IN-SITU IR AND TEMPERATURE-PROGRAMMED DESORPTION MASS-SPECTROMETRY STUDY OF NO ABSORPTION AND DECOMPOSITION BY SILICA-SUPPORTED 12-TUNGSTOPHOSPHORIC ACID

Silica supported 12-tungstophosphoric acid (H3PW12O40 Or HPW), as well as unsupported HPW, were examined for NO absorption and temperature p rogrammed desorption by in situ diffuse reflectance infrared spectrosc opy and TPD-MS. The SiO2 supported samples contained HPW at 2, 4, and 8 wt% W loading levels. X-ray powder diffraction for determination of HPW lattice parameter and crystal size, solid state proton NMR, and ex situ IR were also employed to characterize these catalysts. The resul ts indicate a strong interaction between HPW and the surface hydroxyl groups of the silica support, leading to the formation of an entirely different secondary structure than observed in bulk HPW. In this secon dary structure, water is proposed to be present at H3O+ rather than H5 O2+ as in unsupported HPW. This structure is more compact than that of bulk HPW leading to a shrinkage of the lattice parameter by approxima tely 0.5 Angstrom but still accommodating a substantial fraction of th e original water. Proton NMR supports the proposal that H3O+ is presen t and indicates that silica supported HPW is a much weaker Bronsted ac id than unsupported HPW. NO is absorbed via the same mechanism by both unsupported and supported HPW to produce NOH+ species, although nu(N- O) bands in the IR are shifted to slightly different wave number in th e supported samples. The rate of NO uptake is much lower for the suppo rted catalysts, which may be explained by the much lower acid strength of these materials. Upon temperature programming to 723 K (50 K/min) more than 50% of the absorbed NO decomposes yielding nitrogen and nitr ous oxide as the only reaction products. (C) 1998 Elsevier Science B.V . All rights reserved.