Size-Dependent Electrocatalytic Reduction of CO₂ over Pd Nanoparticles

TLDR

Size effects are known to tune catalytic activity of metal nanoparticles, yet their influence on CO₂ electroreduction remains poorly understood. This study investigates how Pd nanoparticle size (2.4–10.3 nm) affects the activity and selectivity of CO₂ electroreduction. Density‑functional‑theory calculations reveal that CO₂ adsorption and COOH* formation are favored on edge and corner sites of Pd, whereas H* formation is similar across sites, explaining the size‑dependent performance. The Faradaic efficiency for CO rises from 5.8 % at −0.89 V over 10.3 nm particles to 91.2 % over 3.7 nm particles, with an 18.4‑fold current density increase and a volcano‑shaped turnover‑frequency curve that links size to catalytic activity.

Abstract

Size effect has been regularly utilized to tune the catalytic activity and selectivity of metal nanoparticles (NPs). Yet, there is a lack of understanding of the size effect in the electrocatalytic reduction of CO2, an important reaction that couples with intermittent renewable energy storage and carbon cycle utilization. We report here a prominent size-dependent activity/selectivity in the electrocatalytic reduction of CO2 over differently sized Pd NPs, ranging from 2.4 to 10.3 nm. The Faradaic efficiency for CO production varies from 5.8% at −0.89 V (vs reversible hydrogen electrode) over 10.3 nm NPs to 91.2% over 3.7 nm NPs, along with an 18.4-fold increase in current density. Based on the Gibbs free energy diagrams from density functional theory calculations, the adsorption of CO2 and the formation of key reaction intermediate COOH* are much easier on edge and corner sites than on terrace sites of Pd NPs. In contrast, the formation of H* for competitive hydrogen evolution reaction is similar on all three sites. A volcano-like curve of the turnover frequency for CO production within the size range suggests that CO2 adsorption, COOH* formation, and CO* removal during CO2 reduction can be tuned by varying the size of Pd NPs due to the changing ratio of corner, edge, and terrace sites.