CeO2-SiO2 catalysts and their performance in catalytic partial oxidation of methane to syngas

Chinese Journal of Catalysis 35 (2014) 8–20 



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Preparation and characterization of Ni/CeO2‐SiO2 catalysts and their performance in catalytic partial oxidation of methane to syngas Jiubiao Hu a, Changlin Yu a,*, Yadong Bi b,#, Longfu Wei a, Jianchai Chen a, Xirong Chen a School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China

a

b

  A R T I C L E I N F O



Article history: Received 23 July 2013 Accepted 17 September 2013 Published 20 January 2014 Keywords: Sol‐gel process Ceria‐silica composite oxide Supported nickel catalyst Partial oxidation of methane Syngas

A B S T R A C T



Hexahydrated cerium(ΙΙΙ) nitrate (Ce(NO3)3·6H2O) and tetraethyl orthosilicate (C8H20O4Si) were used as the precursors for the synthesis of a series of xCeO2‐(1−x)SiO2 (x = 0, 0.25, 0.5, 0.75, 1) com‐ posite oxides using a sol‐gel process under acidic conditions. The active component, Ni, was loaded on the as‐synthesized composite oxides, producing supported Ni catalysts for catalytic partial oxi‐ dation of methane to syngas. The properties of the as‐synthesized products, such as textural struc‐ ture, reduction behavior, surface acidity, and carbon deposition, were determined using N2 physical adsorption/desorption, X‐ray diffraction, scanning electron microscopy, ultraviolet‐visible diffuse reflectance spectroscopy, temperature‐programmed reduction by H2, temperature‐programmed desorption of NH3, and thermogravimetric analysis. The effects of catalyst composition, calcination temperature, and reaction time on the catalytic performance were investigated. The characteriza‐ tion results showed that these Ni/CeO2‐SiO2 catalysts have large surface area, small CeO2 crystals, weak acidity, and low carbon deposition. Highly dispersed NiO is present and is easy to be reduced. The Ni/CeO2‐SiO2 catalyst with a Ce/Si molar ratio of 1:1, w(Ni) = 10%, and calcined at 700 °C ex‐ hibited good stability and the highest CH4 conversion (~84%) and CO and H2 selectivity (> 87%). © 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

  1. Introduction Syngas is a raw material in the production of numerous chemical products, such as synthetic oils and olefins. Catalytic partial oxidation of methane (POM) is the main method used to produce syngas [1–7]. Designing and developing cheap cata‐ lysts with good catalytic performance and high selectivity for POM are important for the production of syngas from CH4. Supported noble‐metal (Pt [8], Pd [9], and Rh [10]), Ni‐ based [11,12], and Co‐based [5] catalysts are usually used in POM. The support is an important component of the catalyst and significantly affects the catalytic performance. The sup‐

ports generally used for POM catalysts include Al2O3 [13,14], SiO2 [15,16], CeO2 [17], ZrO2 [18], TiO2 [19], and CeZrO2 solid solution [20,21]. Among these, SiO2 shows good stability be‐ cause it does not easily react with the other components at high temperature. Moreover, dispersion of the supported active component over SiO2 is improved as a result of its large surface area. For example, Li et al. [22] reported that a Pd/SiO2 catalyst exhibited good activity in POM because of high dispersion of Pd. After reaction for 7 h at 700 °C, the Pd particle size was still 4–5 nm. Xia et al. [23] reported a spherical, single‐channel Ni/SiO2 catalyst with good Ni dispersion, synthesized by a sol‐gel process. This Ni/SiO2 catalyst showed high activity, se‐

* Corresponding author. Tel/Fax: +86‐797‐8312334; E‐mail: [email protected] # Corresponding author. Tel/Fax: +86‐22‐60214259; E‐mail: [email protected] This work was supported by the National Natural Science Foundation of China (21067004, 21263005, 21003095), Science and Technology Founda‐ tion of Jiangxi Provincial Department of Education (GJJ12344), Program of Jiangxi Provincial Young Scientists Cultivating Object (20122BCB23015), and Young Science and Technology Project of Jiangxi Province Natural Science Foundation (20133BAB21003). DOI: 10.1016/S1872‐2067(12)60723‐2 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 1, January 2014



