Molybdenum trioxide impregnated carbon aerogel for gaseouselemental mercury removal

DOI: 10.1007/s11814-020-0481-x

INVITED REVIEW PAPER

eISSN: 1975-7220

INVITED REVIEW PAPER

†To whom correspondence should be addressed.

E-mail: [email protected], [email protected] Copyright by The Korean Institute of Chemical Engineers.

Molybdenum trioxide impregnated carbon aerogel for gaseous elemental mercury removal

Yang Ling*, Xiaokun Man***, Wenbo Zhang*, Daolei Wang*^,†, Jiang Wu*^,**^,†, Qizhen Liu****, Mingyan Gu*****, Yuyu Lin*****, Ping He*, and Tao Jia*

*College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China

**Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China

***State Grid Shandong Electric Power Company Construction Company, Jinan 250022, China

****Shanghai Environment Monitoring Center, Shanghai 200030, China

*****School of Energy and Environmental Engineering, Anhui University of Technology, Maanshan 243002, China (Received 16 August 2019 • accepted 5 January 2020)

AbstractA novel gaseous elemental mercury (Hg⁰) removal agent was successfully synthesized via impregnation method, by using molybdenum trioxide (MoO3) as the active component and carbon aerogel (CA) as the carrier. The as-prepared samples maintained a large specific surface area and excellent pore structure of the pure carbon aerogel, so that MoO3 was better dispersed to obtain enhanced Hg⁰ removal performance. The maximum efficiency of elemental mercury removal was about 74%, achieved by Mo/C500 sample at 300^oC, while it still had good ability (nearly 60%) in the range of 500-700^oC. The mechanism of mercury oxidation removal was also verified by DFT calculation. This work should help in developing suitable materials for thermocatalytic oxidation of elemental mercury, and also pro- vide some theoretical basis and data support for full-scale application of heavy metal mercury pollution control in coal- fired power plants.

Keywords: Mercury, Carbon Aerogel, Molybdenum Base, DFT

INTRODUCTION

Mercury (Hg) is a kind of heavy metal pollutant with extreme toxicity, persistence and bioaccumulation, the effective control of which has attracted global attention [1,2]. Mercury pollution from coal combustion is the main man-made source of mercury emis- sions. There are three main types of mercury in flue gas from coal- fired power plants [3]: elemental mercury (Hg⁰), oxidized mercury (Hg²⁺) and particulate-bound mercury (Hg^p). Oxidized mercury is soluble in water, so it can be removed by wet flue gas desulfuriza- tion system (WFGD); while particulate-bound mercury can be re- moved by a dust remover, such as electrostatic precipitators (ESP) or fabric filters (FF). However, elemental mercury, which is the largest proportion in the three forms, is highly volatile and insoluble in water, it is difficult to remove by the existing air pollution con- trol devices (APCDs) in power plants [4,5]. In addition, the high temperature environment in the gas flue also poses a challenge for effective removal of mercury.

To remove the mercury in flue gas, the key point is to reduce the percentage of Hg⁰. There are two main methods to reduce the Hg⁰ at present. One is adsorption technology, which uses the active site on the adsorbent surface and fixes the Hg⁰ from the flue gas [6].

Currently, power plants use activated carbon injection to absorb mercury, but it has low utilization and high running cost, and will

change the composition of fly ash, which is not conducive to the recycling of fly ash. The second method is the catalytic oxidation technology, which has been of wide concern to researchers [7-9].

The purpose of this method is to oxidize Hg⁰ into Hg²⁺. While, Hg²⁺

is easy to make soluble in water, and the subsequent WFGD sys- tem can absorb Hg²⁺ so as to achieve the goal of mercury removal.

This method has high utilization rate of mercury removal agent and does not change the composition of fly ash, which is increas- ingly favored by scholars [10-12]. Jampaiah et al. [13] investigated the low-temperature elemental mercury removal over TiO2 nanorods- supported MnOx-FeOx-CrO, which manifested good Hg⁰ removal efficiency at low temperatures in the presence of O2. The authors held that the presence of Mn⁴⁺, Cr⁶⁺, and Fe³⁺ could promote Hg⁰ oxidation due to the strong synergistic interaction between these nanoparticles. Cimino et al. [14] studied the performance of simul- taneous low-temperature selective catalytic reduction (SCR) of NOx

and Hg capture via MnOx catalysts supported on TiO2 or Al2O3. And the authors believed that the higher proportion of Mn⁴⁺ sites led to the much better Hg capture performance of Mn-based sor- bent over TiO2 than Al2O3. However, their work was mostly con- fined to experimental research, lacking theoretical calculation, and the catalyst operates in a narrow temperature window which makes it difficult to cope with complex flue environment.

Molybdenum trioxide (MoO3), an important transition metal oxide due to its rich chemistry that is associated with multiple valence states and its high thermal and chemical stability, has been widely studied [15-18]. Studies have shown that MoO3 has a good effect on catalytic oxidation of elemental mercury [19-21]. However, its

catalytic oxidation mechanism with mercury is less studied, to the best of our knowledge, and its small specific surface area is not con- ducive to the adsorption of reactants. Carbon aerogel (CA), a clas- sic porous material with controllable pore structure, can be widely used in preparation of adsorption catalyst and energy storage areas.

It has large specific surface area and outstanding pore structure, high thermal stability and good mechanical properties after high tem- perature carbonization, providing the possibility to become an ideal carrier and loading the active components [22,23]. In this paper, car- bon aerogel was used as the carrier to prepare molybdenum triox- ide impregnated carbon aerogel catalyst, and the as-prepared samples were tested to remove elemental mercury at different temperature.

The as-prepared sample had good stability and could remove ele- mental mercury over a wide range of temperature. The removal mechanism of elemental mercury was also explored through the aid of DFT (density functional theory) calculations. This paper has posited a new research idea for high temperature mercury pollut- ant control in coal-fired power plants.

EXPERIMENTAL

1. Preparation of Carbon Aerogel and Mo/Cx Samples The resorcinol and formaldehyde were all obtained from Shang- hai Zhanyun Chemical Company; sodium carbonate and ethyl alco- hol were purchased from Sinopharm Group Chemical Reagent Company. The molybdenum trioxide was purchased from Shang- hai Huayuan Chemical Company. All solutions were prepared with deionized water and all chemicals used in this research were ana- lytical grade and used without further purification.

