TiO2âSiO2 SCR

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Impacts of Pb and SO2 poisoning on CeO2-WO3/TiO2-SiO2 SCR catalyst Yue Peng, Dong Wang, Bing Li, Chizhong Wang, Junhua Li, John C. Crittenden, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03309 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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Environmental Science & Technology

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Impacts of Pb and SO2 poisoning on CeO2-WO3/TiO2-SiO2 SCR

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catalyst

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Yue Peng*, a, b, Dong Wanga, Bing Lia, Chizhong Wanga, Junhua Li*, a, John Crittendenb,

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Jiming Haoa

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a

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of Environment, Tsinghua University, Beijing, 100084, China

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b

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State Key Joint Laboratory of Environment Simulation and Pollution Control, School

School of Civil and Environmental Engineering, Georgia Institute of Technology, 800

West Peachtree Street, Suite 400 F-H, Atlanta, Georgia, 30332-0595, United States

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* Corresponding author. Tel.: +86 10 62782030; +86 10 62771093.

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E-mail address: [email protected]; [email protected].

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ACS Paragon Plus Environment


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Abstract

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A CeO2-WO3/TiO2-SiO2 catalyst was employed to investigate the poisoning

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mechanisms of Pb and SO2 during selective catalytic reduction (SCR). The introduction

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of Pb and SO2 suppressed the catalytic performance by decreasing the numbers of

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surface acid and redox sites. Specifically, Pb preferentially bonded with amorphous

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WO3 species rather than with CeO2, decreasing the numbers of both Lewis and

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Brønsted acid sites but exerting less influence on the reducibility. SO2 preferentially

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bonded with CeO2 as sulfate species rather than with WO3, leading to a significant

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decrease in reducibility and the loss of surface active oxygen groups. Although SO2

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provided additional Brønsted acid sites via the interaction of SO42- and CeO2, it had

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little positive effect on catalytic activity. A synergistic deactivation effect of Pb and

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SO42- on CeO2 was found. Pb covered portions of the weakly bonded catalyst sites

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poisoned by SO42-, which increased the decomposition temperature of the sulfate

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species on the catalyst.

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Graphical Abstract

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1. Introduction

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NOx emitted from coal-fired power plants and industrial boilers is a major air pollutant

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with harmful impacts on human health and the environment. The selective catalytic reduction

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(SCR) of NOx with ammonia is widely used for flue gas cleaning at stationary sources.

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Commercial SCR catalysts consist of V2O5/TiO2 with the addition of WO3 or MoO3.1,

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Ammonia reacts with NOx to produce N2 and H2O in the presence of excess O2. Although this

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catalyst exhibits excellent deNOx activity in the temperature range of 350-420 °C, it still faces

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some disadvantages in practical operations.3, 4 In addition to its toxicity to humans, V2O5

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produces large amounts of SO3 from SO2 oxidation at the catalyst working temperatures. The

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deposition and accumulation of sulfate species also lead to catalyst deactivation, pipeline

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corrosion and greater pressure drops.5 Hence, many researchers have focused on the

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development of low- or non-vanadia catalysts and on the improvement of catalyst resistance to

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SO2 and other poisons.6,

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activity of commercial catalysts and could be adopted as an active component in SCR catalyst

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recipes. The CeO2-WO3 catalyst exhibits excellent activity and good N2 selectivity under a high

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gas hourly space velocity (GHSV).8-10 This catalyst also shows better poisoning resistance to

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alkali metals than commercial catalysts.11, 12 This superior resistance is due to its unique dual-

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cycle reaction mechanism: (1) gaseous NH3 is adsorbed on surface polytungstate sites (acid

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cycle) and then (2) the adsorbed NH3 is activated at Ce4+ sites (redox cycle) to further react

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with NOx.13, 14

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Previous works suggested that ceria could improve the deNOx

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In general, poisons in flue gas may deactivate catalysts by chemical deactivation or pore

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blocking. Recently, we investigated different poisons, including alkali and heavy metals, and

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determined the mechanisms by which they deactivate SCR catalysts:15-18 (1) The decreases in

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acid strength and quantity are the major deactivation factors (both decrease surface acidity). (2)

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The loss of reducibility is important but less critical than surface acidity (K2O showed a

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decrease in reducibility, As2O3 slightly promoted reducibility, and PbO had less influence on

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it). (3) The active bridged nitrate/nitrite species exhibit certain advantages with respect to

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activity, whereas inactive bridged and bidentate nitrate species that adsorb on the basic sites of

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catalysts decrease the catalytic activity. However, the mechanisms of catalyst poisoning by

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both acidic and alkaline metals have not been clearly elucidated.

