Date of Award
Open Access Thesis
John R. Monnier
Strong electrostatic adsorption (SEA) is a well-tested procedure for the synthesis of supported monometallic and bimetallic nanoparticles with ultrasmall size (< 2 nm). Where, previous studies have laid the foundation for SEA using a variety of powdered supports, catalysts in this form are not suited for large volume chemical applications where pressure drop is critical to the process economics. Although there is no fundamental difference between the surface functional groups on powdered and extruded supports, the present study examined electrostatic adsorption as it relates to proton diffusion, precursor diffusion, and capillary imbibition.
Three formed alumina spheres and one carbon extrudate were selected for study. The alumina spheres, provided by BASF, were developmental materials with varying pore size distributions between 12 and 24800 angstroms. The carbon, provided by ADM and manufactured by CABOT, was strictly microporous in structure and was included to investigate the differences in mass transport as a function of support material.
Results indicated, a significant limitation in the approach to pH equilibrium comparing formed alumina spheres with the same material crushed and sieved to afindings, the analysis was expanded to include metal adsorption using chloroplatinic acid (CPA) as a Pt source. Where prior work using alumina samples with differing phase and surface areas showed a common max Pt uptake of 1.7 μmol/m2-sup, the crushed alumina spheres in this study had a maximum loading of 0.8 μmol/m2-sup. A similar result was observed for carbon where prior work showed a maximum uptake of 1.8 μmol/m2-sup and the crushed supports in this study had a maximum adsorption of 0.7 μmol/m2. Using unmodified alumina spheres, it was determined that SEA only occurs as a thin outer shell at low concentrations of platinum (CPA), the depth of this shell (~0.3 mm) was independent of average pore diameter.
Temperature programed oxidation (TPO), digestion/ICP, and a series of support wetting experiments were performed to investigate possible contamination and to decouple proton diffusion from metal diffusion and capillary imbibition. TPO indicates that although surface carbon was present it had no measurable effect on the depth of metal adsorption at low concentrations of platinum (CPA). The 36 element ICP analysis indicated significant quantities of Sodium, Nickle, and Zinc, providing a possible explanation for the differences in maximum Pt adsorption. It was concluded that steric hindrance of CPA was the primary factor limiting radial metal penetration, a finding which was supported by the rates of diffusion limited proton exchange. In addition, as the concentration of platinum (CPA) increased, internal metal diffusion increased, while overall metal uptake decreased.
Finally, platinum tetra amine nitrate (PTA) was investigated as an alternative to CPA. Where anionic [PtCl₆]⁻² undergoes SEA in the acidic region, the cationic Pt⁺² species most likely undergoes ion exchange with existing cationic species in the alumina support over a wide range of solution pH. Here metal penetration surpassed the 0.3 mm limit previously mentioned obtaining complete saturation after 7 hours in solution at a low concentration of platinum.
McDonough, C. B.(2021). An Investigation of Strong Electrostatic Adsorption Using Formed Commercial Supports. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/6460