The pure carbon aerogels were synthesized via aqueous sol-gel polymerization of resorcinol (R) and formaldehyde (F). The sodium carbonate (C) was using as a basic sorbent. First, we added 100 ml deionized water into the beaker, and then added resorcinol and formaldehyde to the beaker at the molar ratio of 1 : 2. Carbon aero- gels with different R/C ratios were obtained by changing the amount of sodium carbonate (C) added to the beaker. The beaker was sealed and stirred at room temperature for 2 h. The solution was then placed into an airtight container, heated via water bath at 50 and 80^oC for 1 and 3 days, respectively. The obtained aerogel was then cooled naturally at room temperature. To remove water from their structures, the aerogel was solvent exchanged by immersing in alco- hol for 24 hours. The solvent exchange was performed three times in total, and then the aerogel was treated by freeze drying. The dried organic aerogel was ground evenly and then carbonized in a tubu- lar furnace. Under the protection of N2, the aerogel was heated to 950^oC with a heating rate of 5^oC/min and kept for three hours and then naturally cooled to room temperature. The carbon aerogel was obtained and stored in glass vials for later use. In this paper, the as-prepared carbon aerogel are denoted as Cx, where x represents the molar ratio of resorcinol (R) and sodium carbonate (C). For example, if the R/C ratio of sample was 200, it would be expressed as C200.

The molybdenum trioxide impregnated carbon aerogel (short- hand for Mo/Cx) was synthesized via wet impregnation method and calcined under N2 atmosphere. The molybdenum trioxide (MoO3) was used as the active component, and the mass ratio between

Mo and carbon aerogel was 1 : 1. Taking the preparation of Mo/

C200 as an example, the detailed preparation process is as follows.

First, 100ml deionized water was added into the beaker, and then 1 g as-prepared C200 and 1.5003 g MoO3 were weighed into the beaker. After sufficient stirring, the mixture was ultrasonically oscil- lated for 1.5 h. Then, the beaker was placed into an 80^oC oven, dry- ing under atmospheric pressure for 12 h. The mixture was transferred into a corundum boat and put into a tubular furnace, calcining at 700^oC (heating rate 5^oC/min) for 6 h. After natural cooling to room temperature, the Mo/C200 sample was obtained by grinding. Other samples, such as Mo/C500 and Mo/C800, were prepared accord- ing to a similar method.

2. Characterization

The Brunauer-Emmett-Teller (BET) method (Micromeritics ASAP 2020) was used to find the specific surface area of the sam- ples via analyzing the N2 adsorption/desorption data; the error bars of this method are 1%. The surface structure and morphology of the samples was observed via a scanning electron microscope (SEM, Phillips XL-30 FEG/NEW). The crystallinity of the as-prepared samples was characterized by X-ray diffraction (XRD, BRUKER D8 ADVANCE Diffractometer, Germany) under Cu K radiation, where the scanning range was initiated from 10 to 90^o (scan rate 2^o/min). To identify the properties of elemental composition, X- ray photoelectron spectroscopy (XPS) was carried out with Al K

X-ray radiation operating at 250W (PHI5300, USA).

3. Activity Test

The as-prepared samples were tested for elemental mercury (Hg⁰) removal at different temperature. For each test, 50 mg sample and a certain amount of glass beads (2 mm of diameter) were loaded into a vertical fixed-bed quartz reactor (8 mm of inner diameter, 700 mm of length) and then heated to the given temperature (heat- ing rate 5^oC/min). The N2-O2 mixed gas (95% N2 and 5% O2 by volume, 1.2 L/min) containing Hg⁰ (about 50g/m³, achieved by PSA mercury generator) flowed through the quartz glass tube con- tinuously. Each test lasted three hours. The Hg⁰ concentration at reactor inlet and outlet was measured by an on-line mercury ana- lyzer (Lumex, RA-915-M, Russia), and the elemental mercury re- moval efficiency could be calculated by the following formula:

(1)

where [Hg⁰]0 represents the Hg⁰ concentration (g/m³) at the reac- tor inlet, while [Hg⁰] represents the Hg⁰ concentration (g/m³) at the reactor outlet.

4. Computational Method

Density functional theory (DFT) calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) codes.

The exchange and correlation effects were described by the Per- dew-Burke-Ernzerhof (PBE) functional within generalized gradi- ent approximation (GGA). The convergence criteria were as follows:

maximal force on the atoms 0.05 eV/Å, maximal stress on the atoms 0.1 GPa, maximal atomic displacement 0.002 Å, and maximal energy change per atom 2.0×10^5eV. A 3*3*1 MoO3 (010) surface was used to simulate the reaction surface, and the vacuum slab was set as 15 Å to reduce interference from the adjacent layers. The absorp- tion energy was calculated by the following formula:

Hg⁰⁰Hg⁰ Hg⁰

 ⁰

--- 100%

Fig. 1. Schematic diagram of the experimental system.

Table 1. Summarized N2 physisorption/desorption data Samples BET surface

area (m²/g)

Micropore area (m²/g)

Total pore volume (cm³/g)

Micropore volume (cm³/g)

Mesoporous volume (cm³/g)

BJH average pore diameter (nm)

C200 488 280 0.3579 0.1454 0.2125 04.16

C500 542 332 1.4622 0.1724 1.2898 26.03

C800 522 337 1.5627 0.1749 1.3878 36.41

Mo/C200 204 122 0.1472 0.0628 0.0844 04.17

Mo/C500 234 141 0.5978 0.0728 0.5250 23.80

Mo/C800 204 136 0.5639 0.0704 0.4935 32.25

Fig. 2. N2 adsorption-desorption isotherms of Cx and Mo/Cx samples (x=200, 500 and 800).

Ead=E(AB)E(A)E(B) (2)

where Ead represents the absorption energy. E(AB) is the total energy of B molecule adsorbed by A molecular surface. E(A) and E(B) represent the total energy of isolated A and B molecule, respec- tively. Commonly, a negative adsorption energy indicates that the reaction is spontaneous. The lower the adsorption energy, the more stable the adsorption configuration.

