High Temperature Changes in Alumina Structure and Activity of Platinum-Alumina Catalysts

Waters, R.F.; Peri, J. B.; John, George; Seelig, Herman S.

doi:10.1021/ie50605a032

Cited by 16 publications

(12 citation statements)

References 3 publications

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“…The catalyst based on η‐Al 2 O 3 has the most sintering‐resistant texture as may be judged from the values of the residual surface area and from the amount of small mesopores (Table and Figure ). This contradicts the literature data about relative thermal stability of transition aluminas according to which γ‐Al 2 O 3 should be more stable than η‐ and χ‐Al 2 O 3 . This discrepancy might be caused by special thermal properties of the catalysts under study.…”

Section: Resultscontrasting

confidence: 65%

“…At the same time, the catalysts lost about 60%–70% of their specific surface areas (Table ). The aforementioned changes are the manifestations of sintering, which is inherent in transition alumina‐based systems, including Cr 2 O 3 /Al 2 O 3 , maintained at temperatures above 500 °C . The existence of mesopores in transition alumina‐based catalysts implies a reduced chemical potential of the matter beneath the curved surface of the pore; thermal activation induces matter flux into the pore with subsequent enlargement of the pore sizes and particles’ diameter…”

Section: Resultsmentioning

confidence: 99%

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The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

et al. 2019

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The effect of alumina structure (γ‐, η‐, χ‐Al2O3) on the long‐term stability of industrial‐like Cr2O3/Al2O3 dehydrogenation catalysts under industrial dehydrogenation conditions is studied. It is shown that the type of alumina support determines physicochemical and catalytic stability of the catalyst: η‐Al2O3 is the most stable against irreversible deactivation, whereas χ‐Al2O3 is the least stable. One of the possible reasons of predominant stability of η‐Al2O3‐based catalyst is its relatively high sintering stability under real plant conditions. High‐temperature (>800 °C) calcination, sometimes used to compare stabilities of chromia/alumina catalysts, appears to be unable to simulate industrial aging because of the inconsistency of the phase composition of industrially and artificially aged catalysts.

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Section: Resultscontrasting

confidence: 65%

Section: Resultsmentioning

confidence: 99%

The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

et al. 2019

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“…The catalytic effect of low pressure (<1 atm) water vapor on the transformation of transition aluminas to ␣-Al 2 O 3 has been documented in several studies. [17][18][19][20][21][22][23] Waters et al 17 found that water vapor accelerated the "deactivation" of alumina catalyst supports. The water vapor coarsened the transition alumina crystals and also increased the transformation rate to the lower surface area ␣-Al 2 O 3 , which the authors attributed to the increased mobility of ions.…”

Section: (3) Effect Of Water Vapor On Coarsening and Transformation Rmentioning

confidence: 99%

“…The water vapor coarsened the transition alumina crystals and also increased the transformation rate to the lower surface area ␣-Al 2 O 3 , which the authors attributed to the increased mobility of ions. 17 Yanagida et al 18 studied the effect of water vapor on the kinetics of ␣-Al 2 O 3 formation by heating ␥-Al 2 O 3 and -Al 2 O 3 powders in water vapor atmospheres of 0.0005, 17.5, and 760 mmHg. The 760 mmHg (1 atm) water vapor pressure accelerated the transformation of -Al 2 O 3 to ␣-Al 2 O 3 , at least in the initial stage of the transformation.…”

Section: (3) Effect Of Water Vapor On Coarsening and Transformation Rmentioning

confidence: 99%

Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al₂O₃ from γ‐Al₂O₃

Bagwell¹,

Messing²

1999

Journal of the American Ceramic Society

107

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Isothermal transformation kinetics and coarsening rates were studied in unseeded and ␣-Al 2 O 3 -seeded ␥-Al 2 O 3 powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to ␣-Al 2 O 3 with an activation energy of 567 kJ/mol. Seeding with ␣-Al 2 O 3 increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the ␣-Al 2 O 3 transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to ␣-Al 2 O 3 after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion. (2) Control of Nucleation through HeteroepitaxyMany authors have added a variety of potential nucleation aids such as

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“…The kinetics of the transformation of submicron ~'-A1203 to c~-A1203 has been studied by Steiner et al [36]. The presence of water vapour has been reported to increase the transformation rate of x-A12Oa [37], and kinetic data for the transformation in the presence of water has been published [38]. Rate constants for both sets of data were extrapolated to higher temperatures and the times for 50 % transformation have been plotted in Fig.…”

Section: Transformation To the Equilibrium Structurementioning

confidence: 99%

Formation of metastable phases in flame- and plasma-prepared alumina

McPherson

1973

J Mater Sci

288

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The formation of metastable phases in plasma-and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastaNe to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation of ~,-AI203 rather than c~-Al=O3 because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 gm diameter, but particles larger than this may transform to ~-AI203 during the solidification exotherm oxidation of aluminium chloride, have been reported to consist predominantly of metastable phases 0,-A1203, S-A120~ and 0-A1203) rather than the stable c~-A12Oa form. Plasma-sprayed coatings also consist of metastable forms [6,7] although if deposited onto a heated substrate, some a-A1203 is produced [8].Plummer [1 ] observed that a-A1203 tended to be produced in flame spheroidizedparticleslarger than approximately 15 gm diameter and he suggested that metastable phases were formed at fine particle sizes because their more rapid cooling rate resulted in "quenching in" of the assumed tetrahedral co-ordination of aluminium by oxygen in the liquid state. This would then tend to produce solid phases with cubic close packing of oxygen ions 0'-, 8-, 0-A12Oa) rather than the hexagonal packing of ~-A12Oa. Das and Fulrath [2] also observed that although a metastable form (7-A1203) predominated in particles less than 10 jam diameter spheroidized in a d.c. plasma jet, o~-A1203 was formed at larger particle

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High Temperature Changes in Alumina Structure and Activity of Platinum-Alumina Catalysts

Cited by 16 publications

References 3 publications

The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al₂O₃ from γ‐Al₂O₃

Formation of metastable phases in flame- and plasma-prepared alumina

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High Temperature Changes in Alumina Structure and Activity of Platinum-Alumina Catalysts

Cited by 16 publications

References 3 publications

The Effect of Transition Alumina (γ‐, η‐, χ‐Al2O3) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

The Effect of Transition Alumina (γ‐, η‐, χ‐Al2O3) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al2O3 from γ‐Al2O3

Formation of metastable phases in flame- and plasma-prepared alumina

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The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part II: Industrial‐Like Catalysts and Real Plant Aging Conditions

Effect of Seeding and Water Vapor on the Nucleation and Growth of α‐Al₂O₃ from γ‐Al₂O₃