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TiO2 catalysts: Structure and mechanism study
Molecular Catalysis 479 (2019) 110633
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Simultaneous catalytic oxidation of CO and Hg0 over Au/TiO2 catalysts: Structure and mechanism study
T
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Bo Yanga, , Aohui Penga, Xinzhou Wanga, Qiong Huanga, Mindong Chena, Yuesong Shenb, Haitao Xuc, Shemin Zhub a
Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, PR China b Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China c School of Environmental Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gold Catalytic oxidation Reaction mechanism Carbon monoxide Mercury
Au/TiO2 catalysts with different morphologies were prepared by the modulation of chlorine via a hydrothermal method, which were used for the simultaneous catalytic removal of CO and Hg0. Results showed that the Au/ TiO2 catalyst with a structure of submicron sphere and ascorbic acid modification (denoted as AuV/2 M T) showed 38.2% (41.1%, without H2O) CO conversion at 50 °C and nearly 100% CO conversion at 275-350℃ (225350℃, without H2O). This catalyst also exhibited stable catalytic efficiency of approximately 88% (95.7%, without H2O) for Hg0 removal at 300℃. The high catalytic activity of the AuV/2 M T was ascribed to a series of superior properties, including a high specific surface area, a large amount of adsorbed oxygen species and a large brookite/rutile TiO2 ratio. Moreover, the AuV/2 M T also exhibited excellent stability. The AuV/2 M T continued to exhibit nearly 90.5% CO conversion efficiency and 84.5% Hg0 conversion efficiency after continuous testing with 6 vol.% H2O for 120 h. Therefore, this catalyst is expected to be used in coal-fired power plants. The mechanism of CO removal over AuV/2 M T was in-depth investigated. The presence of chlorine promoted the catalytic oxidation of Hg0 while the presence of CO has no great effect on desorption of Hg0. Moreover, catalytic oxidation of CO over AuV/2 M T catalyst was found to follow L-H mechanisms. The formate was the intermediate species for CO catalytic oxidation, and it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) adsorbed onto the catalyst surface. The oxygen species consumed on the surface of catalyst was supplied by oxygen, and it was the key factor in determining the reaction rate.
1. Introduction
coal-fired power plants [12]. Mercury resides in three states in flue gas: Hg0, Hg2+ and HgP (particle-bound) [13]. Hg2+ and HgP are easily desorbed by desulfurization and precipitator [14]. On the other hand, Hg0 is difficult to remove because of its high vapor pressure [15]. Some technologies, including sorbent injection [16] and catalytic oxidation [17], have been developed to remove Hg0 from the emissions. As is well known, CO and Hg0 coexist in the emissions from coal-fired power plants. The temperature of the exhaust from the coal-fired power plants ranges from 250 to 350 °C. Therefore, the simultaneous catalytic removal of CO and Hg0 in the 250–350 °C emissions of coal-fired power plants has important economic and social significance. Because of its excellent properties, TiO2 is usually used as the carrier and many catalysts have been developed, including CeO2/TiO2 [18–22], V2O5/TiO2 [23–26], and MnO2/TiO2 [27–29]. As an active
The catalytic oxidation of carbon monoxide (CO) has caused considerable concern due to its wide application in the industry [1–3]. Although the catalytic oxidation of CO is a prototypical reaction in catalysis [4–6], the development of practical catalysts for CO removal still faces many problems [7–9]. The catalysts should have the excellent properties simultaneously: high catalytic activity, high stability and resistance to water vapor [10]. As a microscale heavy metal, mercury damages the environment and human health. These effects have been intensified through deterioration of the mercury cycle in natural. The largest proportion of the artificial emissions of mercury was reported to be the combustion of coal [11]. The US Environmental Protection Agency (EPA) has established limits on mercury emissions for latest
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Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.mcat.2019.110633 Received 8 July 2019; Received in revised form 8 September 2019; Accepted 13 September 2019 Available online 21 September 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Molecular Catalysis 479 (2019) 110633
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ingredient, nanometric Au is an excellent electron donor or acceptor. It therefore has the capacity to substantially promote catalytic reaction via the redox cycle process. It also exhibits excellent chemical inertness and can remain stable under various reaction conditions [30]. Therefore, Au nanomaterials are excellent catalysts in catalytic reactions [31,32]. Au-based catalysts have exhibited high catalytic activity for CO removal even at very low reaction temperatures, which has been intensively studied as a matter of fundamental research [33]. Due to the high catalytic activity toward various oxidation and reduction reactions, metal oxide supports such as Fe2O3, TiO2 and CeO2 has attracted particular interest [5,34–36]. In the case of Au-based catalysts over TiO2 surfaces [37], the formation of Au clusters is not only higher than the density on other surfaces, which makes the loading of Au nanoparticles higher [38], but the resistance to sintering of Au clusters is increased [39]. However, the practical application of Au-based catalyst still have difficult problems because of the sensitivity to the preparation conditions and the deactivation during long-term storage and operation [40,41]. With this hypothesis, to enhance the catalytic stability of Au catalyst, we prepared a series of morphology-controlled Au/TiO2 catalysts by an impregnation method. Moreover, the activity toward CO and Hg0 oxidation over these morphology-controlled Au/TiO2 catalysts and its mechanism were also systematically studied.
Fig. 2. Schematic diagram of catalytic testing device.
flowers with a diameter of approximately 8.0 μm. In addition, each micron flower was composed of nanoneedles, and the surface of micron flowers was smooth. Due to the loose flower structure, 6 M T exhibited a larger specific surface area than 2 M T. Au/TiO2 catalysts in this experiment were prepared by the impregnation method. Two grams of the TiO2 (2 M T or 6 M T) carrier was dipped into HAuCl4 solution (2.5 mM, 80 mL) and stirred 65 °C until the solution was transformed into a solid. The product was then dried in air at 80 °C for 24 h and the solid was calcined at 500 °C for 2 h. The catalysts were assigned as Au/2 M T and Au/6 M T, respectively. To investigate the effect of the state of the Au particles on the catalytic activity, ascorbic acid was used as the reducing agent in the preparation of Au/TiO2 catalysts. Seventy milligrams of ascorbic acid were dissolved in 1 mL of H2O to form a solution. The ascorbic acid solution was then added to 80 mL of HAuCl4 solution (2.5 mM) and stirred for 30 min so that the colloidal Au solution was formed. Two grams of the TiO2 catalyst carrier was dipped into the colloidal gold solution, and the resulting mixture was stirred while immersed in a water bath at 65 °C until the solution was transformed into a solid. The product was then dried in air for 24 h at 80 °C and calcined for 2 h at 500 °C. The obtained catalysts were assigned as AuV/2 M T and AuV/6 M T.
2. Experiment 2.1. Catalyst preparation Different morphology of TiO2 with modulated chlorine was synthesized by a hydrothermal mean. In a typical synthsis, 5 mL tetrabutyl titanate (TBOT), 40 mL of concentrated hydrochloric acid (HCl, 12 M) and 20 mL of deionized water were mixed and transferred to a hydrothermal reactor (100 mL). The reactor was heated to 180 °C for 2 h, and the resulting solid was washed three times with deionized water. The TiO2 catalyst carrier was thus obtained and designated as 2 M T. When the reactant amounts were 5 mL TBOT, 10 mL HCl and 50 mL H2O, the obtained TiO2 catalyst carrier was designated as 6MT. As shown in Fig. 1, 2MT consisted of numerous submicron spheres, and its diameter was between 500 and 800 nm. The surface of the submicron spheres was rough and uneven. By contrast, the 6MT consisted of many micron
Fig. 1. FE-SEM photographs of different catalyst carriers. (a) and (c): 2 M T; (b) and (d): 6 M T. 2
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Fig. 3. (a) XRD patterns; (b) N2 adsorption/desorption isotherms; (c) pore size distribution; (d) O2-TPD, (e) H2-TPR and (f) Hg-TPD profiles of different Au/TiO2 catalysts.
2.2. Measurement of catalytic activity
Table 1 Physical and chemical properties of different catalysts. Sample
Au/2 M T AuV/2 M T Au/6 M T AuV/6 M T
O2-TPD
The catalytic activity for simultaneously catalytic removal of CO and Hg0 was measured in a fixed bed reactor (made of quartz tube). The diagram of measurement system is shown in Fig. 2. The Hg0 vapor was generated at 70 °C from a water bath (65 ng·min−1) under N2 gas flowing at 400 mL·min−1; Hg0 at the inlet and outlet of the reactor was measured with an online mercury analyzer (QM210H, GreenCalm). The total gas flow rate was 1000 mL·min−1 and in accordance with a GHSV of 100,000 mL·(g·h)−1. The reactant gas was made up with 600 ppm CO, 10 vol.% O2, Hg0 (65 ng·min−1), 6 vol.% H2O and N2 as balance. The CO concentrations were measured by a flue-gas analyzer (MRU, Germany). The data was recorded after the system is stable. The
SBET (m2·g−1)
Tpeak-1/(cm3·g−1)
Tpeak-2/(cm3·g−1)
Tpeak-3/(cm3·g−1)
1.482 1.501 1.271 1.485
– 0.635 – –
0.851 0.706 0.345 0.596
63.0 76.1 105.4 108.8
3
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Fig. 4. SEM photographs of different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T.
catalytic activity of CO and Hg0 was calculated by Eq. (1) and Eq. (2).
