Controllable CO adsorption determines ethylene and methane productions from CO2 electroreduction

Among all CO2 electroreduction products, methane (CH4) and ethylene (C2H4) are two typical and valuable hydrocarbon products which are formed in two different pathways: hydrogenation and dimerization reactions of the same CO intermediate. Theoretical studies show that the adsorption configurations of CO intermediate determine the reaction pathways towards CH4/C2H4. However, it is challenging to experimentally control the CO adsorption configurations at the catalyst surface, and thus the hydrocarbon selectivity is still limited. Herein, we seek to synthesize two well-defined copper nanocatalysts with controllable surface structures. The two model catalysts exhibit a high hydrocarbon selectivity toward either CH4 (83%) or C2H4 (93%) under identical reduction conditions. Scanning transmission electron microscopy and X-ray absorption spectroscopy characterizations reveal the low-coordination Cu-0 sites and local Cu-0/Cu+ sites of the two catalysts, respectively. CO-temperature programed desorption, in-situ attenuated total reflection Fourier transform infrared spectroscopy and density functional theory studies unveil that the bridge-adsorbed CO (COB) on the low-coordination Cu-0 sites is apt to be hydrogenated to CH4, whereas the bridge-adsorbed CO plus linear-adsorbed CO (COB + COL) on the local Cu-0/Cu+ sites are apt to be coupled to C2H4. Our findings pave a new way to design catalysts with controllable CO adsorption configurations for high hydrocarbon product selectivity. (C) 2020 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

Cu/Metal Oxide Hybrid Nanocrystals as Electrocatalysts for the CO2 Reduction Reaction

Seyedehbehnaz Varandili

The electrochemical CO2 reduction reaction (CO2RR) is envisioned to play a significant role in achieving carbon neutrality while contributing to storing renewable energies. Cu-based materials are among the most promising electrocatalysts. However, 16 different products are obtained from a Cu foil, therefore strategies to make more selective catalysts are needed. This thesis explores Cu/metal oxide interfaces as a mean to direct selectivity towards hydrocarbon products in CO2RR. The aim is to deviate from the scaling relationships which exist on single metal surfaces. Cu/metal oxides hybrid nanocrystals (HNCs) with tunable morphologies and spatial configuration are synthesized via colloidal chemistry. The achieved level of control and tunability is demonstrated to be crucial in understanding the impact of these parameters on the reaction outcome. First, we start by designing a synthesis procedure for Cu/CeO2-x heterodimers (HDs) and then we study their catalytic behavior toward CO2RR. We show that the Cu/CeO2-x interface promotes CO2RR, particularly methane with around 54% selectivity, which is ~5 times higher than those obtained with a physical mixture of isolated Cu and CeO2-x nanocrystals, thus highlighting the important role played by the intimate bonding at the interface. A miscellanea of characterization techniques indicates the higher extent of ceria reduction in the HDs during CO2RR. DFT elucidates the presence of unique catalytic motifs that enable bidentate adsorption of critical intermediates at both Cu and the CeO2-xO-vacancy site, creating an opportunity for reaction engineering beyond scaling relationship limitations. Having assessed the importance of the Cu/ceria interface, we investigate the impact of increasing the Cu/CeO2 interfacial area on the CO2RR outcome. We find that increasing the number of ceria domain around copper inhibits CO2RR. This result highlights that a balance between interfacial area and accessibility to the active catalytic sites must be sought after to exploit synergies arising at the interface between different domains. Finally, we explore Cu/ZnO HNCs as CO2RR pre-catalysts to elucidate the structural and compositional parameters which direct the reaction pathway towards ethanol. Using different sizes of ZnO seeds for the synthesis of Cu/ZnO HNCs, we are able to achieve two different configurations including ZnO decorated Cu core (Cu@ZnO) and Cu decorated ZnO core (ZnO@Cu) HNCs. Upon activation via cathodic voltage, we find that Cu@ZnO HNCs favor methane production while ZnO@Cu exhibits a noticeable selectivity toward ethanol. Operando XAS, XPS, and electron microscopy reveal that both HNCs transform into Cu@CuZn core@shell NCs during the activation step. However, the Zn content in the surface alloy (i.e. degree of alloying) and the presence of surface cationic Zn species are found to be crucial in determining the selectivity of these catalysts, which possibly elucidates some of the contrasting results reported on Cu-Zn across the literature. Overall, this thesis showcases the importance of well-defined and tunable HNCs as platforms to study structure/properties relations and to advance the current knowledge in CO2RR. Furthermore, the library of nanomaterials accessible through the developed synthesis routes might also be utilized in the future to investigate other catalytic reactions and applications where the hybrid interfaces might be relevant.

