Mechanism of Zn salt-induced deactivation of a Cu/activated carbon catalyst for low-temperature denitration via CO-SCR

In the process of industrial flue gas denitration, the presence of heavy metals, especially Zn salts, is known to lead to the deactivation of the denitration catalysts. However, the specific mechanism of the catalyst deactivation remains unclear. In this paper, the mechanism of the ZnCl2- and ZnSO4-induced deactivation of low-temperature denitration catalysts in the carbon oxide (CO) selective catalytic reduction (CO-SCR) reaction was investigated using a Cu/activated carbon (AC) catalyst, in which HNO3/AC was used as the carrier. Cu/AC, ZnCl2–Cu/AC, and ZnSO4–Cu/AC catalysts were prepared by the incipient wetness impregnation method. The physicochemical properties of the catalyst were examined via the Brunauer–Emmett–Teller method, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analyses, which proved the mechanism of catalyst denitrification and enabled the elucidation of the toxicity mechanism of the Zn salts on the Cu/AC catalyst for CO-SCR denitration at low temperatures. The results show that Zn doping reduces the physical adsorption of CO and NO and decreases the concentration of Cu2+ and chemisorbed oxygen (Oβ), leading to the reduction of active sites and oxygen vacancies, thus inhibiting the denitration reaction. Moreover, ZnCl2 is more toxic than ZnSO4 because Cl− not only occupies oxygen vacancies but also inhibits Oβ migration. In contrast, SO42− increases the surface acidity and promotes Oβ supplementation. This study can provide a reference for the development of CO-SCR denitration catalysts with high resistance to Zn salt poisoning.


Introduction
NO x is one of the major air pollutants emitted during fuel combustion. It causes environmental problems such as acid rain, ozone depletion, and photochemical smog, constituting a serious threat to the health of living beings on earth. [1][2][3] As a result of the increasingly restrictive regulations on NO x emissions worldwide, the reduction of NO x emissions has received extensive attention from academia and industry. Currently, selective catalytic reduction technology (SCR) is one of the most widely used methods for the effective removal of NO x emissions from ue gas. Conventional SCR uses NH 3 as the reducing agent, which has good reducibility and catalysts with high denitration efficiency. 4,5 However, NH 3 is a costly and toxic gas whose leakage may cause serious problems, including chemical accidents, catalyst poisoning, and pipeline corrosion. 6 To circumvent these issues, the development of new reducing agents has attracted increasing attention.
The CO has the advantages of strong reducibility, low cost, and easy availability; it can simultaneously remove CO and NO from industrial waste gas at low temperatures, which is considered as a possible technology for large-scale applications. [7][8][9][10] Unfortunately, the currently available CO-SCR denitration catalysts still present some application bottlenecks, such as low catalytic denitration capacity at low temperature, low oxidation resistance, and insufficient anti-toxicity ability against SO 2 , which hinder the practical application of this technology in industry. 11 Consequently, the development of low-temperature denitration catalysts with excellent activity has become the focus of research in this eld. Denitration catalysts are mainly composed of a carrier and an active component. As a catalyst carrier, activated carbon (AC) stands out because of its low cost, high content in oxygen-containing functional groups, huge specic surface area, excellent lowtemperature activity, and stable chemical properties. According to a study, 12 HNO 3 activation treatment was conducted on the surface of AC, and pore volume and specic surface area were signicantly increased. The increase of acidic functional groups on the surface resulted in AC having better NO x adsorption and removal ability. In addition, loading metals on AC enhance catalytic activity, and precious metals such as Pt, 13 Ru, 14 Pd, 15 Rh, 16 and Au 17 have been identied as good catalysts for CO denitration. However, precious metals have limited resources, high cost, low thermal stability, and limited development, rendering transition metals such as Fe, Ni, Co, Mn, and Cu as an attractive alternatives. 