Pushing the limits of electrostatic adsorption: charge enhanced dry impregnation of SBA-15

Charge enhanced dry impregnation (CEDI) is a method to synthesize supported metal nanoparticles which combines the simplicity of incipient wetness impregnation with the small particle size obtained from electrostatic adsorption of metal precursors onto the oxide support. We have explored the utility of CEDI by applying it to a difficult to impregnate support - a largely one dimensional porous SBA-15 silica. Monometallic (N, Pd, Co, Ni, and Cu) catalysts at multiple metal loadings (1-20 wt%) as well as their bimetallic pairs were supported on mesoporous SBA-15 and characterized by high sensitivity powder XRD and in select formulations, with aberration-corrected z-contrast STEM imaging. CEDI applied to SBA-15 without washing is more effective than dry impregnation (DI) of SBA-15. Compared to amorphous silica, CEDI of SBA-15 without washing gives larger particles at lower metal loadings, while at higher metal loadings, the pores of the SBA-15 help render the particle size smaller than amorphous silica. CEDI without washing of SBA-15 with nitrate salts gives somewhat smaller particles than CEDI with chloride salts. CEDI with washing to remove residual counterions of the metal salt typically gives nanoparticles smaller than 2 nm. The smallest particles are always obtained when the counterion is removed by washing, or when a hydroxide salt is employed; ultrasmall particles are obtained without washing in this case.