Enhancement of Oxidation Efficiency of Elemental Mercury by CeO2/TiO2 at Low Temperatures Governed by Different Mechanisms.

This study aims to investigate the photothermal oxidation removal of Hg0 in simulated flue gases using photothermal catalysts at relatively low temperatures of 120-160 °C in two phases: the first phase applied the sol-gel method to prepare TiO2 and CeO2/TiO2 photothermal catalysts and characterized surface properties by specific surface area analysis, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy. The second phase investigated the effects of operating parameters on Hg0 oxidation efficiency at lower temperatures of 100-160 °C. The operating parameters included reaction temperatures and modified concentrations of CeO2. Experimental results indicated that TiO2 prepared by the sol-gel method was mainly in the anatase phase. XPS analysis showed that Ce mostly existed in the form of Ce4+. The content of surface-chemisorbed oxygen increased with the modification amount of CeO2. Photothermal catalytic oxidation results indicated that CeO2/TiO2 had a much higher oxidation efficiency of Hg0 at 120-160 °C than neat TiO2, which increased from 30-60 to >90%. 7%CeO2/TiO2 not only had the best photothermal performance but also maintained high efficiency at a relatively higher reaction temperature of 160 °C.

Introduction

With long-lasting toxicity, bioaccumulation, and high volatility, mercury and its derivates have been known as one of the most severe pollutants in the atmosphere.[1] After officially signing the International Minamata Convention on October 9, 2013, the international consensus on mercury emission reduction has been finally reached. Mercury emissions from coal-fired power plants have been officially announced as the main target. New coal-fired power plants are required to adopt the best available technology (BAT) and best environmental practice (BEP) within 5 years from the entry of the start of the convention. Existing facilities must use the BAT and BEP within 10 years to reduce mercury pollution emissions.[2] Mercury and its derivatives and NO are major air pollutants emitted from coal-fired power plants, which could cause severe adverse effects on the ecological system and human health. There are three forms of mercury including gaseous elemental mercury (Hg0) (emission concentrations in the range of 10–100 μg/m3),[3] gaseous oxidized mercury (Hg2+), and particulate mercury (PHg). Gaseous Hg0 is insoluble in water, so it is difficult to remove Hg0 from the flue gas desulfurization (FGD) system. PHg can be easily removed by electrostatic precipitators (ESPs) because of its adhesion on the surface of the particles. Therefore, the removal of Hg0 is an obstacle to control mercury emitted from coal-fired power plants.[4]

At present, coal-fired power plants are installed mostly with a NO removal device. The removal efficiency of NO is commonly higher than 90% using selective catalytic reduction (SCR). SCR allows NO emissions from stationary sources to comply with its standard.[5] A previous study reported that SCR is an effective technique for low operating costs to catalytically oxidize Hg0.[6] However, SCR is commonly installed in the front of the ESP because of its optimum temperature window of 300–450 °C for thermal catalytic reduction of nitrogen oxides (mainly NO). Additionally, the masking effect of high-concentration particles in the flue gas could reduce the removal efficiency of Hg0 by SCR.[7] Accordingly, this study aims to develop a new photothermal catalyst for SCR that can simultaneously remove Hg0 and NO at relatively low temperatures of 120–160 °C.

Photothermal catalytic oxidation of Hg0 by TiO2 is a promising new air pollution control technology.[8] However, reaction temperatures have a substantial influence on the photothermal activity of TiO2. At lower temperatures of 25–100 °C, the oxidation efficiency of Hg0 using TiO2 has excellent effects. However, its photo-oxidation efficiency is limited at reaction temperatures above 100 °C. Thus, this study modifies TiO2 catalysts with the transition metal Ce in order to improve the photothermal oxidation efficiency of Hg0 at relatively low temperatures of 120–160 °C.

This study was conducted in two phases. The first phase was to prepare CeO2-modified TiO2 (CeO2/TiO2) and characterize the surface properties of CeO2/TiO2 by specific surface area (SSA) analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The second phase was to apply photothermal catalysts to oxidize Hg0 in a photothermal reactor. Under different operating parameters, such as reaction temperatures and influent Hg0 concentrations, the oxidation efficiency of Hg0 was further explored to investigate the effect of the modification amount of CeO2.

