R.D. Sherwood

Catalytic gasification of graphite by nickel in various gaseous environments

Abstract Controlled-atmosphere electron microscopy studies of the nickel/graphite system have revealed that in a strong oxidizing environment carbon gasification occurred mainly by the uncatalyzed route. Under the milder conditions of steam, catalyzed and uncatalyzed reactions occurred at comparable rates. In contrast, the reaction proceeded exclusively via catalyzed attack when experiments were performed in a hydrogen environment. However, in this case activity ceased at temperatures of around 1000 °C due to what is believed to be the formation of a strong nickel-carbon interaction resulting in the transformation of active metal particles, which formed a thin film along the edges of the catalytic channels they had produced at lower temperatures. Regeneration of particles could be achieved by heating in either oxygen at 850 °C or steam at 830 °C. The original activity pattern was restored if these specimens were subsequently heated in hydrogen. Although the precise cause and nature of the interaction between nickel and carbon is not fully understood, the results suggest a novel approach to redispersing large nickel particles.

Direct observation by controlled atmosphere electron microscopy of the changes in morphology of molybdenum oxide and sulfide supported on alumina and graphite

The changes in morphology of monolayer quantities of molybdenum oxide and sulfide phases supported on nonporous alumina and graphite thin films were studied by controlled atmosphere electron microscopy in oxygen and in H2SH2 gas mixtures. When molybdenum oxide was supported on alumina and heated in oxygen at temperatures up to 825 K, a strong oxide-support interaction was observed, resulting in a highly dispersed molybdenum phase. When supported on graphite, the support interaction was weaker and bulk molybdenum oxide formed that became mobile on the surface at temperatures higher than 930 K. On mixed supports containing both alumina and graphite regions, the mobile molybdenum oxide particles on graphite at 930 K were observed to disappear when they contacted alumina edges due to rapid spreading of the molybdenum oxide over the alumina surface. This behavior can be explained by the relatively low surface energies for MoO3 and graphite compared to the high surface energy for Al2O3. The molybdenum oxide-support interaction on alumina was broken by sulfiding in 5% H2SH2 at ca. 750 K, with the formation of crystallites of MoS2 located preferentially at grain boundaries on alumina. Larger particles were obtained at higher sulfidation temperatures. Reoxidation at temperatures higher than 645 K redispersed the crystallites due to spreading of molybdenum oxide over alumina. On the graphite support, a MoS2 “rag phase” was formed from the MoO3 crystallites upon treatment in 5% H2SH2 at 475 K, and molybdenum species did not spread over the graphite surface during reoxidation at temperatures up to 1100 K. Finally, sulfidation and reoxidation of a cobalt-promoted molybdenum/ alumina specimen showed behavior similar to that observed for the unpromoted samples.

A comparison of the catalytic influence of nickel, iron and nickel-iron on the gasification of graphite in various gaseous environments

Abstract Controlled-atmosphere electron microscopy has been used to study the catalytic influence of nickel, iron and nickel—iron on the graphite-steam and graphite-hydrogen reactions. This study has enabled us to identify some characteristics of the alloy catalyst which were not manifested by either of the pure constituents when reacted under similar conditions. When nickel-iron/graphite specimens were reacted in steam, catalysis occurred exclusively by the channeling mode, in contrast to nickel which exhibited a mixture of both edge recession and channeling modes of attack, while iron appeared to be inactive. Confirmation of the higher catalytic activity of the alloy compared to nickel for the gasification of graphite in steam was obtained from bulk flow reactor measurements. The alloy was also found to be a more active catalyst than either of its pure components for the graphite-hydrogen reaction. In this case, two modes of catalytic action were observed: edge recession below 845°C and channeling above this temperature. This change is believed to arise from a modification in the wetting characteristics of alloy particles along graphite edges. It is significant that the channeling action of the alloy, like that produced by active iron particles, persisted up to temperatures in excess of 1100°C, in contrast to the behavior found with nickel, where the catalyst underwent deactivation.

Direct observation of wetting and spreading of iridium particles on graphite

Abstract Redispersion of sintered metal crystallites is one of the most important and least understood processes in the commercial operation of supported metal catalysts. In the present work we have attempted to gain a clearer insight into this process by following the behavior of a catalyst system inside an electron microscope. The system selected for this investigation was iridium/graphite in hydrogen. Wetting and spreading of iridium particles on graphite in hydrogen at 965 °C has been observed directly using controlled-atmosphere electron microscopy. Quantitative measurements of the rate of particle disappearance and the change in contact angle, θ, between the particles and the surface have shown that the spreading can occur below the Tammann temperature of the bulk metal. The results are discussed in terms of a model in which particles have a cherry-like structure, consisting of a “hard core” surrounded by a “viscous” layer. Enhanced atomic mobility in the surface layers may be due to easier surface diffusion as compared to bulk diffusion, possibly due to a higher surface temperature resulting from exothermic processes occurring at the surface.

