Removal of NO by carbonaceous materials at room temperature: A review

Abstract

Carbonaceous materials are considered to be effective materials for selective catalytic reduction (SCR) of NO, especially at room temperature. Carbonaceous materials can not only be used for adsorption applications due to their porosity properties but also can act as supports or reactants, which are globally available. This paper reviews the current state removal of NO by carbonaceous materials. Moreover, the characteristics of carbonaceous materials, mechanisms and kinetics of removal of NO by carbonaceous materials are also discussed. It is observed that carbonaceous material supported metal oxides can remove NO effectively at low temperature. Currently, removal of NO by carbonaceous materials at room temperature has been developed in the lab, but it is still in the research and development stage. In the future, three aspects of the work should be further studied, which is to extend the catalyst life, enhance the catalytic properties and reduce the cost of catalyst preparation. All the tasks are necessary for the practical application of NO removal technology at room temperature.

---

Zheng Zeng

Zheng Zeng received his bachelor's degree of environmental engineering at Hunan University in Changsha, China. Then he worked as a research assistant at key laboratory of environmental biology and pollution control at Hunan University. He is studying for a PhD degree at Arizona State University in Tempe, USA under the guidance of Prof. Jean M. Andino. His research interests include environmental catalysis for removal of NO and catalytic work for CO2 conversion to fuels.

Pei Lu

Pei Lu received his PhD degree of environmental science and technology in 2012 at Hunan University in Changsha, China. Now he is assistant professor and Master Tutor of Hunan University and key laboratory of environmental biology and pollution control at Hunan University. His actual interests include air pollution control techniques, applied catalysis for environmental protection and the development of catalysts.

Caiting Li

Caiting Li received his PhD degree at Hunan University in Changsha, China. He is professor and supervisor of doctoral students of Hunan University. He led more than 30 national and provincial research projects. The technology and equipment of dedusting and desulfurization for coal-fired flue gas has been applied in more than 10 provinces of China. He has published over 90 papers and has over 12 patents in the field of air pollution control.

1. Introduction

Nitrogen oxides (NO, NO₂) emitted from stationary industrial sources or mobile vehicles are harmful atmospheric pollutants that are major causes of acid rain, photochemical smog and depletion of the stratospheric ozone layer. NO and NO₂ also show different properties. NO, as a kind of colorless and odorless gas, is nearly insoluble in water. And as a free radical it has active chemical properties. While NO₂, as a kind of reddish brown and penetrating odorous gas, is easy to dissolve in water. Moreover, NO is easily oxidized to NO₂ at room temperature. Besides, at room temperature, with the influence of O₂, in the case of high concentration, most of NO_x is NO₂. While in the case of low concentration, most of NO_x is NO. For example, with the photolysis rate of 0.164 min⁻¹, its demarcation point is 100 × 10⁻⁹ (volume fraction). Furthermore, NO_x inhalation may result in pulmonary inflammations and the alarm threshold is below 1 ppm. Moreover, a concentration increase of exhaled NO has been correlated to chronic inflammatory airway diseases such as asthma, bronchitis and other respiratory infections including bacterial and viral illness.^1–4

Up to now, many methods have been applied to remove NO. Selective catalytic reduction (SCR) has been regarded as an efficient method to treat NO from a stationary emission source such as power plants, using vanadium on TiO₂ catalysts at 300–400 °C. Its general reaction for SCR of NO is as follows: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O. Selective removal of NO has been also investigated with examples of metal oxides^5–7 and ion-exchanged zeolites.^8–10 However, most of them are applied as a stand-alone, none of which has researched the effect of H₂O and SO₂ on the kinetics or capacities for NO adsorption. Furthermore, the investigations on removal of NO at room temperature from different perspectives have also been carried out. Such as the reaction of the CFO radical with NO molecules,¹¹ the catalysts with variable Ce/Zr ratios,¹² TS-1 catalysts,¹³ and photo-SCR of NO.^1,14

Specially, carbonaceous materials, such as activated coke, carbon nanofibers (CNF), activated carbon, and activated carbon fibers, have been used as effective catalysts for SCR of NO at low temperature (<200 °C).^15–19 Moreover, the presence of oxygen, always present in exhaust gases or air, can enhance the NO adsorption and heterogeneous reactions of NO with carbonaceous materials, which makes the carbonaceous materials suitable to use at room temperature. Thus, in the case of carbonaceous materials, the NO adsorptions maybe play the most important role. Besides, the effects of H₂O, CO, SO₂ in the removal of NO are also taken into consideration. The mechanism of the overall process is complex due to it involving multiple steps including NO sorption, NO reduction, NO oxidation, O₂ gasification, adsorption of H₂O and release of other products.²⁰

Therefore, a comprehensive review on the characteristics, mechanisms and kinetics of NO removal by carbonaceous materials will lead to a better understanding of the removal of NO at room temperature.

2. Removal of NO by carbonaceous materials without external reducing agents

Carbon, especially upon activation, is traditionally one of the most commonly employed materials for adsorption applications due to its porosity properties.²¹ Moreover, since carbon is also an inexpensive reducing agent, carbon based catalysts can act both as a support and a reactant, avoiding the need for an external gaseous reducing agent.²² Therefore, activated carbon has been widely used for catalytic supports due to the fact that it owns many desirable properties, examples of which are as follows: (1) it is cheap; (2) it can be modified in terms of its surface properties to become hydrophobic; (3) it can be used in acid and basic conditions; (4) it can be recycled for metal by burning spent catalysts; (5) it can be changed in pore structure for different purposes.^23–25

2.1 Removal of NO by carbonaceous materials

The interactions of NO with carbons are studied using in situ FTIR since IR spectroscopy provides information on the nature of bonds formed between adsorbents and catalyst surface.²⁶ In the case of the measurements of catalytic activity, the reduction of NO to N₂ can be achieved without the use of an externally supplied redundant. The pure carbons (oxidized and non-oxidized) in the absence of oxygen have very low activity towards NO decomposition. On the contrary, the enhancement of the NO conversion reaction in the presence of oxygen is observed.

The adsorption of NO_x (x = 1,2,3) molecules on single-walled carbon nanotubes (SWCNTs) is investigated in the framework of first-principle calculations using density-functional theory.²⁷ The experimental results show that NO + NO, NO₃ + NO₃ and NO + NO₃ configurations have a stronger binding energy than two NO₂ molecules on the SWCNT surface. The possibility of a surface reaction NO₂ + NO₂ → NO + NO₃ is examined and the relative pathway and barrier is calculated through considering two possible stating points: (a) from a metastable chemisorbed state; (b) from the lowest-energy chemisorbed state. Furthermore, curvature is found to have a sizable effect on the interactions of NO_x molecules with SWCNT surfaces. The adsorption energy increases for smaller SWCNTs, while chemisorption disappears completely on graphene, in agreement with other results.^28–30 Although single NO_x molecules are physisorbed on SWCNTs, molecules can be chemisorbed in pairs on the top of carbon atoms at close sites of SWCNTs.

When it comes to activated carbon fibers (ACFs), the surface properties of ACFs can be controlled by the selection of precursor, activation conditions and post-modification such as heat treatment as well as chemical treatment. For example, nitrogen-doped activated carbon fiber (N-ACF) is obtained by using pyridine-CDV at 1023 K,³¹ but part of the doped nitrogen is unavailable for enhancement of the NO adsorption due to pore blocking by deposited carbons. Further heat-treatment after the pyridine-CDV removes the deposited carbons to increase the nitrogen sites available for NO adsorption as well as the quaternary nitrogen. Thus, the nitrogen-doped ACF is a good application for NO adsorption. Besides, catalytic oxidation of NO to NO₂ over ACFs at room temperature is also carried out.³² Catalytic oxidation of NO to NO₂ over activated carbons PAN-ACF, pitch-ACF and coconut-AC at room temperature are studied to develop a method based on oxidative removal of NO from flue gases. The order of activity of the activated carbons is PAN-ACF < pitch-ACF < coconut-AC. Furthermore, the strong inhibiting effect of humidity on the activity of the as-received fibers is moderated at 850 °C, of which NO conversion is 62% in 80% relative humidity (rh).³³ It is worth discussing the reasons why the catalytic activity of the pitch-based ACF especially in the wet air is enhanced by the calcination. The calcination removes the major oxygen functional groups from the ACF surface, emphasizing the hydrophobic nature of the surface. The defects left by the removal of oxygen functional groups may enhance the catalytic activity of remaining active sites. Moreover, calcination at an optimal temperature also increases the activity of NO reduction in dry air. However, too high calcination temperatures also reduce activity through the progress of partial graphitization, which may eliminate the active site produced by calcinations at optimum temperatures. For example, the calcination at 1100 °C increases the activity, especially in the wet gas, indicating that the calcination moderates the inhibition of humidity on the adsorption of NO, being free from the oxygen functional groups and carrying fairly developed hexagonal planes, which are inherited from condensed aromatic components in the pitch.³⁴

