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Heterogeneous catalysis
Heterogeneous catalysis
Heterogeneous catalysis
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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.

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

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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

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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

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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

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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

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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]

  • Langmuir–Hinshelwood mechanism: The reactant molecules, A and B, both adsorb to the catalytic surface. While adsorbed to the surface, they combine to form product C, which then desorbs.
  • Eley–Rideal mechanism: One reactant molecule, A, adsorbs to the catalytic surface. Without adsorbing, B reacts with absorbed A to form C, that then desorbs from the surface.

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

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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

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

Substances that decrease the reaction rate are called poisons. Poisons chemisorb to the 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:

  • Sintering: when heated, dispersed catalytic metal particles can migrate across the support surface and form crystals. This results in a reduction of catalyst surface area.
  • Fouling: the deposition of materials from the fluid phase onto the solid phase catalyst and/or support surfaces. This results in active site and/or pore blockage.
  • Coking: the deposition of heavy, carbon-rich solids onto surfaces due to the decomposition of hydrocarbons[22]
  • Vapor-solid reactions: formation of an inactive surface layer and/or formation of a volatile compound that exits the reactor.[22] This results in a loss of surface area and/or catalyst material.
  • Solid-state transformation: solid-state diffusion of catalyst support atoms to the surface followed by a reaction that forms an inactive phase. This results in a loss of catalyst surface area.
  • Erosion: continual attrition of catalyst material common in fluidized-bed reactors.[24] This results in a loss of catalyst material.

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

Industrial examples

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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.

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

Process Reactants, Product/s (not balanced) Catalyst Comment
Sulfuric acid synthesis (Contact process) SO2 + O2, SO3 vanadium oxides Hydration of SO3 gives H2SO4
Ammonia synthesis (Haber–Bosch process) N2 + H2, NH3 iron oxides on alumina(Al2O3) Consumes 1% of world's industrial energy budget[2]
Nitric acid synthesis (Ostwald process) NH3 + O2, HNO3 unsupported Pt-Rh gauze Direct routes from N2 are uneconomical
Hydrogen production by Steam reforming CH4 + H2O, H2 + CO2 Nickel or K2O Greener routes to H2 by water splitting actively sought
Ethylene oxide synthesis C2H4 + O2, C2H4O silver on alumina, with many promoters Poorly applicable to other alkenes
Hydrogen cyanide synthesis (Andrussov oxidation) NH3 + O2 + CH4, HCN Pt-Rh Related 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 alumina Produces low-sulfur hydrocarbons, sulfur recovered via the Claus process
Process flow diagram illustrating the use of catalysis in the synthesis of ammonia (NH3)

Other examples

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Solid-Liquid and Liquid-Liquid Catalyzed Reactions

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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 phases Examples given Comment
solid + solution hydrogenation of fatty acids with nickel used for the production of margarine
immiscible liquid phases hydroformylation of propene aqueous phase catalyst; reactants and products mainly in non-aqueous phase

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Heterogeneous catalysis

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Heterogeneous catalysis is a chemical process in which the catalyst operates in a distinct phase from the reactants and products, most commonly involving a solid catalyst that facilitates reactions occurring in the gaseous or liquid phase. This surface-mediated phenomenon relies on the adsorption of reactant molecules onto the catalyst's active sites, where bond breaking and formation occur, followed by desorption of products, enabling the catalyst's regeneration and reuse. Catalysts in this context typically consist of an active metal or component dispersed on a high-surface-area support material, often enhanced by promoters to improve selectivity and stability. Heterogeneous catalysis dominates industrial chemical production, accounting for over 85% of all manufactured chemical products worldwide through its role in enabling efficient, large-scale transformations. It offers advantages such as straightforward separation of the catalyst from reaction mixtures, facilitating continuous processing and reducing operational costs compared to homogeneous alternatives. Key challenges include catalyst deactivation due to poisoning, , or , which necessitates ongoing research into robust designs, including nanostructured and single-atom catalysts. Prominent industrial applications underscore its economic impact: the Haber-Bosch process synthesizes from and using iron-based catalysts at high pressure and temperature, supporting global production and . The employs vanadium pentoxide catalysts to oxidize to , enabling efficient manufacturing essential for fertilizers, dyes, and detergents. In petroleum refining, uses zeolite-based catalysts to convert heavy hydrocarbons into and olefins, contributing to roughly half of global output. Emerging uses extend to , such as automotive exhaust converters that abate pollutants via platinum-group metals.

Fundamentals

Definition and Scope

Heterogeneous catalysis refers to a process in which the catalyst exists in a different phase from the reactants and products, most commonly involving a solid catalyst interacting with gaseous or liquid reactants. This phase separation facilitates the reaction at the interface between the catalyst surface and the reactants, enabling efficient chemical transformations without the catalyst dissolving into the reaction mixture. The key principles of heterogeneous catalysis revolve around surface-mediated reactions, where reactants adsorb onto specific active sites on the catalyst surface, lowering the and thereby enhancing reaction rates while leaving the unchanged. These active sites—often defects, edges, or specific atomic arrangements on the catalyst—serve as localized regions where bonds in the reactants are weakened and reformed into products. Adsorption typically initiates the process, allowing reactants to interact closely with the catalyst surface before proceeding to reaction and desorption steps. The scope of heterogeneous catalysis encompasses gas-solid systems, such as ammonia synthesis, liquid-solid interactions like in solution, and more complex multiphase setups in industrial reactors. It is distinctly differentiated from , where the catalyst shares the same phase (e.g., all in solution) with the reactants, and from biocatalysis, which relies on enzymes or biological systems typically operating in aqueous environments. A basic schematic of the catalyst-reactant interface illustrates reactants approaching and binding to the solid surface:

Reactants (gas/liquid) → Adsorption on active sites → Surface reaction → Desorption → Products ↑ Solid Catalyst Surface

Reactants (gas/liquid) → Adsorption on active sites → Surface reaction → Desorption → Products ↑ Solid Catalyst Surface

This framework highlights the spatial confinement of the reaction to the catalyst's exterior or porous interior. Heterogeneous catalysis underpins approximately 90% of industrial chemical processes by volume, playing a crucial role in energy production (e.g., fuel reforming), chemical (e.g., ), and environmental control (e.g., catalytic converters for emission reduction). Its widespread adoption stems from the ease of catalyst recovery and reuse, contributing to and in large-scale operations.

