Klaas Jan P. Schouten

Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide

The electrochemical reduction of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and molecular electrocatalysts for the reduction of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to determine the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 reduction on other metals and molecular complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.

Two Pathways for the Formation of Ethylene in CO Reduction on Single-Crystal Copper Electrodes

Carbon monoxide is a key intermediate in the electrochemical reduction of carbon dioxide to methane and ethylene on copper electrodes. We investigated the electrochemical reduction of CO on two single-crystal copper electrodes and observed two different reaction mechanisms for ethylene formation: one pathway has a common intermediate with the formation of methane and takes place preferentially at (111) facets or steps, and the other pathway involves selective reduction of CO to ethylene at relatively low overpotentials at (100) facets. The (100) facets seem to be the dominant crystal facets in polycrystalline copper, opening up new routes to affordable (photo)electrochemical production of hydrocarbons from CO(2).

Mechanism of the Catalytic Oxidation of Glycerol on Polycrystalline Gold and Platinum Electrodes

This paper addresses the oxidation mechanism of glycerol on Au and Pt electrodes under different pH conditions. Intermediates and/or reaction products were detected by using an online high‐performance liquid chromatography technique (for soluble products) and online electrochemical mass spectrometry (for CO2). In alkaline media, the main product of glycerol oxidation on the Pt electrode is glyceric acid produced via glyceraldehyde. Glyceric acid is the primary oxidation product on the Au electrode, which is further oxidized to glycolic acid and formic acid at high potentials (≥0.8 V), yielding high current densities. As the pH of the solution is lowered, the glycerol oxidation becomes significantly more sluggish on both Au and Pt electrodes, which results in glyceraldehyde being the main oxidation product under neutral conditions, especially on gold. In acidic solutions, only the Pt electrode shows catalytic activity with a relatively low conversion rate, mainly to glyceraldehyde. At positive potentials corresponding to the formation of a Pt surface oxide, the PtOx surface oxide catalyzes the conversion of glyceraldehyde finally to formic acid and CO2, but only under acidic conditions. Gold catalyzes glycerol oxidation only under alkaline conditions, in contrast to a “real catalyst,” that is, platinum, which catalyzes glycerol oxidation over the entire pH range.

A Step Closer to the Electrochemical Production of Liquid Fuels

The efficient and large-scale synthesis of liquid fuels from renewable sources is one of grand challenges of modern chemistry. One important low-toxicity liquid fuel that can be produced sustainably is ethanol, in particular cellulosic ethanol made from second-generation biomass sources that do not compete with food production. Large-scale plants for producing cellulosic ethanol already exist, demonstrating the viability of this technology. The main advantage of the production of cellulosic ethanol is that we can make extensive use of (photo-)catalytic technology that nature has already developed: the photosynthesis of sugar monomers and cellulose polymers by plants through carbon dioxide fixation, and the subsequent enzymatic “cracking” of cellulose to ethanol. However, from a purely chemical point of view, the production of cellulosic ethanol is inelegant and wasteful. The detour from carbon dioxide via cellulose to ethanol is highly unfavorable energetically; even by the most optimistic estimates, the efficiency of the conversion of photon energy to chemical energy is not more than 1%. Besides the chemical inefficacy, tremendous agricultural investments will be needed if we wish to produce such biofuels on a large scale. A promising long-term alternative for the large-scale production of liquid fuel is to replace nature s catalytic technology by tailored man-made catalytic technology, that is, to convert CO2 to ethanol (or another liquid fuel) through a limited number of sensible intermediates using efficient, durable, and cost-effective synthetic catalysts, which preferably operate at ambient temperature. In contrast to the production of cellulosic ethanol, this new technology does not yet exist, but researchers worldwide are making progress in understanding the intricacies of two key reactions in the process—CO2 reduction and water oxidation. In a recent letter to Nature, the research group lead by Matthew Kanan at Stanford University reports on an exciting new development in a possible final step of such a future technology: the selective conversion of carbon monoxide to ethanol and other oxygenates on a copper electrode. Copper has been known to be a good catalyst for the electrochemical conversion of CO2 and CO to methane and ethylene since the seminal work of Hori in the 1980s. Kanan et al. have now shown that nanocrystalline oxide-derived copper (OD-Cu) electrodes, prepared from the reduction of thick Cu2O layers, produce mainly ethanol, as well as acetate and n-propanol, at low overpotentials ( 0.25 to 0.5 V versus RHE (reversible hydrogen electrode)) with an unprecedented Faraday efficiency of up to 57%. No C1 products were observed, indicating rapid C C coupling at low overpotential. By comparing their OD-Cu to electrodes composed of commercial Cu nanoparticles, Kanan et al. conclude that the exceptional CO reduction activity of OD-Cu is related to the constrained environment of the grain boundaries formed during its synthesis. In addition, OD-Cu has a substantially lower hydrogen evolution activity than Cu nanoparticles, which also contributes to the high selectivity of the OD-Cu catalyst towards hydrocarbons. Another copper surface that mediates the highly selective formation of C2 products through CO reduction is the Cu(100) single-crystal electrode, on which the selective formation of ethylene can be observed at 0.3 V versus RHE. In good agreement with the results of Kanan et al., this selective C2 formation at low overpotential is also observed only in alkaline media, whereas at higher overpotentials, C2 selectivity decreases and methane is formed on Cu(100) as well. The low-potential C C coupling in alkaline media has been explained by the formation of surface-bound CO dimers through a rate-limiting proton-decoupled electron transfer, a reaction step which, according to recent density functional theory (DFT) calculations, strongly prefers (100) surface sites. The subsequent hydrogenation of this dimer leads to the formation of a CH2CHO(ads) intermediate, a kind of oxymetallacycle that binds to the copper surface through both C and O (see Figure 1). The DFT calculations suggest that this intermediate can be either reduced to ethylene, as observed on Cu(100) single-crystal electrodes, or to ethanol, as observed by Kanan et al. The factors determining whether ethylene or ethanol is formed are not yet understood, but presumably the pH and the local surface [*] Dr. K. J. P. Schouten, Dr. F. Calle-Vallejo, Prof. Dr. M. T. M. Koper Leiden University, Leiden Institute of Chemistry P.O. Box 9502, 2300 RA Leiden (The Netherlands) E-mail: m.koper@chem.leidenuniv.nl

CHAPTER 12:Key Intermediates in the Hydrogenation and Electrochemical Reduction of CO2

Mimicking photosynthesis by (re)using carbon dioxide as a carbon feedstock for the production of hydrocarbons would enable a sustainable carbon cycle. Electrochemically, CO2 can be reduced on copper electrodes to hydrocarbons, mainly methane and ethylene, and the integration of this process in a photo-electrochemical device could be a promising way to close the carbon cycle. Understanding the mechanism of this reaction is one of the keys to open up new, sustainable routes to carbon based fuels. In this chapter we aim to obtain more insights in the key intermediates that determine the selectivity of CO2 reduction to various products, by comparing the electrochemical reduction of CO2 to the metal-catalyzed hydrogenation and reduction of CO2 both homogeneously in solution and heterogeneously in the gas phase. We distinguish four main pathways: (1) methane is formed via hydroxycarboxyl (COOH) and carbon monoxide (CO), (2) methanol is formed via formate (HCOO) and formaldehyde, (3) ethylene is formed via the coupling of CO, leading to surface enolates, and (4) CO2 is inserted into existing carbon chains, close to the way CO2 is fixed in nature.