Current scenario of CNG vehicular pollution and their possible abatement technologies: an overview

Abstract

Compressed natural gas is an alternative green fuel for automobile industry. Recently, the Indian government is targeting to replace all the conventional fuel vehicles by compressed natural gas (CNG) automobiles due to its several merits. Still, the presence of a significant amount of CO, CH₄, and NO_x gases in the CNG vehicle exhaust are quiet a matter of concern. Thus, to control the emissions from CNG engines, the major advances are under development of and oxidation is one of them in catalytic converter. In literature, the catalysts such as noble and non-noble metals have been reported for separate oxidation of CO and CH_4.. Experimentally, it was found that non-noble metal catalysts are preferred due to its low cost, good thermal stability, and molding tractability. In literature, several articles have been published for CO and CH₄ oxidation but no review paper is still available. Thus, the present review provides a comprehensive overview of separate as well as simultaneous CO and CH₄ oxidation reactions for CNG vehicular emission control.

Introduction

Internal combustion (IC) engines are used in various fields like transportation, agriculture, power generation, industries, defense, etc. However, the incomplete combustion inside the IC engines emits primary pollutants like carbon monoxide (CO), unburned hydrocarbon (HC), nitrogen oxides (NO_x), and soot/particulate matters into the atmosphere (Veldsink et al. 1995). These primary pollutants are precursors for the formation of secondary pollutants such as peroxyacrylonitrile (PAN), polycylic aeromatic hydrocarbon, acid rain, smog, tropospheric ozone, etc. (Prasad and Singh 2012). Both these pollutants are playing a major role in global warming and climate change. Intially, air pollution was considered a local issue (impacting air quality, public health, ecosystem, storm, cloud burst, drought, etc.), but later on, it is recognized as an intricate problem of regional (acid deposition, atmospheric haze, impact on vegetation, etc.) as well as global influences (greenhouse effect, climate change, and stratospheric ozone depletion) (National Research Council 2010; Task Force on Hemispheric Trasnport of Air Pollution (HTAP) 2007).

It is belived that combustion of conventional fossil fuels such as gasoline and diesel may even get an increase by threefold by 2050 due to the ever increasing number of vehicles on the road (Andrews and Shabani 2012). The vehicles running on fossil fuels have made living easy and convenient; on the other hand, they have also made human life more complex and in danger because of their toxic emissions; so, vehicle is a “necessary evil.” In view of protecting the health and ecology, motivation toward the use of alternative fuels with low emissions for vehicles has been growing from the last few years. Compressed natural gas (CNG) has emerged as an attractive alternative fuel for IC engines (Bhandari et al. 2005; Christensen and Johansson 1998). In the last few years, CNG vehicles on the road has been increasing exponentially around the globe as mentioned in Fig. 1 due to numerous advantages over other fossil fuels. There were more than 24 million CNG vehicles worldwide by August 2017 (Perry 2017).

Fig. 1

Application of CNG-fuelled engines in various fields

From Fig. 2, it can be seen that a number of CNG vehicles is continuously increasing worldwide and reaches to 26.13 million in 2018. Thus, it is expeted that more than 30 million of CNG vehicles will run on the road till 2021.

Fig. 2

Growth of total CNG vehicles in the world from 2000 to 2020 (Perry 2017)

CNG as an alternative eco-friendly fuel

The CNG is a natural gas maintained under pressure of 200–250 bars which remains clear, odorless and non-corrosive. It consists of high content of CH₄ gas which produces engine power when mixed with air and combust in IC engines. The constituent gas composition of CNG also varied depending upon the source of origin, as mentioned in Table 1.

Table 1 Composition of CNG fuel based on its source of origin (Semin 2008)

The CNG can be produced in industry via three typical methods, thermogenic (thermal treatment), biogenic (biological treatment), and mixed (includes thermal as well as biological treatment). CNG produced by thermogenic method contains relatively higher concentration of CH₄ (> 60%) as compared with biogenic methane (60%) and mixed concentration (50–60%) (Faramawy et al. 2016). Depending upon the production of CNG, it consists of some other hydrocarbon and non-hydrocarbons apart from CH₄ in its composition. These constituents are higher hydrocarbons (C₃H₈, C₄H₁₀, etc.), He, diluents (CO₂ and N₂), and contaminants (sulfur, Hg, As, naturally occurring radioactive materials, solid matter, etc. (Faramawy et al. 2016). The CNG has several advantages in comparison with conventional-fuelled vehicles. The comparison of different physical and thermodynamic properties of CNG with fossil fuels is mentioned in Table 2.

Table 2 Comparison of CNG with conventional fuel on the basis of physical and thermodynamic properties (Wright 2015)

From Table 1, it can be observed that CNG consisting of > 90% CH₄ has the lowest C/H ratio among the other HCs present in conventional fuel with a relatively less than 20–30% CO₂ and particulate matter emission that indicates it is an eco-friendly fuel (Nwaoha and Iyoke 2013). It has higher octane rating ~ 130 and auto-ignition temperature of 540 °C as against diesel’s 260 °C which provides CNG additional safety, as shown in Table 2 (Haq et al. 2003; Wright 2015). Moreover, CNG fuel has some other benefits in comparison with gasoline and diesel-fuelled vehicles, as mentioned below:

• CNG is abundant in nature. Furthermore, being in a gaseous form, no distillation is required, thus the production cost of CNG is ~ 50% lower than petrol or diesel.

• It is a safe fuel, being lighter than air; it disperses into the atmosphere in case of any leakage. If its concentration in the air is less than 5% or more than 15%, the gas will not burn even in the presence of a spark (Haq et al. 2003).

• The CNG is adulteration free. When it enters in the engine in gaseous form (and not as a spray or mist like other fuels), it does not contaminate or dilute lubricating oil thus increasing the life of lubricating oils (Bhandarkar and Nijagunappa 2016).

• The CNG reduces chances of wear and tear and prolongs the life of an engine (Hossain 2014).

• The CNG avoids fouling of spark plugs, thereby enhancing plug life and have low operational and maintenance costs (Khan et al. 2015).

• It can be used efficiently in spark ignition (SI) as well as compression ignition (CI) engines (Cho and He 2007; Christensen and Johansson 1998).

• It has superior starting even under severe cold or hot weather conditions, reliable idling, and smooth acceleration (Nwaoha and Iyoke 2013).

• In comparison with diesel/petrol, CNG has wide applications in various fields such as transportation—scooter, car, bus, truck, train, and ship; industries; agriculture—tractor (Kamel et al. 2002), pumping set, and thresher; power generation (Spath and Mann 2000); mining and construction equipment; security—mechanized guns and battle tanks; etc. (Fig. 1).

CNG-allied engines

The substitution of conventional fuel-based (gasoline/diesel) vehicles for road transport can be attained by introducing new vehicles equipped with CNG engines in the market. The following options are available for the conversion of conventional engines by CNG for road transport:

• Dedicated fuel: A dedicated CNG vehicle is one that runs only on CNG fuel. It can be either a new CNG vehicle or modified gasoline engines to CNG. The dedicated CNG vehicles have higher engine efficiencies than either petrol or diesel, which means higher compression ratios due to high octane number of CNG (Kowalewicz and Wojtyniak 2005).

• Bi-fuel: Bi-fuel engines are also called “switchable” systems because it can be switched between gasoline or CNG. Most conversions are done for light-duty (cars and vans) bi-fuel engines because they provide the best experience of both the fuels. These types of vehicles are free from the danger of running out of fuel in the case of limited CNG refueling stations (Kowalewicz and Wojtyniak 2005).

