Enhanced Activity of Nb-modified CeO 2 / TiO 2 Catalyst for the Selective Catalytic Reduction of NO with NH 3

A series of CeO2-Nb2O5/TiO2 catalysts were prepared by an impregnation method and investigated for the selective catalytic reaction (SCR) of NO with NH3. The 15 wt.% CeO2–10 wt.% Nb2O5/TiO2 catalyst (CeNbTi) exhibited the highest activity and resistance to a high gas hourly space velocity and K2O. In the presence of H2O and SO2, it also showed better activity than the CeTi catalyst. The addition of Nb could improve the dispersion of CeO2 and increase the amount of Ce and chemisorbed oxygen species on the catalyst surface, which enhances the catalytic activity of CeTi. The superior SCR activity of CeNbTi might also be attributed to its high redox ability, the enhanced adsorption capacity of NH3 species and the synergistic action between Ce, Nb and Ti species.


INTRODUCTION
Selective catalytic reduction (SCR) of NO x with NH 3 has been a dominant technique to remove NO x in the exhaust gas from stationary and mobile sources (Nakajima and Hamada, 1996;Busca et al., 1998;Du et al., 2018).V 2 O 5 /TiO 2 -based materials are widely adopted because of their high activity and good resistance to SO 2 (Busca et al., 1998;Fu et al., 2014).However, this type of materials is effective only within a narrow temperature range of 300-400°C.Quite a few unavoidable disadvantages still exist, such as the toxicity of vanadium, high conversion of SO 2 to SO 3 with increasing vanadium loadings (Dunn et al., 1998) and low N 2 selectivity at high temperatures (Yates et al., 1996).Consequently, there has been a great interest to modify current catalysts and investigate novel catalysts to substitute vanadium with other transition metals (Mn, Fe, Cu, etc.) (Li et al., 2017) to overcome the above-mentioned disadvantages.

Catalyst Preparation
The CeO 2 -Nb 2 O 5 /TiO 2 catalysts were prepared by an impregnation method.A desired amount of niobium oxalate (C 10 H 5 NbO2 0 ), cerium nitrate (Ce(NO 3 ) 3 •6H2O) and citric acid was dissolved into deionized water, in which the molar ratio of precursor salts/citric acid was 2:1.Next, the solution was mixed with TiO 2 .After exposed to ultrasonic energy for 30 min, the mixture was continuously stirred and heated in water bath at 70°C.At last, the resulted solids were dried at 110°C overnight, followed by calcination at 500°C for 5 h in air.The catalysts were denoted as Ce x Nb y Ti, where x and y represented the mass percentage of CeO 2 and Nb 2 O 5 to TiO 2 , respectively.
The K 2 O-poisoned samples were prepared by impregnating Ce x Ti or Ce x Nb y Ti catalyst with aqueous solution of potassium nitrate.The mixture was evaporated to dryness at 70°C on a water bath, dried at 110°C overnight, and calcined at 500°C in air for 5 h.In this work, the molar ratio of K/Ce was set as 0.1.

Activity Measurement
The SCR activity tests of the catalysts were carried out in a fixed-bed quartz flow reactor (i.d.= 8 mm; L = 600 mm) using 0.19 g catalysts with 60-100 mesh in the temperature range of 150-500°C.The feed gas mixture contained 1000 ppm NO, 1000 ppm NH 3 , 3 vol.%O 2 , 10 vol.%H 2 O (when used), 200 ppm SO 2 (when used) and N 2 as balance gas.Water vapor was generated through the injection of H 2 O in a heated pipe.Under ambient conditions, the total flow rate was 500 mL min -1 and the gas hourly space velocity (GHSV) was 90,000 h -1 .The concentrations of NO, SO 2 and O 2 in the feed gas was tested by a gas analyzer (350 Pro, Testo).The concentrations of NO 2 and N 2 O were recorded by an FT-IR gas analyzer (DX-4000, Gasmet).