[Hg 0]in − [Hg 0]out ×100% [Hg 0]in
(1)
[CO]in − [CO]out ×100% [CO]in
(2)
Hg 0 conversion=
CO conversion=
presence of ascorbic acid promoting the transformation from rutile TiO2 to brookite TiO2. In addition, no peak was observed corresponding to a different valence state of Au, indicating that all of the Au+ ions were transformed to Au0. As shown in Fig. 3(b) and (c), N2 adsorption/desorption isotherms of Au/TiO2 catalysts were made up of mesopores and micropores whose sorption behaviors differed at intermediate relative pressure, and this finding confirmed Langmuir IV adsorption behavior (for the mesopores). The relative pressure of separation decreased gradually with the addition of ascorbic acid. That is, with the addition of ascorbic acid, the pore size of capillary pores and the degree of effect of capillary agglomeration is reduced. Moreover, the porous structures (H1-type in the hysteresis loop) of Au/TiO2 catalysts was beneficial to the diffusion and adsorption of gas molecules during high temperatures, which facilitated the catalytic reaction [42]. Fig. 3(c) shows the pore size distribution of Au/TiO2 catalysts. The most likely pore sizes of four samples were similar, and the pore size distribution was relatively narrow. The ratio of mesopores increased slightly with the presence of ascorbic acid, possibly because of the increased pore size after ascorbic acid ablation. The specific surface areas of the Au/TiO2 catalysts are shown in Table 1. The specific surface areas of the Au/2 M T, AuV/ 2 M T, Au/6 M T and AuV/6 M T catalysts were 63.0 m2/g, 76.1 m2/g, 105.4 m2/g and 108.8 m2/g, respectively. Thus, the specific surface area of 6 M T was much larger than that of 2 M T, and the specific surface area of the Au/TiO2 catalyst increased slightly after ascorbic acid ablation. To investigate oxygen activation and mobility over the Au/TiO2 catalysts, O2-TPD experiments were carried out; the results are shown in Fig. 3(d). As is well known, four states of adsorbed oxygen species exist on the catalyst surface: physically adsorbed oxygen (O2), adsorbed oxygen species (O2− and O−) and surface lattice oxygen (O2-) [43,44]. The physically adsorbed oxygen was removed after the preheating at 300 °C under a helium stream, and the adsorbed oxygen species were easier to desorb than surface lattice oxygen. Two or three peaks were observed in the O2-TPD curves for all of the Au/TiO2 catalysts. The first peak appeared between 50 °C and 350 °C and was attributed to O2−. The second peak between 350 °C and 500 °C was ascribed to O−. The third peak above 500 °C was attributed to O2-. The amount of each oxygen species was calculated from the intensity and area of the peaks;
2.3. Characterization X-ray diffraction (XRD) patterns were illustrated using an X-ray diffractometer. The microstructural nature and element mapping of the catalysts were investigated by scanning electron microscopy, fieldemission scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) graphs were obtained on an AXIS ULTRA DLD instrument. The redox performance of catalysts was analyzed by H2-TPR (the temperature programmed reduction of hydrogen) using an Autochem 2910. The temperature-programmed desorption of oxygen or mercury (O2-TPD or Hg-TPD) was carried out on a CHEMBET-3000. The pore size, pore volume and specific surface area of samples were analyzed by N2-BET method using a surface-area analyzer. In situ diffuse-reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was performed with a Nicolet is50 spectrometer. The detail information was seen in Supporting Information. 3. Results and discussion 3.1. Physical and chemical properties X-ray diffraction patterns of the several catalysts were measured as shown in Fig. 3(a). All of samples exhibited the diffraction patterns of rutile TiO2 (2θ = 27.4°, 36.1°, 41.2°, 54.3°, 56.6°) (PDF-ICDD 71-650), brookite TiO2 (2θ = 25.3°, 30.8°, 48.0°) (PDF-ICDD 29-1360) and Au (2θ = 38.2°, 44.4°, 64.5°, 77.5°) (PDF-ICDD 65-2870). The diffraction peak intensities of rutile TiO2 decreased in the order Au/6 M T > AuV/ 6 M T > Au/2 M T ≈ AuV/2 M T, whereas the intensity ratio of brookite and rutile TiO2 decreased in the order AuV/2 M T > Au/ 2 M T > AuV/6 M T > Au/6 M T. These trends were due to the 4
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Fig. 5. TEM photographs and element mapping of AuV/2 M T catalyst.
adsorbed oxygen species increased with the addition of ascorbic acid and the AuV/2 M T exhibited the greatest mobility of chemisorbed oxygen. Furthermore, the peak temperature of Au/2 M T and AuV/ 2 M T was lower than that of Au/6 M T and AuV/6 M T, indicating that the 2 M T carrier exhibited more excellent redox properties than 6 M T carrier. In other words, the presence of chloride ions was not conducive to the improvement of redox performance. The Hg-TPD was performed over AuV/2 M T catalyst, and the results were exhibited in Fig. 3(f). Binding energy is associated with desorption temperature. Binding energy increases as the desorption temperature increases, so that the Hg-TPD could evaluate the binding strength of Hg [45]. As shown in Fig. 3(f), during the adsorption of Hg + N2, a desorption peak of Hg appeared at 300.5 °C. For the adsorption under Hg + O2, the center of the desorption peak shifts toward high temperature (310.5 °C) compared to that of adsorption under Hg + N2. It demonstrated that the desorption of mercury promoted by O2. However, during the adsorption of Hg + CO + O2, there was no great
the results are included in Table 1. AuV/2 M T had the largest amounts of adsorbed oxygen species, whereas AuV/6 M T and Au/2 M T had relatively few adsorbed oxygen species. Thus, the amount of adsorbed oxygen species increased due to the addition of ascorbic acid and the chemisorbed oxygen over the AuV/2 M T catalyst exhibited the greatest mobility. The redox properties of different catalysts were analyzed by H2-TPR as shown in Fig. 3(e). Further, Table S1 listed the peak (from 200 to 900 °C) temperature obtained from H2-TPR patterns. Au/2 M T indicated two main peaks spanned at 200–650 °C, while the hydrogen reduction of the other catalysts shows a broad peak across the 300–900 °C due to the reduction of the TiO2 surface. As shown in Fig. 3(e), the position of reduction peak shifted to higher temperatures with the addition of ascorbic acid, indicating that the redox properties of Au/TiO2 catalyst decreased. However, the addition of ascorbic acid increased the hydrogen consumption of TiO2 greatly so that the hydrogen consumption of AuV/2 M T and AuV/6 M T was much higher than that of Au/2 M T and Au/6 M T. It was due to the amount of 5
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Fig. 6. (a) survey, (b) Au 4f, (c) Ti 2p and (d) O 1s XPS high-resolution scans spectra of different catalysts.
be beneficial to show the particle size distribution for the different catalysts and link them to their catalytic activity. The two synthetic procedures (impregnation on TiO2 and calcination; colloidal Au reduction by ascorbic acid followed by impregnation of Au particles on TiO2) should be expected to affect the Au particle size significantly. However, considering the economic cost, we chose the optimal catalyst (AuV/2 M T) for TEM characterization. Fig. 5 showed the TEM photographs and element mapping of AuV/2 M T catalyst. It could be seen that the Au nanoparticles were loaded on the TiO2 surface. For Au nanoparticles, there were two orders of magnitude about the size: one was about 40–100 nm and the smaller one was about 8–15 nm.
Table 2 Atomic ratios on the surface of different catalysts. Sample
Au/2 M T AuV/2 M T Au/6 M T AuV/6 M T
Au (At.%)
0.37 0.36 0.34 0.34
Cl (At.%)
0.35 0.34 0.23 0.18
O (At.%)
Ti (At.%)
Oα
Oβ
Ti3+
Ti4+
7.65 10.98 6.58 8.68
60.60 57.06 61.78 59.08
5.52 5.25 5.29 5.30
25.51 26.01 25.78 26.42
change for the position of desorption peak center, which indicated that the presence of CO has no great effect on desorption of mercury. The microstructural property of the Au/TiO2 catalyst was investigated by SEM, the results are illustrated in Fig. 4. Compared with the 2 M T catalyst carrier, no great change in morphology was observed and the average particle size of Au/2 M T and AuV/2 M T increased slightly. The surface of the particles became rough, and many small particles were agglomerated on the carrier surface. By contrast, the micron flower shape could not be observed for the Au/6 M T and AuV/ 6 M T catalysts, possibly because the Au nanoparticles broke the micron flower mechanically during the catalyst preparation process. However, the average particle sizes of the Au/6 M T and AuV/6 M T catalysts were still smaller than those of the Au/2 M T and AuV/2 M T catalysts. Furthermore, the distribution of different elements of the Au/TiO2 catalysts was shown in the elemental mapping images (Figs. S1–S4). As can be seen from Figs. S1–S4, the dispersion of Au, Ti, O and Cl on the catalyst surface was well. Au particle size effects are significant for catalytic oxidation reactions such as CO oxidation as has been reported previously for significantly smaller Au particle size ranges [46]. It would
3.2. Surface analysis The XPS was used to evaluate the surface composition and oxidation states of Au/TiO2 catalysts, which is a crucial factor in the catalytic reaction. Figs. 6 and S5 show the XPS spectra of Au 4f, Ti 2p, O 1s, and Cl 2p over different catalysts. The spectra of all Au/TiO2 catalysts show peaks for O, Ti, Au, Cl, W and Mo. The weak peaks for W and Mo are attributed to impurities in the TiO2 catalyst carrier. As evident in Fig. 6(b), the strong doublet peaks of Au 4f emerging at 87.6 eV and 84.0 eV were attributed to Au0 4f5/2 and Au0 4f7/2, respectively [47]. No peak ascribed to Au3+ was observed, illustrating that the reduction of Au3+ to Au0 was complete during the catalyst preparation. In addition, no great difference was observed among the proportion of Au atoms for all of the Au/TiO2 catalysts (Table 2). Furthermore, trace amounts of chlorine were detected on the Au/TiO2 catalyst surfaces; the peaks for Cl 2p were therefore weak. The proportions of Cl atoms were approximately 1.8–3.5%, which ensured that the Au/TiO2 catalyst still 6
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Fig. 7. (a) CO conversion; (b) Hg0 conversion; (c) CO conversion after H2 pretreatment and (d) stability of AuV/2 M T catalysts at 300 °C. Reaction condition: 600 ppm CO, 10 vol.% O2, 6 vol.% H2O, Hg0 (65 ng·min−1) and N2 balance gas, GSHV of 100 000 mL·(g·h)−1.
