Carbon nanotubes supported nickel as the highly efficient catalyst for

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Carbon nanotubes supported nickel as the highly efficient catalyst for hydrogen production through glycerol steam reforming Shuzhuang Liu, Zhao Yan, Yuanyuan Zhang, Rong Wang, Shi-Zhong Luo, Fangli Jing, and Wei Chu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03095 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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ACS Sustainable Chemistry & Engineering

Carbon nanotubes supported nickel as the highly efficient catalyst for hydrogen production through glycerol steam reforming

Shuzhuang Liu †, Zhao Yan †, Yuanyuan Zhang ‡, Rong Wang †, Shi-Zhong Luo †, Fangli Jing †, ∗, Wei Chu †

†

School of Chemical Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, 610065 Chengdu, China

‡

∗

School of Science, Xihua University, No. 999 Tuqiao Jinzhou Road, 610039 Chengdu, China

Corresponding author. Tel. +86-28-85403836. E-mail: [email protected] (Fangli Jing)

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Different amounts of nickel were loaded on carbon nanotubes (CNTs) by impregnation method, and characterized by N2 adsorption/desorption isotherms, X-ray diffraction, X-ray photoelectron spectroscopy, H2 temperature programmed reduction, H2 chemisorption and transmission electron microscopy to study the porosity, the crystalline phases, the surface property, the reducibility property, the metallic Ni dispersion and morphology. It was found that two types of Ni2+ species differing by the interaction with the CNTs support co-existed, the amount of free NiO was dominated by Ni content, more metallic Ni species could be thus obtained on the catalyst with higher Ni loading. Furthermore, the descriptor between the active phases property and the catalytic performances was established, which suggested that both the surface area and the state of Ni dispersion synergistically determined the catalytic reaction. The catalysts showed the good catalytic performances at a relative lower reaction temperature of 375 oC.

Keywords: Renewable hydrogen, Glycerol, Carbon nanotubes, Nickel catalyst, Steam reforming


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Introduction The combustion of the fossil resources usually accompanies the CO2 emissionwhich is accumulated significantly in the atmosphere during the past decades and may have a profound influence on climate change

1-2

. On the other hand, the increased population of the world also

aggravate the contradiction between the energy demand and the environmental protection, such a state of the art enforces the researcher to seek for the greener and sustainable energy to reduce the dependent on the traditional non-renewable energy resources

3-6

. Take hydrogen as an

example, it is considered as an absolute clean energy as water is the unique product during its thermal combustion. However, current hydrogen production is mainly produced from the reforming of natural gas, leading to a non-sustainability and CO2 emission during the production process 7. In such a context glycerol has received great interests as an alternative hydrogen source because of its high hydrogen/carbon ratio, nontoxicity and convenience in transportation

8

. It should be noted that the production of biodiesel through the

transesterification reaction of triglycerides generates approximately 10 wt.% of glycerol as by-product 9-10. At the same time, the expanded biodiesel market offers the abundant feedstock, which benefits the valorization of glycerol for the value-added chemicals including hydrogen production for environmental and economic reasons

11-12

. Hydrogen production from

bio-sourced glycerol was pioneered by Dumesic 13 through aqueous reforming reaction where the noble metal catalyst (Pt/Al2O3) was applied. Nevertheless, only a few publications on aqueous reforming of glycerol could be found, which may be probably due to the harsh operation conditions under high pressure (usually >2 MPa) and the high cost of the noble metal-based catalysts

14-16

. Instead, the steam reforming of bio-sourced glycerol provides an



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alternative method that allows the reaction to be operated at atmospheric pressure. Steam reforming of glycerol is overall an endothermic reaction which could be represented by equation (1): C3H8O3 + 3H2O ⇒ 3CO2 + 7H2O

∆H0 = 128 kJ/mol

(1)

Where it involves endothermic glycerol decomposition (equation 2) and exothermic water gas shift reaction (equation 3): C3H8O3 ⇒ 3CO + 4H2 CO + H2O ⇔ CO2 + H2

∆H0 = 250 kJ/mol

(2)

∆H0 = -41 kJ/mol

(3)

Besides the main reactions, the side reactions including the methanation and the coke deposition may co-exist. The latter one is regarded as the main factor for catalytic deactivation together with the sintering of active metal sites

