Effect of liquid phase reducing agents on the dispersion of supported Pt catalysts

Silica-supported Pt catalysts were exposed to room temperature, aqueous solutions containing different initial concentrations of formaldehyde, dimethylamine borane, or hydrazine. Platinum sintering was observed upon exposure to each reducing agent; sintering increased with increasing reducing agent concentration until an apparent, limiting dispersion was achieved. Chemisorption results indicate that exposure of 2.2 wt% Pt/SiO2 to a pH 9 solution containing ∼5.6 mmol of hydrazine, formaldehyde, or dimethylamine borane per liter decreased Pt dispersion by ∼30–60%. These results were qualitatively confirmed with STEM and FTIR spectroscopy of adsorbed CO. Sintering of Pt/SiO2 catalysts was observed in solutions at both controlled (pH 9) and uncontrolled pH for three different Pt weight loadings, and upon exposure to both gas phase and liquid phase reducing agents. Significantly less Pt sintering was observed when Pt/Al2O3 and Pt/C catalysts were exposed to similar formaldehyde solutions, strongly suggesting a support dependency. Based on the non-uniform shape of the Pt particles, the prominent sintering mechanism appears to be particle migration and coalescence aided by inherently low Pt–SiO2 metal-support interactions which were further weakened by the reducing environment and, potentially, by the presence of water.

Silica-supported Pt catalysts were exposed to room temperature, aqueous solutions containing different initial concentrations of formaldehyde, dimethylamine borane, or hydrazine. Platinum sintering was observed upon exposure to each reducing agent. Both Pt/C and Pt/Al2O3 catalysts exhibited less sintering upon formaldehyde exposure than that observed for Pt/SiO2, suggesting a support dependency.Figure optionsDownload high-quality image (50 K)Download as PowerPoint slide