CeO2 in CO oxidation and NO + CO model reaction

Applied Surface Science 389 (2016) 1033–1049

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The influence of Mn-doped CeO2 on the activity of CuO/CeO2 in CO oxidation and NO + CO model reaction Changshun Deng, Qingqing Huang, Xiying Zhu, Qun Hu, Wenli Su, Junning Qian, Lihui Dong ∗ , Bin Li ∗ , Minguang Fan, Caiyuan Liang Guangxi Key Laboratory Petrochemical Rescource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China

a r t i c l e

i n f o

Article history: Received 24 May 2016 Received in revised form 3 August 2016 Accepted 6 August 2016 Available online 9 August 2016 Keywords: CO oxidation NO + CO model reaction Mn-doped CeO2 Oxygen vacancies Reaction mechanism

a b s t r a c t This work is mainly focused on the investigation of the influence of Mn-doped CeO2 supported by CuO on the physicochemical and catalytic properties for CO oxidation and NO + CO model reaction. The obtained samples were characterized using N2 -physisorption (BET), XRD, LRS, TEM, EDS-Mapping, ICPAES, XPS, H2 -TPR, O2 -TPD, in situ DRIFTS, CO oxidation, and NO + CO model reaction. The results imply that appropriate doping MnOx into the lattice of CeO2 will cause an obvious change in the properties of the catalyst and the Cu/CeMn-10: 1 catalyst shows the largest specific surface area, the most uniformity of structure, and the most extent of lattice expansion. A few addition of MnOx is more conducive to the generation of low valence manganese ion in the process of calcination, which may contribute to the synergetic introduction. This further results in more Cu+ due to the shifting of redox equilibrium (Cu2+ + Ce3+ ↔ Cu+ + Ce4+ ) to right, as well as more oxygen vacancies. Moreover, the capability of Cu/CeMn10: 1 on desorb/transform/decompose of the adsorbed NO species is more effective than that of Cu/CeO2 . The results of catalytic performance show that Cu+ /Cu0 species play a key role, and the activity is mainly related to the specific surface area, the content of Cu+ and Ce3+ , the reduction, desorption capability of chemisorbed O2 − (and/or O− ) species as well as adsorption behaviors of these catalysts for CO oxidation and NO + CO reaction. Finally, possible reaction mechanisms are tentatively proposed to understand the reactions. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Given that the worsening air pollution and global warming threats are harmful to human health and the environment across the world, it has been a worldwide topic to build a low carbon society [1,2]. CO oxidation is a prototype reaction in heterogeneous catalysis and has been paid huge attention for many years, and two main reasons should be considered. Technologically, CO oxidation is a major reaction in vehicle exhaust control [3], CO2 lasers [4], and other industrial processes [5]; theoretically, CO oxidation is a relatively simple reaction and it is significant for a model system to study the mechanism of heterogeneous catalysis process [6]. On the other hand, nitrogen oxides (NOx ) are one of the main air pollutant particularly correlated to photochemical smog, acid rain, and ozone depletion [7–10]. Chinese legislation of NOx emissions from both stationary and mobile sources is increasingly rigorous [8]. Over the

∗ Corresponding authors. E-mail addresses: [email protected] (L. Dong), [email protected] (B. Li). http://dx.doi.org/10.1016/j.apsusc.2016.08.035 0169-4332/© 2016 Elsevier B.V. All rights reserved.

past several years, noble metals supported on reducible or irreducible supports are used for catalysis and unique and obvious activities can be presented no matter for CO oxidation [11–13] or for NO + CO reaction [14,15]. However, the high prices limit their wide use. In recent years, tremendous attention has been paid to base metals [16,17], especially to copper oxide [18–20]. Ceria has been widely used in energy, environmental, material, and catalysis fields in recent decades due to its outstanding redox ability and high oxygen storage/release capacity (OSC) which is associated with the formation of oxygen vacancies and the Ce4+ /Ce3+ redox couple [2,10,20,21]. On the other hand, however, the practical application of pure ceria is highly discouraged due to its poor thermal stability and low specific surface area. It is common knowledge that the introduction of foreign metal cations into the ceria system will improve its redox behavior and catalytic activity obviously due to the formation of more oxygen vacancies [22–24]. In CuO–CeO2 binary oxides, a strong catalytic synergy interaction between copper oxide and ceria has ever been found [25,26]. It has also been found that MnOx with various labile oxygen plays an important role in the catalytic reaction [27]. MnOx –CeO2 mixed

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oxides possess much higher catalytic activity than pure MnOx or CeO2 ,which is due to a favorable synergistic interaction between manganese oxide and ceria. For example, Machida et al. [28] found that the redox of Mn ions with simultaneous oxygen equilibration with the gas phase should play an important role in facilitating the oxidative adsorption of NO. Li et al. [29] reported that appropriate doping of Mn into CuO CeO2 catalyst was conducive to the formation of a more stable solid solution with a larger specific surface area and smaller particle size, and the redox properties of the catalysts were also increased, which also promoted the selective oxidation performance of CO in hydrogen-rich streams. Xu et al. [30] found that many reasons resulted in the best catalytic activity and the widest reaction window, for MnOx –CeO2 /10% WO3 –ZrO2 catalyst, one of which was attributed to the appropriate surface atomic ratio of Mn: Ce. Kinetic studies also showed that the incorporation of Mn into CeO2 enhanced catalytic activities [31,32]. Although MnOx is widely regarded as dopant and used in many catalytic reactions [27–32], the doping amount of MnOx is relatively large. Researches [20,33] have suggested that a small amount of dopant may cause obvious change on the characteristic of the materials. However, as far as we know, relative studies are few for small amount of MnOx (which is regarded as dopant). Furthermore, copper oxide supported on MnOx –CeO2 mixed oxides used for CO oxidation or NO + CO reaction is scarcely reported. And the study of CO oxidation and NO + CO reaction to which they are simultaneously applied is vacant. The modified storage capacity of CeO2 by MnOx may also improve the activity of supported Cu species for above reactions. Therefore, the influence of Mn-doped CeO2 on the activity of CuO/CeO2 is necessary to be studied. In this work, a series of MnOx -doped CeO2 with different Ce/Mn molar ratios which were 20: 1, 10: 1, 5: 1, 5: 2, 5: 3, 5: 4, and pure CeO2 supported by CuO were synthesized by inverse co-precipitation method, respectively, and the obtained samples were characterized using N2 -physisorption (BET), XRD, LRS, TEM, EDS-Mapping, ICP-AES, XPS, H2 -TPR, O2 -TPD, in situ DRIFTS, CO oxidation, and NO + CO model reaction. This study is mainly focused on: (i) exploring the influence of MnOx with different molar ratio doping into CeO2 on texture, structure, chemical composition, surface state, redox property, and activity of CO oxidation and NO + CO model reaction over CuO supported on MnOx -doped CeO2 catalysts; (ii) investigating the interaction of CO and/or NO with CuO supported on MnOx -doped CeO2 catalysts via in situ DRIFTS technique to understand the nature of CO oxidation and NO + CO model reaction.

2. Experimental 2.1. Catalyst preparation MnOx -doped CeO2 supports with different Ce/Mn molar ratios i.e., 20: 1, 10: 1, 5: 1, 5: 2, 5: 3 and 5: 4 were synthesized by inverse co-precipitation method, respectively. Mn(NO3 )2 (50% solution) and Ce(NO3 )3 ·6H2 O were dissolved in deionized water, then excess ammonia solution (a precipitating agent) was then added into the solution, drop by drop, with vigorously stirring until pH = 10.0. After stirring for 3 h and aging for 24 h, then the mixture solution filtered and washed with deionized water and absolute alcohol for several times, respectively. The obtained solid was dried at 80 ◦ C for about 10 h and then calcined in a muffle furnace at 500 ◦ C for 4 h. The samples were labeled as CeMn-20: 1, CeMn-10: 1, CeMn-5: 1, CeMn-5: 2, CeMn-5: 3, and CeMn-5: 4, respectively. Pure CeO2 was synthesized using the same method. The CuO/CeMn catalysts were synthesized by incipient-wetness impregnating on the support with the Cu(NO3 )2 solution. After vigorously stirring for 2 h, the mixture was evaporated at 100 ◦ C and dried at 80 ◦ C for about

10 h, and then calcined at 450 ◦ C for 4 h. These samples were labeled as Cu/CeMn-20: 1, Cu/CeMn-10: 1, Cu/CeMn-5: 1, Cu/CeMn-5: 2, Cu/CeMn-5: 3, and Cu/CeMn-5: 4, respectively. The synthesis of CuO/CeO2 sample was also similar. The CuO loading in all the catalysts was shown as the weight ratio of CuO/(CeMn) or CuO/CeO2 and fixed at 12 wt.% of the support. 2.2. Catalyst characterization Textural characteristics of these samples were obtained on a Micrometrics TriStar II 3020 analyzer with nitrogen adsorption at 77 K, the specific surface area was expressed by the BrunauerEmmet-Teller (BET) method and the pore distribution was used by the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) patterns were gotten with a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu K␣ radiation (0.15418 nm). The voltage and current are set as 40 kV and 40 mA. Laser Raman spectra (LRS) was detected by a Renishaw RM1000 spectroscope, the laser light wavelength is 514.5 nm. Transmission electron microscopy (TEM) was used by a JEOL JEM-2000EX and a FEI Tecnai G2 Spirit at a voltage of 120 kV. HRTEM images were obtained on a Tecnai G2 F30 S-TWIN (FEI Company). The EDS-Mapping was used by a scanning electron microscopy (SEM, JEOL JSM-6360LV) equipped with an energy-dispersive X-ray spectrometer (EDS). Bulk compositions of these samples were detected by elemental analysis, a inductively coupled plasma atomic emission spectrometer (ICP-AES) system using PerkinElmer Optima 5300 DV with a radiofrequency power of 1300 W. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250Xi high performance electron spectrometer (Thermo Fisher Company), using monochromatic Al Karadiation (15 mA, 15 kV). The electron binding energy is based on C 1 s (284.8 eV), and the sample irradiation area and detecting depth are 2 mm × 1 mm and 2–5 nm, respectively. H2 -TPR was performed by an automated chemisorption analyzer of Finetec Instruments. 50 mg of the sample was heated in N2 with 50 mL/min from ambient temperature to 110 ◦ C and held for 1 h, then cooled to ambient temperature in N2 atmosphere and switched to the stream of 7 vol.% H2 /Ar with 10 mL/min and held for 0.5 h. Then the sample was heated from ambient temperature to 800 ◦ C in 7 vol.% H2 /Ar with a heating rate of 10 ◦ C/min. The consumption of H2 was continuously monitored with a thermal conductivity detector (TCD). O2 -TPD was also performed by an automated chemisorption analyzer of Finetec Instruments. 200 mg of the sample was heated in He with 50 mL/min from ambient temperature to 200 ◦ C and held for 1 h, then cooled to ambient temperature in He atmosphere and switched to pure O2 with 10 mL/min and held for 0.5 h. Then it was purged by He for 0.5 h for removal of residual oxygen. Then the sample was heated from ambient temperature to 700 ◦ C in helium at a heating rate of 10 ◦ C/min. The consumption of O2 was continuously monitored with a thermal conductivity detector (TCD). DRIFTS spectra was performed at ambient temperature on a Nicolet 5700 DRIFTS spectrometer at a resolution of 4 cm−1 and a number of scans: 32. The spectra of empty IR cell was collected in NO or/and CO atmosphere at various target temperatures as background. The catalysts of an amount of small crucible were mounted in a quarts IR cell and pretreated for 1 h at 300 ◦ C in flowing N2 atmosphere and then cooled to ambient temperature. The sample wafers were exposed to a controlled stream of CO-Ar of 10% CO in volume or/and NO-Ar of 5% NO in volume at a rate of 10.0 mL/min for 30 min. 2.3. Catalytic performance tests The activities in CO oxidation were tested in a flow, fixed-bed micro-reactor with reaction gas composition of 1.6 vol.% CO, 20.8

