2 Production from Aqueo - American Chemical Society

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Interface-Promoted Dehydrogenation and Water-Gas Shift toward HighEfficient H2 Production from Aqueous Phase Reforming of Cellulose Jian Zhang, Wenjun Yan, Zhe An, Hongyan Song, and Jing He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04529 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Sustainable Chemistry & Engineering

Interface− −Promoted Dehydrogenation and Water− −Gas Shift toward High− −Efficient H2 Production from Aqueous Phase Reforming of Cellulose Jian Zhang 1‡, Wenjun Yan 3‡, Zhe An 1, Hongyan Song 1, and Jing He 1, 2* 1

State Key Laboratory of Chemical Resource Engineering,

2

Beijing Advanced Innovation

Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, 15 Beisanhuan Donglu, Chaoyang District, Beijing 100029, P. R. China 3

Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences,

27 South Taoyuan Road, Taiyuan 030001, Shanxi, P.R.China Corresponding Author * Tel: +86−10−64434897. Fax: +86−10−64425385. E−mail address: [email protected] or [email protected].

KEYWORDS: hydrogen production, cellulose, supported Ni particles, solid base, interfacial promotion.

ACS Paragon Plus Environment

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ABSTRACT: Hydrogen production from cellulose via aqueous phase reforming (APR) has aroused increasing attention, but its efficiency remains a great challenge. Here, we report an interface− −promoted Ni particle catalyst, in which Ni particles are located on the surface of layered double oxide (LDO) in a narrow distribution and form abundant Ni-support interfaces and interfacial interactions. The Ni− −LDO interfaces and interfacial interactions are enhanced by preparing the supported Ni− −LDO from in situ reduction of Ni− −containing Mg/Al layered double hydroxides (LDHs). A H2 production activity of up to 40.8 mmol·gcatal-1 gcellulose-1 h-1 with a H2 yield of up to 30.9 % has been achieved in this work, which is much higher than the H2 production activity reported so far in literatures. Quasi in− −situ XPS and CO2−TPD results verify that the amount of solid basic sites, abundant Ni− −solid base interfaces, and desired interfacial interactions play a crucial role in the remarkable improvement of H2 production. The basic surface of LDO support is found to be able not only to dissociatively activate the O− −H bond, thus promoting the cleavage of α− −C− −H and C− −C bonds on vicinal Ni sites, but also to suppress the Ni leaching. The stronger interfacial interactions are able not only to facilitate the catalytic oxidation of subcarbonyl species, thus promoting the WGS reaction, but also to prevent the agglomeration of Ni particles.

INTRODUCTION In the past decades, biomass has attracted significant attentions as the renewable carbon resource to produce fuels and chemicals.1-8 As one of clean and important energy of twenty− −first century,9-11 almost 96 % of the H2 amount has been produced from fossil resources and 3.9 % from electrolysis of water till now.12 Photocatalytic water splitting has been regarded as a


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promising pathway to hydrogen fuel in the last decades,13-14 yet its efficiency remains a great challenge. High efficient H2 production from renewable or sustainable resources, such as biomass,7-8 has thus gained more and more attention. Several processes have been explored to produce H2 from biomass, such as catalytic pyrolysis of biomass,15 gasification,16-17 catalytic partial oxidation,18 supercritical water gasification,19-20 photocatalytic reforming 21 and aqueousphase reforming.22-23 Aqueous phase reforming (APR), first reported by Dumesic and co− −workers, has been proposed as an attractive route to produce H2 from oxygenated hydrocarbons at mild temperature (200−260 oC) and modest pressure (2−5 MPa).22-23 In addition, the APR process can produce H2 with a low CO concentration (< 500 ppm),22-23 which is necessary for the application in fuel cells. Many efforts have been reported for the H2 production from methanol, ethylene glycol, glycerol, sorbitol, glucose, or cellulose on Pt, Pd, or Raney− −Ni based catalysts.22-43 About 26.5 % of H2 yield was obtained from cellulose over 5 wt% Pt/C catalyst.24 Since satisfactory H2 yield is hard to achieve on one− −component catalyst, a second metal has often been employed to form PtM (M stands for Ni,25-28 Co,28-31 Fe,28, 32 Mn,33 or Re

34-37

) or PdFe

28

alloys. Alloy formation

tailors the electron density of Pt active sites,44-46 thereby improving the ability for water-gas shift (WGS) reaction. For example, the TOF for H2 production from ethylene glycol at 210 oC could increases from 1.87 min-1 over Pt/Al2O3 to 5.18 or 5.06 min-1 over Pt1Ni1/Al2O3 or Pt1Co5/Al2O3.28 The site time yield for H2 production from glycerol at 230 oC was up to 44.4 min-1 over Pt1Co5 supported on multi− −walled carbon nanotubes, which is almost four times higher than that over Pt supported on multi-walled carbon nanotubes, while the turnover rate for WGS reaction increases from 0.42 min-1 to 13.2 min-1.30 On PtRe/C catalyst, the TOF for H2 production from glycerol was 23.3 min-1 at 225 oC, much higher than 1.8 min-1 on Pt/C catalyst,


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and the ability for WGS reaction increases in 65 times with Re addition.35 Sn addition to Raney− −Ni catalyst could significantly improve the H2 selectivity by decreasing the rate of methane formation as a result of the formation of Ni− −Sn alloy and the Sn location at Ni− −defect sites.23 Another strategy for improving the activity for H2 production is to modify the catalyst or the support

39

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with a WGS promoter, such as CeO2, or use the WGS promoter directly as the

support, such as iron oxide 28, 40. About 24 % of H2 yield was obtained from cellulose over CeO2 modified Raney− −Ni catalyst, higher than 16 % over unmodified Raney Ni catalyst.38 The H2 production rates from APR of ethylene glycol on iron oxide-supported Pd catalyst were surprisingly elevated to several times higher than that on Pd/Al2O3 catalyst owing to the boosting of WGS reaction by the synergistic combination of Pd and iron oxide.28, 40 The H2 production activity could also be promoted by sacrificial CaO 26, 41-42, Ca(OH)2 26, 41-42 or extra KOH 26, 41-42 as a fact that the bases could faciliate CO2 capture and thus induce the WGS reaction and suppress the methanation formation

26, 41-42

or presumably suppress the dehydration pathway to

form alcohols and alkanes in glycerol conversion

35

. In Ni− −catalyzed H2 production system,

alkaline conditions (pH > 8) could not only improve the activity for H2 production but also suppress Ni leaching, which prevented subsequent particle growth via Ostwald ripening.43 Although impressive progress has been achieved in the development of efficient catalyst and/or catalytic system for APR H2 production from biomass feedstock, there are still great challenges in that effective APR H2 production involves not only the cleavage of O− −H and C− −H bonds (dehydrogenation) but also the cleavage of C− −C bonds and subsequent WGS reaction.22-23 Insufficient C− −C bond cleavage leads to coke formation and inefficient WGS reaction leads to CO poisoning. Meanwhile, the cleavage and/or rearrangement of C− −O (single, double, and triple)


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−worker bonds generate alkanes, greatly deteriorating the H2 yield. Iglesia and co−

47

have

reported that C− −C cleavage occurred in oxygenates only at locations vicinal to the C=O group, because such adjacency can weaken C− −C bonds. Pietropaolo and co-worker 48 also reported that the rupture of the O− −H bond is regarded as the fundamental prerequisite for C− −C cleavage. Here we report a highly-efficient APR catalyst for H2 production from cellulose, in which Ni nanoparticles uniformly dispersed on the layered double oxides (LDO). The LDO is supposed to simultaneously serve as a solid base to cooperate with supported Ni. The strong interfacial interactions between Ni particles and LDO facilitate both dehydrogenation and WGS reactions, affording a H2 production activity and a H2 yield higher than reported so far. The LDO− −supported Ni also demonstrates good stability in the APR.

