NO reduction by CO over CuO supported on CeO2-doped TiO2: the effect of the amount of a few CeO2.

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Cite this: DOI: 10.1039/c5cp00745c

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NO reduction by CO over CuO supported on CeO2-doped TiO2: the effect of the amount of a few CeO2 Changshun Deng,a Bin Li,*a Lihui Dong,*a Feiyue Zhang,a Minguang Fan,a Guangzhou Jin,b Junbin Gao,b Liwen Gao,a Fei Zhanga and Xinpeng Zhoua This work is mainly focused on the investigation of the influence of the amount of a few CeO2 on the physicochemical and catalytic properties of CeO2-doped TiO2 catalysts for NO reduction by a CO model reaction. The obtained samples were characterized by means of XRD, N2-physisorption (BET), LRS, UV-vis DRS, XPS, (O2, CO, and NO)-TPD, H2-TPR, in situ FT-IR, and a NO + CO model reaction. These results indicate that a small quantity of CeO2 doping into the TiO2 support will cause an obvious change in the properties of the catalyst and the TC-60 : 1 (the TiO2/CeO2 molar ratio is 60 : 1) support exhibits the most extent of lattice expansion, which indicates that the band lengths of Ce–O–Ti are longer than other TC (the solid solution of TiO2 and CeO2) samples, probably contributing to larger structural distortion and disorder, more defects and oxygen vacancies. Copper oxide species supported on TC supports are much easier to be reduced than those supported on the pure TiO2 and CeO2 surface-modified TiO2 supports. Furthermore, the Cu/TC-60 : 1 catalyst shows the highest activity and selectivity due to more oxygen vacancies, higher mobility of surface and lattice oxygen at lower temperature (which contributes to the regeneration of oxygen vacancies, and the best reducing ability), the most content of Cu+, and the strongest synergistic effect between Ti3+, Ce3+ and Cu+. On the other

Received 5th February 2015, Accepted 19th May 2015

hand, the CeO2 doping into TiO2 promotes the formation of a Cu+/Cu0 redox cycle at high tempera-

DOI: 10.1039/c5cp00745c

tures, which has a crucial effect on N2O reduction. Finally, in order to further understand the nature of the catalytic performances of these samples, taking the Cu/TC-60 : 1 catalyst as an example, a possible

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reaction mechanism is tentatively proposed.

1. Introduction It is known from modern catalytic surface science that the nature and quality of a carrier material is a key part of the catalyst.1 Many researchers have proved that the use of TiO2related catalysts in the selective reduction of NO overcomes the problem of deactivation arising from sulfate formation in the SOx-containing environment and the anatase TiO2 is widely used as the support for the NO reduction reactions.2 However, anatase TiO2 suffers from limited surface area, low abrasion resistance and poor thermal stability.3 Thus, attempting to obtain a titania based mixed oxide support, in order to improve the surface area, mechanical strength, and the thermal stability,

a

Guangxi Key Laboratory Petrochemical Rescource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China. E-mail: [email protected], [email protected] b Department of Chemical Engineering, Beijing Institute of Petro-Chemical Technology, Beijing 102617, China

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has been the object of some very recent investigations.4–7 In addition, active components dispersed on mixed metal oxides often produce superior activity to those supported on single oxide for a number of reactions.8–11 In recent years, TiO2–CeO2 mixed oxides and cerium ion (Ce3+, Ce4+) doped TiO2 have been intensively examined because of their high application potential as electrode materials in electrochromic devices,12 sensing film gas sensors,13 coatings for self-cleaning surfaces,14 photocatalysts,15 and supports of catalysts.16,17 Cerium oxide has attracted much attention due to the optical and catalytic properties associated with the redox pair Ce3+/Ce4+.18 Cerium oxide has the ability to store and release oxygen, and it provokes oxygen vacancies associated with two neighboring Ce3+/Ce4+.19 Cerium oxide and cerium-based oxide in different morphologies have different effects on catalytic activities.20,21 Kim et al.16 prepared the Pd–Cu/TiO2–CeO2 catalysts using the co-precipitation method for the reduction of nitrate ions in water and found that Pd–Cu/TiO2–CeO2 (Ti : Ce = 18 : 1) showed the best activity. Lee et al.17 synthesized a series of MnOx/TiO2–CeO2 catalysts with differing CeO2/TiO2 ratios using a

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wet impregnation method and found that the MnOx(20 wt%)/ CeO2(4 wt%)–TiO2 catalyst exhibited the highest catalytic activity for low-temperature selective catalytic reduction (SCR) of NO with NH3. Romero-Galarza et al.22 prepared anatase-phase TiO2 and Ce-doped TiO2 catalysts using the sol–gel method and found that the Ce-doped Ni/TiO2 catalyst with a Ce/(Ce + Ti) molar ratio of 0.1 was found to be the most active for CO oxidation by NO. Based on the above-mentioned literatures, we found that: (i) few studies are performed using CeO2-doped anatase TiO2 as a support to investigate the influence of doping of a few ceriums into TiO2 on the surface properties and the active species of the catalyst for NO reduction by CO; (ii) the studies are only focused on the molar ratio of Ti/Ce which is less than 20, and no research has been performed to investigate into the properties when the ratio of Ti/Ce is more than 20. In addition, when the addition amount of CeO2 is few, the mixed oxides will keep the anatase-type, i.e., a bit addition of CeO2 is enough to prevent phase transformations from anatase to rutile at high temperature, promote the thermal stability of the catalysts,23 and improve the surface area. Therefore, the CeO2-doped anatase TiO2 should be an essential support to replace the pure anatase TiO2 for many catalytic reactions. Furthermore, a small amount of CeO2 doping may cause an obvious change in the characteristics of the materials. The catalysts containing transition metals, especially copper, show a potential application for the abatement of exhaust gas from stationary and mobile emission sources,24 and complete oxidation of volatile organic compounds.25 Special attention has been paid to this system as a substitute for noble metal containing catalysts. Zou et al.26 found that the strong interaction between the dispersed Cu species and the support Ce1xTixO2 made the catalysts possess much higher oxidation activity and thermal stability when it was used for low-temperature CO oxidation. Si et al.27 found that CuOx impregnated on the co-precipitated WO3– ZrO2 support showed high SCR activity in the range 200–320 1C. Yao et al.28 studied the effect of preparation methods for NO reduction by CO over CuO–CeO2 catalysts and found that the synergistic effect between Cu+ species and surface oxygen vacancies of these CuO–CeO2 catalysts played an important role in this model reaction. For one thing, the catalytic properties of the active copper phase can be greatly influenced by the dispersion behavior. When the active component is dispersed on a support, it presents as a molecular-monolayer at low loadings. When the loading is higher than a whole monolayer, i.e., fully covering the surface of the support, the dispersed component forms a close-packed monolayer and the excess component forms a crystalline phase. For another, the catalytic performance of the copper oxide species can also be greatly influenced by the nature of the supported oxide. For example, in methanol synthesis, it is suggested that the active component is not only Cu+ but also Cu0 and the support plays a major role in controlling the Cu+/Cu0 ratio, which further influences the catalytic activity.29 In the present work, a series of CeO2-doped TiO2 supported by CuO with different Ti/Ce molar ratios which were 10 : 1, 20 : 1, 40 : 1, 60 : 1, 80 : 1, and anatase TiO2 were prepared using the co-precipitation method, respectively. And then, the obtained samples were characterized by means of XRD, N2-physisorption


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(BET), LRS, UV-vis DRS, XPS, (O2, CO, and NO)-TPD, H2-TPR, in situ FT-IR, and a NO + CO model reaction. This study is mainly focused on: (i) exploring the influence of the CeO2 with different ratio doping into anatase TiO2 on the texture, structure, redox property, surface state, and activity of NO reduction by CO over CuO supported on CeO2-doped TiO2 catalysts; (ii) investigating the interaction of CO and/or NO with CuO supported on CeO2doped TiO2 catalysts via an in situ FT-IR technique to understand the nature of a NO + CO model reaction.

