1. Introduction
The use of fossil fuels in the transport sector has increased the greenhouse gas (GHG) emissions during the last decades [
1,
2]. Fossil transport fuels such as gasoline or diesel release exhaust gases composed by carbon dioxide (CO
2), methane (CH
4), nitrous oxide (N
2O), carbon monoxide (CO) and hydrofluorocarbons (HFCs) [
3]. Indeed, the GHG from the transport sector amounts 28% of the total US emissions [
1]. This percentage is lower (19.4%) for European Union, but still important [
4]. The reduction of these emissions are a big challenge for breaking the so called climate change cycle [
5]. Some strategies have been proposed to solve the problem of emissions in the transport sector. Increasing the share of renewable energy and moving towards hydrogen as an energy vector are the preferred actions [
6]. In fact, powering our vehicles through fuel cells or hydrogen internal combustion engines is already technically feasible [
7], but overcoming the overall cost attached to these two technologies is still a challenge in the path towards a low-carbon transport sector [
7]. In this line, other options must be explored. As presented in a recent review [
8], advances in engine technologies are placing additional demands on emission control catalysts. In this sense, the use of catalytic converters to reduce exhaust gases from vehicles and converting them into unharmful compounds is widely extended [
9]. Among catalytic converters, three-way catalytic (TWC) converters have been the state of the art technology since 1970 [
10].
TWC converters are a tool used for reducing the emission of pollutant gases that are present in the exhaust gas released from an internal combustion engine of an automobile [
11]. TWC is chosen over a two-way catalytic converter in automobiles because it can reduce the emissions of the N
2O gases, as well as the emission of CO gas and unburned hydrocarbons. Two-way catalytic converters will only be able to reduce the emissions of CO gas and unburned hydrocarbons, due to the catalysts used and the reactions that occur [
11]. By using the concept of heterogeneous catalysis and rate of reaction, a honeycomb ceramic structure (usually coated with Al
2O
3), which acts as catalyst support, is used. The main reason is that it allows the best possible contact between the flowing exhaust gas and the catalyst surface [
12]. The honeycomb ceramic structure is a catalyst support that provides a large surface area, in order for redox reactions to take place at a higher rate and efficiency. A large surface area, along as temperature and pressure, is one of the main factors that provides a higher rate of reaction [
13]. The structure is then coated with a solution containing different noble metal nitrates such as palladium nitrate, rhodium nitrate and platinum nitrate [
14]. The honeycomb structure facilitates the flow of gas through the converter and controls any pressure drops.
Even though TWC converters are widely used commercially, and fundamental aspects and technology have been described [
15,
16,
17,
18,
19,
20], a review from an educational perspective is still missing. Hence this work is a useful initial resource for newcomers to the field to familiarize them in a direct and clear manner with the main concepts and characteristics of TWC technology. Therefore, the goal of this paper is to offer an overview of the main fundamental working principles of TWC from an educational perspective. To this end, this work is organised as follows. First, the working principles of TWC converters are explained. Afterwards, the most common structures used for the devices are reviewed, along with the typical working conditions for a TWC. The rates of reaction are subsequently provided, focusing finally on pollutant gases reduction.
2. Working Principles of Three-Way Catalytic Converters
Internal combustion engines use the exhaust stroke to expel the spent gases, via the exhaust system, where the harmful emissions are then passed through a muffler-type looking device which is the catalytic converter [
12,
21]. Components of the spent gases consist of unburned hydrocarbons, NO, and CO [
21]. As mentioned above, the main purpose of the catalytic converter is to reduce the original harmful emissions to the most acceptable levels, by the means of catalyst controlled chemical reactions. Therefore, there are needs to be the most careful attention to the engine design, before venting out the gas into the air [
12].
The majority of automobile catalytic converters have a design of a monolithic structure, which is coated with an alumina washcoat. The monolithic structure is referred to as the core of the catalytic converter, where the core is typically a ceramic monolith with a honeycomb configuration. The purpose of the honeycomb structure is to allow for the heterogeneous catalysis to occur on the surface of the honeycomb structure. Metallic monolith is made out of FeCrAl where it has high heat resistance. The washcoat can also consist of aluminum oxide, silicon dioxide, titanium dioxide or a mixture of silica and alumina [
22].
