Quantum chemical modelling of the rate determining step for oxygen reduction on quinones

Two inner-sphere electrocatalytic channels for quinone-mediated reduction of molecular oxygen to form hydrogen peroxide have been addressed by means of density functional theory. Each of the channels comprises an initial rate determining chemical step and a subsequent electrochemical reduction step by which peroxide is produced. The reduction mechanism was determined for 9,10-anthraquinone and 9,10-phenanthrenequinone and the quantum chemical results are compared with experimental results. Two distinctly different structures were determined for the critical chemical step depending on whether the catalytic site is present as HQ* or Q*-. While a superoxo species is formed on HQ*, a van der Waals (vdW) type compound is formed on Q*-. It is shown that the Gibbs energy of activation for the semiquinone/oxygen reaction is largely determined by the entropy term. The results explain the experimentally observed pH dependence of the O2 reduction rate on quinone functionalised electrodes.     

Protective performances of two anti-graffiti treatments towards sulfite and sulfate formation in SO2 polluted model environment

Specific strategies for protection are being developed to counter both the staining and corrosive effects of polluted air in cities, as well as to allow for efficient removal of unwanted graffiti paintings. These protection strategies employ molecules with tailored functionalities, e.g. being hydrophobic, while maintaining porosity for molecular water vapour permeation.The present study employs SO₂ and water to probe the behaviors of two anti-graffiti treatments, a water-base fluoroalkylsiloxane (“Protectosil Antigraffiti” marketed by Degussa) and an organically modified silicate (Ormosil) synthesized from a polymer chain (polydimethyl siloxane, PDMS) and two network forming alkoxides (Zr propoxide and methyl triethoxy silane, MTES) dissolved in n-propanol, on five building materials, comprising limestone, aged lime mortar, hydrated cement mortar, granite, and brick material.The materials were exposed to a synthetic atmosphere for 20 h in a climate chamber, 0.78 ± 0.03 ppm of SO₂ and 95% RH. Diffuse reflectance Fourier transform infrared (DR-FTIR) spectra were registered before and after exposure in the climate chamber in the cases of both treated and untreated samples. DR-FTIR, scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) analyses, suggest the anti-graffiti Ormosil to suppress formation of calcium sulfite hemihydrate (the primary initial product of the reaction of calcium compounds with SO₂ and water) on carbonate materials (limestone and lime mortar).In case of the granite, brick and cement mortar, Ormosil has a negligible influence on the SO₂ capture. While no sulfite formation was detected by DR-FTIR, gypsum is inferred to form due to metal oxides and minority compounds catalysed oxidation of sulfite to sulfate. In case of brick, this understanding finds support from SEM images as well as EDX. A priori presence of gypsum in hydrated cement mortars prevents positive identification by SEM. However, support for sulfur accumulation in hydrated cement mortar is provided by means of EDX.In case of a second anti-graffiti considered, Protectosil, no influence of the anti-graffiti treatment on the SO₂ uptake of any of the building materials was observed.     

Distances between classes in $$W^{1,1}(\Omega ;{\mathbb {S}}^{1})$$

We introduce an equivalence relation on the space \(W^{1,1}(\Omega ;{\mathbb {S}}^1)\) which classifies maps according to their “topological singularities”. We establish sharp bounds for the distances (in the usual sense and in the Hausdorff sense) between the equivalence classes. Similar questions are examined for the space \(W^{1,p}(\Omega ;{\mathbb {S}}^1)\) when \(p>1\).     

Linear cover time is exponentially unlikely

We show that the probability that a simple random walk covers a finite, bounded degree graph in linear time is exponentially small. We conjecture that the same holds for any simple graph.     

Diffusion Limited Aggregation on a Cylinder

We consider the DLA process on a cylinder . It is shown that this process “grows arms”, provided that the base graph G has small enough mixing time. Specifically, if the mixing time of G is at most , the time it takes the cluster to reach the m ^th layer of the cylinder is at most of order . In particular we get examples of infinite Cayley graphs of degree 5, for which the DLA cluster on these graphs has arbitrarily small density. In addition, we provide an upper bound on the rate at which the “arms” grow. This bound is valid for a large class of base graphs G, including discrete tori of dimension at least 3. It is also shown that for any base graph G, the density of the DLA process on a G-cylinder is related to the rate at which the arms of the cluster grow. This implies that for any vertex transitive G, the density of DLA on a G-cylinder is bounded by 2/3.     

Conditioned Diffusions which are Brownian Bridges

Let X _t be a one-dimensional diffusion of the form dX _t=dB _t+(X _t)dt. Let Tbe a fixed positive number and let be the diffusion process which is X _t conditioned so that X ₀=X _T=x. If the drift is constant, i.e., , then the conditioned diffusion process is a Brownian bridge. In this paper, we show the converse is false. There is a two parameter family of nonlinear drifts with this property.     

Quantum chemical modeling of ethene epoxidation with hydrogen peroxide: the effect of microsolvation with water

Quantum chemical calculations were performed to study the mechanism of ethene epoxidation with hydrogen peroxide. The calculations were carried out at the B3LYP/6-311+G(d,p) level of theory. The applicability of this functional to the problem at hand, including basis set effects, was validated by CCSD(T) and CASSCF based multireference MP2 calculations. A mechanism was determined where hydrogen peroxide becomes polarized in the transition state upon binding to the ethene molecule. The distant hydroxide fragment of the attached hydrogen peroxide molecule becomes partly negatively charged, while the other part of the molecule involves a proton and becomes partly positively charged. In the absence of water an activation energy of 139.7 kJ mol(-1) was determined for the isolated H(2)O(2) + C(2)H(4) system. By microsolvating with water, the impact of a hydrogen-bonded network on the activation energy was addressed. A 43.7 kJ mol(-1) lowering of the activation energy, DeltaE(a), was observed when including up to 4 water molecules in the model. This effect results from the stabilization of the proton and hydroxide fragments in the transition state. The findings are discussed in the context of previous theoretical studies on similar systems. Effects of adding or removing a proton to mimic acidic and alkaline conditions are addressed and the limitations of the model in solvating the excess charge are discussed.