Historical Development

The origins of heterogeneous catalysis trace back to the early , when initial observations highlighted the ability of metal surfaces to accelerate chemical reactions without being consumed. In 1817, reported that a hot platinum wire inserted into a mixture of and air induced at the wire's surface, producing heat without a visible , marking one of the first documented catalytic effects in gas-phase reactions. This discovery laid foundational insights into surface-mediated processes. During the 1850s, Henri Sainte-Claire Deville expanded on such phenomena through systematic studies of 's catalytic role in oxidation and decomposition reactions, including the generation of from metal cyanides using platinum sponge, which demonstrated the material's efficacy in promoting reactions at elevated temperatures. A pivotal advancement occurred in the late 19th and early 20th centuries with the development of practical catalytic processes for industrial applications. In 1897, Paul Sabatier and Jean-Baptiste Senderens pioneered the direct hydrogenation of unsaturated organic compounds using finely divided nickel as a catalyst, enabling efficient conversion of carbon monoxide and other gases into valuable products like methane; this work earned Sabatier the 1912 Nobel Prize in Chemistry for his contributions to catalytic hydrogenation theory and practice. Concurrently, Fritz Haber's research from 1909, later industrialized by Carl Bosch between 1910 and 1913, established the Haber-Bosch process for ammonia synthesis, employing iron-based catalysts under high pressure and temperature to fix atmospheric nitrogen—a breakthrough that revolutionized fertilizer production and earned Haber the 1918 Nobel Prize in Chemistry. Irving Langmuir's contributions in the early 20th century further advanced the field through his 1916-1918 development of the Langmuir adsorption isotherm, which mathematically described monolayer adsorption on surfaces, providing a theoretical framework for understanding heterogeneous catalytic mechanisms and earning him the 1932 Nobel Prize in Chemistry. By the mid-20th century, heterogeneous catalysis expanded into specialized materials and environmental applications. In the 1930s and 1940s, pioneering work led to the synthesis and characterization of zeolites—crystalline aluminosilicates with uniform pores—as documented in early studies on and molecular sieving. These materials emerged as key catalysts in the , leveraging their shape-selective properties for refining processes like cracking. The marked a surge in environmental catalysis following the U.S. Clean Air Act of 1970, which mandated sharp reductions in vehicle emissions; this spurred the widespread adoption of three-way catalytic converters using platinum-rhodium and formulations to oxidize and hydrocarbons while reducing nitrogen oxides, debuting commercially in 1975. In the modern era, from the 2000s onward, computational methods and nanoscale innovations have transformed catalyst design. Density functional theory and high-throughput screening enabled the rational prediction of active sites and reaction pathways, accelerating the development of tailored heterogeneous catalysts for energy and sustainability applications. The 2010s introduced single-atom catalysts, where isolated metal atoms anchored on supports maximize atom efficiency and selectivity; the concept gained prominence with Tao Zhang's 2011 report on platinum single atoms anchored on iron oxide (FeOx) for CO oxidation, heralding a shift toward precise, low-loading systems.

Reaction Mechanisms

Adsorption Processes

In heterogeneous catalysis, adsorption serves as the essential initial step, whereby reactant molecules from the gas or liquid phase bind to the surface of a solid , thereby concentrating them at active sites and enabling the formation of reactive intermediates that facilitate subsequent chemical transformations. This process lowers the for reactions by modifying or breaking molecular bonds upon surface interaction, as exemplified in key industrial processes like ammonia synthesis where adsorption on iron surfaces is rate-limiting. Adsorption in catalysis primarily occurs through two distinct mechanisms: physisorption and chemisorption. Physisorption involves weak, non-specific interactions via van der Waals forces, such as London dispersion or dipole-dipole attractions, resulting in low activation energies (often near zero), reversibility, and typical heats of adsorption below 50 kJ/mol; it allows multilayer formation and is prevalent at low temperatures. In contrast, chemisorption entails strong, specific chemical bonds (e.g., covalent or ionic) between the adsorbate and surface atoms, with higher activation energies, heats of adsorption exceeding 80 kJ/mol, and often irreversibility under typical reaction conditions; it is limited to a monolayer and is crucial for activating reactants in catalytic cycles. The extent and efficiency of adsorption are influenced by several key factors, including the catalyst's surface area (which determines the number of available sites), pore structure (affecting and accessibility), temperature (where exothermic adsorption decreases with rising temperature per ), and adsorbate pressure (driving higher coverage at elevated partial pressures). For instance, high surface area materials like zeolites or supported nanoparticles enhance capacity, while optimized pore sizes in mesoporous catalysts improve by facilitating transport to active sites. A foundational model for describing adsorption equilibrium on uniform surfaces with a finite number of identical sites is the Langmuir isotherm, which assumes no adsorbate interactions and monolayer coverage. The fractional surface coverage θ\theta is given by θ=KP1+KP,\theta = \frac{KP}{1 + KP}, where KK is the adsorption equilibrium constant and PP is the partial pressure of the adsorbate. This equation derives from a site balance at equilibrium: the rate of adsorption, proportional to P(1−θ)P(1 - \theta), equals the rate of desorption, proportional to θ\theta, yielding kaP(1−θ)=kdθk_a P (1 - \theta) = k_d \theta, where kak_a and kdk_d are the rate constants; rearranging gives θ=(ka/kd)P1+(ka/kd)P\theta = \frac{(k_a / k_d) P}{1 + (k_a / k_d) P}, with K=ka/kd=e−ΔG∘/RTK = k_a / k_d = e^{-\Delta G^\circ / RT}. Adsorption can further be classified as associative (molecule remains intact) or (molecule breaks into fragments upon binding). For example, adsorption on surfaces like or is typically , where H2_2 splits into atomic (Had_\text{ad}) with a heat of adsorption around 90 kJ/mol, forming a precursor to reactions.

Surface Reaction Steps

In heterogeneous catalysis, the surface reaction steps involve the chemical transformations of adsorbed on the catalyst surface, where bonds are broken and formed at active sites following initial adsorption. These steps occur after reactants have adsorbed onto the surface, enabling interactions that would be energetically unfavorable in the gas phase. The process typically proceeds through elementary reactions, including of adsorbates to bring species together, recombination of adsorbed atoms or molecules (such as O(ad) + CO(ad) → CO₂(ad)), and reactions where an adsorbed species extracts an atom from another adsorbate. These elementary steps are crucial for determining the overall reaction kinetics and selectivity, with often facilitating the migration of intermediates across the catalyst lattice. Two primary mechanisms describe these surface reactions: the Langmuir-Hinshelwood (LH) mechanism, where both reactants adsorb onto the surface before reacting, and the Eley-Rideal (ER) mechanism, where one reactant remains in the gas phase and reacts directly with an adsorbed . In the LH mechanism, the rate-determining step is often the bimolecular reaction between co-adsorbed , leading to the rate law r=kθAθBr = k \theta_A \theta_B, where θA\theta_A and θB\theta_B are the surface coverages of reactants A and B, derived under the assumption of uniform active sites and steady-state conditions. The ER mechanism, in contrast, involves a gas-phase colliding with an adsorbed , resulting in a rate expression like r=kθAPBr = k \theta_A P_B, where PBP_B is the of the gas-phase reactant; this is common in reactions with weak adsorption of one . Adsorption serves as the prerequisite for these mechanisms by positioning reactants near active sites. Active sites, often consisting of atomic ensembles such as edge or corner atoms on metal nanoparticles, play a pivotal role in these reactions by providing coordinatively unsaturated positions that lower barriers and influence selectivity. For instance, sites in Pt nanoparticles facilitate selective bond due to their unique electronic structure, promoting desired pathways while suppressing side reactions. The ensemble size and composition can dictate product distribution; smaller ensembles may favor , whereas larger ones enable complete reactions. A representative example is the oxidation of CO on Pt surfaces, where the LH mechanism dominates under typical conditions. The pathway begins with dissociative adsorption of O₂ to form O(ad) atoms, followed by adsorption of CO at adjacent sites; surface diffusion brings these species together for recombination to CO₂(ad), which then desorbs. The rate is often limited by O₂ dissociation or CO coverage, with edge sites on Pt nanoparticles enhancing activity by stabilizing O(ad) and reducing the recombination barrier to approximately 100-120 kJ/mol. Studies on Pt(111) single crystals confirm this pathway, showing oscillatory behavior due to competing CO poisoning and oxide formation.

