Imprinting isolated single iron atoms onto mesoporous silica by templating with metallosurfactants.

HYPOTHESIS: One of the main drawbacks of metal-supported materials, traditionally prepared by the impregnation of metal salts onto pre-synthesized porous supports, is the formation of large and unevenly dispersed particles. Generally, the larger are the particles, the lower is the number of catalytic sites. Maximum atom exposure can be reached within single-atom materials, which appear therefore as the next generation of porous catalysts. EXPERIMENTS: Herein, we designed single iron atom-supported silica materials through sol-gel hydrothermal treatment using mixtures of a non-ionic surfactant (Pluronic P123) and a metallosurfactant (cetyltrimethylammoniumtrichloromonobromoferrate, CTAF) as porogens. The ratio between the Pluronic P123 and the CTAF enables to control the silica structural and textural properties. More importantly, CTAF acts as an iron source, which amount could be simply tuned by varying the non-ionic/metallo surfactants molar ratio. FINDINGS: The fine distribution of iron atoms onto the silica mesopores results from the iron distribution within the mixed micelles, which serve as templates for the polymerization of the silica matrix. Several characterization methods were used to determine the structural and textural properties of the silica material (XRD, N2 sorption isotherms and TEM) and the homogeneous distribution and lack of clustering of iron atoms in the resulting materials (elemental analysis, magnetic measurements, pair distribution function (PDF), MAS-NMR and TEM mapping). The oxidation and spin state of single-iron atoms determined from their magnetic properties were confirmed by DFT calculations. This strategy might find straightforward applications in preparing versatile single atom catalysts, with improved efficiency compared to nanosized ones.

Improved tribological properties, thermal and colloidal stability of poly-α-olefins based lubricants with hydrophobic MoS2 submicron additives.

HYPOTHESIS: Newtonian liquids, usually used as base oil lubricants, exhibit low viscosity under extreme thermal conditions, needed for the functioning of wind turbines. This is directly affecting the colloidal stability and the tribological properties of the formulations containing additives, such as MoS2. Here, it was hypothesized that the surface hydrophobization of MoS2 particles will allow for an increased colloidal stability of the resulting formulations, for temperatures as high as 80 °C. EXPERIMENTS: The antifriction properties and the thermal stability of the designed formulations were determined on submicron MoS2 particles dispersed in poly-α-olefins (PAO) base oils of different dynamic viscosities (from 32 to 1650 mPa·s at 25 °C). The submicron particles of MoS2 (300-500 nm in diameter) were synthesised by a simple one-pot solvothermal method under mild conditions. The resulting particles were hydrophobized in situ in PAO base oils using alkyltrichlorosilane grafting agents with two chain lengths (C8 and C18). FINDINGS: The covalent grafting of alkylsilanes through Mo-O-Si bonds was confirmed by DFT calculations and FT-IR measurements. Turbiscan optical analysis revealed that thermal and colloidal stabilities can be significantly improved depending on oil viscosity and chain length of the grafting agent. The formulations in the PAO65 oil remained highly stable (TSI < 1), even at 80 °C. Herein, we demonstrate the impact of hydrophobization degree on the tribological properties of the lubricants, which, importantly, could reach ultra-low friction coefficients, less than 0.02.

High added-value products from the hydrothermal carbonisation of olive stones.

Olive stones (OS) were submitted to hydrothermal carbonisation (HTC) in order to evaluate the possibility of producing high added-value products, mainly furfural (FU) and 5-hydroxymethylfurfural (5-HMF) on one hand and hydrochars and carbons on the other hand. Temperature (160-240 °C), residence time (1-8 h), initial pH (1-5.5) and liquid/solid ratio (4-48 w/w) were systematically varied in order to study the main products and to optimise FU production. FU production yield up to 19.9 %, based on the hemicellulose content, was obtained. Other minor, but valuable, compounds such as 5-methylfurfural (5-MF) and some phenolic compounds were also produced. The hydrochar was carbonised at 900 °C, and the resultant carbon material was highly ultramicroporous with a peak of pore size distribution centred on 0.5 nm and a surface area as high as 1065 m2 g-1, typical of most carbon molecular sieves.

Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char.

