MoO3-adjusted δ-MnO2 nanosheet for catalytic oxidation of Hg0 to Hg2+

Applied Catalysis B: Environmental xxx (xxxx) xxxx

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MoO3-adjusted δ-MnO2 nanosheet for catalytic oxidation of Hg0 to Hg2+ Haitao Zhaoa,b,c, Collins I. Ezehd, Shufan Yind, Zongli Xiee, Cheng Heng Pangd, ⁎⁎ ⁎ Chenghang Zhenga, Xiang Gaoa, , Tao Wub,c, a

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo 315100, China New Materials Institute, The University of Nottingham Ningbo China, Ningbo 315100, China d Chemical and Environmental Engineering, The University of Nottingham Ningbo China, Ningbo 315100, China e Catalysis and Materials Discovery, CSIRO Manufacturing, Clayton, VIC, 3168, Australia b c

ARTICLE INFO

ABSTRACT

Keywords: Hg0 catalytic oxidation Molybdenum-manganese catalyst Synergistic effect Defect surface DFT

The challenge of Hg0 emission control has necessitated the development of catalytic oxidation technology. Herein, a combination of experimental and computational approach to explore the potential over Mo-Mn catalysts was conducted. Series of catalysts were synthesized and characterized with varying Mn and Mo loading (1–10%). Among the various compositions, 1.25Mo2 Mn catalyst exhibited the best Hg0 removal performance with Hg0 removal efficiency > 98% (oxidation ratio > 50%, and better stability over 10 h period at 250 °C). Most importantly, the synergistic interaction between MoO3 and defective δ-MnO2 nanosheet was promoted by the adjusted activation energy, bond strength, Brønsted acidic sites and defects, which facilitated the catalytic oxidation of Hg0 to Hg2+. Furthermore, DFT calculations suggested that the role of Mo enabled the reduction in the energy barrier for the desorption step, which was defined as the rate-determining step for catalytic oxidation of Hg0 on Mn-based catalyst.

1. Introduction Recently, atmospheric emission control (NOx [1–4], SOx [5–7] and Hg [8–10] and CO2 [11–15] from coal-fired power plants, particularly Hg, have drawn considerable global attention [16,17]. Hg is a highly toxic air pollutant with severe adverse impact on the ecosystem [18,19] due to its persistence, bioaccumulation and long-range transport [20,21]. Current studies estimated that approximately 13.1% (292 tonnes) of Hg emitted to the atmosphere from anthropogenic sources in 2015 were from stationary combustion of coal (power plants) [22]. Accordingly, mandates on Hg0 emission standards was enacted to be below 0.03 mg/m3 in China, or approximately 0.002 mg/m3 in the U.S. [23,24]. In addition, the Minamata Convention on Hg was officially initiated on 16 August 2017 to prevent and control global Hg emission [25,26]. Consequently, these policies have necessitated the development of novel technology in Hg emission control. Conventionally, Hg0 is commonly controlled via sorbent injection or oxidized mercury (Hg2+) by wet flue gas desulfurization (WFGD) and particulate-bound mercury (HgP) by electrostatic precipitators (ESP) or fabric filters (FF) [20,27,28]. Among these Hg species, Hg0 removal

posed to be the most challenging due to its high volatility and low solubility [29]. The application of sorbent injection was restrained by its secondary pollution and high costs [20,30,31]. Consequently, oxidizing Hg0 into Hg2+ with a suitable catalyst as an alternative approach to sorbent injection appeals to an increasing number of researchers [32]. Major concern has arisen from the use of separate control processes for NOx and Hg0 removal due to large space requirement and high equipment investment [10]. Selective catalytic reduction (SCR) is the most matured technology, which was traditionally aided by vanadium based catalysts [33]. However, vanadium was considered to be highly toxic and only operates at a relatively high and narrow temperature window (between 350–400 °C) [34,35]. This has resulted in the study of other metal based catalysts as alternatives for deNOx-SCR control process [36–39] and synergistic co-benefit of Hg0 removal [39,40]. A series of transition metal oxides, e.g., MnOx, CeOx, FeOx, CuO, V2O5, NiOx and CoOx, have been examined for this purpose with emphasis on lower and broader working temperature window [10,35,41,42]. Among all, MnOx displayed an outstanding performance for NOx removal with 100% NO conversion at 120 °C [20]. MnOx

Corresponding author at: Municipal Key Laboratory of Clean Energy Conversion Technologies, The University of Nottingham Ningbo China, Ningbo 315100, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Gao), [email protected] (T. Wu). ⁎

https://doi.org/10.1016/j.apcatb.2019.117829 Received 18 March 2019; Received in revised form 31 May 2019; Accepted 6 June 2019 Available online 08 June 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Haitao Zhao, et al., Applied Catalysis B: Environmental, https://doi.org/10.1016/j.apcatb.2019.117829

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catalysts was also effective and efficient in removing Hg0 from flue gases by catalytic oxidation and adsorption [43–46]. In order to enhance the Hg0 catalytic oxidation performance, modification of Mn-based catalysts using rare earth metals (e.g., Ce, La and Y) was examined [34]. LaMnO3 was reported to have excellent performance on both Hg0 and NOx removal [35,47,48]. In addition, doping molybdenum (Mo) can significantly enhance the catalytic performance [49], particularly on γ-Al2O3 template [41,50]. The good mechanical properties, fine thermal stability, large adsorption capacity and low-cost nature of γ-Al2O3 made it stand out as an excellent support with outstanding removal performance by both catalytic oxidation and adsorption process [41,50]. Despite the numerous studies conducted so far, there is no systematic study distinguishing Hg0 catalytic oxidation from its adsorptive capture performance. Most studies considered these two mechanisms as the same; and in other instances, adsorption takes precedence over catalytic oxidation. Hypothetically, the latter is more valuable in terms of engineering application considering Hg0 removal process. Hence, it will be beneficial to identify a catalyst with high catalytic oxidation ratio when compared to its total removal efficiency. In this study, a combined experimental and computational investigation was developed to investigate MoO3 adjusted δ-MnO2 nanosheet catalyst for catalytic oxidation of Hg0. This will provide a firsttime insight into the structure-ingredient-reactivity relationship and reaction mechanism of Hg0 catalytic oxidation performance.

approach [55]. The maximum instant mercury removal efficiency (ΔXmax) and catalytic oxidation ratio are calculated using:

Xmax =

[Hg 0]in [Hg 0]out × 100% [Hg 0]in

Catalytic Oxidation Ratio=

Hg 2 + [Hg 0]in

× 100%

(1) (2)

2.4. Computational mechanism study First principles calculations was carried out using the VASP to further study the reaction mechanism [60]. GGA was used with the functional described by Perdew and Wang (cutoff energy, 400 eV) [61]. The δ-MnO2 (101) surface were modeled in a periodic cell with a 14 Å vacuum space in order to curtail the interactions between the created slabs [62]. All the potential adsorption positions of Hg atom on δ-MnO2 (101) surfaces are optimized and the Hg adsorption energy (Eads) is estimated using Eq. (3) [39,54]:

Eads = E(adsorbent + Hg )

(Eadsorbent + EHg )

(3)

where EHg and Eadsorbent are the energy of Hg atom and the adsorbent respectively, and E(adsorbent+Hg) is the total energy.

