Catalytic performance of manganese doped CuO–CeO2 catalysts for selective oxidation of CO in hydrogen-rich gas

Fuel 163 (2016) 56–64

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Catalytic performance of manganese doped CuO–CeO2 catalysts for selective oxidation of CO in hydrogen-rich gas Xiaolin Guo, Jing Li, Renxian Zhou ⇑ Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


We study the effect of calcination CuC(500) CuC-Mn5(400) CuC-Mn5(500) CuC-Mn5(600) CuC-Mn5(700)

100

Temperature (ć)

temperature on CuO–MnOx–CeO2 catalyst for CO-PROX.
CuMnCe mixed oxide calcined at 500 °C shows the highest catalytic activity (T50% = 74 °C).
CuCMn(500) exhibits the broadest temperature window (110–140 °C).
A suitable calcination temperature leads to forming a stable Cu–Mn–Ce– O solid solution. + 4+
More active compounds (Cu /Mn species and oxygen vacancies) generate in CuCMn(500).

80

W: width of operation window (CO conversion > 99.0%)

60

40

20

0

T50%

a r t i c l e

i n f o

Article history: Received 3 May 2015 Received in revised form 13 August 2015 Accepted 17 September 2015 Available online 28 September 2015 Keywords: Hydrogen purification CO selective oxidation CuO–MnOx–CeO2 catalysts Mn doping Calcination temperature

a b s t r a c t Manganese doped CuO–CeO2 catalysts (CuO–MnOx–CeO2) with Mn/Cu molar ratio of 1:5 and variable calcination temperatures were prepared by a hydrothermal method and used for selective oxidation of CO in hydrogen-rich gas, characterized by various techniques. An appropriate calcination temperature shows an essential stimulative effect on CuO–MnOx–CeO2 catalysts for the CO-PROX. The catalyst calcined at 500 °C displays the highest low-temperature catalytic activity (T50% = 74 °C) and the broadest operating temperature ‘‘window” (CO conversion >99.0%, 110–140 °C). XRD and UV-Raman results reveal that an appropriate calcination temperature can promote the formation of Cu–Mn–Ce–O ternary oxide solid solution, adjust the degree of crystallinity of CeO2 and enhance the formation of oxygen vacancies. H2-TPR and XPS demonstrate that calcining at 500 °C improves the active reducing species in both bulk and surface of the catalyst. In situ DRIFTS suggests that Cu+ species which possess strong interaction with ceria also can be facilitated with a suitable calcination temperature. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction With the improving concerns about energy and environment, much more attention had been paid to fuel cells that could convert chemical energy into electricity directly. In the continuing development of fuel cells, polymer electrolyte membrane fuel cell ⇑ Corresponding author. Tel.: +86 571 88273290; fax: +86 571 88273283. E-mail address: [email protected] (R. Zhou). http://dx.doi.org/10.1016/j.fuel.2015.09.043 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

W

(PEMFC) shows some specific characteristics, such as high power density, low operating temperature, rapid start up and long work time [1]. By virtue of these excellent merits, PEMFC becomes the most advanced system for various applications, especially for electric vehicles. Pure hydrogen is regarded as the ideal fuel for the anode of a PEMFC because H2O is the only resultant for the electrode reaction [2]. But the produce and deposit system of the pure hydrogen in the PEMFC limited its use to a great degree. Many attempts are being made to produce hydrogen directly on board

