(672b) Quantitative Understanding of the Aqueous Solid-Liquid Interface During Catalyst Synthesis

The synthesis of heterogeneous catalysts has primarily been considered more of an art than a science.  Systematic experimental studies have been performed to characterize the influence of drying, calcination and reduction on the nanoparticle size and distribution; however, very little quantitative work focuses on the initial adsorption of the transition metal complex (TMC) on the metal oxide surfaces [1].  There has been very little work connecting the influence of the initial adsorption conditions on the final size and distribution of particles [2].  Seminal work by Brunelle introduced ion-exchange as a highly controlled adsorption method because the bulk pH solution conditions can be used to manipulate the surface charge on an oxide surface through protonation/deprotonation of surface hydroxyl groups [3].  More recently, Regalbuto and co-workers have introduced the concept of strong electrostatic adsorption, which can be described by a rather simple interfacial potential model [4,5].  Within this model, electrostatics is the dominant driving force for association between TMC and a charged support surface.  At this stage, the chemical identity of the TMC ligands has no influence and only the size of the ligand influences the maximum adsorption uptake.  In this work, we introduce isothermal titration calorimetry as a quantitative technique to understand catalyst synthesis at solvated neutral and charged solid-liquid interfaces.  Initial experiments demonstrate that the electrostatic adsorption of chloroplatinic acid (CPA) on γ-Al₂O₃ at pH 2.11 is ~70 kJ mol^-1 exothermic with an equilibrium constant of 10⁵ M^-1.  Additional data will be presented for the adsorption of other cationic and anionic Pt TMCs on alumina and silica in order to demonstrate trends in adsorption enthalpy and maximum adsorption capacity.  This talk will focus on a study of the influence of adsorption conditions on the final particle size distribution for Pt-based catalysts synthesized from the adsorption of CPA from aqueous solutions onto g-alumina.  Control of the surface charge of γ-Al₂O₃ relative to its PZC is the primary driving force for adsorption, which influences not only the final particle size and distribution, but also the kinetics of TMC dissocation and the onset of particle formation.  Synchrotron-based high-energy wide-angle scattering has been employed to follow the in-situ growth of Pt nanoparticles under reducing conditions.  Temperature and temporal dependent pair distribution function analysis of catalysts synthesized under different solvated interface conditions demonstrates the structure of treated catalysts is dependent upon the initial solvated interface conditions during the initial adsorption step. 

1. Liu, X., J.G. Khinast, and B.J. Glasser, Ind. Eng. Chem. Res., 49, 2649 (2010).
2. Lambert, J.F. and M. Che, J. Mol. Catal. A: Chem., 162, 5 (2000).
3. Brunelle, J.P., Pure Appl. Chem., 50, 1211 (1978).
4. Hao, X., W.A. Spieker, and J.R. Regalbuto, J. Colloid Interface Sci., 267, 259 (2003)
5. Park, J. and J.R. Regalbuto, J. Colloid Interface Sci., 175,