Catalytic converters use metallic catalysts to promote the desired reactions at lower temperatures [
14]. Common metal catalysts used could be a base metal (such as chromium) and noble metals, such as platinum, palladium, and rhodium [
14]. These catalysts can be used for redox reactions; the reduction of N
2O and the oxidation of unburned hydrocarbons and CO. If platinum, palladium, and rhodium metals are used, these can be a part of the Platinum Group of Metals Solution (PGM Solution), which is also used to coat the honeycomb structure. This would allow for heterogeneous catalysis to take place on the surface of the core, allowing for the production of less toxic gases (CO
2, nitrogen and water vapour) to be produced [
23].
Another component of the catalytic converter is its metal casing that surrounds the core of the catalytic converter. This metal casing directs the exhaust gas flow through the catalyst bed. The metal casing is usually made out of stainless steel, but since low-temperature catalysts are used, stainless steel may not be necessary for the construction of the catalytic converter [
24]. A diagram of a TWC converter is shown in
Figure 1.
The initial step in heterogeneous catalysis is the adsorption of the reactants, where adsorption refers to the binding of molecules to a surface [
12]. Adsorption would occur because of the high reactivity of the atoms/ions on the surface of the solid, facilitating the honeycomb the flow of gas through the converter and controls pressure drop. Since the composition of the exhaust gas also contains oxygen gas and the pollutant gases (CO, N
2O and unburned hydrocarbons), oxygen atoms are made available to react with the other adsorbed pollutant gases. In the case of CO, the oxygen atom reacts with CO yielding CO
2, and so, as the final step in heterogeneous catalysis, CO
2 desorbs from the metal surface and is released as a product out of the catalytic converter. This concept is identical for both the N
2O and the unburned hydrocarbons.
Briefly, in a TWC converter, the simultaneous redox reactions consist of a three-way system which controls emissions by oxidation of the unburned hydrocarbons and CO and by the reduction of the NO
X [
27], (unlike two-way catalytic converters, where only 2 oxidation reactions occurs, so the N
2O are not reduced), so the three-way catalytic converter would have the following functions:
- (1)
Oxidation of unburned hydrocarbons, where oxygen gas is present in the exhaust gas, has its bonds broken and the oxygen atom reacts with the unburned hydrocarbons to produce CO2 and water vapour as the final products. An example would be the oxidation of benzene (Equation (1)):
In this particular reaction, palladium or platinum would be used. Even though palladium and platinum both have similar chemical properties [
28], palladium would be preferred over platinum because of the decline in effectiveness of the converter, which means that platinum deactivates faster than palladium, therefore the converter efficiency would decrease very fast [
29].
- (2)
Oxidation of CO to form CO2 by using either catalysts platinum or palladium nitrate. Oxygen gas that is present in exhaust gases is adsorbed to the surface of the honeycomb ceramic, and so the oxygen bond is weakened and so the oxygen atom reacts with CO to give CO2 (Equation (2)):
Either platinum of palladium can be used as a catalyst for this reaction, as they both have very similar physical and chemical properties.
- (3)
Reduction of N2O to give stable nitrogen and oxygen gas (Equation (3)). Since this is a reduction reaction, rhodium is used instead. Since it is a rare type of noble metal, rhodium is usually alloyed with platinum or palladium.
Rhodium metal is used for this reduction reaction because it is an oxidising agent (a substance that loses electrons), compared to the other reducing agents, palladium and platinum. For these redox reactions the amount of oxygen gas present in the exhaust gas is a critical parameter. If there was a higher concentration of oxygen gas present in the exhaust gas than required, the system is defined as lean, so oxidation of unburned hydrocarbons and CO would more likely occur, as there is less fuel in the automobile. However, if there is a lower concentration of oxygen gas present in the exhaust gas, the system is defined as rich, so reduction of N
2O are more likely to occur as there is a higher concentration of fuel than needed. Therefore, the efficiency of catalytic converters are not always 100% efficient [
11].
In order to achieve the best efficiency of a catalytic converter, the fuel/air ratio, which defines how much fuel or excess air is inside the catalytic converter, can be measured using a feedback control loop system, where the metering of the fuel needs to be precisely measured, and the concentration of the oxygen leaving the catalytic converter is measured using a specific sensor [
27].
