Study on carbon dioxide reduction with water over metal oxide photocatalysts

Various metal oxides with 0.1 wt% Ag loaded as a cocatalyst were prepared by an impregnation method and examined their photocatalytic activity for CO₂ reduction with water. Among all the prepared Ag-loaded metal oxides, Ga₂O₃, ZrO₂, Y₂O₃, MgO, and La₂O₃ showed activities for CO and H₂ productions under ultraviolet light irradiation. Thus, metal oxides involving metal cations with closed shell electronic structures such as d⁰, d¹⁰, and s⁰ had the potential for CO₂ reduction with water. In situ Fourier transform infrared measurement revealed that the photocatalytic activity and selectivity for CO production are controlled by the amount and chemical states of CO₂ adsorbed on the catalyst surface and by the surface basicity, as summarized as follows: Ag/ZrO₂ enhanced H₂ production rather than CO production due to very little CO₂ adsorption. Ag/Ga₂O₃ exhibited the highest activity for CO production, because adsorbed monodentate bicarbonate was effectively converted to bidentate formate being the reaction intermediates for CO production owing to its weak surface basicity. Ag/La₂O₃, Ag/Y₂O₃, and Ag/MgO having both weak and strong basic sites adsorbed larger amount of carbonate species including their ions and suppressed H₂ production. However, the adsorbed carbonate species were hardly converted to the bidentate formate.     

Sub-2 nm mixed metal oxide for electrochemical reduction of carbon dioxide to carbon monoxide

Mixed metal oxide(MMO) represents a critical class of materials that can allow for obtaining a dynamic interface between its components:reduced metal and its metal oxide counterpart during an electrocatalytic reaction.Here,a synthetic method utilizing a MOF-derived micro/mesoporous carbon as a template to prepare sub-2 nm MMO catalysts for CO₂ electro reduction is reported.Starting from the zeolite imidazolate framework(ZIF-8),the pyrolyzed derivatives were used to synthesize sub-2 nm...     

Stable dispersions of silver nanoparticles in carbon dioxide with fluorine-free ligands

Iso-stearic acid, a short, stubby compound with branched, methylated tails has been shown to have high solubility in carbon dioxide. Tail solvation by carbon dioxide makes iso-stearic acid a good choice for use as a ligand to sterically stabilize metallic nanoparticles. Iso-stearic acid coated silver nanoparticles have been stably dispersed in carbon dioxide with hexane cosolvent. Neat carbon dioxide has successfully dispersed iso-stearic acid coated silver nanoparticles that had been deposited on either quartz or polystyrene surfaces. These results are the first reports of sterically stabilized nanoparticles in carbon dioxide without the use of any fluorinated compounds.     

Nitrile oxide cycloadditions in supercritical carbon dioxide

The regioselectivity of dipolar cycloadditions of mesitonitrile oxide to various dipolarophiles in supercritical carbon dioxide can be tuned by changes in density, the magnesium bromide-mediated cycloaddition to pent-1-en-3-ol proceeding with higher stereoselectivity than in most conventional solvents.     

Homopolymerization of propylene oxide and copolymerization of propylene oxide and carbon dioxide with metal salts of acetic acid

The homopolymerization of propylene oxide was first conducted at 80°C in the absence of any solvent by using various metal salts of acetic acid and it was found that Mg(OAc)₂, Cr(OAc)₃, Mn(OAc)₂, Co(OAc)₂, Ni(OAc)₂, Zn(OAc)₂, and Sn(OAc)₂ were effective for the polymerization. The copolymerization of propylene oxide and carbon dioxide was next examined by using these effective metal salts of acetic acid as catalysts. Most of these were effective also for the copolymerization. The nature of the polymer obtained was strongly dependent on the catalyst used. Co(OAc)₂ and Zn(OAc)₂ gave an alternate copolymer of propylene oxide and carbon dioxide, Mg(OAc)₂, Cr(OAc)₃, and Ni(OAc)₂ gave a random copolymer, while Sn(OAc)₂ gave a homopolymer of propylene oxide. Then the copolymerization of propylene oxide and carbon dioxide was kinetically investigated in some detail by using Co(OAc)₂ as a catalyst. On the basis of the results obtained, a plausible mechanism was proposed for both the homopolymerization of propylene oxide and copolymerization of propylene oxide and carbon dioxide.     