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In the flue gas from stationary sources, SO2 and Pb are the most common poisons and, in

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most cases, exist simultaneously. SO2 is generated by the oxidation of sulfur contained in fossil

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fuels. The SO2 concentrations in flue gas can be less than 100 ppm after desulfurization to 5000

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ppm for high-sulfur coal sources in Southwest China.19 Pb is generated from the combustion

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of municipal solid waste and the flue gas from smelting operations. The Pb concentration can

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exceed 3000 ppm in an SCR catalytic convertor that is used in a waste incinerator after

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approximately 2000 h of operation.20 Our recent works examined the Pb poisoning mechanisms

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of CeO2-WO3 catalysts via in situ infrared (IR) and Raman spectra. We found that Pb

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preferentially bonded with WO3 (acid sites) over CeO2 (redox sites),17 whereas SO2

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preferentially bonded with CeO2 to form Ce(SO4)2 and did not form a bond with WO3. In other

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words, SO2 and Pb poison the redox sites and acid sites of CeO2-WO3 catalysts, respectively.

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In this work, SO2 and Pb were used separately and jointly to poison a CeO2-WO3 SCR

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catalyst that was supported on TiO2-SiO2. We employed temperature programmed methods to

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quantify the desorption of NH3, NOx and SO2 on the fresh and poisoned catalysts and proposed

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poisoning mechanisms for Pb and SO2.

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2. Experimental Section

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2.1 Catalyst Preparation

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The CeO2-WO3/TiO2-SiO2 catalyst was prepared using the impregnation method. The

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mass percentages of CeO2 and WO3 were 5.0 wt%, and the support consisted of TiO2 anatase

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and 5 % SiO2 (LUDOX® HS-40). This sample was termed the “fresh” catalyst. The Pb-

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poisoned catalyst was prepared via dipping fresh catalyst into Pb(NO3)2 solutions. The surface

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molar ratio of Pb was 0.20 % (determined from X-ray photoelectron spectroscopy (XPS)

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results), and this sample was termed the “Pb-poisoned” catalyst. The S-poisoned catalyst and

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the Pb and S co-poisoned catalyst were prepared via flue gas treatment of the fresh and Pb-

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poisoned catalysts, respectively, in 1000 ppm SO2/N2 at 50 °C for 10 h. They were termed the

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“S-poisoned” and “Pb&S-poisoned” catalysts. The ratios of S in the S-poisoned catalyst were

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6.15 mg/gcat (determined from inductively coupled plasma-atomic emission spectrometry (ICP-

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AES) results) and 1.30 mole % (determined from XPS results).

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2.2 Activity Measurements

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Catalyst activity was measured in a fixed-bed quartz reactor (inner diameter of 5 mm) with

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100 mg of catalyst (40 to 60 mesh). The feed gas mixture contained 500 ppm NO, 500 ppm

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NH3, and 3 % O2 (in addition to 3 % H2O and 100 ppm SO2 when these gases were used), and

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the balance of the gas was N2. The total flow rate was 200 mL·min-1, and GHSV was

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approximately 120,000 mL·g-1·h-1. The concentrations of the gases (NO, NO2, N2O, NH3, H2O

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and SO2) were continuously monitored using a Fourier transform infrared (FTIR) continuous

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gas analyzer (MultiGas TM 2030).

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The catalytic activity was evaluated using the following equation describing the conversion of NO to nitrogen gas. (Eq. 1 assumes that the reactor is a plug flow reactor.) k 

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V  ln(1  x) W

⑴

where k is the pseudo-first order reaction rate constant (µmol·g-1·s-1), V is the total gas flow

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rate, W is the mass of the catalyst in the reactor, and x is the NO conversion.