Desorption Processes

Desorption represents the final step in the of heterogeneous catalysis, where reaction products detach from surface, thereby regenerating active sites for subsequent reactant adsorption and reaction. This process is essential for maintaining the turnover of , as the release of products prevents site blocking and ensures continuous operation under steady-state conditions. In many catalytic systems, such as ammonia synthesis on iron catalysts, desorption kinetics directly influence the overall by determining the availability of surface sites. Desorption processes in heterogeneous catalysis are broadly classified into thermal desorption, which can be either activated or non-activated, and recombinative (or associative) desorption. Thermal desorption involves the thermally induced release of adsorbed , where non-activated desorption occurs without a significant energy barrier if the adsorption was exothermic and barrierless, allowing spontaneous departure at elevated temperatures; an example is the desorption of from surfaces. Activated thermal desorption requires overcoming an activation barrier, often observed in systems like on , where the process demands higher temperatures (900–1200 K) due to the needed to break strong chemisorption bonds. Recombinative desorption, common for dissociated adsorbates, entails the recombination of atomic on the surface prior to release as a , such as 2H(ads) → H₂(g) on (111), exhibiting second-order kinetics due to the bimolecular nature of the step. The energetics of desorption are governed by the EdE_d, which is typically related to the heat of adsorption by , making desorption endothermic and often rate-limiting at low temperatures. The kinetics are described by the Polanyi-Wigner equation, which models the desorption rate as coverage-dependent: −dθdt=kθn-\frac{d\theta}{dt} = k \theta^n where θ\theta is the surface coverage, k=νexp⁡(−Ed/RT)k = \nu \exp(-E_d / RT) is the rate constant with ν\nu (typically 101310^{13} s⁻¹ for processes), RR is the , TT is , and nn is the desorption order (e.g., n=1n=1 for non-dissociative, n=2n=2 for recombinative). This equation, derived from transition-state theory, highlights how EdE_d values, often 50–200 kJ/mol in catalytic systems like CO on Fe₃O₄, dictate the temperature threshold for desorption. Desorption rates exhibit strong dependence on surface coverage θ\theta, with higher coverage often lowering EdE_d due to adsorbate-adsorbate interactions, leading to shifts in desorption peaks toward lower temperatures in temperature-programmed experiments. Temperature plays a critical role, as increasing TT exponentially accelerates the rate per the Arrhenius form in the Polanyi-Wigner equation, enabling control over desorption in catalytic processes; for instance, in hydrogenation reactions, optimal temperatures balance desorption with preceding surface reactions to maximize yield. A primary technique for studying desorption is temperature-programmed desorption (TPD), which involves adsorbing species on the catalyst at low temperature, then linearly ramping the temperature (e.g., 10 K/s) while monitoring desorbed gases via . TPD spectra reveal desorption peaks whose position, shape, and area provide insights into EdE_d, order nn, and coverage; for example, peak temperature TpT_p approximates Ed≈0.25TpE_d \approx 0.25 T_p (in kJ/mol) via Redhead analysis for first-order processes assuming ν=1013\nu = 10^{13} s⁻¹. This method is widely applied to characterize catalytic surfaces, such as determining binding energies in metal systems.90024-9)

Catalyst Materials and Design

Types of Catalyst Materials

Heterogeneous catalysts are broadly classified into several material types, each offering distinct properties that influence their reactivity, selectivity, and stability in various chemical processes. These include metals, metal oxides, zeolites, and supported systems, with advanced variants such as nanostructured and bimetallic compositions enhancing performance through tailored structures. Metallic catalysts, particularly transition metals, are widely used due to their ability to facilitate adsorption and bond breaking in reactions like and oxidation. Noble metals such as (Pt) and (Pd) excel in processes, where Pt catalyzes the selective reduction of alkenes to alkanes under mild conditions, owing to its high affinity for dissociation. In contrast, base metals like and provide cost-effective alternatives for large-scale applications, such as in for , though they often require higher temperatures to achieve comparable activity. The distinction between noble and base metals arises from differences in electronic structure and resistance to , with noble metals generally exhibiting greater stability in oxidative environments. Metal oxides serve as both active catalysts and supports, leveraging their properties and surface acidity for diverse reactions. For instance, (TiO₂) and aluminum oxide (Al₂O₃) are employed in oxidation and dehydration processes, with TiO₂ promoting photocatalytic reactions through its bandgap that enables electron-hole pair generation under light. Al₂O₃, often used as a support, also acts independently in acid-catalyzed due to its amphoteric nature, balancing Lewis and base sites to stabilize intermediates. These oxides exhibit tunable behavior, where cations like Ce in CeO₂ facilitate and release, enhancing catalytic cycles in automotive exhaust treatment. Zeolites, microporous aluminosilicates, are prized for shape-selective catalysis, confining reactants within their uniform pores to favor specific products. Their framework enables high selectivity in cracking and , as seen in zeolite for in refining, where pore sizes around 0.5-0.6 nm restrict larger molecules. Acidity in zeolites stems from Brønsted and Lewis sites: Brønsted sites, formed by bridging OH groups, protonate substrates for carbocation-mediated reactions, while Lewis sites from extra-framework Al³⁺ coordinate with donors to activate C-H bonds. This dual acidity/basicity allows zeolites to catalyze both acid-driven transformations and bifunctional processes involving metal loading. Supports play a crucial role in dispersing active phases to maximize surface area and prevent . High-surface-area materials like silica (SiO₂) provide inert, thermally stable platforms for metal deposition, with its groups aiding uniform distribution in processes like synthesis over iron/silica. Carbon-based supports, such as or , offer high electrical conductivity and resistance to acidic conditions, ideal for electrocatalysis; for example, carbon supports enhance Pt dispersion in anodes, improving mass transport. These supports not only increase accessibility but also modulate electronic properties through metal-support interactions. Nanostructured catalysts, including nanoparticles and single-atom variants, address limitations of bulk materials by exposing more active sites. Metal nanoparticles, typically 2-10 nm, exhibit size-dependent activity; for , smaller Pd nanoparticles show higher turnover frequencies due to increased edge sites. Single-atom catalysts (SACs), like Pt₁ on CeO₂, achieve atomic efficiency with enhanced stability and activity; in CO oxidation, isolated Pt atoms leverage CeO₂'s oxygen mobility for low-temperature performance, outperforming nanoparticles by activating lattice oxygen. SACs minimize metal use while maximizing utilization, though challenges in synthesis persist for scalability. Bimetallic and alloy catalysts exploit synergistic effects to improve performance beyond monometallic counterparts. In Pt-Ru alloys for fuel cells, Ru enhances CO tolerance by facilitating its oxidation at lower potentials via bifunctional mechanism, where Ru-OH adsorbs and reacts with Pt-bound CO, boosting anode efficiency in proton exchange membrane fuel cells. Such synergies arise from electronic modifications—e.g., Ru donates electrons to Pt, weakening CO binding—and geometric effects that create ensemble sites for reactants. These alloys often show prolonged stability under operating conditions compared to pure metals.

