Catalysts in the Haber process

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

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. 

Many transition metals, or their compounds, are important catalysts. For example, Fe is used as a catalyst in the Haber process to synthesize ammonia ... 

Iron is used to form structural supports for a wide variety of applications, including bridges, and as a catalyst in the Haber process for ammonia production. Iron is also an essential element in the diet, because it is required to make hemoglobin, a component of red blood cells which absorbs oxygen in the lungs and transports it to the tissues. 

In practice this is carried out in two stages at 400 °C over an iron catalyst, and then at 200 °C over a copper one. Finally the carbon dioxide is removed by bubbling the gaseous mixture through either potassium hydroxide or diethanolamine solution. It is important that the oxides of carbon are reduced to a very low level otherwise they may poison the catalyst in the Haber process. 

Carbon monoxide would poison the catalyst used in the Haber process and is therefore removed from the system. 

Transition metals and their compounds are used as catalysts. Catalysts you may already know are Iron In the Haber process (Industrial production of ammonia) platinum in the Ostwald process (Industrial production of nitric acid) and platinum, rhodium and palladium In catalytic converters. 

Another important application of iron is as an industrial catalyst. It is used in catalyst compositions in the Haber process for synthesis of ammonia, and in Fischer-Tropsch process for producing synthetic gasoline. 

The importance of catalysts in chemical reactions cannot be overestimated. In the destruction of ozone previously mentioned, chlorine serves as a catalyst. Because of its detrimental effect to the environment, CFCs and other chlorine compounds have been banned internationally. Nearly every industrial chemical process is associated with numerous catalysts. These catalysts make the reactions commercially feasible, and chemists are continually searching for new catalysts. Some examples of important catalysts include iron, potassium oxide, and aluminum oxide in the Haber process to manufacture ammonia platinum and rhodium in the Ostwald synthesis of nitric... 

Coordinated nitrenes RN2- have been thoroughly investigated in recent years285,286 owing to their formal similarity to carbenes and to their potential importance, e.g. as catalysts in the Haber ammonia process. Theoretical studies have suggested that imido ligands may also prove effective in promoting alkene metathesis.287... 

Nitrogen reacts with H2 in the Haber process for the synthesis of NH3, but the reaction requires high temperatures, high pressures, and a catalyst (Sections 13.8-13.10) ... 

In the Haber process, nitrogen and hydrogen in the correct proportions (1 3) are pressurised to approximately 200 atmospheres and passed over a catalyst of freshly produced, finely divided iron at a temperature of between 350 °C and 500 °C. The reaction in the Haber process is ... 

Table 5.1 shows an application of XPS to the study of the promoted iron catalyst used in the Haber synthesis of ammonia. The sizes of the various electron intensity peaks allows a modest level of quantitative analysis. This catalyst is prepared by sintering an iron oxide, such as magnetite (Fe304) with small amounts of potassium nitrate, calcium carbonate, aluminium oxide and other trace elements at about 1900 K. The unreduced solid produced on cooling is a mixture of oxides. On exposure to the nitrogen-hydrogen reactant gas mixture in the Haber process, the catalyst is converted to its operative, reduced form containing metallic iron. As shown in Table 5.1, the elemental components of the catalyst exhibit surface enrichment or depletion, and the extent of this differs between unreduced and reduced forms. 

In most processes the reaction takes place on an iron catalyst. The reaction pressure is normally in the range of 150 to 250 bar, and temperatures are in the range of 350°C to 550°C. At the usual commercial converter operating conditions, the conversion achieved per pass is only 20% to 30%53. In most commercial ammonia plants, the Haber recycle loop process is still used to give substantially complete conversion of the synthesis gas. In the Haber process the ammonia is separated from the recycle gas by cooling and condensation. Next the unconverted synthesis gas is supplemented with fresh makeup gas, and returned as feed to the ammonia synthesis converter74. 

Explain the dependence of the yield of ammonia upon temperature and pressure in the Haber process, and also explain the role of the catalyst. 

In the Haber process why is the gas mixture cooled after it has left the catalyst chamber ... 

This reaction is the basis for the Haber process, which is critical for the production of fertilizers and therefore critical to the world s food supply. In the Haber process, N2 and H2 react at high pressure and temperature in the presence of a catalyst to form ammonia. In a closed system, however, the reaction does not lead to complete consumption of the N2 and H2. Rather, at some point the reaction appears to stop with all three components of the reaction mixture present at the same time. 

The exact role of promoters is not very well understood in many cases, but it is now generally accepted that it is related to the formation of specific electronic surface states necessary for the given catalytic reaction. It apparently does not matter how that electronic state is produced that is, whether it is formed in the preparation of the native catalyst surface or by the presence of some other component which induces the necessary state. As an example, the presence of small amounts of aluminum and potassium oxides on iron-iron oxide catalyst in the Haber ammonia synthesis greatly improves its activity. Either promoter alone has no significant effect on the process. Why Such questions remain as fodder for further industrial or graduate research. 

Catalysts play an important role in overcoming the activation barrier in ammonia synthesis. It is weU known that strong N=N triple bond and the low sticking coefficient of the molecule nitrogen limit the choice of catalyst. However, the mechanism of ammonia formation on an electrocatalyst seems to be different from that of the conventional catalyst. The information about the conventional catalyst in the Haber-Bosch process and the electrocatalyst in the electrocatalytic membrane reactor are described in this section. 

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