Chompunuch Warakulwit

Dissymmetric carbon nanotubes by bipolar electrochemistry.

Short carbon nanotubes have been modified selectively on one end with metal using a bulk technique based on bipolar electrochemistry. A stabilized suspension of nanotubes is introduced in a capillary containing an aqueous metal salt solution, and a high electric field is applied to orientate and polarize the individual tubes. During their transport through the capillary under sufficient polarization (30 kV), each nanotube is the site of water oxidation on one end and the site of metal ion reduction on the other end with the size of the formed metal cluster being proportional to the potential drop along the nanotube.

Enantioselective recognition at mesoporous chiral metal surfaces.

Chirality is widespread in natural systems, and artificial reproduction of chiral recognition is a major scientific challenge, especially owing to various potential applications ranging from catalysis to sensing and separation science. In this context, molecular imprinting is a well-known approach for generating materials with enantioselective properties, and it has been successfully employed using polymers. However, it is particularly difficult to synthesize chiral metal matrices by this method. Here we report the fabrication of a chirally imprinted mesoporous metal, obtained by the electrochemical reduction of platinum salts in the presence of a liquid crystal phase and chiral template molecules. The porous platinum retains a chiral character after removal of the template molecules. A matrix obtained in this way exhibits a large active surface area due to its mesoporosity, and also shows a significant discrimination between two enantiomers, when they are probed using such materials as electrodes.

Direct synthesis of dimethyl ether from CO2 hydrogenation over Cu–ZnO–ZrO2/SO42−–ZrO2 hybrid catalysts: effects of sulfur-to-zirconia ratios

Sulfated zirconia catalysts were prepared by a direct sulfation method and were admixed with a CuO–ZnO–ZrO2 catalyst for the direct synthesis of DME from CO2 hydrogenation. The effects of sulfur-to-zirconia ratios on the physicochemical properties, activity, selectivity and stability of the catalysts were investigated. The sulfur loading content significantly influenced the structure and surface chemistry of the catalysts. The addition of a small amount of sulfur (5–15 wt%) created numerous mesopores on the catalyst surface, remarkably enhancing the surface area and total pore volume. However, at high sulfur loading (20–30 wt%), the mesopores tended to merge and form a larger pore. The detailed characterization by FT-IR, XANES and NH3-TPD reveals that the sulfated zirconia with low sulfur content (5–10 wt%) mainly contained weak acid sites and acted as Lewis acids. Increasing the sulfur loading (15–30 wt%) resulted in the formation of Bronsted acid sites, thus increasing the acid strengths. The sulfated zirconia catalyst with 20 wt% sulfur loading achieved a superior DME productivity of 236 gDME kgcat−1 h−1 at a reaction temperature and pressure of 260 °C and 20 MPa. However, after 75 h of a time-on-stream experiment, the sulfated zirconia catalyst lost approximately 16.9% of its initial activity while a commercial H-ZSM-5 catalyst was more stable as only a 2.85% reduction was observed.

Site-Selective Synthesis of Janus-type Metal-Organic Framework Composites†

Herein, bipolar electrochemistry is applied in a straightforward way to the site-selective in situ synthesis of metal-organic framework (MOF) structures, which have attracted tremendous interest in recent years because of their significant application potential, ranging from sensing to gas storage and catalysis. The novelty of the presented work is that the deposit can be intentionally confined to a defined area of a substrate without using masks or templates. The intrinsic site-selectivity of bipolar electrochemistry makes it a method of choice to generate, in a highly controlled way, hybrid particles that may have different functionalities combined on the same particle. The wireless nature of electrodeposition allows the potential for mass production of such Janus-type objects.

Wireless Electrografting of Molecular Layers for Janus Particle Synthesis

The chemical functionalization of surfaces with biological, redox-active or photosensitive molecules has been shown to be useful for applications ranging from molecular electronics, catalysis and energy conversion to chemical or biochemical sensors. Among several methods proposed to tailor the properties of conducting surfaces, electrografting of diazonium salts is one of the most popular strategies for tuning the chemical nature of substrate surfaces without losing its bulk properties. This electrochemical reduction of diazonium salts allows a covalent attachment of various functional groups to a wide range of substrates, and especially to carbon surfaces. These grafted organic layers are frequently used to fine-tune the surface properties of such substrates. The grafting is usually carried out by normal electrochemical reduction, meaning that the substrate has to be physically connected to an electrode. However in some situations it might be interesting and even mandatory to modify objects that are suspended in a solution, and thus not being in direct contact with an electrode. This is for example the case for the bulk production of Janus particles. It has been shown recently that this can be achieved by bipolar electrochemistry for the asymmetric deposition of metal layers. Herein we present an original method in which this attractive concept is for the first time used to generate a grafted organic layer, localized on one half sphere of carbon beads, thus leading to Janus particles bearing organic functional groups. The concept of bipolar electrochemistry applied to microspheres has been described by Fleischmann et al. Briefly, when a high electric field polarizes an object with sufficient electrical conductivity suspended in a solution, redox reactions can be carried out at the opposite side of the object, namely reductions at the cathodically polarized side and oxidations at the anodically polarized side (Figure 1). The polarization (DV) of the object is directly proportional to the effective length of the particle. This concept has recently become the driving force for the detec-

