Roger Gadiou

Size-Dependent Hydrogen Sorption in Ultrasmall Pd Clusters Embedded in a Mesoporous Carbon Template

Hydrogen sorption properties of ultrasmall Pd nanoparticles (2.5 nm) embedded in a mesoporous carbon template have been determined and compared to those of the bulk system. Downsizing the Pd particle size introduces significant modifications of the hydrogen sorption properties. The total amount of stored hydrogen is decreased compared to bulk Pd. The hydrogenation of Pd nanoparticles induces a phase transformation from fcc to icosahedral structure, as proven by in situ XRD and EXAFS measurements. This phase transition is not encountered in bulk because the 5-fold symmetry is nontranslational. The kinetics of desorption from hydrogenated Pd nanoparticles is faster than that of bulk, as demonstrated by TDS investigations. Moreover, the presence of Pd nanoparticles embedded in CT strongly affects the desorption from physisorbed hydrogen, which occurs at higher temperature in the hybrid material compared to the pristine carbon template.

Characterization of Carbon Surface Chemistry by Combined Temperature Programmed Desorption with in Situ X-ray Photoelectron Spectrometry and Temperature Programmed Desorption with Mass Spectrometry Analysis

The analysis of the surface chemistry of carbon materials is of prime importance in numerous applications, but it is still a challenge to identify and quantify the surface functional groups which are present on a given carbon. Temperature programmed desorption with mass spectrometry analysis (TPD-MS) and X-ray photoelectron spectroscopy with an in situ heating device (TPD-XPS) were combined in order to improve the characterization of carbon surface chemistry. TPD-MS analysis allowed the quantitative analysis of the released gases as a function of temperature, while the use of a TPD device inside the XPS setup enabled the determination of the functional groups that remain on the surface at the same temperatures. TPD-MS results were then used to add constraints on the deconvolution of the O1s envelope of the XPS spectra. Furthermore, a better knowledge of the evolution of oxygen functional groups with temperature during a thermal treatment could be obtained. Hence, we show here that the combination of these two methods allows to increase the reliability of the analysis of the surface chemistry of carbon materials.

Understanding the mechanism of hydrogen uptake at low pressure in carbon/palladium nanostructured composites

The hydrogen sorption/desorption mechanism below 1 bar at room temperature in porous carbons loaded with nanosized metal particles is not well understood and remains a controversial subject in the literature. The aim of this work is to provide a comprehensive view on the hydrogen sorption/desorption process on C/Pd composites by carefully analysing the phenomena involved during the hydrogen cycles in relation with the material characteristics (amount, size and chemical surface state of the palladium nanoparticles). The C/Pd composites consist of templated microporous carbon in which nanosized palladium particles were homogeneously dispersed. Depending on the synthesis condition, the amount of Pd loaded ranges between 1.4 and 12 wt% and the particle size is ranging from 3 to 15 nm. The presence of a palladium oxide layer on the Pd particle surface is revealed by XPS; the amount of this layer depends on the particle size. The hydrogen sorption/desorption measurements indicate that the total hydrogen amount sorbed on the C/Pd composite exceeds the amount required for the formation of β-palladium hydride. This hydrogen sorption excess is attributed in the literature to the spillover effect. To verify this assumption, we performed a careful exploitation of the hydrogen isotherms along with in situ and ex situ characterizations on the C/Pdx composites and a palladium oxide powder. For the first sorption/desorption cycle, two hydrogen sorption steps were identified and the sorbed hydrogen volume in each step was quantified. The first step which is irreversible is assigned to the reduction of PdO leading to the formation of Pd and water. The second step corresponds to the formation of the palladium hydride (PdHy), a step which is influenced by the presence of water. The processes involved in these two steps are strongly dependent on the Pd particle size. The results presented here clearly demonstrate that the PdO reduction is the predominant phenomenon, explaining the hydrogen uptake excess measured in the first cycle. Although a spillover effect cannot be excluded, the experimental data indicate that its possible contribution would remain significantly much weaker than the contribution due to the PdO reduction. To our knowledge, this is the first time that the effect of the Pd oxide layer on the hydrogen sorption (at low pressure and room temperature) on C/Pdx composites has been experimentally proved and quantified.

Microporous carbon adsorbents with high CO2 capacities for industrial applications

In this study we attempt to investigate the potential use of two zeolite template carbon (ZTC), EMT-ZTC and FAU-ZTC, to capture CO(2) at room temperature. We report their high pressure CO(2) adsorption isotherms (273 K) that show for FAU-ZTC the highest carbon capture capacity among published carbonaceous materials and competitive data with the best organic and inorganic adsorbing frameworks ever-known (zeolites and mesoporous silicas, COFs and MOFs). The importance of these results is discussed in light of mitigation of CO(2) emissions. In addition to these new experimental CO(2) adsorption data, we also present new insight into the adsorption process of the two structures by Monte Carlo simulations: we propose that two separate effects are responsible for the apparent similarity of the adsorption behaviour of the two structures: (i) pore blocking occurring on EMT-ZTC, and (ii) the change of the carbon polarizability due to the extreme curvature of FAU-ZTC.

