Application of Non-Thermal Plasma over Zeolite-Based Materials: Comparison
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沸石基材料被广泛用作净化空气污染物（如 Zeolite based materials are widely used as adsorbents and catalysts for purifying air pollutants (such as NO）的吸附剂和催化剂x以及由于丰富的孔隙结构、规则的孔隙分布和众多的离子交换位点而产生的) and VOC。热处理是去除孔隙中的杂质和促进金属活性物质均匀分散的必要程序，然后应用沸石基吸附剂s due to rich pore structure, regular pore distribution and numerous ion exchange sites. Heat treatment is a necessary procedure to remove impurities in pores and promote the uniform dispersion of metal active substances, and then zeolite based adsorbent/催化剂来净化 NOxcatalyst is used to purify nox/挥发性有机化合物。然而，传统的热场处理（即高温煅烧、高温吹扫等）需要大量的能耗。相比之下，非热等离子体等非常规外场处理显示出高效率、低能耗和低污染的显著优势，在许多领域被用于替代传统热处理。volatile organic compounds. However, the traditional thermal field treatment (i.e. high temperature calcination, high temperature purging, etc.) requires a lot of energy consumption. In contrast, non thermal plasma and other unconventional external field treatments show significant advantages of high efficiency, low energy consumption and low pollution, and are used to replace traditional heat treatment in many fields. 

  • zeolite
  • VOCs
  • NOx
  • non-thermal plasma

1. 引言Introduction

等离子体是一种由放电产生的放热电离气体[25]。通过向反应区域注入能量，电子从原子或分子中释放出来，达到离子、电子和原始物质共存的电中性状态[23]。根据能级、温度和离子密度，等离子体一般分为高温等离子体（核应用）和低温等离子体（包括热等离子体和非热等离子体）[26]。特别是，非热等离子体可以应用于催化剂制备和活化领域，因为相对较低的温度条件（通常为几十到Plasma is an exothermic ionized gas produced by electrical discharge [1]. By injecting energy into the reaction region, electrons are released from the atoms or molecules to achieve an electrically neutral state where ions, electrons, and the original species coexist [2]. According to the energy level, temperature, and ionic density, plasma is generally classified into high-temperature plasma (for nuclear applications) and low-temperature plasma (including thermal plasma and non-thermal plasma) [3]. Particularly, the non-thermal plasma can be applied in the fields of catalyst preparation and activation, because the relatively low-temperature condition (usually tens to 200 °C）不会对材料表面造成热损伤[23]。此外，由于真空系统在大气压下工作，因此可以避免真空系统的成本[27]。具体来说，热等离子体的气体温度接近电子温度（约几十电子伏特），而非热等离子体的气体温度可低至室温[28]。) will not cause thermal damage to the material surface [2]. Additionally, since it works at atmospheric pressure, the cost of the vacuum system can be avoided [4]. Specifically, the gas temperature of thermal plasma is close to the electron temperature (about tens of electron volts), while that of non-thermal plasma can be as low as room temperature [5].
非热等离子体通过在高能物质的蚀刻下产生一些特定的性质或功能，在装饰沸石结构方面显示出巨大的潜力[20]。等离子体活化为沸石主要涉及以下几点：The non-thermal plasma shows great potential in decorating zeolitic structures by generating some specific properties or functions under the etch of energetic species [6]. The activation of plasma to zeolites mainly involves the following points:
(1)
在放电过程中，气体气氛中的分子会被电离产生许多高能电子，这些电子会相互碰撞，将电能转化为粒子的内能和动能，从而提供化学反应的活化能[29]，可以加工常规热场无法激活的材料[18]。During the discharge process, the molecules in the gas atmosphere will be ionized to generate many high-energy electrons, which will collide with each other to convert electric energy into internal energy and kinetic energy of particles, thus providing the activation energy of chemical reactions [7], which can process materials that cannot be activated by the conventional thermal field [8].
(2)
大量高能的电子和其他活性物质会通过蚀刻和烧蚀作用侵蚀沸石的外表面，形成新的孔隙，从而增加孔隙率。Numerous electrons and other active substances with high energy will attack the external surface of zeolites to form new pores through etching and ablation effects, thus increasing the porosity.
(3)
高能的电子可以破坏沸石表面的化学键，形成新的活性基团，增加活性位点的数量。The electrons with high energy can break the chemical bonds on the zeolite surface to form new active groups and increase the active sites amount.
(4)
等离子体放电产生的高能粒子可以促进气体The high-energy particles produced by plasma discharge can promote the excitation, ionization, and interaction of gas-反应物分子的激发、电离和相互作用，形成新的化学键[30]。reactant molecules to form new chemical bonds [9].
与传统的热处理方法相比，非热等离子体具有快速、清洁、节能、方便、成本相对低等优点[18]，同时提高了材料的活性、寿命和选择性[31]Compared to the traditional thermal treatment method, non-thermal plasma is fast, clean, energy-saving, convenient, and relatively low-cost [8] despite improving the activity, life, and selectivity of materials [10];因此，它被视为一种理想的治疗方法。目前，非热等离子体通常用于活化吸附剂 therefore, it is viewed as an ideal treatment method. At present, the non-thermal plasma is generally applied for activating the adsorbents/催化剂或辅助净化过程，以提高VOCs或catalysts or assisting the purification process for enhancing the removal efficiency of the VOCs or NO的去除效率x.用于活化材料的一般非热等离子体技术分别包括介电势垒放电等离子体（ The general non-thermal plasma technologies used for activating materials include dielectric barrier discharge plasma (DBD）、辉光放电等离子体（GDP）和电晕放电等离子体（CDP）[32]。), glow discharge plasma (GDP), and corona discharge plasma (CDP) [11], respectively. DBD是在介质中存在强电场的情况下电子穿透的现象，其中电子被电场加速并获得足够的能量。 is the phenomenon of electron penetration in the presence of a strong electric field in the medium, where the electrons are accelerated by the electric field and obtain enough energy. The GDP是通过向气体中添加足够的电压以引起原子碰撞，产生电离电子以形成电流引起的。CDP发生在大曲率半径的尖端电极附近的不均匀电场中，是由于局部电场强度超过气体电离场强而导致气体电离和激发引起的。 is caused by adding enough voltage to the gas to cause the collision of atoms, producing ionized electrons to form an electric current. The CDP, occurring in an uneven electric field near a tip electrode with a large radius of curvature, is caused by the ionization and excitation of the gas due to the local electric field strength exceeding the gas ionization field strength.