surface of Au/2 M T, AuV/2 M T, Au/6 M T and AuV/6 M T were 7.65%, 10.98%, 6.58% and 8.68%, respectively. Early research had confirmed that chemisorbed oxygen is a kind of active oxygen species which plays a significant role in catalytic oxidation [50]; thus, it could also accelerate the process of the catalytic reaction [51]. The addition of ascorbic acid increased the proportion of Oα, which should increase the catalytic activity. Furthermore, compared with the Au 4f, Ti 2p and O 1s spectra, the Ti 2p and O 1s spectra showed shifts, indicating that the addition of ascorbic acid affected the chemical state of Ti and O atoms instead of Au atoms.
Table 3 Comparison of Au/TiO2 catalysts for CO oxidation. catalyst
AuV/2 M T Au/TiO2 (P25) Au/TiO2 (P25) Au/Y-TiO2 Au/TiO2 (Rutile)
T50 (℃)
Stability
Ref
Time (h)
Deactivation (%) (CO conversion)
Deactivation (%) (Hg0 conversion)
50 < (75*) ∼50
120
5(9*)
3(4*)
24
∼39
\
This work [52]
∼50
48
∼88
\
[41]
∼25 ∼100
48 \
∼11 \
\ \
[41] [53]
3.3. Catalytic performance Fig. 7 displays the CO and Hg0 conversions obtained from Au/2 M T, AuV/2 M T, Au/6 M T, and AuV/6 M T catalysts under different conditions (test under water conditions). In comparison, the corresponding test results in the absence of water are shown in Fig. S6. As shown in Fig. 7(a), the four catalysts exhibited great differences in CO oxidation without additional pretreatment. The Au/6 M T exhibited the lowest catalytic activity for CO oxidation, and the catalytic activity was only 30.1% at 350 °C. However, the catalytic activity of AuV/6 M T could reach 30.7% at 150 °C. The activity was also 93.5% when the temperature was 350 °C. According to the foregoing analysis, the main difference between Au/6 M T and AuV/6 M T was the chemical state of Ti and O atoms, which also changed the crystal form of TiO2, the specific surface area and the amount of adsorbed oxygen species. Similar to the results for Au/6 M T and AuV/6 M T, a great increase in catalytic activity was observed for Au/2 M T upon the introduction of ascorbic acid. The catalytic activities of Au/2 M T and AuV/2 M T were 3.5% and
T50: The temperature for 50% CO conversion. * Test under water conditions.
exhibited high catalytic activity toward Hg0 oxidation without additional chlorine ingress. Fig. 6(c) exhibits the XPS spectra of Ti 2p. The Ti3+ (463.5 eV) and 4+ Ti (458.9 eV and 464.7 eV) were detected to contribute Ti 2p [48]. The Ti 2p peaks of Au/2MT shifted to the higher binding energies with the introduction of ascorbic acid, showing a strong interaction between Au and Ti atoms. The O 1s peaks were deconvoluted into two peaks, one being chemisorbed oxygen (later marked Oα) and the other being lattice oxygen (later marked Oβ) [49]. As can been see from Fig. 6(d), with the introduction of ascorbic acid, the transition of the O 1s peak of Au / 2MT to lower binding energies, suggesting an enhanced interaction between Au and O atoms. Moreover, the atomic ratios of Oα on the 7
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Fig. 8. In situ DRIFTS study of CO oxidation on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. The catalysts were treated under 500 ppm CO/10% O2/N2 in the DRIFTS reaction (F=50 mL·min−1, T =300 °C).
catalysts were pretreated under H2 for 1 h at 500 ℃; the results are shown in Fig. 7(c). The Au/6 M T also showed 84.2% conversion at 350 ℃, which was 54.1% higher than that without pretreatment. In addition, AuV/2 M T still exhibited the best catalytic performance for CO oxidation. It reached 59.1% CO conversion at 50 °C, which was 20.9% higher than that without H2 pretreatment. As is well known, H2 pretreatment could not only reduce the chloride concentration on the catalyst surface but also increase the oxygen vacancy concentration. All of these factors increase the catalytic activity toward CO oxidation. Stability is well known to be an important factor for commercial catalyst. To investigate the stability, AuV/2 M T, which was chosen as the optimum catalyst among the four catalysts developed in this work, was continuously tested for more than 120 h (Fig. 7(d)). No significant attenuation occurred in the catalytic activity after 120 h. The AuV/2 M T continued to exhibit nearly 90% CO conversion efficiency (decreased by about 9%) and 84% Hg0 conversion efficiency (decreased by about 4%) at 300 ℃ after 120 h. Thus, the AuV/2 M T catalyst exhibited excellent stability. Furthermore, the catalytic performance of four catalysts was shown in Fig. S7 when there was no 6 vol.% H2O in the mixed gas. All the catalytic efficiencies were similar but higher than that with 6 vol.% H2O. In other words, water vapor has no obvious effect on the simultaneous catalytic removal of CO and Hg by AuV/2 M T. Catalytic performance of the various Au/TiO2 catalysts from previous literature was summarized in Table 3 [41,52,53]. Apparently, the catalytic activity of the AuV/2 M T catalyst was comparable and the stability was superior to many other Au/TiO2 catalysts. On the other hand, the loading amount of Au for AuV/2 M T catalyst was only 0.37%, which was lower than that for the catalysts in the previous literature. In other words, the AuV/2 M T catalyst was more economical.
38.2%, respectively, at 50 °C. In addition, AuV/2 M T exhibited nearly 100% CO conversion at 275-350℃ (225–350 °C, in the absence of water as shown in Fig. S6). Therefore, higher specific surface areas, greater amounts of adsorbed oxygen species, greater atomic ratios of chemisorbed oxygen and greater brookite/rutile ratios of TiO2 were beneficial for the catalytic oxidation of CO. As shown in Fig. 7(b), Hg0 removal efficiency of Au/2 M T and Au/ 6 M T decreased to 63.5% and 61.3%, respectively, when the reaction time was 3 h. It indicates that Hg0 was partly adsorbed on the catalyst surface, which made the initial removal efficiency of Hg0 relatively high. That is, Au/2 M T and Au/6 M T show low catalytic activity for Hg0 oxidation. Conversely, AuV/2 M T and AuV/6 M T catalysts exhibited a high catalytic efficiency remaining relatively stable of approximately 95.7% and 92.8% for 3 h. After injecting 6 vol.% H2O, the Hg0 removal efficiency of AuV/2 M T and AuV/6 M T still reaching 88% and 84.5% for 3 h. The presence of chlorine on the Au/TiO2 catalyst surface promoted the catalytic oxidation; thus, there was no need to use additional chlorine. The difference in catalytic activities among the four catalysts was due to differences in the chemical states of Ti and O atoms. Further, Fig. S7 exhibited the XPS spectra of Hg 4f for used AuV/ 2 M T catalyst. The peaks of Hg2+ were detected, which is derived from HgO adsorbed on the AuV/2 M T surface. It proved that Hg0 was oxidized over AuV/2 M T to form Hg2+while Hg2+ adsorbed on the catalyst surface. At low temperatures, the presence of Cl hindered the catalytic oxidation of CO but promoted the process of Hg0 oxidation. To adjust the catalytic temperature window, we used H2 pretreatment to decrease the amount of chlorine on the Au/TiO2 catalyst surface so that the CO conversion at low temperatures could be adjusted. All of the Au/TiO2 8
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Fig. 9. In situ DRIFTS study of CO adsorption-desorption on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. The catalysts were pretreated under 500 ppm CO/N2 in the DRIFTS reaction (F =50 mL·min−1, T =300 °C).
