Industrial catalysts
Encyclopedia
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 used 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 catalysts in the industry today.
In the chemical industry and the industrial research, catalysis play an important role. The different catalysts are in constant development to fulfill economic, political and environmental demands. When using a catalyst it is possible to replace a polluting chemical reaction with a more environmental 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 the market development. If not a company’s catalyst is 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 the 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 consider 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 on an improvement can be to develop a catalyst with a longer life time than the previous version. Some of the advantages 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.
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:
CO + H2O ↔ H2 + CO2
The reaction refers to carbon monoxide
(CO) that reacts with water
(H2O) to from 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:
Kp=(pH2 x pCO2)/ (pCO x pH2O)
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 heterogeneously way is used commercially.
The first step in the WGS reaction is the high temperature shift which is carried out at temperatures between 320 °C and 450 °C. As mention before the catalyst is a composition of iron-oxide, Fe2O3(90-95%), and chromium oxides Cr2O3 (5-10%) which have an ideally activity and selectivity at these temperatures. When preparing this 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 life time 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 dominate phase in the catalyst consist of Cr3+ in α-Fe2O3 but the catalyst is still not active. To be active α-Fe2O3 must be reduced to Fe3O4 and CrO3 must be reduced to Cr2O3. 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 done a lot of research on, 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:
CO+(O) →CO2 + (*)
The vacant side is then reoxidized by water, and the oxide center is regenerated:
H2O+(*)→H2+ (O)
The adsorptive mechanism assumes that format species is produced when an adsorbed CO molecule reacts with a surface hydroxyl group:
H2O →OH(ads)+ H(ads)
The format decomposes then in the presence of steam:
COOH(ads)→CO2+H(ads)
2H(ads)→H2
The low temperature process is the second stage in the process, and is designed to take advantage of higher hydrogen equilibrium at low temperatures. The reaction is carried out between 200 °C and 250 °C, and the most commonly used catalyst is based on copper. While the HTS reactor used an iron-chrome based catalyst, the copper-catalyst is more active at lower temperatures thereby yielding a lower equilibrium concentration of CO and a higher equilibrium concentration of H2. The disadvantage with a copper catalysts is that it is very sensitive when it comes to sulfide poisoning, a future use of for example a cobalt- molybdenum catalyst could solve this problem. The catalyst mainly used in the industry today is a copper
-zinc
-alumina (Cu/ZnO/Al2O3) based catalyst.
Also the LTS catalyst has to be activated by reduction before it can be used. The reduction reaction CuO + H2 →Cu + H2O is highly exothermic and should be conducted in dry gas for an optimal result.
As for the HTS catalyst mechanism, two similar reaction mechanisms are suggested. The first mechanism that was proposed for the LTS reaction was a redox mechanism, but later evidence showed that the reaction can proceed via associated intermediates. The different intermediates that is suggested are: HOCO, HCO and HCOO. In 2009 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:
CO + (species derived from H2O) →Intermediate→CO2
Associative mechanism: CO2 produced from the reaction of CO with OH without the formation of an intermediate:
CO + OH →H + CO2
Redox mechanism: Water dissociation that yields surface oxygen atoms which react with CO to produce CO2:
H2O→O (surface)
O (surface) + CO → 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.
For both HTS catalyst and LTS catalyst the redox mechanism is the oldest theory and most published articles support this theory, but as technology has developed the adsorptive mechanism has become more of interest. One of the reasons to the fact that the literature is not agreeing on one mechanism can be because of experiments are carried out under different assumptions.
CO must be produced for the WGS reaction to take place. This can be done in different ways from a variety of carbon sources such as:
-passing steam over coal:
C + H2O = CO +H2
-steam reforming methane, over a nickel catalyst:
CH4 + H2O = CO +3H2
-or by using biomass
. Both the reactions shown above are highly exothermic 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 formated 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).
