Heterogeneous catalysis

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Hydrogenation of ethene on a catalytic solid surface (1) Adsorption (2) Reaction (3) Desorption Hydrogenation on catalyst.svg
Hydrogenation of ethene on a catalytic solid surface (1) Adsorption (2) Reaction (3) Desorption

Heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reagents or products. [1] The process contrasts with homogeneous catalysis where the reagents, products and catalyst exist in the same phase. Phase distinguishes between not only solid, liquid, and gas components, but also immiscible mixtures (e.g., oil and water), or anywhere an interface is present.

Contents

Heterogeneous catalysis typically involves solid phase catalysts and gas phase reactants. [2] In this case, there is a cycle of molecular adsorption, reaction, and desorption occurring at the catalyst surface. Thermodynamics, mass transfer, and heat transfer influence the rate (kinetics) of reaction.

Heterogeneous catalysis is very important because it enables faster, large-scale production and the selective product formation. [3] Approximately 35% of the world's GDP is influenced by catalysis. [4] The production of 90% of chemicals (by volume) is assisted by solid catalysts. [2] The chemical and energy industries rely heavily on heterogeneous catalysis. For example, the Haber–Bosch process uses metal-based catalysts in the synthesis of ammonia, an important component in fertilizer; 144 million tons of ammonia were produced in 2016. [5]

Adsorption

Adsorption is an essential step in heterogeneous catalysis. Adsorption is the process by which a gas (or solution) phase molecule (the adsorbate) binds to solid (or liquid) surface atoms (the adsorbent). The reverse of adsorption is desorption, the adsorbate splitting from adsorbent. In a reaction facilitated by heterogeneous catalysis, the catalyst is the adsorbent and the reactants are the adsorbate.

Types of adsorption

Two types of adsorption are recognized: physisorption, weakly bound adsorption, and chemisorption, strongly bound adsorption. Many processes in heterogeneous catalysis lie between the two extremes. The Lennard-Jones model provides a basic framework for predicting molecular interactions as a function of atomic separation. [6]

Physisorption

In physisorption, a molecule becomes attracted to the surface atoms via van der Waals forces. These include dipole-dipole interactions, induced dipole interactions, and London dispersion forces. Note that no chemical bonds are formed between adsorbate and adsorbent, and their electronic states remain relatively unperturbed. Typical energies for physisorption are from 3 to 10 kcal/mol. [2] In heterogeneous catalysis, when a reactant molecule physisorbs to a catalyst, it is commonly said to be in a precursor state, an intermediate energy state before chemisorption, a more strongly bound adsorption. [6] From the precursor state, a molecule can either undergo chemisorption, desorption, or migration across the surface. [7] The nature of the precursor state can influence the reaction kinetics. [7]

Chemisorption

When a molecule approaches close enough to surface atoms such that their electron clouds overlap, chemisorption can occur. In chemisorption, the adsorbate and adsorbent share electrons signifying the formation of chemical bonds. Typical energies for chemisorption range from 20 to 100 kcal/mol. [2] Two cases of chemisorption are:

  • Molecular adsorption: the adsorbate remains intact. An example is alkene binding by platinum.
  • Dissociation adsorption: one or more bonds break concomitantly with adsorption. In this case, the barrier to dissociation affects the rate of adsorption. An example of this is the binding of H2 to a metal catalyst, where the H-H bond is broken upon adsorption.

Surface reactions

Reaction Coordinate. (A) Uncatalyzed (B) Catalyzed (C) Catalyzed with discrete intermediates (transition states) Catalytic reaction coordinate.jpg
Reaction Coordinate. (A) Uncatalyzed (B) Catalyzed (C) Catalyzed with discrete intermediates (transition states)

Most metal surface reactions occur by chain propagation in which catalytic intermediates are cyclically produced and consumed. [8] Two main mechanisms for surface reactions can be described for A + B → C. [2]

Most heterogeneously catalyzed reactions are described by the Langmuir–Hinshelwood model. [9]

In heterogeneous catalysis, reactants diffuse from the bulk fluid phase to adsorb to the catalyst surface. The adsorption site is not always an active catalyst site, so reactant molecules must migrate across the surface to an active site. At the active site, reactant molecules will react to form product molecule(s) by following a more energetically facile path through catalytic intermediates (see figure to the right). The product molecules then desorb from the surface and diffuse away. The catalyst itself remains intact and free to mediate further reactions. Transport phenomena such as heat and mass transfer, also play a role in the observed reaction rate.

Catalyst design

Zeolite structure. A common catalyst support material in hydrocracking. Also acts as a catalyst in hydrocarbon alkylation and isomerization. Zeolite-ZSM-5-3D-vdW.png
Zeolite structure. A common catalyst support material in hydrocracking. Also acts as a catalyst in hydrocarbon alkylation and isomerization.

