Supported metal catalysts are used in many reactions due to their high surface area, stability, and activity. Strong electrostatic adsorption (SEA) is a method of catalyst synthesis that leverages pH differences between a metal solution and the point of zero charge (PZC) of a support to adsorb metal ions. The resulting catalysts exhibit small particle size, high specific surface area, and high dispersion. However, applications of SEA are limited because of oxidation and reduction of metals, precipitate formation due to hydrolysis or double displacement reactions, and support dissolution. Performing the synthesis at a higher or lower pH, changing the acid or base used for pH adjustment, and using anionic or cationic metal precursors can all be leveraged to tune the metal deposition. We are determining the deposition conditions of iron and platinum onto silica (SiO2) because of their ability to catalyze the reverse water-gas shift (RWGS) reaction and Fischer Tropsch synthesis. Deposition of both metals onto silica via SEA were conducted with anionic and cationic metal precursors in both acidic and basic conditions. Ultimately, it was determined that SEA synthesis is infeasible for iron and the majority of transition metals because under acidic conditions, there are insufficient electrostatic interactions between the metal cations and positively charged support, whereas in basic conditions, the SiO2 support dissolves and the transition metals form insoluble hydroxides. Deposition of chloroplatinic acid-based precursors and other transition metals that are present within anions and stable in acidic environments is a prerequisite for this method to succeed. Our group is investigating alternative precursors to expand the library of catalysts synthesized via SEA and enable applications of the method to a wider variety of supports and reactions.
