The CO interaction with Au+/dealuminated H-mordenite as well as with the nondealuminated samples,
at room temperature, after evacuation at different temperatures (343, 393, 453, and 573 K) has been
studied by in situ Fourier transform infrared spectroscopy. The Au+-CO band at 2192 cm-1 of the
dealuminated sample showed a remarkable thermal stability on raising the temperature (till 473 K) for
the degassed sample at 393 K, compared with the nondealuminated one. It has been revealed from the
X-ray diffraction and X-ray photoelectron spectroscopy (XPS) analyses that the nonframework Al species
were formed and embedded inside the zeolite lattice. The latter species were found to be responsible for
the stability of CO coordinated Au+. The XPS data also showed the easier reduction of Aun+ species in the
nondealuminated sample compared with the dealuminated one, as revealed from the peak area ratio Cl
2p/Au 4f. Adsorption of CO onto Au+/dealuminated H-mordenite, after evacuation at 343 K, produced a
broad band at 1949 cm-1 attributed to bridged bonded CO species. This was confirmed by the relative
increase in intensity of this band (Au-CO-Au) compared with the linearly bonded one at 2192 cm-1 during
thermal evacuation till 473 K. The temperature-programmed reduction profile of Aun+/dealuminated
H-mordenite samples revealed, in concordance with the IR and XPS data, the better stability of the Au+
state in dealuminated samples as compared with nondealuminated ones. |
