Mordehai Halmann

Photoelectrochemical reduction of carbon dioxide to formic acid, formaldehyde and methanol on p-gallium arsenide in an aqueous V(II)-V(III) chloride redox system

Abstract The photoelectrochemical reduction of aqueous carbon dioxide in an acidic aqueous medium of V(II)-V(III) chloride was carried out on a p-type gallium arsenide single-crystal electrode by illumination with visible light, using carbon as the counterelectrode. The photocurrent onset potential was at +0.05 to −0.25 V (vs. SCE). The temperature effect on the current-voltage curves was measured. The point of maximum efficiency of conversion of optical to chemical energy was determined in the range 20–80°C. Assuming 100% faradaic yield for production of formic acid, the efficiencies at 20, 40, 60 and 80°C would be 4.3, 7.5 10.2 and 18.2% respectively. However, in an actual CO 2 photoelectrolysis experiment, carried out in 3.1 M CaCl 2 , 0.87 M HCl and 0.07 M V(II)-chloride, at 60°C with a cell potential of −0.7 V at the p-GaAs electrode (vs. a carbon counterelectrode), current density 8.5 mA cm −2 and a light flux of 58 mW cm −2 , the faradaic yields of formic acid, formaldehyde and methanol were 1.5, 0.3 and 0.14% respectively. The optical to chemical energy conversion efficiency of carbon dioxide reduction derived from these results is 0.2%.

Photoelectrochemical reduction of carbon dioxide in aqueous solutions on p-GaP electrodes: an a.c. impedance study with phase-sensitive detection

Abstract The mechanism of the photoelectrochemical reduction of water and CO2 on p-GaP electrodes in aqueous solutions was studied using photocurrent spectroscopy with chopped light excitation, revealing both a photo-anodic sub-bandgap response and a photo-cathodic current due to the bandgap response. A.c. power curves were recorded for p-GaP in pH 7 phosphate buffer electrolyte using modulated irradiation and a lock-in amplifier. At a light modulation frequency of 150 Hz, CO2 bubbling effected a 15% increase in the photo-cathodic current, relative to that in an Ar flushed solution. This effect disappeared at a light modulation frequency of 1560 Hz. Ruthenium pretreatment of p-GaP resulted in a doubling of both the cathodic photocurrent and the photovoltage. Pretreatment of p-GaP by dipping in a solution of CuSO4 caused a marked enhancement in both the dark current and photocurrent at reverse bias. In preparative reduction of aqueous bicarbonate, using light chopped at 22 Hz and −1.08 V between the p-GaP and a stainless steel counter electrode separated by a cation selective membrane, the production of formic acid was determined. The efficiency of conversion of the incident light energy to the Gibbs energy of formation of the formic acid produced amounted to 0.3%, both with bare and with Cu treated p-GaP.

Photoassisted Carbon Dioxide Reduction and Formation of Two- and Three-Carbon Compounds

We found that aqueous carbon dioxide, in the presence of inorganic minerals with semiconducting properties, underwent photosensitized reduction by ultraviolet and visible light - the main products being formaldehyde and methanol. Effective photoactive materials included naturally occurring minerals such as nontronite, anatase, wolframite, molybdenite, minium, cinnabar and hematite. No appreciable carbon dioxide reduction was observed in the presence of bentonite. The heterogenous photoreduction of carbon dioxide by natural semiconducting minerals could be a precursor of plant photosynthesis. Photosynthetic condensation of dilute aqueous formaldehyde solutions to glyoxal and malonaldehyde was obtained by UV-irradiation in the absence of oxygen. The malonaldehyde concentration reached its maximum after several hours and then declined. The known condensation reactions of malonaldehyde with urea or guanidines to form hydroxy -or amino-pyrimidines may be a potential prebiotic route to pyrimidines.

Formation of dialkoxyphosphinylalkylidene derivatives of galactitol by transacetalation reactions

Abstract Transacetalation reactions of galactitol with 2,2-dialkoxyethylphosphonates gave the expected 1,3:4,6-diacetal as the minor product, but the major product was 1,3:4,5 derivative. With a 3,3-diethoxypropylphosphonate, the 1,3:4,6-diacetal became the major product. The influence of the dialkoxyphosphinyl substituent on the acetal linkage is shown to be stereoelectronic.

Photooxidation of bromide to bromine in Dead Sea water

Abstract The photooxidation of bromide ions to bromine was performed in acidified Dead Sea water under oxygen, using a high-pressure mercury lamp. The reaction was found not to be catalyzed by added powdered TiO2, Fe2O3, or Pb3O4.

The photochemistry of phosphorus compounds. Part 11. Photolysis of dipotassium α-D-glucose 1-phosphate in aqueous solution under argon and oxygen

The stable products of the photolysis of dipotassium α-D-glucose 1-phosphate at 254 nm were determined in either anoxic (under argon) or oxygen-saturated aqueous solutions. The release of orthophosphate was found to be the primary and predominant reaction in both argon and oxygen, with quantum yields of (1.04 ± 0.02)× 10–2 and (3.56 ± 0.01)× 10–2, respectively. Glucose was not observed. Under argon, the most important organic products identified were deoxy-sugars and malondialdehyde. In oxygenated solutions, carbon dioxide and monoxide, glyconate, arabinose, glyoxal, and glycolate were the major products. A tentative mechanism for the primary photochemical step may be the release of orthophosphate from the glucose 1-phosphate dianion, producing a glycosyl radical, which then undergoes further degradation.

