Converting carbon black into an efficient and multi-site ORR electrocatalyst: the importance of bottom-up construction parameters.

To boost efficient energy transitions, alternatives to expensive and unsustainable noble metal-based electrocatalysts for the oxygen reduction reaction (ORR) are needed. Having this in mind, carbon black - Black Pearls 2000 (BP) was enriched in active nitrogen-containing centers, including single-atom Fe-N sites surrounded by Fe nanoclusters, through a synthesis methodology employing only broadly available precursors. The methodical approach taken to optimize the synthesis conditions highlighted the importance of (1) a proper choice of the Fe precursor; (2) melamine as an N source to limit the formation of magnetite crystals and modulate the charge density nearby the active sites, and glucose to chelate/isolate Fe atoms and thus allow the Fe-N coordination to be established, with a limiting formation of Fe0 clusters; and (3) a careful dosing of the Fe load. The ORR on the optimized electrocatalyst (Fe0.06-N@BP) proceeds mostly through a four-electron pathway, having an onset potential (0.912 V vs. RHE) and limiting current density (4.757 mA cm-2) above those measured on Pt/C (0.882 V and 4.657 mA cm-2, respectively). Moreover, the current density yielded by Fe0.06-N@BP after 24 h at 0.4 V vs. RHE was still above that of Pt/C at t = 0 (4.44 mA cm-2), making it a promising alternative to noble metal-containing electrocatalysts in fuel cells.

Carbon Surface-Influenced Heterogeneity of Ni and Co Catalytic Sites as a Factor Affecting the Efficiency of Oxygen Reduction Reaction.

Highly porous carbon black and micro/mesoporous activated carbon were impregnated with cobalt and nickel nitrates, followed by heat treatment at 850 °C in nitrogen. Detailed information about chemistry and porosity was obtained using XPS, XRD, TEM/EDX, and nitrogen adsorption. The samples were used as ORR catalysts. Marked differences in the performance were found depending on the type of carbon. Differences in surface chemistry and porosity affected the chemistry of the deposited metal species that governed the O2 reduction efficiency along with other features of the carbon supports, including electrical conductivity and porosity. While dissociating surface acidic groups promoted the high dispersion of small metal species, carbon reactivity with oxygen and acidity limited the formation of the most catalytically active Co3O4. Formation of Co3O4 on the highly conductive carbon black resulted in an excellent performance with four electrons transferred and a current density higher than that on Pt/C. When Co3O4 was not formed in a sufficient quantity, nickel metal nanoparticles promoted ORR on the Ni/Co-containing samples. The activity was also significantly enhanced by small pores that increased the ORR efficiency by strongly adsorbing oxygen, which led to its bond splitting, followed by the acceptance of four electrons.

Empowering carbon materials robust gas desulfurization capability through an inclusion of active inorganic phases: A review of recent approaches.

Gas-phase desulfurization on carbon materials is an important process attracting the attention of scientists and engineers. When involving physical adsorption, reactive adsorption and catalytic oxidation combined, the process is considered as energy-efficient. Recent developments in materials science directed the attention of researchers to inorganic phases which react with H2S and participate to its oxidation to elemental sulfur. To fully utilize their capability, a developed surface area is needed and this feature is delivered by carbons. This review presents examples of recent advances in this field with focus not only on the activity of inorganic phases, dispersed on the surface or introduced as nanoparticles, but also on the important contribution of a carbon support as providing specific synergistic effects. The active phase promotes the H2S oxidation and participates in the reactions with H2S, while the carbon phase ensures its high dispersion, adds to oxygen activation and to an efficient electron transfer.

Alternative view of oxygen reduction on porous carbon electrocatalysts: the substance of complex oxygen-surface interactions.

Electrochemical oxygen reduction reaction (ORR) is an important energy-related process requiring alternative catalysts to expensive platinum-based ones. Although recently some advancements in carbon catalysts have been reported, there is still a lack of understanding which surface features might enhance their efficiency for ORR. Through a detailed study of oxygen adsorption on carbon molecular sieves and using inelastic neutron scattering, we demonstrated here that the extent of oxygen adsorption/interactions with surface is an important parameter affecting ORR. It was found that both the strength of O2 physical adsorption in small pores and its specific interactions with surface ether functionalities in the proximity of pores positively influence the ORR efficiency. We have shown that ultramicropores and hydrophobic surface rich in ether-based groups and/or electrons enhance ORR on carbon electrocatalysts and the performance parameters are similar to those measured on Pt/C with the number of electron transfer equal to 4.

Proposing an unbiased oxygen reduction reaction onset potential determination by using a Savitzky-Golay differentiation procedure.

