CeO2 catalysts

JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011, P. 872

Hydrogen production by steam reforming of ethanol over copper doped Ni/CeO2 catalysts LIU Qihai (߬݊⍋)1, LIU Zili (߬㞾࡯)2Ҙ, ZHOU Xinhua (਼ᮄढ)1, LI Cuijin (ᴢ㖴䞥)1, DING Jiao (ϕ࿛)1 (1. School of Chemistry and Chemical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, China; 2. School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China) Received 22 March 2011; revised 4 May 2011

Abstract: High surface area CeO2 was prepared by the surfactant-assisted route and was employed as catalyst support. The 0–3 at.% Cu doped Cu-Ni/CeO2 catalysts with 10 wt.% and 15 wt.% of total metal loading were prepared by an impregnation-coprecipitation method. The influence of Cu atomic content on the catalytic performance was investigated on the steam reforming of ethanol (SRE) for H2 production and the catalysts were characterized by N2 adsorption, inductively coupled plasma (ICP), X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed rerduction (TPR) and H2-pulse chemisorption techniques. The activity and products distribution behaviors of the catalysts were significantly affected by the doped Cu molar content based on the promotion effect on the dispersion of NiO particles and the interactions between Cu-Ni metal and CeO2 support. Significant increase in the ethanol conversion and hydrogen selectivity were obtained when moderate Cu metal was doped into the Ni/CeO2 catalyst. Over both of the 10Ni98.5Cu1.5/CeO2 and 15Ni98.5Cu1.5/CeO2 catalysts, more than 80% of ethanol conversion and 60% of H2 selectivity were obtained in the ethanol steam-reforming when the reaction temperature was above 450 ºC. Keywords: copper dopant; Cu-Ni/CeO2 catalyst; ethanol steam reforming; hydrogen production; rare earths

Fuel cells have been considered as an effective generator system in recent years owing to the high conversion efficiency from chemical energy of hydrogen to electricity without emission of any pollutant gases[1]. Proton exchange membrane fuel cells (PEMFCs) were investigated as a candidate for power sources of small generators and vehicles[2,3]. Hydrogen, used as fuel for PEMFCs system, was produced in large part from reforming of hydrocarbons. Among all the possible hydrocarbon fuel, ethanol was the most favorable due to its non-toxicity, natural availability, renewability, high power density, and zero green-house contribution to the atmosphere[4–6]. Consequently, it is important to develop ethanol steam reforming catalysts characterized by high activity at sufficiently low temperatures. There were many studies, in recent years, reporting hydrogen production from ethanol steam reforming (ESR) over various catalysts. Noble metal catalysts, such as Ir, Au, Ru, Pt and Rh[7–10] were investigated as high performance catalysts, and it was considered that they had higher activity and hydrogen selectivity than the base metal ones in the ESR reaction, because base metal catalysts need long pretreatment in reducing atmosphere and rapidly lose activity in contact with air[11]. However, essential cost-efficiency of the base metal systems is the advantage that noble metal ones can never own. Consequently, it is important to develop stable, noble metal-free and highly active catalysts for ESR reaction.

Among the non-noble metal catalysts studied so far, cobaltbased catalysts were reported to show the best performance, giving high hydrogen yield in the SRE reaction[12–15]. Cobalt-based catalysts, however, were found to be inclined toward deactivation because of the carbon deposition, and this deactivation increased with the increasing reaction temperature. Due to the carbon deposition tolerant nature of nano Ni metal particles, Ni catalysts supported on various supports have also been reported to be a sort of promising catalysts for ESR reaction[16–19]. Although Ni catalysts supported on different oxides presented comparable activity, the product distribution was quite different. It was reported that rare earth oxides, such as CeO2, Y2O3, La2O3, offer the greatest potential as catalyst support for Ni catalyst due to their excellent redox and high oxygen storage capacity[20–22]. In addition to the support effect, the doped secondary metal can also significantly affect the catalytic performance of the catalysts. Numbers of studies revealed that bimetallic catalysts often appear to be more active than monometallic ones[23–25]. This is because one of the components can segregate the active particles and prevent them from aggregation by forming an interpolation system between the two metals. Specific interactions between metal species in bimetallic catalysts also often results in completely different adsorption properties and reducibility of the active metals from the monometallic ones[14,17,24]. For example, it has been demon-

