Ljubisa R. Radovic

Evidence for the protonation of basal plane sites on carbon

Abstract The nature of the surface basicity of two series of highly pure, chemically and/or thermally pretreated carbons has been investigated by 1. (a) HCl adsorption, 2. (b) stepped temperature-programmed desorption, 3. (c) electrophoresis and 4. (d) mass titration. Following a comprehensive review of the pertinent literature, it is concluded that two mechanisms are necessary and sufficient to account for the basic properties of carbons. For carbons having low oxygen contents, molar HCl/O adsorption ratios exceeding two indicate that an oxygen-free type of site is involved in the adsorption process. Evidence favoring electron donor-acceptor (EDA) interactions of the type Cπ + H3O+ → Cπ-H3O+ is presented. The postulated Cπ site and its interaction with adsorbed H3O+ ions is described in detail. As the oxygen content of the carbon increases, the molar HCl O ratios decrease sharply and level off at nonzero values, which are small due to the presence of nonbasic surface oxides. When these are removed by heat treatment (at 1073 K), the molar HCl/O adsorption ratio is again found to exceed two at low oxygen coverages, but levels off at ca. 1 for higher oxygen coverages. Of all the basic surface oxides proposed in the literature, only the pyronetype groups can account for this HCl O ratio. Therefore, it is proposed that the basicity of carbon surfaces arises from a combination of EDA and pyrone-type interactions. The predominance of either will be dictated by the oxygen content of the carbon in question.

Importance of carbon active sites in the gasification of coal chars

Abstract 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.

An experimental and theoretical study of the adsorption of aromatics possessing electron-withdrawing and electron-donating functional groups by chemically modified activated carbons☆

Abstract The adsorption of model aromatic compounds (aniline and nitrobenzene) on chemically tailored activated carbons has been systematically investigated. Adsorption experiments at controlled solution pH conditions confirmed that both electrostatic and dispersive adsorbate/adsorbent interactions can have a significant influence on the equilibrium uptakes of ionic and nonionic adsorbate species. For aniline (a weak electrolyte), maximum uptakes were found on oxidized carbon surfaces at solution pH near the adsorbates point of zero charge (pH PZC ). In contrast, nondissociating nitrobenzene uptakes were enhanced on heat-treated surfaces with graphene layers unperturbed by electron-withdrawing functional groups, particularly at solution pH ~ pH PZC . A theoretical model that can successfully account for the observed trends is hereby proposed as a much needed predictor of the experimental conditions and adsorbent surface chemical properties that will maximize the uptake of aromatic compounds by activated carbons.

Importance of catalyst dispersion in the gasification of lignite chars

Lignite chars were prepared in N2 under widely varying conditions of pyrolysis heating rate, temperature, and residence time. Their reactivities were measured by isothermal thermogravimetric analysis in 0.1 MPa air. A major decrease in char reactivity was observed with increasing severity of heat-treatment conditions. The relatively high gasification reactivity of lignite chars, compared to those obtained from higher rank coals, is due to the catalytic effect of the initially very highly dispersed CaO on the char surface. Char deactivation is caused primarily by CaO crystallite growth, measured by X-ray diffraction line broadening. When the reactivities of the various chars are expressed as turnover frequencies, i.e., per unit catalyst site, differences in observed rates of about 200 times are reduced to within 1 order of magnitude. Thus, it has been shown that the commonly observed and heretofore empirically treated lignite char deactivation with increasing severity of pyrolysis conditions can be correlated with a decrease in a measurable fundamental property of the chars: catalyst dispersion.

On the kinetics of carbon (Char) gasification: Reconciling models with experiments

Abstract The gasification reactivity profiles of different carbons (chars) are analyzed from both a theoretical and an experimental point of view. The virtues of and/or problems with utilizing the concepts of total surface area (TSA), active surface area (ASA), and reactive surface area (RSA) to explain or predict gasification rate variations with conversion are discussed. An analysis of several models of char gasification kinetics which predict the evolution of TSA with conversion revealed that the experimentally observed reactivity variations with conversion in carbon dioxide may be explained using just the initial properties of the char. An experimental investigation of char gasification in carbon dioxide using the transient kinetics approach gave a direct measurement of RSA. Gasification rates normalized with respect to RSA were essentially constant over the entire conversion range. A temperature-programmed desorption technique was also used to determine the amount of reactive surface intermediate formed on these chars during gasification in carbon dioxide; the results were in agreement with those obtained from transient kinetics. The application of these two independent but complementary techniques provided the heretofore elusive quantitative experimental explanation of reactivity variations with conversion for char gasification in carbon dioxide.