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

lectivity, and long life in POM. Another porous, core‐shell Ni@SiO2 catalyst, with a narrow particle size distribution, was synthesized by Li et al. [24]. Activity tests showed that the cat‐ alytic performance of this core‐shell Ni@SiO2 catalyst is closely related to the size of the Ni core, the porosity of the SiO2 shell, and core‐shell interactions. The catalyst promoter is also crucial in POM because the reduction of the active component is greatly influenced by the interactions between the promoter and the active component. Moreover, such interactions also affect the stability of the cata‐ lyst. For example, Li et al. [25] found that Ce doping in Ni/SiO2 effectively inhibited the growth of Ni crystallites and improved Ni dispersion. Moreover, CeO2 easily reacts with other oxides to produce composite oxides, such as CeO2‐TiO2, CeO2‐ZrO2, CeO2‐Al2O3, and so on [26,27]. The produced composite oxides always have large surface areas and are rich in oxygen va‐ cancies, enabling better promotion by Ce in POM. In this work, a series of CeO2‐SiO2 composite oxides with large surface areas were synthesized using a sol‐gel process. The active component, Ni, was loaded on the as‐synthesized composite oxides, producing supported Ni catalysts. The effects of the Ce/Si molar ratio in the support, Ni content, calcination temperature, and reaction time on the catalytic performance of Ni/CeO2‐SiO2 in POM were investigated. 2. Experimental 2.1. Catalyst preparation All the chemicals used were analytically pure and obtained from the Sinopharm Chemical Reagent Co., Ltd. The CeO2‐SiO2 composite oxides were synthesized using a sol‐gel process. Stoichiometric hexahydrated cerium(ΙΙΙ) nitrate (Ce(NO3)3· 6H2O) was dissolved in ethanol, and the solution pH was main‐ tained at 2–3 using nitric acid. Stoichiometric tetraethyl ortho‐ silicate (TEOS, C8H20O4Si) was then added dropwise to the so‐ lution under stirring, producing a transparent gel. The pro‐ duced gel was aged naturally overnight, dried at 110 °C, ground, and calcined at 700 °C for 4 h in air, giving CeO2‐SiO2 composite oxide supports. The obtained supports were im‐ pregnated for 12 h with a stoichiometric nickel nitrate (Ni(NO3)2·6H2O) solution, dried, and calcined at different tem‐ peratures for 4 h in air. The obtained catalysts were denoted by wNi/xCeO2‐(1−x)SiO2‐T, where w represents the Ni content (mass fraction, 5%–20%), x refers to the Ce molar fraction (x = 0–1) in the support, and T stands for the calcination tempera‐ ture (600–900 °C) after impregnation with Ni.

2.2. Catalyst characterization The Brunauer‐Emmett‐Teller (BET) surface areas of the samples were obtained from N2 adsorption/desorption iso‐ therms determined at liquid‐N2 temperature (−196 °C) using an automatic analyzer (V‐Sorb 2800P, Beijing Jinaipu Company of Science and Technology). The samples were degassed at 200 °C for 1 h under vacuum before the measurements. X‐ray dif‐ fraction (XRD) measurements were carried out using a Holland



X’Pert Pro MPD X‐ray diffractometer (Philips, the Netherlands) equipped with a monochromatized Co Kα radiation source (λ = 0.1790955 nm). The accelerating voltage and the applied cur‐ rent were 40 kV and 120 mA, respectively. Scanning electron microscopy (SEM) was performed using a Nova Nano SEM230 (20 kV) scanning electron microscope (FEI company, USA). Ultraviolet‐visible (UV‐Vis) diffuse reflectance spectra (DRS) were obtained using a UV‐Vis spectrophotometer (UV‐2550, Shimadzu). The absorption spectra were referenced to BaSO4. Temperature‐programmed reduction by H2 (H2‐TPR) was car‐ ried out using a chemical adsorption instrument (TP5080, Tianjin Xianquan Industry and Trade Development Co., Ltd.). Samples (0.05 g) were placed in a quartz reactor and treated in situ with a flow of dry N2 (30 mL/min) at 300 °C for 0.5 h. Be‐ fore a run, the baseline was stabilized in the gas flow. The re‐ actor was heated in a temperature‐programmed furnace from room temperature to 800 °C (10 °C/min). A 10% (volume frac‐ tion) H2 in N2 mixture was used as the reducing gas. The H2 consumption as a function of the reduction temperature was continuously monitored using a thermal conductivity detector (TCD) cell and recorded. Temperature‐programmed desorp‐ tion of NH3 (NH3‐TPD) was carried out using the same chemical adsorption instrument as that used for H2‐TPR. Before the tests, the samples (0.1 g) were degassed in situ under a N2 stream at 300 °C for 0.5 h and then cooled to room temperature. The samples were then saturated with an NH3 flow (10 mL/min) at room temperature. The physically adsorbed NH3 was purged under a N2 stream until the TCD baseline was stabilized and then TPD was performed from room temperature to 800 °C at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed using a TGA Q50 thermogravimetric analyzer (TA Company, USA) to determine the deposited carbon content. After reaction, about 10 mg of each catalyst was degassed in a N2 stream (40 mL/min) and then tested from room tempera‐ ture to 750 °C at a heating rate of 10 °C/min in an air stream (60 mL/min). 2.3. Catalytic performance tests Catalytic performance tests were carried out using a WFSM‐3060 reaction apparatus (Tianjin Xianqian co, China) equipped with a continuous‐flow fixed‐bed quartz‐tube reactor of inner diameter 8 mm, operated under atmospheric pressure. Before the reaction, about 0.15 g of each catalyst was placed in the quartz reactor and reduced at 750 °C for 1 h in pure H2 stream (20 mL/min), followed by purging with N2 stream (20 mL/min) for 20 min. The reaction mixture CH4/O2 = 20/10 (30 mL/min) was fed to the reactor at a total gas hourly space ve‐ locity of 12000 mL/(h·g). After 30 min the reaction began, the composition of the gaseous products was analyzed using an online GC‐2060 chromatograph (Tengzhou Lunan Analysis Instrument Co, LTD, China) equipped with a TCD and a TDX‐101 packed column. The CH4 conversion, CO selectivity, H2 selectivity, and H2/CO molar ratio were calculated using the following formula, where Ai,out and Fi,out represent the chroma‐ tographic peak area and relative molar correction factor of gaseous component i, respectively. Here, FCH4 = 1, FCO = 38.5,