2. NTP辅助活化材料 Assisted Activation of Materials

非热等离子体可以有效地改变材料的表面性质[25]，因为在放电过程中产生的大量高能电子和其他活性物质攻击沸石表面形成新的孔隙和活性基团。同时，沸石载体上的激发物质和酸碱基之间发生化学反应，从而改变载体的酸碱度[19]。此外，碱性金属的活性成分可以通过与酸性位点的相互作用来调节金属的分散性[35]。此外，通过改变沸石的织构特性，最终，非热等离子体可以帮助沸石基材料提高目标产物的反应性、选择性、稳定性和收率[36]。更重要的是，与传统的高温热处理容易破坏沸石骨架相比，等离子体可以避免由于低温而出现这种情况，保持沸石的稳定性[28]。Non-thermal plasma could effectively modify the surface properties of materials [1] due to the large number of energetic electrons and other active substances produced during the discharge process attacking the zeolite surface to form new pores and active groups. Meanwhile, chemical reactions take place between the excited species and acid-base groups on the zeolite carriers, thus changing the acidity and alkalinity of the carriers [12]. Further, the alkaline metal's active components can regulate the metal dispersion through the interaction with acid sites [13]. In addition, by modifying the textural properties of zeolites, ultimately, non-thermal plasma can assist the zeolite-based materials to improve the reactivity, selectivity, stability, and yield of the target products [14]. More significantly, compared to the traditional thermal treatment at high temperatures that easily destroy the zeolite framework, the plasma can avoid such a situation due to the low temperature, maintaining the stability of zeolites [5].
事实证明，沸石表面会产生羟基等更多的活性基团，伴随着孔隙率的增长和放电后活性位点的相应增加。非热等离子体对材料的活化主要用于辅助It has been proved that more active groups such as hydroxyl will be generated on the surface of the zeolites, accompanied by the growth of porosity and the corresponding increase of active sites after the electric discharge. Material activation by non-thermal plasma is mainly applied in assisting the purification of the VOCs的净化。Liu等[32]发现，. Liu et al. [11] found that the methane conversion rate of the Pd/HZSM-5催化剂经非热等离子体处理后，在400 catalyst grew from °C下甲烷转化率从25%提高到90%，这是活性位点增加所致[37]。25% to 90% at 400 °C after being treated with non-thermal plasma, which resulted from the increase of active sites [15]. Wang等[38]证明，与未处理的催化剂相比，等离子体处理增强了 et al. [16] proved that the plasma treatment enhanced the initial activity of the Pd/Al-MCM-41分子筛催化剂在甲烷燃烧反应中的初始活性（图1 zeolite catalyst in the methane combustion reaction compared to the untreated catalyst (Figure 1A），这是由于催化剂的酸度较高（图), which was caused by the higher acidity of the catalyst (Figure 1B）和) and the better dispersion of the PdO颗粒活性物质的分散性较好。此外，等离子体处理的钯基催化剂稳定性的提高与氧化钯与沸石载体之间更强的相互作用密切相关。Wang等[39]使用非热等离子体显著增大了孔径，使孔径从 particles active species. Moreover, the improved stability of the plasma-treated Pd-based catalyst was proven to be closely related to the stronger interaction between the palladium oxide and the zeolite support. Using the non-thermal plasma, Wang et al. [17] significantly increased the pore size from 0.62 nm增加到to 0.72 nm，比表面积从, the specific surface area from 222.14增加到 to 618.06 m2/g，孔隙体积为, and the pore volume from 0.19 至to 0.36 cm3/g，同时对萘的吸附量大大增加，达到, respectively, accompanied with the greatly increasing adsorption capacity of 167%。此外，还归因于放电区产生的大量活性物质（即高能电子、离子、质子、紫外光子和活性自由基）轰击吸附剂的外表面，通过蚀刻效应促进新孔隙的形成和原有孔隙的重建。另一方面，加速电子在电场中的高能量可以破坏化学键并形成新的键，导致活性位点量增加，从而提高分子筛的活性。 for naphthalene. It was also ascribed to those abundant active substances (i.e., energetic electrons, ions, protons, ultraviolet photons, and active radicals) produced in the discharge region bombarded the outer surface of the adsorbents to promote the formation of new pores and reconstruction of the original pores by etching effect. On the other hand, the high energy of accelerating electrons in the electric field can break chemical bonds and form new bonds, resulting in an increase in the active site amount, thus improving the activity of the zeolites.
Catalysts 13 01461 g001
图Figure 1.（ (A）) Initial activity for methane combustion 甲烷燃烧的初始活性和 （and (B）) The NH3−TPdO/Al-MCM-41催化剂的TPD曲线：（a）常规处理的C-D profiles of PdO/Al−MCM−41 catalysts: (a) C−PdO/Al-MCM-41和（b）等离子体处理的P-−MCM−41 treated conventionally and (b) P−PdO/Al-MCM-41[38]。Co的−MCM−41 treated with the plasma treatment [16]. The TEM图像 images of Co3O4（C）煅烧和（D）等离子体处理的/HZSM−5催化剂 catalysts treated by (C) calcination and (D) plasma;（ (E）) The XPS spectra of O 1 s 的 XPS 谱图（α 氧是表面(alfa oxygen is the lattice oxygen concentration of surface Co 的晶格氧浓度3O4其中，β氧为表面吸附氧的浓度，γ氧为, beta oxygen is the concentration of surface adsorbed oxygen, and gamma oxygen is the lattice oxygen concentration of HZSM−5载体的晶格氧浓度）;（ support); (F）) Methane conversion of 甲烷转化Co3O4通过煅烧和等离子体处理的/HZSM−5催化剂（图中的“C”代表煅烧，“P”代表等离子体）[40]。 catalysts treated by calcination and plasma (“C” in the image stands for calcination and “P” for plasma) [18].
