Olga I. Poddubnaya

Highly stable performance of supercapacitors from phosphorus-enriched carbons

Phosphorus-rich microporous carbons (P-carbons) prepared by a simple H(3)PO(4) activation of three different carbon precursors exhibit enhanced supercapacitive performance in 1 M H(2)SO(4) when highly stable performance can be achieved at potentials larger than the theoretical decomposition potential of water. This ability of P-carbons greatly enhances the energy density of supercapacitors that are capable of delivering 16 Wh/kg compared to 5 Wh/kg for the commercial carbon. An intercept-free multiple linear regression model confirms the strongest influence of phosphorus on capacitance together with micropores 0.65-0.83 nm in width that are the most effective in forming the electric double layer.

Elucidation of the ion binding mechanism in heterogeneous carbon-composite adsorbents

Abstract A two-stage analysis of the mechanism of metal ion binding by a heterogeneous adsorbent was developed. In the first stage, a continuous proton affinity distribution was calculated from potentiometric titration data using the CONTIN method with a Langmuir kernel. Electrostatic effects were accounted for using a diffuse layer model. In the second stage, the parameters obtained from the continuous distribution function (i.e. the number of different types of surface sites, site densities and their protonation constants) were utilized in a discrete distribution to represent the adsorbent in surface complexation and double layer models using the GRFIT speciation code. This information on the surface groups was applied to metal ion potentiometric titration experiments to calculate the surface complexation equilibrium constants of the metal ions and hence elucidate the mechanism of ion binding to these sites. The proposed method was applied successfully to the adsorption of Sr and Cu ions on carbon-composite adsorbents, KAU-mod and SCN-mod. The continuous distribution method (CONTIN) revealed three types of surface sites within these carbon-composite adsorbents with pK values ranging between 3.5–4.1, 5.3–6.3 and 7.7–8.2. The analysis of the metal ion adsorption data using the GRFIT speciation code showed that only the first two surface sites were capable of forming surface complexes with the Sr ions, and that only the first site governed the adsorption of the Cu ions.

Functionalization of Carbon and Silica Gel by Phosphoric Acid

Functionalization of polymer-based carbon (SCS-3) and silica gel (SG60) by phosphoric acid at 800°C was investigated. It was shown that heat treatment of carbon and silica gel in the presence of phosphoric acid at 800°C provides a way of functionalizing materials with phosphorus-containing surface groups. Functionalization of the finished carbon occurs as a surface reaction while destruction of the silica gel structure was observed. Functionalization with phosphoric acid creates new acid surface groups — phosphorus-containing and oxygen-containing in the case of carbon and only phosphorus-containing in the case of silica gel. Enhancement of the amount of acid surface groups by functionalization improves the metal-ion binding properties of both carbon and silica gel.

Comparison of heterogeneous pore models QSDFT and 2D-NLDFT and computer programs ASiQwin and SAIEUS for calculation of pore size distribution

The pore size distributions (PSD) of selected carbons were calculated from their nitrogen adsorption isotherms using both the QSDFT model implemented in ASiQwin version 3.0 software (Quantachrome Instruments) and 2D-NLDFT model implemented in SAIEUS software (Micromeritics). The results showed that both the QSDFT and the 2D-NLDFT methods give similar PSDs despite the different methods for accounting for the heterogeneity of the carbon adsorbent. The characteristic features of the methods and software were discussed and possible improvements were proposed.

Surface heterogeneity of carbon–silica adsorbents studied on the basis of the complex adsorption investigations

Abstract Silica-gel (SG) Si-60 was coated by pyrolytic coke using CVD from methylene chloride. The porous structure of carbon–silica adsorbents (carbosils) was analyzed on the basis of nitrogen adsorption isotherms at 77 K. It has been shown that carbosils are mesoporous adsorbents with complete absence of micropores. Total pore volume and surface area decrease during the carbon deposition on silica surface. Heterogeneity of surface of carbosil samples was estimated by adsorption energy distribution (AEDs) calculated from adsorption isotherms of nitrogen at 77 K and hexane, cyclohexane, benzene and chloroform adsorption isotherms at 473 and 483 K. AEDs were obtained by solving Adsorption Integral Equation with Hill-de Boer kernel function. N2 AEDs show that distribution of silica active surface sites responsible for N2 adsorption at 77 K is not altered during the carbon deposition. However, coke deposition leads to improving of polar–apolar surface composition, resulting in increase of adsorption of apolar (hexane) and decrease of adsorption of polar (chlorophorm) substances. The main changes observed in AEDs are connected with decreasing of silica accessible area at stable carbon structure during CVD of carbon.

