Carbon Active Sites Research Articles

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry—III. The lanthanum modifier and the determination of phosphorus

Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from pyrolytic graphite, and for polycrystalline electrographite tubes with pyrolytic graphite coating in which phosphorus was determined without and with the addition of higher concentrations of lanthanum as the modifier. Lanthanum causes severe pitting and corrosion of the graphite surface already after relatively few determinations, and definite indication was found for the formation of intercalation compounds between lanthanum and graphite. No sign was found, however, for the formation of a dense coating of lanthanum carbide as proposed by several authors. The mechanism for the increase of phosphorus sensitivity is most probably the formation of a thermally stable compound involving lanthanum and phosphorus which leads to vaporization of phosphorus at high enough temperatures to obtain sufficient atomization and useful analytical signals. This is supported by the morphological changes of the graphite surface observed after application of higher lanthanum concentrations, and the resulting increased number of active carbon sites. Phosphorus alone also causes substantial corrosion of graphite, but with a completely different pattern. A very pronounced secondary coating of tube and platform wall is observed in the absence of lanthanum which is most probably supported by the formation and decomposition of compounds between phosphorus and graphite.

Evolution of pore structure and active surface areas of coal and char during hydrogenation

A Belgian coal (Beringen) has been used in a fundamental study of the changes in pore structure and active surface area during hydrogenation. The coal was devolatilized under N 2 and then hydrogenated at constant temperature up to various conversion yields. Reactivities of the chars were determined by isothermal thermogravimetric analysis. Changes in total surface area, calculated from CO 2 adsorption isotherms at 298 K, and in macropore distribution, obtained by mercury penetration, were found to lead to the same conclusion. In the first reaction stage, the preponderant phenomenon was the opening of previously closed micropores, induced by the material consumed by the reaction. This stopped completely when conversion yield exceeded 55 wt% when the maximum surface area was observed. Then removal of material from pore walls became predominant, pores were enlarged and pore walls disappeared gradually with increasing hydrogenation yields. Oxygen chemisorption capacities were measured at 383 K and 0.1 MPa air to give a relative indication of the concentration of carbon active sites. Active surface area results were compared to total surface area values and correlated with reaction rates.

Combined effects of inorganic constituents and pyrolysis conditions on the gasification reactivity of coal chars

A detailed phenomenological study of the gasification behavior of a North Dakota lignite was undertaken and the fundamental parameters that determine char reactivity were investigated. Differences in reactivity of up to three orders of magnitude were obtained by varying the conditions of coal pretreatment and pyrolysis. Pretreatment included demineralization with HCl and HF, ion exchange with ammonium acetate and back exchange with calcium acetate. Pyrolysis temperature, residence time and heating rate were varied in the range 975–1475 K, 0.3 s-1 h and 10 K/min-10 4 K/s, respectively. The observed reactivity differences were rationalized in terms of variations in the concentration of carbon and catalyst active sites.

Pyrolysis of propylene over carbon active sites—I: Kinetics

The influence of carbon active sites on the pyrolysis of propylene was studied in the temperature range 873–1073 K at a starting pressure of 1.6 Pa. Graphon, a graphitized carbon black, was used as a substrate for pyrolysis and subsequent carbon deposition. Pyrolysis was carried out in a static reactor at low pressures where secondary products were less likely to form and complicate the system. The pyrolysis of propylene over the Graphon substrate may be described by the expression d (C 3H 6) dt = (k) (C 3H 6) (ASA) moles/sec where ASA is the active surface area (cm 2) of the carbon measured by propylene chemisorption at 573 K, (C 3H 6) is the concentration of propylene in moles/cm 3, and k is the specific reaction rate constant having a value of 10 9.62 exp (−57 kcal/mole/RT) cm/sec. The carbon active surface greatly enhanced the rate of cracking of hydro-carbons to elemental carbon which was deposited directly on the surface. The carbon was found to deposit on the active sites and become new active sites. Thus, the carbon deposit replicated the active surface area and was autocatalytic.

Chemical oxidation of anthracite with hydrogen peroxide via the Fenton reaction

Solutions of 30% H 2O 2 ranging from pH = 0 to pH = 11.5 have been used to oxidize anthracite at room temperature. The inorganic impurities, primarily pyrite, catalysed the oxidation and reduction of H 2O 2 (the Fenton reaction) to form the hydroxyl radical; the oxidation of the organic matter was minimal and was observed only in strong acidic solutions ( pH < 1.5). After acid demineralization, samples of the same anthracite underwent a significant enhancement of oxidation in both acid and alkaline solutions ( pH = 0.4–11.5). As all the iron had been removed from the surface and the reactions were completed in a much shorter time, the oxidation mechanism must have been of a different nature than that for the untreated anthracite. A qualitative model based on the catalytic decomposition of H 2O 2 by activated carbon sites in the coal surface is used to explain the oxidation of the demineralized anthracite.

Chemisorption of alkanes and alkenes on carbon active sites

Chemisorption of C 1-C 4 hydrocarbons on a graphitized carbon black has been studied volumetrically at 573 K. Isotherms were of the Langmuir type with saturation reached at < 0.7 Pa hydrocarbon pressure. The molecules studied lie flat on the surface and occupy the same sites to varying degrees. The magnitude of chemisorption was affected by surface cleanliness and morphology, molecular size, active site distribution, and molecular accommodation. All the chemisorbed hydrocarbons were tightly bound and did not desorb at adsorption temperature even during evacuation. Upon heating the chemisorbed hydrocarbons thermally cracked before desorbing.

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N 2 under a wide variety of conditions of heating rate (10 K min −1 to 10 4 K s −1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0 2 adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.

Modification of butadiene rubber with 1,3‐dipolar cycloaddition reaction. I. Preparation and properties of the modified polybutadiene

AbstractPolybutadiene of low cis‐content was modified with the precursors of nitrones and nitrile oxide. The carbon‐black‐filled vulcanizates of the modified rubbers containing isoxazolidine and isoxazoline groups showed good tensile properties. The effect of modification was ascribed to the interaction between active sites of carbon black particles and the heterorings.