Enhanced Hydrogen Production from Sawdust Decomposition Using

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Enhanced hydrogen production from sawdust decomposition using hybrid-functional Ni-CaO-CaSiO materials 2

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Guozhao Ji, Xiaoyin Xu, Hang Yang, Xiao Zhao, Xu He, and Ming Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03481 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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Environmental Science & Technology

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Enhanced hydrogen production from sawdust decomposition using hybrid-functional Ni-CaO-Ca2SiO4 materials

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Guozhao Ji †,#, Xiaoyin Xu†,#, Hang Yang†, Xiao Zhao†, Xu He†, Ming Zhao*,†,‡

1 2

5

†

School of Environment, Tsinghua University, Beijing 100084, China

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‡

Key Laboratory for Solid Waste Management and Environment Safety, Ministry of

7

Education, Beijing, 100084, China

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Table of Content:

9

10

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KEYWORDS. Biomass waste, hydrogen production, sawdust, thermal decomposition,

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catalyst-sorbent hybrid

13

ABSTRACT. A hybrid-functional material consisting of Ni as catalyst, CaO as CO2

1

ACS Paragon Plus Environment


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sorbent, and Ca2SiO4 as polymorphic ‘active’ spacer was synthesized by freeze drying

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a mixed solution containing Ni, Ca and Si precursors, respectively, to be deployed

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during sawdust decomposition that generated gases mainly containing H2, CO, CO2

17

and CH4. The catalytic activity showed a positive correlation to the Ni loading, but at

18

the expense of lower porosity and surface area with Ni loading beyond 20 wt.%,

19

indicating an optimal Ni loading of 20 wt.% for Ni-CaO-Ca2SiO4 hybrid-functional

20

materials, which enables ~ 626 ml H2 (room temperature, 1 atm) produced from each

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gram of sawdust, with H2 purity in the product gas up to 68 vol.%. This performance

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was superior over a conventional supported catalyst Ni-Ca2SiO4 that produced 443 ml

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H2 g-sawdust-1 under the same operating condition with a purity of ~ 61 vol.%.

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Although the Ni-CaO bi-functional material in its fresh form generated a bit more H2

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(~ 689 ml H2 g-sawdust-1), its cyclic performance decayed dramatically, resulting in

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H2 yield reduced by 62% and purity dropped from 73 to 49 vol.% after 15 cycles. The

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‘active’ Ca2SiO4 spacer

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Ni-CaO-Ca2SiO4 hybrid-functional material, corresponding to its minor loss in

29

reactivity over cycles (H2 yield reduced by only 7% and H2 purity dropped from 68 to

30

64 vol.% after 15 cycles).

offers

porosity

and

mechanical

strength

to

the

31

32

1. Introduction

33

The declining fossil fuel reserves and the raising concern about greenhouse gas 2


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emission are the major challenges of the world energy and environment. Seeking for

35

alternative clean energies to meet the energy demand and environmental requirement

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are of great interest for sustainable development. Among all the clean energy carriers,

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hydrogen (H2) is one of the promising options due to the high calorific value and

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pollutant-free emission.1 H2 could be generated via thermo-chemical processes from a

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variety of sources including natural gas,2-4 coal,5, 6 chemical waste7 and biomasses.8-11

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Unlike fossil fuels which have limited reserves and require high capital cost to exploit,

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biomass is renewable, as such H2 production from waste biomass cycling such as

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cellulose5, 10, 12-14 and sawdust15-17 via gasification has received growing attentions.

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Another merit is that biomass gasification with CCS is a carbon-negative process,

44

because the released CO2 during biomass gasification with in-situ CO2 capture is less

45

than the CO2 consumed during the process of growth.

