Cristian I. Contescu

Surfaces of nanoparticles and porous materials

Preparation, characterization, and transport properties of nanoparticles and porous solids: synthesis of a polysilazane coating on a silica gel via chemical surface coating comparing liquid and gas phase chlorosilylations - a composition and porosity study preparation of molecular sieves by pillaring of synthetic clays engineering of nanosize superparamagnetic particles for use in magnetic carrier technology acid-base behaviour of surfaces of porous materials electro-optical spectroscopy of colloidal systems NMR studies of colloidal oxides polymer surface dynamics using surface-modified glasses via dynamic contact angle measurements microporous structure of collagen fibres adsorption a onto oxides - the role of diffusion electrokinetic phenomena in porous media and around aggregates transport processes in microemulsions structural effects on diffusivity within aggregates of colloidal zirconia. Adsorption from vapour/gas phase onto nanoparticle and porous solids: characterization of energetically heterogeneous surfaces from experimental adsorption isotherms computer simulations of the structural and thermodynamic properties of adsorbed phases surface heterogeneity effects on adsorption equilibria and kinetics -rationalizations of the Elovich equation single and multicomponent adsorption equilibria of hydrocarbons on activated carbon - the role of microspore size distribution surface and structural properties of modified porous silicas surface chemistry of activated carbon materials - state of the art and implications of adsorption nanodimensional magnetic assembly of confined O2 heat of adsorption of pure gas and multicomponent gas mixtures on microporous adsorbents. Adsorption from the liquid phase onto nanoparticles and porous solids: charge regulation at the surface of a porous solid surface ionization and complexation the surface charge of alkali halides in their saturated solutions ionic adsorbates on hydrophobic surfaces adsorption of metal ions to humic acid hydrous metal oxides as adsorbents for aqueous heavy metals adsorption of ions on alumina protein adsorption onto latex particles adsorption of pharmaceutical organics on porous materials

Surface acidity of carbons characterized by their continuous pK distribution and Boehm titration

The surface acidity of several activated carbons was evaluated using the Boehm and potentiometric titration methods. Experimental conditions (particle size, rate of titration) were varied and several procedures to collect raw data for proton binding isotherms (slow continuous addition of titrant, incremental addition conditioned by a pH stability criterion and batch experiments) were compared. Proton binding isotherms were analyzed at two levels. First, continuous pK distributions were derived and the inventory of surface functionalities was characterized in terms of amount and pK values. Then these data afforded calculations of the base neutralizing capacity of the carbons studied at any pH level. The results were compared with those from Boehm titration of the same samples. Best results were found for titration of finely powdered carbons. Continuous titration denatures the binding isotherms and damps significant features in the pK distribution. On the other hand, titration by incremental addition with a pH stability criterion provides reliable results which can be compared to those retrieved from Boehm titration provided the proper common metric is used. The agreement was quantitative when the equilibrium pH of the supernatant of the Boehm bases was taken as the endpoint for counting neutralized sites. Advantages of analyzing potentiometric titration data in terms of its acidity distribution function with respect to providing a comprehensive characterization of the inventory of surface functionalities as well as its predictive power to assess a carbons base neutralizing capacity under different experimental conditions are discussed.

Acid buffering capacity of basic carbons revealed by their continuous pK distribution

Abstract In a previous paper [Carbon, 1997, 35, 83]it was shown for a series of activated carbons with acidic behavior that surface characterization with a continuous pK distribution was possible based on potentiometric titration provided that several experimental precautions were taken during data collection. The number and strength of acid functionalities calculated from the proton affinity distribution (PAD) were in quantitative agreement with those measured by Boehm titration of the same samples. In this work the surface basicity of an activated carbon was investigated using the same analytical procedures. The factors that may denature the results of potentiometric titration have been analyzed; it is found that incremental addition controlled by a pH stability criterion (the “fully automatic” titration) provides reproducible results even for large particle granulation. From a practical point of view this shows that the results of this optimized analytical method are reliable. The acid buffering capacity of basic carbon, with the quality and quantity of proton binding groups, reveals that the most important proton binding processes occur in the ranges of pH 4–7, 8.4–8.6 and >9.5; the second process accounts for most of the acid buffering capacity of this carbon. The basic properties of the carbon surface are proposed to be due to the combination of redox reactions and proton transfer to/from the surface and the supporting electrolyte.

Topological Defects: Origin of Nanopores and Enhanced Adsorption Performance in Nanoporous Carbon

A scanning transmission electron microscopy investigation of two nanoporous carbon materials, wood-based ultramicroporous carbon and poly(furfuryl alcohol)-derived carbon, is reported. Atomic-resolution images demonstrate they comprise isotropic, three-dimensional networks of wrinkled one-atom-thick graphene sheets. In each graphene plane, nonhexagonal defects are frequently observed as connected five- and seven-atom rings. Atomic-level modeling shows that these topological defects induce localized rippling of graphene sheets, which interferes with their graphitic stacking and induces nanopores that lead to enhanced adsorption of H(2) molecules. The poly(furfuryl alcohol)-derived carbon contains larger regions of stacked layers, and shows significantly smaller surface area and pore volume than the ultramicroporous carbon.

