The surface chemistry of molybdena-alumina catalysts reduced in H2 at elevated temperatures

A. Rédey, Jose Goldwasser, W. Keith Hall

Research output: Contribution to journalArticle

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Abstract

The changes in surface chemistry which occurred as a molybdena-alumina catalyst (8% Mo) was reduced to increasing extents in H2 at temperatures between 500 and 900 °C were characterized by measuring the chemisorptions of NO (at -78 and 24 °C) and of O2 (at -78 °C) and these data were supplemented by studies of the hydrogenation of benzene. Interesting changes occurred as the catalyst was reduced under more severe conditions than those ordinarily used. The average extent of reduction, e Mo, increased monotonically from 1.7 (mainly Mo4+) at 500 °C to 5.0 (Mo0 present) at 900 °C, while the surface area decreased by less than 25%. The oxygen chemisorption increased concomitantly from nearly zero at e Mo = 0.08 to O Mo = 0.5 at e Mo ≈ 3.0; it reached a maximum of O Mo = 0.55 at e Mo ≈ 4 and then decreased slightly at the highest reduction temperature. The NO chemisorption behaved similarly but dropped more precipitously at the highest temperature. The ratio of the two chemisorptions changed from 2NO O ( 4NO O2) at 500 °C (standard preparation) to 2NO O2 above e Mo = 3 (700 °C reduction temperature). NO chemisorbed at -78 °C without evident decomposition, but at room temperature and above, N2O and N2 were evolved in amounts corresponding to oxidation of 20 to 45% of the total Mo by 2e, the amount increasing with the extent of reduction. Moreover, as the extent of reduction increased, it was found that chemisorption of NO on the support became an important fraction of the total. Hydrogenation of benzene did not occur at atmospheric pressure at temperatures up to 250 °C until e Mo ≈ 5 where metallic molybdenum evidently was present. Thus, the sites responsible for hydrogenation of simple olefins were ineffective for benzene and this reaction provides a simple diagnostic tool for the detection of Mo0.

Original languageEnglish
Pages (from-to)82-95
Number of pages14
JournalJournal of Catalysis
Volume113
Issue number1
DOIs
Publication statusPublished - 1988

Fingerprint

Aluminum Oxide
Surface chemistry
chemisorption
Chemisorption
Alumina
aluminum oxides
chemistry
catalysts
Catalysts
hydrogenation
Benzene
Hydrogenation
benzene
Temperature
temperature
Molybdenum
alkenes
Alkenes
molybdenum
atmospheric pressure

ASJC Scopus subject areas

  • Catalysis
  • Process Chemistry and Technology

Cite this

The surface chemistry of molybdena-alumina catalysts reduced in H2 at elevated temperatures. / Rédey, A.; Goldwasser, Jose; Hall, W. Keith.

In: Journal of Catalysis, Vol. 113, No. 1, 1988, p. 82-95.

Research output: Contribution to journalArticle

Rédey, A. ; Goldwasser, Jose ; Hall, W. Keith. / The surface chemistry of molybdena-alumina catalysts reduced in H2 at elevated temperatures. In: Journal of Catalysis. 1988 ; Vol. 113, No. 1. pp. 82-95.
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abstract = "The changes in surface chemistry which occurred as a molybdena-alumina catalyst (8{\%} Mo) was reduced to increasing extents in H2 at temperatures between 500 and 900 °C were characterized by measuring the chemisorptions of NO (at -78 and 24 °C) and of O2 (at -78 °C) and these data were supplemented by studies of the hydrogenation of benzene. Interesting changes occurred as the catalyst was reduced under more severe conditions than those ordinarily used. The average extent of reduction, e Mo, increased monotonically from 1.7 (mainly Mo4+) at 500 °C to 5.0 (Mo0 present) at 900 °C, while the surface area decreased by less than 25{\%}. The oxygen chemisorption increased concomitantly from nearly zero at e Mo = 0.08 to O Mo = 0.5 at e Mo ≈ 3.0; it reached a maximum of O Mo = 0.55 at e Mo ≈ 4 and then decreased slightly at the highest reduction temperature. The NO chemisorption behaved similarly but dropped more precipitously at the highest temperature. The ratio of the two chemisorptions changed from 2NO O ( 4NO O2) at 500 °C (standard preparation) to 2NO O2 above e Mo = 3 (700 °C reduction temperature). NO chemisorbed at -78 °C without evident decomposition, but at room temperature and above, N2O and N2 were evolved in amounts corresponding to oxidation of 20 to 45{\%} of the total Mo by 2e, the amount increasing with the extent of reduction. Moreover, as the extent of reduction increased, it was found that chemisorption of NO on the support became an important fraction of the total. Hydrogenation of benzene did not occur at atmospheric pressure at temperatures up to 250 °C until e Mo ≈ 5 where metallic molybdenum evidently was present. Thus, the sites responsible for hydrogenation of simple olefins were ineffective for benzene and this reaction provides a simple diagnostic tool for the detection of Mo0.",
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