2. Experimental section 2.1.

Catalyst preparation

CeO2-doped TiO2 supports with different Ti/Ce molar ratios, which were 10 : 1, 20 : 1, 40 : 1, 60 : 1, 80 : 1, were prepared using the co-precipitation method. A requisite amount of TiCl4 and Ce(NO3)36H2O was dissolved in the water containing a certain amount of hydrochloric acid, then the mixture solution was slowly dropped to the excess of ammonia solution with vigorous stirring until pH = 10.0. The resulting solution was kept stirring for 3 h and aged for 24 h, then filtered and washed until no Cl could be detected. The obtained solid was dried at 110 1C overnight and then calcined in a muffle stove at 500 1C in flowing air for 5 h. The powders were denoted TC-10 : 1, TC-20 : 1, TC-40 : 1, TC-60 : 1, TC-80 : 1, respectively. Pure TiO2 was prepared following the same method. The CuO/TC catalysts were prepared by incipient-wetness impregnating on the support with the Cu(Ac)2 solution. The mixture was kept under vigorous stirring for 3 h, and then evaporated at 80 1C. The resulting materials were dried at 110 1C overnight and calcined at 450 1C in flowing air for 4 h. These catalysts were denoted Cu/TC-10 : 1, Cu/TC-20 : 1, Cu/TC-40 : 1, Cu/TC-60 : 1, Cu/TC-80 : 1, respectively. The CuO/TiO2 catalyst was also prepared with the same method. For all of these samples, the CuO loading in the catalyst was expressed as the weight ratio of CuO/(TC) or CuO/ TiO2 (which was corresponding to Cu/TiO2) and fixed at 12 wt% of the support. In addition, the CuO/CeO2(1)/TiO2(60) (which was corresponding to Cu/Ce(1)/TiO2(60)) catalyst, where 1 and 60 are the corresponding molar ratio of Ce and Ti, was prepared by stepwise impregnation (first CeO2 and then CuO) under the same condition for comparison, where the precursors of CuO and CeO2 are Cu(Ac)2 and Ce(NO3)36H2O, respectively. 2.2.

Catalyst characterization

2.2.1. X-ray diffraction (XRD) measurement. X-ray diffraction (XRD) patterns were obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu Ka (l = 0.15418 nm) radiation. The 2y scans cover the range 10–801 with a scan rate of 81 min1, and the accelerating voltage and applied current are 40 kV and 40 mA, respectively. 2.2.2. N2-physisorption (BET) measurement. Textural characteristics of these samples were obtained by nitrogen adsorption at 77 K on a Micrometrics TriStar II 3020 analyzer, using the Brunauer– Emmet–Teller (BET) method for the specific surface area and the Barrett–Joyner–Halenda (BJH) method for the pore distribution.


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Prior to each analysis, approximately 0.1 g of the catalyst sample was degassed in a N2–He mixture at 300 1C for 2 h. 2.2.3. Laser Raman Spectra (LRS) measurement. Laser Raman spectra (LRS) measurement was conducted on a Renishaw RM1000 spectroscope with a laser light wavelength of 514 nm. 2.2.4. UV-vis diffuse reflectance spectroscopy (UV-vis DRS) measurement. UV-vis DRS spectra were recorded on a TU-1901 spectrophotometer (Shimadzu) in the wavelength range of 200–800 nm and using BaSO4 as a reference material. 2.2.5. X-ray photoelectron spectroscopy (XPS) measurement. XPS analysis was performed on a Thermo ESCALAB 250 high performance electron spectrometer, using Al Ka radiation (1486.6 eV) operating at an accelerating power of 150 W. All binding energies (BE) were referenced to the adventitious C 1s at 284.6 eV. This reference gave BE values with an accuracy of 0.1 eV. Charge effects were corrected by adjusting the Ce 3d3/2 peak (u 0 0 0 ) to a position of 917.0 eV. 2.2.6. O2, CO, and NO-temperature programmed desorption (TPD) measurement. TPD was carried out on an automated chemisoption analyzer (Finetec Instruments). First, 200 mg (O2-TPD) or 100 mg (CO and NO-TPD) of the sample was heated in He (50 mL min1 for O2-TPD or 30 mL min1 for CO- and NO-TPD) from room temperature to 200 1C (O2-TPD) or 300 1C (CO and NO-TPD) and held for 1 h, subsequently cooled to room temperature in a He atmosphere and switched to pure O2 (10 mL min1) (O2-TPD) or 10 vol% CO/He and 5 vol% NO/He (CO and NO-TPD, respectively) and held for 0.5 h. After that, it was purged by He for 0.5 h for removal of residual gas. Then the sample was heated from room temperature to 700 1C (O2-TPD) or 600 1C and 900 1C (CO and NO-TPD, respectively) in helium at a heating rate of 10 1C min1. The consumption of O2, CO, and NO was continuously monitored using a thermal conductivity detector. 2.2.7. H2-temperature programmed reduction (TPR) measurement. H2-TPR was also carried out on an automated chemisorption analyzer (Finetec Instruments). First, 50 mg of the sample was heated in N2 (50 mL min1) from room temperature to 110 1C and held for 1 h, subsequently cooled to room temperature in a N2 atmosphere and switched to the stream of 7.03 vol% H2/Ar (10 mL min1) and held for 0.5 h. Then the sample was heated from room temperature to 800 1C in N2 at a heating rate of 10 1C min1. The consumption of H2 was continuously monitored using a thermal conductivity detector. 2.2.8. In situ FT-IR adsorption measurement (FT-IR). FT-IR spectra were collected at room temperature on a Nicolet 5700 FT-IR spectrometer equipped with a DTGS as a detector, working in the range of wave numbers 400–4000 cm1 at a resolution of 4 cm1 (number of scans, 32). The spectra of empty IR cell were collected in a NO or/and a CO atmosphere at various target temperatures as a background. The Cu/TC catalysts (B15 mg) were mounted in a quarts IR cell and pretreated for 1 h at 300 1C in a flowing N2 atmosphere. After being cooled to room temperature, the sample wafers were exposed to a controlled stream of CO–Ar (10% of CO by volume) or/and NO–Ar (5% of NO by volume) at a rate of 5.0 mL min1 for 30 min. Desorption/ reaction studies were performed by heating the adsorbed species and the spectra were recorded at target temperatures.


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All of the presented spectra were obtained by subtraction of the corresponding background reference. 2.3.

Catalytic activity tests

The catalytic performances of these samples for NO reduction by CO were determined under the steady state, involving a feed stream with a fixed composition, 5% NO, 10% CO, and 85% He by volume as a diluent. The sample (50 mg) was fitted in a quartz tube and pretreated in a highly purified N2 stream at 110 1C for 1 h to remove the impurities and then cooled to ambient temperature, after that, the mixed gases were switched on. The reactions were carried out at different temperatures with a space velocity of 24 000 mL g1 h1. Two columns (length, 1.75 m; diameter, 3 mm) and two thermal conductivity detectors (T = 100 1C) were used for analyzing the products. Column A with Paropak Q for separating CO2 and N2O, column B packed with 5A and 13X molecular sieves (40–60 M) for separating N2, NO and CO. In addition, 5% NO, 5% CO, and 90% He by volume as a diluent were also regarded as a fixed composition to test at different temperatures with a space velocity of 18 000 mL g1 h1. Other conditions remained the same.

3. Results and discussion 3.1.

XRD, BET, and LRS analyses

Fig. 1(a) shows the XRD patterns of a series of TC and TiO2 samples. For pure TiO2, only the peaks of anatase titania (2y = 25.201, 37.761, 47.921, 53.781, and so on) are detected and the peaks are indexed. It means that the TiO2 sample calcined at 500 1C exists as an anatase structure. For TC samples, only the characteristic peaks of the anatase-type TiO2 appear, and these peaks in TC samples shift to a lower diffraction angle compared with the pure TiO2. Meanwhile, no peaks of crystalline CeO2 can be observed. The ionic radii of Ti4+ and Ce4+ are 0.068 nm and 0.102 nm, respectively. When the larger Ce4+ ions partially substitute for the smaller Ti4+ ions, the anatase TiO2 lattices consequently expand, which should be responsible for the position of the diffraction angle shift to the lower value.30 Thus, it can be concluded that Ce4+ has incorporated into the titania matrix. The XRD peak intensities of the samples become much weaker and wider than for TiO2 alone with the increase of adding Ce content into TiO2. This is attributed to the addition of Ce into the titania lattice, which inhibits the growth of TiO2 crystallites31 and correspondingly leads to a decrease of crystallite size, which is in agreement with the BET results. The surface area and pore volume gradually increase and the average pore diameter decreases with the increase of Ce content (Table 1). Furthermore, from the lower intensity of the diffraction peaks over the Ce-doped TiO2 samples, it is implied that the samples are in poor crystallization and the ceria introduction results in the structural distortion and disorder23 and the decrease of dimension is caused by the number of defects into the crystallites of anatase, generated by the substitution of titanium ions by cerium ions.22 Interestingly, the value of the position of the diffraction angle of TC-60 : 1


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Fig. 1 The XRD patterns of (a) TC and TiO2 samples and (b) Cu/TC and Cu/TiO2 samples.

