Alkali-promoted oxide catalysts, surface

Supported Rhodium Catalysts Alkali Promoters on Metal Surfaces Cobalt-Molybdenum Sulfide Hydrodesulfurization Catalysts Chromium Oxide Polymerization Catalysts... 

The Mittash catalyst is unsupported because iron is cheap and the alumina promoter warrants a high specific surface area. This situation is different with Ru catalysts, which are usually prepared on either oxide or carbon supports. Loading of zeolites with small Ru particles offers an interesting alternative but with lower activity, however (44,45). Pioneering work was performed by Ozaki and co-workers (46), who presented an alkali-promoted Ru catalyst on a carbon support with an activity superior to that of the conventional Fe catalyst operated at identical conditions. Further development of this type of catalyst (47) led to a material that has recently been installed in an industrial plant (Ocelot, Canada). Apart from lower capital costs, this plant also operates with reduced energy consumption, and replacement of the iron catalysts by Ru-based ones remains an interesting option for the future. 

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

Alkalis are the most important electropositive promoters of metal and metal oxide catalysts. They are used in many important industrial catalysts but are also quite suitable for fundamental studies since they can be easily introduced under vacuum conditions on well-characterized model metal surfaces. 

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

To determine the effect of alkali-metal promotion on the surface morphology, the catalysts were investigated by IR spectroscopy.9 DRIFT spectra revealed two intense reflectance bands at 760-680 cm-1 and 650-570 cm-1 for the sodium promoted sample. Moreover, an absorption band at 535 cm, which has previously been assigned to PdC>3,28 was observable in the DRIFT spectra of oxidized Na-PdO/Sn02. 

From the results of both pulse reaction and adsorption experiments, it could be confirmed that Ni has a strong affinity with methane, while alkali promoters with carbon dioxide. The retardation of coke deposition on KNiCa/ZSI catalyst must be ascribed to the abundantly adsorbed CO2 species. This explanation is similar to the suggestion of Horiuchi et al. [5], showing that the surface of the Ni cat2ilyst with basic metal oxides was labile to CO2 adsorption, while the surface without them was labile to CH4 adsorption. Since coke deposition was mainly caused by methane decomposition, the catalyst surface covered with adsorbed CO2 or reactive oxygen species from the dissociation of CO2 would suppress coke deposition. The addition of alkaline promoters also seemed to greatly suppress the activity of supported Ni catalyst for the direct decomposition of methane. 

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... 

In summary, alkali promotion of supported metal catalysts is an interesting subject that does have important technological implications in those cases where the presence of alkali has a pivotal influence on the surface chemistry of the metal phase. Fundamental studies of such systems are certainly justified. However, we should maintain a sense of proportion. Alkalis find relatively limited use as promoters in practical catalysis—indeed in some cases they act as powerful poisons. And we should not lose sight of the fact that what is actually present at the surface of the working catalyst is not an alkali metal, but some kind of alkali surface compound. This chapter deals with the application of alkali promoters to catalysis by metals, as opposed to catalysis by oxides, and, in particular, the technique of electrochemical promotion (EP), which enables us to address some pertinent issues. 

At this point it is important to note that even when alkali promoters react with the catalytic reactants and products on a catalyst surface forming oxides, hydroxides, carbonates, or nitrates, they do not lose their promoting ability [13,88]. This is because the large dipole moment of the alkali cation is maintained when such surface compounds are formed, and thus the alkali cation can still exert its promoting action. 

Catalysts considered in the present discussion cover a wide spectrum of solids reducible multivalent metal oxides as well as non reducible basic compounds Reducible metal oxides possess some inherent problems whereas these problems are less for the alkali ions promoted alkaline earth oxides. Alkaline earth oxides seem to be more suitable for working at low partial pressure of oxygen. By doping alkaline earth oxides with alkali metal compounds it is conceivable that O species can be stabilized for dissociative absorption of methane. Reducible metal oxides will tend to transform into lower valent oxides or even upto metallic state partly under applied reaction conditions specially at low partial pressure of O2. Both activity and selectivity will be deteriorated. But for the non reducible basic oxides structural changes will be quite different. They will tend to reach an equilibrium state in the surface level amongst the oxide, hydroxide and carbonate phases on reacting with evolved H2O and CO. Both the lattice distortion and the formation of O species can occur in the alkali earth oxides in doping with alkali ions as they can not build a mixed oxide lattice. 

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