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3. Results and discussion 3.1. Catalytic performance of the catalysts (CO oxidation and NO + CO reaction) The catalytic performances of these samples in CO oxidation reaction and of fresh and thermal treated samples at 100 ◦ C are shown in Fig. 1(a) and (b), respectively. For Fig. 1(a), it is obvious that pure CeO2 is almost no activity in this temperature window, by contrast, when the molar ratio of Ce to Mn reaches 5: 2, its catalytic activity is improved to a certain extent, suggesting that appropriate doping of MnOx into CeO2 is contributive to the enhancement of catalytic performance in CO oxidation. The CO conversions increase rapidly when CuO was supported on these supports and the CO conversions gradually increase with the increase of MnOx in CeO2 until 10: 1 (the molar ratio of Ce: Mn), that is, Cu/CeMn-10: 1 catalyst displays the best catalytic activity and the completely transformed temperature of CO is 100 ◦ C, which is much lower than that in the literatures [34,35]. Further increasing the content of MnOx leads to the decrease of CO conversions, implying that appropriate doping amount is of benefit to from the enhancement of catalytic performance. It can be seen from Fig. 1(b) that the CO conversion of thermal treated Cu/CeO2 catalyst decreases a lot compared with fresh catalyst demonstrating the thermal stability of pure CeO2 is poor. The more prominent is that CO can be completely transformed at 100 ◦ C for both fresh and thermal treated catalysts when the molar ratio of Ce to Mn is between 10: 1 to 5: 1. Similarly, further increasing the content of MnOx results in the decrease of CO conversions no matter for fresh or for thermal treated catalysts.The above two parts highlighted in yellow color are one paragraph, they should be merged. Catalytic reduction of NO by CO was also performed to evaluate the catalytic performance of these catalysts. The results of NO conversion, CO conversion, N2 selectivity, and N2 yield as a function of reaction temperatures over these samples are presented in Fig. 2. Similarly, the catalytic activity of pure CeO2 is very poor, but the NO conversion and CO conversion of CeMn-5: 2 sample are considerable in Fig. 2(a) and (b). It can be found that when the temperature is below 175 ◦ C, the activity of these catalysts is considerable except CeO2 and CeMn-5: 2 sample, but the selectivity and yield are basically below 20%. Interestingly, the increase of

(a) 100 CeO2 CeMn-5:2 Cu/CeO2

CO conversion (%)

80

Cu/CeMn-20:1 Cu/CeMn-10:1 Cu/CeMn-5:1 Cu/CeMn-5:2 Cu/CeMn-5:3 Cu/CeMn-5:4

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vol.% O2 , and 77.6 vol.% N2 and the total flow rate of the reactants was kept constant at 24,000 mL g−1 h−1 . The reactor was a 4 mm i.d. (6 mm o.d., length 35 cm) quartz tube housed in a furnace. The used amount of catalyst was 50 mg. The catalysts were pretreated in a N2 stream at 100 ◦ C for 1 h before switched to the reaction gas stream and the experimental data were determined after steady state was achieved. They were collected in single measurement. Tail gas was analyzed using gas chromatograph with a thermal conductive detector (TCD). The thermal treatment of catalysts was carried out in a quartz tube and the catalysts were heated in a highly purified N2 stream at 450 ◦ C for 0.5 h. The catalytic tests of these catalysts for NO + CO reaction were carried out with reaction gas composition of 5% NO, 10% CO, and 85% He in volume and with a space velocity of 24,000 mL g−1 h−1 . 50 mg of the sample was fitted in a quartz tube and pretreated in a highly purified N2 stream at 110 ◦ C for 1 h to remove the impurities and then cooled to ambient temperature and the mixed gases were switched on. In addition, 10% H2 O will be introduced into the reaction atmosphere when necessary. Two columns (length, 1.75 m; diameter, 3 mm) and two thermal conductivity detectors (T = 100 ◦ C) were used for analyzing the products (Column A with Paropak Q for separating CO2 and N2 O, column B packed with 5A molecular sieves (40–60 M) for separating N2 , NO, and CO).

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90

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Fresh Thermal treatment at 450 °C for 0.5 h 80

Cu /C e

O 2

Cu /Ce Mn

Cu /C -20 :1

Cu Cu Cu Cu /Ce /C e /Ce /Ce eM Mn Mn Mn Mn n -1 -5: -5: -5: -5: 0:1 4 3 2 1

Fig. 1. The catalytic performance of these samples in CO oxidation reaction (a) and of fresh and thermal treated samples at 100 ◦ C (b). Reaction conditions: 1.6% CO, 20.8% O2 , and balanced N2 ; SV = 24,000 mL g−1 h−1 .

the activity is very slow between 175 and 250 ◦ C, but the selectivity and yield are rising sharply in this temperature range for these catalysts. According to our previous work [20], the reason may be that the reaction of NO reduction by CO over copper-based catalysts proceeds in two sequential steps: firstly, NO is mainly converted to N2 O; secondly, N2 O is further transformed to N2 . When further increasing the temperature up to 300 ◦ C, both the activity (except CO conversion), selectivity, and yield over these catalysts are close to 100%. Furthermore, it can be noticed that Cu/CeMn-10: 1 sample shows the best activity, selectivity, and yield in all samples, which is in good agreement with the result of CO oxidation. In addition, the catalytic stability (under a constant temperature) of Cu/CeMn-10: 1 sample was given in Fig. S1 and the result indicates that the catalytic activity is stable in the current condition. It can be learned from the above results that Mn-doped CeO2 supported by CuO catalysts improve the catalytic activities for both CO oxidation and NO + CO reaction effectively and Cu/CeMn-10: 1 sample displays excellent catalytic activity. However, the reason about the effects on the doping of MnOx and the content of doped MnOx is not clear from the above mentioned. Therefore, in order to gain a clear idea of the reason for the different catalytic performances, comprehensive characterizations of these samples were carried out and the results are listed and discussed in the following sections.

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3.2. Textual characterization (N2 -physisorption)

(a)100

NO conversion (%)

80

60 CeO2 CeMn-5:2 Cu/CeO2 Cu/CeMn-20:1 Cu/CeMn-10:1 Cu/CeMn-5:1 Cu/CeMn-5:2 Cu/CeMn-5:3 Cu/CeMn-5:4

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3.3. Structural characterizations (XRD, LRS, TEM, and EDS-Mapping)

N2 yield (%)

Cu/CeMn-20:1 Cu/CeMn-10:1 Cu/CeMn-5:1 Cu/CeMn-5:2 Cu/CeMn-5:3 Cu/CeMn-5:4

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The N2 adsorption–desorption isotherms and corresponding BJH pore size distribution curves of these supports are displayed in Fig. 3. For Fig. 3(a), CeMn-20: 1 and CeMn-10:1 supports reveal the IV-type isotherms with evident H3-type hysteresis loops. This is the characteristic of mesopores (2–50 nm) with narrow slit-like shapes or plate-like particles, i.e., these supports contain mesopores; other supports exhibit the IV-type isotherms with evident H2-type hysteresis loops, however, which is typical for wormholelike mesostructure and interstice mesoporous structure formed by nanoparticle assembly according to IUPAC [36]. The difference of the type of hysteresis loops suggests that a few amount of doping of MnOx into CeO2 produce notable change compared with pure CeO2 and relative more amount of doping of MnOx into CeO2 . For H3-type supports, they display a pronounced capillary condensation step starting at P/P0 = 0.65, the jumps at p/p0 = 0.65–0.9 are ascribed to the capillary condensation of the mesopores produced by removal of silica walls, and the steeper jumps at P/P0 = 0.9–1.0 can be attributed to the interstitial pores between particles [37]. Moreover, the pore size distribution curves of these supports determined by the BJH method from desorption branch are shown in Fig. 3(b). It is apparent that CeMn-20: 1 and CeMn-10:1 supports display uniform mesopore size distributions and the uniformation of pore size distributions of the latter is better than that of the former. By contrast, the pore size distributions of others are not uniform and the more the amount of doped MnOx , the worse the uniformation of pore size distributions. The textural data of these supports are listed in Table 1. Comparing with pure CeO2 , when Mn2+ is doped into the lattice of CeO2 , both the specific surface area and pore volume increase, which is beneficial to the contact with the reactant molecules and further to improvement of the catalytic performance. These changes may be related to the crystallite size of these samples to some extent. In other words, the incorporation of Mn2+ into the lattice of CeO2 can improve the textural property effectively. Especially, CeMn-10:1 support possesses the largest specific surface area and the second largest pore volume. Another point should be pointed out that the larger pore size and wider pore distribution with the smaller specific surface area and pore volume in the CeMn-5: 4 support are not conducive to the dispersion of active components, which may be one reason for the poor catalytic activity of CO oxidation and NO + CO reaction. Furthermore, as reported in literature [38], the closure point of hysteresis loop for the type IV isotherm is closely related to the size of pores. At low relative pressure, the larger the pore size, the higher the pressure of closure point. That is the reason that the closure point of hysteresis loop of CeMn-5: 4 support appears at P/P0 = 0.75 while those of others are below the value.