X. Guo et al. / Fuel 163 (2016) 56–64

the PEMFC. Up to now, the reforming of hydrocarbons has been regarded as the most practicable means for the mass production of hydrogen-rich gas. Unfortunately, there often contains a small amount of CO in such excess hydrogen streams. Even traces of CO has a significantly negative effect on the Pt anode of PEMFC [3], thus the feed gas must be purified further. The selective oxidation of CO in excess H2 (CO-PROX) has been thought as the most economical and effective approach to eliminate CO (below 10 ppm). Although numerous catalysts have been applied for the reaction, the catalysts could not simultaneously satisfy the demands of conversion requirement and the high selectivity. For example, the precious metal catalysts often exhibit excellent catalytic activity but are too expensive to meet the practical application. As a promising substitute for precious metal catalysts, the CuO–CeO2 mixed oxide catalysts play a significantly important role in the elimination of CO in H2-rich gas on the account of their remarkable catalytic performance and low cost [4,5]. The CuO–CeO2 catalysts possess better catalytic activity and selectivity than Pt-based catalysts at lower reaction temperatures, however, the operating temperature ‘‘window” (CO conversion >99%) for the former in CO-PROX reaction is very narrow (just 5–20 °C) [6–8]. Therefore, researchers made great efforts to explore an approach to broaden the temperature window in which CO could be converted completely over CuO–CeO2 catalysts for CO-PROX reaction. On one hand, Li et al. [9] found that the addition of Mn and Fe could enhance the formation of a more stable solid solution in CuO–CeO2 catalysts and facilitate their catalytic activity for CO-PROX reaction, while the introduction of Co and Cr greatly weakened the interaction between copper and ceria and impaired the redox ability of CuO–CeO2 catalysts. Chen et al. [10] prepared a series of Ce20Cu5NiyOx catalysts with different molar ratio of Cu/Ni for CO oxidation at low temperature and the Ce20Cu5Ni0.4Ox catalyst was the most active of the NiO-promoted CuO–CeO2 catalysts, which was due to that NiO species promoted copper ions doping into the CeO2 and generating more oxygen vacancies in ceria by the formation of a Ni–O–Ce solid solution. On the other hand, the preparation methods affect the catalytic performance of CuO– CeO2 catalysts to a great degree. Our group has reported that the CuO–MnOx–CeO2 catalyst prepared by a hydrothermal method exhibited the best catalytic activity especially at low temperatures versus to the other three catalysts prepared by co-precipitation, impregnation and citrate sol–gel methods [11]. Peng et al. [12] synthetized CuO–CeO2/Ni metal foam (MF) catalysts with hierarchical structures using macroporous nickel foams with copper– cerium hydrosols for the preferential oxidation of CO in excess hydrogen. The catalyst with a Ce/Cu molar ratio of 10 exhibited a high activity and excellent stability because of its high surface area, highly dispersed CuO, and interconnected three-dimensional reticular configuration of nickel metal support. Besides the kinds of the doping elements and synthetic methods, the preparation conditions such as surface treatment and calcination temperature impact the CeO2-based catalysts properties (particle size, surface composition, crystalline phase and oxidation state) greatly. For example, Park et al. [13] prepared a series of CeO2 catalysts with doped/codoped of Co/Cu/Ni and thermal pre-treated them in H2 and N2 condition before CO oxidation test. They found that without the pretreated, the three CeO2 catalysts with Cu-doped, Cu/CO, Cu/Ni-codoped showed a higher CO performance, and upon N2 (or H2-thermal treatment) the T10% of Cu-doped catalyst was lowered by 80 °C, which was attributed to balanced charge pairs of Cu2+/Cu+ and Ce4+/Ce3+ for CO oxidation reactions. Lee et al. [14] have studied the influence of calcination temperature on the catalytic performance of Ce/TiO2 catalysts for selective catalytic oxidation of NH3 to N2 and the results showed that the catalytic activity was mainly dependent on the oxidation

57

states of the Ce species and behavior of oxygen within the catalyst, and the Ce/TiO2 catalyst calcined at 400 °C showed the highest NH3 conversion (96%) and N2 (93%) yield (at 350 °C). Liu et al. [15] have reported the impact of calcination temperature on the structure and catalytic performance of CuOx–CoOy–CeO2 ternary mixed oxide for CO oxidation, and they found that CuOx–CoOy–CeO2 went through continuous structural transformations as the calcination temperature changes, which included three stages, such as the hydroxide dehydration below 400 °C, enhanced interaction between Co3O4 and CeO2 at ca. 600 °C, and the decomposition of Co3O4 to CoO at 700 °C. The catalyst calcined at 600 °C showed the highest catalytic activity for CO oxidation due to a relative enrichment of Cu+ and an enhanced interaction between Co3O4 and CeO2 along with the generation of oxygen vacancies in CeO2. Recently, we have studied the catalytic performance of MnOx doped CuO–CeO2 mixed oxides catalysts with different Mn/Cu molar ratio for CO-PROX, and found that the addition of MnOx with moderate amount significantly improves the catalytic activity of CuO–CeO2 [16]. In the current work, we prepared a series of CuO–MnOx–CeO2 catalysts (Mn/Cu = 1:5) with different calcination temperatures. XRD, BET, H2-TPR, XPS, UV-Raman, and in situ DRIFTS were employed to characterize the samples, in order to obtain some new insight into the relationship between structure and catalytic performance of CuO–MnOx–CeO2 catalysts for CO-PROX reaction in hydrogen-rich gas. 2. Experimental section 2.1. Preparation of catalysts The MnOx–CuO–CeO2 catalysts with Mn/Cu molar ratios of 1:5 were prepared by a hydrothermal method. NH3H2O was added to the mixed ethanol solution of Ce(NO3)3, Cu(NO3)2, Mn(NO3)2 and CTAB (CTAB/Ce molar ratio of 1:1) with constant stirring. The pH value of the mixture was adjusted to 9.0 with NH3H2O, and then the mixture was treated at 100 °C for 1 h in a stainless steel autoclave. The precipitate was separated by filtration, dried at 110 °C for about 4 h. Finally, the MnOx–CuO–CeO2 catalysts were calcined at 400, 500, 600 and 700 °C for 2 h in air and were labeled as CuCMn(400), CuCMn(500), CuCMn(600) and CuCMn(700), respectively. As to be the contrastive group, we also prepared CuO–CeO2 catalyst calcined at 500 °C to evaluate the effect of moderate manganese to the catalyst, which is labeled as CuC(500). The content of Cu was always 5.0 wt.% in the catalysts. 2.2. Catalytic performance tests The activity measurement of the catalysts for selective oxidation of CO were carried out in a fixed-bed micro-reactor (quartz glass, i.d. = 6 mm) at atmospheric pressure. 100 mg of the catalysts with 60–80 mesh (0.3–0.45 mm) was used in the tests and diluted with 200 mg a-alumina particles of the same mesh. The reaction gas component was 50% H2, 1.0% O2, 1.0% CO, 15% CO2 (when used), and 7.5% H2O (when used) and Ar in balance to maintain the total flow rate as 100 ml/min, corresponding to a space velocity of 60,000 ml g1 h1. Before activity measurements, the catalysts were pretreated in oxygen at 150 °C for 0.5 h. The reactor outlet gases were analyzed by an on-line gas chromatograph which is outfitted with a flame ionization detector (FID) and a thermal conductivity detector (TCD). CO and CO2 were separated by a carbon molecular sieve (TDX-01) column, and converted to methane by a methanator and then detected by means of FID. The limit of FID detection for CO is less than 3 ppm. H2 and O2 were separated by a carbon molecular sieve (TDX-01) column and analyzed by means of TCD.