Phosphoric Acid and Steam as Activation Agents for Carbonized Porous Polymer Surfaces

Carbon sorbents derived from the copolymer of 4,4′-bis(maleimidodiphenyl)methane and divinylbenzene are described. The influence of phosphoric acid treatment and the type of carrier gas used in the carbonization process were studied. Solid-phase extraction experiments were conducted to determine their impact on the sorption properties of these new sorbents. The recoveries and breakthrough volumes for phenolic compounds preconcentrated from water were studied.

Ethyl tert-butyl ether synthesis using carbon catalysts from lignocellulose

Porous structure, surface chemistry and catalytic properties of carbon catalysts with various acid groups in ethyl tert-butyl ether synthesis were studied. Carbon catalysts were obtained by phosphoric acid activation followed by sulphonation of lignocellulosic feedstocks of different origin. Porous structure of carbon catalysts was characterized by nitrogen adsorption. Surface chemistry was investigated by potentiometric titration. Carbon catalysts obtained from lignosulphonate and sucrose showed the highest catalytic activity in ethyl tert-butyl ether synthesis (reaction rate at 120℃ is 4.3–5.2 × 10−6 mol·g−1·s−1), is comparable to activity of Amberlyst-15 (5.0 ×10−6 mol·g−1·s−1). Calculated turnover frequency (TOF) of sulphonic groups (pKa = −2.5) equals 0.00116 s−1, polyphosphate groups (pKa = 1.9) 0.01019 s−1, strong carboxylic (pKa = 3.6) 0.00202 s−1 and 0 s−1 for weak carboxylic (pKa = 6) groups. The activity of acid groups with pKa lower than 3.6 is higher than the activity of sulphonic groups of Amberlyst-15 (TOF 0.00115 s−1).

Assessment of the structural evolution of polyimide-derived carbons obtained by phosphoric acid activation using Fourier transform infrared and Raman spectroscopy:

Two series of carbons obtained by carbonization of porous copolymer of 4,4′-bis(maleimidodiphenyl) methane (50 mol%) and divinylbenzene (50 mol%) with and without phosphoric acid (impregnation ratio 1.1) at temperatures 400–1000℃. The carbons were characterized using elemental analysis, nitrogen adsorption, potentiometric titration, Fourier transform infrared and Raman spectroscopy. It has been shown that phosphoric acid causes structural and chemical changes in polyimide copolymer as compared to thermally treated carbons. Structural changes: phosphoric acid promotes transformation of polyimide copolymer to carbon structure at lower temperatures as compared to thermally treated carbons. Phosphoric acid is responsible for formation of highly developed micro/mesoporous structure that is different from that of thermally treated carbons. Chemical changes: phosphoric acid causes elimination of hydrogen and nitrogen, introduction of phosphorus and oxygen as phosphate-like structure. Significant amount of phosphorus imparts acid properties to carbon.

Carbon Materials from Technical Lignins: Recent Advances

Lignin, a major component of lignocellulosic biomass, is generated in enormous amounts during the pulp production. It is also a major coproduct of second generation biofuels. The effective utilization of lignin is critical for the accelerated development of the advanced cellulosic biorefinery. Low cost and availability of lignin make it attractive precursor for preparation of a range of carbon materials, including activated carbons, activated carbon fibers (CF), structural CF, graphitic carbons or carbon black that could be used for environmental protection, as catalysts, in energy storage applications or as reinforcing components in advanced composite materials. Technical lignins are very diverse in terms of their molecular weight, structure, chemical reactivity, and chemical composition, which is a consequence of the different origin of the lignin and the various methods of lignin isolation. The inherent heterogeneity of lignin is the main obstacle to the preparation of high-performance CF. Although lignin-based CF still do not compete with polyacrylonitrile-derived CF in mechanical properties, they nevertheless provide new markets through high availability and low production costs. Alternatively, technical lignin could be used for production of carbon adsorbents, which have very high surface areas and pore volumes comparable to the best commercial activated carbons. These porous carbons are useful for purifying gas and aqueous media from organic pollutants or adsorption of heavy metal ions from aqueous solutions. They also could be used as catalysts or electrodes in electrochemical applications.

Impact of the Carbonization Atmosphere on the Properties of Phosphoric Acid-activated Carbons from Fruit Stones

Activated carbons were obtained by phosphoric acid activation of a mixture of fruit stones (apricot and peach) in two different atmospheres (argon and air) at various temperatures in the 400–1000°C range. The evolution of several characteristic parameters of the resulting carbons (bulk density, yield, BET surface area, ultramicropore, supermicropore and mesopore volumes and cation-exchange capacity) with activation temperature was examined. The above parameters were re-calculated on a volume basis (practical effectiveness) and a volume-yield basis (economic efficiency). It was concluded that carbons obtained in an argon atmosphere exhibit some practical advantages over those obtained in air regarding cation adsorption, although those obtained in air may represent an interesting alternative regarding porous structure.