17-18

. According to the reaction pathway, the

catalyst should possess the functions for breaking the C-C, C-H and H-O bonds as well as the activity for water gas shift reaction to get the hydrogen production. The supported noble metal such as Pt 19, Rh 20, Ru 21-22 and Ir 23 were reported to catalyze this reaction as they showed good catalytic activities and simultaneously restricted the coke deposition. In spite of this, Ni as the transition metal is most commonly studied for both the dry reforming

24-26

and the steam

reforming processes 27-29 due to its high activity in breaking the above-mentioned bonds and its low cost. While for the Ni based catalysts, the improvement in coke resistance and the anti-sintering of Ni sites is always the main challenges in order to obtain a highly efficient catalyst

30-31

. Synthesis of Ni catalysts with controlled micro-structure such as the

core/yolk-shell type offered an efficient strategy

32-34

. Besides the synthesis of the specific

structure, the support effect supplied a more practical way. Various supports such as Al2O3


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based oxides or mixed metal oxides 35-38, SiO2 based porous materials 39-41, CeO2 and/or ZrO2 42-43

, hydroxyapatite

44

and La2O3

40

etc. were widely studied to improve the catalytic

performances for hydrogen production, or to enhance the catalytic stability by suppressing the coke deposition and/or controlling the particle size of active metallic nickel sites. Compared with the conventional supports, carbon nanotubes (CNTs) demonstrated considerable advantages in its intrinsic natures, such as the promotion of electron transfer, high mechanical strength, excellent thermal conductivity, anti-poisoning effect, and the controllable properties like the flexibility of modulating the specific surface area and internal diameter. Moreover, by using CNTs may possibly tune the interactions between metal and the support and modify the chemical composition and the functionalization of the surface. All of these features have been applied to catalysis

45-46

. Particularly the nanoparticles located in the cave are focused and

applied to catalyze various reactions because of the “confinement effect”

47-49

. To our best

knowledge, the dispersion of Ni nanoparticles on CNTs and application to glycerol steam reforming have not ever been reported. Therefore, the CNTs supported Ni catalysts were prepared and utilized for the steam reforming of glycerol. The obtained samples were characterized by various methods to investigate the porosity, the crystalline phases, the surface properties and the active sites dispersion, and to understand their influence on catalytic performances.

Experimental Catalyst preparation The support multi-wall carbon nanotubes were commercially obtained from Chengdu



Organic Chemical Co. (COCC). They were further functionalized by refluxing the CNTs (1 g) in concentrated nitric acid (50 mL, 10.7 M) at 140 oC for 12 h, then filtered and washed with deionized water until neutral, finally dried at 60 oC in a vacuum oven for 12 h. A conventional impregnation method was applied to disperse the active phases. Appropriate amount of Ni(NO3)2·3H2O was dissolved in deionized water, the support was afterwards added under vigorous stirring. The water was removed by a rotary evaporator under reduced pressure condition, the resulting solid was dried at 100 oC for 12 h and calcined at 450 o

C under nitrogen atmosphere for 4 h to get the supported catalysts which were denoted as

xNi/CNTs (where x equaled to 3, 5,10 and 15, representing the mass content of Ni in percentage like x wt.%)

Catalytic characterization The specific surface area, the pore volume and the average pore size were measured by N2 adsorption/desorption isotherms at 77 K on a Micromeritics ASAP 2460 Surface Area Analyzer. The Brunauer-Emmett-Teller (BET) method was applied to calculate the specific surface areas, whereas the Barrer-Joyner-Halenda (BJH) method was used to calculate the average pore sizes and the pore volume. The samples were degassed at 150 °C for 3 h under vacuum (50 Pa) prior to analysis. Crystalline phase analysis was performed on a PANalytical Empyrean diffractometer (CuKα, 40 kV, 35 mA, λ=1.5406 Å). The catalysts were scanned in the range of 10° < 2θ < 80° with steps of 0.026°/s and a acquisition time of 17 s. The average crystallite size was estimated by Scherrer’s equation L = 0.89λ/β(θ)cosθ, where L, λ, θ and β(θ) represented the crystallite


