Phase coexistence of multiple copper oxides on AgCu catalysts during ethylene epoxidation

Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres, multiple phases can coexist on a catalyst s surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual substoichiometric and metastable phases that could play important roles in catalytic processes. For instance, AgCu alloys known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene form an oxide like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy s surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst s enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst s surface, namely, CuO, Cu2O, and some previously unreported form of oxidized Cu, referred to here as CuxOy. Online product analysis, performed during the in situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the CuxOy species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in situ spectromicroscopic methods to understand the complexity of alloy catalyst

Size Dependence of Electrical Conductivity and Thermoelectric Enhancements in Spin-Coated PEDOT:PSS Single and Multiple Layers

This work reveals that the electrical conductivity σ of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film can be significantly increased by spin-coating multiple thin layers onto a substrate. Generally, σ can be improved by more than fourfold for multiple layers, as compared to a single thicker one. A gradual enhancement is observed for pristine PEDOT:PSS films (up to 2.10 ± 0.26 S cm–1 for five-layered films), while a plateau in σ at around 200 S cm–1 is reached after only three layers, when using a PEDOT:PSS solution with 5 vol% dimethyl sulfoxide. By contrast, only a small change in σ is observed for single layers of varying thickness. Accordingly, the thermoelectric power factor is also increased by up to 3.4 times for the multiple layers. Based on atomic force microscopy, X-ray photoelectron spectroscopy, UV–vis, and Raman spectroscopy measurements, two mechanisms are also proposed, involving an increase in percolation by inclusion of smaller grains within the existing ones, respectively, a reorganization of the PEDOT:PSS chains. These findings represent a direct strategy for enhancing the thermoelectric performance of conductive polymer films without additional reagents, while the mechanistic insights explain existing literature results

Transport by molecular motors in the presence of static defects

The transport by molecular motors along cytoskeletal filaments is studied theoretically in the presence of static defects. The movements of single motors are described as biased random walks along the filament as well as binding to and unbinding from the filament. Three basic types of defects are distinguished, which differ from normal filament sites only in one of the motors' transition probabilities. Both stepping defects with a reduced probability for forward steps and unbinding defects with an increased probability for motor unbinding strongly reduce the velocities and the run lengths of the motors with increasing defect density. For transport by single motors, binding defects with a reduced probability for motor binding have a relatively small effect on the transport properties. For cargo transport by motors teams, binding defects also change the effective unbinding rate of the cargo particles and are expected to have a stronger effect.Comment: 20 pages, latex, 7 figures, 1 tabl

In Vitro Aggregation Behavior of a Non-Amyloidogenic λ Light Chain Dimer Deriving from U266 Multiple Myeloma Cells

Excessive production of monoclonal light chains due to multiple myeloma can induce aggregation-related disorders, such as light chain amyloidosis (AL) and light chain deposition diseases (LCDD). In this work, we produce a non-amyloidogenic IgE λ light chain dimer from human mammalian cells U266, which originated from a patient suffering from multiple myeloma, and we investigate the effect of several physicochemical parameters on the in vitro stability of this protein. The dimer is stable in physiological conditions and aggregation is observed only when strong denaturating conditions are applied (acidic pH with salt at large concentration or heating at melting temperature Tm at pH 7.4). The produced aggregates are spherical, amorphous oligomers. Despite the larger β-sheet content of such oligomers with respect to the native state, they do not bind Congo Red or ThT. The impossibility to obtain fibrils from the light chain dimer suggests that the occurrence of amyloidosis in patients requires the presence of the light chain fragment in the monomer form, while dimer can form only amorphous oligomers or amorphous deposits. No aggregation is observed after denaturant addition at pH 7.4 or at pH 2.0 with low salt concentration, indicating that not a generic unfolding but specific conformational changes are necessary to trigger aggregation. A specific anion effect in increasing the aggregation rate at pH 2.0 is observed according to the following order: SO4−≫Cl−>H2PO4−, confirming the peculiar role of sulfate in promoting protein aggregation. It is found that, at least for the investigated case, the mechanism of the sulfate effect is related to protein secondary structure changes induced by anion binding

One-pot versus sequential reactions in the self-assembly of gigantic nanoscale polyoxotungstates

By using a new type of lacunary tungstoselenite {Se2W29O103} (1), which contains a “defect” pentagonal {W(W)4} unit, we explored the assembly of clusters using this building block and demonstrate how this unit can give rise to gigantic nanomolecular species, using both a “one-pot” and “stepwise” synthetic assembly approach. Specifically, exploration of the one-pot synthetic parameter space lead to the discovery of {Co2.5(W3.5O14)(SeW9O33)(Se2W30O107)} (2), {CoWO(H2O)3(Se2W26O85)(Se3W30O107)2} (3), and {Ni2W2O2Cl(H2O)3(Se2W29O103) (Se3W30O107)2} (4), effectively demonstrating the potential of the {Se2W29} based building blocks, which was further extended by the isolation of a range of 3d transition metal doped tetramer family derivatives: {M2WnOm(H2O)m(Se2W29O102)4} (M = Mn, Co, Ni or Zn, n = 2, m = 4; M = Cu, n = 3, m = 5) (5 - 9). To contrast the ‘one-pot’ approach, an optimized stepwise self-assembly investigation utilizing 1 as a precursor was performed showing that the high nuclearity clusters can condense in a more controllable way allowing the tetrameric clusters (5 - 8) to be synthesized with higher yield, but it was also shown that 1 can be used to construct a gigantic {W174} hexameric-cluster {Cu9Cl3(H2O)18(Se2W29O102)6} (10). Further, 1 can also dimerize to {(Se2W30O105)2} (11) by addition of extra tungstate under similar conditions. All the clusters were characterized by single-crystal X-ray crystallography, chemical analysis, infrared spectroscopy, thermogravimetric analysis, and electrospray ionization mass spectrometry, which remarkably showed that all the clusters, even the largest cluster, 10 (50 kD), could be observed as the intact cluster demonstrating the extraordinary potential of this approach to construct robust gigantic nanoscale polyoxotungstates