INTRODUCTION
The global industrialization has led to significant human advancements but also presents serious environmental challenges The combustion of fossil fuels, including coal, oil, and natural gas, releases various air pollutants, notably carbon dioxide (CO2) and nitrogen oxides (NOx) The rising levels of NOx emissions from both vehicles and industrial sources have become a major concern for scientists worldwide.
Nitrogen oxide (NOx) is a general term for the oxides of nitrogen However, the main NOx produced by the combustion process are nitric oxide and nitrogen dioxide
Nitric oxide, primarily generated through combustion processes, constitutes 90-95% of nitrogen oxides (NOx), with nitrogen dioxide making up the remaining 5-10% (Jarvis DJ et al., 2010) This pollutant significantly contributes to atmospheric issues, including acid rain and the formation of tropospheric ozone, which differs from stratospheric ozone that protects the Earth from harmful rays Efforts to mitigate NOx emissions have been implemented at various stages of fossil fuel usage, including fuel control, combustion, and post-combustion processes.
Selective Catalytic Reduction (SCR) effectively minimizes NOx emissions from stationary sources in the post-combustion phase This process utilizes a catalyst to enhance the reaction between ammonia and nitric oxide, in the presence of oxygen, resulting in the formation of nitrogen (N2) and water (H2O).
The general reaction of SCR process is:
Traditionally, V2O5–WO3/TiO2-based catalysts have been utilized, but they only function effectively at high temperatures above 300ºC, making them unsuitable for real-world manufacturing processes that typically operate between 100-200ºC This limitation not only necessitates significant energy consumption but also risks oxidizing NH3 into N2O and NO at temperatures exceeding 400ºC Consequently, researchers have focused on developing more efficient catalysts that perform well at lower temperatures Recent findings indicate that Cr/TiO2, Cu/TiO2, and Mn/TiO2 exhibit high activity at 120ºC, prompting increased interest in low-temperature catalysts for reducing NOx emissions from stationary sources.
This study aims to evaluate the efficiency of a specific low-temperature catalyst in a laboratory setting Additionally, research will be conducted to determine the optimal conditions for effectively utilizing this catalyst to reduce NOx emissions from stationary sources.
To describe the synthesis, preparation and performance test of low temperature catalysts (Mn 20 /TiO 2 , Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 )
To compare the efficiency of three catalysts for NOx reduction from stationary sources
1 How the low-temperature catalysts are synthesized and prepared for SCR process?
2 What is the mechanism of using Mn 20 /TiO 2 , Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 as selective catalytic reduction of NOx with NH 3 ?
3 Compare the efficiency NOx reduction among three catalysts (Mn 20 /TiO 2 ,
Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 )
LITERATURE REVIEW
Nitrogen oxides (NOx)
Nitrogen, primarily found as diatomic molecules (N2), constitutes approximately 80% of the Earth's atmosphere A single nitrogen atom (N) is highly reactive, exhibiting valence states ranging from +1 to +5, allowing it to form various oxides Generally, nitrogen oxides are binary compounds formed between nitrogen and oxygen, including nitrous oxide (N2O), nitric oxide (NO), dinitrogen trioxide (N2O3), nitrogen dioxide (NO2), and dinitrogen tetroxide (N2O4).
In atmospheric chemistry, nitrogen oxides, commonly referred to as NOx, encompass the total concentrations of nitric oxide (NO) and nitrogen dioxide (NO2) These compounds are significant air pollutants produced as byproducts of fossil fuel combustion This research specifically focuses on NO and NO2 as the primary components of NOx.
Nitrogen oxides (NOx), consisting of nitric oxide (NO) and nitrogen dioxide (NO2), originate from both natural and human-made sources Natural emissions arise from stratospheric intrusion, bacterial processes, volcanic activity, and lightning strikes In contrast, anthropogenic NOx emissions are primarily categorized into stationary sources, such as fossil fuel combustion and power generation, and mobile sources, including internal combustion engines.
