Matthew W. Kanan

In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+.

The utilization of solar energy on a large scale requires its storage. In natural photosynthesis, energy from sunlight is used to rearrange the bonds of water to oxygen and hydrogen equivalents. The realization of artificial systems that perform “water splitting” requires catalysts that produce oxygen from water without the need for excessive driving potentials. Here we report such a catalyst that forms upon the oxidative polarization of an inert indium tin oxide electrode in phosphate-buffered water containing cobalt (II) ions. A variety of analytical techniques indicates the presence of phosphate in an approximate 1:2 ratio with cobalt in this material. The pH dependence of the catalytic activity also implicates the hydrogen phosphate ion as the proton acceptor in the oxygen-producing reaction. This catalyst not only forms in situ from earth-abundant materials but also operates in neutral water under ambient conditions.

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 Cu(2)O layers. The CO(2) reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu(2)O layer. Thin Cu(2)O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu(2)O 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 CO(2) at a higher rate than H(2)O. The combination of these features resulted in CO(2) reduction geometric current densities >1 mA/cm(2) 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 CO(2) and H(2)O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.

Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH

The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co(2+) solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and (18)O isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 × RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co(III)-OH and Co(IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.

Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper

The electrochemical conversion of CO2 and H2O into liquid fuel is ideal for high-density renewable energy storage and could provide an incentive for CO2 capture. However, efficient electrocatalysts for reducing CO2 and its derivatives into a desirable fuel are not available at present. Although many catalysts can reduce CO2 to carbon monoxide (CO), liquid fuel synthesis requires that CO is reduced further, using H2O as a H+ source. Copper (Cu) is the only known material with an appreciable CO electroreduction activity, but in bulk form its efficiency and selectivity for liquid fuel are far too low for practical use. In particular, H2O reduction to H2 outcompetes CO reduction on Cu electrodes unless extreme overpotentials are applied, at which point gaseous hydrocarbons are the major CO reduction products. Here we show that nanocrystalline Cu prepared from Cu2O (‘oxide-derived Cu’) produces multi-carbon oxygenates (ethanol, acetate and n-propanol) with up to 57% Faraday efficiency at modest potentials (–0.25 volts to –0.5 volts versus the reversible hydrogen electrode) in CO-saturated alkaline H2O. By comparison, when prepared by traditional vapour condensation, Cu nanoparticles with an average crystallite size similar to that of oxide-derived copper produce nearly exclusive H2 (96% Faraday efficiency) under identical conditions. Our results demonstrate the ability to change the intrinsic catalytic properties of Cu for this notoriously difficult reaction by growing interconnected nanocrystallites from the constrained environment of an oxide lattice. The selectivity for oxygenates, with ethanol as the major product, demonstrates the feasibility of a two-step conversion of CO2 to liquid fuel that could be powered by renewable electricity.

Structure and valency of a cobalt-phosphate water oxidation catalyst determined by in situ X-ray spectroscopy.

A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co(2+) (Co-Pi) has been studied by in situ X-ray absorption spectroscopy. Spectra were obtained for Co-Pi films of two different thicknesses at an applied potential supporting water oxidation catalysis and at open circuit. Extended X-ray absorption fine structure (EXAFS) spectra indicate the presence of bis-oxo/hydroxo-bridged Co subunits incorporated into higher nuclearity clusters in Co-Pi. The average cluster nuclearity is greater in a relatively thick film (∼40-50 nmol Co ions/cm(2)) deposited at 1.25 V vs NHE than in an extremely thin film (∼3 nmol Co ions/cm(2)) deposited at 1.1 V. X-ray absorption near edge structure (XANES) spectra and electrochemical data support a Co valency greater than 3 for both Co-Pi samples when catalyzing water oxidation at 1.25 V. Upon switching to open circuit, Co-Pi undergoes a continuous reduction due to residual water oxidation catalysis, as indicated by the negative shift of the edge energy. The rate of reduction depends on the average cluster size. On the basis of structural parameters extracted from fits to the EXAFS data of Co-Pi with two different thicknesses and comparisons with EXAFS spectra of Co oxide compounds, a model is proposed wherein the Co oxo/hydroxo clusters of Co-Pi are composed of edge-sharing CoO(6) octahedra, the structural motif found in cobaltates. Whereas cobaltates contain extended planes of CoO(6) octahedra, the Co-Pi clusters are of molecular dimensions.

