María A. Grela

Kinetics and mechanism of the thermal decomposition of unsaturated aldehydes: benzaldehyde, 2-butenal, and 2-furaldehyde

The thermal unimolecular decomposition of benzaldehyde (BA), crotonaldehyde (CA), and furfural (FA) have been investigated in a flow reactor at very low pressures by modulated beam mass spectrometry above 1040 K. Each reaction proceeds by a different mechanism. Whereas BA decomposes by C(O)-H bond fission CA readily undergoes decarbonylation to propene via a three-center transition-state reaction. FA decomposition into vinylketene and CO involves ring opening followed by H-atom transfer in the resulting biradical. Overall high-pressure Arrhenius parameters for the three reactions are derived from kinetic data. 26 references, 4 figures, 1 table.

Photoreduction of Cr(VI) using hydroxoaluminiumtricarboxymonoamide phthalocyanine adsorbed on TiO2

Hydroxoaluminiumtricarboxymonoamide phthalocyanine (AlTCPc) adsorbed at different loadings on TiO(2) Degussa P-25 was tested for Cr(vi) photocatalytic reduction under visible irradiation in the presence of 4-chlorophenol (4-CP) as sacrificial donor. A rapid reaction takes place in spite of the presumable aggregation of the dye on the TiO(2) surface. The removal of Cr(vi) is fairly negligible under visible-light irradiation, either without photocatalyst or in the presence of bare TiO(2). The fast capture of conduction band electrons by Cr(vi), which forms a surface complex with TiO(2), inhibits the formation of reactive oxygen species in the reductive pathway. This fact and the easier oxidation of 4-CP as compared to AlTCPc hinder the photobleaching of the dye and make feasible Cr(vi) reduction under visible irradiation. The consumption of Cr(vi) follows a pseudo-first order kinetics; the decay constant depends, in the studied range, on the photocatalyst mass, but it is barely affected by dye loading. The presence of 4-CP is essential, but its concentration has no effect on the Cr(vi) decay rate. Oxidation products of 4-CP, such as hydroquinone, catechol or benzoquinone, are not observed. Direct evidence of the one-electron reduction of Cr(vi) to Cr(v) was obtained by EPR spectroscopy using citric acid as Cr(v) trapping agent. In this case, disappearance of Cr(v) also follows a first order decay, but conduction band electrons do not seem to be involved. The fact that oxidation products of 4-CP are not observed is consistent with the fast dark removal of reaction intermediates by Cr(v), proved by EPR.

Electron Transfer from Photoexcited TiO2 to Chelating Alizarin Molecules: Reversible Photochromic Effect in Alizarin@TiO2 under UV Irradiation

Reduction of alizarin molecules coupled to TiO(2) nanoparticles (A@TiO(2)) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO(2) nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO(2) coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO(2) solar cells is briefly discussed.

Alizarin complexone: an interesting ligand for designing TiO2-hybrid nanostructures

Herein we report the characterization and photoelectrochemical behavior of nanocrystalline TiO2 films modified with some alizarin derivatives. Particularly, alizarin complexone (3-[N,N-bis(carboxymethyl)-aminomethyl]-1,2-dihydroxyanthraquinone) and its Fe(II) and Fe(III) metal complexes have been examined. Based on UV-Visible and Raman spectroscopic studies we proposed that AC forms a tridentate complex with titanium dioxide involving the 2-OH and (methylimino) diacetate groups, while Fe(II) and Fe(III) AC-complexes bind to titanium dioxide through the carbonyl (CO) and hydroxyl (OH) groups at the 1,9 positions of the alizarin framework. These findings allow the design of different nanostructures with diverse photoelectrochemical behavior. Analysis of the j–V curves reveals a sign reversal at −90 mV and −100 mV upon the excitation of AC@TiO2 and [FeIIIAC]@TiO2 photoelectrodes, respectively, whereas [FeIIAC]@TiO2 displays only anodic photocurrents. A description involving both electronic and geometric factors to account for these differences is proposed.

