Abstract
Due to increasingly stringent rules, eliminating pollutants (NOx) emitted by diesel engines in the automobile sector remains an intriguing scientific and technological problem. To meet the strict NOx emission restrictions, a catalytic system with a high level of complexity, unit size, and quantity, as well as higher fuel consumption, is required. As a result, for a reduction in individual exhaust gas emissions, an after-treatment system for a diesel vehicle must employ integrated catalyst technology. For selective catalytic reduction of NOx without any external reductant, a highly effective catalyst “spinel nickel cobaltite” (NiCo2O4) was produced using a polymeric precursor technique. In this work, an exhaust gas treatment system without external reductants using nano-NiCo2O4 as catalyst was designed and fabricated, for NOx control in diesel and petrol engines at low temperature. In order to determine the NOx conversion efficiency in the selective catalytic reduction system, tests were carried out at different engine loads. The system was supposed to be cost-effective due to the nano-NiCo2O4 catalyst’s ability to work at low temperatures. The findings proved the developed SCR system’s potential to reduce NOx emissions. At a high load, the nitric oxide (NO) emissions were reduced by 54 and 96 percent, respectively, without increasing HC, CO, and CO2 emissions or compromising efficiency.
1. Introduction
NOx emissions (NO and NO2) are a significant source of pollution in the atmosphere. They have a wide range of negative consequences for the environment and human health. The main effects of NOx on the environment include acidification of rain, photochemical smog, greenhouse effect, and ozone depletion, while the most severe effect of NOx on people’s health is respiratory tract disorders [1–3]. Although many sources such as agriculture, thermal plants, and industry have been considered as contributors to NOx emissions, the transport sector especially diesel-powered vehicles are the main contributor of NOx emissions. NOX is principally produced by a high-temperature endothermic reaction between nitrogen and oxygen in the fuel-air mixture.
Around 40% of total NOx emissions come from road transport, while diesel-powered cars account for 85% of NOx emissions from the transportation sector. The challenge for vehicle makers and researchers has been to minimize NOx emissions from diesel engines without sacrificing engine performance.
To obtain optimal conversion rates in NOx emissions, numerous research and technological advancements have been done [4]. Pretreatment methods reduce NOx emissions before they are sent to the engine’s exhaust port, whereas after-treatment methods reduce NOx emissions after they have been directed to the engine’s exhaust port. Pretreatment methods for decreasing NOx emissions include exhaust gas recirculation (EGR), electronically controlled fuel injection, engine modification, increased injection time, water spray in the combustion chamber, improved fuel characteristics, and the use of fuel additives. After-treatment technologies including the use of lean NOx trap (LNT) catalysts and the selective catalytic reduction (SCR) systems, which were developed to reduce NOx emissions and are widely used in diesel engines, are the subject of this research [5].
SCR (selective catalytic reduction) of NOx emissions is a common technique nowadays. With a reductant and catalyst in an SCR system, NOx emissions from diesel engines can be reduced significantly [6]. The most common reductant and catalyst used to reduce NOx emissions at high exhaust gas temperatures are ammonia (NH3) and V2O5–WO3/TiO2. However, conversion efficiency remains low at low exhaust gas temperatures below 250°C, and NH3 builds up on the exhaust line and catalyst surfaces. This phenomenon, known as NH3 slip, has a significant detrimental impact on SCR conversion efficiency and can lead to catalyst degradation [7].
Many catalysts, such as Cu zeolites or Mn oxides, have been created with superior low-temperature SCR activity; however, they are all ineffective in most off-gas circumstances because the active sites are substantially destroyed by sulfur dioxide poisoning [8, 9]. Yu et al. created a Cu-SSZ-13 zeolite-metal oxide hybrid catalyst that has improved SO2 tolerance by preferring producing Zn sulfate over Cu sulfate [10]. Unfortunately, commercialization in the field is problematic due to the sophisticated catalyst preparation procedure and the requirement for a very high temperature (650°C) to regenerate inactive catalysts [11].