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(x)) to the efficiency of CO(2) reduction on Sn was evaluated by comparing the activity of Sn electrodes that had been subjected to different pre-electrolysis treatments. In aqueous NaHCO(3) solution saturated with CO(2), a Sn electrode with a native SnO(x) layer exhibited potential-dependent CO(2) reduction activity consistent with previously reported activity. In contrast, an electrode etched to expose fresh Sn(0) surface exhibited higher overall current densities but almost exclusive H(2) evolution over the entire 0.5 V range of potentials examined. Subsequently, a thin-film catalyst was prepared by simultaneous electrodeposition of Sn(0) and SnO(x) on a Ti electrode. This catalyst exhibited up to 8-fold higher partial current density and 4-fold higher faradaic efficiency for CO(2) reduction than a Sn electrode with a native SnO(x) layer. Our results implicate the participation of SnO(x) in the CO(2) reduction pathway on Sn electrodes and suggest that metal/metal oxide composite materials are promising catalysts for sustainable fuel synthesis.

Reaction discovery enabled by DNA-templated synthesis and in vitro selection

Current approaches to reaction discovery focus on one particular transformation. Typically, researchers choose substrates based on their predicted ability to serve as precursors for the target structure, then evaluate reaction conditions for their ability to effect product formation. This approach is ideal for addressing specific reactivity problems, but its focused nature might leave many areas of chemical reactivity unexplored. Here we report a reaction discovery approach that uses DNA-templated organic synthesis and in vitro selection to simultaneously evaluate many combinations of different substrates for bond-forming reactions in a single solution. Watson–Crick base pairing controls the effective molarities of substrates tethered to DNA strands; bond-forming substrate combinations are then revealed using in vitro selection for bond formation, PCR amplification and DNA microarray analysis. Using this approach, we discovered an efficient and mild carbon–carbon bond-forming reaction that generates an enone from an alkyne and alkene using an inorganic palladium catalyst. Although this approach is restricted to conditions and catalysts that are at least partially compatible with DNA, we expect that its versatility and efficiency will enable the discovery of additional reactions between a wide range of substrates.

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.

Pd-Catalyzed Electrohydrogenation of Carbon Dioxide to Formate: High Mass Activity at Low Overpotential and Identification of the Deactivation Pathway

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.

Expanding the Reaction Scope of DNA-Templated Synthesis

The translation of amplifiable information into chemical structure is a key component of nature×s approach to generating functional molecules. The ribosome accomplishes this feat by catalyzing the translation of RNA sequences into proteins. Developing general methods to translate amplifiable information carriers into synthetic molecules may enable chemists to evolve non-natural molecules in a manner analogous to the cycles of translation, selection, amplification, and diversification currently used by nature to evolve proteins. As an initial step towards this goal, we recently examined the generality of DNA-templated synthetic chemistry.[1, 2] We demonstrated the ability of DNA-templated synthesis to direct a modest collection of chemical reactions without requiring the precise alignment of reactive groups into DNA-like conformations. Indeed, the distance independence and sequence fidelity of DNA-templated synthesis allowed the simultaneous, one-pot translation of a model library of more than 1000 templates into the corresponding thioether products, one of which was enriched by in vitro selection for binding to the protein streptavidin and amplified by the polymerase chain reaction (PCR). The range of reactions known to be supported by DNAtemplated synthesis,[2] however, remains a tiny fraction of those used either by synthetic chemists or by nature to generate molecules with desired properties. Many reactions central to the construction of natural or synthetic molecules have yet to be developed in a DNA-templated format despite their known compatibility with water.[3] We describe here the development of several useful DNA-templated reactions, including the first reported DNA-templated organometallic couplings and carbon ± carbon bond forming reactions other than pyrimidine photodimerization.[4, 5] Collectively, these reactions represent an important additional step towards the in vitro evolution of non-natural synthetic molecules by enabling the DNA-templated construction of a much more diverse set of structures than has been previously achieved. We first investigated the ability of DNA-templated synthesis to direct reactions that require a non-DNA-linked activator, catalyst, or other reagent in addition to the principal reactants. To test the ability of DNA-templated synthesis to mediate such reactions without requiring structural mimicry of the DNA backbone, we performed DNA-templated reductive aminations between amine-linked template 1 and benzaldehydeor glyoxal-linked reagents (2 and 3) with millimolar concentrations of NaBH3CN at room temperature in aqueous solutions. Products formed efficiently when the template and reagent sequences were complementary. In contrast, control reactions in which the sequence of the reagent did not complement that of the template, or in which NaBH3CN was omitted, yielded no significant product (Table 1 and Figure 1). While DNA-templated reductive aminations to generate products closely mimicking the