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size, the wavelength of the radiation, the Bragg diffraction angle, and the full width at half maximum (FWHM) 50, respectively. Both H2 temperature programmed reduction and H2 chemisorption measurements were carried out on Micromeritics Autochem II 2920 system equipped with a thermal conductivity detector (TCD). In a typical H2-TPR test, 25 mg sample was firstly pretreated at 200 oC for 3 h and then cooled down to 100 oC in N2 flow (30 mL/min). The nitrogen was afterwards replaced by the reducible gas mixture 10 mol.% H2 in Ar (30 mL/min) in which the sample was heated to 800 oC with a ramp of 5 oC/min. The properties of the active phases such as the nickel dispersion (DNi) and nickel surface area (SNi) were measured by H2-chemisorption. They were calculated using equation 4 and equation 5, respectively, by assuming that one surface nickel atom was occupied by one hydrogen atom (stoichiometry factor, SF=1) and that cross sectional area of one nickel atom (σm) is 6.49×10-20 m2/Ni-atom. Moreover, VH2, Mw, NA and x represented the hydrogen uptake determined by TPD, the molecular weight of nickel atom, the Avogadro constant and the mass content of nickel, respectively

51

. A typical measurement was proceeded as follow: 50 mg

sample was loaded in the quartz reactor and reduced in diluted hydrogen (H2/N2, 1:1 in vol.) at 450 oC for 3 h, then cooled down to the adsorption temperature 50 oC in N2 flow. The sample was saturated in pure H2 atmosphere for 0.5 h, afterwards purged in N2 to remove the weakly adsorbed hydrogen, finally heated until 800 oC with a heating rate of 10 oC/min.

DNi ,% =

2VH2 × Mw × SF

x / 100

×100

(4)



S Ni , m2 / g Ni =

2VH2 × N A × SF × σ m

x / 100

(5)

JEOL JEM-2100F Transmission Electron Microscope was operated at 200 kV to study the morphology of the catalysts. Surface chemical state of catalyst was checked by X-ray photoelectron spectroscopy (XPS) technique on a Thermo Scientific ESCALAB 250Xi spectrometer using Al-Kα radiation (1486.6 eV). The C1s peak of adventitious carbon was fixed to 284.5 eV as a reference.

Catalytic evaluation The catalytic reaction for hydrogen production through steam reforming of glycerol was performed in a conventional stainless steel fixed-bed reactor ( i.d.: 10 mm, length: 200 mm) under atmospheric pressure. In a typical test, 200 mg catalyst diluted with 1.8 g quartz sand (ϕ = 0.30 mm) were loaded into isothermal zone of reactor. After reduction at 450 oC in H2/N2 mixture (1:1 in vol., 60 mL/min) for 2 h, the catalyst was decreased to the reaction temperature. The liquid reactant (glycerol, 10 wt.% in water) was introduced into a 250 °C vaporization chamber with a flow rate of 3.0 mL/h (WHSV= 15 h-1) , and then carried into the reactor under N2 flow (60 mL/min). The gaseous products like H2, CH4, CO and CO2 etc. were analyzed online by a GC-TCD equipped with two packed columns (Porapak N: 4 mm o.d., 2 m length, Molecular sieves 13X: 4 mm o.d., 2 m length). The liquid products were analyzed using an offline GC-FID equipped with a capillary column (KB InnoWAX: 0.35 mm i.d., 30 m length).


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Results and discussion Porous structure of the prepared catalysts The N2 adsorption/desorption isotherms for the different catalysts were gathered in Fig. 1a, all the samples showed the similar adsorption/desorption behaviors, exhibiting the type IV isotherms according to IUPAC with H1 type hysteresis loop 52, which was the typical feature originating from the mesoporous materials. The amount of nitrogen adsorbed did not change a lot until the relative pressure reached 0.8, indicating the absence of micropores

53

. The N2

adsorption volume increased sharply afterwards due to capillary condensation. These results implied that the CNTs based catalysts contained a hybrid structure of the coexistence of mesopores and macropores. It could be further evidenced by the pore size distribution as shown in Fig. 1b which showed three pore size regions, corresponding to the mesopore region between 2 and 3 nm, the mesopore region between 3 and 7 nm and the hybrid pore structure between 7 and 70 nm. The pore size distribution centered at about 3.8 nm was the majority for the all four catalysts, whereas the average pore diameter of the samples varied from10.9 to 17.3 nm, which was strongly dependent on the quantity of loaded nickel. Such a great gap between the two values was probably caused by the presence of large size mesopores (7-50 nm) and the macropores (50-70 nm). The textural properties including the specific surface area, the pore volume and the average pore diameter were collected in Table 1. 3Ni/CNTs showed a surface area of 239.5 m2/g which decreased to 214.4 and 194.9 m2/g when the Ni content increased to 5 wt.% for 5Ni/CNTs and 10 wt.% for 10Ni/CNTs, respectively. The surface area kept almost unchanged when the Ni content rose to 15 wt.% for the sample 15Ni/CNTs. Combining the results with the



variation trends of the pore volume and the average diameter that gradually decreased with the increased Ni loading, it suggested that the nickel oxides may hierarchically and uniformly dispersed on the wall of CNTs. (a )

a b c d

(b)

