J. Lynch

In situ characterisation of cobalt based Fischer-Tropsch catalysts : A new approach to the active phase

Interest in the Fischer-Tropsch synthesis aimed at hydrocarbon production is increasing in the context of clean fuel production. Cobalt containing catalysts are known to be effective in Fischer-Tropsch synthesis, particularly for the production of higher molecular weight fractions.

Nickel contamination of fluidised cracking catalysts : A model study

Abstract Metal poisoning of fluidised cracking catalysts is a major industrial problem. Focusing on nickel, the dispersion, chemical state and toxicity of this metal have been studied for various components of fluid catalytic cracking catalysts such as: silica, kaolin, magnesia, alumina, silica-aluminas and modified Y zeolites. Depending on the alumina content of the solid, nickel can be present in two main forms: nickel oxide-like particles and a nickel aluminate-like phase. The oxide form, reducible at low temperature, is favored on silica-rich supports. Conversely alumina-rich solids favor the formation of a nickel combined phase, reducible at higher temperature. Although both species exhibit dehydrogenation properties, micro activity test results would indicate that the most poisonous phase in fluidised catalytic cracking catalysts are nickel aluminate-like compounds.

Formation of Secondary Pores in Zeolites During Dealumination: Influence of TheCrystallographic Structure and Of the Si/Al RATIO

Summary The parameters affecting the genesis of mesopores in zeolites during dealumination have been investigated. The formation of mesopores is essentially controlled by the structural defects density. Structural defects correspond to: i) framework vacancies, ii) crystalllographic defects and iii) trivalent elements incorporated in the framework. At least two parameters control the structural defects density: the initial Si/Al ratio and the synthesis conditions.

In situ high temperature and high pressure exafs studie of Pt/Al2O3 catalysts. Part I: Reduction and deactivation

EXAFS has been used to follow in situ the structural evolution of a chlorinated and non-chlorinated Pt/Al2O3 catalyst during reduction in the temperature range of 300–500 °C. Smaller metal clusters are formed from the hydrogen reduction of the chlorinated catalyst, in contrast to the larger cluster formed from the non-chlorinated one. At 460 °C, the total hydrogen pressure was raised to 5 atm. and n-heptane was injected over the samples. EXAFS measurements at the Pt edge were carried out while hydrocarbon conversion was monitored with a gas Chromatograph. We observe the rapid formation of a carbon-platinum bond. This is unmodified while turnover rates and selectivities indicate evidence for deactivation. From this structural information supplied by EXAFS, correlated with the data obtained from gas chromatography, we find that our results are consistent with a model proposed by others where deactivation is due to the build-up of a multilayer of carbon.

Bimetallic Reforming Catalysts: EXAFS Investigation of the Particle-Growing Process during the Reduction Step

Abstract EXAFS has been used to study the reduction which leads to the active species of PtRh and PtRe reforming catalysts. The first moments of the formation of the metallic cluster have been followed kinematically at the platinum absorption edge for two catalysts, PtRh/γ-Al 2 O 3 and PtRe/γ-Al 2 O 3 , using two different modes for the EXAFS setup (different synchrotron stations). The experiments have been made in situ , the sample being under flowing hydrogen at high temperature. The main result is the appearance of long metal-metal distances, of the same range order as that of the bond lengths existing in the alumina network. They occur solely for a very low number of metal atoms and are probably due to an intergrowth process with the carrier. The phenomenon seems to be general and disappears with the growth of the particle.