46

The current challenges of biomass gasification include incomplete tar cracking,

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unreformed volatile fragments, and carbon deposition, etc. Nickel (Ni) has been

48

identified as one of the most efficient catalysts in cracking C-C, C-H, and O-H bonds as

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well as enhancing water gas shift reaction,18-20 thus demonstrating high reactivity in tar

50

cracking and light-hydrocarbon reforming. As such, Ni-based catalysts have been

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commonly used for steam reforming of methanol,21, 22 methane,23, 24 glycerol25, 26 and

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ethanol.27, 28

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Another challenge is that the application of H2 such as H2 fuel cells and float glass

54

production generally requires H2 purity up to 99%. However, there is still a gap

55

between the required H2 purity and the purity from biomass gasification. To enhance H2 3



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volumetric fraction from biomass, a feasible strategy is shifting the equilibrium of

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reversible reactions (steam reforming, water gas shift, etc.) towards the H2 side. Since

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carbon dioxide (CO2) is produced with H2 in water gas shift, capturing CO2 may shift

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the water gas shift and thus convert more carbon monoxide (CO) and H2O to H2.

60

Calcium oxide (CaO), owing to its high sorption capacity and low cost, has been widely

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used as CO2 sorbents,29-31 and also applied in sorption enhanced steam reforming to

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promote the H2 yield.32-34 Meanwhile, CaO was found to be able to promote tar

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cracking35-39 and catalyze char gasification,37, 40 both of which enhance the yield of

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gaseous product during biomass pyrolysis. In view of these characteristics, CaO has

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also been used as CO2 acceptor during biomass pyrolysis.9, 15, 41, 42 However, the poor

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cyclic stability of CaO due to high temperature sintering deteriorates the CO2 sorption

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performance.

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Ca2SiO4 has been well investigated for its reversible polymorphic transitions with

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swung temperatures.43 As per our recent research,44 Ca2SiO4 exhibited a strong ability

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to resist particle sintering because the temperature-induced polymorphic transition

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(α↔β) and the resulting volume expansion enabled Ca2SiO4 as an ‘active’ spacer to

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‘automatically’ regenerate the declining porosity. For the hybrid-functional materials,

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Ca2SiO4 hasn't been employed as stabilizer to prevent catalyst being sintered. To

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demonstrate the superior sintering resistance with Ca2SiO4 over an “inert” spacer, the

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most widely reported spacer Ca9Al6O1845-47 was chosen as the reference in this study.

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Furthermore, the drying method for the precursor also has an effect on the material

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structure. Freeze-drying, which sublimates the ice into vapor, avoids the pore-collapse 4


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caused by severe surface tension during direct liquid-gas transition, thus maintains the

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pore and surface areas, which facilitates the mass transfer, catalysis and reactions.44, 48

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The individual roles of Ni catalysts, CaO sorbent and Ca2SiO4 stabilizer have been

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intensively studied in open literatures. However, limited research has been conducted

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in applying a hybrid-functional material with Ni as catalyst, CaO as sorbent and

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Ca2SiO4 as spacer to biomass pyrolysis. This work aimed at synthesizing a

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hybrid-functional material Ni-CaO-Ca2SiO4 by freeze-drying method for enhancing the

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hydrogen production during biomass decomposition. Sawdust, as a typical biomass

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waste, was used as the model biomass. The proximate and ultimate analysis of sawdust

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is given in Table S1.

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2. Material and methods

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2.1 Sample preparation

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5 mmol tetraethyl orthosilicate (TEOS, Si(OC2H5)4) was dissolved into 1 mmol L-1

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nitric acid (HNO3) and stirred at 600 r min-1 at room temperature for 40 min. 45 mmol

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calcium acetate (Ca(C2H3O2)2, MW=176.18) and nickel (II) nitrate hexahydrate

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(Ni(NO3)2·6H2O, MW=290.79) was then added to TEOS solution after TEOS was

95

completely hydrolyzed. The homogenous precursor was frozen at -80 ºC prior to the

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24-h drying under vacuum. In order to examine the effect of Ni loading, the same

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procedure was repeated with varied Ni content (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, 5



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25 wt.% and 30 wt.%) by keeping constant CaO:Ca2SiO4 ratio (3:7). These samples

99

were denoted as Ni05, Ni10, Ni15, Ni20, Ni25 and Ni30, respectively.

