Pore-structure stability and phase transformation in pure and M-doped (M = La, Ce, Nd, Gd, Cu, Fe) alumina membranes and catalyst supports

Kumar, K.N.P.; Tranto, Janne; Kumar, Jyant; Engell, J.

doi:10.1007/bf00274471

Cited by 68 publications

(34 citation statements)

References 32 publications

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“…Apparently, having an excess of La present helps to obtain this wellcrystallized AlLaO 3 structure. Dupree et al 8 also observed an octahedral resonance with a small quadrupolar coupling constant (0.12 MHz) for AlLaO 3 . In their case this pure phase was obtained for a AlLa 0.5 sample treated at 1775°C.…”

Section: Resultsmentioning

confidence: 89%

A Nuclear Magnetic Resonance Study of Amorphous and Crystalline Lanthanum-Aluminates

Iuga¹,

Simon²,

Boer³

et al. 1999

J. Phys. Chem. B

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Lanthanum-aluminates of the composition (1 -x)Al 2 O 3 ‚xLa 2 O 3 (0 < x < 0.7) were prepared by sol-gel synthesis. Subsequent heat-treatment temperatures ranging from 200 to 1200°C were applied. The resulting samples were characterized by X-ray powder diffraction and 27 Al MAS NMR. The most complex spectra were analyzed by MQMAS NMR in order to get insight in the number of sites and the possible distribution of the NMR parameters in the amorphous samples. Analysis of the MQMAS spectra, taking the efficiency of this experiment for different sites into account, helped to resolve ambiguities in the MAS spectra, which could otherwise not be deconvoluted in an unique way. The analyses show that, except for the sample with the lowest La (x ) 0.083) concentration, hardly any pentacoordinated aluminum is observed over the whole temperature treatment range. For all samples the concentration of six-coordinated aluminum decreases as the heating temperature increases until the point where crystallization occurs. The concentration of 4-fold coordinated aluminum shows the opposite behavior. When crystallization occurs, a remarkable transformation between four-and six-coordinated aluminum takes place. Crystallization of the samples occurs at lower temperatures for samples with high amounts of lanthanum. At low La content crystallization is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. Storage of the samples in a water-saturated atmosphere leads to a conversion of four-to six-coordinated aluminum. This shows that the low-coordinated Al atoms are accessible to water molecules and must therefore be situated at the surface.

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Section: Resultsmentioning

confidence: 89%

A Nuclear Magnetic Resonance Study of Amorphous and Crystalline Lanthanum-Aluminates

Iuga¹,

Simon²,

Boer³

et al. 1999

J. Phys. Chem. B

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“…Only for intermediate ionic sizes (i.e., close to the size of Al 3+ ), is the relative stability shifted in favor of the ␣ phase, as seen here for Co. Interestingly, these observations are consistent with the experiments mentioned in the introduction. The dopants reported to retard the -␣ transformation are all significantly larger 4,10-13,15,16 (except B, 14 which is very small and would thus prefer the tetrahedral positions 22 ), whereas Fe 10,17 (at least in its low-spin state) and Cu, 17 which are reported to accelerate the transformation, are intermediate in size, similarly to Co. Thus, the effects of substitutions seen in this work could be part of the explanation for the experimental observations.…”

Section: Concluding Discussionmentioning

confidence: 95%

“…Such studies have mainly been focused on increasing the thermal stability of the metastable aluminas and report results on the transformation kinetics. To summarize these results it can be said that many dopants seem to retard the ␥ to ␣ transformation (e.g., Cr, [10][11][12] Cs, 13 B, 14 La, 4,15 Ce, 4,15 Ba, 16 Sr, 16 Ca, 16 Er, 11 Y 11 ), while some accelerate the transformation (e.g., Fe, 10,17 Cu 17 ), effects which have been partly explained by kinetic arguments. However, the direct influence of the dopants on structural stability has not yet been elucidated.…”

Section: Introductionmentioning

confidence: 97%

Ab initiocalculations on the effects of additives on alumina phase stability

et al. 2005

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The effects of substitutional additives on the properties and phase stability of -and ␣-alumina ͑Al 2 O 3 ͒, are investigated by density functional theory total energy calculations. The dopants explored are 5 at. % of Cr, Mo, Co, and As substituting for Al, respectively, N and S substituting for O, in the and ␣ lattices. Overall, the results show that it is possible to shift, and even reverse, the relative stability between ␣-and -alumina by substitutional additives. The alumina bulk moduli are, in general, only slightly affected by the dopants but density of states profiles reveal additional peaks in the alumina band gaps. We also show that phase separations into pure oxides are energetically favored over doped alumina formation, and we present results on a number of previously unstudied binary oxides.

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“…The previous results of transition aluminas have showed that the impurities or the additives greatly influence the sintering and phase transformations of aluminas. Lanthanum has extremely positive effect of inhibition on the sintering and ␣-phase transformation [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. However, the chemical state of this additive in aluminas and the formation of microstructure of La-aluminas are still of open question, so that the mechanism of stabilization has not been fully established regarding with the nanometer-level microstructural assessment of lanthanum-modified aluminas.…”

Section: Introductionmentioning

confidence: 99%

Formation of featured nano-structure in thermal stable La-doped alumina composite catalyst

Nishio

Ozawa

2009

Journal of Alloys and Compounds

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Pore-structure stability and phase transformation in pure and M-doped (M = La, Ce, Nd, Gd, Cu, Fe) alumina membranes and catalyst supports

Cited by 68 publications

References 32 publications

A Nuclear Magnetic Resonance Study of Amorphous and Crystalline Lanthanum-Aluminates

A Nuclear Magnetic Resonance Study of Amorphous and Crystalline Lanthanum-Aluminates

Ab initiocalculations on the effects of additives on alumina phase stability

Formation of featured nano-structure in thermal stable La-doped alumina composite catalyst

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