RESULTS AND DISCUSSION

1. N₂ Adsorption/Desorption Analysis

To obtain detailed information of the specific surface area and pore structure parameters of as-prepared samples, BET analysis was carried out. As shown in Table 1, the average pore sizes of carbon aerogel C200, C500 and C800 are 4.16, 26.03 and 36.41 nm, respec- tively, all of which belong to mesoporous materials. From Fig. 2, the adsorption isotherm curves of the pure carbon aerogels C500 and C800 are similar, which could be divided into two zones: low pressure zone and high pressure zone (0.8<P/P0<1.0). The isotherm travel appears hysteresis loop in the high pressure section, indicat- ing that the pore diameter of the samples is greater than 3.6 nm, existing capillary condensation and mesoporous structure. Both adsorption and desorption curves are steep, which indicates that the relative pressure is relatively concentrated when the N2 con- denses and evaporates within the C500 or C800 sample. This con- firms that the C500 and C800 samples contain cylindrical channels with both ends open, which also contributes to the adsorption of mercury. On the contrary, C200 does not have this phenomenon.

It has a relatively flat curve at high pressure. The adsorption plat- form appeared in the low-pressure section of all pure carbon aero- gel samples (C200 at P/P0<0.4, C500 and C800 at P/P0<0.6), in- dicating that multi-layer adsorption phenomenon occurred in the samples, with the pore diameter less than 2 nm and a small num- ber of micropores. According to the IUPAC classification, the pure carbon aerogels of C500 and C800 showed a typical IV adsorption isotherm with uniform pore size distribution [24]. The molar ratio of resorcinol (R) and sodium carbonate (C) has an important influ- ence on the pore structure of carbon aerogel samples. The specific surface area and micropore volume of pure carbon aerogel with different R/C ratio (C200, C500 and C800) have little difference.

However, there is a large gap in mesoporous volume and average pore diameter. The micropore and mesoporous volumes of C200 are 0.15 cm³/g and 0.21 cm³/g, respectively. The mesoporous vol- ume is slightly more than the micropore, and the maximum ad- sorption capacity is comparably low, only 231 cm³/g. The meso- porous volumes of sample C500 and C800 are 1.29 and 1.39 cm³/g, respectively, and micropore volume is about 0.17 cm³/g, which also indicates that more mesoporous was formed when the R/C ratio was higher during the carbon aerogel preparation process. The aver- age pore size of C500 and C800 is 26.03 and 36.41 nm, respectively, and the maximum adsorption capacity is 945 and 766 cm³/g, which are much bigger than that of C200. The difference of mesoporous volume and pore diameter in carbon aerogel has great influence on mercury adsorption.

It can be seen from Table 1, that the specific surface area and

pore structure parameters of Mo/Cx samples are significantly dif- ferent from that of pure carbon aerogel without loading molybde- num trioxide. Compared with the pure carbon aerogel of Cx, the average pore size of Mo/Cx showed a slight decrease. While the specific surface area decreased from 488, 542 and 522 m²/g to 204, 234 and 204 m²/g, respectively. The mesoporous volume decreased

Fig. 3. SEM images of (a) C500 carbon aerogel; (b) Mo/C500 before the mercury removal experiment; (c) Mo/C500 after the mer- cury removal experiment.

from 0.21, 1.29 and 1.39 cm³/g to 0.08, 0.53 and 0.49 cm³/g, respec- tively. As shown in Fig. 2, the N2 adsorption-desorption isotherms of Mo/Cx are roughly the same as that of the corresponding pure carbon aerogel, and the hysteresis loop appears in the same pres- sure range. However, the adsorption capacity decreased significantly, and the maximum adsorption capacity of Mo/C500 is only 374 cm³/g. This may be due to the introduction of molybdenum triox- ide blocking part of the carbon aerogel channels.

2. SEM Analysis

Fig. 3 shows the SEM images of the as-prepared C500 carbon aerogel and Mo/C500 sample before/after the mercury removal experiment (reaction temperature 500^oC). As shown in Fig. 3(a), the as-prepared C500 carbon aerogel is loose and porous with obvi- ous pore structure. Carbon nanoparticles are connected with each other and have obvious pore channels. As the MoO3 impregnated (Fig. 3(b)), the Mo/C500 sample is also porous and has obvious pore structure. After the mercury removal experiment (Fig. 3(c)), the sample surface became smoother and the pore channels of the samples were partially blocked. It might be that the reaction prod- ucts accumulated on the carbon nanoparticles, causing the block- age of the pore channels and showing a denser network structure.

3. XRD Analysis

The XRD pattern of the Mo/Cx sample (x=200, 500 and 800) is shown in Fig. 4. Compared with the standard XRD card (JCPDS NO. 47-1081), the obvious diffraction peak in 2 =25.94^o, 53.45^o and 60.46^o could be determined as the diffraction peak of MoO3, which is consistent with the chemical composition of the sample.

Looking in more detail, notice that there exist obvious diffraction peaks at 2 =36.90^o, 53.04^o, 60.19^o and 66.46^o, which is the diffrac- tion peak of MoO2 (JCPDS NO. 32-0671). According to XRD anal- ysis, the existence of molybdenum dioxide (MoO2) is confirmed, the main reason for which is that the Mo⁶⁺ is reduced by carbon aero- gel at high temperature and the reaction is proposed as follows:

2MoO3+C2MoO2+CO2 (3)

According to Fig. 4, the diffraction peaks of Mo/Cx (x=200, 500

and 800) are basically the same, which indicates that the MoO3

and carbon aerogel have relatively complete reactions. MoO2 is gener- ated in different R/C ratio carbon aerogel, but the amount of MoO2 is different, which may further affect the reaction activity for mer- cury removal.

4. XPS Analysis

X-ray photoelectron spectroscopy (XPS) was used to further ana- lyze the surface chemical content of the samples, as shown in Fig.

5. The Mo/C500 samples before and after the Hg⁰ removal experi- ment (reaction temperature 500^oC) were labeled as Mo/C500 fresh and Mo/C500 used, respectively. The fully survey scanned XPS spectra (Fig. 5(a)) agree well with the chemical composition of the tested samples. The surface of pure C500 is mainly composed of carbon peaks, while some weak peaks of oxygen are also observed which may be due to the adsorption of a small amount of water on the surface of the sample. With the impregnation of molybde- num trioxide, the Mo/C500 samples all show strong peaks of oxy- gen and molybdenum. After the elemental mercury removal ex- periment, the mercury peaks in the used Mo/C500 sample are too weak to observed via fully scanned XPS spectra, which demon- strated that less mercury species remained on the surface of the sample after the catalytic reaction. As shown in Fig. 5(b), the C 1s XPS spectra of the three samples (C500, fresh Mo/C500 and used Mo/C500) are very similar, which can be divided into two peaks.

