Heterogeneous catalysts have the ability to accelerate chemical reactions and enhance efficiency in countless industries.
They are employed in the production of synthesis gas and hydrogen, ammonia, methanol, and Fischer-Tropsch synthesis, in the transformation of hydrocarbons (epoxidation of ethylene and propene, ammoxidation of hydrocarbons, and hydroprocessing reactions), as environmental catalysts for catalytic reduction of nitrogen oxides from stationary sources, and in automotive exhaust catalysis.
Table of Contents
1. Synthesis Gas and Hydrogen
In the production of synthesis gas (syngas) and hydrogen, catalysts play an important role in various industrial applications. Fossil fuels like coal, petroleum, heavy oil, tar sands, shale oil, and natural gas, as well as renewable sources such as biomass, can be utilized for syngas production. Syngas, a mixture of hydrogen and carbon monoxide in different ratios, serves as a significant raw material for catalytic syntheses in the chemical industry.
Hydrogen production can be achieved through the water-gas shift reaction (WGS) by converting carbon monoxide (CO) to carbon dioxide (CO2) and subsequent CO2 separation. Direct hydrogen production methods include water electrolysis using electricity from diverse sources like nuclear power, solar, wind, hydro, geothermal, and oceanic energy, as well as combustion in power plants. Heterogeneous catalytic reactions play a role in the development of these new routes due to evolving energy resource.
Syngas is typically manufactured from coal through coal gasification, or from gaseous or liquid hydrocarbons using endothermic steam reforming (SR), exothermic partial oxidation (POX), or a combination of both known as autothermal reforming. Currently, natural gas is the primary feedstock for syngas production.
Nickel is the preferred catalyst for syngas production, although other group 8-10 metals like cobalt (Co) and iron (Fe) exhibit activity as well. Expensive metals such as platinum (Pt), ruthenium (Ru), and rhodium (Rh) demonstrate even higher levels of activity.
Industrial catalysts, often based on Ni/alumina with alkali metal promoters, are available in various forms such as pellets, rings, cylinders with holes, or ceramic foams, and are commonly used in tubular reformers. Ongoing advancements focus on compact reformers, efficient integration with heat exchangers, and heat recovery from the reformed gas.
Another technology for syngas generation is exothermic catalytic partial oxidation (CPOX) of hydrocarbons using metal catalysts, particularly rhodium (Rh). In this process, the fuel (natural gas, vaporized liquid hydrocarbons, alcohols) is premixed with oxygen at a C/O ratio of 1 and fed into a catalytic bed or monolith. Within a few milliseconds at approximately 1000 °C, the fuel is nearly completely converted to synthesis gas.
Autothermal reforming (ATR) combines steam reforming (SR) and partial oxidation (POX), utilizing the internal combustion of the fuel with oxygen to provide the necessary heat for the reforming reaction. CPOX can also be considered a two-stage process, where oxygen is first used to burn a portion of the fuel, followed by steam reforming of the remaining fuel to produce syngas. ATR has been extensively employed in the chemical industry and is now being considered for syngas production in gas-to-liquid (GTL) plants.
ATR processes employ either nickel- or rhodium-based catalysts, often with alumina or magnesium alumina supports for enhanced thermal stability and strength at high operating temperatures.
Several methods have been proposed for syngas production from alternative feeds such as ethanol or other biomass-derived fuels. Notably, CPOX and ATR techniques employing noble metal catalysts have demonstrated high syngas yields when operated at elevated temperatures and with short contact times.
The water-gas shift (WGS) reaction is of paramount importance in the industrial production of hydrogen, ammonia, and other bulk chemicals that utilize syngas, as it enables the adjustment of the CO/H2 ratio. Depending on the desired CO conversion, the WGS reaction is carried out in multiple stages.
The first stage, known as high-temperature shift (HTS), employs FeCr catalysts at temperatures ranging from 280 to 350 °C. Since complete conversion is not favorable at high temperatures due to equilibrium limitations, a second stage called low-temperature shift (LTS) is employed at temperatures between 180 and 260 °C. This stage utilizes CuZn- or CuZnAl-based catalysts to achieve a CO content of 0.05-0.5 vol%.
The production of syngas and subsequent hydrogen is also of significant interest in energy-related catalysis, particularly in the context of fuel cells.
2. Ammonia Synthesis
The Haber-Bosch process involves the synthesis of ammonia using promoted iron metal catalysts, combining nitrogen and hydrogen under conditions of approximately 400 °C and 15 MPa. Reactors with capacities of up to 1000 t/d are employed.