Foundation item: Project supported by the National Natural Science Foundation (21076047) and the Natural Science Foundation of Zhongkai University of Agriculture and Engineering (G3100026) Corresponding author: LIU Zili (E-mail: [email protected]; Tel.: +86-20-39366904) DOI: 10.1016/S1002-0721(10)60558-3

LIU Qihai et al., Hydrogen production by steam reforming of ethanol over copper doped Ni/CeO2 catalysts

strated that, by adding copper or Na, K alkali metal to the Ni-based catalyst, decrease in the reduce temperature of the NiO of the catalyst and increase in the catalyst durability were observed, attributing in part to the interactions between Ni and the doped species[23,26]. In this work we investigated the catalytic performance of a Cu doped Ni catalyst supported on the porous nano CeO2 which was synthesized in the microemulsion solution system. The promoting effect of Cu to the Ni catalyst for the hydrogen production in the ESR reaction was analyzed.

1 Experimental 1.1 Catalyst preparation The CeO2 support employed in this study was prepared by the surfactant-assisted route. 1.0 g of hexadecyl trimethyl ammonium bromide (HTAB) dissolved in 20 ml of H2O was added to a 10 ml 0.5 mol/L cerium nitrate aqueous solution under vigorous stirring and was well mixed to ensure self-assembly. The pH of the mixed solution was adjusted to 10–11 by 25% ammonia solution, and the cerium hydroxide sol-gel was formed. The obtained sol-gel precipitate was filtrated after aging for 48 h at 80 ºC, washed free of chlorine ions by deionized water and was dried at 110 ºC for 12 h. The dried solid was then calcined at 600 ºC for 3 h. The obtained sample was crushed into powders and employed as the catalyst support. Its surface area measured by N2 physisorption was 103 m2/g. The supported Ni and Cu-Ni catalysts were prepared by incipient wetness impregnation method. Prior to impregnation, the CeO2 was dried in an oven at 300 ºC for 2 h. For the incipient wetness method, a defined volume of the solution containing the metal nitrates was taken so that it was completely absorbed by the support material. By this method, the content of introduced nickel and copper can be accurately controlled simply. After the impregnation, the wet sample was dried in air at room temperature for at least 12 h, then dried at 110 ºC for 4 h. After the drying, the samples were calcined at 600 ºC with a heating rate of 10 ºC/min for 3 h. The obtained catalyst powders were pressed into pellets.

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the AutoChem II 2920 V3.05 instrument (Micromeritics Instrument Corporation). H2 pulse chemisorption was carried out to determine the crystallite size and dispersion of the supported nickle metal. Prior to H2 pulse chemisorption, the catalyst was reduced in 5% H2/N2 at a temperature ramping rate of 10 K/min to 800 ºC, and held at 800 ºC for 1 h. Then, the system was purged and cooled to 50 ºC in flowing N2. H2 pulse chemisorption was performed at 50 ºC using H2 of 99.99% purity and repeated at 5 min intervals for 10 times. 1.3 Catalytic test Catalyst tests were performed in a fixed-bed quartz reactor (i.d. 4 mm) at atmospheric pressure. About 150 mg catalysts (catalyst bed length ~12 mm, packed bed volume ~0.6 ml) were loaded into the reactor. The temperature was measured by a chromel-alumel thermocouple in the middle of the catalyst bed. A gas-liquid separator was linked with the outlet of the fixed-bed reactor for the separation of gaseous and liquid products. Prior to each catalytic reaction, the catalyst was flushed by N2 for 10 min, and then reduced in 5% H2-N2 at 650 ºC for 90 min and then cooled down to the initial reaction temperature. The catalysts were tested at the temperature range of 300–600 ºC. The liquid mixed reactant with the volume ratio of H2O:EtOH=7:3 (EtOH:H2O§1:7.6 by molar ratio) was fed into the vaporizer with a flow rate of 2 ml/h by a piston metering pump. The liquid reactant was vaporized at 150 ºC in the vaporizer and diluted with N2 (purity 99.9995%, (C2H5OH+H2O):N2=1:4 by volume ratio) before entering the reaction chamber. The GHSV was around 6000 hí1. The effluent gaseous products were analyzed on-line by an Agilent 6890 gas chromatograph equipped with a TDX-01 column and a thermal conductivity detector. The detection limit of CO, H2, CO2, N2 and CH4 products reached 10 ppm. The liquid products collected in the gas-liquid separator were analyzed off-line on another gas chromatograph equipped with a Porapack Q column by manual injection.