NOx gases contribute to the formation of smog, particularly brown smog in urban areas during the summer months These gases are hazardous not only on their own but also due to their reactions with other atmospheric substances, leading to the creation of ozone (O3) and acid rain Ozone at lower atmospheric levels poses a significant risk to our respiratory health, differing from the protective ozone found in the stratosphere Additionally, increased nitrogen loading in water bodies disrupts the chemical balance, resulting in eutrophication.
Nitrogen oxides, particularly NO2, pose significant health risks due to their potential to form harmful reaction products like ozone and secondary particles Exposure to NOx can lead to serious respiratory issues, as highlighted by the World Health Organization (2003), including decreased lung function, increased susceptibility to bacterial infections, and exacerbation of allergic diseases (Rusznak et al., 1998).
Selective Catalytic Reduction (SCR)
Selective catalytic reduction (SCR) is an effective process for converting nitrogen oxides (NOx) into harmless diatomic nitrogen (N2) using a catalyst Among the various methods, ammonia-based selective catalytic reduction (NH3-SCR) stands out due to its high efficiency, selectivity, and cost-effectiveness, making it a widely adopted technology for flue gas treatment.
The NOx reduction reaction occurs within the catalyst chamber, where ammonia is injected and mixed with the gases prior to entry This process utilizes either anhydrous or aqueous ammonia for selective catalytic reduction, as outlined in the chemical equation provided by Lee and Bai (2016).
However, there are several secondary chemical reactions that are likely to occur, which are:
The result of SCR process affected by several factors, such as reaction temperature, space velocity, and presence of H 2 O and SO 2 (Lee, Bai, 2016).
Catalyst
Catalysts are materials that speed up chemical reactions, and in the selective catalytic reduction (SCR) process, the commonly used commercial catalysts are vanadium pentoxide (V2O5) supported on titanium dioxide (TiO2) in its anatase form However, these vanadia-based catalysts exhibit high activity only within a limited and elevated temperature range.
Page 16 window of 300-400 °C and also is subjected to deactivation by SO 2 (Kompio PGWA, et al, 2012) Moreover, vanadium byproducts formed during catalyst preparation and usage is hazardous to the environment and human health (Chen et al., 2014) Therefore, there have been attempts to develop new catalyst which can operate in low temperature (Nakahjima et al., 1996)
Interest in discovering new catalysts for low-temperature applications has surged over the past decade A review of research published from 1990 to 2015, sourced from the Web of Science using the keyword "Low Temperature Selective Catalytic Reduction," reveals a significant increase in scholarly activity Initially, only a handful of papers were released each year in the early 1990s, but by 2015, the number of publications addressing low-temperature SCR exceeded 380 (Lee, Bai, 2014).
Recent studies have focused on various active metals and supports for low-temperature selective catalytic reduction (SCR) catalysts, including transition metals like Fe, V, Cr, Cu, Co, and Mn, which are supported on materials such as SiO2, Al2O3, or TiO2 (Thirupathi et al., 2011).
Qi and Yang (2003), performed catalyst test of Mn10/TiO2, the reacting condition was T0℃, GHSV= 30,000h-1, [NO]=[NH3]00 ppm, [O2]=2% The result is 90%
In 2004, Pena et al conducted an experiment to evaluate the NOx conversion efficiency of a Mn (20wt%)/TiO2 catalyst The testing conditions included 2% oxygen, a temperature of 100°C, a gas hourly space velocity (GHSV) of 50,000 h⁻¹, and a 1:1 ratio of NO to NH3 concentrations.
Page 17 gasses are 400 ppm) The data revealed that 80% of NOx were being converted throughout the process
In 2006, a study demonstrated that a catalyst composed of 20wt% Mn/TiO2 achieved complete NOx conversion under specific conditions, including a temperature of 100 °C, a gas hourly space velocity (GHSV) of 8,000 h⁻¹, and a 1:1 ratio of NO to NH3 at concentrations of 500 ppm each, with the presence of 5% oxygen.
