Air pollution (volatile organic compound, etc.) and climate change

Air pollution (volatile organic compound, etc.) and climate change

C H A P T E R 2 Air pollution (volatile organic compound, etc.) and climate change Rahma Bensouilah, Sarra Knani, Sahar Mansour and Zouhaier Ksibi La...

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C H A P T E R

2 Air pollution (volatile organic compound, etc.) and climate change Rahma Bensouilah, Sarra Knani, Sahar Mansour and Zouhaier Ksibi Laboratory of Materials Chemistry and Catalysis, Department of Chemistry, Faculty of Sciences of Tunis, Campus University El Manar, 2092 El Manar Tunis, Tunisia

2.1 Introduction Air, this magical invisible gas, is the first vital need of our body. As a matter of fact and because of its abundance, we tend to forget that air is very precious [1]. Moreover, a man could live 30 days without eating, while more than 3 days without drinking, but not even more than 3 min without breathing. So far and over the years, humanity has experienced an exponential evolution of new technologies in different fields (industry, transport, agriculture, etc.) [2]. These advances aim to improve our quality of life but at what price? As a result, air and water pollution will be the cost. However, when it concerns the air, it becomes extremely dangerous because unlike the water we drink, it is impossible to filter all the air we breathe. It is important to know beforehand that the steady increase of air pollutants emissions, generated by human activities, causes major threats to the environment and the living beings [3]. According to the World Health Organization (WHO), urban air pollution is responsible for 3.6 million deaths [4]. In fact, there are two classes of air pollutants: the primary contaminants which are directly emitted into the atmosphere such as nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), particulate matters of different sizes, heavy metals, volatile organic compounds (VOCs) [5] and the secondary pollutants resulting from physicochemical reactions between pollutants under particular conditions effect such as ozone (O3) [6]. The NOx including nitrogen monoxides and especially nitrogen dioxides are mainly formed during the combustion in the vehicles exhaust gas and stationary sources [7]. The NOx were involved in the formation of acid rain and participate indirectly to the degradation of the stratospheric ozone layer and the increase of the greenhouse effect [8]. Moreover, Jouini et al. [9] highlighted the effect of

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these gases on the environment and the health being. Indeed, the authors reported that NOx causes harmful damages in the respiratory branches and contributes to the photochemical smog. Another investigation was conducted by Kuo et al. [10], which showed that the emission of VOCs from natural and industrial sources in the atmosphere leads to serious air pollution such as rain acid, smog, and the production of ground-level ozone. Likewise, ozone is a powerful and aggressive gas; it reduces photosynthesis, and in the human body, it causes inflammation of lungs [11]. In addition, Atkinson [12] deduced that the interaction between NOx and VOCs entails a series of complex photochemical reactions which lead to the formation of tropospheric ozone which in turn is recognized as the major cause of climate change [13]. Setting up of approaches that can effectively and expeditiously remove air pollutants still remains a propitious but arduous challenge. Various techniques have been developed for the reduction of these contaminants, but the catalytic oxidation and the selective catalytic reduction (SCR) appear to be the most promising strategies to remove toxic VOCs and NOx, respectively, from stationary sources and vehicles exhaust gas [1416]. In the following paragraphs, the main characteristics of two air pollutants, NOx and VOCs, and their relationships with the climate are reviewed with the main focus on the catalytic processes and catalysts adopted to reduce their emission, highlighting the most significant advances in this field.

2.2 Air pollutants A VOC is defined according to its chemical and physical properties. Indeed, it is an organic chemical, excluding methane, which consists of at least one element of carbon and one or more other elements such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, silicon, and halogens. According to the legislation of the US Environmental Protection Agency (EPA), a VOC is any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions [17]. These compounds, as their name indicates, are characterized by high volatility under normal conditions of pressure and temperature. Physically speaking, an organic compound is volatile if its boiling point is low and its saturation vapor pressure is high. The European directive no. 1999/13/EC gives another more precise definition: a VOC is “an organic compound having a vapor pressure of 0.01 KPa or more at a temperature of 293.15 K or having a corresponding volatility in the special conditions of use.” Also according to decree no. 2006-623, VOCs include all organic compounds whose boiling point, measured at the standard pressure of 101.3 kPa, is less than or equal to 250 C [18,19]. VOCs are very numerous, and they are a part of a large chemical families such as linear hydrocarbons, saturated or not, as (heptane, hexane, pentane, mineral spirits, etc.); monocyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons such as benzene, toluene, and xylene; oxygenated VOCs as aldehydes, alcohol, ketones, or esters; chlorinated VOCs (tetrachloroethylene, trichloroethylene, dichloromethane); chlorofluorocarbons; plasticizers (dioctyl phthalate, etc.); nitrogen compounds (amines, nitriles, etc.); and sulfur compounds (thiols, dimethyl sulfide, etc.).