18,19 Among them, Cu has a high catalytic activity for the CO-SCR reaction and for NO x decomposition, and different Cu species have been proved to be the active sites for NO x adsorption. [20][21][22] Previous studies 23 have demonstrated the excellent performance of a Cu/AC catalyst in CO-SCR denitration; therefore, this catalyst is expected to enable the denitration process at low temperatures. However, the presence of a large amount of dust and metals (Zn, Pb, Ca, As) in the ue gas may block the catalyst pores, leading to catalyst deactivation. [24][25][26] Thus, the effect of heavy metals on denitration catalysts is attracting increasing research attention. In this context, Qi et al. 27 proved that Pb can reduce the amount of chemisorbed oxygen by covering the surface active sites of the catalyst, thus reducing the surface acidity and reducibility. Su et al. 28 studied the inuence of Pb on a Ce-Mn/AC catalyst, nding that the selective reduction activity of the catalyst was signicantly reduced aer doping PbO due to the signicant reduction of the total pore volume and oxygen functional groups of AC, which increased the oxide crystallization and reduced the content of Mn 4+ and chemisorbed oxygen. Zhu 29 found that Pb doping of a 3Mn10Fe/Ni catalyst altered the content of high-valence metal elements such as Fe 3+ and Mn 4+ , resulting in the reduction of the lattice oxygen concentration, the performance, and the acidity of acid sites. Guo et al. 30 studied the catalytic toxicity of Zn and Pb on Mn/TiO 2 , and the results showed that catalyst deactivation may be caused by the growth of TiO 2 crystals, the reduction of the redox capacity, the reduction of the surface acidity and NO adsorption energy, and the reduction of surface active substances such as Mn 4+ and chemisorbed oxygen. Su et al. 31 investigated the inuence of Zn on the deactivation and toxicity mechanism of an Mn-Ce/AC catalyst and found that doping Zn salts reduced the physical adsorption capacity of the catalyst for NH 3 and NO and increased the crystallization of Mn and Ce oxides, thereby reducing the interaction with the adsorptive gas. Meanwhile, competitive adsorption on the acidic sites of the catalyst surface was observed between Zn salts and NH 3 , which resulted in a reduction of the denitration activity. However, despite the progress made in the investigation of the toxicity on catalysts, the toxicity mechanism for low-temperature CO-SCR denitration catalysts is still unclear. In particular, unveiling the mechanism of catalyst deactivation by Zn is important because Zn is one of the main components in ue gas.
Herein, to explore the toxicity mechanism of Zn salts on low-temperature CO-SCR denitration catalysts, a Cu/AC catalyst was prepared and was poisoned by ZnCl 2 and ZnSO 4 , respectively. The CO-SCR denitration efficiency was studied by comparing the conversion rate of NO. The toxicity mechanism of Zn salts on the Cu/AC catalyst was studied by performing scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy analyses. The presented results can pave the way for the development of CO-SCR denitration catalysts with high resistance to Zn salt poisoning.

Catalyst preparation
To prepare AC, coconut shell AC particles (coconut shell AC, particle size 10-20 mesh, Henan Gongyi Blue Sky water Purication Technology Co., Ltd) were washed with deionized water to neutral to remove ash and suspended solids, ultrasonicated in a water bath at 60 C for 2 h, and then placed in an airblowing drying oven at 110 C for 24 h aer pumping and ltering. 23 Then, HNO 3 /AC was prepared by impregnating the as-prepared AC in an equal volume of a 30% HNO 3 solution (Zhejiang Shiping Chemical Reagent Factory), followed by reuxing for 2 h in a three-necked ask at 80 C and drying for 24 h in a drying oven at 110 C. 23 Three samples with a mass of 40 g HNO 3 /AC were weighed using an analytical balance, and Cu(NO 3 ) 2 $3H 2 O (AR, Tianjin Kermio Chemical Reagent Co., Ltd) as the precursor was dissolved in a certain amount of deionized water and impregnated for 2 h under ultrasonic vibration at a constant temperature of 60 C, and the samples were then drained and dried at 110 C for 24 h. Finally, the samples were calcined under N 2 protection at 450 C for 4 h and labeled Cu/AC. To prepare Zn-poisoned catalysts with a Zn/Cu molar ratio of 1 : 2, 52 mL ZnCl 2 and ZnSO 4 aqueous solutions were impregnated into Cu/AC catalysts and ultrasonicated for 2 h. 30,31 Then, the poisoned catalysts were drained, dried at 110 C for 24 h, and roasted at 450 C for 4 h under N 2 protection. The obtained samples were labeled ZnCl 2 -Cu/AC and ZnSO 4 -Cu/AC, respectively. The chemical composition of Cu/AC catalyst and zinc salt poisoning catalyst is shown in Table 1.

