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THEPORE STRUCTURE OF CADMIUM OXIDE
The Pore Structure of Cadmium Oxide by R. Sh. Mikhail and A. M. Kamel
Downloaded by GEORGETOWN UNIV on September 2, 2015 | http://pubs.acs.org Publication Date: July 1, 1969 | doi: 10.1021/j100727a018
Department of Chemistry, Faculty of Science, Ain Shams University, Abbassh, Cairo, U.A . R. (Received September 90,1068)
Adsorption and desorption isotherms of nitrogen were obtained on a sample of cadmium' hydroxide and on the oxides obtained by heating at 300" for 0.5 and 12 hr and at 500" for 1 hr. Changes in the specific surface area have indicated appreciable sintering to occur at 300°,mainly through surface adhesion and surface diffusion forces, which is followed at 500" by some activation due to the deoxygenation of the lattice and conversion of the oxide into the formula CdOo.8. de Boer's VI-t plots were used to gain an insight on the ranges of size of pores present, and it proved the absence of micropores. The same plots gave valuable information on the setting of capillary condensation on the adsorption branch. The "corrected modelless" method was used to obtain the pore size distribution curves. In spite of the irregularity of the pore shape of the different samples, use of the desorption isotherms and the cylindrical idealization led to reasonable agreement with observed values. The pore size analysis gave interesting conclusions regarding the changes in the pore structure that accompany the sintering process.
Introduction Cadmium oxide, as a nonstoichiometric oxide, and its behavior as degenerate electronic semiconductor have prompted a considerable amount of work on the electronic properties of that substance. Little is known, however, of the surface properties of CdO. The simultaneous effects of surface "activation" and of changes in surface areas by thermal treatment and "sintering" precluded quantitative study. Deoxygenation by thermal treatment adds to the complexity of the system. I n a recent publication by Low and Kamel,' on the thermal decomposition of cadmium hydroxide, it was shown, in a qualitative manner, the close connection between the thermal treatment and the pore structure of cadmium oxide which reflects itself on the activation and sintering behavior of the product. The information available at that time was incomplete, and additional work was required to investigate on quantitative bases the effect of thermal treatment on the pore structure and pore size distribution of cadmium oxide. Information regarding pore shape will also be discussed. Experimental Section Cadmium hydroxide was precipitated from the nitrate with ammonia in the same way and receiving the same treatment of Low and Kamel.' After thorough washing and drying at 100" in air, the material had a composition of CdO. 1.65H20. Nitrogen adsorption-desorption isotherms were obtained on four specimens, using a volumetric apparatus of conventional design. The first specimen was the original cadmium hydroxide degassed overnight at 110" in the adsorption apparatus. This treatment caused the removal of some water from cadmium hydroxide and the formula of the hydroxide became CdO . 1.03H20. Two specimens were obtained by heating in vacuo at
300" for 0.5 and 12 hr, and one specimen was obtained by heating in vacuo at 500" for 1 hr. The four specimens will be designated Cd(OH)2, CdO I, CdO 11, and CdO 111, respectively, in the forthcoming discussion. CdO I and CdO I1 were analyzed thermogravimetrically, and it was found that the parent material under tlhese conditions has lost its total water content to yield the stoichiometric oxide CdO. On the other hand, on heating at 500" in vacuo for 1 hr a material of composition CdOo.swas obtained. Cd(OH)2 is white in color. Upon dehydration at 300" the color turned to brown, while heating at 500" caused the material to change to darker brown.
Results A . Specific Surface Areas. n'itrogen adsorptiondesorption cycles were obtained for the four samples Cd(OH),, CdO I, CdO 11, and CdO 111. The results are plotted in Figure 1 in curves A-D. From these isotherms, the specific surface areas were calculated from the BET equation2 using a molecular area for a The BET surface areas nitrogen molecule as 16.2 iz. (SBET) are summarized in column 2 of Table I. The experimental error in the adsorption measurements was assessed and found to cause an error in the area determination of h4.0%. Evidently, the specific surface area of the parent cadmium hydroxide suffered from a rapid decline by heating at 300". Heating at 300" causes the decomposition of the hydroxide lattice to yield the oxide. This process of dehydration was expected, according to Gregg, to lead to an increase in specific surface area rather than a decrease. Gregga has shown that in a (1) M.J. D. Low and A. M. Kamel, J . Phgs. Chem., 69, 450 (1965). (2) S.Brunauer, P. H. Emmett, and E. Teller, J . Amer. Chem. Soc., 60, 309 (1938). (3) 8. J. Gregg, J . Chem. Soc., 3940 (1963).
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