Thermochemistry of onion-like carbons

The standard enthalpies of formation at 25 °C of onion-like carbons (OLC) with different structural ordering have been investigated by high-temperature oxidation calorimetry. In terms of enthalpy and depending on the degree of structural ordering, OLC can be up to 16 kJ mol⁻¹ less stable than graphite but up to 27 kJ mol⁻¹ more stable than their fullerene allotropes. Furthermore, OLC are approximately 5–9 kJ mol⁻¹ less stable than single-wall carbon nanotubes (SWCNTs). The samples prepared at 1800 °C are energetically less stable than samples made at 1300 and 1500 °C. These changes in energetics may stem from oxygen-containing functional groups bonded to the structure or from the creation of topological defects (polygonization and pentagon formation) whose concentration increases with increasing temperature and whose higher energy is balanced by configurational entropy.     

Studies on formation and siliconization of carbon template of coir fibreboard precursor to SiC ceramics

Formation reaction of carbon template of coir fibreboard samples via pyrolytic decomposition was studied by methods of thermogravimetric, derivative thermogravimetric and differential thermal analysis using different heating rates (5–20 K min⁻¹). The major decomposition reaction at 250–500 °C had activation energy and frequency parameter of 121.84 ± 8.78 kJ mol⁻¹ and 3.2 × 10⁹–2.1 × 10¹¹ min⁻¹, respectively, supporting a free radical mechanism. Siliconization reaction of the carbon template was also studied by non-isothermal DTA technique using varying heating rates (5–20 K min⁻¹); exothermic C-Si reaction was preceded by endothermic melting of Si and it had activation energy of 2793.09 ± 187.65 kJ mol⁻¹; diffusion through solid reaction product was found to control the heterogeneous chemical reaction process. On the basis of these experimental results separate experiments were conducted for preparation of carbon templates and their subsequent siliconization. The correlations between the properties of the coir fibreboard precursor, the carbon template and the siliconized product (SiC) were explained.     

Carbon oxidation characteristics of yttrium manganate catalyst prepared via urea decomposition

YMnO₃ is a hexagonal crystal characterized by high carbon oxidation activity. In this study, carbon black powder has been directly oxidized at temperatures as low as 250 °C with the active oxygen species generated by YMnO₃ catalyst. The activation energies measured for the non-catalyzed and YMnO₃-catalyzed carbon oxidation reactions were 160 kJ mol⁻¹ and 131 kJ mol⁻¹, respectively. During combustion testing of particulate matter in a ceramic form coated with YMnO₃, the captured soot was continuously purified at a temperature of 350 °C.     

Selectively combusting CO in the presence of propylene

Acrylic acid plant capacity can be increased by 30% by substituting propane for nitrogen (in air) thereby increasing the overall gas heat capacity. To minimize propane purge rates, the effluent is recycled but the CO in the recycle stream must be oxidized to avoid poisoning the catalyst. We studied the kinetics of CO combustion in a micro-fluidized bed with a catalyst inventory of 1 g and a 4 cm ID reactor charged with 5 g of catalyst – Pd/zeolite – and 145 g of inert. Experiments were run at temperatures between 90 and 240 °C and at 1 and 3.2 bara. At temperatures below 140 °C propylene conversion was less than 30% while CO conversion approached 90% at gas hourly space velocities near 10,000 h⁻¹. A first order kinetic model characterized the data over the whole range of conditions in which conversion was varied between 2% and 99+%. The rate constant for CO conversion was equal to 0.75 s⁻¹ whereas it equaled 0.04 s⁻¹ for propylene oxidation; the activation energy for CO oxidation was 95 kJ mol⁻¹ K⁻¹ whereas it was 140 kJ mol⁻¹ K⁻¹ for propylene.     

On the stepwise change of activation energies in the hydrodechlorination of chlorobenzene over supported nickel

Kinetics of chlorobenzene hydrodechlorination have been measured over Ni on SiO₂, Al₂O₃, MgO, activated carbon and graphite. A stepwise variation of E_a is analysed using the selective energy transfer model where E_a is identified as the vibrational energy associated with an excitation of the chlorobenzene out-of-plane C–H bending mode. Variation of E_a with vibrational quantum number yields a vibrational frequency of 749 cm⁻¹ and a value (−1.1 cm⁻¹) for the anharmonicity term, which is characteristic of bending vibrational modes. Our analysis suggests that the reacting species are weakly adsorbed on the catalyst: heat of adsorption = −0.31 kJ mol⁻¹.     

