Polar and dispersion interactions at carbon surfaces: further development of the XPS-based model

Enthalpy of immersion (ΔH_i) in water has been measured for a series of ozone oxidised non-porous carbon blacks and, as in our previous studies been found to correlate directly with the total surface oxygen level [O]_T measured by X-ray photoelectron spectroscopy. An equation that allows calculation of either parameter from the other is given and shown to describe behaviour for a wide range of carbon black surfaces which contain ozone-generated or native oxygen functional groups. Using this approach, the surface polarity and the relative hydrophilic character of such surfaces can be predicted. A molar enthalpy for the polar interaction between water and surface oxygen atoms of 17 kJ mol⁻¹ is obtained by assuming a 1:1 co-ordination between water molecules and carbon surface oxygen atoms. The data lead to a predicted value of 37.5 mJ m⁻² for the immersion of oxygen-free carbon black external surface into water. This equates to a value of 2.5 kJ mol⁻¹ for the non-specific dispersion interaction between water and an oxygen-free carbon black surface when a molecular area of 10.5×10⁻²⁰ m² for water is assumed. The same carbon black when oxidised using nitric acid gives a different enthalpy of immersion to the ozone-treated and native oxide materials, this is attributed to differing chemistry of the two surface types, this aspect is discussed. The nitric acid treated carbons do, however, give the same value as the ozonated and native oxide carbons (37.5 mJ m⁻²) for the immersion of an oxygen-free carbon surface into water. A correlation between the point of zero charge (pH_PZC) of the carbons and ΔH_i or [O]_T is also presented. The results from these measurements show extremely good agreement with data from other groups who have used TPD to assess surface oxygen concentration. This gives a firm basis for confident prediction of the thermodynamic properties of carbon surfaces from single measurement techniques.     

Grafting of polyesters onto carbon black. III. Polymerization of β-propiolactone initiated by quaternary ammonium carboxylate groups on the surface of carbon black

By use of carbon black containing quaternary ammonium carboxylate (COO^?N⁺R₄) groups as catalyst, the anionic ring opening polymerization of β-propiolactone (PL) was carried out at 50°C. Although carbon black itself was unable to initiate the polymerization of PL, carbon black containing COO^?N⁺R₄ groups, which was prepared by the reaction of carboxyl groups with corresponding quaternary ammonium hydroxide, was found to be able to initiate the polymerization. The carbon black obtained from the polymerization gave a stable colloidal dispersion in an organic solvent, and it was confirmed that the polyester formed was effectively grafted onto the surface. In addition, the effect of quaternary ammonium countercation on the polymerization was investigated.     

Effect of temperature on tacticity in thermal polymerization of p-methylstyrene by 13C NMR spectroscopy

Poly(p-methylstyrene) (Pp-MS) was synthesized at three different temperatures of 50, 150 and 250?°C via bulk thermal polymerization method. The assignment of all stereosequences at triad and pentad levels for two quaternary aromatic carbons and hexad level for methylene carbon was carried out by ¹³C liquid nuclear magnetic resonance spectroscopy (NMR) in deuterated chloroform at similar conditions. The probability of meso addition (P _m) was calculated from second quaternary aromatic carbon and used to predict the relative intensities of methylene and first quaternary aromatic carbon by Bernoullian and first-order Markov statistical models. The results were compared with experimental data. It is shown that the Bernoullian statistics fit better than first-order Markov model for all three samples. The results indicate that P _m increases by increasing polymerization temperature. The corresponding P _m values determined for synthesized Pp-MSs at 50, 150 and 250?°C were 0.383, 0.392 and 0.404, respectively. It was observed that higher resolutions and better splitting patterns were achievable by increasing the NMR acquisition temperature from 20 to 50?°C. When temperature increased during NMR acquisition, the resolution improved for the first and second quaternary aromatic carbons and methylene carbon, though there was no splitting pattern observed for methyl carbon atom at the para-position of the aromatic ring.     

