Specific Surface Area Measurements Research Articles

Crystal Structure‐ and Morphology‐Driven Electrochemistry of Iron Oxide Nanoparticles in Hydrogen Peroxide Detection

AbstractVarious iron oxide nanoparticles with different morphologies are synthesized and subsequently tested for their conductivity and electrocatalytic activity toward hydrogen peroxide. The morphology and chemical and phase composition of iron oxide nanoparticles are evaluated employing scanning electron microscopy, X‐ray diffraction, 57Fe Mössbauer spectroscopy, and Brunauer–Emmett–Teller specific surface area measurements. The electrochemical properties of the as‐prepared sensors are estimated by electrochemical impedance spectroscopy. It is found that α‐Fe2O3 nanoparticles with the sticks morphology exhibit the best conductivity response among all the tested phases and morphologies. Moreover, it is predicted that conductivity of different iron oxides can be connected with a number of vacancies in their crystal structure. Furthermore, the influence of surface area and porosity of the material on the conductivity can be omitted. Finally, the electrocatalytic activity of iron oxide nanoparticles toward hydrogen peroxide is confirmed by means of cyclic voltammetry. The obtained results perfectly reflect those derived from electrochemical impedance spectroscopy and indicate that glassy carbon electrodes modified with the sticks morphology of α‐Fe2O3 hold a huge potential for hydrogen peroxide detection.

Combined mechanochemical/thermal annealing approach for the synthesis of Co9Se8 with potential optical properties

Synthesis of Co9Se8 phase from metallic cobalt and selenium powders by one-pot mechanochemical synthesis in a planetary ball mill and subsequent thermal treatment in argon atmosphere is reported. Crystal structure and morphology of the products were characterized by X-ray diffraction, specific surface area measurements, and transmission electron microscopy. Our study revealed that only mixture with excess of Co resulted in the formation of higher fraction of the Co9Se8 phase. While milling of Co and Se in 13.5:8 ratio resulted in the synthesis of nanocrystalline hexagonal CoSe (freboldite) and only around 13 wt.% of the targeted Co9Se8 phase, additional isothermal treatment at 450 °C increased yield of the Co9Se8 phase to more than 70%. Optical properties of the product with the highest Co9Se8 fraction showed broad absorption in whole UV–Vis optical region and band-gap energy of 1.93 eV, blue-shifted relative to the bulk Co9Se8. Both UV–Vis and photoluminescence spectra indicated quantum size effect of the finest nanocrystals in Co9Se8 product. The new combined approach represents a simple, fast, and easy up-scalable route for the synthesis of Co9Se8.

Influence of ammonia on properties of TiO2‐MgFe2O4 as high visible‐light active photocatalysts for the degradation of Rhodamine B

AbstractThe influence of ammonia on TiO2‐MgFe2O4 catalysts synthesized via a hydrolysis and co‐precipitation, followed by calcination at 500°C, was studied. Two different catalysts, TiO2‐MgFe2O4 (Am) and TiO2‐MgFe2O4 (W), were prepared using Ti‐precursors, which were synthesized by the hydrolysis of Ti‐butoxide with and without ammonia. The resulting TiO2‐MgFe2O4 catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), specific surface area measurements, X‐ray diffraction, and UV‐vis diffuse reflectance techniques. It was revealed that strong electronic coupling exists between the TiO2 and MgFe2O4 components within the TiO2‐MgFe2O4 catalysts. Photocatalytic activity toward Rhodamine B (RhB) was investigated in aqueous solution under visible light irradiation. TiO2‐MgFe2O4 (W) was found to be an effective catalyst and had several advantages over TiO2‐MgFe2O4 (Am). These results clearly highlight that ammonia had a significant influence on the photocatalytic activity of the TiO2‐MgFe2O4 catalysts. Therefore, the results reported herein indicate that TiO2‐MgFe2O4 is a green, low‐cost, and highly efficient photocatalyst for environmental remediation.

