Active Site of O2 and Its Improvement Mechanism over Ce-Ti Catalyst for NH3-SCR Reaction

The current study on Ce-Ti catalyst was mainly focused on the function of NH3 and NO adsorption sites. In our study, by comparing Ce-Ti (doped catalyst) to Ce/Ti (supported catalyst), the active site of O2 and its improvement mechanism over Ce-Ti catalyst for NH3-Selective catalytic reduction (SCR) reactions were investigated. For Ce-Ti catalyst, a cerium atom was confirmed entering a TiO2 crystal lattice by X-ray diffraction (XRD) and Raman; the structure of Ce-Ti ( represents oxygen vacancy) in Ce-Ti catalyst was confirmed by X-ray photoelectron spectroscopy (XPS) and Photoluminescence spectra (PL spectra). The nature of this structure was characterized by electron paramagnetic resonance (EPR), Ammonia temperature-programmed desorption (NH3-TPD), hydrogen temperature-programmed reduction (H2-TPR), Nitric oxide temperature-programmed desorption (NO-TPD) and In situ DRIFT. The results indicated that oxygen vacancies had a promotive effect on the adsorption and activation of oxygen, and oxygen was converted to superoxide ions in large quantities. Also, because of adsorption and activation of NO and NH3, electrons were transferred to adsorbed oxygen via oxygen vacancies, which also promoted the formation of superoxide ions. We expected that our study could promote understanding of the active site of O2 and its improvement mechanism for doped catalyst.


Introduction
NO x (NO 2 , NO, N 2 O etc.) emitted from mobile or stationary sources causes a series of serious environmental problems such as ozone holes, photochemical smog and acid rain [1], which are harmful to human health.Selective catalytic reduction (SCR) is the mainstream technology that used for denitrification nowadays.V 2 O 5 -WO 3 /TiO 2 catalysts are the most widely used industrial catalyst due to their good catalytic performance and excellent resistance to SO 2 [2].However, there are some defects, such as a narrow temperature window, biological toxicity of V 2 O 5 , generation of larger amount of N 2 O at high temperature, etc. [3].CeO 2 -based catalysts have been proved to possess excellent low-temperature activity for NH 3 -SCR of NO due to their high oxygen storage capacities along with unique redox properties [4].Among all these catalysts, Ce-Ti catalyst has gained wide attention for its excellent properties and lack of secondary pollution.
A lot of relevant work [5][6][7] on Ce-Ti catalyst have been performed.Gao et al. [8] yielded 98.6% NO conversion at around 350 • C with Ce-Ti mixed-oxide catalyst.They attributed the superior performance to the strong interaction between Ce and Ti.Moreover, they thought that high concentration of amorphous Ce on the surface played a vital role in NO reduction.Li et al. [9] studied amorphous Ce-Ti catalyst and put forward the important structure of Ce-O-Ti.In addition, the Ce-O-Ti structure with the interaction between Ce and Ti was confirmed to be the main active site.Li et al. [10] later reported that NH 3 adsorption on Ti and NO adsorption on Ce played a key role in NH 3 -SCR with Ce-Ti catalyst.Based on this, some researchers focused on the transition metal-modified Ce-Ti catalyst and gained good results [11,12].However, little attention was paid to the mechanism of how oxygen participated in the SCR reaction in Ce-Ti catalyst.
In our present study, we also found that Ce-Ti catalyst showed a higher NH 3 -SCR activity than Ce/Ti catalyst, especially in the low-temperature region.Based on previous studies, we focused on the structure of Ce-Ti catalyst, active site of O 2 and its improvement mechanism for NH 3 -SCR reaction by a series of characterization include X-ray diffraction (XRD), PL, electron paramagnetic resonance (EPR), Raman, X-ray photoelectron spectroscopy (XPS), H 2 -TPR, NO-TPD, NH 3 -TPD, H 2 -TPR and In situ DRIFT.We expected that our study could promote understanding of the active site of O 2 and its improvement mechanism for doped SCR catalyst.

Catalytic Activity Tests
Four kinds of catalysts, Ce-Ti-500, Ce-Ti-400, Ce/Ti-500 and Ce/Ti-400, were tested for NO conversion from 120 • C to 390 • C (Figure 1).As shown in Figure 1, the NO conversion mainly increased with the increase of reaction temperature (except Ce/Ti-500 at 400 • C).Among the four kinds of catalysts, the Ce-Ti-400 catalyst showed a superior SCR activity, especially at low temperatures (<300 • C).The NO conversion of Ce-Ti-400 could reach 59.1% and 85.8% at 210 and 240 • C, respectively, and little NO could be detected in outlet gas flow above 270 • C.These results were similar to traditional Ce-Ti catalysts [13,14].Additionally, the formation of N 2 O was <3 ppm, which means the N 2 selectivity was promising.It is worth noting that the NO conversion at low temperatures (<300 • C) increased in the following order: Ce/Ti-500 < Ce/Ti-400 < Ce-Ti-500 < Ce-Ti-400.Therefore, compared with the supported catalysts (Ce/Ti catalysts), the doped catalysts (Ce-Ti catalysts) showed higher NH 3 -SCR activity.
Catalysts 2018, 8, x FOR PEER REVIEW 2 of 14 al.[10] later reported that NH3 adsorption on Ti and NO adsorption on Ce played a key role in NH3-SCR with Ce-Ti catalyst.Based on this, some researchers focused on the transition metal-modified Ce-Ti catalyst and gained good results [11,12].However, little attention was paid to the mechanism of how oxygen participated in the SCR reaction in Ce-Ti catalyst.
In our present study, we also found that Ce-Ti catalyst showed a higher NH3-SCR activity than Ce/Ti catalyst, especially in the low-temperature region.Based on previous studies, we focused on the structure of Ce-Ti catalyst, active site of O2 and its improvement mechanism for NH3-SCR reaction by a series of characterization include X-ray diffraction (XRD), PL, electron paramagnetic resonance (EPR), Raman, X-ray photoelectron spectroscopy (XPS), H2-TPR, NO-TPD, NH3-TPD, H2-TPR and In situ DRIFT.We expected that our study could promote understanding of the active site of O2 and its improvement mechanism for doped SCR catalyst.

