Probing electrosynthetic reactions with furfural on copper surfaces

Synergistic Mechanism for Unconventional Anodic Reaction of Aldehyde Oxidation for Hydrogen Production

Anodic reactions involving non-faradaic processes have significantly expanded the potential application of anodic oxidation half-reactions. Metallic Cu materials can catalyze an unconventional anodic aldehyde oxidation reaction involving the non-faradaic H2 production (AOR-H2). AOR-H2 has distinct advantages of ultra-low thermodynamic potentials and high value-added redox products, etc., but the question of exactly how reduction steps occur during AOR-H2, is something which has long puzzled scientists. Here we illustrate the novel synergistic mechanism of nonelectrochemical/electrochemical redox steps in AOR-H2. Aldehyde undergoes hydration, deprotonation, and spontaneous C-H homolytic cleavage to generate H2, and then is electrochemically oxidized to form carboxylate. Decorating Cu catalysts with metallic Pt species, supported by theoretical calculations, leads to a 12-fold increase in the intrinsic activity of AOR-H2. This work inspires researchers to develop novel cathodic and anodic reactions involving the non-faradaic process for breaking through the limit of existing energy conversion systems.

An alternative to petrochemicals: biomass electrovalorization

Replacing petrochemicals with refined waste biomass as a sustainable chemical source has become an attractive option to lower global carbon emissions. Popular methods of refining lignocellulosic waste biomass use thermochemical processes, which have significant environmental downsides. Using electrochemistry instead would overcome many of these downsides, directly driving chemical reactions with renewable electricity and revolutionizing the way many chemicals are produced today. This review mainly focuses on two furanic platform chemicals that are produced from the dehydration of cellulose, 5-hydroxymethylfurfural and furfural, which can be electrochemically reduced or oxidized to replace fuels and monomers that today are obtained from petrochemicals. Critical parameters such as electrode materials and electrolyte pH are discussed in relation to their influence on conversion efficiency and product distribution.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

Electrocatalytic reduction of furfural to furfuryl alcohol using carbon nanofibers supported zinc cobalt bimetallic oxide with surface-derived zinc vacancies in alkaline medium

Electrocatalytic hydrogenation (ECH) reduction provides an environment-friendly alternative to conventional method for the upgrade of furfural to furfuryl alcohol. At present, exploring superior catalysts with high activity and selectivity, figuring out the reduction mechanism in aqueous alkaline environment are urgent. In this work, zinc cobalt bimetallic oxide (ZnMn2O4) with surface-derived Zn2+ vacancies supported by carbon nanofibers (d-ZnMn2O4-C) was fabricated. The d-ZnMn2O4-C exhibited excellent performance in electrocatalytic reduction of furfural, high furfuryl alcohol yield (49461.1 ± 228 µmol g-1) and Faradaic efficiency (95.5 ± 0.5 %) was obtained. In-depth research suggested that carbon nanofiber may strongly promoted the production of adsorbed hydrogen (Hads), and Zn2+ vacancies may significantly lowered the energy barrier of furfural reduction to furfuryl alcohol, the synergistic effect between carbon nanofiber and d-ZnMn2O4 probably facilitated the reaction between Hads and furfuryl alcohol radical, thereby promoting the formation of furfuryl alcohol. Furthermore, the reaction mechanism was clarified by inhibitor coating and isotope experiments, the results of which revealed that the conversion of furfural to furfuryl alcohol on d-ZnMn2O4-C followed both ECH and direct electroreduction mechanism.

Recent Advances in Electrocatalytic Hydrogenation Reactions on Copper-Based Catalysts

Hydrogenation reactions play a critical role in the synthesis of value-added products within the chemical industry. Electrocatalytic hydrogenation (ECH) using water as the hydrogen source has emerged as an alternative to conventional thermocatalytic processes for sustainable and decentralized chemical synthesis under mild conditions. Among the various ECH catalysts, copper-based (Cu-based) nanomaterials are promising candidates due to their earth-abundance, unique electronic structure, versatility, and high activity/selectivity. Herein, recent advances in the application of Cu-based catalysts in ECH reactions for the upgrading of valuable chemicals are systematically analyzed. The unique properties of Cu-based catalysts in ECH are initially introduced, followed by design strategies to enhance their activity and selectivity. Then, typical ECH reactions on Cu-based catalysts are presented in detail, including carbon dioxide reduction for multicarbon generation, alkyne-to-alkene conversion, selective aldehyde conversion, ammonia production from nitrogen-containing substances, and amine production from organic nitrogen compounds. In these catalysts, the role of catalyst composition and nanostructures toward different products is focused. The co-hydrogenation of two substrates (e.g., CO2 and NOx n, SO3 2-, etc.) via C─N, C─S, and C─C cross-coupling reactions are also highlighted. Finally, the critical issues and future perspectives of Cu-catalyzed ECH are proposed to accelerate the rational development of next-generation catalysts.

Unraveling the reaction mechanisms for furfural electroreduction on copper

Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory-based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining step for the production of the two main products of furfural electroreduction on Cu, i.e., furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu's ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction. As a theoretical guideline, low pH (<1.5) and moderate potential (ca. -0.5 V vs. SHE) conditions are suggested for selective 2-MF production.