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TABLE 2.1 Classification of volatile organic compounds according to the World Health Organization. Volatility

Boiling point range (K)

Compounds

Very volatile

, 323373

Propane, butane, methyl chloride

Volatile

323373 to 513533

Formaldehyde, toluene, acetone, ethanol, 2-propanol

Semivolatile

513533 to 635673

Pesticides, plasticizers, fire retardants

According to their boiling point, VOCs are classified into three categories: very volatile, volatile, and semivolatile compounds. In 1989 the WHO adopted this classification as shown in Table 2.1. However, an organic compound that has high volatility (i.e., low boiling point) will be more likely to be liberated from a product or surface into the air. Very VOCs are so volatile that they are found almost entirely as gases in the air rather than in materials or surfaces. Finally, yet importantly, the semivolatile compounds present in the air constitute a much smaller fraction of the total present indoors, while the majority will be in solids or liquids. The same classification was given also by the standard NF ISO 16000-6 [20,21]. The sources of VOCs are very numerous. The fabrication of solvents and wood combustion are the main sources of the emissions of VOCs [22,23]. Also, combustion plants and fuel use, especially in industry, contribute to their liberation in lesser quantities. Currently, billions of pounds of different VOCs are produced and are widely used as ingredients in many products such as household products, including paints, paint strippers, varnishes, waxes, cleaning products, disinfecting products, cosmetics, degreasing products, aerosol sprays, cleansers, moth repellents, air fresheners, and automotive products. Furthermore, VOCs can be found in office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics, and craft materials, including glues and adhesives, permanent markers, and photographic solutions. Nitrogen oxides (NOx) are represented by a family of seven compounds; their formula depends on the nitrogen (N) valence states as summarized in Table 2.2. In recent years the increasing of the NOx levels in the tropospheric atmosphere is related to industrial revolution and biogenic compounds. In general, NO and NO2 are the most gases emitted in the air, and they are considered as the major air pollutant. The mixture of two gases is mostly denoted as nitrogen oxides. Their emission is mainly related to anthropogenic activities such as industrial boilers, incinerators, gas turbine, and to transport sector, for example, diesel engine. Importantly, the most atmospheric NO2 emitted comes from the oxidation reaction of NO by ozone (O3) [Eq. (2.1)] [24]. NO 1 O3 -NO2 1 O2

(2.1)

The WHO guidelines for NO2 are 40 μg/m3 for annual mean exposure time and 200 μg/m3 for 1-h mean exposure time [25,26]. Indeed, NOx(5NO 1 NO2) exposure has a harmful effects on the human health. It can cause bronchial hyperactivity, cancer, and lung infections for children [2730]. Many published researches combined the theoretical approaches and experimental data for better understanding of the NOx-forming

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TABLE 2.2 Various formulas of nitrogen oxides. Formula

Chemical name

Nitrogen valence state

Properties

N2O

Nitrous oxide

1I

Colorless gas, soluble in water d52

NO

Nitric oxide

1 II

Colorless gas Soluble in water Very toxic gas d 5 1.34

NO2

Nitrogen dioxide

1 IV

Red-brown gas Very toxic Odorous Strong oxidant Poorly soluble in water d 5 1.59

N2O2

Dinitrogen dioxide

1 II

Colorless gas Soluble in water

N2O3

Dinitrogen trioxide

1 III

Blue solid Water soluble Highly toxic d 5 1.783 (g)

N2O4

Dinitrogen tetroxide

1 IV

Transparent Very water soluble Strong oxidant Hypergolic d 5 1.442 (liq, 21 C) Irritant Corrosive

N2O5

Dinitrogen pentoxide

1V

White solid Very water soluble Decomposes in water Strong oxidizer d 5 1.642

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2.3 Climate change

mechanism [3133]. The most cited were thermal NOx, prompt NOx, and fuel NOx. These proposals are developed earlier: • Thermal NOx: This process is considered as the most source of the NOx during combustion of fuels. The mechanism was established by Zeldovich [34] who assumed that at a very hightemperature nitrogen and oxygen combine together according to the following reaction paths [Eqs. (2.2)(2.4)] [35]: N2 1 O -NO 1 N 

