B. H. Davis

Characterization of coprecipitated nickel on silica methanation catalysts by X-ray photoelectron spectroscopy

An X-ray photoemission spectroscopy study of a series of standard nickel compounds (Ni, NiO, Ni(OH)2, NiSiO3) and silica-supported nickel methanation catalysts has been conducted. The binding energies and spectral shapes of the standard samples provide a data base which has been used to understand the catalyst spectra. The activity and thermal stability of coprecipitated nickel catalysts has been attributed to the metal-support interaction. This interaction has been observed in two coprecipitated nickel-on-silica catalysts. The nickel spectra closely resemble those of amorphous NiSiO3. The interaction varies somewhat between the two catalysts studied as evidenced by differences in their reducibility.

Factors influencing the stability of the tetragonal form of zirconia

The p-italicH of the solution that zirconia is precipitated from defines the crystal phase formed after calcination of the material at 400 /sup 0/ to 600 /sup 0/C. A metastable tetragonal form is obtained for either low (less than about 5) or high (greater than about 13) p-italicH. The tetragonal phase formed at high p-italicH is much more stable at the calcination temperature than the material obtained at a low p-italicH is. For a material obtained by precipitation at a low p-italicH, monoclinic crystallites, determined by x-ray line broadening, were obtained that were smaller than the tetragonal crystals that produced them. A crystallite size effect, based on x-ray line broadening, is not responsible for the formation or stabilization of the tetragonal phase.

Alloy formation in Pt-Sn-alumina catalysts: in situ X-ray diffraction study

Changes in PtSnalumina catalysts, prepared from the (Pt3Sn8Cl30)2− complex, during reduction in flowing hydrogen were followed by in situ X-ray diffraction (XRD). For high metal loading (ca. 5% Pt) evidence was found for both PtSn and Pt phases: other PtSn phases, such as Pt3Sn, PtSn2, and PtSn4, were not observed. Metallic Pt was detected for a 0.6 wt% PtAl2O3 catalyst but only a PtSn alloy was observed for a 0.6 wt% Pt catalyst containing Sn. In situ XRD studies therefore support alloy formation with a stoichiometry of Pt: Sn = 1: 1; the Sn in excess of that needed to form this alloy is present in an X-ray “amorphous” form.

Moessbauer spectroscopy study of iron-based catalysts used in Fischer-Tropsch synthesis

Mössbauer spectroscopy investigation of iron-based Fischer-Tropsch (FT) catalysts from the US Department of Energys LaPorte pilot plant demonstration run (August 1992) has revealed that the ratio of iron in the octahedral site to that in the tetrahedral site in magnetite correlates with the water-gas-shift activity. Studies of the catalysts from laboratory-scale fixed-bed reactor indicate that activation of the oxide precursor with CO results in the formation of χ-carbide (Fe5C2) whose amount decreases with increasing SiO2 content in the catalyst. The Fe5C2 is converted into Fe3O4 during FT synthesis. The FT activity of the catalyst is found to follow the trend of χ-carbide variation indicating that χ-carbide is active for the FT synthesis.

Discrepancies in the crystal structures assigned to precipitated zirconia

The prevailing discrepancies in the assignment of crystal structures in zirconia precipitated at pH of 13.5 have been addressed. Higher-angle XRD line splittings of (006) and (600) indicate that the pH 13.5 material is tetragonal. Additional evidence was obtained from Raman spectroscopy to indicate that this material possesses tetragonal symmetry, not the cubic structure. The data clearly demonstrate that the precipitation of zirconyl salts using NaOH at a pH of 13.5 results in the formation of tetragonal phase whether the washing is limited or extensive. It does not appear that conclusive proof is available to demonstrate that the presence of about 3 wt % Na can stabilize the metastable cubic phase at low temperatures. The conclusions of 14.15 need to be supported by data from other characterization techniques.

A study of the oxidation of ferrous hydroxide in slightly basic solution to produce γ-FeOOH

Abstract During the reaction of Fe(II) with oxygen, three regions of nearly constant pH are observed, each separated by two regions where there is an abrupt pH change. Data for the oxidation of a suspension with [Fe(II)] [OH −1 ] = 7 2 and 0.22 M Fe(II) indicate that the oxidation to Fe(III) and the associated hydrolysis to produce a proton are rapid compared to the reaction that consumes the proton generated during the first step. It is suggested that the oxidation occurs in the aqueous solution, and that the slow step is due to the proton reacting with a solid Fe(II) species to produce soluble Fe(II). Fe3O4 is formed at, or near, the first step in the oxidation-pH curve, and then redissolves as oxidation continues. The amount of Fe(II) and Fe(III) in the solid and solution have been determined at several stages of the oxidation, and these are consistent with the amount of oxygen consumed during the reaction.

Electron microdiffraction studies of zirconia particles

A batch of zirconia was prepared at a pH of 2.95 using a sol-gel technique. The crystal structures formed during 500 °C calcination was followed by X-ray diffraction. The tetragonal phase was the major component after the initial calcination period of 15.5 h; however, it gradually transformed to the monoclinic crystal form during 200 h of calcination at 500 °C. Electron microdiffraction was employed in the present investigation to determine the crystal structure of individual particles, and to identify whether these particles contained twin variants. A technique has been developed to get a dispersion of agglomerated particles by condensing and spreading the beam on the agglomerates at 200 kV. The data revealed that some of the individual zirconia particles are featureless and some of them appear to contain single or multiple twin variants.

Crystal structure of hafnia–zirconia mixtures obtained by calcination of hydrous oxide prepared by precipitation

The pH of the solution in contact with a hydrous zirconium oxide plays a dominant role in determining the crystal phase, tetragonal or monoclinic, in the calcined material. The substitution of low concentrations of hafnium for zirconium destabilizes the tetragonal phase so that only the monoclinic phase is formed; the amount of Hf required for destabilization depends upon the pH used for the preparation of the hydrous oxide. While this study has defined a phenomenon, the results do not permit a definition of the mechanism for it.

Effect of pre-heat treatment on a Fischer-Tropsch iron catalyst

Mössbauer spectroscopy was used to investigate the effect of heating the Fischer-Tropsch catalyst 100 Fe/5 Cu/4.2 K/24 SiO2 in two different atmospheres while ramping the temperature of the catalyst from room temperature to 280 °C in 5.5 h prior to pretreatment of the catalyst. Preheating in H2/CO=0.7 gave rise to an iron (Fe2+) silicate, while preheating in helium resulted in the formation of ε′-carbide Fe2.2C. Iron oxides and χ-carbide Fe5C2 were also formed in both preheat treatments.