2 M T > Au/2 M T > AuV/6 M T > Au/6 M T. This trend is attributed to more chlorine being used to control the microstructure of 6 M T, which resulted in many −OH groups being replaced by -Cl groups. The presence of −OH groups, chemisorbed oxygen and especially adsorbed oxygen species were beneficial to the catalytic performance. These results, in conjunction with the O2-TPD and XPS results, indicate that adsorbed oxygen species were the most active oxygen and that AuV/ 2 M T exhibited the highest concentration of −OH groups, chemisorbed oxygen and adsorbed oxygen species, which is why it showed the best catalytic performance among the four catalysts. Fig. 9 presents the in situ DRIFT spectra of CO adsorption-desorption, and Figs. S8–S11 shows the detailed spectra over different catalysts at 300 °C. The catalysts pretreated with CO were purged with a N2 feed for 0.5 h, then 500 ppm CO/N2 was adsorbed for another 0.5 h, and finally purged with 10% O2/N2 for 0.5 h. Notably, the formate species of Au/2 M T and Au/6 M T disappeared immediately, whereas those of AuV/2 M T and AuV/6 M T were still stable after N2 purging for 1 min. All of the CO molecules adsorbed onto Ti4+ ions (Ti4+-CO) or metallic gold (Au0-CO) were consumed; thus, no bands attributed to these species were observed. These results indicate that CO adsorbed onto the catalyst surface had been consumed and transformed into adsorbed CO2 and carbonate species. We also found that the −OH groups of Au/2 M T, Au/6 M T and AuV/6 M T were almost completely consumed, whereas a large amount of −OH groups was still adsorbed onto the AuV/2 M T surface. In addition, all of the bands decreased slightly after N2 purging for 30 min because of the consumption of CO species. When CO purging started, the bands for Ti4+-CO and Au0-CO appeared immediately and other bands for CO species increased slightly. The band for −OH groups decreased slightly after CO purging.
3.4. In situ DRIFTS study In situ DRIFT was conducted to analyze the reaction mechanism of CO oxidation over Au/TiO2 catalysts as shown in Figs. 8–10. The catalysts were treated at 300 °C under 500 ppm CO/10%O2/N2. As shown in Fig. 8, two peaks appeared at 2940 cm−1 and 2880 cm-1; these peaks correspond to the C–H stretch of formate species [54]. The bands at 2370 and 2320 cm-1 are assigned to CO2 [55]. The band at 2180 cm-1 is ascribed to the adsorption of CO onto Ti4+ ions (Ti4+−CO) [55,56]. The band at 2110 cm-1 is assigned to the adsorption of CO onto metallic gold surface (Au0−CO) [57]. The band at 1685 cm-1 is attributed to the stretching vibration of −OH groups [58]. The spectral zone between 1600 and 1300 cm-1 corresponds to carbonates species [59]. Obviously, CO could be adsorbed onto the Au/2 M T and AuV/2 M T surface and transformed into formate species quickly. However, the formate species were consumed gradually over the Au/2 M T surface and maintained a high level over the AuV/2 M T surface. By contrast, CO was adsorbed and transformed into the formate species slowly over the Au/6 M T and AuV/6 M T catalysts and the formate species of AuV/6 M T was higher than that of Au/6 M T. In addition, we noted no great change in the bands of CO2 over Au/2 M T and Au/6 M T the whole time, whereas the bands of CO2 over AuV/2 M T and AuV/6 M T increased gradually in intensity over time. This increase was attributed to the good adsorption capacity of AuV/2 M T and AuV/6 M T for CO2. Furthermore, the stretching vibration of −OH groups was observed over the Au/2 M T and AuV/2 M T surface, whereas no band was observed over Au/6 M T and AuV6 M T. Thus, the concentration of −OH groups over Au/2 M T and AuV/2 M T was higher than that over Au/6 M T and AuV/6 M T. The concentration of −OH groups decreased in the order AuV/ 9
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Fig. 10. In situ DRIFTS study of CO transient-pulse on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. (F=50 mL·min−1, T =300 °C).
same time, the catalyst surface adsorbed a portion of the CO2 generated by the catalytic reaction. To better explore the reaction mechanism for CO oxidation over the Au/TiO2 catalysts, Fig. 10 presents in situ DRIFT spectra of CO transient-pulse on different catalysts. The catalysts were first exposed to 500 ppm CO/N2 for 1 min, followed by N2 for 1 min. Then, 10% O2/N2 was injected for 1 min and N2 for 1 min. For all of the Au/TiO2 catalysts, the bands for Ti4+−CO or Au0−CO appeared after 1 min after CO purging. Similar to the results for in situ DRIFTS spectra of CO oxidation, the bands for formate species over Au/2 M T and AuV/2 M T catalysts appeared and that over Au/6 M T and AuV/6 M T was very weak. It also verified that CO was adsorbed quickly but transformed into the formate species slowly over the Au/6 M T and AuV/6 M T catalysts. CO was easier and quicker to transform into formate species over the Au/ 2 M T and AuV/2 M T catalysts. Because of the transient-pulse, the amount of CO was so small that trace of carbonate species formed and adsorbed on all Au/TiO2 catalyst surface. The bands for Ti4+-CO or Au0-CO disappeared in 1 min and the band for carbonate species also decreased slightly after N2 purging for 1 min. The -OH groups on the catalyst surface were quickly replenished after O2/N2 purging for 1 min. The replenishment rate of -OH groups was also quickest for the AuV/ 2 M T catalyst and slowest for the Au/6 M T catalyst, consistent with the catalytic performance of the catalysts toward CO oxidation. That is, the replenishment and consumption rates of oxygen species on the surface of catalyst were the key factor in the catalytic oxidation of CO.
Fig. 11. Reaction mechanism diagram of simultaneous catalysis oxidation CO and Hg over Au/TiO2 catalyst.
These results suggest that the Au/TiO2 catalyst exhibited excellent adsorption and catalytic oxidation capacity for CO. The bands for -OH groups and carbonates species increased, whereas the bands for Ti4+CO or Au0-CO disappeared after 1 min of O2 purging. These results are attributed to adsorbed CO species on the catalyst surface being oxidized to CO2 by O2 and the catalyst. When the O2 purging continued for 30 min, the bands for carbonate species decreased, indicating weak competitive adsorption between O2 and carbonates species over the Au/TiO2 catalyst. The in situ DRIFTS study of CO adsorption-desorption illustrated that the formate was the intermediate species for CO catalytic oxidation and that it transformed to CO2 quickly by consuming adsorbed oxygen species (O2−, O− and −OH) on the surface of catalyst. Molecular oxygen was mainly used to supplement the oxygen species that were consumed on the catalyst surface, and the oxygen species were the key factors in determining the reaction rate. At the
3.5. Reaction mechanism for removal of CO and Hg0 Combined with after mentioned reactions, the catalytic oxidation of 10
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Appendix A. Supplementary data
CO over the Au/TiO2 catalyst followed L-H mechanisms. The mechanism presented in Fig. 11 and Eq. (3) to Eq. (6). CO first adsorb onto AuV/2 M T to form CO(ads). CO(ads) reacts with –OH, which adsorbs onto the catalyst surface and forms the formate species. The formate species then decompose to generate CO2 and -H+. Then, O2 reacts with -H+ and forms -OH and adsorbs onto the catalyst surface. Finally, oxygen is adsorbed on the surface of the catalyst, oxidizing Ti3+ and Au0 to Ti4+ and Au+, while the lattice oxygen is replenished and regenerated. CO(g) →CO(ads)
(3)
CO(ads) + 2-OH →HCOOH(ads) + -O*(ads)
(4)
HCOOH(ads) →CO2(g) + 2-H+ (ads)
(5)
-H+ (ads) + -O*(ads) →+ -OH(ads)
(6)
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The presence of ascorbic acid promoted the transformation from rutile TiO2 to brookite TiO2, affected the chemical state of Ti and O atoms, and increased the amount of adsorbed oxygen species. These factors were beneficial to enhance the catalytic performance, which is why AuV/2 M T exhibited the best catalytic performance among the developed catalysts. Furthermore, an in situ DRIFTS study of CO adsorption-desorption illustrated that the formate was the intermediate species for CO catalytic oxidation and that it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) on the catalyst surface. Oxygen was mainly used to supplement the oxygen species consumed on the surface of catalyst, and the oxygen species were the key factors determining the reaction rate.
4. Conclusions In this study, Au/TiO2 catalysts with different morphologies were prepared by the modulation of chlorine via a hydrothermal method, and then utilized as catalysts to catalytic removal of CO and Hg0 simultaneously. The Au/TiO2 catalyst with a structure of submicron spheres exhibited excellent catalytic performance for the simultaneous catalytic removal of CO and Hg0. In addition, the AuV/2 M T catalyst also exhibited excellent stability. No significant attenuation occurred in the catalytic activity after continuous testing with 6 vol.% H2O for 120 h. The presence of ascorbic acid promoted the transformation from rutile TiO2 to brookite TiO2, affected the chemical state of Ti and O atoms, and increased the amount of adsorbed oxygen species. Combined with a series of characterization analysis, all these factors conduced to the improvement of catalytic performance. Furthermore, the presence of chlorine promoted the catalytic oxidation of Hg0 while the presence of CO has no great effect on desorption of Hg0. At the same time, the in situ DRIFT spectra results revealed that the catalytic oxidation of CO on Au/ TiO2 catalyst follows the L-H mechanism. The formate was the intermediate species for CO catalytic oxidation and it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) on the catalyst surface. Oxygen was mainly used to supplement the oxygen species consumed on the catalyst surface, and the oxygen species were the key factor determining the reaction rate.
