Modified Cu electrodes were prepared by annealing Cu foil in air and electrochemically reducing the resulting Cu2O layers. The CO2 reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu2O layer. Thin Cu2O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu2O layers formed at 500 °C that were ≥ ∼3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycrystalline Cu to reduce CO2 at a higher rate than H2O. The combination of these features resulted in CO2 reduction geometric current densities >1 mA/cm2 at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Moreover, the activity of the modified electrodes was stable over the course of several hours, whereas a polycrystalline Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that determine the distribution between CO2 and H2O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.

Competing pathways in catalytic reactions often involve transition states with very different charge distributions, but this difference is rarely exploited to control selectivity. The proximity of a counterion to a charged catalyst in an ion paired complex gives rise to strong electrostatic interactions that could be used to energetically differentiate transition states. Here we investigate the effects of ion pairing on the regioselectivity of the hydroarylation of 3-substituted phenyl propargyl ethers catalyzed by cationic Au(I) complexes, which forms a mixture of 5- and 7-substituted 2H-chromenes. We show that changing the solvent dielectric to enforce ion pairing to a SbF6– counterion changes the regioselectivity by up to a factor of 12 depending on the substrate structure. Density functional theory (DFT) is used to calculate the energy difference between the putative product-determining isomeric transition states (ΔΔE‡) in both the presence and absence of the counterion. The change in ΔΔE‡ upon switching from the unpaired transition states in high solvent dielectric to ion paired transition states in low solvent dielectric (Δ(ΔΔE‡)) was found to be in good agreement with the experimentally observed selectivity changes across several substrates. Our calculations indicate that the origin of Δ(ΔΔE‡) lies in the preferential electrostatic stabilization of the transition state with greater charge separation by the counterion in the ion paired case. By performing calculations at multiple different values of the solvent dielectric, we show that the role of the solvent in changing selectivity is not solely to enforce ion pairing, but rather that interactions between the ion paired complex and the solvent also contribute to Δ(ΔΔE‡). Our results provide a foundation for exploiting electrostatic control of selectivity in other ion paired systems.

Furan-2,5-dicarboxylic acid (FDCA) is a biomass-derived diacid that can be used to make polymers including polyethylene furandicarboxylate (PEF), a highly attractive substitute for petroleum-derived polyethylene terephthalate (PET). Current FDCA syntheses require edible fructose as the feedstock, entail a difficult oxidation step that generates undesirable aldehyde impurities, and have moderate yields. As an alternative, carbonate-promoted C–H carboxylation enables the synthesis of FDCA from 2-furoic acid and CO2. This route is potentially advantageous because 2-furoic acid is made from furfural, a feedstock produced commercially from inedible lignocellulosic biomass, and it obviates late-stage oxidation. In the carboxylation reaction, salt mixtures composed of alkali furan-2-carboxylate (furoate) and alkali carbonate (M2CO3) are heated under CO2 in the absence of solvent or catalysts to form furan-2,5-dicarboxylate (FDCA2−), which is subsequently protonated to produce FDCA. Previously, high yields were achieved on small-scale reactions using caesium furoate and Cs2CO3. In this work, we investigate the carboxylation reaction using alkali furoate/M2CO3 salts containing cation blends and describe reaction conditions that provide high yields on a preparative scale. We show that the carboxylation proceeds efficiently with K+/Cs+ blends that have a high K+ content (up to 4 : 1 K+ : Cs+). Removing H2O, which is a by-product of the reaction, is important for suppressing decomposition pathways. The accumulation of the FDCA2− product inhibits the reaction. Integrating these lessons, we demonstrate the carboxylation of furoate on a 1 mol scale using a fixed-bed flow reactor with 89% isolated yield of pure FDCA upon protonation.