2. Experiments and methods

3. Results and discussion

2.1. Preparation of catalysts

3.1. Characterization of as-prepared catalysts

Mn based catalysts were prepared using a mesoporous γ-Al2O3 template (V-SK Co. Ltd.) by the incipient wetness impregnation (IWI) method. Mn(NO3)2•4H2O and (NH4)6Mo7O24•4H2O (SCR Co, Ltd, analytical grade) were selected to synthesize Mo-adjusted Mn based bimetallic catalysts. The detailed procedure for sample preparation was described elsewhere [9].

3.1.1. XRD crystallographic analysis To study the crystallization and monolayer coverage of each catalyst, XRD patterns for a series of Mn, Mo and Mo-adjusted Mn based bimetallic catalysts at different loadings on γ-Al2O3 support were investigated (as shown in Fig. 1). Generally, the prepared catalysts showed similar strong diffraction peaks (at approximately 37°, 46° and 67°) that were attributed to the γ-Al2O3 support. The rest of the peaks account for the formation of possible crystals based on the dopants. Fig. 1 (a) displays the XRD pattern for Mn-doped γ-Al2O3 catalyst. Aside the γ-Al2O3 peaks, the rest of the peaks were identified as a mixture of β-MnO2 (JCPDS 24-0735; with characteristic peaks at 28.7°, 56.7° and 72.4° assigned to 110, 211 and 112 planes respectively) and δ-MnO2 (JCPDS 80-1098; with characteristic peaks at 37.8° and 42.9° assigned to 101 and 111 planes, respectively) crystals indicated by the black and red lines, respectively. Comparing both crystals, it was observed that the peaks for δ-MnO2 (101) surface was higher than that of β-MnO2 (110) surface. Furthermore, the intensity of the peaks increased with increase in Mn addition. In contrast, the intensity of γAl2O3 peaks decreased with increase in Mn-loading; indicating that there was a stronger interaction between the support and catalyst [63]. The most suitable loading ratio for high Mn dispersion was examined and presented in Fig. 2. Fig. 2 shows the evaluated crystallinity of MnO2 on the support via semi-quantitative analysis of the XRD peaks that accounts for MnO2 crystal formation. The semi-quantitative analysis indicated that good dispersion of Mn catalyst and high monolayer coverage on the γ-Al2O3 support was achieved at Mn-loading ≤ 2%. This was mainly attributed to the crystal growth limit in the mesoporous structure. Owing to this, 2% Mn-loading was pre-selected for Mo doping and further investigation. Furthermore, the crystal structure from the addition of Mo component (Mo-loading: 1–10%) on γ-Al2O3 support was investigated (Fig. 1 (b)). Compared to the XRD analysis of Mn catalyst in Fig. 1 (a) (MnO2 crystals were detected at Mn loading above 2%), there was no detected crystal formation on the support despite the increase in Mo loading up to 10%. This result suggests that Mo catalysts are better dispersed than Mn catalysts over the γ-Al2O3 support. Subsequently, the effect of Mo on 2% Mn/γ-Al2O3 catalyst was studied. 2% Mn/γ-Al2O3 catalyst was

2.2. Characterization of catalysts Specific BET surface area, pore volume and average pore size were measured using a Micromeritics ASAP 2020 [51]. XPS analysis was carried out using a Kratos AXIS spectrometer (Ultra DLD) under a vacuum pressure of 10−7 Pa [8]. XRD (Bruker, Germany) was conducted to analyze the crystallinity of the catalysts [52–54]. HRTEM with TEM images were captured using a JEM 2100 microscope [55,56]. In-situ DRIFTS was adopted to investigate the surface acidic sites of the catalysts (by using NH3 as the alkaline probe molecule) [9]. The adsorption and desorption of NH3 and CO2 (as the base and acid probe molecules respectively) on the surface of the sample were studied using Temperature Programmed Desorption (TPD) following similar procedures as the in-situ DRIFTS experiments [9,57]. H2-temperature-programmed reduction (H2-TPR) was conducted using AutoChem 2920 automated chemisorption analyser. This was performed in a flow of H2 (5 vol.%) in Ar (40 mL/min) from 50 to 900 °C [58,59]. 2.3. Hg0 removal performance evaluation The performance of Hg0 catalytic removal was evaluated for the asprepared samples using a specifically designed experimental rig. The Hg0 concentration was accurately controlled by a mercury generator (Tekran 3310, USA). The concentrations of Hg0, total amount of mercury (HgT) and Hg2+ (calculated as [Hg2+] = [HgT] – [Hg0]) were continuously monitored using the mercury analysis system (Tekran 3300RS, USA) at the outlet of the reactor. Hence, the performance of Hg0 catalytic oxidation to Hg2+ for each as-prepared catalyst could be evaluated. The catalytic performance was systematically evaluated using both integrated dynamic-state and steady-state experimental 2

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Fig. 2. XRD semi-quantitative analysis of different loadings of Mn on Al2O3. Table 1 The surface structure of different Mo loadings modified Mn catalysts. Catalysts

Surface area (m2/g)

Total pore volume (cm3/g)

Average pore width (Å)

Mn (g/g)

Mo (g/g)