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

FCO2 = 1.9, FH2 = 0.35. X(CH4) = ([CH4]in–[CH4]out)/([CH4]out+[CO]out+[CO2]out)100% = (ACO,outFCO,out+ACO2,outFCO2,out)/(ACO,outFCO,out+ACO2,outFCO2,out+ACH4,out FCH4,out)100% S(CO) = [CO]out/([CH4]in–[CH4]out)100% = ACO,outFCO,out/(ACO,outFCO,out+ACO2,outFCO2,out)100% S(H2) = [H2]out/2([CH4]in–[CH4]out)100% = AH2,outFH2,out/2(ACO,outFCO,out+ACO2,outFCO2,out)100% n(H2)/n(CO) = [H2]out/[CO]out = AH2,outFH2,out/ACO,outFCO,out 3. Results and discussion 3.1. Catalyst characterization 3.1.1. BET surface area The BET surface area was calculated using the BET equation and the physical adsorption/desorption isotherms of N2; the surface areas of all the supports and catalysts are shown in Table 1. The data in Table 1 show that the surface area is close‐ ly related to the Ce molar fraction. The surface areas of the xCeO2‐(1−x)SiO2 composite oxide supports are larger than that of CeO2 alone. As the Ce molar fraction increases (x value from 0 to 1), the surface area decreases (from 470 to 20 m2/g). The Ce molar fraction is the key factor in obtaining CeO2‐SiO2 com‐ posite supports with large surface area. Increasing the calcina‐ tion temperature (from 600 to 900 °C) decreases the BET sur‐ face area of 0.50CeO2‐0.50SiO2 (from 196 to 96 m2/g). Increas‐ ing the Ni loading only leads to a slight decrease in the surface area (10%Ni/0.50CeO2‐0.50SiO2‐700, 100 m2/g; 20%Ni/ 0.50CeO2‐ 0.50SiO2‐700, 103 m2/g). The Ni is highly dispersed because of the large surface area of the 0.50CeO2‐0.50SiO2‐700 support (149 m2/g). The loaded Ni could enter some of the pores, causing minor decreases in the surface area [25,28–30]. It is notable that even after calcination at 900 °C for 4 h, the surface area of 10%Ni/0.50CeO2‐0.50SiO2‐ 900 still maintains 59 m2/g, which indicates that the high thermal stability and strong anti‐sintering capacity of the catalyst.









3.1.2. XRD (a)

CeO2 (b)

CeO2 NiO

Sample ABET a/(m2/g) SiO2‐700 470 0.25CeO2‐0.75SiO2‐700 260 0.50CeO2‐0.50SiO2‐700 149 0.75CeO2‐0.25SiO2‐700 68 CeO2‐700 20 0.50CeO2‐0.50SiO2‐600 196 0.50CeO2‐0.50SiO2‐800 131 0.50CeO2‐0.50SiO2‐900 96 10%Ni/SiO2‐700 546 10%Ni/0.25CeO2‐0.75SiO2‐700 137 10%Ni/0.50CeO2‐0.50SiO2‐700 100 10%Ni/0.75CeO2‐0.25SiO2‐700 63 10%Ni/CeO2‐700 42 20%Ni/0.50CeO2‐0.50SiO2‐700 103 10%Ni/0.50CeO2‐0.50SiO2‐600 140 98 10%Ni/0.50CeO2‐0.50SiO2‐800 10%Ni/0.50CeO2‐0.50SiO2‐900 59 a Calculated from the linear portion of the BET equation. 

The phase structure of the as‐prepared supports and cata‐ lysts was analyzed using XRD; the XRD patterns are shown in Fig. 1. Figure 1(a) shows that no clear diffraction peak from SiO2 is observed in the single‐component SiO2‐700 support, indicating that SiO2 is amorphous. Strong diffraction peaks from cubic fluorite CeO2 (Ref. code: 00‐034‐0394), at around 2θ = 33.3°, 38.7°, 55.8°, and 66.5°, are observed for the xCeO2‐(1−x)SiO2‐ 700 (x = 0.25, 0.50, 0.75, 1) supports, and the peak intensity increases with increasing Ce molar fraction. Fig‐ ure 1(b) shows that the diffraction peaks of cubic phase NiO (Ref. code: 01‐078‐0423), at around 2θ = 43.7°, 50.8°, and 74.7°, are detected for 10%Ni/0.50CeO2‐0.50SiO2‐700, and, as shown in Fig. 1(c), the peak intensity increases with increasing Ni content (from 10% to 20%). The XRD patterns of the 0.50CeO2‐ 0.50SiO2 and 10%Ni/0.50CeO2‐0.50SiO2 catalysts calcined at different temperature are shown in Fig. 1(d) and 1(e), respectively. The intensity of the CeO2 and NiO diffraction peaks increase with increasing calcination temperature.