One-pot synthesis of LiEuTiO4 as an anode material for lithium-ion batteries operating at 0.8 V

Abstract A single phase of LiEuTiO4 was prepared by a one-pot synthesis, assisted by a solution plasma technique in an ionic liquid. We characterized the sample by X-ray powder diffraction, scanning electron microscopy, specific surface area measurements, X-ray photoelectron spectroscopy, and elemental analysis. Our single-phase material exhibited better electrochemical performance than that of a sample prepared via multi-step synthesis, which we attribute to the smaller particle size and larger specific surface area of the former. The single phase LiEuTiO4 showed an initial discharge capacity of 237.3 mAhg−1 and good capacity retention of 97.0% after 100 cycles. Our samples also had a high specific capacity of 156.2 mAhg−1 at 5 Ag−1. These results demonstrate that one-pot synthesis is effective for preparing LiEuTiO4 with small particle size and high specific surface area offering a potential approach to LiEuTiO4 as a low-operating voltage anode material for lithium ion batteries.

Structural and optical properties of nanostructured copper sulfide semiconductor synthesized in an industrial mill

Chalcogenide nanostructured semiconductor, copper sulfide (CuS) was prepared from copper and sulfur powders in stoichiometric ratio by a simple, fast, and convenient one-step mechanochemical synthesis after 40 min of milling in an industrial eccentric vibratory mill. The kinetics of the mechanochemical synthesis and the influence of the physical properties of two Cu powder precursor types on the kinetics were studied. The crystal structure, physical properties, and morphology of the product were characterized by X-ray diffraction (XRD), the specific surface area measurements, particle size distribution and scanning electron microscopy. The XRD analysis confirmed the hexagonal crystal structure of the product-CuS (covellite) with the average size of the crystallites 11 nm. The scanning electron microscopy analysis has revealed that the agglomerated grains have a plate-like structure composed of CuS nanoparticles. The thermal analysis was performed to investigate the thermal stability of the mechanochemically synthesized CuS. The optical properties were studied using UV-Vis and photoluminescence spectroscopy. The determined optical band gap energy 1.80 eV responds to the value of the bulk CuS, because of agglomerated nanoparticles. In addition, a mechanism of CuS mechanochemical reaction was proposed, and the verification of CuS commercial production was performed.

Ag modified ZnS for photocatalytic water pollutants degradation: Influence of metal loading and preparation method

In this paper, the photocatalytic degradation of organic pollutants was investigated using Ag/ZnS nanoparticles at different noble metal loadings. The photocatalysts were prepared at room temperature by two different methods: photodeposition and chemical reduction. The obtained samples were characterized by Specific surface area measurement, X-ray photoelectron spectroscopy, X-ray Powder diffraction, ultraviolet–visible diffuse reflectance and Raman spectroscopy. The X-ray photoelectron spectroscopy spectra showed that Ag is present on ZnS surface as intermediate state between nanostructured Ag0 and Ag2O. Moreover, the addition of silver caused a significant change of the absorption spectrum of bare ZnS, resulting in higher absorbance in the visible region, due to the Ag surface plasmon band. Methylene blue (MB) was used to evaluate the photocatalytic activity of the prepared samples. The best photocatalytic activity was observed using the sample at 0.1 wt% Ag loading prepared by chemical reduction method. In particular, the almost complete MB degradation was achieved using UV-LEDs as light sources and 6 g L−1 of catalyst dosage. Finally, the optimized photocatalyst was also effective in the degradation of phenol in aqueous solution under UV irradiation.