Catalysts Characterization
The BET surface area of the catalyst samples was measured by N 2 adsorption/desorption analysis at 77 K with ASAP2020-M (Micromeritics Instrument Corp.).Prior to the surface area measurement, the sample was degassed in vacuum at 300°C for 4 h.
Powder X-ray diffraction (XRD) measurements were conducted on a X'Pert PRO diffractometer (Panalytical Corp.) with Cu Ka radiation at 40 kV and 40 mA.
X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250 spectrometer using Al Ka X-rays (hv = 1486.6eV) as a radiation source at 150 W. Binding energies of Ce 3d and O 1s were calibrated using carbon deposit C 1s peak (BE = 284.8eV).
Microstructures of the catalyst samples were observed with a JOEL JEM-2100F Electron Microscope.
The H 2 temperature programmed reduction (H 2 -TPR) and NH 3 temperature programmed desorption (NH 3 -TPD) were performed on FINESORB-3010 chemisorption analyzer (FINETEC Instruments Corp.) with 0.1 g of the catalysts with a thermal conductivity detector (TCD).TPR runs were carried out in a flow of H 2 (10%) in Ar (30 mL min -1 ) from room temperature to 800°C, with a heating rate of 10°C min -1 .For NH 3 -TPD, the sample was pretreated at 500°C in He for 1 h, then it was cooled down and exposed in a 0.5% NH 3 /He (30 mL min -1 ) gas flow for 1 h, followed by flushing with He for 1 h, the sample was then heated up to 700°C with a rate of 10°C min -1 in flowing He.

NH 3 -SCR Performance Effect of CeO 2 and Nb 2 O 5 Loadings
To find the optimal Ce loading, Nb 2 O 5 loading was kept at 10 wt.% TiO 2 and the NO conversion as a function of temperature was compared over various catalysts under different GHSV in Fig. 1.At the GHSV of 90,000 h -1 , Nb 10 Ti exhibited negligible activity in the temperature range of 150-350°C.Afterward, its NO conversion increased with increasing reaction temperature and the maximum NO conversion (92.0%) was obtained at 500°C.After the addition of Ce, NO conversion increased sharply over Nb 10 Ti.Higher Ce loading enhanced SCR activity and widened the temperature window until the mass percentage of CeO 2 /TiO 2 reached 15.Further increasing Ce loading caused the slight decrease in SCR activity below 275°C.In a wide temperature range 250-450°C, 93.9-98.8%NO conversion was obtained at the GHSV of 90,000 h -1 over Ce 15 Nb 10 Ti.As shown in Fig. 1 decreased at 250-400°C.These indicated that Ce 15 Nb 10 Ti catalyst was highly effective for NO reduction at a high GHSV of 200,000 h -1 .The effect of Nb 2 O 5 loading on the NO conversion of Ce-Ti oxide at GHSV of 90,000 h -1 and 200,000 h -1 is exhibited in Fig. 2. As shown in Fig. 2(a), different Nb loadings led to the increase in the catalytic activity of Ce 15 Ti to different extents.When Nb 2 O 5 loading was lower than 20%, the NO conversion was not nearly influenced by Nb 2 O 5 loading and remained nearly 100% in the temperature range of 275-450°C.Ce 15 Nb 10 Ti presented the best catalytic activity and the NO conversion reached up to 98.1% at about 275°C.Further increase in Nb loading resulted in a conspicuous decline in catalytic activity.When the GHSV was increased to 200,000 h -1 , except Ce 15 Nb 10 Ti, the NO conversions of the other samples dramatically decreased at 250-450°C.Ce 15 Nb 10 Ti still maintained high catalytic activity (Fig. 2(b)).

