Laxman N. Mulay

Doped carbon black catalyst supports: I. Characterization by magnetic susceptibility and ESR spectroscopy

Abstract As-received Monarch carbon blacks (MC) from Cabot, graphitized carbons (GMC), desulfurized Carbolac-2, and CSX-203 were characterized by magnetic susceptibility and ESR spectroscopy at room temperature and those in the cryogenic range. The MC samples showed normal Langevin-type diamagnetism, whereas the GMC showed a temperature-dependent Landau diamagnetism. From this, the degeneracy temperature, the effective electron mass of the delocalized electrons, and the Fermi level ( E F ) were estimated, assuming the approximations of a “two-dimensional electron gas” model for graphite. The GMC was doped with boron at the ppm level at heat treatment temperatures (HTT) of 2073 and 2773 K. Boronation at low HTT indicated that boron atoms could enter the interstitial or defect sites, in addition to the substitutional (sp 2 ) sites, whereas boronation at the higher HTT showed that all boron atoms entered at the substitutional sites. ESR studies indicated a lowering of g -values, and half-line widths, and a ten-fold decrease in the spin concentration as a result of boron doping. A transition from Landau to almost Langevin type diamagnetic behavior was observed; a decrease in diamagnetic susceptibility corresponded to a lowering in the E F level of the carbons. Shifts in the E F have been estimated as a function of boron doping. In a sequel to this paper, the preparation of Fe catalysts on the above carbon supports are presented along with their magnetic, Mossbauer, and CO-hydrogenation studies. A rationale for changing the electronic properties of the substrate by boron doping is given, in relation to changing the properties of supported metal ( M 1 ) by “alloying” it with another metal ( M 2 ) to form the socalled “bimetallic” clusters.

Doped carbon black catalyst supports: II. Magnetic, Mössbauer, and catalytic studies of iron/carbon systems

Abstract In continuation of our earlier investigation of carbon-supported Fe catalysts, a systematic study of diamagnetic susceptibility and ESR spectroscopy was carried out on “as-received”, graphitized, and borondoped carbon blacks. This characterization showed the Fermi level ( E F ) of the graphitic carbon decreased with boron doping. These carbon supports were used for dispersing small Fe crystallites, which were further characterized by appropriate in situ magnetization measurements and Mossbauer Spectroscopy. Observations showed that all Fe C catalysts were superparamagnetic yielding particle diameters in the range of 3.5–5.5 nm. From the reaction rates obtained for CO hydrogenation over these catalysts, activation energies, product distributions, and turnover frequencies were computed. These results were compared with Fe Al 2 O 3 and Fe SiO 2 catalysts. The iron dispersed on boronated carbons showed a small increase in turnover frequency for CO hydrogenation as compared with Fe dispersed on undoped carbons. A seven-fold change in the E F level of the carbon supports produced only a twofold change in activity, and consequently a strong interaction between the iron and carbon could not be confirmed.

Novel Applications of organometals: Magnetic, etc. properties of iron dispersions (I) in zeolites and (II) in amorphous glass-like carbons

The first part describes a novel technique for dispersing ultrafine species of iron oxide of controlled particle size (radii < 150 ») within the cage structure of a zeolite matrix, with the aid of organometals such as ferrocene and iron pentacarbonyl. The particle size and magnetic ordering within the iron clusters are elucidated via 57Fe Mossbauer spectrocopy and via their superparamagnetic behavior displayed by magnetization curves obtained as a function of the magnetic field and temperature. The second part of the paper describes the magnetic properties of iron species (Fe3C, Fe3O4, etc.) dispersed in amorphous glass-like carbons. These are obtained by the polymerization of furfuryl alcohol-containg ferrocene derivatives and by the subsequent pyrolysis of the polyfurfuryl alcohol. The technological relevance and applications of the materials studied in Parts I and II of the paper are pointed out.

Aurivillius-Popper mixed superconductors in BiO?CuO?(Sr0.5, Ca0.5)O system

We report the effect of inhomogeneities on the electric resistivity and ac magnetic susceptibility in Aurivillius-like bismuth mixed phase oxides of the BiO--Cuo--(Sr/sub 0.5/, Ca/sub 0.5/)O system and propose a crystal structure of the major phase having highest T/sub c/. Nominal Aurivillius compositions with molar ratios of BiO/(Sr/sub 0.5/, Ca/sub 0.5/)O = 1/2 are superconductors with T/sub c/ ranging from 83 to 107 K, and are accompanied by a large expansion during sintering due to the formation of Kirkendall voids. T/sub c/ increases with decreasing of the c lattice parameter. An oxide BiSrCaCu/sub 2/O/sub x/ (n = 2) shows a maximum T/sub c/ value of 107 K and an onset of superconductivity at a much higher temperature. It seems that the structure of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ consists of an Aurivillius-like phase having two perovskite layers and a Popper mixed phase. The ac magnetic susceptibility showed an overall decrease in susceptibility with time up to 220 days. This appears to be related to the relief of intralattice strain.

