Haber process temperature effects

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

Effect of Temperature and Pressure on the Yield of Ammonia in the Haber Process (PH2 = 3PNl)... 

With the technical development achieved in the last 30 years, pressure has become a common variable in several chemical and biochemical laboratories. In addition to temperature, concentration, pH, solvent, ionic strength, etc., it helps provide a better understanding of structures and reactions in chemical, biochemical, catalytic-mechanistic studies and industrial applications. Two of the first industrial examples of the effect of pressure on reactions are the Haber process for the synthesis of ammonia and the conversion of carbon to diamond. The production of NH3 and synthetic diamonds illustrate completely different fields of use of high pressures the first application concerns reactions involving pressurized gases and the second deals with the effect of very high hydrostatic pressure on chemical reactions. High pressure analytical techniques have been developed for the majority of the physicochemical methods (spectroscopies e. g. NMR, IR, UV-visible and electrochemistry, flow methods, etc.). 

Figure 13.18 shows the effect of temperature and pressure on the amount of ammonia produced by the Haber process. 

Le Chatelier s principle /la-sha-tel-yayz/ If a system is at equilibrium and a change is made in the conditions, the equilibrium adjusts so as to oppose the change. The principle can be appfied to the effect of temperature and pressure on chemical reactions. A good example is the Haber process for synthesis of ammonia ... 

Nitrogen is a diatomic molecule, which is effectively triple-bonded and has a high dissociation energy (940 kj mol" ). It is therefore inert and it only reacts readily with lithium and other highly electropositive elements. The direct combination of nitrogen and hydrogen occurs at elevated temperatures and pressures (400-600°C, 100 atmospheres) and is the basis of the industrially important Haber process for the manufacture of ammonia. 

After trying different substances to see which would be most effective, Carl Bosch (see Chemistry Put to Work The Haber Process, page 615) settled on iron mixed with metal oxides, and variants of this catalyst formulation are stiU used today. These catalysts make it possible to obtain a reasonably rapid approach to equilibrium at around 400 to 500 °C and 200 to 600 atm. The high pressures are needed to obtain a satisfactory equilibrium amount of NH3. If chemists and chemical engineers could identify a catalyst that leads to sufficiently rapid reaction at temperatures lower than 400 °C, it would be possible to obtain the same extent of equilibrium conversion at pressures much lower than 200 to 600 atm. This would result in great savings in the cost of the high-pressure equipment used in ammonia synthesis today. 

Figure 15.9 Effect of temperature and pressure on NH3 yield in the Haber process. Each... 

Discuss the effects of temperature, pressure, and catalysts on the Haber process for the production of ammonia. 

Research with an alkali-promoted (potassium or K2O) ruthenium catalyst has demonstrated that ammonia synthesis can be effected at lower temperatures and pressures than those required by the Haber process. As the price of energy increases, ruthenium catalysis might become increasingly important, because the energy-expensive compression process could be avoided. Another advantage of ruthenium if its diminished susceptibility to poisoning by H2O and CO. Ruthenium catalysts can carry out the direct synthesis of ammonia from N2, CO, and H2O ... 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

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 comparison to water photolysis [1,2] very little research has been directed at the photoreduction of carbon dioxide and at the photooxidation of carbon monoxide. There are several reasons why chemists should be interested in these two processes. CO2 is a natural and abundant raw material it is a major atmospheric pollutant, involved in the greenhouse effect which may ultimately affect the climate and the temperature of our planet [3]. CO is used in many important industrial processes e.g. carbonylation, hydroformylation, Fisher-Tropsch reactions, and it is one of the major contaminants of industrial gases produced during catalytic processes (e.g., Haber-Bosch synthesis of NH3[4]). There are also fundamental reasons for studying CO2 and CO activation. The former is an inert molecule with carbon in its highest oxidation state and therefore its activation is difficult to achieve. Carbon dioxide could either be reduced to... 

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