Charge-Enhanced Dry Impregnation: A Simple Way to Improve the Preparation of Supported Metal Catalysts

A simple variation in the method of incipient wetness can dramatically improve metal dispersion in supported metal catalysts. This very common method, also known as dry impregnation or pore filling, often suffers from the absence of metal precursor-support interaction during metal deposition, which usually leads to extensive sintering and poor distribution of the metal phase after pretreatment. In past work, our group has demonstrated the synthesis of ultrasmall nanoparticles in tight size distributions by inducing strong electrostatic interactions between the dissolved metal precursor and support. We have normally employed “wet impregnation” involving thin solution-support slurries, but now show that electrostatic interactions can be induced at incipient wetness. This is done by acidifying or basifying the impregnating solution to the (surprisingly) great extent needed to charge up the surface. With this simple variation we have demonstrated vast improvements in Pt dispersion over carbon, silica, and alumina supports, as characterized by X-ray diffraction (XRD), Z-contrast imaging, and chemisorption. We present calculations from our Revised Physical Adsorption Model which can be used to anticipate the initial pH needed to sufficiently charge a particular surface, such that this approach may be extended to many other metals and supports.