Admittedly, ACF has been the focus of recent research due to its micro porous texture having a large BET area, significant adsorption characteristics for a number of air pollution species and the ability of regeneration with ease. In addition, ACFs have the flexibility to be applied in reactors, where they can be wrapped or rolled easily. Catalytic oxidation of NO is carried out on ACFs based on various precursors in a packed bed tubular under varying reaction temperatures (30–80 °C), inlet NO (100–400 ppm) and O₂ (5–100% concentration). Maximum conversion of NO is obtained at the reaction temperature of 30 °C. Besides, the performance of the phenolic resin-based ACF is found to be superior to that of the viscose rayon and pitch-based ACF.³⁴

Since NO₂ is easily removed by water, catalytic oxidation of NO to NO₂ at room temperature is a promising way for the removal of NO in flue gas without using ammonia. Besides, with the active carbon pyrolyzed at 700 °C, the best NO₂ sorption capacity is obtained at room temperature in both dry and wet (70% humidity) conditions.³⁵ Moreover, the surface area of the activated carbon is not the dominant factor influencing NO conversion, but is the reaction temperature. The NO conversion markedly decreases with the temperature rise, which also supports our ideas to use the method that catalytic oxidation of NO to NO₂ at such low temperature.

2.2 Effect of O₂, H₂O, CO and SO₂ on NO removal

Firstly, the enhancing effect of oxygen results from the oxidation of NO to NO₂. Moreover, NO₂ has higher reactivity for the reaction with carbon than with NO.³⁶ Although, in the study, the contribution of NO₂ to the enhancement of the NO conversion is only significantly important in the temperature region 300–500 °C,³⁵ the role of NO₂ as an intermediate is suggested to be attributed to the production of free carbon sites which are active for the reaction with NO. Besides, the conclusion that NO₂ reacts with carbon to form N₂ and NO which can be oxidized again to NO₂ is suggested. NO₂ forms from NO and O₂ reacts with carbon at a higher rate than O₂.

On the other hand, the main reason for the oxygen enhancement is the increase of the amount of carbon–oxygen (C(O)) complexes which decompose and produce free active carbon sites.³⁷ The free sites will mainly be attacked by O₂ due to its higher reactivity in the reaction with carbon than NO.³⁶

The decomposition of C(O) complexes creates nascent carbon sites on the carbon black surface. These sites might be more active toward the reaction with NO from those formed from the decomposition of surface complexes generated by the reaction with carbon. Besides, the complexes created by oxygen may be different from those created by NO.³⁸ The higher the decomposition rate, the more active carbon sites there are created. Meanwhile, with increasing the modification time, more C(O) complexes are formed, which in turn decompose to give a higher number of free active carbon sites.³⁶ Stable C(O) complexes seem to be unimportant for the reaction of NO with carbon. However, they play an important role in the formation of more active carbon sites, which might activate the neighboring nascent carbon sites by changing their electronic properties.

Secondly, humidity reduces the adsorption of NO, H₂O being adsorbed in much larger quantities. The fiber of highest activity exhibits the smallest water adsorption and inhibition of NO adsorption.³⁹ The reaction of NO₂ and H₂O resulting in the formation of nitric (HNO₃) and nitrous (HNO₂) acids. The formation of −NO₃ is enhanced by the presence of water, which facilitates the formation of HNO₃via the decomposition of nitrous acid:

3HNO₂ → HNO₃ + 2NO + H₂O.

Besides, the nitrogen-containing groups will likely be oxidized by HNO₂/HNO₃ intermediates and will no longer be available for the adsorption of NO.

The oxidation of the carbon surface by HNO₂/HNO₃ intermediates and the formation of –O–NO₂ and –NO–NO complexes are also investigated:

–C* + HNO₃ → –C(O) + HNO₂

–C(O) + HNO₃ → –C–O–NO₂ + OH⁻

–C(O) + HNO₂ → –C–O–NO + OH⁻

Where −C* is the carbon active site.⁴⁰

For example, a study of the adsorption and reaction of NO in the presence of oxygen and water vapor on activated carbon is presented⁴¹. The experimental results show that on the H₂O–O₂–NO–C system, adsorption, reduction and catalytic oxidation of NO, together with the adsorption of NO₂, occur simultaneously.

Thirdly, some C(O) complexes must be formed on the carbon black surface by the reaction with oxygen. CO is formed from basic C(O) complexes when carbon black is modified by an oxygen pretreatment at 500 °C. The desorption of CO appears to release some carbon sites which are more active sites and thus contribute to the enhancement of the NO conversion. Besides, the presence of CO that can remove oxygen atoms from the surface is beneficial for the enhancement of the reaction between NO and charcoal while the formation of N₂ does not result from the reduction of NO by CO.³⁶ Moreover, CO co-diffuses with the NO and accelerates the rate of NO reduction.⁴² In particular, the rate-determining step of the reaction of NO with carbon black at a steady state is the consumption of surface complexes, which leads to the formation of CO. The experimental result that desorption of product is enhanced at about 40 °C suggests that at low temperatures the reaction of NO with carbon leads to the formation of more unstable surface complexes.³⁶

Finally, the effect of low-concentration SO₂ on the adsorption of NO over activated carbon is studied using adsorption–desorption profiles obtained during thermal analysis-mass spectrometry.⁴³ At 293 K approximately five molecules of NO₂ are adsorbed for each SO₂ molecule adsorbed. In general, more SO₂ is adsorbed in the presence of NO + SO₂ as gaseous reactants than in the presence of only SO₂ as the reactant. NO₂ adsorption on activated carbon involves the catalytic conversion of NO + 1/2O₂ to NO₂ at the carbon surface and that the NO₂ is then condensed within the pores of the carbon while SO₂, being a slightly larger molecule than NO₂, has a critical volume 1.5 times that of NO₂. The presence of SO₂ inhibits adsorption of NO since the SO₂ occupies or blocks some of these sites.

Furthermore, a cheap carbonaceous material such as low rank coal-based carbon (or char) has been used to investigate the effect of SO₂ in NO removal system.⁴⁴ Low SO₂ concentration has a negligible effect on NO reduction due to SO₂, O₂ and H₂O are all adsorbed on the surface of the carbon,⁴⁵ which leads to the following step:

SO₂ + 1/2O₂ + H₂O + C → C–H₂SO₄.

Besides, in the case of NO removal by ammonia in the absence of SO₂, no relationship between the surface area of chars and NO capacity could be found. However, the negative effect of the presence of SO₂ on the NO removal capacity takes place because of the formation of ammonium hydrogen sulfate or ammonium sulfate:

H₂SO₄ + NH₃ → NH₄HSO₄,

NH₄HSO₄ + NH₃ → (NH₄)₂SO₄.

These salts are deposited on the inner surface of the carbons, poisoning the catalytic sites for NO and leading to a decrease in NO conversion. However, for example, SO₂ and NO₂, these compounds are oxidized and hydrated into H₂SO₄ and HNO₃, which can be eluted out from the carbon surface.

