Experimental Research on Mercury Catalytic Oxidation over Ce Modified SCR Catalyst

In order to improve the ability of SCR catalyst to catalyze the oxidation of gaseous elemental mercury, a series of novel Ce modified SCR (Selection Catalytic Reduction, V2O5–WO3/TiO2) catalysts were prepared via two-step ultrasonic impregnation method. The performance of Ce/SCR catalysts on Hg oxidation and NO reduction as well as the catalytic mechanism on Hg oxidation was also studied. The XRD, BET measurements and XPS were used to characterize the catalysts. The results showed that the pore volume and pore size of catalyst was reduced by Ce doping, and the specific surface area decreased with the increase of Ce content in catalyst. The performance on Hg oxidation was promoted by the introduction of CeO2.Ce1/SCR (1%Ce, wt.%) catalyst exhibited the best Hg oxidation activity of 21.2% higher than that of SCR catalyst at 350 C, of which the NO conversion efficiency was also higher at 200–400 C. Furthermore, Ce1/SCR showed a better H2O resistance but a slightly weaker SO2 resistance than SCR catalyst. The chemisorbed oxygen and weak absorbed oxygen on the surface of catalyst were increased by the addition of CeO2. The chemisorbed oxygen and weak absorbed oxygen on the surface of catalyst were increased by the addition of CeO2. The Ce1/SCR possed better redox ability compared with SCR catalyst. HCl was the most effective gas responsible for the Hg oxidation, and the redox cycle (V + Ce 4 V + Ce) played an important role in promoting Hg oxidation.


Introduction
Mercury is considered an important environmental pollutant due to its toxic, bioaccumulative and longdistance transport properties. The global mercury emissions reached 6,500 ton/year in 2014, and it will reach 8,600 tons per year in 2035 according to the ECHMERIT model [1]. In 2013, more than 80 countries and regions, including China, signed the Minamata Convention, which sets the binding standards for mercury-containing products and mercury emissions to atmosphere. Data show that over 3,000 tons of mercury from burning coal are emitted per year globally, and more than 90% of mercury from coal-fired power plants is released into the atmosphere [2]. After coal combustion, mercury mainly exists in coal cinder, fly ash and flue gas, and the content is about 2%, 23.1%∼26.9% and 56.3%∼69.7%, respectively [3]. The average distribution of mercury in flue gas was 56% elemental mercury (Hg 0 ), 34% bivalent mercury (Hg 2+ ) and 10% particulate mercury (Hg P ) [4]. Hg 2+ is soluble and easy to react with limestone, more than 80% of which can be removed by wet limestone-gypsum FGD device [5]. About 90% of Hg P can be captured by electrostatic precipitator, bag filter and other dust removal equipment [6]. However, the air pollution control device (APCDs) has little effect on Hg 0 removal [7]. After long-term screening, SCR catalyst with the component of V 2 O 5 -WO 3 -TiO 2 is the most widely used in coal-fired power plants. It is found that SCR catalyst can catalyze the conversion of Hg 0 to Hg 2+ , which can be removed by subsequent dedusting and FGD equipment [8]. However, researches showed that Hg 0 oxidation over SCR catalyst was limited, highly dependent on the concentration of HCl in flue gas with poor water resistance [9].
Doping metal oxides into SCR catalysts to improve the oxidation effect of mercury without decreasing the denitrification efficiency has become the focus of current research. CeO 2 could provide significant oxygen storage capability through the redox shift between the two oxidation states (Ce 3+ and Ce 4+ ), which is helpful to enhance catalytic activity [10]. It was found that the catalytic activity of Ce/SCR catalyst was higher than that of Fe/SCR, Mn/SCR, Cu/SCR and Co/SCR catalysts [11]. The research of Ce/SCR catalyst is still at the initial stage, the optimal temperature for catalytic performance of Hg 0 oxidation over Ce/SCR catalyst is low due to the large Ce doping amount and the catalyst performs poorly at the normal operating temperature (280 C-420 C) in power plant [12]. Within the operating temperature range of SCR catalyst in power plant, the optimum amount of Ce doping have not been determined at present [11][12][13][14]. Some research such as the effect of flue gas and the mechanism on Hg 0 oxidation over Ce modified SCR catalysts also need to be studied.
In this study, a series of CeO 2 modified support on V 2 O 5 -WO 3 /TiO 2 catalysts were made by ultrasonicassisted impregnation method. The Hg 0 and NO oxidation efficiencies of the catalysts were tested under simulated flue gas. Besides, the effects of individual flue gas components on Hg 0 oxidation were also evaluated. Furthermore, X-ray diffraction (XRD), Brunner−Emmet−Teller (BET) and X-ray photoelectron spectroscopy (XPS) were explored to characterize the catalysts, and the reaction mechanism was discussed based on the experimental and characterization results.