Oxidative degradation of carbon blacks with nitric acid: II. Formation of water-soluble polynuclear aromatic compounds

Furnace black and acetylene black were oxidized with concentrated nitric acid at 100 °C for prolonged periods. The oxidized carbon black was dissolved/dispersed into alkaline solution and was size-fractionated into six fractions by ultrafiltration. The yields of the fractions revealed that oxidized furnace black contains oxygenated polynuclear aromatic compounds with a variety of molecular sizes, but oxidized acetylene black consists of only a great quantity of the largest size fraction, probably carbon black particles, and a scarce amount of the smallest size fraction. With oxidized furnace black, elemental compositions of all fractions except the largest molecular-size fraction were independent of the period of oxidation, suggesting that each fraction possesses a similar molecular structure. Noncarbon constituents such as oxygen and hydrogen increased with decreasing molecular size. The mean molecular weights of fractions were estimated to be in a range from ca. 400 to 1200 and more on the basis of elemental and functional group analyses. ¹³C-NMR and IR analysis showed that the molecules of fractions comprise phenolic, carboxylic, nitro, perhaps quinonic carbonyl groups, and aromatic carbons, but no aliphatic carbons. Ultraviolet and visible spectra of fractions denoted that absorption at higher wavelengths increased with increasing the molecular weights, indicating extension in the conjugated aromatic ring system. On the basis of the experimental results molecular structure models for the fractions were proposed.     

Structural characterization of Saudi Arabian medium crude oil by n.m.r. spectroscopy

Saudi Arabian medium crude oil has been characterized by ¹H and ¹³C n.m.r. spectroscopy. Several structural parameters such as the percentage of aliphatic carbons, aromatic carbons, n-alkanes, naphthenes, branched alkanes and chain length of paraffinic chains were calculated. The aromatic carbons were further classified as those attached to a hydrogen, methyl or alkyl group, or bridgehead carbons.     

Quantitative aspects of solid state 13C n.m.r. of coals and related materials

The quantitative aspects of cross-polarization (CP), which is used in conjunction with dipolar decoupling and magic-angle rotation to obtain high resolution ¹³C n.m.r. spectra of coals, have been studied using a bituminous coal (82 wt% C, dmmf basis) and asphaltenes from an extract of the same coal. The condition for obtaining reliable quantitative data, that rotating frame ¹H relaxation times (T_1p′ these govern the extent of CP) are much longer than the time required to polarize the carbons present (≈1 ms), was met for the asphaltenes. In contrast, about half the protons in the coal have T_1p5 of ≈ ? 1 ms, these times being too short to allow CP of all the carbons. Although the aromaticities obtained for this coal were fairly constant (≈0.75) using (CP) contact times > 0.5 ms, the total peak intensity decreased markedly as the contact time was increased and was much less than that for the asphaltenes. These results indicate that not all the carbons in bituminous coals are observed by CP and, as a consequence, aromaticities reported in the literature for some bituminous coals appear to be low.     

Adsorption of Hg(II) ions from aqueous chloride solutions using powdered activated carbons

Activated carbons were developed from Casurina equisetifolia leaves, by chemically treating with sulfuric acid (1:1) or zinc chloride (25%), at low (425 °C) and high (825 °C) temperatures. The resulting powdered activated carbons were applied for removing mercuric ions from aqueous solution at different agitation times and mercuric ion concentrations. The equilibrium data fitted well the Langmuir adsorption isotherm. The Langmuir adsorption capacities were 12.3 and 20.3 mg g⁻¹ for low temperature carbons and 43.9 and 38.5 mg g⁻¹ for high temperature carbons impregnated with H₂SO₄ and ZnCl₂, respectively. Studies of the effects of carbon dosage, NaCl concentrations and solution pH values were carried out for the more effective, high temperature carbons. Increasing NaCl concentration resulted in a significant decrease in the adsorption efficiency. Adsorption was high from solutions with low and neutral pH values and lower for solutions with alkaline pH values for the high temperature carbons.     

Characterization of vitrinite concentrates. 2. Magic-angle 13C nuclear magnetic resonance studies

The fraction aromaticity determined by ¹³C n.m.r. with cross-polarization and magic-angle spinning of 19 vitrinite concentrates obtained from the Lower Kittaning seam shows a range of values from ≈ 0.65 for the samples of lowest rank (83 wt% C (dmmf) to about 0.83 for those of highest rank (91 wt%C (dmmf)). It was determined that the wt% aromatic carbon correlates to the wt% fixed carbon and is in good agreement with the results reported by other authors. The combination of the ¹³C n.m.r. results with FTIR measurements allows a number of coal parameters to be estimated. The atomic ratios of aliphatic hydrogen to carbon were demonstrated to vary from 1.8–2.0 to between 2.4–2.6 and previous assumptions that a single value can be used in calculating structural parameters for coal of any rank are not strictly valid. The calculation of the aromatic $\text{H}\text{C}$ ratio indicates that in mean structural units there is approximately one aromatic hydrogen atom for every six carbons in vitrinites of carbon content 83 wt%C (dmmf) and that this ratio changes progressively with rank to a value of about one aromatic hydrogen for every four carbons for vitrinites of carbon content 91 wt%C (dmmf).     