Due to thermodynamic preferences, the reaction leading to the desired product NH3 is more favorable at low temperatures and high pressures. To overcome this equilibrium limitation under practical conditions, a loop operation is employed, allowing for the recovery of the easily condensable product gas. The feed gases necessary for the process are obtained from air (nitrogen) and hydrogen through the syngas route.
While numerous systems have been tested over the years, only iron (Fe) and ruthenium (Ru) have proven to be practically useful catalysts.
The ammonia synthesis mechanism is well-known in the field of heterogeneous catalysis. Similar to CO oxidation on platinum (Pt), this reaction has served as a prototype for understanding heterogeneous catalysis by elucidating molecular behavior on the catalytic surface.
It represents one of the successful examples bridging the gap between surface science and industrial heterogeneous catalysis in terms of materials and pressure considerations.
Adsorption of nitrogen on single-crystal iron surfaces leads to surface reconstruction. Dinitrogen dissociates above 630 K, resulting in the formation of complex surface structures, identified as surface nitrides with depths spanning several atomic layers. These surface structures are stable, while the corresponding bulk compound is thermodynamically unstable under the same conditions.
The rate of dissociative adsorption of dinitrogen depends on the surface structure, with the Fe(111) face being the most active due to its low activation energy and high adsorption rate. This crystal face is also the most catalytically active. These observations support the notion that dinitrogen adsorption is an activated process and serves as the rate-determining step in the catalytic cycle.
Conversely, the adsorption of dihydrogen on iron is rapid and characterized by a high sticking coefficient and a minimal activation barrier. This dissociative chemisorption leads to covalently bonded H atoms, which exhibit high mobility on the iron surface.
Following evacuation, atomic nitrogen is shown to be the most stable and prevalent chemisorbed species on Fe(111). It is considered an intermediate in the catalytic reaction, while adsorbed dinitrogen is not deemed a reactive intermediate. The involvement of adsorbed N atoms in the rate-determining step is confirmed through kinetics experiments. Other surface intermediates, such as NH and NH2 species, are less stable and less abundant.
3. Methanol and Fischer-Tropsch Synthesis
3.1. Methanol Synthesis
The synthesis of methanol involves the reaction of synthesis gas according to the following stoichiometry:
CO + 2 H2 → CH3OH
It is widely accepted that this reaction proceeds through the conversion of CO via the water-gas shift reaction, followed by the hydrogenation of carbon dioxide. All of these reactions are characterized as exothermic and limited by equilibrium.
The achievement of a high methanol yield is favored by operating at high pressure and low temperature.
The initial methanol synthesis process was developed by BASF in Germany in 1923, operating at approximately 30 MPa and 300-400 °C over a Zn/Cr2O3 catalyst.
Substantial improvements were made by ICI in the 1960s with the introduction of the more active Cu/ZnO/Al2O3 catalyst, enabling synthesis under milder reaction conditions of 50-100 bar and 200-300 °C.
Presently, the majority of methanol plants employ this advanced low-pressure synthesis method.
Despite intensive research on copper-based catalysts for methanol synthesis over several decades, a consensus regarding the active sites and the reaction mechanism has not been reached.
In terms of the active site, it is understood that metallic copper in close proximity to ZnO is essential for an active and selective catalyst. Various mechanisms have been proposed to explain this synergy, including hydrogen spill-over from ZnO, stabilization of intermediates on ZnO or the interface between Cu and ZnO, and dispersion of Cu on the ZnO surface.
Formate, methoxy, and formyl are the most significant intermediates. A potential reaction mechanism involves the dissociative adsorption of hydrogen, hydrogenation of adsorbed CO to CO2, conversion of atomic hydrogen to formate, subsequent addition of hydrogen to form H2COO, hydrogenation of this species to a methoxy group, and finally hydrogenation of the methoxy group to methanol.
In industrial practice, a variety of reactor types are employed for low-pressure methanol synthesis, typically utilizing fixed beds of catalyst operated in the gas phase.
3.2. Fischer – Tropsch Synthesis
Fischer-Tropsch synthesis (FTS) is the direct conversion of synthesis gas into hydrocarbon chains. The specific composition of the desired product relies on the chain-growth probability, denoted as ‘a,’ which is determined by the catalyst employed.
Various studies investigating FTS support the carbene mechanism, which initiates with the decomposition of CO and incorporates methylene (CH2) species into the growing alkyl chain.
