Nanoparticle Synthesis via Electrostatic Adsorption Using Incipient Wetness Impregnation

In this work charge enhanced dry impregnation (CEDI), a hybrid method of supported nanoparticle synthesis which combines the advantages of electrostatic adsorptionsmall particle size and tight size distributionswith the simplicity of incipient wetness impregnation, is demonstrated for four different metals (Pt, Pd, Co, and Ni) at multiple metal loadings over a common silica support. CEDI is achieved by basifying the impregnating solution sufficiently to charge the silica surface at the condition of incipient wetness. The electrostatic interactions induced between cationic ammine metal precursors and the deprotonated, negatively charged support result in smaller nanoparticles with tighter size distribution in comparison to those for incipient wetness impregnation (or dry impregnation, DI) with no pH adjustment. The method works best when the balancing ion of the precursor salt is hydroxide, such as platinum tetraammine hydroxide, (NH₃)₄Pt­(OH)₂. Using the corresponding chloride salts with CEDI results in larger metal particles, but these are still smaller than DI-derived particles. Washing out the chloride results in very small nanoparticles without appreciable metal loss at metal loadings corresponding to one monolayer of precursor or below. Ammine complexes with nitrate as the counterion give small nanoparticles at lower but still relevant metal loadings (1 or 2 wt %) with no washing; in this way the CEDI synthesis procedure is completely parallel to incipient wetness impregnation but gives much better metal dispersion.