Characterization of Highly Dispersed Metal Catalysts Via Infrared Spectroscopy and X-Ray Diffraction: From Isolated Atoms to Clusters

Heterogeneous catalysis is a booming industry, driven by its ease of use and scalability. Recent focus on isolated atom metal-supported catalysts has led to various synthesis techniques being studied for industrial-scale production, aiming to enhance selectivity, activity, and uniformity of desired reactions. This project aims to study the effect of various synthesis techniques as they pertain to the size of the supported metal particles via several characterization methods. Various synthesis techniques were assessed, such as Dry Impregnation (DI) and Switched Solvent Synthesis (SwiSS). Several analytical techniques were used to study the effect on particle size, including Temperature Programmed Reduction (TPR), Fourier Transform Infrared Spectroscopy (FTIR), and X-Ray Diffraction (XRD). For catalyst preparation, each batch was made using 50 mg of Aerosil 300 SiO₂ support (300 m²/g) at 2% weight loading, corresponding to a nominal areal loading of 0.377 Pd atoms/nm². Aqueous dry impregnation of the Pd(NH₃)₄(NO₃)₂ precursor was performed for each sample. For the SwiSS samples, the appropriate solvent was added afterward in a 3:1 molar ratio (i.e., switched solvent to stock solution ratio.) The solvents used were acetone and diethylamine, which were chosen due to previous preliminary investigations, their similar boiling points, but different properties such as dielectric constants and dipole moments. Samples were reduced at 170° C in 10% H₂/N₂. Particle sizes were estimated with the Scherrer equation from the XRD data, and a quantitative analysis of the FTIR data was performed. It was found that 170° C is a sufficient reduction temperature, and that the SwiSS method resulted in smaller particles compared to dry impregnation. With the present data and analysis from the FTIR, there was not found to be a meaningful difference between the ratios of terminally bound (Pt-CO) to bridge-bound CO (Pt₂-CO, Pt-₃CO), though there is a noticeable difference in the shape of the peaks, so more information may be revealed with more thorough analysis.