The local environment at polarized solid–liquid interfaces provides a unique medium for chemical reactions that could be exploited to control the selectivity of non-faradaic reactions. Polarized interfaces are commonly prepared by applying a voltage to an electrode in an electrolyte solution, but it is challenging to achieve high surface charge densities while suppressing faradaic reactions. Ferroelectric materials have permanent surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid–liquid interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of a Rh porphyrin-catalyzed carbene rearrangement. The addition of ferroelectric BaTiO3 nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar magnitude as performing the reaction at an electrode–electrolyte interface polarized by a voltage. The results demonstrate that colloidal suspensions of BaTiO3 nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

Copper catalyzes the electrochemical reduction of CO to valuable C2+ products including ethanol, acetate, propanol, and ethylene. These reactions could be very useful for converting renewable energy into fuels and chemicals, but conventional Cu electrodes are energetically inefficient and have poor selectivity for CO vs H2O reduction. Efforts to design improved catalysts have been impeded by the lack of experimentally validated, quantitative structure–activity relationships. Here we show that CO reduction activity is directly correlated to the density of grain boundaries (GBs) in Cu nanoparticles (NPs). We prepared electrodes of Cu NPs on carbon nanotubes (Cu/CNT) with different average GB densities quantified by transmission electron microscopy. At potentials ranging from −0.3 V to −0.5 V vs the reversible hydrogen electrode, the specific activity for CO reduction to ethanol and acetate was linearly proportional to the fraction of NP surfaces comprised of GB surface terminations. Our results provide a design principle for CO reduction to ethanol and acetate on Cu. GB-rich Cu/CNT electrodes are the first NP catalysts with significant CO reduction activity at moderate overpotential, reaching a mass activity of up to ∼1.5 A per gram of Cu and a Faradaic efficiency >70% at −0.3 V.

Electrochemical reduction of CO2 to formate (HCO2–) powered by renewable electricity is a possible carbon-negative alternative to synthesizing formate from fossil fuels. This process is energetically inefficient because >1 V of overpotential is required for CO2 reduction to HCO2– on the metals currently used as cathodic catalysts. Pd reduces CO2 to HCO2– with no overpotential, but this activity has previously been limited to low synthesis rates and plagued by an unidentified deactivation pathway. Here we show that Pd nanoparticles dispersed on a carbon support reach high mass activities (50–80 mA HCO2– synthesis per mg Pd) when driven by less than 200 mV of overpotential in aqueous bicarbonate solutions. Electrokinetic measurements are consistent with a mechanism in which the rate-determining step is the addition of electrochemically generated surface adsorbed hydrogen to CO2 (i.e., electrohydrogenation). The electrodes deactivate over the course of several hours because of a minor pathway that forms CO. Activity is recovered, however, by removing CO with brief air exposure.

Uncovering new structure–activity relationships for metal nanoparticle (NP) electrocatalysts is crucial for advancing many energy conversion technologies. Grain boundaries (GBs) could be used to stabilize unique active surfaces, but a quantitative correlation between GBs and catalytic activity has not been established. Here we use vapor deposition to prepare Au NPs on carbon nanotubes (Au/CNT). As deposited, the Au NPs have a relatively high density of GBs that are readily imaged by transmission electron microscopy (TEM); thermal annealing lowers the density in a controlled manner. We show that the surface-area-normalized activity for CO2 reduction is linearly correlated with GB surface density on Au/CNT, demonstrating that GB engineering is a powerful approach to improving the catalytic activity of metal NPs.

CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable surface features that bind CO strongly. OD-Cu electrodes prepared by H2 reduction of Cu2O precursors reduce CO to acetate and ethanol with nearly 50% Faradaic efficiency at moderate overpotential. Temperature-programmed desorption of CO on OD-Cu revealed the presence of surface sites with strong CO binding that are distinct from the terraces and stepped sites found on polycrystalline Cu foil. After annealing at 350 °C, the surface-area corrected current density for CO reduction is 44-fold lower and the Faradaic efficiency is less than 5%. These changes are accompanied by a reduction in the proportion of strong CO binding sites. We propose that the active sites for CO reduction on OD-Cu surfaces are strong CO binding sites that are supported by grain boundaries. Uncovering these sites is a first step toward understanding the surface chemistry necessary for efficient CO electroreduction.