The Influence of H 2 O and SO 2
The effect of H 2 O on the SCR activities of Ce 15 Ti and Ce 15 Nb 10 Ti was investigated and the results are shown in Fig. 5.The presence of H 2 O led to the decrease in the NO conversions over both samples.However, Ce 15 Nb 10 Ti exhibited higher SCR activities than Ce 15 Ti.In the temperature range of 350-450°C, the NO conversion of Ce 15 Nb 10 Ti decreased slightly and still maintained about 90%.It should be noted that H 2 O had a positive impact on the activities of both catalysts at 500°C.The similar phenomena were observed over other Ce-Ti oxide catalysts (Gao et al., 2010a;Jiang et al., 2015b).The negative effect of H 2 O might be ascribed to the competitive adsorption between H 2 O and reactants or the positioning of oxygen vacancies (Gao et al., 2010a).The positive effect of H 2 O at 500°C was possibly due to the inhibition of NH 3 oxidation (Gao et al., 2010a).
Fig. 6 presents the effect of SO 2 on the SCR activities of Ce 15 Ti and Ce 15 Nb 10 Ti.Before SO 2 was added to the feed gas, the NO conversions over Ce 15 Ti and Ce 15 Nb 10 Ti were close to 100% at 350°C.In the presence of SO 2 , the NO conversion over Ce 15 Ti gradually decreased with time to 42.1% after 11 h.As for Ce 15 Nb 10 Ti, although its NO conversion declined, 52.6% NO conversion was still obtained after 11 h.It could be seen that Ce 15 Nb 10 Ti exhibited higher SO 2 resistance than Ce 15 Ti.The deactivation by SO 2 of Ce 15 Ti and Ce 15 Nb 10 Ti might be attributed to the generation of sulfate species, including NH 4 HSO 4 , Ce(SO 4 ) 2 and Ce 2 (SO 4 ) 3 (Xu et al., 2009;Wang et al., 2012;Guo et al., 2017).After SO 2 was cut off from the feed gas, a little activity was recovered over both catalysts.It meant that NH 4 HSO 4 might volatilize or decompose (Xu et al., 2009), thereby resulting in regaining their partial activities after the removal of SO 2 .The deactivation by SO 2 of Ce 15 Ti and Ce 15 Nb 10 Ti primarily originated from the formation of Ce(SO 4 ) 2 and Ce 2 (SO 4 ) 3 with high thermal stability.They could disrupt the Ce 4+ /Ce 3+ redox cycle and inhibit the formation and adsorption of surface nitrate species (Xu  et al., 2009).The addition of Nb might play a certain role in hindering the formation of Ce(SO 4 ) 2 and Ce 2 (SO 4 ) 3 , thereby improving the resistance to SO 2 of Ce 15 Ti.

Characterization of Catalysts BET and XRD Results
The BET surface area and pore characterization of TiO 2 , Ce 15 Ti, Nb 10 Ti and Ce 15 Nb 10 Ti are listed in Table 1.The surface area increased in the following sequence: Ce 15 Nb 10 Ti < Nb 10 Ti < Ce 15 Ti < TiO 2 .The addition of Nb led to the decrease of the surface area of Ce 15 Ti and TiO 2 and the increase of their average pore diameters.However, Ce 15 Nb 10 Ti possessed best SCR activity.It implied that the BET surface area was not a key factor to achieve excellent SCR activity for Ce 15 Nb 10 Ti.
The XRD patterns of different samples are shown in Fig. 7.Only anatase TiO 2 was detected over TiO 2 and Nb 10 Ti.
As for Ce 15 Ti and Ce 15 Nb 10 Ti, besides anatase TiO 2 , the diffraction peaks of cubic CeO 2 were also found.No visible phase of Nb species was observed on Nb 10 Ti and Ce 15 Nb 10 Ti, which suggested that Nb species are highly dispersed on the catalyst surface.The addition of Nb seemed to induce the slight decrease in the intensity of the characteristic peaks of anatase TiO 2 and cubic CeO 2 .This meant that niobium oxide could lower the crystallinity of CeO 2 and thus enhanced the dispersion of CeO 2 on the catalyst surface.In comparison with XRD and activity results, it could be speculated that cubic CeO 2 could be more susceptible to K 2 O than highly dispersed CeO 2 .Therefore, Ce 15 Nb 10 Ti showed better activity than Ce 15 Ti in the presence of K 2 O.