Tyre pyrolysis char (TPC), produced when manufacturing pyrolysis oil from waste tyre, was used as raw material to prepare activated carbons (ACs) by KOH activation. KOH to TPC weight ratios (W) between 0.5 and 6, and activation temperatures from 600 to 800 °C, were used. An increase in W resulted in a more efficient development of surface area, microporosity and mesoporosity. Thus, ACs derived from TPC (TPC-ACs) with specific surface areas up to 814 m(2) g(-1) were obtained. TPC, TPC-ACs and a commercial AC (CAC) were tested for removing Tetracycline (TC) in aqueous phase, and systematic adsorption studies, including equilibrium, kinetics and thermodynamic aspects, were performed. Kinetics was well described by the pseudo-first order model for TPC, and by a pseudo second-order kinetic model for ACs. TC adsorption equilibrium data were also fitted by different isotherm models: Langmuir, Freundlich, Sips, Dubinin-Radushkevich, Dubinin-Astokov, Temkin, Redlich-Peterson, Radke-Prausnitz and Toth. The thermodynamic study confirmed that TC adsorption onto TPC-ACs is a spontaneous process. TC adsorption data obtained in the present study were compared with those reported in the literature, and differences were explained in terms of textural properties and surface functionalities. TPC-ACs had similar performances to those of commercial ACs, and might significantly improve the economic balance of the production of pyrolysis oil from waste tyres.

Kinetics of the hydrothermal treatment of tannin for producing carbonaceous microspheres.

Aqueous solutions of condensed tannins were submitted to hydrothermal carbonization (HTC) in a stainless steel autoclave, and the kinetics of hydrothermal carbon formation was investigated by changing several parameters: amount of tannin (0.5; 1.0; 1.5; 2.0 g in 16 mL of water), HTC temperature (130, 160, 180 and 200°C) and reaction times (from 1 to 720 h). The morphology and the structure of the tannin-based hydrothermal carbons were studied by TEM, krypton adsorption at -196°C and helium pycnometry. These materials presented agglomerated spherical particles, having surface areas ranging from 0.6 to 10.0 m(2) g(-1). The chemical composition of the hydrothermal carbons was found to be constant and independent of reaction time. HTC kinetics of tannin were determined and shown to correspond to first-order reaction. Temperature-dependent measurements led to an activation energy of 91 kJ mol(-1) for hydrothermal conversion of tannin into carbonaceous microspheres separable by centrifugation.

Epoxy resin/carbon black composites below the percolation threshold.

A set of epoxy resin composites filled with 0.25-2.0 wt.% of commercially available ENSACO carbon black (CB) of high and low surface area (CBH and CBL respectively) has been produced. The results of broadband dielectric spectroscopy of manufactured CB/epoxy below the percolation threshold in broad temperature (200 K to 450 K) and frequency (20 Hz to 1 MHz) ranges are reported. The dielectric properties of composites below the percolation threshold are mostly determined by alpha relaxation in pure polymer matrix. The glass transition temperature for CB/epoxy decreases in comparison with neat epoxy resin due to the extra free volume at the polymer-filler interface. At room temperature, the dielectric permittivity is higher for epoxy loaded with CBH additives. In contrast, at high temperature, the electrical conductivity was found to be higher for composites with CBL embedded. The established influence of the CB surface area on the broadband dielectric characteristics can be exploited for the production of effective low-cost antistatic paints and coatings working at different temperatures.

Graphene-like structure of activated anthracites.

The structure of a series of activated carbons prepared from anthracite by chemical activation has been studied using wide-angle x-ray scattering, molecular dynamics and Raman spectroscopy. The BET surface areas of the investigated samples are in the range 1500-3430 m(2) g(-1) and the average pore sizes vary from 0.75 to 1.35 nm. The diffraction measurements were carried out to a maximum value of the scattering vector K(max) = 22 Å(-1). The obtained diffraction data have been converted to a real space representation in the form of the pair correlation function. The structure of the studied samples consists of one or two graphite-like layers, stacked without spatial correlations. The size of the ordered layer region is approximately 24 Å. The atomic arrangement within an individual layer has been described in terms of paracrystalline ordering, in which lattice distortions are propagated proportionally to the square root of inter-atomic distances. The paracrystalline structure has been simulated by introducing the Stone-Thrower-Wales, mono-vacancy and di-vacancy defects, randomly distributed in the network. These defects lead to the formation of a defected network with the presence of non-hexagonal rings in which distortion of the structure extends outside of a defect region. Computer generated structural models have been relaxed at room temperature using the reactive empirical bond order potential for intra-layer interactions and the Lennard-Jones potential for inter-layer interactions. For such generated models the structure factors and the pair correlation functions were computed. A good agreement between the simulation results and the experimental data in both reciprocal and real space provides evidence for the correctness of the proposed models. The Raman data support the validity of these models. Porosity of the activated anthracites is discussed in relation to their defective structure.