Metals such as iron, cobalt, and ruthenium can be used as catalysts for FTS. However, due to its high cost, ruthenium finds limited industrial applicability, leaving iron and cobalt as the primary catalyst choices.
Iron catalysts suffer from kinetic hindrance caused by water, a co-product. Conversely, they exhibit advantageous activity for the water-gas shift reaction, enabling the utilization of synthesis gas mixtures containing carbon dioxide or depleted in hydrogen.
In comparison to iron, cobalt catalysts demonstrate lower reaction temperatures for activity and can maintain durability for up to five years on stream, as opposed to approximately six months for iron. However, cobalt is more expensive than iron. Additionally, different promoters (Pt, Pd, Ru, Re, K) can be utilized alongside the active component.
Alumina, silica, and titania are viable carrier materials for these catalysts. Typical chain-growth probabilities range from 0.5 to 0.7 for iron and 0.7 to 0.8 for cobalt. Currently, the development of cobalt catalysts aims to maximize the chain-growth probability, potentially reaching values as high as 0.95.
Since the obtained product mixtures from these catalysts require further processing to achieve desired fractions like diesel and gasoline fuels, it has been proposed to combine Fischer-Tropsch catalysts with hydrocracking catalysts within a single reactor.
Considering that liquid products can occupy the pore system of working catalysts, leading to diffusion-related resistances even with small catalyst particles, catalyst efficiency significantly declines when the characteristic catalyst dimensions exceed 100 mm.
Furthermore, the higher diffusion coefficient of hydrogen compared to carbon monoxide within the porous catalyst results in an elevated H2/CO ratio. This, in turn, increases the probability of chain termination and consequently decreases the length of the hydrocarbon chains produced.
The pursuit of catalysts with exceptionally high chain-growth probabilities has facilitated the development of advanced low-temperature FTS, wherein synthesis gas and liquid products coexist under reaction conditions.
Industrial FTS reactors typically operate at pressures of 2-4 MPa and temperatures ranging from 220 to 240 °C. Presently, two reactor types are employed for low-temperature FTS: a cooled fixed-bed reactor predominantly used by Shell and a slurry bubble column developed by Sasol.
4. Hydrocarbon Transformations
Selective hydrocarbon oxidation reactions include various significant categories of heterogeneously catalyzed reactions that are extensively employed in large-scale industrial settings for the synthesis of bulk chemicals.
Exemples are oxidative dehydrogenation of alkanes, ammoxidation of alkenes, aromatics and alkanes, and epoxidation of alkenes.
4.1. Epoxidation of Ethylene and Propene
The catalytic epoxidation of ethylene using dioxygen is catalyzed by silver metal and results in the formation of ethylene oxide, an important intermediate for glycols and polyols synthesis. However, the selectivity of the process is limited by the occurrence of total oxidation reactions for both the reactant and the desired product.
To optimize the selectivity for ethylene oxide, the catalyst must be fine-tuned. The active phase consists of large silver particles supported on low surface area alpha-alumina (a-Al2O3) and promoted by alkali metal salts. The addition of chlorine-containing compounds such as vinyl chloride to the reaction feed has a positive impact on the process.
Under the reaction conditions, the chlorine additive readily undergoes combustion on the silver surface, leading to the adsorption of chlorine on the metal surface.
Oxygen can be adsorbed on transition metals in different states, including molecular dioxygen, adsorbed atomic oxygen, and subsurface atomic oxygen. On an Ag(111) surface, molecular oxygen is stable at temperatures below approximately 220 K but dissociates at higher temperatures.
Initially adsorbed oxygen atoms on the external surface of silver can migrate to subsurface lattice positions. Subsurface oxygen atoms have been observed on transition metals like Rh, Pd, and Ag.
When exposed to ethylene, the surface oxygen atoms interact with the p-electrons of ethylene, causing a transfer of electron density from the surface oxygen atom to the positively charged silver atom. This renders the surface oxygen atoms electrophilic, leading to a preferential reaction with the part of the reactant molecule that possesses the highest electron density.
This situation is more likely to occur at high oxygen coverages, which aligns with experimental findings that increasing oxygen coverage significantly enhances epoxidation selectivity. Consequently, the selectivity for epoxidation decreases as the oxygen coverage decreases.
The influence of alkali metal and chlorine modifiers on the catalytic process is complex. Chlorine serves a dual purpose: it inhibits vacant silver sites and enhances the electron deficiency of silver.
The epoxidation of propene using dioxygen is challenging due to the heightened reactivity of the methyl group towards nucleophilic attacks.