Table 1

Textural properties of the supports

Samples

BET surface area (m2 g1)

Pore volume (cm3 g1)

Average pore diameter (nm)

TC-10 : 1 TC-20 : 1 TC-40 : 1 TC-60 : 1 TC-80 : 1 TiO2

118.199 95.594 80.996 59.138 37.445 19.277

0.487 0.397 0.308 0.239 0.157 0.102

16.483 16.608 15.191 16.146 16.812 21.239

is the lowest, it means that the anatase TiO2 lattices of TC-60 : 1 have the most extent of expansion, which indicates that the band lengths of Ce–O–Ti are longer than other TC samples, probably contributing to larger structural distortion and disorder, more defects and oxygen vacancies. And the possible reason may be attributed to the formation of Ce3+ in the process of synthesis (the following XPS results have proved the existence of Ce3+ in Cu/TC samples), since Ce3+ is bigger than Ce4+. Moreover, the most extent of lattice expansion of TC-60 : 1 support means that the content of Ce3+ in the support is the most compared with


other TC supports. In addition, as reported elsewhere,32 a simple reduction of two Ce4+ cations to two Ce3+ cations with one oxygen vacancy formed led to a lattice expansion. Fig. 1(b) shows the XRD patterns of a series of Cu/TC and Cu/TiO2 samples. It can be seen that no characteristic peaks of crystalline CuO can be observed for Cu/TC-10 : 1 and Cu/TC-20 : 1, indicating that the CuO species are highly dispersed on the surface of the two samples above. But very weak characteristic peaks of crystalline CuO can be observed for other samples, which suggests that the CuO species are not completely highly dispersed on their surface. In addition, the characteristic peaks of crystalline CuO are gradually weakened and even disappeared for Cu/TC-20 : 1 and Cu/TC-10 : 1 with the increase of Ce, demonstrating that the surface area gradually increases with the increase of Ce (which is in agreement with the BET results), which contributes to better and better dispersion on the surface of the supports. It should be noted that the characteristic peak of crystalline CuO of Cu/TC-60 : 1 is the weakest in the presence of the characteristic peak of crystalline CuO of several samples, which probably attributes to the most extent of expansion of the lattices. LRS as complementary surface characterization of XRD is performed on these samples, and the corresponding results are exhibited in Fig. 2. We can see from Fig. 2(a) that the pure TiO2 shows several Raman bands at 395, 515, and 638 cm1 assigned to the B1g(1), A1g + B1g(2), and Eg(3) vibration modes of anatase TiO2, respectively.33 When Ce are added into TiO2, the Raman bands shift to a higher and higher wavenumber direction slightly and the intensities become weaker and weaker with the increase of Ce compared with anatase TiO2 (which can be seen in the inset), and only the Raman vibration modes of anatase TiO2 can be detected at the corresponding positions for TC samples. The Raman band of CeO2 (463 cm1) is absent in TC samples. All of which further confirm that Ce4+ has been incorporated into the lattice of anatase TiO2. The Raman results of the copper-based catalysts are presented in Fig. 2(b). From this figure, we can find that the Raman bands corresponding to CuO are absent for all the copper-based catalysts, indicating that copper oxide species are in the form of a commendably dispersed state or/and a clustered state on the surface of TC supports, and it is impossible to discern any signal related to the crystalline copper species. In addition, the Raman bands of Cu/TiO2 are very similar to those of anatase TiO2 basically-without-shift, which suggests that the interaction between CuO and anatase TiO2 is weak. While the Raman bands of Cu/TC catalysts shift to a low-wavenumber direction slightly compared with TC supports, indicating that the incorporation of Ce4+ into the lattice of anatase TiO2 may enhance the interaction between Cu2+ and Ti–O–Ce solid solution. On the other hand, either TC supports or Cu/TC samples display broad Raman bands compared with anatase TiO2 and Cu/TiO2, respectively, which is related to either the small size or the formation of oxygen vacancies.34 Moreover, the FWHM values of TC supports and Cu/TC samples are larger than those of anatase TiO2 and Cu/TiO2, respectively. As Ilieva et al.35 concluded, such an increase in the FWHM value was attributed to the small crystal size and/or defect formation in the support structure. Considering the XRD


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Fig. 3 The (a) N2 adsorption–desorption isotherms, and (b) BJH pore size distribution curves of these supports. Fig. 2

The Raman spectra of these supports (a) and catalysts (b).

results, these observations in the shift and width in this case reveal the influence of phonon confinement, oxygen vacancies, and inhomogeneous strain (related to the presence of reduced states of cerium). 3.2.

N2-physisorption analysis

The N2 adsorption–desorption isotherms and corresponding BJH pore size distribution curves of the synthesized supports are shown in Fig. 3. Noting from Fig. 3(a) all of the carriers exhibit the IV-type isotherms with evident H3-type hysteresis loops, which indicates that they contain mesopores (2–50 nm) with narrow slit-like shapes or plate-like particles according to IUPAC.36 In the low-pressure range of P/P0 r 0.8, the adsorption branches of the six samples slowly ascend; while in the high-pressure range of P/P0 4 0.8, the adsorption branches steeply ascend. The results show that all the samples possess dense mesopores.37 In the low-pressure range, the micropores in the samples play a dominant role in the adsorptive capacity. In the high-pressure range, the mesopores in the samples draw


a decisive influence on the adsorptive capacity, which indicates that the mesopores in these samples are densely distributed. Hysteresis loops emerge because of a capillary condensation effect on the materials. The adsorptive capacity gradually increases with the increase of Ce, which matches with the BET results of the increase of surface area and pore volume. In addition, the TC-60 : 1 support exhibits a hysteresis loop at the lowest pressure and the area of the hysteresis loop is the biggest in the low-pressure range, which also probably attributes to the most extent of expansion of the lattices, indicating that it has a higher and more uniform ratio of mesopores compared with other supports which is in agreement with the pore size distribution results. The corresponding pore size distribution curves are determined using the BJH method from the desorption branch of the isotherms for the supports. It can be seen from Fig. 3(b) that the pore size of these samples is located in the mesoporous range (2–50 nm) except the TC-20 : 1 support. The intensities and areas of the pore size distribution curves, to some degree, all increase with the increase of Ce content, which may be related to the emancipation of gaseous nitrogen oxides during


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the thermal decomposition of the nitrate precursors calcined at 500 1C.38 Specially, the TC-60 : 1 support presents the highest ratio of mesopores.


3.3.

UV-vis DRS analysis

Information on the surface coordination and electronic states of the metal ions by measuring the d–d and f–d electron transitions and oxygen–metal ion charge transfer bands can be acquired from the UV-vis DRS measurement. As shown in Fig. 4(a), pure TiO2 powders show absorption spectra consisting a single broad intense absorption at around 400 nm (i.e. in the UV range) occurring due to the charge-transfer (CT) from the valence band (mainly formed by 2p orbitals of the oxide anions) to the conduction band (mainly formed by 3d t2g orbitals of the Ti4+ cations).39 Pure CeO2 exhibits three bands at B255, 285, and 342 nm.40 The former two are attributed to Ce3+ ’ O2 and Ce4+ ’ O2 ligand to metal charge transfer transitions (LMCT) while the latter is attributed to an interband transition (IBT). Relative to pure TiO2, the strong absorption observed for the CeO2-doped TiO2 supports in the region 200–400 nm must be assigned to O–Ce charge transfer transitions involving a number of Ce4+ ions along with O2 to Ce3+ charge transfer transitions. The red shift of the absorption edges to the visible region for cerium doped TiO2 samples is observed. The spectra shift more to the visible region with the increasing amount of Ce in Ce/TiO2 samples. Moreover, it has been reported that the doping of various transitional metal ions into TiO2 can shift its optical absorption edge from UV into the visible light range (i.e., red shift).41 The red shift of the adsorption edges can be attributed to the incorporation of Ce4+ cations, which substitutes some Ti4+ cations42 and they form solid solutions and lead to a decrease of the symmetry and consequently the strain development at the titanium sites. From the report of Bensalem et al., the shift of the absorbance in varying degrees can be explained as a consequence of change of particle size.43 It should be noted that the extent of the red shift of the TC-60 : 1 support is larger than that of the TC-40 : 1 support rather than smaller, which is probably attributed to the largest expansion of the lattices of the TC-60 : 1 support. On the other hand, the diffuse adsorption spectra of the CuO supported catalysts are exhibited in Fig. 4(b). In the UV region, the intensity of the absorption of all the catalysts is larger than those of corresponding supports, indicating the occurrence of charge transfer transitions between CuO and supports. In the visible region, the CuO-based catalysts show broad and weak d–d bands in varying degrees at around 400 to 650 nm, indicating that the coordination environment of copper species is influenced by support structures. It can be observed through further inspection that the bands gradually broaden and weaken with the increase of Ce compared with that of Cu/TiO2, which can be due to the decrease in grain size. Interestingly, the Cu/TC-60 : 1 catalyst has the lowest d–d absorption band. In summary, the observations from the UV-vis DRS are very consistent with the XRD, BET, and LRS results.