除了改变沸石的孔隙结构和表面性质外，非热等离子体的另一个重要应用是引入一定量的质子酸位点，以提高沸石的酸度和活性[41]。含有高动能的碳、氧、氮的自由基和活性离子很容易与分子筛中的酸性或碱性基团发生反应[42]，从而改变分子筛的酸性或碱性[19]。In addition to changing the pore structure and surface properties of zeolites, another important application of non-thermal plasma is to introduce a certain amount of protonic acid sites to improve the acidity and activity of zeolites [19]. Free radicals and active ions containing carbon, oxygen, and nitrogen possessing high kinetic energy can easily react with acidic or alkaline groups in the zeolites [20], thus changing the acidic or alkaline properties of the zeolites [12]. Xia等[43]发现，与常规处理得到的 et al. [21] found that compared to the Fe-Mo/HZSM-5分子筛相比，非热等离子体处理分子筛的 zeolite obtained by the conventional treatment, the Brönsted酸度高达265%以上，甲烷转化率从11%显著提高到24%[44]。对于 acidity of the non-thermal plasma-treated zeolite was up to more than 265%, which significantly enhanced methane conversion from 11% to 24% [22]. For the Mo-Fe/HZSM-5分子筛， zeolite, Zhu等[45]还发现，等离子体处理催化剂的 et al. [23] also found that the Brönsted酸强度和密度均高于未处理的样品，因此甲烷转化率提高。 acid intensity and density of the plasma-treated catalyst were higher than that of the untreated sample, and thus the methane conversion was increased. Liu等[32]证实，非热等离子体处理显著提高了 et al. [11] confirmed that the non-thermal plasma treatment significantly increased both the Brönsted and Lewis acids contents of the Pd/HZSM-5分子筛的B zeolite. The concentrönsted和Lewis酸含量。两个酸性位点的浓度分别是新鲜样品的1.13倍和1.21倍。结果，处理后的沸石在甲烷燃烧中表现出比新鲜剂高50%的催化活性和更强的稳定性[37]。除了上述等离子体辅助甲烷脱除的应用外，竹内等[46]证明，对于异丁烷催化裂化，等离子体处理ation of both the acidic sites was 1.13 times and 1.21 times higher than that of the fresh sample, respectively. As a result, the treated zeolite showed 50% higher catalytic activity and stronger stability than the fresh agent in methane combustion [15]. In addition to the above applications for plasma-assisted methane removal, Takeuchi et al. [24] proved that for the iso-butane catalytic cracking, the plasma treatment for 5 min比常规 could enhance the activity by 10% compared to the treatment by conventional calcination at 600 °C煅烧2 h处理可使活性提高10%，这归因于等离子体处理后HY分子筛中可以有效形成Lewis酸位点。 for 2 h, which was attributed to the fact that the Lewis acid sites could be effectively formed in the HY zeolite after plasma treatment.
The non-thermal plasma treatment can also increase the dispersion and stability of the metal active components loaded on the materials, thus providing more reactive centers [19][12]. Liu et al. [32][11] also found that after treatment with the non-thermal plasma, the particle size of the PdO on the surface of the Pd/HZSM-5 catalyst decreased from 21.0 to 16.0 nm and the Pd dispersion increased by 2.5 times, effectively avoiding the aggregation of PdO particles, and blocking the pores of ZSM-5. Thereby, the catalyst showed higher catalytic activity and better stability than that without the plasma treatment in the methane combustion. It was concluded that the plasma treatment could make the PdO cluster (alkaline oxide) better attracted to the increased Brönsted and Lewis acidic sites, facilitating the PdO dispersion on the zeolite. In addition, the enhanced interaction between the PdO and acidic sites made the PdO more stable during the reaction, significantly improving the stability of the catalyst [37][15]. Cao et al. [40][18] found that compared to the calcination, the plasma treatment could promote the formation and dispersion of small-size Co3O4 on Co3O4/H-ZSM-5 (TEM shown in Figure 1C,D). Moreover, it was obvious that the content of the adsorbed oxygen species Oβ for 20-P (5.2) was more than 7 times higher than that for 20-C (0.7), which indicated that plasma could improve the number of lattice oxygen of Co3O4 on the catalyst surface (XPS spectra shown in Figure 1E). Therefore, the catalytic activity of methane was simultaneously improved (the conversion rate of 100% at 550 °C) (Figure 1F). Wang et al. [47][25] reported a high performance of the plasma-enhanced Zn/MCM-41 catalyst for acetylene hydration, showed that plasma treatment could effectively enhance the interaction between the Zn active sites and MCM-41 carrier, promote the dispersion of the Zn active component, and inhibit the loss of Zn species.
There is also some research on the activation materials by non-thermal plasma to assist NOx purification, although are not as widespread as the VOCs removal. Yoon et al. [48][26] found that the active sites on the NaY catalyst increased via non-thermal plasma treatment, which greatly improved the NOx conversion rate of the catalyst from 20% to 55% at 225 °C in the lean NOx reduction [49][27]. Zhang et al. [50][28] proved that the dispersion of the Pd active component on the surface of the Pt/NaZSM-5 catalyst increased markedly from 4.2% to 63.3% after plasma treatment. Correspondingly, the particle size decreased significantly from 27.0 to 1.8 nm, and the NO redox activity enhanced observably from almost zero to 61.3% at 400 °C.

