Mechanism and Performance of the SCR of NO with NH 3 over Sulfated Sintered Ore Catalyst

Wangsheng Chen 1,2, Fali Hu 1,2, Linbo Qin 1,2, Jun Han 1,2,* , Bo Zhao 1,2, Yangzhe Tu 1,2 and Fei Yu 3 1 School of Resource and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China; chenwangsheng@wust.edu.cn (W.C.); hufali@hotmail.com (F.H.); qinlinbo@wust.edu.cn (L.Q.); zhaobo87@wust.edu.cn (B.Z.); tuyangzhe123@hotmail.com (Y.T.) 2 Industrial Safety Engineering Technology Research Center of Hubei Province, Wuhan University of Science and Technology, Wuhan 430081, China 3 Department of Agricultural and Biological Engineering, Mississippi State University, Mississippi State, MS 39762, USA; fyu@abe.msstate.edu * Correspondence: hanjun@wust.edu.cn; Tel.: +86-27-6886-2880


Introduction
Nitrogen oxide (NO x ) is one of the major atmospheric pollutants and is mainly generated from the combustion of fossil fuel, which has serious harmful effects on human health and the ecological environment.In 2016, the total emission of NOx from the iron and steel industry was about 1.04 million tonnes [1].The sintering process in iron-making plants uses coal or coke as fuel, which is a major emission source of NOx.About 35-50% of the total NOx emission from the iron and steel industry is attributed to the sintering process [2][3][4].The new ultra-low emission standard of air pollutants for the iron and steel industry will be issued by Chinese government and require that NOx concentration in the sintering flue gas should be below 50 mg/m 3 .Hence, it is urgent to address the treatment of sintering flue gas.
Selective catalytic reduction (SCR) over V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalyst has been widely applied in power stations because of its high denitration efficiency [5][6][7].However, there are still shortcomings such as the toxicity of the catalyst, and the high operation costs [8].In particular, the reaction temperature of the current commercial catalysts is higher than the temperature of the sintering flue gas [9].Therefore, the sintering flue gas must be heated to the reaction temperature of the catalyst by using additional fuels.

