Significant quantum effects in hydrogen activation

Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ∼190 K and for D2 up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation.

On the thermal stabilization of carbon-supported SiO2 catalysts by phosphorus:evaluation in the oxidative dehydrogenation of ethylbenzene to styrene and a comparison with relevant catalysts

A strategy to enhance the thermal stability of C/SiO2 hybrids for the O2-based oxidative dehydrogenation of ethylbenzene to styrene (ST) by P addition is proposed. The preparation consists of the polymerization of furfuryl alcohol (FA) on a mesoporous precipitated SiO2. The polymerization is catalyzed by oxalic acid (OA) at 160 °C (FA:OA = 250). Phosphorous was added as H3PO4 after the polymerization and before the pyrolysis that was carried out at 700 °C and will extend the overall activation procedure. Estimation of the apparent activation energies reveals that P enhances the thermal stability under air oxidation, which is a good indication for the ODH tests. Catalytic tests show that the P/C/SiO2 hybrids are readily active, selective and indeed stable in the applied reactions conditions for 60 h time on stream. Coke build-up during the reaction attributed to the P-based acidity is substantial, leading to a reduction of the surface area and pore volume. The comparison with a conventional MWCNT evidences that the P/C/SiO2 hybrids are more active and selective at high temperatures (450–475 °C) while the difference becomes negligible at lower temperature. However, the comparison with reference P/SiO2 counterparts shows a very similar yield than the hybrids but more selective to ST. The benefit of the P/C/SiO2 hybrid is the lack of stabilization period, which is observed for the P/SiO2 to create an active coke overlayer. For long term operation, P/SiO2 appears to be a better choice in terms of selectivity, which is crucial for commercialization.

Effect of preparation conditions on the formation of the active phase of carbon fiber catalytic systems for the low-temperature oxidation of carbon monoxide

We studied the effects of the composition of impregnating solution and heat treatment conditions on the activity of catalytic systems for the low-temperature oxidation of CO obtained by the impregnation of Busofit carbon-fiber cloth with aqueous solutions of palladium, copper, and iron salts. The formation of an active phase in the synthesized catalysts at different stages of their preparation was examined with the use of differential thermal and thermogravimetric analyses, X-ray diffraction analysis, X-ray photoelectron spectroscopy, and elemental spectral analysis. The catalytic system prepared by the impregnation of electrochemically treated Busofit with the solutions of PdCl, FeCl, CuBr, and Cu(NO ) and activated under optimum conditions ensured 100% CO conversion under a respiratory regime at both low (0.03%) and high (0.5%) carbon monoxide contents of air. It was found that the activation of a catalytic system at elevated temperatures (170-180°C) leads to the conversion of Pd(II) into Pd(I), which was predominantly localized in a near-surface layer. The promoting action of copper nitrate consists in the formation of a crystalline phase of the rhombic atacamite CuCl(OH). The catalyst surface is finally formed under the conditions of a catalytic reaction, when a joint Pd(I)-Cu(I) active site is formed. © 2014 Pleiades Publishing, Ltd.

Experiments on the generation of activated carbon from biomass

Activated carbon is generated from various waste biomass sources like rice straw, wheat straw, wheat straw pellets, olive stones, pistachios shells, walnut shells, beech wood and hardcoal. After drying the biomass is pyrolysed in the temperature range of 500-600 °C at low heating rates of 10 K/min. The activation of the chars is performed as steam activation at temperatures between 800 °C and 900 °C. Both the pyrolysis and activation experiments were run in lab-scale facilities. It is shown that nut shells provide high active surfaces of 1000-1300 m/g whereas the active surface of straw matters does hardly exceed 800 m/g which might be a result of the high ash content of the straws and the slightly higher carbon content of the nut shells. The active surface is detected by BET method. Besides the testing of a many types of biomass for the suitability as base material in the activated carbon production process, the experiments allow for the determination of production parameters like heating rate and pyrolysis temperature, activation time and temperature as well as steam flux which are necessary for the scale up of the process chain. © 2006 Elsevier B.V. All rights reserved.

Activation of proteinase-activated receptor 2 stimulates soluble vascular endothelial growth factor receptor 1 release via epidermal growth factor receptor transactivation in endothelial cells