Fig. 4 UV-vis DRS spectra of these (a) supports and (b) catalysts.

3.4.

XPS analysis

XPS was performed to further investigate the surface compositions and elementary oxidation states of these catalysts. Due to the charging effects during XPS analysis, the binding energy scale is calibrated using adventitious carbon (284.6 eV).42 The XPS spectra of Ti 2p, Ce 3d, Cu 2p, Cu LMM, and O 1s for these Ce doped Cu/TiO2 catalysts are displayed in Fig. 5. The Ti 2p1/2 and Ti 2p3/2 spectra of these catalysts are displayed in Fig. 5(a). The Ti 2p1/2 peak shifts from 464.1 eV in Cu/TiO2 to 463.9 eV in Cu/TC-10 : 1 and Ti 2p3/2 peak shifts from 458.4 eV in Cu/TiO2 to 458.1 eV in Cu/TC-10 : 1 slightly, respectively, with the increase of Ce. This slight shifting represents a spot of the intermediate oxidation state of Ti from Ti4+ to Ti3+,44 which demonstrates that the incorporation of Ce has a strong effect on the oxidation state of Ti and forms oxygen vacancies. The complex spectrum of Ce 3d is numerically fitted with eight components with the assignments defined in Fig. 5(b). The two groups of spin-orbital multiplets, corresponding to 3d3/2 and 3d5/2, are denoted u and v and extend in the binding energy range of 880–920 eV. Schulz et al.45 reported that the bands labeled u 0 and v 0 represent the 3d104f1 initial electronic state corresponding to Ce3+, while the


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Fig. 5

XPS spectra of (a) Ti 2p, (b) Ce 3d, (c) Cu 2p, (d) Cu LMM, and (e) O 1s for these samples.



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Paper Table 2

PCCP The surface (XPS) compositions of these samples


The relative percentage of ions by XPS Samples

Ce3+/(Ce3+ + C4+) (%)

Cu+/(Cu+ + Cu2+) (%)

Sshake-up/SCu

Cu/TiO2 Cu/TC-80 : 1 Cu/TC-60 : 1 Cu/TC-40 : 1 Cu/TC-20 : 1 Cu/TC-10 : 1 Cua

— 27.99 35.01 27.48 23.38 17.78 —

19.31 25.63 37.79 34.38 30.59 25.63 —

32.50 29.72 26.60 26.74 26.98 53.26 55.00

a

2p3/2

(%)

O00 /(O 0 + O00 ) (%) 29.07 22.16 20.88 26.78 25.26 38.07 —

The value of shake-up peaks to Cu 2p3/2 peak of pure CuO.48

other six bands labeled u 0 0 0 and v 0 0 0 , u00 and v00 , u and v are related to Ce4+. Therefore, the chemical valence of cerium on the surface of these samples is mainly in a +4 oxidation state, and a small amount of Ce3+ co-exists, which is consistent with the results of the LRS and UV-vis DRS. Furthermore, the content of Ce3+ can be estimated from the area of u 0 and v 0 , according to the following equation:28 Su0 þ Sv0 Ce3þ ð%Þ ¼ P ðSu þ Sv Þ The relative contents of Ce3+ calculated for these catalysts are given in Table 2. It can be seen that the percentage of Ce3+ for Cu/TC-80 : 1 is 27.99%, when the ratio of Ti and Ce reaches 60 : 1, the percentage of Ce3+ is the most (35.01%), which is in agreement with the XRD results. With the further increase of Ce, however, the percentage of Ce3+ gradually decreases to 17.78% when the ratio of Ti and Ce reaches 10 : 1. Nagai et al.46 concluded that the enhancement of homogeneity of the Ce and Zr atoms could ease the valence change of Ce (Ce4+ - Ce3+). Therefore, the presence of Ce3+ was partly attributed to the relative homogeneous Ti–O–Ce solid solution (XRD patterns and Raman spectra of these samples do not exhibit extra peaks, indicating no phase segregation occurred). On the other hand, according to the Raman results, this phenomenon can be explained by the synergistic effect of CuO and Ti–O–Ce solid solution, that is Ti3+ + Cu2+ 2 Ti4+ + Cu+ and Ce3+ + Cu2+ 2 Ce4+ + Cu+, which leads to the formation of more surface oxygen vacancies. For the spectra of Cu 2p (Fig. 5(c)), all of the catalysts exhibit the main peaks of Cu 2p1/2 at B952.4 eV and Cu 2p3/2 at B933.3 eV and the corresponding shake-up peak between 946 and 938 eV, which is the characteristic of Cu2+ species.28,32 Moreover, a overlap peak centered at B932.2 eV can be detected, indicating the presence of Cu+ species.32,34 The existence of Cu+ species is mainly due to the redox equilibrium (Ti3+ + Cu2+ 2 Ti4+ + Cu+ and Ce3+ + Cu2+ 2 Ce4+ + Cu+) shifting to the right. In order to clarify the valence state of copper, the Auger LMM lines of Cu were also investigated over these mentioned catalysts, as shown in Fig. 5(d), and the overlapped peaks were fitted by Gaussian curves. It can be observed that for all the catalysts, surface copper species mainly exist as Cu2+ (568.8 eV) and Cu+ (570.7 eV).47 In addition, as can be seen, the Ti 2s (565.0 eV) peak is very near to the Cu-LMM


peaks in this system. In this way, it is difficult to quantify the ratio of Cu+ and Cu2+. Thus, the Cu 2p3/2 peaks were fitted by Gaussian curves and the relative content of Cu+ can be represented by the area ratio between Cu+ and Cu+ + Cu2+. The results are given in Table 2 and they are similar to the change of Ce3+ in different catalysts. In order to ensure the credibility of the Gaussian fitting, the area ratios of shake-up peaks to Cu 2p3/2 peak are also given in Table 2. The value of shake-up peaks to Cu 2p3/2 peak of pure CuO is listed for comparison. The smaller the area ratio, the larger the relative content of Cu+. Similarly, the ratio decreases from 32.50% to the lowest (26.60%) when the ratio of Ti and Ce reaches 60 : 1, with a further increase of Ce, however, the percentage gradually increases to 53.26% when the ratio of Ti and Ce reaches 10 : 1, which is in consistent with the results of Cu+/(Cu+ + Cu2+). It should be noted that both of the relative contents of Ce3+ and Cu+ are the largest for the Cu/TC-60 : 1 catalyst, which is mainly attributed to the redox equilibrium (Ti3+ + Cu2+ 2 Ti4+ + Cu+ and Ce3+ + Cu2+ 2 Ce4+ + Cu+) or the strong synergistic effect of CuO and Ti–O–Ce solid solution. In other words, this result can be interpreted by the difference of surface oxygen vacancy concentration. It is easy to understand that the formation of surface oxygen vacancies is accompanied by an increase of the relative contents of Ti3+, Ce3+, and Cu+. The high-resolution spectra for the O 1s ionization features of the samples are numerically fitted, with two components representing the primary O 1s ionization feature and chemically shifted O 1s feature from the chemisorbed surface species, which is shown in Fig. 5(e). The main peak at B529.6 eV (O 0 ) is attributed to the characteristic lattice oxygen bonding to the metal cations while the shoulder with the higher binding energy at B531.1 eV (O00 ) is considered as the adsorbed oxygen and the oxygen in the carbonate and hydroxyl groups.28,42 Similarly, the proportions of O00 /(O 0 + O00 ) are listed in Table 2. The proportion of O00 /(O 0 + O00 ) gradually decreased from 29.07% to 20.88% with the increase of Ce from Cu/TiO2 to Cu/TC-60 : 1 and gradually increased from 20.88% to 38.07% with a further increase of Ce from Cu/TC-60 : 1 to Cu/TC-10 : 1, which demonstrates that, on the one hand, the increase in the concentration of lattice oxygen replies the incorporation of copper into the lattice with the capping oxygen, similar results have also been reported elsewhere;32,34 on the other hand, the concentration of lattice oxygen in the Cu/TC-60 : 1 sample is the most compared with other samples. The result is probably


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attributed to the most extent of lattice expansion of the TC-60 : 1 support, which has been mentioned in XRD results.


3.5.