Preparation and Synthesis Methods

Heterogeneous catalysts are typically prepared by depositing active metal or metal oxide components onto high-surface-area supports, such as alumina or silica, to achieve optimal dispersion and activity. Common methods focus on controlling the structure and to maximize catalytic performance, with techniques ranging from traditional impregnation to advanced colloidal approaches. These synthesis routes aim to produce uniform distributions of active sites while ensuring scalability for industrial applications. Impregnation is one of the most widely used techniques for preparing supported metal catalysts, involving the adsorption of metal precursor solutions onto porous supports. In wet impregnation, excess solution is used, allowing the precursor to penetrate the support pores before and ; however, this can lead to uneven distribution if not controlled. Dry impregnation, or incipient wetness, employs a volume of solution equal to the support's pore volume, promoting uniform loading by and minimizing waste. For example, nitrate is often impregnated onto alumina supports for Fischer-Tropsch synthesis, achieving metal loadings up to 20 wt%. The method's simplicity and scalability make it suitable for , though limitations include potential aggregation during , which affects . Precipitation methods, particularly , enable the simultaneous formation of mixed oxide supports and active phases from aqueous solutions of metal salts. By adding a precipitating agent like or under controlled and temperature, hydroxides or carbonates form and are filtered, washed, dried, and calcined to yield homogeneous compositions. This technique is ideal for high-loading catalysts, such as Cu/ZnO/Al₂O₃ used in synthesis, where precise (around 6-7) ensures small particle sizes (e.g., 3 nm CuO crystallites). Advantages include high dispersion and strong metal-support interactions, but challenges arise from waste generation during washing and sensitivity to local variations, impacting batch-to-batch reproducibility. Advanced techniques like sol-gel synthesis produce porous structures with tailored microstructures through and of metal alkoxides, forming a sol that gels into a network. This method allows incorporation of active metals during gelation, followed by drying to form xerogels or aerogels with high surface areas (up to 700 m²/g). For instance, silica-supported catalysts for are prepared by adjusting the water-to-precursor ratio, yielding mesoporous materials with uniform pores. Benefits include low-temperature processing and homogeneity, but the process is complex and prone to cracking during drying, limiting scalability. Colloidal synthesis offers precise control over morphology, often via seed-mediated growth where small particles (e.g., 5 nm ) serve as sites for controlled overgrowth using molecular precursors and stabilizers. This results in anisotropic structures like nanorods (5 nm , 40-100 nm ) deposited onto supports such as alumina-silica for Fischer-Tropsch catalysts, enhancing stability and activity. The approach excels in achieving narrow size distributions below 10 nm, crucial for high surface-to-volume ratios and activity, though removal steps can introduce variability. Limitations include difficulties in scaling beyond lab quantities due to stabilizer handling. Following synthesis, activation steps are essential to transform precursors into active forms. Calcination involves heating the dried material (typically 300-600°C in air) to decompose precursors, remove volatiles, and stabilize the structure, as seen in converting impregnated nitrates to oxides on alumina supports. Reduction, often with hydrogen at 300-500°C, then reduces metal oxides to metallic particles, enhancing dispersion; for example, low-temperature calcination (250-300°C) followed by H₂ reduction yields highly active cobalt particles for syngas conversion. These steps must be optimized to prevent sintering, which enlarges particles beyond optimal sizes (<10 nm). Key control parameters include particle size distribution, targeted below 10 nm to maximize active site exposure and turnover frequency, achieved by adjusting precursor concentration, pH, or stabilizers in synthesis. Promoter addition, such as potassium in iron-based ammonia synthesis catalysts, enhances electron donation and selectivity by modifying surface basicity; for instance, K₂O doping (1-2 wt%) improves N₂ activation on Fe sites. These additives are incorporated during impregnation or coprecipitation to fine-tune activity without altering bulk structure. Challenges in preparation include ensuring reproducibility across batches, as minor variations in drying rates or pH can lead to inconsistent dispersions, and scaling from lab to industrial levels, where advanced methods like sol-gel face equipment and cost barriers while maintaining uniformity. Addressing these requires standardized protocols and in-line monitoring to bridge the gap between research and production.

Characterization and Analysis

Surface Structure Techniques

Surface structure techniques are essential for elucidating the atomic and molecular arrangements on catalyst surfaces, which directly influence adsorption, reaction pathways, and overall catalytic performance in heterogeneous systems. These methods provide insights into surface composition, active site geometries, and dynamic changes under reaction conditions, enabling the rational design of more efficient catalysts. By probing the interface between the solid catalyst and reactants, such techniques bridge the gap between material synthesis and mechanistic understanding, revealing how surface defects, facets, and coordinative environments dictate selectivity and activity. Spectroscopic approaches, such as , are widely used to determine the oxidation states, elemental composition, and chemical environments of surface atoms in heterogeneous catalysts. XPS operates by irradiating the sample with X-rays and measuring the kinetic energy of emitted photoelectrons, which provides depth-sensitive information up to about 10 nm, making it ideal for analyzing supported metal particles and oxide layers. For instance, ambient-pressure XPS (AP-XPS) variants allow studies under near-reaction conditions, capturing changes in surface oxidation during catalysis. Complementing XPS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identifies adsorbed species and their bonding modes on catalyst surfaces by measuring vibrational spectra in a reflective setup suitable for powders. DRIFTS is particularly valuable for operando studies, where it detects transient intermediates like CO or NO on metal sites, revealing adsorption geometries that correlate with reaction kinetics. Microscopic techniques offer direct visualization of surface features at high resolution. Scanning tunneling microscopy (STM) achieves atomic-scale imaging of catalyst surfaces by scanning a sharp tip to measure tunneling currents, enabling the observation of single atoms, clusters, and reconstruction under ultrahigh vacuum or elevated pressures. In heterogeneous catalysis, STM has been pivotal in studying model systems like Pt nanoparticles on oxides, showing how surface facets evolve during reactions. Transmission electron microscopy (TEM), including high-resolution variants, characterizes particle size distributions, morphologies, and lattice structures by transmitting electrons through thin samples. Aberration-corrected TEM provides sub-angstrom resolution for identifying atomic arrangements in supported catalysts, such as the dispersion of active metals on carbon supports. Diffraction-based methods probe long-range order and local atomic coordination. X-ray diffraction (XRD) determines the crystallite structure, phase purity, and average particle sizes in polycrystalline catalysts through analysis of Bragg reflections, often using the for size estimation from peak broadening. In situ XRD tracks structural transformations, like phase changes in metal oxides during redox cycles. (EXAFS) spectroscopy elucidates coordination environments around absorbing atoms by analyzing oscillations in X-ray absorption spectra beyond the edge, providing bond lengths and neighbor counts without requiring long-range order. EXAFS is crucial for single-atom catalysts, where it confirms low-coordination sites in frameworks like zeolites or metal-organic frameworks. A notable application is the use of nuclear magnetic resonance (NMR) spectroscopy to identify active sites in zeolite catalysts, where solid-state techniques like magic-angle spinning NMR probe framework aluminum distributions and metal incorporations. For example, 27Al and 29Si NMR distinguish tetrahedral sites responsible for Brønsted acidity in proton-form zeolites, linking site density to cracking activity. These insights have guided the design of shape-selective catalysts for hydrocarbon processing. Despite their power, surface structure techniques face limitations, including the need for ultrahigh vacuum in many ex situ measurements, which may not reflect operando conditions and lead to surface reconstructions. In situ and operando variants, such as high-pressure STM or AP-XPS, mitigate this but often compromise resolution or require specialized setups. Such structural details ultimately inform performance metrics, like turnover frequencies tied to specific site types.

