Surface acidity of carbons characterized by their continuous p K distribution and Boehm titration

Abstract

The surface acidity of several activated carbons was evaluated using the Boehm and potentiometric titration methods. Experimental conditions (particle size, rate of titration) were varied and several procedures to collect raw data for proton binding isotherms (slow continuous addition of titrant, incremental addition conditioned by a pH stability criterion and batch experiments) were compared. Proton binding isotherms were analyzed at two levels. First, continuous p K distributions were derived and the inventory of surface functionalities was characterized in terms of amount and p K values. Then these data afforded calculations of the base neutralizing capacity of the carbons studied at any pH level. The results were compared with those from Boehm titration of the same samples. Best results were found for titration of finely powdered carbons. Continuous titration denatures the binding isotherms and damps significant features in the p K distribution. On the other hand, titration by incremental addition with a pH stability criterion provides reliable results which can be compared to those retrieved from Boehm titration provided the proper common metric is used. The agreement was quantitative when the equilibrium pH of the supernatant of the Boehm bases was taken as the endpoint for counting neutralized sites. Advantages of analyzing potentiometric titration data in terms of its acidity distribution function with respect to providing a comprehensive characterization of the inventory of surface functionalities as well as its predictive power to assess a carbon's base neutralizing capacity under different experimental conditions are discussed.