Catalyst Preparation
Cerium nitrate which was the precursor of CeO x will react with ammonium metavanadate precursor or ammonium tungstate precursor to produce cerium vanadate or cerium tungstate precipitation, affecting the uniformity of V 2 O 5 and CeO 2 on the surface of TiO 2 . Therefore, modified catalysts were prepared by multi-step ultrasonic impregnation method based on the common commercial SCR catalysts (containing 1%V 2 O 5 , 9%WO 3 and 90%TiO 2 ). TiO 2 was added into mixed solutions of ammonium metavanadate, ammonium metatungstate and oxalic acid. The slurry was exposed to an ultrasonic bath for 4 h, dried at 105 C for 4 h and calcined at 500 C for 4 h in air to obtain SCR catalyst. Ce(NO 3 ) 3 was dissolved in deionized water at different concentrations. The powder of SCR catalysts was impregnated in Ce(NO 3 ) 3 solution for 4 h, dried at 105 C for 4 h and calcined at 500 C for 4 h. Finally, CeO 2 doped catalyst V 2 O 5 -WO 3 /TiO 2 was obtained and abbreviated as Ce x /SCR. X represents the mass fraction of Ce in the sample, x = 0.5%, 1%, 3%, 5%. 80-100 mesh particles were selected by stainless steel sieve.

Catalytic Activity Test
The schematic of the fixed-bed reactor system used for Hg 0 oxidation was showed in Fig. 1. The reactor system consists of four parts: mercury generation, simulated flue gas mixing, temperature control and mercury analysis.The elemental mercury was generated by a PSA Cavkit 10.534 mercury generator. The water vapor was generated by an IAS Hovocal gas generator. The composition and concentration of simulated flue gas were controlled by mass flow controllers (MFCs). The fixed-bed reactor was made of a quartz tube heated in a vertical tubular furnace. The catalyst sample was placed between the quartz cotton and glass fiber filter membrane, which were loaded in the quartz reactor. The concentrations of Hg 0 and Hg 2+ in simulated flue gas were online monitored by Thermo CEMS. To avoid Hg contamination, all of the tubes, joints, and valves that Hg passed through were made of Teflon. To avoid condensation of water vapor, all pipelines with H 2 O (g)-containing gas passing through were heated up to 120 C. The tail gas was treated by an activated carbon trap before being released to the air.
The inlet Hg 0 concentration was set at 30 μg/Nm 3 and the total flow rate of simulated flue gas was controlled at 2 L/min. The concentration of Hg 0 and Hg 2+ at the outlet was recorded after adsorption balance, which was defined as the fluctuation of concentration less than 3%. The time for the balance was more than 2 h. The total Hg 0 oxidation rate (O xi ) within a certain time was defined as follows: where C t Hg 2þ and C t Hg T were the outlet Hg 2+ and total mercury concentration at the reaction time, respectively.