Catalytic Activity Tests
Four kinds of catalysts, Ce-Ti-500, Ce-Ti-400, Ce/Ti-500 and Ce/Ti-400, were tested for NO conversion from 120 °C to 390 °C (Figure 1).As shown in Figure 1, the NO conversion mainly increased with the increase of reaction temperature (except Ce/Ti-500 at 400 °C).Among the four kinds of catalysts, the Ce-Ti-400 catalyst showed a superior SCR activity, especially at low temperatures (< 300 °C).The NO conversion of Ce-Ti-400 could reach 59.1% and 85.8% at 210 and 240 °C, respectively, and little NO could be detected in outlet gas flow above 270 °C.These results were similar to traditional Ce-Ti catalysts [13,14].Additionally, the formation of N2O was <3 ppm, which means the N2 selectivity was promising.It is worth noting that the NO conversion at low temperatures (<300 °C) increased in the following order: Ce/Ti-500 < Ce/Ti-400 < Ce-Ti-500 < Ce-Ti-400.Therefore, compared with the supported catalysts (Ce/Ti catalysts), the doped catalysts (Ce-Ti catalysts) showed higher NH3-SCR activity.

XRD and Raman Analysis
Figure 2 displays the XRD patterns of catalysts prepared by different methods.As shown in Figure 2, Ce-Ti-400 and Ce-Ti-500 catalysts only showed the anatase type of TiO2 (2θ = 25.2°,37.7°, 47.8°, 53.8°, 55.0°).No representative peaks of Ce species were found.The doped catalyst (Ce-Ti-400) had lower XRD peak intensity than that of supported catalyst (Ce/Ti-400).According to a previous study on Ce-Ti catalyst [15], the anatase peak intensity of catalysts decreased with the increase in Ce doping level, suggesting that Ce interacted with Ti.For Ce-Ti-500, only anatase was detected.However, large amounts of rutile were detected in Ce/Ti-500.As we know, Ce doping could inhibit phase transformation from anatase to rutile [16], which could be an evidence of Ce doped into the          , 55.0 • ).No representative peaks of Ce species were found.The doped catalyst (Ce-Ti-400) had lower XRD peak intensity than that of supported catalyst (Ce/Ti-400).According to a previous study on Ce-Ti catalyst [15], the anatase peak intensity of catalysts decreased with the increase in Ce doping level, suggesting that Ce interacted with Ti.For Ce-Ti-500, only anatase was detected.

XRD and Raman Analysis
However, large amounts of rutile were detected in Ce/Ti-500.As we know, Ce doping could inhibit phase transformation from anatase to rutile [16], which could be an evidence of Ce doped into the TiO 2 lattice for Ce-Ti-400 catalyst.Energy Dispersive Spectroscopy (EDS) results showed that the cerium ratio on the surface of Ce-Ti-400 catalyst was 5.98%, while that of Ce/Ti-400 was 10.29%.The above data could also prove that Ce had entered TiO 2 lattice.In addition, TiO 2 (anatase) is considered to be the best supporter of SCR catalysts because it improves dispersion and reducibility of rhodium particles [17].
Catalysts 2018, 8, x FOR PEER REVIEW 3 of 14 TiO2 lattice for Ce-Ti-400 catalyst.Energy Dispersive Spectroscopy (EDS) results showed that the cerium ratio on the surface of Ce-Ti-400 catalyst was 5.98%, while that of Ce/Ti-400 was 10.29%.The above data could also prove that Ce had entered TiO2 lattice.In addition, TiO2 (anatase) is considered to be the best supporter of SCR catalysts because it improves dispersion and reducibility of rhodium particles [17].The Raman spectra were used to investigate the structure of samples further.Raman spectra of Ce-Ti-400 and Ce/Ti-400 are showed in Figure 3. Referring to the standard Raman spectrum, the peaks at 640 cm −1 , 515 cm −1 , 398 cm −1 , 198 cm −1 and 147 cm −1 are representative peaks of anatase TiO2.The anatase bands at 515 and 398 cm −1 could be attributed to the Ti-O-stretching vibration, and the bands at 640 cm −1 could be attributed to O-Ti-O-bending vibrations [18,19].Obviously, the Raman spectra of Ce/Ti-400 coincides with the standard anatase TiO2 Raman spectrum well.However, in the Raman spectra of Ce-Ti-400, only a weak peak at 144 cm −1 could be found.This phenomenon showed that the original symmetrical structure of Ti-O-Ti was destroyed by Ce doping.Thus, we could infer that in Ce-Ti-400 sample, the new structure of Ce-O-Ti formed [9].