• Dual fuel: A dual-fuel engine utilizes a mixture of CNG and diesel, with diesel as a “pilot” ignition source. The diesel is injected directly into the combustion chamber, while gas is introduced into the air intake by carburetion or by gas injection. The engines run mostly on diesel at low loads, i.e., idling engine condition, but using a mixture of two fuels (~ 80–90%) at higher load condition (Kowalewicz and Wojtyniak 2005).

Challenges of CNG vehicles

In spite of several advantages, CNG vehicles have the following challenges also and some of them are listed below:

• Fuel storage: CNG tanks requires a much larger volume to store the same mass of natural gas as compared with other fuels and also needs a very high pressure of ~ 200 bars or 2900 psi (Pascoli et al. 2001).

• Lack of fuelling stations: CNG filling stations have limited availability. If car users might drive to locations that are not equipped with CNG stations, they move toward more convenient fuel that is easily accessible everywhere (Sierzchula et al. 2012).

• Exponential growth of the CNG-fuelled vehicles would necessitate a new gas pipelines, new refueling stations, and other infrastructure.

• Emissions of undesired gases like unburned hydrocarbon (CH₄), CO, and formaldehyde (HCHO) in the exhaust that is directly affecting the environment and ambient air quality (Corrêa and Arbilla 2005; Trivedi and Prasad 2016).

CNG vehicular emissions

CNG vehicles emit a significant amount of CH₄ as a slip and CO and HCHO gases due to partial oxidation. These gases are harmful to human beings, the environment, and the ecosystem. The possible mechanism for the formation of CO and HCHO is explained by Eq. (1). The oxidation reactions involving intermediate species like HCHO finally lead to formation of CO.

$$ {\mathrm{CH}}_4\to {{\mathrm{CH}}_3}^{\mathrm{o}}\to {{\mathrm{CH}}_3}^{\mathrm{o}}+{\mathrm{O}}_2\to \mathrm{HCHO}\to \mathrm{CO} $$

(1)

In real situation, the complete combustion of CH₄ is not possible. Thus, a significant amount of unburned CH₄ is present in the exhaust gases of the engines. Under ideal conditions, the combustion of CH₄ should produce CO₂, H₂O, and energy, as represented by Eq. (2).

$$ \mathrm{C}{\mathrm{H}}_4\left(\mathrm{g}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\ \left(\mathrm{l}\right)\kern1.75em {\Delta \mathrm{H}}_r^o=-890\ \mathrm{kJ}\ {\mathrm{mol}}^{-1} $$

(2)

A typical automobile exhaust contains approximately 0.5–6.4% CO, 300–1000 ppm hydrocarbon (HCs), and 50–3000 ppm NO_x. The variation in the concentration of vehicles’ exhaust gases depends upon the running condition of the vehicles (Parida 1992). Generally, CNG engines run at lean conditions to maintain low combustion temperature and have low CO emissions. The emissions from the CNG automobiles depend upon the operating conditions of engine such as rich or lean, acceleration, deceleration, cruising, road conditions, traffic conditions, etc. The lean and rich conditions are decided by the value of lambda (λ), which is a ratio of actual air/fuel to stoichiometric air/fuel. When λ = 1, air/fuel mixture is stoichiometric; for λ < 1, the mixture is rich in fuel; and while λ > 1, the mixture is lean in fuel (Cho and He 2008; Ly 2002). The lean-burn engines can work on high compression ratios and thus offer better performance in comparison with conventional stoichiometric engines. So, lean-burn engines are preferred at present due to their inherently lower exhaust CO, HC, and NO_x emissions compared with stoichiometric engines. Lean-burn engines are also able to provide power and torque levels similar to those of conventional engines. They are more fuel efficient than conventional engines. Lean-burn engines’ concepts are used in both light-duty as well as heavy-duty vehicles. The emissions from IC engines under rich and lean conditions are represented in Fig. 3.

Fig. 3

Emission from IC engines (Ly 2002)

The emission characteristics are in general for all types of fuels used in the SI engines. It is clearly seen from the figure that high hydrocarbon (HC) emissions occur from the vehicles at fuel-rich condition (λ < 1) and decreases to a minimum at lean of stoichiometry, but it increases again in the lean-burn region (λ > 1) where the oxidation temperature is lowered by the dilution with excess oxygen and nitrogen. The CO emission from the vehicles is the least in lean-burn conditions. However, NO_x emission is high on the lean side of stoichiometry. The presence of excess air causes thorough combustion at a lower temperature, resulting the least CO as well as low NO_x formation. As the fuelling moves further into the lean-burn region, NO_x emissions fall sharply and CO emissions are at a minimum. However, HC emissions increase and engine power falls. Ultimately, the lean misfire and flame-out regions are reached, where HC emissions are very high and the power drops sharply.

Adverse impact of CH₄ and CO emissions

CH₄ (known as direct “greenhouse gas”) has tetrahedron structure (Fig. 4), and it is a colorless as well as odorless gas. In general, hydrogen sulfide (which smells like rotten egg) is usually mixed with CNG in order to detect during leakage (O’Brien et al. 2004). In a perspective of climate effect, CH₄ is 23 times more potent than CO₂ in trapping heat in Earth’s atmosphere (Trivedi and Prasad 2016) .

Fig. 4

CH₄ and its structure

CO is a colorless, odorless, tasteless, and non-irritating poisonous gas produced during the burning of organic fuel. It is 96.5% as heavy as air and is not appreciably soluble in water. It has also been labeled as “the unnoticed poison of the twenty-first century” and is frequently called “the silent killer,” because it gives no clear warning to its victims. Inhaling even relatively small amounts of CO can lead to hypoxic injury, neurological damage, and even death. It does not only affect human beings but also vegetation.

Effect on human health

Epimiological and clinical studies provide evidence for a progression of some of the adverse health effects of CO in humans. CO is a deadly and treacherous gaseous pollutant. CO poisoning interferes with the normal functioning of hemoglobin as a transporting agent for oxygen in blood.

The oxygen-carrying capacity of the blood depends upon the affinity of hemoglobin in the red blood cells. Moreover, it is found that hemoglobin has 210 times higher affinity for CO as compared with oxygen. Hemoglobin reacts with CO and forms a stable product called carboxy-hemoglobin, as shown in Fig. 5a. The formation of carboxy-hemoglobin reduces oxygen delivery capabilities of the blood to the body’s organs (like the heart and brain) and tissues. The symptoms of diseases at various concentrations of CO in the atmosphere are demonstrated in Fig. 5b. The high concentration of CO > 0.1% can seriously affect human aerobic metabolism ensuing respiratory failure and resulting in death (Prasad and Singh 2012). In many countries, the most common type of lethal air poisoning is known as CO poisoning (Omaye 2002). People with several types of heart disease already have a reduced capacity for pumping oxygenated blood to the heart, which can be exaggerated by inhaling CO in air. Vulnerable people can be affected more than healthy ones: infant, pregnant lady, and others. While, low concentration of CH₄ emission does not have a significant impact on human health directly.