3

dV/dD pore volume (m /g ⋅nm)

Vol. adsorbed (cm3 /g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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a

b c d

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P 0)

1

10

100

Pore diameter (nm)

Fig. 1 N2 adsorption/desorption isotherms (a) and the pore size distribution (b) (Sample information: 3Ni/CNTs, b: 5Ni/CNTs, c: 10Ni/CNTs, d: 15Ni/CNTs)

Table 1 Textural properties of the calcined samples Sample

SBET, m2/g

Pore volume, cm3/g

Average pore diameter, nm

3Ni/CNTs

239.5

0.8519

17.3

5Ni/CNTs

214.4

0.7038

15.1

10Ni/CNTs

194.9

0.6515

13.1

15Ni/CNTs

197.6

0.5021

10.9

The crystalline phase analysis for the calcined and spent catalysts The crystalline phases for the support CNTs and the Ni-loaded catalysts were measured by XRD technique and the patterns were depicted in Fig. 2. Two evident diffraction peaks at 25.9o


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and 42.6o could be found for CNTs. When the NiO was loaded on the support, several new diffraction peaks ascribed to the NiO could be easily found at 36.9 o, 43.6 o and 62.9o, and the intensity of those increased as the Ni content changed from 3 wt.% to 15 wt.%. While for the reduced catalysts (Fig. 3), the diffraction peaks from NiO disappeared and the ones from metallic Ni appeared instead, locating at 2θ angles of 44.6o, 51.9o and 76.5o, which was just caused by the pre-reduction in hydrogen flow prior to the catalytic reaction. The crystalline phases did not change any more for the spent catalysts according to the diffraction results in Fig. 4, only the variation in intensity of diffraction peak from metallic Ni could be observed. The diffraction peaks assigned to the support CNTs were still obvious, even the diffraction peak at 42.6o which was attached to the diffraction peak of Ni at 44.6 o became isolated as the nickel content increased.

Fig. 2 XRD patterns for the support and the calcined xNi/CNTs catalysts (a: CNTs, b: 3Ni/CNTs, c: 5Ni/CNTs, d: 10Ni/CNTs, e: 15Ni/CNTs)



Fig. 3 XRD patterns for the reduced xNi/CNTs catalysts (a: 3Ni/CNTs, b: 5Ni/CNTs, c: 10Ni/CNTs, d: 15Ni/CNTs)

Fig. 4 XRD patterns for the spent xNi/CNTs catalysts (a: 3Ni/CNTs, b: 5Ni/CNTs, c: 10Ni/CNTs, d: 15Ni/CNTs)


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The average crystallite size estimated by Scherrer’s equation were reported in Table 2, the results showed that the NiO nanoparticles had the closed value of about 12 nm for the four samples except 5Ni/CNTs before the catalytic reaction. The particle size of metallic Ni exhibited a smaller value after reduction for 3Ni/CNTs compared with that of the corresponding fresh one. While for the rest, evident increase was observed, suggesting that the aggregation of the reduced metallic Ni could take place even during the reduction process. The sintering of active sites became graver so that bigger particles were formed under the reaction conditions, which could be clearly found from the changes in crystallite size of metallic Ni (Table 2). The sample 5Ni/CNTs showed the minimum loss of crystallite size (5.0%), and the sample 15Ni/CNTs had the maximum value of that.

Table 2 Variation in crystallite size for the calcined, reduced and spent catalysts Catalyst

Estimated Crystallite size a, nm

Delta (∆) b, %

Calcined (LNiO)

Reduced (LNi-R)

Spent (LNi-S)

3Ni/CNTs

12.6

11.5

12.2

6.1

5Ni/CNTs

10.4

12.1

12.7

5.0

10Ni/CNTs

12.1

14.2

15.5

9.2

15Ni/CNTs

12.6

14.5

15.9

9.7

a b

Calculated by Scherrer’s equation based on NiO[111] plane and Ni[111] plane.