NO conversion rate of Mn/TiO 2 was 83% by performing at T 0℃, [NO] r0 ppm, [NH 3 ]0 ppm, [O 2 ]=3% Fang et al., (2014)
Qi and Yang (2003) first introduced the use of MnOx and CeO2 as low-temperature catalysts for the selective catalytic reduction (SCR) of NO with NH3 Despite advancements, the optimal molar ratio of Mn/(Mn+Ce) remains inconsistent, with various preparation methods resulting in significantly different temperature ranges for maximum NO conversion (Chen et al., 2014) Notably, the most effective catalyst achieved nearly 100% NO conversion at 120ºC with a molar ratio of 0.3 and a high space velocity of 42,000 h–1 (Qi, Yang, 2003).
METHODOLOGY
Overview of Research Design
The main objective of this study is to outline the detailed process to evaluate the performance of low temperature catalysts and also implement the result sample catalysts
The process includes synthesis, preparation, and performance test were recorded and described in methodology section As the researcher used Mn 20 /TiO 2 ,
Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 as sample catalysts, the data obtained from the performance test could also be analyzed and compared Therefore, the comparison of the performance of three catalysts is another outcome of this study
The researcher meticulously documented the experimental design and summarized the findings in this thesis To evaluate the performance of the catalysts, performance tests were conducted under consistent conditions The outlet NO concentration for each catalyst was measured using a specific NO detector at 30-minute intervals over a duration of 2 hours.
Moreover, to answer the research question outlined in an introduction part, different methodology will be performed to obtain answers (as shown in table 1)
Table 1 Methodology to Answer Research Questions
Research questions How to obtain answer
4 How the low-temperature catalysts are synthesized and prepared for SCR process?
Study the previous researches for reference
5 What is the mechanism of using Mn 20 /TiO 2 ,
Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 as selective catalytic reduction of NOx with NH 3 ?
Record the condition, catalyst used
observe the system and draw the mechanism diagram
6 Compare the efficiency NOx reduction among three catalysts (Mn 20 /TiO 2 ,
Record the NO reduction rate
Materials and Equipment
The materials for catalyst synthesis are listed in the table 2
Table 2 Materials for Catalyst Synthesis
Chemical: Mn/TiO 2 Mn-Ce/TiO 2
Cerium (III) nitrate hexahydrate (Ce(NO 3 ) 3 6H 2 O)
Aqua 0.5M Sodium carbonate (Na 2 CO 3 )
Figure 1 The magnetic stirrer: it is for stirring catalysts precursers in a synthesis process
Resulting catalyst from the synthesis
Figure 2 The pressure plumber to press catalyst powder into a hard piece
Continuous Flow Tubular Reactor System
Methodology
Three kinds of catalyst, denoted as Mn 20 /TiO 2 , Mn 20 Ce 10 /TiO 2 , Mn 20 Ce 20 /TiO 2 were synthesized by co-precipitation method with 0.5M sodium carbonate (Na 2 CO 3 ) as a precipitant
To prepare the catalyst solution, the necessary precursors listed in Table 3 were combined with 70ml of water in a beaker The mixture was then heated on a magnetic stirrer at 100-115°C with a stirring frequency of 300 rpm Once the temperature reached 60°C, 0.5 M sodium carbonate was gradually added until the pH level was approximately reached.
10 The resulting precipitate was filtered and washed with 2000ml of DI water The catalysts were first dried overnight followed by calcination at 350°C for 6 hours The overall process was shown in figure 3
Table 3 The Amount of Precursor in Each Catalyst
(CH 3 COO) 2 Mn.H 2 O) (Ce(NO 3 ) 3 6H 2 O TiO 2
Figure 3 The Catalyst Synthesis Process
Once the sample catalysts were calcinated, they would be transformed into a pellet type This preparation was to crush and sieve catalysts
The catalyst powder was first wrapped in a 2x2 cm plastic film and then compressed under pressure for 2 minutes After hardening, the packed catalyst was ground and sifted through two layers of a sieve.
