Thomas Baird

Carbon formation on iron and nickel foils by hydrocarbon pyrolysis—reactions at 700°C

Abstract Structural and morphological studies of carbons produced by pyrolysis of various hydrocarbons over iron and nickel foils have been carried out by high resolution electron microscopy. Kinetic and analytical measurements showed that the nature of the deposited carbon was a function of the rate of deposition rather than of the identity of the hydrocarbon gas phase. The carbon deposition rate was independent of pressure for methane and propane but dependent for propylene and butadiene above 1 Torr. The presence of residual metal in the deposited carbon in a finely dispersed form was established by oxidation. These results are compared with those of other workers and a carbon growth mechanism proposed that is consistent for both platelet and filamentary graphitic carbons.

Modified zinc oxide absorbents for low-temperature gas desulfurisation

The hydrogen sulfide absorption capacity of zinc oxide doped with first-row transtion-metal oxides (ca. 5% metal oxide loading) has been determined using a pulse reactor. The doped oxides were prepared either by impregnation of ZnO with the transition-metal nitrates or by comprecipitation of the transition-metal and zinc nitrates with ammonium/sodium carbonate. These absorbent precursors were then calcined to give the mixed oxides.Transmission electron microscopy studies of the impregnated and calcined absorbents revealed that the transition-metal oxides were finely dispersed over the ZnO as γ-Fe2O3, Co3O4 and CuO from the respective nitrate salts of these metals. The basal planes were the predominat exposed faces of the hexagonal ZnO in all the absorbents, irrespective of whether they were prepared by the coprecipitation or impregnation route. CuO and Co3O4 were not seen as separate phases in the respective calcined coprecipitated absorbents, but the particle morphology was noticeably changed after sulfidation and crystalline ZnS was detected by electron diffraction. Oxides prepared by the coprecipitation route had higher surface areas and a greater capacity for H2S removal than their impregnated counterparts. Doping with iron salts had little effect on the H2S uptake of ZnO, irrespective of whether an impregnation or a coprecipitation route had been used, but doping with copper or cobalt salts resulted in a marked enhancement in the H2S uptake in each case.The reaction of the mixed oxides with H2S was restricted to ca. 0.6 monolayers on average, based on calculations from surface areas, and is therefore largely confined to the surface of the oxides. However, the resulting sulfide is not present entirely as an adsorbed layer; some reaction occurs locally in depth to form ZnS crystallites sufficiently large to give an electron diffraction pattern. The main role of the transition-metal oxide is to increase the total surface area available for reaction with H2S.

Cobalt-zinc oxide absorbents for low temperature gas desulfurisation

The hydrogen sulfide absorption capacity of a series of cobalt-zinc oxides with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 was determined using a continuous flow absorption apparatus. The reaction of the mixed oxides with H 2 S amounted to ca. 3 monolayers, and is therefore largely confined to the surface of the oxides. The sulfur uptake was found to be proportional to the surface area of the oxides with a Co/Zn ratio ≤40/60, indicating that lattice diffusion played a major role in the rate determining step, and that the main function of the cobalt was to increase the surface area. At high cobalt concentrations, the sulfur uptake increased more than proportionately with surface area and the reaction was virtually stoichiometric for the oxide with a Co/Zn ratio of 100/0. This was associated with a change in the oxide structure from a bulk biphasic ZnO and Co 3 O 4 absorbent with a ZnCo 2 O 4 surface spinel at Co/Zn ratios ≤30 to a monophasic zincian or pure Co 3 O 4 structure at higher cobalt loadings. Analysis of the sulfided mixed oxides showed that microcrystalline membraneous sheets containing cobalt, zinc and sulfur developed on sulfiding. XPS studies of the sulfided oxides indicated that H 2 S reduced the surface spinel found at Co/Zn ratios ≤30/70 and the zincian/pure Co 3 O 4 found at higher cobalt concentrations to CoO and ZnO prior to the formation of their sulfides. The results are interpreted in terms of a surface reconstruction occurring during sulfiding.

Characterisation of cobalt–zinc hydroxycarbonates and their products of decomposition

A series of cobalt–zinc hydroxycarbonate precursors with nominal Co/Zn atomic ratios of 0/100, 10/90, 20/80, 30/70, 40/60, 50/50, 70/30, 90/10 and 100/0 have been synthesized from their mixed metal nitrates and ammonium carbonate by a coprecipitation route. X-Ray and electron diffraction studies of the precursors revealed that hydrozincite, Zn 5 (CO 3 ) 2 (OH) 6 , was the major phase at Co/Zn ratios≤30/70 and spherocobaltite, CoCO 3 , predominated at Co/Zn ratios of 50/50 to 90/10. The Co/Zn 100/0 precursor formed only the metastable basic carbonate Co(CO 3 ) 0.5 (OH) 1.0 0.1H 2 O. UV–VIS–NIR diffuse reflectance spectroscopy revealed that the cobalt was present in the 2+ oxidation state in an octahedral environment in all the precursors. Decomposition of the Co/Zn precursors at 350 °C resulted in the formation of ZnO as the major phase at low Co loadings and Co 3 O 4 as the major phase at high loadings. The highest surface areas were attained from the decomposition of basic cobalt carbonate or spherocobaltite containing little or no zinc in solid solution. XPS studies of the oxides revealed that only Co 3+ and Zn 2+ ions were present at the surface at Co/Zn ratios≤30/70 indicating the presence of the ‘surface spinel’, ZnCo 2 O 4 . Co 2+ was detected at higher Co loadings.