∆ = (LNi-S-LNi-R)×100/LNi-R

Morphology TEM analysis on the selected 5Ni/CNTs and 10Ni/CNTs (Fig. 5) showed that the active phases were located on both internal and external walls of carbon nanotubes. 5Ni/CNTs (Fig. 5a) exhibited the particle size distribution from 1 to 8 nm, presenting an average value of 4.76 nm.



While the sample 10Ni/CNTs (Fig. 5b) had a wider distribution between 2 to 18 nm, presenting an average value of 5.33 nm. Furthermore, the particles agglomerated to bigger ones during the reaction according to the TEM analysis for the spent 5Ni/CNTs (Fig. 5c) and 10Ni/CNTs (Fig. 5d), increasing by 50.6% to 7.17 nm and by 66.8% to 8.89 nm, respectively. The result that 5Ni/CNTs performed better in anti-sintering than 10Ni/CNTs was consistence with that obtained by XRD, although the precise values were not exact the same due to the two different analysis techniques.

Fig. 5 TEM images of the selected catalysts and the particle size distribution (a: 5Ni/CNTs and its particle size distribution, b: 10Ni/CNTs and its particle size distribution, c: Spent 5Ni/CNTs, d: Spent 10Ni/CNTs)

Reducibility The reducibility of the prepared catalysts were examined by H2-TPR measurements, and


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the collected hydrogen consumption profiles were shown in Fig. 6. All the catalysts exhibited very wide hydrogen consumption peaks in the temperature range from 160 to 750 oC, involving main two reduction processes of Ni2+ cations which were differed upon the interaction between the active phases and support. The weak interaction was due to the existence of free NiO, corresponding to the reduction process occurred below 450 oC, whereas strong interaction was probably due to the formation of Ni(OH)2 on the surface functionalized CNTs. As a result, the corresponding reduction came to the high temperature region centered at around 550 oC.

Fig. 6 TPR profiles for the Ni/CNTs catalysts with different nickel content (a: 3Ni/CNTs, b: 5Ni/CNTs, c: 10Ni/CNTs, d: 15Ni/CNTs)



The quantification of hydrogen consumption was reported in Table 3, the overall hydrogen consumption increased from 2.41 mmol/gcat. for 3Ni/CNTs to 2.76 mmol/gcat. for 15Ni/CNTs, showing a linear relationship between the hydrogen consumption and the increased Ni content. (Fig. 7). The similar trend could be also observed for the hydrogen consumption from the reduction of free NiO that took place at below 450 oC. According to the quantified results in Table 1, the sample 3Ni/CNTs had the minimum hydrogen consumption (0.09 mmol/gcat.) in this range, therefore, no distinguishable reduction peak was found from the TPR profile. As the Ni content increased in the sample, the reduction peaks became visible, even the isolated peak and shoulder peak attached to the main reduction process could be observed as for 15Ni/CNTs. One can state from these relations that the hydrogen consumption was in proportion to the quantity of loaded Ni. Furthermore, the free NiO was much easier to form over the catalysts with higher Ni content, the reducibility of the catalyst was consequently improved just because the reduction of free NiO to metallic nickel required lower temperature. Such a truth was also helpful for the improvement of reduction degree that increased sharply from 17.6% for the sample 3Ni/CNTs to 81.8% for the sample 15Ni/CNTs. In additional, the Ni surface density was defined as the ratio between the amount of reduced Ni under operation conditions and the specific surface area of the corresponding catalyst. The values still respected well the linear relationship as a function of Ni content (see Fig. 7), it gave a maximum one of 10.58 µmol/m2cat. for the highest Ni content catalyst 15Ni/CNTs, and gave a minimum one of 0.37 µmol/m2cat. for the lowest Ni content catalyst 3Ni/CNTs.


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Table 3 Quantification results of TPR results TPR

Reduction features under operation conditions a

H2 consumption,

H2 consumption,

Ni reduction degree b,

Ni surface density c,

mmol/gcat.

mmol/gcat.

%

µmol/m2cat.

