(347b) The Production of Highly Dispersed Metals Via “Strong Electrostatic Adsorption”
Metal coordination complexes such as chloroplatinic acid, [PtCl6]-2 and copper tetraammine, [(NH3)4Cu]+2 appear to adsorb onto common catalyst supports such as alumina, silica, and carbon via an electrostatic mechanism [1-3]. The charged complexes are attracted to an oppositely charged surface, which is created by either the protonation or deprotonation of hydroxyl groups that populate the surface.
The practical usefulness of this mechanism for the synthesis of supported catalysts hinges on whether the high dispersion of the monolayer-adsorbed precursor phase is maintained as the metal complex is reduced. To explore this hypothesis, a series of cationic noble and base metal ammines (Pt(II), Pd(II), Cu(II), Co(II), Ni(II), Ru(II), Ru(III), Rh(III)) was adsorbed at the high pH SEA condition over silica, and a series of anionic metal chlorides (Pt(IV), Pd(II), Cu(II)) was prepared at the low pH condition over alumina and compared to dry impregnation. The SEA-prepared materials consistently gave better dispersions of the reduced metals.
This method of preparation represents a simple way to prepare well dispersed metal particles over oxide and carbon with relatively cheap, common metal precursors. The method can also be extended for the ?nano-engineering? of promoted and bimetallic catalysts.
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