Hu Zhou

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.

The electrochemical CO2 reduction reaction (CO2RR), with water as a hydrogen source, has been attracting great attention due to its promising applications for carbon recycle utilization and renewable electricity storage. In order to drive the process economically, highly efficient catalysts are urgently needed to overcome the constraints of high overpotential, low Faradaic efficiency, and current density of CO2RR. This Perspective summarizes the performance of Pd-containing nanostructures toward CO2RR and related reaction mechanisms. The product selectivity of the Pd catalysts strongly depends on the structure and composition, and the dynamic evolution of active phases induced by the applied potential and reaction intermediate of CO2RR. Introducing a second metal can effectively suppress the decay in the catalytic performance of a Pd catalyst and further improve the activity and selectivity of CO2RR. The electrochemical promotion of catalysis effect drastically improves the production rate of formate over Pd nanoparticles, which demonstrates the advantage of the coupled thermo- and electrocatalytic CO2 reduction. The challenges and tentative strategies for the further application of Pd-containing catalysts in CO2RR are also discussed.

Active-phase engineering is regularly utilized to tune the selectivity of metal nanoparticles (NPs) in heterogeneous catalysis. However, the lack of understanding of the active phase in electrocatalysis has hampered the development of efficient catalysts for CO2 electroreduction. Herein, we report the systematic engineering of active phases of Pd NPs, which are exploited to select reaction pathways for CO2 electroreduction. In situ X-ray absorption spectroscopy, in situ attenuated total reflection-infrared spectroscopy, and density functional theory calculations suggest that the formation of a hydrogen-adsorbed Pd surface on a mixture of the α- and β-phases of a palladium-hydride core (α+β PdH x @PdH x ) above −0.2 V (vs. a reversible hydrogen electrode) facilitates formate production via the HCOO* intermediate, whereas the formation of a metallic Pd surface on the β-phase Pd hydride core (β PdH x @Pd) below −0.5 V promotes CO production via the COOH* intermediate. The main product, which is either formate or CO, can be selectively produced with high Faradaic efficiencies (>90%) and mass activities in the potential window of 0.05 to −0.9 V with scalable application demonstration.

The electron transfer from TiO₂-V_O to Pd NPs plays a crucial role in promoting ORR performance of Pd-based electrocatalysts.