(2.2)

O2 1 N -NO 1 O

(2.3)

N2 1 O2 -2NO

(2.4)

The rate of NOx formation increases with temperature and initial oxygen concentration [31]. On the other hand, the formation of NO2 can take place in the exhaust line of the diesel engine, where the temperature is lower [Eq. (2.5)]. Therefore it seems that a balance between NO and NO2 formation can exist depending on the temperature. Thus at high temperature, NO is the major formed compound, but at lower temperature, the NO2 prevails. 2NO 1 O2 -2NO2

(2.5)

• Prompt NOx: Prompt (Fenimore) mechanism is a complicated process. The formation of NOx occurred by reaction between hydrocarbon fragments, for example, C, CH, and CH2, and atmospheric nitrogen (N2) to form species such as NH, HCN, and CN. These latter are subsequently oxidized to produce NOx [Eq. (2.6)] [36]. N2 ðairÞ 1 HC-HCN 1 N 1 O -NO

(2.6)

However, previous work showed that Fenimore mechanism leads to the formation of a very negligible amount of NOx to the overall NOx emissions, during combustion of petrodiesel [37]. • Fuel NOx: Nitrogen that is chemically bound in the fuel is oxidized during the combustion process. The main reaction path for fuel NOx formation implies the generation of intermediate nitrogen compounds such as HCN or NH3. These molecules react afterward with oxygen to form NOx. This mechanism formation is only problematic with nitrogen-containing fuel, but since the nitrogen level in diesel and biodiesel is very low, this process formation is negligible.

2.3 Climate change Climate change is a phenomenon closely related to air pollution. In recent years, the temperature of the surface earths has dramatically increased. Christensen et al. [38]

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suggest that by 2100, the average of the air temperatures in the Mediterranean basin will increase from 2.2 C to 5.1 C, and the rainfalls will decrease from 4% to 27%. Hence, periods of drought, heat wave, and flood are expected. Unfortunately, such weather changes could induce the collapse of ices and thereby a rise of the sea level, water scarcity and higher forest fire, especially in dry lands. As a consequence, the human health and the ecosystem are altered [39,40]. Such fact is not only related to the emission of the anthropogenic air pollutants or greenhouse gases (CO2, CH4, etc.) but also to the production a secondary pollutants after physical and chemical interaction between primary pollutants. For instance, tropospheric ozone (O3) is a secondary pollutant released into atmosphere through a photochemical process in which VOCs are oxidized by nitrogen oxides [Eqs. (2.7)(2.10)] [30]. It thereby spreads in the atmosphere at large scales at high concentration, especially in warmer air zones. CO 1 NO 1 O2 -CO2 1 NO2

(2.7)

NO2 1 hν-NO 1 O

(2.8)

O 1 O2 -O3

(2.9)

CO 1 2O2 1 hν-O3 1 CO2

(2.10)

According to Sitch et al. [41], O3 concentration could reach by 2100, 40 ppb on all regions and exceeds 70 ppb over some continents such a North America, central and southwestern Africa during the Northern Hemisphere summer. However, such high level is very toxic especially for the ecosystems. Tropospheric O3 causes potential damages to forests and trees. A study carried out by Mills et al. [42] indicates that tropospheric ozone reduces the biomass and the yields of more than 30 crops and 80 plant species developed in 16 European countries in the period between 1994 and 2006. Excessive emission of NO2 onto the atmosphere and climate change also increased the problems of acidification and eutrophication. The main cause of both of these phenomena is the formation of nitric acid (HNO3, acid rain), from nitrogen dioxides and water [Eq. (2.11)], which is deposited immediately over the grounds and aquatic environment. 3NO2 1 H2 O-2HNO3 1 NO

(2.11)

Indeed, the extra azote and oxygen depletion stimulates the growth of seaweed, causing thereby extinction of some plants species and a loss of biodiversity. The deposition of HNO3 leads to a lack of land minerals such as Ca21 and Mg21, which reduces fertility of the soil [43].

2.4 Catalytic treatment of volatile organic compound and NOx A huge number of scientific papers have been published on the catalytic processes for VOCs and NOx removal. In this section, the main techniques used to reduce the emission of both of this pollutant are presented, and the numerous catalytic materials elaborated for this goal are summarized.