Alcohol ethoxylation kinetics: Proton transfer influence on product distribution in microchannels

The influence of the reactor type on the product distribution of the base catalyzed ethoxylation of fatty alcohols was studied.Commonly, proton exchange equilibrium is assumed when modeling this reaction to calculate the product distribution. The model is applicable for ethoxylates produced in semibatch, but cannot explain the products obtained from a continuous microstructured reactor.In this work, a non-equilibrium model is proposed to explain the observed distributions. The model is better suited to fit the distribution curves, and was used to determine the kinetic parameters of this reaction.For the propagation reaction, an activation energy E_A,P = 74 kJ mol⁻¹ was found, which is in good agreement to literature data. For the proton transfer, activation energies in the range of 56 kJ mol⁻¹ to 68 kJ mol⁻¹ were observed.     

Sp3/sp2 character of the carbon and hydrogen configuration in micro- and nanocrystalline diamond

Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp² carbon bonds determined by Raman spectroscopy from the 1332 cm^− 1 diamond peak and the sp² 1550 cm^− 1 G band. Hydrogen concentration appears to be proportional to the sp² bonds ratio. The 3000 cm^− 1 CH_x stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp² phase structure and concentration, as hydrogen outdiffuses.     

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.     

The effects of the surface oxidation of activated carbon,the solution pH and the temperature on adsorption of ibuprofen

A commercial microporous–mesoporous granular activated carbon was modified by oxidation with either H₂O₂ in the presence or absence of ultrasonic irradiation, or NaOCl or by a thermal treatment under nitrogen flow. Raw and modified materials were characterized by N₂ adsorption–desorption measurements at 77 K, Boehm titrations, pH measurements and X-ray photoelectron spectroscopy. Ibuprofen adsorption kinetic and isotherm studies were carried out at pH 3 and 7 on raw and modified materials. The thermodynamic parameters of adsorption were calculated from the isotherms obtained at 298, 313 and 328 K. The pore size distribution of carbon loaded with ibuprofen brought out that adsorption occurred preferentially into the ultramicropores. The adsorption of ibuprofen on pristine activated carbon was found endothermic, spontaneous (ΔG° = −1.1 kJ mol⁻¹), and promoted at acidic pH through dispersive interactions. All explored oxidative treatments led mainly to the formation of carbonyl groups and in a less extent to lactonic and carboxylic groups. This then helped to enhance the adsorption uptake while decreasing adsorption Gibbs energy (notably −7.3 kJ mol⁻¹ after sonication in H₂O₂). The decrease of the adsorption capacity after bleaching was attributed to the presence of phenolic groups.     

Template-free synthesis of N-doped porous carbons and their gas sorption properties

Nitrogen-rich carbon precursors are prepared and subsequently used for the preparation of N-doped porous carbons (NPCs). Two carbon precursors (CPs), CP-60 and CP-40, are prepared at different temperatures, 60 °C and 40 °C, respectively. The obtained CP materials are almost nonporous based on the N₂ adsorption/desorption analysis. These nonporous CP materials are converted into NPC materials through a carbonization at 800 °C. Porous NPC-60-800 and NPC-40-800 are prepared from CP-60 and CP-40, respectively. In contrast to the CP materials, the NPC materials exhibit higher surface areas. The surface areas of NPC-60-800 and NPC-40-800 are 422.9 m² g⁻¹ and 520.8 m² g⁻¹, respectively. The respective N-contents of NPC-60-800 and NPC-40-800 materials are 4.30 and 3.54 wt.% based on the elemental analysis. The types of N-containing groups of NPC materials are investigated by X-ray photoelectron spectroscopy (XPS) analysis. The NPC materials are good sorbents for CO₂ storage. The heats of CO₂ adsorption (Q_st) are 48.4 kJ mol⁻¹ for NPC-60-800 and 47.0 kJ mol⁻¹ for NPC-40-800. Both materials are also good H₂ sorbents at 77 K: 1.23 wt.% (NPC-60-800) and 1.22 wt.% (NPC-40-800).     