300

Temperature (°C)

Fig. 2. The results of (a) NO conversion (%), (b) CO conversion (%), (c) N2 selectivity (%), and (d) N2 yield (%) over these catalysts as a function of reaction temperatures. Reaction conditions: 5% NO, 10% CO, and balanced He; SV = 24,000 mL g−1 h−1 .

Fig. 4 shows the XRD patterns of these samples. We can find that the crystalline form gradually becomes weak with the increase of the content of MnOx , and only the diffraction peaks belonged to cubic fluorite-type CeO2 (PDF-ICDD 34-0394) can be observed. The characteristic peaks ascribed to MnOx are absent, and the diffraction angles of peaks of CeMn samples shift slightly compared to CeO2 , suggesting that Mnx+ have been well doped into the lattice of CeO2 and the uniform solid solutions can be formed [20,39]. The diffraction peaks of these CeMn mixed oxides are broadened resulted from crystallite nanodimensions effect compared with CeO2 [39,40], which can be supported by the crystallite size of these samples calculated from the diffraction peak of the (1 1 1) plane by Debye–Scherrer equation (listed in Table 1). Usually, the

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049 (a)

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Fig. 3. The (a) N2 adsorption-desorption isotherms, and (b) BJH pore size distribution curves of these supports. Table 1 Textural data, crystallite size, the FWHM of the main Raman line, and the peak area ratio in Raman of these samples. BET surface area (m2 g−1 )

Supports CeO2 CeMn-20:1 CeMn-10:1 CeMn-5:1 CeMn-5:2 CeMn-5:3 CeMn-5:4 a b

63.6 93.3 96.5 95.2 88.0 76.4 71.9

Pore volume (cm3 g−1 ) 0.159 0.415 0.383 0.315 0.328 0.352 0.362

Average pore diameter (nm)

Crystallite size by XRD (nm) a

10.0 17.8 15.9 11.9 14.9 18.4 20.1

13.11 (14.16) 11.53 (11.21)a 11.49 (11.67)a 9.76 (9.72)a 8.93 (11.26)a 9.58 (12.69)a 9.19 (11.76)a

FWHM of Reman line (cm−1 ) b

32.94 (27.59) – 45.25 (39.96)b – 54.48 (41.41)b – 58.18 (44.06)b

A592 /AF2g 0.087 – 0.311 – – – –

The crystallite size of CuO supported catalysts. The FWHM of Reman line (cm−1 ) of CuO supported catalysts.

(a) CeMn-5:4

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Fig. 4. The XRD patterns of (a) CeMn and CeO2 supports and (b) Cu/CeMn and Cu/CeO2 catalysts.

crystallite size of these CeMn mixed oxides is smaller than that of CeO2 , which may be resulted from the introduction of Mnx+ into the lattice of CeO2 , and further suppresses the crystal growth of the cubic phase [20,40]. In addition, noting from Fig. 4(a) that the

peak of (1 1 1) plane shifts to high angle direction gradually compared to pure CeO2 when the molar ratio of Ce/Mn is more than 10: 1, which suggests that the lattice shrinks gradually and should be attributed to the effect that the ionic radius of Mn4+ (0.56 Å) (Mn3+ (0.62 Å) or Mn2+ (0.67 Å)) is smaller than that of Ce4+ (0.92 Å), the incorporation of Mnx+ into the lattice of CeO2 results in the contraction and distortion of the lattice [3,22,29,39]. But what makes it interesting is that the peak of (1 1 1) plane shifts to low angle direction compared to pure CeO2 when the molar ratio of Ce/Mn is 20: 1 and 10: 1, implying their lattices expand. The changes in the lattice parameter have significant influence on the concentration of oxygen vacancies and the strong interaction of MnOx with the ceria phase [41,42]. It has been reported that a simple reduction of two Ce4+ cations to two Ce3+ cations with one oxygen vacancy formed results in a ceria cell lattice expansion [43], since Ce3+ is bigger than Ce4+ (the following XPS results have proved the existence of Ce3+ in Cu/CeMn samples). therefore, it is suggested that Mnx+ doped into the lattice of CeO2 can interact with ceria following the reaction Ce4+ + Mn2+ ↔ Ce3+ + Mn3+ or Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ . Particularly, the lattice of CeMn-10: 1 support has the most extent of expansion, which indicates that the content of Ce3+ is probably the most. After supporting CuO, the diffraction peaks of cubic fluorite phase of ceria can also be observed for all catalysts in Fig. 4(b). Two very weak peaks, characteristic of a CuO phase (tenorite), are also detected at 35.5◦ and 38.8◦ for all samples with 12% copper amounts, indicating that bulk CuO crystallites are formed on these catalysts. The data (Table 1) show that the crystallite size of CeO2 , CeMn-5: 2, CeMn-5: 3, and CeMn-5: 4 samples increases a little after the introduction of copper oxide species. The phenomenon may be due to the second calcination of these samples after the impregnation of copper oxide species. Similar phenomenon has also appeared in previous literature [39]. The crystallite size of CeMn-20: 1, CeMn-10: 1, and CeMn-5: 1 samples is almost unchanged after supported by CuO, however, which indicates that they possess better thermal stability than others. This is consistent

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with the results of thermal treated catalysts of CO oxidation to some extent. Raman spectroscopy were performed to detect the surface information of the samples to make up for lack of XRD. It can be seen from Fig. 5(a) that the Raman spectrum of CeO2 exhibits a main band at 464 cm−1 corresponding the F2g vibration model of the cubic fluorite type CeO2 lattice [37,39] and two weak bands around 252 and 592 cm−1 can be also observed, respectively. The broad band at 592 cm−1 corresponds to non-degenerate LO mode of ceria due to relaxation of symmetry rules, which is often linked to oxygen vacancies in the ceria lattice [44]. The other weak band at 252 cm−1 can be ascribed to the displacement of oxygen atoms from their ideal fluorite lattice positions [45]. Furthermore, the central locations of strong bands deviate from the 464 cm−1 of the pure CeO2 , which also suggests that dopant ions have been incorporated into the ceria lattice to form solid solutions. This is because the incorporated ions into the ceria lattice will result in the distortion of lattice, which has effect on the polarizability of the symmetrical stretching mode of [Ce–O8 ] vibrational unit and leads to the shift from that in the pure CeO2 [46]. These results are consistent with the analysis results of XRD that a solid solution is formed. When the molar ratio of Ce/Mn is more than 10: 1, a band appears at 654 cm−1 , which is due to Mn−O lattice vibrations in Mn2 O3 [47]. Besides, according to the literatures [29,48], the relative concentration of oxygen vacancies in the solid solutions can be represented by the area ratio of peaks 592 and 464 cm−1 (noted as A592 /AF2g ) (Table 1) and the results suggest that the incorporation of Mnx+ into the lattice of CeO2 increases the concentration of oxygen vacancies. For these catalysts (Fig. 5(b)), the Raman lines of CuO can be observed and appear at 298, 439, and 629 cm−1 [49], respectively, indicating that bulk CuO crystallites are formed on the surface of these supports. This is consistent with the results of XRD. In addition, the main bands of these catalysts red shifts slightly compared with their supports. This can be ascribed to the following two reasons. Firstly, the existence of interaction between support and active species [50]. Secondly, the possible reason is that the monolayer copper (rCu 2+ = 0.72 Å) is incorporated into the surface/subsurface layers of these supports. Another place should be pointed out that the FWHM (Table 1) decrease after the impregnation of copper oxide species. Graham et al. [51] have observed a correlation between the Raman line-width and the inverse particle size of ceria, pointing out that the defects in ceria lattice have also to be considered. This is in agreement with the results of XRD. The transmission electron microscopy (TEM) imagines and selected area electron diffraction (SAED) images of Cu/CeO2 and Cu/CeMn-10: 1 catalysts are shown in Fig. 6. They are composed of weakly aggregated particles with irregular shape and rough surface. The particle diameter of Cu/CeO2 catalyst is about 14.5 nm and of Cu/CeMn-10: 1 catalyst is around 10.2 nm, respectively, which is similar to the crystallite size determined by XRD. It is obvious that appropriate doping of Mnx+ into CeO2 increases the specific surface area to great extent. It can be seen from Fig. 6(c) that three type of periodicity of lattice fringes (∼0.307, 0.191, and 0.273 nm) can be observed, which corresponds to the (1 1 1) and (2 2 0) planes of CeO2 and (1 1 0) plane of CuO, respectively. However, the interplanar spacing corresponding to the (1 1 1) plane of CeO2 in Cu/CeMn10: 1 catalyst is between 0.312 and 0.317 nm, which is bigger than that in Cu/CeO2 catalyst. The appearance of this phenomenon is mainly due to the expansion of lattice when a few Mnx+ ions are doped into the lattice of CeO2 , which is in good accordance with XRD results. On the other hand, the crystal phase of CuO on the surface of CeO2 sample is more clear and structured than that of CeMn-10: 1 sample, indicating that the dispersing ability of CuO on the surface of CeO2 sample is more poor than that of CeMn10: 1 sample. In other words, the increased specific surface area is more conducive to the dispersion of CuO. The continuous rings in