XPS Results
To identify the chemical state of surface species, Ce 15 Ti and Ce 15 Nb 10 Ti were analyzed by XPS.The photoelectron   8(a), for Ce 15 Ti, the binding energies of two peaks were 464.5 and 458.8 eV, respectively.This indicated that Ti existed in Ti 4+ over Ce 15 Ti (Du et al., 2012;Jiang et al., 2014).After the addition of Nb, the binding energies moved to higher values and increased to 464.8 and 459.1 eV.Du et al. (2012) found the similar phenomena over V-Sb-Nb/TiO 2 catalyst.Referring to the handbook of XPS (Wagner et al., 1979), Ti still existed in the form of Ti 4+ after the addition of Nb.Furthermore, it could be also seen that the intensity of Ti 2p peaks of Ce 15 Nb 10 Ti was obviously lower than those of Ce 15 Ti and Nb 10 Ti.This indicated that there was the strong interaction among Ce, Nb and Ti species.
As exhibited in Fig. 8(b), the Ce 3d XPS peaks denoted as u, uʺ, u‴ and v, vʺ, v‴ could be attributed to Ce 4+ , while uʹ and vʹ could be assigned to Ce 3+ (Burroughs et al., 1976;Jiang et al., 2015a, b).It could be seen that the intensities of Ce 3+ characteristic peaks increased after the doping of Nb, accompanied with a decline in the intensities of Ce 4+ characteristic peaks.Similar phenomena were also observed on Ce-Nb oxide catalyst (Qu et al., 2013).The Ce 3+ ratio of Ce 15 Nb 10 Ti (41.3%), calculated by Ce 3+ /(Ce 3+ + Ce 4+ ), was obviously higher than of Ce 15 Ti (36.5%).The increase in the amount of Ce 3+ resulted from the fact that the interaction between Nb and Ce species could promote the reduction of Ce 4+ to Ce 3+ .Higher Ce 3+ ratio is indicative of more oxygen vacancies, which were beneficial to adsorb and activate reactant species (Liu et al., 2013;Geng et al., 2017;Jiang et al., 2017).
The O 1s XPS spectra of Ce 15 Ti and Ce 15 Nb 10 Ti could be fitted into three overlapping peaks (see Fig. 8(c)).They were referred to the lattice oxygen at 528.5-529.0eV (denoted by Oβ), the chemisorbed oxygen at 528.7-529.5 eV (denoted by Oα) from oxide defects or hydroxyl-like groups, and the surface oxygen by hydroxyl species and/or adsorbed water species at 529.0-530.5 eV (denoted by Oγ) (Dupin et al., 2000;Eom et al., 2008;Liu et al., 2017).The relative ratio of Oα calculated by Oα/(Oα + Oβ + Oγ) on Ce 15 Nb 10 Ti (35.1%) was much higher than that on Ce 15 Ti (30.5%).It suggested that the amount of chemisorbed oxygen increased after the introduction of niobium.High Oα ratio is considered to be beneficial to the NO oxidation to NO 2 in the SCR reaction and thereafter facilitate the "fast SCR" reaction (Kang et al., 2007;Wu et al., 2008).These XPS results confirmed that the increase of Ce 3+ was accompanied by the increment in oxygen vacancies and active oxygen species, which played a positive role in the SCR activity of Ce 15 Nb 10 Ti.

TEM Results
The HR-TEM micrographs of different catalysts are displayed in Fig. 9.The microscope gave a clear view on the morphology and crystal structure of Ce 15 Ti and Ce 15 Nb 10 Ti.It could be seen that the two samples stayed in the form of oval-shaped anatase crystal particles.No fringes attributed to Nb species were observed over the two samples.It meant that Nb species dispersed well on support TiO 2 for the two samples.For Ce 15 Ti, there were three kinds of lattice fringes.0.286 nm, 0.332 nm and 0.357 nm of lattice fringes matched CeO 2 (200) phase (Corrêa et al., 2011), rutile (110) phase (Zhou et al., 2017) and anatase (101) phase (Sutradhar et al., 2014), respectively.It should be noted that rutile phase was not determined in XRD.The reason might be that rutile TiO 2 was well dispersed and existed as an amorphous or highly dispersed species in Ce 15 Ti.The detected rutile phase indicated little sintering of anatase TiO 2 in Ce 15 Ti.However, only 0.283 nm (matching CeO 2 (200) phase; Corrêa et al., 2011) and 0.358 nm (matching anatase (101) phase; Sutradhar et al., 2014) of lattice fringes could be observed on the Ce 15 Nb 10 Ti surface.This showed that the addition of Nb could inhibit the sintering of anatase TiO 2 .This was beneficial to increase the dispersion of CeO 2 on anatase TiO 2 , allowing to improve the catalytic activity.
H 2 -TPR Analysis Fig. 10 illustrates the H 2 -TPR profiles of Ce 15 Ti and Ce 15 Nb 10 Ti to estimate the reducibility of two catalysts.The onset reduction temperature of the peaks of two catalysts was 315°C.There was one broad peak at 550°C on the H 2 -TPR profile of Ce 15 Ti in the test temperature range, which was similar to that of CeO 2 /TiO 2 catalyst (Chen et al., 2012).This peak could be ascribed to the reduction of surface oxygen species of ceria (Chen et al., 2012;Ma et al., 2012).As for Ce 15 Nb10Ti, it was obvious that a new reduction peak appeared at about 520°C.This might originate from the difference in the type and strength of the interaction among Ce, Ti and Nb species.The lower the temperature of reduction peak was, the more easily the catalyst was reduced (Ma et al., 2015).In addition, it was found that the amount of H 2 consumption over Ce 15 Ti was less than that over Ce 15 Nb 10 Ti (1:2.27).It implied more surface oxygen species on Ce 15 Nb 10 Ti, which was in accordance with the XPS results.These could the reason why the Ce x Nb y Ti catalysts exhibited excellent activity than Ce 15 Ti.NH 3 -TPD Analysis NH 3 -TPD was performed to characterize the surface acidity of the catalysts and the results are seen in Fig. 11.Both Ce 15 Ti and Ce 15 Nb 10 Ti possessed two desorption peaks in the temperature range of about 100-400°C and 450-675°C.The areas of both NH 3 desorption peaks over Ce 15 Nb 10 Ti were obviously larger than those over Ce 15 Ti, especially that of low-temperature desorption peak.The low-temperature and high-temperature peaks belonged to the weakly and strongly bonded NH 3 , respectively (Cai et al., 2014).It was reported that the NH 3 bound to Brønsted acid sites are less thermally stable than the coordinated NH 3 molecules bound to Lewis acid sites and could desorb at lower temperatures (Zhao et al., 2016).It meant that the introduction of Nb could increase the amount of Brønsted and Lewis acid sites.Furthermore, the amount of Brønsted acid sites increased more dramatically.The increase in the Brønsted and Lewis acidity helped to enhance the adsorption capacity of NH 3 species on the catalyst, thereby favoring the SCR reaction.