CeO2 NiO

(c) 20%

Intensity

Table 1 Surface area of composite oxide supports and catalysts.



(e) CeO2 NiO

(d) CeO2 900 oC o

1 0.75

1

0.50 0.25

0.50 0.25

x=0

x=0

0.75

800 C 10% 700 oC w=0

600 oC

900 oC 800 oC 700 oC

600 oC

10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 90 2/( o ) 2/( o ) 2/( o ) 2/( o ) 2/( o ) Fig. 1. XRD patterns of composite oxide supports and catalysts. (a) xCeO2‐(1−x)SiO2 supports with different x values calcined at 700 °C; (b) 10%Ni/xCeO2‐(1−x)SiO2 catalysts with different x values calcined at 700 °C; (c) Ni/0.50CeO2‐0.50SiO2 catalysts with different Ni contents calcined at 700 °C; (d) 0.50CeO2‐0.50SiO2 supports calcined at different temperatures; (e) 10%Ni/0.50CeO2‐0.50SiO2 catalysts calcined at different temperatures.



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20



Table 2 Average crystallite size and CeO2 lattice parameters in composite supports and catalysts. Average crystallite size b (nm) Integral FWHM width a (rad) CeO2 lattice parameter c (nm) CeO2 NiO CeO2 NiO SiO2‐700 — — — — — 0.25CeO2‐0.75SiO2‐700 0.01609 — 10.32 — 0.5396 0.50CeO2‐0.50SiO2‐700 0.01613 — 10.30 — 0.5408 0.75CeO2‐0.25SiO2‐700 0.01282 — 12.96 — 0.5403 CeO2‐700 0.00665 — 24.97 — 0.5414 0.50CeO2‐0.50SiO2‐600 0.01753 — 8.068 — 0.5392 0.50CeO2‐0.50SiO2‐800 0.01068 — 13.24 — 0.5402 0.50CeO2‐0.50SiO2‐900 0.00726 — 22.89 — 0.5407 10%Ni/SiO2‐700 — 0.01415 — 12.25 — 10%Ni/0.25CeO2‐0.75SiO2‐700 0.01292 0.01419 12.86 12.42 0.5412 10%Ni/0.50CeO2‐0.50SiO2‐700 0.01452 0.00973 11.44 18.12 0.5405 10%Ni/0.75CeO2‐0.25SiO2‐700 0.01078 0.01388 15.41 12.70 0.5404 10%Ni/CeO2‐700 0.00656 0.01365 25.32 12.92 0.5416 20%Ni/0.50CeO2‐0.50SiO2‐700 0.01722 0.00843 9.641 20.89 0.5413 10%Ni/0.50CeO2‐0.50SiO2‐600 0.01971 0.01013 8.427 17.39 0.5410 10%Ni/0.50CeO2‐0.50SiO2‐800 0.01383 0.00916 12.01 19.67 0.5396 0.01023 0.00875 16.24 20.74 0.5406 10%Ni/0.50CeO2‐0.50SiO2‐900 a Obtained from X‐ray line broadening analyzed by X’pert Highscore Plus software. b Calculated from X‐ray line broadening of CeO2(111) and NiO(200) reflection analyzed by Scherrer formula. c Calculated from CeO2(111) plane reflection by using Bragg equation. —: Not determined because of the phase nature.  Sample

The Scherrer equation, D = 0.89λ/βcosθ, was used to esti‐ mate the average crystallite size of the samples, where β is the width in radians of the XRD peak at half‐height for the plane peak, λ is the X‐ray wavelength in nanometers (λ = 0.1790955 nm), θ is the angle between the incident and diffracted beams in degrees, and D is the average crystallite size of the powder sample in nanometers. The strongest diffraction peaks of CeO2 (2θ = 33.3°) and NiO (2θ = 50.8°) were used in the calculation. The obtained results are summarized in Table 2. The CeO2 and NiO particles are nanoscale. According to the literature [26,31], a mixture of CeO2 and SiO2 first forms a Ce9.33(SiO4)6O2 inter‐ mediate phase. During calcination, Ce9.33(SiO4)6O2 decomposes to nanoscale CeO2 particles, and the produced CeO2 is highly dispersed on the SiO2 matrix. The average crystallite size of CeO2 in catalysts is slightly larger than that in supports; this may be caused by the second calcination. High‐temperature (900 °C) calcination causes an increase in the size of the CeO2 and NiO crystallites, suggesting that aggregation or sintering of these particles occurs, and this could also be a major factor in the decrease in the BET surface area. The average crystallite size of CeO2 in CeO2‐700 and 10%Ni/CeO2‐700 is 0.5414 and