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X. Guo et al. / Fuel 163 (2016) 56–64

In order to avoid the influence of CO2 in the feed gas, the CO conversion was calculated based on the CO decrease as follows:

90

CO conversion (%)

CO conversion ¼

100

½COin  ½COout  100ð%Þ ½COin

The O2 selectivity was calculated from the oxygen consumed by CO oxidation as follows:

0:5ð½COin  ½COout Þ O2 selectivity ¼  100ð%Þ ½O2 in  ½O2 out

80 70 60

CuC(500) CuCMn(400) CuCMn(500) CuCMn(600) CuCMn(700)

50 40 30 20 10 0 100

2.3. Characterization of catalysts

3. Results and discussion 3.1. Catalytic performance of CuO–MnOx–CeO2 catalysts The catalytic performance of CuO–MnOx–CeO2 catalysts is shown in Fig. 1. Compared with CuC(500) without manganese doping, CuCMn(500) shows higher catalytic activity for CO-PROX in excess-H2 stream and the operating temperature ‘‘window” (CO conversion >99%) is obviously widened from 120–130 °C to 110–140 °C. In order to investigate the effect of calcination temperature on the CuO–MnOx–CeO2 catalyst, we evaluate the catalytic performance of a series of CuCMn catalysts whose heating ramp of calcination is 400, 500, 600 and 700 °C. As shown in Fig. 1, with increasing the calcination temperature, the lowtemperature catalytic activity of the CuCMn catalysts for COPROX gradually increases and CuCMn catalyst calcined at 500 °C exhibits the highest catalytic activity, for which the temperature

90

O2 selectivity (%)

X-ray powder diffraction patterns were recorded on a Rigaku D/ Max 2550 PC powder diffractometer using nickel-filtered Cu Ka radiation, and the range of scan was 20–80°. Sizes of the nanocrystals were calculated according to Scherrer’s equation based on the half-width at halfmaximum (FWHM) of the fluorite (1 1 1) diffraction peak. X-ray photoelectron spectra were recorded on a PHI5000c spectrometer equipped with Al Ka radiation source (1486.6 eV) operating at 12.5 kV. The surface charging effect was corrected by fixing the C 1s peak at a binding energy of 284.8 eV. H2 temperature-programmed reduction (H2-TPR) was carried out in a quartz fixed-bed micro-reactor equipped with TCD. Before H2-TPR measurement, 0.05 g of the catalyst was pretreated in a helium flow at 300 °C for 0.5 h and cooled to room temperature. Then the sample was exposed to 5% H2/Ar with a flow rate of 40 ml/min and ramped from 40 to700 °C (10 °C/min). The textural properties were determined by N2 adsorption/desorption using Tristar II 3020 apparatus. Prior to the measurement, the sample was gassed at 250 °C for 4 h under vacuum, and N2 adsorption was carried out at 196 °C UV-Raman spectra were recorded on a UV-HR Raman spectrograph with a He–Gd laser of 325 nm excitation wavelength. The samples were in powder form to avoid diffusion problems, and the spectra acquisition consisted of two accumulations of 30 s with a resolution of 4 cm1. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data were collected on a Nicolet 6700 apparatus equipped with an MCT detector. The DRIFTS cell was fitted with temperature controlled parts and CaF2 window. Spectra were collected at the resolution of 32 scans and 4 cm1. Prior to infrared experiment, the sample was pretreated with 10% O2/Ar at 300 °C for 0.5 h and then cooled to 25 °C in order to remove the contaminants from the catalyst surface. The composition of feed stream was 1% CO, 1% O2, 50% H2 and Ar in balance.