For proper and fair comparison of the performance of Oxygen reduction reaction (ORR) electrocatalysts an un-biased method to determine an onset potential value is needed. Here we report an easy mathematical approach based on the second derivative of linear sweep voltammetry curves, referred to as a second order discrete differentiation method (SODDM) that allows to accurately provide the onset potential. Analysis of the published results showed that the reported values might be affected by an intrinsic human error associated with the application of the most common approaches addressed as a tangent method or those relaying on a visual estimation of the onset potential based on the shape of a linear scan voltammetry (LSV) curve. We have also demonstrated that by using SODDM, electrochemical data collected on different instruments by different researchers leads to comparable results in terms of the ORR onset potential values.

TiO2/S-Doped Carbons Hybrids: Analysis of Their Interfacial and Surface Features.

Hybrids containing approximately equal amounts of P25 TiO2 and S-doped porous carbons were prepared using a water-based slurry mixing method. The materials were extensively characterized by adsorption of nitrogen, potentiometric titration, thermal analysis in air and in helium, XRD, XPS and SEM. The collected results showed the significant blockage of carbon micropores by TiO2 particles deposited on their outer surface. The formation of a new interface, especially for the S-rich samples, might also contribute to the porosity alteration. Analysis of surface chemistry suggested the presence of Ti-S bonds with an involvement of sulfur from thiophenic species in the carbon phase. The latter, especially when polymer-derived, was mainly deposited on the TiO2 nanoparticles. Formation of Ti-S stabilized sulfur and increased the ignition temperature of the hybrids, especially those with a high content of sulfur, in comparison with the ignition temperature of carbons. The surfaces of hybrid with S-containing carbons was also thermally very stable and of basic chemical nature. The formation of interfacial structures Ti-C was detected by XPS analysis suggesting a partial reduction of the Ti.

Barium titanate perovskite nanoparticles as a photoreactive medium for chemical warfare agent detoxification.

Barium titanate nanoparticles (BTO-NPs) in the size range 8-12 nm, prepared by gel collection, are found to be a photoreactive detoxifier for Chemical Warfare Agent vapors, specifically, the sulfur mustard surrogate (2-chloroethyl ethyl sulfide). The relatively monodisperse, uniformly spherical BTO-NPs, initially dispersed in alcohol solvents, form a stable and porous aggregated structure reminiscent of a nanostructured material with voids/pores of an average diameter of 4.6 nm and a relatively narrow distribution of their sizes (2.5-8.7 nm). Due to the interparticle porosity and a polar, chemically active surface, signifcant amounts of CWA surrogate and its decomposition products were adsorbed on the BTO-NPs. The recorded weight uptake on the perovskite was the highest among a series of materials and nanocomposites known for their detoxification activity and tested at the same conditions (169 mg/g, compared to 117 mg/g for zinc oxide and <100 mg/g for other transition metal oxides). Besides adsorption, BTO nanomaterial acts simultaneously as an efficient heterogeneous catalyst by degrading the toxic vapors to alcohols, sulfides and thiols - molecules of significantly lower toxicity than the CWA surrogate. Hydrolysis and dehydrohalogenation were the predominant detoxification pathways, via the formation of the intermediate cyclic sulfonium, whether under light or in the dark. Ambient light irradiation promoted the photo-oxidation and photo-degradation by radical intermediates formed. With an unhindered oxygen rich surface, underlying highly polarizable lattice structure, and large accessible surface area, barium titanate nanoparticles are investigated as a potentially useful medium for photoreactive detoxification of chemical warfare agent vapors.

Mustard Gas Surrogate Interactions with Modified Porous Carbon Fabrics: Effect of Oxidative Treatment.

Removal of chemical warfare agent (CWA) surrogates by highly porous carbon textiles was investigated. The carbon cloth was modified by oxidation in a mixture of concentrated sulfuric and nitric acid. This process did not affect textile structural integrity. The surface properties of the modified textiles were investigated, and their capabilities to remove 2-chloroethyl ethyl sulfide (CEES) and diethylsulfide (EES), two mustard gas surrogates, were evaluated. The oxidized carbon textiles have a highly active surface that has the ability to form radical species. This enhances the degradation of the surrogates, and so the detoxification efficiency. The reaction products detected suggest differences in degradation mechanisms which depend on the type of fabric surface features. Thus, the oxidized surfaces eliminate CEES mainly through dehydrohalogenation, while the nonoxidized surfaces act via hydrolysis. Only the oxidized carbon has a surface active enough to react with the less reactive surrogate EES, by cleavage of the C-S bond. The surface functional groups promote not only the radical formation but also contribute to a strong adsorption of the CWA surrogates, which enhance the decomposition of these toxic species.

Carbon Textiles Modified with Copper-Based Reactive Adsorbents as Efficient Media for Detoxification of Chemical Warfare Agents.