Acknowledgements We would like to acknowledge the financial support from the National Natural Science Foundation of China (Nos.21501097, 21272118and21577065), the Natural Science Foundation of Jiangsu Province (No.BK20170954), the Key Research and Development Program of Jiangsu Province (No. BE2018074), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 18KJB430019), the Startup Foundation for Introducing Talent of NUIST (No.2017r073) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. 11
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Simultaneous catalytic oxidation of CO and Hg0 over Au/TiO2 catalysts: Structure and mechanism study
T
⁎
Bo Yanga, , Aohui Penga, Xinzhou Wanga, Qiong Huanga, Mindong Chena, Yuesong Shenb, Haitao Xuc, Shemin Zhub a
Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, PR China b Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China c School of Environmental Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gold Catalytic oxidation Reaction mechanism Carbon monoxide Mercury
Au/TiO2 catalysts with different morphologies were prepared by the modulation of chlorine via a hydrothermal method, which were used for the simultaneous catalytic removal of CO and Hg0. Results showed that the Au/ TiO2 catalyst with a structure of submicron sphere and ascorbic acid modification (denoted as AuV/2 M T) showed 38.2% (41.1%, without H2O) CO conversion at 50 °C and nearly 100% CO conversion at 275-350℃ (225350℃, without H2O). This catalyst also exhibited stable catalytic efficiency of approximately 88% (95.7%, without H2O) for Hg0 removal at 300℃. The high catalytic activity of the AuV/2 M T was ascribed to a series of superior properties, including a high specific surface area, a large amount of adsorbed oxygen species and a large brookite/rutile TiO2 ratio. Moreover, the AuV/2 M T also exhibited excellent stability. The AuV/2 M T continued to exhibit nearly 90.5% CO conversion efficiency and 84.5% Hg0 conversion efficiency after continuous testing with 6 vol.% H2O for 120 h. Therefore, this catalyst is expected to be used in coal-fired power plants. The mechanism of CO removal over AuV/2 M T was in-depth investigated. The presence of chlorine promoted the catalytic oxidation of Hg0 while the presence of CO has no great effect on desorption of Hg0. Moreover, catalytic oxidation of CO over AuV/2 M T catalyst was found to follow L-H mechanisms. The formate was the intermediate species for CO catalytic oxidation, and it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) adsorbed onto the catalyst surface. The oxygen species consumed on the surface of catalyst was supplied by oxygen, and it was the key factor in determining the reaction rate.
1. Introduction
coal-fired power plants [12]. Mercury resides in three states in flue gas: Hg0, Hg2+ and HgP (particle-bound) [13]. Hg2+ and HgP are easily desorbed by desulfurization and precipitator [14]. On the other hand, Hg0 is difficult to remove because of its high vapor pressure [15]. Some technologies, including sorbent injection [16] and catalytic oxidation [17], have been developed to remove Hg0 from the emissions. As is well known, CO and Hg0 coexist in the emissions from coal-fired power plants. The temperature of the exhaust from the coal-fired power plants ranges from 250 to 350 °C. Therefore, the simultaneous catalytic removal of CO and Hg0 in the 250–350 °C emissions of coal-fired power plants has important economic and social significance. Because of its excellent properties, TiO2 is usually used as the carrier and many catalysts have been developed, including CeO2/TiO2 [18–22], V2O5/TiO2 [23–26], and MnO2/TiO2 [27–29]. As an active
The catalytic oxidation of carbon monoxide (CO) has caused considerable concern due to its wide application in the industry [1–3]. Although the catalytic oxidation of CO is a prototypical reaction in catalysis [4–6], the development of practical catalysts for CO removal still faces many problems [7–9]. The catalysts should have the excellent properties simultaneously: high catalytic activity, high stability and resistance to water vapor [10]. As a microscale heavy metal, mercury damages the environment and human health. These effects have been intensified through deterioration of the mercury cycle in natural. The largest proportion of the artificial emissions of mercury was reported to be the combustion of coal [11]. The US Environmental Protection Agency (EPA) has established limits on mercury emissions for latest
⁎
Corresponding author. E-mail address: [email protected] (B. Yang).
https://doi.org/10.1016/j.mcat.2019.110633 Received 8 July 2019; Received in revised form 8 September 2019; Accepted 13 September 2019 Available online 21 September 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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ingredient, nanometric Au is an excellent electron donor or acceptor. It therefore has the capacity to substantially promote catalytic reaction via the redox cycle process. It also exhibits excellent chemical inertness and can remain stable under various reaction conditions [30]. Therefore, Au nanomaterials are excellent catalysts in catalytic reactions [31,32]. Au-based catalysts have exhibited high catalytic activity for CO removal even at very low reaction temperatures, which has been intensively studied as a matter of fundamental research [33]. Due to the high catalytic activity toward various oxidation and reduction reactions, metal oxide supports such as Fe2O3, TiO2 and CeO2 has attracted particular interest [5,34–36]. In the case of Au-based catalysts over TiO2 surfaces [37], the formation of Au clusters is not only higher than the density on other surfaces, which makes the loading of Au nanoparticles higher [38], but the resistance to sintering of Au clusters is increased [39]. However, the practical application of Au-based catalyst still have difficult problems because of the sensitivity to the preparation conditions and the deactivation during long-term storage and operation [40,41]. With this hypothesis, to enhance the catalytic stability of Au catalyst, we prepared a series of morphology-controlled Au/TiO2 catalysts by an impregnation method. Moreover, the activity toward CO and Hg0 oxidation over these morphology-controlled Au/TiO2 catalysts and its mechanism were also systematically studied.
Fig. 2. Schematic diagram of catalytic testing device.
flowers with a diameter of approximately 8.0 μm. In addition, each micron flower was composed of nanoneedles, and the surface of micron flowers was smooth. Due to the loose flower structure, 6 M T exhibited a larger specific surface area than 2 M T. Au/TiO2 catalysts in this experiment were prepared by the impregnation method. Two grams of the TiO2 (2 M T or 6 M T) carrier was dipped into HAuCl4 solution (2.5 mM, 80 mL) and stirred 65 °C until the solution was transformed into a solid. The product was then dried in air at 80 °C for 24 h and the solid was calcined at 500 °C for 2 h. The catalysts were assigned as Au/2 M T and Au/6 M T, respectively. To investigate the effect of the state of the Au particles on the catalytic activity, ascorbic acid was used as the reducing agent in the preparation of Au/TiO2 catalysts. Seventy milligrams of ascorbic acid were dissolved in 1 mL of H2O to form a solution. The ascorbic acid solution was then added to 80 mL of HAuCl4 solution (2.5 mM) and stirred for 30 min so that the colloidal Au solution was formed. Two grams of the TiO2 catalyst carrier was dipped into the colloidal gold solution, and the resulting mixture was stirred while immersed in a water bath at 65 °C until the solution was transformed into a solid. The product was then dried in air for 24 h at 80 °C and calcined for 2 h at 500 °C. The obtained catalysts were assigned as AuV/2 M T and AuV/6 M T.
2. Experiment 2.1. Catalyst preparation Different morphology of TiO2 with modulated chlorine was synthesized by a hydrothermal mean. In a typical synthsis, 5 mL tetrabutyl titanate (TBOT), 40 mL of concentrated hydrochloric acid (HCl, 12 M) and 20 mL of deionized water were mixed and transferred to a hydrothermal reactor (100 mL). The reactor was heated to 180 °C for 2 h, and the resulting solid was washed three times with deionized water. The TiO2 catalyst carrier was thus obtained and designated as 2 M T. When the reactant amounts were 5 mL TBOT, 10 mL HCl and 50 mL H2O, the obtained TiO2 catalyst carrier was designated as 6MT. As shown in Fig. 1, 2MT consisted of numerous submicron spheres, and its diameter was between 500 and 800 nm. The surface of the submicron spheres was rough and uneven. By contrast, the 6MT consisted of many micron
Fig. 1. FE-SEM photographs of different catalyst carriers. (a) and (c): 2 M T; (b) and (d): 6 M T. 2
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Fig. 3. (a) XRD patterns; (b) N2 adsorption/desorption isotherms; (c) pore size distribution; (d) O2-TPD, (e) H2-TPR and (f) Hg-TPD profiles of different Au/TiO2 catalysts.
2.2. Measurement of catalytic activity
Table 1 Physical and chemical properties of different catalysts. Sample
Au/2 M T AuV/2 M T Au/6 M T AuV/6 M T
O2-TPD
The catalytic activity for simultaneously catalytic removal of CO and Hg0 was measured in a fixed bed reactor (made of quartz tube). The diagram of measurement system is shown in Fig. 2. The Hg0 vapor was generated at 70 °C from a water bath (65 ng·min−1) under N2 gas flowing at 400 mL·min−1; Hg0 at the inlet and outlet of the reactor was measured with an online mercury analyzer (QM210H, GreenCalm). The total gas flow rate was 1000 mL·min−1 and in accordance with a GHSV of 100,000 mL·(g·h)−1. The reactant gas was made up with 600 ppm CO, 10 vol.% O2, Hg0 (65 ng·min−1), 6 vol.% H2O and N2 as balance. The CO concentrations were measured by a flue-gas analyzer (MRU, Germany). The data was recorded after the system is stable. The
SBET (m2·g−1)
Tpeak-1/(cm3·g−1)
Tpeak-2/(cm3·g−1)
Tpeak-3/(cm3·g−1)
1.482 1.501 1.271 1.485
– 0.635 – –
0.851 0.706 0.345 0.596
63.0 76.1 105.4 108.8
3
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Fig. 4. SEM photographs of different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T.
catalytic activity of CO and Hg0 was calculated by Eq. (1) and Eq. (2).