Figure 4 The Catalyst Preparation Process
The catalyst’s performance test was carried out in a fixed bed quartz tube reactor with the inner size of 6mm at atmospheric pressure The feed gases are NO
(200 ppm), NH 3 (200 ppm), air (as a balance gas to yield the total gas flow rate of
800 ccm) The used catalysts yield GHSV of 20,000h -1 The catalytic reactor was pre-heated at 150°C before used Analysis was performed after the reaction system remained steady for at least 20 min
NOx conversion rate was calculated as follows:
Figure 5 SCR system for catalyst’s performance test
The reaction mechanism of the catalyst is illustrated in Figure 5 Initially, ammonia, nitric oxide, and air are injected into the mixing tube, where the gas mixture flows into a heated reactor and through the catalyst bed During this process, the catalyst facilitates the reaction between NH3 and NO, resulting in the formation of N2 and water The treated gas is then released as the outflow The NO conversion rate is monitored in the outflow gas every 30 minutes over a period of 2 hours using an NO detector for further analysis.
RESULTS AND DISCUSSION
Result
4.1.1 The Synthesis of the Catalyst
The synthesis of all three catalysts with varying precursor loadings was successfully achieved using the co-precipitation method Following filtration and washing with 2000 ml of distilled water, the catalysts were dried overnight in an oven and subsequently calcined for 6 hours.
Figure 7 The synthesized catalyst (after calcination), from left to right
(Mn 20 Ce 20 /TiO 2 , Mn 20 Ce 10 /TiO 2 , Mn 20 /TiO 2 )
Final pH value was recorded The pH value of the aqueous catalyst is shown below:
In this study, all three catalysts were prepared by the same method as described in the methodology part earlier The result yielded the catalyst into the pellet type
Figure 8 The Transformation of Catalyst from Powder into Pellet Type
4.1.3 Performance test of the catalysts
The stability of all three catalysts was assessed in a flow gas reactor over a two-hour period, with conversion rates recorded every 30 minutes, as illustrated in Figure 9.
Figure 9 the stability of Mn 20 /TiO 2 catalyst in NOx conversion
Figure 10 The stability of Mn 20 Ce 10 /TiO 2 catalyst in NOx conversion
Figure 11 The stability of Mn 20 Ce 20 /TiO 2 catalyst in NOx conversion
In a flow reactor machine, the Mn 20 Ce 10 /TiO 2 catalyst achieved an impressive 89.55% NO conversion rate after two hours, significantly outperforming Mn 20 /TiO 2 and Mn 20 Ce 20 /TiO 2, which only converted 82.46% and 84.87% of NO, respectively, as illustrated in Figure 12.
Figure 12 The Average NO conversion (%) of each catalyst
Mn20/Tio2 Mn20 Ce10/Tio2 Mn20 Ce20/TiO2
The bar chart of figure 12 suggests that Mn 20 Ce 10 /TiO 2 performed best in converting NO with the conversion rate of 89.55% While Mn 20 /TiO 2 and
Mn 20 Ce 20 /TiO 2 has lower potential with only 82.46% and 84.88% of NO conversion rate, respectively.
Discussion
The objectives of the research are to record the synthesis process of the low temperature catalyst and compare three catalysts’ performances, which are Mn 20 /TiO 2 ,
Mn 20 Ce 10 /TiO 2 , and Mn 20 Ce 20 /TiO 2
The successful synthesis of the catalysts resulted in consistent morphology across all samples As illustrated in Figure 7, the catalysts exhibited a darker color following the calcination process.