The Ni particles supported on LDO was prepared by the calcination of Ni− −containing layered double hydroxides (LDHs) under reduction atmosphere. LDHs, also known as hydrotalcite− −like materials, are a class of anionic clays that accommodate a wide range of divalent (M2+) and high− −valence (M3+ or M4+) metal cations in brucite− −like layers.49 The metal cations are distributed uniformly at atomic level within the brucite− −like layers.50 So LDHs material proves to be the superior precursor for supported metal catalyst with the metal particles in highly homogeneous dispersion.51-57

MATERIALS AND METHODS Materials


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Mg(NO3)2·6H2O, Ni(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, Na2CO3, and concentrated NH3·H2O (25 % ~ 28 wt%) were all of analytical purity and used without further purification. Microcrystalline cellulose was purchased from Alfa Aesar. Preparation NiMgAl− −LDHs, MgAl− −LDHs, Mg(OH)2, and AlO(OH) The NiMgAl− −LDHs with Ni/Mg/Al molar ratio of 0.73/3/1 was synthesized by the co− −precipitation

method.58

Typically,

a

solution

of

Ni(NO3)2·6H2O

(0.046

mol),

Mg(NO3)2·6H2O (0.19 mol), and Al(NO3)3·9H2O (0.063 mol) in 200 mL of deionized water and a solution of NaOH (0.12 mol) and Na2CO3 (0.0315 mol) in 200 mL of deionized water were simultaneously added drop wise into a four− −necked flask containing 200 mL of deionized water under constant pH (10.0), and then the mixture was aged at 85 oC for 18 h. Then the solid was filtrated, washed thoroughly with deionized water till the filtrate is neutral. Finally, the asprepared NiMgAl− −LDHs were dried overnight at 60 oC. According to the results of inductively coupled plasma emission (ICP− −ES) analysis, Ni0.84Mg3.23Al− −LDHs was produced. Similar procedures were used, except the addition of Ni(NO3)2·6H2O, for the synthesis of MgAl− −LDHs with Mg/Al molar ratio of 2/1, 3/1, 4/1, or 5/1, producing Mg2.10Al− −LDHs, Mg3.08Al− −LDHs, Mg4.03Al− −LDHs, or Mg5.00Al− −LDHs. Mg(OH)2 was prepared following the same procedure but without introducing Ni(NO3)2 and Al(NO3)3. AlO(OH) was synthesized by adjusting the pH of the Al(NO3)3 solution to 9.0 by concentrated ammonium hydroxide and then aged at 150 oC for 24 h. Ni2+/MgAl− −LDHs, Ni2+/Mg(OH)2, and Ni2+/AlO(OH)


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−LDHs, Ni2+/Mg(OH)2, and Ni2+/AlO(OH) were prepared by incipient wetness Ni2+/MgAl− impregnation of MgAl− −LDHs, Mg(OH)2, and AlO(OH) with Ni2+ solution. Typically, 1 g of MgAl− −LDHs was impregnated with 1 mL of aqueous solution containing 1.7 mmol of Ni(NO3)2·6H2O, giving a 9.1 wt% Ni loading on MgAl− −LDHs. 1 g of Mg(OH)2 was impregnated with 1 mL of aqueous solution containing 3.0 mmol Ni(NO3)2·6H2O, giving a 15.0 wt% Ni2+ loading on Mg(OH)2. 1 g of AlO(OH) was impregnated with 1 mL of ethanol solution containing 3.6 mmol of Ni(NO3)2·6H2O, giving a 17.4 wt% Ni2+ loading on AlO(OH). In 2 h impregnation at room temperature, the solid was dried at 100 oC overnight. Ni− −loaded MgAl− −LDO, Ni− −loaded MgO, and Ni− −loaded γ−Al2O3 Ni− −loaded MgAl− −LDO, Ni− −loaded MgO, and Ni− −loaded γ− −Al2O3 were produced by calcination of Ni− −MgAl− −LDHs, Ni2+/MgAl− −LDHs, Ni2+/Mg(OH)2, and Ni2+/AlO(OH) under reduction atmosphere. Ni0.84Mg3.23Al− −LDHs was calcined at 800 oC for 5 h under H2 flow with a heating rate of 2 oC min-1. According to the ICP− −ES result, the Ni content in the resulting sample was 19.5 wt%. The samples were denoted 19.5Ni− −Mg3Al− −LDO− −800− −5h. Calcination of 9.1 wt% Ni2+/Mg2.10Al− −LDHs, 9.1 wt% Ni2+/Mg3.08Al− −LDHs, 9.1 wt% Ni2+/Mg4.03Al− −LDHs, 9.1 wt% Ni2+/Mg5.00Al− −LDHs, 15.0 wt% Ni2+/Mg(OH)2, or 17.4 wt% Ni2+/AlO(OH) at 700 oC for 1 h under H2 with a heating rate of 2 19.1Ni/Mg3Al− −LDO− −700− −1h,

o

C min-1 afforded 19.6Ni/Mg2Al− −LDO− −700− −1h,

19.3Ni/Mg4Al− −LDO− −700− −1h,

19.6Ni/Mg5Al− −LDO− −700− −1h,

20.0Ni/MgO− −700− −1h, or 19.7Ni/Al2O3−700− −1h. For comparison, AlO(OH), Mg2.10Al− −LDHs, Mg3.08Al− −LDHs, Mg4.03Al− −LDHs, Mg5.00Al− −LDHs, or Mg(OH)2 was also calcined at 700 oC for 1 h under H2 with a heating rate of 2 oC min-1 and the resulting sample was denoted Al2O3−LDO− −700− −1h, Mg2Al− −LDO− −700− −1h, Mg3Al− −LDO− −700− −1h, Mg4Al− −LDO− −700− −1h,


