Dispersion ammonia synthesis

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

A complex nanostructured catalyst for ammonia synthesis consists of ruthenium nanoclusters dispersed on a boron nitride support (Ru/BN) with barium added as a promoter (33). It was observed that the introduction of barium promoters results in an increase of the catalytic activity by 2—3 orders of magnitude. The multi-phase catalyst was first investigated by means of conventional HRTEM, but this technique did not succeed in identifying a barium-rich phase (34). It was even difficult to determine how the catalyst could be active, because the ruthenium clusters were encapsulated by layers of the boron nitride support. By HRTEM imaging of the catalyst during exposure to ammonia synthesis conditions, it was found that the... 

Structural promotion A highly dispersed support can provide and (or) stabilize a high surface area of the catalyst supported by it. A typical example is ammonia synthesis where the thermal sintering of the iron catalyst is inhibited by alumina (although the phase configuration is different). 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts, whose active ingredients are major amounts of other substances called promoters, which increase the activity. Examples of supported catalysts are the automobile-muffler catalysts mentioned above, the platinum-on-alumina catalyst used in petroleum reforming, and the vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

The reverse of reaction (2.1) is methanation. Used to remove residual CO traces from ammonia synthesis feedstocks, it was also developed as an important source of substitute natural gas (SNG) in the synthetic fuels industry. Since this reaction is exothermic, equilibrium yields are better at low temperatures (300-500 C). Thus, high activity is critical. Nickel must be highly dispersed. Preparational methods are required to produce small nickel crystallites. This high metal area must be maintained in the presence of extreme exothermicity, so that sintering must be avoided. This is partially accomplished through proper catalyst design, but process reactor type must also be considered. Recycle, fluidized, and slurry reactors are appropriate. 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

Early attempts to approach this problem were made by Kobosev and co-workers in the 1930s from the viewpoint of atomic dispersion and active ensembles. They studied the behavior of catalysts containing very small amounts of supported metal and were able to derive the number of atoms within the ensembles that were active for specific reactions (one atom for SO2 oxidation, two atoms for benzene hydrogenation, three atoms for ammonia synthesis, four atoms for acetylene oligomerization) (2a-c). These results as well as later ones have been reviewed by GiFdebrand (3). 

Example 10.19 Discharge of supercritical ammonia and subsequent dispersion Ammonia is employed in a process for NH3 synthesis at pi = 221 bar and Ti = 573.15 K. A leak of cross sectional area Fl = 490 mm opens, but can be isolated after 10 min. The jet emanating from the leak is supposed to have a length of 30 m in the horizontal direction. At which maximum distance from the point of release do we reach a maximum concentration of 150 ppm (ERPG-2 value, vid. Table 2.35) ... 

When activated carbon is used as support, the support degradation by Ru-catalyzed methanation imder ammonia synthesis conditions can occur, so it affects the fife of catalysts. Therefore, many researchers have tried to use metal oxides to replace the activated carbons as the supports for ruthenium catalysts. Supported catalysts with high dispersion and high activity can be obtained when noble metals in precursor forms are supported on hardly reduced metal oxide. The oxides which are commonly used as ruthenium catalyst support include oxides of alkaline earth metals, lanthanide,and alumina. ... 

The SEM of Ru-based ammonia synthesis catalyst prepared by isovolume-impregnation is shown in Fig. 6.28. It can be seen from Fig. 6.28 that the active components are dispersed on support in three ways (i) certain sized active component is dispersed on support unevenly (Fig. 6.28 (a)), (ii) Active component of atom-like state is dispersed on support in cluster (three-dimensional) or raft like (two-dimensional) (Fig. 6.28(b)). (iii) Active component is distributed on the support like accumulated snow (Fig. 6.28(c)), which is an amorphous phase amorphous phase by XRD analysis. Among them, sample 3 has the highest activity in which active component is accumulated like snow on the support after reduced with H2 at 200°C for 2h. The sample 1 and 2 are not reduced in H2 and are just dealt with in vacuum or dried at low temperatures. Their activities are lower, which are only 1/5 to 2/5 of sample 3. It can be seen that the reduction of catalyst with H2 is necessary during the preparation process. 

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