Elucidating the active motifs in a reconstructed AgPdF nanoalloy for the formate oxidation reaction

Abstract

The surface reconstruction of catalysts under the reaction environment may have a dominating influence on the catalytic mechanism; this effect has been intensively investigated. Compared with AgPd, an AgF adlayer on an AgPd nanoalloy surface, also referred to as AgPdF, experimentally demonstrates a higher formate oxidation reaction (FOR) current and durability. However, the way in which the surface of AgF adlayer evolves on AgPd during the FOR in aqueous solution is still ambiguous. In this work, the formation of an AgF(111) adlayer on AgPd(111) is carried out to simulate the surface of the AgPdF nanoalloy, and the Pourbaix diagram is prepared for the surface of the AgPdF nanoalloy. The results indicate that the 1/3 ML OH-adsorbed AgF adlayer(AgF–OH motif) is the most thermodynamically stable structure over the anode sweep from 0.23 to 0.83 V vs. RHE during the FOR. The fluorination could also improve the durability against Pd dissolution. The free energy diagram calculated through density functional theory (DFT) shows that the existence of the AgF–OH motif could weaken the adsorption of HCOO* and H* and thus improve the FOR performance. Experimentally, AgF was successfully grown on an AgPd nanoalloy to form AgPdF. Compared with AgPd, AgPdF exhibits a higher catalytic current (3.05 A mg_Pd⁻¹) and less sensitivity to the upper limit potentials. In situ Raman and XPS characterization was performed on samples extracted under formate oxidation reaction (FOR) working potentials. The observed redshift in the Ag–F vibrational frequency and the emergence of an Ag–OH peak in the Raman spectra confirmed hydroxide adsorption. Complementary XPS analysis revealed additional oxidation of Ag in the operational catalyst compared to the as-prepared AgPdF.