The proteinase-activated receptor 2 (PAR-2) expression is increased in endothelial cells derived from women with preeclampsia, characterized by widespread maternal endothelial damage, which occurs as a consequence of elevated soluble vascular endothelial growth factor receptor-1 (sVEGFR-1; commonly known as sFlt-1) in the maternal circulation. Because PAR-2 is upregulated by proinflammatory cytokines and activated by blood coagulation serine proteinases, we investigated whether activation of PAR-2 contributed to sVEGFR-1 release. PAR-2–activating peptides (SLIGRL-NH2 and 2-furoyl-LIGRLO-NH2) and factor Xa increased the expression and release of sVEGFR-1 from human umbilical vein endothelial cells. Enzyme-specific, dominant-negative mutants and small interfering RNA were used to demonstrate that PAR-2–mediated sVEGFR-1 release depended on protein kinase C-ß1 and protein kinase C-e, which required intracellular transactivation of epidermal growth factor receptor 1, leading to mitogen-activated protein kinase activation. Overexpression of heme oxygenase 1 and its gaseous product, carbon monoxide, decreased PAR-2–stimulated sVEGFR-1 release from human umbilical vein endothelial cells. Simvastatin, which upregulates heme oxygenase 1, also suppressed PAR-2–mediated sVEGFR-1 release. These results show that endothelial PAR-2 activation leading to increased sVEGFR-1 release may contribute to the maternal vascular dysfunction observed in preeclampsia and highlights the PAR-2 pathway as a potential therapeutic target for the treatment of preeclampsia.

Catalytic systems based on carbon fiber materials for the low-temperature oxidation of carbon monoxide

New heterogenized catalytic systems for the low-temperature oxidation of CO were synthesized by supporting solutions of Pd, Cu, and Fe salts on carbon fibrous materials (carbopon and busofit). The carbon supports were studied by elemental analysis, SEM, TGA, and TPD. The effects of the nature of the support, the concentration and composition of the active component, and the conditions of preparation on the efficiency of the catalytic system were studied. It was ascertained that attenuation of hydrophilic properties of the support led to the decrease in system activity. The investigation of the catalysts by XPS showed that sample treatment in the reaction medium results in redistribution of the components of the active phase in the near-surface layer of the catalyst. The catalytic system based on carbon fibrous material carbopon prepared by supporting active components (Pd, Cu, and Fe salts) in three stages with intermediate activation in the reaction medium ensures 95% conversion of CO under respiratory conditions, and is promising for the design of the main element of breathing masks on its basis.

Selective oxidation of crotyl alcohol over Pd(111)

The selective oxidation of crotyl alcohol has been explored over a Pd(111) model catalyst. At low temperatures, the alcohol adsorbs intact with the C=C bond parallel to the surface. Activation likely proceeds through an allyl alkoxide intermediate that follows two distinct reaction channels. Over the clean surface, ∼90% of the alcohol oxidizes to surface bound crotonaldehyde above 200 K, which subsequently all decarbonylates to propene and CO at room temperature. The minor reaction channel involves C-O scission to 2-butene and water. While some of these undesired reactively formed alkene products desorb around 300 K, the majority dehydrogenate to irreversibly bound carbon above 380 K. This latter decomposition pathway is unlikely to be important at the low temperatures utilized in liquid-phase crotyl alcohol oxidation over supported palladium catalysts. Adsorbed CO persists until 430 K and is likely responsible for site-blocking and deactivation of dispersed metallic Pd clusters. Coadsorbed oxygen suppresses crotonaldehyde decarbonylation and promotes its release from the surface. © 2007 American Chemical Society.

Synthesis and characterisation of polyacrylonitrile and its derived carbon materials

Carbon is a versatile material which is composed of different allotropes, and also come in with different structures. Carbon nanofibres (CNFs) is one dimensional carbon nanomaterials, which have exhibited superior mechanical properties, great specific area, good electrical conductivity, good biocompatibility, and ease of modification. In addition to the lower cost associated to compare with carbon nanotubes (CNTs), CNFs have been attracted in numerous applications, such as reinforcement materials, filtrations, Li-ion battery, supercapacitor as well as tissue engineering, just to list a few. Therefore, it is a great deal to understand the relationship between the fabrication conditions and the characteristics of the resulted CNFs. In this project, electrospun PAN NFs were used as precursor material to fabricate carbon nanofibres. In order to produce CNFs with good morphology, the processing parameters of PAN nanofibres by electrospinning was optimized toward to the morphology at solution concentration of 12 wt%. The optimized processing parameters at given concentration were 16 kV, 14 cm and 1.5 mL/h, which led to the formation of PAN NFs with average fibre diameter of approximately 260 nm. Along with the effect of processing parameter study, the effect of concentration on the morphology was also carried out at optimized processing parameters. It was found that by increasing concentration of PAN solution from 2 to 16%, the resulted PAN transformed from beads only, to beaded fibres and finally to smooth fibres. With further increasing concentration the morphology of smooth fibres remain with increase in the fibre diameter. Electrospun PAN NFs with average fibre of 306 nm was selected to be converted into CNFs by using standard heating procedures, stabilisation in air at 280 °C and carbonization in N2. The effect of carbonization temperature ranging from 500 to 1000 °C was investigated, by using SEM, FTIR, Raman, and Impedance spectroscopy. With increasing carbonization temperature from 500 to 1000 °C, the diameter of NFs was decreased from 260 to 187, associated with loss of almost all functional groups of NFs. It was indicated by Raman results, that the graphitic crystallite size was increased from 2.62 to 5.24 nm, and the activation energy obtained for this growth was 7570 J/mol. Furthermore, impedance results (i.e. Cole-Cole plot) revealed that the electrical characteristic of CNFs transitioned from being insulating to electrically conducting in nature, suggested by the different electrical circuits extracted from Cole-Cole plots with carbonization temperature from 500 to 800 °C. The carbonization on PAN NFs with diameter of ~431nm was carried out by using novel route, microwave plasma enhance chemical vapour deposition (MPECVD) process. To compare with carbonized PAN NFs by using conventional route, MPECVD was not only able to facilitate carbonization process, but more interestingly can form carbon nanowalls (CNWs) grown on the surfaces of carbonized PAN NFs. Suggested by the unique morphology, the potential applications for the resulted carbon fibrous hybrid materials are supercapacitor electrode material, filtrations, and etc., The method developed in this project required one step less, compared with other literature. Therefore, using MPECVD on stabilised PAN NFs is proposed as economical, and straightforward approach towards mass production of carbon fibrous hybrid materials containing CNWs.