XPS and PL Analysis
Figure 4 shows XPS spectrum for Ce3d, Ti2p and O1s of Ce-Ti 400 and Ce/Ti 400.From the XPS spectrum of Ce 3d in Figure 4a, two multipliers (U and V) could be found after fitting.The XPS peaks labeled U, U'', U''', V, V'', and V''' were attributed to Ce 4+ species, while the u', v' peaks were assigned to Ce 3+ species.Compared to Ce/Ti-400, it could be clearly seen that peaks of Ce-Ti-400 slightly shifted The Raman spectra were used to investigate the structure of samples further.Raman spectra of Ce-Ti-400 and Ce/Ti-400 are showed in Figure 3. Referring to the standard Raman spectrum, the peaks at 640 cm −1 , 515 cm −1 , 398 cm −1 , 198 cm −1 and 147 cm −1 are representative peaks of anatase TiO 2 .
The anatase bands at 515 and 398 cm −1 could be attributed to the Ti-O-stretching vibration, and the bands at 640 cm −1 could be attributed to O-Ti-O-bending vibrations [18,19].Obviously, the Raman spectra of Ce/Ti-400 coincides with the standard anatase TiO 2 Raman spectrum well.However, in the Raman spectra of Ce-Ti-400, only a weak peak at 144 cm −1 could be found.This phenomenon showed that the original symmetrical structure of Ti-O-Ti was destroyed by Ce doping.Thus, we could infer that in Ce-Ti-400 sample, the new structure of Ce-O-Ti formed [9].
Catalysts 2018, 8, x FOR PEER REVIEW 3 of 14 TiO2 lattice for Ce-Ti-400 catalyst.Energy Dispersive Spectroscopy (EDS) results showed that the cerium ratio on the surface of Ce-Ti-400 catalyst was 5.98%, while that of Ce/Ti-400 was 10.29%.The above data could also prove that Ce had entered TiO2 lattice.In addition, TiO2 (anatase) is considered to be the best supporter of SCR catalysts because it improves dispersion and reducibility of rhodium particles [17].The Raman spectra were used to investigate the structure of samples further.Raman spectra of Ce-Ti-400 and Ce/Ti-400 are showed in Figure 3. Referring to the standard Raman spectrum, the peaks at 640 cm −1 , 515 cm −1 , 398 cm −1 , 198 cm −1 and 147 cm −1 are representative peaks of anatase TiO2.The anatase bands at 515 and 398 cm −1 could be attributed to the Ti-O-stretching vibration, and the bands at 640 cm −1 could be attributed to O-Ti-O-bending vibrations [18,19].Obviously, the Raman spectra of Ce/Ti-400 coincides with the standard anatase TiO2 Raman spectrum well.However, in the Raman spectra of Ce-Ti-400, only a weak peak at 144 cm −1 could be found.This phenomenon showed that the original symmetrical structure of Ti-O-Ti was destroyed by Ce doping.Thus, we could infer that in Ce-Ti-400 sample, the new structure of Ce-O-Ti formed [9].

XPS and PL Analysis
Figure 4 shows XPS spectrum for Ce3d, Ti2p and O1s of Ce-Ti 400 and Ce/Ti 400.From the XPS spectrum of Ce 3d in Figure 4a, two multipliers (U and V) could be found after fitting.The XPS peaks labeled U, U'', U''', V, V'', and V''' were attributed to Ce 4+ species, while the u', v' peaks were assigned to Ce 3+ species.Compared to Ce/Ti-400, it could be clearly seen that peaks of Ce-Ti-400 slightly shifted

XPS and PL Analysis
Figure 4 shows XPS spectrum for Ce3d, Ti2p and O1s of Ce-Ti 400 and Ce/Ti 400.From the XPS spectrum of Ce 3d in Figure 4a, two multipliers (U and V) could be found after fitting.The XPS peaks labeled U, U", U"', V, V", and V"' were attributed to Ce 4+ species, while the u', v' peaks were assigned to Ce 3+ species.Compared to Ce/Ti-400, it could be clearly seen that peaks of Ce-Ti-400 slightly shifted to lower binding energy.As we know, lower binding energy means higher cloud density [20].This change corresponded with previous inference of Ce-O-Ti structure.By calculation, the amount of Ce 3+ /(Ce 3+ + Ce 4+ ) of Ce-Ti-400 and Ce/Ti-400 were 32.63% and 19.11%, respectively.Obviously, Ce-Ti-400 has a higher amount of Ce 3+ /(Ce 3+ + Ce 4+ ).It could be inferred that the interaction between Ce and Ti promote the increase of Ce 3+ .The increase of Ce 3+ could bring more oxygen vacancies, which is beneficial to activity enhancement [21].
461.4eV and 462.3-467.2eV could be assigned to Ti [22].Obviously, cloud density of both Ce and Ti increased.Through previous result of Raman, we confirmed the structure of Ce-O-Ti in Ce-Ti-400 sample.Normally, due to the different electronegativity of Ce ion and Ti ion, the density of Ce and Ti cloud density could not increase at the same time in Ce-O-Ti structure.Therefore, combined with the increasing cloud density of both Ce and Ti, we could infer that for fresh catalysts, oxygen vacancies existed in Ce-O-Ti structure.Thus, Ce-□-Ti possibly existed in Ce-Ti-400 catalyst.
Figure 4c shows the XPS spectrum of O 1s of Ce-Ti-400 and Ce/Ti-400.The two kinds of peaks in these spectra represented two kinds of surface oxygen species.Normally, the peak at 531.0-531.9eVwith lower binding energy could be assigned to lattice oxygen (denoted as Oβ), and the peak 529.0-530.0eV with higher binding energy could be assigned to chemisorbed oxygen (denoted as Oα) [23,24].It is worth noting that peaks of Ce-Ti-400 slightly shifted to lower binding energy and this result means O of Ce-Ti-400 also has higher cloud density.Normally, oxygen vacancy was charged oxygen vacancy [25].In addition, combined with the increasing cloud density of both Ce and Ti in this study, it could also be judged that oxygen vacancy was charged oxygen vacancy.Considering the strong electron absorption ability of O and adsorbed oxygen on catalysts, it is possible that electrons on the oxygen vacancy will produce a certain amount of delocalization, which led to the increase of cloud intensity of Oα and Oβ on catalyst.Figure 5 shows PL spectrum of Ce-Ti-400 and Ce/Ti-400.From previous report, peak 1 at 556 nm was assigned to the surface defect, peak 2 at 633 nm might be attributed to the transfer of excited electrons [26,27].Obviously, the intensity of Ce-Ti-400 was lower than that of Ce/Ti-400.As we have shown, surface defects were electron-trapping centers in catalysts.In addition, a lower PL intensity represented a larger number of surface defects in general [28].Oxygen vacancy was the main surface defect for these two catalysts.That means more oxygen vacancies existed on Ce-Ti-400.From our previous study [22], formation of oxygen vacancies was due to the interaction between doping Ce and Ti.On the one hand, the formation of oxygen vacancies could promote low-temperature SCR activity [29]; on the other hand, oxygen vacancies were the adsorption center of oxygen [30].The absorbed oxygen could be transformed to superoxide ions, which were important active species for the SCR reaction.The analysis of above-mentioned aspects with the influence of NH3 and NO to oxygen vacancies would be the key to the effect of oxygen vacancies on denitrification at low temperature.Further study of the above issues are carried out in this paper.