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2.3 Catalyst Characterization

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The Brunauer-Emmett-Teller (BET) surface areas of the samples were measured using a

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Micromeritics ASAP 2020 apparatus. XPS was performed using an ESCALab 220i-XL

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electron spectrometer from VG Scientific with 300 W of AlKα radiation. The elemental content

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was measured with an IRIS Intrepid II XSP ICP-AES apparatus (Thermo Fisher). The

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temperature programmed desorption (TPD) of NH3, SO2 and NO was measured in a quartz

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reactor using 100 mg samples. Each catalyst was first pretreated using N2 (200 mL·min-1) at

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350 °C for 0.5 h and then cooled to 100 °C for the NH3-TPD analysis or to 30 °C for the SO2-

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and NO-TPD analysis. The sample was saturated with the gas to be tested (500 ppm) for 1 h

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and then purged with N2 to remove physically adsorbed species. The concentrations of the

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gases were measured using the MultiGas TM 2030. For analysis, the catalyst was heated to

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780 °C for SO2-TPD, 400 °C for NH3-TPD and 500 °C for NO-TPD with N2 (200 mL·min-1)

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gas and a temperature increase of 10 °C·min-1. Temperature programmed reduction (TPR) with

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H2 was performed using a chemisorption analyzer (ChemiSorb 2720 TPx) for a 10 %/90 %

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H2/N2 gas flow (50 mL·min-1) and was heated at a rate of 10 °C·min-1 up to 1000 °C. The H2-

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TPR profiles were detected continuously with a thermal conductivity detector (TCD). Each

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sample (20-30 mg) was first pretreated in N2 at 300 °C for 0.5 h. The other experimental details

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and results can be found in the Supporting Information (SI).

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3. Results and Discussion

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3.1 Texture and SCR Performance

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Fig. S1 shows the X-ray diffraction (XRD) patterns of the fresh and poisoned catalysts.

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All the samples exhibited peaks attributed to the TiO2 anatase and CeO2 cubic fluorite phases

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(the small shoulder at 28.5°). The peak locations and intensities did not change when the

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catalyst was poisoned by Pb and/or SO2. The results show that WO3 existed in an amorphous

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phase on the catalyst surface and that CeO2 was highly dispersed in the form of microcrystals.

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The XRD patterns show good agreement with the Raman results (Fig. S2), and the BET surface

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areas of the catalysts are summarized in Table 1. The surface areas of the fresh and the Pb-

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poisoned catalysts are quite similar and are larger than those of the S-poisoned and Pb&S-

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poisoned catalysts, i.e., Pb poisoning does not change the catalyst surface area, whereas SO2

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significantly decreases the surface area due to the coverage of SO42- on catalyst surface. In our

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previous work, Pb poisoning on a CeO2-WO3 catalyst slightly decreased the surface area17 due

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to the uneven dispersion of Pb and its high loading (0.51 % vs 0.20 %) on CeO2-WO3.

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Therefore, we proposed that trace amounts of Pb do not significantly decrease the catalyst

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surface area.

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Fig. 1(a) shows the XPS spectra of Ce 3d. The peaks located at 886.3 and 904.6 eV could

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be attributed to the 3d104f1 state of Ce3+, while the other peaks in the curves of Ce 3d spectra

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could be attributed to the 3d104f0 state of Ce4+.21, 22 The peak intensity at 886.3 eV (ν’, Ce3+)

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was lower than that at 882.8 eV (ν, Ce4+) in the fresh and Pb-poisoned catalysts. In contrast, in

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the S-poisoned and Pb&S-poisoned catalysts, the Ce3+ peak intensity was greater. These results

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indicate that S poisoning could increase the percentage of Ce3+ on the catalyst surface. Previous

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works proposed that SO2 is adsorbed on CeO2 even at room temperature.23 Further, SO2 could

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be oxidized to SO42- using only the oxygen supplied by CeO2, which would lead to the partial

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reduction of Ce4+. To confirm the presence of S species on the poisoned catalyst, S 2p spectra

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were measured and are displayed in Fig. 1(b). The peaks at 168.1 and 169.4 eV are attributed

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to S6+ and correspond to the presence of SO42- on the catalyst surface.23 Fig. 1(c) displays the

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Pb 4f spectra for the Pb-poisoned and Pb&S-poisoned catalysts. No significant difference was

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found between the spectra of the two catalysts, suggesting that SO2 has less poisoning influence

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on the tungsten valence. Because of the overlap of W 4f with Ti 3s (Fig. S3), we cannot discuss

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the influence of Pb on tungsten species on the basis of XPS spectra.24 Based on our previous