γ-Al2O3 0Mo2 Mn 1Mo2 Mn 1.25Mo2 Mn 2.5Mo2 Mn 5Mo2 Mn 7.5Mo2 Mn 10Mo2 Mn

242.1 190.0 194.4 207.6 209.6 209.9 209.5 189.6

0.48 0.46 0.44 0.46 0.45 0.42 0.42 0.37

79.7 99.4 90.9 89.1 86.4 81.2 80.4 78.9

0 0.020 0.019 0.018 0.021 0.017 0.019 0.018

0 0 0.009 0.011 0.019 0.044 0.062 0.097

3.1.2. BET surface area and pore structure The effects of crystal formation as active sites on the surface area, pore volume and pore width of catalysts with different loadings were further investigated. The results are summarized in Table S1, S2 and Table 1 for individual Mn catalysts, individual Mo catalysts and Mo-Mn combined catalysts, respectively. Apparently, the surface area of γ-Al2O3 (242 m2/g) decreased with addition of Mn and Mo components. For individual Mn catalysts, loadings higher than 2% Mn resulted in a relatively low surface area below 190 m2/g. Specifically, the surface area for 4% Mn was 25 m2/g lower than 2% Mn, with similar pore volume of 0. 46 cm3/g. However, the pore size experienced an increase from 99 to 112 Å. This was possibly caused by the formation of crystal structure at a higher Mn loading as depicted in the XRD study. Among all of the Mn catalysts, Mn loading of 2% was seen to have one of the highest surface area and pore volumes, which were notable factors for the adsorption process. By contrast, variation in Mo loading (1% to 10%) had no significant effect on the surface area of the support (with the highest reduction ratio of less than 9%). Specifically, surface area increased from 221 m2/ g (for 1.25% Mo sample) to 227 m2/g (for 5% Mo sample). Moreover, the pore volume of 1.25% Mo sample expanded from 0.48 cm3/g (for support) to 0.52 cm3/g with a 15 Å pore size increase. This finding associates the enhanced morphological characteristics to the dispersive property of 1.25% Mo on the support. Beyond this composition, the porosity of the catalyst witnessed a significant reduction. Besides, the surface area for Mn-Mo combined catalysts (Moloading < 10%) slightly increased and displayed better surface characterization when compared with the individual 2% Mn catalyst. This enhancement was maximum when Mo-loading was 5%. Nevertheless, Mo-loadings between 1.25–7.5% showed considerable increase in surface area greater than 200 m2/g. This agrees with XRD results indicating that Mo introduction promoted active component dispersion

Fig. 1. XRD patterns of (a) Mn catalysts, (b) Mo catalysts and (c) Mn-Mo combined catalysts with range of loadings.

impregnated with Mo catalysts with load range from 1 to 10% (i.e. MnMo combined catalysts). Based on the XRD results for all samples of Mn-Mo combined catalyst, γ-Al2O3 diffraction peaks and patterns were the only observed peaks (Fig. 1 (c)). Increasing the composition of Mo in the catalyst did not result in any crystal formation, rather it further decreased the intensity of the main Al2O3 peaks. This indicates that Mo component on Mn catalyst enhanced the dispersion of Mn catalyst, thus achieving high monolayer coverage on the support. This depicts that Mn loading of 2% in Mn-Mo combined catalyst was well-suited for the enhanced dispersion of Mn and Mo catalyst on the support without the display of any crystalline peaks. 3

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Fig. 3. In situ DRIFT spectra of (a) γ-Al2O3, (b) 2 Mn/γ-Al2O3, (c) 1Mo2 Mn/γ-Al2O3, (d) 1.25Mo2 Mn/γ-Al2O3, (e) 2.5Mo2 Mn/γ-Al2O3, (f) 5Mo2 Mn/γ-Al2O3, (g) 7.5Mo2 Mn/γ-Al2O3, (h) 10Mo2 Mn/γ-Al2O3.

on the γ-Al2O3 support. It is important to highlight that the 1.25Mo2 Mn sample had the largest pore volume (0.46 cm3/g, similar with that of the support) while the pore size increased by less than 10 Å when compared to the support. The result could be explained as the dispersion of about 5 Å of thin layer catalysts around the mesoporous γAl2O3. In general, BET and XRD tests demonstrated that Mo has great potential to disperse Mn on γ-Al2O3 support, which will enable the provision of more available sites for adsorption and/or catalytic activities.

3.1.3. In situ DRIFT characterization of surface acid sites In situ DRIFT analysis was conducted to determine the acidity of the surface sites on the as-prepared catalysts. According to previous research, Hg0 can be treated as a base with the tendency to interact with acidic sites on the catalysts [64]. This justifies the essence of this experiment and the results are shown in Fig. 3. Each sample displayed bands at 1248 and 1610 cm−1, which were attributed to the characteristic bending vibrations of coordinated NH3 on the Lewis acid sites; and bands at 1470 and 1688 cm−1 that can be ascribed to the bands of 4

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NH4+ bound to the Brønsted acidic sites [65,66]. It was evident that the total peak area of the acidic sites decreased with increase in temperature. At about 250 °C, the disappearance of the peaks for both acidic sites were observed. Comparatively, Brønsted acidic sites have higher intensity than Lewis acidic sites. Nonetheless, the former decreased at a higher rate than the latter with increase in temperature. Some of the Lewis acidic sites were still detected at 300 °C with the disappearance of Brønsted acidic peaks. This validates the hypothesis that Lewis acidic sites are more stable than the Brønsted acidic sites [66]. Compared to the individual Mn sample, Mn-Mo combined samples have higher peak intensity for both acidic sites. Particularly, at Mo composition of 1.25% or over 2.5%, a dramatic increase in the peak area of the acidic sites was witnessed. This indicated that Mo addition was beneficial to the increase of both acidic sites, which will likely favor Hg0 adsorption. However, this varies with temperature. A breakdown of the total peak areas for the bands of Brønsted and Lewis acidic sites at different temperatures is illustrated in Fig. 4 (a) and (b), respectively. An insight into the impact of temperature on both Brønsted and Lewis acidic sites is shown in Fig. S1 (a) and (b) of supporting document, respectively. Specifically, for temperatures lower than 200 ℃, Brønsted acidic sites dominates the surface acid sites. It was also observed from Fig. 4 that the increasing rate of the peak intensity with Mo was higher for Brønsted acidic sites than for the Lewis acidic sites. In other words, it can be deduced that Mo addition induces defective sites that benefits the increase of Brønsted acidic sites than Lewis acidic sites. This phenomenon corroborates our previous study that reported the possible conversion of Lewis acidic sites to Brønsted acidic sites [9]. Occurrence of these defective sites subsequently exhibited good catalytic activity and the ratio of the acidic sites will determine the catalyst resistance to deactivation [9,67]. The ratio of the peak area for Brønsted acid over Lewis acid at different Mo loading is illustrated in Fig. S1 (c). From this figure, it was observed that 1.25Mo2 Mn sample showed the optimum configuration

Fig. 4. Semi-quantitative in-situ DRIFT analysis of peak intensities: (a) Brønsted acid peaks, and (b) Lewis acid peaks.

Fig. 5. (a) Hg0 and HgT dynamic transientstate analysis of Mn-Mo catalysts with ranges of Mo loadings. (HgT is represented by the line with solid symbol and Hg0 is represented by the line with hollow symbol). (b) Hg2+ dynamic transient-state analysis of Mn-Mo catalysts with ranges of Mo loadings. (Experimental condition: heat from 25 to 700 °C, heating rate of 1 °C/min, in an atmosphere of Hg0 (30 μg/m3) blended with O2 (5%) in N2).