Catalytic oxidation of graphite by iridium and rhodium

Abstract Controlled atmosphere electron microscopy has been used to investigate the catalytic influence of iridium and rhodium on the graphite-oxygen reaction. Both systems were found to exhibit a similar behavioral pattern. As the temperature was raised, the initial catalytic action was followed by a short period of inactivity before a second, more intense catalytic attack occurred at temperatures >1000 °C. It is probable that the two activity regions correspond to the existence of oxides, IrO 2 and Rh 2 O 3 at the lower temperatures and the respective metals at temperatures >1000 °C. This pattern is quite different than that found for platinum and palladium, which are present in the metallic state throughout the gasification sequence and as a consequence exhibit only a single continuous catalytic action on the graphite-oxygen reaction. Another aspect revealed from these experiments is the manner by which iridium and rhodium particles supported on graphite sinter in an oxidizing environment. Atomic migration accounts for the growth of inactive particles below the Tammann temperature for the metal (1110 °C for iridium; 910 °C for rhodium); above this temperature the overriding mode of growth is via particle migration. Particle mobility on graphite can occur below the Tammann temperature as the catalytic channeling action induces motion into active particles.

Morphologies of Sn and PtSn phases on thin films of alumina and graphite

Transmission electron microscopy studies of Sn and Pt-Sn morphologies were conducted on model catalysts, prepared by the vacuum evaporation of metal onto thin films of γ-alumina and sheets of graphite. All supported Sn samples showed nonwetting metallic particles following treatment in hydrogen. Small (<20 nm) particles assumed a core/shell morphology on alumina, due to the existence of a thin oxidic tin-support phase surrounding or beneath a metallic core, whereas the metallic particles on graphite showed uniform contrast. Upon oxidation, metallic Sn particles transformed into toroidal-shaped particles, indicative of an interaction between tin oxide and the substrates. Upon addition of Pt to the supported Sn samples, Pt-Sn alloy formation was observed under reducing conditions. On alumina, the particles were round and surrounded by a light apron. Graphite-supported particles were round and featureless. Upon oxidation of all specimens, the alloy phase was destroyed, as the tin was preferentially oxidized to SnO2 while the platinum remained metallic, forming a characteristic core/shell appearance. The presence of tin retarded the catalytic oxidation of the graphite substrate by platinum at 770 K. A fraction of the surface tin oxide layer formed during the oxidation treatment of Pt-Sn/Al2O3 did not re-alloy with the platinum upon reduction, due to interaction with the alumina support.

Chromatographic separations of fullerenes : discovery and characterization of C60 mono-epoxide

Abstract A semi-preparative scale high-pressure liquid chromatographic method for the separation of the fullerene extract components has been developed. During the course of these separations, a new fullerene species was isolated and characterized by mass spectrometry, 13C NMR, and infrared and UV-Vis absorption spectroscopy. Mass spectrometry identifies this component as the mono-oxygen adduct of C60 (C60O) and 13C NMR strongly supports the epoxide structure rather than the isomeric oxido-annulene structure. In addition, the effects of different solvents on the optical absorption spectra of purified C60 and C70 and crystalline properties of C60 fullerite powder are reported.

Oxidation studies of various petroleum cokes

Abstract In situ scanning transmission electron microscopy has been used to evaluate various coke samples ranging from very Isotropic to ultrapremium needle-like in terms of their oxidation characteristics. Three major modes of attack were identified: 1. a) low-temperature (375°C) shallow pitting, which predominated in isotropic materials 2. b) high-temperature (525°C) deep pitting, which appeared to be associated with highly anisotropic structures and 3. c) edge recession, which occurred with all cokes but was most apparent with anisotropic structures. A rationale is presented which accounts for the behavior of isotropic and anisotropic cokes in oxygen

C 60 And C 70 : Here’s Looking at you

In this paper we will describe the production, separation and characterization of the new all carbon molecules, C 60 and C 70 . High performance liquid chromatography HPLC is used to obtain purified samples of C 60 and C 70 , which are subsequently characterized by electron impact and chemical ionization mass spectrometry, IR and UV-visible absorption spectroscopy, NMR, ESR, scanning tunnelling microscopy, transmission electron microscopy and x-ray absorption fine structure measurements.

The effect of crystal structure upon the activity of iron in steam gasification

A previous publication reported on the behavior of iron particles supported upon graphite in a hydrocarbon/steam environment. That work concentrated on the temperature range above ca. 1273 K. Catalytic behavior for the same system in the temperature range from room temperature to 1273 K is the subject of this note. Experimental procedures are the same in both studies. Kinetic studies were made using controlled-atmosphere (transmission) electron microscopy (CAEM). In summary, it has been observed that ..gamma..-Fe/sup 0/ is less active than ..cap alpha..-Fe/sup 0/ in the gasification of graphite in a steam/hydrocarbon atmosphere. This effect may be due to differences in carbon solubility, rate of carbon diffusion, or H/sub 2//H/sub 2/O adsorption between the two phases. Additional experimental work is planned to probe the importance of these properties. 8 references.