Determination of the denitration activity of the catalyst
The CO-SCR denitration experiment and Zn salt poisoning of the Cu/AC catalyst were conducted in a xed-bed reactor. The reaction device is shown in Fig. 1. In the experiment, 8 g of catalyst sample (Cu/AC, ZnCl 2 -Cu/AC, and ZnSO 4 -Cu/AC, respectively) was placed in the reactor, and the reaction temperature was set by adjusting the furnace temperature, and the simulated ue gas was injected into the reactor. The simulated ue gas consisted of an NO ow rate of 4 mL min À1 , a CO ow rate of 40 mL min À1 , an O 2 volume concentration of 9%, an N 2 total ow rate of 500 mL min À1 , and a gas hourly space velocity of 3000 h À1 . The denitration tail gas was detected using a Testo-340 ue gas analyzer (German Detu Instrument Company). The catalytic denitration activity was evaluated using NO conversion and N 2 selectivity according to the following calculation method (eqn (1) and (2)).

Catalyst characterization
The surface morphology of the catalysts was observed by SEM (Tescan VEGAS SBH). The distribution and content of metal elements on the catalyst surface were analyzed using a Tescan VEGAS SBH (EDS) device. BET/Barrett-Joyner-Halenda (BJH) experiments were performed on an N 2 adsorption and desorption tester (qds-evo). The specic surface area, pore volume, and average pore size of the catalysts were measured at 77 K according to the N 2 adsorption isotherm. XRD (TTR18 kW Cu target) was used to analyze the crystal phase pattern of the supported metal oxides. The surface atomic states of the catalysts were analyzed by XPS (PHI5000 Versaprobe-II). Al K a X-ray radiation (hv ¼ 1486.6 eV) was used to measure the surface atomic states of the catalysts at 50 W, and the binding energy (BE) was calibrated using the C 1s BE value of 284.8 eV. The changes in the functional groups on the catalyst surface were investigated using an FTIR spectrometer (Nicolet iS 10) in a range of 4000-400 cm À1 .

Results and discussion
3.1 CO-SCR denitration activity Fig. 2(a) shows the NO conversion of the Cu/AC and Zn saltpoisoned catalysts at different reaction temperatures. The Cu/ AC catalyst has a high denitration rate, with the maximum NO conversion rate reaching 80% at 200 C. When the temperature exceeds 250 C, the reduction of denitration efficiency of Cu/AC catalyst is caused by the competitive adsorption of CO and NO. Meanwhile, the denitration rate of ZnCl 2 -Cu/AC and ZnSO 4 -Cu/AC decreases to 44% and 60%, respectively, at the same temperature, which suggests that ZnCl 2 and ZnSO 4 are toxic to the Cu/AC catalyst. In addition, the denitration rate of the ZnCl 2 -Cu/AC catalyst is always lower than that of the ZnSO 4 -Cu/AC catalyst at the same temperature, indicating that the denitration activity decays faster aer ZnCl 2 doping. Fig. 2(b) shows the N 2 selectivity at different temperatures, which follows the order Cu/AC > ZnSO 4 -Cu/AC > ZnCl 2 -Cu/AC. According to a study, 31 the reason was analyzed that ZnSO 4 can improve the surface acidity, thereby enhancing the N 2 selectivity. The different toxicity and denitration activity of the two Zn salts is most likely caused by the difference in their constituent anions.