Fig. 5

a Formation of carboxy-hemoglobin. b Effect of exposure to various concentrations of CO (O’Brien et al. 2004; Prasad and Singh 2012)

Effects on vegetation

It is found that CO can influence the plant seed germination, inducing the adventitious rooting process (Fortunato et al. 2001). CO poisoning causes the curling of plant leaves, decreasing leaf size, and premature aging of the chlorophyll, as shown in Fig. 6. It also reduces the cellular respiration system of plants (Ghaffari et al. 2008; Xuan et al. 2008). The long-term CO exposure decreases the length of primary roots and also the distance from the seed to the tip of the leaf. Whereas, the direct adversarial impact of CH₄ was not observed on the plants, but as a greenhouse gas, it can leave footprints by burning the plant leaves.

Fig. 6

Effect of CO emissions on vegetation (Prasad and Singh 2012)

Effects on environment

CO is only a weak direct greenhouse gas but has an important indirect effect on global warming. It reacts with hydroxyl (OH) radicals in the atmosphere, reducing their abundance. The increasing CO in the environment increases the global warming level and ozone layer depletion (Ogur and Kariuki 2014). It is one of the main reactive trace gases. In the atmosphere, the CO’s level concentration ranges from 15 to 30 ppm; it affects the earth’s radiation directly as the oxidation of CO would result in supplying more CO₂ from the greenhouse gas (Guo et al. 2016; Rudolf 1994). While, CH₄ is a direct greenhouse gas which is responsible for global warming as well as climate change problems.

Global warming and climate change are interchangeable terms according to the U.S. Environmental Protection Agency (EPA 2014). Then, climate change differs from global warming according to the National Academy of sciences (USA). Global warming is related to increasing temperature while the term “climate change” measures any significant change in the climate in addition to the temperature such as precipitation or wind. Among the various countries, India is also a major contributor of greenhouse gases. Issues concerning climate change in India in the past few years include situations like heavy raining at certain places causing floods; on the other hand, drought-prone situation was observed in some areas. The problems that arise due to global warming as well as climate changes in India are discussed below:

• Melting of Gangotri glaciers was a result of an increase in atmospheric temperature (Fig. 7a, b). If the rate of glaciers’ melting increases, flooding is likely in the river valleys fed by these glaciers, followed by a diminished flow, resulting in a scarcity of water for drinking and agricultural irrigation (Kaltenborn et al. 2010; Bajracharya et al. 2006).

• The agriculture in India is highly dependent upon the natural climate (o’Brien et al. 2004). The natural imbalance affects the monsoon which creates the problem in securing water for irrigating crops, resulting in below-average crop yields (Fig. 7c). The drought-prone regions in India are southern and eastern Maharashtra, Northern Karnataka, Andhra Pradesh, Odisha, Gujrat, Telangana, and Rajasthan (Fig. 7d).

• Increased precipitation is likely to come in the form of fewer rainy days but more days of extreme rainfall events, with increasing amounts of rain in each event, leading to significant flooding (Brunetti et al. 2001). For example, in June 2013, a heavy rainfall was observed in Uttarakhand due to cloud burst; a similar problem was also observed in 2014 in Jammu and Kashmir, in August 2017 in Northern India, and so on (Fig. 7d).

Fig. 7

Impacts of global warming as well as climate change

The above-mentioned examples reflect that the natural resources have been distorted by climate change, thereby increasing the risk of several disasters. This kind of natural calamities forced people to escape from their houses and made them environmental migrant, for example, submerged Dwarka, 120 ft underwater in the Bay of Cambay (ARAI 2011) was discovered by marine scientists in early 2002 (Tripati et al. 2002).The city is 5 miles long and 2 miles wide; carbon dating estimates the site to be a whopping 9500 years old, as shown in Fig. 7f.

The last 5 years (2015–2019) were the warmest on the record of India, and frequency of natural distaster is increasing, for example, Cyclone Amphan (2020). The world is tackling with climate-risk mitigation challenges; the biggest example is Covid-19, a major health crisis. The climate changes are playing a crucial role in the current pandemic because rising temperature and the high frequency of extreme weather is associated with various infectious diseases. It is a serious concern, and the government is doing efforts to prevents us from this pandemic and natural disaster (Nambi Appadurai 2020).

Legislations of CO and CH₄ emissions

The legislation has been strictly enacted increasingly into effect to set a specific limit to pollutant emissions from vehicles. Nowadays, HC emissions’ limit from both stationary and non-stationary sources are becoming more stringent (Heynderickx et al. 2010). Recently, emission standards have been executed for the CNG vehicles’ emission especially for heavy duty with a target to lessen it. Initially, European countries has set a specific limit of gaseous emissions for vehicular emissions to control air pollution. India is following the European norms for both light- and heavy-duty vehicles.

In 1970, the Environmental Protection Agency setup to control automobile pollution, and it is quiet in development for CNG vehicles. Further, a national auto fuel policy was broadcasted in October 2003 in which Indian emission standards are revised according to the European Union standards by introducing Euro 2–4 as Bharat Stage (BS II-IV) (Kumar et al. 2015). The Bharat stage emission standards according to the European Union for light-duty vehicles are mentioned in Table 3.

Table 3 Indian standards as per European norms for light duty vehicles (Heynderickx et al. 2010)

The emission standards for CNG vehicles run parallel to the diesel and gasoline vehicles with certain modifications. In the case of CNG vehicles, the mass emission standards are the same as gasoline except that the HC is replaced by non-methane hydrocarbons (NMHCs), where NHMC = 0.3 × HC (Kumar et al. 2015). Nowadays, Euro 6 has been implemented for particulate matter, NO_x, and HC except CH₄ for light-duty vehicles. According to the U.S. Environmental Protection Agency legislation, CH₄ can be regulated at 0.1 g bhp⁻¹ h⁻¹ for heavy-duty engines and 0.05 g mile⁻¹ for pickup trucks and vans (Raj 2016). Currently, the European emission standard (Euro VI) regulated the CH₄ emission and NMHC separately at different test cycles, as presented in Table 4. To meet the emission standards posed by the government, it is imperative to control these emissions of harmful gases up to a desired level. Thus, in this regard, several techniques are available in the literature.

Table 4 European emission standards for heavy duty natural gas vehicle (Worldwide Emissions Standards 2016)

Abatement technologies of CO and CH₄ emissions

Since, the concentration of CO and CH₄ in the exhaust of a CNG vehicle is always below their lower combustible limits, i.e., 4.3 and 12.0%, respectively. Hence, it is not possible to flare them up by the applying flame only. Therefore, to remove them from the exhaust containing lean mixtures of CO and CH₄, an oxidation catalyst is required. Presently, the techniques used to control CO and CH₄ emissions from CNG vehicles are classified into two different ways such as pre-combustion and post-combustion. The detailed discussion of these techniques is as follows:

Pre-combustion techniques

Pre-combustion technique is a conventional technique in which a treatment technique was applied before combustion. It includes basically engine-design modifications and technological advancement, fuel modifications using some additives, etc. These are expensive and not preferable recently. So, researchers focused toward other alternative method, i.e., know a post-combustion technique.