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−LDO− −700− −1h, or MgO− −700− −1h. The calcined/reduced samples were preserved in a N2 Mg5Al− atmosphere for catalytic test, characterizations or pre− −treatment prior to characterizations. Characterizations Powder X− −ray diffraction (XRD) patterns were recorded on a Shimadzu XRD− −6000 diffractometer using Cu Kα monochromatized radiation (λ = 0.1541 nm) with a scanning angle (2θ) range of 3°−80° at a scan speed of 10° min-1. The quantitative analysis for Ni, Mg and Al was performed using a Shimadzu ICPS− −7500 inductively coupled plasma emission spectrometer (ICP− −ES). Before measurements, 5 mg of the sample was dissolved by nitric acid and transferred to a volumetric flask and diluted to 10 mL with deionized water. The specific surface area was determined by Brunauer–Emmett–Teller (BET) method on a Micromeritics ASAP 2460. Before measurements, the samples were degassed at 120 oC under high vacuum for 6 h. The scanning electron microscope (SEM) images were taken on a Zeiss Supra55 (Zeiss Ltd., Germany). Transmission electron microscopy (TEM) images were recorded on a JEOL 2100 high− −resolution transmission electron microscope with 200 kV accelerating voltage. The scanning transmission electron microscopy (STEM) images and element mapping analysis were taken on a Tecnai G2 F20 S− −TWIN operated at 300 kV. The samples for TEM or STEM measurements were pre-treated in absolute ethanol under ultrasonic conditions before depositing on a Cu microgrid, and covered by a carbon coating of several nanometers to eventually prevent magnetization. Quasi in− −situ surface elemental analysis was performed using an AXIS ULTRA DLD X− −ray photoelectron spectroscopy (XPS) spectrometer equipped with the monochromated Al− −K X− −ray source (1486.6 eV) at a pass energy of 40 eV. Prior to measurement, the calcined and reduced samples were pre-treated at 400 °C under H2 atmosphere for 1 h (40 mL min-1),


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followed by flushing with Ar (40 mL min-1) at 450 °C for 0.5 h to remove the chemisorbed hydrogen in the pretreatment chamber of the spectrometer. Then the pre− −treated sample was transferred to the analysis chamber without exposed to air. C 1s peak at 284.8 eV was used as a calibration peak. The Ni dispersion was measured on a Micrometric ChemiSorb 2750 chemisorption instrument with a thermal conductivity detector (TCD) by the method and procedure reported in literature

59

. Typically, about 100 mg of calcined/reduced Ni− −loading

sample was loaded in a U− −type quartz reactor and pre-treated online at 400 °C for 1 h under a flow of 10 vol% H2/Ar mixture (40 mL min-1), followed by flushing with a flow of Ar (40 mL min-1) at 450 °C for 0.5 h to remove the chemisorbed hydrogen. Afterward, the sample was cooled to 90 °C in a flow of Ar (40 mL min-1). Then pulse N2O was introduced into the carrier gas until N2O peak reached saturation to completely oxidize the surface metallic Ni. The sample was then cooled to room temperature and followed up by the H2−temperature programmed reduction (H2−TPR). H2−TPR was carried out with a heating ramp rate of 10 °C min-1 in 10 vol% H2/Ar mixture to 500 °C, with a total flow rate of 40 mL min-1. The amount of consumed H2 was recorded by TCD. It was assumed that one hydrogen molecule reduced one surface NiO to Ni. The Ni dispersion is defined as the fraction of Ni atoms exposed to the surface, and was calculated as follows:

D Ni =

N surface amount of consumed H 2 = ×100%. N total N total (by ICP)

(1)

The surface basicity was measured by CO2−temperature programmed desorption (CO2−TPD) on the Micrometric ChemiSorb 2750 chemisorption instrument with TCD by the method and procedure reported in literature

55

. In a typical CO2−TPD measurement, 100 mg of Ni− −loading


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samples and MgAl-LDO samples were loaded in a U-type quartz tube reactor and pre-treated at 400 oC for 1 h under a flow of 10 vol% H2/Ar mixture (40 mL min-1), followed by flushing with Ar (40 mL min-1) at 450 °C for 0.5 h to remove the chemisorbed hydrogen. Afterward, the sample was cooled to 80 °C in a flow of Ar (40 mL min-1) and then CO2 (20 mL min-1) was fed into the reactor until saturation. A flow of Ar (40 mL min-1) was subsequently fed for 0.5 h to desorb the weakly physically-adsorbed CO2. CO2−TPD was carried out with a heating ramp rate of 10 °C min-1 under a flow of Ar (40 mL min-1) to 500 °C. In situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) of ethanol adsorption/desorption were recorded on a VERTEX 70 (BRUKER) spectrometer equipped with a high temperature cell fitted with KBr windows and a mercury− −cadmium− −telluride (MCT) detector, with a resolution of 2 cm-1 using 128 scans. The Ni− −loaded sample, MgAl− −LDO, MgO, and Al2O3 sample, was pre− −treated under a flow of 10 vol% H2/Ar mixture (40 mL min-1) at 400 °C for 1 h, followed by purging with a flow of Ar (40 mL min-1) for 0.5 h at 450 °C and cooled in Ar to 30 °C. In the cooling, the background spectra were collected at 300 °C, 260 °C, 200 °C, 150 °C, 100 °C, and 30 °C, respectively. Ethanol stream (0.199 vol% in Ar) was introduced at 30 °C into the cell and the spectra were recorded consecutively in the ethanol adsorption until the DRIFTS spectra unchanged. The DRIFTS chamber was flushed with Ar for 20 min, and then the spectrum for ethanol adsorption was collected. The temperature was increased under Ar with a heating rate of 10 oC min-1 and maintained at 100 °C for 10 min. Then the spectrum for ethanol desorption at 100 °C was recorded. The DRIFT spectrum for ethanol desorption at 150 °C, 200 °C, 260 °C, or 300 °C was also recorded.


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In situ Fourier transform infrared spectra (FTIR) of CO adsorption in transmission mode were recorded on an IS50 FT− −IR (NICOLET) spectrometer equipped with a high temperature cell fitted with BaF2 windows and a MCT detector, with a resolution of 2 cm-1 using 64 scans. About 20 mg of 19.5Ni− −Mg3Al− −LDO− −800− −5h or 19.1Ni/Mg3Al− −LDO− −700− −1h was pressed into a self− −supported disk and then loaded into the chamber. Prior to CO adsorption, the sample was pre− −treated in a flow of 10 vol% H2/Ar (40 mL min-1) at 400 °C for 1 h, and then cooled to 250 °C in Ar. After being cooled to 250 °C, the background spectrum was collected. Subsequently, CO stream (1 vol% in Ar, 10 mL min-1) was introduced into the cell for 30 min. In the CO adsorption, the spectra were recorded at intervals. Catalytic test The reaction was conducted in a homemade batch reactor with a mechanical stirrer. In a typical experiment, 140 mg of catalyst, 50 mL of deionized H2O and 1 g of cellulose in the reactor were thoroughly purged with N2, and then 0.4 MPa N2 was sealed in the reactor at 30 °C. The reactor was heated to 260 °C and kept at 260 °C for 4 h. The reactor was then quenched to room temperature. The gas product was quantified by wet gas flowmeter and collected in air bag for product analysis on gas chromatography (GC) (Shimadzu, 2014C) equipped with a TDX− −1 column and TCD. The liquid intermediates were analyzed at 40 ºC by high performance liquid chromatography (HPLC) (Shimadzu, LC− −20AD, RI detector, Waters Sugar− −PakTM 6.5 X 300 mm Column). Ultra− −pure water was used as the mobile phase at a flow rate of 0.5 mL min-1. The products detected in the gas phase were H2, CO2, and CH4. The activity for H2, CH4, or CO2 production was defined as the moles of H2, CH4, or CO2 formed per mole of surface Ni. It was calculated as follows:


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Activity =

amount of produceded H 2 , CH 4 , or CO 2 (mmol) . mass of catalyst (mg) × Ni dispersion × Ni loading / 58.69

Page 12 of 45

(2)

Catalyst recycles The spent catalyst was recovered simply by centrifugation, then thoroughly washed with deionized water, and dried overnight at 80 °C. Then the recovered catalyst was calcined at 550 °C for 4 h at ambient atmosphere with a heating rate of 5 oC min-1 from room temperature to 550 °C, and reduced at 600 °C for 1 h under H2 flow. The regenerated catalyst was reused in the APR of cellulose.