The main C 1s peak at 284.8 eV is dominated by C-C bond in the carbon aerogel, while the peaks at about 286.14-286.21 eV are charac- teristics of the oxygen bound species C-O [25]. From the high res- olution XPS spectra of O 1s in Fig. 5(c), the fresh and used Mo/

C500 samples show a similar distribution which all have two peaks.

The ones at about 530.86-530.93 eV are referred to the feature peaks of lattice oxygen [26]. While the higher binding energy peaks at about 532.05-532.33 eV are ascribed to the surface absorbed oxy- gen, such as -OH group and chemisorbed oxygen-containing spe- cies [27,28]. The C500 sample has a weak and broad peak around 533.01 eV caused by the adsorbed H2O on the sample surface [29].

By comparison, the as-prepared molybdenum trioxide impregnated carbon aerogel has greater concentration of chemically adsorbed oxygen, which facilitates the catalytic reaction with the mercury adsorbed on the catalyst surface [28]. As shown in Fig. 5(d), the Mo 3d3/2-3d5/2 doublet [30] gives the most intense peaks in fresh or used Mo/C500 sample. Its relative position is dependent mainly upon the oxidation sate. The binding energy at 233.03 and 236.2 eV of fresh Mo/C500 sample commonly ascribed to Mo⁶⁺ predom- inates [31]. Besides, a weak contribution of Mo 3d5/2 peaks at energy 229.9 and 235.09 eV is also observed, indicating some amount of Mo-species in lower oxidation number [32]. This result further confirms that part of the molybdenum has been reduced by the carbon aerogel under calcination process. The Mo⁶⁺ state also domi- nates in the XPS spectra of used Mo/C500 sample (233.08 and 236.25 eV), accompanied by a small contribution of Mo⁴⁺ (229.71 and 235.09 eV). However, the contribution of Mo⁴⁺ state is relatively decreased compared to the fresh Mo/C500. Considering the set cata- lytic reaction condition (500^oC, 5% O2 in N2 by volume as the car- rier gas), the oxygen in the gas stream can repair the oxygen vacancy in the Mo/C500 sample, thus partially raising the oxidation sate of these lower oxidation number Mo-species [26]. As shown in the Fig. 4. XRD patterns of Mo/Cx samples (x=200, 500 and 800). (a)

Mo/C200, (b) Mo/C500, (c) Mo/C800.

high-resolution XPS spectra of Hg 4f (Fig. 5(e)), the peaks at 104.76 and 101.78 eV are assigned to Hg 4f5/2 and Hg 4f7/2 states of HgO [33]. In addition, there is no obvious peak of elemental mercury [34,35], suggesting that mercury mainly exists in the form of Hg²⁺

after the catalytic reaction [36].

5. Elemental Mercury Removal Experiment

5-1. Elemental Mercury Removal Performance of Mo/Cx Samples Fig. 6 shows the elemental mercury (Hg⁰) removal efficiency of Fig. 5. XPS spectra of C500, fresh Mo/C500 and used Mo/5600 samples: (a) fully survey scanned XPS spectra; (b) C 1s; (c) O 1s; (d) Mo 3d

and (e) Hg 4f.

Mo/Cx samples (x=200, 500 and 800) at 500^oC. The efficiency curve of all samples first rose in a short time, then gradually fluctuated and decreased to a stable value. The phenomenon of sharp rising peak at the beginning may because when the carrier gas pipeline starts to change from the bypass to the main road, it takes a little while for the stream to get through the main road, so the elemen- tal mercury concentration is low at the moment. Then it gradually reaches a stable state and the elemental mercury removal efficiency of Mo/Cx samples is all above 40% at 500^oC. With the increase of R/C ratio, the Hg⁰ removal efficiency first increased and then de- creased. Mo/C500 sample had the highest Hg⁰ removal efficiency, reaching 65% at the beginning, and could still remain at nearly 60% after three hours. However, the Hg⁰ removal efficiency of Mo/

C200 was only about 43%, which may be due to the existence of a large number of micropores in its pore structure and is prone to blockage and sintering during the reaction. The Hg⁰ removal effi- ciency of Mo/C800 sample was about 50%, possibly because the specific surface area of C800 carbon aerogel is lower than that of C500 (Table 1). The adsorption and removal of elemental mer- cury by demercuration agent is a combined result of physical and chemical adsorption. When molybdenum oxide was added, the adsorption of Hg⁰ improved significantly at a certain reaction tem- perature [37], enhancing the oxidation efficiency for Hg⁰.

5-2. Effect of Reaction Temperature

Fig. 7 shows the Hg⁰ removal efficiency of Mo/C500 sample at different temperatures. The temperature was selected as 120, 300, 500, 600 and 700^oC (while 120^oC is the flue gas exhaust tempera- ture in some coal-fired power plants). When the reaction tempera- ture is 120^oC, the Hg⁰ removal efficiency is about 60%. The ad- sorption of mercury by the Mo/C500 sample could be dominated to physical adsorption at this temperature. The main influencing factor of physical adsorption is van der Waals interaction. The adsorption is without selectivity, so the effect of molybdenum tri- oxide is not obvious. As the reaction temperature rises to 300^oC, the Hg⁰ removal efficiency of Mo/C500 sample reaches the maxi- mum about 74%. This may be because with the increase of tem-

perature, the effect of chemisorption gradually emerges. The elem- ental mercury could form a certain chemical bond with the oxy- gen-containing functional groups on the Mo/C500 sample surface.

At the same time, the increase of temperature would enhance the activity of metal oxides and promote the release of lattice oxygen.