Nanoparticulate metal complexes prepared with compressed carbon dioxide: correlation of particle morphology with precursor structure

Nanoparticles of metal complexes are rare because of the limited methods for their preparation. Described in this communication is the use of precipitation with compressed antisolvent (PCA) to process nanoparticles of neutral metal-salen complexes. PCA is a semicontinuous technique utilizing supercritical carbon dioxide (scCO2) as the precipitant. The scCO2 dissolves into a solution of the complex and simultaneously extracts the solvent, resulting in formation of nanoparticles of uniform morphology. Spectroscopic and analytical methods were used to support the assignment that the nanoparticles are composed of metal complexes. Scanning electron microscopy reveals that planar complexes, such as [NiIIsalen], afford rodlike particles with average diameter and length of 85 and 700 nm, respectively. Complexes with nonplanar molecular structures produce nanoparticles with varied structures; for instance, PCA of the six-coordinate [Ru(salen)(NO)(Cl)] complex yields spherical nanoparticles with average diameters of 50 nm. These findings suggest a correlation between the structures of molecular precursors and the morphology of the processed particles, which possibly could be used as a bottom-up approach to the preparation of functional nanoparticles.     

Stable colloidal dispersions of a lipase-perfluoropolyether complex in liquid and supercritical carbon dioxide

The technique of hydrophobic ion pairing was used to solubilize the lipase from Candida rugosa in a fluorinated solvent, perfluoromethylcyclohexane (PFMC), in complex with a perfluoropolyether (PFPE) surfactant, KDP 4606. The enzyme-surfactant complex was determined to have a hydrodynamic diameter of 6.5 nm at atmospheric pressure by dynamic light scattering (DLS), indicating that a single lipase molecule is stabilized by surrounding surfactant molecules. The complex formed a highly stable colloidal dispersion in both liquid and supercritical carbon dioxide at high CO2 densities (>0.92 and 0.847 g/mL, respectively), with 4% by volume PFMC as a cosolvent, yielding a fluid that was orange, optically translucent, and very nearly transparent. DLS demonstrated aggregation of the enzyme-surfactant complexes in CO2 at 25 and 40 degrees C and various pressures (2000-5000 psia) with hydrodynamic diameters ranging from 50 to 200 nm. The mechanism by which the enzyme-surfactant particles aggregate was shown to be via condensation due to very low polydispersities as characterized by the size distribution moments. Interparticle interactions were investigated with respect to density and temperature, and it was shown that on decreasing the CO2 density, the particle size increased, and the stability against settling decreased. Particle size also decreased as the temperature was increased to 40 degrees C, at constant CO2 density. Nanoparticle aggregates of an enzyme-surfactant complex in CO2, which are nearly optically transparent and stable to settling, are a promising new alternative to previous types of dispersions of proteins in CO2 that either required water/CO2 microemulsions or were composed of large particles unstable to settling.     

Diffusion coefficients of metal acetylacetonates in supercritical carbon dioxide

Binary diffusion coefficients, D₁₂, of the metal acetylacetonates, palladium(II) acetylacetonate and cobalt(III) acetylacetonate, were measured from 308.2 to 343.2 K over the pressure range from 9 to 40 MPa at infinite dilution in supercritical carbon dioxide using the chromatographic impulse response method. The effects of pressure, temperature, density, and viscosity on D₁₂ values were examined. It was observed that the D₁₂ values of palladium(II) acetylacetonate and cobalt(III) acetylacetonate were larger than those of lipids with similar molecular weights, such as arachidonic acid and monoolein, respectively. Furthermore, the measured D₁₂ data of each metal acetylacetonate were well correlated by the hydrodynamic equation D₁₂/T as a function of carbon dioxide viscosity.     

