Strong electrostatic adsorption of Pt onto SiO₂ partially overcoated Al₂O₃—Towards single atom catalysts

It is frequently desired to synthesize supported metal catalysts that consist of very small clusters or single atoms. In this work, we combine strong electrostatic adsorption (SEA) of H₂PtCl₆ and engineered oxide supports to ultimately produce very small Pt clusters, including a large fraction of single Pt atoms. The supports are synthesized by depositing controlled amounts of SiO₂ onto Al₂O₃ (SiO₂@Al₂O₃) that has been previously grafted with bulky organic templates. After the templates are removed, the oxide supports are largely negatively charged, like SiO₂, but have small patches of positively charged Al₂O₃, derived from the regions previously covered by the template. The overall point of zero charge of these materials decreases from pH 6.4 for 1 cycle of SiO₂ deposition to a SiO₂-like <2 for materials with more than 5 cycles of SiO₂ deposition. SEA at pH 4 on templated SiO₂@Al₂O₃ deposits from 1 wt. % to 0.05 wt. % Pt as the amount of SiO₂ increases. Pt loadings drop to near zero in the absence of a template. The resulting Pt nanoparticles are generally <1 nm and have dispersion near 100% by CO chemisorption. Finally, CO DRIFTS shows that the CO nanoparticles become increasingly well defined and have a higher percentage of Pt single atoms as the amount of SiO₂ increases on the SiO₂@Al₂O₃ particles. Overall, this method of synthesizing patches of charge on a carrier particle appears to be a viable route to creating extremely highly dispersed supported metal catalysts.