0.5416 nm, respectively, which are slightly larger than that of CeO2 in the CeO2‐SiO2 composite. This could be attributed to lattice constriction of CeO2 when Si4+ enters the CeO2 lattice and forms the CeO2‐SiO2 composite oxide, because the Si4+ radius (r = 0.042 nm) is smaller than the Ce4+ radius (r = 0.087 nm) [26]. Scheme 1 shows a proposed mechanism for the formation of the CeO2‐SiO2 composite oxide. The formation process could include several steps such as hydrolysis of TEOS, condensation, calcination, and dehydration [32,33]. First, under acidic condi‐ tions, hydrolysis of TEOS takes place, producing orthosilicic acid. The orthosilicic acid is transformed into a silicate polymer by condensation and dehydration. Ce3+ could be adsorbed by the hydroxy (–OH) groups in the silicate polymer. Drying and calcination produce a CeO2‐SiO2 composite oxide with good dispersion and a large BET surface area. 3.1.3. SEM The morphology of typical supports and catalysts was ana‐ lyzed using SEM; the SEM images are shown in Fig. 2. As shown in Fig. 2(a), the SiO2‐700 support consists of a large number of aggregated particles. It can be seen from Fig. 2(b) that there are OH

OC2H5 H5C2O

Si OC2H5

Hydrolyzation

OC2H5 OH O Si O Hydrogen bond

HNO3

OH

Covalent bond H

O

O

Ce3+

Ce3+

Condensation -H2O Physical adsorption

OH

Si O

H H

H

HO Si OH

OH O

O

OH

-H2O

O Si O O

Ce3+

O Si O

OH Si O

OH OH

OH OH

Ce3+

Ce3+

OH Si O O

Drying Calcination

Ce3+

Scheme 1. Proposed mechanism of CeO2‐SiO2 composite oxide formation.

O

O

O

Ce

Si O

O

O

 



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

(a)

(b)

(c)

Fig. 2. SEM images of typical supports and catalysts. (a) SiO2‐700; (b) 0.50CeO2‐0.50SiO2‐700; (c) 10%Ni/0.50CeO2‐0.50SiO2‐700. 

no obvious morphological differences between SiO2‐700 and the 0.50CeO2‐0.50SiO2‐700 composite support. Figure 2(c) shows that after Ni loading (w = 10%), small particles appear on the surface of 0.50CeO2‐0.50SiO2‐700. These are NiO parti‐ cles that were reduced to metallic Ni0 by H2 before POM.

3.1.5. H2‐TPR The H2‐TPR profiles of the catalysts are shown in Fig. 4. The reduction of the CeO2‐SiO2 composite support was first tested to determine whether its reduction peaks overlap with the (a)

H2 consumption

w = 20% w = 15%

0

(5) (4) (3) 297

200

300

x = 0.25

w=0

x=0

700

800

reduction peaks of NiO. Figure 4(a) shows that 0.50CeO2‐ 0.50SiO2‐ 700 has a weak H2 consumption peak at around 490 °C. This peak results from the reaction between H2 and surface lattice oxygen on CeO2 or adsorbed oxygen [28–31,36]. The reduction peak at 780 °C is ascribed to reduction of bulk CeO2 containing large crystallites [36–41] because SiO2 cannot be reduced below 800 °C [36]. The NiO reduction peaks appear at 325–580 °C, and they become stronger with increasing Ni con‐ tent.



x = 0.75

w = 5%

400 500 600 Wavelength (nm)

Fig. 3. UV‐Vis DR spectra of catalysts. (1) 10%Ni/SiO2‐700; (2) 10%Ni/0.25CeO2‐0.75SiO2‐700; (3) 10%Ni/0.50CeO2‐0.50SiO2‐700; (4) 10%Ni/0.75CeO2‐0.25SiO2‐700; (5) 10%Ni/CeO2‐700; (6) CeO2‐700.

x=1

x = 0.50

(2) (1)

(b)

w = 10%

100 200 300 400 500 600 700 8000 Temperature (oC)

(6)

F(R)

3.1.4. UV‐Vis DRS DRS was used to identify Ni species on the catalyst surface. Fig. 3 shows the UV‐Vis DR spectra of typical catalysts. The UV‐Vis DR spectrum of pure CeO2 is also shown. It can be seen from this figure that CeO2 has an obvious absorption peak at around 343 nm, and NiO absorption peaks are observed in the region from 297 to 350 nm. The characteristic absorption peaks of NiO are attributed to charge transfer by octahedral Ni2+ among NiO lattices [25,34,35]. A comparison of Fig. 3(a) with Fig. 3(b)–(e) shows that the peaks in the UV‐Vis DR spec‐ tra of the Ni species in 10%Ni/xCeO2‐(1−x)SiO2‐700 (x = 0.25, 0.50, 0.75, 1) shift to slightly longer wavelengths, indicating that less energy is needed for charge transfer, as a result of interactions between Ni species and the xCeO2‐(1−x)SiO2‐700 composite support [25,30]. These interactions also affect the reduction performance of the catalysts.