3. NTP Cooperative Catalysis Process

In addition to the above-mentioned activation level of material modification, during the process of activating zeolite materials by non-thermal plasma, reaction gas can be accelerated to activated when injected in the discharge region, NOx/VOCs can be cooperatively removed, and the conversion rate can be improved. The basic principle of this technology is that a high-intensity electric field provides energy for the electrons so that electrons can obtain high kinetic energy. Then the high-speed moving electrons transfer energy to molecules through collision, so that some gas molecules in the gas are excited and ionized. Through direct generation or ionization, many active particles such as ions, excited atoms, molecules, electrons, and free radicals are generated, and these active particles undergo a series of complex physicochemical reactions with the NOx or VOCs molecules to achieve the purpose of removal [51][29]. The energy of the active particles in the plasma is higher than that of most chemical bonds, which indicates that these extremely high-energy active particles can completely promote the fracture of the old bonds of reactants, thus generating new chemical bonds [30][9].
The NTP has been widely applied for purifying the NOx via the routes of direct decomposition [52][30], oxidation combined with absorption [53][31], adsorption combined with in-situ decomposition [4][32], selective catalytic reduction [54][33], etc., the core roles of which in above actions are mainly decomposition, oxidation, and reduction. The NO decomposition by the NTP is realized through colliding and making N-O bonds broken by the electrons or high-energy species generated from the NTP to produce N2 and O2 [52][30]. The NO oxidation by the NTP is to use the oxidizing species O, O3, or OH generated by NTP to oxidize NO to NO2 or nitrate [53][31]. NO reduction by NTP is to use the N atom generated by NPT to react with NO to produce N2 and O2; or introduce NH3, HC, or CxHy into the reaction gas to reduce NO to N2 and O2 [54][33]. The specific reaction process can be seen in Equations (1)–(3). The possible ways of combining the discharge plasma with the catalyst are shown schematically in Figure 2A,B [55][34]. The single-stage method is also known as the plasma-driven catalyst (PDC) system [19][12]. In this method, the catalyst is placed directly in the plasma reactor. The PDC system will induce both the gas-phase chemical reaction and the catalytic reaction on the catalyst surface. On the other hand, in the two-stage method, the plasma reactor is followed by the catalyst bed. This method is referred to as plasma-enhanced selective catalyst reduction (PE-SCR) method. In this case, NOx is oxidized by plasma and then reduced by the catalyst.
NTP decomposition of NO: 2NO + e→N2 + O2
NTP oxidation of NO: NO + O+M→NO2 + M, NO + O3→NO2 + O2
NTP reduction of NO: NO + N→N2 + O
Catalysts 13 01461 g002
Figure 2. Combination of plasma with a catalyst: (A) single−stage (PDC) system; (B) two−stage (PE−SCR) system. (C) The infrared spectrum of the exhaust is not treated and treated with plasma [56][35]. (D) NOx conversions for the plasma alone, catalyst alone, and in−plasma catalysis (IPC) cases [57][36]. (E) NOx conversion as functions of temperature over (■) Co−HZSM5 catalyst alone, (●) discharge over Co−HZSM-5 catalyst, and (▲) discharge over quartz pellets [58][37].
It has been demonstrated that the main pathway of plasma to deal with NOx is HC-SCR, NH3-SCR, CH4-SCR, etc. by producing electrons or free radicals. Coupling the NTP technology with the NH3-SCR system by Byun et al. [59][38], the low-temperature reaction activity of NOx was greatly improved under the synergistic effect, and the reaction temperature window was also widened from 300–400 °C to 100–500 °C. This tendency could be attributed to the fact that the OH and H radicals generated from the NTP will react rapidly to decompose NH3 at low temperatures to produce NH2 radicals, followed by selective non-catalytic reduction through the reaction of NO with NH2 to produce N2. The NTP in collaboration with the NH3-SCR technology was also used by Schmidt et al. to remove NOx from diesel exhaust [56][35]. It was regarded that the conversion of NO is mainly triggered by the electron impact dissociation of molecular oxygen and molecular nitrogen. By comparing the infrared spectra of the gas mixture before and after being treated with plasma, it was found that the byproducts are probably formed due to the action of the plasma. With this method, the species produced by plasma can be found since new absorption lines would occur in the spectrum as depicted in Figure 2C. It has been reported by Singh et al. [60][39] that the reaction system cooperated the NTP with NH3-SCR could produce some long-lived and short-lived active substances, such as the gas-phase electronic excited states of NOx, hydrocarbon fragments, some oxygen-containing derivatives, and vibration-excited substances. These active species, which exist in gas or surface phases, are a key factor for the significant synergistic effect of the NTP and SCR catalysts. Fan et al. [57][36] studied the removal of NOx by C2H2-SCR on an H-MOR catalyst in the DBD plasma. The results showed that the synergistic effect of the catalyst and DBD significantly increased the low-temperature catalytic activity of NOx from 54.0% to 91.4% (Figure 2D). A common understanding about the mechanism of C2H2-SCR is plenty of HCN by-products, derived from key surface intermediate species, such as -CN, -NCO, -NCaObHc, which would react with NOx and O2 to produce N2. By introducing hydrocarbon, Niu et al. [58][37] studied the synergistic effect of the non-thermal plasma on the denitrification efficiency of the Co/ZSM-5 catalyst in HC-SCR and found that when 500 ppm NOx was introduced, the NOx removal rate of Co-HZSM-5 catalyst applied plasma discharge was better than Co-HZSM5 catalyst alone, which increased by 30% at most in the temperature range of 150–450 °C. The synergistic effect showed that the excess oxygen generated by NTP led to the massive conversion of NO to NO2, which is then effectively reduced to N2 by hydrocarbons (Figure 2E).