Glyoxal and malonaldehyde formation by ultraviolet irradiation of aqueous formaldehyde

Abstract Irradiation of dilute aqueous formaldehyde (5 × 10 −2 –10 −3 M) in the absence of oxygen by ultaviolet light from high- or low-pressure mercury lamps resulted in the formation of glyoxal and of malonaldehyde. The concentration of malonaldehyde reached a maximum after several hours and then declined. This maximal malonaldehyde concentration was proportional to the initial formaldehyde concentration. At initially 0.05 M formaldehyde (pH 9.4 and 36°C) malonaldehyde reached maximally 3.4 × 10 −5 M. In the range of pH 8.0–11.6, the maximal malonaldehyde concentration was reached at pH 9.4. Quantum yields of glyoxal and malonaldehyde after irradiation of 0.01 M formaldehyde (in 0.01 M NaHCO 3 , 27°C, at 254 nm, under argon, for 195 min) were 7 × 10 −3 and 1.5 × 10 −3 , respectively. In the presence of acetone (0.01 M), the chemical and quantum yields of glyoxal were enhanced, while those of malonaldehyde decreased. The known reaction of malonaldehyde with urea to form pyrimidines may be a model of a prebiotic synthesis of pyrimidines.

Reaction of phosphonated acetals. Acid-catalysed hydrolysis of dialkyl 2,2-dialkoxyethylphosphonates

The kinetics of the acid-catalysed hydrolysis of diethyl 2,2-diethoxyethylphosphonate(I) of diethyl 2,2-dimethoxyethylphosphonate(II), and of dimethyl 2,2-dimethoxyethylphosphonate(III) haye been studied in aqueous dioxan solution containing different acid catalysts. Plots of log k1 against H0 had slopes of 0.98; Plots of (log k1+H0) against log aH2O had slopes W=–0.1. The deuterium solvent isotope effect kD/kH for compound(II) in 1M-sulphuric acid at 30° was 2.8. According to the hypotheses of Zucker–Hammett and of Bunnett, the results obtained are in agreement with an A-1 mechanism. The measured second-order rate constants at 25° were: k2 3.7 × 10–4 l mols–1 for compound(I) and 1.7 × 10–4 l mol s–1 for compounds(II) and (III). Low values of the energy (15–17 kcal mol–1) and entropy (–25 to –27 cal mol–1 K–1) of activation were observed. The substituents at the central carbon atom show the expected polar effects, without any measurable steric effects. The observed values of the energy and entropy activation are correlated with the polarity of the phosphoryl group. An effect of intramolecular catalytic influence is introduced. The products of hydrolysis were identified as the corresponding aldehydes; that of compounds(II) and (III) is the novel dimethyl formylmethyl phosphonate.

Reaction of phosphonated acetals. Part 3. Acid-catalysed hydrolysis of diethyl 3,3-diethoxypropylphosphonate

The kinetics of the acid-catalysed hydrolysis of diethyl 3,3-diethoxypropylphosphonate (I) have been studied in aqueous dioxan solutions, at different acid concentrations and temperatures. The reactions follow an A1 type mechanism with the inductive effect of the phosphonated substituent having only a minor influence. The measured second-order rate constant at 30° is 0.032 l mol–1 s–1. Plots of log kobs. against log CH3O+ are not linear. The deuterium solvent isotope effect kD2O/kH2O is 2.32. The energy of activation is 92 kJ mol–1 and the entropy of activation of 21 J mol–1 K–1. General acid catalysis was not observed. An extremely rapid decrease in the rates of acid-catalysed hydrolysis was observed in going from compound (I) to diethyl 2,2-diethoxyethylphosphonate, (II), and to diethyl diethoxymethylphosphonate, (V). Compound (V) does not undergo regular acetal acid-catalysed hydrolysis.

Reaction of phosphonated acetals. Part 2. Synthesis and acid-catalysed hydrolysis of the cyclic acetals 2-diethylphosphonomethyl-1,3-dioxolan, 2-diethylphosphonomethyl-1,3-dioxan, and bis-O-(diethylphosphonoethyl)pentaerythritol

Phosphonated cyclic acetals were prepared by transacetalation of diethyl 2,2-diethoxyethylphosphonate (I) with ethylene glycol, propane-1,3-diol, or pentaerythritol, to form 2-diethylphosphonomethyl-1,3-dioxolan (III), 2-di-ethylphosphonomethyl-1,3-dioxan (IV), and bis(diethylphosphonomethyl)pentaerythritol (V), respectively. The structures of the products were confirmed by 1H and 13C n.m.r. and by mass and i.r. spectrometry. The kinetics of acid-catalysed hydrolysis of the cyclic acetals were measured in water containing 4% dioxan (v/v) as a function of acid concentration and temperature, and with respect to the deuterium solvent isotope effect. The results are compared with those of the open-chain acetals (I) and diethyl 2,2-dimethoxyethylphosphonate (II). The Zucker–Hammett hypothesis gives for compounds (III)–(V) a linear dependence of log kobs on log CH3O+, with slopes between 1.7 and 3.2. The Bunnett W parameters have values of 0.7–2.7, and kD2O/kH2O is ca. 2. The reaction of the dioxan derivative (IV) is five times slower than that of the corresponding dioxolan derivative (III). These criteria are not in accordance with an A1 reaction, as found for acetals (I) and (II), nor do they agree too well with an A2 reaction. In view of these results we tentatively suggest for the cyclic acetals (III) and (IV) an A–SE2 mechanism. In the proposed mechanism the initial protonated substrate is partially stabilised by hydrogen bonding between the acetal oxygen and the phosphoryl oxygen atoms. As a result the oxonium ion is involved in a reversible reaction and hence proton transfer to the oxygen atom is the rate-determining step of the hydrolysis.