343 326

(c) 900 oC 800 oC 700 oC

100 200 300 400 500 600 700 8000 Temperature (oC)

600 oC 100 200 300 400 500 600 700 800 Temperature (oC)

Fig. 4. H2‐TPR profiles of catalysts. (a) Ni/0.50CeO2‐0.50SiO2‐700 with different Ni contents; (b) 10%Ni/xCeO2‐(1−x)SiO2‐700 with different x values; (c) 10%Ni/0.50CeO2‐0.50SiO2 calcined at different temperatures.

Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

The effects of the Ce/Si molar ratio on the reduction per‐ formance of the catalysts are shown in Fig. 4(b). Two reduction peaks are observed at 365–670 °C for 10%Ni/SiO2‐700 and at 250–470 °C for 10%Ni/CeO2‐700. The reduction temperature of NiO supported on the xCeO2‐(1−x)SiO2‐700 composite sup‐ port (x = 0.25, 0.50, 0.75) is lower than that for Ni/SiO2‐700, suggesting that Ni is more easily reduced over xCeO2‐ (1−x)SiO2‐700. The interactions between CeO2 and NiO could benefit NiO reduction. Moreover, oxygen vacancies in CeO2‐SiO2 could increase the lattice oxygen mobility, making the reduc‐ tion of Ni species easier [20]. Figure 4(c) shows the influence of the calcination tempera‐ ture on the reduction performance of 10%Ni/0.50CeO2‐ 0.50SiO2. An increase in the calcination temperature causes the reduction peak of NiO to shift toward higher temperature. This shift indicates that reduction of NiO is difficult; this could be a result of the strong interactions between CeO2 and SiO2 formed during high‐temperature calcination [29,42]. Increases in the size of the CeO2 and NiO crystallites (see Table 2) could also help to make NiO reduction difficult.

3.1.6. NH3‐TPD The surface acidity of typical supports and catalysts was tested using NH3‐TPD; the TPD profiles are shown in Fig. 5. For SiO2‐700, two NH3 desorption peaks are observed at around 90 °C and 450 °C, corresponding to weak and strong acid sites, respectively. The TPD profile of 10%Ni/SiO2‐700 is consistent with that of SiO2‐700. Compared with SiO2‐700, the 0.50CeO2‐ 0.50SiO2‐700 has a smaller weak acid desorption peak, and the strong acid desorption peak almost disappears, indicating that the acidity is weakened by the formation of the composite support; this could be caused by the alkalinity of Ce. Ni loading (10%) on 0.50CeO2‐0.50SiO2‐700 causes no obvious difference in the TPD profile. In POM, carbon species and coke are easily deposited on strong acid centers [2]. The weak acidity of the 10%Ni/0.50CeO2‐0.50SiO2‐700 catalyst could contribute to its low carbon deposition in POM.

3.1.7. TGA Typical catalysts were analyzed after POM using TGA; the TG profiles are shown in Fig. 6. The mass losses below 130 °C, in the region 130–350 °C, and above 600 °C are produced by

Intensity

450

SiO2-700 10%Ni/SiO2-700

0.50CeO2-0.50SiO2-700 10%Ni/0.50CeO2-0.50SiO2-700 100

101 100

(3)

99

(2)

98 97

7.1%

(1)

96 95 94 93

100

200

300 400 500 600 Temperature (oC)

700

800

Fig. 6. TG profiles of the catalysts after reaction. (1) 10%Ni/SiO2‐700; (2) 10%Ni/0.50CeO2‐0.50SiO2‐700; (3) 20%Ni/0.50CeO2‐0.50SiO2‐700.

desorption of adsorbed water and amorphous carbon, combus‐ tion of filamentous carbon, and graphitic carbon combustion, respectively [43,44]. However, a mass increase is also observed in the region 350–570 °C; this is caused by oxidation of Ni0. The smaller mass losses below 130 °C for 10%Ni/0.50CeO2‐ 0.50SiO2‐700 and 20%Ni/0.50CeO2‐0.50SiO2‐700 compared with that for 10%Ni/SiO2‐700 imply that smaller amounts of water and amorphous carbon are adsorbed on these catalysts, and their more obvious mass increases indicate that there are larger numbers of active Ni0 centers in these two catalysts after reaction. Above 450 °C, no obvious mass loss is observed for 10%Ni/0.50CeO2‐0.50SiO2‐700, whereas above 440 °C, a mass loss of about 7.1% occurs for 20%Ni/0.50CeO2‐0.50SiO2‐700, suggesting that there is a large amount of deposited carbon on 20%Ni/0.50CeO2‐0.50SiO2‐700. Previous research [45–47] showed that the average coke formation rate is closely related to the crystallite size and dispersion of Ni. Smaller Ni crystal‐ lites and higher dispersion of Ni could significantly decrease coke formation. Moreover, Fig. 6 also shows that carbon depo‐ sition increased with increasing Ni content. The increase in Ni content increases the Ni crystallite size, and large Ni crystallites easily become active centers for carbon deposition reactions.