Post-combustion technique

This method is the most promising and economical. The concept of catalytic converter comes under this category. The converter is placed before the silencer without appreciable change in engine design that can convert harmful gases into non-polluted ones using a catalyst concept (Kalam et al. 2009); fo example, the lean mixture of CO and CH₄ in the vehicle exhaust can be catalytically converted into CO₂ and H₂O vapor and released in the atmosphere (Eqs. 3 and 4).

$$ \mathrm{CO}+0.5{\mathrm{O}}_2\kern0.5em \to \kern0.5em {\mathrm{CO}}_2\kern0.5em -{\Delta \mathrm{H}}_r=-393.5\ \mathrm{kJ}\ {\mathrm{mol}}^{-1} $$

(3)

$$ {\mathrm{CH}}_4+1.5{\mathrm{O}}_2\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\kern0.5em -{\Delta \mathrm{H}}_r=-890\ \mathrm{kJ}\ {\mathrm{mol}}^{-1} $$

(4)

Eugene Houdry, a French mechanical engineer introduced the catalytic converter for the first time (Bera and Hegde 2010; Pundir 2007). In 1950, the problem of smog aroused in Los Angeles. Then, Houdry found the exact problem of air pollution which was mainly due to automobile exhaust emission. Thus, he searched out a special company named oxy-catalyst for development of catalytic converter for gasoline engine-an idea ahead of its time for which he attained a patent (US2742437). The idea of catalytic converter was really good but anti-knock agent tetra-ethyl lead would be a “poison” for converter as it forms a coating on the catalyst’s surface, effectively disabling it. Later, the catalytic converter was modified by Keith at the Engelhard Corporation in 1973. Beginning in 1979, a mandated reduction in NO_x required the design development and use of a three-way catalyst (TWC) for simultaneous abatement of CO, HC, and NO_x (Heck and Farrauto 2001; Pardiwala et al. 2011). According to the above-mentioned background, the catalytic converters are of three categories: oxidation/two-, three-, and four-way converters.

The two-way/oxidation catalytic converter is applicable only for CO and HC emissions emitted from diesel-fuelled vehicles. It could convert CO and HCs into CO₂ and H₂O in the presence of air that is present in combustion chamber. Sometimes, these converters are also applicable for spark ignition engines such as gasoline in the USA market automobiles through 1981. Such catalytic converter is ineffective for controlling emission of NO_x if present in the exhaust stream. The most common reactions for two-way converters are similar to above mentioned equations (Eqs. 3 and 4). The required oxygen for oxidation reactions is available either by operating engine in lean condition or by injecting secondary air to the catalytic converter in rich-burn condition. In 1975, the two-way catalytic converters are used in rich operating condition only for CO and HC control. The NO_x was control by exhaust gas recirculation (EGR) techniques. Later, when the NO_x emission standards were made strict from 1981, the reduction catalysts were also needed in the converter. Therefore, to overcome drawback of two-way converter, the three-way converters were designed.

Three-way catalytic converter

Three-way catalytic converters (TWC) can control all three primary pollutants (CO, HCs, and NO_x) simultaneously in a single unit when an engine running slightly below the stoichiometric point (rich condition), as shown in the Fig. 8. The TWC contains precious metal (platinum (Pt), palladium (Pd), and Rh) catalysts. The oxidation reaction is similar to two-way converters. The reduction reaction for NO_x control is as follows: 2NO_x → xO₂ + N₂. As mentioned earlier, the conversion efficiency of the three pollutants present in the exhaust depends on fuel-air equivalence ratio (Fig. 9). It is very clearly seen from the figure that in excessive fuel condition when the engine is running in rich condition, the NO_x reduction is in favor over TWC. In the case of the lean-burn condition (more oxygen is present than required), only the oxidizing reactions are in favor. In today’s world, people like to run the engine having an A/F ratio of ~ 20:1 or higher due to fuel economy concern. The reduction of NO_x is very difficult in oxygen-rich atmosphere. Unfortunately, the temperature of the engine increases in lean condition which makes the thermal NO_x emissions rise up.

Fig. 8

Three-way catalytic converter

Fig. 9

Conversion efficiency of pollutants depends on equivalence ratio

Thus, TWC catalysts are not appropriate in lean engine condition without an additional NO_x controlling device. Therefore, to abate all three pollutants under lean-burn engine exhaust, two separate devices for oxidation of CO, HC, and soot and selective catalytic reduction of NO_x are needed. Thus, the scientists moved toward the double-layer systems. In the double-layer system, two layers of the catalysts are in sequence and behave like a combination of oxidation and reduction catalysts. To control pollution of CNG vehicle, it is imperative to oxidize CO and CH₄ using an efficient catalyst because direct combustion (flame combustion) of lean mixture of CO and CH₄ is not possible at any temperature due to combustible limitations of their concentrations in the exhaust. Thus, the catalytic oxidation is a widely practiced method for treatment of exhaust gases from automobiles at present.

Catalytic oxidation of lean CO-CH₄ mixture

The catalytic oxidation reaction occurred on the bed of catalyst when air-fuel mixture passed through it on a particular temperature. Heat liberates during reaction which produces products after complete combustion, such as CO₂ and H₂O. The total oxidation of CH₄ within the range of the vehicular exhaust temperature is a very tedious task due to a strong C–H bond. Thus, it is vital to investigate an appropriate catalyst which can oxidize CH₄ along with CO at within the range of 150–450 °C. In literature, many catalysts such as platinum group metal (PGM) catalysts (Gelin et al. 2003; Mahara et al. 2017; Zhang et al. 2014; Zorn et al. 2010), perovskite (Alifanti et al. 2007; Ciambelli et al. 2001; Forni and Rossetti 2002; Kucharczyk 2015; Liu et al. 2013; Najjar et al. 2011; Wang et al. 2012), hydrotalcite (Cheng et al. 2008; Genty et al. 2015; Jiang et al. 2010; Liu et al. 2014; Martínez-Lozano et al. 2007; Mokhtar et al. 2010; Saber and Zaki 2014; Takehira et al. 2004), mixed metal oxides (Biabani-Ravandi and Rezaei 2012; Dongsheng et al. 2010; Heo et al. 2014; Li et al. 2009; Trivedi and Prasad 2016), and spinel (Cunningham et al. 1994; Jansson 2000; Tang et al. 2009; Tao et al. 2015; Trivedi and Prasad 2017b; Trivedi and Prasad 2018) is available for individual CO and CH₄ oxidation. The PGM catalysts are highly active for oxidation, but their application is limited due to high cost, rare availability, sensitive to poisons, and sintering at high temperatures. Therefore, search for the less or PGM-free catalysts are of worldwide research and commercial importance. So, to fulfill the gap in literature, our motivation is to search an efficient catalyst for oxidation of CO-CH₄ mixture. The catalysts for separate as well as total oxidation of CO and CH₄ are broadly classified into two categories: precious and transition metal-based catalysts.

• Precious metal catalysts:

• Platinum group metal and

• Gold

• Transition metal catalysts:

• Perovskite,

• Hydrotalcite,

• Mixed metal oxides, and

• Spinel

The above-mentioned catalysts have shown a promising activity to control CO and CH₄ emissions. In the following sections, works done by various researchers are summarized.

Precious metal catalysts

The precious metal-based catalysts include PGM and gold catalysts. A lot of research has been carried out (Gelin et al. 2003; Cohn 1965; Lampert et al. 1997; Stasinska et al. 2008; Colussi et al. 2012; Hussain et al. 2015; An et al. 2014), and a comprehensive review (Li and Hoflund 2003) was reported for CO and CH₄ oxidation over precious metal catalysts. A detailed discussion is given in following sections.