Similar to the NOx removal via the NTP technique, the VOCs molecules are excited and ionized by the oxidizing substances produced by NTP (such as atomic oxygen, ozone, hydroxyl radicals, active ions, or high-energy electrons, etc.), so that they can be broken down into small molecular substances, even harmless substances such as CO2 and H2O. Previous studies strongly suggested that the removal of VOCs proceeds mainly on the surface of the metal-loaded zeolites. During the discharge process, dissociative chemisorption of O2 onto metal-loaded zeolites produces reactive surface species (such as O2− and O−) [61][40], which lead to the oxidative removal of pollutants, which was shown in Equations (4) and (5). Here the subscripts (g) and (s) indicate the species in the gas phase and surface, respectively. When plasma is applied to the catalyst, reactive surface oxygen species can be also generated from the lattice oxygen [62][41] and direct interaction with O3(g) [63][42], or O(g) radicals as well [64][43], as shown in Equations (6)–(8). The NTP can also promote the desorption of reaction products, which helps keep the catalytic activity, shown as in Figure 3A [65][44].
O2(g) + Catalyst*→O2(s)− + Catalyst
O2(s)− +e→O(s)− + O−(s)
Lattice-O(s) + plasma→O(s)− + Catalyst
O3(g) + Catalyst*→O(s)− + O2(s) + Catalyst
O(g) + Catalyst*→O(s)− + Catalyst
The following examples all show that the plasma discharge will produce active substances consisting of reactive short-lived intermediates and long-lived radicals that participate in induced VOCs decomposition reactions. Wallis et al. [66][45] studied the non-thermal plasma-assisted catalysis for the destruction of dichloro-methane (CH2Cl2), demonstrating that the combination of the plasma and HZSM-5 catalyst improved dichloro-methane conversion (36%) compared to plasma processing alone (27%), which is attributed to the synergistic action of zeolite surface with oxygen active species dissociated in the plasma. The schematic diagram of the reaction mechanism for CH2Cl2 decomposition in gaseous air has been given in Figure 3B. In a combined zeolite plasma reactor, Oh et al. [67][46] proved that toluene was decomposed not only by direct plasma decomposition in the plasma region but also by oxidation with O3 on the zeolite outside the plasma region. With increasing reactor temperature, toluene decomposition increased, and the formation of CO was promoted owing to the enhanced decomposition of HCOOH. The reaction mechanism of the zeolite-plasma compound system for toluene decomposition was also studied by Teramoto et al. [68][47]. It was found that O3 directly decomposed toluene adsorbed in the inner of the zeolite micropores because it had a long lifetime, while the contributions of short-lived radicals and fast electrons to the decomposition of adsorbed toluene were low. Trinh et al. [69][48] proposed and described the main reaction pathways responsible for the formation of gaseous byproducts in Figure 3C. The gaseous and adsorbed acetone molecules are first dissociated into CH3, CH3CO, and atomic hydrogen (H) radicals by plasma-induced energetic species. Subsequently, the recombination of these fragments in the gas phase leads to the formation of CH3CHO, HCHO, and CH4. Meanwhile, further oxidations of the intermediates by the oxidizing agents such as O atomic, OH radicals, and O2 finally lead to the formation of COx through reactions. About other VOCs, Francke et al. [70][49] described the synergistic application of plasma treatment and NH4-mordinite zeolite for the catalytic removal of butyl acetate and dichloroethene as typical VOCs oxidized to CO2 at 100 °C. This synergy was partly due to the catalytic oxidation with ozone produced in the discharge. In addition, a plausible mechanism for the plasma-driven catalysis of the VOCs was summarized by Kim et al. [71][50] in Figure 3D. The models explaining the catalytic reactions are the Langmuir–Hinshelwood (L–H) model and the Eley–Rideal (E–R) model. In the L–H model, both of the reactants need to be absorbed on the surface and followed by migration to the active site. In the E–R model, only one reactant is adsorbed on the surface and the other exists in the gas phase. The adsorbed surface species can also migrate from one site to the other and eventually disappear via recombination and chemical reactions.
Catalysts 13 01461 g003
Figure 3. (A) The working principle of the cyclic adsorption−plasma catalytic process for VOC removal [65][44]. (B) A schematic diagram of the reaction pathways for plasma destruction of CH2Cl2 in air [66][45]. (C) Reaction pathways for the oxidation of acetone on the Ag surface [69][48]. (D) A plausible mechanism for the plasma-driven catalysis of VOCs [71][50].

4. Influence Parameters of NTP

Whether in the modification process of material structure or the reaction process of pollutant gas, to maximize the performance and simultaneously reduce the energy consumption of non-thermal plasma, it is essential to determine the appropriate parameters such as discharge power, voltage, and time. Above all, the input power is one of the most important parameters affecting the adsorption/catalysis capacity of the plasma-modified materials. With the growth of power, the reactivity of the zeolites will climb up and then decline. Li et al. [72][51] found that with the enhancement of the plasma power, the NOx conversion of NaY zeolite first increased and then decreased, reaching the maximum at 7.6 W owing to the maximum number of high-energy particles produced. However, when the power was too strong, the side reaction of the NOx synthesis from substantial N2 and O2 might occur, which would reduce the NOx conversion rate. Xiang et al. [73][52] investigated the change of the hexanal removal rate of MnOx/SBA-15 with the discharge voltage under the coordination of the NTP and proved that the higher the discharge voltage, the higher the energy density, and the higher the concentration of active particles and O3 produced by the plasma field, which could promote the complete oxidation of hexanal. However, when the discharge voltage was greater than 8 kv, the hexanal removal rate of the MnOx/SBA-15 catalyst no longer changed significantly (Figure 4A).
Catalysts 13 01461 g004
图Figure 4.（ (A）己醛转化率与不同负载催化剂外加电压的关系[73]。（) Relationship between hexanal conversion and applied voltage over different loading catalysts [52]. (B）放电电压对氯苯脱除效率的影响[74]。不同等离子体处理时间的锌基催化剂在乙炔水合中乙炔（) Effect of discharge voltage on chlorobenzene removal efficiency [53]. Conversion of acetylene (C）的转化率和对乙醛（) and selectivity to acetaldehyde (D）的选择性[47]。) in acetylene hydration over Zn−based catalysts with different plasma treatment times [25].