Magnetic susceptibility of ruthenium(III) in tris-ethylenediamine salts. Effect of outer sphere ligands

Abstract The paramagnetic susceptibility of Ru(III) in tris-ethylenediamine salts has been measured as a function of temperature. In all cases the iodide salt exhibits a greater susceptibility than the bromide salt. This difference may be attributed to a larger degree of temperature independent paramagnetism in the iodide complex due to the presence of low lying charge transfer states.

Magnetic characterization of semi‐amorphous nickel catalysis and their methanation activity

Commercial catalysts (A) and (B) containing 43 and 67% wt. Ni supported on alumina have been successfully characterized by using magnetization (σ) versus H (to 20 k Oe) and T (77−600°K) type curves and by electron microscopy. Hysteresis curves yielded coercive force, remanence and σ (saturation following heat treatment of the catalysts between (440−700°C. These parameters are interpreted in terms of the following properties of constituent particles: (a) superparamagnetic (b) single‐domain‐anisotropic and (c) multi‐domain. Needlelike ’b’ and ’c’ type particles are formed when A is heated to about 600° and above 650°C respectively. The formation of ’c’ type particles coincides with a decrease in the catalytic activity of A for the reaction CO+3H2→CH4+H2O. B consisted of a larger fraction of ’a’ type particles which on heat treatment increased Hc and σ (sat) as expected, but was more thermally resistant to the formation of ’c’ type particles. Unlike A, catalyst B did not show a significant decrease in cataly...

The Effect of Gasification on the Diamagnetism of Graphite and Artificial Carbons

Magnetic susceptibility and magnetic anisotropy measurements by the Faraday method were carried out on samples of natural graphite, graphitized carbon composites, stress recrystallized pyrolytic graphite and on a graphitized carbon black (Graphon). The total susceptibility at room temperature ranged from −u200918.75u2009×u200910−6u2009emu/g for graphon to −21.68u2009×u200910−6u2009emu/g for the stress recrystallized pyrolytic graphite. The anisotropy ratios (Ru2009=u2009χ⊥/χ∥) ranged from 1.02 for an extruded lampblack based carbon composite to 40 for the stress recrystallized sample. The temperature dependence of the susceptibility can be accounted for using a two dimensional degenerate electron gas model. From this model the values of To, the degeneracy temperature of the electron gas, the number of effective electrons per carbon atom and αu2009=u2009m/m*, the ratio of the electron rest mass to its effective mass were obtained. Within the spirit and limitations of this model the following estimates for To were obtained; 344°K for natural graphite...

An apparatus for in situ high temperature pretreatment of materials and measurement of their magnetic properties at cryogenic temperatures

An easy to use apparatus with high sensitivity for magnetic measurements is described that incorporates a unique dual purpose cryostat/heater, capable of in situ pretreatments at pressures up to 1 atm and temperatures up to 700 K. Magnetization measurements can be made at temperatures down to 100 K, either under vacuum or in a selected ambient gas, in the same cryostat with fields up to 1.7 T (17 kG). Techniques for measuring field profiles of the 23 cm diameter gradient pole caps are discussed along with the absolute and relative methods for measuring the magnetic susceptibility or magnetization. This system, based on the Faraday method, is particularly suitable for catalytic materials which need to be pretreated prior to magnetic measurements.

A versatile cryostat and a mutual inductance coil for a.c. susceptibility measurements on high Tc superconductors

Abstract An inexpensive cryostat and a mutual inductance coil are described for a.c. susceptibility measurements on high temperature superconductors. A unique feature of the cryostat is that it can be operated from 77 K upwards, whereas conventional gas-flow-type cryostats reach a lowest temperature of about 90 K. Other advantages of the system are also presented.

Magnetic Characterization of Semi-Amorphous Nickel on Alumina Dispersions: Correlations with their Methanation and Chemisorption Activities

Commercial samples of semiamorphous nickel supported on alumina, typically used as methanation catalysts have been characterized by using magnetization (σ) versus field (up to 20 KOe) and temperature (77–600 K) type curves and by electron microscopy. Hysteresis curves yielded coercive force (Hc) remanence (Ir) and saturation magnetizations (σs), following heat treatment of the catalysts between 400–700°C. These parameters are interpreted in terms of the following properties of constituent particles: (a) superparamagnetic (b) single-domain anisotropic and (c) multi-domain. Needlelike “b” and “c” type particles are formed when a sample A with 43 wt % of Ni is heated to about 600 and above 650°C respectively. Another sample B with a higher loading of Ni (67 wt %) consisted of a larger fraction of “a” type particles which on heat treatment increased Hc and σ (sat) as expected, but was more thermally resistant to the formation of “c” type particles. Unlike sample A, B did not show a significant decrease in the methanation activity upon the formation of “c” type particles. The methanation reaction CO + 3H2 → CH4 +H2O was followed using standard gas chromatographic techniques. The distribution of the superparamagnetic particles in the presence of multi-domain particles in various heat treated samples has been calculated and shown to correlate well with their hydrogen chemisorption activities.