Characterizations
The main components of the two catalysts were shown in Table 1.It presented that the main components of both two catalysts were Fe 2 O 3 and CaO.After the introduction of sulfuric acid, the proportion of iron oxide in the catalyst was decreased and sulphate content was significantly increased.The BET surface area, pore-size and pore volume of the catalysts were presented in Table 2.The specific surface area of the SOC was 3.684 m 2 /g, the total pore volume was 0.00714 cm 3 /g, and the average pore diameter was 6.3 nm.The BET surface area, pore size and pore volume of the SSOC were 4.734 m 2 /g, 5.9 nm and 0.00964 cm 3 /g, respectively.It was indicated that the introduction of sulfuric acid would open some micro-pores, which lead to the increase of the total pore volume.At the same time, the sulphate would block some pores.The BET of SSOC was higher than that of SCO, which meant that the sulfation had a positive effect on the pore structure of SOC.The XRD patterns of the catalysts were demonstrated in Figure 1.Several sharp diffraction peaks at 24.1  were also detected in Figure 1.The results indicated that the main components of SOC were α-Fe 2 O 3 and a small amount of α-Fe 3 O 4 .After the acidification with sulfuric acid, it could be seen that the peak of α-Fe 2 O 3 was decreased.Meanwhile, the characteristic peaks (25.4The XRD patterns of the catalysts were demonstrated in Figure 1.Several sharp diffraction peaks at 24.1˚, 33.2˚, 35.6˚, 49.6˚, 54.2˚, 57.1˚ and 62.6˚ were observed, which were assigned to α-Fe2O3 (JCPDS NO. 33-0664), and the characteristic peaks of α-Fe3O4 at 30.1˚, 33.2˚, 49.6˚, 54.2˚ and 57.1˚ were also detected in Figure 1.The results indicated that the main components of SOC were α-Fe2O3 and a small amount of α-Fe3O4.After the acidification with sulfuric acid, it could be seen that the peak of α-Fe2O3 was decreased.Meanwhile, the characteristic peaks (25.4˚, 31.4˚,38.7˚, 52.2˚) of Fe2(SO4)3 were detected, and a small amount of FeSO4 and CaSO4 also appeared.(a) XRD patterns of the catalysts: SOC (a), SSOC (b) and SSOC after tested (c).
X-ray photoelectron spectroscopy (XPS) was used to study the valence states of the catalysts, as shown in Figure 2. It could be seen from Figure 2 (I) that there were obvious peaks of C (1s), O (1s), and Fe (2p) in both of the two catalysts.Moreover, there were obvious S (2p) peaks after the introduction of sulfuric acid.As shown in Figure 2 (II), the binding energies of Fe 2p 2/3 and Fe 2p 1/2 on SOC were mainly centered at about 710.8 and 724.3 eV, which were indications that the iron species were mainly in the form of Fe 3+ in the SOC [20].At the same time, a small number of Fe 2+ existed, which was also consistent with the XRD results.The binding energies of Fe 2p 2/3 and Fe 2p 1/2 of SSOC were higher than those of SOC.The peak of Fe 2p 2/3 appeared at 711.8 eV and the peak of Fe 2p 1/2 appeared at 724.9 eV.After the acidification with sulfuric acid, both Fe2(SO4)3 and FeSO4 appeared in the SSOC, which contributed to Fe 2p peaks shifting to the higher position.In addition, in Figure 2 (III), the peak of S 2p appeared at 169.3 eV, indicating that sulfur compounds existed in the form of SO 2− 4 [21].X-ray photoelectron spectroscopy (XPS) was used to study the valence states of the catalysts, as shown in Figure 2. It could be seen from Figure 2 (I) that there were obvious peaks of C (1s), O (1s), and Fe (2p) in both of the two catalysts.Moreover, there were obvious S (2p) peaks after the introduction of sulfuric acid.As shown in Figure 2 (II), the binding energies of Fe 2p 2/3 and Fe 2p 1/2 on SOC were mainly centered at about 710.8 and 724.3 eV, which were indications that the iron species were mainly in the form of Fe 3+ in the SOC [20].At the same time, a small number of Fe 2+ existed, which was also consistent with the XRD results.The binding energies of Fe 2p 2/3 and Fe 2p 1/2 of SSOC were higher than those of SOC.The peak of Fe 2p 2/3 appeared at 711.8 eV and the peak of Fe 2p 1/2 appeared at 724.9 eV.After the acidification with sulfuric acid, both Fe 2 (SO 4 ) 3 and FeSO 4 appeared in the SSOC, which contributed to Fe 2p peaks shifting to the higher position.In addition, in Figure 2 (III), the peak of S 2p appeared at 169.3 eV, indicating that sulfur compounds existed in the form of SO 2− 4 [21].
existed, which was also consistent with the XRD results.The binding energies of Fe 2p 2/3 and Fe 2p 1/2 of SSOC were higher than those of SOC.The peak of Fe 2p 2/3 appeared at 711.8 eV and the peak of Fe 2p 1/2 appeared at 724.9 eV.After the acidification with sulfuric acid, both Fe2(SO4)3 and FeSO4 appeared in the SSOC, which contributed to Fe 2p peaks shifting to the higher position.In addition, in Figure 2 (III), the peak of S 2p appeared at 169.3 eV, indicating that sulfur compounds existed in the form of SO 2− 4 [21].

Adsorption of NH3
Figure 3 showed the FTIR spectra of NH3 adsorption over the SSOC with 1000 ppm NH3/Ar under different temperatures (250-350 ˚C).Thirty minutes after NH3/Ar introduction, the bands at 1690, 1525, 1460 and 1408 cm −1 were observed.The bands at 1690, 1460 and 1408 cm -1 were symmetric bending vibration of NH + 4 species at Brønsted acid sites [20,22].The bands at 1525 and 3434 cm -1 were assigned to the intermediate species, such as ammonium or amide species (-NH2) [20].Meanwhile, the bands at, 3242 and 3120 cm -1 could be ascribed to the N-H stretching vibration of coordination NH3, and the bands at 1259 cm -1 (symmetric bending vibration of NH3 on Lewis acid sites) and 1102