Catalysts can be characterized through BET surface area, pore volume, and XRD pattern analysis to understand their performance factors However, due to time and resource constraints, comprehensive studies on these characteristics have not been conducted, with only the pH levels of the aqueous catalyst precursors recorded Initially, it was anticipated that all catalysts would have a pH close to 10 to minimize bias; however, Mn 20/TiO2 exhibited a pH of 10.47, which, despite being a minor deviation, is significant for further catalyst analysis This is crucial because the catalytic activity is influenced by the predominance of Mn 4+ species on the catalyst surface, and the Mn valence state is affected by the pH of the TiO2 aqueous slurry during preparation (Kim, Hong, 2012).
After synthesis, the catalysts required transformation into pellet form before being utilized in the SCR process By repeating the synthesis procedure, three series of catalysts were produced with consistent pellet sizes Consequently, it is assumed that this thesis is not influenced by variations in the catalysts' surface area.
All three catalysts demonstrated a stable performance with conversion rates exceeding 80%, as illustrated in graphs 7, 8, and 9 Specifically, Mn 20/TiO2 achieved an NOx conversion rate of 82.46% under conditions of 0°C, a ratio of NH3 to NO of 0 ppm, and a GHSV of 1,000 h^-1 This result aligns with the findings of Pena et al (2004), who reported an 80% NOx conversion rate for Mn 20/TiO2 tested at 100°C, with 400 ppm of NH3 and NO, a GHSV of 1,000 h^-1, and the presence of 2% oxygen.
Research indicates that incorporating cerium into the catalyst significantly improves its performance The study found that a Mn-Ce ratio of Mn(Mn+Ce)=0.3, functioning at 120ºC and a high space velocity of 42,000 h–1, achieved nearly 100% NO conversion (Qi and Yang, 2003) These findings align with Qi and Yang's experiments, demonstrating that cerium enhances NO conversion, even at lower conversion rates.
Mn 20 Ce 10 /TiO 2 and Mn 20 Ce 20 /TiO 2 yielded NOx conversion of 89.55% and 84.88%, respectively
Increasing the cerium content in the catalyst formulation (Mn 20 Ce 20 /TiO 2) led to a decrease in the conversion rate The underlying reasons for this decline remain unclear, as a detailed characterization of the catalysts has not been conducted due to resource limitations.
The data indicates that while this type of catalyst demonstrated strong performance at specific temperatures and operating factors, variations in operating conditions across different studies complicate the comparison of catalyst effectiveness Therefore, a clear assessment of which catalyst performs better can only be made when considering the unique operating conditions reported in each piece of literature However, based on our experiment, we found that among the three catalysts tested at T0°C, with NH3 and NO levels at 0 ppm and a GHSV of 20,000 hours, we can draw some conclusions regarding their performance.
Mn 20 Ce 10 /TiO 2 performed best, followed by Mn 20 Ce 20 /TiO 2 and Mn 20 /TiO 2
CONCLUSION
The successful synthesis and preparation of three types of catalysts—Mn 20 /TiO 2, Mn 20 Ce 10 /TiO 2, and Mn 20 Ce 20 /TiO 2—were utilized in experiments to effectively reduce NOx emissions from stationary sources The conversion rates achieved were 82.46% for Mn 20 /TiO 2, 89.55% for Mn 20 Ce 10 /TiO 2, demonstrating their efficiency in NOx reduction.
To be specific, with the aid of Cerium, catalysts tended to perform better
Among the three catalysts tested, Mn 20 Ce 10 TiO 2, with a 10% addition of Cerium, demonstrated the highest performance In contrast, increasing the Cerium content to 20% in Mn 20 Ce 20 TiO 2 resulted in a noticeable decline in catalyst efficiency.
In conclusion, the research successfully achieved both primary objectives and addressed three research questions through the experiments conducted However, due to time constraints, each catalyst was only tested once, limiting the results to a reference for future studies rather than practical application Further analysis of the three tested catalysts is necessary to understand their performance.