5

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Table 2 Characteristic temperatures of Mn-catalyst promoted by various composition of Mo. Sample

Ta0 (oC)

Tra,peak (oC)

Ta,range (oC)

Td0 (oC)

Trd,peak (oC)

1Mo2 Mn 1.25Mo2 Mn 2.5Mo2 Mn 5Mo2 Mn 7.5Mo2 Mn 10Mo2 Mn

58 63 68 73 65 50

94 159 111 115 130 116

210-344 177-392 248-379 280-368 205, 328* 200*, 308

411 433 441 445 460 400

421 433 455 448 470 410

Table 4 Catalytic capacity analysis for Mn-catalyst promoted by various composition of Mo.

* The temperature when minimum peak occurred.

3.2.1. Qualitative analysis for Hg0 adsorption evaluation To explore the potential of various composition of Mo-modified 2% Mn catalysts for Hg0 adsorption and oxidation, the Hg0-TPSR was investigated using both qualitative and quantitative analysis. Fig. 5 shows the dynamic transient-state results of Hg0 and Hg2+. Fig. 5 (a) illustrates the concentration profile for Hg0 and HgT within the temperature range 25–700 °C. The troughs and peaks represent the adsorption and desorption of mercury species, with the desorption stage commencing from 400 °C. HgT and Hg0 are indicated by the lines with solid and hollow symbols, respectively. From Fig. 5 (a), it was observed that the adsorption and desorption profile for HgT and Hg0 are similar for both individual Mn and Mo-Mn combined catalysts. However, with increased Mo loading, the difference in concentration between Hg0 and HgT varied. Compared to the 0Mo2 Mn sample, it was evident that Mo addition resulted in the difference in concentration of Hg0 and HgT. Specifically, the concentration difference between Hg0 and HgT increased with increase in Mo loading up to 1.25%. Beyond this, the difference in concentration decreased as Mo loading further increased to 10%. Furthermore, the difference between the concentrations of Hg0 and T Hg gives the amount of Hg2+ generated, which is illustrated in Fig. 5 (b). Fig. 5 (b) shows the transient-state concentration profile for generated Hg2+ at varying temperature range. A combined concentration profile for all three mercury species is shown in Fig. S2 of the supporting document. The analysis depicts that Mo loading of 1.25–5% on Mn catalyst generated the highest amount of Hg2+ with the lowest temperature range. Table 2 lists the characteristic temperatures of Hg0 adsorption and desorption processes, which includes the temperature for initial adsorption (Ta0), peak of adsorption rate (Tra, peak), the best effective adsorption range (Ta, range), initial desorption (Td0) and the peak of desorption rate (Trd, peak). Ta0 was used as an effective indicator to evaluate the Hg0

1Mo2 Mn

1.25Mo2 Mn 2.5Mo2 Mn 5Mo2 Mn

7.5Mo2 Mn 10Mo2 Mn

66.30

49.90 64.18 78.84 74.90 51.64

0.9757

0.9717 0.9408 0.9023 0.9509 0.9672

ΔXmax (%)

Rate constant kr (m3/g.s)

86.01

5.0615e

7974/ T

5.0473e

6002/ T

5.0687e

7720/ T

5.0922e

9483/ T

5.0896e

9009/ T

97.63 92.13 86.50 67.17 41.27

5.04825e

ΔS* (min μg/m3)

SHg2+‡ (min μg/m3)

Error (%)

1Mo2 Mn 1.25Mo2 Mn 2.5Mo2 Mn 5Mo2 Mn 7.5Mo2 Mn 10Mo2 Mn

3160.12 3973.53 3297.60 2929.75 2454.51 1082.13

2394.66 1692.72 1411.54 755.80 434.13 353.62

765.46 2280.80 1886.05 2173.95 2020.37 728.52

760.18 2235.26 1832.50 2122.47 2006.42 702.46

0.69 2.00 2.84 2.37 0.69 3.58

3.2.2. Quantitative analysis for evaluating Hg0 catalytic oxidation The maximum instant Hg0 removal efficiency (ΔXmax), activation energy (Ea), instant Hg2+ generation, adsorption area (Sa) and desorption area (Sd) were quantitatively analyzed to study the adsorption and catalytic oxidation performance. The calculated results are shown in Tables 3 and 4, respectively. ΔXmax was developed as an indicator of Hg0 capture performance, and Ea was calculated from the reaction kinetics based on modified Arrhenius equation. The activation energy (Ea) was then obtained from the modified Arrhenius plot of the Hg0-TPSR experimental data with the Equations as below.

r Hg0 =

F X Hg0 W

ln( X Hg0) =

AWC Hg0 Ea 1 + ln R T F

(4) (5)

where, F is the gas flow rate, mol/s; W is the mass of the adsorbent, g; ΔX is the instant Hg0 removal efficiency, wt%; A is the pre-exponential factor; r is the first-order rate of reaction, r Hg0 = kC Hg0 , mol/(g·s). As shown in Table 3, the values of ΔXmax for Mn-Mo catalysts firstly reduced with increase in Mo loading from 1% to 1.25%. Further increase in Mo composition resulted in the decrease of ΔXmax. The catalyst with 1.25% Mo was found to have the largest removal efficiency indicating the best adsorption capacity. Furthermore, among all the samples, 1.25Mo2 Mn had the lowest Ea (49.90 kJ/mol), which suggests that it was more likely to achieve a better adsorption performance at a lower temperature. The Ea reached its peak value (78.84 kJ/mol) as

Table 3 Reaction kinetics for Mn-catalyst promoted by various composition of Mo. r2

Sd (min μg/ m3)

adsorption performance, which also could be applied to predict the activation energy. As shown in Table 2, Ta0 firstly increased from 58 °C to 73 °C with increasing Mo composition from 1% to 5%, and then decreased to 50 °C with further increase in Mo composition from 5% to 10%. As was found in previous studies, the Ta0 of Mo-O catalyst is 31 °C while that of Mn-O catalyst is 77 °C [41]. Thus, it could be concluded that the decrease in Ta0 was adjusted by the amount of Mo. Tra, peak, Ta, range, Td0 and Trd, peak are parameters directly correlated with the potentials of adsorption and desorption capability. Tra, peak and Trd, peak were obtained by performing the differential analysis of the TPSR profiles. The best effective adsorption range, Ta, range, was defined to be the range with a removal efficiency greater than 80%. The removal efficiency is the key parameter indicating its effective adsorption range. Generally, Table 2 presents that the amount of Mo had a significant effect on the characteristic adsorption-desorption temperatures. However, the effect was more pronounced for Tra, peak and Ta, range than for Td0 and Trd, peak. For Mn-Mo samples up to 5% Mo, broad adsorption ranges could be found, with 1.25% Mo showing the widest temperature range of 177–392 °C. While for Mo-loading of 7.5% and 10%, only two small adsorption peaks could be detected. The results indicate that 1.25Mo2 Mn had the largest adsorption capacity.

