The first time a catalyst was used in the industry was in 1746 by J. Roebuck in the manufacture of lead chamber sulfuric acid. Since then catalysts have been in use in a large portion of the chemical industry. In the start only pure components were used as catalysts, but after the year 1900 multicomponent catalysts were studied and are now commonly used in the industry.
In the chemical industry and industrial research, catalysis play an important role. Different catalysts are in constant development to fulfil economic, political and environmental demands. When using a catalyst, it is possible to replace a polluting chemical reaction with a more environmentally friendly alternative. Today, and in the future, this can be vital for the chemical industry. In addition, it's important for a company/researcher to pay attention to market development. If a company's catalyst is not continually improved, another company can make progress in research on that particular catalyst and gain market share. For a company, a new and improved catalyst can be a huge advantage for a competitive manufacturing cost. It's extremely expensive for a company to shut down the plant because of an error in the catalyst, so the correct selection of a catalyst or a new improvement can be key to industrial success. To achieve the best understanding and development of a catalyst it is important that different special fields work together. These fields can be: organic chemistry, analytic chemistry, inorganic chemistry, chemical engineers and surface chemistry. The economics must also be taken into account. One of the issues that must be considered is if the company should use money on doing the catalyst research themselves or buy the technology from someone else. As the analytical tools are becoming more advanced, the catalysts used in the industry are improving. One example of an improvement can be to develop a catalyst with a longer lifetime than the previous version. Some of the advantages an improved catalyst gives, that affects people's lives, are: cheaper and more effective fuel, new drugs and medications and new polymers. Some of the large chemical processes that use catalysis today are the production of methanol and ammonia. Both methanol and ammonia synthesis take advantage of the water-gas shift reaction and heterogeneous catalysis, while other chemical industries use homogenous catalysis. If the catalyst exists in the same phase as the reactants it is said to be homogenous; otherwise it is heterogeneous.
Water gas shift reaction The water gas shift reaction was first used industrially at the beginning of the 20th century. Today the WGS reaction is used primarily to produce hydrogen that can be used for further production of methanol and ammonia.
WGS reaction
The reaction refers to carbon monoxide (CO) that reacts with water (H2O) to form carbon dioxide (CO2) and hydrogen (H2). The reaction is exothermic with ΔH= -41.1 kJ/mol and have an adiabatic temperature rise of 8–10 °C per percent CO converted to CO2 and H2. The most common catalysts used in the water-gas shift reaction are the high temperature shift (HTS) catalyst and the low temperature shift (LTS) catalyst. The HTS catalyst consists of iron oxide stabilized by chromium oxide, while the LTS catalyst is based on copper. The main purpose of the LTS catalyst is to reduce CO content in the reformate which is especially important in the ammonia production for high yield of H2. Both catalysts are necessary for thermal stability, since using the LTS reactor alone increases exit-stream temperatures to unacceptable levels. The equilibrium constant for the reaction is given as:
Low temperatures will therefore shift the reaction to the right, and more products will be produced. The equilibrium constant is extremely dependent on the reaction temperature, for example is the Kp equal to 228 at 200 °C, but only 11.8 at 400 °C. The WGS reaction can be performed both homogenously and heterogeneously, but only the heterogeneous method is used commercially.
High temperature shift (HTS) catalyst
The first step in the WGS reaction is the high temperature shift which is carried out between 350 °C and 450 °C using a catalyst that has an ideal activity and selectivity at these temperatures. The freshly prepared catalyst contains iron(III) oxide Fe2O3, chromium(III) and (VI) oxides Cr2O3 and CrO3 (8-14 wt%), and copper(II) oxide CuO. When preparing the catalyst, one of the most important step is washing to remove sulfate that can turn into hydrogen sulfide and poison the LTS catalyst later in the process. Chromium is added to the catalyst to stabilize the catalyst activity over time and to delay sintering of iron oxide. Sintering will decrease the active catalyst area, so by decreasing the sintering rate the lifetime of the catalyst will be extended. The catalyst is usually used in pellets form, and the size play an important role. Large pellets will be strong, but the reaction rate will be limited. In the end, the dominant phase in the catalyst consist of Cr3+ in α-Fe2O3 but the catalyst is still not active. To be active, the catalyst must undergo reduction in situ to Cr(III)-doped Fe3O4 containing metallic copper nanoparticles. This usually happens in the reactor start-up phase and because the reduction reactions are exothermic the reduction should happen under controlled circumstances. The lifetime of the iron-chrome catalyst is approximately 3–5 years, depending on how the catalyst is handled. Even though the mechanism for the HTS catalyst has been researched extensively, there is no final agreement on the kinetics/mechanism. Research has narrowed it down to two possible mechanisms: a regenerative redox mechanism and an adsorptive(associative) mechanism. The redox mechanism is given below: First a CO molecule reduces an O molecule, yielding CO2 and a vacant surface center:
The vacant side is then reoxidized by water, and the oxide center is regenerated:
The adsorptive mechanism assumes that format species is produced when an adsorbed CO molecule reacts with a surface hydroxyl group:
The format decomposes then in the presence of steam:
Low temperature shift (LTS) catalyst
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