PGM catalysts

PGM metals are well-known oxidation catalysts for controlling CO and CH₄ emissions. A lot of work has been done on this catalyst for various reactions due to its good catalytic activity. The supported Pd and Pt are the most commonly used for oxidation reactions. PGM metal catalysts was first-time used for CO oxidation by Cohn (1965). The performance of the catalysts for the said reactions was improved by using various supports such as alumina, magnesium, silica, etc. (Zorn et al. 2010; Pardiwala et al. 2011). The use of supports makes them economical also. It was shown that Pd-based catalysts are the most active for oxidation of CO and CH₄ than Pt-based catalyst, as shown in Fig. 10 (Ferrandon 2001). The Pd/γ-Al₂O₃ is found to be the most attractive catalyst for CH₄ combustion which possessed the highest activity as well as good thermal stability. The CH₄ oxidation is highly dependent on the active phase of the metal in the catalyst. The contradictory discussion in the literature regarding the active state of Pd catalyst (metallic Pd, PdO, or mixed Pd/PdOx) is reported by many scientists (Burch et al. 1995; Lyubovsky and Pfefferle 1998; Oh and Mitchell 1994). McCarty (1995) studied that PdO is an active phase for CH₄ combustion than metallic Pd. However, Kucharczyk (2015) suggested that the activity of a mixed Pd/PdO phase was higher than PdO or metallic Pd, when PdO were dispersed over the Pd surface.

Fig. 10

Conversion of CO, CH₄, and C₁₀H₈ for Al₂O₃-supported Pd and Pt catalysts (Ferrandon 2001)

Recently, Pt/CeO₂-ZrO₂-Al₂O₃ cryogels was investigated by Osaki (2020) for CO and CH₄ oxidation. The author reported total conversion of CO and CH₄ over the catalyst (Ce/Al = 1/3) were 150 and 650 °C, respectively. He also discussed the factors that influnces the catalytic activity. He mentioned two different conditions: (i) Ce/Al < 1/3, the catalytic activity was decreased due to less amount of Ce in the catalyst which is having good oxygen storage capacity and also good distributor of Pt metal. (ii) Ce/Al > 1/3, the presence of an excess amount of Ce is also not good because it does not provide sufficient interaction between CeO₂ and Pt metal. Thus, optimum amount was 1/3 that provides good catalytic activity but reported temperature for CH₄ oxidation is still very high (Osaki 2020). The contribution of various researchers for CO and CH₄ oxidation over PGM catalyst and its combination with non-noble metals is mentioned in Table 5. The economic reasons and limited resources of PGM-based catalytic converters have encouraged the investigation of alternative cheaper catalysts.

Table 5 Literature review at a glimpse for PGM catalyst for CO and CH₄ oxidation

Gold-based catalysts

The Au-based catalysts are economical than the PGM catalysts for CO and CH₄ oxidation. It was observed that supported Au nanoparticles showed a promising activity for CO oxidation than PGM catalysts. These catalysts can completely combust CO at sub-ambient temperatures which was reported by Haruta et al. (1989). They also studied the effect of support for CO oxidation and reported the activity order as follows: Au/Fe₂O₃ > Au/Co₃O₄ > Au/NiO. Solsona et al. (2006) also studied the effect of Au supported on CoO_x, MnO_x, CuO, Fe₂O₃, and CeO₂ for CO oxidation and reported that the Fe₂O₃ is the best support. The result is also supported by Song et al. (1999). In practical aspect, Au is inert in bulk, its nanoparticles (< 5 nm) are highly active for CO oxidation. The catalysts activity deos not only depend on the particle size but also other factors like preparation method and nature of support material. Qureshi and Jaseer (2018) studies the effect of support on the activity of the Au particles for CO oxidation (Fig. 11).

Fig. 11

Effect of support for CO oxidation (Qureshi and Jaseer 2018)

Figure 11 does not only provide the information about the effect of support but also reflects the effect of morphology and temperature as well. The performance of Au/SiO₂ is not good as compared with Au/TiO₂, but the activity of wormhole silica is quite good than hexagonal silica. Additionally, the low-temperature preped Au/TiO₂ showed the best performance among all other catalysts. Generally, water affects the performance of the catalysts, but in the Au catalysts, its behavior is different which promotes the catalyst activity. Fujitani et al. (2014) studied the role of water on the activity of gold catalyst for CO oxidation by density functional theory calculations. The theory revealed that presence of moisture in the catalyst is advantageous such as the water maintains the cationic state of Au particles (Au³⁺/Au⁺) and also helps in the activation of O₂ molecules. Recently, the geometric effect of Au nanoclusters on room-temperature CO oxidation discussed by Luo et al. (2020). They found that the 2D-layered Au species is more active as compared with the 3D Au nanoclusters.

While, the CH₄ oxidation is also available on gold nanocatalyst, but its conversion was very low at high temperature (Choudhary et al. 2008; Solsona et al. 2006). The transition metal-supported catalysts are also efficient for CH₄ oxidation (Solsona et al. 2006). Waters et al. (1994) reported the oxidation of CH₄ over Au on various supports such as Co₃O₄, NiO, MnO_x, Fe₂O₃, and CeO₂. The reported activity order for CH₄ oxidation is as follows: Au/Co₃O₄ > Au/NiO > Au/MnO_x > Au/Fe₂O₃ >> Au/CeO₂ (Waters et al. 1994). However, Au/Co₃O₄ catalysts could be sintered at very high temperature and decomposition of CO₃O₄ is also possible. Thus, the application of the catalyst is limited for high temperature. The properties of the catalyst could be enhanced by the addition of promoters on transition metal-supported gold catalyst. Liotta et al. (2008) reported the effect of cerium addition on Au/Co₃O₄ catalyst for CO and CH₄ oxidation. It was observed that the Au/CeO₂ showed the best performance for CO oxidation, while the Au/Co₃O₄-CeO₂ catalyst showed good catalytic activity for CH₄ oxidation. In Au/Co₃O₄, Ce acts as a promoter which stabilized the catalyst at high temperature > 600 °C and maintain its activity for CH₄ oxidation (Liotta et al. 2008). The doping of little amount of PGM catalyst in the catalyst could lower down the reaction temperature for CO oxidation, for example, the doping of Pt in Au/Co₃O₄ catalyst could lower down the temperature of up to 50 °C for CO oxidation due to a strong interaction of Pt with Au and Co₃O₄ (Miao and Deng 2001).

The performance of the catalysts also depends on the preparation methods which affects the size of the catalysts. Further, Ivanova et al. (2006) synthesized the Au/γ-Al₂O₃ catalyst by two different methods such as direct anionic exchange and deposition-precipitation. The authors reported that direct anionic exchange method is better than precipitation-deposition (Ivanova et al. 2006). However, the gold catalyst is temperature sensitive and not favorable at high temperature ~ 1000 °C. Thus, the high-temperature stable and economical catalysts were of a major concern for automotive emission control. The glimpse of literature or oxidation of CO and CH₄ over precious metal catalysts is mentioned in Table 6.

Table 6 Literature at a glance for CO and CH₄ oxidation over Au catalysts

Non-noble metal catalysts

The rare availability and high cost encouraged for the search of alternative cheaper transition metal catalysts. So, the researchers found the different transition metal catalysts for vehicular emission control and categorized as perovskite, hydrotalcite, mixed metal oxides, and spinel (Heo et al. 2014; Tao et al. 2015; Trivedi and Prasad 2016; Trivedi and Prasad 2018; Xie et al. 2009). In the field of automotive emission control, few Ph.D. degrees (Ferrandon 2001; Iablokov and Kruse 2011; Kucharczyk 2015) were given by various universities and several comprehensive reviews (Liotta et al. 2013; Mankidy et al. 2014; Prasad and Singh 2012; Rattan and Kumar 2014) were reported for CO and CH₄ oxidation. The comprehensive literature of transition metal oxides for CO and CH₄ oxidation is described in following sections:

Perovskite catalysts

The general stoichiometry structure of perovskite is ABO₃, where “A” and “B” are divalent and tetravalent cations of very different sizes; O is an anion bonds to both cations, as shown in Fig. 12.