3.2. Catalytic performance 3.2.1. Effects of Ni content Table 3 shows the CH4 conversion and CO and H2 selectivity

90

0



102

Mass (%)



200

300 400 500 Temperature (oC)

600

700

800

Fig. 5. NH3‐TPD profiles of typical supports and catalysts.

Table 3 Catalytic performance of Ni/0.50CeO2‐0.50SiO2‐700 with different Ni contents in POM reaction. X(CH4) S(CO) S(H2) S(CO2) n(H2) /% /% /% /% /n(CO) 0.50CeO2‐0.50SiO2‐700 23.7 8.6 4.5 91.4 — 5%Ni/0.50CeO2‐0.50SiO2‐700 71.6 79.3 87.1 20.7 1.93 84.3 87.4 93.3 12.6 2.04 10%Ni/0.50CeO2‐0.50SiO2‐700 15%Ni/0.50CeO2‐0.50SiO2‐700 74.1 80.3 88.4 19.7 2.06 20%Ni/0.50CeO2‐0.50SiO2‐700 70.9 77.8 84.1 22.2 2.11 Reaction conditions: 150 mg catalyst, CH4/O2 = 20/10 (30 mL/min), gas hourly space velocity = 12000 mL/(h∙g), reaction temperature 750 °C, reaction time 190 min. 

Sample



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

Table 4 Catalytic performance of 10%Ni/xCeO2‐(1−x)SiO2‐700 with different x values in POM reaction.

Table 5 Catalytic performance of 10%Ni/0.50CeO2‐0.50SiO2 calcined at differ‐ ent temperatures in POM reaction.

X(CH4) S(CO) S(H2) S(CO2) n(H2) /% /% /% /% /n(CO) 10%Ni/SiO2‐700 76.1 81.9 87.0 18.1 2.12 10%Ni/0.25CeO2‐0.75SiO2‐700 78.3 84.1 89.0 15.9 2.05 10%Ni/0.50CeO2‐0.50SiO2‐700 84.3 87.4 93.3 12.6 2.04 10%Ni/0.75CeO2‐0.25SiO2‐700 82.2 85.0 90.9 15.0 2.03 10%Ni/CeO2‐700 77.2 81.0 87.9 19.0 1.98 Reaction conditions: 150 mg catalyst, CH4/O2 = 20/10 (30 mL/min), gas hourly space velocity = 12000 mL/(h∙g), reaction temperature 750 °C, reaction time 190 min.

X(CH4) S(CO) S(H2) S(CO2) n(H2) /% /% /% /% /n(CO) 10%Ni/0.50CeO2‐0.50SiO2‐600 77.2 83.3 90.3 16.7 1.94 10%Ni/0.50CeO2‐0.50SiO2‐700 84.3 87.4 93.3 12.6 2.04 10%Ni/0.50CeO2‐0.50SiO2‐800 73.2 80.3 87.8 19.7 2.01 10%Ni/0.50CeO2‐0.50SiO2‐900 71.4 78.6 85.1 22.4 2.05 Reaction conditions: 150 mg catalyst, CH4/O2 = 20/10 (30 mL/min), gas hourly space velocity = 12000 mL/(h∙g), reaction temperature 750 °C, reaction time 190 min. 

Sample

Sample

0.50SiO2‐700, which has the same Ni content (10%), shows the highest CH4 conversion and CO and H2 selectivity. The BET surface area, H2‐TPR, and NH3‐TPD results show that 10%Ni/0.50CeO2‐0.50SiO2‐700 has a large surface area and weak acidity, and the loaded Ni is easily reduced to Ni0; these are the main reasons for its high catalytic performance.

for 0.50CeO2‐0.50SiO2‐700‐supported catalyst with different Ni contents. From the data in Table 3, it can be seen that the 0.50CeO2‐0.50SiO2‐700 support gives low activity. The CH4 conversion is 23.7%, and the CO and H2 selectivity is only 8.6% and 4.5%, respectively, indicating that the major reaction on this support is CH4 combustion. A loading of 5% Ni greatly im‐ proves the activity and selectivity: the CH4 conversion is 71.6%, and the CO and H2 selectivity increases to 79.3% and 89.0%, respectively. Loading of 10% Ni gives the highest CH4 conver‐ sion (84.3%), CO selectivity (87.4%), and H2 selectivity (93.3%). With further increases in the Ni content (20% Ni), the activity and selectivity decrease again. A very low Ni content (5%) does not provide enough active Ni0 centers for POM. However, a very high Ni content (20%) leads to large Ni crys‐ tallites (see Table 2) and higher carbon deposition (in Fig. 6), which impair the catalytic performance.

