Highly Sensitive and Selective Formaldehyde Gas Sensors Based on Polyvinylpyrrolidone/Nitrogen-Doped Double-Walled Carbon Nanotubes.

A highly sensitive and selective formaldehyde sensor was successfully fabricated using hybrid materials of nitrogen-doped double-walled carbon nanotubes (N-DWCNTs) and polyvinylpyrrolidone (PVP). Double-walled carbon nanotubes (DWCNTs) and N-DWCNTs were produced by high-vacuum chemical vapor deposition using ethanol and benzylamine, respectively. Purified DWCNTs and N-DWCNTs were dropped separately onto the sensing substrate. PVP was then dropped onto pre-dropped DWCNT and N-DWCNTs (hereafter referred to as PVP/DWCNTs and PVP/N-DWCNTs, respectively). As-fabricated sensors were used to find 1,2-dichloroethane, dichloromethane, formaldehyde and toluene vapors in parts per million (ppm) at room temperature for detection measurement. The sensor response of N-DWCNTs, PVP/DWCNTs and PVP/N-DWCNTs sensors show a high response to formaldehyde but a low response to 1,2-dichloroethane, dichloromethane and toluene. Remarkably, PVP/N-DWCNTs sensors respond sensitively and selectively towards formaldehyde vapor, which is 15 times higher than when using DWCNTs sensors. This improvement could be attributed to the synergistic effect of the polymer swelling and nitrogen-sites in the N-DWCNTs. The limit of detection (LOD) of PVP/N-DWCNTs was 15 ppm, which is 34-fold higher than when using DWCNTs with a LOD of 506 ppm. This study demonstrated the high sensitivity and selectivity for formaldehyde-sensing applications of high-performance PVP/N-DWCNTs hybrid materials.

Magnetite nanoparticles decorated on multi-walled carbon nanotubes for removal of Cu2+ from aqueous solution.

Acid-functionalized multi-walled carbon nanotube (MWCNTs-COOH) was prepared by acid treatment followed by decoration with magnetite (Fe3O4) nanoparticles (Fe3O4/MWCNTs-COOH) by co-precipitation of Fe2+/Fe3+ in the colloidal suspension of MWCNTs-COOH. The adsorption capacity and separation efficiency of these two adsorbents were investigated for the removal of Cu2+ ions in aqueous solution as water treatment adsorbents. The effect of reaction conditions, such as contact time, initial concentration of Cu2+ ions, and adsorbent dosage, on the adsorption capacity of MWCNTs-COOH was investigated. It was found that contact time of 10 min, adsorbent dosage of 0.2 g/L and 15 mg/L as initial concentration of Cu2+ ions are ideal conditions for maximum adsorption capacity (10.45 mg/g). The adsorption capacity of synthesized Fe3O4/MWCNTs-COOH containing different weight percent of Fe3O4 (10, 25, 50 wt%) was explored for removal of Cu2+ ions from aqueous solution and the best results achieved with 25 wt% Fe3O4/MWCNTs-COOH, which exhibited optimum adsorption capacity of 9.50 mg/g and 97% separation efficiency. Further, Langmuir and Freundlich isotherm models were applied to validate experimental data obtained in this work for Cu2+ adsorption.

Prominent electronic and geometric modifications of palladium nanoparticles by polymer stabilizers for hydrogen production under ambient conditions.

A remarkable effect from the modification of electronic and geometric properties of Pd nanoparticles by the use of polymer pendant groups bound to the surface of palladium nanoparticles is reported. The degree of electron promotion to the Pd nanoparticles under ambient conditions was found to be dependent on the availability of the lone pair electrons of the pendant groups.

Electron promotion by surface functional groups of single wall carbon nanotubes to overlying metal particles in a fuel-cell catalyst.

A remarkable promotion: Functional groups added onto single-wall carbon nanotubes (SWNTs) can significantly influence the activity of a noble metal for formic acid oxidation. Phenolate groups on SWNTs under alkaline conditions can double the activity of 20 % w/w Pd compared to unmodified SWNTs. This catalyst has 14 times higher activity than the commercial benchmark catalyst (10 % w/w Pd on Vulcan).

Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst.

Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H2 from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.

¹³C NMR guides rational design of nanocatalysts via chemisorption evaluation in liquid phase.

The search for more efficient heterogeneous catalysts remains critical to the chemical industry. The Sabatier principle of maximizing catalytic activity by optimizing the adsorption energy of the substrate molecule could offer pivotal guidance to otherwise random screenings. Here we show that the chemical shift value of an adsorbate (formic acid) on metal colloid catalysts measured by (13)C nuclear magnetic resonance (NMR) spectroscopy in aqueous suspension constitutes a simple experimental descriptor for adsorption strength. Avoiding direct contact between the (13)C atom and the metal surface eliminates peak broadening that has confounded prior efforts to establish such correlations. The data can guide rational design of improved catalysts, as demonstrated here for the cases of formic acid decomposition and formic acid electro-oxidation reactions.

Formate as a surface probe for ruthenium nanoparticles in solution 13C NMR spectroscopy.

Formic acid adsorption on ruthenium nanoparticles of different sizes allows differentiation of differently bound formate species by solution (13)C NMR spectroscopy (see picture). The chemical shifts are comparable to those of organometallic analogues, thus indicating that formate can act as a probe to distinguish surface features of metallic nanoparticles in solution with good quantification and resolution.