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work on Pb poisoning of the CeO2-WO3 catalyst, Pb preferred to bond with surface WOx

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species rather than CeO2, which resulted in fewer acid sites and lower thermal stability.17

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Fig. 2(a) shows the SCR activity of the fresh and poisoned catalysts in the temperature

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range of 150-450 °C under a high GHSV of 120000 mL·g-1·h-1. The fresh catalyst yielded

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greater than 90 % NO conversion from 300 °C and minor deactivation above 400 °C. This

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deactivation could be due to the limited acidity of the CeO2-WO3 system. Previous results

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suggested that this catalyst cannot provide stable acid sites at such high temperatures8, 25. The

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Pb-poisoned catalyst exhibited slightly lower activity than the fresh catalyst at these

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temperatures, and the surface acidity sites reached their highest values at 350 °C. These results

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indicate that the acid intensity of the Pb-poisoned catalyst is lower than that of the fresh catalyst.

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The S-poisoned and Pb&S-poisoned catalysts had lower SCR activities than the fresh catalyst,

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due to the combined effects of chemical deactivation and physical pore blockage (loss of

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surface areas). Furthermore, we have also calculated the reaction rate constants of the catalytic

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activity at 200 °C, which are listed in Table 1. The activity sequence of the catalyst was as

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follows (normalized by surface area): fresh>Pb-poisoned>S-poisoned>Pb&S-poisoned

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catalyst. Note that we cannot directly compare the extent of poisoning between Pb and SO2,

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because of their different surface loadings (0.20 % Pb and 1.30 % S, according to XPS results).

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Fig. 2(b) shows the influences of H2O and SO2 on the fresh and Pb-poisoned catalysts at

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250 and 350 °C. The deactivation was directly related to the working temperatures. For the

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fresh catalyst, only 10 % of NO conversion was lost at 350 °C when H2O and SO2 were

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introduced, and part of the catalytic activity was recovered after their removal. In contrast,

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more than 20 % of NO conversion was lost at 250 °C, and the activity did not increase upon

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the removal of H2O and SO2. The Pb-poisoned catalyst exhibited even worse activity at this

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temperature. These results indicate that the CeO2-WO3 catalyst still has a SO2 poisoning

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problem, especially at 250 °C. H2-TPR was performed on the catalysts (Fig. 3). A clear 20 °C

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upshift of the reduction peak was observed at 540 °C for the fresh and Pb-poisoned catalysts.

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These shifts could be due to the overlap of Ce4+ reduction with SO42- reduction,26 i.e., SO2

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blocks the reducibility of CeO2 on the catalyst surface. CeO2 provides the major redox sites for

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this catalyst. Therefore, SO2 poisons the redox sites and thus lowers the catalyst reducibility.

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3.2 TPD Profiles of NH3, SO2 and NO

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Fig. 4(a) shows the NH3-TPD profiles of the fresh and poisoned catalysts in the

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temperature range of 100-400 °C. Three peaks can be observed for the fresh catalyst. The peaks

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at 176 °C, 210 °C and 240 °C can be attributed to the weak, medium and strong acidity of the

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catalyst, respectively. The peak intensities of the Pb-poisoned catalyst exhibited extensive

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decreases compared with those of the fresh catalyst. Previous studies proposed that both the

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number of acid sites and their thermal stability decreased when the catalyst was poisoned by

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Pb.17 For the S-poisoned and Pb&S-poisoned catalysts, the acidities were even lower than that

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of the Pb-poisoned catalyst. When we normalized the total acidity of the catalysts to the surface

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area (Table 1), however, the S-poisoned catalyst yielded even greater acidity than the fresh

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catalyst. These results suggest that SO2 could provide additional acid sites on the catalyst

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surface. Furthermore, the weak peak of the fresh catalyst was higher than the medium and

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strong peaks, while in the S-poisoned and Pb&S-poisoned catalysts, the medium and strong

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peaks were equal to or higher than the weak peak. This finding confirms that sulfur existed as

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SO42-, providing stronger surface acid sites.27 To distinguish the acidity styles of the catalysts,

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we obtained in situ IR spectra (Fig. S4(a)) of the fresh and poisoned catalysts at 100 °C. The

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S-poisoned catalyst had more Brønsted acid sites but fewer Lewis acid sites than the fresh and

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Pb-poisoned catalysts. With increasing temperature, the Lewis acid sites disappeared first,

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followed by the Brønsted acid sites (Fig. S4(b)). Therefore, we proposed that Pb decreases the

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numbers of both Lewis and Brønsted acid sites, whereas SO42- decreases the number of Lewis

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(weak) acid sites but increases the number of Brønsted (strong) acid sites. Similar results

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showing that SO2 can promote increases in the number of acid sites were also found for SO2-

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poisoned CeO2 (Fig. S5).