the SAED patterns (Fig. 6(e) and (f)) indicate the presence of the tetragonal phase, as shown in the (2 0 0) facet with the spacing of ∼0.269 nm [52]. Similarly, the interplanar spacing of diffraction pattern in Cu/CeMn-10: 1 catalyst is bigger than that in Cu/CeO2 catalyst. MnOx crystallite planes are not observed in these two catalysts, though MnOx can be detected by ICP-AES analysis (Table 2), probably due to the incorporation of the foreign metal cation into the lattice of CeO2 [3,53], which is consistent with XRD and Raman results. Notably, the (1 1 1) plane is predominated in these two catalysts. The bulk compositions of partial catalysts were determined by ICP-AES, and the results were summarized in Table 2. From this table, the actual molar ratio of Ce/Mn is a little larger than that of theoretical for these catalysts, implying that a small part of Mn2+ is lost during the preparation process. By contrast, the actual loading of CuO is a little lower than that of theoretical for these catalysts. The surface compositions and elementary oxidation states of the samples can be well determined by XPS technology, but the binding energy scale should be calibrated using adventitious carbon (284.8 eV) due to the charging effects during XPS analysis. The XPS spectra of Ce 3d, Mn 2p, Cu 2p, Cu LMM, and O 1s for partial catalysts are shown in Fig. 7. For complex spectrum of Ce 3d in Fig. 7(a), eight components with the assignments are fitted. The two groups of spin-orbital multiplets (3d3/2 and 3d5/2 ) are referred to u and v and stretch in the binding energy range of 878–920 eV. The bands labeled u and v represent the 3d10 4f1 initial electronic state and are related to Ce3+ , the bands labeled u and v , u and v , u and v are referred to Ce4+ [20]. Thus, 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 in accordance with the results of XRD. Moreover, the relative content of Ce3+ can be evaluated by the area of u and v to others, that is the following equation: [37] Ce3+ (%) =

S  + Sv

u

(Su + Sv )

× 100

The results of relative contents of Ce3+ of these catalysts are presented in Table 3. The percentage of Ce3+ for Cu/CeO2 catalyst is 13.97%, while the percentage of Ce3+ for Cu/CeMn-10: 1 catalyst is the most (14.50%), which is consistent with the XRD results. Nagai et al. [54] concluded that the valence change of Ce (Ce4+ → Ce3+ ) is main due to the enhancement of homogeneity of the Ce and Zr atoms. Thus, the presence of Ce3+ was partly ascribed to the relative homogeneous Ce–O–Mn solid solution (Extra peaks are not displayed in XRD and LRS for Cu/CeMn-10: 1 catalyst, suggesting no phase segregation occurs). In addition, according to the LRS results, this phenomenon can be explained by the surface synergistic interaction of CuO and Ce–O–Mn solid solution, i.e., Ce3+ + Cu2+ ↔ Ce4+ + Cu+ , which is conducive to the formation of more surface oxygen vacancies. The valence states of MnOx on partial catalysts are shown in Fig. 8(b). Two main peaks, from Mn 2p3/2 and Mn 2p1/2 , are observed. By performing peak-fitting deconvolution, the Mn 2p3/2 spectra can be fitted with four peaks: 640.3, 641.3, 642.5, and 644.8 eV, which can be ascribed to Mn2+ , Mn3+ , Mn4+ , and Mn-nitrate, respectively [55,56]. Especially, the satellite peak corresponding to Mn2+ appears at 648.7 eV on Cu/CeMn-10: 1 catalyst. The percent of Mn4+ /(Mn2+ + Mn3+ + Mn4+ ) increases gradually with the increase of doped MnOx , which may be ascribed to the very top atomic layer of Mn under oxidative calcination atmosphere. Similar phenomenon can also be found in previous reports [42,55]. Li et al. [29] have been reported that a large number of Mn4+ ions doped into the CuO–CeO2 catalysts in high valence state existing on the surface can improve the catalytic activity of CO oxidation. Interestingly, however, our research is not consistent with this rule. On

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049 (a)

(b) 464

461

462

CeO2

252

Intensity (a.u)

461

Intensity (a.u)

1039

592

CeMn-10:1

654

CeMn-5:2 CeMn-5:4 465

Cu/CeO2 Cu/CeMn-10:1 Cu/CeMn-5:2 Cu/CeMn-5:4

629

298 349

654

468 200

400

600

800

464

1000

467 200

400

600

Raman shift (cm-1)

800

1000

200

400

600

Raman shift (cm-1)

800

1000

Fig. 5. The Raman spectra of partial supports (a) and catalysts (b).

Fig. 6. TEM, HR-TEM, and selected area electron diffraction (SAED) images of Cu/CeO2 (a, c, and e) and Cu/CeMn-10: 1 (b, d, and f) catalysts, respectively.

the contrary, a small amount of doped MnOx is more beneficial to CO oxidation and NO + CO reaction. Two reasons should be taken into account. One is that appropriate doping of MnOx breaks the inherent balance of CeO2 lattice and increases the specific surface area to great extent, which can induce more defects such as oxygen vacancies [53] and is more contributive to the dispersion of

CuO; another is that more Ce3+ has good influence on synergistic interaction which is as follows: Ce3+ + Cu2+ ↔ Ce4+ + Cu+ . Fig. 8(c) displays the XPS spectrum of Cu 2p for partial catalysts. Noting from the figure that two peaks located at 933.9 and 932.4 eV, are assigned to Cu2+ and Cu+ , respectively, which indicates that Cu is main in a +2 oxidation state and a small amount of +1 oxidation

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C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

Table 2 The bulk (ICP-AES) compositions of partial samples. Samples

Cu/CeO2 Cu/CeMn-10: 1 Cu/CeMn-5: 2 Cu/CeMn-5: 4

Cu (wt.%)

CuO (wt.%)

Ce (wt.%)

CeO2 (wt.%)

Mn (wt.%)

MnO2 (wt.%)

Ce/Mn

CuO/(CeO2 + MnO2 ) (%)

7.73 7.72 7.96 7.72

9.68 9.67 9.96 9.67

73.53 70.31 62.18 55.35

90.32 86.37 76.38 67.99

– 2.50 8.64 14.12

– 3.96 13.66 22.34

– 11.04 (10)a 2.83 (2.5)a 1.54 (1.25)a

10.72 (12)b 10.71 (12)b 11.06 (12)b 10.71 (12)b

The theoretical molar ratio of Ce/Mn. The theoretical loading of CuO.

u0, u u'

u''

v'''

(b) Mn 2p v 0, v v'

v''

Mn3+

Mn 2p1/2

Intensity (a.u.)

(a) Ce 3d u'''

Mn4+ Cu/C eMn10:1

Mn 2p3/2 Mn2+

Cu/CeMn-5:4 656

652

648

644

640

Binding Enegry (eV)

Cu/CeMn-5:4

Intensity (a.u.)

Intensity (a.u.)

Mn-nitrate

Cu/CeMn-5:2

Cu/CeMn-10:1

Cu/CeMn-5:2

Satellite Cu/CeMn-10:1

Cu/CeO

2

Ce 3d3/2 920

915

910

Ce 3d5/2

905

900

895

890

Binding Enegry (eV)

Cu/CeO2

885

(c) Cu 2p

656

880

652

648

LMM

Shake-up

Cu2+

Cu+

n-5:4

Intensity (a.u.)

Cu/CeM

640

Cu+

Cu2+ Cu/CeM

644

Binding Enegry (eV)

Cu 2p3/2 (d) Cu

Cu 2p1/2

n-5:2

e Mn Cu/C

Cu/C

Cu/C

Cu/CeMn-10:1

eMn

-5:4

-5:2

1 -10: eMn

Cu/C

eO 2

Cu/CeO

2

960

955

950

945

940

935

Binding Enegry (eV)

580

578

576

574

572

570

568

Binding Enegry (eV)

566

564

O' ( O2- )

(e) O 1s O'' ( O- ) Cu/CeMn-5:4

Intensity (a.u.)

Intensity (a.u.)

a b

Weight ratio by ICP-AES

Cu/CeMn-5:2

Cu/CeMn-10:1

Cu/CeO2

534

533

532

531

530

529

528

Binding Enegry (eV) Fig. 7. XPS spectra of (a) Ce 3d, (b) Mn 2p, (c) Cu 2p, (d) Cu LMM, and (e) O 1s for partial catalysts.

562

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

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Table 3 The surface (XPS) compositions of partial samples. Atomic concentration and atomic ratio by XPS Atomic concentration (at.%)

Cu/CeO2 Cu/CeMn-10: 1 Cu/CeMn-5: 2 Cu/CeMn-5: 4

Atomic ratio (at.%) +

+

2+

O/(Ce + Mn + Cu) 3+

3+

C

Cu

Ce

Mn

O

Cu /(Cu + Cu )

Ce /(Ce

26.81 29.55 23.99 27.56

5.22 4.59 4.23 3.55

15.86 15.26 13.68 11.13

– 1.64 5.12 7.49

52.12 48.97 52.97 50.27

14.39 27.74 20.67 2.77

13.97 14.50 14.21 12.82

state co-exists [20,34]. The generation of Cu+ may be due to the electronegativity of Cu, which is bigger than that of Ce and Mn and further leads to the donate of the electron to Cu2+ . Moreover, Cu+ species together with Ce3+ species is indicative of the redox equilibrium (Cu2+ + Ce3+ ↔ Cu+ + Ce4+ ) shifting to right, which is claimed to be the source of a synergistic interaction on catalytic performance [39]. The Auger LMM lines of Cu were further investigated over these catalysts on clarifying the valence state of copper in Fig. 8(d). It can be seen that surface copper species mainly exist as Cu2+ (568.9 eV) and Cu+ (570.2 eV) for these catalysts[20]. Furthermore, the intensity ascribed to Cu+ increases when the molar ratio of Ce/Mn reaches 10: 1 compared with CeO2 supported by CuO. The intensity belonged to Cu+ decreases, however, with the further increase of doped MnOx . In other words, the content of Cu+ should be the most in Cu/CeMn-10: 1 catalyst. The Cu 2p3/2 peaks were also fitted by Gaussian curves and the relative content of Cu+ can be estimated by the area ratio between Cu+ and Cu+ + Cu2+ . The results are presented in Table 3, which are similar to the change of Ce3+ for these catalysts. The high-resolution spectra for the O 1s ionization features of these catalysts is fitted with two sections referring to the primary O 1s ionization feature and chemically shifted O 1s in Fig. 8(e). The main peak at 529.2 eV (O ) is ascribed to the characteristic lattice oxygen bonding to the metal cations, the shoulder with the higher binding energy at 531.3 eV (O ) is regarded as the chemisorbed oxygen [53]. Furthermore, the surface compositions calculated from the XPS results are given in Table 3. We can find that the O/(Ce + Mn + Cu) ratios for these samples are higher than the nominal ratio of the full oxidation state and that the excess surface oxygen may be related to a high concentration of surface oxygen as an adsorbed layer of CO2 , CO or water [57]. Previous research has been reported that CO2 and CO prefer to adsorb onto the reduced Ce3+ sites to form carbonate-like species rather than onto the oxidized Ce4+ sites [58]. In addition, the ratio of chemisorbed oxygen quantified based on the area ratio of O /(O + O ) in Cu/CeMn-10: 1 catalyst exhibits the highest value, which indicates that the molar ratio of Ce/Mn with 10: 1 is more beneficial for the formation of oxygen vacancies on the oxide surface. And accordant order can be seen in the relative content of surface Ce3+ , which indicates that CO2 and CO can interact with Ce3+ sites more easily than with Ce4+ sites.