Activation of the methyl group can lead to the formation of allyl species or combustion of propylene epoxide. Alternatively, hydrogen peroxide or hydroperoxide can be used as oxidants. When propene reacts with hydrogen peroxide, the desired product, propylene epoxide, and water are formed.
The preferred catalyst for this reaction is titanium silicalite-1 (TS-1), in which the key role is played by tetravalent titanium (Ti4+) coordinated in a four-coordinate fashion.
Although the exact nature of the reaction intermediate is not yet fully understood, it is suggested that hydrogen peroxide coordinates non-dissociatively to a Lewis acidic tetrahedral Ti4+ site.
This coordination induces electron deficiency on the oxygen atoms of the peroxide, which promotes the epoxidation process. A similar reaction pathway has been proposed for the homogeneous epoxidation of propene using peroxides.
4.2. Ammoxidation of Hydrocarbons
Ammoxidation is a chemical process in which ammonia reacts with a reducible organic compound, typically an alkene, alkane, or aromatic, in the presence of dioxygen to produce nitriles.
The ammoxidation of an alkene involves a six-electron oxidation that results in the formation of an unsaturated nitrile and water. This reaction is related to the four-electron oxidation of alkenes, which produces unsaturated aldehydes and water, as well as the two-electron oxydehydrogenation of alkenes to dienes and water.
Catalysts employed for these reactions are typically complex mixed metal oxides containing variable-valence elements, with ammoxidation catalysts being the most intricate. These catalyst materials possess redox properties, meaning they can be easily reduced by ammonia and reoxidized by gaseous dioxygen.
The lattice oxygen within the catalyst is responsible for reacting with ammonia and the hydrocarbon, while the reduced solid is reoxidized by oxygen present in the gas phase.
One of the most significant alkene ammoxidation processes is the conversion of propene to acrylonitrile, known as the Sohio Acrylonitrile Process. Molybdates and antimonates can be used as catalysts for this reaction. The active sites in the catalyst are believed to have a bifunctional nature.
The proposed mechanism involves the initial interaction of ammonia with the bifunctional active site, leading to the formation of an ammoxidation site. The alkene then coordinates to this site, forming an allylic intermediate. Through several rearrangements and oxidation steps, the surface intermediate is eventually converted into the desired nitrile, which then desorbs from the catalyst.
This process results in the formation of a reduced surface site, which is restored to its fully oxidized state by lattice oxygen (O2-) provided by neighboring reoxidation sites. These sites then dissociate dioxygen to generate lattice oxygen.
The newly formed lattice oxygen diffuses to the oxygen-deficient reduced surface site, while vacancies simultaneously penetrate through the solid’s lattice to reach the reoxidation sites. The communication between these sites occurs via a common solid-state lattice capable of facilitating the transport of electrons, anion vacancies, and lattice oxygen.
In recent years, there has been growing interest in selective catalytic oxidation and ammoxidation of alkanes as more cost-effective alternatives to alkenes. Extensive research has focused on multicomponent metal oxide catalysts. Particularly promising results have been achieved with the MoV-TeNbO system, both for the oxidative dehydrogenation of ethane to ethylene and the ammoxidation of propane to acrylonitrile.
4.3. Hydroprocessing Reactions
Hydroprocessing treatments encompass various essential processes in the petroleum industry, such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), hydrometalation (HDM), hydrogenation, and hydrocracking. These processes have significant catalyst consumption.
Crude petroleum contains significant amounts of organosulfur and organonitrogen compounds, particularly in the heavier fractions, which must be eliminated for environmental purposes. These reactions occur in the presence of hydrogen (H2) at high temperatures (approximately 600-700 K) and pressures ranging from 500 kPa to 1 MPa.
Due to the lower reactivity of organonitrogen compounds compared to organosulfur compounds, the reaction conditions for HDN are more severe than those for HDS.
Catalysts used in hydroprocessing are highly dispersed metal sulfides, primarily molybdenum sulfide (MoS2), and sometimes tungsten sulfide (WS2), supported on gamma-alumina (g-Al2O3). These catalysts are typically promoted by cobalt or nickel, depending on the specific application.
The organosulfur compounds found in petroleum include sulfides, disulfides, and aromatic compounds (such as thiophene, benzothiophene, dibenzothiophene, and related compounds). Benzo- and dibenzothiophene are particularly abundant in heavy fuels.
In addition to hydrogenation and hydrodesulfurization, hydroprocessing reactions also involve hydrodenitrogenation, where organonitrogen compounds in the feed react with hydrogen to produce ammonia (NH3) and hydrocarbons. However, supported metal sulfide catalysts are less selective in removing nitrogen compared to sulfur.