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2.4.1 Volatile organic compounds treatment Given the toxicity of the VOCs, many countries put in regulations in order to limit their emissions. The United States sets regulations for air emissions from stationary and mobile sources called the Clean Air Act, which allows the EPA to establish National Ambient Air Quality Standards to protect human health and regulate emissions of air pollutants such as VOCs [44]. Canada defined restrictions for VOC concentrations in Architectural Coatings Regulations (SOR/2009-264) and Automotive Refinishing Products Regulations (SOR/2009-197). Subsection 93(1) of the Canadian Environmental Protection Act in 1999 is a regulation intended to protect the environment and health of Canadians by setting concentration limits [45]. Clean Air Conservation Act amended in 2007 in the Republic of Korea introduced measures to prevent danger and damage for the national health and environment in order to manage and maintain the atmosphere properly. The regulations are presented in Article 44 of Clean Air Conservation Act published by Ministry of Environment [46]. Merchant Shipping (Prevention of Air Pollution) Regulation in Hong Kong (China) imposes limits on the emissions of VOCs in order to implement Annex VI to the “International Convention for the Prevention of Pollution from Ships, 1973” and to align Hong Kong’s local legal requirements relating to air pollution from ships with international standards [47]. The chemical removal techniques, used to eliminate VOCs, are based on reaction between VOCs pollutants and oxidant or radiation. Among them, we quote photochemical oxidation process, ozone oxidation, catalytic oxidation, and plasma treatment. Generally, such processes lead to the formation of carbon dioxide (CO2) and water (H2O) as the end-products [48]. Formaldehyde (HCHO) is one of the major indoor air pollutants. Its emission is mainly originated from plywood, fiberboard, particle board, and other decorative materials. The industrial sector produces about 37 wt.% of formaldehyde solution per year, which corresponds to 20 million tons worldwide. The biggest producers are China (34%), the United States (14%), and Germany (8%) [49]. Many efforts have been devoted for the formaldehyde removal. Physical adsorption method using activated carbon or molecular as adsorbents for formaldehyde is an effective process to degrade the latter pollutant. However, its application remains limited owing to the adsorption capacity weakness and the regeneration of the adsorbent [5052]. The photocatalytic degradation of formaldehyde over titanium dioxide (TiO2) is a promoting route since it allows the degradation of HCHO to CO2 and H2O as predicted by Yang et al. [53]. In order to enhance its photocatalytic activities, some researchers propose to dope titanium dioxide with other metals or nonmetals. Low and Boonamnuayvitaya [54] studied the photocatalytic activity of TiO2 doped with graphene (GR) and Fe31 ions. The authors demonstrated that the presence of graphene enhances significantly the adsorption of the formaldehyde and hindered charge recombination. Upon addition of Fe31 ions, the photodegradation efficiency of the formaldehyde is further improved. Indeed, the preventing of charge recombination by GR boosts the redox reactions of Fe increasing by the way the production of hydroxyl (OH•) and superoxide ðO2 2 Þ radicals. The narrowing of the band gap energy was also observed. Furthermore, the authors emphasized that photocatalytic activity of TiO2 depends on the concentration of GR and Fe31 ions. The photocatalyst with a weight ratio of graphene to

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TiO2 of 1:50 and 0.12 wt.% of Fe31 exhibits the best efficiency for gaseous formaldehyde removal close to 50.3% and 25.5% under UV and visible light irradiation, respectively. Huang et al. [55] also showed the importance of TiO2 doping. N-doped BixTi12xO2, elaborated by solgel method, with an appropriate amount of N and Bi contributes to the enhancement of the photocatalytic oxidation of gaseous formaldehyde. Moreover, the photocatalyst showed a high stability under experimental conditions. The best photodegradation efficiency was found close to 92% for energy saving lamp irradiation at 24 h. Catalytic oxidation is a well-recognized method for VOCs removal. This process can lead to the direct conversion of HCHO to CO2 and H2O and has many advantages such as energy saving, harmless reaction conditions, and simple equipment [56,57]. The catalytic oxidation of formaldehyde can be occurred on catalysts based on noble metals or transition-metal oxide. In general, supported noble metals are mostly used in order to ensure the high loading of metal particles, allowing thereby an important HCHO conversion at low temperature. The noble metals mainly used are platinum (Pt), silver (Ag), gold (Au), and rhodium (Rh). Zhang et al. [58] studied the catalytic oxidation of formaldehyde over 1 wt.% Pt/TiO2. The activity tests revealed that HCHO conversion reached 100% at a gas hourly space velocity (GHSV) of 50,000 h21 at room temperature. As a result, a complete oxidation of the formaldehyde to CO2 was reached. Sun et al. [59] reported Rh/TiO2 with loading of 1.5 wt.% and subnanometer size. The catalytic tests showed the high stability and activity of the catalyst leading to total conversion of HCHO at room temperature. The production of formates as predominant intermediates for HCHO oxidation was also evidenced. According to the authors, both TiO2 and Rh subnanometer play a crucial role for enhancing HCHO oxidation. Indeed, titanium dioxide seems to provide active sites for the adsorption of the formaldehyde. The Rh species promote the dissociative adsorption of O2 to O atom, and this latter contributes to the transformation of HCHO to formate. On the other hand, the dissociate O atoms can react with water to form hydroxyl species (OH) [Eq. (2.12)] which in turn favor the decomposition of formates to CO2. Oads 1 H2 Oads -2OH