3. Structure of Three-Way Catalytic Converters
To construct a catalytic converter, the following components must be considered:
- (1)
Catalyst Core (substrate).
Typically, for a catalytic converter, the core is an open-channel ceramic monolith or either a metallic honeycomb substrate that provides an area for the catalyst [
30]. The most commonly used structure, the honeycomb ceramic, is designed to maximize the surface area (
Figure 2) [
31]. A larger surface area would result in a higher reaction rate [
32]. Cordierite (2MgO-2Al
2O
3-5SiO
2) is the most commonly used type of monolith because it has a high surface area, large open frontal area, low heat capacity, and good mechanical strength [
30,
33].
Figure 2 displays how the catalyst is placed on the monolith channel.
- (2)
Washcoat
A coat of supported catalysts is applied to the surface of the honeycomb ceramic structure. A thin coating of a wet-based slurry is directly applied on the highly porous material, which contains alumina as well as different metal oxides or zeolites [
36]. The honeycomb ceramic structure is usually coated with γ-Al
2O
3, due to its high resistance towards higher temperatures [
14], which contains the catalyst materials of about 0.1–0.15%, 20% cerium oxide, and stabilisers such as barium oxide. Rare earth and alkaline ions can improve stabilisation [
14]. Cerium oxide is mixed with the catalyst materials as it is used to improve the thermal stability of alumina, and can store and release O
2 in, respectively, lean and rich conditions [
37]. This thin coating of the wet based slurry is then dried up and calcined, which is the heating of solids to a high temperature to remove volatile substances.
There has been research done to directly apply the metal catalysts onto the surface of the honeycomb ceramic without the involvement of drying and calcination. According to [
38], this method involves the use of electrolysis (the electroplating technique), where a washcoat of γ-Al
2O
3 on nickel oxide (NiO) catalyst is electroplated onto the FeCrAl substrate; the honeycomb ceramic is submerged into a solution and alumina ions are directly transferred to the honeycomb surface. A schematic diagram showing the set-up of the experiment is shown in
Figure 3. As a result, [
38] concluded that the CO emissions were below 7 ppm when a catalytic converter of this experiment was used.
Washcoat materials are selected in order to form a rough, irregular surface, which will increase the surface area compared to the smooth surface of the substrate. This coat will protect the sintering of catalytic metal particles even at high temperatures that can go up to 1000 Degrees Celsius [
39].
- (3)
Catalyst Solution
Noble metals are the most commonly used heterogeneous catalysts, as they can be made more thermally resistant to reduced low-temperature activity. Another excellent characteristic is that they provide equivalent catalytic activation with smaller volumes than do base metals. Therefore, a small number of noble metals in the form of a solution, such as palladium, platinum and rhodium nitrates, is used to coat on the surface of a honeycomb ceramic [
14]. This type of heterogeneous catalyst solution, called a PGM Solution, is a solution of platinum group metals that are highly resistant to chemical attacks and have very high-temperature resistance and corrosion resistance [
40], therefore deemed to be the most appropriate catalyst solution in a catalytic converter [
41]. The PGM Solution contains platinum, palladium and rhodium, where its chemical and physical properties are shown in
Table 1, and is coated onto the honeycomb ceramic structure, supported by a coat of supported catalysts of thermal resistant metals [
41].
The quantity of platinum, palladium and rhodium used in auto catalysts depends on the vehicle type, manufacturer, country, year and additional factors [
40]. In TWC Converters, the ratio of Pt/Rh is 5 to 1, and the ratio of Pd/Rh is 7 to 1 [
40].
As demand for TWC converters are increasing, the demand for PGM is also increasing [
40]. About 15–20% of the world demand for platinum comes with recycling from used catalytic converters, however, the amount of platinum required is not enough to meet the growing world demand, hence the shrinking in platinum reserves and the increase in platinum price. Therefore, the high value of PGM has stimulated the recovery of PGM from used catalytic converters [
40], and it is widely practiced. One of the common methods of PGM recovery is shown in
Figure 4. Recovering PGM is very important as it provides supplementary source to the mining of these metals, therefore protecting the environment by limiting the number of waste disposal, saving natural resources exploitation, limiting electricity consumption and diminishing pollutant emission [
40].