2 Results and discussion

1.2 Catalyst characterization

2.1 Characterization of catalysts

The compositions of the prepared samples were analyzed by means of inductively coupled plasma-atomic emission spectroscopy (ICP, IRIS ADVANTAGE. ER/S). The structure of the catalysts and its alteration during heating pretreatment were determined by X-ray diffraction (Beijing Purkinje General, XD-3), using Cu KĮ radiation with 1.5406 nm incidence wavelength. BET surface areas (SBET) were measured by nitrogen absorption (TriStar II 3020 V1.01) at 77 K after degassing at 373 K for 2.0 h. The TEM images of the catalysts were recorded using a JEOI JEM-100CXII electron microscope (0.14 nm resolution at 200 kV) fitted with an energy-dispersive X-ray spectrometer EDAX. The TPR and H2 pulse chemisorption of the catalysts were carried out on

The results of elemental analysis, average metallic crystallite size, metallic dispersion and the BET surface area of the Cu-Ni/CeO2 catalysts with different Cu atomic contents are summarized in Table 1. According to the results of the elemental analysis, the contents of the doped Cu and the total metal loading fit well that aimed for in the catalyst preparation procedure. By comparison, an obvious decrease in the specific surface areas of the catalysts is found when the total metal loading is increased from 10 wt.% to 15 wt.% onto CeO2. This indicates that physical blocking of the support surface with metal containing particles occurred while the supporting metal was up to 15 wt.%, probably due to the formation of large oxide aggregates, which limits the access

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Table 1 Physicochemical properties of the samples prepared in the present study Total metal

Cu content/

Metallic crystallite size/

Metallic dispersion/

Surface area/

loading/wt.%*

at.%*

nm**

%**

(m2/g cat.)

CeO2

–

–

–

–

103

10Ni100/CeO2

9.36

–

32.65

5.56

79

10Cu0.5Ni99.5/CeO2

9.12

0.63

20.77

8.93

77

10Cu1.0Ni99.0/CeO2

9.68

1.21

9.26

12.42

75

10Cu1.5Ni98.5/CeO2

9.41

1.69

10.64

14.88

74

10Cu3.0Ni97.0/CeO2

10.04

3.33

13.09

12.15

72

15Ni100/CeO2

15.57

–

48.06

3.22

64

Samples

15Cu0.5Ni99.5/CeO2

15.02

0.43

30.94

5.98

62

15Cu1.0Ni99.0/CeO2

14.89

0.98

16.48

5.73

63

15Cu1.5Ni98.5/CeO2

15.13

1.55

10.92

7.12

60

15Cu3.0Ni97.0/CeO2

14.78

3.19

25.98

6.85

57

* The total matal loading and the cope atomic content were determined by inductively coupled plasma atomic emission spectrometry; ** The metallic crystallite size and dispersion of catalysts were calculated from H2 pulse chemisorption

to the internal surface of the metal particles[27]. On both 10 wt.% and 15 wt.% metal loading catalysts, it was seen that the doped Cu led to no significant change of the BET surface areas of the catalyst, but it dramatically influenced the metallic crystallite size and dispersion of the catalysts. The Cu doped Cu-Ni/CeO2 catalyst gave the smaller metallic crystallite size and higher metallic dispersion than that of undoped Ni/CeO2 catalyst. The smallest metallic crystallite size and the highest metallic dispersion appeared on the catalyst with 1.5% molar percentage of Cu composition. Fig. 1 shows the XRD patterns of the CeO2 support, the Ni/CeO2 and the Cu doped Cu-Ni/CeO2 catalysts with 15 wt.% of total metal loading. The NiO diffraction peaks at around 2ș=45° could be found in the 15Ni100/CeO2 catalyst. When 0.5% mole ratio of Cu was doped in the catalysts, however, the NiO diffraction became extremely faint and a slight shift to lower diffraction degree was observed. This indicated that the interaction system between Cu and Ni metals might be formed, which could facilitate the dispersion of NiO on the surface of CeO2 support in the crystal that cannot be tested by XRD[28]. The XRD patterns of the 15Cu1.0Ni99.0/CeO2 and 15Cu1.5Ni98.5/CeO2 catalysts were the same as the XRD

