TITANIA-SUPPORTED BIMETALLIC CATALYST SYNTHESIS BY PHOTOCATALYTIC CODEPOSITION AT AMBIENT-TEMPERATURE - PREPARATION AND CHARACTERIZATION OF PT-RH, AG-RH, AND PT-PD COUPLES

We have examined the possibilities of preparing some bimetallic catalysts supported on TiO2 (Degussa P-25) by UV illumination (λ > 300 run, radiant flux, 60 MW cm-2), at room temperature for 24 h, of aqueous suspensions containing 500 mg of this oxide, and cations of the desired metals in amounts corresponding to metal loadings between 0.5 and 2.5 wt% if complete reduction were achieved. In the case of the H2PtC16RhCl3 couple, Rh deposition almost did not take place. For the mixture AgNO3RhC13 at pH 2.6, Rh deposition was substantial but still incomplete. In contrast, Rh deposition was complete in aqueous ammonia (pH 11). TEM and STEM-EDX analysis of the solid recovered in this latter case showed that Rh formed 1- to 3-nn particles and was also found in 8- to 30-nm particles containing ca. 80 wt% Ag (with respect to Rh) presumably as a result of its deposition on more rapidly crystallized Ag. Solids prepared from H2PtC16 and Pd(NO3)2 were studied in more detail because both metals were completely deposited, at least at levels ≤1 wt%. They were heterogeneous, since Pd and Pt formed ca. 1- or 3-nm crystallites, respectively; however, most of the crystallites were found to contain both metals with a high and varied proportion of Pd. An XPS Ar+-sputtering depth profile of the I wt% Pt-1 wt% Pd/TiO2 sample showed a similar behavior for Pt and Pd, which might indicate the formation of a solid solution in some particles. As this formation is attributed to similar reduction rates for both metallic salts, it is inferred that alloys are obtained only in cases where the salts and conditions can be chosen to fulfill this requirement. This limits the capabilities of the photoassisted codeposition method for preparing effective bimetallic supported catalysts. © 1991.