3.2. Hg0 adsorption and oxidation performance evaluation

Ea (kJ/mol)

Sa (min μg/ m3)

* difference between Sa and Sd obtained from Fig. 5 (a). ‡ directly integrate the results in Fig. 5 (b), which was obtained by subtracting the amount of detected Hg0 from the total amount of mercury (HgT) observed.

for improved Brønsted acid sites over Lewis acid sites. As a result, 1.25Mo2 Mn sample represents an excellent ability of this catalyst to adsorb Hg0 with a relatively low Mo-loading. However, increasing temperature for the same sample can lead to a decrease in the Brønsted/ Lewis acidity ratio. Evidently, 1.25Mo2 Mn configuration has the most promising number of acidic sites among the synthesized samples.

Sample

Sample

6211/ T

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Fig. 7. Long-time evaluation of catalytic oxidation of Hg0 over the 1.25Mo2 Mn catalysts: effect of Hg0 concentration (testing temperature at 250 °C).

about 30%, was observed as temperature increased from 300 to 400 °C. This can be partly attributed to the denaturing of available active acidic sites at high temperatures as presented in the in-situ DRIFTS result. In comparison to the removal performance, the catalytic oxidation performance of 1.25Mo2 Mn showed a sinusoidal trend, particularly with higher performance at high temperatures. This corresponds with the concentration profile of Hg2+ presented in Fig. S3. Maximum catalytic oxidation was observed at a temperature of 250 °C with an oxidation ratio of above 40%. A further increase in temperature beyond 250 °C was undesirable as this led to a deteriorating effect in the catalytic oxidation performance. Moreover, given that the kinetics of catalytic oxidation was predicted to be first order, 1.25Mo2 Mn catalyst was shown to have the highest rate constant at 250 °C as shown in Fig. S4. The above results also provide insights into further selected temperature evaluation. Furthermore, the adsorption capacity as well as the oxidation capacity was studied by evaluating the concentration of mercury species at different temperatures. The concentration profile of mercury species over the 1.25Mo2 Mn catalyst is presented in Fig. S3 (supporting document). From this figure, the concentration profiles for HgT and Hg2+ were seen to be similar in trend and magnitude within the considered adsorption temperature range (25–400 °C), while the concentration of Hg0 flattened out to around 0.1 μg/m3. Both profiles gradually increased up to a peak temperature of 250 °C before witnessing a gradual decrease. It is suggested that the removal of Hg0 was mainly due to its oxidation to Hg2+.

Fig. 6. Steady-state analysis of catalytic oxidation of Hg0 over the 1.25Mo2 Mn catalysts.

Mo-composition increased to 5%. These results are in close agreement with the trend of Ta0, which was also supported by the revelation of Mo regulating effect. Table 3 also presents the kinetic rate constant as a function of temperature for each catalyst sample. The pre-exponential factor was observed to be similar across all the catalysts indicating a similar reaction trend within the catalysts. The catalytic performance analysis of these samples was conducted by integrating the TPSR results to estimate the amount of Hg2+ generated, the adsorption area (Sa) and desorption area (Sd), which represent the amount of Hg0 adsorbed and desorbed, respectively (see Table 4). Since the experimental temperature had been increased up to approximately 700 °C, the residual Hg0 in the samples could be assumed to be negligible. Hence, the significant difference between Sa and Sd were considered as the amount of Hg2+ formed from catalytic oxidation. An alternative calculation of Hg2+ is to directly integrate the results in Fig. 5 (b), which was obtained by subtracting the amount of detected Hg0 from the total amount of mercury (HgT) observed. The two sets of results showed a similar trend, with the 1.25% Mo sample displaying the largest amount of Hg2+ generated, followed by the sample with 5% Mo. Afterwards, the amount of Hg2+ generated steadily declined. These adopted methods (using graphical and numerical results) to estimate Hg2+ generation (as shown in Table 4 and Fig. 5 (b)) were precise with an error margin estimated to be less than 4%. This approach provides crucial information for evaluating Hg2+ generation via catalytic oxidation of Hg0. Obviously, both the adsorption and catalytic effects contributed to the capture of Hg0. With the increasing amount of Mo, the contributing share of catalytic oxidation effect increased from 24% to 82% during the transient-state analysis. This can be associated to the availability of activated O-species to facilitate the oxidation process. The results suggest the presence of Mo has positive effect on catalytic oxidation of Hg0. However, the 1.25Mo2 Mn sample (with 56% share of catalytic oxidation) achieved the highest total Hg0 removal efficiency with consideration to both adsorption and catalytic effects.

3.2.4. Effect of Hg0 concentration on performance activity Fig. 7 shows the effect of varying Hg0 concentration in the flue gas on the removal and catalytic oxidation of Hg0 over 1.25Mo2 Mn catalyst. As shown in Fig. 7, varying Hg0 inlet concentration showed similar performance in terms of removal of Hg0 and oxidation of Hg0 to Hg2+. Pertaining to removal performance, over 90% total removal efficiency was observed throughout the 10 h test. Hg0 concentrations of 30 μg/m3 showed the best performance of about 98.5%, which was stably maintained throughout the 10 h test period. Though higher Hg0 concentrations of 40 μg/m3 and 55 μg/m3 also resulted in a relatively stable removal performance, but a minor decrease in the removal efficiency was observed within the 10 h test. In terms of catalytic oxidation, the variation in Hg0 concentration did not result in a distinct change trend in the oxidation performance. However, it was discerned that the catalytic oxidation performance at 40 μg/m3 and 55 μg/m3 was lower than that at 30 μg/m3. This can be attributed to the coverage of active sites by excess Hg species resulting from high concentration dosage.

3.2.3. Effect of operating temperature on performance activity The effect of the operating temperature on the removal performance of Hg0 and the catalytic oxidation performance over the 1.25Mo2 Mn catalyst was examined and the results are shown in Fig. 6. Overall, 1.25Mo2 Mn catalyst achieved the highest Hg0 removal efficiency, particularly within temperature range of 100–300 °C. Maximum removal performance was achieved at a temperature of 175 °C (removal efficiency, almost 100%). Above 300 °C, Hg0 removal efficiency decreased with increase in temperature. Specifically, a drop in performance, by

3.2.5. Effect of space velocity on performance activity Apart from Hg0 inlet concentration, removal and catalytic performance are also affected by space velocity of the reactor design, which is an important parameter for practical application. As shown in Fig. 8, 7

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Fig. 8. Long-time evaluation of catalytic oxidation of Hg0 over the 1.25Mo2 Mn catalysts: effect of space velocities scenarios (testing temperature at 250 °C).