Synthesis and structural characterization of ceria nanoparticle agglomerates with shape inherited from an oxalate precursor

In this work, cerium dioxide nanoparticles (CeNPs) were synthesized by thermal decomposition of cerium oxalate decahydrate Ce2(C2O4)3·10H2O. Although a large amount of water was removed along with CO2, the product of decomposition retained the shape of the precursor crystal, i.e. a pseudomorph formed. The Ce2(C2O4)3·10H2O precursor crystals were of plate-like shape; this shape was inherited by CeO2 formed upon decomposition of the precursor. CeO2 plates with a thickness of less than 15 µm were transparent to visible light. X-ray diffraction analysis, transmission electron microscopy and specific surface area measurements indicated that the plates consisted of CeNPs 4–6 nm in size. The influence of the annealing temperature on the growth of crystallites of the pseudomorph was studied. Fast coarsening of the crystallites started at a temperature of 500 °C.

Elimination of pesticides by natural and modified clays

The objective of this study is the variation of natural and modified clays and their use in industrial waste treatment. For this purpose, we carried out several elimination tests of the effluents by adsorption by using two natural clays from different deposits, namely bentonite located in Maghnia and Djabel Debbagh of Ain Berbar and bentonite coupled with cellulose. The adsorption test was performed on a Decis pesticide. The clays that we tested, in the natural state and in the modified state, are the object of a mineralogical and physicochemical characterization by various techniques of analysis: X-ray diffraction, chemical composition, spectroscopy infrared, cation exchange capacity, scanning electron microscopy, measurement of specific surface area, and various adsorption tests concerning kinetics and capacity, as well as kinetic and thermodynamic modeling and adsorption isotherms. The results showed the practical use of clays for the decontamination of waters contaminated by organic pollutants.

Interrelations of structure, electric surface charge, and hydrophobicity of organo-mica and –montmorillonite, tailored with quaternary or primary amine cations. Preliminary study of pyrimethanil adsorption

Abstract In this work, organo-montmorillonite (organo-Mt) and organo-synthetic mica (organo-mica), obtained by quaternary or primary amine cations (octadecyltrimethylammonium (ODTMA) or octadecyl-amine (ODA), respectively) exchanged at 50% and 100% of their respective CEC, were thoroughly characterized and tested for pyrimethanil uptake. The interlayer entrance of both surfactants for Mt loaded at 50% and 100% CEC indicated a close basal space (of around 1.5 and 1.8 nm, respectively). However, for synthetic mica (Na-Mica-4) the basal space shifted to higher values (3.5 and 3.9 nm and 4.7 and 5.0 nm, for ODTMA and ODA at 50% and 100% CEC, respectively) than those of organo-Mts, keeping the interlayer surface partially free for ODTMA-Mica-4 samples. Tg analysis revealed that the actual ODA loadings were around 40% and 80% for both clays and for ODTMA exchanged Mts, while the actual ODTMA loading for organo-mica samples did not exceed 10.6%. The decrease of negative zeta potential evidenced the presence of ODA or ODTMA at the external surfaces of organo-Mt, while the zeta potential curves for organo-mica samples described the system as a mixture of individual surfactants and Na-Mica-4 sample. Total specific surface area (TSSA) and contact angle (CA) measurements revealed a general hydrophobicity increment with the presence of both surfactants. Interrelations of these parameters point out the different affinity and steric hindrance of the polar head of both surfactants with the solid surfaces, which also conditioned pyrimethanil adsorption.

Mesoporous pyrophyllite–titania nanocomposites: synthesis and activity in phenol photocatalytic degradation

Pyrophyllite–TiO2 nanocomposite PTi750 was successfully synthesized using a sol–gel method at ambient temperature based on exfoliation of the pyrophyllite layered clay by incorporation of the TiO2 precursor titanium(IV) t-butoxide. PTi750 exhibited higher photocatalytic activity in phenol degradation compared with commercial TiO2 Aeroxide P25. Ag-photodeposited PTi750 was more photoactive than PTi750, exhibiting detoxification, total degradation, and good mineralization of polluted solution and excellent stability after five reuses at optimal conditions in terms of the parameters pH, H2O2 concentration, and photocatalyst amount. The nanocomposites were investigated using several techniques, viz. diffuse-reflectance ultraviolet–visible (UV–Vis) spectrophotometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis, X-ray fluorescence spectroscopy, Fourier-transform infrared spectroscopy, and Brunauer–Emmett–Teller (BET) specific surface area measurements.