Carbon textile swatch was oxidized and impregnated with copper hydroxynitrate. A subsample was then further heated at 280 °C to form copper oxide. The swatches preserved their integrity through the treatments. As final products, they exhibited remarkable detoxification properties for the nerve agent surrogate dimethyl chlorophosphate (DMCP). Based on the amount of reactive copper phases deposited on the fibers, their adsorption capacities were higher than those of the bulk powders. After 1 day exposure to DMCP (1:1 weight ratio adsorbent/DMCP), 99% of the initial amount of DMCP was eliminated. A synergistic effect of the composite components was clearly seen. GC-MS results showed that the main surface reaction product was chloromethane. Its formation indicated hydrolysis as a detoxification path. Surface analyses showed phosphate bonding to the fibers and formation of copper chloride. The appearance of the latter species results in a clear textile color change, which suggests the application of these fabrics not only as catalytic protection agents but also as sensors of nerve agents.

Mixed CuFe and ZnFe (hydr)oxides as reactive adsorbents of chemical warfare agent surrogates.

Two sets of zinc-iron and copper-iron mixed (hydr)oxides were prepared by a simple co-precipitation method. Either nitrate or chloride was a source of the metals. The decontamination ability of the materials was tested in closed vials saturated with vapors of 2-chloroethyl ethyl sulfide (CEES) or dimethyl chlorophosphate (DMCP), a blister agent and a nerve agent surrogate, respectively. In both cases, the weight uptakes on the mixed oxides were superior to the ones reported for the pure metal oxides or hydroxides. When exposed to CEES for 5days, zinc-iron (hydr)oxides show much higher activity than the copper-iron ones. The products of reactions in the vessel headspace were investigated by GC/MS and on the surface by FTIR. Ethyl vinylsulfide and chloromethane are the main products of the reactive adsorption of CEES and DMCP, respectively. This indicates that CEES is mainly degraded by dehydrochlorination and DMCP- by hydrolysis.

Smart textiles of MOF/g-C3N4 nanospheres for the rapid detection/detoxification of chemical warfare agents.

Smart textiles consisting of cotton, Cu-BTC MOF and oxidized graphitic carbon nitride, g-C3N4-ox, nanospheres were synthesized and tested as nerve agent detoxification media and colorimetric detectors. Combining Cu-BTC and g-C3N4-ox resulted in a nanocomposite (MOFgCNox) of heterogeneous porosity and chemistry. Upon the deposition of MOFgCNox onto cotton textiles, a stable fabric with a supreme photocatalytic detoxification ability towards the nerve gas surrogate, dimethyl chlorophosphate, was obtained. The detoxification process was accompanied by a visible and gradual color change, which can be used for the selective detection of chemical warfare agents and for monitoring their penetration inside a protective layer. These smart textiles adsorbed almost 7 g of CWA surrogate/its detoxification products per gram of Cu. The superior performance was linked to the high dispersion of the MOF crystals on the fibers, and a specific texture promoting the availability of the active copper centers.

Removal of hydrogen sulfide at ambient conditions on cadmium/GO-based composite adsorbents.

Cadmium-based materials with various hydroxide to carbonate ratios and their composites with graphite oxide were synthesized by a fast and simple precipitation procedure and then used as H2S adsorbents at ambient conditions in the dark or upon a visible light exposure. The structural properties and chemical features of the adsorbents were analyzed before and after hydrogen sulfide adsorption. The results showed that the high ratio of hydroxide to carbonate led to an improved H2S adsorption capacity. In moist conditions cadmium hydroxide was the best adsorbent. Moreover, it showed photoactive properties. While the incorporation of a graphene-based phase slightly decreased the extent of the improvement in the H2S adsorption capacity in moist conditions caused by photoactivity, its presence in the composites enhanced the performance in dry conditions. This was linked to photoactivity of CdS that can split H2S resulting in the formation of water in the system. The graphene-based phase enhanced the electron transfer and delayed the recombination of photoinduced charges. Carbonate-based materials showed a very good adsorption capacity in dark conditions in the presence of moisture. Upon the light exposure, CdS likely photocatalyzes the reduction of carbonate ions to formates/formaldehydes. Their deposition on the surface limits the number of sites available to H2S adsorption.

Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR.

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.

Simultaneous acquisition of pulse EPR orientation selective spectra.

