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Journal ArticleDOI

Correlation between the surface acidity and activity of bismuth tungstate catalysts

01 Jan 1977-Reaction Kinetics and Catalysis Letters (Kluwer Academic Publishers)-Vol. 6, Iss: 1, pp 113-118
TL;DR: In this paper, the surface acid strength distribution in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 has been investigated.
Abstract: The surface acid strength distribution in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 has been investigated. The results are interpreted attributing higher acidities to W−O−W bonds and lower acidities to W=O bonds. An apparent correlation is observed between the surface acidities of the catalysts and their activities in the oxidation and ammoxidation of propylene and 1-butene.
Citations
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TL;DR: In this paper, the authors investigated Bismuth tungstate catalysts with Bi/W ratios 2/3, 1/1, 2/1 and 6/1 for various heat treatments by room temperature X-ray analysis.
Abstract: Bismuth tungstate catalysts with Bi/W ratios 2/3, 1/1, 2/1, and 6/1 have been investigated after various heat treatments by room temperature X-ray analysis. The activity and selectivity of Bi2WO6 catalyst are suggested to be connected with the presence of corner sharing W—O octhedra in the bulk of the solid that give rise to tetragonal pyramidal W—O sites at the surface. The very low activity of Bi2(WO4)3 is attributed to edge-shared octahedral coordination of W while the same of Bi6WO12 is due to tetrahedral coordination of W. The inactivity of Bi2W2O9 may be associated with its typically low oxidation power. The promoting influence of Bi2O3 is to enforce a structural situation on W—O so that W may be present in a tetragonal pyramidal 5-configuration. Wismutwolframkatalysatoren mit einem Bi/W-Verhaltnis von 2:3, 1:1, 2:1, 6:1 wurden nach verschiedenen Warmebehandlungen bei Raumtemperatur rontgenographisch untersucht. Es wird angenommen, das die Aktivitat und Selektivitat des Bi2WO6-Katalysators mit der Anwesenheit eckenverknupfter W—O-Oktaeder im Volumen des Festkorpers zusammenhangen, die tetragonal-pyramidale W—O-Platze an der Oberflache ermoglichen. Die sehr niedrige Aktivitat von Bi2(WO4)3 wird der kantenverknupften oktaedrischen Koordination von W zugeschrieben, wahrend die des Bi6WO12 auf die tetraedrische Koordination des W zuruckgefuhrt wird. Die Inaktivitat von Bi2W2O9 kann mit seiner typisch niedrigen Oxydationsfahigkeit zusammenhangen. Der fordernde Einflus von Bi2O3 liegt darin, das eine strukturelle Situation fur W—O erzwungen wird, so das W in einer tetragonal-pyramidalen 5er-Konfiguration vorliegen kann.

4 citations

Journal ArticleDOI
TL;DR: In this article, the structures of bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 have been studied by X-ray diffraction.
Abstract: The structures of bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 have been studied by X-ray diffraction. The presence of corner sharing W−O octahedra in the bulk of Bi/W=2/1 and 1/1 compositions has been established. The Bi/W=2/3 catalyst appears to contain edge-shared W−O octahedra whereas W in the Bi/W=6/1 sample form WO4 tetrahedra. An apparent correlation has been obtained between the structures of the catalysts and their activities in the oxidation and ammoxidation of propylene and 1-butene.
Journal ArticleDOI
TL;DR: In this article, electron spin resonance absorptions have been studied in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1.
Abstract: Electron spin resonance absorptions have been studied in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1. The first three systems exhibit a mean g value of 2.07±0.04 while the last one does not show any ESR signal. All the systems are diamagnetic. The signals at g=2.03–2.11 are attributed to oxygen ion-radicals chemisorbed (O2−) on W6+. An attempt has been made to correlate these whith the catalytic activities of the bismuth tungstates in the oxidation of propylene and 1-butene.
References
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TL;DR: The surface acidic sites and the formation of CO and CO2 were attributed to the pressence of bonds of covalent type in Bi2WO6 which has a rather polar structure as mentioned in this paper.

24 citations

Journal ArticleDOI
01 Dec 1975
TL;DR: The method, it is shown, can serve as a systematic and unified approach to the solution of transient problems in electrical machines and help a better understanding of the transient conditions.
Abstract: The paper suggests the application of flowgraph method to the transient analysis of electrical machines. The method of approach has been outlined, based on the concept of flowgraph approach to Laplace transforms and representation of differntial equations by the elementary differential operators. The concepts' of sequence flow-graph and conjugate flowgraph are introduced to facilitate fiowgraph representation under transient conditions. Transient analysis of a 3-phase inducation machine at starting with balanced sinusoidal potential applied simultaneously to the terminals has been carried out; the flow-graph of the transient condition being drawn by the assembly of some 'unique and non-redundant' elementry flowgraph transform operators. The method, it is shown, can serve as a systematic and unified approach to the solution of transient problems in electrical machines and help a better understanding of the transient conditions

2 citations