When the temperature reaches to 500^oC, the Hg⁰ removal effi- ciency drops notably. It could reach 70%, and then gradually decline to about 65%. It is probably because when the reaction tempera- ture is higher, the mercury removal products accumulated on the surface of Mo/C500 sample, caused the blockage and collapse of the tunnel, hindering the further process of mercury removal reac- tion. Besides, the sublimation phenomenon takes place partially on molybdenum trioxide at high temperature [38], leading to a decrease of active ingredients. In addition, with the increasing tem- perature, the reaction product (HgO) will gradually decompose [39], which leads to the secondary release of elemental mercury. All these lead to the decrease of the total elemental mercury removal effi- ciency. Therefore, it is important to operate at the optimum tem- perature during the practical industrial application. In the range of 500-700^oC, the initial value of Hg⁰ removal efficiency is around 50%.

As time goes on, the Hg⁰ removal efficiency gradually reaches about 60%. three hours later, it can still stay around 60%, indicating that the Mo/C500 sample has a good stability, which could be possibly used for elemental mercury removal from flue gas at high tem- perature.

5-3. The Stability of Mo/C500 Sample

The stability of samples is a crucial property for practical appli- cation. As shown in Fig. 8, a long time gas phase Hg⁰ removal ex- periment by Mo/C500 sample was taken for five cycles. The flue gas temperature was set to 500^oC and each test lasted three hours.

In the first test, the efficiency curve rose first in a short time, reach- ing over 70%. Then gradually fluctuated down to a stable value of about 60%. In the subsequent experiments, the Hg⁰ removal effi- ciency stayed around 60%, with little difference in each test. The experimental result shows that the Mo/C50 sample has a good sta- bility at 500^oC and can remove elemental mercury effectively over Fig. 6. The minutely elemental mercury removal efficiency of Mo/

Cx (x=200, 500 and 800) at 500^oC.

Fig. 7. The minutely elemental mercury removal efficiency of Mo/

C500 sample at different temperature.

a long period of time, which is helpful for industrial application.

6. Proposed Reaction Mechanism

The orthorhombic molybdenum trioxide crystal has a rare lay- ered structure, as shown in Fig. 9. This layered structure is com- posed of planar chains, which are formed by distorted MoO6 octa- hedron. The MoO6 octahedrons in each layer are connected in one direction by common edges, while connected by sharing the top oxygen atoms in the other direction, and then the layered struc- ture is expanded infinitely in the two-dimensional space [26,40].

The terminal oxygen atoms of Mo=O bonds on the (010) plane, which is the most stable surface [41-43], resemble oxygen anion radicals and may act as active sites for the adsorption of reactant [44,45]. On the other hand, the carbon aerogels are porous nano- materials composed of spherical carbon nanoparticles connected with each other, which is very suitable as a catalyst carrier to make up for the low specific surface area [46] of molybdenum oxide. It is beneficial to provide sufficient diffusion channel and reaction space for the active components [22], thus affecting the effect of mercury removal.

Based on the above discussion, a possible elemental mercury removal mechanism can be speculated. When the temperature is relatively low (120^oC), MoO3 has a very weak adsorption for mer-

cury. At this time, the elemental mercury is mainly captured by the carbon aerogel and the catalytic action is unsubstantial between Hg⁰ and MoO3. As the temperature increases (300^oC), the chemi- cal adsorption of mercury occupies a dominant position at this time. In fact, the local environment of the terminal oxygen atoms on MoO3 (010) plane has been changed because of the surface relaxation phenomenon and behaves by different reactivity from the lattice oxygen. On one side of these surface oxygen atoms is Mo⁶⁺ which owns a certain electrophilicity, and the other side faces the atmosphere. Therefore, there is some electron transfer from the surface oxygen atom to the neighboring molybdenum atom [26,43], which results in increased surface oxygen activity and forms electrophilic oxygen species. From this base, the elemental mercury in the flue gas may be adsorbed on the MoO3 surface and inter- acts with surface oxygen to form Mo-O-Hg state. Then, part of the Mo-O bonds break, so the mercury oxide (HgO) is swept down- stream with the carrier gas. Meanwhile, there is also a small amount of mercury remaining on the surface of the catalyst in the form of oxidation state according to XPS characterization. Molybdenum trioxide is partially reduced to some low oxidation state such as Mo⁴⁺ or Mo⁵⁺ because of losing surface oxygen. Next, the oxygen in the carrier gas (95% N2 and 5% O2 by volume in this experiment) will fill the oxygen vacancy on the surface and hence regenerate the catalyst under suitable heating conditions [26].

To verify the repair effect of the oxygen in carrier gas on oxy- gen vacancy of catalyst surface, contrastive tests with or without oxy- gen in carrier gas were performed, and the corresponding results are shown in Fig. 10. As shown in the XPS spectra of O 1s in Fig.

10(a), the peaks around 530.97-531.01 eV are caused by the lattice oxygen in MoO3, while the peaks around 532.07-532.68 eV are attributed to the surface adsorbed oxygen in the sample surface.

After the catalyst is exposed to the carrier gas, which is the mix- ture of nitrogen and oxygen (95% N2 and 5% O2 by volume), the lattice oxygen concentration is significantly increased, from 78.1%

to 80.5%, compared with pure N2 as the carrier gas. It proves that the oxygen in flue gas can repair the oxygen vacancy left on the catalyst surface. As shown in the XPS spectra of Mo 3d (Fig. 10(b)), the strong pair of peaks at about 233.11-233.12 and 236.26-236.28 eV are ascribed to the Mo⁶⁺ in MoO3. While the weak peaks at about 229.76-229.81 and 235.09-235.11 eV are ascribed to the Mo⁴⁺

caused by the oxygen vacancy on the catalyst surface. The concen- tration of Mo⁴⁺ is 2.1 and 1.4% in the absence and presence of oxygen, respectively. It indicates that the presence of oxygen can repair the oxygen vacancy on the catalyst surface, thus allowing Mo⁴⁺ to be oxidized to Mo⁶⁺. As shown in Fig. 10(c), a long-term experiment over ten hours further confirms the importance of oxy- gen in flue gas. In the absence of oxygen, the Hg⁰ removal efficiency of the catalyst gradually decreased over time, and only 25% remained after ten hours. On the one hand, the reactive oxygen species on the catalyst surface are consumed by elemental mercury, because there is no subsequent oxygen supplement. On the other hand, the catalyst itself has a certain adsorption effect on elemental mer- cury, but with the increase of the adsorption amount of mercury, the adsorption sites on the catalyst surface are completely occu- pied, so that more elemental mercury cannot be fixed. The above reasons together lead to the continuous decline of Hg⁰ removal Fig. 8. The stability of Mo/C500 sample with five cycles at 500^oC.