Rice straw as precursor of activated carbons: activation with ortho-phosphoric acid.

Highly mesoporous activated carbons (ACs) with a mesopore fraction ranging from 42 to 73% were obtained by activation of rice straw (RS) with ortho-phosphoric acid (PA). Due to such a high mesoporosity, these ACs can be successfully used for pollutant removal in aqueous phase. The ACs were prepared at activation temperatures (T) ranging from 350 to 500 degrees C, using PA to RS weight ratios (R) from 0 to 1.6 and activation times from 0 to 2 h. They were characterised by nitrogen adsorption at -196 degrees C, SEM-EDX, and methylene blue adsorption. RS is a very heterogeneous material with a variable content of mineral matter: using the product of activated carbon yield multiplied by surface area (CxS(BET)) as the performance criterion, the best AC was produced at T=450 degrees C and R>or=1. These conditions lead to S(BET) higher than 500 m(2) g(-1) and a CxS(BET) around 270 m(2) g(-1).

X-ray microtomography studies of tannin-derived organic and carbon foams.

Tannin-based rigid foams of different bulk densities and their carbonized counterparts were investigated for the first time by X-ray microtomography. This method allowed acquisition of three-dimensional pictures of such highly porous materials. Through mathematical treatment of the images, extremely useful physical characteristics such as porosity, fraction of open cells, connectivity, tortuosity, and pore-size distribution were determined as a function of the foam's density. The obtained information was compared with independent data derived from pycnometry measurements and scanning electron microscope image analysis. The agreement was shown to be acceptable in the limit of the accuracy of the laboratory microtomograph (4 microm). Moreover, recalculating properties like permeability were shown to be quite possible based on the results of standard microtomography data.

Tannin-based rigid foams: a survey of chemical and physical properties.

Tannin-based rigid foams, prepared from 95% natural material, are suggested for replacing synthetic phenol-formaldehyde foams in various applications. For that purpose, a few physical properties were measured and reported here: resistance to fire and chemicals, absorption of various liquids, permeability, thermal conductivity and mechanical (compressive and tensile) strength. Modifying the composition through the use of boric and/or phosphoric acid allowed substantial increase of fire resistance. The materials were also found to present good resistance to strong acid and bases, and to solvents. High affinity for water, but limited one for organic liquids, was also evidenced. Finally, slightly anisotropic mechanical properties were measured. The materials present a brittle behaviour, whether tested in compression or traction; nevertheless, their strengths, as well as their thermal conductivities, are fully comparable with those of their phenolic counterparts. We show that such materials of vegetable origins can compete with synthetic ones for most of traditional applications.

Arsenic removal by iron-doped activated carbons prepared by ferric chloride forced hydrolysis.

Ferric chloride forced hydrolysis is shown to be a good method for increasing the iron content of activated carbons (ACs). Iron content increased linearly with hydrolysis time, and ACs with iron content as high as 9.4wt.% at 24h hydrolysis time could be prepared. The increase in iron content did not produce any modification in the textural parameters determined by nitrogen adsorption at 77K. Iron-based nanoparticles, homogeneous in size and well-dispersed in the carbon matrix, were obtained. Nanoparticles forming iron (hydr)oxide agglomerates at the outer surface of the carbon grains at hydrolysis times higher than 6h were also produced. The AC obtained after 6h of ferric chloride forced hydrolysis removed 94% of the arsenic present in a groundwater from the State of Chihuahua (Mexico), whereas the commercial AC used as precursor allowed the removal of only 14%. The lower performance in arsenic removal observed for AC prepared using long forced hydrolysis time (24h) is probably due to the existence of iron (hydr)oxides nanoparticles agglomerates, which once hydrated could prevent diffusion of arsenate (HAsO(4)(-)) towards the inner surface of the AC grain.

2-Steps KOH activation of rice straw: an efficient method for preparing high-performance activated carbons.