EPFL2021

Metal-organic framework derived copper catalysts for CO2 to ethylene conversion

Jun Li, Ning Wang

The electrochemical reduction of CO2 to ethylene provides a carbon-neutral avenue for the conversion of CO2 to value-added fuels and feedstocks, so contributing to the storage of intermittent renewable electricity. The exploration of efficient electrocatalysts with high ethylene selectivity and productivity is highly desirable but remains challenging. Here, we present a Cu-based catalyst derived from a metal-organic framework (Cu-MOF) which shows enhanced performance due to its porous morphology, complex oxidation states and strong lattice strain. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy are utilized to track the evolution of the crystal structure and oxidation states during the reaction, and the results reveal that Cu2+ ions are rapidly reduced to Cu+ and then slowly to Cu-0, resulting in a Cu@CuxO core@shell structure. The tensile strain caused by the distorted grain is beneficial for the activation of CO2. Cu+/Cu-0 interfaces formed through stabilized Cu+ facilitate *CO-CO dimerization, promoting conversion to C2+ products and suppressing conversion to C-1 products. The optimized catalyst exhibits a 51% Faraday efficiency (FE) for ethylene and a 70% FE for C2+ products, with 20 h operational stability in an H-cell configuration, and a partial ethylene current density of 150mA cm(-2) in a flow-cell configuration.

ROYAL SOC CHEMISTRY2020

Engineering the ZrO2–Pd Interface for Selective CO2 Hydrogenation by Overcoating an Atomically Dispersed Pd Precatalyst

Alimohammad Bahmanpour, Yuan-Peng Du, Florent Emmanuel Héroguel, Oliver Kröcher, Jeremy Luterbacher, Mounir Driss Mensi, Luka Milosevic

Tailoring the interfacial sites between metals and metal oxides can be an essential tool in designing heterogeneous catalysts. These interfacial sites play a vital role in many renewable applications, for instance, catalytic CO2 reduction. Postsynthesis deposition of metal oxide on supported metal catalysts can not only create such interfacial sites but also prevent particle sintering at high temperature. Here, we report a sol–gel-based strategy to synthesize an atomically dispersed “precatalyst”. In contrast to the deposition on catalysts containing preformed nanoparticles, overcoating this material before reductive treatment can inhibit particle growth during thermal activation steps, yielding highly accessible, sintering-resistant Pd clusters that are less than 2 nm in diameter. This synthetic approach allows us to engineer interfacial sites while maintaining high metal accessibility, which was difficult to achieve in previous overcoated materials. Notably, engineering the Pd–ZrO2 interface into an inverted interface with an amorphous ZrO2 overcoat might facilitate a C–O cleavage route instead of a mechanism containing a bicarbonate intermediate during CO2 hydrogenation. We also observed that carbon deposition occurring on methanation sites could be a key factor for improving CO selectivity. Alteration of the reaction pathway, along with the deactivation of certain sites, led to 100% CO selectivity on the ZrO2@Pd/ZrO2 catalyst. This work demonstrated that overcoated materials could represent a promising class of heterogeneous catalysts for selective CO2 conversion over noble metals, which have higher rates and thermal stability compared to state-of-the-art Cu-based catalysts.

2020

Cu/Metal Oxide Hybrid Nanocrystals as Electrocatalysts for the CO2 Reduction Reaction

Seyedehbehnaz Varandili

EPFL2021