Om P. Mahajan

Carbon as a support for catalysts: I. Effect of surface heterogeneity of carbon on dispersion of platinum

Differences in the degree of dispersion of platinum supported on a graphitized carbon black, subjected to varying levels of carbon burn-off in air, have been found. Dispersion increases with the extent of prior gasification of the carbon support. This increase cannot be explained in terms of total surface area increase of the carbon support as a result of gasification. Rather gasification increases the surface heterogeneity of the carbon support, which in turn increases the potential energy barrier for the diffusion of platinum species across the carbon surface during sample preparation which uses temperatures up to 500 °C.

Surface-Treated Activated Carbon for Removal of Ammonia from Water

Abstract Adsorption of phenol from dilute solutions has been studied on porous and nonporous carbons, as well as on ion-exchange resins. At a given equilibrium concentration, uptake of phenol on nonporous carbons per unit area is determined by the nature of the carbon surface. Phenol uptake on porous activated carbons decreases sharply upon surface oxidation. However, progressive elimination of chemisorbed oxygen from the oxidized carbon upon heat treatment at increasing temperatures in N2 increases the phenol adsorption capacity. The capacity is further enhanced if following heat treatment in N2 at 950 [ddot]C the samples are reacted with H2 at 300 [ddot]C. The mechanism of phenol adsorption on carbons has been discussed. Activated carbons are more effective adsorbents for phenol than commercial ion-exchange resins.

Low Temperature Air Oxidation of Caking Coals: Fourier Transform Infrared Studies

Fourier transform infrared spectroscopy has been used to characterize the oxidation of a coking coal. The results demonstrate that the most important initial products of oxidation are carbonyl and carboxylic acid groups. Bands associated with carbon-oxygen single bonds, as in ethers or phenols, do not become prominent until the later stages of the oxidative process. Upon reaction with potassium in tetrahydrofuran a number of changes in the spectrum of both the oxidized and unoxidized coal become apparent. This reagent cannot be considered specific for cleavage of ether bonds, but can also lead to products usually associated with air oxidation.

Reactivity of heat-treated coals in steam

Abstract Reactivities of seventeen 40 × 100 mesh (U.S.) coals charred to 1000 °C have been measured at 910 °C in 0.1 MPa of a N2H2O mixture containing water vapour at a partial pressure of 2.27 kPa. Char reactivity decreases, in general, with increasing rank of the parent coal. The chars show a 250-fold difference in their reactivities. Results suggest that gasification of chars in air, CO2 and steam involves essentially the same mechanism and that relative gasification rates are controlled by the same intermediate oxygen-transfer step. Removal of inorganic matter from raw coals prior to their charring or from chars produced from raw coals decreases the reactivities of lower-rank chars, whereas reactivities of higher-rank chars increase. Addition of H2 to steam has a marked retarding effect on char reactivity in most cases. However, in a few cases H2 acts as an accelerator for gasification. The effect of particle size, reaction temperature and water-vapour pressure on char reactivity is considered.

Unification of coal-char gasification reaction mechanisms

Shapes of burn-off versus time (t) plots for various chars reacted in different gases are quite similar. These plots can be normalized into a single characteristic curve using a dimensionless time scale such that tτ0.5 equals one at a fractional burn-off (BO) of 0.5. The shape of the characteristic curve has been explained qualitatively on the basis of changes in porosity and surface area occurring during gasification of microporous chars. For each gasification medium, BO versus tτ0.5 data for different chars, up to a maximum BO of 0.7, can be correlated by a cubic equation of the form: BO = a(tτ0.5) + b(tτ0.5)2 + c(tτ0.5)3. Up to a BO of 0.7, different char gasification reactions can also be described reasonably well by a first-order equation. However, the correlation is not as good as that provided by the cubic model.

