J.V. Sanders

Reactions on ZSM-5-type zeolite catalysts

Catalytic reactions and sorption measurements have been carried out with ZSM-5 and silicalite catalysts which are distinguished by variation in skeletal heteroatom concentration. The catalysts were used in both the hydrogen-exchanged and sodium-exchanged forms. Sorption measurements were made with the hydrocarbons n-hexane, 3-methylpentane, and 2,2-dimethylbutane, and with the bases ammonia, n-butylamine, t-butylamine, and 4-methylquinoline. Catalytic reactions were carried out on both unpoisoned and base-poisoned catalysts using methanol, propylene, and 3-methylpentane reactants. In addition, the behavior of ethylene and ethanol reactants was also explored. The ease of base sorption and hydrocarbon sorption has been assessed in terms of effective molecular size in relation to the channel size of the catalyst, and this factor is also used as a basis for explaining the effectiveness of bases for poisoning the catalytic reactions. Temperature-programmed desorption (TPD) measurements with ammonia have been used to assess the energetic distribution of sorption sites for bases, and very strong binding sites with a TPD maximum for ammonia at about 780 °K have been identified as the probable sites used in the conversion processes. The main features of the catalytic conversion process are discussed. It is concluded that sorbed C3,C4 olefinic residues are general intermediates leading to aromatic formation. Under most circumstances, ethylene was relatively unreactive, and it is inferred that a sorbed C2 residue, which is related to ethylene by sorption and desorption, is an unlikely general intermediate. A mechanism is suggested for the formation of sorbed C3,C4 olefinic residues, and for initial carbon-carbon bond formation from methanol. Catalyst self-poisoning was observed with all catalysts except hydrogen-exchanged ZSM-5.

The characterization of ag sols by electron microscopy, optical absorption, and electrophoresis

Abstract Several differently prepared silver sols and a gold sol were examined using electron microscopy optical absorption and microelectrophoresis techniques. The effects of 4-dimethyl-amino-pyridine and pyridine addition to the sols were also studied. In all cases of amine adsorption onto the sol particles, an accompanying decrease in the magnitude of the electrophoretic mobility of the particles was observed, and a long wavelength (>500 nm) absorption band was formed. The solution species responsible for these changes is an aggregate composed of the primary sol.

Morphology and activity of MoS2 on various supports : genesis of the active phase

The morphology and hydrodesulfurization (of thiophene) activity of unpromoted MoS2 supported on alumina, silica, titania, and zirconia have been examined. In the oxidic catalyst, no surface molybdenum phases could be identified by transmission electron microscopy. For all supports except titania, an additional bulk molybdenum oxide phase appeared above a particular Mo loading. In the case of the alumina support, bulk Al2(MoO4)3 also appeared. In the sulfided catalysts, the bulk species retained their original morphology and were surrounded by a skin of MoS2 several layers thick. The core of the bulk MoO3 was reduced to Mo metal. On the surface of the SiO2 and Al2O3, the MoS2 was present as flakes up to five layers thick, sitting vertically on the support. For TiO2 and ZrO2 the MoS2 was present as “islands,” which expanded with increased Mo loading, eventually encapsulating the support particles. The reactivity behavior also fell into two groups, corresponding to the two forms of MoS2 morphology. The specific activity of MoS2 supported on TiO2 and ZrO2 was higher than that for the case of Al2O3 and SiO2 supports.

The relationship of structure and activity of NiMo sulfides to composition of the precursor oxides

Abstract Mixed oxides of Ni and Mo formed from coprecipitated powders with a variety of compositions from pure MoO 3 to pure NiO have been examined by X-ray diffraction and electron microscopy before and after their conversion to sulfides and use as a hydrodesulfurization (HDS) catalyst. The Mo-rich oxides are a mixture of MoO 3 and NiMoO 4 . Sulfiding transforms the MoO 3 into MoO 2 crystals coated with a thin skin of MoS 2 , together with some flakes of MoS 2 ; the NiMoO 4 transforms to highly dispersed MoS 2 . In Ni-rich samples, Mo up to at least 30% cannot be detected directly in the electron microscope but is thought to be incorporated within the NiO, producing disorder within the crystals. Conversion of this material produces crystals of nickel sulfides of various compositions, coated with a layer of MoS 2 . Electron micrographs of the most active catalysts contain sets of black fringes with a 6.15-A spacing taken to be lattice images of layers of MoS 2 . Nickel is homogeneously associated with this highly dispersed MoS 2 , but is not resolvable by electron microscopy.