NO-TPD and NH3-TPD
NO-TPD experiments were carried out to investigate the NO adsorption/desorption active sites and behaviors.Figure 6 showed the NO-TPD results of Ce-Ti-400 and Ce/Ti-400.From Figure 6, the curve of TiO2 did not show any consumption peak of NO.However, CeO2 showed two peaks centered at the tempura range from 200 to 300 °C (Peak I) and from 300 to 400 °C (Peak II).For Ce- Figure 4b shows the XPS spectrum of Ti2p of Ce-Ti-400 and Ce/Ti-400.The main peaks at 455.8-461.4eVand 462.3-467.2eV could be assigned to Ti 4+ [22].Obviously, cloud density of both Ce and Ti increased.Through previous result of Raman, we confirmed the structure of Ce-O-Ti in Ce-Ti-400 sample.Normally, due to the different electronegativity of Ce ion and Ti ion, the density of Ce and Ti cloud density could not increase at the same time in Ce-O-Ti structure.Therefore, combined with the increasing cloud density of both Ce and Ti, we could infer that for fresh catalysts, oxygen vacancies existed in Ce-O-Ti structure.Thus, Ce--Ti possibly existed in Ce-Ti-400 catalyst.
Figure 4c shows the XPS spectrum of O 1s of Ce-Ti-400 and Ce/Ti-400.The two kinds of peaks in these spectra represented two kinds of surface oxygen species.Normally, the peak at 531.0-531.9eVwith lower binding energy could be assigned to lattice oxygen (denoted as O β ), and the peak 529.0-530.0eV with higher binding energy could be assigned to chemisorbed oxygen (denoted as O α ) [23,24].It is worth noting that peaks of Ce-Ti-400 slightly shifted to lower binding energy and this result means O of Ce-Ti-400 also has higher cloud density.Normally, oxygen vacancy was charged oxygen vacancy [25].In addition, combined with the increasing cloud density of both Ce and Ti in this study, it could also be judged that oxygen vacancy was charged oxygen vacancy.Considering the strong electron absorption ability of O and adsorbed oxygen on catalysts, it is possible that electrons on the oxygen vacancy will produce a certain amount of delocalization, which led to the increase of cloud intensity of O α and O β on catalyst.
Figure 5 shows PL spectrum of Ce-Ti-400 and Ce/Ti-400.From previous report, peak 1 at 556 nm was assigned to the surface defect, peak 2 at 633 nm might be attributed to the transfer of excited electrons [26,27].Obviously, the intensity of Ce-Ti-400 was lower than that of Ce/Ti-400.As we have shown, surface defects were electron-trapping centers in catalysts.In addition, a lower PL intensity represented a larger number of surface defects in general [28].Oxygen vacancy was the main surface defect for these two catalysts.That means more oxygen vacancies existed on Ce-Ti-400.From our previous study [22], formation of oxygen vacancies was due to the interaction between doping Ce and Ti.On the one hand, the formation of oxygen vacancies could promote low-temperature SCR activity [29]; on the other hand, oxygen vacancies were the adsorption center of oxygen [30].The absorbed oxygen could be transformed to superoxide ions, which were important active species for the SCR reaction.The analysis of above-mentioned aspects with the influence of NH 3 and NO to oxygen vacancies would be the key to the effect of oxygen vacancies on denitrification at low temperature.Further study of the above issues are carried out in this paper.
and Ti.On the one hand, the formation of oxygen vacancies could promote low-temperature SCR activity [29]; on the other hand, oxygen vacancies were the adsorption center of oxygen [30].The absorbed oxygen could be transformed to superoxide ions, which were important active species for the SCR reaction.The analysis of above-mentioned aspects with the influence of NH3 and NO to oxygen vacancies would be the key to the effect of oxygen vacancies on denitrification at low temperature.Further study of the above issues are carried out in this paper.

NO-TPD and NH3-TPD
NO-TPD experiments were carried out to investigate the NO adsorption/desorption active sites and behaviors.Figure 6 showed the NO-TPD results of Ce-Ti-400 and Ce/Ti-400.From Figure 6, the curve of TiO2 did not show any consumption peak of NO.However, CeO2 showed two peaks centered at the tempura range from 200 to 300 °C (Peak I) and from 300 to 400 °C (Peak II).For Ce-

