Dilute Limit Alloy Pd-Cu Bimetallic Catalysts Prepared by Simultaneous Strong Electrostatic Adsorption: A Combined Infrared Spectroscopic and Density Functional Theory Investigation

Well-defined single-atom catalytic (SAC) sites with unique geometric and electronic properties are at the forefront of catalyst research. One type of SAC is a so-called dilute limit alloy (DLA), where single-metal atom sites are supported on (or in) the surface of a second metal. This contribution explores the simple and scalable approach of strong electrostatic adsorption (SEA) to synthesize silica-supported DLA of palladium on copper. The catalysts were characterized using X-ray diffraction, temperature-programmed reduction, and transmission electron microscopy images to confirm the presence of tightly distributed metal nanoparticles. The isolated Pd sites on Cu were probed using transmission Fourier-transform infrared (FTIR) spectroscopy during adsorption of CO. Through spectral curve fitting to extract Gaussian peak parameters, coupled with density functional theory (DFT) calculations, the nature and structure of the Pd sites were determined. In addition to typical vibrational features observed for CO on copper, three additional peaks at 2046, 2021, and 1908 cm-1 were observed for the DLA Pd-Cu system. The DFT analysis of the same system showed that these peaks are consistent with CO linearly adsorbed on an embedded palladium atom on a Cu(100) surface, a Pd on a Cu(111) surface, and a bridge CO between Pd and Cu with the Pd embedded on a Cu(211) surface. Experimental and computation trends in CO vibration wavenumbers could be well interpreted using the Dewar-Chatt-Duncanson (DCD) model. The promise of the SEA approach for producing DLAs and the utility of this combined FTIR/DFT approach for their characterization will be discussed in light of these findings.

Automated summary

This contribution explores the synthesis of silica-supported dilute limit alloy (DLA) of palladium on copper using strong electrostatic adsorption (SEA). The catalysts were characterized and the nature and structure of the isolated Pd sites on Cu were determined. The study also discussed the potential of the SEA approach for producing DLAs and the utility of the combined FTIR/DFT approach for their characterization.