Nanocrystalline Pb films prepared by reducing PbO2 precursors have up to 700-fold lower H+ reduction activity than polycrystalline Pb foil electrodes but maintain the ability to reduce CO2. As a result, these “oxide-derived” Pb (OD–Pb) electrodes have higher Faradaic efficiency for CO2 reduction to HCO2– in aqueous solutions with almost no competitive H+ reduction. Even with very low CO2 concentrations in N2-saturated NaHCO3 solution, OD–Pb converts CO2 derived from HCO3– decomposition to HCO2– with almost quantitative Faradaic efficiency while Pb foil has less than 10% efficiency. Electrokinetic data suggest that the difference in selectivity between the two electrodes is caused by a difference in the coverage of a surface layer—likely a metastable Pb oxide—that is passivating for H+ reduction but active for CO2 reduction.Keywords: CO2; electroreduction; energy; formate and Pb; fuel

Correction for ‘Electrostatic control of regioselectivity via ion pairing in a Au(I)-catalyzed rearrangement’ by Vivian M. Lau et al., Chem. Sci., 2014, 5, 4975–4979.

The rearrangement of 3-substituted aryl alkynyl sulfoxides catalyzed by cationic Au(I) complexes was studied with different counterions in solvents spanning a range of dielectric constants (ε). Pulsed-gradient diffusion NMR experiments demonstrated strong ion pairing in low-ε solvents. The regioselectivity of the reaction was insensitive to ε when ion pairing was weak but increased monotonically as ε was decreased in the regime of strong ion pairing. DFT calculations of putative product-determining transition states indicated that the product resulting from the more polar transition state is favored due to electrostatic stabilization in the presence of strong ion pairing.

Gold films produced from gold oxide precursors (“oxide-derived Au”) were compared to polyhedral Au nanoparticles for electrocatalytic alkaline O2 reduction. Despite having no detectable abundance of (100) facets, oxide-derived Au exhibited 4e− selectivity and surface-area-normalized activity that rivaled cubic Au nanoparticles with high (100) abundance. The activity of oxide-derived Au likely arises from active sites at the surface terminations of defects that are trapped during gold oxide reduction.

An intramolecular reaction catalyzed by Rh porphyrins was studied in the presence of interfacial electric fields. 1-Diazo-3,3-dimethyl-5-phenylhex-5-en-2-one (2) reacts with Rh porphyrins via a putative carbenoid intermediate to form cyclopropanation product 3,3-dimethyl-5-phenylbicyclo[3.1.0]hexan-2-one (3) and insertion product 3,3-dimethyl-2,3-dihydro-[1,1′-biphenyl]-4(1H)-one (4). To study this reaction in the presence of an interfacial electric field, Si electrodes coated with thin films of insulating dielectric layers were used as the opposing walls of a reaction vessel, and Rh porphyrin catalysts were localized to the dielectric–electrolyte interface. The charge density was varied at the interface by changing the voltage across the two electrodes. The product ratio was analyzed as a function of the applied voltage and the surface chemistry of the dielectric layer. In the absence of an applied voltage, the ratio of 3:4 was approximately 10:1. With a TiO2 surface, application of a voltage induced a Rh porphyrin–TiO2 interaction that resulted in an increase in the 3:4 ratio to a maximum in which 4 was nearly completely suppressed (>100:1). With an Al2O3 surface or an alkylphosphonate-coated surface, the voltage caused a decrease in the 3:4 ratio, with a maximum effect of lowering the ratio to 1:2. The voltage-induced decrease in the 3:4 ratio in the absence of TiO2 was consistent with a field–dipole effect that changed the difference in activation energies for the product-determining step to favor product 4. Effects were observed for porphyrin catalysts localized to the electrode–electrolyte interface either through covalent attachment or surface adsorption, enabling the selectivity to be controlled with unfunctionalized Rh porphyrins. The magnitude of the selectivity change was limited by the maximum interfacial charge density that could be attained before dielectric breakdown.

Carbon dioxide reduction is an essential component of many prospective technologies for the renewable synthesis of carbon-containing fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. We show that the reduction of thick Au oxide films results in the formation of Au nanoparticles (“oxide-derived Au”) that exhibit highly selective CO2 reduction to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycrystalline Au electrodes and several other nanostructured Au electrodes prepared via alternative methods require at least 200 mV of additional overpotential to attain comparable CO2 reduction activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•– intermediate on the surfaces of the oxide-derived Au electrodes.