NO-TPD and NH 3 -TPD
NO-TPD experiments were carried out to investigate the NO adsorption/desorption active sites and behaviors.Figure 6 showed the NO-TPD results of Ce-Ti-400 and Ce/Ti-400.From Figure 6, the curve of TiO 2 did not show any consumption peak of NO.However, CeO 2 showed two peaks centered at the tempura range from 200 to 300 • C (Peak I) and from 300 to 400 • C (Peak II).For Ce-Ti-400 and Ce/Ti-400, Peak I and Peak II were detected at similar temperature range.Also, Ce-Ti-400 and Ce/Ti-400 had similar peak intensity ratio to CeO 2 .It could be inferred that Ce sites might be the main adsorption site to NO for Ce-Ti-400 and Ce/Ti-400.It is worth noting that Peak I and Peak II of Ce-Ti-400 had higher peak intensity than those of Ce/Ti-400.The NO-TPD peaks above 150 • C are due to the decomposition of relatively thermal stable nitrate, such as bidentate nitrate [31].The formation of nitrate species is essential in the process of NH 3 -SCR reaction.It has been confirmed that nitrate species could significantly improve the low-temperature SCR activity [32].Therefore, Ce-Ti-400 catalyst has better oxidation effect on NO and promoted denitrification at low temperature.EDS analysis results (Figures S3 and S4) show that more cerium exists on the surface of Ce/Ti-400 than on Ce-Ti-400.The amount of cerium (Ce sites) on the surface of catalyst is not the reason for the increase of NO oxidation.Ti-400 and Ce/Ti-400, Peak I and Peak II were detected at similar temperature range.Also, Ce-Ti-400 and Ce/Ti-400 had similar peak intensity ratio to CeO2.It could be inferred that Ce sites might be the main adsorption site to NO for Ce-Ti-400 and Ce/Ti-400.It is worth noting that Peak I and Peak II of Ce-Ti-400 had higher peak intensity than those of Ce/Ti-400.The NO-TPD peaks above 150 °C are due to the decomposition of relatively thermal stable nitrate, such as bidentate nitrate [31].The formation of nitrate species is essential in the process of NH3-SCR reaction.It has been confirmed that nitrate species could significantly improve the low-temperature SCR activity [32].Therefore, Ce-Ti-400 catalyst has better oxidation effect on NO and promoted denitrification at low temperature.EDS analysis results (Figures S3 and S4) show that more cerium exists on the surface of Ce/Ti-400 than on Ce-Ti-400.The amount of cerium (Ce sites) on the surface of catalyst is not the reason for the increase of NO oxidation.Figure 7 showed the ammonia desorption results of Ce-Ti-400 and Ce/Ti-400.The different acidic sites, including both Brønsted and Lewis acidic sites, could be investigated by NH3-TPD [33].In the experimental temperature range, for the CeO2 sample, one NH3-TPD peak for weak acidic sites between about 170 to 300 °C was observed.For Ce/Ti-400 catalyst, one NH3-TPD peak for strong acidic sites between about 300 to 500 °C was observed.Both strong and weak peaks could be detected on the NH3-TPD curve of TiO2 sample.Therefore, Ti sites might be the acidic sites in Ce-Ti-400.Li [10] drew the same conclusion that TiO2 plays the role in providing surface Lewis acid sites.One may argue that weak NH3 desorption peak on Ce-Ti-400 was similar to that of CeO2.It is worth noting  In the experimental temperature range, for the CeO 2 sample, one NH 3 -TPD peak for weak acidic sites between about 170 to 300 • C was observed.For Ce/Ti-400 catalyst, one NH 3 -TPD peak for strong acidic sites between about 300 to 500 • C was observed.Both strong and weak peaks could be detected on the NH 3 -TPD curve of TiO 2 sample.Therefore, Ti sites might be the acidic sites in Ce-Ti-400.Li [10] drew the same conclusion that TiO 2 plays the role in providing surface Lewis acid sites.One may argue that weak NH 3 desorption peak on Ce-Ti-400 was similar to that of CeO 2 .It is worth noting that no desorption peak similar to CeO 2 was detected on the curve of Ce/Ti-400.Therefore, Ce sites were not adsorption sites of NH 3 .From calculation of the peak area, a larger area of Ce-Ti-400 means more acidic sites existed in Ce-Ti-400 than Ce/Ti-400.More acidic sites were favorable for the adsorption of NH 3 and considered to improve the SCR reaction [3].

H2-TPR and EPR Analysis
H2-TPR is widely used in evaluating the redox capacity of SCR catalyst.In our experiment, we used 5%O2 + 95%N2 to treat the sample for 2h and detected H2-TPR signal.Figure 8 showed the H2-TPR profiles of Ce-Ti-400 and Ce/Ti-400.TiO2 had no obvious H2 consumption peaks in the curve.Different from the curve of TiO2, CeO2 showed two peaks centered at about 450 °C and 550 °C.Therefore, Ti sites were not the redox center of this reaction.Redox center of the catalyst was related to Ce sites.For Ce-Ti-400, two weak peaks located at 255 °C and 393 °C were likely assigned to the reduction of surface oxygen [34].The third peak located at 614 °C could be assigned to the reduction of Ce 4+ to Ce 3+ [35,36].Considering that chemisorbed oxygen was easily oxidized, the first peak for Ce-Ti-400 might relate to chemisorbed oxygen, and the second peak might relate to surface active lattice oxygen.For Ce/Ti-400, three peaks appeared in a similar position as Ce-Ti-400.In our test, molar weight of Ce for CeO2 was 0.00058mol, for Ce-Ti-400 and Ce/Ti-400 was 0.00012 mol.In other words, Ce species of Ce-Ti-400 and Ce/Ti-400 were four fifths less than that of CeO2.However, the peak area of Ce-Ti-400 and Ce/Ti-400 was larger than that of CeO2.Thus, the interaction of Ce and Ti could significantly improve redox of catalyst.Compared with Ce/Ti-400, the first two peaks of Ce-Ti-400, which represented surface oxygen, was larger than Ce/Ti-400.Combined with the activity of catalysts and discussion above, the first peak showed that the structure of Ce-□-Ti had a stronger ability to adsorb and activate oxygen; the second peak showed that surface active lattice oxygen was more active, i.e., oxygen vacancies were easier to form in Ce-Ti catalyst due to the Ce-O-Ti structure.Oxygen vacancies of SCR catalysts could affect its performance of store and release superoxide ions, which was important for the NO removal [37].In low-temperature denitrification, superoxide ions were quite important because they could oxidize NO and promote low-temperature SCR reaction.Therefore, EPR spectra were used to further research properties of superoxide ions.

