SUPPRESSED HYDROGEN CHEMISORPTION OF ZEOLITE ENCAGED METAL-CLUSTERS -DISCRIMINATION BETWEEN THEORETICAL-MODELS ON THE BASIS OF RU NAY/

The effects of thermal treatment and zeolite proton concentration on t he chemical state and metal particle size of zeolite Y supported ruthe nium (3.0 wt%) have been investigated using H-2-TPR, H-2-TPD, TPMS, FT IR, TEM, and EXAFS. Heating in Ar of the precursor after ion exchange, [Ru(NH3)(6)](3+)/NaY, up to 400 degrees C leads to nearly 100% autore duction of the ruthenium, as evidenced by H-2-TPR and TPMS. Heating in O-2 results in the formation of volatile ruthenium oxides. After auto reduction, the Ru clusters are extremely small, their coordination num bers, derived from EXAFS, are 0.6 for Ru/HY and 0.8 for Ru/NaY. Subseq uent treatment at 500 degrees C in flowing H-2 induces Ru agglomeratio n to particles which are about the size of the zeolite Y supercages, a s indicated by TEM and EXAFS. The Ru-Ru distances are contracted compa red to bulk Ru metal. Washing of autoreduced Ru/NaY with NaOH, thus re moving the protons formed during autoreduction, results in Ru agglomer ation to large particles (60-100 Angstrom). Comparison of the hydrogen adsorption of Ru clusters with similar sizes of 10-15 Angstrom reveal s a marked interaction of the Ru clusters with zeolite protons. Increa sing the H+/Ru ratio from 3 for Ru/NaY to 10 for Ru/HY, results ina su ppression of hydrogen chemisorption per Ru atom by 75%. The conclusion that formation of metal-proton adducts affects the electronic structu re of the Ru clusters, thus being one of the main causes of the loweri ng of the heat of hydrogen chemisorption, is supported by FTIR data of adsorbed CO. The most pronounced C-O vibration band in Ru/HY is locat ed at 2099 cm(-1); this band is absent in Ru/NaY. Significant blue-shi fting of the IR bands is in conformity with electron-deficiency of the Ru clusters in Ru/HY. The results confirm that adsorptive properties of zeolite encaged metal clusters can be ''tuned'' by other ions shari ng the same cavities.