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CrO3 supported on sargassum-based activated carbon as low temperature catalysts for the selective catalytic reduction of NO with NH3
Fuel 191 (2017) 511–517
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Full Length Article
CrO3 supported on sargassum-based activated carbon as low temperature catalysts for the selective catalytic reduction of NO with NH3 Sujing Li, Xiaoxiang Wang, Shan Tan, Yun Shi, Wei Li ⇑ Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
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Article history: Received 18 April 2016 Received in revised form 22 November 2016 Accepted 24 November 2016 Available online 5 December 2016 Keywords: Sargassum-based activated carbon Chromium Low temperature NH3-SCR Water and sulfur tolerance
a b s t r a c t Sargassum based activated carbon (SAC) doped with various transition metals was developed via impregnation as a new catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) in the temperature range of 50–250 °C with gas hourly space velocity (GHSV) of 80,000 h1. The samples were characterized by means of Brunauer–Emmett–Teller method (BET), scanning electron microscope (SEM), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The impacts of water (H2O) and sulfur dioxide (SO2) on the SCR activity of the CrO3/SAC catalyst were also discussed. The experimental results showed that the introduction of Cr increased the acid sites formed on the surface of catalysts and enhanced the SCR reaction rate by the valence changing between Cr6+ and its lower oxidized states (Cr5+, Cr3+ and Cr2+). The catalyst with a Cr/SAC mass ratio of 2%:1 exhibited the best NOx-removing performance, with NOx conversion greater than 90% at the temperature of 125–150 °C. Moreover, it had excellent water and sulfur tolerance, making the Cr-doped SAC catalyst as a good candidate for reducing the NOx emission from fired power plants. Ó 2016 Published by Elsevier Ltd.
1. Introduction Selective catalytic reduction with ammonia (NH3-SCR) is an efficient and economic technique for the abatement of NOx from exhaust gases of coal-fired power plants [1]. Currently, V2O5-WO3 (MoO3)/TiO2 is the most widely applied catalyst with a narrow temperature window of 300–400 °C [2–4]. However, the SCR unit is normally suggested to be located behind particulate collector (ESP) and desulfurization (FGD) equipment in power plants, where the flue gas temperature could drop to 50–60 °C [5]. To achieve the desired working temperature (normally 350–500 °C) for the V-based catalysts, reheating the flue gas is required, resulting in the significant energy consumption. In addition, low N2 selectivity as a result of formation of N2O, high conversion of SO2 to SO3 [6,7], the toxicity of V2O5 to the environment, and the adverse effects of vanadium on human health are the urgent problems that need to be addressed. Consequently, to save energy consumption and be eco-friendly, developing new catalysts
⇑ Corresponding author. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.fuel.2016.11.095 0016-2361/Ó 2016 Published by Elsevier Ltd.
that would keep high activities under low temperature has become imperative and has attracted a lot of attentions [8,9]. Carbon materials are regarded as promising candidates for low temperature SCR catalysts. In addition, transition metal oxides (Ce, Mn and Cr) supported on carbon materials have shown an excellent catalytic performance and SO2 resistance [10–13]. In our previous studies [14], sargassum-based activated carbon (SAC) was proved to be a good carbon-based material with high nitrogen content and rich surface functional groups, demonstrating to be a suitable support for SCR catalysts. Also, nitrogen modified sargassum-based activated carbon exhibited an excellent SCR activity with a maximum NOx conversion of 87% at 150 °C. The utilization of sargassum-based activated carbon as a SCR catalyst support is an effective approach of combining waste disposal, biomass energy recovery and air pollution control in one process. However, it is necessary to increase the maximum NOx conversion at lower temperature with higher GHSV by different modification methods, for further reducing the energy consumption on reheating the flue gas. In this work, various metals had been doped on the SAC to further improve its catalytic performance at low temperature. Various chromium oxide loadings over catalysts as a promoter can lead to an increase in SCR activity [15]. The NOx removal rate
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for a series of alumina-supported CrO3 and Fe2O3 catalysts prepared by Gorge et al. increased by 10% with the addition of small amounts of Cr [16]. Chen et al. reported that addition of Cr into the MnOx lattice was the crucial factor to enhance the SCR activity over the Cr-Mn catalysts [17]. Donovan et al. also found that titanium-supported Mn, Cr and Cu performed well at low temperature due to the increase of acid sites on the surface [18]. Pasel et al. found that the loading of transition-metal oxide (Fe2O3, Cr2O3 and CuO) on the commercial active carbons led to an increase in SCR activity [15]. Chromium was used as a promoter to improve the performance of Pt/ZSM-35 by Yu et al. and an increase of NO conversion from 80.5% to 94.7% was obtained at 120 °C [19]. They found that Cr addition not only enhanced the adsorption of NOx but also promoted the formation of surface NH4+ species. Although chromium was often applied as a promoter for metal oxides, seldom literature investigated its enhancing effect on the carbon supports during SCR and the optimum content of Cr doping was rarely studied. In the present study, various metals (Mn, Cr, Ce) were selected as the active components to improve the catalytic performance of sargassum-based activated carbon catalyst and the effect of different Cr doping amounts on the SCR activity of SAC catalyst was also studied. Moreover, a series of analytical techniques, including SEM, BET, XPS, NH3-TPD and H2-TPR, were employed to get insight into the enhancing role of Cr. In addition, the impacts of SO2 and H2O on NH3-SCR activity over metal-modified catalysts were investigated. 2. Experimental 2.1. Catalyst preparation The activated carbon was prepared using a mixture of H3PO4 and air-dried sargassum with a mass ratio of 2:1 in a tubular quartz reactor, which was described in detail in our previous publication [14]. The precursor was activated by being heated up to 500 °C with a heating rate of 10 °C/min for 1 h under a stream of nitrogen (400 mL/min). Then, the sample was cooled down to room temperature. The resulted activated carbon was washed with 1 mol/L HCl solution, followed by deionized water. It was dried and sieved to obtain a particle size range of 40–60 mesh, named as SAC. Metal modified catalyst was prepared by impregnating SAC with various metal salt solution (C4H6MnO44H2O, CeSO44H2O, CrO3). The slurry was mixed via sonication for 2 h and then held at ambient temperature for 12 h. Afterwards, the impregnated samples were heated under a stream of nitrogen up to 500 °C for 1 h before cooled down to room temperature. The metal modified catalysts were denoted as M(x)/SAC, where M stood for the metal, including Mn, Ce, Cr while x represented the different mass ratio of Metal/SAC. For the SAC and Metal/SAC, they are both based on sargassum. The proximate and ultimate analyses were carried out on the sargassum, as shown in Table 1.