3.2.3. Effects of calcination temperature Table 5 shows the catalytic performance over the catalysts calcined at different temperatures. The highest CH4 conversion and product selectivity were obtained over 10%Ni/ 0.50CeO2‐ 0.50SiO2‐700. A very high calcination temperature results in an obvious decrease in the BET surface area and growth of CeO2 and NiO crystallites. Moreover, after very‐high‐temperature calcination, reduction of NiO becomes difficult, and the number of Ni0 active centers decreases. The catalytic performance of 10%Ni/0.50CeO2‐0.50SiO2‐900 was therefore poor.



3.2.2. Effects of Ce/Si molar ratio The effects of Ce/Si molar ratio on the catalytic performance in POM are shown in Table 4. 10%Ni/SiO2‐700 and 10%Ni/ CeO2‐700 show low catalytic activity, but 10%Ni/ 0.50CeO2‐

(2)

70 60

0 100 200 300 400 500 600 700 800 900 Time on stream (min) 100 (d)

(c) (2)

30 20

(1)

10 0 100 200 300 400 500 600 700 800 900 Time on stream (min)



90

(1)

(2)

70 60 50

0 100 200 300 400 500 600 700 800 900 Time on stream (min)

(2)

70 60

2.5

(1)

90 80

(b)

80

0 100 200 300 400 500 600 700 800 900 Time on stream (min)

n(H2)/n(CO)

CO2 selectivity (%)

40

(a)

80

50

50

(1)

H2 selectivity (%)

CH4 conversion (%)

90

3.2.4. Stability tests The changes in CH4 conversion, and CO and H2 selectivity with increasing reaction time over 10%Ni/0.50CeO2‐0.50SiO2‐ 700 and 10%Ni/SiO2‐700 are shown in Fig. 7. For 10%Ni/SiO2‐



CO selectivity (%)





(e)

(1)

2.0 (2) 1.5 1.0

0 100 200 300 400 500 600 700 800 900 Time on stream (min)

Fig. 7. Comparison of catalytic performance of 10%Ni/0.50CeO2‐0.50SiO2‐700 (1) and 10%Ni/SiO2‐700 (2) in POM reaction. (a) CH4 conversion; (b) CO selectivity ; (c) CO2 selectivity; (d) H2 selectivity; (e) H2/CO molar ratio. Reaction conditions: 150 mg catalyst, CH4/O2 = 20/10 (30 mL/min), gas hourly space velocity = 12000 mL/(h·g), temperature 750 °C, reaction time 900 min.



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

700, when the reaction time is prolonged to 390 min, the CH4 conversion and CO and H2 selectivity decrease sharply, sug‐ gesting poor stability. However, for 10%Ni/0.50CeO2‐ 0.50SiO2‐ 700, there is almost no change in the CH4 conversion and CO and H2 selectivity during POM for 900 min, implying good sta‐ bility.





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[12] Xia W S, Chang G, Hou Y H, Weng W Z, Wan H L. Acta Phys‐Chim

Sin, 2011, 27: 1567

A series of CeO2‐SiO2 composite oxide supports with large surface area were synthesized using a sol‐gel process. After Ni loading, the produced Ni/CeO2‐SiO2 catalysts exhibited high catalytic performance in POM. The catalytic performance of Ni/CeO2‐SiO2 was closely related to the Ni content, Ce/Si molar ratio, and calcination temperature. The experimental results showed that the highest CH4 conversion and product selectivity and good stability were obtained over Ni/CeO2‐SiO2 calcined at 700 °C, with a Ce/Si molar ratio of 1:1 and a Ni content of 10%. The large BET surface area, weak acidity, ease of NiO reduction, and low carbon deposition are the main reasons for its high catalytic performance.

[13] Bayrakdar E, Altınçekiç T G, Öksüzömer M A F. Fuel Process Tech‐

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Graphical Abstract Chin. J. Catal., 2014, 35: 8–20 doi: 10.1016/S1872‐2067(12)60723‐2 Preparation and characterization of Ni/CeO2‐SiO2 catalysts and their performance in catalytic partial oxidation of methane to syngas Jiubiao Hu, Changlin Yu *, Yadong Bi *, Longfu Wei, Jianchai Chen, Xirong Chen Jiangxi University of Science and Technology; Tianjin University of Technology

O2 Ce(NO 3)3

Calcination Impregnation

o

750 C Ni0

Ni0

Ni/CeO2-SiO2

1 atm

3.0 2.8

Con. (CH4) Sel. (CO) Sel. (CO2) Sel. (H2) n(H2)/n(CO)

2.6 2.4 2.2 2.0

n(H2)/n(CO)

CO+H2

CH4

100 90 80 70 60 50 40 30 20 10 0

Conversion & Selectivity (%)

TEOS

1.8 0 100 200 300 400 500 600 700 800 900 Time on stream (min)

1.6



A catalyst consisting of Ni (10%) loaded on a CeO2‐SiO2 composite support synthesized using a sol‐gel process exhibited high perfor‐ mance in the catalytic partial oxidation of methane to syngas, as a result of its large BET surface area, weak acidity, high dispersion, easy of Ni reduction, and low carbon deposition.



Jiubiao Hu et al. / Chinese Journal of Catalysis 35 (2014) 8–20

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