Characterization
XRD measurements were carried out on polycrystalline X-ray diffractometer (Smartlab, Rigaku, Japan) to examine the crystallinity and dispersivity of crystal pieces using Cu Ka radiation (9 KW) in the range of 3-90 . The specific surface area, pore volume and pore diameter of catalysts were tested on automatic nitrogen adsorption analyzer (Micrometritics ASAP2010). The surface area and pore size was calculated by Brunauer−Emmett−Teller (BET) method and Barrett−Joyner−Halenda (BJH) method, respectively. To investigate the elemental states, XPS was carried out by X-ray photoelectron spectrometer (PHI QuANTRO SXM, ULVAC-PHI, Japan). The observed spectra were calibrated with the C1s binding energy (BE) value of 284.6 eV.  Fig. 2 showed the Hg 0 catalytic oxidation efficiencies over various catalysts with a space velocity of 60000 h −1 in the temperature range of 150-400 C. The simulated flue gas consisted of 10 ppm HCl, 12% CO 2 , 5% H 2 , 300 ppm NO, 400 ppm SO 2 and N 2 as balance gas. The results indicated that Ce doping did not change the optimum Hg 0 catalytic oxidation temperature over SCR catalyst, which was still 350 C. It was clearly found that the addition of CeO 2 resulted in enhancement of Hg 0 oxidation activity in varying degrees. With the increase of Ce doping, the Hg 0 oxidation efficiency over Ce x /SCR catalyst first increased and then decreased at 350 C. Ce 1 /SCR performed the best mercury oxidation and approximately 84.24% mercury oxidation efficiency was obtained. It was worth noting that the Hg 0 oxidation rate of Ce 5 /SCR decreased remarkably, even less than the Hg 0 oxidation rate of SCR when the temperature exceeded 350 C. It was presumed that CeO 2 -TiO 2 performed optimal Hg 0 catalytic oxidation at about 150 C [15], and excessive Ce doping lead to poor Hg 0 oxidation over Ce 5 /SCR at high temperature.

Effect of SO 2
The comparison of effect of SO 2 on Hg 0 oxidation efficiency over Ce 1 /SCR and SCR catalyst was measured in Fig. 3. The results indicated that SO 2 had a slightly promotional effect on Hg 0 oxidation over SCR catalyst. SO 2 was oxidized by chemisorbed oxygen to form SO 3 , which constituted new chemisorption sites for Hg 0 and reacted with Hg 0 to produce HgSO 4 [15]. However, SO 2 exhibited a weak inhibiting effect on Ce 1 /SCR catalytic activity. This could be due to that SO 2 reacted with Ce x O y to form Ce(SO 4 ) 2 and Ce 2 (SO 4 ) 3 , which reduced the participation of Ce x O y in Hg 0 oxidation process and covered the surface of Ce 1 /SCR catalyst, reducing the contact area between the active components and the reaction gas. Because Ce content in Ce 1 /SCR catalyst was only 1%, SO 2 had no obvious inhibition on Hg 0 oxidation over Ce 1 /SCR catalyst.

Effect of H 2 O
The results that the effect of H 2 O on Hg 0 oxidation efficiency over Ce 1 /SCR and SCR catalyst was shown in Fig. 4. H 2 O inhibited Hg 0 oxidation due to the competitive adsorption between H 2 O and Hg 0 [13]. However, Ce 1 /SCR catalyst had a superior performance of H 2 O resistance than SCR catalyst. It was speculated that the active sites on the surface increased or the adsorption of water vapor decreased by doping Ce into SCR, or the transition between Ce 4+ /Ce 3+ was likely to offset part of the inhibitory effect of H 2 O.  Fig. 5 showed the effect of NO on Hg 0 oxidation over Ce 1 /SCR catalyst. It indicated that NO had a slight enhancing effect on Hg 0 oxidation. This probably because NO reacted with chemisorbed oxygen to form NO x species, which could enhance Hg 0 oxidation.

NO Reduction Activity over Various Catalysts
Catalytic activity evaluation was carried out using a flow-through powder reactor system referred in Pang et al. [16]. NO concentration was tested for 30 min at each steady state. The NO conversion was calculated using the equation below: (2) where NO in represents the inlet NO concentration maintained at 500 ppm and NO out represents the outlet average NO concentration.
The NO conversion over SCR and Ce 1 /SCR catalyst at various temperature were showed in Fig. 6. The NO conversion over SCR catalyst was noticeably enhanced by Ce doping. SCR catalyst performed best NO conversion at 400 C while Ce 1 /SCR catalyst performed best at 350 C. It was speculated that Ce had a good effect on catalytic oxidation at low temperature, which decreased the optimal denitrification temperature. At operating temperature (300-400 C) of catalyst in power plant, NO conversion over Ce 1 /SCR catalyst was greater than 90%, which had a small difference from the optimal denitrification efficiency. Therefore, Ce 1 /SCR can be well applied in power plant, which will have a better simultaneous removal performance of NO and Hg 0 than SCR.