CONCLUSIONS
Ce 15 Nb 10 Ti possessed excellent catalytic activity even at a high GHSV of 200,000 h -1 in the temperature range of 275-450°C.It also exhibited a higher resistance than Ce 15 Ti to K 2 O, H 2 O and SO 2 .The Ce-Ti oxides with and without Nb were characterized using BET, XRD, XPS, TEM, H 2 -TPR and NH 3 -TPD.The results indicate that the introduction of Nb could improve the dispersion of Ce species on the TiO 2 support.Compared with cubic CeO 2 , highly dispersed CeO 2 was likely to have a higher resistance to K. XPS results reveal that the interaction between Nb and Ce species could promote the transformation of Ce 4+ into Ce 3+ .As a result, the amount of Ce 3+ increased, accompanied by an increase in oxygen vacancies and active oxygen species, which play a positive role in the SCR reaction over Ce-Ti oxide.H 2 -TPR results demonstrate the enhanced reducibility of the Ce-Ti oxide after being doped with Nb.NH 3 -TPD results reveal the increase in surface acidity, especially due to the increase in the number of Brønsted acid sites, leads to a marked increase in the amount of NH 3 absorbed on the surface of Ce 15 Nb 10 Ti.
Based on these results, the excellent SCR performance of Ce 15 Nb 10 Ti could be attributed to the high dispersion of CeO 2 and Nb 2 O 5 on the TiO 2 surface, the increased amount of Ce 3+ and surface chemisorbed oxygen, the improvement in surface acidity and reducibility, and the synergetic interaction among Ce, Nb and Ti species.
Fig. 3 compares the N 2 O concentration over Ce 15 Ti and Ce 15 Nb 10 Ti.The yield of N 2 O increased with increasing reaction temperatures over the two samples.Their N 2 O concentrations were very close below 300°C.When the reaction temperature was higher than 300°C, the introduction of Nb could inhibit the generation of N 2 O distinctly.The N 2 O concentration over Ce 15 Nb 10 Ti was only about 10 ppm at 450°C.It indicated that Nb could improve the N 2 selectivity of Ce 15 Ti especially at high temperatures.The Influence of K 2 O Fig. 4 displays the influence of K 2 O on the catalytic activities of Ce 15 Ti and Ce 15 Nb 10 Ti.K 2 O could inhibit the SCR activity of Ce 15 Ti remarkably and its NO conversion decreased from 95.9% to 43.4% at 350°C due to K 2 O doping.Although the NO conversions also dropped over Ce 15 Nb 10 Ti in the presence of K 2 O, 89.3-98.0%NO conversion was still obtained at 275-450°C.It was clear that the addition of Nb could significantly enhance its resistance to K 2 O of Ce-Ti oxide.

Table 1 .
Physical properties of various samples.