Tin Oxide Dependence of the CO₂ Reduction Efficiency on Tin Electrodes and Enhanced Activity for Tin/Tin Oxide Thin-Film Catalysts

The importance of tin oxide (SnO_<i>x</i>) 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 SnO_<i>x</i> 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 SnO_<i>x</i> 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 SnO_<i>x</i> layer. Our results implicate the participation of SnO_<i>x</i> in the CO₂ reduction pathway on Sn electrodes and suggest that metal/metal oxide composite materials are promising catalysts for sustainable fuel synthesis

Carbonate-Promoted Hydrogenation of Carbon Dioxide to Multicarbon Carboxylates

CO₂ hydrogenation is a potential alternative to conventional petrochemical methods for making commodity chemicals and fuels. Research in this area has focused mostly on transition-metal-based catalysts. Here we show that hydrated alkali carbonates promote CO₂ hydrogenation to formate, oxalate, and other C₂₊ carboxylates at elevated temperature and pressure in the absence of transition-metal catalysts or solvent. The reactions proceed rapidly, reaching up to 56% yield (with respect to CO₃^2–) within minutes. Isotope labeling experiments indicate facile H₂ and C–H deprotonations in the alkali cation-rich reaction media and identify probable intermediates for the C–C bond formations leading to the various C₂₊ products. The carboxylate salts are in equilibrium with volatile carboxylic acids under CO₂ hydrogenation conditions, which may enable catalytic carboxylic acid syntheses. Our results provide a foundation for base-promoted and base-catalyzed CO₂ hydrogenation processes that could complement existing approaches

CO₂ Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu₂O Films

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

Controlling H⁺ vs CO₂ Reduction Selectivity on Pb Electrodes

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

Aqueous CO₂ Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles

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

An Electric Field–Induced Change in the Selectivity of a Metal Oxide–Catalyzed Epoxide Rearrangement

The rearrangement of <i>cis</i>-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

A Direct Grain-Boundary-Activity Correlation for CO Electroreduction on Cu Nanoparticles

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

Grain-Boundary-Dependent CO₂ Electroreduction Activity

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

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure–function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125–325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement

Electrostatic Control of Regioselectivity in Au(I)-Catalyzed Hydroarylation

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 2<i>H</i>-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 (ΔΔ<i>E</i>^‡) in both the presence and absence of the counterion. The change in ΔΔ<i>E</i>^‡ upon switching from the unpaired transition states in high solvent dielectric to ion paired transition states in low solvent dielectric (Δ­(ΔΔ<i>E</i>^‡)) was found to be in good agreement with the experimentally observed selectivity changes across several substrates. Our calculations indicate that the origin of Δ­(ΔΔ<i>E</i>^‡) 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(ΔΔ<i>E</i>^‡). Our results provide a foundation for exploiting electrostatic control of selectivity in other ion paired systems