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For comparison purpose, three types of materials Ni-Ca2SiO4, Ni-CaO, and

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CaO-Ca2SiO4 were also synthesized in a similar manner with the different recipe ratio.

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The recipe details of each hybrid material were listed in Table S1. All of the

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freeze-dried precursors were calcined at 850 ºC (ramping rate 5 ºC min-1) in air for 1h.

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Reduction for activating Ni-catalysts was implemented under 5 vol. % H2 in nitrogen

105

(N2) for 1 hour at 750 ºC (ramping rate 5 ºC min-1).

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Since lignocellulose accounts for over 90% of plant biomass.49 Sawdust, which

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has rich lignocellulose content, was selected as biomass feedstock in this work. The

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sieved particles (< 250 µm) were dried in oven at 60 ºC prior to the pyrolysis test.

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2.2 Characterization

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The crystalline structure of the hybrid materials was assessed using wide-angle X-ray

111

diffraction (XRD, D/Max 2500V+/PC, Cu Kα, 2θ=10~80°, step interval= 0.02°).

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Scanning electron microscopy (SEM, Zeiss MERLIN VP Compact) was used to gain

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insight of morphology. N2 physisorption using a Quantachrome autosorb iQ-C

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instrument was carried out to detect the surface area, pore volume and mean pore size

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of the materials.

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2.3 H2 production test in TG-MS

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The as-synthesized hybrid materials were mixed with sawdust with a constant mass 6


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ratio 2:1, and the mixed complex was decomposed at a ramping rate of 40 ºC min-1 to

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850 ºC under the purging argon (Ar) at 500 ml min-1. A modified thermogravimetric

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analyzer (SDT Q600) coupled with a mass spectrometer (Hiden HPR20) (TG-MS)

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recorded the mass variation and gases yield during the pyrolysis, allowing the catalyst

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activity and selectivity to be assessed. A semi-quantitative method was adopted to

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interpret the online MS data. (Details and limitation discussion can be referred to from

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our previous published works13, 14, 20, 50) The key m/z numbers used to represent the

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major gas species were given in Table S2. The volumetric gas production, ml

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g-sawdust-1, should be evaluated for normal conditions (room temperature, 1 atm).

127

128

3. Results and discussion

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3.1 Material characterization

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The N2 adsorption isotherms were plotted in Figure S1 for the hybrid-functional

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materials with varied Ni loadings. Samples with Ni loading less than 20 wt.%

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presented Type II isotherms which indicated a wide range of pore sizes of the

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materials. The hysteresis implied the presence of meso-pores in these materials.51 The

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sample Ni30, which contains 30 wt.% Ni, displayed type III isotherm with indistinct

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hysteresis, suggesting the macro-porous structure of Ni30. Table 1 listed the textural

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properties from N2 adsorption measurements. The BET surface area was maintained

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similar to the value (~50 m2 g-1) of CaO-Ca2SiO4 when the Ni loading was increased 7



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from 5 to 20 wt.%. Meanwhile, the pore volume and average pore size were close to

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0.4 ml g-1 and 3.72 nm with just minor deviations. However, for the samples with Ni

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loadings exceeding 20 wt.%, dramatic changes of the BET surface area and pore

141

volume were present. The surface area of Ni25 dropped to 28.83 m2 g-1, being only

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half of the surface areas of lower Ni loading samples. The pore volume was deceased

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significantly by 40%. With 30 wt.% Ni loading, Ni30 provided the lowest surface area,

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and the average pore size (62.88 nm) indicated that Ni30 became macro-porous

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material.