(2.12)

A comparative study carried out by Zhang and He [60] on various TiO2 supported noble metals (Pt, Pd, Au, Rh) for HCHO oxidation. The catalytic test showed that the activity of the catalysts increases following this order: Pt/TiO2cRh/TiO2 . Pd/ TiO2 . Au/TiO2cTiO2. Importantly, Pt/TiO2 catalysts allowed total conversion of the formaldehyde into CO2 and H2O at low temperature in a GHSV of 50,000 h21. The activity is high because the formate species are produced on this material during HCHO oxidation and then oxidized to CO species by O2. Contrariwise, Rh/TiO2 converts only 20% of HCHO, and the other materials were found inactive toward the oxidation of the formaldehyde due to their low efficiencies to adsorb formate species. Metal oxide supports, such as MnO2, CeO2 or their composites, are mostly used for the catalytic oxidation of HCHO. Ma et al. [61] elaborated Ag/CeO2 by one-step hydrothermal method and impregnation method in order to examine the effect of the morphological properties on the HCHO oxidation efficiency. According to the authors, the total oxidation of the formaldehyde is reached with Ag/CeO2 nanospheres at 110 C and under high GHSV. It is worth noting that Ag/CeO2 nanospheres present a specific reaction rate per

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second and per unit of surface area 3.6 times better that Ag/CeO2 nanoparticles. Obviously, a synergetic interaction between Ag and CeO2 nanospheres promotes the activation of the surface chemisorbed oxygen species which therefore boost the decomposition of intermediate species, mainly the formate, to CO2 and H2O. Another work carried out by Tan et al. [62] aimed to study the formaldehyde oxidation dependency on the morphological properties of ceria supports (i.e., nanocubes, nanorods, and nanooctahedron). For this purpose, palladium (Pd) nanoparticles were loaded on different shape of CeO2 support prepared by hydrothermal method. The authors group demonstrates that Pd/CeO2 nanocubes exhibit the highest catalytic activity for HCHO oxidation and the greatest durability against humidity. Moreover, it was found that (100) crystal planes of CeO2 nanocubes are the most active faces to adsorb and activate HCHO. Interestingly, 600 ppm of formaldehyde could be totally oxidized to CO2 on {100} Pt/CeO2 nanocubes at a GHSV of 10,000 h21 at room temperature. According to the authors, the (100) crystal planes on the ceria nanocubes activate oxygen species that boost the reducibility of Pd, resulting in excellent HCHO oxidation activity [63,64]. The synthesis of solid solution supports is an efficient way to enhance HCHO oxidation at low temperature. Indeed, Tang et al. [65] reported an Ag/MnOxCeO2 catalyst with total oxidation of the formaldehyde to carbon dioxide and water at 373K. This catalyst removed the formaldehyde easily because of the continuous transfer of oxygen atoms between metal and solid solution owing to the high dispersion of silver particles and the good interaction between CeO2 and MnOx. The use of noble metal as catalyst for HCHO is an effective way to ensure its total removal from indoor atmosphere. Nevertheless, this process is very expensive especially on an industrial scale. So, Zhang et al. [66] proposed to study the total oxidation of the formaldehyde on different crystal structures of MnO2. The different kinds of manganese oxides (α, β, γ, and δ) were elaborated via hydrothermal method. The catalytic activity of α-, β-, γ-, and δ-MnO2 toward HCHO removal was investigated as a function of temperature at a GHSV of 100,000 mL/gcat h and 170 ppm formaldehyde inlet concentration. The authors reported that the best performance was obtained with δ-MnO2 which can totally oxidize the formaldehyde at 80 C. Furthermore, they mentioned that both the tunnel structure and the presence of active lattice oxygen species can be the main factors that manage the high activity of δ-MnO2 catalyst. Mixed oxides are used for further removal of formaldehyde. For instance, MnOxCeO2 catalyst prepared by coprecipitation method and heat treated at 773K presents a high catalytic performance for the formaldehyde oxidation. Indeed, the conversion rate of formaldehyde with this material reached 100% at 373K with a GHSV of 21,000 mL/gcat h. The enhancement of the oxidation ability of the catalyst was attributed to the formation of a MnOxCeO2 solid solution, presence of manganese with high oxidation states (Mn41), and surface enrichment by the lattice oxygen species [67].

