Adsorbents for methane storage made by phosphoric acid activation of peach pits Original Research Article

Powdered peach pit was impregnated with various concentrations of phosphoric acid and then heated to temperatures between 350 and 900 °C. The resulting chars were characterized by nitrogen isotherms at 77 K and mercury porosimetry. Methane isotherms were measured gravimetrically at 298 K to 3.5 MPa. Although many of the chars had relatively high 77 K nitrogen BET surface areas and micropore volumes, they consistently adsorbed less methane at 298 K than expected. A linear relationship between surface area and methane uptake for many different carbons had previously been observed [1]. With increasing heat treatment temperature, these chars followed more closely the predicted relationship. From reflectance IR absorption and temperature-programmed desorption, it is apparent that these chars cannot be considered as only carbon because of their high oxygen and hydrogen content. Only at temperatures above 700 °C do they begin to show the methane uptake expected from their surface area and micropore volume. The adsorption potential of the char pore towards methane at 298 K appears to be lower than that of a truly carbonaceous adsorbent. Alternatively, the adsorption potential to nitrogen at 77 K may be greater for these chars than the adsorption potential of a microporous carbon. The chars produced in this study by phosphoric acid treatment of peach pit do not adsorb sufficient methane to be considered as suitable adsorbents for natural-gas storage applications.