H2-TPR and EPR Analysis
H 2 -TPR is widely used in evaluating the redox capacity of SCR catalyst.In our experiment, we used 5%O 2 + 95%N 2 to treat the sample for 2h and detected H 2 -TPR signal.Figure 8 showed the H 2 -TPR profiles of Ce-Ti-400 and Ce/Ti-400.TiO 2 had no obvious H 2 consumption peaks in the curve.Different from the curve of TiO 2 , CeO 2 showed two peaks centered at about 450 • C and 550 • C. Therefore, Ti sites were not the redox center of this reaction.Redox center of the catalyst was related to Ce sites.For Ce-Ti-400, two weak peaks located at 255 • C and 393 • C were likely assigned to the reduction of surface oxygen [34].The third peak located at 614 • C could be assigned to the reduction of Ce 4+ to Ce 3+ [35,36].Considering that chemisorbed oxygen was easily oxidized, the first peak for Ce-Ti-400 might relate to chemisorbed oxygen, and the second peak might relate to surface active lattice oxygen.For Ce/Ti-400, three peaks appeared in a similar position as Ce-Ti-400.In our test, molar weight of Ce for CeO 2 was 0.00058mol, for Ce-Ti-400 and Ce/Ti-400 was 0.00012 mol.In other words, Ce species of Ce-Ti-400 and Ce/Ti-400 were four fifths less than that of CeO 2 .However, the peak area of Ce-Ti-400 and Ce/Ti-400 was larger than that of CeO 2 .Thus, the interaction of Ce and Ti could significantly improve redox of catalyst.Compared with Ce/Ti-400, the first two peaks of Ce-Ti-400, which represented surface oxygen, was larger than Ce/Ti-400.Combined with the activity of catalysts and discussion above, the first peak showed that the structure of Ce--Ti had a stronger ability to adsorb and activate oxygen; the second peak showed that surface active lattice oxygen was more active, i.e., oxygen vacancies were easier to form in Ce-Ti catalyst due to the Ce-O-Ti structure.Oxygen vacancies of SCR catalysts could affect its performance of store and release superoxide ions, which was important for the NO removal [37].In low-temperature denitrification, superoxide ions were quite important because they could oxidize NO and promote low-temperature SCR reaction.Therefore, EPR spectra were used to further research properties of superoxide ions.Figure 9 shows EPR spectra profiles of Ce/Ti-400 and Ce-Ti-400.In our experiment, we used different gases to treat the sample for 2h and detected EPR signal.The signals at g = 2.0036 was attributed to the superoxide ions [38].Compared to Ce-Ti-400 (Figure 9b), EPR intensity of Ce/Ti-400 (Figure 9a) shows little change when atmosphere changed, and peak intensity was much lower.It suggests that very few superoxide ions existed in the sample Ce/Ti-400, and there are obviously more superoxide ions in Ce-Ti-400 catalyst.As confirmed above, the structural differences between Ce-Ti-400 and Ce/Ti-400 are mainly reflected in the Ce-O-Ti (Ce-□-Ti) structure in Ce-Ti-400.Therefore, we could draw the following conclusions: for doped Ce-Ti catalyst, the Ce site was the active site of NO adsorption and Ti site was the active site of NH3 adsorption, and oxygen vacancy was the active site of O2 adsorption in the structure of Ce-□-Ti.There have been a lot of papers about the activation of NO and NH3 [39][40][41].However, regarding how chemisorbed oxygen was activated, most of these papers stayed at the increase of chemisorbed oxygen and superoxide ions.No study regarding the change of activated oxygen in the atmosphere of NH3 and NO on Ce-Ti catalyst has yet been reported.
As shown in Figure 9b, the intensity of superoxide ions decreased as O2 + NO > NO + O2 + NH3 > O2 + NH3 > O2.A small number of superoxide ions were observed when only O2 was injected.It indicated that when only O2 existed, the formation of superoxide ions was limited.For the NH3 + O2 test, the peak of superoxide ions increased clearly.During the process of activation of NH3 adsorption to NH2 in SCR reaction, electron transfer existed [42].In addition, NH3 species were electron-donating groups and able to donate electrons to oxygen vacancies [43,44].In the structure of Ce-□-Ti, oxygen was adsorbed on the oxygen vacancies, which promoted the increase of active site electronegativity.Combined with EPR test results, it could be determined that electrons from NH3 were transferred to O2 finally and superoxide ions were produced.It is worth noting that TiO2 showed little SCR activity and produced little superoxide ions.From the result of NH3-TPD, a considerable amount of adsorption sites of NH3 exist on TiO2.Considering few oxygen vacancies on TiO2 and denitrification activity of Ce-Ti-400, we could infer that oxygen vacancies were important to the activation of NH3.
It also can be seen form Figure 9b that for the Ce-Ti-400 catalyst, the superoxide ion intensity of NO + O2 test was higher than that of NH3 + O2.Therefore, similar to the NH3 electron transfer mechanism, NO also could transfer electron to oxygen vacancies and produce superoxide ions.In addition, NO showed greater ability to transfer electron.It is worth noting that in the gas composition of NO + O2 + NH3, the superoxide ion intensity of Ce-Ti-400 decreased.In other words, only when NH3, NO and O2 co-existed would superoxide ions be consumed.From this phenomenon, the following inferences can be drawn: (1) It has been reported that superoxide ions could oxidize NO in SCR reaction [32] and this process would consume superoxide ions.However, compared to NO + O2 + NH3, superoxide ions signal of NO + O2 was stronger.For low-temperature SCR reaction, NH3 and NO adsorption took the leading role, exactly L-H mechanism [45].Therefore, the reason for when NH3, NO and O2 co-existed the signal of superoxide ions decreased was that the reaction on activated Figure 9 shows EPR spectra profiles of Ce/Ti-400 and Ce-Ti-400.In our experiment, we used different gases to treat the sample for 2h and detected EPR signal.The signals at g = 2.0036 was attributed to the superoxide ions [38].Compared to Ce-Ti-400 (Figure 9b), EPR intensity of Ce/Ti-400 (Figure 9a) shows little change when atmosphere changed, and peak intensity was much lower.It suggests that very few superoxide ions existed in the sample Ce/Ti-400, and there are obviously more superoxide ions in Ce-Ti-400 catalyst.As confirmed above, the structural differences between Ce-Ti-400 and Ce/Ti-400 are mainly reflected in the Ce-O-Ti (Ce--Ti) structure in Ce-Ti-400.Therefore, we could draw the following conclusions: for doped Ce-Ti catalyst, the Ce site was the active site of NO adsorption and Ti site was the active site of NH 3 adsorption, and oxygen vacancy was the active site of O 2 adsorption in the structure of Ce--Ti.There have been a lot of papers about the activation of NO and NH 3 [39][40][41].However, regarding how chemisorbed oxygen was activated, most of these papers stayed at the increase of chemisorbed oxygen and superoxide ions.No study regarding the change of activated oxygen in the atmosphere of NH 3 and NO on Ce-Ti catalyst has yet been reported.
As shown in Figure 9b, the intensity of superoxide ions decreased as O 2 + NO > NO + O 2 + NH 3 > O 2 + NH 3 > O 2 .A small number of superoxide ions were observed when only O 2 was injected.It indicated that when only O 2 existed, the formation of superoxide ions was limited.For the NH 3 + O 2 test, the peak of superoxide ions increased clearly.During the process of activation of NH 3 adsorption to NH 2 in SCR reaction, electron transfer existed [42].In addition, NH 3 species were electron-donating groups and able to donate electrons to oxygen vacancies [43,44].In the structure of Ce--Ti, oxygen was adsorbed on the oxygen vacancies, which promoted the increase of active site electronegativity.Combined with EPR test results, it could be determined that electrons from NH 3 were transferred to O 2 finally and superoxide ions were produced.It is worth noting that TiO 2 showed little SCR activity and produced little superoxide ions.From the result of NH 3 -TPD, a considerable amount of adsorption sites of NH 3 exist on TiO 2 .Considering few oxygen vacancies on TiO 2 and denitrification activity of Ce-Ti-400, we could infer that oxygen vacancies were important to the activation of NH 3 .
NH3 and NO with superoxide ions was much faster, which reflected the consumption of superoxide ions.(2) In the atmosphere of NH3, NO and O2, signal of superoxide ions still stronger than that in the atmosphere of O2.For one thing, the adsorption of NO and NH3 promoted the formation of superoxide ions; for another, the existence of superoxide ions would consume NO and NH3 continually.Therefore, a favorable dynamic balance was formed in this process.

