A calorimetric, IR, XANES and EXAFS study of the adsorption of NH₃ on Ti-silicalite as a function of the sample pre-treatment

The interaction of NH₃ with Ti-silicalite-1 (TS-1) was studied by means of adsorption microcalorimetry IR, XANES and EXAFS techniques. A specific interaction with Ti(IV) sites was revealed and quantitatively and energetically described, in comparison with silicalite taken as a reference material. The adsorption was found to be largely reversible but a minor amount of ammonia was irreversibly held at the surface of TS-1. The energetic spectrum obtained by the heat of adsorption versus coverage plot is quite wide in both TS-1 (40<q_ads<130kJmol^-1) and silicalite (40<q_ads<90kJmol^-1), indicating not only that different processes occur at the surface but that a few highly energetic sites are present on TS-1. The standard pre-treatment of the samples in ammonium acetate aqueous solution lowers the whole adsorption capacity of TS-1 surface (as well of silicalite at about the same degree), but not the number of NH₃ molecules adsorbed per Ti atom, which is close to two for both untreated and treated TS-1 samples. The most energetic sites are preferentially suppressed by the ammonium acetate treatment on both TS-1 and silicalite, as shown by the heat of adsorption values much lower for the treated samples than for the untreated ones. Parallel IR and XANES-EXAFS results were found to support these findings.