[Hg 0]in − [Hg 0]out ×100% [Hg 0]in
(1)
[CO]in − [CO]out ×100% [CO]in
(2)
Hg 0 conversion=
CO conversion=
presence of ascorbic acid promoting the transformation from rutile TiO2 to brookite TiO2. In addition, no peak was observed corresponding to a different valence state of Au, indicating that all of the Au+ ions were transformed to Au0. As shown in Fig. 3(b) and (c), N2 adsorption/desorption isotherms of Au/TiO2 catalysts were made up of mesopores and micropores whose sorption behaviors differed at intermediate relative pressure, and this finding confirmed Langmuir IV adsorption behavior (for the mesopores). The relative pressure of separation decreased gradually with the addition of ascorbic acid. That is, with the addition of ascorbic acid, the pore size of capillary pores and the degree of effect of capillary agglomeration is reduced. Moreover, the porous structures (H1-type in the hysteresis loop) of Au/TiO2 catalysts was beneficial to the diffusion and adsorption of gas molecules during high temperatures, which facilitated the catalytic reaction [42]. Fig. 3(c) shows the pore size distribution of Au/TiO2 catalysts. The most likely pore sizes of four samples were similar, and the pore size distribution was relatively narrow. The ratio of mesopores increased slightly with the presence of ascorbic acid, possibly because of the increased pore size after ascorbic acid ablation. The specific surface areas of the Au/TiO2 catalysts are shown in Table 1. The specific surface areas of the Au/2 M T, AuV/ 2 M T, Au/6 M T and AuV/6 M T catalysts were 63.0 m2/g, 76.1 m2/g, 105.4 m2/g and 108.8 m2/g, respectively. Thus, the specific surface area of 6 M T was much larger than that of 2 M T, and the specific surface area of the Au/TiO2 catalyst increased slightly after ascorbic acid ablation. To investigate oxygen activation and mobility over the Au/TiO2 catalysts, O2-TPD experiments were carried out; the results are shown in Fig. 3(d). As is well known, four states of adsorbed oxygen species exist on the catalyst surface: physically adsorbed oxygen (O2), adsorbed oxygen species (O2− and O−) and surface lattice oxygen (O2-) [43,44]. The physically adsorbed oxygen was removed after the preheating at 300 °C under a helium stream, and the adsorbed oxygen species were easier to desorb than surface lattice oxygen. Two or three peaks were observed in the O2-TPD curves for all of the Au/TiO2 catalysts. The first peak appeared between 50 °C and 350 °C and was attributed to O2−. The second peak between 350 °C and 500 °C was ascribed to O−. The third peak above 500 °C was attributed to O2-. The amount of each oxygen species was calculated from the intensity and area of the peaks;
2.3. Characterization X-ray diffraction (XRD) patterns were illustrated using an X-ray diffractometer. The microstructural nature and element mapping of the catalysts were investigated by scanning electron microscopy, fieldemission scanning electron microscopy and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) graphs were obtained on an AXIS ULTRA DLD instrument. The redox performance of catalysts was analyzed by H2-TPR (the temperature programmed reduction of hydrogen) using an Autochem 2910. The temperature-programmed desorption of oxygen or mercury (O2-TPD or Hg-TPD) was carried out on a CHEMBET-3000. The pore size, pore volume and specific surface area of samples were analyzed by N2-BET method using a surface-area analyzer. In situ diffuse-reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was performed with a Nicolet is50 spectrometer. The detail information was seen in Supporting Information. 3. Results and discussion 3.1. Physical and chemical properties X-ray diffraction patterns of the several catalysts were measured as shown in Fig. 3(a). All of samples exhibited the diffraction patterns of rutile TiO2 (2θ = 27.4°, 36.1°, 41.2°, 54.3°, 56.6°) (PDF-ICDD 71-650), brookite TiO2 (2θ = 25.3°, 30.8°, 48.0°) (PDF-ICDD 29-1360) and Au (2θ = 38.2°, 44.4°, 64.5°, 77.5°) (PDF-ICDD 65-2870). The diffraction peak intensities of rutile TiO2 decreased in the order Au/6 M T > AuV/ 6 M T > Au/2 M T ≈ AuV/2 M T, whereas the intensity ratio of brookite and rutile TiO2 decreased in the order AuV/2 M T > Au/ 2 M T > AuV/6 M T > Au/6 M T. These trends were due to the 4
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Fig. 5. TEM photographs and element mapping of AuV/2 M T catalyst.
adsorbed oxygen species increased with the addition of ascorbic acid and the AuV/2 M T exhibited the greatest mobility of chemisorbed oxygen. Furthermore, the peak temperature of Au/2 M T and AuV/ 2 M T was lower than that of Au/6 M T and AuV/6 M T, indicating that the 2 M T carrier exhibited more excellent redox properties than 6 M T carrier. In other words, the presence of chloride ions was not conducive to the improvement of redox performance. The Hg-TPD was performed over AuV/2 M T catalyst, and the results were exhibited in Fig. 3(f). Binding energy is associated with desorption temperature. Binding energy increases as the desorption temperature increases, so that the Hg-TPD could evaluate the binding strength of Hg [45]. As shown in Fig. 3(f), during the adsorption of Hg + N2, a desorption peak of Hg appeared at 300.5 °C. For the adsorption under Hg + O2, the center of the desorption peak shifts toward high temperature (310.5 °C) compared to that of adsorption under Hg + N2. It demonstrated that the desorption of mercury promoted by O2. However, during the adsorption of Hg + CO + O2, there was no great
the results are included in Table 1. AuV/2 M T had the largest amounts of adsorbed oxygen species, whereas AuV/6 M T and Au/2 M T had relatively few adsorbed oxygen species. Thus, the amount of adsorbed oxygen species increased due to the addition of ascorbic acid and the chemisorbed oxygen over the AuV/2 M T catalyst exhibited the greatest mobility. The redox properties of different catalysts were analyzed by H2-TPR as shown in Fig. 3(e). Further, Table S1 listed the peak (from 200 to 900 °C) temperature obtained from H2-TPR patterns. Au/2 M T indicated two main peaks spanned at 200–650 °C, while the hydrogen reduction of the other catalysts shows a broad peak across the 300–900 °C due to the reduction of the TiO2 surface. As shown in Fig. 3(e), the position of reduction peak shifted to higher temperatures with the addition of ascorbic acid, indicating that the redox properties of Au/TiO2 catalyst decreased. However, the addition of ascorbic acid increased the hydrogen consumption of TiO2 greatly so that the hydrogen consumption of AuV/2 M T and AuV/6 M T was much higher than that of Au/2 M T and Au/6 M T. It was due to the amount of 5
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Fig. 6. (a) survey, (b) Au 4f, (c) Ti 2p and (d) O 1s XPS high-resolution scans spectra of different catalysts.
be beneficial to show the particle size distribution for the different catalysts and link them to their catalytic activity. The two synthetic procedures (impregnation on TiO2 and calcination; colloidal Au reduction by ascorbic acid followed by impregnation of Au particles on TiO2) should be expected to affect the Au particle size significantly. However, considering the economic cost, we chose the optimal catalyst (AuV/2 M T) for TEM characterization. Fig. 5 showed the TEM photographs and element mapping of AuV/2 M T catalyst. It could be seen that the Au nanoparticles were loaded on the TiO2 surface. For Au nanoparticles, there were two orders of magnitude about the size: one was about 40–100 nm and the smaller one was about 8–15 nm.
Table 2 Atomic ratios on the surface of different catalysts. Sample
Au/2 M T AuV/2 M T Au/6 M T AuV/6 M T
Au (At.%)
0.37 0.36 0.34 0.34
Cl (At.%)
0.35 0.34 0.23 0.18
O (At.%)
Ti (At.%)
Oα
Oβ
Ti3+
Ti4+
7.65 10.98 6.58 8.68
60.60 57.06 61.78 59.08
5.52 5.25 5.29 5.30
25.51 26.01 25.78 26.42
change for the position of desorption peak center, which indicated that the presence of CO has no great effect on desorption of mercury. The microstructural property of the Au/TiO2 catalyst was investigated by SEM, the results are illustrated in Fig. 4. Compared with the 2 M T catalyst carrier, no great change in morphology was observed and the average particle size of Au/2 M T and AuV/2 M T increased slightly. The surface of the particles became rough, and many small particles were agglomerated on the carrier surface. By contrast, the micron flower shape could not be observed for the Au/6 M T and AuV/ 6 M T catalysts, possibly because the Au nanoparticles broke the micron flower mechanically during the catalyst preparation process. However, the average particle sizes of the Au/6 M T and AuV/6 M T catalysts were still smaller than those of the Au/2 M T and AuV/2 M T catalysts. Furthermore, the distribution of different elements of the Au/TiO2 catalysts was shown in the elemental mapping images (Figs. S1–S4). As can be seen from Figs. S1–S4, the dispersion of Au, Ti, O and Cl on the catalyst surface was well. Au particle size effects are significant for catalytic oxidation reactions such as CO oxidation as has been reported previously for significantly smaller Au particle size ranges [46]. It would
3.2. Surface analysis The XPS was used to evaluate the surface composition and oxidation states of Au/TiO2 catalysts, which is a crucial factor in the catalytic reaction. Figs. 6 and S5 show the XPS spectra of Au 4f, Ti 2p, O 1s, and Cl 2p over different catalysts. The spectra of all Au/TiO2 catalysts show peaks for O, Ti, Au, Cl, W and Mo. The weak peaks for W and Mo are attributed to impurities in the TiO2 catalyst carrier. As evident in Fig. 6(b), the strong doublet peaks of Au 4f emerging at 87.6 eV and 84.0 eV were attributed to Au0 4f5/2 and Au0 4f7/2, respectively [47]. No peak ascribed to Au3+ was observed, illustrating that the reduction of Au3+ to Au0 was complete during the catalyst preparation. In addition, no great difference was observed among the proportion of Au atoms for all of the Au/TiO2 catalysts (Table 2). Furthermore, trace amounts of chlorine were detected on the Au/TiO2 catalyst surfaces; the peaks for Cl 2p were therefore weak. The proportions of Cl atoms were approximately 1.8–3.5%, which ensured that the Au/TiO2 catalyst still 6
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Fig. 7. (a) CO conversion; (b) Hg0 conversion; (c) CO conversion after H2 pretreatment and (d) stability of AuV/2 M T catalysts at 300 °C. Reaction condition: 600 ppm CO, 10 vol.% O2, 6 vol.% H2O, Hg0 (65 ng·min−1) and N2 balance gas, GSHV of 100 000 mL·(g·h)−1.