Reduction of SO2 on different carbons

The reduction of SO₂ on four carbons (graphite, charcoal, activated carbon and coke) was studied under steady-state conditions and when the kinetics was chemically controlled in a reactor operated under differential conditions. The reaction showed second-order kinetics: first order with respect to carbon and first order with respect to the partial pressure of SO₂. The reactivity of the different carbons, as measured by the second-order rate constants, followed the sequence of decreasing crystallinity: graphite<coke (7.34)<coke (11.73)<charcoal. The difference in reactivity between graphite and charcoal was determined by ΔH^≠, while for cokes it increased with the ash content because of a favorable ΔS^≠. The main reaction products for all carbons were CO₂ and sulfur in the ratio 2:1, considering the sulfur as S₂, which was shown to be formed through the same path. CO, COS, and CS₂ were also detected, and the product distribution depended on the carbon and whether the reaction was diffusion controlled or chemically controlled. Analysis of product ratios strongly suggested that CO, COS and CS₂ were produced from consecutive reactions of the primary products. CO was formed from CO₂ by a slow Boudouard reaction that occurred partially and under conditions of non-equilibrium. Complexed sulfur reacted with CO to form COS and CS₂. There was an interaction between the active site of reduction and the site where sulfur is inserted.     

Influence of Carbon Content on Ceramic Injection Molding of Reaction‐Bonded Silicon Carbide

Reaction‐bonded silicon carbide (RBSC) was prepared by ceramic injection molding (CIM) technique with feedstocks containing silicon carbide (SiC), a wax‐based organic system and different amounts of carbon black. As a critical effect of the reaction sintering process, carbon was introduced from the carbon black and the decomposition product of the organic polymers, respectively. This study described the influence of carbon content on the mixing and injection process firstly and then emphasized the debinding process since it played a large role in the process of the pyrolysis of organic. Results indicated that the preferable thermal debinding was performed in N₂ and the optimal performance was obtained for RBSC with 7 wt.% of carbon black, with the density of 2.98 g/cm³, apparent porosity of 0.24%, bending strength of 301.59 MPa and fracture toughness of 4.18 MPa·m^1/2.     

Ammonia-tailoring of GAC to enhance perchlorate removal. II: Perchlorate adsorption

Thermal tailoring of a bituminous granular activated carbon (GAC) by ammonia gas at 500-800 °C for 60 min achieved as much as 4-fold improvement in perchlorate adsorption as determined by rapid small-scale column tests (RSSCTs). When treating natural groundwater that contained 70-78 ppb perchlorate, the bed volumes to 4 ppb perchlorate breakthrough increased from 1100 bed volumes for commercial bituminous carbon up to 4400 bed volumes for carbon that was tailored in ammonia at 700 °C. Increased perchlorate adsorption corresponded to increased positive surface charges with R² 0.956. The increase in positive surface charge was closely related to the increase in nitrogen content after ammonia tailoring. The ammonia-tailored activated carbons also exhibited a moderate increase in TOC removal and higher selectivity for erchlorate. The work herein interfaces with a companion paper by the same authors in which the ammonia-tailored carbon surfaces are characterized relative to physical-chemical features [1].     

Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons

The interactions of nitrogen oxides with carbons differing in the chemical structure of surface functional groups were studied using in situ FTIR combined with the measurements of catalytic activity. Microporous carbon samples with similar pore size distribution were prepared from cellulose. The structure and coverage of adsorbates during reactions at temperatures between 295 and 573 K are determined by FTIR. No significant changes in NO_x reaction with carbon surface were found by oxidation of the carbonized film. During the study of the reaction of NO/O₂ mixture with carbons, the infrared absorption bands for the surface species formed are similar to the IR bands observed after the reaction of carbon samples with NO₂. For both reactions, surface species, including C-NO₂, C-ONO, C-NCO and anhydride structures are formed. Catalytic NO_x reduction by carbons has been investigated in the temperature range 295-623 K in the flow reactor equipped with an FTIR gas analyzer. As the surface of carbon is exposed to NO₂ gaseous NO is formed. The reduction of NO₂ to N₂ without the use of an externally supplied reductant can be achieved with microporous carbons. Significant NO₂ conversion to N₂ occurred at 623 K on both oxidized and non-oxidized carbons.     

Hydrodechlorination activity of catalysts based on nitrogen-doped carbons from low-density polyethylene

Nitrogen-doped carbons (NDCs) were prepared by co-pyrolysis of low density polyethylene (LDPE) and tested in the aqueous phase catalytic hydrodechlorination (HDC) of 4-chlorophenol (4-CP). The NDCs were obtained as clusters of non-porous spherical particles with carbon black-like structure. Pyridine and 1,10-phenanthroline were used as nitrogen precursors. Both precursors allowed incorporating a significant content of nitrogen into the carbon matrix (0.1–1.7%, w), with a preferential allocation at the outer regions in the case of pyridine. Different pyrolysis conditions were tested to address their effect on the amount of nitrogen incorporated to the carbon structure. Nitrogen showed different bonding configurations: pyridine-like, pyrrole-like and quaternary. Those NDCs prepared with phenanthroline showed higher content of quaternary and lower of pyrrolic nitrogen in their structure. The NDCs exhibited some activity in HDC, showing their potential as metal-free catalysts. Pd/NDCs catalysts were also prepared and tested. They yielded a much higher activity than Pd supported on non-doped carbons used as blanks, thus suggesting a synergistic between the NDCs and Pd. The NDCs-supported Pd catalysts prepared from 1,10-phenanthroline exhibited a remarkable activity of 55.6 mmol g_Pd⁻¹ min⁻¹, which may be related to the higher amount of nitrogen of the support with a higher contribution of quaternary nitrogen species.     

Ultrasound-promoted N-propargylation of imidazole by alkaline-doped carbons

The synthesis of N-propargyl imidazoles via alkylation of imidazole with propargyl bromide by sonochemical and thermally activated reactions over two alkaline promoted carbons (Na and Cs-Norit) as catalysts is reported. In this green, solvent free procedure, we produce exclusively N-propargyl imidazoles in very high yields (>80%) when the Cs⁺-Norit carbon is employed under ultrasound activation. The basicity of the alkaline cation increases the conversion and ultrasound activation affords a remarkable increase in the yields. The catalysts were characterized by thermal analyses, X-ray photoelectron spectroscopy and nitrogen adsorption isotherms.     

Difficult carbonaceous materials and their infra-red and Raman spectra. Reassignments for coal spectra

Infra-red and Raman spectra of intractable carbonaceous materials are difficult to obtain. For coals, carbon blacks, and some chars infra-red spectra have been obtained with relative ease. Only recently have good infra-red transmission spectra of activated carbons been obtained. More difficult materials have now been successfully studied by the transmission infra-red method, most notably ground graphite (non-crystalline), through the use of efficient and extensive grinding. Intense infra-red bands are observed at about 1590 and 1360 cm⁻¹ for ground graphite, carbon blacks, and some activated carbons. Laser-Raman spectra of coals, carbons, and graphites have two lines at about the same frequencies as the infra-red bands. However, the similarity of these laser-Raman spectra indicates in the case of coal that we may be observing the spectrum of carbonized coal rather than of coal, due to degradation of the sample by the laser beam. These new spectral results necessitate reassignment of some bands in the infra-red spectra of coals. Graphitic structures (non-crystalline) are believed to be responsible for the 1600 cm⁻¹ band in coals and the broader 1360 cm⁻¹ band, which fit closely the broad band contour in the infra-red spectra of coals from 1800 to 900 cm⁻¹. The intensities of the 1600 and 1360 cm⁻¹ bands in ground graphite are more than sufficient to account for the band intensities observed in the spectra of coals and chars. Diamond-like structures such as quaternary carbon atoms are weak absorbers and cannot be responsible for either of these bands.     