Photogeneration of carbon dioxide from polypropylene film stabilized by hindered amines

Infrared spectrometric measurements have been used to measure the amount of carbon dioxide generated by UVA irradiation of polypropylene film stabilized with 0.5% of three different hindered amine light stabilizers (HALS). The measurements were made in situ, using a specially constructed glass cell fitted with CaF₂ windows. In each case the amount of photogenerated carbon dioxide was less than that from a HALS-free polypropylene film of similar thickness. The amount of photogenerated CO₂ was greatest in oxygen that had been pre-saturated with water and the amount of CO₂ evolved depended on the grade of HALS. In dry oxygen, although the differences between the films containing different HALS were much reduced the amount of CO₂ continued to be smaller than that from the HALS-free polymer. The sensitivity of CO₂ photogeneration to the presence of HALS provides new evidence of the relevance of the CO₂ photogeneration method to the diagnosis of photosensitivity of polymers and the influence of stabilizing additives.     

The effect of alkaline earth metal oxide on nickel catalyst for carbon dioxide reforming of methane

A study on the effect of alkaline earth metal oxide on the activity and stability of nickel catalyst for carbon dioxide reforming of methane was performed. When CaO, SrO, and BaO were used as supports, there was no difference in catalytic activity between the catalysts made by coprecipitation and impregnation. However, when MgO was used as a support, the catalyst prepared by coprecipitation showed superior activity than that made by impregnation. The higher activity of the catalyst made by coprecipitation was due to the formation of solid solution consisting of nickel and magnesium. The formation of the solid solution enhanced the dispersion of nickel and the resistance to coke formation.     

Reactions of carbon dioxide with metal carbonyl anions

The reactivity of a series of metal carbonyl anions with CO₂ has been found to parallel their relative nucleophilicities. The highly nucleophilic species, C₅H₅Fe(CO)^?₂, reacts readily to give the dimer, (C₅H₅Fe(CO)₂)₂, and carbonate while Co(CO)^?₄ is unreactive. The reaction of ¹³CO₂ with C₅H₅Fe(CO)^?₂ results in the formation of the ¹³CO enriched dimer.     

Carboxylations of alkali metal phenoxides with carbon dioxide

The reaction mechanism of the Kolbe-Schmitt reaction of phenol and 2-naphthol has been investigated. An alkali metal phenoxide-CO2 complex is not an intermediate that can be easily transformed into a carboxylic acid, such as salicylic acid (SA) and p-hydroxybenzoic acid (pHBA). A direct carboxylation of phenoxide with CO2 takes place even at room temperature, and is competitive with the formation of the CO2 complex. The resulting complex decomposes thermally (above ca. 100 degrees C) to phenoxide, which then undergoes further competitive reactions. Experiments using a carbon-13 labeled complex support a mechanism of direct carboxylation, and not the mechanism via a CO2 complex. The reactivity, C-13 NMR and MOPAC/PM3 calculations suggest a new carbonate-like structure for the CO2 complex.     

Retention behavior of metal particle dispersions in aqueous and nonaqueous carriers in thermal field-flow fractionation

Until quite recently, theories on thermophoresis of particles predicted very low thermophoretic velocities of metal particles in liquids. This prediction was based on the understanding that the very high thermal conductivities of metals relative to most liquid media resulted in quite low temperature gradients across the metal particle thereby leading to low net force on the particle. In this paper, we report the retention behavior of submicrometer size metal particles of silver (Ag), gold (Au), palladium (Pd) and platinum (Pt) suspended in both aqueous and organic (specifically, acetonitrile and tetrahydrofuran) carrier liquids in thermal field-flow fractionation (ThFFF). The dependence of the metal particle retention on various factors such as particle composition, amount of added electrolyte, carrier liquid composition, field strength, channel thickness, and carrier flow-rate is evaluated and discussed. A comparison in particle retention behavior among equal-sized metal, latex and silica particles is also provided.     

Hydrogenation of carbon dioxide over iron oxide catalyst

Hydrogenation of carbon dioxide was carried out over reduced Fe₃O₄ catalyst. Effect of reduction temperature of the catalyst, reaction temperature, space velocity, and component of feed gas were examined on the catalyst. From various experiments, the proper conditions for the hydrogenation of carbon dioxide are proposed. In addition, oxidation of the catalyst took place simultaneously during the hydrogenation of CO₂.