For SCR NOx reduction, a variety of catalysts have been studied, including platinum group metals (PGM), perovskite-type oxides, spinel-type oxides, and mixed transition metal oxides [12–15]. In a low-temperature selective catalytic reduction, low-cost spinels were used [16].
However, in this work, the catalyst used is capable of reducing NOx at low temperatures less than reported temperature of ABS [17] without any external reductant. Further, NiCo2O4 was chosen as the preferable catalyst in this investigation because of its high activity at low exhaust temperatures, environmental benefits when compared to conventional catalyst toxic effects, and great performance in NOx reduction [18, 19]. NOx conversion efficiency was studied using NiCO2O4 as a catalyst at two different engines with various loading conditions. Furthermore, using a basic physical mixing strategy is a cost-effective and practical option that can be quickly implemented in the industry. This catalyst reduces additional fuel expenses and carbon dioxide emissions by operating at low temperatures without the use of an external reductant. As a result, it provides enormous economic and environmental benefits.
2. Materials and Methods
2.1. Synthesis of Nano-NiCo2O4
The polymeric precursor technique [20] was used to make NiCo2O4. The starting components, nickel nitrate (Ni(NO3)2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), and chitosan, were employed exactly as purchased. 14.5 g cobalt nitrate and 7 g nickel nitrate were dissolved in distilled water and stirred continuously for 20 minutes. Then, 5% of the chitosan solution was added, and the mixture was stirred constantly at 80°C until it formed a gel, which was then heated at 80°C for 4 hours at a rate of 5°C/min to generate a black powder. The black powder was pounded in an agate mortar and then calcined for 6 hours in an air environment at 300, 500, 700, and 900°C. The schematic representation of synthesis of nano-NiCo2O4 is shown in Figure 1.
2.2. Engine
The process begins by choosing the engine. Here, a twin-cylinder diesel engine, three-cylinder petrol engine, is used for testing the emission. The twin-cylinder diesel engine is a stationary engine with constant speed at different load conditions. The various emissions like HC, CO, NOx, CO2, and O2 are noted down at different load conditions.
2.3. Fabrication of SCR System
The honeycomb structure is brought in and the synthesized catalyst (nickel cobalt oxide (NiCo2O4)) is coated into it using a binder solution of polyvinyl alcohol (PVA). The honeycomb structure is then welded around the edges of a cylindrical metal (Figure 2).
2.4. Experimental Setup
Figure 3 shows a schematic illustration of the experimental setup used to determine the effectiveness of NOx conversion utilizing fabricated SCR systems. A specially designed exhaust system and a generator motor that can be operated at various loads makes up the test setup. Experiments were carried out at different engine loads (0 kW, 4 kW, 8 kW, and 12 kW, using two different engines). We chose a twin-cylinder diesel engine and s-presso three-cylinder petrol engine to test the fabricated SCR system with a speed rate of 1500 rpm. The specifications of the engines are given in Table 1. NOx conversion ratios were determined in the tests to determine the performance of the catalyst generated at low exhaust gas temperatures.
2.5. Characterization
X-ray diffraction (XRD) was used to determine the crystalline structure and phase purity of the products using an X-ray diffractometer (Bruker Instrument (D8 Advance)) with Cu-Kα () radiation ranging from 10 to 80° at ambient temperature. The sample’s formation and morphology were studied using scanning electron microscopy (SEM) (FEI QUANTA FEG 200 HR) and Fourier transform infrared spectroscopy (FTIR spectrometer, JASSCO Model 4100). The fabricated catalytic converter is put through its paces with different loads on an emission analyzer. The base readings and the final readings are made as a graph, and from the graph, we can identify the NOx reduction percentage and reduction of other emissions.
3. Results and Discussion
3.1. Characterization of NiCo2O4
The synthesized NiCo2O4 sample’s FTIR spectrum is shown in Figure 4. The spectrum of NiCo2O4 spinel exhibits two separate sharp bands in the region of 550 to 650 cm-1, as indicated in the previously published literature. The presence of a metal–oxygen bond in a metal oxide spinel is shown by these two bands. The stretching vibrations of Ni–O bond were assigned to the band at 571 cm-1, while the stretching vibrations of Co–O bond were attributed to the band at 664 cm-1 in NiCo2O4 spinel [21–23].