Adsorption of NH 3
Figure 3 showed the FTIR spectra of NH 3 adsorption over the SSOC with 1000 ppm NH 3 /Ar under different temperatures (250-350 • C).Thirty minutes after NH 3 /Ar introduction, the bands at 1690, 1525, 1460 and 1408 cm −1 were observed.The bands at 1690, 1460 and 1408 cm −1 were symmetric bending vibration of NH + 4 species at Brønsted acid sites [20,22].The bands at 1525 and 3434 cm −1 were assigned to the intermediate species, such as ammonium or amide species (-NH 2 ) [20].Meanwhile, the bands at, 3242 and 3120 cm −1 could be ascribed to the N-H stretching vibration of coordination NH 3 , and the bands at 1259 cm −1 (symmetric bending vibration of NH 3 on Lewis acid sites) and 1102 cm −1 (NH 3 species adsorbed on Lewis acid sites) also appeared [20,23,24].The intensities of the peaks of NH + 4 species (1690, 1460 and 1408 cm −1 ) and -NH 2 species (1525 cm −1 ) first increased and then decreased in the temperature range of 250-350 • C.There were peaks at 960 and 930 cm −1 (NH 3 of gaseous state or weak adsorption state) under 300 • C. Yu et al. [25] stated that the following reactions probably took place in the reaction: According to Equations ( 1)-( 3), the more functional groups formed at the surface of the catalyst, the higher the reaction activity.Thus, the optimum adsorption activity of NH 3 over the SSOC was 300 • C, as shown in Figure 3.
The dependence of FTIR spectra over the SOC and SSOC on the reaction time at 1000 ppm NH 3 /Ar and 300 • C was presented in Figure 4. Figure 4a demonstrated that there were 6 bands in the range of 1690-1100 cm −1 .The bands at 1690, 1405 and 1454 cm −1 were related to the symmetric bending vibration of NH + 4 species, and the band at 1525 cm −1 was ascribed to the intermediate products of ammonium or -NH 2 species.Moreover, the band at 3242 cm −1 (N-H stretching vibration of coordinated NH 3 ) appeared at 10 min.With the feed of NH 3 , the adsorption band at 1259 cm −1 (symmetric bending vibration of NH 3 on Lewis acid sites) appeared at 30 min, and the band at 1107 cm −1 (NH 3 of gaseous state or weak adsorption state) appeared at 60 min.Hence, there were both Lewis acid sites and Brønsted acid sites on the surface of SOC.The FTIR spectra over SSOC dependence of the reaction time was presented in Figure 4b, and the bands of NH 3 adsorption were basically the same as those of SOC.Moreover, the bands at 3434 cm −1 (-NH 2 groups) and 3128 cm −1 (coordination NH 3 on Lewis acid sites) were observed because the surface acidity of SSOC was strengthened.There were two new weak bands that appeared at 965 and 927 cm −1 , where NH 3 was in a gaseous state or weak adsorption state.These weakly adsorbed or gaseous NH 3 could rapidly adsorbed on the acid sites once the adsorbed NH 3 species was consumed.It could also be seen that the strength of NH 3 adsorption peak was enhanced after SOC acidification with sulfuric acid, which was the reason that the surface acidity of SSOC was enhanced after the acidification.The influence of the acidity of SSCO on the dentiration efficiency is presented in Figure 5.The different acidity of SSOC was achieved by sulfating with 1, 3 and 5 mol/L sulfuric acid.In this experimental run, the mass of catalyst and the volume of sulfuric acid were the same.Only the concentration of sulfuric acid was varied.It was demonstrated the maximum denitration efficiency occurred at the catalyst treated by 5 mol/L sulfuric acid, and its denitration efficiency was 92.3% at 300 • C. At the same time, the denitration efficiency of the catalysts sulfated by 1 and 3 mol/L was 56.6% and 68.5% at the same reaction temperature.Moreover, the experimental results proved that the optimum reaction temperature for all catalysts was 300 • C.These sulfates on the surface of SSOC provided more Brønsted acid sites (Peaks at 1690, 1525 cm −1 were increased), which promoted the adsorption capacity for NH 3 .Hence, the catalyst denitration performance was also improved [26,27].
treated by 5 mol/L sulfuric acid, and its denitration efficiency was 92.3% at 300 ˚C.At the same time, the denitration efficiency of the catalysts sulfated by 1 and 3 mol/L was 56.6% and 68.5% at the same reaction temperature.Moreover, the experimental results proved that the optimum reaction temperature for all catalysts was 300 ˚C.These sulfates on the surface of SSOC provided more Brønsted acid sites (Peaks at 1690, 1525 cm -1 were increased), which promoted the adsorption capacity for NH3.Hence, the catalyst denitration performance was also improved [26,27].