RESULTS AND DISCUSSION Ni-LDO interfaces Figure 1 shows the XRD patterns of as− −prepared precursors and Ni− −loaded oxides. The (003), (006), (009), (110), and (113) reflections at about 11.7o, 23.6o, 35.4o, 60.3o, and 61.2o characteristic of hydrotalcite− −like structure are clearly observed in the XRD patterns of Ni0.84Mg3.23Al− −LDHs and MgxAl− −LDHs (Figure 1A, a− −e). The basal spacing of the LDHs is estimated to be nearly 0.73 nm, resulting from carbonate anion (CO32-) as the interlayer anion. The (001), (100), (101), (102), (110), and (111) reflections characteristic of brucite structure for Mg(OH)2 are clearly observed at about 18.5o, 32.9o, 38.0o, 50.9o, 58.7o, and 62.1o (JCPDS: 44− −1482) (Figure 1A, f). The (020), (120), (031), and (051) reflections characteristic of bohmite structure for AlO(OH) are clearly observed at about 14.5o, 28.2o, 38.3o, and 48.9o (JCPDS: 21− −1307) (Figure 1A, g). In each case, no reflections for other phases are observed (Figure 1A).


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−LDHs at 800 °C for 5 h under reduction atmosphere (Figure 1B, a) Calcination of Ni0.84Mg3.23Al− produced both of Mg-Al mixed oxide (MgxAl− −LDO) phase with characteristic reflections around 36.6°, 43.5°, and 63.1° (JCPDS: 45− −0946) and metallic Ni phase with reflections at 44.5°, 51.9°, and 76.4° (JCPDS: 04− −0850). Similarly, after calcination at 700 °C for 1 h under reduction atmosphere of the samples with Ni loaded by impregnation, the diffractions of metallic Ni phase are also observed at 44.5°, 51.9° and 76.4° in the XRD patterns (Figure 1B, b− −g). The reflections characteristic of Mg/Al LDO are also clearly observed around 36.6°, 43.5°, and 63.1° (Figure 1B, b− −e). The (111), (200), and (220) reflections characteristic of MgO are clearly observed around 36.9°, 42.9°, and 62.3° (Figure 1B, f). The (311), (400), and (441) reflections characteristic of Al2O3 can be clearly observed around 37.4°, 45.8°, and 63.06° (JCPDS: 04− −0880) (Figure 1B, g). In each case, no spinel phase has been produced (Figure 1B), as a fact that no (311) reflection at 36.85o (JCPDS: 21− −1152) for MgAl2O4 spinel is observed.

Figure 1. XRD patterns of (A) Ni0.834Mg3.23Al− −LDHs (a), Mg2.10Al− −LDHs (b), Mg3.08Al− −LDHs (c), Mg4.03Al− −LDHs (d), Mg5.00Al− −LDHs (e), Mg(OH)2 (f), and AlO(OH) (g); XRD patterns of (B) fresh, (C) spent, and (D) regenerated 19.5Ni− −Mg3Al− −LDO− −800− −5h (a), 19.6Ni/Mg2Al− −LDO− −700− −1h

(b),

19.1Ni/Mg3Al− −LDO− −700− −1h

(c),


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−LDO− −700− −1h (d), 19.6Ni/Mg5Al− −LDO− −700− −1h (e), 20.0Ni/MgO− −700− −1h (f), 19.3Ni/Mg4Al− and 19.7Ni/Al2O3−700− −1h (g). In the TEM and HRTEM images for 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 2, a), produced by calcination of Ni0.84Mg3.23Al− −LDHs at 800 °C for 5 h under reduction atmosphere, Ni particles are observed to be well dispersed in a narrow distribution of particle size with the maximum at 7~8 nm. For comparison, the Ni particles have also been controlled in a narrow size distribution with the maximum at 7~8 nm (Figure 2, b− −g) by reducing the reduction temperature to 700 °C and shortening the reduction time to 1 h for the samples with Ni− −loaded by impregnation. In each case, Ni nanoparticles are observed to be dispersed with (111) facet exposed on MgxAl− −LDO (Figure 2, a− −e), MgO (Figure 2, f), or Al2O3 (Figure 2, g).


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Figure 2. TEM images, HRTEM images, and size distribution of metallic Ni particle for (a) −LDO− −800− −5h, 19.5Ni− −Mg3Al−

(b)

19.6Ni/Mg2Al− −LDO− −700− −1h,

(c)

19.1Ni/Mg3Al− −LDO− −700− −1h,

(d)

19.3Ni/Mg4Al− −LDO− −700− −1h,

(e)

19.6Ni/Mg5Al− −LDO− −700− −1h, (f) 20.0Ni/MgO− −700− −1h, and (g) 19.7Ni/Al2O3−700− −1h.


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From HAADF images (Figure 3), the Ni particles in uniform size and homogeneous −LDO− −800− −5h (Figure 3, a). high− −density distribution are clearly observed for 19.5Ni− −Mg3Al− From the element mapping images and line scan profiles (Figure 3), the Ni element in 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 3, a) can be observed in a periodic distribution with regular spacing and similar peak width, while the spacing between two adjacent maximum of the Ni

element

distribution

in

19.1Ni/Mg3Al− −LDO− −700− −1h

(Figure

3,

b)

or

19.6Ni/Mg5Al− −LDO− −700− −1h (Figure 3, c) is observed to be less regular. The results indicate that not only the size distribution of Ni particles is uniform but also the dispersion of Ni particles on LDO support is homogeneous in 19.5Ni− −Mg3Al− −LDO− −800− −5h, which is supposed to result from the merits of LDH precursor strategy. In each case, Mg element is observed in a continuous and homogeneous distribution.