80 70 60 50 40 70

80

90 100 110 120 130 140 150 160 170 180 190 200

Temperature ( ) Fig. 1. The catalytic performance of CuO–MnOx–CeO2 catalysts.

of 50% CO conversion (T50) is merely 74 °C (Table 1) and the operating temperature ‘‘window” is the broadest (100–140 °C). However, as the calcination temperature continues to rise, the catalytic activity for the selective oxidation of CO falls rapidly, especially for CuCMn catalyst calcined at 700 °C, for which there is even no temperature window occurring at the whole tested temperature. It suggests that an appropriate calcination temperature is in favor of the interaction between the active components in CuO– MnOx–CeO2 catalyst and improves the catalytic activity in the COPROX. Moreover, no methane production is observed in the range of test temperature. In addition, as shown in Fig. 1, the selectivity of O2 shows a contrary trend compared with CO conversion. As the calcination temperature elevates from 400 to 700 °C, the temperature of 100% O2–CO selectivity gradually broadens from 110 to 140 °C. It indicates that the competing adsorption of hydrogen and carbon monoxide exists in the CO-PROX reaction system. CO molecules adsorb on copper–ceria interfacial regions firstly and react with lattice oxygen nearby to produce CO2 along with the reduction of copper cations from Cu2+ to Cu+, so that the H2–O2 reaction could not occur until carbon monoxide desorbed [17,18]. In order to exploring the effect of adding H2O and CO2 on the catalytic performance for CO-PROX, we introduce 7.5% H2O, 15% CO2 and 7.5% H2O + 15% CO2 respectively to the standard reaction gas, and investigate the catalytic performance of CuCMn(500)

Table 1 Structure/texture characteristics of CuO–MnOx–CeO2 catalysts.

a

Catalysts

Cell parameter (nm)

D(1 1 1)a (nm)

SBET (m2 g1)

T50 (°C)

CeO2(500) CuC(500) CuCMn(400) CuCMn(500) CuCMn(600) CuCMn(700)

0.5499 0.5413 0.5407 0.5410 0.5409 0.5408

– 9.6 4.4 8.7 10.3 30.5

– 52 106 68 23 9

– 82 77 74 76 98

From line broadening of CeO2 (1 1 1) peak.

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X. Guo et al. / Fuel 163 (2016) 56–64

100

Conversion & Selectivity (%)

80 60 40 20

90

80 70 60 50

CO conversion O2 selectivity

40 30

80

0

6

12

18

70

without CO2 or H2O with 15% CO2 with 7.5% H2O with 15% CO 2 + 7.5% H2O

60 50

80

100

120

30

36

42

48

54

60

Fig. 3. Catalytic stability test for CO-PROX reaction over the CuCMn(500) catalyst in H2-rich streams with 15% CO2 and 7.5% H2O at 140 °C.

40 60

24

Reaction time (h)

140

160

180

Temperature ( oC) Fig. 2. Effect of CO2 and H2O addition on the performance of CuCMn(500) catalyst.

under the four reaction gas, which is shown in Fig. 2. The existences of H2O and CO2 both severely suppress the CO oxidation activity for CuCMn(500), which leads to the temperature window reducing from 40 °C (110–140 °C) to 20 °C and shifting to the higher temperature (140–150 °C). The low-temperature oxidation activity for CuCMn(500) in the addition of 15% CO2 is much higher than that of 7.5% H2O, meaning that H2O inhibits the catalyst more strongly than CO2. However, the selectivity of CuCMn(500) for CO selectivity oxidation is increased by the addition of H2O and CO2 in a certain degree. Moreover, CO2 exhibits an advantage in improving the selectivity of the catalyst than H2O. The inhibiting effects caused by CO2 on the catalytic performance is due to that the competitive adsorption of CO and CO2 on the ceria surface limits the redox capability via hindering the formation of partially reduced CuOx that are the most active interfacial sites for CO oxidation [8,19]. The negative effect of H2O is mainly because of the presence of adsorbed molecular water which blocks the copper active sites [8]. In general, the negative effect of H2O is much more severe than CO2. Fig. 3 shows the catalytic stability of CuCMn(500) for CO-PROX reaction in H2-rich streams with 15% CO2 and 7.5% H2O. From Fig. 3, we can see that at first 99.4% CO conversion and about 60% O2–CO selectivity maintain on the catalyst at 140 °C. After 29 h stability test, the CO conversion drops due to the accumulated adsorption of CO2 and H2O on the catalyst. However, when stopping adding CO2 and H2O to the feed gas, the CO conversion arises to 99.4% instantly and remains constant during the next four hours (shown in the dashed box). With the introduction of CO2 and H2O again, the CO conversion drops to about 99.1% and maintains for more than 20 hours, which means that the existence of H2O undermines the stability of CuCMn(500) much more greatly than that of CO2 due to the blocking effect of adsorbed molecular water and the accumulated carbonate species [6,8,20].

that all the catalysts mainly show the distinct characteristic peaks of fluorite-type oxide structure of CeO2. There is no peaks of manganese oxides observed in all the catalysts with Mn doping but shows two weak peaks of CuO in 35.6° and 38.7° in the XRD patterns of CuO–MnOx–CeO2 catalysts with relative high calcination temperatures (>500 °C). No peaks of manganese oxides indicate that MnOx may be highly dispersed and part of them enter into the CuO–CeO2 mixed oxides framework forming solid solution. Compared with CuC(500), CuCMn(500) shows a bigger surface area and smaller crystalline size. Meanwhile, it exhibits a much broader FWHM (peak width at half height) and a relative lower diffraction intensity representing a lower crystallinity degree, maybe due to the addition of Mn poorly crystallized [21]. Among the four CuO–MnOx–CeO2 catalysts with different calcination temperature, as the calcination temperature increases, the diffraction peaks intensity of CuO–MnOx–CeO2 catalysts obviously increase indicating the occurrence of bigger particle size (Table 1). Simultaneously the peak shape gradually sharpens and narrows on account of the increased degree of crystallinity. The weak peaks of CuO in 35.6° and 38.7° observed at CuCMn(600) and CuCMn(700) imply that calcining at a higher temperature leads to the aggregation of copper species and impair the interaction between CuOx and CeO2 species, which undermines the catalytic performance of the CuO–MnOx–CeO2 catalysts for CO-PROX. Furthermore, the