500 ppm NO, 500 ppm NH3, 5v/v% O2, 100 ppm or 300 ppm SO2 (when used), 5% or 10% H2O (when used), N2 as carrier gas with a GHSV of 80,000 h1. Water vapor was generated by passing N2 through a heated bottle containing deionized water. Prior to catalytic tests, the catalysts were heated up to 500 °C for 30 min under N2 to purify the sample surface. The reactor was then cooled down to 30 °C and N2 was switched to reaction gas mixture. For each experiment, fresh samples were used and their catalytic activities were estimated within the temperature range of 50–250 °C under steady-state conditions. The outlet concentration of reactants and products were continuously monitored by a Nicolet iS50 FTIR spectrometer coupled with a heated FTIR gas cell. The NOx conversion and N2 selectivity were obtained by using the following Eqs. (1)–(3): NOx conversion ¼
C NOx ðinÞ C NOx ðoutÞ 100% C NOx ðinÞ
ð1Þ
C NOx ¼ C NO þ C NO2 þ 2C N2 O ð2Þ 2 C N2 O 100% N2 selectivity ¼ 1 C NOx ðinÞ þ C NH3 ðinÞ C NOðoutÞ C NO2 ðoutÞ C NH3 ðoutÞ ð3Þ
2.3. Catalyst characterization The morphologies of Cr/SAC catalysts with different chromium doping contents were studied by using scanning electron microscopy (philips model FEI model Quanta 400 SEM). Nitrogen adsorption isotherms were measured at liquid nitrogen temperature (77 K) using a Quantachrome Autosorb-1C apparatus. The specific surface area was calculated by using the BET method. The total pore volume and micropore surface area were calculated using the t-plot method. X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo ESCALAB 250 electron spectrometer equipped with an Al-Ka X-ray source (hv = 1486.6 eV). The XPS spectra were calibrated by taking the graphitic peak as 284.6 eV. Peak analysis was performed using Casa-XPS Version 2.1.9 software with all spectra Shirley-background subtracted prior to fitting. NH3-TPD was performed using quadrupole mass spectrometer to record the signals of NH3 and NOx. Prior to the TPD experiments, the samples (50 mg) were pretreated at 400 °C in a flow of He (30 mL/min) for 30 min and cooled down to 80 °C. Then the samples were exposed to a flow of NH3 for 2 h, followed by Ar purging for 1 h. Finally, the temperature was raised to 1000 °C in a He flow at a rate of 10 °C/min. Prior to the H2-TPR experiments, 50 mg samples were pretreated at 400 °C in a flow of He for 30 min and cooled down to 50 °C. The temperature was then raised to 1000 °C at a rate of 10 °C/min in a flow of H2/Ar (30 mL/min). 3. Results and discussion 3.1. Effect of different doping metals on the NH3-SCR activity of the catalysts
2.2. Catalytic activity test NH3-SCR activity of the catalyst was tested in a fixed-bed quartz tube reactor, which was described in detail in our previous study [14]. The reaction conditions were set as following: 0.10 g catalyst,
Different species (Mn, Ce, Cr) and loading amount of transitional metal were employed to seek out the catalyst with the highest NOx conversion in the temperature range of 50–250 °C. With the introduction of metal species, a promotional effect is observed
Table 1 Proximate analysis and ultimate analysis of raw sargassum (wt%. dry basis) [14]. Sample
Sargassum
Proximate analysis (wt%)
Ultimate analysis (wt%)
Water
Ash
Volatiles
Fixed carbon
C
H
N
O
6.7
6.6
54.7
32
40.3
5.4
2.5
51.8
513
80
60 40 SAC Mn(1%)/SAC Mn(3%)/SAC Mn(5%)/SAC Mn(7%)/SAC
20 0 50
75
100
125
150
175
200
225
NOx conversion (%)
80
N2 selectivity(%)
100
NOx conversion(%)
100
N2 selectivity (%)
S. Li et al. / Fuel 191 (2017) 511–517
60
40 SAC Cr(1%)/SAC Cr(2%)/SAC Cr(3%)/SAC Cr(4%)/SAC
20
250
0
o
Temperature ( C)
50
75
100
125
150
175
200
225
250
Temperature (o C) Fig. 1a. Effect of different Mn loadings on NOx conversion (solid) and N2 selectivity (open) of Mn(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
100
Fig. 1c. Effect of different Cr loadings on NOx conversion (solid) and N2 selectivity (open) of Cr(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
100
40 SAC Ce(0.5%)/SAC Ce(1%)/SAC Ce(2%)/SAC Ce(3%)/SAC
20 0 50
75
100
125
150
175
200
225
80
percentage (%)
60
N2 selectivity(%)
NOx conversion(%)
80
0% H2O
60
5% H2O
40
20
N2 Selectivity
250
o
Temperature ( C) Fig. 1b. Effect of different Ce loadings on NOx conversion (solid) and N2 selectivity (open) of Ce(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
among all the M(x)/SAC catalysts within the temperature range of 50–250 °C. Fig. 1 illustrates the detailed information about the SCR catalytic activity of different metal modifying SAC catalysts at the temperature range of 50–250 °C. As shown in Fig. 1a, Mn(5%)/ SAC exhibited the highest NOx conversion at the temperature range of 50–250 °C and achieved the maximum NOx conversion of 73.32% at 125 °C when the loading amount of Mn ranged from 1% to 7%. From Fig. 1b, Ce(0.5%)/SAC had the highest catalytic activity of 70.24% at 125 °C when the loading amount of Ce ranged from 0.5% to 3%. Also, it was observed that the NOx conversion efficiency showed a decreasing trend with the increase of Ce amount from 0.5% to 3%. Hence, higher Ce loading on the SAC was not beneficial for improving its NOx conversion efficiency. A series of Cr(x)/SAC were prepared with different Cr/SAC mass ratios. Fig. 1c depicts the catalytic performance of Cr(x)/SAC catalysts with different mass ratios. SCR activity of the Cr(x)/SAC samples increased with the mass ratio increasing from 1% to 2%. Cr (2%)/SAC had a highest NOx conversion of 92.3% at 125 °C when the loading amount of CrO3 ranged from 1% to 4%. Besides, NOx removal efficiency of Cr(2%)/SAC was much higher than that of the SAC. NOx conversion dramatically decreased when the impregnation ratio further increased to 4%. N2 selectivities were maintained at above 90% for all the Cr(x)/SAC samples at the whole
0% H2O
10% H2O
NOx Conversion
0
0
50
100
150
200
250
Time (min)
300
350
400
450
Fig. 2a. Impact of H2O on the SCR activity of the Cr(2%)/SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 5% or 10% H2O (if added) + 5% O2 + N2 balance, GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
working temperature window. Hence, the results indicated that the Cr addition enhanced the SCR activity of SAC sample in the temperature range of 50–250 °C and 2% was the optimal mass ratio for Cr doping on the SAC.