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To further investigate the possible interactions between Pb and S, we performed SO2-TPD

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experiments on the fresh and Pb-poisoned catalysts in the range of 30-780 °C (Fig. 4(b)). Three

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peaks were observed for the fresh catalyst located at 100, 510 and 690 °C, which can be

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attributed to the weakly adsorbed SO2, moderately bonded sulfate species, and strongly bonded

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sulfate species, respectively.28, 29 For the Pb-poisoned catalyst, the moderately bonded sulfate

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species significantly decreased at 510 °C, while the peak at 690 °C increased. We also

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quantified the desorbed SO2 (Table 1), which suggested that the total SO2 desorption

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normalized by surface area (2.56 VS. 2.44 mmol·m-2) remained nearly unchanged.

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Fig. 4(c) shows the NO-TPD of the fresh and poisoned catalysts. The fresh catalyst

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exhibited two NO desorption peaks at 100 and 220 °C and two NO2 desorption peaks at 100 °C

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and 300 °C, respectively. The lower-temperature peaks could be attributed to weakly adsorbed

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NO or NO2, whereas the higher-temperature peaks could be attributed to nitrite or nitrate

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species.30 For the Pb-poisoned catalyst, the NO desorption peak above 250 °C significantly

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decreased, while the NO2 desorption peak at 200 °C increased. To further quantify the

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percentage of NO2 in the total NOx (Table 2), we note that an increase in NO2 occurred on the

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Pb-poisoned catalyst. The results indicate that Pb could improve the oxidation of NO to NO2

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and the adsorption of nitrate species to the catalyst. For the S-poisoned catalyst, the amount of

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NO2 significantly decreased, and most of the NO desorbed in the range of 50 to 350 °C. The

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SO2-poisoned catalytic surface exhibited relatively low reducibility and weak NO adsorption

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strength. The decrease of NO and NO2 adsorption could be due to the loss of reducibility and

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the occupation of SO42- on CeO2 sites, they prohibited the NO oxidation to NO2 and the

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adsorption of NO and NO2 on CeO2 sites (as nitrite or nitrate species). For the Pb&S-poisoned

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catalyst, the percentage of NO2 was lower than that on the Pb-poisoned catalyst but slightly

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higher than that on the S-poisoned catalyst. Therefore, Pb could improve NO oxidation,

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especially at low temperature, and SO42- could suppress the NO oxidation and weaken the NO

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adsorption strength. Fig. S6 also shows the NO+O2-TPD of the catalysts, and the results agree

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with the results of the NO-TPD.

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3.3 TPD Profiles of SO2+NH3

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Recent studies have suggested that the types of sulfur species are interchangeable

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depending on whether NH3 is involved.23, 31 SO2+NH3-TPD experiments were employed to

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further elucidate the sulfur species on the fresh and Pb-poisoned catalysts. Fig. 5(a) shows the

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TPD results for catalysts first saturated by SO2 and then then purged by NH3. The NH3

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desorption peaks were de-convoluted into two peaks: the peak lower than 100 °C was attributed

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to the weakly adsorbed or physical adsorbed NH3, and the peak at 250 °C was attributed to the

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chemical adsorbed NH3. The NH3 desorption profile of the fresh catalyst was quite similar to

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that of the Pb-poisoned catalyst. However, the surface acidity of the Pb-poisoned catalyst was

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slightly lower than that of the fresh catalyst. The SO2 desorption peaks of the two catalysts

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were similar to the results of SO2-TPD. Quantitative results show that the total chemically

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adsorbed NH3 decreased slightly, possibly due to the competitive adsorption between SO2 and

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NH3 on the catalyst surface. Based on previous studies, only ceria was consistent with this

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feature. Ceria could provide some of the weak Lewis acid sites from the in situ IR spectra

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results and could bond with gaseous SO2 to form Ce(SO4)2 on the surface due to its excellent

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oxygen content and release capacity.

