Ripudaman Malhotra

Single crystal metals encapsulated in carbon nanoparticles

Single-domain microcrystals of LaC2 encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as α-LaC2, the phase of LaC2 that is stable at room temperature. Bulk α-LaC2 is metallic and hydrolytic. Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC2 single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.

Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO2) with H20 and hydrogen chloride (HCl) on surfaces that simulate polar stratospheric clouds [ice and nitric acid (HNO3)—ice and sulfuric acid] are studied at temperatures relevant to the Antarctic stratosphere. The reaction of ClONO2 on ice and certain mixtures of HNO3 and ice proceeded readily. The sticking coefficient of ClONO2 on ice of 0.009 � 0.002 was observed. A reaction produced gas-phase hypochlorous acid (HOCl) and condensed-phase HNO3; HOC1 underwent a secondary reaction on ice producing dichlorine monoxide (Cl2O). In addition to the reaction with H20, ClONO2 reacted with HCl on ice to form gas-phase chlorine (Cl2) and condensed-phase HNO3. Essentially all of the HCl in the bulk of the ice can react with ClONO2 on the ice surface. The gaseous products of the above reactions, HOCl, Cl20, and Cl2, could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the formation of condensed-phase HNO3 could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.

Mechanisms of hydrogen transfer and bond scission of strongly bonded coal structures in donor-solvent systems

Abstract Five possible non-ionic mechanisms for hydrogen transfer leading to scission of strongly bonded coal structures are considered. Cleavage of diphenyl ether, diphenylmethane, and 1,2′-dinaphthylmethane was studied in a number of aromatic and hydroaromatic solvents. Relative cleavage rates and product selectivity data show that a substantial portion of the cleavage takes place as a consequence of direct H-atom transfer from solvent derived cyclohexadienyl radical intermediates to the ipso position of the substrate in a single biomolecular step. This reaction is given the name radical hydrogen transfer or RHT.

Aromatic substitution. 43. Perfluorinated resinsulfonic acid catalyzed nitration of aromatics

Alkylbenzenes are nitrated in high yields with butyl nitrate or acetone cyanohydrin nitrate catalyzed by a perfluorinated resinsulfonic acid (Nafion-H) catalyst . The reactions are very clean and do not require any aqueous basic workup . Nitrations were also carried out with nitric acid, under azeotropic distillation conditions to remove water and thus prevent dilution of the acid , and with dinitrogen tetroxide.

Magnetic separation of GdC2 encapsulated in carbon nanoparticles

Abstract Single crystal particles of α-GdC2 have been encapsulated into carbon polyhedral shells by the carbon arc-discharge technique. These particles range in size from 20 to 50 nm and are paramagnetic in nature. The phase and stoichiometry of these crystals have been determined using a combination of techniques, such as X-ray diffraction and high-resolution electron microscopy. As reported for α-LaC2 earlier, the crystal structure of bulk α-GdC2 is also tetragonal, and it is metallic and undergoes hydrolysis. However, the encapsulation of the nanoscale crystals in carbon shells prevents their hydrolysis indefinitely. These nanometer-sized particles, which can be preferentially extracted using powerful magnetic fields, may have useful applications in several fields of science.

Organic compounds on crack surfaces in olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii

Abstract Organic compounds associated with thin carbonaceous films on crack surfaces have been detected by thermal-desorption photoionization mass spectrometry in large single crystals of olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii. Alkalis, silicon, aluminum and halogens are also present in the 3–4 nm thick carbonaceous films. The organics probably were not derived from the upper mantle or lower crust or from environmental biogenic contamination after eruption and cooling. It is likely that the carbonaceous films and organics were deposited or formed on crack surfaces created during eruption and cooling of the host alkali basalts. Whether the organics were produced abiotically by FischerTropsch-like reactions involving volcanic gases and fresh-fractured surfaces where reduced carbon was deposited, or whether the organics represent biogenic material that was assimilated into the magmatic system prior to or during magma ascent, cannot be ascertained at this time due to their low abundance.

Silicon carbide formation by annealing C60 films on silicon

Silicon carbide films were grown on (100) silicon substrates by deposition of 200-nm-thick C60 films, followed by annealing. The predeposited C60 is progressively destroyed by annealing, and carbon reacts with silicon to produce SiC. The reaction starts at the interface and continues by diffusion of silicon through the already formed SiC. At the lower temperatures (700 °C), the reaction is localized at the interface. Diffusion of silicon and formation of stoichiometric SiC requires annealing at 800 °C for t⩾100 min and at 900 °C for t⩾25 min. The stoichiometric films are uniform with a grain size of 20–40 nm. A diffusion coefficient of silicon in SiC of 4×10−15 cm2/s at 900 °C was determined. Because the diffusion of silicon is faster through preferential paths in the SiC film, such as grain boundaries and other crystalline defects, pits and voids are produced in the silicon substrate when the C60 predeposited film covers larger areas.