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Table 1. Surface area, pore size and pore volume of materials with varied Ni loading Material

BET surface area (m² g‾¹)

Pore volume (ml g‾¹)

Pore size (nm)

CaO-Ca2SiO4

49.915

0.454

3.719

Ni05

52.901

0.434

3.717

Ni10

45.107

0.351

3.723

Ni15

45.332

0.408

3.719

Ni20

56.948

0.385

3.72

Ni25

26.832

0.241

3.939

Ni30

16.323

0.224

62.882

147 148

The XRD patterns of materials with varied Ni loadings presented the peaks of

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NiO, CaO and Ca2SiO4, demonstrating the combinations of these three components

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(Figure S2). As expected, the peaks corresponding to NiO became increasingly 8


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evident with Ni loading increment. A possible impurity in the materials could be SiO2

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which may resulted from incomplete reaction between TEOS and Ca(C2H3O2)2, but

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SiO2, in the tested range of this work, cannot be detected from these XRD patterns

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owing to its amorphous property.52

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156 157

Figure 1. XRD patterns of all functional materials

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Figure 1 also showed the XRD patterns of synthesized Ca2SiO4 and all the

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functional materials (CaO-Ca2SiO4, Ni-CaO and Ni-Ca2SiO4). The hybrid-functional

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material was also displayed in Figure 1 at the bottom for comparison. Ni-CaO did not

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showed any unexpected peaks, suggesting it was produced without the presence of

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any impurities, at least at the XRD detection level. Ca2SiO4 was synthesized by 9



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keeping the stoichiometry ratio of TEOS:Ca(C2H3O2)2=1:2 (Table S1), but the tiny

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peaks of CaO in the Ca2SiO4 XRD patterns implied that the TEOS and Ca(C2H3O2)2

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might not reach complete reaction. In view of this incomplete reaction, Ni-Ca2SiO4

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may contain a small quantity of CaO, and the ratio of CaO:Ca2SiO4 in CaO-Ca2SiO4

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and Ni-CaO-Ca2SiO4 could be slightly higher than 7:3.

168 169

Figure 2. SEM images of hybrid-functional materials with varied Ni loading. (a) 5

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wt.% Ni loading, (b) 10 wt.% Ni loading, (c) 15 wt.% Ni loading, (d) 20 wt.% Ni

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loading, (e) 25 wt.% Ni loading, and (f) 30 wt.% Ni loading. 10


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Figure 2 displayed the SEM images of hybrid-functional materials with varied Ni

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loadings. The hybrid-functional material with 5 wt.% Ni loading was well dispersed

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and showed the most uniform particle sizes (~ 50 nm) among all the hybrid-functional

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materials. With increased Ni loading up to 20 wt.%, some grown particles were

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observed, but particles were still dispersed with distinct gaps in between to form the

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meso-porous structure. However, when Ni loading exceeded 25 wt.%, particles started

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to agglomerate, and meso-pores was vanishing attributed to the coalescence of

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particles, which was consistent with the findings in the N2 adsorption test (Table. 1

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and Figure S1).

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3.2 Effect of Ni loading on catalyst activity

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The sawdust were mixed with hybrid-functional materials with various Ni loading and

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decomposed in TG-MS. The evolved H2, CO and CO2 generation rates were plotted in

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Figure 3. The details of converting the MS signal to volumetric flow rate was

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provided in our previous work 14. The H2 generation started to be observable from 300

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ºC and peaked around 400 ºC regardless of the Ni loading. However, the comparison

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of peak intensities suggested that the hybrid-functional material with 20 wt.% Ni

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loading produced the most H2. Since the major catalytic component in the

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hybrid-functional material is Ni, with higher content of Ni, there would be more

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active sites in the material, and thus higher catalytic activity could be expected. This

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resulted in the H2 and CO production rates in the order of Ni05Ni10>Ni15 (Figure 3c). Since the total mass of material used in each test was

215

the same, lower Ni loadings in the material also means higher fractions of CaO in it,

216

and in turn more CO2 sorption in this material. The released CO2 at 650 ºC came from

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the CO2 captured before this temperature. Since Ni05 contained the most CaO,

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consequently it released the most CO2 finally.