Highly effective adsorption of crystal violet dye from contaminated water using graphene oxide intercalated montmorillonite nanocomposite

Herein, graphene oxide intercalated montmorillonite nanocomposites were prepared by a facile chemical route and then used for the adsorption of crystal violet dye from contaminated water. Structural characterization of the nanocomposites were performed using Fourier transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, zeta potential, X-ray diffraction, specific surface area and pore volume measurements. The isothermal data obtained using batch adsorption technique were fitted using Langmuir and Freundlich equations and it was found that the experimental data is well described by the Langmuir isotherm model with a very high adsorption capacity of 746.27 mg g−1. The kinetics of the adsorption process showed rapid dynamics and conformed to pseudo-second-order model with a correlation coefficient of R2 > 0.99. The influence of interaction time and initial dye concentration on the adsorption efficiency were also investigated. Additionally, thermodynamic studies revealed that the adsorption process was spontaneous and endothermic. Further, the results indicated that the synthesised nanocomposites adsorb crystal violet dye efficiently (∼96%) with a small decrease in removal efficiency even after five cycles of adsorption and could be employed in wastewater treatment for the removal of cationic dyes.

Preparation of cobalt substituted zinc ferrite nanopowders via auto-combustion route: an investigation to their structural and magnetic properties

CoxZn1−xFe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4) nanopowders were fabricated via auto-combustion synthesis followed by calcined treatment. The structural, morphological, compositional and magnetic properties of the as-synthesized samples were decided by X-ray diffraction (XRD), field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, specific surface area and Physical Property Measurement System analyses, respectively. The XRD patterns revealed all annealed cobalt substituted zinc nanoferrites display a single phase cubic spinel structure, the decrease in lattice constant with increasing Co2+ ions concentration is related to the lattice shrinkage originated from the replacement of Zn2+ ions (ionic radii of 0.82 A) by Co2+ ions (ionic radii of 0.78 A); the increase of crystallite size with increasing Co2+ ions content can be attributed to the less exothermic for the formation of cobalt ferrite than that for zinc ferrite. The M–H curves revealed that there are unsaturated magnetization and negligible hysteresis loops for all samples with lower cobalt concentration (x = 0, 0.1, 0.2, and 0.3), implying a superparamagnetic behavior; while the Co0.4Zn0.6Fe2O4 nanoparticles (x = 0.4) show ferromagnetism at room temperature. The M–T relations inferred the substitution of cobalt ions can remarkably enhance Curie temperature of the as-prepared Co–Zn ferrite nanoparticles. At room temperature lower cobalt-substituted zinc nanoferrites tend to show superparamagnetism while higher cobalt-substituted zinc nanoferrites prefer to present ferromagnetism.

Preparation of a PANI/ZnO Composite for Efficient Photocatalytic Degradation of Acid Blue.

Polyaniline/zinc oxide (PANI/ZnO) composite photocatalysts were prepared from neutral media by in situ chemical oxidation of aniline (ANI) in the presence of different amounts of diethylene glycol (DEG). The PANI/ZnO composite photocatalysts were synthesized to efficiently remove organic dye (acid blue, AB25) from model wastewater. The PANI/ZnO composite photocatalysts were studied with the intention of efficient removal of organic dye (acid blue, AB25) from wastewater to obtain low-cost heterogeneous catalysts that offer high catalytic activity and stability. The conductive PANI polymer, which absorbs Vis irradiation, was used in this work as ZnO absorbs only ultraviolet (UV) irradiation; thus, the composite photocatalysts’ activity was broadened into the Vis region. Characterization of the composite photocatalysts was done by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, electric conductivity, UV-Vis spectroscopy, and by specific surface area (SBET) measurements. The composites’ photocatalytic activity under solar irradiation was validated by monitoring degradation of the AB25 dye. This study presented that it was possible both to prepare PANI and to prevent ZnO dissolution if in situ polymerization starts from neutral media with the addition of DEG. Additionally, efficient removal of AB25 dye, about 90% in 60 min, was achieved. The first-order rate constants of the photodegradation of AB25 by PANI/ZnO 0.02/0.024/0.04 DEG (and pure ZnO)) were computed to be 0.0272/0.0281/0.0325 (and 0.0062) min−1, indicating that the morphology and surface of the photocatalysts have significantly influenced the catalytic activity.