Immunological synapse: a mathematical model of the bond formation process:mathematical modelling of the immature synapse

The cell:cell bond between an immune cell and an antigen presenting cell is a necessary event in the activation of the adaptive immune response. At the juncture between the cells, cell surface molecules on the opposing cells form non-covalent bonds and a distinct patterning is observed that is termed the immunological synapse. An important binding molecule in the synapse is the T-cell receptor (TCR), that is responsible for antigen recognition through its binding with a major-histocompatibility complex with bound peptide (pMHC). This bond leads to intracellular signalling events that culminate in the activation of the T-cell, and ultimately leads to the expression of the immune eector function. The temporal analysis of the TCR bonds during the formation of the immunological synapse presents a problem to biologists, due to the spatio-temporal scales (nanometers and picoseconds) that compare with experimental uncertainty limits. In this study, a linear stochastic model, derived from a nonlinear model of the synapse, is used to analyse the temporal dynamics of the bond attachments for the TCR. Mathematical analysis and numerical methods are employed to analyse the qualitative dynamics of the nonequilibrium membrane dynamics, with the specic aim of calculating the average persistence time for the TCR:pMHC bond. A single-threshold method, that has been previously used to successfully calculate the TCR:pMHC contact path sizes in the synapse, is applied to produce results for the average contact times of the TCR:pMHC bonds. This method is extended through the development of a two-threshold method, that produces results suggesting the average time persistence for the TCR:pMHC bond is in the order of 2-4 seconds, values that agree with experimental evidence for TCR signalling. The study reveals two distinct scaling regimes in the time persistent survival probability density prole of these bonds, one dominated by thermal uctuations and the other associated with the TCR signalling. Analysis of the thermal fluctuation regime reveals a minimal contribution to the average time persistence calculation, that has an important biological implication when comparing the probabilistic models to experimental evidence. In cases where only a few statistics can be gathered from experimental conditions, the results are unlikely to match the probabilistic predictions. The results also identify a rescaling relationship between the thermal noise and the bond length, suggesting a recalibration of the experimental conditions, to adhere to this scaling relationship, will enable biologists to identify the start of the signalling regime for previously unobserved receptor:ligand bonds. Also, the regime associated with TCR signalling exhibits a universal decay rate for the persistence probability, that is independent of the bond length.

On the stability of conventional and nano-structured carbon-based catalysts in the oxidative dehydrogenation of ethylbenzene under industrially relevant conditions

Relevant carbon-based materials, home-made carbon-silica hybrids, commercial activated carbon, and nanostructured multi-walled carbon nanotubes (MWCNT) were tested in the oxidative dehydrogenation of ethylbenzene (EB). Special attention was given to the reaction conditions, using a relatively concentrated EB feed (10 vol.% EB), and limited excess of O2 (O 2:EB = 0.6) in order to work at full oxygen conversion and consequently avoid O2 in the downstream processing and recycle streams. The temperature was varied between 425 and 475 °C, that is about 150-200 °C lower than that of the commercial steam dehydrogenation process. The stability was evaluated from runs of 60 h time on stream. Under the applied reactions conditions, all the carbon-based materials are apparently stable in the first 15 h time on stream. The effect of the gasification/burning was significantly visible only after this period where most of them fully decomposes. The carbon of the hybrids decomposes completely rendering the silica matrix and the activated carbon bed is fully consumed. Nano structured MWCNT is the most stable; the structure resists the demanding reaction conditions showing an EB conversion of ∼30% (but deactivating) with a steady selectivity of ∼80%. The catalyst stability under the ODH reaction conditions is predicted from the combustion apparent activation energies. © 2014 Elsevier Ltd. All rights reserved.