2.3 Theories of NO removal by carbonaceous materials

There is basically no NO adsorption at room temperature when oxygen is absent. The presence of surface oxygen groups on carbon may assist the NO adsorption slightly when oxygen gas is absent. However, when oxygen is present, both physical and chemical adsorption of NO⁴⁶ are significantly enhanced, which leads to the conclusion that NO may co-adsorb physically with O₂ on the carbon surface. Moreover, carbons with small micropores have higher amounts of chemisorbed NO than the carbons with relatively larger micropores. Furthermore, NO chemisorption on AC is believed to be a result of NO₂ reduction by carbon along with carbon surface oxidation.⁴⁷

It is assumed that the AC acts as a catalyst to convert NO to NO₂ in the presence of O₂ and then NO₂ adsorbed on the carbon surface. With the process of carbonisation and activation by KOH, active carbon samples of well-developed surface areas with a prevalence of micropores could be obtained. Moreover, the beneficial effect of the activated carbon surface modification with amines on adsorption of NO₂ at ambient temperature is demonstrated.^48–51 However, the process of NO oxidation on AC has not been fully understood yet. It is suggested that NO is oxidized by oxygen to NO₂ in the gas phase and then NO₂ is adsorbed on the carbon surface.⁵² But the result that the homogeneous oxidation of NO in the gas phase is too slow to count for the observed NO oxidation on AC is pointed out later. The second proposed route is that NO reacts with the surface oxygen atoms which are formed by the reaction between gas phase O₂ and AC,⁵³ the overall reaction is 2NO + O₂ = (NO)₂ + O₂ = 2NO₂. But pre-adsorption of O₂ on AC does not increase NO adsorption. The third proposed route is related to the micropores of AC. If O₂ and NO molecules co-adsorb in the narrow micropores, they will have to be in very close contact with each other. Since the reaction is first-order with respect to NO and half-order to O₂, the overall reaction can be proposed as NO + 1/2(O₂) → NO₂. Meanwhile, NO₂ on the surface is known to oxidize NO to accelerate NO oxidation additionally. The other possible way for NO₂ formation is through 4NO → 2(NO)₂ → 2NO₂ + N₂. This reaction, if it occurred, plays a negligible role in the studied condition due to the observations that nearly no NO conversion is observed when oxygen is absent and no noticeable N₂ formation is detected.⁴⁹

When it comes to the kinetics, some researchers reported them from different perspectives:

(a) The apparent reaction rate is of the first order of the NO concentration and the 0.25th order of the oxygen concentration. The relationship can be expressed by the following equation: R = k_cP_NOP^b_O2 (F/W). At room temperature, the apparent rate constant k_c is 0.94, the order b is 0.042, 0.31 and 0.16 for the coconut-ACF, the pitch-ACF and the PAN-ACF, respectively.³¹

(b) The conversion in the concentration range 20–380 ppm in dry air can be described by a Langmuir-type equation, where NO is assumed to be adsorbed in a dimer form: V = kK[NO]²/(1 + K[NO]²) while first-order kinetics is suggested below 20 ppm.³⁹ (1) second-order kinetics: micropores or active sites can concentrate NO to form its dimer as an intermediate in the reduction over the carbon surface. The dimer is oxidized to NO₂, which stays on the surface until the adsorption is saturated. (2) first-order kinetics: NO of very low concentration reduces the chance of dimer formation before the oxidation, giving first-order kinetics.

(c) The total amount of NO adsorption appears to follow the Langmuir equation resulted from O₂ with rather low concentration. The Freundlich equation fits better for the entire oxygen concentration range. In contrast, the amount of saturated adsorption increases by an order of 0.3 in NO in the concentration range of 100–500 ppm. Adsorbate-interactive adsorption of O₂ and NO appears to take place, accelerating adsorption of NO as well as oxidation of adsorbed NO. The major adsorbed species is the oxidized form of NO in the stationary state. The adsorption is enhanced by an order of 0.3 in NO while the oxidation is of 1.2 order suggest (NO–NO₂) type complex of intermediate over the ACF which produces NO₂ as the gaseous product. NO–NO intermediates can be ruled out because NO₂ is the principle species on the ACF surface. Single NO₂ and dimeric (NO₂)₂ are also excluded as intermediates because such intermediates may provide zero order in NO, being rate-determining by NO₂ desorption. Hence, the following sequence of adsorption, oxidation and desorption may take place over the ACF surface where (NO–O–NO₂) ad is assumed as an adsorbed species.

NO → NO_ad → NO_2(ad) → (NO–O–NO₂)_ad → NO_2g(steady),

NO + O₂ → NO_2g(slow).

It must be noted that (NO)₂ formation can be excluded in the gas phase as no oxidation takes place without the ACF. Such steps proceed over the surface. The last step depends much on the concentration of (NO–O–NO₂)_(ad).⁵⁴

With the help of the above findings, some experimental results can be elucidated clearly. For example, the adsorption and reduction behavior of NO₂ taking place over P-ACF is also discussed. The results suggest two kinds of NO₂ adsorption sites. Site 1 adsorbs NO₂ weakly and it desorbs NO₂:

Site 1: NO₂ → NO_2(ad) (slow) → Heating → NO_2(g).

Site 2 adsorbs NO₂ and participates in oxygen transfer, which produces adsorbed NO₃ and liberates NO into the gas phase. Oxidation of adsorbed NO₂ appears slow by taking place only in the initial stage. Adsorbed NO₃ may produce both NO and NO₂ when heated, leaving one or two oxygens on the surface, which produce CO and CO₂ by further heating:

Site 2: NO₂ → NO_2(ad) (rapid) → NO_(g),

NO_2(ad) → NO_3(ad),

NO_2(ad) → Oxidation O₂ → NO_3(ad),

NO_3(ad) → Heating → NO_2(g), NO_(g), CO_(g), CO_2(g).⁵⁵

3. Removal of NO by carbonaceous materials with external reducing agents

3.1 Removal of NO by carbonaceous materials with NH₃

NO adsorption and reduction with NH₃ over activated coke at low temperatures are studied. The literature has shown that there are a large number of oxygen-containing functional groups on the AC surface.^52,56 The formation of the chemically adsorbed species of NO is inhibited by the competitive adsorption of NH₃.

On comparing the results of NO–O₂ co-adsorption and NO–NH₃–O₂ co-adsorption, a similar deduction can be made on the basis of the obvious decrease of NO and NO₂ desorption in the presence of NH₃. It is possible that there are at least two types of adsorption sites on the AC surface, one adsorbs both NO and NO₂, and another only NO₂. Two reaction mechanisms proposed for SCR on metal oxides may be used to describe the SCR reaction on AC: (1) a reaction of the gas phase NO with adsorbed NH₃ to form an active intermediate species which is subsequently decomposed into N₂ and H₂O (Elay-Rideal (ER) mechanism); (2) the adsorption of NO at sites adjacent to that of NH₃, followed by reaction between them to form the products (Langmuir-Hinshelwood (LH) mechanism).

Additionally, among the reactants, NO, NH₃ and O₂, NH₃ adsorption is expected to be the rate-limiting step. This suggestion is supported by the following facts: (1) the increase of oxygen-containing functional groups on the AC surface increases NO conversion and NH₃ adsorption capacity and decreases the NO adsorption capacity;^52,56 (2) the presence of the basic surface N, either in the carbon structure or incorporated as an additive or surface modifier, increases NO conversion; (3) the SCR reaction can proceed via both the LH and ER mechanisms, the NO adsorption being unnecessary in the latter way.⁵⁷

Furthermore, the influence of treated carbon with sulfuric and nitric acids (H₂SO₄ and HNO₃) on the activity of the carbon catalyst for NO reduction with NH₃ is investigated.⁵⁸ The transient behavior of NO reduction over the original and the treated carbons is found to vary with the reaction pattern before the steady state is reached. This suggests that the number of oxygen-containing sites significantly affects the reaction pattern before the steady state is reached. Nitric acid, an acid with a stronger electrochemical tendency as an oxidizing reagent than sulfuric acid, introduces oxygen functional groups on the surface. The rate of NO reduction with NH₃ increases with the population of acidic sites on the carbon surface and the adsorption of NH₃ on the acidic sites is one of the important steps of this heterogeneous reaction.^52,56 The reaction scheme can involve the interaction of NH₃ with the acidic part of carboxyl groups, for example, COH to CO⁻(NH₄)⁺. Following the adsorption of NH₃, NO molecules would diffuse to the carboxyl group and interact with the C=O part to form C(ONO). The complexes formed on the carboxyl group will then interact with each other to produce N₂ and H₂O.

On the other hand, another scheme involving the interaction of NH₃ with phenolic hydroxyl groups to form CO⁻(NH₄)⁺ complexes on the carbon surface may also occur. In the absence of oxygen, the adsorption of NO might occur on basic surface oxides such as pyrone-like structures.⁵⁸ In the presence of oxygen, the C(ONO) complexes might be generated from the interactions of NO₂ with some carbon sites as well as NO with the basic groups. The rate of NO reduction in the presence of oxygen is expected to be higher than that in the absence of oxygen because of the formation of NO₂ and the larger amount of basic surface oxides. Besides, the formation of CO⁻(NH₄)⁺ on hydroxyl groups is dominant in the presence of oxygen.

In conclusion, (1) in the absence of the oxygen system, the catalytic activity of the original carbon is improved by nitric acid treatment while the carbon is deactivated by the treatment with sulfuric acid. Besides, the formation of these two complexes on one carboxyl site followed by the interactions may be the main route in the reaction. (2) in the presence of oxygen system, the activity of the original carbon is significantly enhanced by treatments with both acids, especially with nitric acid. Moreover, the formation of CO⁻(NH₄)⁺ on hydroxyl groups to interact with C(ONO) formed on neighboring sites may be dominant.