In Situ DRIFT (NH3 Adsorption after NO + O2 Pre-Adsorption)
To further investigate the NO oxidation on the catalyst, In situ DRIFT was used in our experiment.Figure 10 showed the In situ DRIFT spectra of Ce/Ti-400 (a) and Ce-Ti-400 (b).Catalysts were pretreated in the atmosphere of 500 ppm NO + 5% O2 at 240 °C.All curves were obtained by deducting the original catalyst signal.From Figure 10a, the spectra of Ce/Ti-400 showed little change after NO and O2 were injected.From Figure 10b, we could find that a peak at 1621 cm -1 and 1603 cm - 1 appeared immediately after NO and O2 were injected.These peaks might be assigned to nitrate or nitrite.As time went on, the generation of other species was not found and the size of peak at 1621 cm -1 and 1603 cm -1 basically unchanged.Combined with the above experimental results, we could draw the following conclusions: Ce-Ti-400 could oxidize NO fast; the final product of NO was nitrate; the amount of nitrate from oxidation was stable.Therefore, Ce-Ti-400 showed stronger ability to oxidize NO than Ce/Ti-400, which could promote the SCR reaction at low temperature [46].Combined It also can be seen form Figure 9b that for the Ce-Ti-400 catalyst, the superoxide ion intensity of NO + O 2 test was higher than that of NH 3 + O 2 .Therefore, similar to the NH 3 electron transfer mechanism, NO also could transfer electron to oxygen vacancies and produce superoxide ions.In addition, NO showed greater ability to transfer electron.It is worth noting that in the gas composition of NO + O 2 + NH 3 , the superoxide ion intensity of Ce-Ti-400 decreased.In other words, only when NH 3 , NO and O 2 co-existed would superoxide ions be consumed.From this phenomenon, the following inferences can be drawn: (1) It has been reported that superoxide ions could oxidize NO in SCR reaction [32] and this process would consume superoxide ions.However, compared to NO + O 2 + NH 3 , superoxide ions signal of NO + O 2 was stronger.For low-temperature SCR reaction, NH 3 and NO adsorption took the leading role, exactly L-H mechanism [45].Therefore, the reason for when NH 3 , NO and O 2 co-existed the signal of superoxide ions decreased was that the reaction on activated NH 3 and NO with superoxide ions was much faster, which reflected the consumption of superoxide ions.(2) In the atmosphere of NH 3 , NO and O 2 , signal of superoxide ions still stronger than that in the atmosphere of O 2 .For one thing, the adsorption of NO and NH 3 promoted the formation of superoxide ions; for another, the existence of superoxide ions would consume NO and NH 3 continually.Therefore, a favorable dynamic balance was formed in this process.To further investigate the NO oxidation on the catalyst, In situ DRIFT was used in our experiment.Figure 10 showed the In situ DRIFT spectra of Ce/Ti-400 (a) and Ce-Ti-400 (b).Catalysts were pretreated in the atmosphere of 500 ppm NO + 5% O 2 at 240 • C. All curves were obtained by deducting the original catalyst signal.From Figure 10a, the spectra of Ce/Ti-400 showed little change after NO and O 2 were injected.From Figure 10b, we could find that a peak at 1621 cm −1 and 1603 cm −1 appeared immediately after NO and O 2 were injected.These peaks might be assigned to nitrate or nitrite.As time went on, the generation of other species was not found and the size of peak at 1621 cm −1 and 1603 cm −1 basically unchanged.Combined with the above experimental results, we could draw the following conclusions: Ce-Ti-400 could oxidize NO fast; the final product of NO was nitrate; the amount of nitrate from oxidation was stable.Therefore, Ce-Ti-400 showed stronger ability to oxidize NO than Ce/Ti-400, which could promote the SCR reaction at low temperature [46].Combined with the discussion in this paper, we know that the increase of NO oxidation was related to the structure of Ce--Ti.