3.2. Effect of H2O and SO2 on NH3-SCR activity of the Cr(2%)/SAC catalyst Figs. 2a and 2b show the impact of water on the SCR activity of Cr(2%)/SAC and SAC catalyst at 125 °C. Since water content in the flue gases is around 2–18% [1], 5% and 10% H2O were added in the feeding gas mixture, respectively. The NOx conversion of the catalyst shows an abrupt decrease from 91.9% to 83.1% while it decreases from 51% to 27% for SAC with the addition of 5% H2O. This result proves that H2O has an inhibitory effect on the SCR activity of the Cr(2%)/SAC and SAC catalyst, and the inhibition of water might be due to the competition adsorption between H2O and reactants (NH3 and/or NO) [20,21]. Previous reports [7,22– 24] also found that the addition of H2O inhibited the SCR reaction on other catalysts at low temperatures. The NO conversion of Cr (2%)/SAC only slightly decreases to 80.2% with the H2O content increasing from 5% to 10%. However, NO conversion is restored
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100
100 0% H2O
5% H2O
0% H2O
10% H2O
80
Percentage(%)
Percentage(%)
80
60
40
20
0 ppm SO2
60
100 ppm SO2 300 ppm SO2 5% H O+100 ppm SO 2 2
40
20
N2 Selectivity
N2 selectivity
NOx Conversion
0
0
50
100
150
200
250
300
350
400
450
NOx conversion
0
0
Time(min) Fig. 2b. Impact of H2O on the SCR activity of the SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 5% or 10% H2O (if added) + 5% O2 + N2 balance, GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
completely to the previous level for both of Cr(2%)/SAC and SAC catalysts when switching off H2O. This indicates that the inhibitory effect of H2O is reversible on the Cr(2%)/SAC catalyst and the Cr doping increases the water tolerance of SAC greatly. In addition, N2 selectivity is maintained above 95% during the whole experiments. This indicates that the Cr(2%)/SAC catalyst has a good water tolerance and high N2 selectivity. The impact of SO2 on the SCR activity of the Cr(2%)/SAC and SAC catalysts at 125 °C was also investigated as shown in Figs. 2c and 2d. NO conversion over Cr(2%)/SAC and SAC catalyst exhibit almost no decrease (only 2%) after introducing 100 ppm SO2. There is only about 5% decrease when adding 300 ppm SO2 into the reaction mixture, indicating that a good SO2 tolerance of the Cr(2%)/SAC catalyst. However, the NO conversion displays a decreasing tendency with the coexistence of 5% H2O and 100 ppm SO2 in a few minutes and it decreases to about 71.5% and 34% for Cr(2%)/SAC and SAC respectively after 150 min. It was observed that there was a thin layer (about 3 mm in height) of white crystal formed on catalysts with the coexistence of H2O and SO2, while nothing was observed on the surface of the catalysts with the presence of H2O or SO2. As a result, the coexistence of H2O and SO2 would result in the formation and deposition of ammonium-sulfate salts, such as NH4HSO4
100
Percentage (%)
80
60 0 ppm SO2
100 ppm SO2 300 ppm SO2 5% H2O+100 ppm SO2
40
200
300
Time(min)
400
500
Fig. 2d. Impact of SO2 on the SCR activity of the SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 100 ppm or 300 ppm SO2 (if added) + 5% H2O (if added) + 5% O2 + N2 balance. GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
and (NH4)2S2O7 on the catalyst surface, blocking the active sites on the catalysts and deactivating the catalysts [3,4,18,25]. 3.3. SEM analysis Fig. 3 displayed the SEM images of the Cr(x)/SAC samples with the mass ratio of 0, 1%, 2%, 4%. Compared with SAC, the Cr-doped SACs have rougher morphologies. However, it is difficult to distinguish the differences among the Cr-doped SACs with different Cr loading amount. The flocculent species might be the chromium oxides. 3.4. BET analysis The BET data of the Cr(x)/SAC catalysts were provided in Table 2. From BET analysis, we can see that the surface area of catalysts decreased as the amount of doping chromium increased. SAC has the largest BET surface area, total pore volume and average pore size of all samples. Metal doping on the SAC resulted in a large decrease in surface area, which was in line with previous publications [15,26,27]. The blocking effects of chromium ions on the active carbon might lead to this decrease. The total pore volume and the average pore size are about 0.14 cm3/g and 4.8 nm for all Cr-doped catalysts. However, in spite of their lower surface area, all the Cr(x)/SAC samples showed higher activity towards NO reduction. This agrees with previous findings that the catalytic activity is more dependent on surface chemistry than surface area [14]. Moreover, the suitable particle size and uniformly dispersed particle is an important affecting factor for NH3-SCR of NO [28], although large surface area, pore volumes and average pore size are beneficial in minimizing mass transfer limitations to promote SCR reaction [29]. 3.5. XPS analysis
20
N2 selectivity NOx conversion
0
100
0
100
200
300
Time (min)
400
500
Fig. 2c. Impact of SO2 on the SCR activity of the Cr(2%)/SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 100 ppm or 300 ppm SO2 (if added) + 5% H2O (if added) + 5% O2 + N2 balance. GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
XPS was performed to investigate the surface binding energies and valence state of various catalysts. The photoelectron peak pertaining to O1s is shown in Fig. 4(a). For the Cr(2%)/SAC catalyst, there is almost no surface lattice oxygen (Ob) around 528.9– 529.2 eV, and the oxygen species are mainly assigned to the surface adsorbed oxygen (Oa) at 531.5 eV, such as O2 2 or O pertaining to defect-oxygen or hydroxyl-like groups [16]. It has been generally accepted that Oa species play an important role in the oxidation
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Fig. 3. SEM images of Cr(x)/SAC samples (a) SAC (b) Cr(1%)/SAC (c) Cr(2%)/SAC (d) Cr(4%)/SAC.