(O2, CO, and NO)-TPD analyses

To further investigate the adsorption and activation of oxygen on the catalysts, O2-TPD measurements were carried out, and the results are displayed in Fig. 6. Generally, the surface adsorbed oxygen species undergo the following transformation procedures with electron gain: O2(ad) - O2(ad) - O(ad) O2(ad/lattice). O2(ad) refers to physically adsorbed oxygen, which can usually be removed by purging argon before the analysis. The oxygen adsorbed species of O2(ad) and O(ad) are weakly bonded to catalyst surfaces and are easier to desorb. O2(ad/lattice) is the surface lattice oxygen and is difficult to be extracted.49 As shown in Fig. 6, all samples exhibit large intense peaks at around 100 1C and have small broad peaks centered at around 260 1C, referring to the peroxy species O2(ad) and surface adsorbed monatomic species O(ad), respectively, which can be correlated with the superior activity of the catalysts for redox reactions. The peaks above 550 1C are assigned to surface lattice oxygen, which is generally not correlated with the reactions due to their high temperature desorption. The intensity and area of the peaks of the peroxy species O2(ad) are largely enhanced compared with pure TiO2 when Ce4+ are doped into TiO2. On the other hand, the spectrum of the O 1s of XPS spectroscopy can also be employed to gain further insight into the existence of the oxygen species presented on the surface of the Cu/TC samples. As shown in Fig. 5(e), two peaks are identified for the O 1s spectra of all samples, indicating the existence of several kinds of surface oxygen species which differ

Fig. 6

O2-TPD profiles of these catalysts.


in their chemical states. The species at binding energies of B531.1 (O00 ) and B529.6 (O 0 ) eV as measured for all samples are ascribed to surface O and lattice O2 species, respectively.50,51 It can be seen in Table 2 that the proportion of O00 /(O 0 + O00 ) is the least, indicating that relatively more copper incorporates into the lattice with the capping oxygen, which increases the surface defects and oxygen vacancies due to different valence states and ionic radii. One may recall that XPS is sensitive to only the outermost B20 Å of the solid, i.e. the spectra describe only the surface properties of the systems. Therefore, in combination of the O2-TPD results, the XPS results strongly suggest that the superficial oxygen species exists as surface O on the surface of the Cu/TC catalysts. Furthermore, the intensity and area of the peaks of the surface adsorbed monatomic species O(ad) are also a little increased compared with pure TiO2. Both of which indicate that the formation of Ti–O–Ce bonds produces an appreciable change in the electronic properties of the Ce4+ doped catalysts and increases the surface defects and oxygen vacancies. Interestingly, the temperatures of O2(ad) and O(ad) of the Cu/TC-60 : 1 sample are the lowest, which are 93 1C and 251 1C, respectively. Generally, TiO2 is classified as a support which exhibits the strong metal–support interaction with metals. Bernal et al.52 reported that the temperatures of oxygen desorption from the metal catalyst surface increased when a strong interaction between the metal particles and support occurred. The addition of Ce into the TiO2 lattice to form Ti–O–Ce bonds may lead to a decrease of the metal–support interaction and result in a decrease of oxygen desorption temperature. In other words, the adsorbed oxygen on the Ce-doped TiO2 with optimum Ti/Ce ratios can be dissociated to atomic species which desorbed at lower temperature. And, this is similar to the report of Comsup et al.53 about Si-modified TiO2. Moreover, the presence of Ce in TiO2 can decrease the strength of surface lattice oxygen bonding, resulting in higher mobility of lattice oxygen, which contributes to NO reduction by CO. In order to study the property of the catalysts for CO and NO adsorptions, CO-TPD and NO-TPD tests were carried out, respectively. The CO-TPD profiles are displayed in Fig. 7. For the Cu/TiO2 sample, there is only one weak and broad CO desorption peak at about 249 1C, while for Cu/TC samples, there are two peaks appearing at lower temperatures, indicating different types of activated adsorption sites with different binding strengths and an obvious growth of CO adsorption capacity when Ce are doped into TiO2. According to the literature,54 CO is hardly adsorbed on pure CuO, so the remarkably increased desorption peaks are probably owing to the desorption of CO adsorbed on more Cu+ and/or Cu0 on the surface, whose presence has been confirmed by XPS results. For the Cu/TC-60 : 1 sample, both the area and intensity are the largest with the increase of Ce from Cu/TiO2 to Cu/TC-60 : 1 and the changing trend of areas and intensities of these catalysts is in agreement with the content of Cu+ calculated by XPS. On the other hand, the desorption temperature of the Cu/TC-60 : 1 sample is the lowest compared with other samples, which should be ascribed to the most content of Cu+ on the surface and the synergistic effect between Ti3+, Ce3+ and Cu+.


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Fig. 7 CO-TPD profiles of these catalysts.

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molecules than that of pure TiO2 oxides supported by CuO, which is the prerequisite of catalytic reaction. A possible reason is that Ce-doped TiO2 oxides species have more unpaired electrons than pure TiO2 that can back-donate to the antibonding orbital of the adsorbed NO species to weaken the N–O bond, and further lead to the dissociation of the adsorbed NO species.57 Furthermore, the desorption temperature of the Cu/TC-60 : 1 sample is the lowest compared with other samples for bridged nitrates and chelating nitrates and nitrites, which should be related to the synergistic effect between between Ti3+, Ce3+ and Cu+. From O2-TPD, CO-TPD and NO-TPD results, it can be seen that the Cu/TC-60 : 1 sample can offer higher mobility of lattice oxygen, the most adsorbed sites of either CO or NO species, and have the best catalytic activity among the catalysts (which has been given in back reactivity part), that is in the redox reaction, the reactant CO is adsorbed at the surface of the catalyst firstly, and then reacts with the adsorbed NO to produce N2 and CO2, so it is the most adsorbed sites of CO and NO species that improves the reaction of NO reduction by CO. 3.6.

Fig. 8 NO-TPD profiles of these catalysts.

The NO-TPD profiles are displayed in Fig. 8. As the temperature increased from 40 to 900 1C, the desorption spectra are dominated by four peaks for NO desorption, suggesting that NO is adsorbed on different sites. Accordingly, the first desorption peak at about 85 1C is due to bridged nitrates, the second peak at about 320 1C is attributed to chelating nitrates and nitrites,55 and the last two at about 556 1C and 860 1C are ascribed to the decomposition of nitrites and nitrates, respectively.56 It is obvious that the areas and intensities of these Cu/TC samples are sharply increased compared with the Cu/TiO2 sample when Ce are doped into TiO2 for bridged nitrates and chelating nitrates and nitrites. This result implies that Ce-doped TiO2 oxides supported by CuO, with unpaired electrons, are more beneficial to preferentially adsorb NO


H2-TPR analysis

The H2-TPR test was performed to study the reducibility of CuO supported on a series of TC and TiO2 supports. Fig. 9(a) shows the TPR profiles of the Cu/TC, Cu/TiO2, and pure CuO samples. For the Cu/TiO2 sample, the former two peaks at B182 1C and B209 1C are assigned to the stepwise reduction of surface dispersed CuO species, i.e., Cu2+ - Cu+ and Cu+ - Cu0;47 the third peak at B226 1C should be related to the reduction of the crystalline CuO. Similar results have been obtained by Dong et al.58 The positions of the peaks gradually shift to a lower temperature range with the increase of Ce until TC-40 : 1, which can be interpreted by two respects. On the one hand, the increasingly strong synergistic effect between CuO and TiO2 and CuO and CeO2 through Ti3+ + Cu2+ 2 Ti4+ + Cu+ and Ce3+ + Cu2+ 2 Ce4+ + Cu+, respectively, makes copper oxide species easily reduced; on the other hand, the introduction of Ce gradually increases the surface area of supports and consequently enhances the dispersion of CuO on the supports, which decreases the accumulation of the crystalline CuO and also easily contributes to the reduction of surface CuO. Furthermore, the peak areas of the crystalline CuO show a decreasing trend and afterward the peak disappears when further increasing the Ce content. In addition, for the crystalline CuO, the peak area of Cu/TC-60 : 1 is the smallest (which can be seen from Table 3), demonstrating that the crystalline CuO quantity of the surface of the sample is the least. The above mentioned are in agreement with the XRD results of the intensity of the crystalline CuO and the lowest intensity of the Cu/TC-60 : 1 sample. With a further increase of Ce to TC-10 : 1, however, the positions of the peaks gradually shift to a higher temperature range. Although the addition of Ce plays a better role, this not only causes the increase of surface area but also leads to the gradual increase of pore volume of the supports, which results in partial CuO entering into the interior of the channels when the samples are prepared during the impregnation process,


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Fig. 9 The TPR results for (a) these catalysts and (b) copper oxides species on different supports.