Fig. 9. Raman spectra of (a) 1.25% MoO3/γ-Al2O3 catalyst, (b) 2% MnO2/γAl2O3 catalyst, (c) 1.25% MoO3-2% MnO2/γ-Al2O3 catalyst.

variation in space velocity resulted in a significant change in the removal and catalytic performance over the 10 h test. Pertaining to removal performance, high total removal efficiency was observed at space velocities of 3.9 × 104 ml/(h·g) and 5.85 × 104 ml/(h·g) over the 10 h test. Moreover, space velocity of 3.9 × 104 ml/(h·g) showed a more stable removal performance of about 98% over the stipulated time frame. Similarly, space velocity of 5.85 × 104 ml/(h·g) also showed a relatively stable performance (> 95%). However, there was a minor drop in the removal performance after the 10 h of test. On the contrary, high space velocity of 7.8 × 104 ml/(h·g) presented a constantly deteriorating Hg0 removal performance to below 90% after 10 h of test. This can be attributed to the balance between rate of active sites coverage and rate of catalytic oxidation. At a catalytic oxidation ratio of 30% (for 7.8 × 104 ml/(h·g) space velocity after 10 h), limited amount of Hg0 was oxidized to Hg2+, resulting in the subsequent decrease in total removal efficiency. In terms of catalytic performance, the variation in space velocity has a significant effect on the oxidation performance of Mn-Mo catalyst. For space velocity of 3.9 × 104 ml/(h·g), a stable oxidation performance over the 10 h period was achieved after about 1 h. Furthermore, the catalytic oxidation ratio reached above 52% during the 10 h test. This was not the case for higher space velocities. At space velocity of 5.85 × 104 ml/(h·g), the oxidation performance gradually increased and plateaued at about 45% after 8 h into the test. A strong relationship between removal and catalytic performances can be determined through the studies conducted. For instance, an increase in catalytic performance over the 10 h test has been observed in Figs. 7 and 8. This suggests that the formation of Hg2+ increased over time, and consequently increased the disappearance of Hg0. Therefore, an increase in Hg0 removal efficiency should be expected through time. Through this study, it was obvious that the variation in Hg0 inlet concentration did not significantly affect the Hg0 removal and oxidation performances. Meanwhile, space velocity of flue gas greatly affected the Hg0 removal and oxidation performances. These two factors are considered important in the removal and catalytic performance because they are correlated to the number of Hg0 atoms competing for the catalyst’s active sites and to be oxidized to Hg2+. Low Hg0 concentration and space velocity indicate a low number of Hg0 atoms compete for the catalyst’s active sites and subsequently oxidized to Hg2+, thus making the removal and oxidation processes more effective. Therefore, low Hg0 inlet concentration (e.g. 30 μg/ m3) at space velocity (e.g. 3.9 × 104 ml/(h·g)) presents the best Hg0 removal and catalytic oxidation performance over 1.25Mo2 Mn catalyst with a high removal efficiency (> 98%), high oxidation ratio (> 52%) and better stability over the 10 h period.

examine the variations in vibrational and rotational modes associated with 1.25Mo2 Mn catalyst. In this test, a comparative study was conducted between 2% Mn-, 1.25% Mo- and 1.25%Mo-2%Mn- catalysts. As shown in Fig. 9 (b), 2%Mn-catalyst did not show any significant peaks. Only fluctuations with similar intensity along 200-1500 cm−1 were observed. From literature, MnO, MnO2 and Mn2O3 are the most common crystals formed in Mn-catalysts. Peaks for MnO were found at 1030 cm−1, 648 cm−1, 544 cm−1 and 306 cm−1 bands in Raman spectrum for Mn/γ-Al2O3 catalyst [68]. Meanwhile, Mn2O3 species

3.3. Raman spectroscopy surface study

Fig. 10. XPS spectra of (a) Mn in MnO2/γ-Al2O3 and MnO2-MoO3/γ-Al2O3 (fresh and spent) catalysts, and (b) Mo in MoO3/γ-Al2O3 and MnO2-MoO3/γAl2O3 (fresh and spent) catalysts.

Raman spectroscopy, depicted in Fig. 9, is a suitable approach to 8

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were detected at 697 cm−1, 653 cm−1 and 311 cm−1 bands; whereas MnO2 was Raman inactive [68]. However, these peaks were not detected in the Raman spectra shown in Fig. 9 (b). Therefore, this suggests that there was no formation of crystal structures of Mn species for 2% Mn catalyst. This agrees well with the XRD results indicating that 2% Mn was well-dispersed on the support. Thereby, no crystal formation on the catalyst was expected to be found. On the other hand, 1.25% Mo catalyst exhibited both a sharp and weak band on a high-frequency- and a low-frequency-Raman region respectively (Fig. 9 (a)). Based on previous studies conducted on dehydrated Mo/γ-Al2O3 catalyst, the sharp band at ∼1000 cm−1 accounted for the Mo = O stretching mechanism, whereas that at ∼300 cm−1 was associated with the Mo = O bending mechanism [69,70]. In the Mo/γ-Al2O3 catalyst with 1.25% Mo loading, the presence of the signal bands at ∼300 and ∼1000 cm−1 indicated that there were shorter Mo = O bonds and a greater structural distortion associated with the catalyst [54]. Furthermore, the absence of ∼210–220 cm−1 band in the Raman spectra also supports the occurrence of a highly distorted and isolated dehydrated surface species [54]. This explains why no crystal formation was observed in XRD study of Mo/γ-Al2O3 catalyst. Doping of 1.25% Mo to 2% Mn catalyst led to a change in the spectral line as shown in Fig. 9 (c). Hypothetically, 2% Mn and 1.25% Mo spectral line should be expected in the 1.25Mo2 Mn Raman result. However, this was not the case as three major peaks at ∼600 cm−1, ∼950 cm−1 and ∼1050 cm−1 were observed. These peaks were similar peaks to δ-MnO2 in previous work [71]. This indicates that the addition of Mo dopants to Mn catalyst contributed to the exposure of δ-MnO2, which further improved performance of Hg0 catalytic oxidation.

