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 NH, with Ti-silicalite-1 (TS-1) was studied by means of adsorption microcalorimetry IR, XANES and EXAFS techniques. A specific interaction with T1(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) < 130 kJ mol(-1)) and silicalite (40 < q(ads) 90 kJ mol(-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 las 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. (C) 1999 Elsevier Science B.V. All rights reserved.