Table 2 BET data for the Cr(x)/SAC catalysts.
(a) Total pore volume, v/(cm3/g)
Average pore size, d/(nm)
SAC Cr(1%)/SAC Cr(2%)/SAC Cr(3%)/SAC Cr(4%)/SAC
604.0 136.7 111.7 103.6 90.4
0.9 0.16 0.13 0.15 0.11
6.3 4.8 4.8 5.6 4.7
of NO due to its high mobility. Moreover, the surface hydroxyl groups could act as Brønsted sites, which are in favor of facilitating the NH3 absorption. This effect may be an important reason for a better catalytic activity of the Cr(2%)/SAC catalysts compared with others in previous studies. As we can see from Fig. 4(b), Cr(2%)/SAC catalyst consists of two characteristic binding energies of Cr ions locating at 587.6 eV (Cr2P1/2) and 577.8 eV(Cr2P3/2). Chromium ions may exist in different oxidation states in solid materials, and exhibit different catalytic properties. Hence, the Cr2p3/2 of Cr(2%)/SAC catalyst is separated into four peaks by the peak-fitting deconvolution technique corresponding to Cr2+ (575.6–575.7 eV), Cr3+ (576.6– 576.7 eV), Cr5+ (578.3–578.9 eV) and Cr6+ (579 eV) [19,30–32]. It was thought that Cr addition could facilitate the SCR performance by the valence change between Cr6+ and lower oxidized states (Cr5+, Cr3+ and Cr2+). The area of Cr6+ peak was largest among all the Cr valences, indicating that more Cr6+ was easy to turn into lower valence. The changing of valence state would promote the generation of oxygen vacancy on the surface of catalytic, which was in favor of the SCR reaction [30,31].
531.5
Intensity (a.u.)
Surface area, A/(m2/g)
545
540
535
530
525
Binding Energy (eV)
(b)
Intensity(a.u.)
Sample
3.6. NH3 temperature-programed desorption NH3-TPD was performed to investigate the amount and strength of surface acid sites [33]. The area and position of desorption peaks correlate with the acid amount and acid strength,
595
590
585
580
575
570
Binding Energy(eV) Fig. 4. The XPS spectrum of O1s (a) and Cr2p (b) for the Cr(2%)/SAC.
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Intensity (a.u)
Intensity (a.u)
SAC Cr(2%)/SAC
SAC
Cr(2%)/SAC
Cr(2%)/SAC
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature ( C) 0
SAC
100
200
300
400
500
600
Temperature ( C) 0
Fig. 5. NH3-TPD profiles of SAC and Cr(2%)/SAC.
Fig. 6. H2-TPR profiles of SAC and Cr(2%)/SAC.
respectively. And the peak at low temperature usually represents the weak acid sites while the peak at high temperature represents the strong acid sites. Both of weak acid sites and strong acid sites have a crucial effect on the SCR activity. As shown in Fig. 5, two main NH3 desorption peaks are detected in the region below 250 °C and above 500 °C, respectively [34,35], assigning to the desorption of ammonia from weak and strong acid sites, respectively. The NH3-TPD spectrum of the SAC was investigated and one peak appeared at around 200 °C as well as another at 610 °C, corresponding to NH3 desorbed by weak and strong acid sites, respectively. For the Cr(2%)/SAC catalyst, the peak represented the weak acid sites appeared at 256 °C and the peak represented the strong acid sites appeared at 715 °C. Compared with SAC catalyst, the desorption peaks of Cr(2%)/SAC shifts to a higher temperature range, suggesting that the strength of the acid sites on Cr(2%)/SAC is stronger, indicating the doping of Cr could increase the strong acid sites of the SAC. Combining with the valence change between Cr6+ and its lower oxidized states (Cr5+, Cr3+and Cr2+) as shown in Section 3.5, these might result in the improvement of the SCR performance over the SAC.
4. Conclusions The catalytic activity of the CrO3 modified sargassum based activated carbon in selective catalytic reduction of NO by NH3 was studied. The optimal Cr/SAC mass ratio for modification of the Cr(x)/SAC catalyst is 2%:1. With the introduction of Cr species, there is an attractive increase of NOx conversion from 56.91% to 92.3% at 125 °C and above 95% of N2 selectivity is achieved during the whole experiment for Cr(2%)/SAC catalyst. Compared with SAC catalyst, the desorption peaks of Cr(2%)/SAC shift to a higher temperature range, suggesting that the incorporated Cr ions can enhance the acidity of the materials. And the chromium doping on the SAC facilitates the SCR performance by the valence change between Cr6+ and lower oxidized states (Cr5+, Cr3+and Cr2+). Both of them result in an improvement of the SCR performance over the catalysts. In addition, Cr(2%)/SAC has an excellent water and sulfur tolerance, making the Cr-doped SAC catalyst as a good candidate for reducing the NOx emission from fired power plants. Acknowledgements
3.7. H2 temperature-programed reaction H2-TPR experiments were carried out over the SAC and Cr(2%)/ SAC catalysts to investigate the reducibility of metal species in the catalyst as shown in Fig. 6. The reduction profiles of bulk CrO3 reported in the literature [30] consist of reduction peaks at 280 °C, 462 °C, and 585 °C. The hydrogen reduction of Cr6+ to Cr5+, Cr5+ to Cr3+ and Cr3+ to either Cr2+ or to the metallic state occurs at about 180, 462, 585 °C, respectively [36]. For Cr(2%)/ SAC catalyst, an intense reduction peak centered at about 280 °C is emerged. This peak can be ascribed to the reduction of Cr6+ to Cr5+or other lower oxidized state species. Moreover, the hydrogen consumption of samples is obtained by integrating the curves at the temperature range of 250–650 °C. It’s obvious that Cr(2%)/ SAC has higher hydrogen consumption than those of SAC samples, attributing to the reduction of more surface Cr6+ species and acidic functional groups, as illustrated by XPS and TPD. In addition, The peak at the higher temperature peak above 400 °C can be attributed to carbon gasification [35,37]. The peak of Cr(2%)/SAC is observed at 579.5 °C while that is observed at 550.78 °C for SAC. It appears that the presence of Cr species on the catalyst promotes carbon gasification, as indicated by the stronger TPR intensity of the peak at 400–600 °C, comparing with the SAC support.
