The adsorption of carbon dioxide on magnesia (prepared by hydrolysis of magnesium methoxide) has been studied by measurements of the i.r. spectra of carbon dioxide which had been adsorbed a t various temperatures, supplemented by gravimetric measurements of adsorption isotherms a t temperatures ranging from -97.9 to 500 " C . Three types of adsorption were encountered : (I) a rapid physical adsorption ; (11) a slow chemisorption involving the formation of a bidentate carbonate ion; and (111) a still slower chemisorption which resulted in the formation of a carbonate ion similar to that present in bulk magnesium carbonate. (I) and (11) occur a t temperatures up to and including room temperature ; and (11) and (111) a t temperatures from that of the room upwards.
The kinetics of the isothermal decomposition in vacuu of calcite (two series of experiments, A and B) and of magnesite (one series of experiments, C) has been studied with the aid of an automatically recording sorption balance, over the approximate ranges 720" to 780" C for calcite and 540" to 600" C for magnesite. In all cases the course of the decomposition can be represented by the equation (w/wo)" = -kt + a, where W/WO = the fraction still undecomposed at time t ; k, a and n are constants. The value of (1n) varies slightly from one experiment to another, but its average value is 0-74 for series A, 0.72 for series B and 0.58 for series C. The decomposition probably occurs at an interface advancing from the outside of the lump, or of the whole sample, to the interior: this mechanism would require (1n) = 0-67 in the ideal case, and the deviations from this value are perhaps due jointly to self-cooling, to impedance in the escape of carbon dioxide and to a somewhat slow rate of formation of decomposition nuclei. The value of the energy of activation E of the decomposition are, for calcite (cal mole-1) : series A, 35,000 f 4,800 ; series By 41,600 d= 2,900, so that they do not differ significantly from the energy of decomposition (40,000) ; for magnesite E is 35,600 f 3,300, and so appears to be somewhat higher than the energy of decomposition (ca. 27,000). The rate of decomposition of both substances agrees reasonably with the Polanyi-Wigner expression.
If an adsorbent yields a type I1 isotherm (BDDT classification), then the value of the monolayer capacity, and therefore of the specific surface, calculated from the isotherm will be incorrect unless the micropore contribution is allowed for. A method for the evaluation of the micropore contribution of a carbon black to the adsorption isotherm of nitrogen at -195°C is described. The micropores are filled with nonane at -195°C by adsorption from the vapow phase, and the excess pumped off at room temperature. The adsorption isotherm of nitrogen at -195°C is determined on the sample, both before and after thus filling the micropores. The volume of the micropores is then given by the distance between the (parallel) portions of the two isotherms corresponding to the multilayer region.
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