A versatile chemical method for the formation of macroporous transition metal alloys from cyanometalate coordination polymers †

A facile two-step synthetic route to macroporous metal and metal alloy frameworks from hydrogel-forming coordination polymers known as cyanogels is described. The polymerization of a chlorometalate and cyanometalate in aqueous solution results in the formation of a cyanogel that will auto-reduce at elevated temperatures to form metal alloys under an inert atmosphere. Cyanogels are versatile precursors to macroporous metals due to the large number of metal alloy systems that can be produced. This synthetic route is advantageous due to the production of uncontaminated final products of refractory metals at low temperatures. Transient reactive liquid sintering is shown to be the physical process through which macroporous metal forms from the cyanogel precursor

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO_2 Fixation

Two major energy-related problems confront the world in the next 50 years. First, increased worldwide competition for gradually depleting fossil fuel reserves (derived from past photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand

High-Efficiency Conversion of CO2 to Oxalate in Water Is Possible Using a Cr-Ga Oxide Electrocatalyst

Electrochemical transformation of CO2 into commodity chemicals such as oxalate is a strategy for profitably remediating high atmospheric CO2 levels. Electrocatalysts for oxalate generation, however, have required prohibitively large applied potentials, forcing the use of nonaqueous electrolytes. Here, a thin film comprised of alloyed Cr and Ga oxides on glassy carbon is shown to electrocatalytically generate oxalate from aqueous CO2 with high Faradaic efficiencies at 690 mV overpotential. Oxalate is produced at a surface anion site via a CO-dependent pathway; the process is highly sensitive to the hydrogen-bonding environment and avoids the commonly invoked CO2•– intermediate. Ultimately, this catalytic system accomplishes efficient CO2 to oxalate conversion in protic electrolyte

Enhanced Carbon Dioxide Reduction Activity on Indium-Based Nanoparticles

Nanoparticles of indium, indium hydroxide, and indium oxide were synthesized and evaluated for their electrocatalytic abilities in the reduction of CO2 to formate. These nanoparticles were characterized with several microscopic and spectroscopic techniques in order to investigate their structure and surface features. Their electrochemical behavior was also probed through voltammetry and bulk electrolysis. faradaic efficiencies approaching 100% were achieved on these particles at potentials as positive as −1.3 V vs. Ag/AgCl, which represents a significant decrease in the overpotential compared to that observed using bulk indium electrodes. The nanostructuring of the particles and the partial surface oxidation of the indium nanoparticles to yield catalytic surface indium hydroxide species are implicated in the high efficiencies at low overpotentials

Mechanistic insights into C2 and C3 product generation using Ni3Al and Ni3Ga electrocatalysts for CO2 reduction

Thin films of Ni3Al and Ni3Ga on carbon solid supports have been shown to generate multi-carbon products in electrochemical CO2 reduction, an activity profile that, until recently, was ascribed exclusively to Cu-based catalysts. This catalytic behavior has introduced questions regarding the role of each metal, as well as other system components, during CO2 reduction. Here, the significance of electrode structure and solid support choice in determining higher- versus lower-order reduction products is explored, and the commonly invoked Fischer–Tropsch-type mechanism of CO2 reduction to multi-carbon products is indirectly probed. Electrochemical studies of both intermetallic and non-mixed Ni–Group 13 catalyst films suggest that intermetallic character is required to achieve C2 and C3 products irrespective of carbon support choice, negating the possibility of separate metal sites performing distinct yet complementary roles in CO2 reduction. Furthermore, Ni3Al and Ni3Ga were shown to be incapable of generating higher-order reduction products in D2O, suggesting a departure from accepted mechanisms for CO2 reduction on Cu. Additional routes to multi-carbon products may therefore be accessible when developing intermetallic catalysts for CO2 electroreduction

Hydrogen Bonded Pyridine Dimer: A Possible Intermediate in the Electrocatalytic Reduction of Carbon Dioxide to Methanol

Previously, electrogenerated pyridinyl was implicated as a catalyst for the reduction of CO2 to methanol. However, recent quantum mechanical calculations of both the homogeneous redox potential for the pyridinium/pyridinyl redox couple (900 mV more negative than experimentally reported) and the pKa of the reduced pyridinyl species (~27) have led to the proposal that the homogeneous reduction of pyridinium does not play a role in the observed catalytic reduction of CO2 to methanol. In contrast, a more complete consideration of the reaction including the realization that pyridinium reduction is tightly coupled to H2 evolution, produces a calculated redox potential in agreement with the experimental findings. In reexamining this system, it is found that aqueous solutions containing a near equimolar mixture of pyridine and pyridinium (i.e., solution pH near the pyridinium pKa = 5.2) contain a substantial concentration of a hydrogen-bonded dimer formed by the generation of a N-H•••N bond containing one strong NH bond and one elongated NH bond. This species has been identified by X-ray diffraction of crystals grown in aqueous media from pyridine/pyridinium mixtures, and can be observed directly in solution using Raman spectroscopy. DFT (density functional theory) calculations indicate that the pKa for this species is ~22, a value that is consistent with a proton exchange capability. This suggests that this hydrogen bonded dimer may be the pre-electrocatalyst for the observed activation of CO2

Mechanisms of charge transfer at the chemically derivatized interface: the Ni/[NiII(CN)FeII/III(CN)5]2-/1- system as an electrocatalyst

The [Ni(NC)Fe(CN)5]2-/1- derivatized nickel electrode represents an electrocatalytic surface for a variety of oxidations and reductions. This surface exhibits a unique dependence of redox potential on supporting electrolyte cation, which allows for a direct analysis of the effect of surface redox potential on the electrocatalytic rate constant. Two electrocatalytic systems have been evaluated with respect to surface redox potential: the one-electron reduction of Fe3+(aq) and the two-electron (two-proton) oxidation of ascorbic acid. Mediated charge transfer is found to be an operational electron-transfer mechanism in both cases, with the bimolecular surface species to solution species charge transfer being rate limiting. In the case of Fe3+(aq) reduction Marcus theory is found to yield a good description of the relationship between surface redox potential and the electrocatalytic rate constant. Pseudo-first-order rate constants as large as 0.15 cm/s (in LiNO3 supporting electrolyte) have been observed for this reaction. The ascorbic acid oxidation rate constant is found to be ∼10-3 cm/s. This rate constant is independent of surface redox potential, suggesting that the transfer of the second electron is rate limiting

Diffuse Reflectance Spectro: Electrochemistry As a Probe of the Chemically Derivatized Electrode Interface. the Derivatized Nickel Electrode

Diffuse reflectance spectroelectrochemistry has been employed to directly monitor the interface charge-transfer (CT) behavior of surface-bound [NiII(NC)FeII/III(CN)5]2-/- on a nickel electrode. The technique is shown to be species specific and sensitive to the amount of surface-confined material and the oxidation state of the surface-attached species. It is therefore of utility in observing the time-dependent behavior of the surface species under transient potential conditions. This technique is compared with chronocoulometry carried out on the same system. The two techniques are used to obtain values of apparent diffusion coefficients for the derivatized surface. In the short-time limit both techniques are shown to follow the Cottrell equation. However, it is necessary to incorporate time-dependent diffusion coefficients to obtain agreement for long-time data. The reflectance technique is shown to be superior to chronocoulometry in that it can discriminate against current not associated with the surface species of interest