Performance Evaluation Methods

Performance evaluation in heterogeneous catalysis involves quantitative assessment of catalyst activity, selectivity, and stability to determine efficacy under reaction conditions. Key metrics focus on intrinsic rates and overall productivity, ensuring comparisons across studies are standardized and reliable. Experimental setups simulate industrial processes, while computational methods predict performance based on fundamental interactions. These approaches enable optimization of catalysts for practical applications. Activity is primarily quantified using turnover frequency (TOF), defined as the number of product molecules formed per active site per unit time, expressed as TOF=moles of productmoles of active sites×time\text{TOF} = \frac{\text{moles of product}}{\text{moles of active sites} \times \text{time}}. This metric isolates the intrinsic reactivity of catalytic sites, independent of catalyst loading or reactor geometry, and is essential for comparing site-specific performance. Space-time yield complements TOF by measuring productivity per unit reactor volume per unit time, providing a practical indicator of process efficiency in scaled systems. Standardization of active site determination, often via techniques like chemisorption, is critical to avoid overestimation or underreporting of TOF values. Selectivity, the fraction of converted reactant forming the desired product relative to total products, and yield, the amount of desired product relative to initial reactant, are evaluated in fixed-bed reactors where steady-state flow allows precise monitoring of effluent composition via gas chromatography or mass spectrometry. Conversion, standardized as X=reactant inlet−reactant outletreactant inletX = \frac{\text{reactant inlet} - \text{reactant outlet}}{\text{reactant inlet}}, quantifies reactant utilization and is routinely reported alongside selectivity to assess overall process viability. Stability is tested by tracking these metrics over extended periods, typically in continuous operation, to identify deactivation onset and long-term performance. Testing setups include batch reactors, which use finite reactant volumes for initial screening of catalyst behavior in closed systems, versus continuous flow reactors like fixed-bed configurations that mimic industrial steady-state conditions for reliable kinetic data. In-situ and operando spectroscopy, such as infrared or X-ray absorption under working conditions, enables real-time monitoring of active species and reaction intermediates to correlate performance with dynamic surface changes. Computationally, density functional theory (DFT) calculates binding energies of adsorbates, guided by the , which posits optimal catalysis at intermediate adsorption strengths to balance activation and desorption. This volcano-shaped relationship between binding energy and activity informs catalyst design without exhaustive experimentation.

Deactivation and Regeneration

Mechanisms of Catalyst Deactivation

Heterogeneous catalysts deactivate over time due to various mechanisms that reduce active site availability or alter surface properties, leading to diminished reaction rates. These processes are broadly classified into chemical, thermal, and mechanical categories, with deactivation manifesting as a gradual or abrupt loss of catalytic activity. Understanding these mechanisms is essential for designing more stable catalysts, as deactivation remains a primary limitation in industrial applications. Poisoning occurs through the strong, often irreversible chemisorption of impurities on active sites, blocking access for reactants. Common poisons include sulfur compounds, which adsorb dissociatively on metal surfaces like , forming stable sulfides that reduce the ensemble size required for reactions such as . For instance, exposure to H₂S can decrease nickel catalyst activity by several orders of magnitude at concentrations as low as 1–10 ppb, with the poisoning effect being site-specific and dependent on the poison's adsorption energy. Coking, or carbon deposition, involves the formation of carbonaceous species that coat active sites or plug pores, particularly in hydrocarbon processing. These deposits arise from side reactions like dehydrogenation or polymerization, yielding structures such as filamentous carbon or polymeric coke. In reactions involving hydrocarbons, coke buildup can encapsulate metal particles, reducing surface area and hindering mass transport; for example, steam reforming catalysts may form carbon filaments above 450°C, exacerbating deactivation in coke-sensitive processes. Sintering refers to the thermal agglomeration of catalyst particles, which decreases metal dispersion and surface area. This process is driven by mechanisms like Ostwald ripening, where smaller particles dissolve atomically and redeposit onto larger ones via surface or vapor-phase diffusion, favored at high temperatures (>500°C). can accelerate sintering by enhancing metal oxide mobility; studies on supported catalysts show up to 70% loss in surface area after 50 hours at 750°C. Particle migration and coalescence also contribute, especially for larger nanoparticles. Phase transformation involves chemical or structural changes in the catalyst under reaction conditions, converting active phases to less active or inactive ones. This can include oxidation of metallic sites to oxides or solid-state transitions, such as the γ- to α-alumina shift at approximately °C, which drastically reduces from over 200 m²/g to about 1 m²/g. Such transformations alter electronic properties and site geometry, often irreversibly, and are influenced by environments or high temperatures. Fouling results from the physical accumulation of extraneous materials on the catalyst surface or within pores, impeding reactant . This includes deposition of particulates like dust or reaction byproducts that block access to active sites; for example, fly ash in combustion-related catalysis can rapidly plug pore networks, leading to increases and flow restrictions. Unlike , fouling is often mechanical and reversible through cleaning, but it severely impacts mass transfer-limited systems. Attrition involves mechanical breakdown of catalyst particles due to or handling, generating fines that reduce effective inventory and increase emissions. Deactivation kinetics are commonly modeled to predict catalyst lifetime, with the first-order model describing activity aa (relative to initial activity a0a_0) as a=e−kta = e^{-kt}, where kk is the deactivation rate constant and tt is time. More general approaches, like the power-law expression, account for parallel deactivation pathways: −dadt=kdam-\frac{da}{dt} = k_d a^m, where mm reflects the order (e.g., m=1m=1 for poisoning). These models integrate experimental data to quantify rates, often revealing that deactivation accelerates with temperature via Arrhenius dependence. Regeneration strategies can mitigate these effects by reversing certain mechanisms, such as coke removal via oxidation.