Methodologies

Experimental Materials and Catalyst Preparation

The chemical reagents used to prepare TiO2 and Ce/TiO2 and Hg0 measurement are summarized in Table S1 with the purity and manufacturer. The photothermal catalysts used to oxidize Hg0 were prepared by the sol–gel method. They were prepared with titanium isopropoxide, isopropanol, and acetic acid as precursors. Titanium isopropoxide (20 mL) was dissolved into a mixture of isopropyl alcohol (40 mL) and acetic acid (40 mL) and stirred vigorously at room temperature. After reacting for 10 h, 60 mL of deionized (DI) water was added to the solution. The sol was then aged at 65 °C to form a gel. The obtained wet gel was further dried at 105 °C for 10 h and finally calcined at 500 °C for 3 h to obtain the photothermal catalysts. After cooling to room temperature, TiO2 was ground into a powder, mixed with DI water to a specific ratio (1.5 g TiO2/20 mL H2O), immersed in the glass beads, and then dried at 105 °C. In the same manner, TiO2 coated on glass beads can be prepared by repeatedly coating on the surface of glass beads.[9] The preparation of CeO2-modified TiO2 followed the same procedure as the preparation of TiO2, except that Ce(NO3)3·6H2O solution with different concentrations was dropped to the mixture of titanium isopropoxide, isopropyl alcohol, and acetic acid in the preparation of 1–40%CeO2-modified TiO2.

Analytical Instruments and Experimental Apparatus

In this study, the instruments used for the surface characteristic analysis of photothermal catalysts are summarized in Table S2. The crystalline characteristic of the photothermal catalyst was measured using an XRD analyzer (Siemens, D5000). The CuKα radiation was used as an X-ray source, and the scanning step was set as 0.5 °/s. The acceleration voltage and the electrical current were set as 40 kV and 40 mA to calculate the mass ratio (fA) of anatase in TiO2 using the following formula (eq )[10]where IR is the peak intensity of rutile (27.46°) and IA is the peak intensity of anatase (25.36°). The grain size of anatase and rutile phases can be estimated with the Scherrer equation (eq )where λ is the X-ray incident wavelength of 0.154 nm and B is the full width at half-maxima. θ is the diffraction angle of rutile and anatase, which are 27.46 and 25.36°, respectively. The chemical state of Ti, Ce, and O in photothermal catalysts was measured at room temperature using XPS (VG Scientific, ESCALAB 250). The excitation source of X-rays was calibrated to its measurement range with C1s = 284.8 eV before measuring. The Brunauer–Emmett–Teller (BET) SSA of each sample was measured using a SSA analyzer under 77 K liquid nitrogen. The samples must be subjected to degas (heated outgassing) before analyzing. Photoluminescence (PL) was performed using laser light with a wavelength of 325 nm and with a scanning wavelength range of 300–900 nm.

A photothermal catalytical reaction system was designed for this study, which consisted of a mercury generator, a mixing chamber, a catalytic reactor, and a mercury on-line measuring instrument (NIC, EMP-2) (Figure ). The Hg0 vapor was generated via a dynamic calibrator with high purity N2 as a dilution gas; the gas flow rate was set to 900 mL/min. The gas stream was first mixed in a mixing chamber prior to entering the catalytic reactor. The reaction was performed in a circular double-casing reactor with a length of 450 mm and an inside diameter of 28 mm. A near-UV lamp was placed in the inner tube of the reactor with an illumination wavelength of 365 nm and an illumination intensity of 15 W. Photothermal catalysts coated on the surface of glass beads were placed between the inner wall of the reactor and the outer wall of the outer tube. The reaction time of the inflowing gas and the photothermal catalyst was 0.80 s. The outer wall of the reactor was wrapped with a heating tape to control the reaction temperatures (120, 140, and 160 °C). In this study, the photothermal catalyst was coated on the smooth glass beads and placed in the catalytic reactor. The reaction parameters investigated in this study included reaction temperatures and weight percentages of CeO2 over TiO2. After passing through the photothermal catalysts in the reactor, the difference between the inlet and outlet Hg0 concentrations could be used to determine the removal efficiency of Hg0 (ηremoval) as shown in (eq ).[11,12]

Figure 1

Setup of the photothermal catalytic reaction system.