3.5. Redox behaviors and desorption studies (H2 -TPR and O2 -TPD) H2 -TPR characterization were performed to investigate the reducibility of these catalysts. The shape of these TPR profiles is very similar and all of them exhibit three reduction peaks, as shown in Fig. 8. The former two peaks are mainly assigned to the stepwise reduction of surface dispersed copper oxide species (Cu2+ → Cu+ and Cu+ → Cu0 ) [19,33,39], the latter one may be mainly ascribed to the reduction of crystalline CuO [20]. However, in our case, the intensity of the second peak is much larger than that of the first, which should be correlated to strong interaction between the sup-

4+

+ Ce )

4+

2+

Mn /(Mn

+ Mn

3+



4+

+ Mn )



23.19 26.96 23.40 23.76

500

50

245

361

238

256

100

200

503 300

400

500

600

Cu/CeMn-5:4

242

Cu/CeMn-5:3

219

181

236

Cu/CeMn-5:2

204 218

Cu/CeMn-5:1

189 206

170

CeMn-5:2

233

188

167

2.47 (1.79) 2.28 (1.81) 2.30 (1.83) 2.28 (1.85)

CeO2

194

175



O / (O + O )

– 27.72 34.52 41.00

H2 consumption (a.u.)

Samples

Cu/CeMn-10:1

194 213

Cu/CeMn-20:1

185 158

199

317

Cu/CeO2 CuO

100

200

300

400

500

600

Temperature (°C) Fig. 8. The H2 -TPR results for these catalysts.

port and copper species. Therefore, the H2 -TPR results are in good agreement with the XRD and LRS results. It can be seen from the inset that only one peak appears at 503 ◦ C for CeO2 in the temperature range, which is ascribed to the reduction of surface oxygen of CeO2 [59] and two peaks appear at 238 and 361 ◦ C for CeMn-5: 2 support, which are attributed to the reduction of MnO2 to Mn2 O3 and Mn2 O3 to Mn3 O4 , respectively [60]. After supporting CuO, however, the reduction temperatures are much lower than that of their supports, no matter for CeO2 or CeMn-5: 2 support, and pure crystalline CuO, which suggests that dispersed CuO is easier to be reduced due to the interaction between support and active species [59]. The others should be similar. In addition, the reduction temperatures of Cu/CeMn-10: 1 catalyst are the lowest except Ce/CeO2 catalyst, indicating that appropriate doping of MnOx into CeO2 is more conducive to the reduction of catalyst. It is obvious that the actual H2 consumption (see Table 4) of these catalysts shows an increasing trend with the increase of MnOx , which is easy to understand that this is mainly ascribed to more and more reduction of doped MnOx . But what makes it interesting is that the actual H2 consumption of Cu/CeMn-10: 1 catalyst is less than that of Cu/CeMn-20: 1 catalyst. One of the possible reasons should be that there are more low valence copper and manganese species (eg: Cu+ , Mn2+ , and Mn3+ ) in Cu/CeMn-10: 1 catalyst, this point can be well proved by the XPS results. The above mentioned implies that Cu/CeMn-10: 1 catalyst possesses more excellent reducibility than the others, which is strongly correlated to the outstanding catalytic activities of CO oxidation and NO + CO reaction. Furthermore, it should be noted that the actual H2 consumption of these catalysts is higher than the theoretical H2 consumption. This excess hydrogen uptake should be due to the possibility of partial reduction of the supports through donation of

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C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

Table 4 The information of H2 -TPR and O2 -TPD over these catalysts. H2 consumption (␮mol g−1 )

Samples

Cu/CeO2 Cu/CeMn-20:1 Cu/CeMn-10:1 Cu/CeMn-5:1 Cu/CeMn-5:2 Cu/CeMn-5:3 Cu/CeMn-5:4

O2 desorption

Actual H2 consumption

Theoretical H2 consumption

A/T

Area of the first peak (99–135 ◦ C)

Area of the second peak (216–279 ◦ C)

1768 2315 2067 2376 2883 3184 3791

1348 1348 1348 1348 1348 1348 1348

1.31 1.72 1.53 1.76 2.14 2.36 2.81

2709 3893 4027 3967 2849 3783 2522

1300 2080 2313 2056 1938 2221 980

O1 O2

Cu/CeMn-5:4

O3

Intensity (a.u.)

Cu/CeMn-5:3

Cu/CeMn-5:2

Cu/CeMn-5:1

Cu/CeMn-10:1

Cu/CeMn-20:1

Cu/CeO2

100

200

300

400

500

600

Temperature (°C) Fig. 9. The O2 -TPD results for these catalysts.

mobile oxygen to copper oxide species [39], which suggests that the surface layer of the corresponding support takes part in the reduction. O2 -TPD experiments were performed to further investigate the nature of the surface oxygen species possibly involved on oxidation and redox reactions. According to the literature [34], the desorption peaks below 400 ◦ C are generally assigned to superficial oxygen species, and they are weakly bound to the surface. Furthermore, such species are known to take part in oxidation reactions through a superficial mechanism [61]. Noting from Fig. 9 that all catalysts exhibit three characteristic peaks and they are main centered at about 100, 250, and 380 ◦ C, respectively. The first peak (O1-oxygen) is attributed to the physically adsorbed oxygen species weakly bound to the surface and easily desorbed at low temperatures; the second peak (O2-oxygen) is related to O2 − (and/or O− ) species formed by the adsorbed O2 on the surface vacancies, which is in good agreement with the XPS results; the third peak (O3-oxygen) may be occasioned by the lattice oxygen O2− species on the sample surface [19,20,34]. Notably, extra peaks are appeared with the increase of MnOx , which should be correlated to the lattice oxygen O2− species of MnOx . The emphasis is that the former two desorption peaks must be further studied due to the possibility of participating in the oxidation and redox reactions. The oxygen-supplying ability is determined by the number of oxygen-supplying centers and activity [19]. The information of the former two desorption peaks is given in Table 4. It is interesting

to find that the amount of O1-oxygen species is sequentially consistent with the specific surface area (see Table 1), implying that the large specific surface area is conducive to the adsorption of physically adsorbed oxygen. Obviously, the amount of O2-oxygen species increases significantly with the introduction of MnOx and that of Cu/CeMn-10: 1 catalyst is the largest and then decreases. Apparently, a close association between the concentration of O2 − (and/or O− ) oxygen species and catalytic activity implies that the large amount of reactive O2-oxygen species is more conducive to the improved oxidation and redox activity. The XRD and TEM results have pointed out that the Cu/CeMn-10: 1 catalyst possesses the maximum lattice expansion, and the ionic radii between Ce4+ and Mnx+ are different remarkably, which leads to lattice distortion probably in Ce-O-Mn solid solution. This is close correlated to the concentration of oxygen vacancies. The LRS results suggest that the concentration of oxygen vacancies increases significantly when appropriate Mnx+ are doped into CeO2 compared to pure CeO2 . The XPS results demonstrate that the Cu/CeMn-10: 1 catalyst accounts for the largest proportion of Ce3+ . All these results result in a high density of oxygen vacancies, which may play a significant role in promoting the activation and the formation of chemisorbed O2 − (and/or O− ) oxygen species. In other words, the Cu/CeMn-10: 1 catalyst displays the best catalytic activity of CO oxidation and NO + CO reaction, which is probably due to the most concentration of oxygen vacancies to great extent in our current research work.

3.6. Possible forming cause of the structural characteristics of different supports and comparison of the activities of copper oxide supported on different supports Ce(NO3 )3 and Mn(NO3 )2 (the precursors of CeO2 and MnOx ) were mixed in an aqueous solution. Excess ammonia solution (a precipitating agent) was then added into the solution, drop by drop, with vigorously stirring. Ce(OH)3 and Mn(OH)2 precipitations would be formed and they were evenly mixed in an alkaline environment. After filtering, washing, and drying, they can distribute uniformly in the form of powder. Then, Ce(OH)3 and Mn(OH)2 are decomposed into H2 O and respective oxides, similar to the form of explosion concurrently, in the process of calcination until decomposition temperature. This leads to the formation of cerium oxide or manganese oxide fine powder samples and they are mixed uniformly with particle diameter magnitudes of several Å. At this moment, the cerium oxide exists as Ce(1−n) 4+ Cen 3+ O(2−n/2) and Ce3+ takes up a larger proportion because the Ce3+ is precipitated by ammonia and quickly decomposed out with the main form of trivalent cerium oxide at the time of pyrolysis which results in not all Ce3+ oxidising into Ce4+ . Manganese oxide exists as Mn(1−k−m) 4+ Mnk 3+ Mnm 2+ O(2−k/2−m) and Mn2+ has a larger share, and its origin is in similar cause with Ce3+ .