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:
CO+2H2→CH3OH (l)
CO2+3H2→CH3OH (l) +H2O (l)
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.:
H2→2H*
CO2*+ H*→HCOO*
HCOO*+3H*→CH3OH+ O*
CO+ O*→CO2
H2 + O*→H2O
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):
The reaction is given as:
CH3OH(l)+ H2O (l) →CO2+ 3H2 ΔH= +131 KJ/mol
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 roposed for the Cu/ZnO/Al2O3:
CH3OH+ H2O →CO2+ 3H2
CH3OH→CO+ 2H2
CO+ H2O →CO2+H2
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:
2CH3OH→CH3OCHO+ 2H2
CH3OCHO+H2O→HCOOH+CH3OH
HCOOC→CO2+H2
When methanol is almost completely converted CO is being produced as a secondary product via the reverse water-gas shift reaction.
The second way to produce hydrogen from methanol is by methanol decomposition:
CH3OH(l)→ CO + 2H2 ΔH= +128 KJ/mol
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 a production of by-product as dimethyl ether, methyl format, methane and water is 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 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 :
CH3OH(l) + 1/2 O2 → CO2 + 2H2 ΔH=-155 KJ/mol
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 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:
(s+p)CH3OH(l) +sH2O(l) + 1/2pO2→ (s+p)CO2 +(3s+2p)H2
s and p 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.
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 an promoted iron catalyst, where the promoters can be K2O (Potassium oxide), Al2O3 ( Aluminium oxide) and CaO (Calcium oxide ) and the basic catalytic material is Fe. The most common is to use fixed bed reactors for the synthesis catalyst.
The main ammonia reaction is given below:
N2+ 3H2↔ 2NH3
The produced ammonia can be used further in production of nitric acid via the Ostwald process
.
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
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 used 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 catalysts in the industry today.
In the chemical industry and the industrial research, catalysis play an important role. The different catalysts are in constant development to fulfill economic, political and environmental demands. When using a catalyst it is possible to replace a polluting chemical reaction with a more environmental 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 the market development. If not a company’s catalyst is 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 the 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 consider 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 on an improvement can be to develop a catalyst with a longer life time than the previous version. Some of the advantages 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
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...
, while other chemical industries use homogenous catalysis
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid 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 (WGS) reaction
The water gas shift reactionWater gas shift reaction
The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...
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:
CO + H2O ↔ H2 + CO2
The reaction refers to carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO) that reacts with water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
(H2O) to from carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H2). The reaction is exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
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:
Kp=(pH2 x pCO2)/ (pCO x pH2O)
- (3) Kp=e((4577.8K/T-4.22))
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 heterogeneously way is used commercially.
High temperature shift (HTS) catalyst
The first step in the WGS reaction is the high temperature shift which is carried out at temperatures between 320 °C and 450 °C. As mention before the catalyst is a composition of iron-oxide, Fe2O3(90-95%), and chromium oxides Cr2O3 (5-10%) which have an ideally activity and selectivity at these temperatures. When preparing this 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
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
of iron oxide. Sintering will decrease the active catalyst area, so by decreasing the sintering rate the life time 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 dominate phase in the catalyst consist of Cr3+ in α-Fe2O3 but the catalyst is still not active. To be active α-Fe2O3 must be reduced to Fe3O4 and CrO3 must be reduced to Cr2O3. 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 done a lot of research on, there is no final agreement on the kinetics/mechanism. Research has narrowed it down to two possible mechanisms: a regenerative redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
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:
CO+(O) →CO2 + (*)
The vacant side is then reoxidized by water, and the oxide center is regenerated:
H2O+(*)→H2+ (O)
The adsorptive mechanism assumes that format species is produced when an adsorbed CO molecule reacts with a surface hydroxyl group:
H2O →OH(ads)+ H(ads)
- (7) CO(ads)+ OH(ads)→COOH (ads)
The format decomposes then in the presence of steam:
COOH(ads)→CO2+H(ads)
2H(ads)→H2
Low temperature shift (LTS) catalyst
The low temperature process is the second stage in the process, and is designed to take advantage of higher hydrogen equilibrium at low temperatures. The reaction is carried out between 200 °C and 250 °C, and the most commonly used catalyst is based on copper. While the HTS reactor used an iron-chrome based catalyst, the copper-catalyst is more active at lower temperatures thereby yielding a lower equilibrium concentration of CO and a higher equilibrium concentration of H2. The disadvantage with a copper catalysts is that it is very sensitive when it comes to sulfide poisoning, a future use of for example a cobalt- molybdenum catalyst could solve this problem. The catalyst mainly used in the industry today is a copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
-zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
-alumina (Cu/ZnO/Al2O3) based catalyst.