Strategies for Regeneration

Regeneration of heterogeneous catalysts aims to restore activity lost due to reversible deactivation mechanisms, such as coke deposition or mild poisoning, through targeted techniques that minimize structural damage to the catalyst. These strategies are broadly classified as in-situ (performed within the reactor) or ex-situ (offline treatment), with the choice depending on the deactivation type and process economics. Oxidative and chemical methods address surface fouling, while physical approaches target sintering, and preventive measures incorporate design elements to extend catalyst life. One common method for removing coke deposits involves oxidative regeneration, where carbonaceous residues are burned off using oxygen or air at temperatures typically between 400°C and 600°C, converting coke to CO and CO₂. This approach is effective for catalysts in hydrocarbon processing, as it restores surface area without excessive , though careful control of oxygen is required to avoid over-oxidation of active metals. For instance, in (FCC) units, continuous in-situ oxidative regeneration occurs in a regenerator operating at 500–560°C, allowing seamless catalyst circulation between reaction and regeneration zones. Chemical cleaning techniques are employed to eliminate poisons that adsorb strongly on active sites, such as species. A representative example is the use of oxide (ZnO) beds to irreversibly adsorb H₂S, preventing its migration to downstream catalysts in processes like hydrotreating or Fischer-Tropsch synthesis, where sulfur levels must be reduced below 50 ppb. This ex-situ or guard-bed approach effectively regenerates the primary catalyst by sacrificial removal of contaminants, with ZnO regeneration possible via high-temperature reduction if needed. Physical regeneration methods focus on reversing sintering, where metal particles agglomerate and lose dispersion. Oxychlorination is a widely adopted technique for platinum-based catalysts, involving treatment with a chlorine-oxygen mixture at 500–550°C to form volatile PtClₓ species that redisperse upon subsequent reduction, restoring up to 80–90% of original dispersion on supports like alumina. This in-situ or ex-situ process is standard in catalytic reforming, where sintered Pt is redispersed every 6–12 months to maintain activity. Pressure swing regeneration, applicable in some gas-phase processes, exploits pressure changes to desorb weakly bound species or volatiles, such as in sorption-enhanced reactions, enabling faster cycling without thermal input. Preventive strategies integrated during catalyst design or operation help mitigate deactivation from the outset. Adding promoters, such as tin (Sn) to Pt/Al₂O₃ or molybdenum (Mo) for sulfur resistance, modifies surface ensembles to reduce coke formation or poison adsorption, extending operational life by 20–50%. Continuous operation modes, like fluidized beds in FCC, inherently incorporate regeneration to avoid downtime, with catalyst inventory continuously refreshed to balance activity. Economically, regeneration strategies involve trade-offs between treatment frequency and full catalyst replacement costs, which can be substantial in large-scale operations. For example, regenerating FCC catalysts in-situ avoids the significant expense of replacement while recovering 70–90% activity, though repeated cycles may necessitate periodic full refresh to prevent irreversible losses; overall, effective regeneration optimizes lifespan against and yield penalties.

Applications and Examples

Gas-Phase Industrial Processes

Heterogeneous catalysis plays a pivotal role in gas-phase industrial processes, enabling efficient conversion of reactants under controlled conditions to produce essential chemicals and mitigate emissions. One of the most significant applications is the synthesis of ammonia via the Haber-Bosch process, where nitrogen and hydrogen gases react over iron-based catalysts to form NH₃, supporting global fertilizer production. The Haber-Bosch process operates at high pressures of 200-400 atm and temperatures of 400-500°C, utilizing promoted (Fe₃O₄) as the primary to achieve industrially viable rates despite the reaction's thermodynamic limitations. This endothermic, (N₂ + 3H₂ ⇌ 2NH₃) requires careful optimization of conditions to maximize yield, with the iron often promoted by potassium, aluminum, and calcium oxides to enhance activity and stability. Ruthenium-based alternatives, such as Ru supported on carbon or metal oxides, offer higher activity at milder conditions (e.g., 300-450°C and 4-15 MPa), though their higher cost limits widespread adoption to niche, low-pressure applications. These catalysts demonstrate superior dissociation but require promoters like cesium to mitigate inhibition by . In production, heterogeneous catalysis facilitates the selective partial of impurities to , preventing in downstream units. Palladium-based catalysts, typically supported on alumina or silica, are employed for this gas-phase reaction (C₂H₂ + H₂ → C₂H₄), operating at 100-200°C and near-atmospheric to achieve high selectivity (>90%) toward while minimizing over- to . The Pd active sites enable dissociative adsorption of and , with promoter metals like silver or tuning the electronic properties to suppress oligomerization and coke formation. This process is critical in operations, where levels must be reduced below 5 ppm for polymer-grade . Steam methane reforming (SMR) represents a cornerstone for , converting into (CO + H₂) via an endothermic gas-phase reaction over catalysts supported on alumina (Ni/Al₂O₃). The primary reaction, CH₄ + H₂O → CO + 3H₂, occurs at approximately 800°C and 20-30 atm in tubular reactors, with the nickel particles (typically 10-20 nm) catalyzing C-H bond breaking while the alumina support provides thermal stability and dispersion. Side reactions like the water-gas shift (CO + H₂O → CO₂ + H₂) further adjust the H₂/CO ratio, making SMR the dominant route for approximately 76% of global as of 2024, though carbon deposition remains a challenge managed by excess steam. Ethylene oxide (EO) production exemplifies selective in gas-phase heterogeneous catalysis, where reacts with oxygen over silver catalysts supported on low-surface-area α-alumina (Ag/α-Al₂O₃) to yield the (C₂H₄ + ½O₂ → C₂H₄O). This exothermic process runs at 220-280°C and 10-20 atm, achieving selectivities of 80-90% under optimized conditions, with and promoters enhancing oxygen adsorption and suppressing total to CO₂. The α-Al₂O₃ support (with surface area <5 m²/g) minimizes acid-catalyzed side reactions like isomerization, while silver particle sizes (1-10 μm) influence the electrophilic oxygen species responsible for epoxidation. EO is a key intermediate for ethylene glycol and surfactants, underscoring the process's economic scale. Environmental applications of gas-phase heterogeneous catalysis include automotive three-way catalytic converters, introduced in 1975 to comply with U.S. emission standards, which simultaneously convert CO, hydrocarbons (HC), and NOx in exhaust gases using platinum-group metals (Pt, Pd, Rh) supported on ceramic monoliths coated with γ-Al₂O₃. These converters operate at 400-800°C, where Pt and Pd oxidize CO and HC to CO₂ and H₂O (e.g., 2CO + O₂ → 2CO₂), while Rh reduces NOx to N₂ (e.g., 2NO + 2CO → N₂ + 2CO₂), achieving >90% conversion efficiency near stoichiometric air-fuel ratios. The synergy among metals, stabilized by ceria-zirconia oxygen storage components, enables transient operation during engine load changes, significantly reducing urban air pollutants since their mandated adoption.