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The integrated yields of the H2, CO, CO2 CH4, and H2O were summarized in

220

Figure 4. The sawdust mixed with Ni20 (20 wt.% Ni loading) was the most

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productive material in generating H2 and CO among all the hybrid-functional

222

materials. A general consideration of syngas quality for clean energy production is the 13



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H2 purity as well as the lower heating value (LHV). The LHV of the product gas is

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defined as:53, 54

LHV = (107.98 X H 2 + 126.36 X CO + 358.18 X CH4 )

225 226

where

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respectively. Figure 4 inset displayed the H2 fraction in the total gas and the LHV

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from per kilogram sawdust. The material with 20 wt.% Ni loading showed optimal H2

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purity and heating value over those with other Ni loadings. The H2 volumetric fraction

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could reach to 68% and the LHV is 9334 kJ kg-1. Hence Ni20 was selected to

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represent the hybrid-functional material Ni-CaO-Ca2SiO4 in further experiments.

X H2

,

X CO

and

(1)

X CH 4

are the molar percentage of H2, CO and CH4,

232

233 234 235

Figure 4. Influence of Ni loading on the cumulative yield of major gases. Flow rate was calibrated at 20 ºC and 1 atm. Inset: Influence of Ni loading on the H2 14


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fraction and lower heating value.

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3.3 Effect of sawdust:hybridfunctional-material ratio on the gas yield

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The effect of sawdust:hybrid-functional-material ratio on the pyrolysis product was

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also systematically investigated by varying the mass ratio of sawdust to Ni20

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(msaw:mmat) from 1:1 to 1:3. As demonstrated in Figure 5, increasing quantity of the

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hybrid-functional material from msaw:mmat=1:1 to msaw:mmat=1:2 significantly

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enhanced the H2 yield by 89%. However, further increasing the mass of

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hybrid-functional material to msaw:mmat=1:3 failed to result in higher H2 yield. On the

244

contrary, 32% reduction of H2 was observed. Therefore, 1:2 was the optimum mass

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ratio of sawdust to the hybrid-functional-material for the purpose of producing H2.

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This ratio value was consistent with the value in Zhao et al.’s work for cellulose

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decomposition using CaO, or Ni-Co.50

248 249

Figure 5. The cumulative gas yield under varied msaw:mmat ratio. Flow rate was

250

calibrated at 20 ºC and 1 atm. 15



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3.4 Activity comparison against other functional material

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The catalytic effect of hybrid-functional material (20 wt.% Ni loading) on the sawdust

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decomposition was further compared with cases using three functional materials

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(Ni-Ca2SiO4, Ni-CaO and CaO-Ca2SiO4). Figure 6a showed the H2 generation rates

255

from the hybrid functional materials. In addition, the case of pyrolyzing only sawdust

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without any catalyst was also plotted as basis. The two cases (No Catalyst,

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CaO-Ca2SiO4) in the absence of Ni merely produced little H2. The other three cases in

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the presence of Ni (Ni-CaO-Ca2SiO4, Ni-Ca2SiO4 and Ni-CaO) generated remarkable

259

peaks for H2, demonstrating high activity of Ni in O-H and C-H cleavage. The H2

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generation rate of the three Ni-containing materials followed the order of

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Ni-CaO>Ni-CaO-Ca2SiO4>Ni-Ca2SiO4, which is also the order of CaO mass fraction

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in each material. This order strongly suggested that H2 generation rate had a positive

263

correlation with CaO fraction. This is because the CO2 capture (Eq. (2)) shifted the

264

equilibrium of water gas shift reaction (Eq. (3)), and favored the H2 production

265

CaO + CO2 ⇌ CaCO3

(2)

266

CO + H2O ⇌ CO2 + H2

(3)

267

The CO generation rate with temperature by these hybrid functional materials

268

evolved very differently (Figure 6b). For the case with sawdust alone, there was only

269

one CO generation peak in identical temperature range as H2 peak. For other cases

270

with functional materials, secondary peaks were all observed at various temperatures.