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When we reversed the order of gas introduction (Fig. 5(b)), physical adsorption of SO2

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was observed on the fresh and Pb-poisoned catalysts at temperatures lower than 200 °C.

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However, the SO2 desorption peak at 550 °C for the fresh catalyst disappeared, and the SO2-

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TPD shape observed for the fresh catalyst was similar to that observed for the Pb-poisoned

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catalyst. The results indicate that the relatively weak SO2 adsorption sites were occupied by

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pre-adsorbed NH3 and that the adsorption sites could also be occupied by Pb doping. Moreover,

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we quantified the adsorption of NH3 and SO2 during the TPD process and their ratios, as shown

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in Table 3. The increase in the NH3/SO2 ratio (1.04 to 1.34) calculated from Fig. 5(a) could be

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attributed to the low SO2 adsorption on the Pb-poisoned catalyst, whereas the decrease in the

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NH3/SO2 ratio (1.78 to 1.59) calculated from Fig. 5(b) could be attributed to the low NH3

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adsorption on the Pb-poisoned catalyst. Combined with the NH3-TPD and SO2-TPD results,

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Pb can occupy portions of both the NH3 and SO2 adsorption sites, and this occupation cannot

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be mitigated by purging with another gas. Two types of SO2 adsorption sites exist on the fresh

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CeO2-WO3 catalyst: weakly adsorbed and strongly adsorbed sites. The weakly adsorbed sites

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can be covered by Pb and by NH3. However, these species have less influence on the strongly

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adsorbed sites.

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3.4 The Poisoning Mechanisms of Pb and S

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In our previous works, we studied the reaction mechanisms and roles of active components

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of CeO2-WO3 catalysts by in situ IR and Raman spectroscopy.14 The results suggested that

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polymeric WO3 on the catalyst surface could serve as acid sites to capture gaseous NH3, while

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CeO2 could serve as redox sites to activate the adsorbed NH3. Combined with studies of Pb and

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S poisoning, the results suggest the following. (1) Pb prefers to bond with amorphous surface

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WO3 species on the CeO2-WO3 catalyst, decreasing the numbers of both Lewis and Brønsted

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acid sites. However, Pb actually has a slightly positive effect on catalyst reducibility. (2) SO2

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is converted to SO42- during the gradual poisoning process and prefers to bond with surface

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CeO2, leading to the formation of stable sulfate species. These species can serve as additional

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stable Brønsted acid sites but block the redox sites of CeO2, lowing the percentage of active

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oxygen on the surface and decreasing its reducibility. (3) Pb and SO42- also exhibit synergistic

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deactivation effects on the CeO2 surface. Pb covers a portion of the weakly bonded sites that

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are pre-poisoned by SO42- on the catalyst. These newly formed sulfate species on the catalysts

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require even higher decomposition temperatures than Ce(SO4)2.

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We also studied the surface-adsorbed species and reducibility of fresh and Pb-poisoned

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catalysts after running the SCR process for 24 h with high concentrations of SO2 and H2O (Fig.

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S7). Compared with the fresh catalyst, the Pb-poisoned catalyst has more weakly adsorbing

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NH3 and stable SO2 adsorption sites. Simultaneously, the reducibility of the Pb-poisoned

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catalyst also decreases due to the highly stable sulfate species on the catalyst surface. Pb

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covered portions of the weakly bonded catalyst sites poisoned by SO42-, which increased the

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decomposition temperature of the sulfate species on the catalyst. These results further support

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the discussion of the impacts of poisons on CeO2-WO3 catalysts. Combined with our previous

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studies of K,11 Ca,18 As32 and Pb17 poisoning on ceria-based catalysts, we proposed that the

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CeO2-WO3 catalyst shows better resistance to K, Ca and Pb than the commercial V2O5-

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WO3/TiO2 catalyst and the CeO2-MoO3 catalyst shows better resistance to arsenic than does

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CeO2-WO3. However, the catalysts suffer from the impacts of SO2 on the catalyst redox sites.

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After the introduction of relatively high SO2 and H2O concentrations for 24 h or longer, sulfate

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species form irreversibly on the catalyst surface. These species cannot easily decompose at

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standard SCR reaction temperatures and significantly suppress the catalytic activity. Therefore,

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we suggest that these catalysts are not as good as commercial V2O5-WO3/TiO2 catalysts (Fig.

