Local structure of calcium species dispersed on carbon: influence of the metal loading procedure and its evolution during pyrolysis

An X-ray absorption fine structure (XAFS) study has been carried out to analyze the characteristics of calcium species dispersed on carbon after the loading step and during subsequent pyrolysis treatments. The results are in full agreement with those previously obtained from a temperature-programmed desorption (TPD) study. The combination of both techniques (TPD and XAFS) allows a deeper understanding of the calcium species formed after the loading process and the changes ocurring during the pyrolysis step. The effect of the catalyst loading method on the nature of calcium species and on its dispersion and catalytic activity after pyrolysis have been studied. Three calcium loading procedures have been used: ion exchange, ion exchange at fixed pH (pH = 10), and impregnation. The results show that the largest dispersion results using the ion-exchange procedure without controlling the pH. Moreover, this study confirms a previous one in which it was found that only the ion-exchanged calcium is effective as catalyst and that the use of a high pH during the ion-exchange process should be avoided. In fact, a high pH generates CaCO3, which favors the sintering of calcium species during the pyrolysis and, hence, the loss of contact between the catalyst and the carbon.