which has been proved by XPS results. Thus, the reduction of CuO is hindered and correspondingly the reduction temperature will increase. On the other hand, LRS and XPS results have mentioned that the existence of Ce3+ in Cu/TC samples and the relative percentage of Ce3+ is the most in Cu/TC-60 : 1 sample, respectively, which is easier to promote the reduction of CuO due to a synergistic effect. It is implied that suitable doping content of Ce can promote a greater extent of reduction of CuO. It is well-known that the peak position and shape of the H2-TPR profile are highly sensitive to the nature of the catalyst. So, curve fitting was employed using Gaussian curves to gain an insight into the surface species of the catalysts. According to


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Table 3, the H2 consumption of the peak2 (from the reduction of Cu+ - Cu0) is larger than that of the peak1 (from the reduction of Cu2+ - Cu+), it may be ascribed to the reduction of Cu+ to Cu0 occurring before the end of the former one due to the influence of H2 spillover as reported elsewhere.59 On the surface of metallic copper, H2 can be disassociated into more active H and thus promotes the reducibility of Cu+. In addition, the H2 consumption was also calculated and listed in Table 3. It can be seen that the ratios of actual H2 consumption (denoted as A) to theoretical H2 consumption (denoted as T) of the catalysts are all less than 1.0, which imply that Cu2+ has not been reduced completely. On the one hand, some Cu2+ ions enter into the TiO2 lattice, which has been proved by XPS results, and form Cu–O–Ti solid solutions (a TiO2-rich Cu–O–Ti solid solution is hard to detect by XRD due to the similar ion radius Cu2+ (0.073 nm) and Ti4+ (0.068 nm)), the reduction temperature of which is higher than the temperature used in this experiment. Similar reasons have been reported by Chen et al.8 On the other hand, there exists partial Cu+ on the surface of the catalysts, which also results in the abatement of H2 consumption. It can be concluded from the above analysis that although the H2 consumption of the Cu/TC-10 : 1 sample is the most, its reduction temperature is slightly higher compared with the other ratio of Ce doped catalysts due to lesser relative content of Ce3+ and Cu+, so the reducibility is not the best. In contrast, the Cu/TC-60 : 1 sample has a better reducibility with a similar reduction temperature, more dispersed CuO, less crystalline CuO, and more total H2 consumption compared with the Cu/TC-40 : 1 sample due to the most relative content of Ce3+ and Cu+. So, the reducibility of the Cu/TC-60 : 1 sample is the best of all the catalysts. In order to investigate the distinct properties of the TC-60 : 1 support, the TPR profiles of Cu/TiO2 and Cu/Ce(1)/TiO2(60) samples are studied for comparison, as shown in Fig. 9(b). In the profile of the Cu/TiO2 sample, the three peaks at B182 1C, B209 1C, and B226 1C belong to the reduction of Cu2+ to Cu+, Cu+ to Cu0, and CuO small particles, respectively. Three similar peaks can be seen from the profile of the Cu/Ce(1)/TiO2(60) sample, but, however, all the reduction temperatures are higher and the first and third peak areas are larger than that of the Cu/TiO2 sample. The results suggest that the reduction behavior of CuO has been affected by the CeO2 addition. For the Cu/Ce(1)/TiO2(60) sample, on the one hand, since CeO2 are supported on the TiO2 surface and the ionic radius of Ce4+ is larger than that of Cu2+, which may lead to a stronger synergistic effect between CeO2 and CuO, so the first peak area of the Cu/Ce(1)/TiO2(60) sample is larger than that of the Cu/TiO2 sample. But on the other hand, the dispersion capacity of a few CeO2 supported TiO2 is lower than that of TiO2, consequently, the dispersion of CuO on the Ce(1)/TiO2(60) support is also lower than that of the pure TiO2 support. As a result, the interaction between CuO and Ce(1)/TiO2(60) supports weakens and the crystalline CuO increases, which leads to a higher reduction temperature. Interestingly, the reduction temperature of the Cu/TC-60 : 1 sample is even lower than that of the Cu/TiO2 and Cu/Ce(1)/TiO2(60) samples. The results suggest


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Table 3

The peaks areas of the H2-TPR profiles and the H2 consumption (mmol g1) of the catalysts


Dispersed

Crystalline

Samples

Peak1

Peak2

Peak1 + Peak2

Peak3

Total peak areas

Cu/TiO2 Cu/TC-80 : 1 Cu/TC-60 : 1 Cu/TC-40 : 1 Cu/TC-20 : 1 Cu/TC-10 : 1 CuO (0.0108 g)

6971.25 7510.80 8326.34 8087.21 8805.75 5770.18 59998.83

14233.44 10767.16 14086.25 13584.00 14346.70 20226.17 —

21204.69 18277.96 22412.59 21671.21 23152.45 25996.35 —

1775.41 4080.10 1362.67 1663.37 — — —

22980.10 22358.06 23775.26 23334.58 23152.45 25996.35 59998.83

that the low-temperature reduction of the copper oxide species in the Cu/TC-60 : 1 sample should not be simply attributed to the influence of the surface cerium oxide species, i.e., the CeO2-doped TiO2 support obviously provides different surface properties compared with pure TiO2. The possible reasons may be as follows: (i) The electron charge density (ECD) is influenced by the CeO2 doping. The electrons of the oxygen in the Ce–O–Ti link are strongly attracted by Ti4+ ions, which causes a delocalization of ECD around the Ce4+ ion and an enrichment of the ECD around the Ti4+ ion.60 Consequently, on the surface of the support, the Ti4+ ion is the electronegative center and the Ce4+ ion is the electropositive center. Thus, it is reasonable to conclude that Cu2+ is preferentially linked with the oxygen anion from the Ti–O bond, forming a Ti–O Cu–O link. Furthermore, for the enrichment of the ECD around the Ti4+ ion, the interaction between the Cu2+ and the lattice oxygen of the Ti–O bond on the TC-60 : 1 support should be stronger than that on the pure TiO2 support. Accordingly, the Cu–O bond of CuO will be weaker and the CuO species are easier to be reduced. (ii) The coordination structures of the Cu2+ ions are changed on the surface of the TC-60 : 1 support. As discussed by Zhu et al.,61 the (001) plane of the anatase TiO2 is considered as the preferentially exposed plane. The configuration of the Cu2+ ion on the TiO2 (001) plane is depicted in Fig. 6. The Cu2+ ions incorporate into the surface octahedral vacancies and form an octahedral coordination structure, as shown in Fig. 10(a). Consequently, for the influence of the different ECD and radii between the two kinds of atoms, the length of the M(Ti, Ce)–O bond will be different from each other, as such, the octahedral coordination structures of the Cu2+ ions will be distorted, which are relatively more unstable than before, as shown in Fig. 10(b). Accordingly, the Cu2+ species are easily reduced to Cu+ and Cu0. In summary, for the Cu/TC-60 : 1 sample, it has an optimum ratio of Ti/Ce, the Cu2+ ion on the surface of the TC-60 : 1 support is a distorted octahedral coordination structure. Additionally, the CuO reducibility is affected by the ECD. These may be the reasons for the copper oxides supported on TC-60 : 1 are most easily to be reduced among all of the samples. 3.7.

Catalytic activity and selectivity of NO reduction by CO

The NO conversion, N2 selectivity and CO conversion results as a function of temperature for these catalysts are given in Fig. 11.


Actual H2 consumption (mmol g1)

Theoretical H2 consumption (mmol g1)

A/T

1.041 1.012 1.077 1.057 1.048 1.178

1.348 1.348 1.348 1.348 1.348 1.348

0.772 0.751 0.799 0.784 0.777 0.874

Fig. 10 Tentative model of the surface-dispersed copper oxide species formed on the (001) plane of (a) the anatase TiO2 surface and (b) the CeO2-doped anatase TiO2 surface.

It can be seen that NO conversions (Fig. 11(a)) only reach 10–30% for all the samples and the N2 selectivities (Fig. 11(b)) are also very low at the region of 150–200 1C. A further increase of the temperature results in a dramatic increase of both the NO conversion and the N2 selectivity at 250 1C except Cu/TiO2 and Cu/TC-10 : 1 samples, indicating that the surface area of supports is not the main factor responsible for the NO conversion. CO conversion (Fig. 11(c)) results are similar to NO conversions. In addition, according to the literature,28,47,58 Cu+ can adsorb CO molecules efficiently, which is beneficial to the enhancement of the activity for the reaction involving CO. Therefore, more Cu+ are probably generated after 200 1C. It should be noted that the Cu/TC-60 : 1 sample has the best CO and NO conversions and N2 selectivity among all the catalysts. From the XRD and LRS results we know that the TC-60 : 1 support has the most extent of lattice expansion and has major Ti3+ and Ce3+ (UV-vis DRS and XPS results), which can lead to more creation of oxygen vacancies, simultaneously, the Ce–O–Ti bands are weaker than those of other supports. Consequently, active copper oxide species are easier to interact with the support, which leads to a stronger synergistic effect. Moreover, it has been mentioned in the H2-TPR results that partial Cu–O–Ti solid solutions have been formed which may also lead to the creation of oxygen vacancies. And the Ce-doped TiO2 with optimum Ti/Ce ratios can provide higher mobility of the surface and lattice oxygen at lower temperature, the most adsorbed sites of either CO or NO species, which has been mentioned in the TPD results, and contribute to the regeneration of oxygen vacancies. Thus, it can be concluded that the Cu/TC-60 : 1


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Fig. 12 XRD patterns of the 14% Cu/TC-60 : 1 catalyst after reaction (AR) at 200, 250, and 300 1C.

