Effect of undercoordinated Ag(111) defect sites on the adsorption of ethanol

Abstract

In recent years, the use of silver-based materials for selective and highly active ethanol reactivity in single atom catalysis and the ethanol oxidation reaction in direct fuel cells highlights the importance of silver (Ag) in an ethanol economy. Understanding the interaction of ethanol with Ag(111) and the natural defects found on extended Ag(111) is critical to the overall understanding of more complex catalytic processes including ethanol activation over Ag-based catalysts. The research herein aims to characterize the interaction of ethanol molecules on undercoordinated defect sites of Ag(111) to mimic active sites found on Ag nanoparticle catalysts. The interaction between ethanol and Ag(111) was studied using temperature programed desorption (TPD), x-ray photoelectron spectroscopy, and density functional theory (DFT). Molecular ethanol adsorption and desorption from Ag(111) and the distinction between undercoordinated Ag(111) adsorption sites were determined using TPD in correlation with DFT. Complete analysis of TPD data for ethanol adsorbed to terrace sites was used to calculate a kinetic prefactor (3.4 × 1015) and desorption energy (0.54 eV). A better understanding of defect-dependent behavior for ethanol on silver can lead to a greater insight into high surface area nanoparticle catalysts used in industries, catalytic converters, and photo-, electro-, and heterogeneous catalysis. The results suggest that ethanol preferentially adsorbs to undercoordinated sites on Ag(111), resulting in higher binding energies for these molecules (Redhead first order approximation for desorption energies is terrace, 0.54 eV; step edge, 0.57 eV; and kink sites, 0.61 eV). Furthermore, alteration of the silver surface can lead to a redistribution of these sites.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111)

“…6,28 DFT calculations showed trends in the adsorption energies for ethanol adsorbed to step edge and kink sites supporting the TPD desorption features. 28 Previous results on MeOH/Au(111) assigned higher temperature desorption features to repulsive molecular interactions on terrace sites, 5 yet due to the combination of TPD sputtering experiments with DFT calculations, the higher temperature desorption features are interpreted as step edge and kink site defects in this paper. The identification of CO adsorption on Au(111) step edges and kink sites after Ar + sputtering has also been described using TPD, STM, and DFT.…”

“…Previous DFT studies of a single ethanol molecule adsorbed on Ag(111) indicate that vdW interactions between the ethanol carbon tail and Ag surface are weakened at defect sites. 28 Due to the similarities in the desorption behaviour of alcohols on Ag and Au, it is expected that comparable effects occur at defect sites on Au(111) as well. Interestingly, the rate of increase of desorption energies is the similar for terrace and step edges according to Fig.…”

“…At near monolayer coverages, the terrace desorption feature dominates, but small, high-temperature desorption features have previously been identified and characterized as desorption of molecules adsorbed to step edge (coordination number (CN) = 7-8) and kink sites (CN = 5-6), as compared to terrace (CN = 9) for EtOH on Au(111) 6 and Ag(111). 28 Fig. 2 highlights distinct adsorption sites corresponding to undercoordinated step edge (CN = 7 shown in the red circle) and kink (CN = 6 shown by the blue square) sites.…”

“…The assignment of ethanol adsorption to defect sites on Au(111) and Ag(111) has been previously reported. 6,28 TPD experiments were conducted for each sample following Ar + ion sputtering to induce defects to aid in the identification of undercoordinated adsorption sites. 6,28 DFT calculations showed trends in the adsorption energies for ethanol adsorbed to step edge and kink sites supporting the TPD desorption features.…”

“…6,28 TPD experiments were conducted for each sample following Ar + ion sputtering to induce defects to aid in the identification of undercoordinated adsorption sites. 6,28 DFT calculations showed trends in the adsorption energies for ethanol adsorbed to step edge and kink sites supporting the TPD desorption features. 28 Previous results on MeOH/Au(111) assigned higher temperature desorption features to repulsive molecular interactions on terrace sites, 5 yet due to the combination of TPD sputtering experiments with DFT calculations, the higher temperature desorption features are interpreted as step edge and kink site defects in this paper.…”