Fischer–Tropsch Synthesis with Cu-Co Nanocatalysts Prepared Using Novel Inorganic Precursor Complex

The structural properties and activities of Cu-Co catalysts used in Fischer-Tropsch synthesis are explored according to their method of preparation. Impregnation, co-precipitation, and a novel method of thermal decomposition were applied to an inorganic precursor complex to generate the Cu-promoted alumina- and silica-supported cobalt catalysts. The precursors and the catalysts obtained by their calcination underwent powder x-ray diffraction, thermal gravimetric analysis, specific surface area measurement using the Brunauer-Emmett-Teller method, scanning electron microscopy, and Fourier Transform InfraRed (FT-IR) spectroscopy. The catalytic performance of all calcined catalysts used in Fischer-Tropsch synthesis was investigated at 280 to 360 °C. The Cu-Co/SiO2 catalyst was prepared by thermal decomposition of [Cu(H2O)6][Co(dipic)2].2H2O/SiO2, which served as an optimal precursor for synthesis gas conversion into light olefins. The results highlight the advantages of this novel procedure over impregnation and co-precipitation approaches for effective and durable preparation of cobalt catalysts for Fischer-Tropsch synthesis.

Adsorption of phenol and chromium (VI) pollutants in wastewater using exfoliated graphite

Phenol and Cr(VI) are two of the most common organic and heavy metal-based pollutants found in industrial effluents. Both pollutants pose considerable health risks if left untreated. Activated carbon adsorption is generally used for the physical removal of these types of pollutants during wastewater treatment. This work investigates the use of thermally exfoliated graphite as an alternative adsorbent material for the removal of phenol and Cr(VI) in wastewater. The well-developed surface pore structure and high adsorption capacities reported in literature make this material an ideal candidate for investigation. The effectiveness of the exfoliation process was characterised using x-ray diffraction whilst the particle morphology, surface structure and adsorption surface area were determined using scanning electron microscopy and Brunauer-Emmett-Teller (BET) specific surface area measurements. Upon expansion, the particle morphology of expandable graphite changed from flakes to worm-like, accordion structures. This change was accompanied by an increase in BET surface area from 2.4 to 22.4 g/m2. Batch experiments using simulated wastewater revealed that expandable graphite had negligible adsorption affinity towards both pollutants. However, exfoliated graphite had adsorption capacities of 0.73 mg/g and 0.55 mg/g for Cr(VI) and phenol. The equilibrium adsorption isotherms for both pollutants were best described by the Langmuir adsorption model and had adsorption constants of 0.84 and 0.32 L/mg for the two pollutants. The adsorption capacities obtained were much lower than those reported for the same pollutants when using activated carbon as an adsorbent. This is most likely due to the high BET surface areas recorded for activated carbon. Additional modification of the exfoliated graphite is required before satisfactory adsorption capacities can be achieved for large-scale wastewater treatment applications.