3Ni/CNTs

2.41

0.09

17.6

0.37

5Ni/CNTs

2.48

0.45

52.8

2.10

10Ni/CNTs

2.69

1.28

75.1

6.57

15Ni/CNTs

2.76

2.09

81.8

10.58

Catalyst

a

The values were calculated from the hydrogen consumption below 450 oC by deconvolution treatment.

b

Ni reduction degree, % = Amount of reduced Ni below 450 oC × 100/Amount of Ni loading

c

Calculation based on the operation conditions (reduction temperature: 450 oC)

Fig. 7 Hydrogen consumption and Ni surface density as a function of Ni content (a: 3Ni/CNTs, b: 5Ni/CNTs, c: 10Ni/CNTs, d: 15Ni/CNTs)

The property of the active phase Ni determined by H2-chemisorption The metallic Ni is the active site for the glycerol steam reforming to produce hydrogen, the properties of metallic Ni such as the dispersion and surface area played important roles in



Page 18 of 33

determining the catalytic activity and the products distribution. The H2-chemisorption results listed in Table 4 showed that the hydrogen uptake changed from 78.3 µmol/g for 3Ni/CNTs to 107.2 µmol/g for 15Ni/CNTs, which was good agreement with the abovementioned TPR results. While the Ni dispersion and surface area exhibited the inverse relationship to the Ni content, as shown in Fig. 8. The Ni dispersion decreased significantly by 37.1% from 30.6% to19.2%, and the Ni surface area lost sharply by 37.3% from 102.0 m2/gNi to 64.0 m2/gNi as the Ni content varied from 3 wt.% to 5 wt.%. When Ni content continued increasing from 10 wt.% to 15 wt.%, such a trend became much more gentler, decreased by 20.8% and 20.6%, respectively. Based on these results, one can speculate that the Ni dispersion and surface area were likely limited to a value. That was saying that the particle size existed a limited value, which was directly related to the well known “confinement effect” of CNTs 54. On the other hand, the Ni dispersion and surface area were profoundly influenced by the particle size, the highest values of them on 3Ni/CNTs with the lowest Ni content probably indicated that the smaller Ni nanoparticles dispersed on support. Furthermore, the 3Ni/CNTs had the smallest reduction degree, which meant that the smaller nanoparticles were more difficult to reduce 55.

Table 4 The hydrogen chemisorption results Catalyst

H2-chemisorption H2 uptake VH2, µmol/g

Ni dispersion DNi, %

Ni surface area SNi, m2/gNi

3Ni/CNTs

78.3

30.6

102.0

5Ni/CNTs

81.9

19.2

64.0

10Ni/CNTs

90.0

10.6

35.2

15Ni/CNTs

107.2

8.4

27.9


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Fig. 8 Influences of Ni content on Ni dispersion and surface area

Surface analysis by XPS The chemical state of surface elements was analyzed on the 5Ni/CNTs and 10Ni/CNTs samples through XPS technique. The Ni2p3/2 spectra zones for the calcined and spent catalysts were collected in Fig. 9, showing the binding energy (BE) from ca. 870 to ca. 845 eV. The BE values of different type of Ni species and their quantification results were reported in Table 5. For the calcined catalysts (Fig. 9a and Fig. 9b), the split peaks between ca. 858.5 and ca. 850.2 eV were assigned to Ni2+, and the peak between ca. 870.0 and ca. 858.5 eV (centered at ca. 861.0 eV) appeared at about 5.7 eV higher BE side, which was considered as a satellite peak of the aforementioned one

56-58

. The split peaks comprised two overlapped ones by

deconvolution treatment, corresponding to two different chemical state of Ni2+. The peak at ca. 853.7 eV was ascribed to the free NiO [Ni2+(I)], whereas the one at ca. 855.5 eV was assigned



Page 20 of 33

to the Ni(OH)2 [Ni2+(II)] formed through the strong interaction between the NiO phase and the support CNTs 59-62. Such a surface composition could be further confirmed by the O1s spectra as shown in Fig. 10 where the peak at ca. 529.3 eV was assigned to the origination from the metal oxides NiO, and the one at ca. 531.5 eV attributed to the hydroxyl component

63-64

.

