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. Open image in new window

Water-selective zeolite membranes: The yield of many gas-phase industrial reactions is limited by the formation of water as a by-product. Li et al. harnessed the water-sieving properties of NaA zeolite crystals by forming them into continuous defect-free membranes within tube reactors (see the Perspective by Carreon). These membranes can let water pass but reject gases such as hydrogen, carbon monoxide, and carbon dioxide (CO ₂ ). When these membranes were used in CO ₂ hydrogenation to form methanol with water as a by-product, substantial increases were observed in both the CO ₂ conversion and methanol yield. Science , this issue p. 667 ; see also p. 624