In the case of room temperature, reduction of very low concentration NO with NH₃ is investigated over activated carbon fibers (ACF) derived from pitch (P) and poly(acrylonitrile) (PAN).⁵⁹ The oxygen-containing groups are not all catalytically active, especially for the reaction in humid conditions at room temperature. The reactive surface C–O complex is a possible active site. Moreover, a surface C–O complex produced through the NO and C reaction is the intermediate, releasing CO or CO₂ for the regeneration of the active carbon sites.⁶⁰ It should be noted that the adsorbed NO is reduced with NH₃ without producing C–O groups on the ACF surface under the present conditions. NH₃ can stay on the ACF surface or be dissolved in the adsorbed H₂O at room temperature. Such NH₃ species present in sufficient quantities on the ACF surface can react with NO₂ derived from NO on the ACF surface. Sufficient adsorption of NH₃ appears to take place to accomplish the reduction of NO over the ACF surface with a minimum amount of oxygen functional groups at room temperature.

In the case of the process of the reaction, the NO–NH₃ reaction over the ACF appears to change the conversion due to the time course.³³ Firstly, higher NO conversion could be had in the initial stage, then a gradual decrease of the conversion at the transient stage and stable conversion at the stationary stage. The removal of NO consists of its reduction with NH₃ and adsorption in the initial stage, the adsorption decreasing its contribution to the removal by approaching the saturation in the transient stage. Finally, only the stationary NO–NH₃ reaction progresses in the stationary stage. The lower NO concentration lengthens the initial stage since it takes longer for saturated adsorption. The higher temperature shortens the initial stage due to the smaller saturation occurring in a shorter time. However, the whole activity is not very high in the wet air but it appears practically applicable, indicating a way of development for a unique technology to reduce NO to a very low concentration in the atmosphere.

It is good news that heat-treatment can change the situation above since the adsorption of NO followed by oxidation to NO₂ drastically increases over the ACF with the higher heat-treatment temperature. Strong inhibition of humidity and promotion of oxygen are recognized. The heat-treatment removes the oxygen functional group, reducing the inhibition by humidity. High hydrophobicity of P-ACF due to large carbon layers may explain its higher resistance against humidity especially after the heat-treatment.³³

Other good news is that regeneration of initial activity of a P-ACF for NO–NH₃ reactions at room temperature is also investigated.⁶¹ The ACF used in the reaction of NO–NH₃ for 6 h is regenerated for 3–6 h by flowing 4% O₂ with NH₃ or 4%O₂ alone in He. The flow rate and concentration of NH₃ in air are 100 ml min⁻¹ and 100 ppm, respectively. The regeneration temperature is controlled to 25, 30 or 40 °C by heating the reactor. This is the condition for regeneration. Besides, the key point of the regeneration is that the adsorbed NO and NH₃ react with each other in flowing wet air to regenerate the active sites for adsorption and reduction of both substrates. Moreover, the best temperature of 30 °C is defined by the minimum leakage of adsorbed species. The reactivity of adsorbed species increases naturally with temperature unless they desorb from the ACF surface. Of course, longer regeneration time allows the present procedure to be completed at 25 °C.

The advantage of the regeneration switch procedure could be applicable to remove the low concentration of NO in the closed space. A Langmuir–Hinshel reaction through the surface migration of the adsorbed substrates is observed in the regeneration reaction. NH₃ can be adsorbed chemically and physically. The condensed H₂O phase on ACF may also provide sites for chemical and physical adsorption of NH₃. Elongated time allows the reaction of both NH₃ species with oxidatively adsorbed NO in the regeneration stage by the aid of oxygen.³³

In the real world, NO is not the only thing that exists in the flue gas. Thus, we should take other matters into account. Recently, a study investigated the development and potential application of ACF functionalized with NH₃ for the control of NO and particulate matter (PM) in diesel engine exhausts.⁶² In the case of NO removal, a tubular reactor packed with ACF is used to experimentally study the oxidation of NO at room temperature. Tests are conducted at ACF functionalized with three aqueous ammonia concentrations (3, 5, 10 M), three basic reagents (ammonia, pyridine, amine) and three NO concentrations (100, 300, 500 ppm). The relatively larger NO adsorption is observed on ACF-NH₃/5M due to the presence of more N-containing groups. In addition, nitrogen doping on ACF at room temperature is found to be more effective than at high temperatures. Moreover, the prepared catalysts are observed to be promising for capturing particulate matter from the engine exhaust as well. Therefore, the study shows significant potential for the ACF based filters in capturing both homogeneous and heterogeneous pollutants emitted from automobiles.

3.2 Removal of NO by carbonaceous materials with urea

Urea has much higher storage capacities for reducing agents than NH_3,^63,64 which attract some researchers to make full use of it. The reduction of NO with urea supported on activated carbons at room temperature is attempted.⁶⁵ Concentrations of NO in air, urea on the carbon, the amount of carbon material and humidity are varied to investigate their effects on the NO reduction. The reduction of NO with urea supported on activated carbon appears to proceed through the following steps:

NO + 1/2O₂ → NO₂,

NO₂ + NO + (NH₂)₂CO → 2N₂ + CO₂ + H₂O.

The above reaction equations could be combined as the following reaction equation:

2NO + 1/2O₂ + (NH₂)₂CO → 2N₂ + CO₂ + 2H₂O.

Hence, the oxidation of NO into NO₂ is the first and key step, over carbon materials, which requires more open surface of the carbon. However, urea must, meanwhile, be impregnated on the carbon to react with NO₂, which occupies or covers the surface of carbon. Thus, the amount of urea on carbon defines the capacity of NO reduction, restricting the adsorption of O₂ or oxidation of NO. The amount of urea must be optimized to balance the conversion and period of NO reduction.

When it comes to other reaction parameters, the oxidation of NO becomes slower when humidity is higher since water vapor may retard the adsorption of O₂ to oxidize NO into NO₂. Moreover, smaller concentrations of urea are preferred for smaller concentrations of NO since the oxidation of NO becomes much slower when its concentration is lower.

Interestingly, the regenerated ACF can show the slightly lower, but basically the same activity toward NO removal, which is consistent with the good news above.

Therefore, such a reaction can be applied to reduce NO present in open atmosphere with significantly high efficiency.

Furthermore, since NO₂ plays an important role in the NO–NH₃ reaction, the reactivity of NO₂ in air with urea supported on ACF is also examined at room temperature.⁶⁶ NO₂ is believed to be oxidized into NO₃ over ACF and some of NO₂ present on ACF as NO_3(ad): NO₂ + 1/2O₂ → NO_3(ad). Major reactions occurring on ACF are believed to be reactions as follows:

2NO₂ → NO_3(ad) + NO,

NO_3(ad) + (NH₂)₂CO → 1.5N₂ + CO₂ + 2H₂O,

NO₂ + NO + (NH₂)₂CO → 2N₂ + CO₂ + 2H₂O.

Such steps above are summed to the reaction: 6NO₂ + 4(NH₂)₂CO → 7N₂ + 4CO₂ + 2H₂O. Moreover, H₂O in air enhances the reaction (NO_3(ad) + NO₂ + H₂O → 2HNO₃) to produce aqueous HNO₃. Thus, oxygen and water extend the breakthrough time of NO₂ beyond the complete consumption of urea over ACF. It is good news that humidity does not retard the reaction of NO₂ with urea, but they slightly accelerate the oxidation reactions. Therefore, with all of the findings above, such reactivity is very useful to remove NO_x in the open atmosphere such as the inner-space of building, busy traffic cross-sections or near high ways.

3.3 Removal of NO by carbonaceous materials with CH₃OH

It has been demonstrated that for NO to effectively be removed, even in the presence of water vapor and excess of oxygen, it is possible if it is reduced with methanol.⁶⁷ The effect of various metal additives on the catalytic performance of carbon during SCR of NO with methanol has been studied with/without oxygen. In the absence of oxygen NO does not adsorb on the surface of carbon at room temperature and it does not decompose with high rate at this temperature. NO reacts with methanol decomposition products, resulting in the formation of NCO species^68,69 on the surface of carbon-supported catalysts, which may be correlated with the efficiency of carbon-supported catalysts in the NO reduction with methanol.

CH₃OH_(g) → CH₃OH_(ad) → CH₃O_(ad) + H_(ad),

CH₃O_(ad) → H₂CO_(ad) + H_(ad),

2H_(ad) → H_2(g),

H₂CO → H_2(g) + CO_(ad),

CH₃OH_(ad) → CO_(ad) + 2H_2(g),

CO_(ad) → CO_(g),

NO_(g) → NO_(ad) → N_(ad) + O_(ad) → NCO_(ad).