Liquid-Phase Industrial Processes

Heterogeneous catalysis in liquid-phase processes involves reactions where solid catalysts interact with liquid reactants, often in solid-liquid or immiscible liquid-liquid systems, enabling efficient transformations in industries such as and . These systems leverage the high surface area of solid catalysts to accelerate reactions like hydrotreating and , while addressing challenges such as and . Key examples include for fuel purification, olefin for plastics production, and selective for food applications, where catalyst and reactor configuration play critical roles in overcoming barriers. Hydrodesulfurization (HDS) is a vital liquid-phase for removing impurities from feedstocks, converting organosulfur compounds like dibenzothiophenes into (H₂S) to meet stringent environmental regulations on fuel content, typically below 10 ppm. Industrial HDS employs cobalt-promoted catalysts supported on alumina (CoMo/Al₂O₃), which operate under moderate conditions of 300–400°C and 2–6 MPa in fixed-bed trickle-flow reactors, where liquid hydrocarbons flow downward over the catalyst bed while is co-fed. The active sites on the edges of MoS₂ slabs, promoted by , facilitate two primary pathways: direct desulfurization (DDS) via C-S bond cleavage and (HYD) to saturate aromatic rings before desulfurization, with DDS being more selective for refractory species. This is essential for producing and , significantly reducing SOₓ emissions from engines. Polymerization of olefins, particularly to , exemplifies heterogeneous catalysis in liquid or phases using Ziegler-Natta catalysts. Discovered in the 1950s by and , these catalysts consist of (TiCl₄) supported on (MgCl₂), activated by alkylaluminum cocatalysts like triethylaluminum (AlEt₃), enabling stereospecific at ambient to moderate temperatures (50–80°C) and pressures (1–5 MPa) in or gas-liquid systems. The MgCl₂ support mimics the crystalline structure of TiCl₃ used in early generations, providing active Ti³⁺ or Ti²⁺ sites that coordinate with π-electrons for isospecific insertion, yielding high-molecular-weight isotactic with exceeding 98%. This breakthrough revolutionized the industry, enabling the production of millions of tons annually for , automotive, and consumer goods, with catalyst activity reaching 30–60 kg PP/g cat in modern formulations. Hydrogenation of edible oils represents a classic liquid-phase application, converting unsaturated vegetable oils into semi-solid fats for and shortenings using as a heterogeneous catalyst. Developed by Murray Raney in 1926, this sponge-like catalyst, prepared by leaching aluminum from a Ni-Al with , provides high surface area (up to 100 m²/g) for selective of carbon-carbon double bonds at 120–180°C and 0.1–0.5 MPa pressure in batch or continuous stirred-tank reactors. The process improves oil stability, , and texture by partially saturating polyunsaturated fatty acids while minimizing trans-fat formation in optimized conditions, with billions of pounds of oils processed annually worldwide. 's insolubility allows easy separation and reuse, making it industrially dominant despite alternatives like . In solid-liquid heterogeneous catalysis, limitations significantly influence reaction efficiency, particularly in systems where reactants must diffuse from the bulk liquid to active sites on the surface. External , governed by and , can reduce rates in fixed-bed reactors, where larger pellets (1–10 mm) minimize but exacerbate diffusion barriers, often quantified by the ; internal pore , assessed via the Thiele modulus (Φ), further limits effectiveness in microporous supports when Φ > 1, yielding η < 1. reactors mitigate these issues by suspending fine particles (10–100 μm) in the liquid, enhancing gas-liquid-solid contact and coefficients (k_L a up to 0.1–1 s⁻¹), though they require for recovery and are suited for exothermic reactions like HDS. Fixed-bed trickle-bed reactors, conversely, offer continuous operation for high-throughput processes but demand careful design to avoid flooding or channeling. Deactivation in liquid environments, often from coke deposition or , can be briefly addressed through periodic regeneration. Liquid-liquid heterogeneous catalysis employs phase-transfer mechanisms with insoluble solid-supported catalysts to facilitate reactions across immiscible phases, such as aqueous-organic systems. Supported phase-transfer catalysts (PTCs), typically salts immobilized on polymers, clays, or carbon materials, transfer anionic reactants (e.g., halides or nucleophiles) from aqueous to organic phases via ion-exchange, enabling efficient biphasic reactions without soluble PTCs. These heterogeneous systems, exemplified by salts on silica or , promote processes by allowing easy catalyst recovery through , reducing waste, and maintaining activity over multiple cycles in transformations like or oxidation. Advantages include enhanced and selectivity in industrial liquid-liquid extractions, with recent advances in chiral supported PTCs for asymmetric synthesis.

Emerging and Specialized Applications

Heterogeneous catalysis has expanded into emerging applications that address challenges, such as production and carbon capture, leveraging to enhance and selectivity. In electrocatalysis, supported on carbon (Pt/C) remains a benchmark for the (ORR) in fuel cells, where it facilitates the four-electron reduction of O₂ to water under acidic conditions. The activity of Pt-based catalysts follows a volcano plot, correlating optimal performance with the of oxygen intermediates; Pt sits near the peak, balancing adsorption strength to minimize , typically around 0.4 V versus the . Efforts to reduce Pt loading while maintaining have led to core-shell alloys like Pt₃Ni, which shift the d-band center for improved kinetics and stability exceeding 100,000 cycles in accelerated tests. Photocatalysis represents another frontier, with titanium dioxide (TiO₂) enabling water splitting for hydrogen production under ultraviolet irradiation, driven by its wide band gap of approximately 3.0–3.2 eV that generates electron-hole pairs for redox reactions. In the seminal demonstration, TiO₂ anatase electrodes achieved stoichiometric H₂ and O₂ evolution from water, highlighting its potential for solar-driven fuel generation, though quantum yields remain below 10% due to rapid charge recombination. Band gap engineering, such as doping with nitrogen or coupling with narrower-gap semiconductors like CdS, extends absorption into the visible spectrum, boosting H₂ production rates to over 100 μmol·g⁻¹·h⁻¹ in modified TiO₂ systems. These modifications enhance charge separation, as evidenced by prolonged photoluminescence lifetimes, making TiO₂-based photocatalysts viable for scalable photoelectrochemical cells. In biomass conversion, acidic zeolites catalyze the of to glucose, offering a sustainable route to platform chemicals from lignocellulosic feedstocks without homogeneous acids. H-type zeolites like H-β or H-ZSM-5 provide Brønsted acid sites that cleave β-1,4-glycosidic bonds, achieving glucose yields up to 50% at 150–200°C in media, surpassing traditional processes in recyclability. For production, (Co) and iron (Fe)-based catalysts in Fischer-Tropsch synthesis convert derived from into liquid hydrocarbons, with Co favoring high selectivity to diesel-range paraffins (chain length C₁₀–C₂₀) at 200–250°C and 20–30 bar. Fe catalysts, often promoted with , excel in handling CO₂-contaminated , yielding oxygenated compounds alongside hydrocarbons, with Anderson-Schulz-Flory chain growth probabilities around 0.8–0.9 for applications. CO₂ utilization via heterogeneous catalysis mitigates greenhouse emissions by converting it to value-added products like through with H₂. Cu/ZnO catalysts, typically stabilized with Al₂O₃, promote the reaction at 200–300°C and 30–50 bar, with active sites at the Cu-ZnO interface facilitating intermediates for selectivity over 90%. These catalysts achieve space-time yields up to 1 g·mL⁻¹·h⁻¹, drawing from industrial processes but optimized for CO₂ feeds, where ZnO enhances CO₂ adsorption and spillover of . Recent advances include metal-organic framework (MOF)-derived catalysts, pyrolyzed in the to yield high-surface-area metal nanoparticles embedded in porous carbon, improving dispersion and resistance to in reactions like CO₂ reduction. For instance, ZIF-8-derived Co/N-doped carbon exhibits ORR activity comparable to Pt/C, with half-wave potentials around 0.85 V, due to synergistic metal-nitrogen sites. Similarly, enzyme-mimetic nanomaterials, such as nanozymes, replicate or activities in heterogeneous systems, catalyzing oxidative transformations with turnover frequencies exceeding 10⁴ s⁻¹, offering robust alternatives to fragile biocatalysts in industrial settings. These developments underscore heterogeneous catalysis's role in bridging fundamental mechanisms to innovative, sustainable technologies.

