Aluminas have been used extensively as adsorbenu and active catalysrs and catalyst supponsm the pas. Already in 1197 the aluminadyzed dehydration of ettllnoi was dtscavered by Dutch chermsts: and S;rbatier [3] remewed the use of dumlnas as active cazaiysrs far vanous reacttons UI the second decade of thu century. She that time the applicazions of aluuuas m dycic pmcesses have mcreased tremendously. In tndustrral cualytic pmcesses, alumuus are mostiy used as catalyst suppons [4]. Oxides a d mued oxides ap well as tracuuion mauls and noble meare supported oa alumma. Thuscb. romaa-elumana catalysts are ktng used for the conversion of parafdns to olailnrc hydrocarbons, 10 hydrodealkplation of aromatics. and to a lesser exzm in catalyzic reforming. The larter process LS also caralyzed by molybdena-alumina, a cavlyst system whid is also active for malang toluene and ocher aromatics from satwed hydrocarc bons. It also dyzes the Isomerhation of pm. Great efions are presently be-made to nudy the surface c...

Catalytic Aluminas: Surface Models and Characterization of Surface Sites

Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

Copper-Zinc Oxide-Alumina Methanol Catalysts Revisited

Publisher Summary This chapter discusses the specific poisoning of a catalyst surface. A probe molecule other than the reactant is preadsorbed as a poison and its effect on the catalytic activity studied. From the nature of interaction of this probe molecule with the surface sites one may obtain information regarding the chemical nature of these sites and from the number of poisoning molecules necessary to bring the activity to zero one may estimate an upper limit of the active site density. Besides the obvious difficulty to discriminate between surface sites that purely absorb the probe molecule and those that are involved in the catalytic process as active sites, there are a number of other limitations making it difficult to obtain clear-cut information on the nature of active sites. The chapter describes the proposals that are indispensable for a proper application of this method and the precautions that have to be kept in mind while drawing conclusions from specific poisoning experiments. The chapter illustrates two examples—namely, the dehydration of alcohols and the isomerization of olefins on alumina.

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces

Small amounts of gallium ions were added to y-alumina and their influence on the structural properties of the system Co-(Mo)/y-Al203 was studied. It is shown that, due to the presence of Ga3+ ions, a "surface" spinel CoAl204 is formed with a larger amount of Co2+ in tetrahedral sites as compared to the spinel formed on gallium-free alumina. A decrease of the segregated Co304 is also observed. A possible effect of gallium ions on molybdenum is discussed. It is also reported that different preparation methods (single or double impregnation) lead to the formation of different surface species. Cobalt aluminate, molybdate monolayer, and Co3O4, depending on the Co content, are formed on doubly impregnated specimens. Cobalt aluminate and cobalt molybdate are the main species formed on singly impregnated specimens. Finally brief consideration is given to how the Co and Mo species, present in the oxide form, change in the sulfided form.

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Effect of gallium ions and of preparation methods on the structural properties of cobalt-molybdenum-alumina catalysts

An infra-red investigation has been carried out of CO adsorbed onto Ni2+/η-Al2O3 and Ni2+/γ-Al2O3 catalysts. These contain a “surface spinel” phase in which nickel ions are present both in octahedral and tetrahedral sites. Their relative concentrations are primarily determined by the atmosphere (damp or dry) in which the preparation at 600°C has been carried out. Upon CO adsorption four bands appear in the 2250–2150 cm–1 region due to molecules linked to coordinatively unsaturated surface cations. The relative intensities are related to the preliminary treatment of the sample and are therefore discussed in terms of the possible surface configurations and of the relative amounts of tetrahedral and octahedral sites occupied. The infra-red spectrum of CO adsorbed on hydrogen reduced samples has also been investigated and the importance of the preliminary treatment on the reducibility has been confirmed.

Surface configurations and infra-red studies on nickel oxide supported on η- and γ-Al2O3

A study has been made of the catalytic oxidation of propene over the oxides WO3, WO2.95, WO2.90, WO2.72 and WO2, which were selected because they possess specific features of chemical and structural interest rather than for their catalytic ability. It was found that the oxides WO2.95, WO2.90 and WO2.72 all selectively produce acrolein in small amounts. The oxides WO3 and WO2 were non-selective and rather inactive. The results are discussed in terms of a mechanism involving both variable valence in the crystal and the specific structural geometry of these compounds.

The Catalytic Activity of Several Tungsten Oxides for the Oxidation of Propene

The catalytic properties of copper oxides (CuO and Cu20), and CuO—MgO and CuO—ZnO solid solutions, have been investigated. Structural studies (X-ray diffraction, \"LTV. and VIS. reflectance spectroscopy and magnetic susceptibility measurements) allow the systems to be fully characterized. Their catalytic activity towards the decomposition of N20 has been tested in the range 250° to 500 °C. It was thus possible to confirm that Cu20 is very active for this reaction, whereas CuO has only very low activity. For the CuO—MgO and CuO—ZnO solid solutions, it has been shown that, under repeated vacuum conditioning treatments and catalytic experiments, a segregation of Cu20 occurs. This species is responsible for the catalytic activity. The CuO—ZnO specimens are more active than the CuO—MgO ones because of a greater Cu20 segregation in the former system. The role of the matrix in the segregation process is discussed.

The Catalytic Activity of Copper Ions in Copper Oxides and in their Solid Solutions with ZnO and MgO for the Decomposition of N2O

Es wurde die Adsorption von O2 und C3H6 an den Wolfram-Oxiden WO3, WC\"2,96, WO2.90, W02,72 und WO2 untersucht. Die Ergebnisse werden in Bezug zu früheren katalytischen Untersuchungen (N20-Zerfall und CsHe-Oxydation) erläutert. Es wird die besondere Rolle der strukturellen Anordnung der reduzierten Phasen bei katalysierten Umsetzungen mit Beteiligung von Sauerstoff gezeigt. Introduction The decomposition of N2O and the oxidation of propene over

M. Schiavello

Papers

Effect of gallium ions and of preparation methods on the structural properties of cobalt-molybdenum-alumina catalysts

Surface configurations and infra-red studies on nickel oxide supported on η- and γ-Al2O3

The Catalytic Activity of Several Tungsten Oxides for the Oxidation of Propene

The Catalytic Activity of Copper Ions in Copper Oxides and in their Solid Solutions with ZnO and MgO for the Decomposition of N2O

Adsorption of Oxygen and Propene over a Series of Tungsten Oxides