The effect of activated carbon surface moisture on low temperature mercury adsorption

Experiments with elemental mercury (Hg⁰) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27°C) to determine the role of surface moisture in capturing Hg⁰. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg⁰ adsorption capacity. About 75-85% reduction in Hg⁰ adsorption was observed when both carbon samples’ moisture (∼2 wt.% as received) was removed by heating at 110°C prior to the Hg⁰ adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg⁰ under these conditions. The common effect of moisture on Hg⁰ adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg⁰ is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H₂O is closely associated with surface oxygen complexes and the removal of the H₂O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg⁰ or eliminates the reactive surface conditions that favor Hg⁰ adsorption.     

Graphitic mesoporous carbons synthesised through mesostructured silica templates

In this paper the fabrication and characterization of graphitizable and graphitized porous carbons with a well-developed mesoporosity is described. The synthetic route used to prepare the graphitizable carbons was: (a) the infiltration of the porosity of mesoporous silica with a solution containing the carbon precursor (i.e. poly-vinyl chloride, PVC), (b) the carbonisation of the silica–PVC composite and (c) the removal of the silica skeletal. Carbons obtained in this way have a certain graphitic order and a good electrical conductivity (0.3 S cm⁻¹), which is two orders larger than that of a non-graphitizable carbon. In addition, these materials have a high BET surface area (>900 m² g⁻¹), a large pore volume (>1 cm³ g⁻¹) and a bimodal porosity made up of mesopores. The pore structure of these carbons can be tailored as a function of the type of silica selected as template. Thus, whereas a graphitizable carbon with a well-ordered porosity is obtained from SBA-15 silica, a carbon with a wormhole pore structure results when MSU-1 silica is used as template. The heat treatment of a graphitizable carbon at a high temperature (2300 °C) allows it to be converted into a graphitized porous carbon with a relatively high BET surface area (260 m² g⁻¹) and a porosity made up of mesopores in the 2–15 nm range.     

Synthetic carbons activated with phosphoric acid III. Carbons prepared in air

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer in an air atmosphere at various temperatures in the 400-900 °C interval. The carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration, copper adsorption from solution and physical adsorption of N₂ at −196 °C and CO₂ at 0 °C. It was shown that, similarly to synthetic phosphoric acid activated carbons obtained in argon, the synthetic carbons activated with phosphoric acid in air possess an acidic character and show considerable cation-exchange properties. The contribution of oxygen-containing surface groups along with phosphorus-containing groups to CEC is higher for carbons obtained in air. Three types of surface groups were identified on carbons prepared at temperatures up to 600 °C, and four types on carbons prepared at higher temperatures. These groups were assigned to ‘super-acidic’ (pK<0), phosphorus-containing (pK=1.1-1.2), carboxylic (pK=4.7-6.0) and phenolic (pK=8.1-9.4) groups. The cation-exchange capacity was at a maximum for the carbon prepared at 800 °C. Copper adsorption by synthetic phosphoric acid activated carbons obtained in air at temperatures lower than 800 °C is higher than for similar carbons obtained in argon. The increase is due to additional formation of oxygen-containing surface groups. Calculated copper binding constants revealed the importance of phosphorus-containing and carboxylic groups for adsorption of copper from aqueous solution. All carbons show a multimodal pore size distribution including simultaneously micropores and mesopores, but the porous texture is not a prime factor in determining the cation-exchange capacities of these carbons. Synthetic phosphoric acid activated carbons show a greater development of porosity when obtained in air as compared to carbons carbonized in argon.     

Characterisation of separated melamine—formaldehyde adducts (methylolmelamines) and adduct mixtures by h.p.l.c. and by n.m.r. and u.v. spectroscopy

Most of the individual methylolmelamines that occur in melamine–formaldehyde adduct mixtures have been separated by preparative high-performance liquid chromatography (h.p.l.c.) and recovered in amounts sufficient to allow their characterisation by high-field ¹H- and ¹³C-n.m.r. Particular attention has been paid to the chemical shifts of the aromatic azine carbons of the melamine nuclei. These carbon shifts are shown broadly to be in agreement with some values previously published, but additional assignments have been made. It is shown that a satisfactory quantitative analysis of the methylolmelamines in a melamine–formaldehyde adduct mixture can be carried out either by h.p.l.c. or by ¹³C-n.m.r.     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.