Fig. 9. Schematic of MoO3 crystal structure: (a) bulk MoO3 and (b) MoO3 (010) surface.

efficiency. However, when oxygen is present, the Hg⁰ removal effi- ciency is very stable, remaining at more than 72% after ten hours.

As mentioned, the presence of oxygen can make up for the oxygen vacancy on the sample surface, forming some surface adsorbed oxygen species, so that elemental mercury is continuously oxidized into the mercuric oxide. Fig. 10(d) also shows the repair effect of oxygen on catalyst. In the absence of oxygen, the Hg⁰ removal effi- ciency of the catalyst decreased slowly, leaving only 50% after three hours. And then, 5% oxygen was injected continuously, increas- ing the Hg⁰ removal efficiency and stabilizing about 74% in three hours, which further illustrates the repair effect of oxygen on the surface of catalyst.

In addition, according to the DFT calculations, the feasibility of the above conjecture can be further verified (Fig. 11). Initially, the Hg atom is adsorbed on MoO3 (010) surface, which reacts with one surface lattice O atom and forms an O-Hg bond on the surface.

This process (AB) is thermodynamically spontaneous, releas- ing heat by 0.148 eV. Secondly, the formed mercuric oxide (HgO)

desorbed from the MoO3 surface (BC), which requires an energy barrier of 4.806 eV and is endothermic by 4.782 eV. Due to the desorption of HgO, an oxygen vacancy is formed on the catalyst surface. Thirdly, the formed oxygen vacancy can strongly accom- modate O2 (CD), while the adsorption energy is 2.326eV. The surface with oxygen vacancy defect is repaired, forming a special structure of MoO3 with an additional surface reactive oxygen hang- ing. And then, another Hg atom can be adsorbed on the surface reac- tive oxygen species (DE), and the adsorption energy is 0.138 eV. Once more, the second HgO is desorbed from the surface (EF), which has to overcome the energy barrier by 2.225 eV and the reaction is endothermic by 0.727 eV. Finally, the MoO3

(010) surface is regenerated and the reaction goes to the next cycle.

The whole reaction (ABCDEF) is a net endothermic process, and the net heat absorption is 2.897 eV. Heat is needed for the process of reaction, which is consistent with the experimental phenomenon. The most difficult step is the desorption of the first HgO from clean MoO3 surface, and its higher energy barrier (4.806 Fig. 10. XPS spectra of Mo/C500 sample after the mercury removal experiment (3 hours) for (a) O 1s and (b) Mo 3d; (c) the Hg⁰ removal efficiency of Mo/C500 sample for 10 hours; (d) the Hg⁰ removal efficiency of Mo/C500 sample in pure N2 for 3 hours, then treat with N2+5% O2 to regenerate for 3 hours (For each test, the catalyst amount was 50 mg, the reaction temperature was 300^oC, and the car- rier gas was pure N2 or N2+5% O2).

eV) restricts the whole reaction. Comparatively speaking, the sec- ond HgO is easier to desorb which just has to overcome the energy barrier by 2.225 eV. The oxygen vacancy defect on MoO3 surface is very important, because it can easily absorb oxygen from flue gas and form a surface reactive oxygen species. During the whole cycle, the catalyst itself does not consume, but only needs to pro- vide a certain amount of oxygen, and the element mercury in flue gas could be continuously catalyzed oxidized into mercury oxide.

CONCLUSIONS

Molybdenum trioxide impregnated carbon aerogel was success- fully synthesized via aqueous sol-gel polymerization and wet im- pregnation method. The carbon aerogel samples have large spe- cific surface area, pore volume and stable pore structure, provid- ing sufficient active sites for MoO3 loading. During the calcination process, part of Mo⁶⁺ is transformed into Mo⁴⁺ because of the reduc- tion by carbon aerogel. The as-prepared samples have good effi-

ciency for gaseous elemental mercury removal and cycling stability at high temperature environment, and the maximum efficiency could achieve about 74% (Mo/C500 sample, 300^oC). It still has good ability to remove elemental mercury (~60%) within the range of 500-700^oC. The as-prepared sample has good stability and can remove elemental mercury at high temperature in a long time. The most difficult step in reaction is the first HgO desorbed from clean MoO3 surface. The oxygen vacancy defect on the MoO3 surface plays a key role during the thermally catalytic oxidation process.

This work would give some help in developing suitable materials for thermocatalytic oxidation of elemental mercury, and also pro- vide some theoretical basis and data support for full-scale applica- tion of heavy metal mercury pollution control in coal-fired power plants.

ACKNOWLEDGEMENTS

This work was partially sponsored by National Natural Science Foundation of China (50806041, 51106133, 51606115), Natural Sci- ence Foundation of Shanghai (18ZR1416200).

SUPPORTING INFORMATION

Additional information as noted in the text. This information is available via the Internet at http://www.springer.com/chemistry/

journal/11814.

REFERENCES

1. J. H. Pavlish, E. A. Sondreal, M. D. Mann, E. S. Olson, K. C. Gal- breath, D. L. Laudal and S. A. Benson, Fuel Process. Technol., 82, 89 (2003).

2. N. Saman, H. Kong, S. S. Mohtar, K. Johari, A. F. Mansor, O. Has- san, N. Ali and H. Mat, Korean J. Chem. Eng., 36, 1069 (2019).

3. C. E. Romero, Y. Li, H. Bilirgen, N. Sarunac and E. K. Levy, Fuel, 85, 204 (2006).

4. X. Zhou, J. Wu, Q. Li, Y. Qi, Z. Ji, P. He, X. Qi, P. Sheng, Q. Li and J. Ren, Chem. Eng. J., 330, 294 (2017).

5. Y. Ling, C. Zhang, J. Wu, W. Xu, Y. Qi, P. He, L. Zhao, Y. Guan, Z.

Zhang and Y. Tian, Catal. Commun., 116, 91 (2018).

