Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

/pdf/surface-chemistry-and-spectroscopy-of-chromium-in-inorganic-5dnw014r22.pdf

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

The structure of active centers and the ethylene polymerization mechanism on the Cr/SiO2 catalyst: a frontier for the characterization methods.

https://dspace.library.uu.nl/bitstream/handle/1874/21357/weckh_99_olefinpolymerization.pdf?sequence=1

Olefin polymerization over supported chromium oxide catalysts

Pure (MnO x ), mixed (A y Mn 1− y O x ) and composite ((A,B) y Mn 1− y O x ) manganese oxide based catalysts were prepared, where A is either Ni II , Cu II , Bi III or Ce IV , A and B are various combinations of them, and y =0.4. Subsequently, they were characterized for the crystalline bulk structure by X-ray powder diffractometry, the electron availability by magnetic susceptibility measurements, the surface area by BET-analysis of nitrogen adsorption isotherms, and the surface chemical composition and oxidation state by X-ray photoelectron spectroscopy. The H 2 O 2 decomposition activity was determined by oxygen-gasometry of the reaction kinetics at 20–35°C. Results thus obtained have helped to conclude that (i) the mixed and composite catalysts are generally more active than the pure catalyst, and (ii) of the mixed and composite catalysts, those assuming mixed bulk phases with the host oxide are relatively more active. The optimal catalytic surface has been found to be that exposing ionic sites of different oxidation states and residing on a crystalline bulk of ca. 50% electron availability. These properties were found to be accomplished by surfaces of NiMnO x , on which the H 2 O 2 decomposition required a much lower activation energy (11.6 kJ mol −1 ) as compared to that (77.1 kJ mol −1 ) necessary for the reaction on the pure catalyst (MnO x ).

Promotion of the hydrogen peroxide decomposition activity of manganese oxide catalysts

The Phillips supported chromium catalyst is used to produce some 40–50% of the world's high-density polyethylene. The chemistry of this important catalyst is reviewed from an industrial perspective, on the basis of more than half a century of commercial experience, as well as published research. Polymer characteristics are used as an analytical tool to probe and better understand the active-site population on the catalyst.

A Review of the Phillips Supported Chromium Catalyst and Its Commercial Use for Ethylene Polymerization

Magnetic and optical studies were performed on coprecipitated chromium and aluminium hydroxide gels after calcination in air at temperatures between 50 and 1150°C. The results show that magnetic dilution was not achieved and that surface clusters of chromium oxide were formed. The magnetism, e.s.r. and optical spectra of oxidised samples are discussed in terms of a mixed-valency phase of partially-oxidised chromium containing Cr3+ and Cr6+ species. This new model of γ-phase chromium is shown to be consistent and an acceptable part of the extensive mixed-valency chemistry of chromium.

Magnetic and optical studies of chromium oxides. Part 3.—Calcination of coprecipitated chromium and aluminium hydroxide gels

The Cr–silica catalyst system is commercially important in the polymerisation of ethene. The nature of the chromium species present on the support following activation, as well as the nature of the polymerisation active oxidation state, have been investigated extensively but results have not always been conclusive. This investigation attempts to identify the nature of the surface species and the oxidation state of the chromium on both the activated and hydrogen-reduced samples. Temperature-programmed reduction was employed to identify the number of surface species on the support. These species were identified with the aid of EPR and UV–VIS diffuse reflectance spectroscopy.The surfaces of the activated Cr–silica catalysts were generally covered by chromate-like surface species, accompanied by dichromate-like species and chromium aggregates, the proportions of which were dependent upon chromium loading and activation conditions. These samples were mostly oxidised to CrVI during activation with traces of CrIII remaining after the treatment. Samples which were reduced in hydrogen indicated that reduction of CrVI to CrIII had occurred, but traces of CrVI always remained. The reduction process was accompanied by an increase in the amount of aggregation of the chromium.

Characterisation of Cr–silica polymerisation catalysts

Characterisation of modified Cr—silica catalysts

Chromium(6+) oxide was supported on γ-alumina and silica by aqueous impregnation producing two series of samples (SiO2, 1–18% Cr; Al2O3, 1–12% Cr) dried by vacuum desiccation at 25 °C and calcined in air at temperatures from 100 to 600 °C. Fast-atom bombardment–secondary-ion mass spectroscopy studies have revealed contrasting development of chromium oxide clusters, so that clusters on silica are thin and widely spread relative to clusters of uneven thickness on alumina. These surface models are explained in terms of the long-range chromium oxide–support interaction evident in alumina-supported material.

The structure of ethylene polymerisation catalysts. Part 1.—Fast-atom bombardment–secondary-ion mass spectrometry studies of chromium clusters

The Cr/silica catalyst system, which is commercially important in the polymerisation of ethane, is made much more active by the inclusion of small quantities of titanium The nature of any new surface species which may be formed has been studied by investigation of a series of titanium- and magnesium-modified Cr/silica catalystsTemperature-programmed reduction (TPR) experiments on the modified catalysts showed evidence for two new surface species These species exhibited increased thermal stability with respect to the surface chromate and dichromate-like species already present and may approximate to aggregates of titanium or magnesium chromate and dichromate

Alan Ellison

Papers

Magnetic and optical studies of chromium oxides. Part 3.—Calcination of coprecipitated chromium and aluminium hydroxide gels

Characterisation of Cr–silica polymerisation catalysts

Characterisation of modified Cr—silica catalysts

The structure of ethylene polymerisation catalysts. Part 1.—Fast-atom bombardment–secondary-ion mass spectrometry studies of chromium clusters

Characterisation of Cr/silica catalysts. Part 2.—Ti- and Mg-modified catalysts