Co-Adsorption of NO and O 2
Figure 6 indicated the DRIFTS spectra of co-adsorption of NO and O 2 under 250-350 • C, and 1000 ppm NO + 15% O 2 /Ar.After 30 min, there were 4 bands appeared in the range of 1620-1290 cm −1 and a weak band appeared at 1014 cm −1 .The band at 1607 cm was ascribed to bridged nitrate species and adsorbed NO 2 molecules [28,29].The bands at 1485 and 1417 cm −1 were assigned to bidentate nitrates and monodentate nitrates respectively [30][31][32].In addition, the bands at 1293 and 1014 cm −1 were related to nitro compounds [31].The variation trend of the intensities of these bands were same as NH 3 adsorption in the temperature range of 250-350 • C. The adsorption process can be explained by the following formula: According to Equations ( 4)-( 10), all the peaks in Figure 6 were important to the SCR reaction.The intensity of peaks at 300 • C was the highest, which mean that the optimum adsorption temperature for NO was also 300 • C.
Figure 7 showed the DRIFTS spectra over SOC and SSOC at different reaction times under 300 • C, 1000 ppm NO + 15% O 2 /Ar.Similar with the spectra in Figure 6, after 10 min, there were 4 bands that appeared in the range of 1620-1300 cm −1 and a weak band appeared at 1022 cm −1 .The bands intensities were gradually increased with the adsorption time.It could be seen that in Figure 7b, the adsorption intensity of SSOC was obviously higher than that of SOC, especially for the nitro compounds (1290 cm −1 ) and the nitrate species (1490 cm −1 ).The results indicated that the adsorption capacity of NO was improved after the acidification with sulfuric acid.It had been shown that the introduction of SO 2− 4 enhanced the adsorption of NO [26].
capacity of NO was improved after the acidification with sulfuric acid.It had been shown that the introduction of SO 2− 4 enhanced the adsorption of NO [26].The results showed that the peak intensities of the adsorbed species related to NO2 molecules and bridged nitrate species (1607 and 1618 cm -1 ) were decreased obviously with purging by Ar, which indicated NO2 molecules and bridged nitrate species absorbed at the surface of catalysts were unstable.At the same time, it was found that the intensities of bidentate nitrates, monodentate nitrates and nitro compounds were stable even after an Ar purge.Adsorbance/a.u.

Reaction between NH3 and NO
The reaction at the surface of catalyst by introducing NH3 and NO + O2 into an in-situ reactor was also studied.Figure 8 shows that NH + 4 species (1695 cm -1 ), amide species(-NH2) or ammonium (1533 cm -1 ), and several weak adsorption bands at 1252, and 3242 cm -1 (NH3 adsorbed on Lewis acid site) were detected on the surface of SOC after NH3 introduction at 30 min.On the surface of SSOC catalyst, NH + 4 species at 1695, 1427 and 1454 cm -1 and weakly adsorbed NH3 or gaseous NH3 (966, 925 cm -1 ) were also detected.There were two bands at 3434 cm -1 (-NH2 groups) and 3127 cm -1 (coordination NH3 on Lewis acid sites) that appeared at the same time.After switching to NO + O2, the adsorbed species of NH3 over SOC and SSOC gradually disappeared and the adsorbed species of The results showed that the peak intensities of the adsorbed species related to NO 2 molecules and bridged nitrate species (1607 and 1618 cm −1 ) were decreased obviously with purging by Ar, which indicated NO 2 molecules and bridged nitrate species absorbed at the surface of catalysts were unstable.
Catalysts 2019, 9, 90 9 of 13 At the same time, it was found that the intensities of bidentate nitrates, monodentate nitrates and nitro compounds were stable even after an Ar purge.