6. W. Xu, A. Hussain and Y. Liu, Chem. Eng. J., 346, 692 (2018).

7. H. Li, W. Zhang, J. Wang, Z. Yang, L. Li and K. Shih, Waste Man- age., 74, 253 (2018).

8. H. Li, L. Zhu, S. Wu, Y. Liu and K. Shih, Int. J. Coal Geol., 170, 69 (2017).

9. H. Xu, J. Jia, Y. Guo, Z. Qu, Y. Liao, J. Xie, W. Shangguan and N.

Yan, J. Hazard. Mater., 342, 69 (2018).

10. J. Mei, C. Wang, L. Kong, X. Liu, Q. Hu, H. Zhao and S. Yang, Environ. Sci. Technol., 53, 4480 (2019).

11. J. Yang, W. Zhu, S. Zhang, M. Zhang, W. Qu, H. Li, Z. Zeng, Y.

Zhao and J. Zhang, Chem. Eng. J., 360, 1656 (2019).

12. Y. Zhu, X. Han, Z. Huang, Y. Hou, Y. Guo and M. Wu, Chem. Eng.

J., 337, 741 (2018).

13. D. Jampaiah, A. Chalkidis, Y. M. Sabri, E. L. Mayes, B. M. Reddy and S. K. Bhargava, Catal. Today, 324, 174 (2019).

14. S. Cimino and F. Scala, Ind. Eng. Chem. Res., 55, 5133 (2016).

Fig. 11. Mechanism diagram of Hg⁰ catalytic oxidation: (a) Reaction path diagram; (b) schematic diagram of the reaction cycle (For the details of structures, please see the Supplementary Material.).

15. J. Han, X. Ji, X .Ren, G. Cui, L. Li, F. Xie, H. Wang, B. Li and X.

Sun, J. Mater. Chem. A, 6, 12974 (2018).

16. Z. Xie, Y. Feng, F. Wang, D. Chen, Q. Zhang, Y. Zeng, W. Lv and G. Liu, Appl. Catal. B-Environ., 229, 96 (2018).

17. Y. Zhang and S.-J. Park, J. Catal., 361, 238 (2018).

18. Q. Lu, Z. Ali, H. Tang, T. Iqbal, Z. Arain, M. Cui, D. Liu, W. Li and Y. Yang, Korean J. Chem. Eng., 36, 377 (2019).

19. S. Zhao, Z. Li, Z. Qu, N. Yan, W. Huang, W. Chen and H. Xu, Fuel, 158, 891 (2015).

20. N. Usberti, S. A. Clave, M. Nash and A. Beretta, Appl. Catal. B- Environ., 193, 121 (2016).

21. B. Zhao, J. Han, L. Qin, W. Chen, Z. Zhou and F. Xing, New J.

Chem., 42, 20190 (2018).

22. H. Tian, J. Wu, W. Zhang, S. Yang, F. Li, Y. Qi, R. Zhou, X. Qi, L.

Zhao and X. Wang, Chem. Eng. J., 313, 1051 (2017).

23. J. Wu, S. Yang, Q. Liu, P. He, H. Tian, J. Ren, Z. Guan, T. Hu, B. Ni and C. Zhang, Environ. Sci. Technol., 50, 5370 (2016).

24. T. Xiong, F. Dong, Y. Zhou, M. Fu and W.-K. Ho, J. Colloid Inter- face Sci., 447, 16 (2015).

25. L. Zhang, X. Yang, R. Cai, C. Chen, Y. Xia, H. Zhang, D. Yang and X. Yao, Nanoscale, 11, 826 (2019).

26. X. Chan, N. Akter, P. Yang, C. Ooi, A. James, J. A. Boscoboinik, J. B. Parise and T. Kim, Mol. Catal., 466, 19 (2019).

27. Y. Suffren, F.-G. Rollet and C. Reber, Comment. Inorg. Chem., 32, 246 (2011).

28. X. Sun, J. Wu, Q. Li, Q. Liu, Y. Qi, L. You, Z. Ji, P. He, P. Sheng, J.

Ren, W. Zhang, J. Lu and J. Zhang, Appl. Catal. B-Environ., 218, 80 (2017).

29. Q. Yuan, L. Chen, M. Xiong, J. He, S.-L. Luo, C.-T. Au and S.-F.

Yin, Chem. Eng. J., 255, 394 (2014).

30. A. Cimino and B. D. Angelis, J. Catal., 36, 11 (1975).

31. M. Kolodziej, E. Lalik, J. Colmenares, P. Lisowski, J. Gurgul, D.

Duraczyńska and A. Drelinkiewicz, Mater. Chem. Phys., 204, 361 (2018).

32. W. Crünert, A. Y. Stakheev, R. Feldhaus, K. Anders, E. S. Shpiro and K. M. Minachev, J. Phys. Chem., 95, 1323 (1991).

33. L. Zhao, C. Li, S. Li, Y. Wang, J. Zhang, T. Wang and G. Zeng, Appl. Catal. B-Environ., 198, 420 (2016).

34. D. Liu, C. Lu and J. Wu, Energy Fuel, 32, 8287 (2018).

35. N. D. Hutson, B. C. Attwood and K. G. Scheckel, Environ. Sci. Tech- nol., 41, 1747 (2007).

36. J. Wu, K. Xu, Q. Liu, Z. Ji, C. Qu, X. Qi, H. Zhang, Y. Guan, P. He and L. Zhu, Appl. Catal. B-Environ., 232, 135 (2018).

37. H. Chang, Q. Wu, T. Zhang, M. Li, X. Sun, J. Li, L. Duan and J.

Hao, Environ. Sci. Technol., 49, 12388 (2015).

38. L. Lietti, I. Nova, G. Ramis, L. Dall'Acqua, G. Busca, E. Giamello, P. Forzatti and F. Bregani, J. Catal., 187, 419 (1999).

39. K. C. Galbreath and C. J. Zygarlicke, Environ. Sci. Technol., 30, 2421 (1996).

40. K. Inzani, T. Grande, F. Vullum-Bruer and S. M. Selbach, J. Phys.

Chem. C, 120, 8959, (2016).

41. M. Shetty, B. Buesser, Y. Román-Leshkov and W. H. Green, J. Phys.

Chem. C, 121, 17848 (2017).