Preparation of sulfur-doped microporous carbons for the storage of hydrogen and carbon dioxide

Structurally well ordered, sulfur-doped microporous carbon materials have been successfully prepared by a nanocasting method using zeolite EMC-2 as a hard template. The carbon materials exhibited well-resolved diffraction peaks in powder XRD patterns and ordered micropore channels in TEM images. Adjusting the synthesis conditions, carbons possess a tunable sulfur content in the range of 1.3–6.6 wt.%, a surface area of 729–1627 m² g^?1 and a pore volume of 0.60–0.90 cm³ g^?1. A significant proportion of the porosity in the carbons (up to 82% and 63% for surface area and pore volume, respectively) is contributed by micropores. The sulfur-doped microporous carbons exhibit isosteric heat of hydrogen adsorption up to 9.2 kJ mol^?1 and a high hydrogen uptake density of 14.3 × 10^?3 mmol m^?2 at ?196 °C and 20 bar, one of the highest ever observed for nanoporous carbons. They also show a high CO₂ adsorption energy up to 59 kJ mol^?1 at lower coverages (with 22 kJ mol^?1 at higher CO₂ coverages), the highest ever reported for any porous carbon materials and one of the highest amongst all the porous materials. These findings suggest that S-doped microporous carbons are potential promising adsorbents for hydrogen and CO₂.     

Thermochemistry of nanodiamond terminated by oxygen containing functional groups

The standard enthalpies of formation at 25 °C of nanodiamonds terminated by oxygen containing functional groups have been investigated by high-temperature oxidation calorimetry. Depending on the amount of oxygen containing functional groups, the nanodiamonds (plus oxygen and hydrogen as represented in the surface functional groups) can be up to 52 kJ mol⁻¹ more stable in enthalpy than graphite, which means that less heat is evolved during oxidation of nanodiamonds terminated by oxygen containing functional groups, since their surface carbon is already partially oxidized. The stability of the nanodiamonds terminated by oxygen containing functional groups increases (enthalpy of formation becomes more negative) with increasing surface area within the studied range, reflecting the dominant effect of higher content of surface functional groups over the destabilizing effect of higher surface-to-volume ratio typical for nanoparticles.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Effective synthesis of well graphitized high yield bamboo-like multi-walled carbon nanotubes on copper loaded α-alumina nanoparticles

A high-yield bamboo like multiwalled carbon nanotubes (CNTs) were successfully synthesized on copper substituted alumina nanoparticles by thermal chemical vapor deposition (CVD) technique under atmospheric pressure. The obtained products were characterized by various techniques like FESEM with EDX, HRTEM and Raman spectroscopy, which reveals the formation of CNTs and are of bamboo shaped (stacking arrangement) multiwalled type with graphene layers having a diameter between 4 and 9 nm. The appearance of two peaks at 1597 cm^− 1 and 1302 cm^− 1 in Raman spectra are noticed as G-band and D-band for graphitic nature and defects due to bending & curvature of bamboo like carbon nanotubes (b-CNTs), respectively. The influence of reaction parameters such as time, temperature and flow rate was also studied to increase the carbon yield.     

Facile synthesis of CuO mesocrystal/MWCNT composites as anode materials for high areal capacity lithium ion batteries

CuO mesocrystal entangled with multi-wall carbon nanotube (MWCNT) composites are synthesized through a facile scalable precipitation and a followed oriented aggregation process. When evaluated as anode materials for lithium ion batteries, the CuO-MWCNT composites exhibit high areal capacity and good cycling stability (1.11 mA h cm⁻² after 400 cycles at the current density of 0.39 mA cm⁻²). The excellent electrochemical performance can be ascribed to the synergy effect of the unique structure of defect-rich CuO mesocrystals and the flexible conductive MWCNTs. The assembled architecture of CuO mesocrystals can favor the Li-ion transport and accommodate the volume change effectively, as well as possess the structural and chemical stability of bulk materials, while the entangled MWCNTs can maintain the structural and electrical integrity of the electrode during the cycles.     