2.4.2 NOx abatement NOx abatement is of paramount importance to ensure air quality which meets environmental regulations. Various technologies have been developed for NOx removal including precombustion control which aims to purify the fuel from nitrogen compound and combustion modification, which is based on the change of the operating conditions in order to

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reduce the production of NOx [24]. However, both of these methods are not totally efficient since the total removal of NOx emissions is not obtained [68]. Postcombustion method is an alternative technique that deals with NOx in exhaust gases from mobile and stationary sources. The main challenges of this technology are the remove of the nitrogen oxides from flue gas and to ensure their total decomposition. The first approach can be reached by adopting absorption or adsorption processes, whereas the second one seems to be problematic since that nitrogen oxides are thermodynamically unstable and their decomposition to benign products, that is, N2 and O2 [Eq. (2.13)], requires highly activated energy (364 kJ/mol). So, in order to overcome this drawback, the use of an efficient catalyst is indispensable. 2NOx 2N2 1 xO2

(2.13)

Today, SCR method is a promising way for the NOx abatement. This method requires moderate operating temperature, and it relies on the catalyst materials. Reducing agent such hydrogen (H2) or ammonia (NH3) or hydrocarbons (HC) in presence of excess oxygen are also needed to boost NOx reduction with a high selectivity for N2. A former published researches carried out on HC-SCR of NOx in oxidizing atmosphere revealed that zeolite-based catalysts, such as copper-ion exchanged zeolite (Cu-ZSM-5) or proton exchanged zeolite (H-ZSM-5), are very effective for NO conversion to N2 [69,70]. Also, acidic oxides metals such as alumina (Al2O3), zirconia (Zr2O3), silica (SiO2), and their based materials were found to boost the SCR of NO to N2 [7173]. It is noteworthy that the performance of these materials cited below depends essentially on the operating conditions, that is, temperature, GHSV, reducing agent, the acidity of the support, and the nature of the metal. Some theoretical studies proposed a mechanism for the NO abatement on zeolites and alumina [7476]. Obviously, the most likely mechanism over Cu-ZSM-5 is the adsorption/dissociation process. Once NO is adsorbed on the active sites, it undergoes dissociation to form N(ads) and O(ads). The adsorbed oxygen reacts with hydrocarbons to form carbon dioxide (CO2) and two of N(ads) react to generate N2 [77]. The oxidationreduction paths can also occur especially onto alumina materials. First, the NO is oxidized due to the presence of oxygen, producing thus some reactive intermediates such as NO2(ads) and NO3(ads). As for hydrocarbons (i.e., reducing agent), they are transformed to hydrocarbon oxygenate such as acetate or organo-nitrogen species (CN) which react with NO2 to form N2 and CO2 [78]. However, the HC-SCR of NO over zeolites or alumina-based catalysts has many practical drawbacks. At high temperature, zeolites materials are characterized by low hydrothermal stability [79]. The unselective combustion of reductant is kinetically more favored than the selective NOx reduction leading to a decrease of reductant species needed for the NO2 removal. Other than the adsorption of NO2 and other pollutants, such as hydrocarbons, it can poison the surface of the catalyst, inhibiting thereby the NO dissociation [80]. Platinum group metals (PGM) are known to be very active catalysts at moderate temperature. Obuchi et al. [81] studied the NOx conversion over a series of PGM (platinum, iridium, palladium, rhodium, and ruthenium) supported Al2O3 and compared their activities to that of PGM/ZSM-5. The authors showed that Pt/Al2O3 and Rh/Al2O3 were the most active catalysts for the SCR of NO diesel exhaust gas by propene. The NO conversion of platinum is nearly 40% and 13% for Rh at low temperature regions (525K). Accordingly,