In the experiments, the oxidation efficiency of Hg0 was studied using TiO2 and 1–20%CeO2/TiO2 in a lower temperature environment. Different photothermal catalysts were applied to conduct the photocatalytic oxidation efficiency of Hg0 by TiO2 and 1–20%CeO2/TiO2 at lower temperatures (120–160 °C) in a N2 atmosphere.

Results and Discussion

Surface Property Analysis

Figure shows the XRD patterns of the photothermal catalysts and that the peaks of TiO2 were located at 2θ = 25.3° (101), 37.8° (004), 48.0° (200), 53.9° (105), 55.1° (211), 62.7° (204), 70.3° (220), 75.1° (215), and 82.8° (224). According to the code of 001093 in AMCSD (American Mineralogist Crystal Structure Database),[13] it was found that these peaks corresponded to the anatase phase, while the rutile’s peaks did not appear. Thus, the TiO2 prepared by the sol–gel method was mainly in the anatase phase. Table S3 summarizes that the grain size of TiO2 and 1–40%CeO2/TiO2 calculated with the Scherrer equation ranged from 4 to 12 nm. In addition, from the XRD spectrum of each photothermal catalyst, it could be known that 1–20%CeO2/TiO2 peaked at 2θ = 28.5° (111), 33.1° (200), 47.5° (220), 57.1° (311), 70.41° (400), 77.8° (331), 79.1° (420), and 88.5° (422), respectively, and did not exhibit the peaks of CeO2.[14] It might be attributed that CeO2 was added in such a small amount and did not form an independent crystal.[15] When the modification amount of CeO2 reached up to 40%, the peak intensity at 2θ = 25.4, 37.8, and 48.1° attenuated substantially. Likewise, the intensity of peaks in 40%CeO2/TiO2 representing the fluorite CeO2 crystallite was far weaker than those of CeO2, which was probably attributed to the relatively lower contents of CeO2 or TiO2 on the surface of CeO2/TiO2s than those of neat and CeO2 or TiO2.

Figure 2

XRD spectra of TiO2, 1–40%CeO2/TiO2, and CeO2.

To investigate the form of CeO2 distributed on TiO2s, the transmission electron microscopy (TEM) morphologies of neat TiO2 and 7%CeO2/TiO2 are displayed in Figure . The lattice space of TiO2 nanoparticles was measured to be about 0.35 nm (see Figure a), which was assigned to the fringe space of the (101) plane in anatase. After the modification, the lattice space of 7%CeO2/TiO2 granules was 0.35 nm as well, while the typical lattice spaces of 0.31 nm and 0.38 nm corresponding to the planes of (111) and (101) of cubic fluorite CeO2 were not found.[16] In addition, the XRD result had demonstrated that independent CeO2 crystals were not formed on the surface of 1–20%CeO2/TiO2. Previous literature reported that both the ion radius of Ce3+ (0.103 nm) and Ce4+ (0.102 nm) are larger than the radius of Ti4+ 0.064 nm and it is difficult for Ce3+ and Ce4+ to substitute the Ti4+ or insert into the crystal cell of TiO2.[17] Thus, the added CeO2 was probably amorphously distributed on the surface of TiO2.

Figure 3

TEM morphologies of (a,b) TiO2 and (c,d) 7%CeO2/TiO2.

The SSA is one of the important physical parameters that affect the adsorption activity of the photothermal catalysts. The SSAs of various photothermal catalysts are summarized in Table The BET SSA of TiO2 was 72.1 m2/g, and those of 1–40%CeO2/TiO2 were 88.8, 104.9, 117.5, 119.3, 122.8, 123.1, 124.9, 129.1 m2/g, respectively. The commercial CeO2 has a SSA of only 5.6 m2/g. It can be seen that the SSA of CeO2/TiO2 prepared by the sol–gel method was significantly improved and increased with the addition of Ce(NO3)3·6H2O during the preparation. The distribution pattern of the BET SSAs varied significantly between TiO2 and 7%CeO2/TiO2, as shown in Figure S1. The surface areas of meso- and micropores in 7%CeO2/TiO2 were much higher than those in TiO2, revealing that a large number of meso- and micropores were generated because of the modification of CeO2, thereby leading to the increase in BET SSAs of CeO2/TiO2s.