16

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Figure 3. HAADF image with the corresponding Mg (red) and Ni (green) element mappings (from the yellow frame regions), line scan profiles of Mg and Ni elemental distribution and the structure

schematic

drawing

for

(a)

−LDO− −800− −5h, 19.5Ni− −Mg3Al−

(b)

19.1Ni/Mg3Al− −LDO− −700− −1h, and (c) 19.6Ni/Mg5Al− −LDO− −700− −1h. In the Ni 2p3/2 XPS spectra (Figure 4A), the binding energies (BE) assigned to Ni0 and Niδ+ 6062

are observed at 852.12 and 855.24 eV for 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 4A, a), while

at 851.81~_852.01 eV and 854.52~855.07 eV for Ni/MgxAl− −LDO− −700− −1h (the samples with Ni− −loaded by impregnation) (Figure 4A, b− −e). Both of the BE values originating from Ni0 and Niδ+ in 19.5Ni− −Mg3Al− −LDO− −800− −5h are higher than that in each Ni/MgxAl− −LDO− −700− −1h, indicative of more electron-deficiency for the Ni0 and Niδ+ sites in 19.5Ni− −Mg3Al− −LDO− −800− −5h. As the Ni particles are produced by reduction under H2 atmosphere and thereafter no exposing to air, the Niδ+ sites could be assigned to metallic Ni interacting with the oxygen site on the LDO surface. According to the deconvoluted area, the Niδ+ sites were quantified, summarized in Table 1. The Niδ+ species accounts for almost 82 % in 19.5Ni− −Mg3Al− −LDO− −800− −5h, higher than that in each Ni/MgxAl− −LDO− −700− −1h (64 %~78 %), which is consistent with STEM results (Figure 3) that more Ni atoms are located at the metal/support interface and interacting with support surface in 19.5Ni− −Mg3Al− −LDO− −800− −5h. In the O 1s XPS spectra (Figure 4B), two contributions are distinguished at 530.02 eV and 530.99 eV after deconvolution for 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 4B, a) while 530.33~530.53 eV and 531.62~532.03 eV for the samples with Ni− −loaded by impregnation (Figure 4B, b− −e), which are assigned to surface O2- species and M− −O (M stands for Mg or Al) species according to the previous reports.63-64 Both of the BE values originating from the O2- and M− −O species in 19.5Ni− −Mg3Al− −LDO− −800− −5h are lower than that


17


Page 18 of 45

−LDO− −700− −1h, in accord with the Ni 2p3/2 XPS results, indicating that the in each Ni/MgxAl− electron transfer from Ni to O species at the Ni− −LDO interfaces is more abundant in 19.5Ni− −Mg3Al− −LDO− −800− −5h. The results indicate that not only Ni− −LDO interfaces are more abundant but also the Ni− −LDO interface interaction is stronger in 19.5Ni− −Mg3Al− −LDO− −800− −5h.

Figure 4. Quasi in− −situ XPS spectra for (A) Ni 2p3/2 and (B) O 1s of (a) 19.5Ni− −Mg3Al− −LDO− −800− −5h, 19.1Ni/Mg3Al− −LDO− −700− −1h,

(b) (d)

19.6Ni/Mg2Al− −LDO− −700− −1h, 19.3Ni/Mg4Al− −LDO− −700− −1h,

and

(c) (e)

19.6Ni/Mg5Al− −LDO− −700− −1h. The surface basicity of LDO− −supported Ni particle has been determined by CO2−TPD technique. The CO2−TPD curves are presented in Figure 5 and estimated basic amounts are given in Table 1. A broad peak from CO2 desorption is observed between 100 and 500 °C (Figure 5) in each case, which can be deconvoluted into three or two contributions at the region with a maximal temperature < 170 °C, 170~290 °C, and > 290 °C. The contributions have been


18

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identified as the adsorption of CO2 on weak (CO2 adsorption to weak OH groups), medium− −strong (bidentate carbonates formed on metal− −oxygen pairs), and strong (CO2 bonded with low− −coordination oxygen anions) base sites.55, 65-66 On LDO− −supported Ni samples (Figure 5, a− −e), almost no weak basic sites are detected, which is different from either MgO (Figure 5, f) or Al2O3 (Figure 5, g) supported Ni sample. With the increasing of Mg/Al molar ratio, the CO2 desorption shifts to low temperature (Figure 5, a− −c) while the total amount of medium− −strong and strong base sites increases (Table 1, entries 1− −3). Further with the increasing of Mg/Al molar ratio to 5, however, the CO2 desorption shifts to higher temperature (Figure 5, d) while the total amount of medium− −strong and strong base sites decrease (Table 1, entry 4). For either MgO (Table 1, entry 5) or Al2O3 (Table 1, entry 6) supported Ni sample, the amount of medium-strong and strong base sites is much less than that for LDO− −supported Ni samples, meaning that MgxAl− −LDO materials are better than MgO or Al2O3 as solid base supports. Mg3Al− −LDO− −700− −1h (Table 1, entry 7), without Ni loaded, displays similar medium− −strong and strong base sites to 19.1Ni/Mg3Al− −LDO− −700− −1h (Table 1, entry 2). The temperature for CO2 desorption from 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 5, e) is similar to that from 19.1Ni/Mg3Al− −LDO− −700− −1h (Figure 5, b). But the amount of medium− −strong base sites on 19.5Ni− −Mg3Al− −LDO− −800− −5h (Table 1, entry 8) is slightly higher than that on 19.1Ni/Mg3Al− −LDO− −700− −1h (Table 1, entry 2), which is consistent with their difference in the percentage of surface O2- species in the XPS spectra.


19


Page 20 of 45

Figure 5. CO2−TPD profiles deconvoluted into two or three peaks corresponding to weak base (I, blue), medium− −strong base (II, magenta) and strong base (III, green) site, for (a) 19.6Ni/Mg2Al− −LDO− −700− −1h,

(b)

19.1Ni/Mg3Al− −LDO− −700− −1h,

(c)

19.3Ni/Mg4Al− −LDO− −700− −1h,

(d)

19.6Ni/Mg5Al− −LDO− −700− −1h,

(e)

19.5Ni− −Mg3Al− −LDO− −800− −5h, (f) 20.0Ni/MgO− −700− −1h. Dot −1h, and (g) 19.7Ni/Al2O3−700− dash lines represent the deconvoluted peaks associated with different basic strengths.

Table 1. Physiochemical properties of Ni loaded oxides and MgAl-LDO.

Entry Samples

Ni

Specific

Dispers

surface area(c) of

D Ni(a) /nm

Percentage Medium Niδ+ /Strong


20

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ion(b)/

/(m2 g-1)

sites (d)/%

base sites(e)/(m

%

mol CO2/g) 0.33/0.42

19.6Ni/Mg2Al−LDO 7.4

1

13.0

171.9 (152.1)

64

−700− −1h 0.63/0.34

19.1Ni/Mg3Al−LDO 7.6 (14.4)

2

13.0

168.7 (161.2)

73

−700− −1h 0.59/0.57

19.3Ni/Mg4Al−LDO 3

7.5

12.1

165.1 (157.8)

78

7.8

12.5

185.3 (175.8)

75

20.0Ni/MgO−700−1h 7.2

12.8

157.7 (142.3)

ND

0.24/0.26

7.8

13.0

158.5 (164.1)

ND

0.27/0.11

-

-

ND

-

0.59/0.39

7.9 (8.0)

12.3

171.3 (156.5)

82

0.74/0.26

−700− −1h 0.48/0.54

19.6Ni/Mg5Al−LDO 4 −700− −1h 5

19.7Ni/Al2O3−700−1 6 h Mg3Al−LDO−700−1 7 h 19.5Ni−Mg3Al−LDO 8 −800− −5h (a)

The average size of Ni nanoparticles was evaluated by counting and averaging TEM images.

