Activation/Inhibition Effects during the Coelectrodeposition of PtAg Nanoparticles: Application for ORR in Alkaline Media

PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag⁺ ions predominates, thus an increased Ag content in the co‐deposit is accomplished. The mechanism for this anomalous co‐deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt²⁺ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface.     

Evaporation of single component viscous liquids in a metal falling film evaporator

Heat transfer for single component falling film evaporation has been investigated in a stainless steel single tube falling film evaporator. The tube had a heated length of 2,500 mm. Propylene glycol and cyclohexanol have been used as evaporating media. Liquid film running down the tube, is formed on the inner side of the tube. For the distribution of liquid two different devices were examined. Process equipment was operated in pump-around mode with the distillate being condensed and recycled. Results show that none of the available correlations for heat transfer in falling film evaporation is able to describe the results qualitatively as well as quantitatively. Using different film distribution devices, a significant influence of the Reynolds number for the transition from laminar to turbulent flow is seen. However, differences between experimental results and correlations in literature cannot be explained only by usage of different film distributions, in particular when the correlation is based on measurements with a different tube length. A model approach is presented for cyclohexanol as evaporating medium with a flat weir as film distributor.     

Generalised acyclic edge colourings of graphs with large girth

The r-acyclic edge chromatic number of a graph G is the minimum number of colours required to colour the edges of G in such a way that adjacent edges receive different colours and every cycle C receives at least min{|C|,r} colours. We prove that for any integer r?4, the r-acyclic edge chromatic number of any graph G with maximum degree Δ and with girth at least 3(r-1)Δ is at most 6(r-1)Δ.     

Thiaozonide formation by singlet oxygen cycloaddition to 2,5-dimethylthiophene

Singlet oxygen reacts with 2,5-dimethylthiophene (1) exclusively by (4+2)-cycloaddition to yield the thiaozonide 2. The structure of this product is inferred from its ¹H- and ¹³C NMR spectra. Neat thiaozonide 2 decomposes violently at room temperature. In aprotic non-polar and in protic polar solvents it is slowly transformed into cis-sulfine 3c and cis-2,5-dione 4c, which rearranges to the trans-isomer 4t. A mechanism for the transformation reaction is proposed.     

Singlet oxygen photooxygenation of furans : Isolation and reactions of (4+2)-cycloaddition products (unsaturated sec.-ozonides)

Tetraphenylporphin-photosensitized oxygenations of furan (19), 2-methylfuran (26), 2-ethylfuran (39), furfurylalcohol (24), 2-acetylfuran (40), 2-methoxyfuran (42), 2,5-dimethylfuran (30), furfural (25) and 5-methylfurfural (41) in non-polar aprotic solvents yield the corresponding monomeric unsaturated secondary ozonides due to a (4+2)-cycloaddition of singlet oxygen to these furans. With the exception of the ozonide derived from 25, the ozonides were isolated and characterized (¹H- and ¹³C-NMR spectra, etc.). In non-polar aprotic solvents, the ozonides derived from 19, 26 and 39 undergo thermal rearrangements to the corresponding cis-diepoxides and epoxylactones. Ozonide 31, derived from 30, however, dimerizes, only above about 60° is a cis-diepoxide formed from either 31 or its dimer. Rose bengal-photosensitized oxygenations of the furans in alcohols (MeOH, EtOH, i-PrOH) also produce the corresponding ozonides as the primary products of (4+2)-cycloadditions of singlet oxygen to these furans. However, reactions of the alcohols with the ozonides are too fast to allow the ozonides to be isolated. Instead, the same products are obtained as are isolated from reactions carried out by dissolving the ozonides in the alcohols. Depending on the structure of the ozonide, three pathways are available to ozonide/alcohol (ROH) interactions:(1) addition of ROH to yield alkoxy hydroperoxides; one out of several possible isomers is formed in a completely stereoselective and regiospecific reaction; (2) elimination of a bridgehead proton by ROH as a base, as observed with the ozonide derived from 19 to give hydroxy butenolide (78) in yields between 20 and 60%, and (3) ROH-attack on a carbonyl side-chain under elimination of the corresponding alkyl ester, as observed with furfural photooxygenation which yielded hydroxy butenolide (78) in high yields (95%). Interaction of ozonide 31 with tert.-butyl alcohol (t-BuOH) yields quantitatively cis-3-oxo-1-butenylacetate (81) by a Baeyer-Villiger-type rearrangement with vinyl group migration Hydrogenbonding between the alcohol and the peroxy group of the ozonides assist the heterolysis of the C—O bonds in the ozonides; the most stabilized cation develops. Front-attack of ROH on this cation explains the stereoselectivity as well as the regiospecificity of the alkoxy hydroperoxide formation; with a bulky alcohol like t-BuOH, ROH-attack on the cation is sterically hindered thus allowing a rearrangement to occur. 1,3-Dipolar cycloaddition of p-nitrophenyl azide to ozonide 31 proceeds stereoselectively to one of the isomers 87a/87b. Finally, kinetic results of furan photooxygenation in methanol show the following order of furan-reactivity towards singlet oxygen: 30 > 42 > 26 > 19 > 41 > 25, with absolute rate constants ranging from 1.8 × 10⁸ (with 30) to 8.4 × 1O⁴ M^-1P^-1 (with 25).