The case for induced bond scission during coal pyrolysis

Bond scissions during coal pyrolysis are generally considered to result solely from the thermolysis of weak bonds in coal structures. Evidence is presented from published data showing that a significant amount of bond scission under pyrolysis cannot be due to simple thermolysis and must be induced by some other means. The evidence includes the following observations: 1, polymeric models consisting of no weak linkages are volatilized in the 450 to 550 °C range; 2, polymeric coal models designed specifically to degrade by thermolysis of weak bibenzylic linkages show nonetheless a significant fraction of the cleavage of strong CarCal linkage; 3, under pyrolysis conditions bibenzyl type structures immobilized on silica yield benzene and ethylbenzene in addition to the expected toluene; 4, pyrolysis of O-benzylated coals shows besides the expected cleavage of the benzyl ether linkage products from the scission of the strong phenyl-C bond. By analogy with coal liquefaction, the scission of strong bonds results from β-scission of radical intermediates, and from hydrogenolysis engendered by either free H-atoms or cyclohexadienyl radicals. These reactive cyclohexadienyl radicals themselves may be formed from the scavenging of radicals (produced by weak-bond thermolysis) by dihydroaromatic structures or from reverse radical-disproportionation between aromatic and hydroaromatic structures. Finally, the importance of induced bond scissions was illustrated by showing how they help rationalize the observed selective production of oils as opposed to gas in the hydropyrolysis of coals when the pyrolysis temperature is maintained below a certain threshold.

The effect of carbon additives on the mesophase induction period of Athabasca bitumen

Abstract The ability of solid carbonaceous material to retard the formation of coke during thermal cracking and hydrocracking of heavy hydrocarbons is well known. In this study, we used in-situ microscopy (hot-stage) to obtain additional mechanistic information on whether fine coke and fullerene soot particles retard the growth of mesophase during thermal cracking of Athabasca bitumen, thus reducing the possibility of fouling in preheaters and furnaces. The findings from this study could also have application in other non-catalytic thermal processes such as visbreaking and coking. In the absence of additives in the Athabasca bitumen feed, the formation of mesophase occurred after 61 and 67 min (measured from room temperature) at reaction temperatures of 450°C and 440°C, respectively. The addition of solid coke (ca. 5 wt.%) from a commercial delayed coking operation shortened the mesophase formation time to almost 45–50 min under similar conditions. The coke, having surface area of only 1.65 m 2 /g, resulted in enhanced bitumen fluidity and large-textured mesophase. These observations were rationalized based on the ability of delayed coker coke to release hydrocarbons into the bulk fluid during thermal cracking. Light hydrocarbons released from coke may have changed the solvating power of the liquid phase in bitumen and promoted phase separation, resulting in a shorter induction period. In contrast, adding small amounts of fine fullerene soot (ca. 1 and 5 wt.%) delayed the appearance of mesophase significantly under similar conditions. The ability of fullerene soot to physically absorb the mesophase precursors into its pore structure led to an increase in the apparent viscosity of the bulk phase, which is known to reduce mesophase size and prolong the induction period. Consistent with this, the induction period was prolonged an additional 10 min when the soot surface area was increased from 152 to 208 m 2 /g. The increase in induction period is significant with respect to reaction times and suggests that these fullerene soot materials could be effective in allowing for increased severity and liquid products yield from visbreaking, with less likelihood of fouling in the preheater tubes and furnace walls.

Characteristics of CANMET coprocessing distillates at different coal concentrations

Abstract The effect of feed coal concentration on the characteristics of gross and fractionated coprocessing distillates was investigated. Elemental analysis, 1H n.m.r., high performance liquid chromatography (h.p.l.c.) and field ionization mass spectrometry (f.i.m.s.) were used in this study. The distillates (205–525 °C) were obtained by coprocessing Forestburg subbituminous coal at three concentration levels, 3.7, 23.9 and 39.5 wt% (based on a maf slurry feed) and Cold Lake vacuum bottoms from Alberta in a nominal 1 kg/h bench-scale unit. Gross distillate characteristics as determined by elemental and 1H n.m.r. analyses indicate that as coal concentration is increased, more coal-derived liquids contribute to the distillate. For detailed analysis, distillates from 3.7 and 23.9 wt% coal concentration were separated into five different fractions using a Polar Amino Cyano (PAC) column. Analysis of the fractions indicated that increasing coal concentration from 3.7 to 23.9 wt% results in an increase in the amount of polar materials and a subsequent decrease in the amount of saturated compounds. The wt% of monoaromatics and polyaromatics was not affected by the amount of coal present in the slurry feed. The number and weight average molecular weights of the distillates boiling between 205 °C–525 °C and their hydrocarbon-type fractions decreased as the coal concentration increased. Each hydrocarbon-type fraction was analysed by 1H n.m.r. and f.i.m.s. to determine the effect of coal concentration on compound-type distribution. The results show that increasing coal concentration has a significant effect on compound-type distribution and, in part, coal may enhance the upgrading of bitumen. Based on preliminary isotopic mass balance measurements that take advantage of the difference between the 13 C 12 C ratio of coal and bitumen, the amount of coal-derived carbon in the coprocessing distillate was estimated.