According to the quantification calculation, the proportion of free NiO [Ni2+(I)] in 10Ni/CNTs was a little bit higher than that in 5Ni/CNTs (34.7% vs 32.5%, respectively). The two types of nickel species also confirmed the two reduction processes proposed by H2-TPR characterization. The spectra for the spent catalysts (Fig. 9c and Fig. 9d) involved a more complex chemical state of nickel species. A new peak at ca. 852.7 eV was assigned to the metallic Ni0 [Ni0(III)], together with the peaks at ca. 854.2 eV [Ni2+(I)] and ca. 853.3 eV [Ni2+(II)], once can easily concluded that the catalysts were only partially reduced after hydrogen pretreatment. Furthermore, it should be noted that the binding energy of Ni2+(I) shifted to the higher side by 0.1 for 5Ni/CNTs and by 0.8 for 10Ni/CNTs after reaction through comparing the calcined and spent samples. On the contrary, the binding energy of Ni2+(II) for the both catalysts declined 0.2~0.3 eV, implying the weakened interaction. In addition, an ill-defined peak at ca. 851.7 [Nix+(IV)] could be found for the both spent samples, which might come from some kind of transition state of Ni species.


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Fig. 9 Ni2p3/2 spectra for the calcined and spent 5Ni/CNTs and 10Ni/CNTs catalysts (a: 5Ni/CNTs calcined, b: 10Ni/CNTs calcined, c: 5Ni/CNTs spent, d: 10Ni/CNTs spent)

Fig. 10 O1s spectra for the calcined (a) 5Ni/CNTs and (b) 10Ni/CNTs catalysts



Page 22 of 33

As far as the atomic ratio was concerned, both Ni2+(I) and Ni2+(II) still coexisted on the surface but the proportion of them decreased quite a lot after reaction, the Ni0(III) increased considerably instead due to the partial reduction of Ni2+ species. When comparing the surface composition of the two spent catalysts, it could be found that the 10Ni/CNTs sample had higher content of Ni0(III) (38.7% vs 35.0% for 5Ni/CNTs) and lower content of Ni2+(I) (7.9% vs 13.2% for 5Ni/CNTs). These results suggested that more free NiO in 10Ni/CNTs with higher Ni content were reduced and thus leaded to the increase of surface Ni0, which was good consistent with the TPR quantification results.

Table 5 Quantification for the XPS results Catalyst

Binding energy, eV

Atomic ratio, %

Ni2+(I) a

Ni2+(II) b

Ni0(III) c

Nix+(IV) d

I/II/III/IV

5Ni/CNTs

853.7

855.5

-

-

32.5/67.5

10Ni/CNTs

853.8

855.6

-

-

34.7/65.3

5Ni/CNTs-spent

853.8

855.2

852.6

851.6

13.2/45.8/35.0/5.9

10Ni/CNTs-spent

854.6

855.4

852.8

851.8

7.9/45.7/38.7/7.7

a Bulk NiO b NiO interacted with CNTs support c Metallic Ni d Ill-defined nickel oxides

Catalytic evaluation on glycerol steam reforming reaction The catalytic steam reforming of glycerol for the hydrogen production was carried out at reaction temperature of 375 oC after the pretreatment in H2/N2 mixture (1:1 in vol.), the catalytic activity and the products distribution were gathered in Table 6. The evident differences


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could be observed for the conversion to gas and the product distribution as the Ni content varied. Over the catalyst with higher Ni loading, more gaseous products could be generated, which was strongly determined by the Ni surface density, exhibiting linear relationship as a function of that (see Fig. 11). The similar trend could be also obtained for hydrogen selectivity. Recalling the characterization results on textural properties, crystalline phase, active phase properties by surface analysis, reducibility and chemisorption, high content of Ni had the catalyst lose the surface area, but it gave more amount of Ni species interacting weakly with the CNTs support. These Ni species were easily to reduce under the reaction conditions, more amount of metallic Ni was consequently formed on catalyst surface, leading to higher reduction degree. The higher Ni surface density was therefore achieved. Correlating these information with catalytic performances, one can reasonably state that the surface area and the dispersion of Ni active sites affected synergistically the hydrogen selectivity. On the other hand, the selectivity of CO fell off overall, while the selectivity of CO2 climbed up from 13.8% to 56.1% when the Ni content as well as the Ni surface density increased. Such a product distribution further confirmed the important role of Ni surface content, the high value of which not only helped the C-C and C-H bonds cleavage (equation 2), favoring to generate the gas rich products, but also promoted the water gas shift reaction (equation 3), resulting in the formed CO consecutively react with water and finally enhance the hydrogen productivity. Methane could be found in the products because of the methanation of CO and/or CO2, the selectivity of it increased in a very narrow range implied that the methanation could be effectively suppressed on Ni/CNTs catalysts. More importantly, the competitive catalytic performances were achieved over the CNTs supported nickel catalysts at reaction temperature of 375 oC which was rather low as the operation



Page 24 of 33

temperature for this reaction was usually higher than 500 oC 8, 65-68.

