274095 Thermodynamic Investigation of the Buried Solid-Liquid Interface During Heterogeneous Catalyst Synthesis
Catalyst synthesis via wet deposition is comprised of four unit operations: (i) support and precursor selection, (ii) transition metal complex (TMC) deposition, (iii) drying, and (iv) calcination. Systematic experimental studies have been performed to characterize the influence of drying and calcination on the final physical properties (nanoparticle size and shape) of dispersed nanoparticle catalysts; however, very little work focuses on the adsorption of the TMC on the chosen support under well-described solution conditions. There are three adsorption mechanisms that occur at the solid-liquid interface: outer-sphere, inner-sphere, and strong electrostatic adsorption (SEA). SEA is the dominant adsorption mechanism when the bulk pH solution leads to the formation of a net surface charge due to protonation/deprotonation of surface hydroxyls in the presence of the appropriate cationic/anionic TMC. The further the pH is from the point of zero charge (PZC), the stronger the adsorption affinity. Therefore, the weight loading of Pt can be controlled simply by varying pH and the contact time. Prior studies mainly utilize a qualitative analysis between the initial bulk solution pH and the adsorption uptake.
This study will provide an experimental approach to determining the effect of bulk solution pH on the uptake of Pt precursors on amphoteric metal oxide supports. We initially studied the effect of contact time and precursor concentration at various pH values to determine conditions where SEA is most favorable. We then provide a direct experimental measurement of the heat of adsorption using isothermal titration calorimetry (ITC) of platinum complexes at various bulk solution pH values. ITC was used to determine the ΔHads, equilibrium binding constant, K, and number of binding sites, n in a single experiment. The results show the difference in binding for adsorption conditions within the electrostatic adsorption regime, therefore making it possible to develop a correlation between the initial solution pH and adsorption energy and final catalyst morphology. In order to determine the correlation between solution pH and nanoparticle size, the catalysts were then calcined and reduced at 400°C and 500°C and studied using TEM and high energy x-ray diffraction utilizing pair distribution function (PDF) analysis. Total scattering measurements were performed at the Advanced Photon Source (11-ID-B beamline) at Argonne National Laboratory, which allows for the size of the nanoparticles to be determined in-situduring reduction.