Water adsorption on coals

Abstract Adsorption of water on six coals of rank varying from anthracite to HVC bituminous has been studied at 0 °C and 20 °C. From BET plots, the calculated surface areas vary from 17.5m 2 /g for the low volatile bituminous coal to 82.5m 2 /g for the HVC coal of 45.4% volatile matter. The fraction of the total surface area (measured by carbon dioxide at 25 °C) covered by water in a monolayer varies from 12% for the low volatile bituminous coal to 60% for the HVC coal. In the monolayer, water adsorbs on hydrophilic sites provided by oxygen functional groups on the coal surface and by mineral matter. The heat of adsorption of water on the coals is closely equal to the heat of liquefaction (10.6 kcal/mol) ∗ at all adsorption pressures studied, that is above about 0.03 relative pressure. Hysteresis is observed on all the isotherms down to essentially zero relative pressure of water. Reasons for the hysteresis are discussed.

Reactivity of heat-treated coals in hydrogen

Abstract Reactivities of eighteen 40 × 100 mesh U.S. coals charred to 1000 °C have been measured in H2 at 2.7 MPa and 980 °C. The char-hydrogen reaction usually occurs in two stages: a slow induction period followed by a constant-rate region. Reactivities of various chars in the initial stage (Ri) decrease, in general, with increasing carbon content of the parent coals, whereas reactivities in the constant-rate region (Rc) are essentially independent of the rank of the parent coals. Reactivities of chars in H2 differ markedly from those in air and CO2. Results of surface-area measurements of chars and activation energies for the hydrogasification reaction suggest that during the induction period the reaction is diffusion-controlled whereas in the constant-rate region it is chemically controlled. Upon removal of mineral matter, Ri values generally decrease but Rc values show a random variation. Removal of mineral matter from coals prior to their carbonization brings about profound changes in surface area and porosity of chars. The effect of char particle size on reactivity is considered.

Low-temperature air oxidation of caking coals. 1. Effect on subsequent reactivity of chars produced

Abstract The effect of preoxidation of two highly caking coals in the temperature range 120–250 °C on weight loss during pyrolysis in a N2 atmosphere up to 1000 °C and reactivity of the resultant chars in 0.1 MPa air at 470 °C has been investigated. Preoxidation markedly enhances char reactivity (by a factor of up to 40); the effect on char reactivity is more pronounced for lower levels of preoxidation. For a given level of preoxidation, the oxidation temperature and the presence of water vapour in the air used during preoxidation have essentially no effect on weight loss during pyrolysis and char reactivity. An increase in particle size of the caking coals reduces the rate of preoxidation as well as subsequent char reactivity. Preoxidation of caking coals sharply increases the surface area of the chars produced. Compared to heat treatment in a N2 atmosphere, pyrolysis in H2 of either the as-received or preoxidized coal results in a further increase in weight loss and a decrease in subsequent char reactivity.

Differential scanning calorimetry studies on coal. 1. Pyrolysis in an inert atmosphere

Abstract Results of thermal changes involved during the pyrolysis of twelve US coals of various ranks in a helium atmosphere at 5.6 MPa (gauge) and temperatures up to 580 °C are reported. Thermal effects during pyrolysis of coals ranging in rank from anthracite to HVC bituminous are endothermic in nature over the temperature range investigated. Exothermic heats are observed only in the case of sub-bituminous and lignitic coals. The net thermal effects, that is the resultant of endothermic and exothermic heats, go from endothermic to exothermic with increase in carbon content, a transition occurring around 66% carbon and another in the reverse direction at about 75% carbon. A maximum in exothermicity occurs around 71% carbon and in endothermicity at about 81% carbon. Results have been compared with published DTA data on coals. The fallacy in the interpretation of published DTA thermograms of coals, where weight changes accompany thermal effects, is discussed.

Measurement of swelling of coals in organic liquids: a new approach

Abstract The sorption of organic molecules on coals of various rank was studied using gravimetric and pycnometric techniques. The pycnometric technique was insensitive to sorption due to swelling. Hence, a comparison of sorption values obtained by these two techniques provides an accurate measure of solvent-induced swelling in porous samples. In this study the maximum degree of benzene and tetralin-induced swelling occurred in a coal of 75 wt% carbon content. Compressibility was also found to be at a maximum in this coal. These results suggest that at 75 wt% carbon content the size and/or flexibility of the chain segments between crosslinks in coal are at a maximum.