Time-resolved photoacoustic calorimetry of aqueous peroxodisulfate photolysis in the presence of nitrite anions

The photodissociation of aqueous peroxodisulfate in the presence of nitrite anions was studied between 282 and 292 K by time-resolved photoacoustic calorimetry at 266 nm. Aqueous peroxodisulfate was chosen as the calorimetric reference, thus enabling the measurement of the enthalpy and volume changes associated with the reaction of the photoproduced sulfate radicals with nitrite anions to form SO4−2 and NO2 in the submicrosecond time range. The enthalpy change obtained (−192 ± 19 kJ mol−1) is consistent with available thermochemical data for related species. The interpretation of the measured reaction volume (−13.8 ± 0.7 mL mol−1) points to the ionic charge accumulation on the sulfate anion as the dominating change causing the contraction of the solvent. The comparative analysis of our experimental result and data for partial molar volumes and electrostriction volumes of electrolytes, reinforces the concept that particularly in the case of water, more realistic models for the solvent are needed involving structure rather than continuous features.

Rate of the reaction between oxygen monofluoride and ozone

The upper limit to k_4 (FO+O_3 → products) ⩽ 4.0× 10^(−18) cm^3/molecule s, derived from a reassessment of the F_2-photosensitized decomposition of O_3, severely restricts the possibility that stratospheric ozone might be depleted in cycles involving F/FO_x radicals. A lower limit to k_3 (FO_2+O_3 → products) ⩾ 5 × 10^(−16) cm^3/molecule s, is also estimated.

Thermochemical kinetics of bromine nitrate, bromine nitrite, halogen hydroperoxides, dichlorine pentoxide, peroxycarboxylic acids, and diacyl peroxides

Heats of formation of BrONO_2, BrONO, BrOOH, FOOH, FOOCl, CF_3C(O)OOH, HC(O)OOH, CH_3C(O)OOH, and [CH_3C(O)O]_2 are estimated from bond contributions taken from J. Phys. Chem.,100, 10150 (1996). They agree within ±2 kcal/mol with recent experimental or ab initio data. The resulting BDE(O-O) = 36 kcal/mol value in diacetyl peroxide requires the concerted assistance of exothermic C-C(O) weakening in the transition state of its decomposition into free radicals. It also implies the existence of a previously unrecognized 12 kcal/mol nonbonded repulsion in acyl anhydrides. The formation of chloryl chlorate with ΔH_f(O_2ClOClO_2) = 50 kcal/mol, a marginally stable species toward dissociation into (ClO_3+OClO), may account for observations made in the [O(^3P+OClO] system at low temperatures.

The spontaneous room temperature reduction of HAuCl4 in ethylene glycol in the presence of ZnO: a simple strategy to obtain stable Au/ZnO nanostructures exhibiting strong surface plasmon resonance and efficient electron storage properties

The room temperature spontaneous reduction of HAuCl4 in ethylene glycol in the presence of pre-formed ZnO nanoparticles is investigated by UV-vis spectroscopy. Analysis by HRTEM demonstrated that the synthesized nanostructures consist of small ZnO nanoparticles (5.0 nm) in contact with bigger (15 nm) spherical Au nanoparticles. The electronic communication between Au and ZnO blocks is confirmed by UV excitation of the colloid. These experiments indicate that ZnO nanoparticles efficiently transfer the electrons to Au nanoparticles in contact, inducing a 15 nm blue shift in the plasmon band. Titration experiments using the methylviologen couple (MV2+/MV˙+) are presented and analyzed to quantify the enhancement in the electron storage capability in the new nanostructure and the negative shift in the Fermi level caused by the Au loading in ZnO.

Application of photoacoustic calorimetry to the determination of volume changes in reactions involving radical anions in aqueous solutions

The particular features of the application of photoacoustic calorimetry to the determination of volume changes in reactions involving radical anions in aqueous solutions are addressed. Analysis of recent literature data on volume changes of redox pairs, including our previous results on the couples NO2(-)/NO2 and SO4(2-)/(SO4)*-, together with known values of molal volume changes for ionization reactions, allow us to derive an empirical correlation for these categories of reactions. Finally, the pertinence of describing the volume changes of complex processes as a simple sum of separable terms is critically examined.

Very low pressure pyrolysis of phenylacetic acid

A Kinetic study of the very low pressure thermolysis of phenylacetic acid above 900 K reveals that its decomposition occurs in a concerted manner, yielding toluene and carbon dioxide, probably via a four-centre transition state [reaction (1)] [graphic omitted] with log[k_∞(1)/s^(–1)]=[(13 ± 0.3)–(12 200 ± 500)]/T.