Heterogeneous catalysis
Encyclopedia
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 are solids and the great majority of reactants are gases or liquids. Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Heterogeneous catalysis has attracted Nobel prize
s for Fritz Haber
and Carl Bosch
in 1918, Irving Langmuir
in 1932, and Gerhard Ertl
in 2007.
to the catalyst surface and adsorb onto it, via the formation of chemical bonds. After reaction, the products desorb from the surface and diffuse away. Understanding the transport phenomena and surface chemistry such as dispersion
is important. If diffusion rates are not taken into account, the reaction rates for various reactions on surfaces
depend solely on the rate constants and reactant concentrations.
For solid heterogeneous catalysts, the surface area of the catalyst is critical since it determines the availability of catalytic sites. Surface areas can be large, for example some mesoporous silicates
have areas of 1000 m2/g. The most common approach to maximizing surface area is by the use of catalyst support
s, which are the materials over which the catalysts are spread.
The oxidation of hydrocarbons to water and carbon dioxide
. This process can occur with any of the hydrocarbons, but most commonly is performed with petrol or diesel.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, heterogeneous catalysis refers to the form of catalysis
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....
where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
, liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
, vs gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
, but also immiscible liquids, e.g. oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
and 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...
. The great majority of practical heterogeneous catalysts are solids and the great majority of reactants are gases or liquids. Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Heterogeneous catalysis has attracted Nobel prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
s for Fritz Haber
Fritz Haber
Fritz Haber was a German chemist, who received the Nobel Prize in Chemistry in 1918 for his development for synthesizing ammonia, important for fertilizers and explosives. Haber, along with Max Born, proposed the Born–Haber cycle as a method for evaluating the lattice energy of an ionic solid...
and Carl Bosch
Carl Bosch
Carl Bosch was a German chemist and engineer and Nobel laureate in chemistry. He was a pioneer in the field of high-pressure industrial chemistry and founder of IG Farben, at one point the world's largest chemical company....
in 1918, Irving Langmuir
Irving Langmuir
Irving Langmuir was an American chemist and physicist. His most noted publication was the famous 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N. Lewis's cubical atom theory and Walther Kossel's chemical bonding theory, he outlined his...
in 1932, and Gerhard Ertl
Gerhard Ertl
Gerhard Ertl is a German physicist and a Professor emeritus at the Department of Physical Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany...
in 2007.
Concepts
In heterogeneous catalysis, the reactants diffuseSurface diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters at solid material surfaces. The process can generally be thought of in terms of particles jumping between adjacent adsorption sites on a surface, as in figure 1...
to the catalyst surface and adsorb onto it, via the formation of chemical bonds. After reaction, the products desorb from the surface and diffuse away. Understanding the transport phenomena and surface chemistry such as dispersion
Dispersion (materials science)
In materials science, dispersion is the fraction of atoms of a material exposed to the surface. In general:where D is the dispersion, NS is the number of surface atoms and NT is the total number of atoms of the material...
is important. If diffusion rates are not taken into account, the reaction rates for various reactions on surfaces
Reactions on surfaces
By reactions on surfaces it is understood reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants...
depend solely on the rate constants and reactant concentrations.
For solid heterogeneous catalysts, the surface area of the catalyst is critical since it determines the availability of catalytic sites. Surface areas can be large, for example some mesoporous silicates
Mesoporous silicates
Mesoporous silicates are silicates with a special morphology.-Background:Porous inorganic solids have found great utility as catalysts and sorption media because of their large internal surface area, i.e. the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales...
have areas of 1000 m2/g. The most common approach to maximizing surface area is by the use of catalyst support
Catalyst support
In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed. The reactivity of heterogeneous catalysts occurs at the surface atoms. Consequently great effort is made to maximize the surface area of a catalyst by distributing it over...
s, which are the materials over which the catalysts are spread.