Influence of Surface Chemistry on the Adsorption of Oxygenated Hydrocarbons on Activated Carbons

The objective of this work was to study the adsorption of different oxygenated hydrocarbons (methanol, ethanol, 1 and 2-butanol, methyl acetate) on activated carbons from organic mixtures with cyclohexane. Three activated carbons prepared by thermal and chemical treatments of a commercial carbon were employed for this purpose. Their textural properties were found to be similar, whereas their surface chemistries were modified, as shown by temperature-programmed desorption coupled to mass spectrometry (TPD-MS) and X-ray photoelectron spectroscopy (XPS). The adsorption isotherms were obtained by depletion method, and the analysis of adsorbed species was evaluated by TPD-MS to obtain new insight into the interactions between the different hydrocarbons and the carbon surface. Ethanol leads to a high-energy interaction between its hydroxyl function and the oxygenated surface groups and also to a lower energy interaction between the aliphatic part of the molecule and the carbon material. The desorption activation energy for this hydrophilic interaction is high (50 to 105 kJ/mol), and it is related to the nature of the carbon surface groups. The relative importance of these two interactions depend on the size of the alcohol/methanol is similar to ethanol, whereas butanols lead to more dispersive interactions. Methyl-acetate cannot undergo this kind of strong interaction and behaves like cyclohexane, having desorption activation energies ranging between 25 and 45 kJ/mol no matter the molecule and the carbon surface chemistry.

High resolution solid state 2D NMR analysis of biomass and biochar.

Solid state NMR methods are required to analyze biomass as a function of its chemical or biological treatment for biofuels, chemicals, or biochar production. The native polymers network in lignocellulosic biomass and other solid materials, such as coal, coke, or biochar, can hardly be analyzed by liquid state NMR due to their poor swelling ability without chemical modification. A (1)H-(13)C two-dimensional heteronuclear correlation (HETCOR) experiment with frequency-switched Lee-Goldburg (FSLG) irradiation is performed on a high field spectrometer (750 MHz). This method leads to previously unattained resolution for biomass and biochar and offers a unique ability to reveal their chemical composition. The formation of aromatic moieties from carbohydrates and lignin thermal conversion is clearly distinguished. This method can be applied to all other carbonaceous materials.

Inorganic–Organic Thin Implant Coatings Deposited by Lasers

The lifetime of bone implants inside the human body is directly related to their osseointegration. Ideally, future materials should be inspired by human tissues and provide the material structure-function relationship from which synthetic advanced biomimetic materials capable of replacing, repairing, or regenerating human tissues can be produced. This work describes the development of biomimetic thin coatings on titanium implants to improve implant osseointegration. The assembly of an inorganic-organic biomimetic structure by UV laser pulses is reported. The structure consists of a hydroxyapatite (HA) film grown onto a titanium substrate by pulsed-laser deposition (PLD) and activated by a top fibronectin (FN) coating deposited by matrix-assisted pulsed laser evaporation (MAPLE). A pulsed KrF* laser source (λ = 248 nm, τ = 25 ns) was employed at fluences of 7 and 0.7J/cm(2) for HA and FN transfer, respectively. Films approximately 1500 and 450 nm thick were obtained for HA and FN, respectively. A new cryogenic temperature-programmed desorption mass spectrometry analysis method was employed to accurately measure the quantity of immobilized protein. We determined that less than 7 μg FN per cm(2) HA surface is adequate to improve adhesion, spreading, and differentiation of osteoprogenitor cells. We believe that the proposed fabrication method opens the door to combining and immobilizing two or more inorganic and organic materials on a solid substrate in a well-defined manner. The flexibility of this method enables the synthesis of new hybrid materials by simply tailoring the irradiation conditions according to the thermo-physical properties of the starting materials.

Mechanism of Chloride Inhibition of Bilirubin Oxidases and Its Dependence on Potential and pH

Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 β.

Modulation of the Reactivity of a WO3/Al Energetic Material with Graphitized Carbon Black as Additive

Although pyrotechnic performance is fundamental, the strong mechanical and electrostatic intrinsic sensitivities of nanothermite energetic composites represent an obstacle to their development. The addition of a ternary component to the classical binary energetic composite appears to be a promising idea to overcome the problem. A carbon black additive (V3G) was used on a WO3/Al nanothermite. The effect of the pristine and modified carbon particles on the mechanical and electrical sensitivities of the composites was measured together with the pyrotechnic properties. The results show a complete desensitization to friction with a ball-milled carbon when the combustion velocity is slightly reduced.

Tuning the Photocatalytic Activity and Optical Properties of Mesoporous TiO2 Spheres by a Carbon Scaffold

The photoelectrochemical response and catalytic efficiency towards phenol photooxidation of mesoporous titania particles with spherical morphology have been explored. The catalysts were synthesized in two different arrangements using carbon spheres in a dual role as support and morphology director: hollow spherical titania particles and dense structures where the titania shell is surrounding a carbon core. Although the synthesized titania hollow spheres exhibited a similar photoelectrochemical behavior and optical properties than commercial P25, they showed a better photocatalytic response towards phenol photo-oxidation in terms of pollutant mineralization. This behavior cannot be explained in terms of the crystallinity (found to be higher for P25) and has been attributed to both confinement effects in the mesoporosity of these catalysts as well as to the spherical morphology of titania particles. The spherical arrangement of the titania surface would favor the fast motion of the charge carriers and minimize recombination processes. On the other hand, no clear contribution of the carbon phase to the enhanced photocatalytic response, since quite similar performance is observed for the hollow spheres and the core/shell composite. However, separation and filtration of the catalysts become easier for the carbon/titania composite, thereby improving the so-called practical efficiency.