271

The temperatures for the first peak in all cases were close to 390ºC. The 16


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Ni-CaO-Ca2SiO4 generated more CO than Ni-CaO in the first peak range (300-450

273

ºC). They had equal Ni fraction. The major difference was the CaO fraction between

274

them. Ni-CaO contained more CaO which enhanced the water gas shift further in the

275

forward direction by capturing CO2, thus more CO was converted (Eq. (3)). On the

276

other hand, the presence of Ca2SiO4 in Ni-CaO-Ca2SiO4 may provide better stability

277

of the Ni catalytic activity, which could also be the reason of higher CO generation

278

rate.

279 280

Figure 6. Influence of different functional materials on the generation of major gases.

281

(a) H2, (b) CO, and (c) CO2. Flow rate was calibrated at 20 ºC and 1 atm. 17



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Ni-CaO-Ca2SiO4 and Ni-Ca2SiO4 presented secondary CO peaks around 590 ºC

283

(Figure 6b). The secondary peaks of Ni-CaO and CaO-Ca2SiO4 appeared at relatively

284

higher temperature from 660 to 680 ºC. When Ni was together with Ca2SiO4, the

285

activity of Ni could be better maintained owing to the stability of Ca2SiO4. So the

286

secondary CO peak from Ni-CaO-Ca2SiO4 and Ni-Ca2SiO4 could probably come from

287

tar cracking. The tar cracking activity of Ni-CaO and CaO-Ca2SiO4 was not as high as

288

Ni-CaO-Ca2SiO4 and Ni-Ca2SiO4 due to the absence of stabilizer or catalyst. So the

289

CO generation peak of Ni-CaO and CaO-Ca2SiO4 may not come from tar cracking.

290

An interesting point is in Figure 6c only Ni-CaO and CaO-Ca2SiO4 showed CO2

291

secondary peaks. These CO2 generation came from CaCO3 calcination reaction (Eq.

292

(4))

293

294

CaCO3 ⇌ CaO + CO2

(4)

The released CO2 could further react with char by Boudouard reaction

295

CO2 + C ⇌ 2CO

296

Hence the CO generation of Ni-CaO and CaO-Ca2SiO4 appeared at 660 to 680 ºC

297

was a result of Eqs. (4) and (5). However many open literatures reported CO2

298

desorption at 800 ºC or above, 29, 31 so whether CO2 could be released from CaCO3 at

299

the temperature range of 660 to 680 ºC remains an issue. In order to address this point,

300

the fully carbonated CaO-Ca2SiO4 was heated for decarbonation under varied CO2

301

volume fractions, and the decarbonation temperature was derived from the most rapid

302

mass loss (Figure S3). The decarbonation temperature was 620 ºC when the CO2 18


(5)

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303

fraction is zero. The CO2 fraction during sawdust decomposition in the TG-MS

304

system was evaluated in the order of 10-5 which is close to 0, demonstrating the

305

possibility of releasing CO2 from CaCO3 between 660 and 680 ºC. The occurrence of

306

Reaction (5) was also investigated by calculating the driving force term in Langmuir

307

Hinshelwood Hougen Watson (LHHW) kinetics. The driving force in Figure S4

308

proved Reaction (5) could occur from 600 ºC and above.

309

310

311

Figure 7. The mass variation and mass loss rate of sawdust with different

312

functional materials. Solid lines are mass and dashed lines are mass loss rate.