Photocatalytic Water Disinfection under Solar Irradiation by d-Glucose-Modified Titania

Modified titania photocatalysts were synthesized by the pressure method using titanium(IV) oxide from Grupa Azoty Zakłady Chemiczne “Police” S.A., Police, Poland, and d-glucose solution. Characterization of obtained composites was performed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis, and measurements of zeta potential and specific surface area (SSA). The possibility of using glucose-titania composites as photocatalysts for simulated solar-assisted disinfection against gram-negative Escherichia coli and gram-positive Stapchyloccocus epidermidis bacteria were examined in two reaction systems, i.e., for suspended and immobilized photocatalysts (on the concrete). It was found that an increase in the d-glucose concentration, i.e., higher carbon content, led to a decrease in antibacterial properties. The sample obtained from 1% of d-glucose solution at 100 °C (TiO2-1%-G-100) showed superior photocatalytic activity under UV-Vis irradiation toward both bacteria species. Water disinfection was more efficient for suspended photocatalyst than that for supported one, where complete disinfection was reached during 55–70 min and 120 min of irradiation, respectively. For the first time, it has been shown that titania modified with monosaccharides can be efficiently used for water disinfection, and the immobilization of photocatalyst on the concrete might be a prospective method for public water supplies.

Synthesis and Characterization of Platinum-Praseodymium Oxide Nanocatalysts for Methanol Oxidation

Direct methanol fuel cells (DMFCs) are promising energy conversion devices, which can provide high efficiency and energy density, and as such, have been studied intensively. Due to the poisoning of Pt catalysts during methanol oxidation, various co-catalysts have been studied. Ruthenium has been studied extensively and good results have been obtained. More recently various oxide materials have been proposed and studied. Of these materials, rare earth metal oxides have been shown to be promising co-catalysts, although of the various oxides, CeO2 has perhaps received most attention. In this study, praseodymium oxide containing catalyst materials have been synthesised and characterized. Praseodymium oxide containing nanocatalysts were obtained via deposition of praseodymium hydroxide onto a carbon support material and calcination of the resulting material. Thereafter, platinum nanoclusters were deposited onto the praseodymium oxide containing composite material by impregnation of the Pt precursor into the support material and further reduction with hydrogen. The studied materials were electrochemically characterized using rotating disk electrode, cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy methods. Physical properties of the materials were characterized using X-ray diffraction, thermogravimetric analysis, scanning electron microscopy with EDX, inductively coupled plasma mass spectrometry and specific surface area measurements. The studied materials exhibit high currents for methanol oxidation even at low Pt loadings. Compared to a commercial Pt-C(Vulcan) material, the overpotential was drastically reduced, and the peak potential for methanol oxidation is at E = 0.15 V vs mercury-mercurous sulfate electrode. The good results of the study are a continuation of the investigation of nanocatalysts containing rare earth metal oxides for electrooxidation of alcohols. Acknowlegements This work was supported by the EU through the European Regional Development Fund Project TK141 “Advanced materials and high-technology devices for energy recuperation systems” (2014-2020.4.01.15-0011) and by the Estonian Research Council (institutional research grant IUT20-13, and PUT1581). P. Valk thanks Estonian Students Fund in USA for financial support.