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

Then, the pyrolysis samples are activated at higher temperature and the Ce(1−n) 4+ Cen 3+ O(2−n/2) can get some oxygen from the air resulting in part Ce3+ oxidation into Ce4+ during this stage: Ce(1−n) 4+ Cen 3+ O(2−n/2) + (z/4)O2 → Ce(1−(n−z)) 4+ Ce(n−z) 3+ O(2−(n−z)/2)

Similarly, Mn(1−k−m) 4+ Mnk 3+ Mnm 2+ O(2−k/2−m) can also obtain some oxygen from the air resulting in part increase of manganese valence. Here, we mark manganese oxide which has gotten some oxygen from the air as MnOx due to the complexity of changeable valence Mn. In addition, CeO2 can also obtain some oxygen from the MnOx to oxidise Ce3+ into Ce4+ due to the small particles of CeO2 and MnOx , which are well-dispersed and in close contact: MnOx → Mn(1−y) 2x+ Mny (2×−1)+ O(x−y/2) + (y/4)O2

Ce(1−(n−z)) 4+ Ce(n−z) 3+ O(2−(n−z)/2) + (y/4)O2 → Ce(1−(n−z−y)) 4+ Ce(n−z−y) 3+ O(2−(n−z−y)/2) Labeling (n−z−y) = ␸, Ce(1−(n−z−y)) 4+ Ce(n−z−y) 3+ O(2−(n−z−y)/2) can be written as Ce(1−␸) 4+ Ce␾ 3+ O(2−␸/2) . The above mentioned structures also generate corresponding oxygen vacancies and the solid solution of CeO2 and MnOx after activation can be represented by the following constitutional formulae: [Mn(1−y) 2x+ Mny (2×−1)+ ][O(x−y/2) ϒ y/2 ] [Ce(1−␸) 4+ Ce␸3+ ][O(2−␸/2) ϒ 2/␸ ] The higher cerium oxide content in the solid solution leads to the obtainment of more oxygen from CeO2 to MnOx , which contributes to the lower valence manganese ion and oxygen vacancies. The LRS and XPS results can support these ideas. Although cerium oxide gets oxygen from MnOx , however, it can also obtain oxygen from air to convert Ce3+ into Ce4+ , thus the MnOx content has little influence on the CeO2 structure. On the contrary, the solid solution has a major effect on MnOx structure. This result is in good accordance with the above XRD, LRS, TEM, XPS, H2 -TPR, and O2 -TPD results. They show that for CeO2 –MnOx solid solution, the increasing CeO2 content results in the increase of the epiphase low valence manganese ion (Mn2+ and Mn3+ ) content (XPS result), when MnOx content is too low to provide sufficient oxygen for Ce3+ oxidation into Ce4+ and the transformation of Ce(OH)3 into cerium oxide is rapid, thus Ce3+ content also increases and the structural reflection is lattice expansion and the increase of oxygen vacancies (XRD, LRS, TEM, and XPS results). After supporting CuO, there exists strong interaction between CuO and unsaturated structure support due to more low valence metal ions when doping MnOx content is low, and thus the reducibility is stranger and the content chemisorbed O2 − (and/or O− ) oxygen species is more (H2 -TPR and O2 -TPD results). In accordance with the above speculation, Ce3+ content should be the most in pure CeO2 or CuO supported CeO2 sample, but the experiment result seems be not compatible with the above mentioned. N2 -physisorption results have pointed out that the structure of pure CeO2 is different from that of low MnOx doped CeO2 and the specific surface area of pure CeO2 is smaller than that of low MnOx doped CeO2 , which is not conducive to the dispersion of CuO. On the other hand, the formation process of pure CeO2 may be different from that of low MnOx doped CeO2 due to no impurity. Another focus is the comparison of the activities of copper oxide supported on different supports. Here, taking as example for Cu/CeO2 and Cu/CeMn-10: 1 catalyst due to potential structural changes. When discussing this cause, we imagined that the surface structure of these supports would be a critical reason. It is generally

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accepted that CeO2 , a typical fluorite structure, can be described as an array of cations and the face-centered cubic lattice with oxygen ions occupying the tetrahedral interstitial sites can be formed[62]. The octahedral sites are empty in a perfect fluorite structure and are deemed to locate in the exposed (1 1 1) surface. The crystal structure of pseudocubic CeMn-10: 1 sample is similar to pure CeO2 , thus it can be proposed that CeMn-10: 1 sample has the similar vacant sites on the (1 1 1) plane. For Cu/CeO2 sample, Cu2+ can be incorporated into the surface lattice with the form of occupying the vacant site on the (1 1 1) plane. The capping oxygen is accompanied for charge compensation, as shown in Fig. 10(a). Consequently, the incorporation gives rise to the generation of synergetic interaction between copper and CeO2 (1 1 1) surface. Liu et al. [63] have also proposed that the Cu2+ located at the interfacial region between the finely dispersed CuO clusters and CeO2 can penetrate into the cerium oxide lattice by occupying the vacant sites of the cerium ions on the (0 0 1) plane. The incorporation of Cu2+ into the surface vacant sites of CeO2 (1 1 1) crystal plane displays a trigonal bipyramid symmetry, whose stability is worse than that of the octahedral coordination structure (Fig. 11(b)). For another, for Cu/CeMn-10: 1 sample, a few doping MnOx results in lattice expansion and distortion due to different ionic radii between cerium and manganese ions, simultaneously, there exists more low valence metal ions (eg. Cu+ , Mn2+ , and Mn3+ ) on the surface compared with Cu/CeO2 sample, thus the Cu2+ on the surface of Cu/CeMn-10: 1 sample is a distorted trigonal bipyramid coordination structure (Fig. 11(c)) compared to CeO2 and this can increase the association with the adjacent cerium ions and further contributes to the generation of the electronic interaction, which can benefit a stronger synergetic interaction between copper- and ceria-rich phase. It has been reported that the redox-type mechanism of the supported copper species between Cu2+ and Cu0 is promising over the simple NO + CO reaction [64] and this is probably applicable to CO oxidation. The reduction temperature of Cu2+ and support surface oxygen can be decreased by the stronger interaction between the incorporated copper species and the ceria-rich surface compared with pure CeO2 surface. Moreover, the larger specific surface area increases the dispersion of copper component and further favors the facility for achieving partial reduction of copper oxide, which entities at the interface zone. Furthermore, the generation of the lower valence metal ions brings more oxygen vacancies, which possesses more chemisorbed O2 − (and/or O− ) oxygen species using for redox of reaction. All of these may be the main reasons in explaining the different activities. In addition, the quick change of copper valence in ceria-rich catalyst and strong electron state interaction are also greatly conducive to the higher activity for CO oxidation and NO + CO reaction. The N2 -physisorption, XRD, LRS, TEM, XPS, H2 -TPR, and O2 -TPD can support these ideas. 3.7. Effect of H2 O In order to evaluate the stability of catalysts, NO + CO model reaction at 200 ◦ C was performed in the presence of 10% H2 O (Vol.) on these representative catalysts, as shown in Fig. 11. It can be observed that when H2 O is introduced into the reaction atmosphere, the NO conversion, N2 selectivity, and N2 yield of the two catalysts decrease firstly, which is attributed to the NO and H2 O competition for the Cu active sites [65]. This is in agreement with the results reported by Stegenga et al. [66], who performed Cu–Cr/C catalysts in NO reduction by CO in the presence of H2 O and reported that NO and CO conversions were greatly declined by the presence of H2 O. Then, they maintain balance in the atmosphere of H2 O, similar results can be seen in the research of Cai et al. [52]. When H2 O is removed in the reaction system, the NO conversion, N2 selectivity, and N2 yield recover to some extent, but they are lower than that of the initial values. Moreover, aside from the activity of the

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C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

Fig. 10. The diagram of Cu2+ and Cu+ incorporated in the vacant sites on the (1 1 1) plane of the Cu/CeO2 catalyst (a) and Cu/CeMn-10: 1 catalyst (c), and the corresponding coordination status of Cu2+ on the (1 1 1) crystal plane of the Cu/CeO2 catalyst (b).

100

10% H2O in 10% H O out 2

90 80

(%)

70 60 50 40 30 20

Cu/CeO2

Cu/CeMn-10:1 NO conversion N2 selectivity

NO conversion N2 selectivity

N2 yield

10

N2 yield

0 1

2

3

4

5

6

7

8

9

10

11

12

Reaction time (h) Fig. 11. Effect of H2 O on NO conversion, N2 selectivity, and N2 yield for Cu/CeMn-10: 1 and Cu/CeO2 catalysts at 200 ◦ C.

fresh catalyst that Cu/CeMn-10: 1 sample is more excellent than that of Cu/CeO2 sample, the decrease extent of catalytic activity for the former is smaller than that for the latter in the presence of H2 O and the recovery extent of catalytic activity for the former is larger than that for the latter after the remove of H2 O, which suggests that appropriate doping MnOx into the lattice of CeO2 can enhance the H2 O resistance performance of Cu/CeO2 catalyst effectively. 3.8. CO and/or O2 interaction with these representative catalysts (in situ DRIFTS) To further study the influences between the support and the surface dispersed copper oxide species, the in situ DRIFTS spectra of CO adsorption were recorded, as shown in Fig. 12. Regarding the exposure of Cu/CeO2 in CO atmosphere at ambient temperature, it is widely reported that the bands ascribed to different vibration modes of monodentate carbonates (␯(C O), ␯s (CO3 2− ), and ␯as (CO3 2− )) appear at about 1045, 1314 and 1473 cm−1 generally, respectively; the band attributed to hydrogencarbonates with ␦(C O· · ·H) vibration mode often appears at about 1213 cm−1 ; the bands ascribed to vibration modes (␯s (COO− ) and ␯as (COO− )) of carboxylates appear at about 1389 and 1548 cm−1 [20,33,40]. It can also be found that the band at 1045 cm−1 disappears due to the thermal desorption and reduction when the temperature increase to 175 ◦ C, and a new band ascribed to the coordination of COx to the reduced ceria appears at 1066 cm−1 , which suggests that