Reaction between NH 3 and NO
The reaction at the surface of catalyst by introducing NH 3 and NO + O 2 into an in-situ reactor was also studied.Figure 8 shows that NH + 4 species (1695 cm −1 ), amide species(-NH 2 ) or ammonium (1533 cm −1 ), and several weak adsorption bands at 1252, and 3242 cm −1 (NH 3 adsorbed on Lewis acid site) were detected on the surface of SOC after NH 3 introduction at 30 min.On the surface of SSOC catalyst, NH + 4 species at 1695, 1427 and 1454 cm −1 and weakly adsorbed NH 3 or gaseous NH 3 (966, 925 cm −1 ) were also detected.There were two bands at 3434 cm −1 (-NH 2 groups) and 3127 cm −1 (coordination NH 3 on Lewis acid sites) that appeared at the same time.After switching to NO + O 2 , the adsorbed species of NH 3 over SOC and SSOC gradually disappeared and the adsorbed species of NO x appeared.These bands were ascribed to NO 2 molecules and bridged nitrate species (1602 and 1618 cm −1 ), bidentate nitrates (1488 and 1498), monodentate nitrates (1413 and 1419 cm −1 ) and nitro compounds (1290, 1295, 1020 and 1011 cm −1 ).Comparing Figure 8a with Figure 8b, it could be seen that there were more nitrite species and NH 3 species adsorbed at SSOC than those at SOC.This could be explained that the SSOC contained more active sites, which resulted from the sulfates on SSOC, and Equations ( 7)-( 10) occurred [33].The reaction of amide species (-NH 2 ) and NH + 4 species on the surface of catalysts with the gaseous NO was E-R mechanism, and the reaction between adsorbed state NO 2 and adsorbed NH 3 followed L-H mechanism.Therefore, the reaction between NO and NH 3 over SOC and SSOC had two mechanisms.
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 13 surface of catalysts with the gaseous NO was E-R mechanism, and the reaction between adsorbed state NO2 and adsorbed NH3 followed L-H mechanism.Therefore, the reaction between NO and NH3 over SOC and SSOC had two mechanisms.2.2.4.SCR performance SCR performance were carried out in a fixed-bed reactor, which was made of quartz with an inner diameter of 20 mm and a length of 1000 mm.In this experiment, the mass of the catalysts samples was 13.49 g, the flow rate of the flue gas which simulated sintering flue gas was about 600

SCR Performance
SCR performance were carried out in a fixed-bed reactor, which was made of quartz with an inner diameter of 20 mm and a length of 1000 mm.In this experiment, the mass of the catalysts samples was 13.49 g, the flow rate of the flue gas which simulated sintering flue gas was about 600 mL/min, the simulated sintering flue gas contained 300 ppm NO, 15% O 2 and balance N 2 .The temperature in the reactor was kept at 100 • C-350 • C, with the condition of a 0.5-1.0NH 3 /NO ratio and 5000 h −1 GHSV.The NO x concentrations in simulated flue gas at the inlet and outlet of the reactor were continuously recorded by a gas analyzer (PG-350, Horiba, Kyoto, Japan) with an accuracy of ±1.0%.The NO x conversion was calculated according to the following equations: Figure 9 presented the denitration performance of SOC and SSOC in the temperature range of 100 • C-350 • C. It was found that the reaction temperature had a great effect on the NH 3 -SCR denitration performance of SOC and SSOC.The optimum reaction temperature was 300 • C, which conformed well with the in-situ DRIFTS results.The NO x conversion of SOC was only 27% at 300 • C, 1.0 NH 3 /NO ratio and 5000 h −1 GHSV, and the NO x conversion of SSOC was 92% at the same condition.It was found that the denitration performance of SSOC was greatly improved and the optimum reaction temperature was 300 • C.
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 13 denitration performance of SOC and SSOC.The optimum reaction temperature was 300 ˚C, which conformed well with the in-situ DRIFTS results.The NOx conversion of SOC was only 27% at 300 ˚C, 1.0 NH3/NO ratio and 5000 h -1 GHSV, and the NOx conversion of SSOC was 92% at the same condition.It was found that the denitration performance of SSOC was greatly improved and the optimum reaction temperature was 300 ˚C.
The stability test of SSOC was shown at Figure 10.The reaction continued for 24 h at 300 ˚C, 1.0 NH3/NO ratio, GHSV = 5000 h -1 .It could be seen that the NOx conversion was stable at about 92%, which indicated that SSOC has a good denitration stability.Compared with the SOC, the denitration performance had been greatly improved after the acidification with sulfuric acid.The adsorption of NH3 and NO on SSOC was obviously improved, which subsequently promoted the NH3-SCR reaction.The stability test of SSOC was shown at Figure 10.The reaction continued for 24 h at 300 • C, 1.0 NH 3 /NO ratio, GHSV = 5000 h −1 .It could be seen that the NO x conversion was stable at about 92%, which indicated that SSOC has a good denitration stability.Compared with the SOC, the denitration performance had been greatly improved after the acidification with sulfuric acid.The adsorption of NH 3 and NO on SSOC was obviously improved, which subsequently promoted the NH 3 -SCR reaction.