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Full Length Article
CrO3 supported on sargassum-based activated carbon as low temperature catalysts for the selective catalytic reduction of NO with NH3 Sujing Li, Xiaoxiang Wang, Shan Tan, Yun Shi, Wei Li ⇑ Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 18 April 2016 Received in revised form 22 November 2016 Accepted 24 November 2016 Available online 5 December 2016 Keywords: Sargassum-based activated carbon Chromium Low temperature NH3-SCR Water and sulfur tolerance
a b s t r a c t Sargassum based activated carbon (SAC) doped with various transition metals was developed via impregnation as a new catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) in the temperature range of 50–250 °C with gas hourly space velocity (GHSV) of 80,000 h1. The samples were characterized by means of Brunauer–Emmett–Teller method (BET), scanning electron microscope (SEM), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The impacts of water (H2O) and sulfur dioxide (SO2) on the SCR activity of the CrO3/SAC catalyst were also discussed. The experimental results showed that the introduction of Cr increased the acid sites formed on the surface of catalysts and enhanced the SCR reaction rate by the valence changing between Cr6+ and its lower oxidized states (Cr5+, Cr3+ and Cr2+). The catalyst with a Cr/SAC mass ratio of 2%:1 exhibited the best NOx-removing performance, with NOx conversion greater than 90% at the temperature of 125–150 °C. Moreover, it had excellent water and sulfur tolerance, making the Cr-doped SAC catalyst as a good candidate for reducing the NOx emission from fired power plants. Ó 2016 Published by Elsevier Ltd.
1. Introduction Selective catalytic reduction with ammonia (NH3-SCR) is an efficient and economic technique for the abatement of NOx from exhaust gases of coal-fired power plants [1]. Currently, V2O5-WO3 (MoO3)/TiO2 is the most widely applied catalyst with a narrow temperature window of 300–400 °C [2–4]. However, the SCR unit is normally suggested to be located behind particulate collector (ESP) and desulfurization (FGD) equipment in power plants, where the flue gas temperature could drop to 50–60 °C [5]. To achieve the desired working temperature (normally 350–500 °C) for the V-based catalysts, reheating the flue gas is required, resulting in the significant energy consumption. In addition, low N2 selectivity as a result of formation of N2O, high conversion of SO2 to SO3 [6,7], the toxicity of V2O5 to the environment, and the adverse effects of vanadium on human health are the urgent problems that need to be addressed. Consequently, to save energy consumption and be eco-friendly, developing new catalysts
⇑ Corresponding author. E-mail address: [email protected] (W. Li). http://dx.doi.org/10.1016/j.fuel.2016.11.095 0016-2361/Ó 2016 Published by Elsevier Ltd.
that would keep high activities under low temperature has become imperative and has attracted a lot of attentions [8,9]. Carbon materials are regarded as promising candidates for low temperature SCR catalysts. In addition, transition metal oxides (Ce, Mn and Cr) supported on carbon materials have shown an excellent catalytic performance and SO2 resistance [10–13]. In our previous studies [14], sargassum-based activated carbon (SAC) was proved to be a good carbon-based material with high nitrogen content and rich surface functional groups, demonstrating to be a suitable support for SCR catalysts. Also, nitrogen modified sargassum-based activated carbon exhibited an excellent SCR activity with a maximum NOx conversion of 87% at 150 °C. The utilization of sargassum-based activated carbon as a SCR catalyst support is an effective approach of combining waste disposal, biomass energy recovery and air pollution control in one process. However, it is necessary to increase the maximum NOx conversion at lower temperature with higher GHSV by different modification methods, for further reducing the energy consumption on reheating the flue gas. In this work, various metals had been doped on the SAC to further improve its catalytic performance at low temperature. Various chromium oxide loadings over catalysts as a promoter can lead to an increase in SCR activity [15]. The NOx removal rate
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for a series of alumina-supported CrO3 and Fe2O3 catalysts prepared by Gorge et al. increased by 10% with the addition of small amounts of Cr [16]. Chen et al. reported that addition of Cr into the MnOx lattice was the crucial factor to enhance the SCR activity over the Cr-Mn catalysts [17]. Donovan et al. also found that titanium-supported Mn, Cr and Cu performed well at low temperature due to the increase of acid sites on the surface [18]. Pasel et al. found that the loading of transition-metal oxide (Fe2O3, Cr2O3 and CuO) on the commercial active carbons led to an increase in SCR activity [15]. Chromium was used as a promoter to improve the performance of Pt/ZSM-35 by Yu et al. and an increase of NO conversion from 80.5% to 94.7% was obtained at 120 °C [19]. They found that Cr addition not only enhanced the adsorption of NOx but also promoted the formation of surface NH4+ species. Although chromium was often applied as a promoter for metal oxides, seldom literature investigated its enhancing effect on the carbon supports during SCR and the optimum content of Cr doping was rarely studied. In the present study, various metals (Mn, Cr, Ce) were selected as the active components to improve the catalytic performance of sargassum-based activated carbon catalyst and the effect of different Cr doping amounts on the SCR activity of SAC catalyst was also studied. Moreover, a series of analytical techniques, including SEM, BET, XPS, NH3-TPD and H2-TPR, were employed to get insight into the enhancing role of Cr. In addition, the impacts of SO2 and H2O on NH3-SCR activity over metal-modified catalysts were investigated. 2. Experimental 2.1. Catalyst preparation The activated carbon was prepared using a mixture of H3PO4 and air-dried sargassum with a mass ratio of 2:1 in a tubular quartz reactor, which was described in detail in our previous publication [14]. The precursor was activated by being heated up to 500 °C with a heating rate of 10 °C/min for 1 h under a stream of nitrogen (400 mL/min). Then, the sample was cooled down to room temperature. The resulted activated carbon was washed with 1 mol/L HCl solution, followed by deionized water. It was dried and sieved to obtain a particle size range of 40–60 mesh, named as SAC. Metal modified catalyst was prepared by impregnating SAC with various metal salt solution (C4H6MnO44H2O, CeSO44H2O, CrO3). The slurry was mixed via sonication for 2 h and then held at ambient temperature for 12 h. Afterwards, the impregnated samples were heated under a stream of nitrogen up to 500 °C for 1 h before cooled down to room temperature. The metal modified catalysts were denoted as M(x)/SAC, where M stood for the metal, including Mn, Ce, Cr while x represented the different mass ratio of Metal/SAC. For the SAC and Metal/SAC, they are both based on sargassum. The proximate and ultimate analyses were carried out on the sargassum, as shown in Table 1.