Effect of alumina supports on the properties of supported nickel catalysts

Abstract The objective of this study is to compare the properties and performance of a series of alumina supported nickel catalysts where the only variable is the origin of the alumina. Three commercial aluminas were subjected to a battery of characterization techniques including dissolution studies, X-ray diffraction, nickel adsorption, and measurements of the pH of their point of zero charge. Nickel catalysts formed on these supports were characterized by temperature-programmed reduction, temperature-programmed reaction of carbon monoxide, and temperature-programmed surface reaction. The performance of the catalysts was assessed by CO/H2 methanation studies. The results from the characterization and performance studies demonstrated that two types of nickel can exist on these alumina supports: a surface nickel and an incorporated nickel. The extent of partitioning between these two states is controlled by the dissolution properties of each alumina when in contact with nickel electrolytes at low pH.

Crown ethers in graphene

Crown ethers are at their most basic level rings constructed of oxygen atoms linked by two- or three-carbon chains. They have attracted attention for their ability to selectively incorporate various atoms or molecules within the cavity formed by the ring. However, crown ethers are typically highly flexible, frustrating efforts to rigidify them for many uses that demand higher binding affinity and selectivity. Here we present atomic-resolution images of the same basic structures of the original crown ethers embedded in graphene. This arrangement constrains the crown ethers to be rigid and planar. First-principles calculations show that the close similarity of the structures should also extend to their selectivity towards specific metal cations. Crown ethers in graphene offer a simple environment that can be systematically tested and modelled. Thus, we expect that our finding will introduce a new wave of investigations and applications of chemically functionalized graphene.

Hydrogen confinement in carbon nanopores: extreme densification at ambient temperature.

In-situ small-angle neutron scattering studies of H(2) confined in small pores of polyfurfuryl alcohol-derived activated carbon at room temperature have provided for the first time its phase behavior in equilibrium with external H(2) at pressures up to 200 bar. The data were used to evaluate the density of the adsorbed fluid, which appears to be a function of both pore size and pressure and is comparable to the density of liquid H(2) in narrow nanopores at ∼200 bar. The surface-molecule interactions responsible for densification of H(2) within the pores create internal pressures that exceed the external gas pressure by a factor of up to ∼50, confirming the benefits of adsorptive storage over compressive storage. These results can be used to guide the development of new carbon adsorbents tailored for maximum H(2) storage capacities at near-ambient temperatures.

Thermal Treatment Effects on Charge Storage Performance of Graphene-Based Materials for Supercapacitors

Graphene materials were synthesized by reduction of exfoliated graphite oxide and then thermally treated in nitrogen to improve the surface area and their electrochemical performance as electrical double-layer capacitor electrodes. The structural and surface properties of the prepared reduced graphite oxide (RGO) were investigated using atomic force microscopy, scanning electron microscopy, Raman spectra, X-ray diffraction pattern analysis, and nitrogen adsorption/desorption studies. RGO forms a continuous network of crumpled sheets, which consist of large amounts of few-layer and single-layer graphenes. Electrochemical studies were conducted by cyclic voltammetry, impedance spectroscopy, and galvanostatic charge-discharge measurements. The modified RGO materials showed enhanced electrochemical performance, with maximum specific capacitance of 96 F/g, energy density of 12.8 Wh/kg, and power density of 160 kW/kg. These results demonstrate that thermal treatment of RGO at selected conditions is a convenient and efficient method for improving its specific capacitance, energy, and power density.

Modern approaches to studying gas adsorption in nanoporous carbons

Conventional approaches to understanding the gas adsorption capacity of nanoporous carbons have emphasized the relationship with the effective surface area, but more recent work has demonstrated the importance of local structures and pore-size-dependent adsorption. These developments provide new insights into local structures in nanoporous carbon and their effect on gas adsorption and uptake characteristics. Experiments and theory show that appropriately tuned pores can strongly enhance local adsorption, and that pore sizes can be used to tune adsorption characteristics. In the case of H2 adsorbed on nanostructured carbon, quasielastic and inelastic neutron scattering probes demonstrate novel quantum effects in the motion of adsorbed molecules.

Microstructure-Dependent Gas Adsorption: Accurate Predictions of Methane Uptake in Nanoporous Carbons

We present a framework for rapidly predicting gas adsorption properties based on van der Waals density functional calculations and thermodynamic modeling. Utilizing this model and experimentally determined pore size distributions, we are able to accurately predict uptakes in five activated carbon materials without empirical potentials or lengthy simulations. Our results demonstrate that materials with smaller pores and higher heats of adsorption can still have poor adsorption characteristics due to relatively low densities of highly adsorbent pores.