The review by Busca et al (1998) delves into the chemical and mechanistic aspects of the selective catalytic reduction (SCR) of nitrogen oxides (NOx) using ammonia over oxide catalysts It provides a comprehensive analysis of the processes involved, highlighting the efficiency and effectiveness of these catalysts in reducing harmful emissions The findings are crucial for understanding the environmental impact of NOx and improving catalytic technologies.
2 C Rusznak, S Jenkins, P R Mills, R J Sapsford, J L Devalia, R J Davies, Mechanism of pollution-induced allergy and asthma, „Revue Franỗaise d’Allergologie et d’Immunologie Clinique” 1998, nr 38 (7), s S80–S90
3 Chen L, Si ZC, Wu XD, et al (2014) Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review J Rare Earth 32: 907-917
4 Clean Air Technology Center (1999) Technical Bulletin: Nitrogen oxides (NOx), why & how they are controlled United states
5 F Nakahjima, I Hamada, The state-of-the-art technology of NOx control,
6 Health aspects of air pollution with particulate matter, ozone and nitrogen dioxide (2003) Copengahen: World Health Organization, Regional Office for Europe
7 Hiromichi Arai, Hisashi Fukuzawa, Research and development on high temperature catalytic combustion, In Catalysis Today, Volume 26, Issues 3–4,
1995, Pages 217-221, ISSN 0920-5861, https://doi.org/10.1016/0920- 5861(95)00142-8.
(http://www.sciencedirect.com/science/article/pii/0920586195001428)
8 Jarvis DJ, Adamkiewicz G, Heroux ME, et al Nitrogen dioxide In: WHO Guidelines for Indoor Air Quality: Selected Pollutants Geneva: World Health
Organization; 2010 5 Available from: https://www.ncbi.nlm.nih.gov/books/NBK138707/
9 Kapteijn F, Singoredjo L, Andreini A, et al (1994) Activity and Selectivity of Pure Manganese Oxides in the Selective Catalytic Reduction of Nitric-Oxide with Ammonia Appl Catal B-Environ 3: 173-189
Kim and Hong (2012) investigated the enhancement of Mn/TiO2 catalyst activity by manipulating the pH levels and the valence state of manganese during the catalyst preparation process Their findings, published in the Journal of the Air & Waste Management Association, highlight the significance of these parameters in optimizing catalyst performance for environmental applications The study provides valuable insights for improving catalytic processes, which can lead to more efficient air and waste management solutions.
11 Kompio PGWA, Bruckner A, Hipler F, et al (2012) A new view on the relations between tungsten and vanadium in V2O5-WO3/TiO2 catalysts for the selective reduction of NO with NH 3 J Catal 286: 237-247
12 Lee, T., & Bai, H (2016) Low temperature selective catalytic reduction of NOx with NH3 over Mn-based catalyst: A review AIMS Environmental Science, 3(2), 261-289 doi:10.3934/environsci.2016.2.261
The article titled "Rare Earth Containing Catalysts for Selective Catalytic Reduction of NOx with Ammonia: A Review," authored by Lei Chen, Zhichun Si, Xiaodong Wu, Duan Weng, Rui Ran, and Jun Yu, was published in the Journal of Rare Earths, Volume 32, Issue 10, in 2014 This review discusses the effectiveness of rare earth elements in enhancing catalysts used for the selective catalytic reduction of nitrogen oxides (NOx) with ammonia The findings highlight the potential of these catalysts to improve environmental outcomes by reducing harmful emissions The article spans pages 907 to 917 and is accessible via its DOI link.
(http://www.sciencedirect.com/science/article/pii/S1002072114601629)
Manganese oxide catalysts have been studied for their effectiveness in reducing NOx emissions with ammonia at low temperatures In the research conducted by Min Kang, Eun Duck Park, Ji Man Kim, and Jae Eui Yie, published in Applied Catalysis A: General, Volume 327, Issue 2, the findings highlight the potential of these catalysts in environmental applications The study, detailed on pages 261-269, emphasizes the importance of this catalytic process in addressing air pollution challenges.