The NCO species are capable of reacting with NO, NO₂ or O₂:

NCO_(ad) + NO_(g) → N_2(g) + CO_2(g),

NCO_(ad) + NO_2(g) → N_2(g) + CO_2(g) + O_(ad),

2NCO_(ad) + O_2(g) → N_2(g) + CO_2(g).

NO₂ chemisorption is stronger on the non-oxidized than on the oxidized carbon surface. Catalytic conversion of NO/CH₃OH/O₂ over carbon-Cu catalyst results in the formation of surface nitrites and nitrates. NO₂ is also converted to HNO₃ apparently as a result of the presence of water (from methanol decomposition):

3NO_2(ad) + H₂O_(g) → 2HNO_3(ad) + NO_(g),

2NO_2(ad) + H₂O_(g) → 2HNO_3(ad) + HNO_2(g).

Therefore, this is a new way to remove NO, which could be well applied to the room temperature removal of NO.

4. Removal of NO by carbonaceous material supported metals or metal oxides

Many researchers have studied air pollution removal using carbonaceous materials by metal electroplating, impregnation, sputtering or controlling the pore structures. For example, the formation of copper electroplating over ACFs⁷⁰ can be represented by the following equations:

The anode oxidation: Cu → Cu²⁺ + 2e⁻,

The cathode reduction: Cu²⁺ + 2e⁻ → Cu/ACFs.

Elemental compounds that have been attempted as catalysts for SCR of NO are Rb, Ac, Th, Hf, Nb, Md, Tc, Ru, Rh, Rd, Ag, Os, Ir, Pt, Ds, Sg, Ga, Ge and so on.²¹ It could be found that most of the catalysts investigated are transition metals.

4.1 Removal of NO by carbonaceous material supported metals

Since the discovery of multi-wall carbon nanotubes (MWCNTs) in the early 1990s and the successive production of single-wall carbon nanotubes (SWCNTs), the research on these new carbon forms has become one of the most studied fields of nanoscience. The use of CNTs in the abatement of highly oxidizing substrates like NO_x seems paradoxal.⁷¹ Moreover, CNTs have been reported to possess higher oxidation resistance compared to other carbonaceous materials⁷² that have a large specific surface area because of the graphite edge sites being exposed at the wall of the fibers. Recently, there has been a strong interest in CNTs, particularly in their potential usage as a support material in heterogeneous catalysis. Ni catalysts supported on CNF show high reactivity and longevity for the methane decomposition into CNF,⁷³ which inspires the development of CNF formed by methane decomposition over various Ni–M/SiO₂ (M refers to different metal added). The reducing ability of CNF for NO depended significantly on the specific area and the degree of graphitization of CNF, while metal species (Ni, Co, Pt, Pd and Cu) do not work as catalysts for the reduction of NO in the presence of oxygen.⁷⁴

On the contrary, metals play an important role in another study. The catalytic removal of NO over rhodium particles supported on carbon nanotubes (Rh/CNT) are studied in the absence and presence of oxygen,⁷¹ which gives evidence for the stoichiometric reduction. The strong metal-support interaction and outstanding catalytic activity of Rh/CNT catalysts in the absence of excess oxygen result from carbon bearing the reducing agent without the use of additional reducing agents. Besides, the formation and desorption of CO_x significantly prevents the saturation of the active centers. Compared to the classic SCR process, the N–O bond scission step proceeds independent of any reducing agent on Rh/CNT catalysts. Since the net reaction includes a stoichiometric consumption of a reducing agent, it must be regarded as a catalytic reduction of NO_x. Furthermore, the experimental results also show that pretreatment of the support material enhances the catalytic activity of rhodium catalysts supported on CNTs. Thus, the way may improve the activity of room temperature removal of NO. Nevertheless, although Rh/CNT has many advantages and special activities, there are also problems for existing technology. Rh/CNT catalysts exhibit high steady-state activity in the abatement of NO_x for a very limited time since the lifetime of the catalysts are determined by the stoichiometric consumption of the support material. Besides, they are not at all suitable for removal of NO in the presence of excess oxygen.

On the other hand, transition metal (Cu, Co, Fe and Ni) is a very important group for SCR of NO. The reactions of NO with diesel soot, fullerene, carbon nanotubes and activated carbons doped with transition metals are researched.⁷⁵ The experimental results show that the reactions of NO with these materials are still completely described by the overall reactions (2C + 2NO → 2CO + N₂, C + 2NO → CO₂ + N₂). Nevertheless, the reaction (C + 4NO → CO₂ + 2H₂O) accounts for the formation of N₂O and CO₂ between 300 and 500 °C when NO reacts with activated charcoal A(Pt). Moreover, in the case of activated carbon doped with copper, it has been reported that catalysis is caused by the dissociative chemisorption of NO on Cu (2Cu + 2NO → 2CuO + N₂), followed by the reduction of CuO by the activated carbon (CuO + C → Cu + C(O)).⁷⁶ Furthermore, in the case of the Cu/ACFs,⁷⁰ the copper metal plays a reductant and major role in the NO reduction. Initially, the copper metal converts to Cu₂O, and comes to change into CuO with further oxidation. From the experimental results, the mechanism of NO reduction over Cu/ACFs can be partly presented as follows:

NO + ACF–Cu → ACF–Cu₂O + 1/2N₂

NO + ACF–Cu₂O → ACF–CuO + 1/2N₂.

It can be concluded that a mechanism of continuous reduction and oxidation of the metal may explain the catalytic effect of a variety of transition elements, such as Fe, Cu and Ni, which will contribute to the application of transition metals for SCR of NO at room temperature.

Besides, a theory is supported that catalyst activity is maintained while the metal is kept in the reducing state.⁷⁷ ACFs are treated by a Ni-electroplating technique in order to remove NO.²² Ni-electroplating leads to an increase of nickel content and a decrease of specific surface, micropore volumes and microporosity with treatment time. Nevertheless, NO conversion of ACFs is improved due to the catalytic reaction of nickel deposited on ACFs.⁷⁸ Moreover, a bimetallic Pt/K catalyst exhibits an excellent performance in the simultaneous reduction of NO and N₂O.⁷⁷ The experimental result that two metals carry out a synergistic effect may be explained by the ability of K to maintain Pt in the reduced state. Although the temperature of complete and stable conversion of both gases is 350 °C, the theory will be applicable at room temperature when it comes to N₂O coexisting with NO in flue gases.

As for simultaneously volatile organic compound (VOC) control and NO reduction, some investigations indicate that activated carbon impregnated with transition metal (Cu, Co, Fe and Ni) catalysts have the potential to simultaneously remove VOC and NO.⁷⁹ The purpose to use VOC, like toluene, as a reducing agent is attempted. From the experimental results, we can conclude that the appearance of the oxidized state of the transition metals on activated carbon will decrease the capacity of VOC adsorption and the catalyst activity. Furthermore, Co/AC shows high activity during the process and the support activated carbon will be burning off during the catalytic oxidation, which reduces the amount of activated carbon solid waste. Therefore, the Co/AC continuous injection system that reduces energy loss and saves the treatment charge for AC solid waste and other pollutants has the potential for application in an air pollution control system.

4.2 Removal of NO by carbonaceous material loaded metal oxides

Carbonaceous materials promoted with metal oxides show high activity in NO reduction with the considerations of the influence of ash, the influence of promotion with metal oxides, the influence of oxygen and humidity. Although the experimental temperature is much more than 30 °C, many theories can be used at room temperature in the same way or in the way with little modification. Some of them are as follows:

(a) Catalysts are described for the low-temperature (120–220 °C) reduction of NO with a carbonaceous adsorbent Ambersorb 572 (A-572) as a support. NO decomposition in He by A-572/5% CuO, A-572 is investigated; reduction of NO with CO in He by A-572/5% CuO, A-572/5% CuO/1% CeO₂, A-572/5% NiO, A-572/5% CuO/5% Cr₂O₃, A-572/5% CuO/5% Fe₂O₃, A-572/5% CoO, A-572/5% MnO, Charcoal/5% CuO/1% CeO₂ is investigated; reduction of NO with CO in 5% O₂ in He by A-572/5% CuO/5% Fe₂O₃ is investigated; reduction of NO with n-hexane in He by A-572/5% CuO/1% CeO₂, A-572/5% CeO₂ is investigated; reduction of NO with n-hexane in 20% O₂ by A-572/5% CuO/1% CeO₂, Kureha^b/5% CuO/1% CeO₂, A-572/5% MnO, A-572/5% CoO is also investigated. The A-572/5% CuO/1% CeO₂ and the A-572/5% CuO/5% Fe₂O₃ are excellent catalysts for the reaction of CO and NO. Besides, A-572/5% CoO leads to good catalyst for reduction by CO and hexane in the presence of O_2.⁸⁰ What we can learn from the investigation is that they take all the experimental conditions into account to accomplish these experiments.