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the substrate plays a key role in enhancing the catalytic reduction of NOx with high selectivity to N2. Aluminum phosphate (AlPO4) is a novel support developed by Fujii et al. [82] in order to investigate Pt/AlPO4 activity for SCR of the NO by propene. The study emphasized that the activity of 0.5 wt.% Pt/AlPO4 is much higher than that of 0.5 wt.% Pt/ Al2O3, and the maximal conversion of NO to N2 occurs at very low temperature. The authors Kim et al. [83] investigated the effect of the Al2O3 phases (α-Al2O3 and γ-Al2O3) on the catalytic activity of Pt for the SCR of NOx by CH4. Experimental results showed that the conversion of NOx is important with the Pt/γ-Al2O3 owing to high adsorption efficiency of NO onto γ-Al2O3 as revealed the Fourier transform infrared spectroscopy (FTIR) spectra. Despite the positive results obtained with the HC-SCR process, the production of CO2 besides to N2 and H2O and the low conversion rate of NO make it an inefficacious way for the NOx abatement. NH3-SCR is a process in which ammonia is a reductant that reacts with NOx in the presence of a catalyst to form N2 and H2O without using excess of oxygen [84]. A great effort has been made worldwide to develop a catalyst, for NH3-SCR of NOx, with high performance and selectivity toward N2 at low temperature. A huge number of studies showed the catalytic performance of noble metals (Pt, Rh, Ir) and Fe, Cu exchanged zeolites for the selective reduction of NO by NH3 [8596]. However, the use of these materials presents many drawbacks that hinder their commercialization. Noble metals are very expensive, and they are easily poisoned by sulfur dioxide content in flue gas [97]. Iron and copper-based zeolite catalysts exhibit very low activity, especially at low temperature, due to hydrothermal aging and their poisoning by hydrocarbon coking [98,99]. For the practice use, vanadium-based catalysts are (V2O5MoO3/TiO2, V2O5WO3/TiO2) are the main commercial catalysts for NOx removal from stationary and automobile sources [100,101]. These materials are very interesting owing to their SO2 tolerance and their effectiveness for NH3-SCR reaction. Dong et al. [102] studied the effect of pH on the activity performance of the V2O5WO3/TiO2. The authors demonstrated that the number of active sites for the catalytic reduction of NO at low temperature increased by boosting the surface acidity. According to them, the enhancement of the precursor solution acidity during the preparation process favors the formation of polymeric vanadium species and an adequate V41(31)/V51 ratio which in turn led to the decreasing of the apparent activation energy [103]. Casagrande et al. [104] compared the catalytic performance of V2O5MoO3/TiO2 to that of binary systems V2O5/TiO2 and MoO3/TiO2 for NO abatement. The results obtained showed that ternary catalyst exhibits the best activity in the NH3-SCR reaction. Moreover, the temperature window of the SCR over V2O5MoO3/TiO2 is widened and shifted to lower temperature. Obviously, a synergetic interaction between vanadium (V), molybdenum (Mo) improves the availability of the catalyst lattice oxygen increasing thereby the NO conversion. The effect of Mo/V ratio on the NO conversion was carried out by Qiu et al. [105]. The study emphasized that the highest catalytic activity is reached with Mo/V ratio of eight with a complete conversion of NO to N2 in the temperature range between 623K and 723K. A high SO2 tolerance and hydrothermal resistance were also evidenced by the authors. In contrast the increasing of the loading amount of V2O5MoO3 while keeping the Mo/V ratio constant, led to the crystallization of V2O5. Hence, the interaction between

