The rational preparation of niobia promoted and supported platinum catalysts

In this study a rational preparation is employed for the wet impregnation of platinum complexes onto niobia, and for the selective partitioning of platinum complexes onto either the promoter phase or the support phase in promoted oxide support. This work elucidate the adsorption mechanism of wet impregnation in catalytically significant supported metal oxide systems: the adsorption of PTA and CPA onto niobium pentoxide; and the adsorption of PTA and CPA over Nb promoted Al2O3. A variety of experiments were performed to prove that the mechanism of platinum complexes (PTA or CPA) adsorption onto niobium pentoxide is controlled by electrostatic adsorption when surface of niobia is dominated by either positively or negatively charged hydroxyl groups; while it is controlled by ion exchange/surface grafting when surface of niobia is dominated by neutral hydroxyl groups. Furthermore, it was experimentally determined that a strong electrostatic adsorption (SEA) method of impregnation leads to not only a 100% dispersion of the platinum particles after drying in air and reduction at 275°C in H2 but also a 100% adsorption of the platinum precursors from solution with 200 ppm of Pt; whereas a dry impregnation approach leads to a lower dispersion and larger platinum particles. A most interesting finding is the "SMSI" effect of niobia as a support on platinum particles. A variety of characterizations were performed to prove that this effect occurs when strong interaction between platinum precursor and niobia support occurs in preparation step; after reduction, SMSI is observed as a coating of Nb suboxides over Pt panicles, a change of oxidation state of both Nb and Pt on the surface of catalyst, and a change of reduction temperature of surface Nb suboxides. These phenomena are not observed in samples prepared by dry impregnation. The selective adsorption nature of PTA and CPA over alumina supported niobia was also studied. A comparison of PTA onto SEA support and DI support suggests an advantage of SEA over DI on adsorption selectivity of Pt over the promoter phase.;A variety of characterizations were performed to prove that Nb, as a promoter, is a good candidate to lower the reduction temperature and produce ultra small particles of Pt on the end catalyst.