500 ppm NO, 500 ppm NH3, 5v/v% O2, 100 ppm or 300 ppm SO2 (when used), 5% or 10% H2O (when used), N2 as carrier gas with a GHSV of 80,000 h1. Water vapor was generated by passing N2 through a heated bottle containing deionized water. Prior to catalytic tests, the catalysts were heated up to 500 °C for 30 min under N2 to purify the sample surface. The reactor was then cooled down to 30 °C and N2 was switched to reaction gas mixture. For each experiment, fresh samples were used and their catalytic activities were estimated within the temperature range of 50–250 °C under steady-state conditions. The outlet concentration of reactants and products were continuously monitored by a Nicolet iS50 FTIR spectrometer coupled with a heated FTIR gas cell. The NOx conversion and N2 selectivity were obtained by using the following Eqs. (1)–(3): NOx conversion ¼
C NOx ðinÞ C NOx ðoutÞ 100% C NOx ðinÞ
ð1Þ
C NOx ¼ C NO þ C NO2 þ 2C N2 O ð2Þ 2 C N2 O 100% N2 selectivity ¼ 1 C NOx ðinÞ þ C NH3 ðinÞ C NOðoutÞ C NO2 ðoutÞ C NH3 ðoutÞ ð3Þ
2.3. Catalyst characterization The morphologies of Cr/SAC catalysts with different chromium doping contents were studied by using scanning electron microscopy (philips model FEI model Quanta 400 SEM). Nitrogen adsorption isotherms were measured at liquid nitrogen temperature (77 K) using a Quantachrome Autosorb-1C apparatus. The specific surface area was calculated by using the BET method. The total pore volume and micropore surface area were calculated using the t-plot method. X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo ESCALAB 250 electron spectrometer equipped with an Al-Ka X-ray source (hv = 1486.6 eV). The XPS spectra were calibrated by taking the graphitic peak as 284.6 eV. Peak analysis was performed using Casa-XPS Version 2.1.9 software with all spectra Shirley-background subtracted prior to fitting. NH3-TPD was performed using quadrupole mass spectrometer to record the signals of NH3 and NOx. Prior to the TPD experiments, the samples (50 mg) were pretreated at 400 °C in a flow of He (30 mL/min) for 30 min and cooled down to 80 °C. Then the samples were exposed to a flow of NH3 for 2 h, followed by Ar purging for 1 h. Finally, the temperature was raised to 1000 °C in a He flow at a rate of 10 °C/min. Prior to the H2-TPR experiments, 50 mg samples were pretreated at 400 °C in a flow of He for 30 min and cooled down to 50 °C. The temperature was then raised to 1000 °C at a rate of 10 °C/min in a flow of H2/Ar (30 mL/min). 3. Results and discussion 3.1. Effect of different doping metals on the NH3-SCR activity of the catalysts
2.2. Catalytic activity test NH3-SCR activity of the catalyst was tested in a fixed-bed quartz tube reactor, which was described in detail in our previous study [14]. The reaction conditions were set as following: 0.10 g catalyst,
Different species (Mn, Ce, Cr) and loading amount of transitional metal were employed to seek out the catalyst with the highest NOx conversion in the temperature range of 50–250 °C. With the introduction of metal species, a promotional effect is observed
Table 1 Proximate analysis and ultimate analysis of raw sargassum (wt%. dry basis) [14]. Sample
Sargassum
Proximate analysis (wt%)
Ultimate analysis (wt%)
Water
Ash
Volatiles
Fixed carbon
C
H
N
O
6.7
6.6
54.7
32
40.3
5.4
2.5
51.8
513
80
60 40 SAC Mn(1%)/SAC Mn(3%)/SAC Mn(5%)/SAC Mn(7%)/SAC
20 0 50
75
100
125
150
175
200
225
NOx conversion (%)
80
N2 selectivity(%)
100
NOx conversion(%)
100
N2 selectivity (%)
S. Li et al. / Fuel 191 (2017) 511–517
60
40 SAC Cr(1%)/SAC Cr(2%)/SAC Cr(3%)/SAC Cr(4%)/SAC
20
250
0
o
Temperature ( C)
50
75
100
125
150
175
200
225
250
Temperature (o C) Fig. 1a. Effect of different Mn loadings on NOx conversion (solid) and N2 selectivity (open) of Mn(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
100
Fig. 1c. Effect of different Cr loadings on NOx conversion (solid) and N2 selectivity (open) of Cr(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
100
40 SAC Ce(0.5%)/SAC Ce(1%)/SAC Ce(2%)/SAC Ce(3%)/SAC
20 0 50
75
100
125
150
175
200
225
80
percentage (%)
60
N2 selectivity(%)
NOx conversion(%)
80
0% H2O
60
5% H2O
40
20
N2 Selectivity
250
o
Temperature ( C) Fig. 1b. Effect of different Ce loadings on NOx conversion (solid) and N2 selectivity (open) of Ce(x)/SAC samples. Conditions: 500 ppm NO + 500 ppm NH3 + 5% O2 + N2 balance, GHSV = 80,000 h1.
among all the M(x)/SAC catalysts within the temperature range of 50–250 °C. Fig. 1 illustrates the detailed information about the SCR catalytic activity of different metal modifying SAC catalysts at the temperature range of 50–250 °C. As shown in Fig. 1a, Mn(5%)/ SAC exhibited the highest NOx conversion at the temperature range of 50–250 °C and achieved the maximum NOx conversion of 73.32% at 125 °C when the loading amount of Mn ranged from 1% to 7%. From Fig. 1b, Ce(0.5%)/SAC had the highest catalytic activity of 70.24% at 125 °C when the loading amount of Ce ranged from 0.5% to 3%. Also, it was observed that the NOx conversion efficiency showed a decreasing trend with the increase of Ce amount from 0.5% to 3%. Hence, higher Ce loading on the SAC was not beneficial for improving its NOx conversion efficiency. A series of Cr(x)/SAC were prepared with different Cr/SAC mass ratios. Fig. 1c depicts the catalytic performance of Cr(x)/SAC catalysts with different mass ratios. SCR activity of the Cr(x)/SAC samples increased with the mass ratio increasing from 1% to 2%. Cr (2%)/SAC had a highest NOx conversion of 92.3% at 125 °C when the loading amount of CrO3 ranged from 1% to 4%. Besides, NOx removal efficiency of Cr(2%)/SAC was much higher than that of the SAC. NOx conversion dramatically decreased when the impregnation ratio further increased to 4%. N2 selectivities were maintained at above 90% for all the Cr(x)/SAC samples at the whole
0% H2O
10% H2O
NOx Conversion
0
0
50
100
150
200
250
Time (min)
300
350
400
450
Fig. 2a. Impact of H2O on the SCR activity of the Cr(2%)/SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 5% or 10% H2O (if added) + 5% O2 + N2 balance, GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
working temperature window. Hence, the results indicated that the Cr addition enhanced the SCR activity of SAC sample in the temperature range of 50–250 °C and 2% was the optimal mass ratio for Cr doping on the SAC.