313

Figure 7 plotted the TGA curves during the sawdust decomposition with and 19



314

without these functional materials. The case of only sawdust showed a single mass

315

loss near 390 ºC, which was in consistence with MS result in Figure 6. This

316

significant weight loss was observed in all cases in Figure 7, corresponding to the

317

volatile decomposition. The sawdust mixed with material containing CaO all showed

318

a remarkable additional weight loss around 660 ºC. This implied that reactions of

319

Eqns. (4) and (5) which reduce mass were occurring at the same time.

320

Ni-CaO-Ca2SiO4 and Ni-Ca2SiO4 showed a slight weight loss at about 590 ºC

321

attributed to the tar cracking by Ni. But the weight loss at this temperature range was

322

not noticeable in the case of Ni-CaO. These three materials all contained Ni, but

323

Ni-CaO failed in tar cracking. This implied that the catalytic activity of Ni could only

324

be maintained when it was loaded on stabilizers such as Ca2SiO4.

325

The integrated gas yields of H2, CO, CO2 and CH4 were summarized in Figure 8.

326

The hybrid-functional material Ni-CaO-Ca2SiO4 generated 41% higher H2 gas than

327

conventional supported Ni-Ca2SiO4 catalyst, and the H2 purity in the product gas was

328

also 11% higher (Figure 8 inset). The bi-functional material Ni-CaO performed better

329

than the hybrid-functional material Ni-CaO-Ca2SiO4 in producing H2. Meanwhile

330

Ni-CaO generated less CO than Ni-CaO-Ca2SiO4, indicating the enhancement of

331

water gas shift reaction by more CaO in Ni-CaO. The H2 purity in the product gas

332

(Figure 8 inset) followed the same order as the H2 accumulative yield, with the highest

333

purity up to 73 vol.% for Ni-CaO.

20


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334 335

Figure 8. Influence of different functional materials on the cumulative yield of

336

major gases and H2 purity in the product gas. Flow rate was calibrated at 20 ºC and 1

337

atm.

338

3.5 Cyclic stability demonstration

339

The functional materials were not designed to be one-off in practical application, thus

340

the cyclic stability of the material is of greater importance from an industrial

341

perspective. Although the hybrid-functional material Ni-CaO-Ca2SiO4 produced a bit

342

less H2 than Ni-CaO, due to the lower quantity of CaO in Ni-CaO-Ca2SiO4,

343

incorporating Ca2SiO4 spacers greatly stablized the material, maintaining the catalytic

344

activity and sorption capacity of the material over long-term operation. To

345

demonstrate

346

Ni-CaO-Ca2SiO4 and Ni-CaO, were treated for 15 carbonation-decarbonation cycles

347

prior to the catalytic decomposition of sawdust. The details of the 15 cyclic test was

348

plotted in Figure S5. After the 15 cycles, these two materials were mixed with

the

stablization

with

Ca2SiO4

spacers,

21


the

two

materials,


349

sawdust for decomposition tested again in TG-MS. The accumulated gases yields

350

before and after the cyclic carbonation-decarbonation were compared in Figure 9.

351 352 353

Figure 9. The cyclic stabilities of Ni-CaO-Ca2SiO4 and Ni-CaO in decomposing sawdust. Flow rate was calibrated at 20 ºC and 1 atm.

354

Both materials showed decreased H2 yield and increased CO2 yield after the

355

cycles. Although the fresh bi-functional material Ni-CaO exhibited outstanding

356

performance in generating H2 prior to the carbonation-decarbonation cycles (Figure 8),

357

after the 15 cycles, its H2 yield decreased by 62%. Meanwhile, H2 purity in the

358

produced gas reduced from 73 vol.% down to 49 vol.%. However, the reduction of H2

359

yield for Ni-CaO-Ca2SiO4 was only 7% from 626 to 582 ml g-1, demonstrating superb

360

cyclic stability. The H2 purity before and after the cycles were 68 and 64 vol.%. After

361

the 15 cycles, the H2 yield by Ni-CaO-Ca2SiO4 was more than two times higher than

362

that generated by Ni-CaO. A deeper understanding of the different long-term stability 22


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363

could be performed by looking at the evolution of gas generation rates (Figure S6).