Table 6 Catalytic activity and gaseous products distribution at reaction temperature of 375 oC Catalyst

Conversion to gas (xg) a, %

Selectivity, % H2

CO

CO2

CH4

3Ni/CNTs

55.9

38.2

39.3

13.8

2.8

5Ni/CNTs

59.2

40.3

40.5

15.6

3.0

10Ni/CNTs

65.8

54.9

27.7

34.2

3.8

15Ni/CNTs

86.4

72.9

24.8

56.1

5.6

Reaction conditions: mcatal.=200 mg, atmospheric pressure, temperature=375 oC, feed flowrate=3.0 mL/h a

in in , Where FGly is the molar flowrate of glycerol at the inlet, αi are the numbers of C x g = ∑ α i Fi / 3 FGly

atoms in product i molecule and Fi is the molar flowrate of gaseous product i originated from glycerol.

Fig. 11 Effect of Ni surface density on catalytic performances


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The influence of reaction temperature on catalytic performances was further investigated on the sample 10Ni/CNTs with a medium glycerol conversion, and the results were reported in Table 7. It was obvious that reaction temperature showed significant effect on the products distribution, the conversion of glycerol increased from 63.3% at 350 oC to 80.7% at 450 oC, the selectivties of hydrogen and CO2 also exhibited the similar trend. However, the selectivity of CO gave the contrary trend, dropping sharply by around 90% from 41.1% to 4.4% as the reaction temperature varied from 350 oC to 450 oC. Such catalytic results allowed us to speculate that the higher reaction temperature in the studied range was helpful for the crack reaction of glycerol, and promoted the subsequent water gas shift reaction to produce hydrogen. The methanation reaction was simultaneously suppressed even prohibited when the temperature was higher than 375 oC.

Table 7 The influence of reaction temperature on catalytic performances Reaction temperature, oC

Conversion to gas (xg), %

Selectivity, % H2

CO

CO2

CH4

450

80.7

83.6

4.4

76.3

-

425

78.2

77.3

11.5

66.6

-

400

74.3

74.5

12.6

61.7

-

375

65.8

54.9

27.7

34.2

3.9

350

63.3

47. 0

41.1

17.9

4.4

Reaction conditions: mcatal.=200 mg, atmospheric pressure , feed flowrate=3.0 mL/h

Conclusions Carbon nanotubes supported nickel catalysts were prepared by impregnation method, and



the content of Ni has profound influences on the solid properties including textural properties, reducibility, surface chemical state and active phase dispersion. NiO species was favored to form on the surface of a catalyst with a high Ni content as shown from XRD, and NiO weakly interacted with the CNTs support and therefore can be easily reduced under the reaction conditions according the TPR and XPS results. The maximum values of Ni dispersion and Ni surface area calculated from hydrogen chemisorption were obtained on 3Ni/CNTs with a minimum content of Ni, over which the lowest reduction degree could be found. This could be probably caused by the formation of smaller particle size. The NiO species was partially reduced, and the amount of which was dependent on the content of active phases. The Ni surface density played an important role in determining the reactivity and the product distribution as a linear relationship can be found between it and the conversion of glycerol. In other words, both the surface area and the active metallic Ni dispersion co-affected the catalytic performances, the well-dispersed metallic Ni was favored to promote the consecutive glycerol decomposition and the reverse water gas shift reactions for hydrogen production. Higher reaction temperature was beneficial to a high selectivity of the reaction towards the hydrogen production in our tested range. Furthermore, the sample 15Ni/CNTs exhibited a strong ability to achieve the gaseous products (86.4%) and high selectivity of hydrogen (72.9%) when the reaction was conducted at the temperature as low as 375 oC.

Acknowledgments The authors thank the financial supports from National Natural Science Foundation of China (NSFC, No. 21603153), Science and Technology Department of Sichuan Province (No.


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2016HH0026) and the Fundamental Research Funds for the central Universities (No. YJ201544). F. Jing also acknowledged the support from “The 1000 talent plan” of Sichuan Province.

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Abstract graphic

The renewable hydrogen product was generated by steam reforming of bio-sourced glycerol over Ni/CNTs catalyst.


Carbon nanotubes supported nickel as the highly efficient catalyst for

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