Fig. 1 XRD patterns of CeO2 (1), 15Ni100/CeO2 (2), 15Cu0.5Ni99.5/CeO2 (3), 15Cu1.0Ni99.0/CeO2 (4), 15Cu1.5Ni98.5/CeO2 (5) and 15Cu3.0Ni97.0/ CeO2 (6)

pattern of the 15Cu0.5Ni99.5/CeO2 sample, demonstrating neither nickel oxide, nor copper oxide peaks. This likely meant that the nickel and copper dispersed finely on the support of the catalysts because of the segregation effect of the Cu-Ni interation system[23]. As the Cu content increased to 3 at.%, slight diffraction peaks corresponding to nickel oxide(2ș=45°) and copper oxide (2ș=37° and 2ș=42°) appeared in the XRD patterns, indicating that some large metallic oxide particles within the XRD detection formed. The results of TEM measurements for the 15Ni100/CeO2 and 15Cu1.5Ni98.5/CeO2 catalysts are shown in Fig. 2. In

Fig. 2 TEM images of the 15Ni100/CeO2 (a) and 15Cu1.5Ni98.5/CeO2 (b) catalysts

LIU Qihai et al., Hydrogen production by steam reforming of ethanol over copper doped Ni/CeO2 catalysts

the images, the particles of bulk nickel metal are easily seen in Fig. 2(a) for the 15Ni100/CeO2 catalyst. From the TEM images, the size of the particles of the 15Cu1.5Ni98.5/ CeO2 catalyst is quite smaller and uniformly distributed than the 15Ni100/CeO2 catalyst. This is expected since the doped Cu could promote the dispersion of Ni metal on the CeO2 surface, as stated above according to the result of the H2 pulse chemisorption and XRD data. Based on the XRD and TEM results, it is deduced that a Cu-Ni alloy is probably formed in the catalyst, and the bimetallic system likely contains a range of metal particles with somewhat different Cu-Ni ratios, leading to the surface migration and aggregation blocking effect[29]. Fig. 3 shows the TPR profiles of the 15Ni100/CeO2, 15Cu1.0Ni99.0/CeO2, 15Cu1.5Ni98.5/CeO2 and 15Cu3.0Ni97.0/CeO2 catalysts. Except the curve (2), all the profiles presented two reduction peaks. The peaks at 252 or 254 ºCon the TPR curves, that did not appear on the 15Ni100/CeO2 catalyst, are reasonable to assign to the reduction of CuO species. With the increase of the copper content, the intensity of these peaks increased, but no obvious change on the reduction temperature was found. The reduction peaks presented at above 350 ºC were logically attributed to the reduction of NiO[16]. On the catalysts with less than 1.5% Cu atomic ratio, these NiO reduction peaks were charactered by the cloven reduction peaks, indicating that at least two forms of NiO species assembled on the CeO2 support. The lower NiO reduction peaks are due to the reduction of the finely dispersive NiO on the CeO2 surface, and the higher ones are probably owing to the reduction of those NiO species that strongly interact with CeO2 by entering into the lattice of CeO2 when treated at high temperature[26]. It is interesting that the cloven NiO reduction peaks merged into one when the Cu mole ratio in the catalyst was increased higher than 1.5%. This is probably because the doped Cu prevented NiO species from transferring to form polymorphs and the NiO particles can be dispersed in-situ with the size of nano particles in the porous of CeO2 support. This illuminates that the copper doped in the catalysts significantly promoted the dis-