(http://www.sciencedirect.com/science/article/pii/S0926860X07003249)
Nitrogen oxides (NOx) significantly impact our health and environment, influencing the transport of ozone across regions The U.S Environmental Protection Agency (EPA) has introduced new regulations aimed at reducing nitrogen oxide emissions to improve air quality and protect public health This initiative underscores the importance of addressing NOx pollution for better living conditions and respiratory health.
Peủa, D A., Uphade, B S., and Smirniotis, P G (2004) conducted a study on TiO2-supported metal oxide catalysts aimed at the low-temperature selective catalytic reduction of nitrogen oxides (NO) using ammonia (NH3) Their research, published in the Journal of Catalysis, focuses on the evaluation and characterization of first-row transition metals, providing significant insights into the effectiveness of these catalysts in reducing NO emissions efficiently.
17 Qi G, Yang R T Performance and kinetics study for lowtemperature SCR of
NO with NH3 over MnOx-CeO2 catalyst J Catal., 2003, 217(2): 434
18 Qi G, Yang RT A superior catalyst for low-temperature NO reduction with NH3 Chem Commun (Camb) 2003 Apr 7;(7):848-9 PubMed PMID:
19 Sloss, L L (1992) Nitrogen oxides control technology fact book Park Ridge, NJ: Noyes Data Corporation
The study by Smirniotis et al (2001) investigates the low-temperature selective catalytic reduction (SCR) of nitrogen oxides (NO) using ammonia (NH3) with manganese (Mn), chromium (Cr), and copper (Cu) oxides supported on Hombikat TiO2 This research highlights the effectiveness of various metal oxides in enhancing the catalytic process for reducing NO emissions at lower temperatures, contributing to advancements in environmental chemistry and pollution control technologies.
21 Sounak Roy, M.S Hegde, Giridhar Madras, Catalysis for NOx abatement, In Applied Energy, Volume 86, Issue 11, 2009, Pages 2283-2297, ISSN 0306-
(http://www.sciencedirect.com/science/article/pii/S0306261909000968)
22 Thirupathi B, Smirniotis PG (2011) Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn,
Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures Appl Catal B-Environ 110: 195-206
The article by Thirupathi Boningari and Panagiotis G Smirniotis discusses the detrimental effects of nitrogen oxides (NOx) on both the environment and human health It emphasizes the role of manganese-based materials in the effective reduction of NOx emissions Published in *Current Opinion in Chemical Engineering*, Volume 13, the study highlights the significance of addressing NOx pollution to improve air quality and public health.
(http://www.sciencedirect.com/science/article/pii/S2211339816300661)
24 V.I Pârvulescu, P Grange, B Delmon, Catalytic removal of NO, In Catalysis Today, Volume 46, Issue 4, 1998, Pages 233-316, ISSN 0920-5861, https://doi.org/10.1016/S0920-5861(98)00399-X.
(http://www.sciencedirect.com/science/article/pii/S092058619800399X)
Wenqing Xu, Yunbo Yu, Changbin Zhang, and Hong He conducted a study on the selective catalytic reduction of nitrogen oxides (NO) using ammonia (NH3) over a cerium-titanium dioxide (Ce/TiO2) catalyst This research was published in Catalysis Communications, Volume 9, Issue 6, in 2008, spanning pages 1453 to 1457 The study focuses on the effectiveness of the Ce/TiO2 catalyst in reducing NO emissions, which is crucial for environmental protection and air quality improvement.
(http://www.sciencedirect.com/science/article/pii/S1566736707005389)
26 WHO (2003) Health Aspects of Air Pollution with Particulate Matter, Ozone and Nitrogen Dioxide, Report on a WHO Working Group.