CuCeMn(500)

CuO CuCMn(700)

CuCe(500)

CeO 2 (500) 27.5

28.0

28.5 29.0 2 Theta (degrees)

29.5

CuCMn(600) CuCMn(500) CuCMn(400)

CuC(500) CeO2(500)

3.2. Structural/texture properties of CuO–MnOx–CeO2 catalysts 20

XRD patterns of CuO–MnOx–CeO2 catalysts are exhibited in Fig. 4, and the BET specific surface areas, cell parameters and crystalline size of the catalysts are listed in Table 1. It can be observed

Intensity (a.u.)

O2 selectivity (%)

0 100

90

Intensity (a.u.)

CO conversion (%)

100

30

40

50

60

70

2 Theta (degrees) Fig. 4. XRD patterns of CuO–MnOx–CeO2 catalysts.

80

60

X. Guo et al. / Fuel 163 (2016) 56–64

diffraction peaks in the patterns of the samples with copper or/and manganese doping are observed to shift to lower 2h angles (inset in Fig. 4) compared with CeO2(500), indicating the presence of structural strains. On the other hand, from Table 1, it also can be seen that the lattice parameters of ceria in all the CuO–MnOx–CeO2 catalysts are smaller compared with that of CuC(500) (0.5413 nm), while the cell parameter (0.5499 nm) of pure CeO2(500) is much larger than that of the catalysts with copper or/and manganese doping. The ionic radius of Mnx+ (Mn4+: 0.056 nm, Mn3+: 0.062 nm, Mn+: 0.067 nm) and Cux+ (Cu2+: 0.073 nm, Cu+: 0.077 nm) are all smaller than that of Ce4+ (0.096 nm), so the decrease of lattice parameters means that a few Mnx+ and Cux+ enter into ceria lattice and substitute Ce4+ partly, leading to the shrinkage of unit cell and the formation of Cu–Mn–Ce–O solid solution. The results demonstrate that doping manganese and calcined at an appropriate calcination temperature both can adjust the texture and structure properties such as SBET and the degree of crystallinity, which benefits to enhancing the formation of solid solution and the interaction between various active components, ultimately promoting catalytic performance for the CO-PROX. In contrast to XRD results which afford us information related to the cation sublattice, Raman spectroscopy of the catalysts is dominated by oxygen lattice vibrations. Fig. 5 shows the UV-Raman patterns of CuO–MnOx–CeO2 catalysts. Three characteristic peaks could be observed in all the catalysts. The strong band at 454 cm1 is attributed to the F2g Raman vibration mode of the cubic fluorite-structure phase and the broad band at 584 cm1 is attributed to the oxygen vacancies generated by the partial substitution of some other metal ions to Ce4+ [22,23]. In addition, the weak band at 327 cm1 belongs to the rearrangement of oxygen atoms from their ideal fluorite lattice locations [24]. Usually, the area ratio of peaks 584 and 454 cm1 (noted as A584/A454) can represent the relative concentration of oxygen vacancies [25]. According to the passage [26], the high concentration of oxygen vacancies is bound to result in a considerably enhanced activity of the catalysts. Comparing CuCMn(500) with CuC(500), CuCMn(500) shows a stronger peak at 584 cm1 and a weaker peak at 454 cm1 than CuC(500). It is obvious that A584/A454 for CuCMn(500) is much bigger than CuC(500), which means Mn doping in CuC(500) could increase the quantity of oxygen vacancies of the catalyst. From Fig. 3, it also can be seen that with increasing the calcination temperature, the intensity of the band at 454 cm1 increases apparently, while the intensity of the band at 584 cm1 decreases. The area ratio of peaks 584 and 454 cm1 follows the

454 cm-1

584 cm-1

-1

327 cm

Intensity (a.u.)