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persion of the nickel via aggregation blocking effect. Additionally, it was reported that small amount of Cu entering into the lattice of CeO2 can improve the oxygen mobility of the CeO2 carrier by surface modification[30]. 2.2 Catalytic performance Table 2 shows the catalytic performance of the 10CuNi/CeO2 and 15Cu-Ni/CeO2 catalysts with various Cu atomic contents at the reaction temperature of 350 ºC. It is seen from Table 2 that Cu had tremendous effect on the ethanol conversion and the products distribution. Over both the 10 wt.% and 15 wt.% metal loading catalysts, with the molar percentage of Cu increasing from 0 to 1.5%, the conversion of ethanol increased dramatically from below 18% to around 40%. However, the 3.0 at.% Cu content seemed to fail to further promote the increase of ethanol conversion, for no significant change on the ethanol conversion was found when the Cu molar content was increased form 1.5 to 3.0 at.%. The catalyst with Cu1.5Ni98.5 composition was the most active for both 10 wt.% and 15 wt.% metal loading. At 3.0 at.% Cu concentration the activity decreases slightly, most probably due to the blocking of external surface of the CeO2 by larger metal aggregates, which was confirmed by a decrease of metallic crystallite size and dispersion of catalysts when Cu molar content was up to 3.0%, as shown in Table 1. It is shown in Table 1 and Fig. 3 that the reducibility of the metallic oxide particles was strongly affected by their crystallite size: larger oxide particles require relatively higher reduction temperatures. Since no metal phase was observed neither by XRD nor by TEM measurements in the case of the samples containing 1.5 at.% Cu metal, it is quite reasonable to assume that in this case much smaller active metal particles were formed during the preparation procedure. It is observed in Fig. 3 that with Cu content increasing from 1 to 1.5 at.% on the catalyst, the maximum reduction temperature shifted almost 170 ºC towards lower temperatures. For catalysts doped with Cu metal, a significant amount of reducible CuO species likely serves as metallic nuclei to facilitate reduction of NiO and block the aggregation of small NiO particles, which lead to increased ESR reaction activity in Table 2 Catalytic performance of the as-synthesised series CuNi/CeO2 catalysts at 350 ºC Catalysts

Et(OH)

Selectivity/%

tent/at.% conversion/%

H2

-

16.87

45.05 9.52

12.07 7.24 26.12

10Cu0.5Ni99.5/CeO2 0.63

28.25

48.59 8.85

11.71 6.87 23.98

10Cu1.0Ni99.0/CeO2 1.21

35.25

50.72 8.65

13.83 5.82 20.98

10Cu1.5Ni98.5/CeO2 1.69

38.22

55.71 6.37

12.61 5.22 20.09

10Cu3.0Ni97.0/CeO2 3.33

37.73

53.62 5.67

14.98 6.11 19.62

10Ni100/CeO2

15Ni100/CeO2

Fig. 3 TPR profiles of the 15Ni100/CeO2 (1), 15Cu1.0Ni99.0/CeO2 (2), 15Cu1.5Ni98.5/CeO2 (3) and 15Cu3.0Ni97.0/CeO2 (4) catalysts

Cu con-

-

CO

CH4

CO2 CH3CHO

17.86

44.19 10.35 15.45 4.36 25.65

15Cu0.5Ni99.5/CeO2 0.43

27.63

50.42 7.85

12.65 5.47 23.61

15Cu1.0Ni99.0/CeO2 0.98

33.35

54.23 5.84

13.22 5.62 21.09

15Cu1.5Ni98.5/CeO2 1.55

40.63

51.54 6.58

16.65 5.47 19.76

15Cu3.0Ni97.0/CeO2 3.19

40.25

49.62 3.97

18.31 6.32 21.78

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agreement with previous findings[26]. As for the products selectivity, it is seen from the Table 2 that, all the catalysts presented a higher than 45% hydrogen selectivity. As Cu was doped to the catalyst, an increase in the selectivity of the hydrogen was observed, and an around 10% of the highest increase rate in the hydrogen selectivity can be obtained. In addition to H2, moderate CO, CO2, CH4 and CH3CHO were produced over these Cu-Ni/CeO2 catalysts. Higher than 20% of CH3CHO selectivity obtained in the reaction indicates that dehydrogenation of ethanol might predominate at the reaction temperature of 350 ºC, and it is likely the first step for the hydrogen production of the ESR reation. To further investigate the catalytic behavior of the catalysts, the ESR reaction at the temperature range from 300 to 600 ºC was performed over the 10Ni98.5Cu1.5/CeO2 and 15Ni98.5Cu1.5/CeO2 catalysts, as they showed the highest catalytic activity, and the results are listed in Tables 3 and 4. Two catalysts showed excellent catalytic performance with more than 80% of ethanol conversion and 60% of H2 selectivity in the ethanol steam-reforming when the reaction temperature was above 450 ºC. Over the two catalysts, the selectivity to hydrogen was increasing in parallel with the ethanol conversion. The ethanol conversion was increased alone near to 100% with the temperature increasing up to 600 ºC, and high yields to H2 and CH4 were obtained. In all cases the main reaction which took place under the experimental conditions used, was ethanol steam reforming. When catalysts operated at lower temperature (<400 ºC), high selectivity to CH3CHO was observed due to the dehydrogenation of ethanol[14]. The selectivity to CH3CHO dramatically decreased Table 3 Catalytic behavior of the 10Cu1.5Ni98.5/CeO2 catalyst at different temperatures* T/ºC