(b) A complete mechanism for describing the low-temperature (125 °C) SCR of NO with NH₃ over carbon supported Mn₃O₄ is discussed as Table 1 showed,⁸¹ which will help us to better understand the specific interactions among NH₃, NO, O₂ and a managanese-based catalyst.

Table 1 SCR process of NO

Stages	SCR by aminooxy groups	SCR by ammonium groups
	Steady-state	Pseudo-steady-state
1	Ammonia adsorption on oxygen atoms	Ammonium formation on the hydroxyl groups
2	The formation of aminooxy groups	Ammonium ions react with gas-phase NO2
3	Aminooxy groups react with gas-phase NO or NO2
4	Dehydroxylation of the octahedral phase
5	Surface oxidation of the oxygen vacancies
Over SCR process	6NO + 4NH3 → 5N2 + 6H2O	4NO + 4NH3 + O2 → 4N2 + 6H2O

---

(c) The SCR of NO with NH₃ is studied using Fe₂O₃, Cr₂O₃ and CuO loaded active carbons as catalysts in the presence and absence of oxygen at low temperature.⁸² Introduction of transition metal dramatically increases the amounts of strongly bound NH₃ and NO, but especially those of NH₃. Catalytic experiments have shown that activation with nitric acid results in higher catalytic activity than pretreatment with air since catalysts pretreated in nitric acid are provided with more oxygen containing surface groups such as ethers, anhydrides or carboxylic groups. Strong differences are observed between the three different metal free supports due to their different ash contents. In the presence of oxygen, Fe containing and Cr catalysts are the most active and selective ones in the temperature range between 140 and 180 °C. In the absence of oxygen, N₂O is an important reaction intermediate on Fe and Cr containing active carbon supports. In particular, Cu containing catalysts reveal a surprising catalytic behavior with very high catalytic activity at 70 °C, which may be well used at room temperature.

(d) More importantly, being a major poison, SO₂ is often avoided for SCR of NO with NH₃ at low temperature. However, the V₂O₅/CNT catalyst with high NO catalytic activities tolerant to SO₂ are reported.⁸³ Besides, it seems that the reacting temperature behaves like a switch, capable of turning on and off the poisoning reactions. When the experimental temperature is higher than 200 °C, SO₂ enhances the SCR activity, while it is lower than 200 °C, SO₂ deactivates the SCR activity. It could be elucidated that on the surface of V₂O₅/CNTs in the presence of SO₂ at temperatures above 200 °C, the formed sulfate species act as new acid sites for NH₃ adsorption and activation. Meanwhile, the ammonium ion reacts continuously with NO to avoid the formation and accumulation of excess ammonium sulfate salts on the catalyst surface, thus prohibiting the deactivation of the catalysts, which ensures that the V₂O₅/CNTs will be promoted but not poisoned by SO₂. More importantly, the V₂O₅/CNTs catalysts show a great stability with a NO conversion maintaining at 92% under the SO₂-containing conditions after being operated for 100 h.

Admittedly, while different kinds of metal oxides supported on carbonaceous materials have been prepared for SCR of NO with NH₃, the study of the effect of the preparation method on metal oxides supported on carbonaceous materials is rare, especially with a wide variety of complementary characterization techniques. Special attention should been paid to the effect of the preparation methods. They include the following: (1) way of introducing the metal; (2) concentration of the precursor solution and time of contact with the monoliths in the case of impregnation; (3) use or not of a chemical pre-treatment of the support; (4) the final drying procedure. Obviously, significant differences present on the catalysts depending on their preparation procedure, both in their physical–chemical properties and catalytic activity. From one of the experimental results,⁸⁴ the extrapolation of conclusions related to metal activation procedures from powders to monolithic catalysts should be taken with care. Furthermore, integration of the metal precursor before extrusion does significant implications like more hard to activate the carbon. Thus, some results reported in the paper will inspire researchers to seek more suitable catalysts for SCR of NO at room temperature by changing some preparation methods.

In the case of room temperature, most of the NO are present in the off-gas, owing to the limited solubility of NO.⁸⁵ Many other studies have also been attempted. Ultrafine Ru particles are dispersed in micropores of ACF. The dimerized NO is in equilibrium with NO₂ and N₂O in the micropore of ACF at 30 °C. The reaction equations are as follows:

3(NO)₂ = 2N₂O + 2NO₂.

The loaded Ru particles accelerate the following reaction:

N₂O → N₂ + O_chem.

NO₂ is chemisorbed on the carbon wall to produce NO and O_chem and the produced NO can be available for this NO reduction cycle repeatedly. Thus, the yield is 80% and the half-life of the reduction reaction is 3 min. Meanwhile, the oxygen is not evolved at room temperature. Furthermore, a cellulose-based ACF, evacuated and then immersed in the Ru³⁺, Pt⁴⁺ or Cu²⁺ solution and then dried after complete adsorption of the metal ions, is used in the SCR of NO. All of them are noticeably effective for NO reduction at 303 K. However, the NO removal of Fe–ACF is lower due to the surface area in micropores where NO can be adsorbed on it.⁸⁶

Therefore, this excellent catalysis of the fine Ru particles in the carbon nanospace should show a new direction not only in room temperature SCR of NO but also in surface and environmental sciences.

Furthermore, in order to solve the serious environmental problems of China,⁸⁷ the authors have studied NO purification abilities of ACF, ACF modified by HNO₃, ACF loaded La₂O₃, ACF loaded CeO₂ at low temperature and longer catalytic durabilities have been found.^20,21,88 Besides, urea-MO/ACF (La₂O₃ and CeO₂ are referred to MO) is considered to be a promising catalyst for SCR of NO at room temperature^89,90 due to the mechanisms described as follows:

2MO + O₂ → 2MO₂˙,

MO₂˙ + NO → NO₂ + MO,

2MO₂˙ + 2(NH₂)₂CO + 2NO₂ → 2MO + 2CO₂ + 3N₂ + 4H₂O,

6MO₂˙ + 4(NH₂)₂CO + 6NO → 6MO + 4CO₂ + 7N₂ + 8H₂O.

The catalytic centers may be the metal oxides, which result from their electron transfer abilities, showed as reactions above. (NH₂)₂CO could be treated as a reducing agent like the action of NH₃ on SCR. Moreover, urea may play another important role, since active carbons activated by KOH and urea could be used as electrode materials in acidic and alkaline capacitors.⁹¹ The reasons why we choose rare-earth elements as supported metals are as follows: (a) lanthanides, such as La and Ce, are in abundant supply in China, which can keep catalysts cheap; (b) the metal special electronic structure. For example, the electron structure of Ce is 4f¹5d¹6s². The outer d-election level is empty while the inner one is filled and it is therefore argued that the adsorption of NO on Ce sites through the N atom is not reasonable as the required bonds cannot be formed, whereas vacant oxygen anion sites seem to be the optimum adsorption sites; (c) more importantly, La₂O₃/ACF, CeO₂/ACF and NiO–CeO₂/ACF also show high catalytic activity at low temperature (30 °C–120 °C), with urea loading on the prepared catalysts as reductant.^92,93 In particular, the addition of lanthanide could greatly improve the dispersibility of other metal oxides on the carrier and increase its catalytic activity.^94,95

Besides, the enhanced NO₂ adsorption can be achieved at room temperature on activated carbon impregnated with copper.⁹⁶ Moreover, the positive impact of the Zr(OH)₄ addition into the CeO₂ fluorite structure on the adsorption of NO₂ at room temperature is demonstrated. And mixed oxides show a better NO₂ adsorption capacity than the parent materials (CeO₂ and Zr(OH)₄).⁹⁷ So in recent studies, selective catalytic reduction (SCR) of NO by urea-CeO₂–CuO/ACF is carried out at room temperature.⁹⁸ The experimental results show that 10% urea-9% CeO₂/ACF could yield the highest NO conversion of 85% among the series of urea-CeO₂/ACF that were prepared. The desirable mass ratio of CeO₂ and CuO was 1 : 1, and it could yield over 90% NO conversion when ACF was loaded with 10% urea. Furthermore, the influence of SO₂ and H₂O is the target of our research. Since the capacity depends on the nature of the metal deposit and its acidity, and is also affected by the presence of moisture and surface functional groups.⁹⁹ To tackle such a bottleneck, a concrete understanding of surface chemistry should precede. It could be found that the removal efficiency of NO reduces with increasing H₂O concentration, and adsorption capacity and the removal efficiency of NO reduces with increasing SO₂ concentration, which is consistent with previous research.¹⁰⁰

Another key challenge is to know if the catalyst could have been easily introduced into the market, the suitability of this new product should be economically evaluated, taking into account their thermal stability, long performance, spatial velocity influence and mechanical properties.