Fig. 12

Structure of perovskite catalyst

Perovskite oxides build up by incorporation of A cations into BO₆ octahedron, where A is rare earth (La, Ce, Pr, Nd), alkali, or alkaline earth metal (Cs, Sr, Ba, Ca, Ra) of larger ions (radius of A ∼ 0.90 Å) and B site was consisted of various transition metal cations with relatively smaller radius (radius of B ∼ 0.51 Å) (Forni and Rossetti 2002; Labhsetwar et al. 2006; Shinjoh 2006). Many perovskites can be designed by the partial substitution of A and B cations with other hetero-valent cation which induces the structural distortions and/or B site valence transformations and altering their physiochemical properties for various applications with improved activity (Alifanti et al. 2007; Ciambelli et al. 2001). The perovskite catalysts are thermally stable, having excellent redox properties, good mechanical strength, low cost, and large variety in comparison with the other oxides. Limited work is available in the literature for CO and CH₄ oxidation under the same experimental condition over perovskite catalysts (Arandiyan 2015; Kucharczyk 2015; Liu et al. 2013; Najjar et al. 2011; Wang et al. 2012).

The oxidation of CO and CH₄ over LaMnO₃ under the same experimental condition was reported by several authors (Kucharczyk 2015; Machocki et al. 2004; Seiyama 1992). Kucharczyk (2015) studied oxidation of CO over LaMnO₃ and found that a high temperature of 295 °C is required for the complete oxidation reaction. The performance of the perovskite catalyst could be improved by substitution of A and B sites by small amount of noble metal for the said reaction. Thus, the authors observed that the substitution on A site in the perovskite with small amount (Pd, 0.05–0.2 wt.%) of noble metal improves the activity of the catalyst for CO oxidation than the substitution of Pd metal on B site of perovskite. The substitution decreased the total oxidation temperature up to 45 °C (Kucharczyk 2015). While, the complete CH₄ oxidation over LaMnO₃ was also reported by Machocki et al. (2004). Further, modification in LaMnO₃ catalyst by Ag was done to improve the performance of the catalyst for CH₄ oxidation. The activity order of the catalysts modified by Ag for the same reaction was as follows: La_0.7Ag_0.3MnO₃ > La_0.9Ag_0.1MnO₃ > LaMnO₃. However, the reported total oxidation temperature (> 400 °C) for CO and CH₄ over perovskite is relatively high than the other metal oxides (Wei et al. 2010). The perovskite is prominent for soot oxidation (Mishra and Prasad 2015), but it is not suited for CH₄ oxidation as well (Chen et al. 2010; Machocki et al. 2004; Wei et al. 2010) due to their low surface area. The literature survey at a glance over perovskite catalyst for said reaction is mentioned in Table 7.

Table 7 Literature review at a glance of perovskite catalysts for CO and CH₄ oxidation

Hydrotalcite

Hydrotalcite (HT) and their compounds (HTlcs), called layered double hydroxides which is represented by general compound Mg₆Al₂(CO₃)(OH)₁₆·4(H₂O). Generally, HTlcs are denoted by double-layered hydrotalcite (LDH). The chemical expression of these material is [M(П)_1–x M(Ш)_x(OH)₂]^x+[(Aⁿ⁻)_x/n·mH₂O]^x−, where M(П) represents any divalent metal cation, M(Ш) any trivalent metal cation, and Aⁿ⁻ an anion; the value of x is equal to the molar ratio of M(П)/(M(П) + M(Ш)) which ranges between 0.2 and 0.4 (Vaccari 1998). The structure of hydrotalcite catalyst is shown in Fig. 13.

Fig. 13

Structure of hydrotalcite catalyst

Recently, HTlcs have been receiving a considerable attention as heterogeneous catalyst due to their high surface area and good catalytic activity (Cavani et al. 1991; Martínez-Lozano et al. 2007; Takehira et al. 2004). Mokhtar et al. (2010) prepared spinel catalyst via CoMnMgAl hydrotalcite precursor for CO oxidation. The authors found that catalyst with Co/Mn = 4 showed the best catalytic performance for complete CO oxidation at 160 °C. Genty et al. (2015) investigated X₆Al₂HT hydrotalcite (X = Fe, Cu, Zn, Ni, Co, Mn, or Mg) precursor for the same reaction and found that X₆Al₂ nano-oxides is the best one, but the maximum conversion was limited to 50% at very high temperature of 249 °C. Further, multi-oxide catalysts of Zn, Cu, and Ti with different ratios were obtained from LDH precursors for CO oxidation and a maximum conversion of 95% at 275 °C was reported. The formation of different phases was confirmed by XRD analysis.

The CH₄ oxidation over hydrotalcite catalysts was also studied by some researchers. Cheng et al. (2008) studied CH₄ combustion over Cu–Co/X–Al (X: Fe, Mn, La, Ce) hydrotalcite-like compounds and observed total oxidation at 496 °C. The Co_xMg_3–x/Al oxides were produced by calcination of Co_xMg_3–x/Al hydrotalcite precursor (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) at 800 °C by Jiang et al. (2010). The activity, thermal stability, and homogeneity of the catalyst markedly depend on the concentration of Co in the hydrotalcite. They found 90% conversion at 600 °C over 1.5CoMgAlO. Recently, Liu et al. (2013, 2014) observed the CH₄ oxidation by utilizing Co_xMg_6–xMn₂LDO as a catalyst. The maximum conversion was limited to 90% < 495 °C over Co_4.5Mg_1.5Mn₂LDO (Saber and Zaki 2014). Literature at a sight for CO and CH₄ oxidation over hydrotalcite catalyst is presented in Table 8.

Table 8 Literature review at a glance of hydrotalcite catalysts for CO and CH₄ oxidation (GHSV, T₁₀₀, T₅₀,T_90~95)

With the aforementioned background, it is concluded that LDH showed good catalytic performance for CO and CH₄ combustion. The Co–Mn–Mg–Al and Cu–Co/X–Al (X = Fe, Mn, La, Ce) catalysts showed the total oxidation of CO and CH₄, respectively. However, the temperature for CH₄ oxidation is relativity higher than Co–Mn-mixed oxide and spinel catalysts as reported in the literature.

Mixed oxides

The mixed metal oxides showed the good catalytic performance in comparison with individual metal oxides for the said reaction (Ferrandon 2001). A lot of literature is available for CO oxidation, while the studies on total CH₄ combustion are also limited over mixed oxides (Biabani-Ravandi and Rezaei 2012; Li et al. 2009). Ferrandon (2001) studied the oxidation of both CO and CH₄ over supported mixed metal oxides in his thesis work. The researchers synthesized the Al₂O₃-supported metal oxides (Cu, Mn, Fe, Co, and Ni) by incipient wetness impregnation methods and checked their performance for CO, CH₄, and C₁₀H₈ oxidation (Ferrandon 2001). Generally, CuO seems to have a high activity than MnO_x for the said reaction (Grisel and Nieuwenhuys 2001). The comparison of supported metal oxides for CO and CH₄ oxidation is shown in Fig. 14.

