Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

Sol-gel prepared mixtures of silica or zirconia with titania are shown to have significantly higher activities than pure titania for the complete photocatalytic oxidation of ethylene. These higher activities are only apparent when the respective catalysts are stabilized by sintering. The differences become even more pronounced when the catalysts are used in a tubular reactor. Optimum mixture concentrations are found to be 12 wt % zirconia and 16 wt % silica in titania. Both catalyst types exhibit activity maxima with respect to sintering temperature. It is hypothesized that the maxima arise from opposing effects of densification and phase transformation versus beneficial sintering. A comparison of our catalyst compositions with literature in this area suggests that the increase in activity due to the addition of silica or zirconia may be a result of higher surface acidity. However, isoelectric point measurements employing the unsintered and sintered catalysts show no conclusive increase in surface acidity...

Enhanced Photocatalytic Performance of Titania-Based Binary Metal Oxides: TiO2/SiO2 and TiO2/ZrO2

The present review discusses the most important aspects to take into consideration to improve the properties of γ-Al2O3 as a support for catalytic applications. We show that the synthetic route to γ-Al2O3 is the starting point that determines the micro- and macrostructure of the oxide and, consequently, allows control of the support characteristics. The relevance of the adequate structural characterization of the oxide as well as of its surface sorption behavior through the proton-affinity distributions, are considered. The usefulness of the latter during supported catalyst preparation, for catalytic active sites characterization, and for stability evaluation after calcination are discussed for the Co/γ-Al2O3 system in particular. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

γ-Alumina as a Support for Catalysts: A Review of Fundamental Aspects

We have proposed a new hypothesis that the acid sites on binary metal oxides are formed by an excess of a negative or positive charge in the mixed oxides and that whether the charge is excess or not, and whether it is positive or negative, are determined by the coordination numbers and valences of the positive and negative elements in the model structures pictured according to two postulates. The validity of the hypothesis is, in fact, 90% for the 31 kinds of binary oxides tested, much higher than the validity (48%) of Thomas’ hypothesis. The hypothesis explains well the mechanism of the acidity generation of binary oxides and predicts whether the acid sites will be of the Bronsted or the Lewis type.

A New Hypothesis Regarding the Surface Acidity of Binary Metal Oxides

Chemistry of zirconia and its use in chromatography

Eighteen binary metal oxides consisting of TiO2–MmOn, ZnO–MmOn and Al2O3–MmOn (MmOn: metal oxide) were prepared by the usual co-precipitation method, their acid amounts and strengths being determined by n-butylamine titration using various acid-base indicators. The acid strengths of fourteen of the tested binary oxides of molar ratio 1 : 1 were found to be remarkably higher than those of the component single oxides. High acid strengths were as follows: H0≤−8.2 for TiO2–SiO2, H0≤−5.6 for TiO2–Al2O3 and Al2O3–ZrO2 and H0≤−3 for TiO2–CdO, TiO2–SnO2 and ZnO–SiO2. The acid amounts of sixteen binary oxides were larger than those of the component oxides. The effect of the composition of binary oxides on acidity was examined for TiO2–Al2O3, ZnO–Al2O3 and Al2O3–ZrO2. The acidity maxima appearing for TiO2–Al2O3 and ZnO–Al2O3 were found to be of molar ratio\simeq9:1 and for Al2O3–ZrO2\simeq3:2. A fairly good correlation has been demonstrated between the observed highest acid strengths and the average electronegativi...

Acidic properties of Binary Metal Oxides

A new hypothesis regarding the surface acidity of binary metal oxides

The catalytic activity and the sintering property of iron oxide catalysts with and without potassium additives have been studied. The potassium oxide acts as a promoter and is effective under given conditions in increasing the activity of the catalyst from four to ten times. The effects of the promoter can be classified thus: (1) Structural promoter: The increase in the apparent bulk density and the shrinkage of the catalyst tablet upon calcination during the preparation of the catalyst are prevented by potassium additives. The differences in the specific surface area and in the pore volume of the catalyst between with and without potassium promoter are slight. These effects on iron oxide appear to be caused largely by the formation of K2Fe22O34. (2) Synergetic promoter: The potassium promoter changes the apparent energy of activation of the dehydrogenation reaction from 28 to 43 kcal/mol. The increase in the catalytic activity is due to the increase in the pre-exponential factor. The increase in the fact...

Effect of Potassium Promoter on Iron Oxide Catalysts for Dehydrogenation of Ethylbenzene to Styrene

Acidic properties of binary metal oxides

The exchange reactions of oxygen atoms between oxygen and carbon dioxide on metal oxides were studied by the static batch method in the temperature range from 150 to 600°C. The order of the catalytic activity of the oxides in the reaction was found to be; (higher activity) Co3O4>MnO2>NiO>Fe2O3>Cr2O3>V2O5>TiO2. A very close similarity of the change in the reactivity of these oxides was found in the exchange between oxygen and oxides. There is an approximately linear relationship between the catalyst activity of the metal oxides and the heat of the formation of these oxides. At any temperature examined, carbon dioxide exchanged their oxygen atoms with those of oxide rapidly. The rate of isotopic exchange decreased in the following order on any oxide examined: CO2-Oxide>O2-Oxide>O2-CO2 on oxide. The exchange between oxygen and carbon dioxide was observed only when the exchange between oxygen and oxide took place. According to these results, it was concluded that the exchange between oxygen and carbon dioxide...

Tadamitsu Kiyoura

Papers

Acidic properties of Binary Metal Oxides

A new hypothesis regarding the surface acidity of binary metal oxides

Effect of Potassium Promoter on Iron Oxide Catalysts for Dehydrogenation of Ethylbenzene to Styrene

Acidic properties of binary metal oxides

The Exchange Reaction of Oxygen Atoms between Oxygen and Carbon Dioxide on Metal Oxides of the Fourth Period of the Periodic System