Fig. 11 The results of (a) NO conversion (%), (b) N2 selectivity (%) and (c) CO conversion (%) over these catalysts as a function of reaction temperatures. Feed composition: NO 5%, CO 10%, and He 85% by volume, GV = 24 000 mL g1 h1.

sample has the best catalytic performances of all these catalysts, which may be related to the structure, reduction behavior, Cu+ content, synergistic effect of the surface of low valence ions, and surface oxygen vacancy concentration of the sample.


Furthermore, the XRD, BET, LRS, UV-vis DRS, XPS, H2-TPR, and TPD results can support these ideas. The difference in N2 selectivity at low and high temperatures suggests that different mechanisms possibly work at low and high temperatures,62,63 which might be resulted from the change of active species at high temperature. XRD measurement, which is a simple and powerful tool for distinguishing the surface phases of copper related catalysts, was combined to track whether the Cu/TC catalysts were changed during the reaction process. As shown in Fig. 12, XRD results of the 14% Cu/TC-60 : 1 (CuO/(TC-60 : 1)) sample after reaction (denoted as AR) suggest that the copper oxides species are changed in the reaction atmosphere. It can be seen from Fig. 12 that the intensity of the crystalline CuO gradually decreases with the increase of the reaction temperature, which implies that the crystalline CuO are gradually reduced. Moreover, Cu2O appears and no Cu0 metal can be detected after reaction at 200 1C. With increasing temperature, the Cu2O species disappears and is further reduced to Cu0 metal by CO after reaction at 300 1C. Cu0 metal is not detected after reaction at 250 1C maybe due to the too low content of Cu0 metal. These results suggest that the surface copper oxide should be changed to Cu+/Cu0 species at high temperature (4200 1C), which might be responsible for the N2 selectivity enhancement.64 The results of different NO conversion (%) and N2 selectivity (%) (the high content of CO denoted as reaction A, the low content of CO denoted as reaction B) over Cu/TC-60 : 1 catalyst are displayed in Fig. 13. It is mentioned earlier that Cu+ species is generated after 200 1C and Cu+ can adsorb CO molecules efficiently, which is beneficial to the enhancement of the activity for the reaction involving CO. So, for the reaction A, the NO conversion and N2 selectivity are higher due to more CO content compared with the reaction B. At low temperature (o200 1C), the low NO conversion is due to the Cu2+ species, but the low N2 selectivity is due to that NO is mainly reduced to N2O at low temperature.28,44 At high temperature (4200 1C),


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Fig. 13 The results of different NO conversion (%) and N2 selectivity (%) over the Cu/TC-60 : 1 catalyst as a function of reaction temperatures. Feed composition: NO 5%, CO 10%, and He 85% by volume, GV = 24 000 mL g1 h1; NO 5%, CO 5%, and He 90% by volume, GV = 18 000 mL g1 h1, respectively.

the N2 selectivity enhances remarkably, demonstrating that N2O is further reduced to N2. In addition, it should be noticed that the reaction B also has excellent activity for NO reduction by CO, which attests that the Cu/TC-60 : 1 catalyst has a wide range of activity with some different ratio of NO/CO for NO reduction by CO. 3.8.

CO or/and NO interaction with the Cu/TC-60 : 1 catalyst

In order to further understand the interaction of reactants with the catalysts, which can provide the information about the changes of the surface adsorbed species, CO or/and NO adsorption in situ FT-IR was carried out under the simulative reaction conditions for the Cu/TC-60 : 1 catalyst. 3.8.1. Single CO interaction with the Cu/TC-60 : 1 catalyst. Fig. 14 shows the in situ FT-IR results of CO adsorption on the Cu/TC-60 : 1 catalyst. The results of CO adsorption on the catalyst with the time proceeding from 0 to 30 min at room temperature are displayed in Fig. 14(a). The peaks at 1638 cm1 and 1368 cm1 can be attributed to the bidentate bicarbonate65 and bidentate formate,66 respectively, and the latter maybe originates from the interaction between CO molecules and the surface hydroxyls; two peaks at 1552 cm1 and 1470 cm1 which are attributed to surface carbonate species can also be observed at room temperature.65,67 Their intensities all get stronger with the increase of the time. Interestingly, in the higher wavenumber region, a peak, whose intensity increases with the time proceeding, at 2103 cm1 can be seen, which should be ascribed to the adsorption of linear Cu+–CO species,28,47 demonstrating the presence of Cu+ on the surface of the Cu/TC-60 : 1 catalyst. Fig. 14(b) shows the results of CO adsorption on the catalyst with the temperature proceeding from 50 to 325 1C. Four peaks in the range of 1300–1700 cm1 are as similar as the adsorption results at room temperature and the species belonged to are identical to those of room temperature. But their intensities get weaker with the temperature increasing and disappear completely at 250 1C due to their decompositions.


Fig. 14 In situ FT-IR results of CO (10% in volume) adsorption on the Cu/TC-60 : 1 catalyst (a) from 0 to 30 min and (b) from 50 to 325 1C.

Identically, the adsorption of linear Cu+–CO species appears at 2103 cm1. While, this band shifts to 2120 cm1 with the temperature increasing to 325 1C. On the other hand, increasing the temperature to 200 1C results in the maximum intensity of Cu+–CO, and further increasing the temperature to 325 1C leads to the greatly weakness of this species. Generally, the adsorption of CO molecules on Cu2+, Cu+, and Cu0 gives rise to peaks with characteristic vibrational frequencies at about 2220–2150 cm1, 2160–2080 cm1, and below 2130 cm1 respectively, and at room temperature, Cu+–CO is the most stable adsorption.68 From these information, it is deduced that copper species go through stepwise reduction, namely from Cu2+ to Cu+ to Cu0, similar to the results of Spoto et al.69 and Dong et al.70 In addition, increasing the temperature to 100 1C results in the increase of Cu+–CO species accompanied by the increasing enhancement of gaseous CO2 at 2360 and 2339 cm1, which indicates that Cu2+ are reduced continuously to Cu+ and further to Cu0 by CO. 3.8.2. Single NO interaction with the Cu/TC-60 : 1 catalyst. Fig. 15 shows the in situ FT-IR results of NO adsorption on the Cu/TC-60 : 1 catalyst. The results of NO adsorption on the


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catalyst with the time proceeding from 0 to 30 min at room temperature are displayed in Fig. 15(a). Bridging bidentate nitrates exhibit a NQO stretching mode at 1622 cm1, and a remarkable NO2 symmetric vibration band at 1075 cm1;28,42,71 the peaks at 1483 and 1196 cm1 are assigned to bridging monodentate nitrates;47 the linear nitrites give a vibration band at 1315 cm1;28,42 the weak peak at 1902 cm1 should be assigned as chemisorbed NO on Cu2+ species.47 Their intensities all get stronger with the increase of the time. Fig. 15(b) shows the results of NO adsorption on the catalyst with the temperature proceeding from 50 to 325 1C. The above mentioned peaks can also be observed at low temperature, but, bridging bidentate nitrates and the linear nitrites disappear as the temperature increases to 200 1C and bridging monodentate nitrates disappear as the temperature increases to 250 1C due to their poor stability. Simultaneously, at 150 1C, two new peaks appear at 1570 and 1286 cm1, which should be attributed to

Fig. 15 In situ FT-IR results of NO (5% in volume) adsorption on the Cu/TC-60 : 1 catalyst (a) from 0 to 30 min and (b) from 50 to 325 1C.