Catalyst preparation
In this experiment, the sintered ore was sampled from a sintering workshop in Wuhan.The sintered ore was dried, milled and sieved to 0.15-0.25 mm, which was denoted as SOC.The SSOC

Catalyst Preparation
In this experiment, the sintered ore was sampled from a sintering workshop in Wuhan.The sintered ore was dried, milled and sieved to 0.15-0.25 mm, which was denoted as SOC.The SSOC was prepared by using an impregnation method.Firstly, 100 g sintered ore (0.15-0.25 mm) was weighed and put into a beaker, then 50 mL sulfuric acid solution with a concentration of 5 mol/L was added and stirred simultaneously for 30 min.After filtration and washing with the deionized water, the mixture was dried at 105 • C and then calcined at 500 • C for 3 h in the air atmosphere.Finally, the prepared SSOC were naturally cooled to the room temperature, then crushed and sieved to 0.15-0.25 mm.

Catalyst Characterization
The main chemical composition of the SOC and SSOC were analyzed by X-ray fluorescence spectroscopy (XRF) (ARL SMS-XY, Thermo Fisher Scientific Corp., Waltham, MA, USA).The specific surface area, pore volume and pore size distribution of the catalysts were measured by an automated adsorption analyzer (Micromeritics ASAP 2020, Micromeritics Corp., Norcross, GA, USA).The catalysts samples were firstly degassed at 240 • C for 4 h before the test, and the adsorption medium was liquid nitrogen.The nitrogen adsorption and desorption were analyzed at −196 • C using BET analyzer (Micromeritics ASAP 2020, Micromeritics Corp., Norcross, GA, USA).The surface area was calculated by using the BET method according to nitrogen adsorption data in the relative pressure (P/P0) range of 0.01-1.X-ray diffraction (XRD; Rigaku RINT2000, Tokyo, Japan) was performed using CuKα radiation (λ = 1.54056Å) to detect the crystalline phases of the samples.The analysis of XRD was referred to International Centre for Diffraction Data (ICDD).X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Shimazu Corp., Kyoto, Japan) was used to determine the valence states of the surface atoms of the catalysts with Al Kα radiation.
In-situ DRIFTS experiments were carried out in a FTIR spectrometer (FT-IR, Bruker Tensor II, Bruker optics Corp., Karlsruhe, Germany) equipped with an in-situ cell and a mercury-cadmium-telluride detector [34][35][36][37].For the adsorption of NH 3 (or NO + O 2 ), the catalyst was exposed to a 20 mL/min NH 3 (or NO + O 2 ), which resulted in the variation with adsorption time of the DRIFT spectra, and argon purging was subsequently performed.In the reaction mechanism studies, the catalyst was pretreated in a flow of 20 mL/min NH 3 for 40 min, then was shifted NO + O 2 at 300 • C to get the DRIFT spectra.All spectra were recorded by accumulating 100 scans at a spectra resolution of 4 cm −1 .

Figure 3 .
Figure 3. DRIFT spectra of SSOC in the condition of 1000 ppm NH3 at 250-350 ˚C for 30 min.Figure 3. DRIFT spectra of SSOC in the condition of 1000 ppm NH 3 at 250-350 • C for 30 min.

Figure 5 .
Figure 5.The influence of acidity of SSOC on SCR performance.Figure 5.The influence of acidity of SSOC on SCR performance.

Figure 7 .
Figure 7. DRIFT spectra of SOC (a) and SSOC (b) exposed to 1000 ppm NO and 15% O2 at 300 ˚C for a different time.

Figure 7 .
Figure 7. DRIFT spectra of SOC (a) and SSOC (b) exposed to 1000 ppm NO and 15% O 2 at 300 • C for a different time.
DRIFT spectra of SSOC in the condition of 1000 ppm NO and 15% O 2 at 250-350 • C for 30 min.