The role of substitutional boron in carbon oxidation

Carbon oxidation has been very thoroughly investigated. It is a reasonably well understood heterogeneous gas/solid reaction. Yet there are many practical and fundamental {open_quote}details{close_quote} that need to be sorted out. There are also some very important fundamental issues that are not understood. In this paper, the influence of boron as an inhibitor or catalyst of carbon oxidation is discussed.

Oxidation inhibition effects of phosphorus and boron in different carbon fabrics

Abstract To improve oxidation inhibition, elemental boron and two phosphorus compounds were doped into an activated carbon cloth and a carbon felt. The hypothesis was that P can block active sites by virtue of the formation of C–P–O or C–O–P bonds at graphene edges while substitutional B can alter the chemical reactivity of the residual free active sites by reducing the electron density in the graphene layer. To increase the final dopant concentration, the carbon felt was activated in nitric acid. The crystallinity of activated carbon cloth was improved by heat treatment and substitutional B; that of carbon felt was also improved, but not necessarily due to substitutional B. In all cases the oxidation reactivity is suppressed by heat treatment and in the presence of dopants. The oxidation inhibition mechanism in P-doped samples appears to be active sites blockage because of a proportional increase of oxidation inhibition with increasing P loading. The results for B-doped samples are consistent with our previous studies in which B was found to exhibit both a catalytic and an inhibiting effect on carbon oxidation. Samples doped with both P and B showed the most effective oxidation inhibition and their oxidation behavior is described in detail.

On the importance of the electrokinetic properties of carbons for their use as catalyst supports

Abstract Equilibrium adsorption of molybdenum on different carbon supports resulted in widely varying catalyst uptakes. The results were rationalized on the basis of varying degrees of electrostatic interaction between the catalyst precursor and the supports. This interaction is known to be dependent on the surface charge of the support (which in turn depends on the pH of the catalyst precursor solution) and the charge of the ionic precursor. The carbon supports were subjected to widely varying thermal and chemical pretreatments. Their surface charge (electrokinetic mobility as a function of pH) was determined by electrophoresis. This study and a critical analysis of the literature illustrate the heretofore mostly neglected fact that, for achieving controlled catalyst uptake and/or a high degree of catalyst dispersion, it is not sufficient to create adsorption (or catalyst anchoring) sites on the carbon support surface; these must also be made accessible to the catalyst precursor. If the isoelectric point of the support is known, the precursor can be chosen and/or the solution pH can be modified to favor catalyst precursor/support interaction (e.g., adsorption), and thus maximize initial catalyst dispersion.

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

Abstract 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-104 K/s, respectively. The observed reactivity differences were rationalized in terms of variations in the concentration of carbon and catalyst active sites.

On the porous structure of coals: Evidence for an interconnected but constricted micropore system and implications for coalbed methane recovery

An experimental and theoretical study of adsorption and diffusion of carbon dioxide and methane in coals of widely varying rank was carried out. Low pressures adsorption isotherms of CO2 were obtained and analyzed using Dubinins theory of volume filling of micropores. High-pressure adsorption isotherms of CH4 were obtained and analyzed using tracer pulse chromatography in conjunction with an appropriate adsorption/diffusion model. A preliminary129Xe NMR analysis of chemical shifts experienced by xenon atoms in particles of different sizes is also reported.The heretofore undocumented and/or underestimated effects of activated diffusion of CO2 at 273–298 K complicate the elucidation of the true microporous structure of coals, especially its dependence on coal rank. Activated diffusion of both CO2 and methane at room temperature does not allow reliable estimates of coalbed gas content to be made. A model of an interconnected network of pores which includes randomly distributed, numerous and ultramicroporous constrictions (at any size scale) is consistent with all these experimental and theoretical findings.