3.2. Effect of H2O and SO2 on NH3-SCR activity of the Cr(2%)/SAC catalyst Figs. 2a and 2b show the impact of water on the SCR activity of Cr(2%)/SAC and SAC catalyst at 125 °C. Since water content in the flue gases is around 2–18% [1], 5% and 10% H2O were added in the feeding gas mixture, respectively. The NOx conversion of the catalyst shows an abrupt decrease from 91.9% to 83.1% while it decreases from 51% to 27% for SAC with the addition of 5% H2O. This result proves that H2O has an inhibitory effect on the SCR activity of the Cr(2%)/SAC and SAC catalyst, and the inhibition of water might be due to the competition adsorption between H2O and reactants (NH3 and/or NO) [20,21]. Previous reports [7,22– 24] also found that the addition of H2O inhibited the SCR reaction on other catalysts at low temperatures. The NO conversion of Cr (2%)/SAC only slightly decreases to 80.2% with the H2O content increasing from 5% to 10%. However, NO conversion is restored
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100
100 0% H2O
5% H2O
0% H2O
10% H2O
80
Percentage(%)
Percentage(%)
80
60
40
20
0 ppm SO2
60
100 ppm SO2 300 ppm SO2 5% H O+100 ppm SO 2 2
40
20
N2 Selectivity
N2 selectivity
NOx Conversion
0
0
50
100
150
200
250
300
350
400
450
NOx conversion
0
0
Time(min) Fig. 2b. Impact of H2O on the SCR activity of the SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 5% or 10% H2O (if added) + 5% O2 + N2 balance, GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
completely to the previous level for both of Cr(2%)/SAC and SAC catalysts when switching off H2O. This indicates that the inhibitory effect of H2O is reversible on the Cr(2%)/SAC catalyst and the Cr doping increases the water tolerance of SAC greatly. In addition, N2 selectivity is maintained above 95% during the whole experiments. This indicates that the Cr(2%)/SAC catalyst has a good water tolerance and high N2 selectivity. The impact of SO2 on the SCR activity of the Cr(2%)/SAC and SAC catalysts at 125 °C was also investigated as shown in Figs. 2c and 2d. NO conversion over Cr(2%)/SAC and SAC catalyst exhibit almost no decrease (only 2%) after introducing 100 ppm SO2. There is only about 5% decrease when adding 300 ppm SO2 into the reaction mixture, indicating that a good SO2 tolerance of the Cr(2%)/SAC catalyst. However, the NO conversion displays a decreasing tendency with the coexistence of 5% H2O and 100 ppm SO2 in a few minutes and it decreases to about 71.5% and 34% for Cr(2%)/SAC and SAC respectively after 150 min. It was observed that there was a thin layer (about 3 mm in height) of white crystal formed on catalysts with the coexistence of H2O and SO2, while nothing was observed on the surface of the catalysts with the presence of H2O or SO2. As a result, the coexistence of H2O and SO2 would result in the formation and deposition of ammonium-sulfate salts, such as NH4HSO4
100
Percentage (%)
80
60 0 ppm SO2
100 ppm SO2 300 ppm SO2 5% H2O+100 ppm SO2
40
200
300
Time(min)
400
500
Fig. 2d. Impact of SO2 on the SCR activity of the SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 100 ppm or 300 ppm SO2 (if added) + 5% H2O (if added) + 5% O2 + N2 balance. GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
and (NH4)2S2O7 on the catalyst surface, blocking the active sites on the catalysts and deactivating the catalysts [3,4,18,25]. 3.3. SEM analysis Fig. 3 displayed the SEM images of the Cr(x)/SAC samples with the mass ratio of 0, 1%, 2%, 4%. Compared with SAC, the Cr-doped SACs have rougher morphologies. However, it is difficult to distinguish the differences among the Cr-doped SACs with different Cr loading amount. The flocculent species might be the chromium oxides. 3.4. BET analysis The BET data of the Cr(x)/SAC catalysts were provided in Table 2. From BET analysis, we can see that the surface area of catalysts decreased as the amount of doping chromium increased. SAC has the largest BET surface area, total pore volume and average pore size of all samples. Metal doping on the SAC resulted in a large decrease in surface area, which was in line with previous publications [15,26,27]. The blocking effects of chromium ions on the active carbon might lead to this decrease. The total pore volume and the average pore size are about 0.14 cm3/g and 4.8 nm for all Cr-doped catalysts. However, in spite of their lower surface area, all the Cr(x)/SAC samples showed higher activity towards NO reduction. This agrees with previous findings that the catalytic activity is more dependent on surface chemistry than surface area [14]. Moreover, the suitable particle size and uniformly dispersed particle is an important affecting factor for NH3-SCR of NO [28], although large surface area, pore volumes and average pore size are beneficial in minimizing mass transfer limitations to promote SCR reaction [29]. 3.5. XPS analysis
20
N2 selectivity NOx conversion
0
100
0
100
200
300
Time (min)
400
500
Fig. 2c. Impact of SO2 on the SCR activity of the Cr(2%)/SAC catalyst at 125 °C. Reaction conditions: 500 ppm NO + 500 ppm NH3 + 100 ppm or 300 ppm SO2 (if added) + 5% H2O (if added) + 5% O2 + N2 balance. GHSV = 80,000 h1. (j) NOx conversion; (d) N2 selectivity.
XPS was performed to investigate the surface binding energies and valence state of various catalysts. The photoelectron peak pertaining to O1s is shown in Fig. 4(a). For the Cr(2%)/SAC catalyst, there is almost no surface lattice oxygen (Ob) around 528.9– 529.2 eV, and the oxygen species are mainly assigned to the surface adsorbed oxygen (Oa) at 531.5 eV, such as O2 2 or O pertaining to defect-oxygen or hydroxyl-like groups [16]. It has been generally accepted that Oa species play an important role in the oxidation
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Fig. 3. SEM images of Cr(x)/SAC samples (a) SAC (b) Cr(1%)/SAC (c) Cr(2%)/SAC (d) Cr(4%)/SAC.