364

For Ni-CaO-Ca2SiO4, the H2 generation rate before and after 15 cycles did not show

365

any evident difference throughout the entire decomposition process. But for Ni-CaO,

366

the H2 generation rate at all temperatures was lower after 15 cycles, especially the H2

367

generation peak at 400 ºC was reduced from >300 ml min-1g-1 to less than 100 ml

368

min-1g-1. This was likely due to the catalyst deactivation during cycles. The material in

369

the absence of spacer failed to resist the sintering and lost some surface area and

370

porous structure, thus the exposed catalytic sites must have been significantly reduced.

371

As shown in Figure S6 (b), both materials presented reduction of CO genreation rate

372

at both two CO peaks after 15 cycles, but this decline was more remarkable for

373

Ni-CaO, especiall at the second CO peak. CO2 generation rate in Figure S6 (c) for

374

both Ni-CaO-Ca2SiO4 and Ni-CaO displayed some increases because of the reduced

375

CO2 capture efficiency after cycles, but the rise for Ni-CaO-Ca2SiO4 was very

376

marginal. Two increments of CO2 genreation rate were obvious for Ni-CaO ranging

377

from 410 to 550 ºC and from 700 to 800 ºC. This reduced CO2 sorption was also

378

probably owing to the sintering effect. Since CO2 in-situ sorption shows positive

379

correlation with H2 production, the poorer CO2 capture performance of Ni-CaO after

380

cycles (Figure S6 (c)) was also partially responsible for the H2 generation decline

381

(Figure S6 (a)).

382

In spite of the lower CaO quantity in hybrid-functional material Ni-CaO-Ca2SiO4,

383

the presence of Ca2SiO4 kept exellent long term stability and proved the necessity and

23



384

advantage of having spacers in the hybrid-functional material. The long term stability

385

of Ca2SiO4 was also compared to the widely used stabilizer Ca9Al6O18. In the receipe

386

to synthesize Ni-CaO-Ca9Al6O18, the TEOS silica precursor was replaced by

387

Al(NO3)3.9H2O with proper quantity to form Ni-CaO-Ca9Al6O18, which has the

388

identical ratio of mNi:mCaO:mstabilizer. The comparison of gas yield before and after 15

389

cycles in Table S4 indicated a better stability of Ca2SiO4 than Ca9Al6O18, which was

390

likely due to the polymorphic transitions of Ca2SiO4. Figure S7 displayed the SEM

391

images of the functional materials before and after the 15 cycles. Ni-CaO showed

392

serious sintering effect. All the surface roughness and porous structure disappeared

393

after 15 cycles. The morphology change again confirmed the analysis above that the

394

catalyst deactivation and sorption decay was caused by sintering. However, the

395

presence of spacers Ca2SiO4 and Ca9Al6O18 evidently maintained the porous structure

396

after 15 cycles.

397 398 399

AUTHOR INFORMATION

400

Corresponding Author

401

* Phone: +86 10 6278 4701; Email: [email protected] (MZ)

402

Author Contributions

403

#

404

Funding Sources

These authors contributed equally.

24


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405

National Natural Science Foundation of China (grant number: 51506112), the Tsinghua

406

University Initiative Scientific Research Program (grant number: 20161080094) and

407

China Postdoctoral Science Foundation (grant number: 2017M610910).

408 409

ACKNOWLEDGEMENTS

410

This work was supported by the National Natural Science Foundation of China (grant

411

number: 51506112) and the Tsinghua University Initiative Scientific Research Program

412

(grant number: 20161080094). G. Ji is grateful for the support by the International

413

Postdoctoral Exchange Fellowship Program and China Postdoctoral Science

414

Foundation (grant number: 2017M610910).

415 416

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