电压是影响等离子体催化系统目标的一个影响因素，以低能源成本实现可接受的去除效率。 Voltage is an influential factor that affects the goal of a plasma catalytic system to achieve an acceptable removal efficiency with low energy cost. Zhu等[74]发现， et al. [53] found that synergy between the NTP和催化剂之间的协同作用使转化效率提高了 and catalyst enhanced the conversion efficiency by 18-40%，特别是在5 kV的放电电压下。在低电压下，高能电子和反应物质的数量相对较低。此外，氧气的浓度由–40%, especially at a discharge voltage of 5 kV. At low applied voltage, the numbers of energetic electrons and reactive species are relatively low. In addition, the concentration of oxygen resulting from O产生3分解率低，紫外线辐射太弱，无法有效活化反应器中的催化剂。因此，催化剂表面的 decomposition was low, and UV radiation was too weak to effectively activate the catalyst in the reactor. Consequently, the concentration of O原子浓度较低。当放电电压增加时，气体放电变得更加强烈，催化活性增加，从而产生更高的去除效率（图 atoms on the catalyst surface was low. When the discharge voltage increased, the gas discharge became more intense, and catalytic activity increased, resulting in higher removal efficiency (Figure 4B）。
非热等离子体的放电时间对反应也表现出重要影响，具体而言，最佳放电时间不仅可以提高沸石的反应性，还可以降低能耗。). The discharge time of non-thermal plasma also exhibits an important effect on the reaction, and specifically, the optimal discharge time can not only improve the reactivity of zeolites but also can reduce energy consumption. The catalytic performance of the Zn-based catalysts with different oxygen plasma treatment times for acetylene hydration was studied by Wang[47]研究了不同氧等离子体处理次数的Z [25]. The activity and selectivity of the catalysts were shown in基催化剂对乙炔水合的催化性能。催化剂的活性和选择性如图 Figure 4C，,D所示。当t=. When t = 30 min时，反应12 h后乙炔转化率最高，对乙醛的选择性约为80%。与t = 30 min相比，反应12 h后，催化剂的转化率和选择性降低，当, it showed that the highest conversion of acetylene was over 95% and selectivity to acetaldehyde was about 80% after 12 h of reaction. The conversion and selectivity of the catalyst were reduced after 12 h of reaction when t = 60、, 90 120 min时。可以解释为，等离子体处理的长周期可能导致活性组分的团聚或催化剂结构的破坏，从而阻碍了催化剂实现更好的催化性能。Wang等[39]证实，随着等离子体放电时间的增加， compared to t = 30 min. It may be explained that the long period of the plasma treatment may lead to the agglomeration of the active components or the destruction of the catalyst structure, thus obstructing the better catalytic performance of the catalyst to be achieved. Wang et al. [17] verified that with the increase of plasma discharge time, the naphthalene adsorption capacity of the NaY分子筛的萘吸附能力迅速增加，在 zeolites increased rapidly, achieving the maximum at 25 min时达到最大值，与未改性的NaY相比提高了56%。可归因于产生丰富的酸位点和表面化学官能团，可促进萘吸附到沸石的外表面。然而，当放电时间超过25 min时，NaY的吸附能力没有明显变化，这主要是由于活性位点的数量达到饱和，不再增加。, which was improved by 56% compared to the unmodified NaY. It could be attributed to the generation of plentiful acid sites and surface chemical functional groups, which can facilitate the adsorption of naphthalene to the external surface of zeolite.

References

  1. Ejtemaei, M.; Sadighi, S.; Rashidzadeh, M.; Khorram, S.; Back, J.O.; Penner, S.; Noisternig, M.F.; Salari, D.; Niaei, A. Investigating the Cold Plasma Surface Modification of Kaolin- and Attapulgite-Bound Zeolite A. J. Ind. Eng. Chem. 2022, 106, 113–127.
  2. Du, M. Study on the Reduction of NOx by NH3 Catalyzed by Low Temperature Plasma and Manganese Based Oxides. Master’s Thesis, Beijing University of Chemical Technology, Beijing, China, 2020.
  3. Liu, C.; Wang, J.; Yu, K. Floating double probe characteristics of non- thermal plasmas in the presence of zeolite. J. Electrostat. 2002, 54, 149–158.
  4. Mirzaei, H.; Almasian, M.R.; Moosavian, S.M.A.; Sid Kalal, H. Plasma modification of a natural zeolite to improve its adsorption capacity of strontium ions from water samples. Int. J. Environ. Sci. Technol. 2019, 16, 6157–6166.
  5. Liu, C.; Vissokov, G.P.; Jang, B.W.-L. Catalyst preparation using plasma technologies. Catal. Today 2002, 72, 173–184.
  6. Hanna, A.R.; Fisher, E.R. Tailoring the surface properties of porous zeolite constructs using plasma processing. Microporous Mesoporous Mater. 2020, 307, 110467–110478.
  7. Veerapandian, S.K.P.; Geyter, N.D.; Giraudon, J.-M.; Morin, J.-C.; Tabaei, P.S.E.; Weireld, G.D.; Laemont, A.; Leus, K.; Voort, P.V.D.; Lamonier, J.-F.; et al. Effect of non-thermal plasma in the activation and regeneration of 13X zeolite for enhanced VOC elimination by cycled storage and discharge process. J. Clean. Prod. 2022, 364, 132687–132696.
  8. El Roz, M.; Lakiss, L.; Valtchev, V.; Mintova, S.; Thibault-Starzyk, F. Cold plasma as environmentally benign approach for activation of zeolite nanocrystals. Microporous Mesoporous Mater. 2012, 158, 148–154.
  9. Wang, J.; Tang, H.; Yi, X.; Zhao, S.; Gao, F.; Yang, Z. Oxygen plasma-catalytic conversion of NO over MnOx: Formation and reactivity of adsorbed oxygen. Catal. Commun. 2017, 100, 227–231.