The XRD spectrum of the as-synthesized NiCo2O4 nanoparticle is shown in Figure 5. The synthesized sample’s XRD pattern is indexed to spinel NiCo2O4 (JCPDS card no. 73-1702). No other peaks corresponding to NiO or CoO was found which indicates the formation of pure phase NiCo2O4 [24].
The surface morphology of synthesized NiCo2O4 nanostructures was examined using SEM. The SEM image in Figure 6 clearly shows the formation of hexagonal-shaped nanostructures of NiCo2O4 [25].
3.2. NOx Reduction Analysis
The NOx emissions acquired from two different types of engines at different engine loads are illustrated below. The temperature of combustion has a big impact on nitrogen oxide production. At high engine load and high combustion temperature, the highest production of NOx occurs [26–28]. When the engine is under maximum load, high combustion temperatures are reached. High engine load produces more NOx than low engine load. The temperature is the key factor for the highest NOx reduction at higher loads.
In the case of a twin-cylinder diesel engine, the engine rpm was fixed as 1500 rpm but its load is varied from 0 to 12 kW. NOx emissions before and after the SCR catalyst are shown in Tables 2 and 3. There is a considerable reduction in NOx after treatment with NiCO2O4-modified SCR without an external reductant. On varying load conditions, the catalyst NiCo2O4 showed superior NOx conversion at all loads, particularly at 12 kW load conditions. Similarly, the overall conversion percentages of HC, CO, and CO2 were affected by the load. If the temperature of the SCR catalytic converter is between 120°C and 200°C, the most efficient NOx reduction is achieved. The reason for the NOx reduction at low temperature is due to synthesized nanonickel cobaltite which possesses higher oxygen vacancies in the surface-active site of the catalyst NiCo2O4. As reported by Trivedi, our catalyst showed good redox property in reducing the NOx at low temperatures due to the surface-active sites [18]. Figure 7 clearly depicts the reduction in NOx with and without SCR catalyst on varying loads.
Similar reports are observed (Tables 4 and 5) when the same SCR catalyst was examined in petrol engine. The efficiency of the SCR catalyst (synthesized nickel cobaltite) was studied using s-presso GDI petrol engine. The engine was run with 1500 rpm speed, but the load is varied from 0 to 16 kg. On varying the load, the NOx reduction occurs maximum at lower temperature from 250 to 320°C. Similar to diesel engine study, HC, CO, and CO2 also reduced drastically. This further confirms that the catalyst NiCo2O4 (nickel cobaltite) has surface-active sites which is more effective in reducing NOx at low temperatures.
Since NOx is a key pollutant for many environmental issues, even though other pollutants are also significantly reduced in both engines, our focus is on NOx reduction. The catalyst’s ability to reduce NOx without H2 may be caused by redox couples of Ni3+/Ni2+ (0.58 V/0.49 V) and Co3+/Co2+ (0.53 V/0.51 V) as well as oxygen vacancy-rich NiCo2O4 nanoparticles, which boost the catalyst’s reactivity and number of active sites [29].
4. Conclusion
The spinel catalyst NiCo2O4 was successfully synthesized and fabricated on alumina monolith for NOx reduction study. Two types of engines have been selected, and NOx reduction efficiency was investigated. In the studies, no external reductant was added but the reduction was maximum at lower temperatures. This may be due to the surface-active centers of nanospinel NiCo2O4 and its morphology. After further investigations over lengthy periods of time on stream from CNG cars, such as water vapors and S-based chemicals, the spinel catalyst NiCo2O4, which is affordable, highly active, and thermally stable, may be suggested for use as a noble metal alternative in emission management.
Data Availability
All data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors thank KCG College of Technology for providing all required facilities to carry out the experiments.