Adsorption and Oxidation of CO on Ceria Nanoparticles Exposing Single-Atom Pd and Ag: A DFT Modelling.

Vladimir A Nasluzov,Konstantin M Neyman,Svetlana S Laletina,Aleksey M Shor,Elena A Ivanova-Shor

doi:10.3390/ma14226888

Abstract

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32− species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

Highlights

COx intermediates formed upon calculated as follows: Eb (CO) adsorption and oxidation on single M = Pd and
The stability of the COx moieties anchored to the ceria-supported M atom is found to increase in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP
Except for the Pd atom saturated by four O atoms of the ceria surface O4 -site, which is unable to properly adsorb CO, the doping of the ceria nanoparticle with Pd and Ag atom increases its propensity to bind the CO molecule with respect to bare ceria material

Summary

Introduction

As an active reducible support, ceria facilitates the dispersion of metals and MOx phases on the surface [7,14,15,16,17] and provides lattice O atoms to oxidise reactants [2,9,17,18,19,20]. Supported transition metals can enhance the redox performance and oxygen storage capacity of ceria [23]. High catalytic efficiency is achieved using a nanostructured ceria support via enhanced metal–support interaction, which improves the dispersion of metal particles and suppresses their sintering at elevated temperatures [3,4,7,8,9,11,19,24,25,26,27]

Objectives

Results

Conclusion