(b)

Ni dispersion was measured by N2O pulse chemisorption.

(c)

Specific surface area was

determined by Brunauer–Emmett–Teller (BET) method.(d) Interfacial sites were measured by the


21


Page 22 of 45

Ni 2p3/2 in the XPS spectra, according to the percentage of the deconvoluted area for the Niδ+ species in total area for the Ni species. (e) Solid base sites were measured by CO2–TPD. The data in parentheses are for the spent samples. ND = not determined. Catalytic H2 Production from Cellulose The LDO− −supported Ni particles, with similar Ni particle size and Ni dispersity, were applied as the catalysts for catalytic H2 production from cellulose. On each LDO− −supported Ni catalyst, a H2 production activity of more than 232.5 mmol H2/mmolsurface Ni in 4 h from 1 g cellulose (25.3 mmol·gcatal-1 gcellulose-1 h-1) was observed, which is much higher than reported so far in the literature to our best knowledge. Especially, not only a higher H2 production activity (396.7 mmol H2/mmolsurface Ni in 4 h from 1 g cellulose, 40.8 mmol·gcatal-1gcellulose-1h-1) but also a higher H2 yield (30.9 %) than that reported so far have been achieved on 19.5Ni− −Mg3Al− −LDO− −800− −5h. The H2 production activity and H2 yield are clearly observed to increase with increasing amount of medium− −strong and strong base sites (Figure 6), indicating a possible promotion of surface basic sites on the H2 production. Much lower H2 production activity and H2 yield on 20.0Ni/MgO− −700− −1h or 19.7Ni/Al2O3–700–1h are thus not surprising. Almost no H2 production activity and H2 yield were observed over Mg3Al–LDO–700–1h containing almost the same base sites as the 19.1Ni/Mg3Al− −LDO− −700− −1h, indicative of the dominating catalysis of Ni particles in the APR of cellulose. But it is interesting that higher H2 production activity and H2 yield achieved over 19.5Ni− −Mg3Al− −LDO− −800− −5h than over all the catalysts with Ni− −loaded by impregnation, even though the amount of medium–strong and strong base sites on 19.5Ni− −Mg3Al− −LDO− −800− −5h

is

less

than

that

on

19.3Ni/Mg4Al− −LDO− −700− −1h

or

19.6Ni/Mg5Al− −LDO− −700− −1h. This interesting result suggests that the interfaces promote the H2


22

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production, which is consistent with the observations that the Ni− −LDO interfaces are more −LDO− −800− −5h. abundant and the Ni− −LDO interfacial interaction is stronger in 19.5Ni− −Mg3Al− To make the effect of interfacial promotion clear, the number of interfacial Niδ+ sites is plotted vs H2 production activity (Figure 7). The H2 production activity and H2 yield are clearly observed to increase with increasing amount of interfacial Niδ+ sites. On the mixture of Mg3Al− −LDO− −700− −1h with 19.7Ni/Al2O3−700− −1h, the H2 production activity (195.6 mmol/mmolsurface Ni) and H2 yield (16.1 %) is higher than on 19.7Ni/Al2O3−700− −1h but much lower than on 19.5Ni− −Mg3Al− −LDO− −800− −5h, further hinting the promoting effects of basic sites and the significance of desirable Ni− −LDO interfaces. On each supported Ni catalysts, no CO and large amount of CO2 were detected, clearly showing the excellent WGS ability of Ni particles supported on the solid base surface. CH4 was also detected over each supported Ni catalysts. This is not much surprising because Ni-based catalysts have been widely used in the methanation of CO

67-68

or CO2 53, 69-70. On highly efficient supported Ni catalyst, the conversion of CO2 to

methanation exceeded 90 % at 265 oC,53 a temperature similar to the reaction temperature (260 o

C) of this work. In addition, the direct catalytic pyrolysis could be another source of CH4 in the

case of insufficient C–H cleavage.71 The liquid intermediates over 19.5Ni− −Mg3Al− −LDO− −800− −5h and 19.1Ni/Mg3Al− −LDO− −700− −1h were also determined (Table 2). The main intermediate over 19.5Ni− −Mg3Al− −LDO− −800− −5h and 19.1Ni/Mg3Al− −LDO− −700− −1h was glucose (Table 2), which has been considered as the key intermediate for the cellulose conversion under hydrothermal conditions

24

. The selectivity for dehydrogenated intermediates (comenaldehyde and

glycolaldehyde) is higher over 19.5Ni− −Mg3Al− −LDO− −800− −5h than 19.1Ni/Mg3Al− −LDO− −700− −1h (Table 2), indicative of the promoting effects of desirable Ni-LDO interfaces in dehydrogenation, in accordance with the results observed with gas products.


23


Page 24 of 45

Figure 6. The H2, CH4, CO2 production activity and H2 yield as a function of solid base sites over the catalyst with Ni loaded by impregnation.

Figure 7. The H2 production activity as a function of interfacial Niδ+ sites.

Table 2. The liquid intermediates for the APR of cellulose over Ni loaded MgAl-LDO. (a)

Entr

C yield

Selectivity (c) /%

Samples (b)

y

1

19.5Ni− −Mg3Al− −LD

%

64.1

GLU SOR CMA

THFA LA

PA

GLA

EA

37.1

1.2

3.2

16.9

5.3

12.3

19.4

4.6


24

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O−800−5h

19.1Ni/Mg3Al−LD 2

61.2

45.3

13.6

13.2

2.1

7.8

5.4

4.9

7.7

O− −700− −1h (a)

Conditions: cellulose 1.0000 g, catalysts 140 mg, initial pressure 0.4 MPa N2 (30 oC), T = 260

o

C, 50 mL H2O. (b) C yield = yield of total C in liquid product. (c) Products: GLU = glucose; SOR

= sorbitol; CMA = comenaldehyde; THFA = tetrahydrofurfuryl alcohol; LA = levulinic acid; PA = propionic acid; GLA = glycolaldehyde; EA = ethanol. Selectivity was determined by HPLC using an external standard method. To figure out the role of the solid bases in promoting H2 production activity, DRIFT spectra of ethanol adsorption at 30 oC and desorption at varied temperatures were recorded (Figure 8). On 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 8A), the ethanol adsorption at 30 oC gives absorption bands at 2975, 2933, 2879, 2747, 1459, 1387, 1277, 1116, and 1031 cm-1. According to a previous report,72 the bands at 2975, 2933, and 2879 cm-1 can be assigned to the asymmetric stretching modes of the CH3 group and CH2 group in adsorbed ethoxide. The asymmetric stretching modes of the CH2 group probably might overlap with the symmetric stretching modes of the CH3 group at 2879 cm-1. The bands at 1459 and 1387 cm-1 can be assigned to CH2 and CH3 δ modes of un− −dissociated adsorbed ethanol, the bands at 1116 and 1031 cm-1 assigned to the C− −O stretching in adsorbed ethoxide, and the bands at 2747 and 1277 cm-1 assigned to the stretching modes of C− −H group and C− −O group in η2−adsorbed acetaldehyde. These results evidence that the adsorption of ethanol on 19.5Ni− −Mg3Al− −LDO− −800− −5h occur through the activation of O− −H bond followed by step− −by− −step dehydrogenation to form ethoxide and acetaldehyde. With increasing temperature to desorption, the absorption at room temperature