A study of the adsorption sites on thoria by scanning transmission electron microscopy and fourier-transform infrared spectroscopy. Adsorption and desorption of water and methanol

A study of the adsorption sites of thoria has been made using scanning transmission electronic microscopy (STEM) and Fourier-transform infrared spectroscopy (FTIR). The STEM study showed that the thoria surface was composed mainly of (110)(211) and (111) faces in about equal proportions. The infrared study of activated ThO2 at 873 K indicated the existence of two remaining hydroxyl groups at 3660 and 3510 cm–1. With water adsorption, a third one appeared at 3745 cm–1 which was thermally less stable. Methanol adsorption gave rise to three kinds of species: (i) one reversibly adsorbed at room temperature and linked by hydrogen bonds in which thoria acts as a proton receiver; (ii) two methoxy species characterized by ν(CO) bands at 1127 cm–1(species I) and 1060 cm–1(species II). Species I desorption bands led to methanol whilst species II, much more thermally stable, gave methanol, dimethyl ether and carbon monoxide. Species I, which corresponds to the hydroxyl species at 3745 cm–1, are of the Th—OMe (Th—OH) type, and are formed on the (110) faces. Species II, which correspond to hydroxyl species at 3660 cm–1, are of the [graphic omitted] type, and are formed on the (211) faces. The third hydroxyl species, characterized by the ν(OH) band at 3510 cm–1, is probably due to species of the [graphic omitted] type appearing on (111) faces. The stability and reactivity of the methoxy species are shown to depend on their structure, and therefore on the local arrangement of the surface.

A beam hardening correction for X-ray microtomography

We propose a correction of the transmitted intensities in X-ray microscopy allowing improved quantification for polychromatic sources, particularly in tomographic reconstruction. The correction is based on the application of Beers law for each energy present in the source spectrum. Knowledge of the source spectrum and the average specimen composition are used. The correction is demonstrated on a chemically homogeneous specimen (a glass capillary filled with glass beads) and on a polyphasic oil reservoir rock sample. In the former, detection of 20 μm defects is demonstrated. In the latter, different minerals, quartz, feldspath and kaolinite, can be discriminated and their distributions mapped.

Interaction Between Pt(acac)2 and Alumina Surfaces Studied by XAS

The anchoring and decomposition mechanisms of platinum(II) bis-acetylacetonate on alumina surfaces are studied by X-ray absorption spectroscopy at the platinum LIII edge. A distinction is made between highly reactive surfaces which are partially dehydroxylated and exhibit coordinatively unsaturated surface sites, and deactivated surfaces which are covered by a monolayer of OH groups. The samples are studied after three stages of a wet impregnation synthesis: after drying at room temperature, drying at 120 °C and calcination at 350 °C. The XANES signal and the filtered EXAFS signals of the first and second coordination shells around platinum were analyzed. Two different mechanisms are discussed for the two types of supports.

In situ EXAFS studies of modifications to supported metallic catalysts under reactive atmospheres

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to study the structural evolution of monometallic (Pt/Al2O3) and bimetallic (Pt-Re/Al2O3) catalysts with metal loadings representative of those used industrially under conditions close to those seen during preparation (calcination and reduction), during catalysis (under a hydrogen-hydrocarbon mixture), and during regeneration (simulated by cycles of reduction and oxidation). During the catalytic reforming ofn-heptane, the formation of bonds to carbon species is observed directly by EXAFS. Despite the fact that only platinum-carbon bonds are observed, bimetallic systems show significant differences compared with monometallic systems with, in the case of Pt-Re, a reduction in the temperature range over which carbon bonding is observed, and structural modifications of the metallic particles. Under the same conditions, no carbon bonding is observed for the Pt-Sn system. In the case of Pt-Sn/Al2O3, the EXAFS results, confirmed by transmission electron microscopy, demonstrate the high resistance to sintering of this bimetallic system compared to the monometallic system.

Characterization of the Secondary Pore System in Dealuminated HY Zeolites Comparison between Isomorphous Substitution and Hydrothermal Treatment

Abstract Dealumination of HY zeolites by self steaming/acid leaching takes place nearly homogeneously throughout the solid, down to a scale of 20 nm, with creation of a secondary pore system consisting of large spherical pores connected by narrow restrictions. In contrast, isomorphous substitution occurs progressively from the exterior of the zeolite grains, but up to about 50% dealumination, preserves their microporous nature.