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

Abstract

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.

The preparation of silica supported, dilute limit PdAu alloys via simultaneous strong electrostatic adsorption

“…The detailed catalyst synthesis procedures were illustrated in our previous investigation. 18–20 Uptake results, plotted in Fig. 1, show volcano-shaped uptake characteristic of electrostatic adsorption.…”

“…We have recently employed the co-SEA synthesis of dilute Pd-in-Cu alloys on silica has been successfully achieved, demonstrated by rigorous fitting and DFT interpretation of the Fourier-transform infrared (FTIR) spectra of adsorbed carbon monoxide. 20 In this work, we extend co-SEA to the synthesis of a series of silica supported DLA nanoparticles of Pd isolated in Au. With a comprehensive battery of characterization, we combine in situ IR spectroscopy and chemisorption to probe the isolation of surface Pd sites in Au ensembles and the fraction of surface Pd.…”

“…We have recently employed the co-SEA synthesis of dilute Pd-in-Cu alloys on silica has been successfully achieved, demonstrated by rigorous fitting and DFT interpretation of the Fourier-transform infrared (FTIR) spectra of adsorbed carbon monoxide. 20…”

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

“…(a) Fitted spectra (final N 2 purging) for monometallic Pd and Cu and bimetallic Pd–Cu systems and (b) three additional peak assignments for isolated palladium atom sites on diluted Pd–Cu single-atom alloy. Reproduced with permission from ref . Copyright 2022 American Chemical Society.…”

“…For example, the number of surface Pd sites in the dilute PdAu bimetallic nanoparticles is determined by quantified pulses of CO by assuming all the Pd sites are equal for adsorption of CO. 186 However, it is inferred by a comparative study that multiple types of adsorption configurations could exist in the dilute single-atom PdCu alloy nanoparticles, which gives different CO/Pd ratios in the adsorption models (see Figure 19). 185 Therefore, the precise measurements of the metal sites in bimetallic nanoparticles relies on the identification of the adsorption configuration (mono-or dicarbonyl configuration) of the probe molecule at the measurement conditions (temperature, partial pressure of the probe molecule etc.) by performing rigorous control experiments on the monometallic samples.…”