Effects of both pre-treatment and number of steps in KOH activation of raw rice straw (RS) on textural and adsorption properties of RS-derived activated carbons (ACs) were investigated. Three pre-treatment protocols were tested: mechanical, chemical by NaOH pulping, and a combination of both. Activation of RS-based materials was investigated, at a constant temperature of 800 degrees C, according to two ways: a 1-step simultaneous carbonisation-activation with KOH, and a 2-steps process: carbonisation followed by activation by KOH. 2-Steps KOH activation was more efficient in producing activated carbons with high surface areas and high methylene blue adsorption than 1-step KOH activation. Additionally, 2-steps KOH activation increased the yield of activated carbons having moderate ash content (8-14%). Surface areas as high as 1917 m(2)/g were obtained when the combined mechanical-chemical pre-treatment method and a 2-steps activation process were carried out.

Synthesis, characterization and performance in arsenic removal of iron-doped activated carbons prepared by impregnation with Fe(III) and Fe(II).

Arsenic removal from natural well water from the state of Chihuahua (Mexico) is investigated by adsorption using a commercial activated carbon (AC). The latter is used as such, or after oxidation by several chemicals in aqueous solution: nitric acid, hydrogen peroxide, and ammonium persulphate. Raw and oxidised activated carbons are fully characterised (elementary analysis, surface chemistry, pore texture parameters, pH(ZC), and TEM observation). Adsorption of As is measured in the aforementioned water, containing ca. 300 ppb of arsenic: removal of As is poor with the raw AC, and only the most oxidised carbons exhibit higher performances. By contrast, iron-doped ACs are much more efficient for that purpose, though their As uptake strongly depends on their preparation conditions: a number of samples were synthesised by impregnation of raw and oxidised ACs with HCl aqueous solutions of either FeCl(3) or FeCl(2) at various concentrations and various pH. It is shown that iron(II) chloride is better for obtaining high iron contents in the resultant ACs (up to 8.34 wt.%), leading to high As uptake, close to 0.036 mg As/g C. In these conditions, 100% of the As initially present in the natural well water is removed, as soon as the Fe content of the adsorbent is higher than 2 wt.%.

Activated carbons prepared from wood particleboard wastes: characterisation and phenol adsorption capacities.

The problems of valorisation of particleboard wastes on one hand, and contamination of aqueous effluents by phenolic compounds on the other hand, are simultaneously considered in this work. Preparation of activated carbons from a two steps thermo-chemical process, formerly designed for generating combustible gases, is suggested. The resultant carbonaceous residue is activated with steam at 800 degrees C. Depending on the preparation conditions, surface areas within the range 800-1300 m(2)/g are obtained, close to that of a commercial activated carbon (CAC) specially designed for water treatment and used as a reference material. The present work shows that particleboard waste-derived activated carbons (WAC) are efficient adsorbents for the removal of phenol from aqueous solutions, with maximum measured capacities close to 500 mg/g. However, most of times, the adsorption capacities are slightly lower than that of the commercial material in the same conditions, i.e., at equilibrium phenol concentrations below 300 ppm. Given the extremely low cost of activated carbons prepared from particleboard waste, it should not be a problem to use it in somewhat higher amounts than what is required with a more expensive commercial material. Phenol adsorption isotherms at 298 K were correctly fitted by various equations modelling type I and type II isotherms for CAC and WAC, respectively. Phenol adsorption isotherms of type II were justified by a 3-stages adsorption mechanism.

Influence of the demineralisation on the chemical activation of Kraft lignin with orthophosphoric acid.

The preparation of activated carbons (ACs) from the thermal decomposition of mixtures of orthophosphoric acid (PA) and either as-received softwood Kraft lignin, KL, or demineralised one, KL(d), has been investigated. Activation with PA has been studied for a PA/lignin ratio of 1 (dry ash-free basis) and 1h carbonisation time at final temperatures of 400, 500 and 600 degrees C. The yield, surface area, porosity, surface chemistry and methylene blue adsorption capacity have been determined. All ACs were found to be essentially microporous, with surface areas higher than 800 m(2)/g and a maximum value of nearly 1200 m(2)/g for the carbon prepared at 600 degrees C from KL. In order to study the influence of temperature on the properties of the ACs prepared from KL and KL(d), the latter precursors were analysed by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and X-ray diffraction (XRD). We have concluded that the very different characteristics of the ACs obtained from KL and KL(d) are due to the presence or not of mineral matter during carbonisation, but mainly to the demineralisation process itself, which produces polymerisation of the raw lignin. Methylene blue adsorption was found to be higher for ACs prepared from KL, mainly because of their higher ash and sulphur contents.