Catalytic properties of rare earths

Viswanathan B (1984) Solid state and catalytic properties of rare earth orthocobaltites - a new generation catalysts. J Sci Ind Res 44 66-74... 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

Recently there has been a growing interest in catalytic properties of rare earths (R) and related compounds (Edelmann 1996, Hogerheide et al. 1996, Inumaru and Misono 1995, Taube 1995, Yasuda 1995, Imamoto 1994). Since rare-earth elements have specific electron configurations based on 4f orbitals, many intriguing reactions mediated by them which cannot be achieved by d-block transition metal compounds are expected to occur ... 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

Catalytic investigations on rare earth oxides of mostly Russian workers have been reviewed up to about 1973 by Minachev (1973), and catalytic properties of rare earth oxides in many reactions have been compiled more recently by Rosynek (1977). To evaluate catalysis on rare earth oxides in this chapter we will present selected examples of important catalytic results in some detail at the expense of a more exhaustive listing. Reactions are classified into several groups for ease of presentation following closely the reaction categories employed by Rosynek (1977). 

T.T. Bakumenko, Catalytical properties of rare and rare-earth elements, AN USSR, Kiev, 1963 (in Russian). 

Rabo, J.A., C.A. Angell and V Schomaker, 1968, Catalytic and structural properties of rare-earth exchanged forms of type Y zeolite, in Proc. 4th Int. Congr. on Catalysis, Moscow (Nauka, Moscow) preprint 54, pp. 96—113. 

Structural, physical and chemical properties of bulk rare earth oxides can be found in Chapter 27, Volume 3 of this series and have been compiled with a view towards catalysis in a recent review by Rosynek (1977). An important parameter for the catalytic behavior of rare earth oxides is their basicity. The basicities of rare earth oxides resemble those of the alkaline-earth oxides, and scale directly with the respective cation radii. Thus, La203 shows the strongest basicity and SC2O3 the weakest, with sesquioxide basicities decreasing smoothly along the lanthanide series going from La to Lu. This periodic trend allows one to study the influence of subtle variations in basicity on catalytic behavior in a class of related materials with similar electronic and geometrical structure. 

Mild reaction conditions using catalytic triflates of rare earth metals were also developed. This was based on the better Lewis acid properties of the catalysts, their ready availability and easy handling. An alternative is the use iron(iii) triflate. In carbohydrate chemistry, iron(iii) triflate has only been used for oxidative C-C bond cleavage, thioglycosylation of peracetylated glycosides and type I Ferrier rearrangement of glucal. ... 

In addition, catalytic amonnts of rare earth metal triflates have been snccessfiiUy nsed, while stoichiometric amonnts of conventional Lewis acids have been employed in many cases. From the viewpoint of snstainable green chemistry, it is remarkable that rare earth metal triflates can be recovered and rensed withont loss of activity. These properties will lead to really enviromnental-friendly chemical processes nsing rare earth metals as catalysts. 

To imderstand the catalytic role of rare earth species, first principles calculations, especially those based on the density functional theory (DFT), have been extensively conducted. The properties ofbulk, surfaces, supports. 

In this chapter, we show that computational simulations based on first-principles DFT calculations have been extensively performed to study the various geometric, electronic, and catalytic properties of bulk and surfaces of rare earth Ce02. In particular, the DFT calculations corrected by on-site Coulomb interaction have been shown to reproduce available experimental results, indicating that the DFT + U method is reliable to explain and predict the catalytic activities of rare earth Ce02-based materials. 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Sodium forms of zeolites X and Y are known to be Inactive for alkylation. Calcium Introduction (catalyst 1) has resulted In a catalyst with some activity. Selectivity of the sample was not high about 57% of the alkylate were octanes with a ratio of TfV to DW of 2 1. The yield and quality of the alkylate were Improved, If Na" " cations were replaced with cations of rare-earth elements (catalysts 2 and 3). Product yield for catalysts 2 and 3 were 86.0% and 77.5% respectively with a TMP content In C0-fractionof about 85%. Unfortunately the stabilities of these two catalysts were rather low In both cases, and alkylates yields and quality declined after 3 or 4 runs. For example the percentage of unsaturated hydrocarbons In the hydrocarbon product for catalyst 3 Increased from 18 up to 30%, and TMP concentration decreased to 35% after several runs. Catalyst 4 has proved to be the most active and stable catalyst and the yields and quality of alkylates obtained over It have been the same even after many reaction-regeneration cycles. Further Increase of calcium content In the catalyst (catalyst 5) deteriorated Its catalytic properties. 

A rare earth metal-exchanged Y-type (REY) zeolite catalyst was found to be an effective catalyst for the catalytic cracking of heavy oil. The influence of the reaction conditions and the catalytic properties of REY zeolite on the product yield and on gasoline quality have been described above. In this section, a reaction pathway is proposed for the catalytic cracking reaction of heavy oil, and a kinetic model for the cracking reaction is developed [14,33]. 

Rare earth elements, such as yttrium, cerium and samarium, are well known to stabilize tetragonal Zr02 at lower temperatures, although the tetragonal Zr02 is generally stable at and above 1373 K. In the present study the effect of rare earth element addition on the catalytic properties of the amorphous Ni-Zr alloy-derived catalysts has been examined in order to improve the catalytic activity of the catalysts. 

For a better understanding of the role of rare earth elements in the catalytic properties of the amorphous alloy-derived catalysts, characterization of the catalysts was carried out. Figure 3shows the change in the number of surface nickel atoms,... 

The decomposition products identified following reaction are not necessarily the primary compounds which result directly from the rate limiting step. Particularly reactive entities may rapidly rearrange before leaving the reaction interface and secondary processes may occur on the surfaces of the residual material which often possesses catalytic properties. The volatile products identified [144] from the decomposition of nickel formate were changed when these were rapidly removed from the site of reaction. The primary products of decomposition of thorium formate were identified [17] as formaldehyde and carbon dioxide, but secondary processes occurring on the residual thoria yielded several additional compounds. The oxide product similarly catalysed interactions between the primary products of decomposition of zinc acetate [145]. During the decomposition of rare earth oxalates, carbon monoxide disproportionates extensively to carbon dioxide and carbon [81,82]. 

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