Synthesis of Doped Catalyst
Doped catalyst was synthesized by a sol-gel method.1.19 g cerous nitrate was added to 20 mL of tetrabutyl titanate under continuous stirring.80 mL ethanolic solution was added to dilute the solution.After stirring for 30 min, mixed solution of 20 mL ethanolic, 8 mL glacial acetic acid and 8 mL water was added in.Then the gel was aged under air for 2 days.After being heated at 120 • C for 6 h, the dried gel was calcined at 400 • C for 3 h.Doped catalyst calcined at different temperature was denoted as Ce-Ti-X (X represents the temperature of calcination).Catalyst without mixing cerium was donated as TiO 2 .

Synthesis of Supported Catalyst
The preparation method of TiO 2 supporter was similar to the above without cerous nitrate.Then 3 g of supporter and 0.757 g cerous nitrate were added into 50 mL water under continuous stirring.After being heated at 60 • C for 4 h and dried at 120 • C for 6 h, the dried gel was calcined at 400 • C for 3 h.The supported catalyst calcined at different temperature was denoted as Ce/Ti-X (X represents the temperature of calcination).

Catalyst Activity Tests
The activity of different catalysts was carried out in a fixed-bed reactor (Quartz tube, inner diameter 5 mm).The temperature range was from 120 • C to 390 • C. 0.2 g catalyst sieving 60-80 mesh under normal pressure was used in this reaction.The reactant gas consisted of 500 ppm NO, 500 ppm NH 3 , 5% O 2 and N 2 as a balance.The total flow rate was 500 mL/min.In addition, all the samples were kept on stream for 1 h to reach steady state at each temperature.The concentration of detected gas (NO, N 2 O and O 2 ) in the inlet and outlet of the fixed-bed was measured by an in situ FT-IR gas analyzer (SERVOPRO 4900, Servomex, Sussex, UK).The NO conversion ratio could be calculated by the following equation: NO conversion (%) = ([NO] in − [NO] out )/NO in × 100%

Characterization
All the catalysts involved in this study were fresh catalysts, i.e., all kinds of catalysts were characterized immediately after calcination (catalysts for EPR were pretreatment at specific gas composition).The specific characterization was as follows: XRD patterns were collected at XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd., (Beijing, China).Raman spectra were recorded with a Raman Spectrometer (Perkin-Elmer400F, PerkinElmer, Waltham, MA, USA).In situ DRIFTS experiments were carried out by a Nicolet IZ10 FTIR spectrometer (ThermoFisher, Waltham, MA, USA).XPS experiments were performed on a PHI-5000C ESCA system (PerkinElmer, Waltham, MA, USA) with Mg Kα radiation.Photoluminescence spectra (PL) were carried out on a Labram-HR800-type spectrophotometer (Jobin Yvon Co., Palaiseau, France).The EPR measurements were made with a Bruker EMX-10/12-type spectrometer in the X-band.Ammonia temperature-programmed desorption (NH 3 -TPD), Nitric oxide temperature-programmed desorption (NO-TPD) and hydrogen temperature-programmed reduction (H 2 -TPR) experiments were measured by a chemisorption analyzer (Quanta Chrome Instruments, Boynton Beach, FL, USA).Catalysts were pre-heated at 120 • C for 12 h before tests.About 0.3 g of catalyst was used.Experiments were carried out under a flow of He (70 mL•min −1 ) for 1h at 400 • C. Subsequently, TPD was performed by ramping the temperature at 10 • C•min −1 in He (60 mL•min −1 ).Then the NH 3 and NO adsorption was conducted under a flow rate of 70 mL•min −1 at room temperature.In the H 2 -TPR experimental,

Figure 2
Figure 2 displays the XRD patterns of catalysts prepared by different methods.As shown in Figure 2, Ce-Ti-400 and Ce-Ti-500 catalysts only showed the anatase type of TiO 2 (2θ = 25.2 • , 37.7 • , 47.8 • , 53.8 • , 55.0 • ).No representative peaks of Ce species were found.The doped catalyst (Ce-Ti-400) had lower XRD peak intensity than that of supported catalyst (Ce/Ti-400).According to a previous study on Ce-Ti catalyst[15], the anatase peak intensity of catalysts decreased with the increase in Ce doping level, suggesting that Ce interacted with Ti.For Ce-Ti-500, only anatase was detected.

Figure 2 .
Figure 2. X-ray diffraction (XRD) patterns of catalysts prepared by different methods.

Figure 2 .
Figure 2. X-ray diffraction (XRD) patterns of catalysts prepared by different methods.

Figure 2 .
Figure 2. X-ray diffraction (XRD) patterns of catalysts prepared by different methods.

Figure 7
Figure7showed the ammonia desorption results of Ce-Ti-400 and Ce/Ti-400.The different acidic sites, including both Brønsted and Lewis acidic sites, could be investigated by NH 3 -TPD[33].

1 .
Synthesis of Doped CatalystDoped catalyst was synthesized by a sol-gel method.1.19 g cerous nitrate was added to 20 mL