25


Page 26 of 45

decreased and vanished at 260 oC, and meanwhile two new bands appear at 1440 and 1587 cm-1 since 200 oC. These bands have been assigned to the OCO νs, and OCO νas modes of adsorbed acetate.72 Similar results are observed on 19.1Ni/Mg3Al− −LDO− −700− −1h (Figure 8B), except that the absorption at room temperature vanishes at 300 oC and the bands assigned to acetate (1437 and 1579 cm-1) appear since 260 19.1Ni/Mg3Al− −LDO− −700− −1h

is

19.3Ni/Mg4Al− −LDO− −700− −1h

(Figure

o

slower

C. It indicates that the dehydrogenation on than

8C),

on with

19.5Ni− −Mg3Al− −LDO− −800− −5h. more

basic

sites

than

On either

19.1Ni/Mg3Al− −LDO− −700− −1h or 19.5Ni− −Mg3Al− −LDO− −800− −5h, the band at 1653 cm-1, which has been assigned to η1-adsorbed acetaldehyde,72 is observed even at room temperature, in addition to the absorption resulting from dissociated and un− −dissociated ethanol. The absorption at room temperature vanishes at 150 oC and the bands assigned to acetate (1432 and 1590 cm-1) appear since 200 oC. On 19.7Ni/Al2O3−700− −1h (Figure 8D), ethanol is adsorbed in ethoxide and un-dissociated form, and no adsorbed acetaldehyde is observed at room temperature. The bands assigned to acetate (1413 and 1576 cm-1) appear since 200 oC. The results clearly indicate that the solid base sites play a significant role in enhancing dehydrogenation. For comparison, the ethanol adsorption on bare Mg3Al− −LDO− −700− −1h (Figure 8E) and Al2O3−700− −1h (Figure 8F) was also investigated. The adsorption of ethanol on bare Mg3Al− −LDO− −700− −1h or Al2O3−700− −1h occur through the activation of O− −H bond in ethoxide or un− −dissociated form, with no adsorbed acetaldehyde observed at room temperature and acetate appeared since 200 oC. It is clear that the form of ethanol adsorption and subsequent transformation of adsorbed ethanol are connected with both of surface basicity and Ni− −LDO interfaces. More population of basic sites promotes the dehydrogenation of adsorbed ethanol to acetaldehyde. With similar amount of basic sites (19.1Ni/Mg3Al− −LDO− −700− −1h or 19.5Ni− −Mg3Al− −LDO− −800− −5h), more abundant


26

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Ni− −LDO interfaces make the dehydrogenation of adsorbed ethanol to acetaldehyde easier. The basic sites of LDO surface cause the dissociation of O− −H in ethanol, which facilitate the cleavage of α− −C− −H on vicinal Ni sites, just as observed with the adsorption of isopropanol on −octanol on supported Cu nanoparticles 74-75. supported Ni13 cluster 73 and 1−

Figure 8. DRIFTS spectra of ethanol adsorption at 30 oC for 40 min (a) followed by desorption at 100 oC (b), 150 oC (c), 200 oC (d), 260 oC (e), and 300 oC (f) on (A) 19.5Ni− −Mg3Al− −LDO− −800− −5h,

(B)

19.1Ni/Mg3Al− −LDO− −700− −1h,

(C)

19.3Ni/Mg4Al− −LDO− −700− −1h, (D) 19.7Ni/Al2O3−700− −1h, (E) Mg3Al− −LDO− −700− −1h, and (F) Al2O3−700− −1h. Ethanol activation models are shown in inserts.


27


Page 28 of 45

The color of reaction mixture was monitored in the APR (Figure 9). The cellulose was dispersed in water to form a white suspension. After the APR reaction, white cellulose vanished and transparent solutions were formed. The color of the solution after APR reaction is deeply yellow on the catalyst with a basic amount of only 0.38 mmol CO2/g. With the increasing of base sites, the color of the solution after APR reaction becomes lighter and lighter and even colorless, indicating that the coke resistance increases with the increase in the amount of solid base sites. Coke formation is supposed to result from insufficient C− −C bond cleavage. In the case of Mg/Al− −LDO as support, C− −C bond cleavage has been effectively promoted by the solid base sites. According to the mechanism investigation by Iglesia 47 and Pietropaolo 48, the activation of O− −H bond and followed formation of aldehyde species observed in this work on LDO supported Ni catalysts are contributed to the effective α− −C− −H cleavage and C− −C bond cleavage, giving rise to higher H2 production efficacy.

Figure 9. The color of APR reaction solution as a function of solid base sites of the supported Ni catalysts with Ni-loaded by impregnation.


28

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In the FT-IR spectra of CO adsorption (Figure 10), the bands around 2062 (2067) cm-1 assigned to subcarbonyl nickel species Ni(CO)n (n = 2 or 3),76 1923 cm-1 assigned to bridged carbonyl species adsorbed on metallic Ni,70 1378 (1382) and 1602 (1606) cm-1 due to the carbonate species as a result of oxidation of adsorbed CO,77-78 and 2849 cm-1 originating from formate

species

71-72

are

observed

in

4

min

adsorption

at

250

°C

on

both

19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 10A) and 19.1Ni/Mg3Al− −LDO− −700− −1h (Figure 10B). The absorption intensity of carbonate species, the intermediate for WGS reaction, gradually increases with increasing adsorption time, indicating the continuous oxidation of adsorbed CO. But the increase (1602 cm-1) is more obvious on 19.5Ni− −Mg3Al− −LDO− −800− −5h than on 19.1Ni/Mg3Al− −LDO− −700− −1h (Figure 10C), indicative of better WGS catalytic activity of 19.5Ni− −Mg3Al− −LDO− −800− −5h. It is proposed that the abundant Ni− −LDO interfaces in 19.5Ni− −Mg3Al− −LDO− −800− −5h facilitated the transformation of CO produced by the dehydrogenation and C− −C bond cleavage of cellulose. The Niδ+ sites interacting with support oxygen at Ni− −LDO interfaces promoted the catalytic oxidation of subcarbonyl species, especially on the Ni sites located in the interfacial region between Ni particle and solid base support.