For example, a proper utilization of carbon-based briquettes is investigated.¹⁰¹ Catalytic briquettes show a high thermal stability which is negatively affected by oxidation and impregnation steps although both steps are essential to get a higher NO reduction. Long-term performance tests guarantee a stable catalytic conversion under the experimental conditions without signs of deactivation. Spatial velocity influences catalytic activity decreasing the NO reduction. When it comes to the mechanical strength, it can be concluded that mechanical properties depend significantly on the activation and oxidation process since the textural and surface chemistry properties develop during these processes. Finally, a simple economic assessment should be carried out to provide a rough cost production. They include raw materials, energy, facility mortgages, operation and transportation costs. However, despite the evaluation above, the evidence is not enough to ensure that it could be perfectly introduced into the catalyst market since more aspects such as lifetime operation, flow dynamics, mass transfer in a fixed bed reactor and even the consumer requirements should be taken into consideration, respectively.

5. Conclusions

In order to establish a desirable way for NO reduction at room temperature, some catalysts have been used for removal of NO. Depending on the type of catalysts, reaction conditions and the catalyst preparation methods, the NO conversion mechanisms could be varied. Therefore, the characteristics of the carbonaceous materials, the mechanisms and kinetics of removal of NO by carbonaceous materials should be discussed, respectively. Moreover, it is observed that carbonaceous material supported metal oxides can remove NO efficiently at room temperature, so the effect of the catalytic sample components on NO removal should also be emphatically researched in the latter experiments. Besides, the effect of other parameters, such as how easy it is to make and economic considerations, should also be taken into consideration, which are useful to evaluate if the product and technique can have practical applications and be easily introduced into the market. Therefore, in order to make the NO removal technology a practical application, an important task is to enhance the catalytic properties and life of the catalyst and reduce the cost of catalyst preparation in future studies.

Acknowledgements

This work is part of a Project supported by the National Natural Science Foundation of China (51108169,71072134), the National High Technology Research and Development Program of China (863 Program, No. 2011AA060803), the Scientific and Technological Project (2011SK3219, 20103002) and Major Special Project of Hunan Province in China (2010XK6003), Hunan Natural Science Foundation (No. 11JJB007), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0916), Innovative Research Team of Hunan Natural Science Foundation (No. 09JJ7002), Project of Young teacher grown of Hunan University.