A self-healing polymer network based on reversible covalent bonding

A self-healing polymer network for potential coating applications was designed based on the concept of the reversible Diels–Alder (DA) reaction between a furan functionalized compound and a bismaleimide. The network allows local mobility in a temperature window from ca. 80 °C to 120 °C by shifting the DA equilibrium towards the initial building blocks. Changing the spacer length in the furan functionalized compound leads to tailor-made properties. Elastomeric model systems were chosen to evaluate the kinetic parameters by Fourier transform infrared spectroscopy. For the DA reaction a pre-exponential factor ln(A_DA in kg mol^?1 s^?1) equal to 13.1 ± 0.8 and an activation energy (E_DA) of 55.7 ± 2.3 kJ mol^?1 are found. For the retro-DA reaction, ln(A_rDA) and E_rDA are 25.8 ± 1.8 s^?1 and 94.2 ± 4.8 kJ mol^?1, respectively. The enthalpy and entropy of reaction are calculated as ?38.6 kJ mol^?1 and ?105.3 J mol^?1 K^?1. The kinetic results are validated by micro-calorimetry. Non-isothermal dynamic rheometry provides the gel-point temperature of the reversible network. The sealing capacity is evaluated by atomic force microscopy for micro-meter sized defects. Repeatability of the non-autonomous healing is checked by micro-calorimetry, ruling out side-reactions below 120 °C.     

Kinetic modelling of the liquid-phase dimerization of isoamylenes on Amberlyst 35

Selectivity, conversion, yield and kinetics of the liquid-phase dimerization of 2-methyl-1-butene and 2-methyl-2-butene mixture have been studied in a batch stirred tank reactor in the temperature range 60–100 °C catalysed by the acid resin Amberlyst 35 and using ethanol as a selectivity enhancer. Selectivity to dimers showed a maximum at R_IA/EtOH = 20. Obtained diisoamylenes consisted mainly of 3,4,4,5-tetramethyl-2-hexene, 2,3,4,4-tetramethyl-1-hexene and 3,4,5,5-tetramethyl-2-hexene. LHHW type kinetic model was derived for the dimerization reaction as a whole. The kinetic model assumes that three active sites take part in the rate-limiting step of dimerization; the apparent activation energy for the dimerization reaction being 65 kJ mol⁻¹. A pseudohomogeneous kinetic model was used to describe the dimerization reaction network. The apparent activation energy for each dimerization reaction was found to be in the range 61–81 kJ mol⁻¹.     

Kinetics analysis of phenol and benzene decomposition in supercritical water

Decomposition of phenol and benzene was studied in supercritical water (SCW) at 370–450 °C and 25 MPa over very short residence times (0.5–100 s). The study of simple model compounds such as phenol and benzene is an essential preliminary step to elucidate the primary mechanism of char and gas formation from lignin compounds. A quantitative detailed chemical kinetics model for the primary pathways of phenol and benzene decomposition in SCW was determined using the reaction pathways for its decomposition under supercritical conditions. The activation energy of benzene decomposition (91.16 kJ mol⁻¹) in SCW is much higher than that of phenol (54.17 kJ mol⁻¹) under similar experimental conditions. This emphasized the importance of the substituent group (hydroxyl group) in the benzene ring to enhance its decomposition rate. In addition, the reaction rate parameters, which are deduced for the overall reaction network of its decomposition under similar conditions, show good agreement with each another. Hence, the reaction rates of these reaction pathways are successfully described in this study.     

Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework

The preparation, characterization and CO₂ uptake performance of N-doped porous carbon materials and composites derived from direct carbonization of ZIF-8 under various conditions are presented for the first time. It is found that the carbonization temperature has remarkable effect on the compositions, the textural properties and consequently the CO₂ adsorption capacities of the ZIF-derived porous materials. Changing the carbonization temperature from 600 to 1000 °C, the composites and the resulting porous carbon materials possess a tuneable nitrogen content in the range of 7.1–24.8 wt%, a surface area of 362–1466 m² g⁻¹ and a pore volume of 0.27–0.87 cm³ g⁻¹, where a significant proportion of the porosity is contributed by micropores. These N-doped porous composites and carbons exhibit excellent CO₂ uptake capacities up to 3.8 mmol g⁻¹ at 25 °C and 1 bar with a CO₂ adsorption energy up to 26 kJ mol⁻¹ at higher CO₂ coverages. The average adsorption energy for CO₂ is one of the highest ever reported for any porous carbon materials. Moreover, the influence of textural properties on CO₂ capture performance of the resulting porous adsorbents has been discussed, which may pave the way to further develop higher efficient CO₂ adsorbent materials.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.