  10. Fahmy, A.; Elzaref, A.; Youssef, H.; Shehata, H.; Wassel, M.; Friedrich, J.; Poncin-Epaillard, F.; Debarnot, D. Plasma O2 modifies the structure of synthetic zeolite-A to improve the removal of cadmium ions from aqueous solutions. Turk. J. Chem. 2019, 43, 172–184.
  11. Liu, C.; Yu, K.; Zhang, Y.; Zhu, X.; He, F.; Eliasson, B. Characterization of plasma treated Pd/HZSM-5 catalyst for methane combustion. Appl. Catal. B Environ. 2004, 47, 95–100.
  12. Huang, X.; Yu, K.; Zhu, C.; Li, H. Progress in the reciprocal effect between plasma and catalysts. Chem. Eng. 2011, 25, 46–48+53.
  13. Taaca, K.L.M.; Vasquez, M.R. Fabrication of Ag-exchanged zeolite/chitosan composites and effects of plasma treatment. Microporous Mesoporous Mater. 2017, 241, 383–391.
  14. He, F.; Liu, C.; Eliasson, B.; Xue, B. XPS characterization of zeolite catalyst in plasma catalytic methane conversion. Surf. Interface Anal. 2001, 32, 198–201.
  15. Liu, C.; Yu, K.; Zhang, Y.; Zhu, X.; He, F.; Eliasson, B. Remarkable improvement in the activity and stability of Pd/HZSM-5 catalyst for methane combustion. Catal. Commun. 2003, 4, 303–307.
  16. Wang, Z.; Liu, Y.; Shi, P.; Liu, C.; Liu, Y. Al-MCM-41 supported palladium catalyst for methane combustion: Effect of the preparation methodologies. Appl. Catal. B Environ. 2009, 90, 570–577.
  17. Wang, H.; Xu, Q.; Zhou, X.; Liang, J.; Yuan, H.; Yang, D.; Wang, W. Highly efficient adsorptive removal of persistent organic pollutants using NPD-acid combined modified NaY zeolites. Chem. Eng. J. 2022, 431, 133858–133867.
  18. Cao, X.; Zhou, R.; Rui, N.; Wang, Z.; Wang, J.; Zhou, X.; Liu, C.-J. Co3O4/HZSM-5 catalysts for methane combustion: The effect of preparation methodologies. Catal. Today 2017, 297, 219–227.
  19. Kwak, J.H.; Peden, C.H.F.; Szanyi, J. Non-thermal plasma-assisted NOx reduction over Na-Y zeolites: The promotional effect of acid sites. Catal. Lett. 2006, 109, 1–6.
  20. Furukawa, K.; Tian, S.R.; Yamauchi, H.; Yamazaki, S.; Ijiri, H.; Ariga, K.; Muraoka, K. Characterization of HY zeolite modified by a radio- frequency CF4 plasma. Chem. Phys. Lett. 2000, 318, 22–26.
  21. Xia, Q.; Liu, C.-J.; Zhang, Y.-P.; Yu, K.-L.; Eliasson, B.; Xue, B. A plasma enhanced acidity of solid acid. In Proceedings of the CHEMRAWN XIVth Conference on Green Chemistry, Boulder, CO, USA, 9–13 June 2001.
  22. Yu, K.-L.; Xia, Q.; Liu, C.-J.; Li, G.; Eliasson, B.; Xue, B. On the plasma enhanced Bronsted acidity of solid acids. In Proceedings of the 4th International Symp, On Green Chemistry in China, Jinan, China, 9–12 May 2001.
  23. Zhu, X.; Yu, K.; Cheng, D.; Zhang, Y.; Xia, Q.; Liu, C. Modification of acidity of Mo-Fe/HZSM-5 zeolite via argon plasma treatment. Front. Chem. Eng. China 2008, 2, 55–58.
  24. Takeuchi, N.; Electrochem, J. Surface Characterization of Zeoite Catalyst Heat-treated by Microwave Cold Plasma. Soc. Jpn. 1995, 63, 164–166.
  25. Wang, Q.; Zhang, S.; Yu, Y.; Dai, B. High-performance of plasma-enhanced Zn/MCM-41 catalyst for acetylene hydration. Catal. Commun. 2020, 147, 106–122.
  26. Yoon, S.; Panov, A.G.; Tonkyn, R.G.; Ebeling, A.C.; Barlow, S.E.; Balmer, M.L. An examination of the role of plasma treatment for lean NOx reduction over sodium zeolite Y and gamma alumina Part 2. Formation of nitrogen. Catal. Today 2002, 72, 251–257.
  27. Yoon, S.; Panov, A.G.; Tonkyn, R.G.; Ebeling, A.C.; Barlow, S.E.; Balmer, M.L. An examination of the role of plasma treatment for lean NOx reduction over sodium zeolite Y and gamma alumina: Part 1. Plasma assisted NOx reduction over NaY and Al2O3. Catal. Today 2002, 72, 243–250.
  28. Zhang, Y.; Ma, P.; Zhu, X.; Liu, C.; Shen, Y. A novel plasma-treated Pt/NaZSM-5 catalyst for NO reduction by methane. Catal. Commun. 2004, 10, 35–39.
  29. Xu, J.; Tang, T.; Sheng, X.; Zhang, Y.; Zhang, Q.; Guo, F. DBD plasma activation-enhanced low temperature propylene-SCR of NOx over a low-load Mn/ZSM-5 catalyst under lean-burn conditions. J. Environ. Chem. Eng. 2022, 10, 107009.
  30. Wang, J.; Yi, H.; Tang, X.; Zhao, S.; Gao, F.; Zhang, R.; Yang, Z. Products Yield and Energy Efficiency of Dielectric Barrier Discharge for NO Conversion: Effect of Oz Content, NO Concentration, and Flow Rate. Energy Fuels 2017, 31, 9675–9683.
