Metal oxide catalysts, role

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

A relevant aspect to consider for this goal is to develop metal-oxide catalysts showing ordered interfaces. Onishi and Iwasawa remarked the role of the interfacial chemistry on metal-oxides on the reactivity. Also in metal nanoparticles supported on metal-oxides, the interface between the two plays a relevant catalytic role. ... 

Reaction-induced dispersion may be used as a substitute for conventional preparation methods for supported metal oxides (Wachs and Cai, 2001) it constitutes a particular case of solid—solid wetting, which is proposed to play an important role in catalyst preparation (Leyrer et al., 1990). Industrially relevant mixed metal oxide catalysts can be prepared by reaction-induced dispersion at temperatures that are significantly... 

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

But how ubiquitous actually are alkalis in the promotion of reactions catalyzed at metal surfaces An examination of recent authoritative sources [6,7] shows that the majority of medium-to large-scale processes do not employ alkali promoters, even when one includes nonmetallic (i.e., metal oxide) catalysts. In a number of cases (e.g., steam reforming of naphtha) it seems clear that the role of alkali is simply to reduce the acidity of the oxide support. There are famous cases, of course, where the presence of alkali species on the catalytically active metal surface is critically important to the chemistry. Notable are ethene epoxidation (Ag-Cs), ammonia synthesis (Fe-K), acetoxylation of ethene to vinyl acetate (Pd, Pd/Au-K), and Fischer-Tropsch synthesis (Fe, Co, Ru-K). The first three are major industrial... 

Exploration of alkaline earth/metal oxide catalysts and other metal/metal oxide catalysts has been continued at Union Carbide. As an example, after over 350 hours of methane coupling with a 5 wt% barium carbonate on titanium oxide (with ethyl chloride in the feed gas), a C2 yield of 22%, a Cj selectivity of 58%, and an ethylene/ethane ratio of 8 1 were obtained. The coupling catalysts were comparable in selectivity, activity, and Cj yield to the better literature catalysts, but provide hundreds of hours of stable operation in the oxidation of methane to Cj s. These catalysts require the presence of a small amount of halides, either as a catalyst component or as a periodic or continuous additive to the catalyst. The chloride appears to serve three distinct roles, resulting in suppression of carbon dioxide formation, increased rates to Cg products, and higher ethylene-to-ethane product ratios. There have been numerous other recent reports. 

In the case of supported metal oxide catalysts, the role of the support is to disperse the active phase and to create new active surface species by host (active phase) - guest (support) interaction. The dispersion of the active phase plays a fundamental role, and very often a maximum of strength of the active sites is observed when the monolayer coverage is reached. 

Another important process in which catalyst deactivation by coke deposits plays an important role is propane dehydrogenation, which can be performed with a variety of materials, including metal and metal oxide catalysts. Different in situ and operando spectroscopies have been applied to these catalysts, including UV-vis, Raman, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies [4, 115, 140],... 

The likely role of amide species produced by oxidation of Lewis-site coordinated ammonia finds support from the studies concerning ammonia activation and oxidation on metal oxide catalysts. 

Ml phase " represents the clearest example of a multifunctional catalyst in which each element, in close geometrical and electronic synergy with the surrounding elements, plays a specific role in turn, as an isolated active site, in every reaction step for the alkane transformation into the partial oxidation product desired. The flexibility of the structure allows modification of the catalyst composition and hence its catalytic behavior. Moreover, this type of mixed-metal oxide catalyst has the ability to catalyze other different oxidation reactions starting from alkanes, such as propane oxidation to acrylic acid, " oxidative dehydrogenation of ethane to ethylene, and n-butane selective oxidation. ... 

The interesting role which solid electrolytes can play in the study of heterogeneous catalysis was first recognized by Wagner, who proposed the use of solid electrolyte cells for the measurement of the activity of oxygen on metal and metal oxide catalysts. This technique, first used to study the mechanism of SO2 oxidation on noble metals, was subsequently called solid electrolyte potentiometiy (SEP). It has been used in conjunction with kinetic measurements to study the mechanism of several catalytic reactions on metals and, more recently, metal oxides. It is particularly suitable for the study of oscillatory catalytic reactions. - The SEP literatnre has recently been reviewed. ... 

The catalysis science of supported metal oxide catalysts, especially supported vanadia catalysts, has lagged behind their industrial development. In the 1970s, two models were proposed for the active metal oxide component a three-dimensional microcrystalline phase (e.g., small metal oxide crystallites) or a two-dimensional surface metal oxide overlayer (e.g., surface metal oxide monolayer). In the 1980s, many studies demonstrated that the active metal oxide components were primarily present as two-dimensional surface metal oxide overlayers, below monolayer coverage, and that the surface metal oxide overlayers control the catalytic properties of supported metal oxide catalysts. The synergistic interaction between the surface vanadia overlayer and the underlying oxide support prompted Ceilings to state. . that neither the problem of the structure of suppored vanadium oxide nor that of the special role of TiOa as a support have definitely been solved. Further work on these and related topics is certainly necessary. In more recent years, many fundamental studies have focused on the molecular structural determination of the surface vanadia phase and to a lesser extent the molecular structure-reactivity relationships of supported vanadia catalysts. " ... 

As evident in the preceding discussions on the metal and metal oxide catalysts, the heat of adsorption plays a key role in characterizing a catalyst. The heat of adsorption can also be used to gain an understanding of the nature of catalytic reactions. Three different experimental methods can be used for the determination of heats of adsorption adsorption experiments (isotherms), the calorimetric method, and temperature-programmed desorption (TPD). In the first method, the adsorption isotherms obtained at different temperatures are used. The values of d In P/dT at constant v (volume of gas adsorbed) are calculated as a function of v through... 

Defect sites present in the oxide are not covered by metal particles, as in a conventional metal/oxide catalyst. The oxide/metal system is an attractive model to investigate the role of the oxide in a catalytic process, and also can be used to study reaction mechanisms... 

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