thet-ranges of 0.5–0.7 and above 0.7 nm. The constancy ofVa(mlg−1) values abovet=1.2 nm points out the absence of effective capillary condensation in the high relative pressure region. Variations of BET-surface area and the reciprocal of average pore radius, ¯r−1 (nm−1), as functions of the heat-treatment temperature indicate that MnCl2-loaded samples display a higher thermal stability than CuCl2-loaded samples; the latter exhibit some aggregation cementation behaviour. At temperatures below 300 °C, the heat of immersion data are not as predicted from changes in water loss and surface-area parameters. Thus, MnCl2-loaded samples display lower heat of immersion values than the respective CuCl2-loaded samples, despite the higher BET areas of the former samples; an effect which may be tentatively interpreted in terms of the capability of the smaller sized Mn2+ ions to preserve more water molecules via such processes as complex formation, ion hydration and crystallization, which contribute to lowering the integral heat measured. This apparent discrepancy in the behaviour of integral heat of immersion (Jg−1) data with water loss (gg−1) at temperatures below 300°C is clarified by considering the change in the heat of immersion per unit area,hi (J m−2) with the temperature of treatment.