  31. Tang, X.; Wang, J.; Yi, H.; Zhao, S.; Gao, F.; Huang, Y.; Zhang, R.; Yang, Z. N2O Formation Characteristics in Dielectric Barrier Discharge Reactor for Environmental Application: Effect of Operating Parameters. Energy Fuels 2017, 31, 13901–13908.
  32. Yu, Q.; Gao, Y.; Tang, X.; Yi, H.; Zhang, R.; Zhao, S.; Gao, F.; Zhou, Y. Removal of NO from flue gas over HZSM-5 by a cycling adsorption-plasma process. Catal. Commun. 2018, 110, 18–22.
  33. Tang, X.; Wang, J.; Yi, H.; Zhao, S.; Gao, F.; Chu, C. Nitrogen Fixation and NO Conversion using Dielectric Barrier Discharge Reactor: Identification and Evolution of Products. Plasma Chem. Plasma Process. 2018, 38, 485–501.
  34. Kim, H.H.; Takashima, K.; Katsura, S.; Mizuno, A. Low-temperature NO, reduction processes using combined systems of pulsed corona discharge and catalysts. J. Phys. D Appl. Phys. 2001, 34, 604613.
  35. Schmidt, M.; Basner, R.; Brandenburg, R. Hydrocarbon assisted NO oxidation with non-thermal plasma in simulated marine diesel exhaust gases. Plasma Chem. Plasma Process. 2013, 33, 323–335.
  36. Fan, H.-Y.; Shi, C.; Li, X.-S.; Yang, X.-F.; Xu, Y.; Zhu, A.-M. Low-temperature NOx Selective Reduction by Hydrocarbons on H-Mordenite Catalysts in Dielectric Barrier Discharge Plasma. Plasma Chem. Plasma Process. 2009, 29, 43.
  37. Niu, J.; Yang, X.; Zhu, A.; Shi, L.; Sun, Q.; Xu, Y.; Shi, C. Plasma-assisted selective catalytic reduction of NOx by C2H2 over Co-HZSM-5 catalyst. Catal. Commun. 2006, 7, 297–301.
  38. Byun, Y.; Ko, K.B.; Cho, M.; Namkung, W.; Shin, D.N.; Koh, D.J. Effect of hydrogen generated by dielectric barrier discharge of NH3 on selective non-catalytic reduction process. Chemosphere 2009, 75, 815–818.
  39. Singh, D.; Kumar, A.; Kumar, K.; Singh, B.; Mina, U.; Singh, B.B.; Jain, V.K. Statistical modeling of O3, NOx, CO, PM2.5, VOCs and noise levels in commercial complex and associated health risk assessment in an academic institution. Sci. Total Environ. 2016, 572, 586–594.
  40. Naydenov, A.; Mehandjiev, D. Complete oxidation of benzene on manganese dioxide by ozone. Appl. Catal. A Gen. 1993, 97, 17.
  41. Ogata, A.; Kim, H.H.; Futamura, S.; Kushiyama, S.; Mizuno, K. Effects of catalysts and additives on fluorocarbon removal with surface discharge plasma. Appl. Catal. B Environ. 2004, 53, 175.
  42. Konova, P.; Stoyanova, M.; Naydenov, A.; Christoskova, S.; Mehandjiev, D. Catalytic oxidation of VOCs and CO by ozone over alumina supported cobalt oxide. Appl. Catal. A Gen. 2006, 298, 109.
  43. Roland, U.; Holzer, F.; Kopinke, F.-D. Improved oxidation of air pollutants in a non-thermal plasma. Catal. Today 2002, 73, 315.
  44. Veerapandian, S.K.P.; Geyter, N.D.; Giraudon, J.-M.; Lamonier, J.-F.; Morent, R. The Use of Zeolites for VOCs Abatement by Combining Non-Thermal Plasma, Adsorption, and/or Catalysis: A Review. Catalysts 2019, 9, 98.
  45. Wallis, A.E.; Whitehead, J.C.; Zhang, K. The removal of dichloromethane from atmospheric pressure nitrogen gas streams using plasma-assisted catalysis. Appl. Catal. B Environ. 2007, 72, 282–288.
  46. Oh, S.M.; Kim, H.H.; Einaga, H.; Ogata, A.; Futamura, S.; Park, D.W. Zeolite-combined plasma reactor for decomposition of toluene. Thin Solid Films. 2006, 506–507, 418–422.
  47. Teramoto, Y.; Kim, H.-H.; Negishi, N.; Ogata, A. The role of ozone in the reaction mechanism of a bare zeolite-plasma hybrid system. Catalysts 2015, 5, 838–850.
  48. Trinh, Q.H.; Gandhi, M.S.; Mok, Y.S. Adsorption and plasma-catalytic oxidation of acetone over zeolite-supported silver catalyst. Jpn. J. Appl. Phys. 2015, 54, 01AG04.
  49. Francke, K.P.; Miessner, H.; Rudolph, R. Plasmacatalytic processes for environmental problems. Catal. Today 2000, 59, 411–416.
  50. Kim, H.H.; Ogata, A.; Futamura, S. Oxygen partial pressure-dependent behavior of various catalysts for the total oxidation of VOCs using cycled system of adsorption and oxygen plasma. Appl. Catal. B Environ. 2008, 79, 356–367.
  51. Li, D.; Tang, X.; Yi, H.; Ma, D.; Gao, F. NOx decomposition over modified NaY zeolite by dielectric barrier discharge plasma in the presence of excess oxygen. Acta Sci. Circumstantiae 2015, 35, 4088–4094.
  52. Xiang, D.; Zhao, G.; Ye, D. Study on factors influencing Hexanal oxidation by plasma-assisted MnOx/SBA-15 catalysis. Sci. Online 2010, 5, 355–359.
  53. Zhu, R.; Mao, Y.; Jiang, L.; Chen, J. Performance of chlorobenzene removal in a nonthermal plasma catalysis reactor and evaluation of its byproducts. Chem. Eng. J. 2015, 279, 463–471.
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