Table 1

Specific Surface Areas of TiO2 and 1–40%CeO2/TiO2

photocatalysts	BET surface area (m2/g)	Langmuir surface area (m2/g)	t-plot micropore area (m2/g)	t-plot external surface area (m2/g)
TiO2	72.1	115.7		74.2
1%CeO2/TiO2	88.8	139.6	12.1	76.7
3%CeO2/TiO2	104.9	168.1	0.0	104.9
5%CeO2/TiO2	117.5	186.1	2.3	115.2
7%CeO2/TiO2	119.3	188.7	3.0	116.2
10%CeO2/TiO2	122.8	194.7	1.7	121.0
15%CeO2/TiO2	123.1	204.6	1.4	127.6
20%CeO2/TiO2	124.9	198.2	1.5	123.4
40%CeO2/TiO2	129.1	204.6	1.4	127.6
CeO2	5.6	8.4	3.4	2.1

The chemical composition of the photothermal catalysts was measured by XPS. The survey spectra of pristine TiO2 and 7%CeO2/TiO2 are first investigated. The main XPS peaks of the two photothermal catalysts were attributed to Ti and O. The difference between their profiles was little significant. The intensity of the binding energy peak representing Ce 3d was much weaker than that of Ti and O peaks in 7%CeO2/TiO2, as shown in Figure . Figure shows the XPS spectra of Ti 2p. It was observed that two well-defined characteristic peaks attributing to Ti 2p1/2 at about 464 eV and Ti 2p3/2 at about 458 eV appeared for all CeO2/TiO2 samples, indicating that Ti was mainly in the form of Ti4+.[18−22] As the modification amount of CeO2 increased, the binding energies of surface Ti 2p1/2 and Ti 2p3/2 in these photothermal catalysts overall moved toward a higher energy direction. For instance, the binding energies of Ti 2p1/2 and 2p3/2 increased to 465.1 and 460.2 eV, respectively, when the amount of CeO2 reached up to 20%, which suggested an alteration of the chemical state of the Ti atom on the surface. It was probably because accompanying the surface modification of CeO2, the terminal oxygen atom of Ti on the surface of TiO2 would be connected with a Ce atom to form Ti–O–Ce or more oxygen vacancies will be generated on the surface, thereby causing the trapping of outer layer electrons of the Ti atom by the nearby Ce atom or oxygen vacancy.[23]

Figure 4

Low-resolution XPS survey spectra of TiO2 and 7%CeO2/TiO2, and the inset of the Ce 3d profile of 7%CeO2/TiO2.

Figure 5

Ti 2p XPS spectrum of each catalyst.

Figure shows the Ce 3d XPS spectra of 1–20%CeO2/TiO2. Previous literature indicated that the major difference of the Ce 3d XPS features between CeO2 and Ce2O3 is that the Ce 3d XPS peak of CeO2 consists of three pairs of spin–orbit doublets, while that of Ce2O3 has only two pairs of spin–orbit doublets.[24] The Ce 3d XPS feature of 1%CeO2/TiO2 exhibits four deconvolution subpeaks, which are assigned to two pairs of spin–orbital doublets; however, as the modification amount increased, more than two pairs of spin–orbital doublets gradually emerged in the Ce3d XPS spectra of 3–20%CeO2/TiO2, implying that their Ce(IV) oxide mostly existed in a relatively high weight ratio of CeO2 over TiO2. Additionally, the bind peaks labeled u1 and v1 in Ce 3d XPS spectra of 1–20%CeO2/TiO2 represent the initial electronic states of Ce 3d104f1, and the corresponding Ce valence was Ce3+; while the peaks labeled u, v, u2, v2, u3, and v3 were the electronic state of Ce 3d104f0, and the corresponding Ce valence is Ce4+.[25] The transformation between CeO2 and Ce2O3 is believed to be an important factor for oxidizing Hg0 because active oxygen as the primary oxidant of Hg0 was generated during the transformation of Ce valence.[26] The semiquantitative analysis of XPS could determine the mass ratio of different electronic states of Ti 2p, Ce 3d, and different forms of O 1s (Table ).