Direct oxidation of methane to methanol on Fe–O modified graphene

Introduction of functional groups to graphene can be used for the rational design of catalysts for the oxidation of hydrocarbons to alcohols. We have employed the PBE-D2 level of theory to study the direct oxidation of CH4 to CH3OH on a Fe–O active site generated on graphene by the decomposition of nitrous oxide (N2O) over Fe-embedded graphene. Restricted and unrestricted spin state of systems were also taken into account. The calculations show that FeO/graphene provides excellent reactivity for the oxy-functionalization of methane to methanol. The oxygen-centered radicals (O−˙) on the catalyst can activate the strong C–H bond of methane leading to its homolytic cleavage. The C–H bond activation requires an energy of 17.5 kcal mol−1, which is comparable with the barrier on traditional effective catalysts. Comparing the molecular adsorption complex, the formation of the iron coordinated fragments of C–H bond activation on the graphene support is found to be less energetically stable than on the Fe sites in the zeolite support. As a result, the conversion of the grafted species to the methanol product in the second step of the reaction is much more facile than for Fe-exchanged zeolite catalysts. An activation energy of 16.4 kcal mol−1 is required to yield the methanol product. Fe–O modified graphene materials could be promising catalysts for the partial oxidation of methane with N2O as an oxidant.

Asymmetric synthesis using chiral-encoded metal

The synthesis of chiral compounds is of crucial importance in many areas of society and science, including medicine, biology, chemistry, biotechnology and agriculture. Thus, there is a fundamental interest in developing new approaches for the selective production of enantiomers. Here we report the use of mesoporous metal structures with encoded geometric chiral information for inducing asymmetry in the electrochemical synthesis of mandelic acid as a model molecule. The chiral-encoded mesoporous metal, obtained by the electrochemical reduction of platinum salts in the presence of a liquid crystal phase and the chiral template molecule, perfectly retains the chiral information after removal of the template. Starting from a prochiral compound we demonstrate enantiomeric excess of the (R)-enantiomer when using (R)-imprinted electrodes and vice versa for the (S)-imprinted ones. Moreover, changing the amount of chiral cavities in the material allows tuning the enantioselectivity.

Controlled purification, solubilisation and cutting of carbon nanotubes using phosphomolybdic acid

We describe a straight forward procedure that allows an easy purification, an efficient solubilisation and, if desired, a controlled cutting of carbon nanotubes. The method uses aqueous solutions of phosphomolybdic acid combined with ultrasound, leading to the adsorption of phosphomolybdate on the nanotubes. This generates a negative surface charge allowing the formation of very stable suspensions. Prolonged exposure results in a controlled shortening of the tubes. Single- as well as multi-wall carbon nanotubes can be treated this way, which facilitates their further processing.

A novel method for dengue virus detection and antibody screening using a graphene-polymer based electrochemical biosensor

Dengue fever is a major disease that kills many people in the developing world every year. During early infection, a patient displays a high temperature without other signs. After this stage, and without proper treatment, serious damage to internal organs can happen, which occasionally leads to death. A rapid technique for the early detection of dengue virus (DENV) could reduce the number of fatalities. This study presents a new technique for the detection, classification and antibody screening of DENV based on electrochemical impedance spectroscopy (EIS). We found that the charge transfer resistance (Rct) of a gold electrode coated with graphene oxide reinforced polymer was influenced by virus type and quantity exposed on the surface. Molecular recognition capability established during the GO-polymer composite preparation was used to explain this observation. The linear dependence of Rct versus virus concentrations ranged from 1 to 2×103pfu/mL DENV with a 0.12 pfu/mL detection limit.

Reaction mechanism of methanol to formaldehyde over Fe- and FeO-modified graphene.

We employed periodic DFT calculations (PBE-D2) to investigate the catalytic conversion of methanol over graphene embedded with Fe and FeO. Two possible pathways of dehydrogenation to formaldehyde and dehydration to dimethyl ether (DME) over these catalysts were examined. Both processes are initiated with the activation of methanol over the catalytic center through O-H cleavage. As a result, a methoxo-containing intermediate is formed. Subsequently, H-transfer from the methoxy to the adjacent ligand leads to the formation of formaldehyde. Conversely, the activation of the second methanol over the intermediate gives DME and H2O. Over Fe/graphene, the dehydration process is kinetically and thermodynamically preferable. Unlike Fe/graphene, FeO/graphene is predicted to be an efficient catalyst for the dehydrogenation process. Oxidative dehydrogenation over FeO/graphene takes place through two steps with free energy barriers of 5.7 and 10.2 kcal mol(-1).