29


Page 30 of 45

Figure 10. In-situ FTIR spectra of CO adsorption versus adsorption time at 250 oC on (A) 19.5Ni−Mg3Al−LDO−800−5h and (B) 19.1Ni/Mg3Al−LDO−700−1h, and (C) the intensity ratio of 1602 (1606 ) cm-1 (carbonate) to 2062 (2067 ) cm-1 (Ni−carbonyl). Stability and Regeneration To investigate the stability and regeneration, the spent catalyst was recovered. In the XRD patterns (Figure 1C), the (003), (006), (009), (110), and (113) reflections characteristic of hydrotalcite− −like structure are clearly observed for spent Ni− −loaded MgxAl− −LDO (Figure 1C, ae). The MgO transformed to Mg(OH)2 for spent 20.0Ni/MgO− −700− −1h (Figure 1C, f), while no changes of Al2O3 support are detected for spent 19.7Ni/Al2O3−700− −1h (Figure 1C, g). In comparison with fresh samples (Figure 1B), the reflections of metallic Ni phase are observed to become more visible (Figure 1C, b-g) for spent Ni/MgxAl− −LDO, 20.0Ni/MgO− −700− −1h, and 19.7Ni/Al2O3−700− −1h (Ni-loaded by impregnation), while almost no changes for spent 19.5Ni− −Mg3Al− −LDO− −800− −5h (Figure 1C, a). This means that the Ni particles in the sample with Ni-loaded by impregnation become larger in the APR reaction, while well retain in the sample by direct calcination/reduction of Ni-containing LDH. In the TEM images (Figure 11), no obvious agglomeration is observed for spent 19.5Ni− −Mg3Al− −LDO− −800− −5h, and the size of Ni particles is maintained at 6−10 nm with the maximum at 8.0 nm (Figure 11A). But for spent 19.1Ni/Mg3Al− −LDO− −700− −1h, larger Ni particle is clearly observed and the particle size increased to 9−21 nm with the maximum at 14.4 nm (Figure 11B). The results suggest that the strong

interfacial

interaction

between

Ni

particles

and

solid

base

supports

in

19.5Ni− −Mg3Al− −LDO− −800− −5h could effectively restrict the migration of Ni particles, thus avoiding the aggregation and growth of Ni particles. The APR reaction causes only slight


30

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decrease in the specific surface area for each sample (Table 1). The regeneration by calcination −LDHs or Mg(OH)2 to under ambient atmosphere followed by H2 flow well recuperates MgxAl− MgxAl− −LDO or MgO, while makes no effects on the reflections of metallic Ni phase (Figure 1D). In the SEM images (Figure 12), the regenerated samples display lamellar morphology. But the lamellar stacking is denser than the spent ones while looser than fresh counterparts. The regenerated samples were then reused in the APR H2 production of cellulose. Over 19.5Ni− −Mg3Al− −LDO− −800− −5h, the H2 production activity only decreases by 9 % (from 144.1 to 130.7 mmol g-1) in fifth run, while decreases by 75 % (from 131.8 to 32.9 mmol g-1) over 19.1Ni/Mg3Al− −LDO− −700− −1h (Figure 13). The difference in the catalytic stability between 19.5Ni− −Mg3Al− −LDO− −800− −5h and 19.1Ni/Mg3Al− −LDO− −700− −1h can be well explained with the stability of Ni particle size. The Ni leaching was also detected by ICP− −ES technique after first run. The Ni concentration in the liquid effluent is 5.64 ppm for 19.5Ni− −Mg3Al− −LDO− −800− −5h and 8.40 ppm for 19.1Ni/Mg3Al− −LDO− −700− −1h (total volume of the liquid effluent were 50 mL), accounting for only about 1 % and 2 % of the Ni amount.

Figure 11. Comparison of Ni particle sizes before and after catalytic APR for (A) 19.5Ni− −Mg3Al− −LDO− −800− −5h and (B) 19.1Ni/Mg3Al− −LDO− −700− −1h.


31


Page 32 of 45

Figure 12. SEM images of fresh (left), spent (middle), and regenerated (right) −LDO− −800− −5h (a) and 19.1Ni/Mg3Al− −LDO− −700− −1h (b). 19.5Ni− −Mg3Al−

Figure

13.

Variation

of

H2

production

versus

catalytic

run

with

(a)

19.5Ni− −Mg3Al− −LDO− −800− −5h and (b) 19.1Ni/Mg3Al− −LDO− −700− −1h.

CONCLUSIONS In summary, interface− −promoted Ni catalyst has been developed in this work for the H2 production directly from the microcrystalline cellulose. This interface-abundant structure was achieved through the in situ topological transformation of Ni containing Mg/Al− −LDHs precursor. The activity for H2 production was up to 40.8 mmol·gcatal-1gcellulose-1h-1 with a H2 yield of up to


32

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30.9 % and the H2 yield only decrease by 9 % even in fifth run, showing superior performance to the catalysts reported so far. The basic surface of LDO support is able not only to dissociatively activate the O− −H bond, thus promoting the cleavage of α− −C− −H and C− −C bonds on Ni particles, but also to inhibit the Ni leaching. The stronger interfacial interactions are able not only to facilitate the catalytic oxidation of subcarbonyl species, thus promoting the WGS reaction, but also to prevent the agglomeration of Ni particles. The Ni particle sizes are not included in this paper considering that the effects of Ni particle size could be multiple and complex. The particle size could affect not only the population of interfacial sites but also the intrinsic activity, such as the activity for C−C bond cleavage. But the effect of Ni particle size on H2 production from APR of cellulose deserve to be investigated in more detail and reported in our later research. AUTHOR INFORMATION Corresponding Author * Tel: +86− −10− −64434897. Fax: +86− −10− −64425385. E− −mail address: [email protected] or [email protected]. Author Contributions ‡

Jian Zhang and Wenjun Yan contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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Financial support by National Natural Science Foundation of China (21521005 and 91634120), the National Key R&D Program of China (2017YFA0206804), and 973 Project (2014CB932104) is gratefully acknowledged. REFERENCES (1) Zhang, X.; Zhang, Q.; Wang, T.; Ma, L.; Yu, Y.; Chen, L. Hydrodeoxygenation of ligninderived phenolic compounds to hydrocarbons over Ni/SiO2–ZrO2 catalysts. Bioresource Technol. 2013, 134, 73–80. (2) Liu, Y.; Chen, L.; Wang, T.; Zhang, Q.; Wang, C.; Yan, J.; Ma, L. One-Pot Catalytic Conversion of Raw Lignocellulosic Biomass into Gasoline Alkanes and Chemicals over LiTaMoO6 and Ru/C in Aqueous Phosphoric Acid. ACS Sustain. Chem. Eng. 2015, 3, 1745– 1755. (3) Shi, N.; Liu, Q.; Zhang, Q.; Wang, T.; Ma, L. High yield production of 5hydroxymethylfurfural from cellulose by high concentration of sulfates in biphasic system. Green Chem. 2013, 15, 1967–1974. (4) Liao, Y.; Liu, Q.; Wang, T.; Long, J.; Ma, L.; Zhang, Q. Zirconium phosphate combined with Ru/C as a highly efficient catalyst for the direct transformation of cellulose to C6 alditols. Green Chem. 2014, 16, 3305–3312. (5) Zhang, Z.; Song, J.; Han, B. Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids. Chem. Rev. 2017, 117, 6834–6880.


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Graphical abstract

Synopsis: The effect of interfacial sites in Ni-LDO catalyst was investigated for high-efficient production of renewable H2 from aqueous phase reforming of cellulose.


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