42. R. Coquet and D. J. Willock, Phys. Chem. Chem. Phys., 7, 3819 (2005).

43. Y.-H. Lei and Z.-X. Chen, J. Phys. Chem. C, 116, 25757 (2012).

44. F. Wang and W. Ueda, Chem. Eur. J., 15, 742 (2009).

45. M. Witko and R. Tokarz-Sobieraj, Catal. Today, 91-92, 171 (2004).

46. J. Haber and E. Lalik, Catal. Today, 33, 119 (1997).

Supporting Information

Molybdenum trioxide impregnated carbon aerogel for gaseous elemental mercury removal

Yang Ling*, Xiaokun Man***, Wenbo Zhang*, Daolei Wang*^,†, Jiang Wu*^,**^,†, Qizhen Liu****, Mingyan Gu*****, Yuyu Lin*****, Ping He*, and Tao Jia*

*College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China

**Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China

***State Grid Shandong Electric Power Company Construction Company, Jinan 250022, China

****Shanghai Environment Monitoring Center, Shanghai 200030, China

*****School of Energy and Environmental Engineering, Anhui University of Technology, Maanshan 243002, China (Received 16 August 2019 • accepted 5 January 2020)

The catalyst efficiency is affected by many factors, such as cata- lyst content, initial mercury concentration in flue gas, etc. In the lab-scale study, it is necessary to reduce the amount of catalyst in

each test in order to make a more intuitive distinction between each samples. Efficiency is a relative concept, and the focus of this paper is to study the reaction mechanism of catalytic oxidation of

Fig. S1. Schematic of Model A structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Fig. S2. Schematic of Model B structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Hg⁰. In each test of this paper, the catalyst content was only 50 mg.

As shown in the Fig. S9, if the catalyst dosage per test is increased to 100 mg, the efficiency of Hg⁰ removal can reach up to 90.3%. In

Fig. S3. Schematic of Model TS1 structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Fig. S4. Schematic of Model C structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Fig. S5. Schematic of Model D structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

addition, the initial concentration of Hg⁰ in the flue gas also affects the removal efficiency of catalyst. In this paper, the Hg⁰ concentra- tion at the reactor inlet is about 50g/m³, and the carrier gas flow

is 1.2 L/min. Generally speaking, lower initial mercury concentra- tion and faster flow rate will lead to lower efficiency of mercury removal.

Shen et al. [1] developed some catalysts for the Hg⁰ removal via

Fig. S6. Schematic of Model E structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Fig. S7. Schematic of Model TS2 structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

Fig. S8. Schematic of Model F structure: (a) Side view from (001) facet; (b) side view from (010) facet; (c) side view from (100) facet; (d) 3D view.

TiO2 doped with WO3 and V2O5, which maximum Hg⁰ removal efficiency reached to 87% after 60 min. While in their paper, the initial mercury concentration was up to 100g/m³ and the cata- lyst content was up to 200 mg per test. Wu et al. [2] studied the

removal properties of mercury by cerium-doped functional acti- vated carbon, and the Hg⁰ removal efficiency was up to 88% after 3 hours. The catalyst content was 0.125 cm³, while the initial mer- cury concentration was up to 5*10³g/m³. Ma et al. [3] designed the MnO2/CeO2-MnO2 for elemental mercury removal from coal- fired flue gas, and the highest efficiency was up to 95.6% after 600 min with the catalyst content of 30 mg. However, the flue gas veloc- ity was only 500 mL/min, so the contact time between flue gas and catalyst was improved, which was beneficial to the reaction. More- over, the initial mercury concentration was not given in their paper.

The absolute efficiency of the catalyst is not the point. The focus of this paper is to explore the mechanism of catalyst removal of mer- cury, which will provide theoretical support for the subsequent ap- plication of power plants.

We have supplemented the experiment on the effects of some common gas species (CO2, CO, SO2, NO) on the Hg⁰ removal per- formance (Fig. S10). Initially, the carrier gas of nitrogen and oxy- gen mixture (95% N2 and 5% O2 by volume) is used as the control experiment.

When the carrier gas contains 500 ppm of CO2, the efficiency of catalyst remains basically the same. While when the carrier gas contained 500 ppm of CO, the efficiency decreased significantly, leaving only 51.6%. This is because the MoO3 has a certain catalytic oxidation effect on CO, therefore, CO might compete with Hg⁰ for the active site on the catalyst surface, causing the competitive reac- tion and leading to the decrease in the efficiency of mercury removal.

When the carrier gas contains 500 ppm of SO2, the Hg⁰ removal effi- ciency also dropped, reach to 58.5%. According to the previous study [4], carbonaceous materials have a stronger adsorption effect on SO2 than Hg⁰, so SO2 will compete with mercury for adsorp- tion. Besides, the adsorbed SO2 could react with the surface acti- vated oxygen, which also leads to the decline of the active sites for Hg⁰ removal. When the carrier gas contains 500 ppm of NO, the Hg⁰ removal efficiency drops slightly to 70.1%, which means that the existence of NO can inhibit the removal of mercury.

By the way, this paper focuses on exploring the mechanism of Hg⁰ removal via molybdenum-based catalyst in the theoretical level, providing some theoretical support for the practical application in power plants. The action mechanism between catalysts and differ- ent flue gas components will be reported in detail in our next study.

REFERENCES

1. H. Shen, I.-R. Ie, C.-S. Yuan, C.-H. Hung and W.-H. Chen, Envi- ron. Sci. Pollut. R., 23, 5839 (2016).

2. S. Wu, P. Yan, W. Yu, K. Cheng, H. Wang, W. Yang, J. Zhou, J. Xi, J.

Qiu, S. Zhu and L. Che, Fuel Process. Technol., 196, 106167 (2019).

3. Y. Ma, B. Mu, D. Yuan, H. Zhang and H. Xu, J. Hazard Mater., 333, 186 (2017).

4. P. He, J. Wu, X. Jiang, W. Pan and J. Ren, Appl. Surf. Sci., 258, 8853 (2012).

Fig. S9. The Hg⁰ removal efficiency of Mo/C500 sample with differ- ent catalyst contents (For each test, the reaction temperature was 300^oC, the experimental time was 3 hours, and the car- rier gas was N2+5% O2).

Fig. S10. The Hg⁰ removal efficiency of Mo/C500 sample in the pres- ence of some common gas species (For each test, the cata- lyst amount was 50mg, the reaction temperature was 300^oC, and the experimental time was 3 hours).