Catalysts are not active towards reactants across their entire surface; only specific locations possess catalytic activity, called active sites. The surface area of a solid catalyst has a strong influence on the number of available active sites. In industrial practice, solid catalysts are often porous to maximize surface area, commonly achieving 50–400 m2/g. [2] Some mesoporous silicates, such as the MCM-41, have surface areas greater than 1000 m2/g. [10] Porous materials are cost effective due to their high surface area-to-mass ratio and enhanced catalytic activity.

In many cases, a solid catalyst is dispersed on a supporting material to increase surface area (spread the number of active sites) and provide stability. [2] Usually catalyst supports are inert, high melting point materials, but they can also be catalytic themselves. Most catalyst supports are porous (frequently carbon, silica, zeolite, or alumina-based) [4] and chosen for their high surface area-to-mass ratio. For a given reaction, porous supports must be selected such that reactants and products can enter and exit the material.

Often, substances are intentionally added to the reaction feed or on the catalyst to influence catalytic activity, selectivity, and/or stability. These compounds are called promoters. For example, alumina (Al2O3) is added during ammonia synthesis to providing greater stability by slowing sintering processes on the Fe-catalyst. [2]

Sabatier principle can be considered one of the cornerstones of modern theory of catalysis. [11] Sabatier principle states that the surface-adsorbates interaction has to be an optimal amount: not too weak to be inert toward the reactants and not too strong to poison the surface and avoid desorption of the products. [12] The statement that the surface-adsorbate interaction has to be an optimum, is a qualitative one. Usually the number of adsorbates and transition states associated with a chemical reaction is a large number, thus the optimum has to be found in a many-dimensional space. Catalyst design in such a many-dimensional space is not a computationally viable task. Additionally, such optimization process would be far from intuitive. Scaling relations are used to decrease the dimensionality of the space of catalyst design. [13] Such relations are correlations among adsorbates binding energies (or among adsorbate binding energies and transition states also known as BEP relations) [14] that are "similar enough" e.g., OH versus OOH scaling. [15] Applying scaling relations to the catalyst design problems greatly reduces the space dimensionality (sometimes to as small as 1 or 2). [16] One can also use micro-kinetic modeling based on such scaling relations to take into account the kinetics associated with adsorption, reaction and desorption of molecules under specific pressure or temperature conditions. [17] Such modeling then leads to well-known volcano-plots at which the optimum qualitatively described by the Sabatier principle is referred to as the "top of the volcano". Scaling relations can be used not only to connect the energetics of radical surface-adsorbed groups (e.g., O*,OH*), [13] but also to connect the energetics of closed-shell molecules among each other or to the counterpart radical adsorbates. [18] A recent challenge for researchers in catalytic sciences is to "break" the scaling relations. [19] The correlations which are manifested in the scaling relations confine the catalyst design space, preventing one from reaching the "top of the volcano". Breaking scaling relations can refer to either designing surfaces or motifs that do not follow a scaling relation, or ones that follow a different scaling relation (than the usual relation for the associated adsorbates) in the right direction: one that can get us closer to the top of the reactivity volcano. [16] In addition to studying catalytic reactivity, scaling relations can be used to study and screen materials for selectivity toward a special product. [20] There are special combination of binding energies that favor specific products over the others. Sometimes a set of binding energies that can change the selectivity toward a specific product "scale" with each other, thus to improve the selectivity one has to break some scaling relations; an example of this is the scaling between methane and methanol oxidative activation energies that leads to the lack of selectivity in direct conversion of methane to methanol. [21]

Catalyst deactivation

Catalyst deactivation is defined as a loss in catalytic activity and/or selectivity over time.

Substances that decrease reaction rate are called poisons. Poisons chemisorb to catalyst surface and reduce the number of available active sites for reactant molecules to bind to. [22] Common poisons include Group V, VI, and VII elements (e.g. S, O, P, Cl), some toxic metals (e.g. As, Pb), and adsorbing species with multiple bonds (e.g. CO, unsaturated hydrocarbons). [6] [22] For example, sulfur disrupts the production of methanol by poisoning the Cu/ZnO catalyst. [23] Substances that increase reaction rate are called promoters. For example, the presence of alkali metals in ammonia synthesis increases the rate of N2 dissociation. [23]

The presence of poisons and promoters can alter the activation energy of the rate-limiting step and affect a catalyst's selectivity for the formation of certain products. Depending on the amount, a substance can be favorable or unfavorable for a chemical process. For example, in the production of ethylene, a small amount of chemisorbed chlorine will act as a promoter by improving Ag-catalyst selectivity towards ethylene over CO2, while too much chlorine will act as a poison. [6]

Other mechanisms for catalyst deactivation include:

In industry, catalyst deactivation costs billions every year due to process shutdown and catalyst replacement. [22]

Industrial examples

In industry, many design variables must be considered including reactor and catalyst design across multiple scales ranging from the subnanometer to tens of meters. The conventional heterogeneous catalysis reactors include batch, continuous, and fluidized-bed reactors, while more recent setups include fixed-bed, microchannel, and multi-functional reactors. [6] Other variables to consider are reactor dimensions, surface area, catalyst type, catalyst support, as well as reactor operating conditions such as temperature, pressure, and reactant concentrations.