CuC(500)

CuCMn(700)

CuCMn(600)

CuCMn(500)

CuCMn(400)

200

400

Raman shift

600

800

(cm-1)

Fig. 5. UV-Raman patterns of CuO–MnOx–CeO2 catalysts.

sequence of CuCMn(500) (2.08) > CuCMn(400) (2.07) > CuCMn (600) (1.90) > CuCMn(700) (1.68), which indicates that the amount of oxygen vacancies in CuO–MnOx–CeO2 catalysts is strongly affected by the calcination temperature. When the calcination temperature is higher than 500 °C, the intensity of the band at 454 cm1 increases apparently, which is related to the structure change of the crystal phase caused by the sintering of the catalysts. Moreover, it is noteworthy that the peak position near 454 cm1 for the CuO–MnOx–CeO2 catalysts shows some migration as response to the change of calcination temperature. Specifically, the peak position gradually shifts from 454 cm1 to 463 cm1 (shown with dotted lines in Fig. 5) with the calcination temperature increasing from 400 to 700 °C. The shift of the Raman peak near 454 cm1 in CeO2y nanoparticles progressively to higher energy indicates the increase of their particle size and the crystallinity degree [27], which is consistent with the results of XRD. To get the information about the elements on the surface of the catalysts, XPS spectra of the Cu 2p, Ce 3d and Mn 2p in Fig. 6 are analyzed, and the data of surface species is listed in Table 2. According to the literature [11], it can be known that there are two characteristic peaks in the XPS spectra of Cu 2p3/2. The one centered at about 933.1 eV is the characteristic peak of Cu2+/CuO and the other one centered at 932.2–933.1 eV is regarded as the characteristic peak of Cu+/Cu2O. From Fig. 6(a), it is found that CuCMn(400) and CuCMn(500) catalysts all possess a lower binding energy of Cu 2p3/2 (about 932 eV) and have no shake-up peak compared with CuC(500) catalyst, representing the presence of more Cu+ species in the manganese doped catalysts, which promotes catalytic performance for the CO-PROX. But for CuCMn (700) catalyst, the Cu 2p3/2 peak shifts to higher binding energy regions, indicating that the amount of Cu2+ species in the catalyst calcined at 700 °C increases obviously. As shown in Table 2, the Cu+ content in CuOx species on the surface of CuC(500), CuCMn (400) and CuCMn(500) catalysts is 25.5, 24.7 and 29.5%, respectively, much higher than that in CuCMn(700) catalyst (10.5%). Moreover, the appropriate calcination temperature also promotes the dispersion of CuOx species, which results in the decrease of CuOx species content on the surface of catalysts and enhances the interaction between CuOx and CeO2. However, the exceedingly high calcination temperature would cause the increase of the CuOx species content on the surface due to the aggregation of CuO species, which weakens the interaction between CuOx and CeO2. Fig. 6(b) shows the binding energy of Mn 2p3/2 in the CuO– MnOx–CeO2 catalyst. There are three oxidation states for the manganese cations which are Mn2+, Mn3+ and Mn4+ occurring in the XPS spectra of the Mn 2p2/3 [11], and their binding energy are 640.9, 641.8 and 642.5 eV, respectively. Mn 2p3/2 peaks of CuCMn(500) catalyst shift to higher binding energy regions comparing with that of other catalysts, indicating that the catalyst calcined at 500 °C possess the most Mn4+ species, probably due to the formation of more Cu–Mn–Ce–O solid solutions. Moreover, it is shown in Table 2 that the content of MnOx species on the surface of CuCMn(500) is also the most. The more manganese species in CuO–MnOx–CeO2 catalysts turn to higher oxidation states, the more oxygen species in ceria transfer into the lattice of manganese oxide for charge balance, which promotes the formation of more oxygen vacancies. On the other hand, high valence state of manganese oxides is more easily reduced by CO. Thus, the reducibility of the catalyst could be promoted by the existence of more Mn4+ species on the surface, which enhances the activity of CuO–MnOx– CeO2 catalysts for the CO-PROX. Comparing with other two catalysts, CuCMn(700) catalyst has lower binding energy value of Mn 2p3/2, suggesting the existence of more Mn3+/Mn2+ species, which weakens the reduction ability of copper species. Fig. 6(c) shows the XPS spectra of the Ce 3d. We can see that the Ce 3d spectrum curves consist of two series spin–orbit lines u and

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X. Guo et al. / Fuel 163 (2016) 56–64

(a) Cu 2p3/2

933.1

(b) Mn 2p

CuCMn(700)

Intensity (a.u.)

932.2

Intensity (a.u.)

CuCMn(500) CuCMn(400)

CuCMn(700)

CuC(500) CuCMn(500) CuCMn(400)

928

930

932

934

936

938

940

942

630

944

635

640

Banding Energy (eV)

645

650

655

660

Binding energy (eV)

(c) Ce 2d

v'''

u' u''

u

v''

v'

CuCMn(700)

Intensity (a.u.)

v

CuCMn(500)

CuCMn(400) CuC(500)

875

880

885

890

895

900

905

910

915

Binding Energy (eV) Fig. 6. XPS spectra for CuO–MnOx–CeO2 catalysts.

Table 2 Surface atom ratios of Cu, Mn and Ce species in the CuO–MnOx–CeO2 catalysts derived from XPS analysis.

CuC(500) CuCMn(400) CuCMn(500) CuCMn(700)

Cu/(Cu + Ce + Mn) (at.%)

Mn/(Cu + Ce + Mn) (at.%)

Mn/Cu

23.8 18.1 19.0 23.9

– 5.7 7.1 5.7

– 0.32 0.37 0.24

Cu (%) Cu+

Cu2+

25.5 24.7 29.5 10.5

72.5 75.3 70.5 89.5

Ce 3d5/2 in Ce (%) 14.9 16.3 17.0 11.4

Intensity (a.u.)