Roughness-Dependent Electro-Reductive Coupling of Nitrobenzenes and Aldehydes on Copper Electrodes

The electro-reductive coupling of nitro and carbonyl compounds enables a facile, environmentally friendly and energy benign transformation toward value-added nitrones or imines, but the selectivity is still challenging. Here, the surface roughness of Cu electrodes is introduced for the first time as the determinant to switch products from nitrones to imines owing to the controllable reduction of nitroarenes to hydroxylamines or amines on tailored Cu^I /Cu⁰ interfaces. The roughness-dependent selectivity, that is the decrease of nitrones and the increase of imines with enhanced roughness, is visible in the electro-reductive coupling of nitrobenzene and furfural. Thus, the high selectivity of nitrone (98 %) and imine (80 %) can be achieved on a surface smooth Cu foil and the one electrochemically roughened in the presence of I^- , respectively. Such roughness-dependence of nitrone/imine selectivity on Cu electrodes is further verified in a wide substrate scope, highlighting the promise of surface/interfacial engineering for electrochemical synthesis.

Promoted electrocatalytic hydrogenation of furfural in a bi-phasic system

The promoted electrocatalytic hydrogenation of biomass-derived furfural to 2-methylfuran is for the first time identified in a water/oil bi-phasic system, in which the oil phase can quickly separate hydrophobic products from the electrode/electrolyte interfaces, resulting in a beneficial equilibrium toward hydrodeoxygenation.

Insight into the Interaction of Furfural with Metallic Surfaces in the Electrochemical Hydrogenation Process

Electrocatalytic hydrogenation of furfural on metal surfaces has become an important research subject due to the potential of the reaction product 2-methylfuran as a renewable energy resource. Identifying effective determinants in this reaction process requires a thorough investigation of the complex electrode-electrolyte interactions, which considers a variety of the influential components. In this work, in operando electrochemical Raman Spectroscopy and Molecular Dynamics simulations were utilized to investigate different characteristics of the interface layer in the electrocatalytic hydrogenation of furfural. Hereby, the influence of applied potentials, electrode material, and electrolyte composition were investigated in detail. The studied parameters give an insight into furfural's binding situation, molecular orientation, and reaction mechanism.

An experimental guide to in operando electrochemical Raman spectroscopy

Electrochemical Raman spectroscopy can provide valuable insights into electrochemical reaction mechanisms. However, it also shows various pitfalls and challenges. This paper gives an overview of the necessary theoretical background, crucial practical considerations for successful measurement, and guidance for in situ/in operando electrochemical Raman spectroscopy. Several parameters must be optimized for suitable reaction and measurement conditions. From the experimental side, considerations for the setup, suitable signal enhancement methods, choice of material, laser, and objective lens are discussed. Different interface phenomena are reviewed in the context of data interpretation and evaluation.

Graphical Abstract

Mechanistic Differences between Electrochemical Hydrogenation and Hydrogenolysis of 5-Hydroxymethylfurfural and Their pH Dependence

Hydrogenation and hydrogenolysis are two important reactions for electrochemical reductive valorization of biomass-derived oxygenates such as 5-hydroxymethylfurfural (HMF). In general, hydrogenolysis (which combines hydrogenation and deoxygenation) is more challenging than hydrogenation (which does not involve the cleavage of carbon-oxygen bonds). Thus, identifying factors and conditions that can promote hydrogenolysis is of great interest for reductive valorization of biomass-derived oxygenates. For the electrochemical reduction of HMF and its derivatives, it is known that aldehyde hydrogenation is not a part of aldehyde hydrogenolysis but rather a competing reaction; however, no atomic-level understanding is currently available to explain their electrochemical mechanistic differences. In this study, combined experimental and computational investigations were performed using Cu electrodes to elucidate the key mechanistic differences between electrochemical hydrogenation and hydrogenolysis of HMF. The results revealed that hydrogenation and hydrogenolysis of HMF involve the formation of different surface-adsorbed intermediates via different reduction mechanisms and that lowering the pH promoted the formation of the intermediates required for aldehyde and alcohol hydrogenolysis. This study for the first time explains the origins of the experimentally observed pH-dependent selectivities for hydrogenation and hydrogenolysis and offers a new mechanistic foundation upon which rational strategies to control electrochemical hydrogenation and hydrogenolysis can be developed.

Bifunctional Pt/Au Janus electrocatalysts for simultaneous oxidation/reduction of furfural with bipolar electrochemistry

The sustainable conversion of biomass-derived compounds into high added-value products is a very important contemporary scientific challenge. In this context, we report here the simultaneous electro-oxidation/-reduction of a biomass-derived compound in a one-pot approach using bipolar electrochemistry. Bifunctional Pt/Au Janus electrocatalysts are employed for a selective conversion of furfural into both, furfuryl alcohol and furoic acid, which can't be achieved when using non-Janus particles. The results emphasize the benefits of bipolar electrochemistry in the frame of electrosynthesis processes.

In situ reconfiguration of plasma-engineered copper electrodes towards efficient electrocatalytic hydrogenation

Defect engineering of Cu via O2-plasma is introduced to accomplish efficient electrocatalytic hydrogenation, in which the in situ reduction of CuOx to defective Cu promotes the kinetics.