High resolution pulse EPR methods are usually applied to resolve weak magnetic electron-nuclear or electron-electron interactions that are otherwise unresolved in the EPR spectrum. Complete information regarding different magnetic interactions, namely, principal components and orientation of principal axis system with respect to the molecular frame, can be derived from orientation selective pulsed EPR measurements that are performed at different magnetic field positions within the inhomogeneously broadened EPR spectrum. These experiments are usually carried out consecutively, namely a particular field position is chosen, data are accumulated until the signal to noise ratio is satisfactory, and then the next field position is chosen and data are accumulated. Here we present a new approach for data acquisition of pulsed EPR experiments referred to as parallel acquisition. It is applicable when the spectral width is much broader than the excitation bandwidth of the applied pulse sequence and it is particularly useful for orientation selective pulse EPR experiments. In this approach several pulse EPR measurements are performed within the waiting (repetition) time between consecutive pulse sequences during which spin lattice relaxation takes place. This is achieved by rapidly changing the main magnetic field, B(0), to different values within the EPR spectrum, performing the same experiment on the otherwise idle spins. This scheme represents an efficient utilization of the spectrometer and provides the same spectral information in a shorter time. This approach is demonstrated on W-band orientation selective electron-nuclear double resonance (ENDOR), electron spin echo envelope modulation (ESEEM), electron-electron double resonance (ELDOR)--detected NMR and double electron-electron resonance (DEER) measurements on frozen solutions of nitroxides. We show that a factors of 3-6 reduction in total acquisition time can be obtained, depending on the experiment applied.

Studying supramolecular assemblies by ESEEM spectroscopy: inclusion complexes of cyclodextrins.

Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate intramolecular and intermolecular complexes of cyclodextrins (CDs) with a nitroxide group. The interaction with solvent molecules (D(2)O) was followed through the (2)H modulation depth. Competition experiments with adamantane-type guests were used to confirm complexation. The shielding of the nitroxide group from solvent upon inclusion into a CD cavity made this technique more sensitive to complexation than cw EPR spectroscopy. ESEEM analysis of a series of CDs mono and bis spin-labeled on the primary rim of the cavity showed that only one compound formed a self-inclusion complex. This suggests that significant linker length/flexibility is required for formation of inclusion complexes in functionalized CDs. DEER (double electron-electron resonance) experiments confirmed that the self-inclusion complex of the spin-labeled CD was intra- rather than intermolecular.

Self-assembly of pluronic block copolymers in aqueous dispersions of single-wall carbon nanotubes as observed by spin probe EPR.

The self-assembly of Pluronic block copolymers in dispersions of single-wall carbon nanotubes (SWNT) was investigated by spin probe electron paramagnetic resonance (EPR) spectroscopy. Nitroxide spin labeled block copolymers derived from Pluronic L62 and P123 were introduced in minute amounts into the dispersions. X-band EPR spectra of the SWNT dispersions and of native polymer solutions were measured as a function of temperature. All spectra, below and above the critical micelle temperature (CMT), were characteristic of the fast limit motional regime. The temperature dependence of the 14N isotropic hyperfine coupling, aiso, and the rotational correlation time, tauc, were determined. It was observed that, below the CMT, EPR does not distinguish between chains adsorbed on SWNT and free chains. Above CMT, substantial differences were observed: in the native solution, the Pluronics spin labels experience only one environment, Sm, assigned to spin labels in the corona of the Pluronic micelle, whereas in the SWNT dispersions, in addition to Sm, a second population of nonaggregated, individual chains, Si, is observed. The relative amounts of Sm and Si were found to depend on the relative concentrations of the Pluronic and SWNT. Furthermore, the aggregates formed in the SWNT dispersions do not show the typical increase in chain-end mobility as a function of temperature, observed in the post-CMT regime of the native Pluronic solutions. This suggests a larger dynamical coupling among aggregated chains in the presence of the SWNT as compared to the native micelles. The overall findings are consistent with the formation of a new type of aggregates, composed of a SWNT-polymer hybrid.

Aggregation and self-assembly of amphiphilic block copolymers in aqueous dispersions of carbon nanotubes.

The self-assembly (SA) of amphiphilic block copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) was investigated in dispersions of single-walled and multiwalled carbon nanotubes (SWNT and MWNT, respectively) as a function of temperature. Differential scanning calorimetry (DSC) was used for characterization of the thermal behavior of the combined polymers-nanostructures system, and spin-probe electron paramagnetic resonance (EPR) was employed for probing the local dynamic and polarity of the polymer chains in the presence of nanostructures. It was found that SWNT and MWNT modify the temperature, enthalpy, and dynamic behavior of polymer SA. In particular, SWNT were found to increase the cooperativity of aggregating chains and dominate aggregate dynamics. MWNT reduced the cooperativity, while colloidal carbon black additives, studied for comparison, did not show similar effects. The experimental observations are consistent with the suggestion that dimensional matching between the characteristic radius of the solvated polymer chains and the dimensions of additives dominate polymer SA in the hybrid system.