surface of Au/2 M T, AuV/2 M T, Au/6 M T and AuV/6 M T were 7.65%, 10.98%, 6.58% and 8.68%, respectively. Early research had confirmed that chemisorbed oxygen is a kind of active oxygen species which plays a significant role in catalytic oxidation [50]; thus, it could also accelerate the process of the catalytic reaction [51]. The addition of ascorbic acid increased the proportion of Oα, which should increase the catalytic activity. Furthermore, compared with the Au 4f, Ti 2p and O 1s spectra, the Ti 2p and O 1s spectra showed shifts, indicating that the addition of ascorbic acid affected the chemical state of Ti and O atoms instead of Au atoms.
Table 3 Comparison of Au/TiO2 catalysts for CO oxidation. catalyst
AuV/2 M T Au/TiO2 (P25) Au/TiO2 (P25) Au/Y-TiO2 Au/TiO2 (Rutile)
T50 (℃)
Stability
Ref
Time (h)
Deactivation (%) (CO conversion)
Deactivation (%) (Hg0 conversion)
50 < (75*) ∼50
120
5(9*)
3(4*)
24
∼39
\
This work [52]
∼50
48
∼88
\
[41]
∼25 ∼100
48 \
∼11 \
\ \
[41] [53]
3.3. Catalytic performance Fig. 7 displays the CO and Hg0 conversions obtained from Au/2 M T, AuV/2 M T, Au/6 M T, and AuV/6 M T catalysts under different conditions (test under water conditions). In comparison, the corresponding test results in the absence of water are shown in Fig. S6. As shown in Fig. 7(a), the four catalysts exhibited great differences in CO oxidation without additional pretreatment. The Au/6 M T exhibited the lowest catalytic activity for CO oxidation, and the catalytic activity was only 30.1% at 350 °C. However, the catalytic activity of AuV/6 M T could reach 30.7% at 150 °C. The activity was also 93.5% when the temperature was 350 °C. According to the foregoing analysis, the main difference between Au/6 M T and AuV/6 M T was the chemical state of Ti and O atoms, which also changed the crystal form of TiO2, the specific surface area and the amount of adsorbed oxygen species. Similar to the results for Au/6 M T and AuV/6 M T, a great increase in catalytic activity was observed for Au/2 M T upon the introduction of ascorbic acid. The catalytic activities of Au/2 M T and AuV/2 M T were 3.5% and
T50: The temperature for 50% CO conversion. * Test under water conditions.
exhibited high catalytic activity toward Hg0 oxidation without additional chlorine ingress. Fig. 6(c) exhibits the XPS spectra of Ti 2p. The Ti3+ (463.5 eV) and 4+ Ti (458.9 eV and 464.7 eV) were detected to contribute Ti 2p [48]. The Ti 2p peaks of Au/2MT shifted to the higher binding energies with the introduction of ascorbic acid, showing a strong interaction between Au and Ti atoms. The O 1s peaks were deconvoluted into two peaks, one being chemisorbed oxygen (later marked Oα) and the other being lattice oxygen (later marked Oβ) [49]. As can been see from Fig. 6(d), with the introduction of ascorbic acid, the transition of the O 1s peak of Au / 2MT to lower binding energies, suggesting an enhanced interaction between Au and O atoms. Moreover, the atomic ratios of Oα on the 7
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Fig. 8. In situ DRIFTS study of CO oxidation on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. The catalysts were treated under 500 ppm CO/10% O2/N2 in the DRIFTS reaction (F=50 mL·min−1, T =300 °C).
catalysts were pretreated under H2 for 1 h at 500 ℃; the results are shown in Fig. 7(c). The Au/6 M T also showed 84.2% conversion at 350 ℃, which was 54.1% higher than that without pretreatment. In addition, AuV/2 M T still exhibited the best catalytic performance for CO oxidation. It reached 59.1% CO conversion at 50 °C, which was 20.9% higher than that without H2 pretreatment. As is well known, H2 pretreatment could not only reduce the chloride concentration on the catalyst surface but also increase the oxygen vacancy concentration. All of these factors increase the catalytic activity toward CO oxidation. Stability is well known to be an important factor for commercial catalyst. To investigate the stability, AuV/2 M T, which was chosen as the optimum catalyst among the four catalysts developed in this work, was continuously tested for more than 120 h (Fig. 7(d)). No significant attenuation occurred in the catalytic activity after 120 h. The AuV/2 M T continued to exhibit nearly 90% CO conversion efficiency (decreased by about 9%) and 84% Hg0 conversion efficiency (decreased by about 4%) at 300 ℃ after 120 h. Thus, the AuV/2 M T catalyst exhibited excellent stability. Furthermore, the catalytic performance of four catalysts was shown in Fig. S7 when there was no 6 vol.% H2O in the mixed gas. All the catalytic efficiencies were similar but higher than that with 6 vol.% H2O. In other words, water vapor has no obvious effect on the simultaneous catalytic removal of CO and Hg by AuV/2 M T. Catalytic performance of the various Au/TiO2 catalysts from previous literature was summarized in Table 3 [41,52,53]. Apparently, the catalytic activity of the AuV/2 M T catalyst was comparable and the stability was superior to many other Au/TiO2 catalysts. On the other hand, the loading amount of Au for AuV/2 M T catalyst was only 0.37%, which was lower than that for the catalysts in the previous literature. In other words, the AuV/2 M T catalyst was more economical.
38.2%, respectively, at 50 °C. In addition, AuV/2 M T exhibited nearly 100% CO conversion at 275-350℃ (225–350 °C, in the absence of water as shown in Fig. S6). Therefore, higher specific surface areas, greater amounts of adsorbed oxygen species, greater atomic ratios of chemisorbed oxygen and greater brookite/rutile ratios of TiO2 were beneficial for the catalytic oxidation of CO. As shown in Fig. 7(b), Hg0 removal efficiency of Au/2 M T and Au/ 6 M T decreased to 63.5% and 61.3%, respectively, when the reaction time was 3 h. It indicates that Hg0 was partly adsorbed on the catalyst surface, which made the initial removal efficiency of Hg0 relatively high. That is, Au/2 M T and Au/6 M T show low catalytic activity for Hg0 oxidation. Conversely, AuV/2 M T and AuV/6 M T catalysts exhibited a high catalytic efficiency remaining relatively stable of approximately 95.7% and 92.8% for 3 h. After injecting 6 vol.% H2O, the Hg0 removal efficiency of AuV/2 M T and AuV/6 M T still reaching 88% and 84.5% for 3 h. The presence of chlorine on the Au/TiO2 catalyst surface promoted the catalytic oxidation; thus, there was no need to use additional chlorine. The difference in catalytic activities among the four catalysts was due to differences in the chemical states of Ti and O atoms. Further, Fig. S7 exhibited the XPS spectra of Hg 4f for used AuV/ 2 M T catalyst. The peaks of Hg2+ were detected, which is derived from HgO adsorbed on the AuV/2 M T surface. It proved that Hg0 was oxidized over AuV/2 M T to form Hg2+while Hg2+ adsorbed on the catalyst surface. At low temperatures, the presence of Cl hindered the catalytic oxidation of CO but promoted the process of Hg0 oxidation. To adjust the catalytic temperature window, we used H2 pretreatment to decrease the amount of chlorine on the Au/TiO2 catalyst surface so that the CO conversion at low temperatures could be adjusted. All of the Au/TiO2 8
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Fig. 9. In situ DRIFTS study of CO adsorption-desorption on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. The catalysts were pretreated under 500 ppm CO/N2 in the DRIFTS reaction (F =50 mL·min−1, T =300 °C).