Cu/CeO2 can be reduced by CO during the heating process [67]. Furthermore, the bands attributed to these carbonates and carboxylates enhance with further raising the temperature, the main reason may be that CO and CO2 are favored in adsorbing on the reduced state Ce3+ sites rather than on the oxidized state Ce4+ sites [40,58]. In addition, previous study has been reported that no CO adsorption can be viewed at 2050–2200 cm−1 for CeO2 , thus the peak at about 2100 cm−1 in the spectra should be attributed to the vibration of Cu+ –CO at room temperature [40], which indicates the presence of some Cu+ species in the samples at ambient temperatures. This is in agreement with the XPS results, which suggests the presence of Cu+ ions on the samples surface. While, this band shifts to 2111 cm−1 with the temperature increasing to 225 ◦ C. On the other hand, increasing the temperature to 100 ◦ C results in the maximum intensity of Cu+ –CO, and further increasing the temperature to 225 ◦ C results in the disappearance of this species. Generally, the adsorption of CO molecules on Cu0 , Cu+ , and Cu2+ will generate peaks with characteristic vibrational frequencies at below 2130 cm−1 , 2080–2160 cm−1 , and 2150–2220 cm−1 , respectively, and at ambient temperature, the adsorption of Cu+ –CO is the most stable [19,20]. From these information, it can be derived that stepwise reduction is went through for copper species, i.e., from Cu2+ to Cu+ to Cu0 . For Cu/CeMn-10: 1, the bands ascribed to monodentate carbonates, hydrogencarbonates, carboxylates, and Cu+ –CO are also observed at ambient temperature and the trend changed with temperature is similar to that of Cu/CeO2 . Interestingly, the peak intensity of gaseous CO2 increases gradually with increasing temperature after 150 ◦ C for Cu/CeMn-10: 1, while it decreases gradually for Cu/CeO2 . Sun et al. [68] have reported that the reduction of Mn4+ → Mn3+ appears at about 240 ◦ C and that of Mn3+ → Mn2+ appears after 260 ◦ C in CO reducing atmosphere. Thus the increasing adsorption of gaseous CO2 may comes from the increasing Ce3+ with increasing temperature, one of the reasons is probably attributed to the synergetic introduction of Ce4+ + Mn2+ ↔ Ce3+ + Mn3+ or Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ , which is in agreement with the XRD results. In short, appropriate doping MnOx into CeO2 is conductive to the more generation of Ce3+ compared with pure CeO2 , which may contributes to the activity of CO oxidation. The in situ DRIFTS spectra of CO and O2 co-adsorption were also tested when simulating the CO + O2 reaction for further investigation of the surface reaction mechanism, as shown in Fig. 13. For the two representative samples, in CO and O2 co-adsorption, the bands of monodentate carbonates, hydrogencarbonates, carboxylates, and Cu+ –CO are also observed at similar wavenumbers at ambient temperature. But there are a few different changes. Firstly, the band ascribed to the coordination of COx to the reduced ceria

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049 (a)

1389

0.5

(b)

1473 1548

1045

1387 1483 1552

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1327 gaseous CO2

1213

1066

2111

Kubelka-Munk (a.u.)

Kubelka-Munk (a.u.)

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300 °C 275 °C 250 °C 225 °C 200 °C 175 °C 150 °C 125 °C 100 °C 75 °C 50 °C 25 °C

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Cu+-CO

2500

gaseous CO2

300 °C 275 °C 250 °C

1209

Cu+-CO

1500 2000 Wavenumber (cm-1)

2500

2080

2160

Fig. 12. In situ DRIFTS spectra of CO (1.6% in volume) adsorption with the representative samples: (a) Cu/CeO2 , (b) Cu/CeMn-10: 1.

(b)

0.5

Kubelka-Munk (a.u.)

1327

1391

1469 1538

1332

1390

1471 1540

275 °C 250 °C

250 °C

225 °C 175 °C 150 °C 125 °C 100 °C

2101

225 °C 200 °C 1050

1220

175 °C 150 °C 125 °C 100 °C

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50 °C 25 °C 1500

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2100

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(a)

Cu+-CO

Wavenumber (cm-1)

25 °C 2500

1000

1500

2000

Wavenumber (cm-1)

Cu+-CO

2500

Fig. 13. In situ DRIFTS spectra of CO (1.6% in volume) and O2 (20.8% in volume) co-adsorption with the representative samples: (a) Cu/CeO2 , (b) Cu/CeMn-10: 1.

and appeared at 1066–1070 cm−1 does not appear. This may be ascribed to excess O2 in CO and O2 co-adsorption, which makes Ce4+ difficult to reduce to Ce3+ . Another place to prove it reasonable is that it is hardly found the adsorption of CO2 at about 2360 cm−1 . Similar reason leads to the disappearance of hydrogencarbonates after 175 ◦ C. Another point must be firstly explained is that the peaks of CO adsorption on Cu+ appear at ambient temperature, as indicated by the peak at 2100 cm−1 . However, the intensity of the peak is much weaker than that of CO adsorption, which implies that O2 molecules preferentially adsorb on the surface of the sample and occupies surface vacancies. Therefore, the adsorption of CO is restrained. In this case, free gaseous CO can react with the adsorbed O2 − species slowly. The peak at 2100 cm−1 suddenly disappears after 100 ◦ C, which suggests that gaseous CO may be completely oxidized to CO2 by excess O2 . This is in consistence with the catalytic activity of CO oxidation. Furthermore, the intensities of these carbonates and carboxylates increase gradually and reach the maximum at 100 ◦ C, and then decrease gradually with increasing temperature for the representative samples. Combining with the catalytic activity of CO oxidation, there exists CO and CO2 molecules and more and more CO can react with the adsorbed O2 − species with increasing temperature to 100 ◦ C, thus the intensities of carbonates and carboxylates increase gradually. After 100 ◦ C, there almost only exists CO2 molecules and no extra CO react with the adsorbed O2 − species, which occupies adsorption sites. On the other hand, excess O2 makes Ce4+ difficult to reduce to Ce3+ . Thus, the intensities of carbonates and carboxylates decrease gradually.

It is worth noting that the weakening extent of the intensities of carbonates and carboxylates of Cu/CeMn-10: 1 is greater than that of Cu/CeO2 . one of possible reasons is that more O2 − species can be adsorbed on Cu/CeMn-10: 1 than that on Cu/CeO2 , indicating that Cu/CeMn-10: 1 possesses more oxygen vacancies compared with Cu/CeO2 . This is in good agreement with the LRS results. 3.9. CO and/or NO interaction with these representative catalysts (in situ DRIFTS) Similarly, In order to further investigate the interaction of reactants on the catalysts, CO adsorption in situ DRIFTS was recorded under the simulative reaction conditions for getting the information about the changes of the surface adsorbed species. The results are similar to 1.6% CO adsorption in volume and were given in Fig. S2. Major difference is that the intensities of various peaks is much stronger due to more CO adsorption. Fig. 14 shows the in situ DRIFTS spectra of NO interaction with the representative catalysts. Liu et al. [69] found that NO molecules interacted with the dispersed copper oxide species preferentially and formed several kinds of nitrate or nitrite-like species when comparing the interaction of NO molecules with Ce0.67 Zr0.33 O2 support and 0.33CuO/Ce0.67 Zr0.33 O2 catalyst with in situ DRIFTS. Furthermore, we also found that the NO adsorption of CuO/CeO2 catalyst was identical to that of CuO/Ce0.67 M0.33 O2 (M = Zr4+ , Sn4+ , Ti4+ ) catalysts in situ DRIFTS [39], which further supported the results of Liu et al. Moreover, noting from Fig. 15 that the adsorbed

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049 O

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Cu -NO 2+

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1000

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275 °C 300 °C

M M

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1800

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2000

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Fig. 14. In situ DRIFTS spectra of NO (5% in volume) adsorption with the representative samples: (a) Cu/CeO2 , (b) Cu/CeMn-10: 1.

NO species display their vibration bands at similar wavenumber over the Cu/CeO2 and Cu/CeMn-10: 1 catalysts, which further supports the conclusion. For some supported copper-based catalysts, combining with previous reports [20,33,39,69], in the present work, the bands ascribed to bridging bidentate nitrate exhibited with a remarkable NO2 symmetric vibration and a N O stretching model appear at 1009–1016 cm−1 and 1606–1608 cm−1 , respectively. The two bands attributed to chelating bidentate nitrate appear at 1205–1211 cm−1 and 1540–1543 cm−1 , respectively. The band ascribed to linear nitrite with the NO2 asymmetric vibration appears at 1233–1239 cm−1 . The two bands assigned to monodentate nitrate appear at 1280–1283 cm−1 and 1501–1513 cm−1 , respectively. The band referred to bridging monodentate nitrate appears at 1574–1578 cm−1 . Furthermore, chemisorbed NO on Cu2+ species also displays a band at 1891–1893 cm−1 . Increasing the temperature gradually, bridging bidentate nitrate, linear nitrite, monodentate nitrate, and bridging monodentate nitrate decrease gradually, and chemisorbed NO on Cu2+ species also decrease gradually and disappear at 225 ◦ C. Simultaneously, two new peaks assigned to chelating bidentate nitrate appear at 1205–1211 and 1540–1543 cm−1 and first increase and then decrease, with increasing temperature to 300 ◦ C (but it is difficult to be completely desorbed/transformed/decomposed). Two points should be illustrated, firstly, these species go through rearrangement rather than desorption or decomposition; secondly, the adsorption of chelating bidentate nitrate is relatively stable under NO atmosphere at high temperature. However, the weakening extent of these nitrate and nitrite of Cu/CeMn-10: 1 is greater than that of Cu/CeO2 , especially for monodentate nitrate (it disappears after 225 ◦ C), suggesting that the adsorption/desorption behavior of Cu/CeMn-10: 1 is better than that of Cu/CeO2 catalyst. The possible reason may be that when NO molecules are adsorbed on the surface of the catalyst, there will be a kind of electronic interaction, i.e., the back-donation of the d-electron from the copper cation interact with an antibonding orbital of NO, which maybe weaken the N O bond of NO [70]. The Cu content is much less than the Ce, thus the electrons captured by Cu2+ mainly come from Ce and there exists more Ce3+ in Cu/CeMn-10: 1 compared with Cu/CeO2 due to the synergetic introduction of Ce4+ + Mn2+ ↔ Ce3+ + Mn3+ or Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ , which makes more electrons donated to Cu2+ . In other words, the easy extent of the electrons donating to Cu2+ in the representative catalysts through Cu–O Ce link decreases in accordance with the order: Cu/CeMn-10: 1 > Cu/CeO2 . Furthermore, the more electrons around Cu2+ are good for the back-donation of the d-electron