Figure 6

Ce 3d XPS spectra of (a) TiO2 and 1–20%CeO2/TiO2, (b–f) peak deconvolution of the Ce 3d.

Table 2

Mass Ratio of Each Element of TiO2 and 1–20%CeO2/TiO2

	Ti 2p		Ce 3d		O 1s
photocatalysts	Ti 2p1/2 (%)	Ti 2p3/2 (%)	Ce3+ (%)	Ce4+ (%)	Oα (%)	Oβ (%)
TiO2	25	75	0	0	99	1
1%CeO2/TiO2	25	75	47	53	88	12
3%CeO2/TiO2	24	76	47	53	82	18
5%CeO2/TiO2	28	72	46	54	84	16
7%CeO2/TiO2	24	76	28	72	66	34
10%CeO2/TiO2	25	75	46	54	52	47
15%CeO2/TiO2	25	75	23	77	68	31
20%CeO2/TiO2	26	74	41	59	76	24

For O 1s (Figure ), two kinds of oxygen species were observed in TiO2. The peak at around 529.1 eV corresponded to lattice oxygen (denoted as Oα), while the one at about 531.0 eV was regarded as surface-chemisorbed oxygen (denoted as Oβ).[20] The O 1s also shifted to a high binding energy with the addition of CeO2. Because surface-chemisorbed oxygen (Oβ) was considered to have one of the main roles in oxidizing Hg0,[21] the proportion of Oβ gradually increased with the modification amount of CeO2, implying that the addition of CeO2 could contribute to the oxidation of Hg0.

Figure 7

XPS spectra of (a) TiO2 and (b–h) 1–20%CeO2/TiO2 O 1s.

Figure depicts the fluorescence spectrum of TiO2 and 1–20%CeO2/TiO2. It showed that TiO2 had a fluorescence peak at 550 nm. This peak was thought to be caused by the migration of Ti3+ electrons to O–.[27] The PL peak of CeO2/TiO2 decreased obviously compared to that of TiO2, meaning that the recombination rate of photogenerated electrons and holes in TiO2 was significantly reduced by the addition of CeO2. The addition of CeO2 is beneficial to prolonging the separation time of TiO2 photogenerated electron–hole pairs. It was speculated that the electrons of TiO2 migrate from its conductive band to the conductive band of Ce and the holes of Ce migrate to the valence band of TiO2, thereby increasing the accumulation of electrons in the Ce band and the TiO2 valence band. Effective separation of CeO2/TiO2 electrons and holes promoted the activity of photothermal catalysts.

Figure 8

PL spectra of TiO2 and 1–40%CeO2/TiO2.

Photothermal Catalytic Oxidation Efficiency of Hg0

The experiments investigated the effect of reaction temperatures on the oxidation efficiency of Hg0 with these photothermal catalysts. The experimental parameters were summarized as follows: influent Hg0 concentration of 75 μg/m3; reaction temperatures of 120, 140, and 160 °C; reaction time of 120 min, and near-UV light (λ = 365 nm) was used as a light source. Figure illustrates that the photothermal-oxidation efficiencies of Hg0 by different catalysts were ordered from high to low as: η120°C > η140°C > η160°C. Each catalyst had a photothermal efficiency of more than 90% at 120 °C. As the reaction temperature increased, the photothermal efficiency decreased significantly. In particular, the oxidation efficiency of TiO2 was only about 20% at 160 °C. Nevertheless, the oxidation efficiency of Hg0 using 1–20%CeO2/TiO2 was much higher than that of TiO2. The first stage of CeO2/TiO2 for the removal of Hg0 was physical adsorption. As the reaction progressed, once the adsorption reached equilibrium, Hg0 was converted to its catalytic oxidation state of Hg2+.[28] Under photothermal catalytic conditions, the lattice oxygen (Oα) of the catalyst then participated in the oxidation of Hg0. The lattice oxygen (Oα) was mainly derived from the change of the Ce valence state, and the reaction formula was as follows (eq )

Figure 9

Comparison of photothermal catalytic oxidation efficiency of Hg0 with TiO2 and 1–20%CeO2/TiO2 at a flow concentration of 75 μg/m3 at (a) 120, (b) 140, and (c) 160 °C.