.1 XRD
The XRD patterns of the raw and modified SCR catalysts were displayed in Fig. 7. TiO 2 phase other than rutile TiO 2 phase was detected in all samples. The diffraction line of all samples were narrow and sharp, which indicated the high crystallinity. The characteristic peaks of V 2 O 5 and WO 3 were hardly detected, which were due to widely dispersion and poorer crystalline on the surface. CeO 2 was not observed when the loading of Ce was lower than 3% (wt.%). There were weak characteristic CeO 2 peaks at 28.38 when the loading of Ce was 5%, indicating a well-dispersed cluster with a small population of CeO 2 may exist.

BET
The pore structure parameters of different samples were summarized in Table 1. As can be seen from the result, the BET surface area, BJH pore volume and average pore volume of Ce x /SCR catalyst were lower than those of SCR. The BET surface area of Ce x /SCR decreased with the increase of Ce doping amount. The pore volume and pore diameter of Ce x /SCR catalyst did not show obvious relationship with Ce doping amount. The Nitrogen adsorption-desorption isotherms of Ce x /SCR were the IV adsorption-desorption isotherm specified by IUPAC. H1 hysteresis loop indicated mesoporous materials, which might facilitate mass transfer in the catalytic reaction (Fig. 8a). As displayed in Fig. 8b, Ce doping had no significant effect on the pore size distribution of catalysts. There were much mesoporous of 2∼50 nm in Ce x /SCR catalysts, indicating strong interactions between catalyst surface and adsorbate.

Hg 0 Oxidation Reaction
The effects of HCl and O 2 on mercury oxidation over Ce 1 /SCR catalyst were shown in Fig. 9. Ce 1 /SCR catalyst showed less than 5% mercury oxidation efficiency whether 5% O 2 was introduced into gas stream or not. Considering the measurement error, O 2 had little effect on Hg 0 oxidation. That was probably because the level of reaction between O 2 and Hg 0 was low, generating unstable Hg 2+ which coule convert to Hg 0 easily.  0.1 ppm HCl balanced in N 2 resulted in Hg 0 oxidation efficiency of 13.02%, which is higher than the 2.73% Hg 0 oxidation efficiency under pure N 2 condition. Hg 0 oxidation efficiency of 64.68% was observed when HCl concentration further increased to 1 ppm. HCl exhibited a decisive effect on Hg 0 oxidation over Ce 1 /SCR catalyst. 0.1 ppm HCl resulted in Hg 0 oxidation efficiency of 50.42% with the aid of 5% O 2 , indicating more HCl can be oxidized to form active chlorine species in the presence of O 2 . That might because O 2 replenished the consumed chemisorbed oxygen, regenerated the lattice oxygen and hence maintained the high surface oxygen concentration [15]. The reaction between HCl and Ce 1 /SCR catalysts may occur through Cl þ Hg 0 ¼ HgCl HgCl þ Cl ¼ HgCl 2 (6) where Cl denoted an active chlorine species for oxidizing Hg 0 , and O represented chemisorbed or lattice oxygen on the surface of Ce 1 /SCR catalysts.
The entire reaction can be written as follows:

XPS
To determine the oxidation states of the element in these catalysts and to further explain the mechanism of Hg 0 oxidation over Ce 1 /SCR catalyst, the catalysts were investigated by XPS technique. Ce 1 /SCR catalyst was pretreated in the gas composed of 50 μg/m 3 , 10 ppm HCl and 5% O 2 at 350 C for 4 h to get sCe 1 /SCR catalyst. Fig. 10 showed the O1s XPS spectra for SCR, Ce 1 /SCR and sCe 1 /SCR catalyst. The peak appeared at low binding energy (529-530.5 eV) could be ascribed to be the lattice oxygen (denoted as O b ) [17], while the binding energy of 531.0-532.9 eV was ascribed to the chemisorbed oxygen and weakly bonded oxygen species (denoted as O a ) [18]. O a has been thought to be the most active oxygen and played an important role in oxidation reaction [18][19]. The surface atomic concentrations of catalysts are given in Table 2. It could be found that O a relative concentration of Ce 1 /SCR calculated by O a /(O a + O b ) was higher than that of SCR. The reason was that Ce doping created labile oxygen vacancies, which may conduce to the improvement of chemisorbed oxygen and was helpful for mercury oxidation. The Ce3d spectra of sCe 1 /SCR and Ce 1 /SCR catalysts were presented in Fig. 11. The bands labeled u1 and v1 represent the characteristic peaks of Ce 3+ with 3d104f1 initial electronic state, whereas the peaks labeled u, u2, u3, v, v2, and v3 represent Ce 4+ with 3d104f0 electronic state [20]. Table 2 showed a slight change in the Ce valence state ratios after the tests under a flue gas flow. This implied that the redox shift between Ce 3+ and Ce 4+ happened during the Hg 0 catalytic oxidation process. In comparison with Ce 1 /SCR, the relative concentration of Ce 3+ species decreased in sCe 1 /SCR, which might be due to sufficient O 2 in flue gas to oxidize Ce 3+ to Ce 4+ .
The Hg4f XPS profile spectrum of sCe 1 /SCR catalyst was presented in Fig. 12. The binding energies at 101.5 and 110.5 eV was corresponded to HgCl 2 and HgO, respectively. No Hg 0 was observed on the surface of sCe 1 /SCR catalyst, which probably because HCl accelerated Hg 0 oxidation.  Figure 10: O1s XPS spectra for SCR,Ce 1 /SCR and Ce 1 /SCR

Mechanism
Previous studies have shown that the interconversion of V 4+ /V 5+ participates in the reaction process of Hg 0 catalytic oxidation over SCR. Based on the results above, a possible mechanism for Hg 0 oxidation can be proposed as follows: HCl was adsorbed onto the active sites of catalyst and reacted with chemisorbed oxygen from V 2 O 5 to form active surface chlorine species, which then would react with gas-phase or weakly bonded Hg 0 to produce HgCl 2 . The redox process of V 4+ /V 5+ was contributed by the shift of CeO 2 /Ce 2 O 3 . The missing lattice oxygen of CeO 2 would be replaced by O 2 . The redox cycle (V 4+ + Ce 4+ ↔ V 5+ + Ce 3+ ) played an important role in promoting Hg 0 oxidation.

Conclusion
Ce doping can significantly improve the catalytic activity for Hg 0 oxidation of SCR catalyst Ce 1 /SCR (1%Ce, wt.%) displayed the best performance with an Hg 0 oxidation efficiency of 21.2% higher than that of SCR catalyst in simulated coal-fired flue gas at 350 C. Ce 1 /SCR also enhanced NO conversion efficiency at 200-400 C. H 2 O in flue gas had an inhibitive effect on Hg 0 oxidation. However, Ce 1 /SCR showed a better H 2 O resistance but a slightly weaker SO 2 resistance than SCR catalyst. NO had a slight promotion effect on the Hg 0 oxidation over Ce 1 /SCR. The XPS results indicated that Ce 1 /SCR possed more chemisorbed oxygen and better redox ability compared with SCR catalyst. The catalytic mechanism was that adsorbed HCl was oxidized by chemisorbed oxygen to form active chlorine which would react with gas-phase or weakly bonded Hg 0 to form HgCl 2 . HCl played an essential role in Hg 0 oxidation, and the redox cycle (V 4þ þ Ce 4þ $ V 5þ þ Ce 3þ ) had an important role in promoting Hg 0 oxidation.

Conflicts of Interest:
The authors declare that they have no conflicts of interest to report regarding the present study.