Surface morphology and load
Fig . 3 shows the SEM images of the Cu/AC and Zn salt-poisoned catalysts. As can be seen from Fig. 3(a) and (b), the Cu/AC catalyst has a good pore structure, and metal oxide particles of different sizes are distributed on the catalyst surface. Aer magnication by 5000 times (Fig. 3(b)), the pore structure, which is benecial for the adsorption of the reaction gas, is still clearly visible. Fig. 3(c)-(e) show that most of the surface wall of the ZnCl 2 -Cu/AC catalyst collapses, and the pore structure is seriously damaged, indicating that the AC structural strength decreases aer ZnCl 2 doping. Fig. 3(f)-(h) shows that the ZnSO 4 -Cu/AC catalyst matrix is damaged to a certain extent, the surface is rough, the pore structure is almost invisible, and only large metal oxide particles are attached at certain positions, with local accumulation. This agglomeration phenomenon occurs on the catalyst surface aer ZnSO 4 doping, which is not  conducive to the adsorption of CO and NO gas, leading to a decrease in the denitration activity. 27 Next, the elemental composition of the catalysts was investigated via EDS characterization. The results are shown in Fig. 4, in which the EDS spectra (a), (b), and (c) correspond to Cu/AC surface spectrogram 1, ZnCl 2 -Cu/AC surface spectrogram 2, and ZnSO 4 -Cu/AC surface spectrogram 3, respectively. The obtained elements and their contents are summarized in Table 2, which shows that the surface of the three catalysts contains Cu, 9.09%, 14.58%, and 9.87%, respectively, con-rming the successful loading of the active component Cu. Similarly, Zn was successfully loaded on the surface of the ZnCl 2 -and ZnSO 4 -doped catalysts at an amount of 3.53% and 4.37%, respectively, and the pores on the catalyst surface were clearly observed by SEM aer doping the Zn salt. According to the denitration curve in Fig. 2(a), the denitration activity decreased signicantly, inferring that zinc salt affected the denitration activity of the catalyst by reducing the adsorption performance of the catalyst. Table 3 shows the BET characterization results of the Cu/AC and Zn salt-poisoned catalysts. The specic surface area of the Cu/ AC catalyst is 631 m 2 g À1 , whereas that of ZnCl 2 and ZnSO 4 decreases to 538 and 600 m 2 g À1 , respectively. The specic surface area and pore structure of the catalyst affected the SCR denitrication activity to a certain extent, which was consistent with the denitrication activity shown in Fig. 2. This result is consistent with the denitration activity shown in Fig. 2. Fig. 5(a) shows that the relative pressures of the adsorption-desorption curves of the Cu/AC and Zn salt-poisoned catalysts are between 0.4 and 0.8. According to the IUPAC, all samples exhibited typical type IV adsorption isotherms and type H4 hysteresis rings, 32 which are consistent with a narrow aperture and stem from the layered structure. The results suggest the presence of a large number of micropores and mesopores in the catalyst,  but the total pore volume and average pore size decrease aer Zn doping. Fig. 5(b) shows the average pore size distribution of the catalysts. The corresponding pore size distribution curve was determined according to the BJH method using the adsorption branch of the isotherm. The average pore size of Cu/ AC, ZnCl 2 -Cu/AC, and ZnSO 4 -Cu/AC is 1.96, 1.95, and 1.90 nm, respectively. The specic surface area, total pore volume, and average pore size decreased slightly aer doping zinc salt, which can be mainly attributed to the Zn salt particles covering the catalyst surface, blocking the pores, and reducing the contact area between the catalyst and the adsorbed gas. 30,31 In addition, the total pore volume decreases the most for the ZnCl 2 -doped catalyst, which may be caused by the collapse of the catalyst surface. The average pore diameter of the ZnSO 4 -Cu/AC catalyst is the smallest, mainly due to the blockage of the catalyst pore structure by metal particles. Nevertheless, the total pore volume is large, most likely because SO 4 2À generates new pores and removes ash from the original pores during the poisoning process, 33 which is consistent with the SEM results. The decrease in the specic surface area and the average pore size of the catalyst is not conducive to the gas mass transfer, adsorption, and activation during the reaction process, resulting in the deactivation of the catalyst. 