Industrial catalysts

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. 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 containing metallic copper nanoparticles. there are in total three mechanisms that are proposed for the water-gas shift reaction over Cu(111), given below. Intermediate mechanism (usually called associative mechanism): An intermediate is first formed and then decomposes into the final products: Associative mechanism: CO2 produced from the reaction of CO with OH without the formation of an intermediate: Redox mechanism: Water dissociation that yields surface oxygen atoms which react with CO to produce CO2: It is not said that just one of these mechanisms is controlling the reaction, it is possible that several of them are active. Q.-L. Tang et al. has suggested that the most favorable mechanism is the intermediate mechanism (with HOCO as intermediate) followed by the redox mechanism with the rate determining step being the water dissociation. • passing steam over coal: • steam reforming methane, over a nickel catalyst: • or by using biomass. Both the reactions shown above are highly endothermic and can be coupled to an exothermic partial oxidation. The products of CO and H2 are known as syngas. When dealing with a catalyst and CO, it is common to assume that the intermediate CO-Metal is formed before the intermediate reacts further into the products. When designing a catalyst this is important to remember. The strength of interaction between the CO molecule and the metal should be strong enough to provide a sufficient concentration of the intermediate, but not so strong that the reaction will not continue. CO is a common molecule to use in a catalytic reaction, and when it interacts with a metal surface it is actually the molecular orbitals of CO that interacts with the d-band of the metal surface. When considering a molecular orbital(MO)-diagram CO can act as an σ-donor via the lone pair of the electrons on C, and a π-acceptor ligand in transition metal complexes. When a CO molecule is adsorbed on a metal surface, the d-band of the metal will interact with the molecular orbitals of CO. It is possible to look at a simplified picture, and only consider the LUMO (2π*) and HOMO (5σ) to CO. The overall effect of the σ-donation and the π- back donation is that a strong bond between C and the metal is being formed and in addition the bond between C and O will be weakened. The latter effect is due to charge depletion of the CO 5σ bonding and charge increase of the CO 2π* antibonding orbital. When looking at chemical surfaces, many researchers seems to agree on that the surface of the Cu/Al2O3/ZnO is most similar to the Cu(111) surface. Since copper is the main catalyst and the active phase in the LTS catalyst, many experiments has been done with copper. In the figure given here experiments has been done on Cu(110) and Cu(111). The figure shows Arrhenius plot derived from reaction rates. It can be seen from the figure that Cu(110) shows a faster reaction rate and a lower activation energy. This can be due to the fact that Cu(111) is more closely packed than Cu(110). ==Methanol production==

Production of methanol is an important industry today and methanol is one of the largest volume carbonylation products. The process uses syngas as feedstock and for that reason the water gas shift reaction is important for this synthesis. The most important reaction based on methanol is the decomposition of methanol to yield carbon monoxide and hydrogen. Methanol is therefore an important raw material for production of CO and H2 that can be used in generation of fuel. BASF was the first company (in 1923) to produce methanol on large-scale, then using a sulfur-resistant ZnO/Cr2O3 catalyst. The feed gas was produced by gasification over coal. Today the synthesis gas is usually manufactured via steam reforming of natural gas. The most effective catalysts for methanol synthesis are Cu, Ni, Pd and Pt, while the most common metals used for support are Al and Si. In 1966 ICI (Imperial Chemical Industries) developed a process that is still in use today. The process is a low-pressure process that uses a Cu/ZnO/Al2O3 catalyst where copper is the active material. This catalyst is actually the same that the low-temperature shift catalyst in the WGS reaction is using. The reaction described below is carried out at 250 °C and 5-10 MPa: Both of these reactions are exothermic and proceeds with volume contraction. Maximum yield of methanol is therefore obtained at low temperatures and high pressure and with use of a catalyst that has a high activity at these conditions. A catalyst with sufficiently high activity at the low temperature does still not exist, and this is one of the main reasons that companies keep doing research and catalyst development. A reaction mechanism for methanol synthesis has been suggested by Chinchen et al.: Today there are four different ways to catalytically obtain hydrogen production from methanol, and all reactions can be carried out by using a transition metal catalyst (Cu, Pd): Steam reforming The reaction is given as: Steam reforming is a good source for production of hydrogen, but the reaction is endothermic. The reaction can be carried out over a copper-based catalyst, but the reaction mechanism is dependent on the catalyst. For a copper-based catalyst two different reaction mechanisms have been proposed, a decomposition-water-gas shift sequence and a mechanism that proceeds via methanol dehydrogenation to methyl formate. The first mechanism aims at methanol decomposition followed by the WGS reaction and has been proposed for the Cu/ZnO/Al2O3: The mechanism for the methyl format reaction can be dependent of the composition of the catalyst. The following mechanism has been proposed over Cu/ZnO/Al2O3: When methanol is almost completely converted CO is being produced as a secondary product via the reverse water-gas shift reaction. Methanol decomposition The second way to produce hydrogen from methanol is by methanol decomposition: As the enthalpy shows, the reaction is endothermic and this can be taken further advantage of in the industry. This reaction is the opposite of the methanol synthesis from syngas, and the most effective catalysts seems to be Cu, Ni, Pd and Pt as mentioned before. Often, a Cu/ZnO-based catalyst is used at temperatures between 200 and 300 °C but by-products of production like dimethyl ether, methyl format, methane and water are common. The reaction mechanism is not fully understood and there are two possible mechanism proposed (2002) : one producing CO2 and H2 by decomposition of formate intermediates and the other producing CO and H2 via a methyl formate intermediate. Partial oxidation Partial oxidation is a third way for producing hydrogen from methanol. The reaction is given below, and is often carried out with air or oxygen as oxidant : The reaction is exothermic and has, under favorable conditions, a higher reaction rate than steam reforming. The catalyst used is often Cu (Cu/ZnO) or Pd and they differ in qualities such as by-product formation, product distribution and the effect of oxygen partial pressure. Combined reforming Combined reforming is a combination of partial oxidation and steam reforming and is the last reaction that is used for hydrogen production. The general equation is given below: {{NumBlk|:|(s+p)\ce{CH3OH(l)} + s\ce{H2O(l)} + \ce{1/2\mathit{p}O2 ->}\ (s+p)\ce{CO2} + (3s+2p)\ce{H2}|}} and are the stoichiometric coefficients for steam reforming and partial oxidation, respectively. The reaction can be both endothermic and exothermic determined by the conditions, and combine both the advantages of steam reforming and partial oxidation. == Ammonia synthesis ==

Ammonia synthesis was discovered by Fritz Haber, by using iron catalysts. The ammonia synthesis advanced between 1909 and 1913, and two important concepts were developed; the benefits of a promoter and the poisoning effect (see catalysis for more details). Ammonia production was one of the first commercial processes that required the production of hydrogen, and the cheapest and best way to obtain hydrogen was via the water-gas shift reaction. The Haber–Bosch process is the most common process used in the ammonia industry. A lot of research has been done on the catalyst used in the ammonia process, but the main catalyst that is used today is not that dissimilar to the one that was first developed. The catalyst the industry use is a promoted iron catalyst, where the promoters can be K2O (potassium oxide), Al2O3 (aluminium oxide) and CaO (calcium oxide) and the basic catalytic material is iron. The most common is to use fixed bed reactors for the synthesis catalyst. The main ammonia reaction is given below: The produced ammonia can be used further in production of nitric acid via the Ostwald process. == See also ==