Theory-guided enhancement of CO2 reduction to ethanol on Ag-Cu tandem catalysts via particle-size effects

In the CO2 reduction reaction, the design of electrocatalysts that selectively promote alcohols over hydrocarbons (e.g., ethanol over ethylene) hinges on the understanding of the pathways and specific sites that forms alcohols. Herein, theoretical considerations guide state-of-the-art synthesis of well-defined catalysts to show that higher selectivity toward ethanol is achieved on Cu(110) edge sites compared to Cu(100) terraces. Specifically, we study the catalytic behavior of Cu nano-cubes (Cucub) of different sizes in the framework of tandem catalysis with CO-producing Ag nanospheres. We predict and experimentally find that the smaller Cucub possess higher selectivity for ethanol in view of their larger edge-to-faces ratio and of the fact that ethylene is produced at terraces while ethanol is selectively produced at step edges. These results call for synthetic developments toward Cu nanostructures exposing only edge sites, such as hollow cubic nanocages, to further increase ethanol selectivity. More generally, this study encourages the application of well-defined nano catalysts as a bridge between theory and experiments in electrocatalysis.This work was financially supported by Gaznat S.A. J.R.P. acknowledges the H2020 Marie Curie Individual Fellowship grant SURFCAT with Agreement No. 837378. This publication was created as part of NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation. The theoretical effort was supported by Spanish MICIUN’s RTI2018-095460–B-I00, Ramón y Cajal RYC-2015-18996, and María de Maeztu MDM-2017-0767 Grants, and partly by Generalitat de Catalunya via 2017SGR13. M.J.K. and F.C.V. are thankful to Red Española de Supercomputación (RES) for supercomputing time at SCAYLE (Projects QS-2019-3-0018, QS-2019-2-0023, and QCM-2019-1-0034). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences

Elucidating the Facet-Dependent Selectivity for CO2 Electroreduction to Ethanol of Cu-Ag Tandem Catalysts

Despite being desirable high-value products of the electro-chemical CO2 reduction reaction (CO2RR), alcohols are still obtained with lower selectivity compared to hydrocarbons and the reaction pathways leading to their formation are still under debate. In this joint experimental-computational work, we exploit structural sensitivity effects to elucidate the ethanol-producing active sites on Cu-Ag CO2RR tandem catalysts. Specifically, methane-selective Cu nano-octahedra (Cu-oh), enclosed by (111) facets, and ethylene-selective Cu nanocubes (Cu-cub), enclosed by (100) facets, are mixed with CO-selective Ag nanospheres (Ag-sph) to form Cu-oh-(A)g and Cu-cub-Ag bimetallic catalysts. Ethanol is selectively enhanced via the *CHx-*CO coupling pathway at the terraces of Cuoh-Ag in the CO-enriched environment generated by the Ag-sph. Conversely, on Cu-cub-Ag, ethanol is selectively produced via the same pathway at the edges and corners of Cu-cub, while ethylene continues to be produced at the terraces. The terraces being the predominant surfaces on the catalysts, such facet dependence explains the higher ethanol-to-ethylene ratio on the Cuoh-Ag. These findings illustrate how tandem catalysis and structure-sensitive effects can be combined to obtain notable changes in the selectivity of electrochemical reactions

Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO₂ Reduction at Commercially Viable Current Densities

Despite substantial progress in the electrochemical conversion of CO2 into value-added chemicals, the translation of fundamental studies into commercially relevant conditions requires additional efforts. Here, we study the catalytic properties of tailored Cu nanocatalysts under commercially relevant current densities in a gas-fed flow cell. We demonstrate that their facet-dependent selectivity is retained in this device configuration with the advantage of further suppressing hydrogen production and increasing the faradaic efficiencies toward the CO2 reduction products compared to a conventional H-cell. The combined catalyst and system effects result in state-of-the art product selectivity at high current densities (in the range 100-300 mA/cm2) and at relatively low applied potential (as low as-0.65 V vs RHE). Cu cubes reach an ethylene selectivity of up to 57% with a corresponding mass activity of 700 mA/mg, and Cu octahedra reach a methane selectivity of up to 51% with a corresponding mass activity of 1.45 A/mg in 1 M KOH.ChemE/Materials for Energy Conversion & Storag

Nanocrystals as Precursors in Solid-State Reactions for Size- and Shape-Controlled Polyelemental Nanomaterials

Solid-state reactions between micrometer-size powders are among the oldest, simplest, and still widely used methods for the fabrication of inorganic solids. These reactions are intrinsically slow because, although the precursorsare "well mixed" at the macroscale, they are highly inhomogeneous at the atomic level. Furthermore, their products are bulk powders that are not suitable for device integration. Herein, we substitute micrometersize particles with nanocrystals. Scaling down the size of the precursors reduces the reaction time and temperature. More importantly, the final products are nanocrystals with controlled size and shape that can be used as active materials in various applications, including electro- and photocatalysis. The assembly of the nanocrystal precursors as ordered close-packed superlattices enables microscopy studies that deepen the understanding of the solid-state reaction mechanism. We learn that having only one of the two nanocrystal precursors dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the nanocrystal precursor that is the most stable at the reaction temperature. Considering the variety of controlled nanocrystals available, our findings open a new avenue for the synthesis of functional and tunable polyelemental nanomaterials

Insights into Reaction Intermediates to Predict Synthetic Pathways for Shape-Controlled Metal Nanocrystals

Understanding nucleation phenomena is crucial across all branches of physical and natural sciences. Colloidal nanocrystals are among the most versatile and tunable synthetic nanomaterials. While huge steps have been made in their synthetic development, synthesis by design is still impeded by the lack of knowledge of reaction mechanisms. Here, we report on the investigation of the reaction intermediates in high temperature syntheses of copper nanocrystals by a variety of techniques, including X-ray absorption at a synchrotron source using a customized in situ cell. We reveal unique insights into the chemical nature of the reaction intermediates and into their role in determining the final shape of the metal nanocrystals. Overall, this study highlights the importance of understanding the chemistry behind nucleation as a key parameter to predict synthetic pathways for shape-controlled nanocrystals

Colloidal Nanocrystals as Electrocatalysts with Tunable Activity and Selectivity

Correlating the catalyst activity, selectivity, and stability with its structure and composition is of the utmost importance in advancing the knowledge of heterogeneous electrocatalytic processes for chemical energy conversion. Well-defined colloidal nanocrystals with tunable monodisperse size and uniform shapes are ideal platforms to investigate the effect of these parameters on the catalytic performance. In addition to translating the knowledge from single-crystal studies to more realistic conditions, the morphological and compositional complexity attainable by colloidal chemistry can provide access to active catalysts which cannot be produced by other synthetic approaches. The sample uniformity is also beneficial to investigate catalyst reconstruction processes via both ex situ and operando techniques. Finally, colloidal nanocrystals are obtained as inks, a feature which facilitates their integration on different substrates and cell configurations to study the impact of interactions at the mesoscale and the device-dependent reaction microenvironment on the catalytic outcome. In this Review, we discuss recent studies in selected electrochemical reactions and provide our outlook on future developments on the use of well-defined colloidal nanocrystals as an emerging class of electrocatalysts