34 3.4 Phase structure Fig. 6 shows the XRD patterns of the Cu/AC and Zn saltpoisoned catalysts. The diffraction peak observed at 20 -30 can be attributed to the (002) crystal plane of graphite microcrystals with a layered structure (JCPSF 13-0148). 35

Valence state of the element
The active component valence and element concentration of the catalysts were studied by XPS, and the results are shown in Table 4. Fig. 7(a) shows the O 1s XPS spectra of the Cu/AC and Zn salt-poisoned catalysts. The tted peaks can be assigned to the three types of oxygen atoms: lattice oxygen O a (approx. 529.0-530.0 eV), chemisorbed oxygen O b (approx. 531.3-531.9 eV), and hydroxyl O g (approx. 532.7-533.5 eV). 38 As can be extracted from Table 4 and Fig. 7(a), Zn doping leads to   39 In addition, a higher percentage of O b contributes to the oxidation of NO to NO 2 , and the SCR activity is enhanced by a "rapid SCR" reaction (CO + NO + NO 2 / 2N 2 +2CO 2 ). As shown in Table 4, the concentration of O b decreases signicantly aer Zn doping, indicating that the Zn salt can inhibit the formation of O b , which reduces the denitration efficiency of the catalyst. Aer ZnCl 2 doping, the concentration of O b is lower than that of the ZnSO 4 -doped catalyst, which is detrimental to the SCR denitration reaction and in accord with the results of the CO-SCR denitration activity. Fig. 7(b) shows the XPS spectra of the Cu/AC and Zn saltpoisoned catalysts. The peaks at binding energies of $933.2 and $953.1 eV correspond to Cu 2p 3/2 and Cu 2p 1/2 of Cu 0 , respectively. The energy difference between the two spin states is 19.9 eV due to spin-orbit coupling. Meanwhile, the peaks at $934.2 and $954.2 eV correspond to Cu 2p 3/2 and Cu 2p 1/2 of Cu 2+ , respectively, 40 and those at $932.1 and $951.5 eV can be attributed to Cu 2p 3/2 and Cu 2p 1/2 of Cu + , respectively. 41 Most of the NO adsorbed on Cu 2+ can be quickly converted to N 2 , and Cu 2+ has a good catalytic effect on the removal of NO; therefore, the high catalytic activity of Cu 2+ is conducive to the adsorption and activation of NO and CO. Fig. 7(b) shows that aer ZnCl 2 and ZnSO 4 doping, the Cu 2+ content on the surface of the Cu/AC catalyst decreases from 44.37% to 15.95% and 23.42%, respectively, indicating that the Zn salt doping decreases the proportion of Cu 2+ , thus reducing the denitration capacity. 42 In addition, the Cu 2+ content on the ZnCl 2 -Cu/AC catalyst exhibits the lowest Cu 2+ content and the highest toxicity. 43 According to previous studies, SO 4 2À ions can promote the continuous 31 which is in agreement with the lower toxicity of the ZnSO 4 -doped catalyst compared with the ZnCl 2 -Cu/AC catalyst. Fig. 7(c) shows the spectra of Zn 2p XPS spectra as a Zn saltpoisoned catalysts. The peaks at $1022.2 and $1045.3 eV correspond to Zn 2p 3/2 and Zn 2p 1/2 , respectively, 30 indicating that Zn is in the divalent state in the Zn salt-poisoned catalysts. It is clear from the gure that the Zn 2+ content is lower in ZnCl 2 -Cu/AC than in ZnSO 4 -Cu/AC. This is consistent with the EDS result, suggesting that Zn 2+ occupies the active site and oxygen vacancies, 42 thus affecting the denitration activity. Fig. 8 shows the FTIR spectra of the Cu/AC and Zn salt-poisoned catalysts. The absorption peak at 3414 cm À1 is generally attributed to the stretching vibration of carboxyl and O-H functional groups. 44,45 The peak at 1618 cm À1 is due to the C]O stretching vibration of the aliphatic group. 46 The absorption peak at 1390 cm À1 is ascribable to the C-H stretching vibration of naphthenes and aliphatic hydrocarbons. 