In situ nanoscale studies of copper-based catalysts for carbon dioxide electroreduction

The urgency of climate change demands the simultaneous removal of carbon dioxide from the atmosphere and the transition to renewable energy sources. This aim is realizable through electrochemical reduction of carbon dioxide (CO2RR), which is a promising route for synthesizing renewable carbon-based fuels and chemicals.Among many existing CO2RR catalysts, copper is the only earth-abundant metal that produces energy-rich molecules. Practical application is hindered by its poor product selectivity and modest long-term catalyst stability. To overcome these limitations, an atomistic understanding of the catalytic processes and identification of CO2RR active sites on Cu are needed.Current research implies that the active sites form upon the catalysts' evolution during the reaction. However, fast Cu oxidation hinders their identification in conventional ex situ studies. The following questions remain unanswered: What are the morphology and chemical composition of the active sites? How do they form?This thesis explores the in situ evolution of Cu-based CO2RR catalysts. We study this phenomenon down to the nanoscale through a synergy of in situ microscopy and spectroscopy studies. We employ graphene as a 2D protecting layer on Cu to study the Cu nanoscale structural evolution in situ via electrochemical scanning tunneling microscopy (EC-STM). We use in situ Raman and X-ray photoelectron spectroscopies to study the evolution of the Cu catalysts' oxidation state.First, EC-STM studies reveal the formation of few nanometer-sized cuboids on Cu catalysts as the main structural evolution during CO2RR. We show that this evolution occurs in aqueous electrolytes over a wide range of pH and compositions. Second, we show that metallic Cu is the active phase. Reduction of the Cu oxide, formed either through chemical synthesis or through the exposure of Cu catalysts to air, is unavoidable prior to CO2RR.To understand the nanocuboid formation mechanism, we present a comparative study of the Cu nanocuboids formed upon CO2RR and the Cu2O cubes prepared through electrochemical cycling synthesis. Morphological and chemical composition differences between them allow us to conclude on distinct formation mechanisms. We show that the metallic Cu nanocuboids form during a potential-driven re-organization of atoms in the topmost surface layers.Furthermore, we present EC-STM studies revealing the atomistic details of the Cu nanocuboid formation. By comparing these processes with the multilayer mound growth in Cu homoepitaxy, we show that the nanocuboids are (100) facet multilayers formed upon reduction of Cu oxides. We propose that the nanocuboids are CO2RR active sites, ultimately forming on any Cu catalyst regardless of its initial macroscopic shape and oxidation state.The in situ identification of active sites, presented here, offers a unique framework for the re-interpretation of the existing literature, invites further fundamental theoretical and experimental research, and paves the way for rational catalyst design. Moreover, this thesis further demonstrates the importance of in situ studies across different length scales in understanding key catalytic processes in renewable energy schemes.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

Mimicking Enzymatic Active Sites on Surfaces for Energy Conversion Chemistry

Klaus Kern, Magalí Alejandra Lingenfelder, Sebastian Stepanow

Metal organic supramolecular chemistry on surfaces has matured to a point where its underlying growth mechanisms are well understood and structures of defined coordination environments of metal atoms can be synthesized in a controlled and reproducible procedure. With surface-confined molecular self-assembly, scientists have a tool box at hand which can be used to prepare structures with desired properties, as for example a defined oxidation number and spin state of the transition metal atoms within the organic matrix. From a structural point of view, these coordination sites in the supramolecular structure resemble the catalytically active sites of metallo-enzymes, both characterized by metal centers coordinated to organic ligands. Several chemical reactions take place at these embedded metal ions in enzymes and the question arises whether these reactions also take place using metal organic networks as catalysts. Mimicking the active site of metal atoms and organic ligands of enzymes in artificial systems is the key to understanding the selectivity and efficiency of enzymatic reactions. Their catalytic activity depends on various parameters including the charge and spin configuration in the metal ion, but also on the organic environment, which can stabilize intermediate reaction products, inhibits catalytic deactivation, and serves products and therefore ensures the selectivity of the enzyme. Charge and spin on the transition metal in enzymes depend on the one hand on the specific metal element, and on the other hand on its organic coordination environment. These two parameters can carefully be adjusted in surface confined metal organic networks, which can be synthesized by virtue of combinatorial mixing of building synthons. Different organic ligands with varying functional groups can be combined with several transition metals and spontaneously assemble into ordered networks. The catalytically active metal centers are adequately separated by the linking molecules and constitute promising candiates for heterogeneous catalysts. Recent advances in synthesis, characterization, and catalytic performance of metal organic networks are highlighted in this Account. Experimental results like structure determination of the networks, charge and spin distribution in the metal centers, and catalytic mechanisms for electrochemical reactions are presented. In particular, we describe the activity of two networks for the oxygen reduction reaction in a combined scanning tunneling microscopy and electrochemical study. The similarities and differences of the networks compared to metallo-enzymes will be discussed, such as the metal surface that operates as a geometric template and concomitantly functions as an electron reservoir, and how this leads to a new class of bioinspired catalysts. The possibility to create functional two-dimensional coordination complexes at surfaces taking inspiration from nature opens up a new route for the design of potent nanocatalyst materials for energy conversion.

Amer Chemical Soc2015

Emergence of Potential-Controlled Cu-Nanocuboids and Graphene-Covered Cu-Nanocuboids under Operando CO2 Electroreduction

Karla Banjac, Fernando Pablo Cometto, Magalí Alejandra Lingenfelder, Thanh Hai Phan

The electroreduction of CO2 (CO2RR) is a promising strategy toward sustainable fuels. Cu is the only Earth-abundant and pure metal capable of catalyzing CO2-to-hydrocarbons conversion with significant Faradaic efficiencies; yet, its dynamic structure under operando CO2RR conditions remains unknown. Here, we track the Cu structure operando by electrochemical scanning tunneling microscopy and Raman spectroscopy. Surprisingly, polycrystalline Cu surfaces reconstruct forming Cu nanocuboids whose size can be controlled by the polarization potential and the time employed in their in situ synthesis, without the assistance of organic surfactants and/or halide anions. If the Cu surface is covered by a graphene monolayer, smaller features with enhanced catalytic activity for CO2RR can be prepared. The graphene-protecting layer softens the 3D morphological changes that Cu-based catalysts suffer when exposed to aggressive electrochemical environments and allows us to track the kinetic roughening process. This novel strategy is promising for improving Cu long-term stability, and consequently, it could be used as a platform to ultimately control product selectivity.