2 M T > Au/2 M T > AuV/6 M T > Au/6 M T. This trend is attributed to more chlorine being used to control the microstructure of 6 M T, which resulted in many −OH groups being replaced by -Cl groups. The presence of −OH groups, chemisorbed oxygen and especially adsorbed oxygen species were beneficial to the catalytic performance. These results, in conjunction with the O2-TPD and XPS results, indicate that adsorbed oxygen species were the most active oxygen and that AuV/ 2 M T exhibited the highest concentration of −OH groups, chemisorbed oxygen and adsorbed oxygen species, which is why it showed the best catalytic performance among the four catalysts. Fig. 9 presents the in situ DRIFT spectra of CO adsorption-desorption, and Figs. S8–S11 shows the detailed spectra over different catalysts at 300 °C. The catalysts pretreated with CO were purged with a N2 feed for 0.5 h, then 500 ppm CO/N2 was adsorbed for another 0.5 h, and finally purged with 10% O2/N2 for 0.5 h. Notably, the formate species of Au/2 M T and Au/6 M T disappeared immediately, whereas those of AuV/2 M T and AuV/6 M T were still stable after N2 purging for 1 min. All of the CO molecules adsorbed onto Ti4+ ions (Ti4+-CO) or metallic gold (Au0-CO) were consumed; thus, no bands attributed to these species were observed. These results indicate that CO adsorbed onto the catalyst surface had been consumed and transformed into adsorbed CO2 and carbonate species. We also found that the −OH groups of Au/2 M T, Au/6 M T and AuV/6 M T were almost completely consumed, whereas a large amount of −OH groups was still adsorbed onto the AuV/2 M T surface. In addition, all of the bands decreased slightly after N2 purging for 30 min because of the consumption of CO species. When CO purging started, the bands for Ti4+-CO and Au0-CO appeared immediately and other bands for CO species increased slightly. The band for −OH groups decreased slightly after CO purging.
3.4. In situ DRIFTS study In situ DRIFT was conducted to analyze the reaction mechanism of CO oxidation over Au/TiO2 catalysts as shown in Figs. 8–10. The catalysts were treated at 300 °C under 500 ppm CO/10%O2/N2. As shown in Fig. 8, two peaks appeared at 2940 cm−1 and 2880 cm-1; these peaks correspond to the C–H stretch of formate species [54]. The bands at 2370 and 2320 cm-1 are assigned to CO2 [55]. The band at 2180 cm-1 is ascribed to the adsorption of CO onto Ti4+ ions (Ti4+−CO) [55,56]. The band at 2110 cm-1 is assigned to the adsorption of CO onto metallic gold surface (Au0−CO) [57]. The band at 1685 cm-1 is attributed to the stretching vibration of −OH groups [58]. The spectral zone between 1600 and 1300 cm-1 corresponds to carbonates species [59]. Obviously, CO could be adsorbed onto the Au/2 M T and AuV/2 M T surface and transformed into formate species quickly. However, the formate species were consumed gradually over the Au/2 M T surface and maintained a high level over the AuV/2 M T surface. By contrast, CO was adsorbed and transformed into the formate species slowly over the Au/6 M T and AuV/6 M T catalysts and the formate species of AuV/6 M T was higher than that of Au/6 M T. In addition, we noted no great change in the bands of CO2 over Au/2 M T and Au/6 M T the whole time, whereas the bands of CO2 over AuV/2 M T and AuV/6 M T increased gradually in intensity over time. This increase was attributed to the good adsorption capacity of AuV/2 M T and AuV/6 M T for CO2. Furthermore, the stretching vibration of −OH groups was observed over the Au/2 M T and AuV/2 M T surface, whereas no band was observed over Au/6 M T and AuV6 M T. Thus, the concentration of −OH groups over Au/2 M T and AuV/2 M T was higher than that over Au/6 M T and AuV/6 M T. The concentration of −OH groups decreased in the order AuV/ 9
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Fig. 10. In situ DRIFTS study of CO transient-pulse on different catalysts: (a) Au/2 M T, (b) AuV/2 M T, (c) Au/6 M T and (d) AuV/6 M T. (F=50 mL·min−1, T =300 °C).
same time, the catalyst surface adsorbed a portion of the CO2 generated by the catalytic reaction. To better explore the reaction mechanism for CO oxidation over the Au/TiO2 catalysts, Fig. 10 presents in situ DRIFT spectra of CO transient-pulse on different catalysts. The catalysts were first exposed to 500 ppm CO/N2 for 1 min, followed by N2 for 1 min. Then, 10% O2/N2 was injected for 1 min and N2 for 1 min. For all of the Au/TiO2 catalysts, the bands for Ti4+−CO or Au0−CO appeared after 1 min after CO purging. Similar to the results for in situ DRIFTS spectra of CO oxidation, the bands for formate species over Au/2 M T and AuV/2 M T catalysts appeared and that over Au/6 M T and AuV/6 M T was very weak. It also verified that CO was adsorbed quickly but transformed into the formate species slowly over the Au/6 M T and AuV/6 M T catalysts. CO was easier and quicker to transform into formate species over the Au/ 2 M T and AuV/2 M T catalysts. Because of the transient-pulse, the amount of CO was so small that trace of carbonate species formed and adsorbed on all Au/TiO2 catalyst surface. The bands for Ti4+-CO or Au0-CO disappeared in 1 min and the band for carbonate species also decreased slightly after N2 purging for 1 min. The -OH groups on the catalyst surface were quickly replenished after O2/N2 purging for 1 min. The replenishment rate of -OH groups was also quickest for the AuV/ 2 M T catalyst and slowest for the Au/6 M T catalyst, consistent with the catalytic performance of the catalysts toward CO oxidation. That is, the replenishment and consumption rates of oxygen species on the surface of catalyst were the key factor in the catalytic oxidation of CO.
Fig. 11. Reaction mechanism diagram of simultaneous catalysis oxidation CO and Hg over Au/TiO2 catalyst.
These results suggest that the Au/TiO2 catalyst exhibited excellent adsorption and catalytic oxidation capacity for CO. The bands for -OH groups and carbonates species increased, whereas the bands for Ti4+CO or Au0-CO disappeared after 1 min of O2 purging. These results are attributed to adsorbed CO species on the catalyst surface being oxidized to CO2 by O2 and the catalyst. When the O2 purging continued for 30 min, the bands for carbonate species decreased, indicating weak competitive adsorption between O2 and carbonates species over the Au/TiO2 catalyst. The in situ DRIFTS study of CO adsorption-desorption illustrated that the formate was the intermediate species for CO catalytic oxidation and that it transformed to CO2 quickly by consuming adsorbed oxygen species (O2−, O− and −OH) on the surface of catalyst. Molecular oxygen was mainly used to supplement the oxygen species that were consumed on the catalyst surface, and the oxygen species were the key factors in determining the reaction rate. At the
3.5. Reaction mechanism for removal of CO and Hg0 Combined with after mentioned reactions, the catalytic oxidation of 10
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Appendix A. Supplementary data
CO over the Au/TiO2 catalyst followed L-H mechanisms. The mechanism presented in Fig. 11 and Eq. (3) to Eq. (6). CO first adsorb onto AuV/2 M T to form CO(ads). CO(ads) reacts with –OH, which adsorbs onto the catalyst surface and forms the formate species. The formate species then decompose to generate CO2 and -H+. Then, O2 reacts with -H+ and forms -OH and adsorbs onto the catalyst surface. Finally, oxygen is adsorbed on the surface of the catalyst, oxidizing Ti3+ and Au0 to Ti4+ and Au+, while the lattice oxygen is replenished and regenerated. CO(g) →CO(ads)
(3)
CO(ads) + 2-OH →HCOOH(ads) + -O*(ads)
(4)
HCOOH(ads) →CO2(g) + 2-H+ (ads)
(5)
-H+ (ads) + -O*(ads) →+ -OH(ads)
(6)
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The presence of ascorbic acid promoted the transformation from rutile TiO2 to brookite TiO2, affected the chemical state of Ti and O atoms, and increased the amount of adsorbed oxygen species. These factors were beneficial to enhance the catalytic performance, which is why AuV/2 M T exhibited the best catalytic performance among the developed catalysts. Furthermore, an in situ DRIFTS study of CO adsorption-desorption illustrated that the formate was the intermediate species for CO catalytic oxidation and that it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) on the catalyst surface. Oxygen was mainly used to supplement the oxygen species consumed on the surface of catalyst, and the oxygen species were the key factors determining the reaction rate.
4. Conclusions In this study, Au/TiO2 catalysts with different morphologies were prepared by the modulation of chlorine via a hydrothermal method, and then utilized as catalysts to catalytic removal of CO and Hg0 simultaneously. The Au/TiO2 catalyst with a structure of submicron spheres exhibited excellent catalytic performance for the simultaneous catalytic removal of CO and Hg0. In addition, the AuV/2 M T catalyst also exhibited excellent stability. No significant attenuation occurred in the catalytic activity after continuous testing with 6 vol.% H2O for 120 h. The presence of ascorbic acid promoted the transformation from rutile TiO2 to brookite TiO2, affected the chemical state of Ti and O atoms, and increased the amount of adsorbed oxygen species. Combined with a series of characterization analysis, all these factors conduced to the improvement of catalytic performance. Furthermore, the presence of chlorine promoted the catalytic oxidation of Hg0 while the presence of CO has no great effect on desorption of Hg0. At the same time, the in situ DRIFT spectra results revealed that the catalytic oxidation of CO on Au/ TiO2 catalyst follows the L-H mechanism. The formate was the intermediate species for CO catalytic oxidation and it transformed to CO2 quickly by consuming the oxygen species (O2−, O− and −OH) on the catalyst surface. Oxygen was mainly used to supplement the oxygen species consumed on the catalyst surface, and the oxygen species were the key factor determining the reaction rate.
Acknowledgements We would like to acknowledge the financial support from the National Natural Science Foundation of China (Nos.21501097, 21272118and21577065), the Natural Science Foundation of Jiangsu Province (No.BK20170954), the Key Research and Development Program of Jiangsu Province (No. BE2018074), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 18KJB430019), the Startup Foundation for Introducing Talent of NUIST (No.2017r073) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. 11
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