Rt(OH)

Selectivity/%

conversion/%

H2

CO

CH4

CO2

CH3CHO

300

16.87

55.07

5.52

8.97

4.26

26.18

350

38.22

55.71

6.37

12.61

5.22

20.09

400

66.63

65.72

5.41

10.78

4.53

13.56

450

81.17

75.47

5.37

11.81

5.26

2.09

500

88.22

73.91

6.70

12.98

6.41

0

550

95.11

72.61

7.21

12.33

7.85

0

600

97.33

70.74

7.11

13.78

8.37

0

* The reaction time was 3 h

Table 4 Catalytic behavior of the 15Cu1.5Ni98.5/CeO2 catalyst at different temperatures* T/ºC

Rt(OH)

Selectivity/%

conversion/%

H2

CO

CH4

CO2

CH3CHO

300

17.86

52.09

3.45

17.45

3.36

23.65

350

40.63

51.54

6.58

16.65

5.47

19.76

400

63.35

57.26

5.86

21.22

6.66

8.99

450

80.81

63.63

5.32

19.29

7.57

4.2

500

88.25

70.62

6.97

14.31

6.32

1.77

550

94.02

72.81

7.52

11.72

7.96

0

600

97.65

70.64

8.85

12.28

8.23

0

* The reaction time was 3 h

with the temperature increasing, and no CH3CHO was detected when the temperature was up to 550 ºC, indicating that the dehydrogenation of CH3CHO completely proceeded at this temperature. The main difference of the catalytic behavior between the 10Ni98.5Cu1.5/CeO2 and 15Ni98.5Cu1.5/CeO2 catalysts was their selectivity to CH4 product. The selectivity of the 15Ni98.5Cu1.5/CeO2 to CH4 was higher than that of 10Ni98.5Cu1.5/CeO2 catalyst at the temperature lower than 500 ºC. The high CH4 selectivity of the 15Ni98.5Cu1.5/CeO2 catalyst is likely attributed to the methanation reaction of CO and CO2(CO+3H2ĺCH4+H2O, CO2+4H2ĺCH4+2H2O), as nickel was found to also have high catalytic activity for the methanation of CO/CO2 reaction[31–33]. This is further testified by the lower CO and CO2 distribution of 15Ni98.5Cu1.5/CeO2 catalyst than that of 10Ni98.5Cu1.5/CeO2 one below 500 ºC.

3 Conclusions Hydrogen production by steam reforming of ethanol was investigated over the 10 wt.% and 15 wt.% metal loading Cu-Ni/CeO2 catalysts with 0–3.0% Cu atomic composition. The doped Cu had tremendous effect both on the activity and products distribution of the catalyst. Significant increase in the ethanol conversion and hydrogen selectivity were obtained when moderate Cu metal was doped into the Ni/CeO2 catalyst. Among the catalysts studied, the one with Cu1.5Ni98.5 active metal composition was the most active and selective to hydrogen for both 10 wt.% and 15 wt.% metal loading. Over the 10Ni98.5Cu1.5/CeO2 and 15Ni98.5Cu1.5/CeO2 catalysts, more than 80% of ethanol conversion and 60% of H2 selectivity was obtained in the ethanol steam-reforming when the reaction temperature was higher than 450 ºC. The doped Cu influenced the catalytic performance of the Ni/ CeO2 catalyst in the ESR reaction based on the increase of dispersion of the NiO particles and the modified surface of CeO2 support by forming the interaction system between Cu-Ni metal and support.

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