Also the LTS catalyst has to be activated by reduction before it can be used. The reduction reaction CuO + H2 →Cu + H2O is highly exothermic and should be conducted in dry gas for an optimal result.
As for the HTS catalyst mechanism, two similar reaction mechanisms are suggested. The first mechanism that was proposed for the LTS reaction was a redox mechanism, but later evidence showed that the reaction can proceed via associated intermediates. The different intermediates that is suggested are: HOCO, HCO and HCOO. In 2009 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:
CO + (species derived from H2O) →Intermediate→CO2
Associative mechanism: CO2 produced from the reaction of CO with OH without the formation of an intermediate:
CO + OH →H + CO2
Redox mechanism: Water dissociation that yields surface oxygen atoms which react with CO to produce CO2:
H2O→O (surface)
O (surface) + CO → 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.
For both HTS catalyst and LTS catalyst the redox mechanism is the oldest theory and most published articles support this theory, but as technology has developed the adsorptive mechanism has become more of interest. One of the reasons to the fact that the literature is not agreeing on one mechanism can be because of experiments are carried out under different assumptions.
Carbon Monoxide
CO must be produced for the WGS reaction to take place. This can be done in different ways from a variety of carbon sources such as:
-passing steam over coal:
C + H2O = CO +H2
-steam reforming methane, over a nickel catalyst:
CH4 + H2O = CO +3H2
-or by using biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
. Both the reactions shown above are highly exothermic and can be coupled to an exothermic partial oxidation. The products of CO and H2 are known as syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
.
When dealing with a catalyst and CO, it is common to assume that the intermediate CO-Metal is formated 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
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
(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
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
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
BASF
BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
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
Imperial Chemical Industries
Imperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...
) 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:
CO+2H2→CH3OH (l)
CO2+3H2→CH3OH (l) +H2O (l)
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.:
- (18) CO2→CO2*
H2→2H*
CO2*+ H*→HCOO*
HCOO*+3H*→CH3OH+ O*
CO+ O*→CO2
H2 + O*→H2O
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:
CH3OH(l)+ H2O (l) →CO2+ 3H2 ΔH= +131 KJ/mol
Steam reforming
Steam reforming
Fossil fuel reforming is a method of producing hydrogen or other useful products from fossil fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to...
is a good source for production of hydrogen, but the reaction is endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
. 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 roposed for the Cu/ZnO/Al2O3:
CH3OH+ H2O →CO2+ 3H2
CH3OH→CO+ 2H2
CO+ H2O →CO2+H2
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:
2CH3OH→CH3OCHO+ 2H2
CH3OCHO+H2O→HCOOH+CH3OH
HCOOC→CO2+H2
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:
CH3OH(l)→ CO + 2H2 ΔH= +128 KJ/mol
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 a production of by-product as dimethyl ether, methyl format, methane and water is 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 :
CH3OH(l) + 1/2 O2 → CO2 + 2H2 ΔH=-155 KJ/mol
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:
(s+p)CH3OH(l) +sH2O(l) + 1/2pO2→ (s+p)CO2 +(3s+2p)H2
s and p 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 an 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 catalysisCatalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
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 an promoted iron catalyst, where the promoters can be K2O (Potassium oxide), Al2O3 ( Aluminium oxide) and CaO (Calcium oxide ) and the basic catalytic material is Fe. The most common is to use fixed bed reactors for the synthesis catalyst.
The main ammonia reaction is given below:
N2+ 3H2↔ 2NH3
The produced ammonia can be used further in production of nitric acid via the Ostwald process
Ostwald process
The Ostwald process is a chemical process for producing nitric acid, which was developed by Wilhelm Ostwald . It is a mainstay of the modern chemical industry. Historically and practically it is closely associated with the Haber process, which provides the requisite raw material,...
.