- (4)
Metal Casing
Due to mechanical vibrations inside the car and thermal stresses, the metal casing is essential [
43]. Furthermore, a metal casing is used to direct the exhaust gas flow, since gas flow is a scalar quantity (only has a magnitude but no direction). High thermal stress can alter the shape of the substrate easily, so a metal casing is required to hold the ceramic together. At high thermal stress, this can result in permanent plastic deformations [
43]. The metal casing can be produced through the canning process. A thinned wall metal casing is desired [
44], as this can allow better heat transfer to the surroundings, to prevent the catalytic converter from reaching temperatures of up to 1000 °C. However, the question of durability and efficiency is considered, as high temperatures can result in deformation of the catalytic converter [
45,
46].
In another design, the option of using a support mat is considered, where it is placed in between the honeycomb ceramic and the steel casing acting as thermal insulation. A schematic diagram is shown in
Figure 5. A support mat purpose is to act as a mechanical support of the monolith, provide thermal insulation and minimize the potential for shell deformation [
45]. Shells may be a variety of grades of stainless steel and are an integral part of the exhaust system. Therefore, the designer should take the thermal expansion differences and yield strengths of various materials into consideration [
45]. Substrate and the steel shell of the catalytic converter have different thermal coefficients of thermal expansion, so there is a gap present between both sections. The gap expands and contracts as the converter cycles in temperature during its use [
45]. Gap expansion can be minimized by adjusting the material of the stainless-steel shell, where it is suggested to use ferritic steel shells (SS409), where is has about 50% lower thermal expansion compared to austenitic (SS310) [
45,
47].
5. Rate of Reaction
The rate of reaction is defined as the change in the concentration of reactants and products in a certain amount of time, or in simpler terms, the speed of a chemical reaction [
12]. The reactants, which in the case of a catalytic converter, would be the exhaust gas that is directly emitted from the combustion engine are conducted inside the catalytic converter, where these reactions produce the products, which produces the non-toxic gases.
To determine how each reaction happens and the conditions that are required to optimize the reaction, the factors that affect the rate of reaction need to be known. The key factors that would affect the rate of reaction would be the temperature of the reaction, pressure, the concentration of reactants, surface area, and catalysts [
12].
If the temperature of the reaction is increased, the kinetic energy of the molecules (moving energy) would increase. An increase in kinetic energy means that the molecules would gain more energy to move around, as kinetic energy is the energy of an object has because of its motion. This would allow for molecules to move around more frequently and collide with each other at faster speeds, so the reaction rate would increase. So, a high temperature is more favorable for a chemical reaction if the reaction needs to be sped up.
When the exhaust gas temperature is between 450 K and 500 K, NO conversion efficiency of less than 10% and concentration of N
2O generated is less than 0.01% [
61]. It is because as there is a lower temperature in under these temperature conditions, the activity of the catalyst inside the porous medium is too low, so the reaction progresses slowly, making the reaction progress slowly. However, at a temperature at 600K, the NOx conversion efficiency reaches 68%—highlighting the fact that as the temperature increases, the activity of the catalyst i.e., the kinetic energy of the catalyst increases. A high temperature is favourable.
For a general gaseous reaction, if the pressure increases, this would force the gas particles closer together and so there are more gas molecules in a given volume; allowing for more frequent collisions to occur. Since the rate of reaction is dependent on the number of collisions, the rate of reaction would increase. Therefore, the products of a reaction are produced in a much shorter amount of time. The chances of collision occurring are greater. The effect of pressure on the catalytic activity is investigated by changing the pressure and observing the methane and CO
2 conversion. Rising the pressure from 1 to 2 to 4 Bar leads to an increase in methane, CO and HCHO conversions, and there is higher residence time. This makes sense as the residence time is the time required to process one (reactor) volume of feed under specific feed conditions. Therefore, if the pressure increases, the gas concentration in a given volume increases, giving a higher residence time [
62].
Concerning the surface area, as it increases, ‘more molecules are exposed’ to the surroundings, so collisions between molecules inside the catalytic converter would occur more frequently, so more products are produced at a higher rate, meaning that the rate of reaction would increase [
62].