Catalysts

CuCMn(700)

3+

CuCMn(600)

CuCMn(500)

CuCMn(400)

v. Ce 3d3/2 spin–orbit components corresponding to the series of u mainly consist of three characteristic peaks labeled as u (900.6– 901.0 eV), u00 (907.5–907.7 eV) and u000 (916.6–916.9 eV). And Ce 3d5/2 spin–orbit components corresponding to the series of v also mainly include three characteristic peaks which are marked as v (882.2–882.6 eV), v00 (889.1–889.3 eV) and v000 (898.2–898.5 eV). The above three pairs of peaks are attributed to the characteristic peaks of Ce4+ species. Moreover, the spectral line labeled as u0 (903.5–904.2 eV) and v0 (885.1–885.8 eV) belongs to the Ce3+ species [22]. The relative percentages of cerium species are obtained by the area ratios of Ce4+3d5/2 (v, v00 and v000 )/Ce3+ 3d5/2 (v0 ). According to Table 2, compared with CuC(500) catalyst, CuCMn(400) and CuCMn(500) catalysts possess more Ce3+ species, which indicates that more copper/manganese species enter CeO2 lattice in the manganese doped catalysts and a portion of Ce4+ is reduced to Ce3+ due to charge balance, while the appearance of Ce3+ is related to oxygen vacancies generating in the catalysts [22]. The content of Ce3+ species in CuCMn(700) catalyst is obviously decreased due to

β

α 100

150

200

γ 250

CuC(500)

300

350

400

Temperature ( oC) Fig. 7. H2-TPR profiles of CuO–MnOx–CeO2 catalysts.

the interaction of the various active components being weakened, which is in accordance with the result of UV-Raman. 3.3. Redox property of catalysts In order to study the redox ability of the catalysts, the CuO– MnOx–CeO2 catalysts were investigated by H2-TPR and the results are displayed in Fig. 7. Three reduction peaks (a, b and c) related to the reduction of copper species are observed at the temperature range of 100–300 °C for CuC(500) catalyst without doping man-

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2359

2339

CuC(500)

2.0

110 C

2111 2121

o

2124

210 oC 190 oC 170 oC 150 C o 130 C

2121

o

K-M

K-M

o

CuCMn(400)

2.0

CO2(g)

210 o C 190 o C 170 ooC 150 C 130 o C

110 C 2122

2119 o

90 C

90 o C

2065

70 o C

o

70 C o 30 C

30 o C Initial

2400

2300

2200

2100

Initial

2300

2400

2000

Wavenumber (cm-1)

2000

1800

CuCMn(700)

o

2.0

210 oC 190 oC 170 oC 150 C o 130 C

2118

o

110 C 2121

1900

Wavenumber (cm -1)

2115

CO2(g)

o

210 C o 190 C o 170 C o 150 C

2105

K-M

K-M

2123

2100

CuCMn(500)

2.0

CO2(g)

2200

2111

o

130 C o

o

90 C

110 C

2123

2108

o

90 C o

70 C

o

70 C o 30 C Initial

o

30 C Initial

2400

2250

2100

1950

1800

-1

Wavenumber (cm )

2400

2300

2200

2100

2000

1900

1800

-1

Wavenumber (cm )

Fig. 8. DRIFTS spectra of CuO–MnOx–CeO2 catalysts measured under the simple CO + O2 + H2 reaction gas. An indication in the right part, the bottom spectrum corresponds to the initial one before exposure to the reaction gas; the rest from bottom to top corresponds to spectra measured every 20 °C from 30 to 210 °C.

ganese. The first peak at about 160 °C (peak a) is assigned to the reduction of the CuOx species that have strong interaction with the ceria support and the third peak at about 280 °C (peak c) belongs to the reduction of Cu+ species which is considered as the active site to adsorb CO [28,29]. And the weak peak at about 212 °C (peak b) which is overlapping between peak a and peak c represents the reduction of dispersive CuOx species on the surface of ceria, including isolated Cu2+ ions which have weak interaction with ceria and the small size copper clusters with two- and three-dimensional structure [26]. As shown Fig. 7, for CuCMn (500) catalyst, the reduction temperature of peak a shifts slightly to high-temperature region and the hydrogen consumption of peak a gets increased obviously, 218 lmol/gcat for CuC(500) and 282 lmol/gcat for CuCMn(500), respectively. According to the literature [30], it can be known that the coexistence of CeO2 and MnOx can mutually promote each other’s reduction by enhancing the mobility of oxygen species. But it is difficult to distinguish the reduction peaks of copper and manganese species because the two peaks are basically overlapping with each other. It suggests that the reduction from Mn4+ to Mn3+ may occur in the low temperature region to increase the hydrogen consumption of peak a of CuCMn(500). However, manganese oxide is much harder to be reduced than copper oxide because the free energy of formation of manganese oxide is lower than copper oxide [31,32], which results in the slight increase of the reduction temperature of peak a for CuCMn(500). Moreover, hydrogen consumption of peak c for CuCMn(500) catalyst is also higher than that of CuC(500), 299 and 224 lmol/gcat, respectively, which means that the peak c should also include the reduction of MnOx species. In addition, with the increase of the calcination temperature from 400 °C to 500 °C,

