Iron catalysts, heterogeneous

A heterogeneous catalyst is a catalyst present in a phase different from that of the reactants. The most common heterogeneous catalysts are finely divided or porous solids used in gas-phase or liquid-phase reactions. They are finely divided or porous so that they will provide a large surface area for the elementary reactions that provide the catalytic pathway. One example is the iron catalyst used in the... 

OMS materials are also green in that they are heterogeneous catalysts. Heterogeneous catalysts do not leach into the system and are therefore completely recoverable. In addition, doping of OMS materials with iron allows separation of... 

A heterogeneous catalyst is in a different phase or state of matter than the reactants. Most commonly, the catalyst is a solid and the reactants are liquids or gases. These catalysts provide a surface for the reaction. The reactant on the surface is more reactive than the free molecule. Many times these homogeneous catalysts are finely divided metals. Chemists use an iron catalyst in the Haber process, which converts nitrogen and hydrogen gases into ammonia. The automobile catalytic converter is another example. 

Heterogeneous iron catalyst (Fe -containing ashes) is stable for at least 6 hours in aqueous solutions (50 mM Na2S04, pH 2.8) similar to those found in textile effluents. This implies that, imder the heterogeneous catalyst approach, iron ion is better eontrolled making more efficient the heterogeneous Fenton-like process than in the homogeneous eatalyst approach. 

Taylor (37) describes the shape of adsorption isobars for hydrogen on an iron catalyst and curves showing desorption, readsorption when the temperature is increased stepwise. These observations are best explained by the existence of an a priori heterogeneity. This heterogeneity may well be connected with the existence of abnormal lattice distances on the surface. 

Since iron carbonyls are not only involved in chemical synthesis (of e.g., iron nanoparticles) but also can occur in heterogenous iron catalysts during reaction conditions (e g., Fischer Tropsch reactions), we wiU briefly discuss the Mossbauer parameters of this class of compounds. 

A heterogeneity was demonstrated for the surfaces of iron, tungsten, and also for oxide catalysts consisting of various phases, in accordance with poisoning experiments on these catalysts. One of the important functions of promoters is to develop active sites on the surface of certain catalysts, as, e.g., on the promoted iron catalysts used in the ammonia synthesis. 

The results for nickel and iron catalysts imply that the reaction scheme given in Figures 48 and 49 do not represent the FTS adequately. A hybrid mechanism of both CO and CH insertion into growing chains has been proposed for nickel catalysts. On iron catalysts the monomer building blocks are proposed to be heterogeneous. Thus, isotopic transient kinetic studies have provided a more detailed understanding of the Fischer-Tropsch reaction. 

Fe/silica catalyst was prepared by the impregnation of FeCls bHzO onto silica geKKiesel gd 60). The catalyst was subsequently dried at ISO C for 2h and calcined at 400 for 3h in an air stream. The pr iared catalyst was analyzed by the spectroscopic methods of X-ray fluorescence. X-ray diffraction. X-ray photodectron and Mossbauer. For the reaction, cycloheaxne, pyridine, acetic acid, zinc and the corresponding heterogeneous iron catalyst were placed in a 125 ml Erlenmeyer flask. The reaction mixture was stirred vigorously under air (1 atm) at room temperature for 16 hr. Quantitative analysis of reaction mixture was performed on a gas chromatograph. 

The Haber-Bosch process has been known and used for over a century, and considerably little has changed over such a long period of time. In early work, Mittasch developed a highly active heterogeneous iron catalyst prepared from magnetite (Fe304) very similar catalyst formulations are still used in modern ammonia synthesis. It was also demonstrated that catalysts prepared from magnetite had superior catalytic activity in comparison to catalysts prepared from other iron oxides. 

CO and H O—the water-gas shift reaction—occurs by attack of water On a CO ligand that is coordinated to a heterogeneous iron catalyst " in the commercial processes and to a soluble complex in many homogeneous systems. ... 

Efforts to develop catalysts for the synthesis of ammonia from nitrogen and hydrogen at low temperatures illustrate this effect. The Haber-Bosch process is the well-known reaction of nitrogen with hydrogen at high temperature and pressure to form ammonia in the presence of heterogeneous iron catalysts supported on alumina, as shown in Equation 14.1. The yield of the reaction is limited by thermodynamics. Because this reaction is exothermic (but entropically unfavorable), the yield could be higher if a catalyst were developed that would allow the reaction to be conducted at lower temperatures. 

In 1920s, the studies on the catalysts for ammonia sjmthesis were performed sporadically in BASF, instead, the company mainly focused on the organic synthesis under high pressm-es and the new fields in heterogeneous catalysis. Dm-ing the development of ammonia synthesis catalysts, researchers provided valuable information about the dm-ability, thermal stability, sensitivity to poisons, and in particular to the concept of promoter. Mittasch smnmarized the roles of various additives as shown in Fig. 1.9. The hypothesis of successful catalyst is multi-component system proposed by Mittasch was confirmed to be very successful. Iron-chromium catalysts for water gas shift reaction, zinc hromium catalystfor methanol synthesis, bismuth iron catalysts for ammonia oxidation and iron/zinc/alkali catalysts for coal hydrogenation were successively developed in BASF laboratories. 

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