The role of nickel in the activity of unsupported NiMo hydrodesulfurization catalysts

A series of unsupported nickel-molybdenum sulfide catalysts covering the range 0 to 100 atom % nickel has been prepared by continuous coprecipitation. The catalyst containing 55 atom % nickel showed the highest specific activity for thiophene hydrodesulfurization. Chemical and structural analyses of the catalysts appear consistent with the suggestion that the hydrodesulfurization sites are coordinatively unsaturated Mo/sup 3 +/ cations at anion vacancies on the edges of the MoS/sub 2/ flakes, while edge-intercalated nickel serves as an olefin hydrogenation site, and reducing Mo/sup 4 +/ to Mo/sup 3 +/, in accordance with the structural model of Cossee and Farragher. Highly dispersed Ni/sub 3/S/sub 2/ is suggested to play a role in hydrogen activation, transferring protons and electrons to the MoS/sub 2/, and this may form the origin of the synergistic mechanism. The likely roles of the promoter are summarized.

The influence of defects and surface structure on the catalytic activity of silver films

Abstract Formic acid vapor has been decomposed catalytically to hydrogen and carbon dioxide on silver films of four types, differing in the concentrations of four types of defects and in surface structure. When the rate is expressed as A exp ( −E RT ), values of A and E for the zero order reaction depend upon structural details. A and E change together to give an almost constant rate, i.e. compensation occurs. The concentrations of defects in the films have been determined by transmission electron microscopy, and the surface structure from replicas. It is concluded that points and lines of emergence of defects at the surface are unimportant, and that, with one possible exception, changes in values of A and E are related to the varying proportion of the surface which is flat and parallel to {111} planes.

Radiation-induced dissolution of colloidal manganese oxides☆

Abstract The reductive dissolution of several well-characterized suspensions of manganese oxides (α-, δ-, γ-MnO 2 , and Mn 5 O 8 ) under steady γ-irradiation at pH 3 has been studied. Radiolytically produced reducing radicals (e.g., (CH 3 ) 2 COH, CH 3 CHOH, HO 2 , and viologen radicals) can be quantitatively captured by the oxide suspension to release Mn 2+ ions into the solution. The radiolytically produced H 2 O 2 similarly leads to quantitative dissolution of the oxide. The rate-determining step in the dissolution process is slower than the diffusion-controlled limit. The stoichiometry of the dissolution process under complete scavenging of the radicals is determined by the oxidation state of the oxide ( x in MnO x ).

Structure and properties of evaporated metal films

Abstract Metal films particularly of nickel and tungsten, deposited on glass have been studied by electron microscopy in transmission and by replication, and by the measurement of gas adsorption and electrical resistance. From the transmission electron micrographs crystal widths have been obtained and the presence of gaps between the crystals established for nickel films examined as deposited at 273 °K. On sintering, the first stage is the removal of intercrystalline gaps while in later stages surface asperities are removed and crystal growth occurs. The effect of sintering on gas adsorption and electrical resistance has been studied. From the extinction contours on individual nickel crystals in the transmission electron micrographs the detailed crystal shape has been deduced. Films deposited when the substrate was at an elevated temperature showed an area for gas adsorption considerably below that for films which had been sintered at that temperature subsequent to deposition at 273 °K.

The surface structure of films of silver on glass

Abstract Films of silver were prepared by condensation in vacuum onto glass substrates in the temperature range −160 ° to 300 °C and examined by electron microscopy. Their surface structure, revealed by decorating the films with gold, depends on the conditions of preparation, and can be understood in terms of the processes occurring during the growth and annealing of the films. These results are significant in relation to the interpretation of the surface chemistry of evaporated metal films.

Structural analysis of transferred monolayers of high-molecular-weight polyvinyloctadecyl ether

Abstract The pressure area ( π A ) characteristics of high-molecular-weight polyvinyloctadecyl ether, 7-hydroxy-4-heptadecyl coumarin, and mixtures of the two at the air/water interface are reported, along with limiting areas for these systems. Electron diffraction results of cast monolayer films of these materials are also present. Analysis of the diffraction rings reveal that the molecular packing on the cast film is in the form of a hexagonal array with some long-range ordering present.