Hydroprocessing reactions are carried out in various types of reactors, with the most common being fixed-bed reactors operated in the trickle-flow regime. These reactors involve the co-current flow of gas and liquid, either in an upward or downward direction.
5. Environmental Catalysis
5.1. Catalytic Reduction of Nitrogen Oxides from Stationary Sources
Fossil fuels like coal, oil, and gas are commonly burned or gasified for energy conversion. To reduce emissions, particularly of nitrogen oxides (NOx) from power plants, Western European countries and Japan have implemented measures since 1980.
Selective catalytic reduction (SCR) with ammonia in the presence of oxygen is the preferred method for removing NOx from exhaust gases in power plants, industrial boilers, and gas turbines.
The standard SCR reaction is most effective when NOx is primarily derived from high-temperature combustion processes with low levels of nitrogen dioxide (NO2). However, in exhaust streams with higher concentrations of NO2, the fast SCR reaction, which is at least ten times faster than the standard SCR reaction, may dominate.
At temperatures above approximately 450 °C, ammonia reacts with oxygen in an undesired parallel reaction, resulting in the production of N2, N2O, or NO. Conversely, at temperatures below 200 °C, ammonia and NOx can form solid deposits of ammonium nitrate and nitrite.
Initially, Pt catalysts were used for the SCR of nitrogen oxides. However, due to the high selectivity towards nitrous oxide (N2O) of these catalysts, base metal catalysts have been developed.
Vanadium-based catalysts supported on titania (preferably in the anatase form) and promoted with tungsten or molybdenum oxide exhibit excellent catalytic properties for SCR.
Anatase is the preferred support for SCR catalysts for two main reasons. Firstly, it undergoes only moderate sulfation under real exhaust gas conditions, and its catalytic activity may even increase after sulfation. Secondly, vanadium can form highly active structures with a large surface area when deposited in thin layers on the anatase support.
SCR reactors can be configured in various ways depending on factors such as fuel type, flue gas composition, NOx limits, and other considerations.
Iron-exchanged zeolites (such as MFI and BEA) are promising catalysts for nitrogen oxide removal. Although these catalysts have shown some deactivation, particularly due to mercury, they appear to be well-suited for “clean” exhaust gases found in nitric acid plants.
5.2. Automotive Exhaust Catalysis
Internal combustion engines found in cars are a significant contributor to the release of harmful air pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), unburnt hydrocarbons (HC), and diesel engines additionally emit soot.
To effectively reduce these pollutants, the combustion process needs to be modified. In addition to primary measures, meeting current emission standards set by regulations requires the implementation of secondary methods involving the use of catalysts for exhaust purification.
The choice of catalytic system for treating the exhaust gas depends primarily on the type of fuel used (gasoline, diesel, biofuels) and the specific operating conditions.
Generally, there are three categories: stoichiometrically operated gasoline engines, lean-operated gasoline engines, and diesel engines. Each of these engine types produces different primary pollutants, namely CO/NOx/HC, NOx, and NOx/soot, respectively.
5.2.1. Three-Way Catalyst
The primary type of catalytic converter commonly found in automobiles is known as the three-way catalyst (TWC) and is specifically designed for stoichiometrically operated gasoline engines.
TWC systems have been utilized in gasoline engines since the 1980s and typically consist of a combination of platinum/rhodium (Pt/Rh) or palladium/rhodium (Pd/Rh) with a mass ratio of approximately 5/1. The total loading of these precious metals is approximately 1.7 grams per liter.
The main function of the TWC is to simultaneously convert nitrogen oxides (NOx), carbon monoxide (CO), and unburnt hydrocarbons (HC) into harmless nitrogen (N2), carbon dioxide (CO2), and water (H2O). The catalytic components are supported by a cordierite honeycomb monolith coated with a high surface area γ-alumina (γ-Al2O3).
Additionally, the washcoat layer of the TWC contains thermal stabilizers like lanthanum oxide (La2O3) and an oxygen-storage component called cerium oxide (CeO2).
The catalytic conversion process of the TWC specifically occurs within a narrow range of oxygen content, which closely aligns with stoichiometric combustion conditions. This means that the air coefficient (l) should fall within the range of 0.99 to 1.01.
To achieve these optimal conditions, an oxygen sensor is utilized to measure the air coefficient of the exhaust stream. This information is then used by the engine management system to regulate the air/fuel ratio accordingly.