Schematic representation of a heterogeneous catalytic system from the subnanometer to industrial scale. Heterrogenous catalysis across scales.png
Schematic representation of a heterogeneous catalytic system from the subnanometer to industrial scale.

Some large-scale industrial processes incorporating heterogeneous catalysts are listed below. [4]

ProcessReactants, Product/s (not balanced)CatalystComment
Sulfuric acid synthesis (Contact process)SO2 + O2, SO3vanadium oxidesHydration of SO3 gives H2SO4
Ammonia synthesis (Haber–Bosch process)N2 + H2, NH3iron oxides on alumina(Al2O3)Consumes 1% of world's industrial energy budget [2]
Nitric acid synthesis (Ostwald process)NH3 + O2, HNO3unsupported Pt-Rh gauzeDirect routes from N2 are uneconomical
Hydrogen production by Steam reforming CH4 + H2O, H2 + CO2Nickel or K2OGreener routes to H2 by water splitting actively sought
Ethylene oxide synthesisC2H4 + O2, C2H4O silver on alumina, with many promotersPoorly applicable to other alkenes
Hydrogen cyanide synthesis (Andrussov oxidation)NH3 + O2 + CH4, HCNPt-RhRelated ammoxidation process converts hydrocarbons to nitriles
Olefin polymerization Ziegler–Natta polymerization propylene, polypropylene TiCl3 on MgCl2 Many variations exist, including some homogeneous examples
Desulfurization of petroleum (hydrodesulfurization)H2 + R2S (idealized organosulfur impurity), RH + H2S Mo-Co on aluminaProduces low-sulfur hydrocarbons, sulfur recovered via the Claus process
Process flow diagram illustrating the use of catalysis in the synthesis of ammonia (NH3) Haber-Bosch-En.svg
Process flow diagram illustrating the use of catalysis in the synthesis of ammonia (NH3)

Other examples

Solid-Liquid and Liquid-Liquid Catalyzed Reactions

Although the majority of heterogeneous catalysts are solids, there are a few variations which are of practical value. For two immiscible solutions (liquids), one carries the catalyst while the other carries the reactant. This set up is the basis of biphasic catalysis as implemented in the industrial production of butyraldehyde by the hydroformylation of propylene. [31]

Reacting phasesExamples givenComment
solid + solutionhydrogenation of fatty acids with nickelused for the production of margarine
immiscible liquid phases hydroformylation of propene aqueous phase catalyst; reactants and products mainly in non-aqueous phase

See also

Related Research Articles

<span class="mw-page-title-main">Catalysis</span> Process of increasing the rate of a chemical reaction

Catalysis is the increase in rate of a chemical reaction due to an added substance known as a catalyst. Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.

<span class="mw-page-title-main">Haber process</span> Industrial process for ammonia production

The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia. It converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using an iron metal catalyst:

<span class="mw-page-title-main">Surface science</span> Study of physical and chemical phenomena that occur at the interface of two phases

Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid–gas interfaces. It includes the fields of surface chemistry and surface physics. Some related practical applications are classed as surface engineering. The science encompasses concepts such as heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, and adhesives. Surface science is closely related to interface and colloid science. Interfacial chemistry and physics are common subjects for both. The methods are different. In addition, interface and colloid science studies macroscopic phenomena that occur in heterogeneous systems due to peculiarities of interfaces.

Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated at the adsorbent surface. Examples include macroscopic phenomena that can be very obvious, like corrosion, and subtler effects associated with heterogeneous catalysis, where the catalyst and reactants are in different phases. The strong interaction between the adsorbate and the substrate surface creates new types of electronic bonds.

<span class="mw-page-title-main">Adsorption</span> Phenomenon of surface adhesion

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

<span class="mw-page-title-main">Hydrogenation</span> Chemical reaction between molecular hydrogen and another compound or element

Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons.

Desorption is the physical process where adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding energy that keep it attached to the surface.

Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis. Via scanning tunneling microscopy, it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the solid–gas interface are in some cases related to catalysis.

Temporal Analysis of Products (TAP), (TAP-2), (TAP-3) is an experimental technique for studying the kinetics of physico-chemical interactions between gases and complex solid materials, primarily heterogeneous catalysts. The TAP methodology is based on short pulse-response experiments at low background pressure (10−6-102 Pa), which are used to probe different steps in a catalytic process on the surface of a porous material including diffusion, adsorption, surface reactions, and desorption.