46 The absorption peak at 1128 cm À1 is due to the C-O stretching vibration of the C-O-C bonds of functional groups such as lactone, phenol, and ether on the AC surface. 46,47 The stretching vibration peaks of the carboxyl and O-H groups at 3412 cm À1 weaken aer the Zn salt doping, which may be caused by substituting H atoms of the -OH group by Zn resulting in a decrease in the number of active sites for the adsorption reaction. 31 Furthermore, the peak corresponding to the asymmetric vibration of the lactone group at 1621 and 1625 cm À1 weakens signicantly. Teng et al. 48 found that NO adsorption occurs in the C]O moiety and surface oxygen functional groups can improve the dispersion of active components and form CO adsorption sites, whereas Zn 2+ decreases the number of C]O groups. The intensity of the C-O stretching vibration peak at 1138 and 1140 cm À1 decreases, which might be due to the occupation of oxygen vacancies or active sites. 28 In summary, the catalyst deactivation aer Zn salt doping is attributed to the occupation of oxygen vacancies by Zn 2+ that reduces the number of oxygen-containing functional groups such as carboxyl and lactone groups, which in turn decreases the surface acidity and active sites. Simultaneously, the introduced Zn interacts strongly with Cu species, disrupting the Cu-O-H and Cu-C]O structures. 49 Moreover, Cl À occupies the surface active sites of the catalyst, severely inhibiting the uidity of O b and enhancing the toxicity, which is consistent with the XPS results. Additionally, SO 4 2+ increases the Brønsted acidity in the catalyst surface and offsets the destruction of oxygen-containing functional groups by Zn 2+ . 49 Therefore, the denitrication rate of ZnCl 2 -Cu/AC is lower than that of ZnSO 4 -Cu/AC, and the gap becomes larger with the increase in temperature, which may be one of the reasons for the higher toxicity of ZnCl 2 than ZnSO 4 .

Mechanism of catalytic denitration
The low-temperature denitration via CO-SCR reaction over the Cu/AC catalyst surface is a representative heterogeneous catalytic system that follows the Langmuir-Hinshelwood reaction mechanism. 50 Accordingly, the denitration mechanism depicted in Fig. 9 for the CO reduction of NO over the Cu/AC catalyst was proposed. The active component Cu is uniformly dispersed on the AC, which increases the specic surface area of the catalyst, as shown in Table 4, and enhances the adsorption of NO and CO. This conclusion indicates that Cu is uniformly distributed on the surface of the Cu/AC catalyst in Fig. 2, which improves the denitration efficiency up to 80%. CO is adsorbed on the catalyst surface to reduce -O-Cu 2+ to ]Cu d+ and generate CO 2 (eqn (6)- (8)). The adsorbed NO dissociates into N and O, and the dissociated N atom combines with an NO molecule to form N 2 O (eqn (9)-(11)). 51 It has been reported that the dissociation of NO is a key step for the CO-induced elimination of NO. 52 The release of the active sites promotes the adsorption of CO, and nally, N 2 O and ]Cu d+ react to generate N 2 and -O-Cu 2+ (eqn (12)), which further promotes the denitration reaction. In the reaction of CO with NO, the Cu 2+ 4 Cu d+ exchange may alter the valence state of the catalyst surface, and more oxygen vacancies may be available for the conversion process, promoting the conversion of NO to N 2 . This is consistent with Pan's proposal of the Mars-van Krevelen mechanism (redox process). 53 3.8 Mechanism of Zn salt poisoning on the Cu/AC catalyst On the basis of clarifying the denitrication mechanism, the toxicity mechanism of Zn salt on Cu/AC catalyst was further studied. According to the results presented in Table 2 and Fig. 3, the Cu/AC catalyst has a good pore structure with the active component Cu evenly dispersed on the surface of the catalyst, which is advantageous for the adsorption and loading of the reactive gas. However, the Zn salt doping on the surface of the catalyst destroys the pore structure of the catalyst. As can be extracted from Table 3 and Fig. 5, the specic surface area, total pore volume, and average pore size of the Zn