the temperature of the three reduction peaks for the CuO–MnOx– CeO2 catalysts obviously decreases, which demonstrates that raising the calcination temperature to a certain extent can specifically facilitate the formation of active copper/manganese species and strengthen the mutual effect between the active components and support, thus improve the redox property of the catalyst and its catalytic ability for selective oxidation of CO in excess H2. When the calcination temperature is higher than 500 °C, the hydrogen consumption of peak a obviously decreases and that of peak c shows a contrary tendency. The reduction temperature of crystalline phase CuO (about 255 °C) is a little lower than Cu2 O (about 280 °C) [33]. Therefore, the results indicate that the exceedingly high calcination temperature would lead to the aggregation of CuO species and the decrease of Cu+ species, which is consistent with the result of XRD. Fig. 8 presents DRIFTS spectra (2400–1800 cm1) of CuO– MnOx–CeO2 catalysts under CO + O2 + H2 reaction gas. As mentioned in the literature [9], the two characteristic peaks at 2359 and 2339 cm1 are the rotational vibration absorption peaks of CO2 (g), whose intensity is related with the oxidation of CO. The strong bands at the region of 2200–2000 cm1 are attributed to carbonyl species adsorbed on CuOx species. Usually, the lower wave-number band at 2065 cm1 is attributed to the vibration absorption peak of CO adsorption at copper clusters with small particle size, and the higher wave-number bands at 2130– 2110 cm1 are regarded as CO adsorption at Cu+ species [34]. However, the strong interaction between CuOx species and CeO2 could make the bands shift to lower wavenumber [8,35,36]. There are two strong peaks at the region of 2200–2080 cm1 observed over the catalysts, in which the band at 2111 cm1 is attributed to CO

X. Guo et al. / Fuel 163 (2016) 56–64

chemisorbed on Cu+ species that interact strongly with CeO2, and the band at 2121 cm1 is relevant to CO–Cu+ with weak interaction and/or the isolated copper species on the surface of CeO2. Compared with CuC(500), the intensity of the band at 2121 cm1 is obviously increased over CuCMn(500) catalyst, which indicates that doping manganese promote the interaction between copper and ceria due to the formation of more Cu–Mn–Ce–O solid solutions. For CuO–MnOx–CeO2 catalyst, as the calcination temperature increases from 400 to 500 °C, the intensity of the bands at 2111 cm1 and 2121 cm1 both get increased, representing the promoted interaction between Cu species and CeO2. But with further increasing the calcination temperature, the intensity of the band related to CO–Cu+ species is weakened sharply, especially for that of the bands at 2121 cm1, indicating that the interaction between Cu species and CeO2 is obviously suppressed due to the aggregation of CuOx species. In addition, from Fig. 8, it can be also seen that the bands related to CO–CuOx species obviously increases in the intensity when the reaction temperature rises up to 70– 90 °C over CuC (500), CuCMn(400) and CuCMn(500) catalysts, while the oxidation reaction of CO is significantly promoted at this temperature (Fig. 1(A)). As the reaction temperature is further increased, the CO oxidation continues to be accelerated along with the decrease of the bands for Cu+-carbonyl species. Ultimately, this bands disappear at 210 °C on account of CO desorbing from catalyst surface completely. It is noteworthy that a small red shift occurs to the peak for Cu+-carbonyl species as the reaction temperature higher than 110 °C, which is on account of the reduction of Cu+ to Cu0 by H2 [37]. However, Cu0 can accelerate the dissociation and oxidation of H2 and results in the sharp decrease of the oxygen to CO selectivity at high temperature (>110 °C, Fig. 1(B)).

4. Conclusions In this study, we investigated the catalytic performance of a series of CuO–MnOx–CeO2 catalysts with different calcination temperatures for CO-PROX in excess H2 and characterized them with BET, XRD, XPS, H2-TPR, UV-Raman and In situ DRIFTS techniques. The results show that the catalytic performance of CuO–CeO2 catalysts for CO-PROX is improved by Mn doping and an appropriate calcination temperature (500 °C). The operating temperature ‘‘window” for CO conversion above 99.0% is 110–140 °C for CuCMn (500), while which for CuCMn(700) does not occur at the whole test temperature range. This result is attributed to that an appropriate calcination temperature enhances the interaction between CuOx/MnOx species and CeO2, further facilitates the formation of more Cu+/Mn4+ species and oxygen vacancies in the solid solution of Cu–Mn–Ce–O. However, the exceedingly high calcination temperature (higher than 500 °C) would weaken the interaction between CuOx and CeO2 species, which leads to the aggregation of CuOx species and the decrease of Cu+ species content and impairs the catalytic ability of the catalysts for selective oxidation of CO.

Acknowledgement We gratefully acknowledge the financial supports from Nature Science Foundation of China (No. 21477109).

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