Experimental Results with the Vacuum Microbalance

Nitrogen adsorption isotherms at —195° were determined gravi-metrically on chromia gel activated at 126, 200, 275, 350, 425, and 450°. The amorphous chromias all exhibited Type-I isotherms whereas the microcrystalline a-Cr203 prepared by activation in hydrogen at 450° gave a Type-IV isotherm with a hysteresis loop (27). The surface 

A very large fraction of the porosity of amorphous chromias is in inicropores (27, 28). From the slope of the Lippens and de Boer t plot beyond the relative pressure at which micropores are filled, we estimate the external surface area to be only about 2 m /gm. As the activation temperature is increased, the adsorption of nitrogen at low P/Po decreases relatively, particularly after activation in helium at 425°. Thus, the average diameter of the micropores appears to increase somewhat as the activation temperature is increased from 200 to 425°. 

Chemisorption studies usually involved about 230 mg of ( 203. We estimate total uncertainties in weighing, adsorption of poisons, and so on, as amounting to about 0.05 mg. 

The chemisorption of carbon monoxide as a function of the conditions of activation is shown in Fig. 7. We have defined chemisorption as the 

Carbon monoxide isotherms at 25° open circles, activation temperature, 350° solid circles, activation tem erature, 200°. 

---