Fig. 14

Conversion of a CO and b CH₄ over supported metal oxides

It can be clearly seen from the figure that Ni possesses poor activity for the reaction. The activity order of metals is as follows: Cu > Mn > Fe > Co > Ni. Further, the combination of ceria with different MO_x (M = Cu, Mn, Fe, Co, and Ni) was tested for CO and CH₄ oxidation by Dongsheng et al. (2010). The authors reported the effect of H₂O vapor on the performance of the catalysts. The light-off temperatures (T₅₀) for CO and CH₄ oxidation were 150 and 514 °C, respectively, over the best catalyst Ce_0.9Co_0.1O_2–δ composition. In the presence of H₂O vapor, light off temperatures for CO and CH₄ were raised to 17 and 5 °C which results the deactivation of the catalysts (Dongsheng et al. 2010). Literature review at glance of Co–Mn-mixed oxides is presented in Table 9.

Table 9 Literature review at a glance of CO and CH₄ oxidation over mixed oxides

From the aforementioned background, it was observed that the oxidation temperature for CH₄ oxidation is very high in comparison with CO. Till now, the lowest reported temperature for CH₄ oxidation over Co–Mn-mixed oxides (Co:Mn = 5:1) was 340 °C for maximum 90% conversion (Li et al. 2009). In the mixed oxide catalysts, the insertion of Mn in Co₃O₄ improves the activity of reactive ion present in octahedral site which assist dihydroxylation steps (Li et al. 2009).

Spinel catalysts

The spinel comprises a large group of ternary compounds with general formula, AB₂O₄ where A and B are divalent and trivalent cations, respectively. It occurs in nature as minerals and also synthesize in the laboratory with specific properties for special applications. Natural-occurring spinel is ruby MgAl₂O₄, a pink gemstone. The structure of spinel is a cubic crystal system, as shown in Fig. 15. The spinel structure was firstly reported by Bragg (1915). The unit cell of the spinel structure consists of the A in the tetrahedral site and B in the octahedral site. The spinel consists of a cubic close-packed array of oxide ions, in which one eighth of the tetrahedral sites and one half of the octahedral sites are occupied by cations. Tetrahedral sites’ cations are surrounded by four oxygen atoms while octahedral site cations are surrounded by six oxygen atoms.

Fig. 15

Spinel structure (AB₂O₄)

Due to its robustness in structure, it is a stable compound for oxidation reactions. The presence of cations of different metals occupy the position in tetrahedral as well as octahedral site that means the variable valance cations are present on the catalyst surface to facilitate reactions. Thus, the spinels have a good application in various fields of science and engineering due to its outstanding characteristics like good catalytic activity, excellent electrical, magnetic, and optical properties (Tatarchuk et al. 2016). On the basis of position of cations in the structure, the spinels are classified in three different categories: normal, inverse, and mixed spinels. The structure of all types of spinels with examples is given in Table 10.

Table 10 Different types of spinels

The brief discussion on the structure and application of different categories of spinels are given below:

• a Normal spinel

In the normal spinel, the divalent cations occupy the A sites (in tetrahedral voids) and only trivalent cations occupy the B sites (in octahedral voids) and the distribution is represented by the formula (A²⁺)_t[B₂³⁺]_oO₄ (see Table 10). An example of normal spinel is Co₃O₄ (Hastings and Corliss 1953). In the Co₃O₄ structure, Co²⁺ occupies 8 tetrahedral sites, Co³⁺ occupies 16 octahedral sites, and 32 sites are occupied by O²⁻ ions. The other representation of spinel is CoO·Co₂O₃. These can be used as magnetic nanoparticles in micro-batteries, nanowires, in electrode, as carbides, in catalysis, etc. The other application of spinel is as superconductors, as an electronic ceramics, and also good for oxidation reactions. The spinel is good catalyst for CO oxidation and sometimes used for CH₄ oxidation. It can lower down the CO and CH₄ reaction temperature. Among various catalysts, CO₃O₄ is a good catalyst and has a promising activity for oxidation of both gases (Cunningham et al. 1994; Dou et al. 2017; Garbowski et al. 1990; Meng et al. 1997; Shelef et al. 1968; Yu Yao 1974). The temperature for CO oxidation over Co₃O₄ was ≤ 54 °C (Grillo et al. 2004). The performance of the catalysts is highly dependent on morphology, support used, preparation methods, etc., for example, the total oxidation temperature of CO over nanorod Co₃O₄ was reduced to − 23 °C in comparison with nanosphere catalyst (Xie et al. 2009).

The activity of the catalysts over supported catalysts were also checked by several researchers because real application-supported catalysts would be highly stable than bulk catalyst (Jansson 2000; Zhang et al. 2017). The performance of the catalysts varies when the different supports were used. The total oxidation of CO over Co₃O₄/Al₂O₃ was reported at room temperature (Jansson 2000). When the support was CeO₂, the temperature for total oxidation was high ~ 80 °C (Zhang et al. 2017). As discussed earlier, spinel (Co₃O₄) has a good catalytic performance for CH₄ as well and its performance also depends on parameters such as support, precipitants, promoters, etc. A numerous studies are available for CH₄ oxidation over supported catalysts. McCarty et al. (1997) compared the activity of the various oxides for CH₄ oxidation and reported the sequence in order of their performance: Co₃O₄ > CuO > NiO > Mn₂O₃ > Cr₂O₃. The authors checked the performance of the catalyst using various supports (ZrO₂, MgO, TiO₂, and Al₂O₃) for CH₄ combustion. They found that ZrO₂ and Al₂O₃ supports are themselves active with metal oxide whereas MgO and TiO₂ are inactive supports for the reaction (Xiao et al. 2001). Recently, Tang et al. (2009) reported the total conversion temperature of CH₄ over Co₃O₄-SnO₂ is 500 °C. It was good observation because the total oxidation of CH₄ is not an easy task and it of course would require little bit high temperature. The reason behind the complete conversion is high mobility of oxygen species due to a strong interaction between cobalt and tin oxides (Tang et al. 2009). Thus, the main advantage of supported catalysts is that it can reduce the cost of bulk catalysts as well as participate to improve its performance.

Similarly, the authors studied the effect of precipitants on the performance of CeO₂-supported Co₃O₄ catalyst (Wu et al. 2015). Wu et al. (2015) found that precipitating agents affected the textural (crystallite size and specific surface area) as well as catalytic properties of both. The addition of promoters affects the lights off characteristics of catalyst for CH₄ combustion. Xu et al. (2014) studied the effect of Sm addition on the performance of Co₃O₄ for CO and CH₄ oxidation. The authors found that the presence of Sm in the catalysts increases the conversion of CH₄ 55% to 100% at temperature of more than 500 °C and decreases the temperature for CO oxidation from 190 to 125 °C. The reason behind the good performance of the catalysts is that Sm modified the spinel and formed amorphous SmCoO₃ dispersed on the catalyst surface, which enhanced the concentration of Co³⁺ cation on the catalyst surface. The Sm could also form a more active oxygen species on the catalyst surface (Xu et al. 2014). Wang et al. (2020) recently synthesized a core-shell catalyst to enhance the performance of Co₃O₄ catalyst for CH₄ oxidation. The hydrothermally prepared core-shell flower-spheroidal Co–Mn catalyst showed better activity along with a low risk of sintering of Co₃O₄ particles. The complete oxidation of CH₄ over Mn-incorporated Co₃O₄ occurred at temperature of 450 °C while 6% less conversion was obtained over Co₃O₄/SiO₂ at the same temperature. The Mn²⁺ ion replaced Co²⁺ in Co₃O₄ spinel that promotes the mobility and production of reactive oxygen species which improves performance.