Classes of heterogeneous catalysts
Although the majority of heterogeneous catalysts are solids, many variations exist.| Reacting phases | Examples | Comment |
|---|---|---|
| solid + gas | Ammonia synthesis from N2 + H2 over iron catalysts | |
| solid + solution | hydrogenation of fatty acids with nickel | used for the production of margarine Margarine Margarine , as a generic term, can indicate any of a wide range of butter substitutes, typically composed of vegetable oils. In many parts of the world, the market share of margarine and spreads has overtaken that of butter... |
| immiscible liquid phases | hydroformylation Hydroformylation Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond... of propene |
catalyst in aqueous phase, reactants and products mainly in nonaqueous phase |
Examples
Many examples exist, the table emphasizes large-scale industrial processes, although diverse examples are known.| Process | Reactants, product(s) | Catalyst | Comment |
|---|---|---|---|
| Sulfuric acid synthesis (Contact process Contact process The contact process is the current method of producing sulphuric acid in the high concentrations needed for industrial processes. Platinum was formerly employed as a catalyst for the reaction, but as it is susceptible to poisoning by arsenic impurities in the sulfur feedstock, vanadium oxide is... ) |
SO2 + O2, SO3 | vanadium oxides | hydration Hydration reaction In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous... of SO3 gives H2SO4 |
| Ammonia synthesis (Haber-Bosch process) | N2 + H2, NH3 | iron oxides on alumina | consumes 1% of world's industrial energy budget |
| Nitric acid synthesis (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,... ) |
NH3 + O2, HNO3 | unsupported Pt-Rh gauze | direct routes from N2 are uneconomical |
| Hydrogen production by 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... |
CH4 + H2O, H2 + CO2 | Nickel on K2O | Greener routes to H2 by water splitting Water splitting Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy. Various techniques for water splitting have been issued in water splitting patents in... actively sought |
| Ethylene oxide Ethylene oxide Ethylene oxide, also called oxirane, is the organic compound with the formula . It is a cyclic ether. This means that it is composed of two alkyl groups attached to an oxygen atom in a cyclic shape . This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered... synthesis |
C2H4 + O2, C2H4O | silver Silver Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal... on alumina, with many promotors |
poorly applicable to other alkenes |
| Hydrogen cyanide synthesis (Andrussov oxidation) | NH3 + O2 + CH4, HCN | Pt-Rh | Related ammoxidation Ammoxidation In chemistry, ammoxidation is an industrial process for the production of nitriles using ammonia and oxygen. The usual substrates are alkenes. It is sometimes called the Sohio process, acknowledging that ammoxidation was commercialized by Standard Oil of Ohio... process converts hydrocarbons to nitrile Nitrile A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called... s |
| Olefin polymerization Ziegler-Natta polymerization | propylene Propylene Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and... , polypropylene Polypropylene Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes... |
TiCl3 on MgCl2 Magnesium chloride Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water... |
many variations exist, including some homogeneous examples 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:... |
| Desulfurization of petroleum (hydrodesulfurization Hydrodesulfurization Hydrodesulfurization is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils... ) |
H2 + R2S (idealized organosulfur impurity), RH + H2S | Mo Molybdenum Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores... -Co Cobalt Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.... on alumina |
produces low-sulfur hydrocarbons, sulfur recovered via the Claus process Claus process The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the scientist Carl Friedrich Claus, the Claus process has become the industry standard.... |
Other examples
- Reduction of nitrileNitrileA nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
s for instance in a synthesis of phenethylaminePhenethylaminePhenylethylamine or phenethylamine is a natural monoamine alkaloid, trace amine, and also the name of a class of chemicals with many members well known for psychoactive drug and stimulant effects. Studies suggest that phenylethylamine functions as a neuromodulator or neurotransmitter in the...
with Raney nickelRaney nickelRaney nickel is a solid catalyst composed of fine grains of a nickel-aluminium alloy, used in many industrial processes. It was developed in 1926 by American]] engineer Murray Raney as an alternative catalyst for the hydrogenation of vegetable oils in industrial processes...
and ammoniaAmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
:
-
- The cracking, isomerisationIsomerisationIn chemistry isomerisation is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms are rearranged e.g. A-B-C → B-A-C . In some molecules and under some conditions, isomerisation occurs spontaneously...
and re-forming of hydrocarbons to form appropriate and useful blends of petrol. - Catalytic converters are often used in automobiles. Three main reactions are catalysed by catalytic converters. The oxidation of carbon monoxideCarbon monoxideCarbon 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...
to carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
.
- The cracking, isomerisation
- 2CO(g) + O2(g) → 2CO2(g)
- The reductionRedoxRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of nitrogen monoxide back to nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
. - 2NO(g) + 2CO(g) → N2(g) + 2CO2(g)
The oxidation of hydrocarbons to water and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. This process can occur with any of the hydrocarbons, but most commonly is performed with petrol or diesel.
- 2 C6H6 + 15 O2 → 12 CO2 + 6 H2O
- Asymmetric heterogeneous catalysis affords enantiomerically enriched compounds using chiral heterogeneous catalysts.
- The vast majority of heterogeneous catalysts are based on metalMetalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s or metal oxidesOxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
, however, some chemical reactions can be catalyzed by carbonCarbocatalysisCarbocatalysis is a form of catalysis that uses heterogeneous carbon materials for the transformation or synthesis of organic or inorganic substrates. The catalysts are characterized by their high surface areas, surface functionality, and large, aromatic basal planes...
-based materials, e.g., oxidative dehydrogenationDehydrogenationDehydrogenation is a chemical reaction that involves the elimination of hydrogen . It is the reverse process of hydrogenation. Dehydrogenation reactions may be either large scale industrial processes or smaller scale laboratory procedures....
s or selective oxidations.
- EthylbenzeneEthylbenzeneEthylbenzene is an organic compound with the formula C6H5CH2CH3. This aromatic hydrocarbon is important in the petrochemical industry as an intermediate in the production of styrene, which in turn is used for making polystyrene, a common plastic material....
+ 1/2 O2 → Styrene + H2O - AcroleinAcroleinAcrolein is the simplest unsaturated aldehyde. It is produced widely but is most often immediately reacted with other products due to its instability and toxicity...
+ 1/2 O2 → Acrylic acidAcrylic acidAcrylic acid is an organic compound with the formula CH2=CHCO2H. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a characteristic acrid or tart smell. It is miscible with water, alcohols,...
See also
- Homogeneous catalysisHomogeneous catalysisIn 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:...
- Temperature-programmed reductionTemperature-programmed reductionTemperature-programmed reduction is a technique for the characterization of solid materials and is often used in the field of heterogeneous catalysis to find the most efficient reduction conditions, an oxidized catalyst precursor is submitted to a programmed temperature rise while a reducing gas...
- Thermal desorption spectroscopyThermal desorption spectroscopyThermal desorption spectroscopy , also known as temperature programmed desorption is a Buzzword for the method of observing desorbed molecules from a surface when the surface temperature is increased...