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S8) for use with flue gas with high SO2 concentrations. They show better activity between 200-

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300 °C only without SO2.

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Supporting Information Available

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Some related figures are shown in the supporting information. This information is available

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free of charge via the Internet at http://pubs.acs.org/.

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Notes

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The authors declare no competing financial interests.

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Acknowledgments

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The authors gratefully acknowledge the financial support from the National Science

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Foundation (21777081), and the Public Projects Foundation of China Ministry of

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Environmental Protection (201509021 and 201509012). The authors also appreciate support

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from the Brook Byers Institute for Sustainable Systems (BBISS) and from the Hightower Chair

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and Georgia Research Alliance at Georgia Institute of Technology. The views and ideas

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expressed herein are solely those of the authors and do not represent the ideas of the funding

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agencies in any form.

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References:

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1.

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and Adsorbed-NOx. Environ. Sci. Technol. 2014, 48, (8), 4515-4520.

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19



398

399 400 401

Page 20 of 27

Table 1. Physicochemical properties of the fresh and poisoned CeO2-WO3/TiO2-SiO2 catalysts. SA

ka

k/SA

NH3ad/SAb

SO2ad/SAc

(m2·g-1)

(µmol·g-1·s-1)

(µmol·m-2·s-1)

(mmol·m-2)

(mmol·m-2)

Fresh

68.6

362

5.28

1.40

2.56

Pb-poisoned

68.4

157

2.30

1.14

2.04

S-poisoned

38.6

76.1

1.96

1.48

-

Pb&S-poisoned

39.3

32.2

0.82

1.13

-

a

calculated at 200 °C calculated from NH3-TPD profiles c calculated from SO2-TPD profiles b

20


Page 21 of 27

402


Table 2. Product analysis of NO-TPD profiles of the fresh and poisoned catalysts. NOad

NO2ad

NO+NO2

NO2/NOxa

(mmol·g-1)

(mmol·g-1)

(mmol·g-1)

(%)

Fresh

13.4

9.45

22.85

41.3

Pb-poisoned

8.63

11.4

20.03

56.9

S-poisoned

21.6

2.74

24.34

11.3

Pb&S-poisoned

16.3

4.26

20.56

20.7

403

21



404

405 406

Page 22 of 27

Table 3 Product analysis of NH3+SO2-TPD profiles of the fresh and Pb-poisoned catalysts.

a

NH3a

SO2a

NH3b

SO2b

(mmol·m2 )

(mmol·m2 )

(mmol·m2 )

(mmol·m2 )

Fresh

2.74

2.65

2.76

1.55

1.04

1.78

Pb-poisoned

2.63

1.96

2.24

1.41

1.34

1.59

calculated from NH3+SO2 TPD profiles (Figure 5(a)) calculated from NH3+SO2 TPD profiles (Figure 5(b))

b

22


NH3/SO2a NH3/SO2b

Page 23 of 27


407 408

Figure 1. XPS spectra of (a) Ce 3d (b) S 2p and (c) Pb 4f of the fresh and poisoned CeO2-

409

WO3/TiO2-SiO2 catalysts.

23



410

Page 24 of 27

10

411 412

Figure 2. (a) NO conversion of the fresh and poisoned catalysts and (b) stability of the fresh

413

and Pb-poisoned catalysts in SO2 and H2O at 250 and 350 °C. Reaction conditions: catalyst

414

amount=100 mg, inlet [NO]=inlet [NH3]=500 ppm, inlet [O2]=3 %, balance N2,

415

GHSV=120,000 mL·g-1·h-1.

24


Page 25 of 27


416 417

Figure 3. H2-TPR profiles of the fresh and poisoned catalysts in the range of 200-900 °C.

25



418 419

Figure 4. (a) NH3-, (b) SO2- and (c) NO-TPD profiles of the fresh and poisoned catalysts.

26


Page 26 of 27

Page 27 of 27


420

421 422

Figure 5. SO2+NH3-TPD profiles of the fresh and Pb-poisoned catalysts: (a) pretreated first

423

with SO2 and then with NH3 and (b) treated in the reverse order.

27


TiO2âSiO2 SCR

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