---

Fe₃O₄. , , . , . CO₂ .

     

Alternating copolymerization of limonene oxide and carbon dioxide

The alternating copolymerization of (R)- or (S)-limonene oxide and CO2 using beta-diiminate zinc acetate catalysts is reported. At 100 psi CO2 and 25 degrees C, the catalyst exhibits a high selectivity for the trans isomer and produces regioregular polycarbonate. The copolymer contains >99% carbonate linkages, a narrow molecular weight distribution, and an Mn value consistent with the [epoxide]/[Zn] ratio.     

Viscoelastic properties of sterically stabilized nonaqueous dispersions

The viscoelastic properties of a dispersion of poly(vinyl acetate) particles sterically stabilized by poly(2-ethyl hexyl methacrylate) and dispersed in Isopar G have been measured as a function of particle concentration and frequency at ambient temperature.At low particle concentrations, it was found that the loss modulus was larger than the storage modulus, while at high particle concentrations, i.e.,w>0.40, the storage modulus was found to be larger than the loss modulus. This inversion from a viscous to an elastic response as a function of particle concentration is attributable to the change in the configurational entropy of the steric barrier as a result of increasing the concentration of particles in the dispersion.     

The preparation of sterically stabilized polymer dispersions

The emulsion polymerizations of styrene (St) and butyl acrylate (BA) stabilized by nonionic polyoxyethylene type emulsifiers did not show the long stationary rate interval. This was discussed in terms of two opposing effects: 1) the decreased monomer concentration at the reaction loci due to the depletion of monomer droplets or depressed monomer droplet degradation and 2) the increased number of polymer particles with increasing conversion. The continuous particle nucleation is attributed to the continuous release of emulsifier from the emulsifier saturated monomer droplets and/or the presence of monomer swollen micelles (microdroplets). The limited particle flocculation operative at lower emulsifier concentrations increases the nonstationary-state polymerization. The particle agglomeration is accompanied by the increased reaction order x (N_p vs. [E]^x) above 0.6. The increased uniformity of monomer emulsion stabilized by Tween 20 by homogenization of monomer emulsion increased the final conversion and the polymerization rate as well. The polymerization rate vs. conversion curve of the homogenized emulsion characterized with broader stationary rate interval reminds the four rate intervals system typical for miniemulsion. The accumulation of polymer and nonionic emulsifier within the monomer phase preserves the monomer droplets up to high conversion. The decreased monomer droplet degradation rises the monomer-starved condition or the depressed transport of both monomer and emulsifier to the reaction loci.     

Colloidal metal dispersions and metal complexes in hydrocarbons

Stable dispersions of colloidal metals in hydrocarbons have been prepared by a novel phase-transfer method. The metals were gold, silver, palladium and ruthenium; the hydrocarbons were n-hexane, cyclohexane and benzene. The phase transfer of colloidal metal particles from an aqueous phase to a hydrocarbon phase was achieved by adding salt to the emulsion of hydrocarbon in the aqueous suspension of metal with sodium oleate. The salts were sodium chloride, magnesium chloride, sodium sulfate, etc. The size distributions of the metal particles in the resulting hydrocarbon suspensions were almost the same as that of the original aqueous suspension. The dispersions of colloidal metals in hydrocarbons were stable for a long period of time without the addition of hydrocarbon-soluble stabilizer. The critical phase-transfer concentrations of various salts were determined. The phase-transfer powers of cations were larger than those of anions. Those of divalent and trivalent cations were exceedingly larger than that of the monovalent cation. The concentration of colloidal metal dispersed in hydrocarbon was achieved by using the phase-transfer method.     

The role of carbon dioxide in chemoselective hydrogenation of halonitroaromatics over supported noble metal catalysts in supercritical carbon dioxide

Chemoselective hydrogenation of halogenated nitrobenzenes over Pt/C catalysts proceeds effectively in supercritical carbon dioxide (scCO2) to produce halogenated anilines with excellent selectivity; the rate of the hydrogenation of nitro groups is markedly enhanced in scCO2 compared to the neat reaction, and the dehalogenation reaction is significantly suppressed.