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the NO2 symmetric and asymmetric vibration of chelating bidentate nitrates and chelated nitrites, respectively,42,72 and raising the temperature further up to 325 1C leads to the decrease of these bands (but it is difficult to be completely desorbed/transformed/decomposed).73 Two respects can be inferred, one is that rearrangement rather than desorption or decomposition occurs among these species; another is that the adsorption of chelating bidentate nitrates and chelated nitrites is stable under NO atmosphere at high temperature. Furthermore, another one peak appeared at 1360 cm1 is attributed to hyponitrites at 200 1C. This is resulted from the formation of oxygen vacancies at CuO-promoted interfacial sites by the strong interaction of copper with the ceria of the TC-60 : 1 support and via electron transfer from a reduced Lewis center (Ce3+ or Cu+) to a NO molecule.32,73 These NO species can dimerize to yield N2O2, which can easily decompose to form N2O at low temperatures.74 The weak peak of Cu2+–NO at 1902 cm1 weakens increasingly with the temperature increasing and disappears completely at 300 1C. 3.8.3. NO and CO co-interaction with the Cu/TC-60 : 1 catalyst. In situ FT-IR was performed under the simulative reaction conditions in order to understand the interaction of reactants with the catalysts, which can provide the information about the changes in the surface adsorbed species and the copper state. As shown in Fig. 16(a), similar to single NO adsorption results, bridging bidentate nitrates appeared at 1623 and 1078 cm1, bridging monodentate nitrates are observed at 1476 and 1199 cm1, linear nitrites and chemisorbed NO on Cu2+ species can also be seen at 1316 and 1905 cm1, respectively. Identically, their intensities all get stronger with the increase of the time. The difference is that the adsorption of linear Cu+–CO species appears at 2104 cm1 when the adsorption time is the 1st min, but afterward it decreases and disappears completely when the adsorption time is the 5th min, indicating that initially CO can be adsorbed on the Cu/TC-60 : 1 catalyst but are quickly substituted by NO (the chemisorbed NO on Cu2+ species starts from the 2nd min) due to its unpaired electron as a result of producing complex types of nitrite-/nitrate-like species which are chemisorbed on the surface of these samples.32,42 Bridging bidentate nitrates (1636 and 1077 cm1), bridging monodentate nitrates (1504 and 1195 cm1), linear nitrites (1314 cm1), and Cu2+–NO species (1917 cm1) can be detected at low temperature from Fig. 16(b), and disappear above 200 1C. At 100 1C, however, chelating bidentate nitrates appear at 1564 cm1 and a new adsorption appears at 1543 cm1 which is attributed to bridged nitro,47 and chelated nitrites cannot be detected at about 1286 cm1, which indicates that the rearrangement among these species in NO and CO atmosphere is easier than that of single NO. A few unlabeled peaks appear at 1570–1480 cm1 above 200 1C, one possible reason is that CO2 are adsorbed on the Cu/TC-60 : 1 catalyst and develop bridging carbonates, which is probably attributed to surface oxygen vacancies. The intensity of CO2 peak at 2360 cm1 increases to a maximum at 200 1C and consequently decreases, which can act as evidence of the development of bridging carbonates. Starting from 150 1C, CO2, N2O, Cu+–CO species, and hyponitrites appear at


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Cu+ species and surface oxygen vacancies; secondly, further increasing temperature to 300 1C leads to the disappearance of gaseous N2O owing to being further reduced to N2 (which has no FT-IR signals), which is supported by the results of activity and selectivity; lastly, as the Cu+ can be reduced to Cu0 remarkably in a single CO atmosphere at 325 1C, and Cu0 can also be reoxidized to Cu+ by NO under the same condition,47 it is reasonable to conclude that in the NO + CO atmosphere, a Cu+/Cu0 redox cycle exists in the Cu/TC-60 : 1 catalyst at high temperature, and combined with the results in NO + CO reaction, Cu+/Cu0 should be closely related to the activity and selectivity in high temperatures. 3.9.

Possible reaction mechanism for NO reduction by CO

Based on the above results and discussion, a possible reaction mechanism for NO reduction by CO is tentatively proposed to further understand the nature of this reaction, as shown in Fig. 17. Taking the Cu/TC-60 : 1 catalyst as an example, when exposing the catalyst to CO and NO mixture gases, NO molecules are preferentially adsorbed on the surface of the catalyst because their unpaired electrons inhibit the adsorption of CO species.28,71 According to the report by Xiong et al.,75 the dissociation of NO is the key step for NO reduction by CO, and an oxygen vacancy can activate the N–O bond to promote this dissociation. Surface oxygen vacancies have been reported to have high reactivity toward N2O dissociation.42 In our case, the above adsorbed NO species can be dissociated with increasing temperature to expose active sites to adsorb CO species. Simultaneously, the contact of CO and the Cu/TC-60 : 1 catalyst can result in the reduction of the catalyst by CO to form some Ti3+ and Ce3+ and more surface oxygen vacancies. On the one hand, the surface Ce3+ can provide more adsorbed-sites to adsorb CO molecules;28 on the other hand, the synergistic effect of Ti3+ + Cu2+ 2 Ti4+ + Cu+ and Ce3+ + Cu2+ 2 Ce4+ + Cu+ is conducive to the formation of more Cu+. Furthermore, Cu2+ can be reduced to Cu+ during the heating procedure. As a result, more CO adsorption sites can be generated due to more Cu+. Fig. 16 In situ FT-IR results of NO (5% in volume) and CO (10% in volume) adsorption on the Cu/TC-60 : 1 catalyst (a) from 0 to 30 min and (b) from 50 to 325 1C.

2360, 2240, 2105, and 1366 cm1, respectively. The appearance of Cu+–CO species indicates that the desorption/conversion/ dissociation of adsorbed NO species exposes active sites to adsorb CO species, and the intensity of the peak increases to a maximum at 200 1C. At the same time, the peak of the bridged nitrates and linear nitrites also decrease sharply at this temperature. Thus, it can be concluded that the large amounts of Cu+–CO species consequently reduce the bridged nitrates and linear nitrites. As temperature increases further, the peak intensity of Cu+–CO adsorption gets weaker and finally disappears at 325 1C. According to the above results and the heating procedure, some interesting phenomena can be observed. Firstly, the appearance of CO2, N2O, Cu+–CO species, and hyponitrites at 150 1C suggests that the reaction between NO and CO takes place remarkably due to the synergistic effect of


Fig. 17 Possible reaction mechanism for NO reduction by CO over the Cu/TC-60 : 1 catalyst.


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CO molecules adsorbed on Cu+ can react with O radicals resulting from the dissociation of NO species on surface oxygen vacancies to produce CO2, the rest of the N radicals are able to recombine with NO or CO to give N2O or NCO, respectively, or combine with another N to generate N2. Moreover, Cu2+ can be further reduced to Cu0 metal and N2O can be further reduced to N2 in higher temperatures and according to the literature,47 the Cu0 species is the active species for N2O decomposition. Thus, the Cu+/Cu0 redox cycle should play an important role in the reduction of N2O to N2. Since N2O are evolved into N2 and O, then neighbouring CO combines with O to form CO2 and new active sites on the surface are regenerated. As a result, a considerable amount of N2 and CO2 can be observed.

4. Conclusions Based on the above experimental results and discussion, it can be concluded that a small quantity of CeO2 doping into the TiO2 support will cause an obvious change in the properties of the catalyst. The present work studies the effect of different Ti/Ce molar ratios supported by CuO on the texture, structure, redox property, surface state, and activity of NO reduction by CO based on a few CeO2. Several major conclusions can be obtained as follows: (i) The surface area of the TC-10 : 1 and Cu/TiO2 supports is the largest and the least, respectively, but neither support supported by CuO has the best catalytic performances, indicating that the influence of the surface area is not main in the present work. (ii) Copper oxide species supported on TC supports are much easier to be reduced than those supported on the pure TiO2 and CeO2 surface-modified TiO2 supports prepared by stepwise impregnation (first CeO2 and then CuO). This phenomenon, especially for the Cu/TC-60 : 1 catalyst, should be due to the changing of the electron charge density (ECD) by the CeO2 doping and the distortion of octahedral coordination structures of Cu2+. (iii) The Cu/TC-60 : 1 catalyst shows the highest activity and selectivity due to more oxygen vacancies, higher mobility of the surface and lattice oxygen at lower temperature (which contributes to the regeneration of oxygen vacancies, and the best reducing ability), the most content of Cu+, and the strongest synergistic effect between Ti3+, Ce3+ and Cu+. On the other hand, the CeO2 doping into TiO2 promotes the formation of a Cu+/Cu0 redox cycle at high temperatures, which has a crucial effect on N2O reduction.

Acknowledgements The financial support from National Basic Research Program of China (973 program, No. 2012CB21500203), the China Postdoctoral Science Foundation (No. 2014M550451), the Nature Science Foundation of Guangxi Province (No. 2014GXNSFBA118036), the Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification


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Technology (Nos. 2013K009, 2013Z001), the Project of Guangxi Postdoctoral Special Foundation (Nos. Y304002007, B41054) is gratefully acknowledged.

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Gold-copper nanoalloys supported on TiO2 as photocatalysts for CO2 reduction by water.

CO Oxidation at the Interface between Doped CeO2 and Supported Au Nanoclusters.

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