Structures and transformation mechanisms of the η, γ and θ transition aluminas

Zhou, Ru; Snyder, Robert L.

doi:10.1107/s0108768191002719

Cited by 759 publications

(633 citation statements)

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“…gamma-, theta-and alpha-alumina). The peaks centred at a chemical shift of approximately 30 ppm, could either indicate the presence of quasi-octahedral [15,23] or pentahedral (AlO 5 ) aluminium coordination in accordance with previously reported values for the chemical shift [17,24,25]. However, the higher value of C Q and ΔC Q more likely indicates the presence of pentahedral coordination.…”

Section: A More Complex Problem?supporting

confidence: 69%

The nature and impacts of fines in smelter-grade alumina

Perander

Žujović

Kemp

et al. 2009

JOM

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Fines in smelter grade aluminas are recognised as a significant process problem in aluminium smelting. However the understanding of the nature of this fine material and how it impacts on the reduction process are less clearly understood. The combination of new analytical methods such as Variable Pressure Scanning Electron Microscopy and very high field Solid State NMR provide new insights into the spatial distribution of phases within aluminas, and suggest how such fine materials are generated, particularly during calcination in the alumina refinery.

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Section: A More Complex Problem?supporting

confidence: 69%

The nature and impacts of fines in smelter-grade alumina

Perander

Žujović

Kemp

et al. 2009

JOM

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“…aluminium and oxygen. Crystallography information of gamma alumina was taken as a defect spinel structure from Zhou and Snyder [14].…”

Section: Coating Characterisationmentioning

confidence: 99%

Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

et al. 2016

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Suspension High Velocity Oxy Fuel Spraying (SHVOF) can be used to produce thermally sprayed coatings from powdered feedstocks too small to be processed by mechanical feeders, allowing formation of nanostructured coatings with improved density and mechanical properties. Here, alumina coatings were produced from sub-micron sized feedstock in aqueous suspension, using two flame combustion parameters yielding contrasting microstructures. Both coatings were tested in dry sliding wear conditions with an alumina counterbody. The coating processed with high combustion power of 101 kW contained 74 wt% amorphous phase and 26 wt% crystalline phase (95 wt% gamma and 3 wt% alpha alumina) while the 72 kW coating contained lower 58 wt% amorphous phase and 42 wt% crystalline phases (73 wt% was alpha and 26 wt % gamma). The 101 kW coating had a dry sliding specific wear rate between 4-4.5 x 10 -5 mm 3 /Nm, 2 orders of magnitude higher than the 72 kW coating wear rate of 2-4.2 x 10 -7 mm 3 /Nm. A severe wear regime dominated by brittle fracture and grain pull out of the coating was responsible for the wear of the 101 kW coating, explained by mean fracture toughness three times lower than the 72 kW coating, owing to the almost complete absence of alpha alumina.

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“…[42] The g-Al 2 O 3 crystal was built up from X-ray diffraction data obtained and interpreted by Zhou et al [43] Subsequently, this crystal was cut along the (110) plane to expose the preferentially exposed surface; that is, the D layer according to the LEIS measurements performed by Stobbe-Kreemers et al [44] To create a structural model for supported monomeric vanadium oxide, a VO 4 moiety was attached to the Al 2 O 3 surface. The distances of the V= O (1) and VÀO (2) coordinations of the VO 4 unit, as found in the EXAFS analysis, were used as input parameters for the Cerius 2 molecular modelling program.…”

mentioning

confidence: 99%

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

Keller

Weckhuysen

Koningsberger

2007

Chemistry A European J

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IntroductionMany parameters affect the catalytic properties of transition-metal oxide catalysts, for example, metal oxide loading, pre-treatment conditions, molecular structure, electronic structure, and the nature of the supporting oxide. [1][2][3][4][5][6][7] Various researchers have investigated the activities and selectivities of metal oxide catalysts. However, little is known about the parameters that are truly responsible for the catalytic action of transition-metal oxides.To study the effect of the support on the catalytic performance, one has to find a tool that is capable of probing the influence of the local environment (e.g., the support or promoters) on the properties of the catalytically active species, that is, a metal particle or metal oxide cluster. Metal-A C H T U N G T R E N N U N G (oxide)-support interactions can be ascribed to a structural (particle/cluster size and shape) effect, but can also have an electronic origin. For supported metal particles, the effect of the support on the catalytic properties has been determined by several techniques, for example, XPS, FT-IR analysis of adsorbed CO, and newly developed delta XANES and atomic XAFS (AXAFS) techniques. [8][9][10][11][12][13][14][15] In particular, the application of AXAFS has, in our opinion, led to a better understanding of metal-support interaction effects. [8,[15][16][17] AXAFS is sensitive to changes in the potential around the X-ray absorbing atom. The potential field around an atom is mainly determined by the first and second coordination spheres. O Grady et al. have shown for Pt/Ru alloys that the AXAFS intensity changes systematically with the composition of the alloy. [18] Thus, when the number of Ru atoms around a Pt atom increases, the intensity of the FT AXAFS peak increases and the peak centroid shifts to lower values of R. Comparison of these effects with the effects observed for a Pt electrode as a function of the applied voltage suggests that the first coordination shell of the Pt in the Pt/Ru alloy influences the electronic properties (inter-A C H T U N G T R E N N U N G atomic potential) of the Pt atom. [18,19] The effect of changes in the coordination around the absorber atom has been further illustrated for Pt by van Dorssen et al., who compared the AXAFS of a Pt foil with that of bulk Na 2 Pt(OH) 6 . The AXAFS intensity was found to be significantly higher in the Abstract: The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and aluminasupported vanadium oxide clusters has been investigated. Using Na 3 VO 4 and V 2 O 5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been dete...

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Structures and transformation mechanisms of the η, γ and θ transition aluminas

Cited by 759 publications

References 12 publications

The nature and impacts of fines in smelter-grade alumina

The nature and impacts of fines in smelter-grade alumina

Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

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