3.4. XPS valence states analysis To further confirm the chemical states of elements in the Mn-Mo combined catalyst, XPS test was conducted. The XPS spectra for metal elements present in the combined catalyst was compared with their individual metal oxides (see Fig. 10). Fig. 10 also presents the XPS spectra for the combined catalyst before and after Hg0 removal, designated as fresh and spent, respectively. The comparison for Mn XPS spectra is shown in Fig. 10 (a) while that for Mo is shown in Fig. 10 (b). It was observed from Fig. 10 (a) that there are two major peaks for both the combined sample and the individual sample, centered at approximately 653 eV and 642 eV respectively. These two peaks were attributed to the Mn 2p state. The first peak was assigned to the Mn 2p1/ 2, while the other peak was attributed to the Mn 2p3/2 of the sample. These are characteristic peaks of Mn4+ [20], indicative of the existence of MnO2 in the combined catalyst. Compared with the individual Mn-based catalyst, it was noticed that the Mn peaks for the combined catalyst became sharper with higher intensity. This was attributed to the increased amount of Mn active site on this catalyst as a result of enhanced dispersion from Mo mediation. This was also supported by the increase in peak intensity in H2-TPR profile for the combined catalyst due to dispersive effect of Mo (Fig. S7, supporting document). This result provides insight into the role of Mo in improving Hg0 removal efficiency. After Hg removal, the peak intensity of the spent catalyst decreased indicating the coverage of the metal sites with Hg species. Accordingly, the XPS spectra for the spent 1.25Mn2 Mn catalyst displayed a peak at about 99 eV (Fig. S5 of supporting document), which was attributed to the binding energy of Hg 4f7/2 [72]. This indicates that mercury existed in the form of Hg2+ on the spent 1.25Mo2 Mn catalyst and can be ascribed to the retained oxidized Hg0 on

Fig. 11. TEM image of (a) MnO2/γ-Al2O3 catalyst, (b) 1.25%MoO3-2%MnO2/γ-Al2O3 catalyst; and HRTEM image of (c) MnO2/γ-Al2O3 catalyst and (d) 1.25%MoO32%MnO2/γ-Al2O3 catalyst (with an inserted figure: top view of structural configuration of the δ-MnO2 (101) surface).

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Al2O3. However, in Fig. 11 (d), the lattice with a spacing of 0.245 nm, which was indexed to the (101) plane of δ-MnO2, was clearly observed. The simulated micro structure of the (101) plane is shown in the inset of Fig. 11 (d). This phenomenon shows that Mo addition promoted a well dispersed Mn site, agreeing with the findings from Raman spectra. In addition, the defective surfaces of (101) plane can be clearly seen in Fig. 11 (d) thus supporting claims of the presence of defective sites on the catalysts as proposed in DRIFTS and TPD tests (as shown in Fig. S6). Conclusively, it can be deduced that Mo and Mn synergistic interaction aids to enhance surficial defects and dispersion of δ-MnO2 nanosheet. 3.6. Reaction mechanism for Hg0 oxidation on MoO3-adjusted δ-MnO2 nanosheet To gain insight into the synergistic interaction between Mo and Mn on the MoO3-adjusted δ-MnO2 nanosheet, DFT computational study was conducted. Hg removal efficiency, indicated by the adsorption performance, on individual Mo, individual Mn and Mo-adjusted δMnO2 catalysts was evaluated. The potential adsorption positions considered (and the characteristic plots) for MnO2 and MoO3 are presented in Fig. S8 (and Fig. S9) and Fig. S10 (and Fig. S11), respectively. This was aimed to determine the most stable adsorption position on these surfaces. A structural representation of the most stable adsorption positions is shown in Fig. 12 and their characteristic energies for Hg adsorption are presented in Table 5. From the results, it was evident that the most stable adsorption position predicted for Hg atom was at the center of three Mn atoms of the perfect δ-MnO2 (101) surface with an adsorption energy of -0. 1694 eV (Fig. 12 (b)). Whereas, the most stable adsorption position for MoO3 surface (Fig. 12 (a)) had an adsorption energy of -0.1008 eV. This established that the δ-MnO2 (101) surface was more suitable than MoO3 for adsorbing Hg. Furthermore, the presence of defects showed a significant increase in adsorption performance of δ-MnO2 (101) (Fig. S9 (b), supporting document). The defected O atom (VO) surface (Fig. 12 (c)) was more beneficial for Hg adsorption than the perfect δ-MnO2 (101) surface. Specifically, the presence of defects increased the adsorption energy from -0. 1694 eV (perfect surface) to -0.2049 eV. This corresponds with findings from literature stating that the presence of defective structures can increase the exposure of active sites and subsequently, improve the catalytic oxidation of Hg0 [20]. Based on this theoretical calculation, the effect of MoO3 modification on the Hg adsorption performance on the δ-MnO2 nanosheet was further studied. Fig. 12 (d) shows the schematic representation of the most stable position. Hg atom was adsorbed on the O vacant sites with an adsorption energy of -0.2511 eV, which is 25% higher than that of the defected δMnO2 surface (Table 5). It is speculated that the presence of Mo enhanced the adsorption performance of δ-MnO2. This was attributed to the adjusted structure (from XRD), exposure of Mn sites (from Raman), improved dispersion (XPS) and enhanced acidic sites (DRIFTS). Therefore, the improved adsorption performance of MoO3-adjusted δ-MnO2 surface can be expounded by the synergistic effect of Mo and Mn. Based on this result, the reaction pathway for Hg0 oxidation by O2 was studied and shown in Fig. 13. The reaction pathway involves a 3step process: adsorption, reaction and desorption. Detailed simulated results are presented in Table S3 and Fig. S12 in the supporting document.

Fig. 12. The top view (left) and side view (right) of the schematic representation of the most stable configurations (a) MoO3, (b) perfect δ-MnO2, (c) Vo-defect δ-MnO2, and (d) MoO3-adjusted δ-MnO2.

the surface of the catalyst as speculated in the TPSR test. Fig. 10 (b) also displayed two major peaks for Mo, centered at around 236 eV and 232.8 eV, and can be assigned to the binding energy of Mo 3d3/2 and Mo 3d5/2 in the sample respectively. Both of these peaks can be assigned to Mo6+ species, which posits the catalyst to have high concentration of surficial oxygen responsible for Hg0 oxidation [20]. The results from this test corresponds with the activity tests. Moreover, no significant change was observed for the XPS spectra for Mo species before and after Hg removal. 3.5. TEM and high-resolution TEM (HRTEM) morphology study TEM and HRTEM were applied to study the detailed crystal facets of the catalyst with respect to the performance of the catalysts. The TEM images of Mn and 1.25Mo2 Mn combined catalyst are shown in Fig. 11 (a) and (b) respectively. A layered structure can be clearly observed in Fig. 11 (b) [73]. This was consistent with the XRD pattern, which showed that 1.25Mo2 Mn was well dispersed on the γ-Al2O3 support, achieving a monolayer structure. Fig. 11 (c) and (d) are HRTEM images of Mn and 1.25Mo2 Mn catalysts, respectively. In Fig. 11 (c), the lattice spacing of 0.200 nm was observed, representing the existence of γ-

Table 5 Characteristic energies for Hg0 adsorption on the most stable configuration for MoO3, δ-MnO2 and MoO3-adjusted δ-MnO2 surfaces. Most stable configuration

E(adsorbent+Hg) (eV)

Eadsorbent (eV)

EHg (eV)

ΔEads (eV)