References

1. I. H. Su and J. C. S. Wu, Catal. Commun., 2009, 10, 1534.
2. W. Xu, S. Zheng, R. A. Dweik and S. C. Erzurum, Free Radical Biol. Med., 2006, 41, 19.
3. K. Kostikas, A. I. Papaioannou, K. Tanou, P. Giouleka, A. Koutsokera, M. Minas, S. Papiris, K. I. Gourgoulianis, D. R. Taylor and S. Loukides, Respir. Med., 2011, 105, 526.
4. S. A. Kharitonov, Vasc. Pharmacol., 2005, 43, 371.
5. B. Klingenberg and M. A. Vannice, Appl. Catal., B, 1999, 21, 19.
6. S. K. Bhargava and D. B. Akolekar, J. Colloid Interface Sci., 2005, 281, 171.
7. I. Sobczak, A. Kusior and M. Ziolek, Catal. Today, 2008, 137, 203.
8. J. Nováková, Appl. Catal., B, 2001, 30, 445.
9. S. I. Naayuthaya, N. Mongkolsiri, P. Praserthdam and P. L. Silveston, Appl. Catal., B, 2003, 43, 1.
10. L. Olsson, H. Sjovall and R. J. Blint, Appl. Catal., B, 2009, 87, 200.
11. P. Behr, E. Shafranovsky and H. Heydtmann, Chem. Phys. Lett., 1995, 247, 327.
12. Q. Yu, L. Liu, L. H. Dong, D. Li, B. Liu, F. Gao, K. Q. Sun, L. Dong and Y. Chen, Appl. Catal., B, 2010, 96, 350.
13. A. Raj, T. H. N. Le, S. Kaliaguine and A. Auroux, Appl. Catal., B, 1998, 15, 259.
14. Y. H. Yu and I. H. Su, Jeffery.C.S. Wu, Environ. Technol., 2010, 31, 1449.
15. K. Knoblauch, E. Richter and H. Jüntgen, Fuel, 1981, 60, 832.
16. M. E. Gálvez, A. Boyano, R. Moliner and M. J. Lázaro, J. Anal. Appl. Pyrolysis, 2010, 88, 80.
17. L. Singoredjo, F. Kapteijn, J. A. Moulijn, J. M. M. Martínez and H. P. Boehm, Carbon, 1993, 31, 213.
18. T. Grzybek, M. Rogóz and H. Papp, Catal. Today, 2004, 90, 61.
19. K. Tsuji and I. Shiraishi, Fuel, 1997, 76, 549.
20. P. Lu, C. T. Li, G. M. Zeng, L. J. He, D. L. Peng, H. F. Cui, S. H. Li and Y. B. Zhai, Appl. Catal., B, 2010, 96, 157.
21. Z. Zeng, P. Lu, L. Mai, Z. Li, J. Guo and C. T. Li, Guangzhou Chem. Ind., 2010, 38, 20.
22. F. Goncalves and J. L. Figueiredo, Appl. Catal., B, 2004, 50, 271.
23. R. Pietrzak, K. Jurewicz, P. Nowicki, K. Babel and H. Wachowska, Fuel, 2010, 89, 3457.
24. G. de la Puente and J. A. Menéndez, Solid State Ionics, 1998, 112, 103.
25. A. F. Carley, P. R. Davies and M. W. Roberts, Surf. Sci., 2003, 406, 587.
26. J. Zawadzki, M. Wisniewski and K. Skowrońska, Carbon, 2003, 41, 235.
27. J. Y. Dai, P. Giannozzi and J. M. Yuan, Surf. Sci., 2009, 603, 3234.
28. K. A. Hossain, M. N Mohd-Jaafar, K. B. Appalanidu, A. Mustafa and F. N. Ani, Environ. Technol., 2005, 26, 251.
29. A. Ricca and C.W. Bauschlicher Jr., Chem. Phys., 2006, 323, 511.
30. C. M. Yang and K. Kaneko, J. Colloid Interface Sci., 2002, 255, 236.
31. Z. C. Guo, Y. S. Xie, I. Hong and J. Kim, Energy Convers. Manage., 2001, 42, 2005.
32. I. Mochida, S. Kawano, M. Hironaka, S. Yatsunami, Y. Korai, Y. Matsumura and M. Yoshikawa, Energy Fuels, 1995, 9, 659.
33. I. Mochida, S. Kawano, N. Shirahama, T. Enjoji, S. H. Moon, K. Sakanishi, Y. Korai and M. Yoshikawa, Fuel, 2001, 80, 2227.
34. S. Adapa, V. Gaur and N. Verma, Chem. Eng. J. (Amsterdam, Neth.), 2006, 116, 25.
35. R. Pietrzak, Energy Fuels, 2009, 23, 3617.
36. J. Yang, G. Mestl and D. Herein, Carbon, 2000, 38, 729.
37. T. Suzuki, T. Kyotani and A. Tomita, Ind. Eng. Chem. Res., 1994, 33, 2840.
38. L. L. Zhu, B. C. Huang, W. H. Wang, Z. L. Wei and D. Q. Ye, Catal. Commun., 2011, 12, 394.
39. I. Mochida, K. Kawabuchi, S. Kawano, Y. Matsumura and M. Yoshikawa, Fuel, 1996, 76, 543.
40. S. Bashkowa and T. J. Bandosz, J. Colloid Interface Sci., 2009, 333, 97.
41. W. Klose and S. Rincón, Fuel, 2007, 86, 203.
42. L. K. Chan, A. F. Sarofim and J. M. Beér, Combust. Flame, 1983, 52, 37.
43. A. M. Rubel and J. M. Stencel, Fuel, 1996, 76, 521.
44. M. T. Izquierdo, B. Rubio, C. Mayoral and J. M. Andrés, Fuel, 2003, 82, 147.
45. E. Richter, Catal. Today, 1990, 7, 93.
46. X. H. Xu, S. H. Liu, H. P. Wang, S. G. Chang and J. W. Fisher, Energy Fuels, 2003, 17, 1303.
47. W. J. Zhang, S. Rabiei, A. Bagreev, M. S. Zhuang and F. Rasouli, Appl. Catal., B, 2008, 83, 63.
48. R. Pietrzak, K. Jurewicz, P. Nowicki, K. Babel and H. Wachowska, Fuel, 2007, 86, 1086.
49. Z. P. Zhu, Z. Y. Liu, S. J. Liu and H. X. Niu, Appl. Catal., B, 1999, 23, 229.
50. K. H. Chuang, C. Y. Lu, M. Y. Wey and Y. N. Huang, Appl. Catal., A, 2011, 397, 234.
51. E. Deliyanni and T. J. Bandosz, Langmuir, 2011, 27, 1837.
52. S. N. Ahmed, R. Baldwin, F. Derbyshire, B. McEnaney and J. Stencil, Fuel, 1993, 72, 287.
53. Z. G. Huang, Z. Y. Liu, X. L. Zhang and Q. Y. Liu, Appl. Catal., B, 2006, 63, 260.
54. I. Mochida, N. Shirahama, S. Kawano, Y. Korai, A. Yasutake, M. Tanoura, S. Fujii and M. Yoshikawa, Fuel, 2000, 79, 1713.
55. N. Shirahama, S. H. Moon, K. H. Choi, T. Enjoji, S. Kawano, Y. Korai, M. Tanoura and I. Mochida, Carbon, 2002, 40, 2605.
56. B. J. Ku, J. k. Lee, D. Park and H. K. Rhee, Ind. Eng. Chem. Res., 1994, 33, 2868.
57. Z. P. Zhu, Z. Y. Liu, S. J. Liu and H. X. Niu, Fuel, 2000, 79, 651.
58. H. Teng, Y. T. Tu, Y. C. Lai and C. C. Lin, Carbon, 2001, 39, 575.
59. I. Mochida, S. Kawano, M. Hironaka, Y. Kawabuchi, Y. Korai, Y. Matsumura and M. Yoshikawa, Langmuir, 1997, 13, 5316.
60. M. J. Illan-Gomez, A. Linares-Solano, L. R. Radovic and C. S. de Lecca, Energy Fuels, 1995, 9, 112.
61. I. Mochida, M. Kishino, S. Kawano, K. Sakanishi, Y. Korai, A. Yasutake and M. Yoshikawa, Fuel, 1998, 77, 1741.
62. R. S. Rathore, D. K. Srivastava, A. K. Agarwal and N. Verma, J. Hazard. Mater., 2010, 173, 211.
63. P. Forzatti, Appl. Catal., A, 2001, 222, 221.
64. P. Forzatti, Catal. Today, 2000, 62, 51.
65. N. Shirahama, I. Mochida, Y. Korai, K. H. Choi, T. Enjoji, T. Shimohara and A. Yasutake, Appl. Catal., B, 2005, 57, 237.
66. N. Shirahama, I. Mochida, Y. Korai, K. H. Choi, T. Enjoji, T. Shimohara and A. Yasutake, Appl. Catal., B, 2004, 52, 173.
67. J. Zawadzki, M. Wisniewski and K. Skowronska, Appl. Catal., B, 2002, 35, 255.
68. T. Bansagi, T. S. Zakar and F. Solymosi, Appl. Catal., B, 2006, 66, 147.
69. R. Dumpelmann, N. W. Cant and D. L. Trimm, Appl. Catal., B, 1995, 6, 291.
70. S. J. Park and B. J. Kim, J. Colloid Interface Sci., 2005, 292, 493.
71. H. Beyer and K. Kohler, Appl. Catal., B, 2010, 96, 110.
72. P. Serp, M. Corrias and P. Kalck, Appl. Catal., A, 2003, 253, 337.
73. K. Otsuka, H. Ogihara and S. Takenaka, Carbon, 2003, 41, 223.
74. H. Ogihara, S. Takenaka, I. Yamanaka and K. Otsuka, Carbon, 2004, 42, 1609.
75. C. J. Tighe, J. S. Dennis, A. N. Hayhurst and M. V. Twigg, Proc. Combust. Inst., 2009, 32, 1989.
76. M. J. Illan-Gomez, A. Linaressolano, L. R. Radovic and C. S. M. Delecea, Energy Fuels, 1996, 10, 158.
77. B. Thirupathi, Panagiotis G. Smirniotis, Appl. Catal., B, 2011, 110, 195.
78. S. J. Park, G. H. Shim and H. Y. Kim, J. Colloid Interface Sci., 2005, 291, 585.
79. C. Y. Lu and M. Y. Wey, Fuel Process. Technol., 2007, 88, 557.
80. K. Jurczyk and R. S. Drago, Appl. Catal., A, 1998, 173, 145.
81. G. Marbán, T. Valdés-Solís and A. B. Fuertes, J. Catal., 2004, 226, 138.
82. J. Pasel, P. Kabner, B. Montanari, M. Gazzano, A. Vaccari, W. Makowski, T. Lojewski, R. Dziembaj and H. Papp, Appl. Catal., B, 1998, 18, 199.
83. S. L. Bai, J. H. Zhao, L. Wang and Z. P. Zhu, Catal. Today, 2010, 158, 393.
84. M. Ouzzine, G. A. Cifredo, J. M. Gatica, S. Harti, T. Chafik and H. Vidal, Appl. Catal., A, 2008, 342, 150.
85. S. W. H. Van Hulle, J. Callens, K. E. Mampaey, M. C. M. van Loosdrecht and E. I. P. Volcke, Environ. Technol., 2012 DOI:10.1080/09593330.2012.665492.
86. Y. Nishi, T. Suzuki and K. Kaneko, J. Phys. Chem. B, 1997, 101, 1938.
87. P. Lu, C. T. Li, G. M. Zeng, X. W. Xie, Z. H. Cai, Y. X. Zhou, Y. P. Zhao, Q. Zhan and Z. Zeng, J. Hazard. Mater., 2012, 199, 200–272.
88. C. T. Li, P. Lu, G. M. Zeng, Q. J. Wang, Q. Li, L. J. He and Y. B. Zhai, China Environ. Sci., 2008, 29, 3280.
89. P. Lu, Z. Zeng, C. T. Li, G. M. Zeng, J. Guo, X. Jiang, Y. B. Zhai and X. P. Fan, Environ. Technol., 2012, 33, 1029.
90. Z. Zeng, P. Lu, C. T. Li, G. M. Zeng, X. Jiang, Y. B. Zhai and X. P. Fan, Environ. Technol., 2012, 33, 1331.
91. K. Jurewicz, R. Pietrzak, P. Nowicki and H. Wachowska, Electrochim. Acta, 2008, 53, 5469.
92. H. F. Cui, C. T. Li, P. Lu, D. L. Peng, J. Guo and L. Chen, China Environ. Sci., 2010, 31, 2575.
93. Z. Zeng, P. Lu, C. T. Li, G. M. Zeng and X. Jiang, J. Coord. Chem., 2012, 65, 1992.
94. J. Guo, C. T. Li, P. Lu, H. F. Cui, D. L. Peng and Q. B. Wen, China Environ. Sci., 2011, 32, 2240.
95. W. W. Zhao, C. T. Li, P. Lu, Q. B. Wen, Y. P. Zhao, X. Zhang, C. Z. Fan and S. S. Tao, Environ. Technol., 2012 DOI:10.1080/09593330.2012.689366.
96. B. Levasseur, E. Gonzalez-Lopez, J. A. Rossin and T. J. Bandosz, Langmuir, 2011, 27, 5354.
97. B. Levasseur, A. M. Ebrahim and T. J. Bandosz, Langmuir, 2011, 27, 9379.
98. X. Jiang, P. Lu, C. T. Li, Z. Zeng, G. M. Zeng, L. P. Hu, L. Mai and Z. Li, Environ. Technol., 2012 DOI:10.1080/09593330.2012.707232.
99. L. M. Le Leuch and T. J. Bandosz, Carbon, 2007, 45, 568.
100. S. C. Ma, J. J. Yao and L. Gao, Environ. Technol., 2012, 33, 1811.
101. A. Boyano, R. Gálvez Moliner and M. J. Lázaro, Catal. Today, 2008, 137, 209.

---