Table 2 BET data for the Cr(x)/SAC catalysts.
(a) Total pore volume, v/(cm3/g)
Average pore size, d/(nm)
SAC Cr(1%)/SAC Cr(2%)/SAC Cr(3%)/SAC Cr(4%)/SAC
604.0 136.7 111.7 103.6 90.4
0.9 0.16 0.13 0.15 0.11
6.3 4.8 4.8 5.6 4.7
of NO due to its high mobility. Moreover, the surface hydroxyl groups could act as Brønsted sites, which are in favor of facilitating the NH3 absorption. This effect may be an important reason for a better catalytic activity of the Cr(2%)/SAC catalysts compared with others in previous studies. As we can see from Fig. 4(b), Cr(2%)/SAC catalyst consists of two characteristic binding energies of Cr ions locating at 587.6 eV (Cr2P1/2) and 577.8 eV(Cr2P3/2). Chromium ions may exist in different oxidation states in solid materials, and exhibit different catalytic properties. Hence, the Cr2p3/2 of Cr(2%)/SAC catalyst is separated into four peaks by the peak-fitting deconvolution technique corresponding to Cr2+ (575.6–575.7 eV), Cr3+ (576.6– 576.7 eV), Cr5+ (578.3–578.9 eV) and Cr6+ (579 eV) [19,30–32]. It was thought that Cr addition could facilitate the SCR performance by the valence change between Cr6+ and lower oxidized states (Cr5+, Cr3+ and Cr2+). The area of Cr6+ peak was largest among all the Cr valences, indicating that more Cr6+ was easy to turn into lower valence. The changing of valence state would promote the generation of oxygen vacancy on the surface of catalytic, which was in favor of the SCR reaction [30,31].
531.5
Intensity (a.u.)
Surface area, A/(m2/g)
545
540
535
530
525
Binding Energy (eV)
(b)
Intensity(a.u.)
Sample
3.6. NH3 temperature-programed desorption NH3-TPD was performed to investigate the amount and strength of surface acid sites [33]. The area and position of desorption peaks correlate with the acid amount and acid strength,
595
590
585
580
575
570
Binding Energy(eV) Fig. 4. The XPS spectrum of O1s (a) and Cr2p (b) for the Cr(2%)/SAC.
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Intensity (a.u)
Intensity (a.u)
SAC Cr(2%)/SAC
SAC
Cr(2%)/SAC
Cr(2%)/SAC
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Temperature ( C) 0
SAC
100
200
300
400
500
600
Temperature ( C) 0
Fig. 5. NH3-TPD profiles of SAC and Cr(2%)/SAC.
Fig. 6. H2-TPR profiles of SAC and Cr(2%)/SAC.
respectively. And the peak at low temperature usually represents the weak acid sites while the peak at high temperature represents the strong acid sites. Both of weak acid sites and strong acid sites have a crucial effect on the SCR activity. As shown in Fig. 5, two main NH3 desorption peaks are detected in the region below 250 °C and above 500 °C, respectively [34,35], assigning to the desorption of ammonia from weak and strong acid sites, respectively. The NH3-TPD spectrum of the SAC was investigated and one peak appeared at around 200 °C as well as another at 610 °C, corresponding to NH3 desorbed by weak and strong acid sites, respectively. For the Cr(2%)/SAC catalyst, the peak represented the weak acid sites appeared at 256 °C and the peak represented the strong acid sites appeared at 715 °C. Compared with SAC catalyst, the desorption peaks of Cr(2%)/SAC shifts to a higher temperature range, suggesting that the strength of the acid sites on Cr(2%)/SAC is stronger, indicating the doping of Cr could increase the strong acid sites of the SAC. Combining with the valence change between Cr6+ and its lower oxidized states (Cr5+, Cr3+and Cr2+) as shown in Section 3.5, these might result in the improvement of the SCR performance over the SAC.
4. Conclusions The catalytic activity of the CrO3 modified sargassum based activated carbon in selective catalytic reduction of NO by NH3 was studied. The optimal Cr/SAC mass ratio for modification of the Cr(x)/SAC catalyst is 2%:1. With the introduction of Cr species, there is an attractive increase of NOx conversion from 56.91% to 92.3% at 125 °C and above 95% of N2 selectivity is achieved during the whole experiment for Cr(2%)/SAC catalyst. Compared with SAC catalyst, the desorption peaks of Cr(2%)/SAC shift to a higher temperature range, suggesting that the incorporated Cr ions can enhance the acidity of the materials. And the chromium doping on the SAC facilitates the SCR performance by the valence change between Cr6+ and lower oxidized states (Cr5+, Cr3+and Cr2+). Both of them result in an improvement of the SCR performance over the catalysts. In addition, Cr(2%)/SAC has an excellent water and sulfur tolerance, making the Cr-doped SAC catalyst as a good candidate for reducing the NOx emission from fired power plants. Acknowledgements
3.7. H2 temperature-programed reaction H2-TPR experiments were carried out over the SAC and Cr(2%)/ SAC catalysts to investigate the reducibility of metal species in the catalyst as shown in Fig. 6. The reduction profiles of bulk CrO3 reported in the literature [30] consist of reduction peaks at 280 °C, 462 °C, and 585 °C. The hydrogen reduction of Cr6+ to Cr5+, Cr5+ to Cr3+ and Cr3+ to either Cr2+ or to the metallic state occurs at about 180, 462, 585 °C, respectively [36]. For Cr(2%)/ SAC catalyst, an intense reduction peak centered at about 280 °C is emerged. This peak can be ascribed to the reduction of Cr6+ to Cr5+or other lower oxidized state species. Moreover, the hydrogen consumption of samples is obtained by integrating the curves at the temperature range of 250–650 °C. It’s obvious that Cr(2%)/ SAC has higher hydrogen consumption than those of SAC samples, attributing to the reduction of more surface Cr6+ species and acidic functional groups, as illustrated by XPS and TPD. In addition, The peak at the higher temperature peak above 400 °C can be attributed to carbon gasification [35,37]. The peak of Cr(2%)/SAC is observed at 579.5 °C while that is observed at 550.78 °C for SAC. It appears that the presence of Cr species on the catalyst promotes carbon gasification, as indicated by the stronger TPR intensity of the peak at 400–600 °C, comparing with the SAC support.
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