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metal oxides and TiO2 support becomes weak, which could affect the catalytic activity of the catalyst and its redox properties. Recently, Xu et al. [106] highlighted the highest durability of V2O5MoO3/TiO2 compared to V2O5WO3/TiO2 during a test carried out at 493K for 30 h in presence of SO2 and H2O. Indeed, V2O5MoO3/TiO2 is able to convert 88% of NOx contrary to V2O5WO3/TiO2. Nevertheless, vanadium species are very toxic, and their presence as active metal in the catalysts contributes to the formation of a large amount of N2O that is not beneficial especially for diesel vehicles [107]. Thus a new process has to be developed to overcome all the drawbacks cited earlier. SCR of NOx by H2 seems to be a promising alternative to HC-SCR and NH3-SCR processes. H2 as a reducing agent promotes the catalytic elimination of NOx at very low temperature (,473K) while producing water without emission of greenhouse gases (e.g., CO2) [108]. Many researches demonstrated the efficiency of supported platinum catalysts toward H2-SCR reaction [109112]. For instance, Costa et al. [109] have reported 95% of NOx conversion over Pt/MgOCeO2 at 423K with a N2 selectivity nearly to 78% at 373K. Yang et al. [113] showed that the conversion of NOx can reach 87% with 56% of selectivity to N2 at 423K over 0.5 wt.% platinum supported on H-ferrierite with a gas mixture of 910 ppm NO, 90 ppm NO2, 5000 ppm H2, 10% O2, and at a GHSV of 36,000 h21. However, other supported platinum catalysts such as Pt/TiO2ZrO2 [114] and Pt/Al2O3 [115] showed that high selectivity toward N2O other than the NOx conversion efficiency at temperature inferior to 473K has to be improved. Therefore some researchers proposed to form plurimetallic platinum supported catalysts. Yu et al. [116] showed the promotional effect of chrome (Cr) to Pt/ZSM-35. Indeed, an enhancement of the catalytic activity of platinum was found and addition of Cr favors the NOx adsorption and formation of NH41 species. Yokota et al. [117] reported that PtMaNo/SiO2 has a high catalytic activity toward H2-SCR reaction with lower N2O byproduct compared to Pt/SiO2, though from a cost point of view the use of Pt with high loading is very expensive. Hence, other metals have been used for H2-SCR of NOx. Palladium (Pd)-based catalysts have been investigated and showed a high activity toward NOx removal with N2 selectivity [118,119]. However, the activity temperature range should be expanded. The addition of V2O5 to 1% Pd/ TiO2Al2O3 increases the NOx conversion that reaches 100% and widens the reaction temperature range to 523K [120]. Ferna´ndez-Garcı´a et al. [121] found that the synergetic effect between Pd and Cr over PdCr/Al2O3 led to higher H2-SCR activity than Pd/Al2O3. According to the authors, such activity is related to the presence of metallic Pd0 that enhances H2-SCR reaction at low temperature via NO adsorption/dissociation paths. The effect of Fe, Co, and K in addition to Pd/Al2O3-TiO2 catalyst was studied by Greenhalgh et al. [122]. The experimental results showed high NOx conversion over the catalyst series. Moreover, among the various added elements, the use of K and Co was found to promote significantly the selectivity of Pd to N2 at low temperature.

2.5 Conclusion and future trends Since the industrial revolution, the declining of natural resources at high rates has been discerned. The damages done to the environmental conditions (i.e. global warming, deforestation and wetlands loses) and their effects on the human health have stimulated

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2.5 Conclusion and future trends

43

researchers to find the main causes of such phenomena in order to remedy them. NOx and VOC are a very dangerous air pollutants caused mainly by the anthropogenic activities. Their presence at high levels into atmosphere affects considerably the climate. Therefore the tendency toward decreasing their emissions was grown. Within the last decades, researchers assess that SCR and catalytic oxidation are a reliable process that can lead to the reduction of NOx and VOC emission, respectively. However, these processes still ineffective and very expensive to achieve the total removal of these pollutants. So, other initiatives are required to limit air pollution with a minimum of cost, especially from heavy industries. For this purpose, the development of sustainable energy sources, for example, solar energy, wind power, is recommended to meet the energy demands of the present and future generation while ensuring a clean and a sustainable environment.

List of acronyms Al2O3 Au Ag Cu-ZSM-5 CH4 CO2 CO CeO2 CN Cr EPA Fe31 GR GHSV H2 H2O HCHO HC HNO3 H-ZSM-5 MnO2 Mo NH3 NOx N2O NO NO2 N2O2 N2O3 N2O4 N2O5 O2 O O3 OH• Pd

alumina gold silver copper-ion exchanged zeolite methane carbon dioxide carbon monoxide ceria organo-nitrogen species chrome Environmental Protection Agency ferric ions graphene gas hourly space velocity hydrogen water formaldehyde hydrocarbons nitric acid proton exchanged zeolite manganese oxides molybdenum ammonia nitrogen oxides nitrous oxide nitric oxide nitrogen dioxide dinitrogen dioxide dinitrogen trioxide dinitrogen tetroxide dinitrogen pentoxide dioxygen oxygen atom ozone hydroxyl radicals palladium

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44

2. Air pollution (volatile organic compound, etc.) and climate change

Pt Rh SiO4 SCR SO2 TiO2 V VOC WHO Zr2O3

platinum rhodium silica selective catalytic reduction sulfur dioxide titanium dioxide vanadium volatile organic compounds World Health Organization zirconia

List of symbols d density

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