The importance of tin oxide (SnOx) to the efficiency of CO2 reduction on Sn was evaluated by comparing the activity of Sn electrodes that had been subjected to different pre-electrolysis treatments. In aqueous NaHCO3 solution saturated with CO2, a Sn electrode with a native SnOx layer exhibited potential-dependent CO2 reduction activity consistent with previously reported activity. In contrast, an electrode etched to expose fresh Sn0 surface exhibited higher overall current densities but almost exclusive H2 evolution over the entire 0.5 V range of potentials examined. Subsequently, a thin-film catalyst was prepared by simultaneous electrodeposition of Sn0 and SnOx on a Ti electrode. This catalyst exhibited up to 8-fold higher partial current density and 4-fold higher faradaic efficiency for CO2 reduction than a Sn electrode with a native SnOx layer. Our results implicate the participation of SnOx in the CO2 reduction pathway on Sn electrodes and suggest that metal/metal oxide composite materials are promising catalysts for sustainable fuel synthesis.

The rearrangement of cis-stilbene oxide catalyzed by Al2O3 was studied in the presence of interfacial electric fields. Thin films of Al2O3 deposited on Si electrodes were used as the opposing walls of a reaction vessel. Application of a voltage across the electrodes engendered electrochemical double layer formation at the Al2O3–solution interface. The aldehyde to ketone product ratio of the rearrangement was increased by up to a factor of 63 as the magnitude of the double layer charge density was increased. The results support a field–dipole effect on the selectivity of the catalytic reaction.

Modified Cu electrodes were prepared by annealing Cu foil in air and electrochemically reducing the resulting Cu2O layers. The CO2 reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu2O layer. Thin Cu2O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu2O layers formed at 500 °C that were ≥ ∼3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycrystalline Cu to reduce CO2 at a higher rate than H2O. The combination of these features resulted in CO2 reduction geometric current densities >1 mA/cm2 at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Moreover, the activity of the modified electrodes was stable over the course of several hours, whereas a polycrystalline Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that determine the distribution between CO2 and H2O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.

The local environment at polarized solid–liquid interfaces provides a unique medium for chemical reactions that could be exploited to control the selectivity of non-faradaic reactions. Polarized interfaces are commonly prepared by applying a voltage to an electrode in an electrolyte solution, but it is challenging to achieve high surface charge densities while suppressing faradaic reactions. Ferroelectric materials have permanent surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid–liquid interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of a Rh porphyrin-catalyzed carbene rearrangement. The addition of ferroelectric BaTiO3 nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar magnitude as performing the reaction at an electrode–electrolyte interface polarized by a voltage. The results demonstrate that colloidal suspensions of BaTiO3 nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

Gold films produced from gold oxide precursors (“oxide-derived Au”) were compared to polyhedral Au nanoparticles for electrocatalytic alkaline O2 reduction. Despite having no detectable abundance of (100) facets, oxide-derived Au exhibited 4e− selectivity and surface-area-normalized activity that rivaled cubic Au nanoparticles with high (100) abundance. The activity of oxide-derived Au likely arises from active sites at the surface terminations of defects that are trapped during gold oxide reduction.

The rearrangement of 3-substituted aryl alkynyl sulfoxides catalyzed by cationic Au(I) complexes was studied with different counterions in solvents spanning a range of dielectric constants (ε). Pulsed-gradient diffusion NMR experiments demonstrated strong ion pairing in low-ε solvents. The regioselectivity of the reaction was insensitive to ε when ion pairing was weak but increased monotonically as ε was decreased in the regime of strong ion pairing. DFT calculations of putative product-determining transition states indicated that the product resulting from the more polar transition state is favored due to electrostatic stabilization in the presence of strong ion pairing.

Correction for ‘Electrostatic control of regioselectivity via ion pairing in a Au(I)-catalyzed rearrangement’ by Vivian M. Lau et al., Chem. Sci., 2014, 5, 4975–4979.