Flame synthesis of nanophosphors using sub-micron aerosols

A flame synthesis approach is described for producing metal oxide nanoparticles. It is based on a novel microjet atomisation technique, which generates solution aerosol droplets in the sub-micron range with a mean diameter of 200 nm. Here, luminescent Eu-doped Y2O3 nanophosphors are successfully synthesised from aqueous metal nitrate precursors delivered to non-premixed CH4/N2-O2 flames. The effect of droplet size and synthesis temperature on the particle formation route is experimentally investigated by comparison with micron-size droplets (mean diameter 3.5 µm) produced by ultrasonic atomisation. Depending on the synthesis temperature, the results show that particles are formed via two different mechanisms: gas-to-particle and droplet-to-particle. Using sub-micron droplets at low temperatures (1150 K), the latter mechanism allows production of luminescent Y2O3:Eu3+ nanoparticles in the cubic phase, with straightforward particle size control from 10 to 100 nm by adjusting the precursor concentration in the 0.01–1 mol/L range. At higher flame temperatures (2750 K), results show that nanoparticles are formed via the gas-to-particle mechanism and have a size of 4–5 nm. In comparison with micron-size droplets, specific surface area measurements of the product powders show that sub-micron droplets form an increased fraction of nanoparticles via the gas-to-particle route. The greater developed surface area of sub-micron droplets enhances vaporisation of metal precursor molecules into the gas phase. Timescale analysis shows that the relative rates of solvent evaporation and bulk solute diffusion are also dependent on the droplet size. Electron microscopy confirms that dense particles are formed from sub-micron droplets subjected to a rapid rate of temperature increase in the flame. For the same synthesis conditions, micron-size droplets tend to form hollow particles, indicating that high peak temperatures near the product melting point (>2700 K) are subsequently required to promote dense particle formation.

Temperature effects on rock engineering properties and rock-fluid chemistry in opal-CT-bearing chalk

In this study, eight tri-axial tests on Cretaceous age outcrop chalk from Aalborg have been performed systematically by injecting MgCl2 for the first time at different temperatures (25, 60, 92, 110 and 130 °C) and for comparison, NaCl at 130 °C. Whole-rock geochemistry, stable isotope measurements, pycnometry, Field Emission Gun Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy, X-Ray Diffraction (XRD) and measurements of Specific Surface Area (Brunauer-Emmett-Teller theory (N2)) were applied to analyse unflooded and flooded cores. Based on analyses of changes in brine composition, mineralogy, specific surface area, solid density, porosity and permeability some conclusions can be drawn on temperature effects on rock engineering properties and rock-fluid chemistry.The MgCl2 flooded cores show systematically higher creep rates at higher temperature and the cores tested at 25 and 60 °C show similar creep rates as the two NaCl flooded cores at 130 °C. All fluid-rock interactions were more pronounced at higher temperature. After flooding with MgCl2 at 110 and 130 °C newly formed magnesite is observed. In the cores tested at 25, 60 and 92 °C magnesite crystals have not been positively identified, but minute increases in MgO in whole-rock geochemistry analyses are seen. Si4+ originating from the dissolution of silica bearing phases (mainly diagenetic opal-CT), has taken part in the re-precipitation of Si-Mg-bearing minerals during MgCl2 injection from 25 to 130 °C, leading to an increase of the specific surface area. This is partly balanced by opal-CT dissolution, which should lower the specific surface area. The flaky Si-Mg-bearing minerals, covering significant portions of the pore surfaces and throats are the main drivers to reduce permeability in cores flooded at high temperatures. Additionally, in NaCl flooded cores where mineralogical changes are minute, the dissolution of parts of the existing opal-CT has lowered the SSA. At high temperatures, the following chemical changes must be carefully acknowledged when porosity reduction is calculated: calcite and opal-CT dissolution and precipitation of new minerals, particularly Mg-bearing minerals. The presence of opal-CT strongly influences the mineralogical alterations in flooded cores, hence the geo-mechanical evolution.

Electrochemical properties of lignin/polypyrrole composites and their carbonized analogues

Lignin/polypyrrole composites were prepared by single-step coating of water-insoluble kraft lignin with polypyrrole. The polymerization of pyrrole was performed with and without the presence of methyl orange, leading to the formation of polypyrrole films of two different morphologies, globular and nanotubular, respectively, on the lignin surface. Such composites were converted to double-carbon composites enriched in inert atoms by pyrolysis in nitrogen atmosphere at 650 °C. The carbonization process was followed by thermogravimetric analysis and confirmed by Raman spectroscopy. The properties of all composites were investigated by FTIR spectroscopy, XPS, EPR, SEM, specific surface area and electrical conductivity measurements. Electrochemical properties were analyzed in terms of their capacitive behavior mainly in acidic solutions, but also in neutral and alkaline media.