5.2.2. Selective Catalytic Reduction (SCR) of NOx by Ammonia
Selective Catalytic Reduction (SCR) is the sole technique capable of converting nitrogen oxides (NOx) into nitrogen (N2), even in highly oxidizing environments.
Due to its ability to effectively reduce NOx emissions in lean-burn engines, SCR has emerged as the preferred technology for NOx abatement.
SCR proves to be suitable for diesel engines, covering the temperature range in which they operate, and demonstrating efficient NOx reduction. Consequently, SCR has become a state-of-the-art solution for heavy-duty vehicles. However, the storage of ammonia (NH3) presents challenges in mobile applications.
To address this issue, an aqueous solution of urea known as AdBlue, with a concentration of 32.5 wt%, is currently utilized. AdBlue is sprayed into the exhaust system, where vaporizing urea-water droplets undergo thermolysis and hydrolysis, resulting in the production of ammonia.
Ongoing research is focused on optimizing the dosing system for AdBlue and developing catalysts that are free of vanadium, such as utilizing Fe-ZSM5 zeolites as substitutes. There is also exploration into alternative reducing agents like hydrocarbons and hydrogen.
5.2.3. NOx Storage Reduction Catalysts
Originally designed for lean spark-ignition engines, NOx storage reduction catalysts (NSC) are now being adapted for use in diesel passenger cars. The NSC process is centered around the periodic adsorption and reduction of nitrogen oxides (NOx).
These catalysts typically contain a combination of platinum (Pt), palladium (Pd), and rhodium (Rh) in a mass ratio of approximately 10/5/1, with a total loading of precious metals amounting to approximately 4 grams per liter. NSCs also incorporate basic adsorbents such as aluminum oxide (Al2O3) at a concentration of 160 grams per liter, cerium oxide (CeO2) at 98 grams per liter, and barium carbonate (BaCO3) at 29 grams per liter (equivalent to BaO).
During the lean phase of engine operation, the exhaust NOx is adsorbed onto the basic components of the NSC, primarily barium carbonate, resulting in the formation of nitrate compounds.
Once the storage capacity of the catalyst is reached, the engine is momentarily operated under rich conditions for a few seconds. This rich phase generates an exhaust stream containing carbon monoxide (CO), hydrocarbons (HC), and hydrogen (H2), which serve as reducing agents for the regeneration of the catalyst. This leads to the conversion of the stored nitrate compounds back into carbonate form.
The presence of the barium component allows for effective NOx adsorption at temperatures above 250 °C. Additionally, significant storage capacity is provided by aluminum oxide and cerium oxide at lower temperatures.
5.2.4. Catalytic CO Oxidation
Catalytic oxidation of carbon monoxide (CO) plays important role in both three-way catalysts (TWC) and NOx storage reduction catalysts (NSC). This reaction has been employed in diesel engines since the 1990s through a system known as the direct oxidation catalyst (DOC).
Additionally, catalytic CO abatement has become a state-of-the-art technology in gas turbine engines powered by natural gas. DOCs typically contain platinum (Pt) as the active component, demonstrating excellent performance in CO oxidation.
To reduce costs, the expensive platinum can be substituted with palladium, which is less active but more affordable. The precious metal loading in a DOC is approximately 3 grams per liter. DOCs are also capable of oxidizing gaseous hydrocarbons (HC) as well as HC adsorbed on soot particles.
5.2.5. Removal of Soot
Diesel particulate filters (DPF) are utilized to effectively remove soot particles from diesel exhaust. These filters operate by compelling the exhaust gas to diffuse through porous walls, mechanically separating the particles and achieving high filtration efficiency.
The application of DPFs necessitates a process called regeneration, which involves the oxidation of the accumulated soot particles. Soot deposits can cause significant backpressure, leading to increased fuel consumption and reduced engine efficiency.
The preferred method for DPF regeneration is the use of continuously regenerating trap (CRT) technology. This approach initiates the oxidation of soot through the production of nitrogen dioxide (NO2), which is generated by oxidizing nitrogen monoxide (NO) on platinum (Pt) catalysts, similar to the processes employed in NSC and SCR technologies.
Pt catalysts can be implemented as a precatalyst or applied as a coating on the DPF. Additionally, fuel-borne catalysts (FBC) known as organometallic compounds, based on elements such as cerium (Ce) or iron (Fe), such as ferrocene, can be added to the fuel to aid in the regeneration process.
- Heterogeneous Catalysis and Solid Catalysts, 3. Industrial Applications; Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.o05_o03