salt-poisoned catalyst decrease, thus signicantly reducing the physical adsorption capacity for CO and NO. Fig. 6 shows that with the Zn salt doping, the crystallization and agglomeration of CuO and Cu 2 O appear on the catalyst surface, not only blocking the pores and occupying the active site but also hindering the participation of some active components in the denitration reaction. As can be seen from Table 4 and Fig. 7, the content of effective active component Cu 2+ on the surface of the catalyst doped with Zn salt decreases, resulting in a decrease in oxygen vacancies and O b , in the interaction with CO and NO, in the rapid SCR (eqn (5)-(9)), and in the concentration of intermediate N 2 O. This inevitably reduces the denitration efficiency of the catalyst. 53 ZnCl 2 occupies the oxygen vacancies and active sites and reacts with -OH and C]O to reduce the oxygencontaining functional groups and the surface acidity of the catalyst (Fig. 8). Cl À not only occupies the oxygen vacancies but also inhibits the uidity of O b and enhances the toxicity. The BET results show that the specic surface area of the catalyst is signicantly reduced aer ZnCl 2 doping. In the ZnSO 4 -Cu/AC catalyst, new pores are generated by the reaction with SO 4 2À and the ash content of the original pores is removed. The average pore size decreases, but the specic surface area and total pore volume increase compared with those of ZnCl 2 -Cu/ AC. The results of the N 2 selectivity and XPS analysis show that ZnSO 4 occupies the oxygen vacancies but increases the surface acidity and promotes the supplement of O b , thus alleviating the toxicity of the catalyst. As a result, ZnCl 2 is more toxic than ZnSO 4 . On the basis of the above characterization results and the denitration mechanism, the mechanism shown in Fig. 10 was proposed to explain the toxicity of ZnCl 2 and ZnSO 4 on the Cu/AC catalyst. On the basis of the aforementioned characterization and analysis results, the following toxicological mechanisms of zinc salts on Cu/AC catalysts were proposed: ZnCl 2 interacts with Cu d+ -OH to form a Cu d+ -O-Zn complex and H + and Cl À react to form HCl (eqn (10)), which then reacts with Cu d+ -C]O to form In conclusion, Zn 2+ occupies the oxygen vacancies on the surface of the catalyst, which inhibits the adsorption of CO and NO. Meanwhile, the dissociation of NO is inhibited and the intermediate N 2 O decreases, resulting in the reduction of active sites and inhibition of the adsorption of CO. The Cu-O-H and Cu-C]O structures are destroyed by Zn salts, and the number of oxygen-containing functional groups, such as carboxyl and lactone, decreases, thereby decreasing surface acidity and active sites. It is worth noting that the presence of Cl À can severely inhibit the O b mobility, resulting in serious toxicity.

Conclusions
The mechanism of zinc salt on Cu/AC catalyst CO-SCR denitri-cation at low temperatures was discussed as follows. Zn 2+ competes with CO and NO for the active sites for CO adsorption, which reduces the physical adsorption capacity for CO and NO. The agglomeration of CuO and Cu 2 O on the catalyst surface and the blockage of the pores by Zn particles damage the pore structure and decrease the specic surface area. Due to the interaction between Zn and Cu oxides, Zn 2+ reacts with Cu-O-H and Cu-C]O, resulting in the reduction of oxygen-containing functional groups and the active sites and oxygen vacancies. The Zn doping decreases the concentration of Cu 2+ and O b and inhibits the denitration reaction. In addition, the toxicity of ZnCl 2 is more serious than that of ZnSO 4 because Cl À not only occupies oxygen vacancies but also inhibits the O b migration. In contrast, SO 4 2À increases the surface acidity and promotes the O b supplementation. In conclusion, in addition to the reduction of the specic surface area, the decrease in the amount of Cu 2+ and O b is the main reason for the deactivation of the CO-SCR catalyst at low temperatures.

Conflicts of interest
There are no conicts to declare.