Mixed Co–Zn–Al oxides as absorbents for low-temperature gas desulfurisation

High-surface-area zinc–cobalt–aluminium oxides have been prepared from coprecipitated hydroxycarbonate precursors. The oxides were tested for their H2S uptake in a microreactor at 303 K. The precursors all adopted a hydrotalcite-type structure, whilst the oxides were also all single-phase materials. Their structures were either based on that of ZnO or Co3O4, which suggested the presence of solid solutions with all the ions dissolved into one phase. A comparison of the H2S uptake of these materials with that of the Co–Zn oxides suggested that the presence of aluminium ions in the oxide gave rise to an increase in their surface areas, but that the modified compounds did not absorb significantly more H2S. Indeed, the more Co-rich cobalt–zinc–aluminium oxides absorbed less H2S than Co–Zn oxides prepared in a similar way.

Synthesis and characterization of catalytic titanias via hydrolysis of titanium (IV) isopropoxide

Abstract Transmission electron microscopy (TEM) studies and nitrogen gas adsorption data were used to investigate textural characteristics of titanias prepared via calcination, at 400°C/3 h, of the hydrolysis products of titanium(IV) isopropoxide in n -heptane or isopropanol solvent. A variety of titanias (anatase form) were produced with different textural characteristics. The nature of these products was controlled largely by the synthesis parameters such as water ratio, the addition of acetic acid, and the solvent used during the hydrolysis. The resultant microstructure of the solids is correlated with the pertinent preparative conditions.

Morphology and CO2 uptake in tobermorite gel

Abstract The products of carbonation of a synthetic tobermorite gel have been examined by transmission electron microscopy, selected area diffraction and infrared spectrophotometry. The relationships between crystallinity and morphology of starting materials and products are discussed in terms of the observed pseudomorphosis.

Mixed cobalt–iron oxide absorbents for low-temperature gas desulfurisation

High surface area cobalt–iron oxides (with Co ∶ Fe ratios of [3 ∶ 1], [1 ∶ 1] and [1 ∶ 2]) have been prepared by the calcination of predominantly single-phase, coprecipitated hydrotalcite-type precursors prepared from mixed-Co2+–Fe3+ aqueous solutions. The oxide structures were all spinels; for the Co–Fe [3 ∶ 1] oxide, this was a Co3O4-type lattice, for the Co–Fe [1 ∶ 2] sample, it was a Fe3O4-type lattice and for Co–Fe [1 ∶ 1] an equal mix of the two was observed. The H2S uptake of all the oxides was tested at 303 K and the data correlated with sorbent preparation and characterisation. The Co–Fe [1 ∶ 1] oxide absorbed the most H2S before breakthrough and further studies into the preparation of this oxide showed that nitrate-free precursors produced at pH 7 and then aged for 30 minutes or more gave rise to the best oxide sorbents after calcination at 623 K rather than 723 K. These observations have been rationalised on the basis that these oxides had lower density and a greater mix of Co2+/3+ and Fe2+/3+ leading to more distortion of the parent oxide lattice. Increased H2S uptake was ascribed to more rapid ion diffusion through the sorbent lattice leading to enhanced replenishment of surface oxide.

Deposition of columnar and laminar forms of carbon on polycrystalline nickel foils

Abstract Two distinct forms of carbon in terms of morphology and crystallinity have been produced on polycrystalline Ni foils by pyrolysis of propene (50 Torr). A thick layer (up to 10 μm) of columnar carbon ( d (0002) = 3.40 A ; E a = 33 Kcal/mole ) was obtained within the temperature region 320–380°C. From 380–800°C a thin layer of laminar graphite ( d (0002) = 3.36 A ; E a = 16 kcal/mole ) was formed. As a consequence of the temperature gradient along the length of the foils in these experiments both forms of carbon could be produced simultaneously. Bulk diffusion and surface diffusion mechanisms have been proposed to account for the growth of the columnar and laminar products respectively. At ~700°C migration of Ni metal particles to the cooler foil edges occurred and resulted in the formation of non-graphitic filaments. Preliminary studies with butadiene have revealed similar deposition characteristics and activation energies to those of propene.

Magnesium hydroxide precipitation as studied by gel growth methods

Abstract The precipitation of magnesium hydroxide from aqueous solutions by reaction with calcium hydroxide has been studied in agarose gel by electron microscopy and diffraction. The first stage, which dominates at very low Mg 2+ concentrations, is the formation of thin single crystals of magnesium hydroxide, some 500 nm or less in diameter. At higher concentrations, these are overgrown to produce larger particles, thicker towards the outside, and generally consisting of several crystallites, with a common c -axis, but imperfectly aligned in the ab plane. The implications for growth mechanisms, and for the production of magnesium hydroxide from seawater, are discussed.