Because metal oxide transfer could provide lattice oxygen (Oα), Hg0 could be oxidized by lattice oxygen (Oα) in the absence of O2 as follows (eq ).[29]

It was pointed out that Ce could adsorb oxygen to produce highly reactive oxygen species and form oxidized species of lattice oxygen (Oα). However, in general flue gas, O2 will provide an oxygen atom to the metal oxide, ensuring that the oxidation of Hg0 could continue. The transfer of O2 to the lattice oxygen (Oα) in the metal oxide is as follows (eq )

Chemically adsorbed oxygen (Oβ) is also known as hydroxyl oxygen (OH), and eq and eq described the oxidation of Hg0 by the hydroxyl oxygen and the regeneration of hydroxyl oxygen on the catalyst surface, respectively.

The effect of different modified concentrations of CeO2/TiO2 on the photocatalytic oxidation of Hg0 at 120–160 °C is shown in Figure . The addition of CeO2 increased the efficiency of photothermal catalytic oxidation of Hg0 at 120 °C. The photothermal oxidation efficiency of Hg0 by 1–20%CeO2/TiO2 was slightly improved with the concentration of CeO2. The original 80% efficiency for neat TiO2 increased to over 90% for 7%CeO2/TiO2. As the reaction temperature increases, the effect of the increase in the modification concentration on the removal efficiency was more obvious. When the reaction temperature reached 140 °C, the photothermal oxidation efficiency of Hg0 by 1, 3, and 20%CeO2/TiO2 decreased significantly, but they were still higher than that of TiO2, while 5, 7, and 10%CeO2/TiO2 retained above 70% of Hg0 removal efficiency. When the reaction temperature increased to 160 °C, the photothermal oxidation efficiency of Hg0 by neat TiO2 was only about 20%, but those for 1–20%CeO2/TiO2 increased by about 10–40%. The photothermal efficiency of 7%CeO2/TiO2 even exceeded 90% at 160 °C. It could be seen that the photothermal removal efficiency of Hg0 gradually improved as the modification amount of CeO2 increased from 1 to 7%, probably because more surface-chemisorbed oxygen was generated on the surface region. The 7%CeO2/TiO2 not only has the best photothermal performance but also maintains high efficiency at a relatively higher reaction temperature of 160 °C. Nevertheless, a further increase in the CeO2 content to 20%CeO2 weakened the photothermal activity of CeO2/TiO2 instead, the reason for which further investigation is needed.

Figure 10

Photothermal catalytic oxidation efficiency of Hg0 by various catalysts at different temperatures.

Conclusions

This study investigated the difference in the SSA between neat TiO2 and CeO2/TiO2s. The SSA of CeO2/TiO2s after upgrading was higher than that before the modification and increased with the increase of the added amount. It is known from the XRD pattern that TiO2 was mainly composed of the anatase crystal form, and there was no peak of rutile. It could be seen from the PL spectrum that the peak of CeO2/TiO2 was significantly lower than the peak intensity of TiO2. It showed that the rate of recombination of photoexcited electrons and holes was reduced. The addition of CeO2 could prolong the recombination time of TiO2-photogenerated electrons and holes and had a significant effect on the improvement of photothermal catalytic activity. The photothermal oxidation efficiency of Hg0 in this study showed that the removal efficiency of Hg0 between TiO2 and 1–20%CeO2/TiO2 decreased with the increase of reaction temperature. Ce-modified TiO2 could effectively improve the oxidation efficiency of elemental mercury. The Ce could adsorb oxygen to generate active oxygen to form a lattice oxygen (Oα)-oxidizing substance. If O2 was present in the general flue gas, O2 would provide oxygen like a metal oxide to ensure that Hg0 oxidation can continue.