In simultaneous CO-CH₄ oxidation, the mechanism will be different over the same catalyst. Both reactants affect the performance of each other. The activity for CO oxidation is likely to be related to the low ΔH of vaporization of O₂. Desorption of the lattice oxygen can be influenced by the Co–O bond strength (Haneda et al. 2003). The simple mechanism for CO oxidation is that the CO reacts with the lattice or pre-adsorbed oxygen, which may further react to form carbonate species. A complete understanding of the sequence of elementary for CO oxidation is still confusing. In regard to catalytic oxidation of CH₄, the lights off characteristics is related to the activity of the catalysts. The lights off characteristics of a catalyst is defined as T₁₀, T₅₀, and T₁₀₀ as the temperature corresponding to the 10, 50, and 100% conversion of reactant, respectively. The rate-controlling step for CH₄ combustion is the C–H bond breaking (Baldi et al. 1998). The summarized report of CO₃O₄ catalyst for CO and CH₄ oxidation is mentioned in Table 11.

Table 11 Light off characteristics for oxidation of CO and CH₄ over Co₃O₄ catalyst

As we know, CO₃O₄ is a good catalyst for CO and CH₄ oxidation. The main limitation of this catalyst is its toxicity and low thermal stability as compared with the other catalysts like perovskite. The other problem of this catalyst is that it showed a maximum possible conversion at a very high temperature. So, it cannot be considered a perfect catalyst for simultaneous oxidation of CO and CH₄ gaseous mixture. Therefore, it is a mandate to identify low cost, eco-friendly, and stable transition metals for partial replacement of normal Co₃O₄ spinel which could make it complete eco-friendly and also improving its catalytic activity, selectivity, as well as stability.

• b Mixed spinel

The mixed spinel is not much popular as inverse and normal spinels. The ionic distribution in the mixed spinel is intermediate between normal and inverse configuration. The distribution in this case is represented by the formula (A²⁺_x B³⁺_1–x)_t [A²⁺_1–x B³⁺_1 + x]_oO₄. The example of mixed cation distribution is NiFe₂O₄ (Chinnasamy et al. 2001). Till now, no report is available over mixed spinel for the oxidation of CO and CH₄.

• c Inverse spinel

In inverse spinel, half of the trivalent cations occupy A sites and the other half of the trivalent cations and total divalent cations randomly occupy B-sites (B³⁺)_t[A²⁺B³⁺]_oO₄. An example of inverse spinel is NiCo₂O₄, in which all the divalent cations of Co occupy only the B sites (Hastings and Corliss 1953). The cobaltites like MCo₂O₄ (M = Ni, Cu, Mn, Zn, Mg, Fe, etc.) are the examples of the inverse spinel catalysts. It is generally regarded as a mixed valance (Co³⁺/Co²⁺and M³⁺/M²⁺) oxide that adopts spinel structure in which the M occupies the octahedral site and Co is distributed over both tetrahedral and octahedral sites.

Recently, inverse spinel have drawn significant attention for many technological applications ranging from catalysts and sensors to electrode material, electrochromic devices and supercapacitor due to their superior physicochemical properties. It was found that only inverse spinel such as NiCo₂O₄ has good catalytic activity for the oxidation of CO as well as CH₄ also. However, few studies are reported in the literature for the oxidation of CO and CH₄ over NiCo₂O₄ catalyst. So far, only three reports are available on NiCo₂O₄ for CO oxidation till now (Gou et al. 2013b; He et al. 2015; Zhu and Gao 2009b) and single paper on CH₄ oxidation (Tao et al. 2015). Zhu and Gao (2009a) prepared a mesoporous cobaltites (MCo₂O₄, M = Cu, Mn, and Ni) by a nanocasting pathway for CO oxidation, where SBA-15 is used as hard template. The authors found that CuCo₂O₄ and MnCo₂O₄ exhibited high activity and robust stability than NiCo₂O₄. The temperature for total CO oxidation over CuCo₂O₄ and MnCo₂O₄ were (69) and 73 °C, respectively. In contrast with the other cobaltites, the NiCo₂O₄ showed the poor activity and continuous deactivation during life/stability test. Further, a series of Ni-Co oxides was prepared by Gou et al. (2013b) for CO oxidation. A stable lamellar structure of NiCo₂O₄ was formed when the Co percentage was between 40 and 60%. The Ni–Co binary oxides are categorized into three categories such as NiO, Co₃O₄, and NiCo₂O₄. The NiO showed the better catalytic activity in comparison with Co₃O₄ and NiCo₂O₄. The lights off temperature for CO oxidation was 75 °C over NiO catalyst (Gou et al. 2013b). He et al. (2015) reported the complete oxidation of CO at low temperature of 115 °C over a hexangular ring–core NiCo₂O₄ porous nanosheets/NiO nanoparticle composite.

The temperature for total oxidation CO was relatively higher over NiCo₂O₄ in comparison with individual constituents of this catalyst. However, the NiCo₂O₄ possesses the best performance for CH₄ oxidation and 90% conversion of CH₄ oxidation was reported at a low temperature of 350 °C (Tao et al. 2015). Thus, it is a topic of attention in recent years to explore NiCo₂O₄ catalyst for CO and CH₄ oxidation. Literature review at a glimpse for separate and simultaneous oxidation of CO and CH₄ over NiCo₂O₄ is mentioned in Table 12.

Table 12 Literature for oxidation of CO, CH₄, and their mixture over NiCo₂O₄ catalyst

Little work is available in literature for simultaneous oxidation of CO-CH₄ mixture over NiCo₂O₄. Trivedi and Prasad (2016, 2017a, b, 2018) studied the oxidation of CO-CH₄ mixture over NiCo₂O₄ and found a very low temperature of 350 °C for complete oxidation reaction. The authors found that it is very stable catalyst and maintained its activity for a long period of 50 h at complete combustion temperature (350 °C) of CO-CH₄ mixture. They also explained the synergistic effect of reactants in the mixture, i.e., CO facilitates oxidation of CH₄ by lowering its complete combustion temperature. Further, the authors appreciated the addition of dopants in the inverse spinel and found the lowest temperature of 320 °C for oxidation of the CO-CH₄ mixture over K–Pd-doped NiCo₂O₄.

Conclusion

The best approach to curb the threat of vehicular pollution and the choice of the appropriate catalyst for the converters is a vital step in terms of activity, selectivity, durability, availability, and cost. The noble metals are completely replaceable with transition metal to abate pollutants nowadays. In the same line, the cobalt-based catalysts exhibit good catalytic performance for CO and CH₄ oxidation under lean conditions. Moreover, Co-based catalyst can effectively remove three major pollutants (CO, HCs, and NO_x) from the exhaust in the temperature region considerably lower than flame or explosion temperatures (150–450 °C). However, the presence of poisoning compounds in exhaust stream can reduce the performance or affect activity of the catalyst. The performance of the catalyst is highly dependent upon some important parameters like supports, preparation methods, precipitants, etc. Among the different types of spinel, NiCo₂O₄ is well studied and a good one for CO-CH₄ oxidation. Still, research is required in order to develop the catalysts for oxidation of CO-CH₄ mixture using newer recently investigated routes. The present paper opens a new horizons or opportunity for the “cobalt based catalyst” as competitive catalysts in methane combustion reaction.