<span class="mw-page-title-main">Sabatier principle</span>

The Sabatier principle is a qualitative concept in chemical heterogeneous catalysis named after the French chemist Paul Sabatier. It states that the interactions between the catalyst and the reactants should be "just right"; that is, neither too strong nor too weak. If the interaction is too weak, the molecule will fail to bind to the catalyst and no reaction will take place. On the other hand, if the interaction is too strong, the product fails to dissociate.

<span class="mw-page-title-main">Catalyst support</span> Porous material with a high specific surface area supporting a catalyst

In chemistry, a catalyst support is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the specific surface area of a catalyst. One popular method for increasing surface area involves distributing the catalyst over the surface of the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of activated carbon, alumina, and silica.

Transition metal oxides are compounds composed of oxygen atoms bound to transition metals. They are commonly utilized for their catalytic activity and semiconducting properties. Transition metal oxides are also frequently used as pigments in paints and plastics, most notably titanium dioxide. Transition metal oxides have a wide variety of surface structures which affect the surface energy of these compounds and influence their chemical properties. The relative acidity and basicity of the atoms present on the surface of metal oxides are also affected by the coordination of the metal cation and oxygen anion, which alter the catalytic properties of these compounds. For this reason, structural defects in transition metal oxides greatly influence their catalytic properties. The acidic and basic sites on the surface of metal oxides are commonly characterized via infrared spectroscopy, calorimetry among other techniques. Transition metal oxides can also undergo photo-assisted adsorption and desorption that alter their electrical conductivity. One of the more researched properties of these compounds is their response to electromagnetic radiation, which makes them useful catalysts for redox reactions, isotope exchange and specialized surfaces.

<span class="mw-page-title-main">Jens Nørskov</span> Danish physicist

Jens Kehlet Nørskov is the Villum Kann Rasmussen professor at the Technical University of Denmark. He is a Danish physicist most notable for his work on theoretical description of surfaces, catalysis, materials, nanostructures, and biomolecules.

Operando spectroscopy is an analytical methodology wherein the spectroscopic characterization of materials undergoing reaction is coupled simultaneously with measurement of catalytic activity and selectivity. The primary concern of this methodology is to establish structure-reactivity/selectivity relationships of catalysts and thereby yield information about mechanisms. Other uses include those in engineering improvements to existing catalytic materials and processes and in developing new ones.

<span class="mw-page-title-main">Heterogeneous catalytic reactor</span> Chemical reactor

Heterogeneous catalytic reactors put emphasis on catalyst effectiveness factors and the heat and mass transfer implications. Heterogeneous catalytic reactors are among the most commonly utilized chemical reactors in the chemical engineering industry.

The electrochemical promotion of catalysis (EPOC) effect in the realm of chemistry refers to the pronounced enhancement of catalytic reactions or significant changes in the catalytic properties of a conductive catalyst in the presence of electrical currents or interfacial potentials. Also known as Non-faradaic electrochemical modification of catalytic activity (the NEMCA effect), it can increase in catalytic activity (up to 90-fold) and selectivity of a gas exposed electrode on a solid electrolyte cell upon application of a potential. This phenomenon is well documented and has been observed on various surfaces (Ni, Au, Pt, Pd, IrO2, RuO2) supported by O2−, Na+ and proton conducting solid electrolytes.

<span class="mw-page-title-main">Heterogeneous gold catalysis</span>

Heterogeneous gold catalysis refers to the use of elemental gold as a heterogeneous catalyst. As in most heterogeneous catalysis, the metal is typically supported on metal oxide. Furthermore, as seen in other heterogeneous catalysts, activity increases with a decreasing diameter of supported gold clusters. Several industrially relevant processes are also observed such as H2 activation, Water-gas shift reaction, and hydrogenation. One or two gold-catalyzed reactions may have been commercialized.

In chemistry, catalytic resonance theory was developed to describe the kinetics of reaction acceleration using dynamic catalyst surfaces. Catalytic reactions occur on surfaces that undergo variation in surface binding energy and/or entropy, exhibiting overall increase in reaction rate when the surface binding energy frequencies are comparable to the natural frequencies of the surface reaction, adsorption, and desorption.

<span class="mw-page-title-main">Philippe Sautet</span> French chemist (born 1961)

Philippe Sautet is a French chemist. He was elected to the French Academy of sciences on 30 November 2010. He was a research director at the CNRS and works in the chemistry laboratory of the École normale supérieure de Lyon where he devoted a large part of his scientific activity to molecular modelling. Now he is a professor at the University of California - Los Angeles.

Dissociative adsorption is a process in which a molecule adsorbs onto a surface and simultaneously dissociates into two or more fragments. This process is the basis of many applications, particularly in heterogeneous catalysis reactions. The dissociation involves cleaving of the molecular bonds in the adsorbate, and formation of new bonds with the substrate.

References

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