MoO3 surface perfect δ-MnO2 surface defected δ-MnO2 surface MoO3-adjusted δ-MnO2 surface

−520.1032 −240.0981 −235.2163 −736.6547

−519.8836 −239.8100 −234.8926 −736.2848

−0.1188 −0.1188 −0.1188 −0.1188

−0.1008 −0.1694 −0.2049 −0.2511

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Fig. 13. Reaction pathway of catalytic oxidation of Hg0 on δ-MnO2 nanosheet (Red block indicate the role of MoO3 on MnO2 surface at the rate-determining desorption step and the details plotted in Fig. S13) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

The first step involves the stable adsorption of Hg0 (Hg0 → Hg(ads)) on defective O atom surface (VO), Hg(ads)-Os. This was the strongest adsorption site as illustrated in Section S8.1 (supporting document) with a relative energy of -207.1 eV. The second step involved the reaction between adsorbed (Hg(ads)) and adsorbed oxygen (Hg(ads) → HgO(ads)) through a series of transitions. During this step, Hg atom strips an O atom (Ostrip1) from the dissociated O2 molecule. As a result, the bond length between Hg(ads) and Ostrip1 shortens from 2.59 Å to 2.16 Å, indicating the formation of HgO(ads). The reaction was also associated with changes in other characteristic bond lengths. The bond length between Hg(ads) and the surface increased from 3.49 Å to 3.88 Å. Contrarily, the bond length between the adsorbed O atom (Ostrip2) and the surface decreased from 2.56 Å (2.45 Å) to 2.02 Å (1.97 Å). The bond length in bracket refers to the second bonding of O(strip2) with adjacent Mn site. This indicates that the bonding of Ostrip2 to the surface was reinforced in this step, while that of Hg(ads)-Os was weakened. During this reaction step, the relative energy peaked to -206.3 eV from -206.9 eV before decreasing to -208.3 eV (Table S3). In the third step, formed HgO on the surface site was desorbed (HgO(ads) → HgO) with the breakage of weakened Hg(ads)-Os and Ostrip1Ostrip2 bonds accordingly. The Ostrip1-Ostrip2 bond length weakened with value changing from 1.30 Å to 2.05 Å after breakage of Hg(ads)-Os bond. The desorption step displayed a relative energy of -206.2 eV (Table S3). Based on the above results, the desorption step can be considered as the rate-determining step for Hg0 catalytic oxidation [49]. The desorption of Hg2+ was more difficult than the adsorption and reaction of Hg0, which was supported by TPSR and XPS analysis. The reaction mechanism of Mo on the desorption step was then conducted to emphasize the role of Mo. The effect of Mo on the reaction

mechanism is designated by the red line in Fig. 13. Further elaboration of the desorption energy is presented in Fig. S13 of the supporting document. Prior to Mo addition, desorption energy transited from −208.27 eV (step 3) to −206.22 eV step 4 and then to −206.60 eV step 5. After Mo addition, the desorption energy transited from −208.27 eV (step 3) to −207.72 eV step 4 and then to −207.67 eV step 5. that Mo exhibiting a promotive effect by reducing the energy barrier for the desorption step. This can be attributed to the synergistic effect between Mo and Mn. As a result, the combined catalyst displays the desorption of HgO than the δ-MnO2 nanosheet, as posited by TPSR experimental test. In summary, Fig. 14 illustrates the mechanism of MoO3-adjusted δMnO2 nanosheet for catalytic oxidation of Hg0 to Hg2+. This proposed that the introduction of Mo resulted in the occurrence of structural distortion (Raman spectroscopy) and introduction of defect surfaces (DRIFTS and HRTEM). Subsequently, this increased the exposure of active sites that facilitated Hg adsorption and catalytic oxidation (DRIFTS and TPD (supporting document)). Moreover, the distributive and synergistic effect of Mo and Mn can be described as the logical explanation for the enhanced catalytic performance (XPS and TPR (supporting document)). Validation of this hypothesis was supported using DFT first principles calculation to show that VO sites were notable for inducing active unsaturated O-species, specifically when properly adjusted. This corresponds with the findings from Raman, DRIFT and HRTEM. In addition, this defective site promoted surficial interaction resulting in facetdependent oxygen activated sites on δ-MnO2 surface as proposed by DRIFTS and TPD analyses. Moreover, the synergistic effect between Mo and Mn resulted in the reduction in energy for the rate-determining desorption step. This result was ascribed to the varying bond strength of desorbed HgO on the surface of MoO3 (010) and VO defect δ-MnO2 (101). This was supported by TPSR analysis which revealed that the varying bond strength on the different surfaces of MoO3 (010) and VO defect δ-MnO2 (101) regulated the activation energy for Hg0 removal. In general, the synergistic interaction between MoO3 and defective δ-MnO2 nanosheet surface was best suited in the 1.25Mo2 Mn catalyst, which resulted in the maximum amount of Hg2+ generated. This was achieved by the MoO3-adjusted properties of activation energy, bond strength, Brønsted acidic sites and defects, which were favorable for the catalytic oxidation of Hg0. In addition, the desorption step, which was defined as the rate-determining step, was promoted by MoO3 introduction. The promoting effect was highlighted by the significant reduction in desorption energy.

Fig. 14. Proposed mechanism for MoO3-adjusted δ-MnO2 nanosheet for Hg0 catalytic oxidation.

4. Conclusion The combination of computational modelling and knowledge-driven 11

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experimentation has provided unprecedented new insight into the synthesis and evaluation of MoO3-adjusted δ-MnO2 nanosheet for Hg0 catalytic oxidation. The amount of Hg2+ generated varied with the adjusted amount of Mo precursor. Among the various compositions, the 1.25Mo2 Mn catalyst exhibited the best Hg0 removal performance with a Hg0 removal efficiency > 98%. Furthermore, the catalyst displayed the most extended working temperature window between 100–325 °C, highest oxidation ratio > 52%, and better stability over 10 h period at 250 °C. It is also demonstrated that the characteristic morphology, dispersive and defective promotional effect of Mo, and the complimentary interaction between Mo and Mn species have a synergistic effect on exposing surficial sites necessary for Hg0 adsorption and oxidation. Moreover, the complimentary interaction between MoO3 and defective δ-MnO2 nanosheet facilitated rapid desorption of HgO.

[19] [20] [21] [22] [23] [24] [25]

Acknowledgements

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Following funding bodies are acknowledged for partially sponsoring this research: National Key Research and Development Projects (2017YFB0603202), National Natural Science Foundation of China (51706114 and 51836006), Ningbo ‘Science and Technology Innovation 2025’ Major Project, Ningbo Natural Science Foundation (2017A610060) and Postdoctoral International Exchange Program.

[27] [28]

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.117829.

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References

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