to an antibonding orbital of NO, and thus the N O bond of NO is weakened, which can enhance the catalytic performance. It is worth noting that another one peak appeared at 1345 cm−1 is assigned to hyponitrites. This is caused by the formation of oxygen vacancies at CuO-promoted interfacial sites by the strong interaction of copper with the ceria of the CeMn-10: 1 support and via electron transfer from a reduced Lewis center (Ce3+ or Cu+ ) to a NO molecule [20,69], which is in good consistence with the XRD, TEM, and XPS results. These NO− species can dimerize to yield N2 O2 − , which can decompose to form N2 O easily at low temperatures [20]. Whereas, it is not obvious for Cu/CeO2 catalyst. In order to investigate the surface reaction mechanism further, CO and NO co-adsorption in situ DRIFTS spectra were carried out under the simulating reaction conditions, the results are displayed in Fig. 15. It is worth noting that neither carbonates nor Cu+ -CO species can be observed below 150 ◦ C, which suggests that NO adsorbs on the surface of the catalysts preferentially and covers the active sites due to its unpaired electron. Thus, complex types of nitrite-/nitrate-like species are produced, which are chemisorbed on the surface of the catalysts [20]. Therefore, the adsorption of CO is restrained. In this case, the free gas CO can react with the adsorbed NO species slowly. Increasing the temperature to 150 ◦ C gradually, because of the desorption, arrangement, and reaction with the free gas CO, the adsorbed NO species disappear except chelating bidentate nitrate. Consequently, CO adsorbtion occurs due to the exposion of the active sites of the catalyst. It should be pointed out that the carbonates and Cu+ –CO species are detected when the temperature is up to 125 ◦ C, which implies that CO have been adsorbed on the surface of the catalyst. In this case, the adsorbed CO species and the free gas NO may react each other. When the temperature is up to 150 ◦ C, interestingly, the intensities of carbonates differ little for both Cu/CeO2 and Cu/CeMn-10: 1. But the intensity of Cu+ –CO species of the latter is much stronger than that of the former, which implies that the content of Cu+ in Cu/CeMn-10: 1 is more than that in Cu/CeO2 or the CuO supported on CeMn-10: 1 can be reduced more readily than that on CeO2 during the heating process. This should be one of the reasons of better catalytic activity for Cu/CeMn-10: 1 compared with Cu/CeO2 . Moreover, Cu+ can be reduced to Cu0 by CO through further increase of temperature, which is conducive to the enhancement of catalytic performance [20,33,39]. In addition, for noble metal catalysts, such as Pt, Rh, and Pt-Rh, the reaction mechanism of NO + CO model reaction has been widely studied in recent decades, and a Langmuir-Hinshelwood (LH) mechanism is acceptable for these reaction systems [71–73].

C. Deng et al. / Applied Surface Science 389 (2016) 1033–1049

O

1279

O

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O

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1371 1480

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1061

250 °C 275 °C 300 °C

1200

1379

1499

1564

1500

gaseous CO CO2

2100

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Fig. 15. In situ DRIFTS spectra of CO (10% in volume) and NO (5% in volume) co-adsorption with the representative samples: (a) Cu/CeO2 , (b) Cu/CeMn-10: 1.

Whereas, it is few reported and controversial for the exploration of the mechanism of this reaction on transition metal oxide catalysts. Li et al. [47] investigated the effect of CO-pretreatment CuO–Mn2 O3 supported ␥-Al2 O3 catalyst on NO + CO reaction and found that NO adsorbed on Mn2+ and CO on Cu+ preferentially during the in situ DRIFTS experiments, implying that a LangmuirHinshelwood (L-H) mechanism is close contact with this reaction due to two types of active sites (Mn2+ and Cu+ ). Whereas, an EleyRideal (E-R) mechanism was indicated over NO + CO reaction in Taniike and Tada’s [74] investigation on the supported unitary component Co2+ -ensemble/␥-Al2 O3 catalyst through DFT calculation and in situ DRIFTS characterization. In our current research, it can be observed that only NO species can be adsorbed on the surface of the representative catalysts when the temperature is below 125 ◦ C and mainly adsorbed CO species can be observed when the temperature overtop 125 ◦ C. In other words, the NO + CO reaction over these catalysts is mainly occurring between adsorbed species and gaseous molecules in the whole range of reaction temperature, indicating an Eley-Rideal (E-R) mechanism, which is also consistent with the reports of Lin Dong and his co-workers [39,69].

3.10. Possible reaction mechanism for CO oxidation and NO + CO reaction Based on the previous characterizations, possible reaction mechanisms for CO oxidation and NO + CO reaction under current condition are tentatively proposed to further understand the nature of the reactions, and taking the Cu/CeMn-10: 1 catalyst as an example, as shown in Fig. 16. For CO oxidation, it can be observed that the Cu+ –CO species appear at 2100 cm−1 in CO stream at room temperature while the Cu+ –CO peak weakens sharply in CO +O2 streams, which demonstrates that O2 first adsorbs on the surface of the catalyst and then forms the O2 − species, giving rise to the conversion of Cu+ to Cu2+ , in turn denting the CO adsorption [19]. Appropriate doping MnOx into CeO2 is conductive to the more generation of Ce3+ due to the synergetic interaction of Ce4+ + Mn2+ ↔ Ce3+ + Mn3+ or Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ , which can adsorb more CO and makes CO more easier react with adsorbed O2 − species. Furthermore, the electrons can migrate more easily from Ce3+ to Cu2+ for the generation of Cu+ compared with Ce4+ at ambient temperature due to the larger electronegativity of Cu than that of Ce, which is supported by the XPS and in situ DRIFTS results of CO adsorption. The above reaction process can be well supported by Liu and Stephanopou-

Fig. 16. Possible reaction mechanisms for CO oxidation (a) and NO + CO reaction (b) over Cu/CeMn-10: 1 catalyst.

los [63] and Caputo et al. [75], which was interpreted by a classical Langmuir-Hinshelwood (L-H) mechanism on the catalytic behavior of CO oxidation over CuO/CeO2 catalyst. Furthermore, this is also similar to that proposed by Dong et al. [19]. The catalytic cycle involves five steps: (1) chemisorption of CO on Cu+ ions on the surface of CuO to form Cu+ –CO species, (2) migration of the chemisorbed CO to the interface of CuO and CeMn-10: 1, (3) O2 activation on the oxygen vacancies in CeMn-10: 1 and formation of active oxygen O2 − , (4) reaction between the chemisorbed CO at interface and the nearby active oxygen, and (5) refill of oxygen vacancies by gas phase O2 . For NO + CO reaction (Fig. 17(b)), it can be found that the reaction over Cu/CeMn-10: 1 catalyst is an Eley-Rideal (E-R) mechanism

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according to the CO and NO co-adsorption in situ DRIFTS results, which is different from CO oxidation. But what makes identical to CO oxidation is that appropriate doping MnOx into CeO2 is conductive to the more generation of Ce3+ due to the synergetic interaction of Ce4+ + Mn2+ ↔ Ce3+ + Mn3+ or Ce4+ + Mn3+ ↔ Ce3+ + Mn4+ , and the electrons can migrate more easily from Ce3+ to Cu2+ for the generation of Cu+ compared with Ce4+ at ambient temperature. Under the mixed atmosphere of CO and NO gases, NO molecules are preferentially adsorbed on the surface of the catalyst to form several kinds of nitrite and nitrate species due to its unpaired electron, and this behavior restrains the adsorption of CO species [20]. Xiong et al. [76] reported that the dissociation of NO was the key step for NO + CO reaction, and oxygen vacancies could weaken the N O bonds to promote their dissociation. Surface oxygen vacancies have been reported to be contributive to N2 O dissociation [53]. In our case, increasing temperature dissociates the above adsorbed NO species and exposes active sites, which adsorbs CO species next. Furthermore, the surface Ce3+ can supply more adsorbedsites to adsorb CO molecules and the synergistic interaction of Ce3+ + Cu2+ ↔ Ce4+ + Cu+ is helpful in the formation of more Cu+ , and Cu2+ can also be reduced to Cu+ during the heating procedure due to excess CO. Consequently, more Cu+ provides more CO adsorption sites. CO molecules adsorbed on Cu+ can react with O radicals to generate CO2 due to the dissociation of NO species on surface oxygen vacancies, the rest N radicals can recombine with NO or CO to form N2 O or NCO, respectively, or combine with another N to produce N2 . Furthermore, Cu2+ can be further reduced to Cu0 metal and N2 O can be further reduced to N2 in higher temperatures. Sun et al. [33] investigated the NO + CO reaction on Cu-based catalysts and indicated that the Cu0 species is the active species for N2 O decomposition. Therefore, the reduction of N2 O to N2 may be close related to the Cu+ /Cu0 redox cycle. Because N2 O is transformed into N2 and O, CO2 can be formed due to the combination of neighbouring CO with O and new active sites on the surface are also regenerated. Consequently, an appreciable quantity of N2 and CO2 can be detected.

4. Conclusions This work studies the effect of different Ce/Mn molar ratios supported by CuO on the texture, structure, chemical composition, surface state, redox property, and activity of CO oxidation and NO + CO model reaction. Several major conclusions can be obtained as follows:

(1) Appropriate doping MnOx into the lattice of CeO2 increases the specific surface area, which is conducive to the dispersion of CuO, and enhances the catalytic activity and thermal stability. (2) Cu/CeMn-10: 1 catalyst exhibits the best catalytic activity of CO oxidation and NO + CO reaction. This is related to larger specific surface area, more uniformity of structure, more Cu+ and Ce3+ (which may generate more oxygen vacancies), better reduction ability, stronger desorption capability of chemisorbed O2 − (and/or O− ) oxygen species, and suitable adsorption/desorption behavior. (3) CO oxidation follows a classical Langmuir-Hinshelwood (L-H) mechanism while NO + CO reaction complies with an EleyRideal (E-R) mechanism.

Notes The authors declare no competing financial interest.

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