References

  1. https://www.sciencedirect.com/science/article/pii/B9780128498910000047
  2. https://academic.oup.com/nsr/article/2/2/150/2096108
  3. https://pubs.acs.org/doi/10.1021/ed200816g
  4. https://www.cademix.org/applications-of-heterogeneous-catalysis-in-industry/
  5. https://www.sciencedirect.com/topics/materials-science/heterogeneous-catalysis
  6. https://pubs.rsc.org/en/content/articlehtml/2015/cs/c5cs00376h
  7. https://pubs.acs.org/doi/10.1021/acsenergylett.4c02954
  8. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/3d-Block_Elements/Group_05:_Transition_Metals/Chemistry_of_Vanadium
  9. https://pressbooks.online.ucf.edu/chemistryfundamentals/chapter/catalysis/
  10. https://pubs.acs.org/doi/10.1021/acsinfocus.7e5032
  11. https://www.essentialchemicalindustry.org/processes/catalysis-in-industry.html
  12. https://pubs.acs.org/doi/10.1021/acs.chemrev.3c00029
  13. https://pmc.ncbi.nlm.nih.gov/articles/PMC8619138/
  14. https://pubs.acs.org/doi/pdf/10.1021/bk-1983-0222.ch001
  15. https://www.researchgate.net/publication/236233411_The_History_of_Catalysis_-_From_the_Beginning_to_Nobel_Prizes
  16. https://www.nobelprize.org/prizes/chemistry/1912/sabatier/facts/
  17. https://www.sciencedirect.com/topics/engineering/haber-bosch-process
  18. https://pubs.acs.org/doi/10.1021/bk-1983-0222.ch002
  19. https://www.wiley-vch.de/books/sample/3527325050_c01.pdf
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC4594121/
  21. https://www.sciencedirect.com/science/article/pii/S2666916120300062
  22. https://www.sciencedirect.com/science/article/pii/S0167299100809908
  23. https://pubs.rsc.org/en/content/articlehtml/2022/ra/d2ra00657j
  24. https://www.nature.com/articles/nchem.1095
  25. https://www.nobelprize.org/uploads/2018/06/ertl_lecture.pdf
  26. https://pubs.acs.org/doi/10.1021/acsomega.0c04744
  27. https://www.sciencedirect.com/science/article/pii/S0920586123000238
  28. https://pubs.acs.org/doi/10.1021/ja02242a004
  29. https://vallance.chem.ox.ac.uk/pdfs/KineticsAtSurfaces.pdf
  30. https://chem.libretexts.org/Courses/New_York_University/CHEM-UA_652%253A_Thermodynamics_and_Kinetics/01%253A_Lectures/1.22%253A_Kinetics_of_Catalysis
  31. https://pubs.acs.org/doi/10.1021/acscatal.4c05188
  32. https://www.mdpi.com/2073-4344/8/10/478
  33. https://doi.org/10.3390/catal8100478
  34. https://pubs.rsc.org/en/content/articlehtml/2017/cs/c7cs00045f
  35. https://doi.org/10.1039/C7CS00045F
  36. https://www.fhi.mpg.de/1072789/rosenthal_thermal_analysis_150208.pdf
  37. https://www.fhi.mpg.de/1071368/christmann_adsorption_221004.pdf
  38. https://pubs.acs.org/doi/10.1021/acsphyschemau.2c00031
  39. https://pubs.acs.org/doi/10.1021/acs.chemrev.7b00776
  40. https://www.mdpi.com/2073-4344/7/11/341
  41. https://link.springer.com/article/10.1007/s42452-023-05416-6
  42. https://pubs.rsc.org/en/content/articlehtml/2022/cy/d2cy01325h
  43. https://pubs.acs.org/doi/10.1021/jacs.9b07484
  44. https://www.sciencedirect.com/science/article/pii/S266710932200286X
  45. https://www.sciencedirect.com/science/article/pii/S0920586123000214
  46. https://www.science.org/doi/10.1126/science.aao2109
  47. https://pubs.rsc.org/en/content/articlehtml/2023/ra/d2ra07644f
  48. https://pubs.acs.org/doi/10.1021/acs.accounts.5c00446
  49. https://pubs.acs.org/doi/10.1021/cr500486u
  50. https://www.researchgate.net/publication/349949786_A_Review_of_Preparation_Methods_for_Heterogeneous_Catalysts
  51. https://pmc.ncbi.nlm.nih.gov/articles/PMC6416638/
  52. https://www.researchgate.net/publication/322239219_A_Seed-Mediated_Approach_for_the_Preparation_of_Modified_Heterogeneous_Catalysts
  53. https://pubs.acs.org/doi/10.1021/acs.iecr.5c03601
  54. https://www.nature.com/articles/s41467-023-38050-2
  55. https://www.sciencedirect.com/science/article/pii/S2542435119305355
  56. https://pubs.acs.org/doi/10.1021/cs3005264
  57. https://pubs.acs.org/doi/10.1021/acs.chemrev.4c00184
  58. https://www.annualreviews.org/doi/10.1146/annurev-chembioeng-080615-034413
  59. https://www.mdpi.com/2073-4344/5/1/145
  60. https://www.mdpi.com/1996-1073/15/6/2061
  61. https://www.mdpi.com/2073-4344/9/4/362
  62. https://pubs.rsc.org/en/content/articlelanding/2021/re/d0re00431f
  63. https://www.nature.com/articles/s41598-019-51784-8
  64. https://www.sciencedirect.com/science/article/abs/pii/S2095495620303351
  65. https://pmc.ncbi.nlm.nih.gov/articles/PMC8790769/
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC8564627/
  67. https://www.sciencedirect.com/science/article/pii/S2949754X23000297
  68. https://www.sciencedirect.com/science/article/abs/pii/S0021951721003791
  69. https://www.nature.com/articles/s41467-024-54784-z
  70. https://www.sciencedirect.com/science/article/abs/pii/S0926337316303988
  71. https://www.sciencedirect.com/science/article/pii/S0360319916333821
  72. https://crsreports.congress.gov/product/pdf/R/R48196
  73. https://pmc.ncbi.nlm.nih.gov/articles/PMC11563198/
  74. https://www.sciencedirect.com/science/article/pii/S0920586123004716
  75. https://edu.rsc.org/feature/the-evolution-of-catalytic-converters/2020252.article
  76. https://pubs.acs.org/doi/10.1021/cr100168g
  77. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/catalytic-converter
  78. https://pubs.acs.org/doi/10.1021/acsomega.5c02980
  79. https://www.nature.com/articles/s41598-024-68289-8
  80. https://onlinelibrary.wiley.com/doi/abs/10.1002/pola.10962
  81. https://www.acs.org/content/dam/acsorg/education/whatischemistry/landmarks/raney-nickel/raney-nickel-booklet.pdf
  82. https://cdn.intechopen.com/pdfs/23539/InTech-A_review_of_mass_transfer_controlling_the_reaction_rate_in_heterogeneous_catalytic_systems.pdf
  83. https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202301425
  84. https://pubs.acs.org/doi/10.1021/jp047349j
  85. https://pubs.acs.org/doi/10.1021/acsami.4c19839
  86. https://pubs.aip.org/aip/apl/article/105/20/202114/27523/Band-gap-engineering-of-TiO2-through-hydrogenation
  87. https://pmc.ncbi.nlm.nih.gov/articles/PMC8002707/
  88. https://pubs.rsc.org/en/content/articlehtml/2014/cy/c4cy00327f
  89. https://www.science.org/doi/10.1126/science.aal3573
  90. https://www.sciencedirect.com/science/article/abs/pii/S2095495620301947
  91. https://pubs.acs.org/doi/10.1021/acscatal.1c01219
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